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. 2026 Aug 14. Online ahead of print. doi: 10.1039/d6na00106h

Deep eutectic solvent-enabled green synthesis of gold nanoparticle-decorated ZIF-L composites for colorimetric detection of trimethoprim

Trung Hieu Vu a,b, Ngoc Xuan Dat Mai a,b, Nhu Hoa Thi Tran b,c, Nae Yoon Lee d, Kieu The Loan Trinh a,b,✉
PMCID: PMC13530891  PMID: 42682959

Abstract

Developing environmentally friendly synthesis strategies for metal–organic frameworks (MOF) and their hybrid nanocomposites is critical to creating sustainable, high-performance materials for analytical sensing applications. In this study, a gold nanoparticle-decorated ZIF-L (ZIF-L/Au) composite was synthesized via a deep eutectic solvent (DES)-assisted green synthesis, thereby providing enhanced stability and improved dispersion of Au nanoparticles and minimizing the use of hazardous organic solvents. The DES-stabilized ZIF-L/Au composite exhibited strong peroxidase-like activity toward ABTS˙+ radical scavenging, effectively decolorizing it from green to transparent. Notably, the presence of trimethoprim (TMP) inhibited this catalytic decolorization, leading to a concentration-dependent restoration of the green color. Under optimized conditions, the ZIF-L/Au composite enabled colorimetric detection of TMP with high sensitivity and selectivity, with a limit of detection of 4.38 nM. This work provides a sustainable strategy for constructing functional MOF-based hybrid nanomaterials and demonstrates their strong potential for practical, on-site monitoring of antibiotic residues.

ZIF-L/Au composite was synthesized via a deep eutectic solvent (DES)-assisted green synthesis, and enabled colorimetric detection of trimethoprim with high sensitivity and selectivity.graphic file with name d6na00106h-ga.webp

Introduction

Food contamination caused by the overuse of antibiotics in agriculture and aquaculture has received considerable attention due to its severe impact on ecosystems and human health.1,2 Excessive and uncontrolled antibiotic application leads to residue accumulation in the food chain, contributing indirectly to the emergence of multidrug-resistant pathogens.3,4 Thus, many countries, including Vietnam, have established strict standards for antibiotic residues in food to ensure the balance of the ecosystem and protect consumer health. In particular, monitoring antibiotic residues is an essential part and receives considerable attention in ensuring product quality. Trimethoprim (TMP), a widely used antibacterial in livestock, aquaculture, and medicine, can easily enter the environment through prolonged and uncontrolled use, thereby leading to serious bacterial resistance.5

The increasing demands of society for speed, convenience, and accuracy promote the development and improvement of rapid detection methods for antibiotic residues. Conventional methods, such as high-performance liquid chromatography (HPLC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), can detect analytes with high specificity; however, they require labor-intensive sample preparation, high operating costs, and sophisticated equipment, which hinder widespread adoption beyond research laboratories and analytical centers.6 Consequently, simple and low-cost analytical strategies, including fluorescence,7 surface-enhanced Raman spectroscopy (SERS),8 electrochemical detection,9 and colorimetric sensing,10 show great potential to support and replace traditional methods under challenging conditions. Among them, colorimetric sensing is particularly appealing due to its simplicity, low cost, and compatibility with smartphone-based analysis, enabling immediate on-site interpretation of results without the need for complex equipment. Sustainable hybrid material platforms, including morphology-controlled nanomaterials and synergistic interfaces, are highly valued for applications in monitoring and removing pollutants. Wu et al. demonstrated that combining rhombohedral α-Fe2O3 nanoparticles with multiwalled carbon nanotubes significantly increased the electrochemically active surface area and electron-transport capacity, owing to synergistic interactions between α-Fe2O3 NPs and MWCNTs.11 Tran et al. improved the sensitivity of the Raman detector by integrating an amino-functionalized MOF-5 film with silver nanoparticles (AgNPs). MOF-5, with its porous structure, helps adsorb the analyte, increasing the analyte density at the hot spot and controlling the distribution of AgNPs, thereby improving the sensitivity of SERS substrates compared to single AgNPs.12

Nanomaterial-based colorimetric probes, particularly those utilizing gold nanoparticles (AuNPs), offer excellent optical and catalytic properties.13 Compared with other metals, AuNPs exhibit unique optical properties stemming from their surface plasmon resonance (LSPR) effect, which is highly influenced by morphology and size. In addition, catalytic ability, large specific surface area, biocompatibility, low toxicity, and excellent interaction with amine groups, thiols, and specific metal ions have led to the development of many colorimetric sensors suitable for various applications.14,15 However, the facile aggregation of AuNPs poses an obstacle to the development of such sensors.

On the other hand, using heterogeneous materials has emerged as a powerful strategy to overcome the limitations of single-component materials. Interface engineering is key to tuning and modulating charge-transfer pathways, increasing active-site density, and improving analyte accessibility. Guo et al. developed a highly sensitive electrochemical sensor based on the NH2-MIL-125(Ti)/Ti3C2Tx heterostructure, constructed using synergistic interface techniques.16 In this process, NH2-MIL-125(Ti), with its hierarchical porous structure, acts as a trap and enricher for analyte molecules, while Ti3C2Tx promotes charge transfer. These studies highlight the potential applications of heterogeneous materials based on framework materials, which are increasingly appreciated in the field of sensing. Moreover, metal–organic frameworks (MOFs) have emerged as an effective solution to address the problems associated with the aggregation of Au NPs and improved catalytic performance due to their properties, such as a large specific surface area and tunable pore size. MOFs serve as effective carriers for AuNPs while supporting analyte adsorption and providing metal oxide-like catalytic sites for MOFs/AuNPs composites.17 Furthermore, MOFs hybridized with AuNPs with glucose oxidase (GOx) activity, when combined with certain metalloporphyrin-based MOFs, produce hydrogen peroxide in the presence of d-glucose, thereby activating a colorimetric response via horseradish peroxidase-mimicking catalytic activity.18 In the family of zeolite imidazolate frameworks (ZIFs), a subgroup of MOFs, ZIF-L, a two-dimensional material, exhibits high surface area, structural stability, and inherent catalytic features, making it suitable for hosting AuNPs and enhancing catalytic activity.19 Furthermore, surface modification and the structural design of artificial recognition sites can enhance specificity and optical signals in visual sensing platforms.20 Similarly, the integration of AuNPs into MOFs creates recognition sites that enhance the selectivity and sensitivity of the colorimetric sensor.

Despite their advantages, MOFs, including ZIF-based materials, are often synthesized using the solvothermal method in toxic organic solvents, such as dimethylformamide, N,N-diethylformamide, N-methyl-2-pyrrolidone, and methanol, which raise environmental and safety concerns, especially for large-scale production.21,22 As a sustainable chemistry approach, deep eutectic solvents (DESs) have emerged as a green and sustainable alternative to organic solvents in the synthesis of MOF materials. DESs are formed by blending two components, specific hydrogen bond donors (HBDs) and acceptors (HBAs) in appropriate proportions to lower their melting points by forming complex hydrogen bond systems. DESs are often formulated from low-cost, less toxic components, such as choline chloride, urea, and citric acid, to create a solvent that replaces organic solvents with low cost and high biodegradability.23 Maia et al. highlighted the role of DES as a structure-directing agent, tailoring and transforming one MOF structure into another. Additionally, the process of removing DES from MOFs after synthesis could become an effective strategy for creating porous structures.24 Besides, the hydrothermal synthesis of ZIF-L shows key limitations in terms of scalability and energy (temperature of 100–150 °C). Meanwhile, stirring the components at room temperature (RT) shows greater optimization in terms of fabrication process, safety, and scalability. Furthermore, the possibility of recovering and reusing DES can be considered to optimize costs and minimize environmental impact.

In this study, we developed a green strategy to synthesize ZIF-L/Au composites using DES as a solvent, which serves as an eco-friendly colorimetric probe for TMP detection. The DES medium facilitates the formation of stable ZIF-L nanosheets while ensuring uniform incorporation of AuNPs. The resulting hybrid material exhibits improved catalytic performance, as evidenced by enhanced peroxidase-like activity in ABTS˙+ radical scavenging compared to single-component counterparts. When applied to TMP analysis, the ZIF-L/Au composite achieved a detection limit of 4.38 nM, well below regulatory limits for TMP residues in aquaculture products, demonstrating its strong analytical potential and suitability for practical applications.

Experimental

Materials

Tetrachloroauric (III) acid (HAuCl4·3H2O, 99.9%), 2-methylimidazole (Hmim, C4H6N2, 99%), and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS, ≥ 98%) were all purchased from Sigma-Aldrich Co., MO, USA. Sodium hydroxide (NaOH, 96%) and ethanol (EtOH, C2H5OH, 99.8%) were provided by Fisher Ltd (UK). Choline chloride (98%) was purchased from Thermo Fisher Scientific Inc. (NYSE: TMO, US). Urea (CH4N2O, ≥99%), hydrogen peroxide (H2O2, 30%), and zinc nitrate hexahydrate (Zn (NO3)2·6H2O, ≥ 99%) were provided by Xilong Scientific Co., Ltd, China. Trimethoprim (≥99.96%) was purchased from BioBasic Inc (Toronto, Canada). De-ionized (DI) water was obtained using a Milli-Q® IQ 7000 ultrapure water purification system (Merck KGaA, Darmstadt, Germany).

Synthesis of ZIF-L/Au composites

The synthesis process of ZIF-L/Au composite material is described in Fig. 1. First, a mixture of 0.1396 g choline chloride, 0.1201 g urea, 0.0297 g Zn(NO3)2·6H2O, and 0.3284 g Hmim was dissolved in 40 mL of DI water. Then, the mixture was continuously stirred at 800 rpm for 20 h. The solution gradually turns white-opaque due to the formation of ZIF-L material. Urea and chloride interact to form a dense network of hydrogen bonds that assist in controlling nucleation and crystal growth. On the other hand, the quantity of DI added to the mixture helps reduce the viscosity of the DES, facilitating the diffusion of Zn2+ and the ligand. In addition, the selective adsorption of DES components on different faces of the ZIF-L crystals directs the growth of the characteristic 2D sheet structure of ZIF-L. HAuCl4 (12 mM) at the investigated ratios (0.5, 0.75, and 1 mL) was added to the mixture under magnetic stirring. After 15 min, 0.5 mL of NaBH4 (12 mM) was rapidly added to the solution, and the color immediately changed from pale yellow to bluish purple and gradually deepened. The reaction was maintained under the same conditions for 30 min, after which the composite material was cleaned of any remaining precursors by centrifugation at 4500 rpm for 10 min. The entire synthesis process was carried out at RT (25 °C), and the solution has about a pH of 9.1. The process was repeated twice before the material was re-dispersed in 30 mL of DI water.

Fig. 1. Schematic illustration of a one-step process for the green synthesis of decorative AuNPs on ZIF-L.

Fig. 1

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Characterization of ZIF-L and ZIF-L/Au composites

Morphological and elemental analyses of the synthesized ZIF-L and ZIF-L/Au composites were carried out using scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS) on a Magellan 400 microscope, FEI Co., Cambridge, UK. Additionally, transmission electron microscopy (TEM, Tecnai G2 F30 S-TWIN, FEI Co., Netherlands) was also conducted. For SEM and TEM, an appropriate amount of materials suspension was dropped on silicon wafers and copper grids, respectively, and then dried overnight at RT. Multiple TEM images were used to measure the size of approximately 200 particles using ImageJ, yielding a statistically significant sample size. Next, the particle size distribution function was constructed from the dataset using a Gaussian distribution. An X-ray diffraction (XRD) machine (Bruker D8 Advance diffractometer) was used to determine the crystallinity of the ZIF-L/Au composite in the range of 5–80°. The optical properties of the materials and analyte detection were recorded using a Cary 60 UV-Vis spectrophotometer at a rate of 400 nm min−1 in the range of 200–1000 nm. A Spectrum Two FT-IR spectrometer (PerkinElmer) was used to identify characteristic functional groups of ZIF-L and ZIF-L/Au composites in attenuated total reflectance (ATR) mode in the region of 500–4000 cm−1. Thermogravimetric analysis was conducted from room temperature to 800 °C using a Q-500 thermogravimetric analyzer (TA Instruments, Tokyo, Japan) under continuous airflow at a heating rate of 10 °C min−1. Surface area and pore size distribution were assessed using the Brunauer–Emmett–Teller (BET) method using an BELSORP-max analyzer. X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo Scientific, Fitchburg, WI, USA) was used to determine the chemical state of the ZIF-L/Au composite.

Evaluation of peroxidase-like activity of the ZIF-L/Au composite

The peroxidase-like activity of the ZIF-L/Au composite was evaluated using the ABTS˙+ assay in DI water. Briefly, ABTS (7 mM) was reacted with K2S2O8 (2.45 mM) at a 1 : 1 ratio (v/v) to generate the ABTS˙+ radical cation. ZIF-L, ZIF-L/Au 050, ZIF-L/Au 075, ZIF-L/Au 100, and AuNPs were added to the ABTS˙+/H2O2-containing system. After 5 min, the absorption intensity at 417 nm was recorded to find the optimal material for TMP detection applications.

Absorption spectra of the ZIF-L/Au 075/ABTS˙+, ZIF-L/Au 075/H2O2, ABTS˙+/H2O2, ZIF-L/Au 075/ABTS˙+/H2O2, and ABTS˙+ systems were recorded to confirm the peroxidase activity of the materials. Subsequently, absorbance at 417 nm was recorded every 10 s for the ZIF-L/Au 075/ABTS˙+/H2O2 system at different H2O2 concentrations (10–170 mM) to construct a plot according to Michaelis–Menten enzyme kinetics.

Colorimetric detection of TMP using the ZIF-L/Au 075 composite

Fig. 2 illustrates the TMP detection procedure based on the color change of the ABTS˙+ indicator, thanks to the peroxidase-like activity of the ZIF-L/Au composite in the presence of H2O2. In detail, the TMP stock solution was initially prepared in ethanol and subsequently diluted with DI to obtain a series of TMP concentrations. The effects of temperature and incubation time on the TMP detection were first evaluated. Typically, the ZIF-L/Au 075 composite and TMP were incubated at various temperatures (25–55 °C) and incubation times (15, 30, 45, 60, and 75 min). After incubation, ABTS˙+ and H2O2 3% were added, and the absorbance at 417 nm was recorded.

Fig. 2. Schematic illustrating the TMP detection based on the ZIF-L/Au composite.

Fig. 2

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To determine the limit of detection of TMP, 1 mL of TMP at various concentrations was mixed with 100 µL of ZIF-L/Au solution and incubated at RT for 45 min before adding 100 µL of ABTS˙+ and 5 µL of H2O2 3% solution. The absorbance of the reaction mixture at 417 nm was further recorded after incubation for 10 min at RT. Each TMP concentration was analyzed four times.

Results and discussion

Characterization of ZIF-L/Au composites

The UV-Vis spectra of the ZIF-L and ZIF-L/Au composites revealed a clear change in the optical properties upon AuNP incorporation compared to pristine ZIF-L (Fig. 3a). Specifically, the absorption in the visible region increases gradually with the increase in the Au content in the composites. A thin absorption ridge at 545 nm is observed for the ZIF-L/Au 075 and ZIF-L/Au 100 composites, which characterizes the plasmonic nature of the Au component. On the other hand, the LSPR spectra of AuNPs provide further evidence for the dispersed/aggregated state of the AuNPs in colloidal form. The plasmon peaks of both ZIF-L/Au 050 and ZIF-L/Au 100 samples are almost completely extinguished. In contrast, the characteristic absorption peak of AuNPs at 545 nm is still clearly visible in ZIF-L/Au 075, indicating good dispersion of AuNPs in solution. This observation is consistent with the XRD results (Fig. 3b), which confirmed the coexistence of both ZIF-L and AuNPs. The characteristic diffraction peaks of ZIF-L in the range of 10–20° remained sharp and intense, demonstrating the high crystallinity of the synthesized framework.25 The ZIF-L crystal mainly grows in the (112) direction, corresponding to a high intensity diffraction peak at 12.8°, with crystallite size calculated using the Debye Scherrer formula being 61.46 nm. Similarly, the crystal size of AuNPs at 38.6 corresponding to the lattice plane (111) was determined to reach 5.26 nm. Additionally, to eliminate the diffractometer effect, instrumental broadening was subtracted from the experimental total breadth using the silicon standard correction. Notably, the absence of the (011) reflection at 7.4° and the presence of the (222) peak at 18.1° reflect the distinctive structural arrangement of ZIF-L related to the bridging of adjacent sodalite layers.26 On the other hand, the FTIR spectra (Fig. 3c) further support the successful formation of ZIF-L, showing characteristic Zn–N stretching vibrations at 424 and 685 cm−1, which confirm coordination between Zn2+ ions and the Hmim linker. To further demonstrate the role of DES in the formation and stabilization of ZIF-L/Au, ZIF-L was synthesized using DES-assisted and DES-free approaches, while keeping all other conditions unchanged. In terms of synthesis efficiency, the ZIF-L sample synthesized in DI achieved 50% of the ZIF-L sample synthesized in DES. FTIR analysis results (Fig. S1a) showed that the characteristic vibrations of ZIF-L in the two samples, with and without DES, showed no changes in position, nor did they exhibit any new vibrational peaks. This demonstrates that DES was completely removed from ZIF-L by centrifugation following synthesis. However, observations of the characteristic diffraction peaks of ZIF-L in the XRD pattern (Fig. S1b) reveal a peak at 7.34 in the ZIF-L sample synthesized in DI, which characterizes the phase transition from ZIF-L to ZIF-8. This is compelling evidence that DES plays a role in maintaining the stability of the material within the ZIF-L structure.27 Thermal behavior of ZIF-L and ZIF-L/Au composites was further evaluated by TGA (Fig. 3d), which demonstrated that both ZIF-L and ZIF-L/Au composites were stable up to 200 °C, followed by minor mass loss (<5 wt%) between 200 and 400 °C. The major decomposition stage, corresponding to the collapse of the ZIF-L framework, occurred between 400 and 550 °C with an overall weight loss of approximately 66 wt%.28

Fig. 3. (a) UV-Vis spectra, (b) XRD patterns, and (c) FTIR spectra of ZIF-L, ZIF-L/Au 050, ZIF-L/Au 075, and ZIF-L/Au 100, and (d) TGA curves of the ZIF-L and ZIF L/Au 075 composites.

Fig. 3

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The morphology of ZIF-L and ZIF-L/Au composites was observed by HRTEM, confirming the successful formation of ZIF-L/Au hybrid structures at all three investigated ratios (ZIF-L/Au 050, ZIF-L/Au 075, and ZIF-L/Au 100) (Fig. 4 and S2–S4). Among them, the ZIF-L/Au 075 composite exhibited a uniform deposition of well-dispersed AuNPs across the ZIF-L surface (Fig. 4a and b). The inset depicts the size distribution of spherical AuNPs with an average calculated size of approximately 5.624 ± 0.05 nm. In addition, the EDX analysis of the ZIF-L/Au 075 sample also indicated the co-existence of two components, Zn and Au, with mass fractions of 24.8 and 12.6%, respectively (Fig. 4c and d). In contrast, the ZIF-L/Au 050 and ZIF-L/Au 100 samples displayed poorer AuNP dispersion and noticeable particle agglomeration, which likely decreased the accessible surface area and reduced the number of active interaction sites for analyte binding (Fig. S3 and S4).

Fig. 4. (a and b) HRTEM images of ZIF-L/Au 075 with the inset figure showing the size distribution and lattice representation of AuNPs. (c and d) EDX mapping and EDX spectrum of the ZIF-L/Au 075 composite at a 20 µm scale.

Fig. 4

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Peroxidase-like activity of ZIF-L/Au composites

The absorption at 417 nm of ABTS˙+ solution indirectly evaluated the peroxidase-like activity of ZIF-L, AuNPs, ZIF-L/Au 050, ZIF-L/Au 075, and ZIF-L/Au 100 in the presence of H2O2 (Fig. 5a). The characteristic absorbance of ABTS˙+ at 417 nm gradually decreases due to the transition to the neutral state of ABTS˙+ from the reduction process carried out by the decomposition products of H2O2. Reduction in absorbance relative to the ABTS˙+ blank varied significantly across different material systems at constant concentrations of ABTS˙+ and H2O2. The ZIF-L/Au 075 sample showed an optimal catalytic activity with ABTS˙+ intensity degradation of up to 53.1%, while the ZIF-L, AuNPs, ZIF-L/Au 050, and ZIF-L/Au 100 samples showed a lower catalytic activity of about 36.7, 8.4, 11.2, and 26.5%, respectively. The synergistic effect between AuNPs and ZIF-L leads to a significant improvement in peroxidase-like activity. However, the high concentration of AuNPs in the ZIF-L/Au 100 composite could easily cause the agglomeration of AuNPs, resulting in the blocking of the physical contact between H2O2 and the catalyst center. In addition, agglomeration significantly reduces the LSPR effect, which indirectly reduces the material's catalytic activity. As shown in Fig. 5b, to demonstrate the peroxidase-like activity of the ZIF-L/Au 075 material, five different reaction conditions were used. Under the same reaction conditions, the sample containing ZIF-L/Au 075, ABTS˙+, and H2O2 showed complete disappearance of the characteristic absorption peaks of ABTS˙+ corresponding to the color change of the solution from blue to colorless. Unlike the remaining reaction conditions, there was invariable change in the intensity of the characteristic absorption peaks of ABTS˙+. The concentration of ABTS˙+ was calculated from the Beer–Lambert equation from the absorbance and molar extinction coefficient of ABTS˙+ at 417 nm (36 000 M−1 cm−1). The variation of concentration with time allowed the determination of the ABTS˙+ reaction rate at different H2O2 concentrations (Michaelis–Menten plot, Fig. 5c). The Lineweaver–Burk plot (Fig. 5d) was constructed to determine the two parameters Km and Vmax of the system and to identify the optimal Au ratio. The Km value of the ZIF-L/Au 075 material system reached 58.63 lower than that of the ZIF-L and ZIF-L/Au 050 material systems (62.38 and 116.7, respectively) (Fig. S5), proving that the affinity of ZIF-L/AuNPs to H2O2 was optimized under the ZIF-L/Au 075 sample ratio conditions. The synergistic effect of ZIF-L and AuNP materials is mainly responsible for the significantly increased affinity of ZIF-L/AuNPs to H2O2, making the reaction possible even in samples containing very low amounts of H2O2. On the other hand, the stability of ZIF-L/Au 075 materials was indirectly evaluated based on UV-Vis absorption spectra at the time of synthesis and after 1 year of storage in the dark at 4 °C (Fig. S6). As demonstrated in numerous previous studies, the LSPR peak of AuNPs is highly sensitive to changes in the dielectric medium.29 Therefore, small changes in AuNPs (agglomeration, concentration depletion, or structural modifications) can alter the shape, intensity, or location of the LSPR peak of the material. Monitoring the LSPR peak of the ZIF-L/Au material over time under dark conditions at 4 °C revealed no significant changes in position, intensity, or shape during the one-year survey period, providing compelling evidence for the material's stability.

Fig. 5. (a) The peroxidase-like activity of single and composite ZIF-L/Au samples with different Au ratios, (b) UV-Vis spectra of different reaction systems, (c) the Michaelis–Menten kinetics of the ZIF-L/Au075/ABTS˙+/H2O2 system at different H2O2 concentrations, and (d) the corresponding Lineweaver–Burk plot.

Fig. 5

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In addition, the BET surface area and pore size distribution of the ZIF-L and ZIF-L/Au (investigated ratio) powders were recorded as shown in Fig. 6a. All four samples exhibited Type IV isotherms characteristic of mesoporous materials formed by interparticle voids, consistent with the leaf-like structure of ZIF-L. The ZIF-L/Au 075 sample had the highest surface area, approximately 31.20 m2 g−1. Table 1 shows the textural properties of ZIF-L and ZIF-L/Au at corresponding ratios. This indicates that the optimal Au content in the ZIF-L/Au 075 sample causes AuNPs to intercalate between the ZIF-L sheets, thereby preventing the close stacking of the nanosheets. Consequently, new mesopores form, increasing the material's surface area. At the same time, a higher number of exposed active sites (AuNPs and Zn2+) enhances the material's catalytic activity and facilitates the diffusion of analytes toward the active sites. Simultaneously, the chemical states of the components provided by the XPS spectrum (Fig. 6b) once again confirm the formation of the ZIF-L/Au composite material corresponding to the presence of distinct regions for N 1s, Au 4f, Au 4d, C 1s, O 1s, and Zn 2p. In detail, Fig. 6c and d show the spin–orbit splitting in higher energy regions than pure Au0 of the Au 4f and Au 4d levels corresponding to the states Au 4f7/2 (85.03 eV), Au 4f5/2 (89.01 eV), Au 4d5/2 (335.21 eV), and Au 4d3/2 (353.04 eV), indicating electron transfer from Au to the ZIF-L framework. The change in the surface electron density of Au enhances the adsorption of H2O2 on the AuNP surface, thereby boosting its peroxidase-like activity. Similarly, the two sharp peaks at 1021.52 and 1044.53 eV characterize the spin–orbit splitting of Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 6e). The imidazole (C double bond), pyridinic nitrogen (C–N Created by potrace 1.16, written by Peter Selinger 2001-2019 ), and graphitic nitrogen (C–NH–) ligands are characterized by the C 1s, N 1s, and O 1s spectra (Fig. 6f–h) at binding energies of 285.55, 398.64, 407.11, and 531.57 eV.

Fig. 6. (a) N2 adsorption–desorption isotherm of ZIF-L and ZIF-L/Au. (b) XPS spectrum of ZIF-L/Au 075. High resolution (c) Au 4f, (d) Au 4d, (e) Zn 2p, (f) C 1s, (g) N 1s, and (h) O 1s XPS spectra.

Fig. 6

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Table 1. BET analysis of ZIF-L, ZIF-L/Au 050, ZIF-L/Au 075, and ZIF-L/Au 100 samples.

Sample Total surface area (m2 g−1) Micropore volume (m3 g−1)
ZIF-L 22.32 0.0202
ZIF-L/Au 050 16.38 0.0199
ZIF-L/Au 075 31.20 0.0285
ZIF-L/Au 100 14.92 0.0167
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Colorimetric detection of TMP

The catalytic discoloration of ABTS˙+ by ZIF-L/Au composites can be inhibited in the presence of TMP molecules. Leveraging this phenomenon, a colorimetric sensor for TMP detection was developed, preserving the typical simplicity of this type of sensor while increasing the friendliness of green-synthesized sensor materials. ZIF-L, with its leaf-like structure, acts as a support that stabilizes and effectively disperses AuNPs. As a result, TMP adsorbs more easily onto the ZIF-L/Au surface through coordination bonding between the amine group in the TMP molecule and the AuNPs.30 The adsorption of TMP molecules on the AuNP surface reduces the active sites, thereby slowing the catalytic reduction of ABTS˙+ and decreasing its free-radical scavenging efficiency. The temperature and incubation time for ZIF-L/Au and TMP were optimized to ensure assay sensitivity (Fig. S7). The characteristic absorbance of ABTS˙+ at 417 nm after 45 min of incubation at RT (25 °C) showed the highest inhibitory effect on the catalytic process. Therefore, this condition was used for the subsequent experimental steps. Evidence from XPS shows that the binding energy of Au 4f7/2 decreased from 85.03 to 83.39 eV and Au 4f5/2 decreased from 89.15 to 84.96 eV (close to the pure Au0 level) after the material was incubated with TMP for 45 min at RT (Fig. S8). Furthermore, this very clear change in peak position and shape demonstrates the formation of a chemical bond between TMP and the ZIF-L/Au material system. On the other hand, the sharp increase in intensity and the shift towards the low-energy region of C 1s also indicate high-density adsorption on the material surface. This is consistent with the UV-Vis absorption spectra results of ZIF-L/Au 075 material and ZIF-L/Au 075 annealed for 45 min with TMP, which showed the disappearance of the characteristic absorption peak of AuNPs (Fig. S9). At the same time, the expanded porous structure of ZIF-L enriches the TMB in the vicinity of the Au sites (increasing the local effective concentration), while electronic modification at the Au surface (observed via XPS) enhances TMB affinity at the reaction centers themselves. As a result, the increasing TMP concentration is associated with the increasing absorbance at 417 nm (Fig. 7). The inset image in Fig. 7a visually confirms the color intensification of the solution at higher TMP concentrations after 10 min of reaction. Based on the absorbance variation at 417 nm, the TMP detection range was determined to be 0–5 ppm, with a linear range of 0–0.05 ppm, corresponding to a correlation coefficient of R2 = 0.9948 (Fig. 7b). The calculated detection limit, based on linearity,31 was determined to be as low as 4.38 nM, approximately 4 times lower than the maximum concentration of TMP residues in fisheries and fishery products regulated in Vietnam. This LOD value is comparable with those of recent assays for detecting TMP residue (Table 2). The analytical performance of sensor platforms heavily relies on the strategic design of nanoparticle morphology and surface interactions. For instance, composite platforms integrating anisotropic oxides with conductive carbon networks, such as shuttle-shaped α-Fe2O3 decorated on multi-walled carbon nanotubes, have been shown to improve the electroactive surface area and electron-transport capacity significantly. This proves that the analytical performance of sensor platforms heavily relies on the strategic design of nanoparticle morphology and surface interactions. Integrating metal nanomaterials with high-surface-area materials helps control density loss, prevent aggregation, and enhance the platform's catalytic efficiency. Analogously, the 2D leaf-like structure of ZIF-L serves as a high-surface-area scaffold that restricts Au nanoparticle aggregation while facilitating rapid electron transport. Overall, research on sensors for detecting TMP has seen a significant shift in recent years, moving from simple colorimetric sensors to electrochemical and fluorescence sensors. For example, Ding et al. developed an electrochemical aptasensor by integrating TMP-specific aptamers with Pd@Ir bimetallic nanoparticles to enhance signal and enable a hybrid chain reaction (HCR) for further amplification, achieving trace sensitivity (LOD = 0.02 nm mL−1).32 Although the sensitivity and selectivity of the substrate are significantly improved by integrating TMP-specific aptamers with Pd@Ir bimetallic nanoparticles, this approach entails a more complex fabrication procedure and requires stricter storage conditions. Also, Wang et al. developed an open droplet microchannel-based magnetosensor for an immunofluorometric assay (OMM-IFA) with an LOD of about 0.30 and 0.02 ng mL−1 in chicken and pork, respectively, over a wide detection range.33 While sensitive, these techniques require expensive instrumentation and sophisticated operations, hindering on-site screening. Due to TMPs having several structural limitations that can impede improvements in sensitivity in colorimetric sensor development, leveraging synergistic properties in suitable material combinations is key to developing colorimetric sensors for TMP detection. Evidence shows that combining a 2D-structured ZIF-L material with an optimized metal-center density and decorating it with gold nanoparticles increases the contact area between TMP and the reaction sites. Simultaneously, the synergistic effect between ZIF-L and AuNPs significantly improves the material's catalytic activity. On the other hand, compared with previously reported nanozyme-based colorimetric sensors, the conversion of ABTS (colorless) to ABTS˙+ (blue) prior to addition to the assay system reduces signal noise. It improves color recognition: the negative sample is colorless, and the positive sample is blue. On the other hand, nanozyme-based colorimetric sensors are typically operated in acidic environments with a pH range of 3.5–4.5, which reduces the durability of MOF materials. Therefore, performing the assay in a neutral environment is crucial to addressing a key gap in affordable, portable microanalysis for MOF-based colorimetric sensors.

Fig. 7. (a) UV-Vis spectra of ZIF-L/Au 075/ABTS˙+/H2O2 with different concentrations of TMP and inset: photos showing the corresponding sample color. (b) Absorbance at 417 nm of the system as a function of TMP concentration and inset: photos showing the linear relationships between TMP concentration and the absorbance. (c) Selectivity profile of the sensor toward TMP against various interfering agents. Statistical significance was determined by one-way ANOVA with Tukey's post-hoc test (p < 0.001 compared to TMP).

Fig. 7

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Table 2. Comparison of the LOD values of ZIF-L/Au 075 composite-based assays with those of recent assays for TMP detectiona.

Material Method LOD Reference
MWCNT/PBnc Electrochemical sensing 60 nM 37
AgNPs/sol–gel mesoporous carbon ceramic composite Electrochemical sensing 120 nM 38
MoS2@CNTs Electrochemical sensing 27 nM 39
Open droplet microchannel-based magnetosensor Immunofluorometric assay 1.03 nM 33
AuNPs/TLC plates TLC-SERS 170 nM 40
Eu(iii)-functionalized iCOF hybrids Fluorescence 139 nM 41
ZIF-L/Au 075 Colorimetry 4.38 nM This work
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a

LOD: limit of detection; MWCNTs: multi-walled carbon nanotubes; PBnc: Prussian blue nanocube; AgNPs: silver nanoparticles; AuNPs: gold nanoparticles; TLC: thin layer chromatography; iCOF: ionic covalent organic framework; SERS: surface-enhanced Raman spectroscopy.

To evaluate sensor specificity, various antibiotics, metal ions, and organic compounds, including penicillin, ciprofloxacin, oxacillin, erythromycin, chloramphenicol, streptomycin, K+, Na+, NH4+, Si2+, and methylene blue, were tested at a concentration of 5 ppm. Each interfering agent exhibited different levels of interaction with the ZIF-L/Au composite, resulting in varied ABTS˙+ scavenging rates. These differences are clearly reflected in the UV-Vis absorption spectra. As shown in Fig. 7c, only TMP exhibited an increase at 417 nm, demonstrating the high specificity of this assay. To confirm the statistical validity of this selectivity, a one-way ANOVA with Tukey's post-hoc analysis was performed (Table S1). The analysis revealed that the current response to TMP was significantly higher than that to all other interfering agents (F value = 750 and p < 0.001), demonstrating the outstanding selectivity of the developed method. However, real-world samples (such as environmental water, pharmaceutical wastewater, or biological fluids) often contain more complex components that generally reduce the sensor accuracy (e.g., background signal interference and false positives). Pre-treatment of the water samples including filtration or centrifugation can mitigate background interference and reduce the impact of certain impurities. Additionally, incorporating capture agents or surface-modifying materials can enhance the sensor's specificity.

Overall, nanostructured materials are increasingly demonstrating their role and potential applications in sensing and biomedical fields. Platforms combined with MOFs, to create multifunctional material structures, have received considerable attention in recent years. MOFs facilitate material distribution, increasing surface interactions and functional integration, thereby improving the sensor's sensitivity and selectivity and expanding its biomedical applications.34 For instance, MOF-coated MnO2 nanosheets showed the ability to co-deliver survivin to inhibit DZ and to co-deliver doxorubicin (DOX) for highly effective chemo-gene combinatorial treatment of cancer.35 Cobalt metal–organic frameworks were synthesized and then used as a carrier for AuPtNPs and as a secondary antibody for detecting carcinoembryonic antigen 15-3 using an electrochemical sensor.36 Similarly, ZIF-L/Au, with its good dispersibility in water and its peroxidase-like activity in neutral environments, shows great potential for expanded applications in biomedical fields. The 2D ZIF sheet acts as a robust matrix that stabilizes AuNPs and provides additional active sites via Zn2+ centers, thereby enhancing catalytic efficiency. However, cytotoxicity remains a limitation that needs careful evaluation in biomedical applications.

Conclusions

In this work, we successfully developed a DES-assisted green synthesis strategy to produce a stable and uniformly Au-decorated ZIF-L composite. The resulting ZIF-L/Au material exhibited strong peroxidase-like activity, efficiently decolorizing ABTS˙+ radicals. In the presence of TMP, this catalytic decolorization was inhibited, enabling sensitive and selective colorimetric detection of TMP. With its simplicity, high selectivity, and environmentally friendly fabrication, the ZIF-L/Au composite shows strong potential for practical on-site monitoring of antibiotic residues. Overall, this study highlights the promise of green-engineered MOF-based hybrids as sustainable and high-performance sensing materials.

Author contributions

T. H. Vu: conceptualization, formal analysis, investigation, validation, visualization, writing – original draft. N. X. D. Mai: formal analysis, investigation, software, writing – review and editing. N. H. T. Tran: formal analysis, investigation, writing – review and editing. N. Y. Lee: validation, formal analysis, writing – review and editing. K. T. L. Trinh: conceptualization, methodology, formal analysis, funding acquisition, project administration, supervision, resources, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

NA-OLF-D6NA00106H-s001

Acknowledgments

This research was funded by Vietnam National University, Ho Chi Minh City (VNU-HCM) under grant number B2025-50-01.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: FTIR spectrum, and XRD pattern of synthesis of ZIF-L using DES-assisted and DES-free methods; SEM image, EDX mapping result, and EDX spectrum of ZIF-L; TEM images, the size distribution of AuNPs, EDX mapping results, and EDX spectrum of ZIF-L/Au 100; TEM images, the size distribution of AuNPs, EDX mapping results, and EDX spectrum of ZIF-L/Au 050; the Michaelis–Menten kinetics follow with H2O2 concentration of ZIF-L/ABTS˙+/H2O2 system, ZIF-L/Au 050/ABTS˙+/H2O2 system, ZIF-L/Au 100/ABTS˙+/H2O2 system and the corresponding Lineweaver–Burk plot; UV-vis spectra of ZIF-L/Au 075 at the time of synthesis and after 1 year of storage; effect of temperature and reaction time on the detection of TMP; XPS spectrum of ZIF-L/Au 075 material before and after incubating for 10 min with TMP at room temperature; statistical significance was determined by one-way ANOVA with Tukey's post hoc test. See DOI: https://doi.org/10.1039/d6na00106h.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

NA-OLF-D6NA00106H-s001
NA-OLF-D6NA00106H-s001.pdf (1.7MB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: FTIR spectrum, and XRD pattern of synthesis of ZIF-L using DES-assisted and DES-free methods; SEM image, EDX mapping result, and EDX spectrum of ZIF-L; TEM images, the size distribution of AuNPs, EDX mapping results, and EDX spectrum of ZIF-L/Au 100; TEM images, the size distribution of AuNPs, EDX mapping results, and EDX spectrum of ZIF-L/Au 050; the Michaelis–Menten kinetics follow with H2O2 concentration of ZIF-L/ABTS˙+/H2O2 system, ZIF-L/Au 050/ABTS˙+/H2O2 system, ZIF-L/Au 100/ABTS˙+/H2O2 system and the corresponding Lineweaver–Burk plot; UV-vis spectra of ZIF-L/Au 075 at the time of synthesis and after 1 year of storage; effect of temperature and reaction time on the detection of TMP; XPS spectrum of ZIF-L/Au 075 material before and after incubating for 10 min with TMP at room temperature; statistical significance was determined by one-way ANOVA with Tukey's post hoc test. See DOI: https://doi.org/10.1039/d6na00106h.

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