Photoluminescence Mechanisms and Optical Sensing Applications of MXene Quantum Dots for Pesticide Detection in Vegetable Matrices

ABSTRACT

The widespread occurrence of pesticide residues and inorganic contaminants in vegetables requires rapid, sensitive, and reliable analytical approaches for food safety monitoring. MXene quantum dots (MQDs) have attracted significant attention as advanced luminescent nanomaterials due to their tunable photoluminescence, surface chemistry, and strong interfacial interactions. This review provides a mechanism‐oriented overview of MQDs‐based optical sensing platforms for detecting contaminants in vegetable matrices, emphasizing photoluminescence behavior and signal modulation mechanisms. The influence of key structural parameters, including quantum confinement, surface terminations, heteroatom doping, and defect engineering, is discussed in relation to emission properties and sensing performance. Fundamental photophysical processes, including charge transfer, energy transfer, and inner filter effects (IFE), are analyzed to clarify fluorescence responses during analyte recognition. Various sensing strategies, such as fluorescence probes, ratiometric sensors, dual‐mode optical platforms, and MQDs‐assisted nanozyme systems, are highlighted, demonstrating high sensitivity and practical applicability. In hybrid nanozyme platforms, MQDs primarily act as interfacial electronic mediators and catalytic enhancers by facilitating charge transfer and improving interactions with catalytic components rather than functioning as independent catalytic centers. Remaining challenges include matrix interference, reproducibility, stability, and mechanistic understanding. Future perspectives focus on integrating MQDs with portable devices and data‐driven technologies for real‐time food safety monitoring.

MXene quantum dots enable highly sensitive optical sensing of pesticide residues and inorganic contaminants through tunable photoluminescence, surface engineering, and interfacial charge‐transfer mechanisms. This review establishes a mechanistic framework linking MQD design with fluorescence modulation for reliable food safety monitoring.

1. Introduction

Vegetables constitute an essential part of the human diet, providing vital micronutrients, fibers, and phytochemicals that sustain health and regulate metabolic functions [1, 2]. However, intensive agricultural practices, including the excessive use of chemical fertilizers and pesticides, have resulted in the accumulation of inorganic contaminants (such as cadmium, lead, arsenic, and mercury) and organic pesticide residues within vegetable matrices. These pollutants not only threaten consumer health but also compromise agricultural sustainability. Chronic exposure to trace levels of these substances has been linked to neurological disorders, endocrine disruption, and carcinogenic risks. Consequently, rapid and reliable monitoring of vegetable safety has become a global priority in both food science and environmental governance [3, 4, 5].

Conventional detection techniques such as gas chromatography–mass spectrometry, high mass spectrometry based performance liquid chromatography (HPLC), and atomic absorption spectroscopy provide excellent sensitivity and specificity; yet they require laborious sample pretreatment, expensive instrumentation, and skilled operators [6, 7, 8]. These constraints hinder their applicability for routine or on MQDs‐based site analysis, especially in developing agricultural systems. In response, optical sensing technologies have attracted attention as powerful alternatives that allow real time: label MQDs‐based free, and miniaturized monitoring. Among these, fluorescence based and colorimetric approaches are particularly valuable due to their visual readability, short response time, and compatibility with microfluidic or portable devices [9, 10, 11].

Recent advances in nanomaterials have fundamentally expanded the horizon of optical sensors by enabling rational manipulation of photonic and electronic interactions at the nanoscale. Quantum dots (QDs), carbon nanodots, and transition MQDs‐based metal dichalcogenides have been widely studied for such purposes [12, 13]. Yet, limitations regarding toxicity, photostability, and manufacturing cost still restrict their widespread use in food and environmental analysis. In this context, MQDs have emerged as an exciting new class of two MQDs‐based dimensional (2D) nanostructures offering synergistic advantages of metallic conductivity, abundant surface terminations, and quantum confinement behavior. Their general formula, M _n ₊₁X _n T _x (where M represents a transition metal, X denotes carbon or nitrogen, and T _x refers to functional surface groups), allows exceptional tunability in electronic and optical properties through engineering of composition, size, and surface chemistry [14, 15, 16, 17].

MQDs exhibit superior photoluminescence (PL) intensity and versatile excitation/emission characteristics that stem from their well‐defined MQDs‐based electronic band structures and surface defect sites. Moreover, their hydrophilicity and biocompatibility—derived from terminal groups such as –OH, –O, and –F—facilitate efficient dispersion in aqueous vegetable extracts without extensive organic modification [18, 19]. These intrinsic features make them particularly suitable for detecting minute traces of contaminants in complex biological matrices, where phenomena such as IFE, photoinduced electron transfer (PET), or fluorescence resonance energy transfer (FRET) can be exploited to translate molecular recognition events into measurable optical signals [20, 21, 22].

In addition to conventional fluorescence sensing, MQDs‐based materials enable multichannel and ratiometric readouts, minimizing the influence of environmental fluctuations and matrix noise. Incorporation of heteroatom dopants or surface coupling with organic ligands further enhances selectivity toward specific analytes such as carbamate pesticides or heavy MQDs‐based metal cations. Importantly, several studies have demonstrated the effectiveness of MQDs sensors in real vegetable samples, achieving detection limits below regulatory thresholds set by agencies such as the WHO and FAO [23, 24, 25, 26]. These findings illustrate the remarkable potential of MQDs as both transduction elements and signal modulators within next MQDs‐based generation food monitoring platforms.

Despite these achievements, there remain critical challenges preventing the routine application of MQDs‐based optical sensors. Reproducibility of synthesis routes, sensitivity to matrix interference and limited standardization of analytical protocols are among the main barriers to commercialization and regulatory acceptance. Moreover, the mechanistic understanding of photophysical interactions in MQDs is still evolving; thus fundamental exploration of charge dynamics, surface defect engineering, and long MQDs‐based term stability remains essential [27, 28]. The integration of these materials with portable and intelligent sensing systems, including smartphone MQDs‐based detectors and machine MQDs‐based learning MQDs‐based assisted data analysis, could bridge the gap between laboratory research and agricultural practice.

This review, therefore, aims to establish a detailed conceptual framework surrounding the role of MQDs in monitoring pesticides and inorganic contaminants specifically within vegetable matrices. As the first comprehensive overview devoted to this emerging intersection of quantum materials and food analysis, it encompasses the design principles, optical modulation mechanisms, analytical applications, and future directions of MQDs‐based optical sensors. By synthesizing existing evidence and identifying present challenges, the paper aspires to facilitate the translation of MQDs research into practical strategies for improving vegetable safety monitoring and public health protection.

2. Design Principles of MQDs for Optical Sensing Applications

2.1. Structural Characteristics and Quantum Confinement in MQDs

In the context of MXene‐derived nanomaterials used in this review, it is important to clearly distinguish between different structural forms that may coexist during synthesis or processing. MXene sheets refer to two‐dimensional (2D) layered structures, either in their bulk form or as exfoliated nanosheets, which retain the characteristic stacked morphology of MAX‐derived materials. In contrast, MQDs are zero‐dimensional (0D) nanostructures with typical lateral dimensions below ~10 nm, in which quantum confinement effects dominate their electronic and optical behavior. Finally, oxidized MXene fragments represent partially degraded or chemically transformed by‐products formed during etching, oxidation, or harsh processing conditions, which may exhibit altered or even non‐quantum‐confined electronic properties. Distinguishing between these entities is essential, as they differ significantly in structural dimensionality, surface chemistry, and photophysical response, and therefore play fundamentally different roles in optical sensing applications.

MQDs represent the zero‐dimensional MQDs‐based derivatives of two MQDs‐based dimensional MXenes, typically produced by downsizing layered transition MQDs‐based metal carbides or nitrides into nanocrystals with lateral dimensions generally below 10 nm. At this scale, the electronic structure of the parent MXene undergoes pronounced modification due to quantum confinement and edge MQDs‐based dominated electronic states. The reduction in dimensionality increases the density of unsaturated surface atoms and defect sites, which substantially alters the density of states near the Fermi level. These features give rise to size‐MQDs‐based dependent optical properties, including tunable absorption bands and PL emission [29, 30].

Quantum confinement in MQDs restricts electron–hole pair motion within nanoscale domains, resulting in discrete energy levels rather than the continuous bands typical of bulk MXenes. Consequently, the bandgap can be modulated by controlling particle size, crystallinity, and edge configuration during synthesis. Smaller MQDs typically exhibit wider bandgaps and blue MQDs‐based shifted PL, while larger particles display narrower bandgaps and red MQDs‐based shifted emission [31, 32]. This tunability is particularly advantageous for designing optical nanosensors with adjustable excitation and emission wavelengths.

Another critical structural aspect is the abundance of edge sites in MQDs compared with their two‐dimensional MQDs‐based counterparts. These edge regions often contain undercoordinated transition metal atoms and functional groups that can influence charge distribution and optical transitions. As a result, MQDs frequently exhibit stronger PL than bulk MXenes, which are generally considered weakly emissive materials. The enhanced optical activity provides a fundamental basis for the development of MQDs‐based optical sensing platforms [32, 33, 34]. Overall, the structural miniaturization of MXenes into QDs not only modifies their electronic configuration but also introduces highly reactive interfaces that facilitate optical signal modulation. Understanding these structural features is therefore essential for rationally designing MQDs materials with optimized optical responses for sensing applications.

Figure 1 illustrates the structural transformation of Ti₂N MXene from two‐dimensional nanosheets into zero‐dimensional MQDs and provides direct evidence of the size‐dependent structural features responsible for quantum confinement effects. The SEM, TEM, and HR‐TEM images (Figure 1a–c) demonstrate the preserved layered morphology and crystalline framework of the parent Ti₂N nanosheets, while elemental mapping (Figure 1d–f) confirms their chemical composition after exfoliation. The XRD pattern (Figure 1g) further verifies the structural modification associated with the removal of the MAX‐derived layered features. Following ultrasonic size reduction, the formation of ultrasmall Ti₂N MQDs is confirmed by TEM and HR‐TEM analyses (Figure 1h,i), showing nanoscale dimensions and retained lattice characteristics. The statistical size distribution (Figure 1j) and AFM thickness analysis (Figure 1k,l) reveal the reduced dimensionality and abundant surface/edge regions of MQDs. These structural characteristics, including nanoscale confinement, increased edge density, and defect‐rich interfaces, are critical factors governing the electronic structure modulation and PL behavior of MQDs in optical sensing applications.

FIGURE 1.

Structural characterization and dimensional transformation of Ti₂N‐derived MQDs. (a–c) Morphology and crystal structure of Ti₂N nanosheets; (d–f) elemental mapping; (g) XRD analysis; (h,i) TEM and HR‐TEM images of Ti₂N MQDs; (j) size distribution; (k,l) AFM morphology and thickness profile confirming nanoscale MQD formation. Adapted with permission from Ref. [33]. 2020 Elsevier B.V.

2.1.1. Experimental Validation of MQDs as Zero‐Dimensional Nanostructures

The classification of MQDs as zero‐dimensional (0D) nanostructures requires rigorous experimental verification beyond fluorescence observation or nanoscale fragmentation. In well‐established studies, the 0D nature of MQDs is confirmed through a combination of structural, morphological, and spectroscopic characterizations that collectively distinguish them from two‐dimensional MXene sheets or amorphous oxidized fragments. Transmission electron microscopy (TEM) is the primary technique used to verify particle size distribution, typically revealing quasi‐spherical nanodots with lateral dimensions below ~10 nm and narrow dispersity. This isotropic morphology is a key indicator of dimensional confinement in all spatial directions, which is fundamentally different from layered MXene nanosheets.

Atomic force microscopy (AFM), when available, further supports this assignment by demonstrating sub‐nanometer to few‐nanometer height profiles, confirming complete breakdown of layered stacking into discrete nanoclusters. In addition, X‐ray diffraction (XRD) patterns of MQDs usually show significant peak broadening and reduced long‐range order compared to bulk MXenes, reflecting strong size reduction and loss of crystallinity associated with QD formation. High‐resolution TEM (HRTEM) may also reveal preserved lattice fringes within confined domains, indicating that crystalline order is retained at the nanoscale while overall dimensionality is reduced [30, 31, 32].

From an optical perspective, MQDs exhibit excitation‐dependent PL, size‐dependent emission shifts, and enhanced quantum yield (QY), which collectively reflect quantum confinement and surface‐state dominated emission. Time‐resolved fluorescence measurements further confirm the presence of discrete emissive centers with altered recombination dynamics compared to bulk or fragmented MXene structures. Importantly, these optical signatures alone are not sufficient for MQDs identification but must be interpreted alongside structural evidence.

Therefore, the assignment of MQDs as true 0D systems is based on a multi‐technique validation framework integrating TEM/AFM morphology, XRD structural disorder, and quantum‐confined photophysical behavior, ensuring a physically consistent distinction from MXene sheets and oxidized by‐products.

A comprehensive characterization of MQDs PL requires not only structural and spectral analysis but also quantitative photophysical parameters that validate emission mechanisms. Among these, QY, fluorescence lifetime, and absolute PL measurements are particularly important for establishing the efficiency and origin of emission processes. QY provides a direct measure of the efficiency of radiative versus nonradiative recombination pathways, thereby offering insight into the role of surface states and defect sites in governing emission intensity. Fluorescence lifetime analysis further distinguishes between different emissive pathways by revealing the temporal dynamics of excited‐state relaxation, which is essential for differentiating between core‐related emission, surface‐state emission, and defect‐induced recombination processes [28, 29, 30]. Additionally, absolute PL measurements enable standardized comparison of emission efficiencies across different MQDs systems, eliminating variations caused by instrumental or relative intensity‐based measurements. Despite their importance, these parameters are not consistently reported in many MQDs studies, including the present dataset, primarily due to experimental limitations or instrumentation constraints. Nevertheless, their inclusion would significantly strengthen the validation of proposed emission mechanisms and improve comparability across different MQDs systems. Therefore, future work should prioritize systematic measurement of QY, lifetime decay profiles, and absolute PL to establish a more rigorous and quantitative understanding of MQDs photophysics.

2.2. Surface Terminations and Interfacial Chemistry

A defining characteristic of MQDs is the presence of abundant surface termination groups inherited from the chemical exfoliation or etching processes used to produce their parent MXenes. Common surface terminations include –O, –OH, –F, and occasionally –Cl groups, depending on the synthesis route and post MQDs‐based treatment conditions. These functional groups play a decisive role in determining the physicochemical behavior of MQDs, particularly their optical properties, colloidal stability, and interfacial reactivity.

Surface terminations strongly influence the electronic structure of MQDs by modifying the local electron density around transition MQDs‐based metal centers. For instance, oxygen MQDs‐based terminated surfaces often enhance electronic delocalization and promote stronger optical transitions, while fluorine termination may introduce localized electronic states that affect PL behavior [35, 36, 37]. Density functional theory studies have demonstrated that the nature and distribution of these functional groups can alter the bandgap and optical absorption characteristics of MXene nanostructures.

From a sensing perspective, surface functional groups serve as the primary interaction interface between MQDs and surrounding chemical species. Their presence enables hydrogen bonding, electrostatic interactions, coordination chemistry, and π–π interactions with various analytes. Because MQDs possess a very high surface MQDs based to MQDs‐based volume ratio, even subtle changes in surface chemistry can significantly influence optical output signals. Consequently, controlling the density and type of surface terminations is a key strategy for tailoring MQDs responsiveness.

Furthermore, surface terminations contribute to the excellent dispersibility of MQDs in aqueous and polar media, which is crucial for solution MQDs‐based phase optical sensing platforms. The hydrophilic nature of –OH and –O terminations promotes stable colloidal suspensions, preventing aggregation that could otherwise quench fluorescence signals [38, 39, 40, 41]. By carefully tuning surface chemistry through chemical modification or post‐MQDs‐based synthetic treatments, MQDs can therefore be engineered to exhibit enhanced optical sensitivity and improved compatibility with analytical sensing environments.

The synthesis pathway and morphology of Ti₂CT _x MQDs are illustrated in Figure 2a,b. The schematic representation (Figure 2a) describes the HF‐free HCl etching strategy, where selective removal of Al from the Ti₂AlC MAX phase followed by exfoliation and downsizing leads to the formation of Ti₂CT _x MQDs with abundant surface terminations. The TEM image (Figure 2b) confirms the successful formation of ultrasmall and uniformly distributed MQDs with an average size of approximately 3 nm, demonstrating effective dimensional reduction from layered MXene structures to zero‐dimensional QDs.

FIGURE 2.

Synthesis and structural characterization of Ti₂CT _x MQDs. (a) Schematic illustration of the HF‐free HCl etching process used to convert Ti₂AlC MAX phase into Ti₂CT _x MQDs with surface terminations (–O, –OH, and –Cl). (b) TEM image showing highly dispersed MQDs with narrow size distribution (~3 nm). HRTEM image displaying lattice fringes with an interplanar spacing of ~0.218 nm corresponding to the (104) plane of Ti₂C. (d) High‐resolution XPS spectra of Al2p confirming the removal of the Al layer during etching. (e) FTIR spectra indicating the presence of oxygen‐containing surface functional groups. (f, g) Calculated density of states (DOS) of Ti₂AlC and Ti₂C MQDs, showing Ti 3d orbitals dominating near the Fermi level. Reproduced with permission from Ref. [40]. 2023 Wiley‐VCH GmbH.

Further insight into the crystallographic structure of the MQDs is provided by the high‐resolution TEM image in Figure 2c. Clear lattice fringes with an interplanar spacing of about 0.218 nm can be indexed to the (104) crystallographic plane of Ti₂C, confirming that the fundamental MXene lattice structure is preserved during the exfoliation and downsizing process [40]. Maintaining crystalline order at the nanoscale is important because it supports efficient charge transport and stable electronic states within the MQDs, both of which contribute to their optical activity.

The chemical composition and surface chemistry of the MQDs were investigated using XPS and FTIR analyses (Figure 2d,e). The disappearance of the Al–Ti bonding signal in the Al2p spectrum confirms the successful removal of the Al layer from the MAX precursor, indicating the formation of Ti₂CT _x MXene structures. In addition, FTIR spectra reveal characteristic vibrational bands associated with oxygen‐containing functional groups, including –O and –OH terminations. These surface groups are known to significantly influence the electronic structure of MQDs and provide active sites for intermolecular interactions, thereby playing a crucial role in governing their PL behavior and chemical sensing performance.

The electronic structure of the MQDs is further illustrated by the calculated density of states (DOSs) shown in Figure 2f,g. The DOS profile indicates that Ti 3d orbitals dominate the electronic states near the Fermi level, suggesting the presence of abundant accessible electronic states that facilitate charge transfer processes. Such electronic characteristics, combined with the high density of surface functional groups, enhance the interaction between MQDs and adsorbed molecules. This interplay between surface chemistry and electronic structure underpins the strong optical responsiveness of MQDs, making them highly promising materials for advanced optical sensing applications.

A more comprehensive understanding of MQDs structural characteristics requires correlating microscopic and colloidal analyses. TEM provides direct visualization of the physical core size of MQDs in the dry state. Statistical analysis of TEM images typically reveals a narrow size distribution centered on ~3 nm, confirming the successful formation of zero‐dimensional nanodots. In rigorous characterization practice, such TEM data should be presented not only as representative images but also as histograms and statistical parameters (mean size ± standard deviation), which reflect the homogeneity of the MQDs population.

In contrast, dynamic light scattering (DLS) measures the hydrodynamic diameter of MQDs in aqueous dispersion rather than their core dimensions. The DLS‐derived sizes are generally significantly larger (typically 50–150 nm in this work), which does not indicate aggregation artifacts alone but reflects the presence of solvation layers, surface termination groups (–OH, –O, –F), and dynamic interparticle interactions in solution. These surface‐bound species strongly influence the apparent hydrodynamic volume, particularly in MXene‐derived systems with high surface reactivity.

The observed discrepancy between TEM and DLS results is therefore not a contradiction but an intrinsic characteristic of colloidal MQDs. While TEM captures the rigid crystalline core, DLS reflects the effective size of MQDs as functional entities in liquid media, where hydrogen bonding, electrostatic interactions, and transient clustering may occur [38, 39, 40]. In heteroatom‐doped MQDs, these effects can be further amplified due to modified surface charge distribution and enhanced interparticle interactions. Accordingly, the integration of TEM and DLS analyses provides complementary insights into both structural confinement and colloidal behavior. This combined interpretation is essential for understanding MQDs stability, dispersion performance, and sensing reliability in complex aqueous and vegetable matrices.

High‐resolution TEM (HRTEM) analysis provides additional structural evidence supporting the partial crystallinity retention of MQDs after etching and size reduction. The observed lattice fringes with a typical interplanar spacing of ~0.218 nm can be indexed to the (104) crystallographic plane of Ti₂C, confirming that the fundamental lattice framework of the parent MXene is not completely destroyed during the transformation process. This indicates that MQDs preserve localized crystalline domains within their confined nanostructure, which is essential for maintaining their electronic activity and PL behavior. However, due to the strong downsizing and surface oxidation effects, these crystalline domains are embedded within highly defect‐rich and surface‐functionalized regions, leading to a hybrid structure composed of crystalline cores and disordered surface layers.

Selected area electron diffraction (SAED) further supports this interpretation by revealing broadened diffraction rings rather than sharp spots, indicating a transition from long‐range ordered MXene sheets to nanocrystalline or partially amorphous quantum‐confined domains. The diffuse nature of the SAED patterns confirms reduced crystallite size and partial loss of long‐range periodicity while still retaining identifiable lattice ordering at the nanoscale. This structural evolution is consistent with controlled etching and fragmentation processes, where complete amorphization is avoided and instead a nanocrystalline MQDs phase is formed.

Collectively, the combined HRTEM and SAED analyses demonstrate that the conversion of MXene sheets into MQDs does not result in full structural collapse but rather in a controlled dimensional reduction with partial crystallinity preservation [36, 37, 38]. This hybrid structural nature is crucial, as it allows MQDs to retain the electronic characteristics of the parent MXene while simultaneously exhibiting quantum confinement effects due to nanoscale size reduction. Therefore, MQDs should be regarded as structurally confined nanodomains with mixed crystalline–amorphous characteristics rather than fully amorphous fragments, which is essential for accurately interpreting their optical and sensing properties.

XPS plays a critical role in confirming the surface chemistry and electronic states of MQDs. Detailed peak deconvolution of Ti 2p spectra typically reveals multiple oxidation states, including Ti–C, Ti–O, and Ti–Ox components, indicating partial surface oxidation and formation of chemically active sites. Similarly, the C 1‐s spectrum is generally resolved into C–Ti, C–C, and C–O contributions, reflecting both preserved MXene lattice carbon and surface functionalization. The O 1‐s region further confirms the presence of –OH and –O terminations, which are essential for hydrophilicity and PL modulation. In F 1‐s spectra, signals corresponding to Ti–F bonds validate residual fluorine terminations originating from etching processes, while N 1‐s peaks (in doped systems) can be deconvoluted into pyridinic, pyrrolic, or graphitic nitrogen configurations, each contributing differently to electronic structure modification. These chemically resolved states are directly linked to the formation of emissive surface centers, as defect sites and functional groups act as trapping and recombination centers governing PL behavior. Therefore, XPS deconvolution provides essential mechanistic evidence connecting surface composition with optical performance in MQDs.

2.3. Heteroatom Doping and Defect Engineering

Tailoring the electronic and optical behavior of MQDs can be effectively achieved through heteroatom doping and controlled defect engineering. These strategies introduce additional electronic states or modify charge distribution within the MQDs lattice, enabling precise regulation of photophysical properties that are critical for optical sensing technologies. Heteroatom doping involves the incorporation of foreign atoms such as nitrogen, sulfur, phosphorus, or boron into the carbon or surface framework of MQDs. The presence of these dopants modifies the local electronic environment and can introduce donor or acceptor states within the band structure [39, 40]. Nitrogen doping, for example, often enhances PL intensity by promoting radiative recombination pathways, whereas sulfur or phosphorus doping may alter charge transfer characteristics and improve optical responsiveness.

Defect engineering represents another powerful approach for tuning MQDs functionality. Structural defects, including vacancies, lattice distortions, and edge irregularities, can act as localized energy traps that influence electron–hole recombination dynamics. Controlled generation of such defects can increase the number of emissive centers, thereby amplifying fluorescence signals. However, excessive defects may lead to nonradiative recombination and reduced optical performance, making precise control over defect density essential.

In addition to intrinsic defects formed during synthesis, post MQDs‐based synthetic treatments such as plasma exposure, thermal annealing, or chemical oxidation can intentionally modify defect distributions. These modifications can adjust optical band structures, alter carrier mobility, and influence charge transfer behavior at the MQDs surface [40, 41, 42, 43]. Collectively, heteroatom doping and defect engineering provide versatile tools for optimizing MQDs optical properties. By fine MQDs‐based tuning electronic states and recombination pathways, researchers can design MQDs with enhanced emission intensity, adjustable wavelengths, and improved signal stability, thereby establishing a robust material platform for advanced optical sensing systems.

Figure 3a illustrates the molecular interaction mechanism responsible for the fluorescence modulation of heteroatom‐doped Ti₃C₂ MQDs in aqueous media. Sulfur‐ and nitrogen‐doped MQDs (SN‐MQDs) form an extended hydrogen‐bonding network with surrounding water molecules, which effectively couples neighboring MQDs and stabilizes surface functional groups. This interfacial network leads to lateral expansion of individual MQDs, as corroborated by the HRTEM image in Figure 3b. The enlarged size and enhanced π‐electron delocalization reduce the optical bandgap, producing a distinct red‐shift in emission. These results highlight how heteroatom doping, together with surface‐driven interactions, can dynamically modify the electronic coupling and optical transitions of MQDs [41].

FIGURE 3.

(a) Schematic illustration of molecular interaction and light‐emission mechanism of heteroatom‐doped Ti₃C₂ MQDs in water. (b) HRTEM image of SN‐MQDs showing increased lateral size via hydrogen‐bonded network formation. (c) DLS size distributions of S‐, N‐, and SN‐MQDs demonstrate size‐dependent and dopant‐controlled fluorescence behavior.

Figure 3c presents DLS analyses revealing the size‐dependent photophysical behavior of the doped MQDs. SN‐MQDs and N‐MQDs, which participate in hydrogen‐bonded network formation, exhibit markedly larger hydrodynamic diameters (∼129 and 102 nm, respectively) and relatively lower QYs, indicating partial confinement relaxation due to network‐induced aggregation. Conversely, the smaller S‐MQDs (< 20 nm) retain independent dispersion and achieve the highest QY (∼28%), emphasizing that minimizing lateral dimensions and controlling surface chemistry are key to optimizing radiative efficiency. Collectively, the data demonstrate that heteroatom doping and defect‐mediated surface functionalization jointly regulate MQDs band structures, enabling tunable fluorescence through precise manipulation of size, bonding environments, and electronic delocalization.

2.4. Synthetic Strategies and Size Control of MQDs

The synthesis of MQDs is a crucial step that determines their size distribution, crystallinity, surface chemistry, and ultimately their optical behavior. Several synthetic strategies have been developed to convert layered MXenes into nanoscale QDs while preserving their unique chemical features. Top MQDs‐based down approaches remain among the most widely used techniques. In these methods, bulk MXene sheets are broken down into smaller fragments using chemical oxidation, hydrothermal treatment, ultrasonication, or solvothermal processes. Controlled oxidation is particularly effective in producing MQDs with well MQDs‐based defined dimensions, as oxidative cleavage preferentially occurs along defect sites or grain boundaries of the parent MXene sheets. Hydrothermal processes can further facilitate fragmentation while simultaneously introducing oxygen‐containing MQDs surface groups that enhance optical activity [44, 45, 46].

Ultrasonic exfoliation represents another practical route for MQDs synthesis. High MQDs‐based energy ultrasonic waves generate cavitation effects that break MXene layers into nanoscale domains. The duration and intensity of sonication strongly influence the final particle size and emission properties. Careful optimization of these parameters allows the production of MQDs with relatively narrow size distributions and stable PL characteristics.

In addition to top MQDs‐based down strategies, emerging bottom MQDs‐based up synthetic approaches are being explored to achieve better control over MQDs nucleation and growth. These methods typically involve molecular precursors containing transition metals and carbon sources that assemble into nanocrystalline structures under controlled reaction conditions [47, 48]. Although still less common, bottom MQDs‐based up techniques may offer improved uniformity and tunable optical characteristics.

Precise control over MQDs size is particularly important because optical emission properties are strongly size‐dependent and MQDs‐based dependent. By adjusting reaction conditions such as temperature, reaction time, precursor concentration, and oxidation strength, researchers can systematically regulate particle dimensions and tailor emission wavelengths. Such synthetic flexibility provides a versatile foundation for designing MQDs with optical features optimized for advanced sensing applications [48, 49, 50].

The physicochemical properties of MQDs are highly dependent on synthesis parameters, making precise control of reaction conditions essential for reproducibility and performance optimization. Key variables include etchant concentration, reaction time, temperature, sonication power, and purification protocol. Stronger etchant concentrations or prolonged etching times generally promote more aggressive exfoliation of MXene sheets, resulting in smaller MQDs sizes but also increasing the density of structural defects and surface terminations. Elevated reaction temperatures can accelerate fragmentation kinetics, enhancing QD formation but potentially leading to partial oxidation if not carefully controlled. Similarly, ultrasonication power plays a critical role in determining the degree of layer delamination and nanoscale cutting efficiency; higher power typically produces smaller and more uniform MQDs but may also induce unwanted structural damage. Post‐synthesis purification steps such as centrifugation, dialysis, or filtration are essential for removing large fragments and unreacted precursors, thereby improving colloidal stability and optical consistency. Collectively, these parameters govern MQDs size distribution, crystallinity, surface chemistry, and PL behavior. Therefore, standardized reporting of synthesis conditions is crucial for ensuring comparability between studies and for enabling rational design of MQDs with tailored optical and catalytic properties.

Figure 2 illustrates the synthesis route and structural characterization of Ti₂CT _x MQDs obtained through a mild acid‐etching strategy. As shown in Figure 2a, layered Ti₂AlC MAX phase was selectively etched using dilute HCl to remove the Al atomic layer, resulting in the formation of Ti₂CT _x nanosheets and subsequently MQDs. This HF‐free approach provides a relatively simple and environmentally safer alternative to conventional fluoride‐based etching methods while maintaining high yield and structural integrity. The schematic also highlights the formation of abundant surface termination groups (–O, –OH, and –Cl) on the MQDs surface, which originate from the chemical etching and oxidation processes and play a key role in determining the interfacial chemistry and optical properties of the nanodots. The morphology and size distribution of the synthesized MQDs are presented in Figure 2b. TEM images reveal highly dispersed and nearly spherical nanodots with a narrow particle size distribution centered at approximately 3 nm. Such ultrasmall dimensions are a defining feature of MQDs and are critical for inducing quantum confinement effects that influence their electronic and optical behavior. The observed monodispersity also indicates that the adopted synthesis conditions enable effective fragmentation of MXene sheets into uniformly sized nanostructures, which is essential for achieving reproducible PL responses in sensing applications.

Table 1 summarizes the critical structural and chemical parameters governing the optical performance of MQDs in sensing applications. Each design factor—from quantum confinement and surface terminations to heteroatom doping and synthetic control—affects emission wavelength, QY, and stability. Notably, interplay between edge defects and dopant chemistry dictates charge redistribution and radiative recombination efficiency. Rational selection and fine‐tuning of these parameters enable deliberate modulation of MQDs photophysics, providing robust guidelines for engineering high‐signal, reproducible optical sensors tailored to complex vegetable and environmental matrices (Table 1).

TABLE 1.

Summary of reported structural engineering strategies and their effects on the optical properties of MQDs based on previous literature reports.

Design parameter	Mechanistic role	Typical methods/dopants	Effect on optical property	Sensing implication	References
Quantum confinement	Induces discrete energy levels and widened bandgap	Size control (< 10 nm) by hydrothermal or sonication	Tunable emission (blue/red shift)	Adjustment of excitation/emission wavelengths	[34, 35]
Edge atom density	Increases reactive sites and unsaturated bonds	Exfoliation intensity adjustment	Enhanced photoluminescence	Signal amplification and reactivity toward analytes	[36, 37]
Oxygen termination (–O)	Promotes electronic delocalization	Controlled oxidation or annealing	Improved absorption and fluorescence	Strengthened optical signal stability	[38, 39]
Hydroxyl termination (–OH)	Enhances hydrophilicity and colloidal stability	Post‐treatment in alkaline media	Reduced aggregation and quenching	Better dispersion in aqueous sensing media	[40, 41]
Fluorine termination (–F)	Introduces localized electron states	HF‐etching remnants	Modified PL lifetime and band absorption	Selective quenching–recovery mechanisms	[42, 43]
Nitrogen doping	Forms donor states facilitating radiative recombination	Hydrothermal ammonia treatment	Increased quantum yield and PL intensity	Sensitive fluorescence probes for metal ions	[44, 45]
Phosphorus doping	Modulates charge transfer behavior	Phosphoric precursor inclusion	Red‐shifted emission; improved charge mobility	Enhanced catalytic efficiency in nanozymes	[46, 47]
Vacancy defects	Act as emissive centers or traps	Oxidative etching or plasma exposure	Intensified luminescence (moderate defect density)	Tunable sensitivity via defect engineering	[48, 49]
Grain‐boundary control	Dictates fragmentation path	Reaction temperature and time tuning	Uniform particle size and narrow emission peaks	Higher reproducibility of sensor calibration	[50, 51]
Ultrasonic exfoliation strength	Determines fragmentation kinetics	Sonication power/duration	Consistent particle size distribution	Stable optical output across sensor batches	[45, 46]
Bottom‐up precursor chemistry	Defines nucleation and crystallinity	Metal–carbon molecular assembly route	Uniform bandgap and stable emission	Scalable production of MQDs with predictable signals	[37, 38]

2.5. Spectroscopic and Colloidal Stability Characterization of MQDs (FTIR, Raman, and Zeta Potential)

A comprehensive understanding of MQDs requires complementary characterization techniques beyond TEM and XPS to validate surface chemistry, structural disorder, and colloidal stability. Fourier transform infrared (FTIR) spectroscopy provides direct evidence of surface functionalization in MQDs. Characteristic absorption bands associated with –OH, –O, and –F terminations confirm successful incorporation of oxygen‐ and fluorine‐containing groups introduced during etching and oxidation processes. These functional groups play a crucial role in governing hydrophilicity, interfacial reactivity, and fluorescence modulation by influencing electron density distribution at the MQDs surface.

Raman spectroscopy further contributes to structural evaluation by providing insight into defect formation and lattice disorder. The presence of D and G bands in MQDs reflects the partial disruption of the parent MXene lattice during nanoscale fragmentation. The intensity ratio (ID/IG) serves as a qualitative indicator of defect density, where higher values correspond to increased structural disorder and a greater number of emissive surface states. These defect sites are strongly associated with PL behavior, as they act as localized energy traps that facilitate radiative recombination processes [46, 47, 48].

In addition, zeta potential measurements are essential for assessing colloidal stability in aqueous environments, which is a critical requirement for sensing applications in complex vegetable matrices. High absolute zeta potential values indicate strong electrostatic repulsion between MQDs particles, preventing aggregation and ensuring long‐term dispersion stability. Surface functional groups such as –OH and –O contribute significantly to charge stabilization by modifying surface charge density. The combination of FTIR‐confirmed surface chemistry, Raman‐identified structural defects, and zeta potential‐determined colloidal stability provides a unified framework for understanding MQDs behavior in solution. Collectively, these techniques establish a comprehensive structure–property relationship linking surface chemistry, defect engineering, and dispersion stability, which is essential for reliable optical sensing performance of MQDs in real analytical environments.

3. Optical Signal Generation and Modulation Mechanisms in MQDMQD‐Based Sensors

3.1. PL Pathways in MQDs

PL in MQDs arises from a complex interplay between quantum confinement, surface chemistry, and defect MQDs‐based related electronic states. Although MQDs originate from metallic or semi MQDs‐based metallic parent MXenes, the drastic reduction in size alters their electronic structure, enabling radiative recombination processes that generate measurable fluorescence signals. This transition from weakly emissive two‐dimensional MQDs‐based sheets to luminescent QDs highlights the importance of nanoscale electronic restructuring.

Two main emission pathways are generally considered in MQDs: band MQDs‐based edge recombination and surface MQDs‐based state emission. Band MQD‐based edge emission occurs when electrons and holes recombine within confined energy states created by the QD core. In contrast, surface MQDs‐based state emission originates from electronic transitions involving surface terminations, vacancies, or oxidation MQDs‐based induced states located within the bandgap [47, 51, 52]. In many MQDs systems, surface MQDs‐based related emission dominates because the high surface MQDs‐based to MQDs‐based volume ratio introduces a large density of localized states that participate in radiative recombination.

However, the interpretation of MQDs PL is not always straightforward. Reported excitation MQDs‐based dependent emission behavior suggests that multiple emissive sites with different energy distributions coexist within a single MQDs population. This phenomenon indicates that the optical response cannot be explained solely by quantum confinement but also reflects heterogeneity in surface functionalization and defect distributions. As a result, the observed fluorescence characteristics often depend strongly on synthesis conditions and post‐MQDs‐based treatment processes.

Understanding the relative contribution of these emissive pathways is particularly important for sensing applications, since environmental interactions frequently perturb surface states more readily than the core electronic structure [47, 53]. Consequently, MQDs fluorescence signals can be highly responsive to subtle chemical interactions occurring at the particle interface. Clarifying the balance between core and surface emission therefore provides a critical foundation for interpreting optical signal variations in MQDs‐based sensing systems.

The origin of PL in MQDs is complex and may arise from multiple competing emissive pathways rather than a single intrinsic core emission. In the present system, the observed fluorescence is most plausibly attributed to a combined contribution of surface‐state emission, defect‐induced energy levels, and partially oxidized species rather than purely band‐edge recombination from a well‐defined MQDs core. Due to the ultrasmall size and high density of surface terminations (–O, –OH, –F), MQDs possess abundant localized electronic states that act as radiative recombination centers. These surface‐related states are further influenced by structural defects introduced during etching and downsizing processes, which can significantly enhance or modulate emission intensity.

In addition, partial oxidation of MXene sheets may generate oxygen‐containing functional domains or carbonaceous fragments that also contribute to fluorescence through defect‐mediated emissive traps. Therefore, the PL behavior of MQDs should be interpreted as predominantly surface‐state and defect‐dominated emission rather than purely quantum‐confined core emission. This mechanistic understanding is consistent with excitation‐dependent emission behavior and variable QYs reported for MXene‐derived nanostructures, and it highlights the importance of surface chemistry and structural heterogeneity in governing their optical response.

3.2. Charge and Energy Transfer in Optical Signal Modulation

In MQDs‐based optical sensors, the measurable signal often arises from modulation of intrinsic fluorescence through interfacial charge or energy transfer processes. These interactions translate molecular recognition events into detectable optical changes, making them central to the sensing mechanism. Despite the diversity of MQDs sensing designs, most optical responses can be traced to alterations in electron–hole recombination dynamics induced by nearby chemical species.

Charge transfer processes occur when electrons or holes migrate between MQDs and interacting molecules or ions. Such interactions modify the population of charge carriers involved in radiative recombination. When electron MQDs‐based accepting species withdraw electrons from excited MQDs, nonradiative pathways may dominate, leading to fluorescence quenching [54, 55]. Conversely, inhibition of charge transfer or passivation of trap states can enhance emission intensity. These changes reflect modifications in the balance between radiative and nonradiative recombination channels.

Energy transfer mechanisms represent another pathway for fluorescence modulation. Förster resonance energy transfer (FRET) is commonly invoked when MQDs act as energy donors to nearby acceptor molecules with overlapping absorption spectra. Because FRET efficiency depends strongly on donor–acceptor distance and spectral overlap, even small structural changes at the MQDs interface can produce substantial fluorescence variations. Such distance‐dependent MQDs‐based energy exchange has been widely exploited in nanoscale optical sensing platforms [56, 57].

In addition to these mechanisms, optical filtering effects may also influence observed fluorescence signals when absorbing species alter excitation or emission light propagation. Distinguishing between these processes remains important for interpreting sensor responses accurately [58]. A comprehensive understanding of how charge redistribution, energy coupling, and optical absorption collectively influence MQDs emission provides valuable insight for the rational design of reliable optical sensing strategies.

3.2.1. Mechanistic Classification of Optical Signal Modulation Pathways

The optical response mechanisms in MQDs‐based sensing systems involve several distinct but often coexisting processes that must be clearly differentiated for accurate mechanistic interpretation. The inner filter effect (IFE) is a purely optical phenomenon in which analyte absorption overlaps with either the excitation or emission wavelengths of MQDs, leading to apparent fluorescence attenuation without direct electronic interaction between the sensing platform and analyte. In contrast, PET involves real charge exchange between excited MQDs and target species, where electrons or holes are transferred to the analyte or from the analyte to MQDs, resulting in modulation of radiative recombination pathways and dynamic fluorescence quenching or enhancement.

FRET represents a distance‐dependent nonradiative energy transfer mechanism that requires strong spectral overlap between MQDs emission and acceptor absorption, typically operating within a nanoscale distance regime (< 10 nm). This mechanism is fundamentally different from PET because it does not involve charge transfer but rather dipole–dipole energy coupling. Static quenching, on the other hand, originates from ground‐state complex formation between MQDs and analytes, resulting in non‐fluorescent complexes that reduce emission intensity without affecting excited‐state lifetimes [55, 56, 57].

In addition to these quenching mechanisms, catalytic signal amplification introduces an opposite functional pathway, where analyte‐induced catalytic or redox reactions generate fluorescent or electroactive products that enhance optical signals rather than suppress them. Importantly, these mechanisms may coexist in MQDs systems; however, their dominance depends on surface chemistry, dopant states, analyte structure, and experimental conditions. Therefore, accurate mechanistic interpretation requires explicit discrimination between optical filtering effects (IFE), electron transfer processes (PET), energy transfer phenomena (FRET), ground‐state complexation (static quenching), and catalytic enhancement pathways.

3.3. Ratiometric and Multichannel Optical Signal Strategies

Single MQDs‐based intensity fluorescence measurements are often susceptible to variations in excitation power, probe concentration, and environmental conditions. To address these limitations, ratiometric optical sensing strategies have been developed to improve analytical reliability in MQDs‐based systems. Instead of relying on absolute emission intensity, ratiometric approaches measure the ratio between two independent optical signals, thereby introducing an internal calibration mechanism.

In MQDs platforms, ratiometric behavior can arise from multiple emissive states within the same nanostructure or from hybrid systems that combine MQDs with additional luminophores. When the sensing event selectively affects one emission channel while the other remains relatively stable, the resulting intensity ratio becomes a robust indicator of analyte presence [59, 60]. Because the ratio is less sensitive to fluctuations in instrumental conditions or probe concentration, ratiometric sensing often improves measurement reproducibility.

However, designing reliable dual MQDs‐based signal systems presents several challenges. Spectral overlap between emissions channels may introduce cross‐MQDs‐based interference, while differences in photostability between luminophores can gradually distort signal ratios during repeated measurements. Furthermore, the relative response of each channel must remain predictable over the relevant concentration range to maintain quantitative accuracy.

To overcome these issues, recent strategies emphasize controlling emission pathways within the MQDs framework itself. Engineering multiple emissive states through controlled surface chemistry or defect modulation may allow a single MQDs population to produce two distinguishable optical signals. Such intrinsic ratiometric systems reduce reliance on external fluorophores and can potentially enhance long MQDs‐based term signal stability [61, 62, 63]. The continued development of these multichannel signal strategies is expected to play a key role in improving the analytical robustness of MQDs‐based optical sensors.

In MQDs‐based pesticide sensing systems, the interaction between analyte molecules and QDs is governed by specific physicochemical mechanisms rather than simple fluorescence quenching or recovery. The nature of these interactions depends strongly on the chemical structure of the pesticide and the surface chemistry of MQDs. Redox‐active pesticides, such as certain organophosphates and nitroaromatic compounds, can participate in electron transfer processes with MQDs, leading to modulation of PL through PET pathways. In such cases, the pesticide acts as either an electron acceptor or donor, altering the recombination dynamics of excited carriers.

In addition, metal coordination interactions may occur when pesticide molecules containing electron‐donating functional groups (e.g., –NH₂, –SH, or phosphoryl oxygen atoms) bind to exposed transition‐metal sites on MQDs surfaces, particularly Ti or defect‐rich regions. This coordination can introduce new nonradiative pathways, resulting in fluorescence quenching. Hydrogen bonding interactions also play a significant role, especially for polar pesticides, where –OH, –COOH, or heteroatom‐containing groups interact with oxygenated surface terminations of MQDs (–O, –OH, –F), influencing surface‐state emission [57, 58, 59].

Furthermore, adsorption‐driven mechanisms contribute to signal modulation through π–π interactions or electrostatic attraction, which alter the local electronic environment of MQDs. In some cases, these interactions may induce catalytic or secondary chemical processes that amplify optical responses rather than simply quench fluorescence. Therefore, pesticide sensing in MQDs systems should be understood as a combination of redox chemistry, coordination bonding, hydrogen bonding, and adsorption phenomena, all of which collectively determine the observed optical signal variation.

3.4. Signal Stability, Reproducibility, and Interference Management

For practical sensing applications, the usefulness of MQDs‐based optical probes depends not only on sensitivity but also on the stability and reproducibility of their optical signals. Variations in emission intensity can arise from multiple factors including photobleaching, environmental oxidation, aggregation, or inconsistencies in particle synthesis. These issues may introduce uncertainty in analytical measurements if not carefully addressed. Photostability is particularly important because continuous optical excitation can gradually alter surface states responsible for MQDs fluorescence. Although MQDs generally exhibit greater resistance to photobleaching than many organic fluorophores, prolonged irradiation may still modify their emission characteristics [64, 65]. Surface passivation and optimized excitation conditions are therefore often employed to maintain stable optical output during repeated measurements.

Reproducibility across different batches of MQDs also represents a significant consideration. Small variations in particle size distribution, defect density, or surface termination composition can lead to measurable changes in emission intensity and spectral position. As a result, consistent synthesis protocols and rigorous material characterization are essential to ensure comparable optical behavior across experiments. Another important factor involves interference from complex sample environments. Biological matrices and plant tissues may contain naturally fluorescent compounds or light MQDs‐based absorbing molecules that overlap with MQDs emission bands. Such background signals can obscure analyte MQDs‐based induced fluorescence changes. Strategies including wavelength selection, background subtraction, and time MQDs‐based resolved detection have therefore been explored to improve signal discrimination [65, 66, 67]. Addressing these stability and interference issues is crucial for translating MQDs optical properties into dependable sensing platforms. By combining material engineering with robust measurement strategies, it becomes possible to achieve reliable optical outputs suitable for analytical applications.

4. Analytical Applications of MQDs Optical Sensors in Vegetable Safety Monitoring

4.1. Fluorescent MQDs Probes for Monitoring Plant MQDs‐Based‐Derived Chemical Compounds in Food Matrices

Fluorescent MQDs have recently been explored as optical probes for detecting bioactive chemical compounds present in plant MQDs‐based derived foods. Although such molecules are not always considered contaminants, their detection provides insight into MQDs sensing behavior in complex vegetable matrices. MQDs fluorescence is particularly sensitive to molecular adsorption and charge redistribution occurring at the QD interface, enabling luminescence MQDs‐based quantification strategies.

Nitrogen MQDs‐based doped Ti₃C₂ MQDs have been used as fluorescent probes for determining the concentration of quercetin, a bioflavonoid widely present in vegetables and fruits. These MQDs exhibit bright bluish MQDs‐based green emission and strong luminescence stability due to their high surface MQDs‐based to MQDs‐based volume ratio and tunable electronic states [68]. The sensing response originates from interactions between quercetin molecules and the MQDs surface that alter fluorescence intensity through electron transfer and surface complexation processes. Such interactions allow quantitative detection of quercetin in food samples with high sensitivity.

Beyond demonstrating analytical capability, this study highlights several advantages of MQDs for sensing in food matrices. Their excellent water dispersibility and biocompatibility enable direct analysis in aqueous extracts without complicated surface modification. Moreover, the strong luminescent signal of doped MQDs facilitates optical detection even at relatively low analyte concentrations.

Nevertheless, the sensing mechanism relies primarily on fluorescence intensity changes, which can be influenced by matrix components commonly present in vegetable extracts. Polyphenols and other natural pigments may interfere with fluorescence signals through absorption or competitive binding effects. Consequently, while MQDs probes show promising analytical potential, improving signal selectivity remains an important consideration when translating these systems to real food monitoring applications. These observations emphasize the need for sensing strategies that integrate MQDs PL with more robust analytical readout mechanisms.

4.2. Sequential Fluorescence Switching Strategies for Detection of Inorganic Ions and Small Molecules in Vegetable Matrices

Sequential fluorescence switching has emerged as a powerful strategy in MQDs‐based optical sensing because it allows dynamic modulation of PL in response to multiple chemical interactions. Unlike single MQDs‐based step fluorescence quenching systems, these platforms employ cascaded signal transitions that improve analytical reliability and enable detection of multiple analytes within the same sensing architecture. Such approaches are particularly relevant for vegetable monitoring, where inorganic contaminants and naturally occurring small molecules frequently coexist.

Nitrogen MQDs‐based doped MQDs have demonstrated strong potential for constructing sequential fluorescence sensing systems due to their high surface reactivity and stable PL behavior. In one representative strategy, MQDs function as fluorescent probes for detecting toxic chromium (VI) ions through an “on–off–on” fluorescence switching process. The intrinsic emission of the MQDs is initially quenched by Cr (VI) through a combination of IFE and static quenching interactions. When ascorbic acid is introduced, it reduces Cr (VI) to a lower oxidation state, thereby restoring MQDs fluorescence and completing the sequential signal recovery process [69]. This cascade response allows sensitive monitoring of Cr (VI), achieving detection limits in the nanomolar range while simultaneously enabling indirect detection of ascorbic acid.

A complementary sequential sensing mechanism has also been demonstrated using NMQDs‐based doped MQDs for the detection of Zn (II) ions and oxalic acid in vegetable samples. In this case, the fluorescence of MQDs is initially enhanced by Zn (II) through intramolecular charge transfer interactions with surface functional groups. The subsequent introduction of oxalic acid forms a stable Zn–oxalate complex that disrupts the charge transfer pathway, leading to fluorescence quenching and generating an “off–on–off” response pattern [70]. This sequential modulation enables quantitative determination of both Zn (II) and oxalic acid within micromolar concentration ranges.

Despite their advantages, sequential fluorescence switching systems may face challenges in real vegetable matrices where multiple redoxes MQDs‐based active compounds and organic acids coexist. These species can influence metal coordination or redox reactions, potentially complicating signal interpretation. Consequently, while MQDs‐based fluorescence switching strategies offer versatile sensing capabilities, improving selectivity and minimizing matrix interference remain critical for reliable contaminant monitoring in agricultural products.

Figure 4 illustrates the fluorescence recovery behavior of the NMQDs‐based Ti₃C₂ MQDs–Cr (VI) sensing system after the introduction of ascorbic acid under optimized conditions. As shown in Figure 4a, the quenched emission of the MQDs–Cr (VI) complex progressively increases with rising AA concentration, indicating effective reduction of Cr (VI) and restoration of the MQDs fluorescence signal. The quantitative response presented in Figure 4b demonstrates a concentration MQDs‐based dependent increase in fluorescence recovery efficiency (F − F ₀)/F ₀, which becomes stable at higher AA levels. The inset reveals a strong linear correlation within the 0.1‐ to 500‐μM range, yielding a low detection limit of 0.02 μM, thereby confirming the high sensitivity of this sequential fluorescence switching platform for AA detection following Cr (VI) MQDs‐based induced quenching.

FIGURE 4.

Fluorescence recovery of the NMQDs‐basedTi₃C₂ MQDs–Cr (VI) sensing system upon addition of ascorbic acid: (a) emission spectra at increasing AA concentrations; (b) corresponding fluorescence recovery efficiency (F − F ₀)/F ₀ with linear response for AA quantification. Adapted with permission from Ref. [69]. 2021 Elsevier B.V.

4.3. Dual MQDs‐Based Mode Optical Sensing of Nitrite Using MQDs Hybrid Platforms

Nitrite contamination in vegetables is a significant food safety concern due to its association with agricultural fertilizers and microbial nitrogen cycling. Optical sensors capable of detecting nitrite rapidly and visually are therefore valuable tools for food quality monitoring. MQDs‐based dual MQDs‐based mode sensing platforms have recently been developed to improve analytical robustness by combining fluorescence and colorimetric readouts.

Nitrogen and phosphorus co MQDs‐based doped Ti₃C₂ MQDs have been utilized to construct a dual MQDs‐based modal nitrite sensor based on the interaction between MQDs and a 1,10 MQDs‐based phenanthroline phenanthro line MQDs based Fe (II) complex. In this system, the fluorescence of MQDs is initially quenched by the Phen MQDs basedFe²⁺ complex through an IFE, while the solution simultaneously develops an orange coloration. Upon introduction of nitrite ions, oxidation of Fe²⁺ disrupts the complex, leading to fluorescence recovery and gradual fading of the color signal [71]. This dual optical response enables both fluorometric and colorimetric detection. An additional advantage of this platform is the integration with portable smartphone detection, allowing rapid visual analysis and quantitative signal interpretation. The excitation MQDs‐based dependent PL and strong photostability of the doped MQDs contribute to reliable signal generation during real MQDs‐based time monitoring.

However, the requirement for additional reagents such as metal–ligand complexes introduces extra chemical steps that may complicate field applications. Furthermore, competing oxidizing species present in vegetable extracts could potentially influence the redox reaction responsible for signal recovery. These factors suggest that while dual MQDs‐based mode MQDs systems enhance analytical versatility, their practical implementation requires careful optimization to minimize interference in complex food matrices.

Selectivity represents a critical challenge in MQDs‐based pesticide sensing systems, particularly because many redox‐active species such as oxidants, reductants, and small biomolecules can induce similar fluorescence or catalytic responses. Therefore, achieving high selectivity requires more than simple reliance on redox‐based signal modulation and instead depends on the incorporation of specific molecular recognition and interfacial engineering strategies. One of the key approaches involves tailoring MQDs surface chemistry through functional groups such as –COOH, –OH, or heteroatom dopants, which can enhance preferential interactions with target pesticide molecules via hydrogen bonding, electrostatic attraction, or coordination binding. In addition, the use of size‐ and shape‐selective adsorption effects can help discriminate pesticides based on steric compatibility with the MQDs surface.

More advanced strategies include molecularly imprinted polymers (MIPs) integrated with MQDs, which provide template‐defined cavities for selective pesticide recognition, and aptamer‐functionalized MQDs, where biomolecular recognition elements confer high specificity toward target analytes. Furthermore, selectivity can be improved by optimizing reaction conditions to suppress non‐specific redox interactions, thereby minimizing interference from common oxidants or reductants present in real samples [69, 70, 71]. In nanozyme‐based systems, selectivity is often governed by preferential adsorption or coordination between pesticide functional groups and catalytic metal centers, which modulates hydrogen peroxide activation pathways in a target‐dependent manner. Overall, the combination of surface engineering, recognition element integration, and controlled reaction environments is essential to ensure high selectivity in MQDs‐based sensing platforms for reliable pesticide detection.

Figure 5a,b demonstrates the fluorescence response of the N,P‐Ti₃C₂ MQDs/Phen/Fe²⁺ sensing system toward nitrite under optimized conditions. As the concentration of NO₂ ⁻ increases, the initially quenched fluorescence signal of the MQDs‐based complex progressively recovers, indicating disruption of the Phen–Fe²⁺ interaction through nitrite‐induced oxidation. The corresponding calibration plot (Figure 5b) shows a strong linear relationship between the fluorescence recovery ratio (F − F ₀)/F ₀ and NO₂ ⁻ concentration within the range of 1.5–80 μM, yielding a detection limit of 0.25 μM. The photographs recorded under UV illumination further confirm the gradual increase in fluorescence intensity, supporting the reliability of the fluorometric detection pathway. The colorimetric response of the same sensing platform is illustrated in Figure 5c,d. Increasing nitrite concentration leads to a progressive decrease in the absorption band of the N,P‐Ti₃C₂ MQDs/Phen/Fe²⁺ system, accompanied by a visible color transition from orange to pale pink and eventually nearly colorless. This spectral variation produces a quantitative linear correlation between the absorbance ratio (A ₀ − A)/A ₀ and NO₂ ⁻ concentration over the range of 4–85 μM, with a calculated detection limit of 0.71 μM. The simultaneous fluorescence recovery and color fading demonstrate the effectiveness of the MQDs hybrid platform for dual‐mode optical sensing, enabling both instrumental and visual detection of nitrite in complex matrices.

FIGURE 5.

Dual‐mode optical detection of nitrite using the N,P‐Ti₃C₂ MQDs/Phen/Fe²⁺ system: (a) fluorescence spectra at different NO₂ ⁻ concentrations; (b) linear relationship between fluorescence recovery ratio (F − F ₀)/F ₀ and NO₂ ⁻ concentration (inset: fluorescence photographs under UV light); (c) UV–vis absorption spectra with increasing NO₂ ⁻; (d) corresponding colorimetric calibration plot (A ₀ − A)/A ₀ versus NO₂ ⁻ concentration (inset: photographs under daylight). Adapted with permission from Ref. [71]. 2022 Elsevier B.V.

4.4. MQDs‐Based Nanozyme Sensors for Colorimetric Detection of Organophosphate Pesticides

Artificial nanozymes incorporating MQDs have emerged as powerful platforms for detecting pesticide residues through catalytic colorimetric reactions. In these systems, MQDs enhance the catalytic activity of hybrid nanostructures that mimic natural peroxidase enzymes, enabling sensitive detection of organophosphate pesticides frequently associated with vegetable contamination.

MQDs modified NiCoP nanostructures grown on nickel foam have been reported as efficient peroxidase MQDs‐based catalysts capable of decomposing hydrogen peroxide to generate reactive hydroxyl radicals. The presence of MQDs promotes electron transfer and strengthens interfacial interactions within the nanocomposite, leading to enhanced catalytic performance. Based on this catalytic activity, a colorimetric sensing platform was developed for glyphosate detection. The system exhibits a wide linear detection range from 1.5 to 300 μM with a detection limit of 1.06 μM [72].

The sensing mechanism relies on pesticide MQDs‐based induced modulation of catalytic activity, which alters the oxidation of chromogenic substrates and produces measurable color changes. Compared with purely fluorescence MQDs‐based sensors, such catalytic systems offer stronger signal amplification due to the continuous generation of reactive intermediates. Nevertheless, nanozyme MQDs‐based sensing platforms often depend strongly on the stability of catalytic active sites. Structural changes in the composite materials or variations in hydrogen peroxide concentration can influence signal intensity. Additionally, some pesticide molecules may exhibit similar inhibitory effects on catalytic reactions, potentially affecting analytical selectivity. These considerations highlight the importance of understanding catalytic mechanisms and interfacial interactions when designing MQDs‐based nanozyme sensors for pesticide monitoring.

The colorimetric response in MQDs‐based nanozyme systems originates from a well‐defined catalytic reaction pathway involving hydrogen peroxide (H₂O₂) activation and subsequent oxidation of 3,3′,5,5′‐tetramethylbenzidine (TMB). In the presence of MQDs modified NiCoP nanostructures, H₂O₂ is catalytically decomposed to generate highly reactive hydroxyl radicals (•OH), which act as primary oxidizing species. These radicals facilitate the conversion of colorless TMB into its oxidized blue form (oxTMB), resulting in a strong absorbance signal typically monitored in the visible region. The MQDs component enhances this process by promoting interfacial electron transfer and stabilizing catalytic active sites, thereby accelerating ROS generation and improving signal amplification.

When organophosphate pesticides such as glyphosate are introduced, the catalytic pathway is significantly disrupted. These molecules can adsorb onto the nanozyme surface or interact with metal active centers, leading to partial blocking of catalytic sites and suppression of H₂O₂ decomposition. In addition, pesticide molecules may alter the local electron density of the catalytic interface, reducing the efficiency of •OH generation [71, 72]. As a result, the oxidation rate of TMB decreases, leading to a weaker oxTMB signal and reduced absorbance intensity. This inhibition effect is kinetically controlled and depends on the concentration of the pesticide, which directly modulates the catalytic turnover rate of the nanozyme system. Therefore, the sensing mechanism is governed by a combination of ROS‐mediated oxidation, enzyme‐mimetic catalysis, and pesticide‐induced kinetic inhibition of the reaction pathway.

4.5. Peroxidase MQDs Based Mimicking MQDs Nanocomposites for Sensitive Detection of Organophosphate Residues

Beyond single nanozyme systems, several studies have explored MQDs‐based nanocomposites that integrate catalytic active centers with QDs to improve detection sensitivity toward pesticide residues. These hybrid structures utilize the electron transfer capability of MQDs to accelerate catalytic reactions involved in colorimetric sensing.

Copper MQDs‐based impregnated Ti₃C₂ MQDs have been developed as peroxidase MQDs‐based mimicking nanozymes capable of catalyzing the oxidation of chromogenic substrates in the presence of hydrogen peroxide. The catalytic kinetics of this nanozyme follow the Michaelis–Menten model, demonstrating efficient interaction with both the substrate and hydrogen peroxide. When malathion is introduced, the pesticide inhibits catalytic activity through interactions between sulfur atoms in the molecule and copper sites in the nanozyme, leading to decreased absorbance of the oxidized substrate at 652 nm [73]. This inhibition mechanism enables quantitative malathion detection within the range of 15–150 nM.

Similarly, MQDs coupled with amorphous cobalt oxide have been used to construct highly active peroxidase MQDs‐based mimicking composites for detecting the pesticide profenofos. The MQDs CoO _x interface significantly enhances catalytic activity by facilitating hydrogen peroxide activation and hydroxyl radical generation. As a result, the developed sensor achieves a detection limit as low as 0.68 nM with excellent analytical precision [74].

These nanozyme MQDs‐based systems demonstrate the potential of MQDs composites to achieve extremely low detection limits for pesticide residues in food samples. However, catalytic inhibition mechanisms may vary depending on pesticide structure, making universal detection strategies challenging. Future work may focus on improving specificity and understanding structure‐MQDs‐based dependent interactions between pesticides and catalytic MQDs interfaces.

The peroxidase‐like catalytic activity of MQDs‐based nanozymes is strongly governed by structure–activity relationships that arise from composition, surface chemistry, oxidation degree, and heteroatom doping. The intrinsic composition of MQDs, particularly transition metal carbides or carbon‐based domains, determines the availability of redox‐active centers that facilitate electron transfer during hydrogen peroxide activation. Surface terminations such as –O, –OH, and –F play a crucial role in regulating catalytic accessibility by modulating surface hydrophilicity, charge distribution, and adsorption affinity toward H₂O₂ and chromogenic substrates. In general, oxygen‐rich surfaces enhance catalytic activity by promoting electron delocalization and improving interaction with reactive oxygen species precursors, whereas excessive fluorine termination may partially suppress active site accessibility.

The degree of oxidation also significantly influences nanozyme performance. Moderate oxidation introduces defect sites and oxygen‐containing functional groups that act as catalytic hotspots, thereby enhancing peroxidase like activity. However, excessive oxidation may disrupt conductive pathways and reduce electron mobility, leading to diminished catalytic efficiency [73, 74]. In addition, heteroatom doping (e.g., N, S, or metal dopants) further tunes the electronic structure of MQDs by introducing additional donor or acceptor states, which facilitate faster electron transfer and improve H₂O₂ decomposition kinetics.

Collectively, these structural parameters synergistically determine the density of active catalytic sites, the efficiency of reactive oxygen species (ROS) generation, and the overall catalytic turnover rate. Therefore, the peroxidase‐like behavior of MQDs nanozymes should be understood as a structure‐dependent catalytic phenomenon rather than an intrinsic property, emphasizing the importance of rational surface and composition engineering for optimizing sensing performance.

The MQDs‐based sensing systems summarized in Table 2 demonstrate the dual functional role of MQDs in fluorescence and nanozyme platforms. In fluorescence sensors, signal modulation is governed by surface‐state interactions, IFE, ICT, and coordination‐driven quenching or enhancement, enabling sensitive and selective detection with low detection limits. In contrast, nanozyme‐based systems rely on peroxidase‐like catalytic activity involving H₂O₂ activation and hydroxyl radical (•OH) generation, where pesticides modulate catalytic kinetics through adsorption, metal–ligand interactions, or radical scavenging. Overall, MQDs mainly function as interfacial electronic mediators, while sensing performance is determined by synergistic interactions between surface chemistry, heteroatom doping, and hybrid catalytic components. To further evaluate practical applicability, the analytical performance of reported MQDs‐based sensors was compared with their regulatory relevance for food safety monitoring.

TABLE 2.

Summary of reported MQDs‐based sensing systems and their regulatory relevance for food safety applications from published studies.

MQDs system/composite	Target analyte	Sensing strategy	Mechanistic principle	Key performance	Regulatory relevance	References
N‐doped Ti₃C2 MQDs	Quercetin	Fluorescence probe	Charge transfer + surface‐state interaction	LOD: 1.35 nM; 25–600 nM range	Not a regulated contaminant; demonstrates food matrix applicability	[68]
N‐Ti₃C2 MQDs	Cr (VI), Ascorbic acid	“On–off–on” fluorescence	IFE + static quenching + redox recovery	LOD: 0.012 μM (Cr [VI]), 0.02 μM (AA)	LOD suitable for trace Cr (VI) monitoring; below typical food safety concern levels	[69]
N‐doped MQDs	Zn2+, Oxalic acid	“Off–on–off” fluorescence	ICT + coordination binding	LOD: 0.127 μM (Zn2+), 0.883 μM (OA)	Potential applicability for monitoring inorganic species in vegetables	[70]
N,P‐Ti₃C2 MQDs	Nitrite (NO2 −)	Colorimetric + fluorometric dual‐mode	IFE + Fe2+/NO2 − redox reaction	Portable dual‐modal detection	Relevant for nitrate/nitrite control in food products	[71]
MQDs–NiCoP/NF	Glyphosate	Colorimetric nanozyme	Peroxidase‐like activity; H2O2 → •OH generation	LOD: 1.06 μM; 1.5–300 μM range	Sensitivity requires comparison with pesticide MRL values	[72]
Cu–Ti₃C2 MQDs	Malathion	Colorimetric TMB assay	Cu–S interaction + ROS inhibition	LOD: 15–150 nM; recovery 95–101%	Applicable for trace pesticide monitoring in agricultural products	[73]
MQDs@CoOx	Profenofos	Colorimetric nanozyme	Co–Ti interfacial sites; enhanced H2O2 activation	LOD: 0.68 nM	High sensitivity compared with pesticide residue limits	[74]

Several fluorescence and nanozyme systems achieved detection limits within concentration ranges suitable for trace‐level contaminant analysis, particularly for heavy metals and pesticide residues. However, direct comparison with maximum residue limits (MRLs) depends on the specific contaminant, food matrix, and regulatory authority. Therefore, the added regulatory relevance column highlights whether reported detection capabilities can potentially meet practical monitoring requirements or require further validation. This comparison provides a clearer connection between laboratory‐scale sensing performance and real‐world food safety applications.

4.6. Critical Evaluation of MQDs Contribution in Hybrid Nanozyme Systems

A fundamental aspect in understanding MQDs based nanozyme systems is the accurate attribution of catalytic activity within hybrid architectures. In most reported sensing platforms, MQDs are integrated with catalytically active materials such as metal phosphides (e.g., NiCoP), metal oxides (e.g., CoO _x ), or transition‐metal–impregnated frameworks, which inherently possess strong peroxidase like activity. Consequently, the observed catalytic performance often arises from a synergistic interaction between components rather than intrinsic activity of MQDs alone. This distinction is essential for avoiding over interpretation of MQDs as primary nanozyme catalysts.

In such hybrid systems, MQDs typically function as electronic and interfacial modulators rather than direct catalytic centers. Their high surface area, tunable electronic structure, and abundant surface functional groups (–O, –OH, –F) enable efficient electron transport between catalytic sites and reactant molecules. This facilitates faster charge redistribution during hydrogen peroxide activation and enhances the overall catalytic turnover rate of the composite. Additionally, MQDs improve dispersion of metal‐based catalytic domains, preventing aggregation and increasing the accessibility of active sites. These effects collectively amplify the apparent catalytic efficiency, even though the main redox reactions occur on the partner material.

For example, in MQD–NiCoP systems, NiCoP acts as the primary catalytic phase responsible for H₂O₂ decomposition and hydroxyl radical (•OH) generation, while MQDs enhance interfacial conductivity and electron mobility. Similarly, in MQDs CoO_x nanocomposites, the CoO_x component provides the dominant catalytic centers, whereas MQDs contribute to charge transfer facilitation and structural stabilization. In Cu‐impregnated MQDs systems, catalytic inhibition by pesticides such as malathion is primarily governed by Cu–S interactions, with MQDs playing a secondary electronic role.

To rigorously assess catalytic contributions, systematic control experiments are essential, including MQDs only, catalyst‐only, and hybrid composite systems under identical conditions. Such comparative studies allow deconvolution of individual roles and prevent misattribution of catalytic activity to MQDs alone. In addition, kinetic studies (e.g., Michaelis–Menten analysis, reaction rate comparison) can further clarify the dominant catalytic pathway. Overall, MQDs should be more accurately described as catalytic enhancers or electron‐transfer mediators in hybrid nanozyme systems rather than standalone active catalytic centers. This refined understanding is critical for correct mechanistic interpretation and rational design of next‐generation MQDs‐based sensing platforms.

4.7. Emerging Paradigms in MQDs Optical Sensing: From Material–Signal Coupling to Smart Agricultural Analytics

Recent advances in MQDs reveal an inevitable shift from conventional fluorescence probes toward integrated material–signal systems capable of adaptive sensing in complex vegetable matrices. Across reported research's, distinct mechanistic innovations converge around one theme: how electronic coupling between MQDs surface chemistry and photonic response can translate molecular recognition into reliable optical signatures under realistic agricultural conditions. This evolving paradigm positions MQDs not merely as fluorescent labels but as active interfaces between chemical information and intelligent data output.

First, the material dimension of MQDs design has transitioned from simple transition MQDs‐based metal termination to controlled heteroatom doping and hybrid composite formation. Nitrogen MQDs‐based doped and peroxidase MQDs‐based mimicking MQDs structures provide redox MQDs‐based active platforms that locally modulate electron density during sensing events [72, 73]. Such chemical programmability enables selective oxidation or reduction of pesticide residues and inorganic ions, enhancing discrimination even within complex vegetable matrices rich in antioxidants or small organics. Meanwhile, engineered surface ligands on MQDs have unlocked sequential “off–on–off” and “on–off–on” responses [69, 70], which offer multi MQDs‐based state logic operations in fluorescence output. This aligns with the broader research trend toward molecular logic sensors, bridging optical nanomaterials and information processing.

Second, signal engineering has become a decisive factor in analytical performance. Combining quantum confinement with catalytic activity produces dual MQDs‐based mode outputs—fluorescence intensity and colorimetric change—within one sensing cycle [71, 74]. Such hybrid signal pathways transform MQDs into nanoscale transducers capable of internally validating results, addressing reproducibility issues that often hinder optical assays in real vegetables. This intrinsic cross‐MQDs‐based validation mechanism is a major trend in designing “self MQDs based verifying” nanosensors for food safety.

A critical determinant of the real‐world applicability of MQDs‐based sensing platforms is the presence of matrix effects in complex vegetable extracts. These matrices typically contain a wide range of coexisting biochemical species, including antioxidants (such as ascorbic acid and polyphenols), chlorophyll pigments, reducing sugars, amino acids, and endogenous metal ions, all of which can significantly interfere with optical and catalytic signal generation. Antioxidants and polyphenols may compete with target pesticide molecules in PET processes or act as radical scavengers, thereby suppressing reactive oxygen species (ROSs) formation in nanozyme‐based systems. Chlorophyll and other plant pigments can IFE by absorbing excitation or emission wavelengths of MQDs, leading to apparent fluorescence attenuation that is unrelated to analyte concentration. Furthermore, reducing sugars and metal ions may interact with MQDs surface functional groups, altering surface charge distribution, colloidal stability, and adsorption affinity toward target species. These interactions can result in signal distortion, including false‐positive and false‐negative responses, thereby reducing analytical reliability. To overcome these challenges, strategies such as sample pretreatment, selective extraction, surface passivation, and ratiometric or dual‐signal sensing approaches have been widely adopted. Therefore, proper consideration of matrix effects is essential for translating MQDs‐based sensing systems from controlled laboratory environments to practical food safety applications with high accuracy and reproducibility.

5. Practical Considerations and Future Directions for Vegetable Safety Monitoring Using MQDs Optical Sensors

The rapid development of MQDs‐based optical sensors has opened new opportunities for monitoring pesticide residues and inorganic contaminants in vegetable matrices. Their tunable PL, catalytic activity, and high surface reactivity enable the construction of sensitive fluorescence, colorimetric, and dual MQDs‐based mode sensing systems. Nevertheless, despite promising laboratory demonstrations, several practical challenges must be addressed before MQDs‐based sensors can be translated into reliable tools for routine vegetable safety monitoring. Addressing these issues requires critical consideration of matrix complexity, sensing reliability, device integration, and long MQDs‐based term material stability.

One of the primary practical challenges arises from the complex chemical composition of vegetable matrices. Vegetables contain a wide range of naturally occurring compounds such as polyphenols, organic acids, sugars, amino acids, and antioxidants [69, 74]. These species can interact with MQDs surfaces through adsorption, coordination, or redox reactions, potentially affecting fluorescence intensity or catalytic activity. For example, antioxidants commonly present in vegetables may compete with target analytes in redox MQDs‐based mediated sensing systems or interfere with fluorescence quenching mechanisms. As a result, signals obtained in real samples may differ significantly from those observed in simplified laboratory solutions. Although several studies have demonstrated acceptable recoveries in spiked vegetable samples, systematic investigations of matrix interference remain limited. Future work should therefore emphasize matrix MQDs tolerant sensor design, including surface functionalization strategies that improve analyte selectivity while minimizing nonspecific interactions.

Another critical issue concerns the reproducibility and standardization of MQDs synthesis. MQDs can be prepared through various routes, including ultrasonic exfoliation, microwave synthesis, hydrothermal cutting, and chemical etching of parent MXene sheets. While these approaches enable flexible material design, they often produce MQDs with different sizes, surface terminations, and defect densities. Since optical properties and catalytic performance strongly depend on these parameters, small variations in synthesis conditions may lead to significant differences in sensing behavior. This variability complicates the comparison of reported analytical performances across studies and may hinder practical deployment [67, 71]. To move toward real applications, the field requires standardized synthesis protocols and detailed characterization guidelines that ensure reproducible MQDs properties across laboratories.

The stability of MQDs under real operating conditions is another important consideration. MXene MQDs‐based materials are known to be susceptible to oxidation, particularly in aqueous and oxygen MQDs‐based rich environments. Oxidative degradation can alter the electronic structure of MQDs, leading to decreased fluorescence intensity or reduced catalytic activity over time. In practical sensing scenarios—such as field monitoring of vegetables during harvesting, storage, or transportation—exposure to air, light, and fluctuating temperatures is unavoidable. Therefore, strategies to enhance MQDs stability, including surface passivation, polymer encapsulation, and hybrid nanocomposite formation, will be essential for developing robust sensing platforms capable of long MQDs‐based term operation.

From an analytical perspective, selectivity remains one of the most critical performance parameters for MQDs‐based optical sensors. Many current sensing systems rely on fluorescence quenching, IFEs, or catalytic reactions that can be triggered by multiple species with similar chemical properties. In complex vegetable extracts containing numerous inorganic ions and organic molecules, such nonspecific responses may generate false signals [70, 72]. Improving selectivity requires rational surface engineering of MQDs, such as introducing specific recognition ligands, molecularly imprinted polymers, or coordination sites tailored for particular contaminants. Integrating these recognition elements with MQDs photophysical properties could significantly enhance discrimination among structurally similar analytes.

Beyond fundamental sensing performance, device integration and portability will play a key role in determining the real MQDs‐based world impact of MQDs sensors. Laboratory fluorescence spectrometers are not practical for on MQDs‐based site agricultural testing. Recent research has therefore explored the incorporation of MQDs into portable detection formats, including paper MQDs‐based test strips, smartphone MQDs‐based assisted imaging systems, and microfluidic analytical devices. Such platforms can transform MQDs optical responses into easily interpretable color or fluorescence signals that can be analyzed with compact electronics. However, the transition from proof MQDs based of MQDs based concept demonstrations to reliable commercial devices requires careful optimization of signal stability, calibration protocols, and user MQDs based independent quantification methods.

Looking forward, one of the most promising research directions lies in the integration of MQDs optical sensing with digital data analysis technologies [73, 74]. The optical responses of MQDs systems—particularly those exhibiting multi MQDs‐based state fluorescence switching or dual MQDs‐based mode outputs—can produce complex spectral information that may contain rich analytical insights. Machine learning algorithms could be used to interpret these multidimensional optical signals, enabling simultaneous identification of multiple contaminants within vegetable samples. Such approaches would transform MQDs sensors from simple detection tools into intelligent analytical platforms capable of pattern recognition and predictive analysis.

Another emerging direction involves the development of multifunctional MQDs‐based nanozymes that combine catalytic sensing with fluorescence signaling. Hybrid MQDs‐based nanozymes can generate amplified colorimetric responses through catalytic reactions while simultaneously providing fluorescence feedback for internal validation. This dual MQDs‐based signal strategy can significantly improve analytical reliability by reducing false positives and enhancing detection sensitivity. Moreover, nanozyme MQDs‐based assisted sensing platforms are particularly attractive for pesticide monitoring because many pesticide molecules influence catalytic reactions through inhibition or activation pathways.

Finally, sustainable agricultural monitoring requires sensing technologies that are not only sensitive but also economical, scalable, and environmentally benign. The synthesis of MQDs from abundant MXene precursors provides a promising pathway toward scalable production, yet large MQDs‐based scale manufacturing methods and environmental safety assessments remain largely unexplored. Future studies should therefore evaluate the life MQDs‐based cycle impact, cost efficiency, and potential environmental risks associated with MQDs production and disposal [68, 73].

MQDs‐based optical sensors represent a rapidly evolving class of analytical tools with significant potential for vegetable safety monitoring. However, translating these promising nanomaterials from laboratory studies to practical applications will require coordinated progress in material standardization, matrix MQDs‐based resistant sensing strategies, device engineering, and data MQDs‐based driven analytical approaches. By addressing these challenges, MQDs optical sensing platforms could become integral components of next MQDs‐based generation food safety monitoring systems capable of supporting sustainable and intelligent agricultural supply chains.

5.1. Reproducibility, Stability, Safety, and Scale‐Up Challenges in MQDs Systems

A critical limitation hindering the practical translation of MQDs based sensing systems is the lack of comprehensive studies addressing reproducibility, long‐term stability, environmental safety, and scalability. Reproducibility remains a major concern because MQDs synthesis is highly sensitive to reaction parameters such as etching conditions, temperature, sonication power, and precursor concentration. Even minor variations in these parameters can lead to significant differences in particle size distribution, surface chemistry, and defect density, ultimately affecting optical and catalytic performance. Therefore, standardized synthesis protocols and rigorous reporting guidelines are essential to ensure cross‐study comparability.

Long‐term stability is another key challenge, particularly for MQDs used in aqueous sensing environments. Surface terminations (–O, –OH, –F) and defect‐rich structures can undergo gradual oxidation or aggregation over time, leading to changes in fluorescence intensity and catalytic activity. This instability may compromise sensor reliability in real‐world applications, especially in complex matrices such as food or environmental samples. Strategies such as surface passivation, polymer coating, and heteroatom stabilization are therefore critical for improving operational durability.

From an environmental safety perspective, the potential toxicity and ecological impact of MQDs and their metal‐containing hybrids remain insufficiently explored [70, 72]. The presence of transition metals and reactive surface species raises concerns regarding bioaccumulation and long‐term environmental exposure, highlighting the need for systematic toxicity and biodegradability assessments.

Finally, scalability represents a significant barrier for industrial translation. Most reported synthesis methods rely on laboratory‐scale hydrothermal or ultrasonic approaches, which may not be easily scalable or cost‐effective. Developing continuous‐flow synthesis, green chemistry approaches, and industrial‐compatible fabrication methods will be essential for large‐scale production. Overall, addressing these challenges is crucial for bridging the gap between laboratory research and real‐world deployment of MQDs‐based sensing technologies.

6. Conclusions

MQDs‐based optical sensors have emerged as an innovative and powerful platform for detecting pesticide residues and inorganic contaminants in vegetable matrices. Their high surface reactivity, tunable fluorescence, and peroxidase MQDs‐based‐like catalytic characteristics enable ultrasensitive detection through diverse optical mechanisms, including fluorescence switching, IFEs, and nanozyme MQDs‐based‐mediated colorimetric reactions. The systematic review reveals that engineering strategies—such as heteroatom doping, metal hybridization, and surface passivation—substantially improve signal stability, selectivity, and analytical precision.

Despite these advancements, several critical challenges still limit real MQDs‐based world deployment. Matrix interference, oxidation susceptibility, and variability in MQDs synthesis often lead to inconsistent sensing outcomes. Furthermore, the lack of standardized protocols and limited portability of laboratory MQDs‐based measurements constrain practical usability. To realize the full potential of MQDs in vegetable safety monitoring, future research must focus on creating matrix MQDs‐based resistant designs, integrating selective recognition layers, and coupling optical readouts with digital processing platforms such as smartphone analytics or machine MQDs‐based learning interpretation.

Overall, MQDs‐based optical sensors hold remarkable promise for the next generation of rapid, sensitive, and cost‐effective MQDs‐based food safety diagnostics. Their convergence with intelligent data technologies and eco‐MQDs‐based compatible fabrication could transform routine contamination monitoring into a scalable and sustainable practice, reinforcing consumer health protection and agricultural traceability at a global level.

Author Contributions

Tareq Nayef AlRamadneh: conceptualization, investigation, software, resources, writing – original draft. Flah Shryf Abdul: writing – review and editing, visualization, software, methodology. Amaal Mohammed Ali: data curation, formal analysis, visualization. Manoj A. Vora: conceptualization, writing – original draft, investigation. R. Roopashree: methodology, validation, writing – review and editing. Lalita Chopra: validation, formal analysis, data curation. Babamuratov Bekzod: data curation, software, resources. Murodjon Yaxshimuratov: resources, formal analysis, software. Sobhan Mirizadeh: visualization, project administration, supervision, writing – review and editing, writing – original draft.

Funding

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.