Near‐Infrared Fluorescent Single‐Walled Carbon Nanotubes for Biosensing

Abstract

The objective of this article is to provide a comprehensive overview of the recent advancements in biosensing using near‐infrared (NIR) fluorescent single‐walled carbon nanotubes (SWCNTs). SWCNTs are cylindrical structures formed by rolling up a graphene layer, with their chiral index (n,m) defining their diameter and electronic, mechanical, and optical properties, making them metallic, semimetallic, or semiconducting. The semiconducting variants feature NIR fluorescence, which offers significant advantages for biological imaging and sensing due to deep tissue penetration and minimal background interference. Moreover, SWCNTs are highly photostable, demonstrating resistance to photobleaching and blinking. Owing to these unique optical properties, SWCNTs have been widely used as optical probes for monitoring a broad spectrum of biological analytes, ranging from small molecules to macromolecules. This review explores the photophysics of SWCNTs, their suitability for biosensing, and strategies for developing effective SWCNT‐based sensors. This review begins with their photophysical properties, highlighting their relevance to biosensing, followed by key technical concepts. Additionally, biosensing principles, methods for optimizing functionalized SWCNTs, and diverse sensing approaches are also covered. Overall, this review intends to provide a foundational understanding of SWCNT‐based biosensing, equipping readers with the knowledge needed to explore and apply these powerful nanomaterials in diverse biosensing applications.

Near‐infrared fluorescent single‐walled carbon nanotubes (SWCNTs) stand out as versatile biosensors due to their spectral overlap with the biological transparency window, surface‐sensitive fluorescence emission, and chemical modularity. This review offers a concise guide to understanding the photophysics of SWCNTs, various functionalization strategies, and sensor design principles, thereby empowering researchers to optimize probe selection and engineer tailored sensing platforms for a variety of biological targets.

1. Introduction

Single‐walled carbon nanotubes (SWCNTs) have emerged as powerful tools in biosensing applications, particularly due to their unique near‐infrared (NIR) fluorescent properties. Before exploring their diverse biosensing applications, it is essential to understand the fundamental nature of SWCNTs, the mechanisms governing their fluorescence in the NIR spectral region, and the distinctive characteristics of their emission spectrum. Carbon nanotubes (CNTs) can be visualized as cylindrical structures formed by rolling sheets of graphene.^{[ 1 ]} The characteristics of these nanotubes are dictated by the specific arrangement of the sp²‐hybridized carbon atoms and whether they are composed of single or multiple graphene layers. CNTs made from just one layer are known as single‐walled carbon nanotubes, while those consisting of multiple concentric layers are termed multi‐walled carbon nanotubes (MWCNTs).^{[ 2 ]} The structure of SWCNTs is typically described using a pair of integers (n, m), known as the chiral index, which defines the rolling angle of the graphene sheet.^{[ 3} , ⁴ , ^{5 ]} This index also determines the nanotube's diameter, which can vary, typically from as small as 0.4 nm up to ≈10 nm, depending on the specific chirality and structural arrangement.^{[ 6 ]} Furthermore, recent literature provides a detailed mechanism for understanding SWCNT formation, highlighting its clear dependence on chirality.^{[ 7 ]} Due to their entirely carbon‐based structure, SWCNTs are inherently hydrophobic, necessitating the use of suitable coatings to make them dispersible in aqueous environments, which is a precondition for biological applications. These coatings typically consist of (bio)molecules that can interact with the nanotube surface through hydrophobic forces or π‐π stacking, while their hydrophilic components enable stable suspension in water.^{[ 8} , ^{9 ]} This dual functionality is essential for integrating SWCNTs into biological systems and consequent biomedical applications.^{[ 10 ]}

The first indication of a successful SWCNT suspension is an absorption spectrum, spanning the visible to NIR range, where distinct structure‐dependent absorption peaks of SWCNTs appear over a broad background. This background arises from unintended chemical reactions or damage, SWCNTs sidewall functionalization, or amorphous carbon. Pristine, well‐dispersed SWCNTs enriched in a specific (n, m) species show minimal background, whereas samples with diverse semiconducting species or aggregated SWCNTs exhibit overlapping or broadened resonant peaks.^{[ 11 ]}

When suspended SWCNTs are illuminated by an excitation source, excitons, i.e., strongly correlated electron‐hole pairs bound by Coulombic attraction, are generated and can diffuse along the nanotube axis.^{[ 12} , ^{13 ]} In semiconducting SWCNTs, the absorption of photons in the visible spectrum generates the excitons,^{[ 13 ]} which can undergo radiative recombination, producing the distinctive NIR fluorescence characteristic of SWCNTs, typically at wavelengths greater than 870 nm^{[ 10} , ^{14 ]} (Figure 1A), which is highly advantageous for biological sensing owing to the reduced scattering, absorption, and autofluorescence emission from interfering biological components (Figure 1B).^{[ 10 ]} Quantum chemical predictions reveal the existence of four singlet and twelve triplet exciton states, though only transitions from the singlet state are optically allowed.^{[ 15} , ^{16 ]} As the singlet state resides at a higher energy than most other excitonic states, a range of non‐radiative decay pathways exist for dark excitons. For instance, in the case of (6,5) SWCNTs, the exciton extends over approximately 2 nm and can diffuse up to 100 nm along the nanotube axis during its lifetime.^{[ 10 ]} Since SWCNTs consist entirely of carbon atoms, exciton dynamics are profoundly influenced by the surrounding coating (often referred to as corona), which consists of molecules or ions adsorbed onto the nanotube surface. This close interaction between the excitons and the nanotube's environment significantly alters their photophysical properties, making SWCNTs highly sensitive probes for chemical and biological sensing applications.^{[ 17} , ^{18 ]}

Figure 1.

A) Bandgap structure of semiconducting SWCNTs, illustrating the origin of their NIR fluorescence, with excitation in the visible range and NIR emission at wavelengths exceeding 870 nm. B) The optical emission spectrum of (6,5) SWCNTs, demonstrating the overlap of SWCNT fluorescence with the tissue transparency window, where biological tissues exhibit minimal absorption, scattering, and autofluorescence. Reproduced with permission from ref. [10] Copyright 2022, John Wiley & Sons.

As implied, the corona of SWCNTs plays a major role in controlling their optical properties. Additionally, it also makes the SWCNTs chemically interactive and more sensitive to their surrounding environment. Therefore, since both the chemical reactivity and optical properties are governed by the same moiety—the corona—a chemical interaction occurring at the surface of the SWCNTs is translated into a corresponding modulation of their optical signals.^{[ 19 ]} This process of converting chemical changes into optical signals is known as optical transduction, which is essential for the effective sensing capabilities of SWCNTs in various applications.^{[ 20 ]}

Optical transduction is fundamentally based on the interaction of exogenously introduced molecules with either the corona or the SWCNTs themselves. This interaction induces changes in the surface composition of the SWCNTs, resulting in alterations to their optical properties. Notably, optical transduction is primarily governed by mechanisms such as charge transfer, dielectric screening, exciton quenching, and Förster resonance energy transfer (FRET). When external molecules interact with either the SWCNT corona or the nanotube surface, charge transfer may occur, altering exciton recombination efficiency.^{[ 21 ]} Dielectric screening modifies the local electrostatic environment, leading to shifts in SWCNT fluorescence emission wavelengths.^{[ 22 ]} Exciton quenching, which can result from direct energy transfer to the surrounding medium or non‐radiative recombination, further influences fluorescence intensity. Additionally, in cases where fluorescent molecules are present near the SWCNT corona, FRET can occur when there is sufficient spectral overlap between the donor's fluorescence emission and the acceptor's absorption spectrum, leading to energy transfer that modulates SWCNT fluorescence in a distance‐dependent manner.^{[ 23 ]} Consequently, these variations in optical signals provide valuable insights into the concentration, as well as the physical and chemical characteristics, of the added molecules. Thus, SWCNTs effectively function as optical sensors, translating chemical interactions into quantifiable optical responses that can be leveraged for various (bio)analytical applications.^{[ 20} , ^{24 ]}

A main advantage of SWCNT‐based sensing for biological applications arises from their fluorescence in the NIR range.^{[ 20} , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ^{31 ]} Compared to the UV‐visible spectrum, the NIR spectral region experiences significantly less background interference, making it particularly advantageous for biological sensing.^{[ 32} , ³³ , ^{34 ]} Biological tissues and fluids exhibit minimal absorption and scattering in the NIR range, allowing for clearer signals and improved sensitivity in detecting target analytes (Figure 1B).

Furthermore, long‐term stability is a crucial factor in ensuring the consistent performance of SWCNTs‐based sensors over extended periods.^{[ 35} , ^{36 ]} Stability can be affected by environmental conditions, chemical degradation, and the structural integrity of the sensing interface. To this end, proper functionalization strategies, such as polymer wrapping, covalent modifications, or stabilizing surfactants, can enhance the stability of these sensors in complex biological and environmental media.^{[ 37} , ^{38 ]} Additionally, structural stabilization plays a key role in preserving the optical and electronic properties of SWCNTs. Preventing aggregation and maintaining nanotube dispersion through controlled surface chemistry and optimized corona phases ensure reliable and reproducible sensing performance over time. Further, for optical probes to be successfully implemented in sensing and real‐time monitoring, they must maintain photostability for the duration required by the sensing process. This stability is essential for consistent data collection, preventing signal degradation, and ensuring accurate detection. High photostability is particularly crucial for real‐time tracking of biological events, as it enables prolonged observation without fluorescence loss. SWCNTs fulfill this requirement due to their minimal photobleaching and blinking, making them highly suitable for extended visualization in complex biological settings. Moreover, they exhibit remarkable stability across a wide range of physiological conditions, including variations in pH, temperature, and ionic strength, further reinforcing their potential as robust optical sensors.^{[ 39 ]} Consequently, the use of SWCNTs as optical probes in the NIR enhances their effectiveness and reliability in various biosensing applications, offering long‐term performance with minimal signal degradation.^{[ 23} , ²⁴ , ²⁸ , ³³ , ³⁴ , ⁴⁰ , ⁴¹ , ⁴² , ^{43 ]} Finally, SWCNTs, when dispersed using suitable corona phases, exhibit excellent biocompatibility and long‐term stability in biological environments.^{[ 44} , ⁴⁵ , ⁴⁶ , ^{47 ]} Functionalizing their surface with hydrophilic groups such as hydroxyl, carboxyl, or amine enhances their solubility in aqueous media while also improving stability and biocompatibility. The addition of biocompatible polymers like poly(ethylene glycol) (PEG) to these functionalized SWCNTs helps minimize oxidative stress and reduce cytotoxicity. Furthermore, noncovalent coatings with biocompatible materials provide an extra layer of protection, preventing nanotube aggregation and further lowering toxicity. These modifications not only improve SWCNTs stability but also enable their use in biosensing applications.

Building on the discussion of SWCNT photophysics and the origin of their fluorescence, this article examines their potential for biosensing and the key strategies for developing effective SWCNT‐based sensors (Scheme 1 ). We introduce fundamental technical concepts related to SWCNT‐based biosensing and outline the core principles underlying their application in biological detection. Additionally, we explore various functionalization strategies that enhance their sensing capabilities and highlight different biosensing approaches utilizing SWCNTs for diverse applications. Overall, this article provides a comprehensive foundation in SWCNT‐based biosensing, equipping readers with the necessary insights to design and implement these nanomaterials in real‐world detection systems.

Scheme 1.

Schematic illustration of biosensing using NIR fluorescent SWCNTs, highlighting key terms in the sensing process, the optical properties of SWCNTs, essential strategies for sensor selection, and different types of SWCNT‐based sensing.

2. Sensor Selection

Given the demonstrated potential of SWCNTs in sensing biologically relevant analytes, we now turn to explore the essential considerations and guidelines for selecting an appropriate sensor tailored to specific biosensing applications. As previously discussed, functionalizing the surface of SWCNTs with an appropriate corona is essential for improving their solution processability, stability, and biocompatibility. In this context, the Swager research group has outlined various strategies for SWCNTs functionalization, employing both covalent and non‐covalent methods while clearly delineating the advantages and limitations of each approach.^{[ 48 ]} The choice of a SWCNTs‐based sensor is influenced by several crucial factors, including the particular analyte of interest, the required sensitivity and selectivity, the environmental conditions, and the context of the intended application. Moreover, for reversible sensing, weak target‐analyte interactions facilitate rapid and complete reversibility, but often at the cost of selectivity, which typically requires stronger binding. Thus, achieving an optimal balance is essential, with the SWCNTs corona—being the only tunable component—playing a pivotal role in fine‐tuning these interactions for modular reversible sensing.^{[ 49 ]} Furthermore, the significance of the corona phase becomes particularly evident when undertaking the critical task of this selection process, as it plays a key role in the sensor's performance for a given application.^{[ 50} , ^{51 ]} To this end, we discuss various strategies to identify the most suitable sensor with the greatest potential for successful application, addressing each approach in detail.

2.1. Rational Design of the Corona of SWCNT‐Based Sensors

The rational design of the corona for SWCNT sensors primarily involves suspending SWCNTs with specific molecular motifs that enhance their responsiveness to target analytes.^{[ 51 ]} In this approach, the corona is chemically engineered to include interactive sites capable of directly engaging with the target analytes. This interaction leads to significant modifications in the chemical composition of the corona following interaction with the analyte. These alterations are then transduced into variations in the fluorescence signals of the SWCNTs, providing valuable insights into both the concentration and the physicochemical characteristics of the analyte. To this end, significant efforts have been devoted to developing rationally designed SWCNT sensors for detecting a plethora of analytes, including important disease biomarkers.

Building on this, the Strano research group pioneered a glucose affinity sensor by constructing dextran‐coated SWCNTs.^{[ 52 ]} When Concanavalin A—a protein with four saccharide binding sites—was introduced to dextran‐coated SWCNTs, it triggered SWCNTs aggregation and a corresponding decrease in their fluorescence. However, upon subsequent binding of glucose, the SWCNTs disaggregated, restoring the fluorescence signal (Figure 2A). This rational corona design enabled highly sensitive and selective glucose detection, showcasing the effectiveness of tailored SWCNTs coronas for biosensing applications. In another study, selective detection of glycans was achieved by designing a specialized corona around SWCNTs, tailored to selectively interact with glycan molecules.^{[ 53 ]} Recombinant lectins, serving as glycan recognition sites, were incorporated into the SWCNTs corona through nickel complexes, establishing a quenching mechanism for fluorescence upon glycan binding. Through this engineered corona, specific glycans such as fucose and N‐acetylglucosamine (GlcNAc) were detected via fluorescence modulation, allowing for highly sensitive and selective detection at the single‐nanotube level. It was notably observed that SWCNTs with intermediate brightness yielded the most robust transduction response, illustrating how a rationally designed corona can optimize sensor performance. Furthermore, the Landry research group^{[ 54 ]} developed catalytically inactive apo‐glucose oxidase‐stabilized SWCNTs sensors capable of rapid (within 1 s) and highly sensitive (up to 40% normalized intensity variation) glucose detection in biological fluids and tissues. Unlike conventional glucose sensors that function through catalytic oxidation, these sensors were reported to operate via direct glucose‐GOx binding, ensuring the preservation of the glucose analyte. The Boghossian research group^{[ 55 ]} developed a reversible, mediator‐free NIR glucose sensor using GOx‐suspended SWCNTs for continuous glucose monitoring. Unlike conventional SWCNT sensors that relied on charge transfer and required mediators, this sensor operated via localized enzymatic doping of SWCNT defect sites, leading to a selective fluorescence increase upon glucose binding. A tunable response was exhibited over a physiologically relevant glucose range (3–30 mm) with a Michaelis–Menten constant of ∼13.9 mm. Notably, fluorescence enhancement was fully reversible upon glucose removal, highlighting its potential for real‐time, non‐invasive glucose sensing.

Figure 2.

A) Schematic illustration depicting the aggregation of dextran‐coated SWCNTs upon interaction with Concanavalin A, leading to a decrease in fluorescence, followed by disaggregation upon glucose addition, which restores fluorescence. Reproduced with permission from ref. [52] Copyright 2006, John Wiley & Sons. B) Fluorescence intensity time‐traces showing fluorescence increase of RAP1 aptamer stabilized SWCNTs upon the addition of lysate from cells that were grown to produce RAP1. C1,C2) Corresponding images of the SWCNTs before and after the addition of cell lysate. Reproduced with permission from ref. [58] Copyright 2017, Nature. D) Schematic of the screening and refinement process to determine the most suitable SWCNTs‐based optical sensor for miRNA detection. The screening process involves multiple selection steps to identify a functional SWCNT‐based sensor. First, DNA sequences with an SWCNT‐binding region and a miRNA recognition unit are tested for their ability to suspend SWCNTs. Next, only those that successfully disperse SWCNTs are evaluated in buffer by introducing complementary miDNA and miRNA targets. DNA sequences that produce a fluorescence response in buffer advance to the next stage, where their sensing performance is tested in serum. Ultimately, a single DNA sequence is identified as a viable sensor based on its fluorescence response in both conditions. The right side of the scheme visually represents the stepwise selection process, with the DNA helix illustrating the corona phase around the SWCNTs. Reproduced with permission from ref. [60] Copyright 2023, American Chemical Society.

In another context, a well‐established strategy for detecting disease markers using the rational design of SWCNTs’ coronas involves suspending SWCNTs with antibodies that selectively bind to antigens indicative of serious diseases. The Heller research group leveraged this approach to develop an advanced SWCNTs‐based optical sensor for the early detection of high‐grade serous ovarian carcinoma (HGSC), as conventional HGSC biomarkers, such as CA‐125 and HE4, are typically undetectable in serum until later stages of the disease. To overcome this challenge, the team engineered an implantable prototype sensor composed of a SWCNT complex functionalized with HE4‐specific antibodies.^{[ 56 ]} This configuration allowed for quantitative HE4 detection by modulating the nanotube's fluorescence emission, achieving nanomolar sensitivity sufficient to differentiate malignant from benign biofluids. Encased in a semipermeable membrane, the sensors were successfully implanted into four ovarian cancer models, enabling real‐time optical detection of HE4 within live animals. This work represents the first instance of an in vivo optical nanosensor applied for noninvasive biomarker detection in orthotopic disease models, highlighting the potential of rationally designed SWCNTs‐based sensors for early cancer diagnosis.

Therapeutic outcomes for prostate cancer patients are often compromised by the challenge of distinguishing indolent from aggressive disease. To address this issue, a quantitative fluorescent nanosensor was developed for detecting the cancer biomarker urokinase plasminogen activator (uPA).^{[ 57 ]} Here, the rational design of the SWCNTs’ corona, achieved through passivation with bovine serum albumin (BSA), was essential for enhancing sensor specificity and stability within complex protein environments like human blood. This tailored corona enabled successful detection by modulating the SWCNT's fluorescence upon interaction with uPA, allowing for precise, quantitative measurements. The engineered sensor demonstrated reliable detection of uPA concentrations in blood products, highlighting its potential as a rapid, point‐of‐care diagnostic tool.

Nanosensor arrays excel in achieving remarkable sensitivity through the strategic placement of sensors in close proximity to target analytes. In this regard, a microfluidic platform was designed for label‐free detection of proteins secreted from individual cells of Escherichia coli and Pichia pastoris.^{[ 58 ]} The array was crafted by employing rational design principles, incorporating a DNA aptamer sequence linked to SWCNTs. A variable chemical spacer was systematically optimized to enhance sensor performance, allowing for the selective detection of unlabelled proteins such as RAP1 GTPase (Figure 2B,C) and HIV integrase, which elicited significant fluorescent responses. Analysis of protein secretion revealed considerable variability, with nutrient‐starved E. coli cells undergoing division exhibiting 66% less protein output compared to their non‐dividing counterparts. Furthermore, the system successfully detected unique proteins produced during T7 bacteriophage infection, underscoring the power of rationally designed nanosensor arrays for the analysis of protein dynamics across diverse cell types. Additionally, the Williams research group employed DNA aptamer‐functionalized SWCNTs for the specific detection of interleukin‐6, achieving a sensitivity threshold of 105 ng mL⁻¹.^{[ 59 ]}

In the context of DNA corona, through the rational design of SWCNT sensors, a systematic approach was employed to target miRNA biomarkers for acute myocardial infarction (AMI).^{[ 60 ]} By integrating a complementary DNA recognition unit onto the SWCNT surface, effective RNA‐DNA hybridization was achieved. Out of five selected miRNA candidates, the optimized sensor, SWCNT‐miDNA208a, demonstrated high specificity and selectivity for the target sequence (Figure 2D), showing significant fluorescence modulation in serum. This approach highlights how rational design facilitates the development of sensitive and specific sensors for early detection of AMI biomarkers.

Enzymes represent another crucial category of biomarkers for disease detection, and the rational design of SWCNTs has improved the monitoring of their activities.^{[ 61 ]} In a study by the Reuel research group, SWCNTs suspended in substrates such as carboxymethylcellulose (CMC), pectin, and BSA were utilized to track the activity of the corresponding target enzymes (Figure 3A1), including cellulase, pectinase, and bacterial protease.^{[ 62 ]} The interactions between these enzymes and their substrates triggered the aggregation of the SWCNTs, leading to detectable changes in their fluorescence properties (Figure 3A2). Additionally, SWCNTs with rationally engineered coronas incorporating polymer‐dendron hybrids featuring enzyme recognition sites (esters and amides) have been utilized to monitor the activities of esterase and amidase.^{[ 63 ]} Furthermore, the conversion of fibrinogen to fibrin, contingent upon thrombin activity, was successfully monitored using fibrinogen‐appended SWCNTs.^{[ 64 ]} Moreover, SWCNTs suspended with myristoylcholine, the substrate for cholinesterase, were also employed to assess cholinesterase activity in blood plasma (Figure 3B) with a limit of detection of 0.0168 ± 0.00005 U L⁻¹. Additionally, SWCNTs suspended by extracellular polymeric substances (EPS) from biofilms were used to fluorometrically monitor EPS degradation caused by hydrolase enzymes.^{[ 65 ]} Together, these innovations highlight the potential of rational design of coronas in optimizing SWCNTs‐based sensors for precise and effective detection of enzyme activities and inhibition.^{[ 66} , ⁶⁷ , ^{68 ]}

Figure 3.

A1) Schematic illustrating the suspension of SWCNTs with the target enzyme's substrate, where enzymatic action induces SWCNT aggregation. A2) Left panel: Time‐dependent fluorescence response of CMC‐stabilized SWCNTs upon cellulase exposure, demonstrating SWCNT aggregation, as shown in digital photographs taken before and after enzyme activity (right panel). Reproduced with permission from ref. [62] Copyright 2018, American Chemical Society. B) Schematic illustration of monitoring cholinesterase activity and inhibition using myristoylcholine‐suspended SWNCTs. Reproduced with permission from ref. [66] Copyright 2024, John Wiley & Sons.

Furthermore, phospholipid polyethylene glycol (PL‐PEG) corona phases were rationally designed for SWCNTs by optimizing factors like chain length, fatty acid saturation, molecular weight, and end‐group functionalization. Demonstrated by the Cho research group, this approach allowed for receptor‐free detection of a broad range of viruses, including Ebola, Lassa, H3N2, H1N1, MERS, SARS‐CoV‐1, and SARS‐CoV‐2, without the need for biological receptors.^{[ 69 ]} Moreover, covalent functionalization of SWCNTs with ssDNA was achieved by anchoring DNA sequences to the SWCNT surface through guanine defects (Figure 4A,B).^{[ 70 ]} Hybridization with complementary nucleic acid strands resulted in the modulation of the SWCNT fluorescence (Figure 4C), with longer capture sequences producing larger fluorescence responses. This approach enhanced detection stability and specificity while enabling the addition of recognition units, making the sensor adaptable to various targets. The functionality of this covalent strategy was validated with sensors for bacterial siderophores and the SARS‐CoV‐2 spike protein, establishing a robust method for developing stable and tunable SWCNTs‐based sensors.^{[ 70 ]}

Figure 4.

A): Schematic representation of SWCNTs‐based sensor design utilizing covalent DNA anchoring. The reaction process was designed to incorporate varying densities of guanine defects, followed by hybridization with complementary ssDNA. Guanine‐rich anchor sequences were covalently attached to the SWCNT surface, while the guanine‐free capture sequence was used to facilitate binding to complementary DNA strands. This capture sequence was further extended with a recognition unit (R), such as an aptamer, for targeted molecular interactions. B) Fluorescence spectra of DNA stabilized SWCNTs before and after defects incorporation using riboflavin (RB) under green light illumination. C) Changes in normalized fluorescence of guanine defects induced SWCNTs following the addition of complementary DNA (A₂₀), noncomplementary DNA (T₂₀), and buffer. Reproduced with permission from ref. [70] Copyright 2023, American Chemical Society.

As evident from the above examples, the rational design of SWCNTs’ coronas has emerged as a transformative approach in the development of highly sensitive and selective biosensors for disease detection. For the rational design of SWCNTs with tailored coronas, it is essential to ensure that the corona's chemical configuration is compatible with the SWCNTs. This requires the presence of hydrophobic or π‐conjugated rings to facilitate strong interactions with the nanotube surface, while also incorporating hydrophilic groups to maintain dispersion in water, particularly for biological applications. Additionally, the analyte binding site within the corona must remain accessible for target interaction, and the binding should occur close to the SWCNT surface to enable efficient optical signal transduction. Moreover, achieving an optimal balance between selectivity and reversibility is crucial, as strong binding enhances specificity, whereas weaker interactions improve reversibility for continuous monitoring. Thus, by tailoring the corona with specific molecular motifs, researchers have successfully enhanced the interaction between SWCNTs and target analytes, leading to significant improvements in optical signal transduction. From glucose sensors to advanced cancer biomarkers, each study emphasizes the versatility and efficacy of rationally designed SWCNTs in various diagnostic applications. The ongoing innovations in sensor design and functionality demonstrate the potential of these nanoscale systems to revolutionize early disease detection and monitoring, paving the way for more effective and personalized healthcare solutions. Further refinements in the rational design of SWCNT coronas, coupled with advancements in machine‐learning efforts,^{[ 71} , ⁷² , ⁷³ , ⁷⁴ , ^{75 ]} are expected to broaden their applicability in biomarker detection, offering new avenues for addressing complex health challenges.

2.2. Screening of Corona Phases for SWCNTs Sensor Selection

In rational design, it is essential to acknowledge that the interaction between an analyte and the corona is often complex and not necessarily specific. Consequently, predicting which corona would optimize interactions with a given analyte remains challenging. Moreover, the size and chemical properties of the recognition unit may not always align with the requirements for stabilizing SWCNTs. Additionally, the precise design of rational corona phases can be chemically challenging and labor‐intensive, making an empirical screening approach essential for identifying optimal compositions. Thus, to identify the most effective sensor, systematic screening of various corona phases through diverse permutations is crucial. This enables exploration of a wide range of coronas, ultimately selecting the one that enhances sensor performance for specific applications, a process referred to as “screening of corona phases for SWCNT sensor selection.” Significant efforts have been directed toward this screening for SWCNT sensors aimed at detecting a broad spectrum of analytes, including key disease markers.

The concept of corona phase molecular recognition (CoPhMoRe) was first introduced by the Strano research group, demonstrating that synthetic heteropolymers, when adsorbed onto SWCNTs, could form a 3D corona phase around the nanotube that enable highly selective recognition of molecules like riboflavin, L‐thyroxine, and estradiol.^{[ 76 ]} Further, the Strano research group utilized the CoPhMoRe technique to identify polymer phases on SWCNTs that enable selective detection of neurotransmitters, particularly dopamine.^{[ 77 ]} By functionalizing SWCNTs with a library of 30 diverse polymers, including phospholipids and nucleic acids, several corona phases were identified that substantially enhanced SWCNT fluorescence in response to neurotransmitter presence (Figure 5A1). Notably, the addition of 100 µm dopamine resulted in a fluorescence increase of 58–80%, achieving a detection limit of 11 nm for (GT)₁₅ DNA‐wrapped SWCNTs (Figure 5A2,‐A3).

Figure 5.

A1): Fluorescence responses of polymer‐stabilized SWCNTs conjugates (x‐axis) to various neurotransmitters (y‐axis) illustrated in a color‐coded heat map. A2) Schematic illustrating fluorescence increase of polymer‐stabilized SWCNTs upon interaction with dopamine. A3) Fluorescence spectra of (GT)₁₅ SWCNTs before (black) and after (red) the addition of dopamine. Reproduced with permission from ref. [77] Copyright 2014, American Chemical Society. B) Screening of different corona phases of SWCNTs for fibrinogen detection. The heat map displays the normalized response of the combined peak from (9,4) and (7,6) SWCNT chiralities to various proteins. Reproduced with permission from ref. [79] Copyright 2016, Nature. C) Screening of different corona phases of SWCNTs for insulin detection. The heat map displays the normalized response of the (10,2) SWCNTs chiralities to various proteins. Reproduced with permission from ref. [80] Copyright 2018, American Chemical Society.

The Strano research group further explored the Interleukin‐6 (IL‐6) family of cytokines, which play regulatory roles in inflammation and various biochemical pathways.^{[ 78 ]} Traditional methods for measuring cytokine levels, such as enzyme‐linked immunosorbent assay (ELISA) and western blotting, often involve lengthy processing times, high costs, specialized machinery, and extensive training. To overcome these challenges, the study focused on understanding the molecular recognition mechanisms of cytokines with synthetic substrates, employing the CoPhMoRe approach to engineer novel carbon nanotube constructs for binding studies. Through library screening, two polymer‐based CoPhMoRe constructs were identified, consisting of SWCNTs complexed with p(AA68‐rand‐BA16‐rand‐CD16) polymer (MK2) and p(SS80‐rand‐BS20) polymer (P14). The constructs exhibited dissociation constants (KD) of 8.38 ng mL⁻¹ and 16.7 µg mL⁻¹, respectively, compared to the natural IL‐6 receptor's KD of ≈0.32 ng mL⁻¹. Remarkably, the MK2 constructs exhibited a nonmonotonic response function upon IL‐6 binding, and comparative binding experiments indicated recognition of the α‐helical structures characteristic of the IL‐6 family.

In the domain of protein detection, the Strano research group developed a specialized screening procedure utilizing SWCNTs to target human blood proteins, successfully identifying a corona phase that selectively recognized fibrinogen (Figure 5B).^{[ 79 ]} Upon binding, SWCNTs fluorescence decreased by over 80% at saturation, with quenching kinetics suggesting sequential binding of the three fibrinogen nodules. Notably, this recognition persisted in serum at clinically relevant fibrinogen concentrations, paving the way for synthetic antibody analogs in medical and clinical applications.

Additionally, the Strano research group conducted high‐throughput screening of a library of poly(ethylene glycol) (PEG)‐conjugated lipids adsorbed onto SWCNTs to identify a corona phase selective for insulin (Figure 5C).^{[ 80 ]} They found that C16–PEG(2000 Da)–ceramide caused a 62% decrease in the fluorescent intensity of (10,2) chirality nanotubes in the presence of 20 µg mL⁻¹ insulin, despite showing no prior affinity in free solution, as confirmed by isothermal titration calorimetry. Testing a panel of human blood proteins and short insulin peptide fragments ruled out nonselective recognition mechanisms. Interestingly, longer α‐ and β‐peptide chains of insulin were detected with lower affinity, indicating that the construct recognized insulin in its native form. Successful insulin recognition and quantification were demonstrated in both buffer and blood serum, showcasing the construct's efficacy in complex environments. These findings laid the groundwork for developing nonbiological synthetic recognition sites and continuous in vivo insulin monitoring, potentially enhancing glycemic control in closed‐loop artificial pancreas systems.

In the area of small molecule sensing, the Sen research group utilized high‐throughput screening of corona phases formed by polymer‐coated SWCNTs to achieve selective detection of nitric oxide (NO).^{[ 81 ]} By functionalizing the (7,6)‐chiral SWCNT surfaces with various ssDNA oligonucleotides, they identified the (AT)₃₀ sequence as effective for NO sensing. The sensor demonstrated a LOD of 1.24 µm and a limit of quantification (LOQ) of 4.13 µm for NO, highlighting the potential of this approach for single‐molecule detection and early disease diagnosis.

In the realm of enzyme activity monitoring, the Bisker research group conducted a library screening of various DNA oligomers to identify optimal corona phases for SWCNTs.^{[ 82 ]} The oligomers (GT)₁₅ and T₃₀ were highlighted as the most effective for facilitating the detection of thiocholine, the product of cholinesterase activity, in blood serum. This detection was achieved through the interaction of thiocholine with the corona surrounding the SWCNTs.

In this regard, given the challenge of screening extensive corona phase libraries in CoPhMoRe, the Strano research group developed a streamlined, self‐templating approach.^{[ 83 ]} By linking steroid molecules (such as progesterone or cortisol) directly to a styrene‐acrylic acid copolymer, they allowed the steroid itself to direct the polymer's structure, enabling more selective sensing without the need for extensive library screening. These sensors proved to be both stable and reversible, performing reliably over repeated hormone detection cycles. The team further validated their method in vivo by implanting the sensors in mice, with a protective hydrogel and cellulose layer to reduce nonspecific absorption. This self‐templating strategy presents a promising pathway for continuous, robust hormone monitoring in live environments. Furthermore, the Cho research group has successfully diagnosed cerebrospinal fluid (CSF) leakage using the CoPhMoRe approach. Specifically, they developed a library of custom‐designed PEG‐lipids and, through high‐throughput screening, identified an optimal corona capable of detecting the CSF biomarker β‐trace protein.^{[ 84 ]}

Further, accurate analyte classification remains a key challenge in sensor technologies. The Analyte Classification and Feature Selection Algorithm (ACFSA) offers a computational framework to identify optimal sensor combinations based on unique fingerprint patterns.^{[ 85 ]} Applied to a library of peptide‐functionalized SWCNTs designed for NIR fluorescent sensing of heavy metal ions, ACFSA effectively optimized sensor selection by analyzing fluorescence response patterns. By leveraging diverse peptide sequences, SWCNTs chiralities, and photochemical modifications, the screening approach enabled the identification of a minimal two‐sensor set with an ultra‐low classification error of 0.02%. This study demonstrates the potential of the peptide‐SWCNTs platform as a versatile tool for facile sensor selection and analyte fingerprinting in biosensing applications.

In summary, innovative approaches to corona phase screening for SWCNT sensors have significantly advanced the selective detection of various analytes, ranging from neurotransmitters, cytokines, and small molecules to proteins and enzymes.^{[ 86 ]} These methodologies demonstrate the potential of rational design and systematic screening in optimizing sensor performance through tailored polymer phases. These developments not only enhance our understanding of molecular recognition mechanisms but also pave the way for the creation of robust, selective sensors applicable in clinical diagnostics and real‐time monitoring of biochemical processes. As research continues to evolve, these breakthroughs are poised to enhance the sensitivity, reliability, and versatility of nanosensor technologies, with promising implications for improving health outcomes and early disease detection.

2.3. Directed Evolution of SWCNTs Based Sensors

Building on the systematic screening of corona phases to determine optimal sensor configurations, directed evolution presents itself as a valuable complementary approach to further enhance SWCNT sensor performance. This powerful strategy not only relies on choosing a sensor but also focuses on refining the properties and functionalities of SWCNT sensors through iterative cycles of mutation and selection. Inspired by natural evolutionary processes, directed evolution empowers researchers to methodically improve the sensitivity, selectivity, and stability of SWCNT‐based sensors across a diverse array of analytes. By utilizing extensive libraries of modified SWCNTs, each evolutionary iteration facilitates the identification of advanced sensor constructs that demonstrate tailored interactions with their target molecules.

To this end, the Boghossian research group concentrated on evolving brighter ssDNA‐SWCNT sensors through the directed evolution of DNA wrapping.^{[ 87 ]} This approach establishes a systematic framework for fine‐tuning the properties of ssDNA‐SWCNTs constructs, even when a well‐defined structure‐function relationship is lacking. Additionally, the group developed and implemented a directed evolution strategy aimed at enhancing the optical sensing capabilities of DNA‐wrapped SWCNTs for the detection of mycotoxins, which are critical for ensuring food safety. Their research successfully engineered sensors capable of detecting aflatoxin B1 (AFB1) and fumonisin B1 (FB1) mycotoxins, utilizing the specific fluorescence responses of (9,4) and (7,5) SWCNTs chirality, respectively. This chirality‐specific responsivity facilitated the multimodal detection of both mycotoxins across different wavelengths of light, even within complex food matrices. Furthermore, the researchers demonstrated that directed evolution not only improved the chiral‐dependent selectivity of the sensors but also enhanced their sensitivity and fluorescence intensity through multiple evolutionary cycles. Further, the optoelectronics properties of DNA‐stabilized SWCNTs could be engineered through a directed evolution approach.^{[ 88 ]} The versatile methodology highlighted in this study holds promise for applications beyond mycotoxin detection, extending to other SWCNT sensors and nanosensors that integrate biological elements.

In the context of directed evolution, the Landry research group introduced a platform for evolving synthetic molecular recognition on SWCNTs signal transducers, specifically targeting the neuromodulator serotonin.^{[ 89 ]} Using an extensive library of approximately 6.9 × 10¹⁰ unique single‐stranded DNA (ssDNA) sequences conjugated to the SWCNTs, researchers developed a reversible probe that exhibited a remarkable ∼200% fluorescence enhancement upon exposure to serotonin, with a dissociation constant (Kd) of 6.3 µm. This probe demonstrated selective responsiveness to serotonin, distinguishing it from analogs, metabolites, and receptor‐targeting drugs, and maintained reactivity even upon repeated exposure to exogenous serotonin in acute brain slice preparations. Moreover, the Jeong research group introduced an innovative strategy to enhance serotonin‐responsive ssDNA‐wrapped SWCNT nanosensors by combining directed evolution with machine learning‐based predictions (Figure 6A,B).^{[ 90 ]} Their iterative optimization process was designed to improve both sensitivity and selectivity. Over three rounds focusing on sensitivity, they achieved an impressive 2.5‐fold increase in fluorescence response compared to the original sequence. Subsequently, in two additional rounds, they shifted their focus to selectivity, resulting in a 1.6‐fold enhancement in differentiating serotonin from dopamine, despite the structural similarities between the two neurotransmitters. This groundbreaking approach enabled high‐throughput screening of mutated sequences, marking a significant advancement in biosensor development. The resulting top‐performing nanosensors, designated as N2‐1 for sensitivity and L1‐14 for selectivity, serve as promising reference sequences for future studies involving serotonin detection.

Figure 6.

A): Directed evolution guided by machine learning. A machine learning model was applied to predict sequences with improved sensor responses, facilitating the directed evolution process. B) The protocol for developing serotonin (5‐hydroxytryptamine, 5HT)‐responsive ssDNA‐SWCNT nanosensors was conducted using ML‐based screening to refine selection. Reproduced with permission from ref. [90] Copyright 2024, Multidisciplinary Digital Publishing Institute.

From the aforementioned examples it is evident that directed evolution serves as an essential approach for enhancing the performance of SWCNTs‐based sensors, facilitating substantial advancements in sensitivity, selectivity, and stability across various applications.

In summary, several key considerations must be taken into account for optimal sensor selection. The corona's chemical composition should ensure stable dispersion while allowing for selective and sensitive interactions with the target analyte. The spatial arrangement of functional groups must support efficient optical transduction without steric hindrance. Striking the right balance between sensitivity and selectivity is essential—strong binding enhances specificity, while moderate interactions enable reversible sensing in dynamic environments. Screening a diverse range of corona phases aids in optimizing fluorescence response while minimizing background interference and ensuring environmental stability. In directed evolution, iterative refinements should aim to enhance binding affinity, fine‐tune fluorescence modulation, and improve sensor robustness for reliable performance in complex biological settings.

3. Sensing Mechanism

With the key considerations for selecting an optimal SWCNT sensor established, the next logical step is to delve into the various sensing mechanisms by which these sensors function to detect specific analytes. In this section, we discuss each of these mechanisms in detail.

3.1. Single Event Sensing

Single‐event sensing with SWCNTs‐based sensors focuses on detecting a specific analyte at a single, defined moment, rather than continuously monitoring it over time. This approach not only provides a snapshot of analyte presence but also enables concentration‐dependent sensing, making it particularly useful for scenarios where knowing the precise amount of an analyte is critical. Such one‐point measurements are valuable for capturing rapid or transient events that require a quick and definitive response, offering both high sensitivity and selectivity.

In this context, SWCNTs have been employed as probes to detect a wide range of analytes, including metal ions,^{[ 91} , ⁹² , ^{93 ]} pathogens,^{[ 94 ]} oncometabolites,^{[ 95 ]} neurotransmitters,^{[ 77} , ⁹⁶ , ⁹⁷ , ⁹⁸ , ^{99 ]} endolysosomal pH levels,^{[ 100 ]} lipid molecules,^{[ 101} , ^{102 ]} and various other small molecules.^{[ 19} , ⁷⁶ , ⁸³ , ⁹⁷ , ¹⁰³ , ¹⁰⁴ , ^{105 ]}

To start with, fluorenylmethyloxycarbonyl‐tyrosine (FmocY) was used to suspend SWCNTs. Thereafter, tyrosine oxidase was added to the SWNCTs dispersion to oxidize FmocY to a melanin‐like substance, resulting in a multicomponent system (Figure 7A1), which could be further used for the detection (and scavenging) of multivalent metal ions like Mg²⁺, Ca²⁺, Cu²⁺, Fe³⁺, Mn²⁺, Hg²⁺, Cr³⁺, Zn²⁺, and Pb²⁺ with limits of detection in the range of sub‐micromolar (Figure 7A2).^{[ 91 ]} Further, by utilizing the metal chelation properties of the melanin‐inspired material and the NIR fluorescence response of SWCNTs, this system enabled metal removal with an efficiency of up to 98%. This work highlights the potential of SWCNT‐based hybrid systems for simultaneous metal detection and removal, offering a promising strategy for environmental remediation and real‐time monitoring in biological and aquatic systems. Moreover, the Budhathoki‐Uprety group developed polymer‐stabilized SWCNTs with potassium‐chelating groups to enable sensitive, sub‐millimolar detection of potassium ions^{[ 92 ]} (Figure 7B). The nanosensors were also validated for biocompatibility and successfully applied to potassium ion detection in serum, highlighting their potential for real‐time biomedical monitoring and disease diagnostics. Additionally, the Strano research group developed sensors for detecting divalent metal ions using the CoPhMoRe methodology with DNA‐SWCNTs, establishing an optimal sensing protocol through experimental parameter optimization.^{[ 93 ]} These DNA‐SWCNTs corona phases were able to differentiate between Mn²⁺, Hg²⁺, Cr²⁺, and Pb²⁺ in buffer solutions. Additionally, the sensors successfully detected Hg²⁺ in fish tissue using a portable NIR reader and a paper‐based barcode strip, achieving a detection limit of 33 nm, well below the average mercury content in fish tissues (≈1.1 µm). Thus, these sensors hold promise for monitoring heavy metals in food safety applications.

Figure 7.

A1): Schematic illustration of tyrosinase‐mediated oxidative polymerization of FmocY, facilitating SWCNTs suspension to create a multicomponent system for metal ion sensing. A2) Fluorescence intensity variations of oxidized Fmoc tyrosine‐suspended SWCNTs in response to different metal‐ion concentrations. Reproduced with permission from ref. [91] Copyright 2022, John Wiley & Sons. B) Schematic representation of fluorescence‐based detection of potassium ions utilizing polymer‐stabilized SWCNTs. Reproduced with permission from ref. [92] Copyright 2024, American Chemical society.

Coming to pathogens, the Kruss research group developed NIR fluorescent nanosensors based on SWCNTs for the rapid detection of clinically relevant bacteria.^{[ 94 ]} These sensors were integrated into hydrogel arrays to detect released metabolites and virulence factors while providing high tissue penetration and ultra‐low background noise. Through remote NIR imaging, the sensors were used to identify and differentiate key pathogens, including Staphylococcus aureus and Pseudomonas aeruginosa, from over 25 cm away. This innovative approach enabled multiplexed detection, highlighting its potential for smart surface applications in combating infectious diseases.

In the context of oncometabolites, the Bisker research group developed a near‐infrared optical nanosensor for the oncometabolite D‐2‐hydroxyglutarate (D2HG),^{[ 95 ]} which is linked to cancers such as glioma and glioblastoma. A library of fluorescent SWCNTs functionalized with ssDNA was screened, identifying (ATTT)₇‐SWCNT as a specific sensor for D2HG, which exhibited increased fluorescence upon interaction. The sensor was shown to distinguish D2HG from related metabolites, with fluorescence modulation observed based on analyte concentration and SWCNT chirality. This work advanced molecular recognition of oncometabolites, enhancing cancer research.

Focusing on neurotransmitters, the Kruss research group has developed chirality‐pure SWCNTs to facilitate their detection.^{[ 97 ]} By employing aqueous two‐phase extraction (ATPE),^{[ 9} , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ^{109 ]} a technique for the separation of multi‐chirality SWNCTs, they successfully isolated specific chiralities, including (6,5), (7,5), (9,4), and (7,6) SWCNTs. Notably, the monochiral (GT)₄₀‐(6,5)‐SWCNTs exhibited a remarkable +140% fluorescence response to dopamine (Figure 8A,B), coupled with an impressive long‐term stability exceeding 14 days, enabling sensitive and selective neurotransmitter detection in NIR applications. Additionally, the group enhanced the selectivity and sensitivity of catecholamine neurotransmitter detection by varying the organic phase around the SWCNTs. Their research revealed that DNA‐functionalized SWCNTs demonstrated a wide range of dissociation constants (Kd), from 2.3 nm for norepinephrine to 9.4 µm for dopamine, with detection limits in the single‐digit nm range. Impressively, certain sensors were able to distinguish between dopamine and norepinephrine at low concentrations (50 nm), showcasing the potential of tailored DNA‐functionalized SWCNTs for discriminating catecholamine neurotransmitters in complex biological environments. Moreover, incorporating defects into SWCNTs has proven to be an effective strategy for modulating their optical responses to neurotransmitters. The Bisker research group recently illustrated that introducing oxygen defects in SWCNTs resulted in divergent optical responses to dopamine and serotonin, whereas pristine SWCNTs exhibited similar optical behaviors for both neurotransmitters (Figure 8C).^{[ 110 ]} Additionally, (GT)₆‐ssDNA, which forms ordered rings around (9,4) SWCNTs, has been shown to enhance dopamine detection (Figure 8D,E).^{[ 96 ]} Beyond non‐covalent surface functionalization, covalent modifications of the SWCNT surface have also been explored for dopamine sensing.^{[ 99 ]} Utilizing a unique method called “systematic evolution of ligands by exponential enrichment” (SELEC), high‐throughput screening of ss‐DNA was established, leading to the sensitive detection of serotonin.^{[ 89 ]} Recently, the Boghossian research group achieved fluorescence enhancement‐based sensing of dopamine in saline using a metal ion pre‐treatment strategy. By treating DNA‐stabilized SWCNTs with aluminum ions, they mitigated interference from other metal ions. This enabled a robust fluorescence enhancement response to dopamine over a dynamic range of 1 nm to 10 µm.^{[ 111 ]} These innovative approaches to neurotransmitter detection highlight the significant potential of SWCNT‐based biosensors in advancing our understanding of neurobiology and improving diagnostic capabilities in complex biological systems.

Figure 8.

A) Fluorescence spectra of (GT)₄₀‐(6,5)‐SWCNTs before and after the addition of dopamine, demonstrating a notable increase in fluorescence following dopamine introduction. B) Concentration‐dependent fluorescence response of (GT)₄₀‐(6,5)‐SWCNTs to varying dopamine levels. Reproduced with permission from ref. [97] Copyright 2021, American Chemical Society. C) Schematic representation illustrating how the introduction of oxygen defects in (6,5)‐enriched SWCNTs results in distinct optical responses to dopamine and serotonin, with dopamine enhancing fluorescence and serotonin decreasing it. Reproduced with permission from ref. [110] Copyright 2024, American Chemical Society. D) Schematic representation depicting the formation of ordered rings of (GT)₆‐ssDNA around the SWCNTs, which ultimately resulted in fluorescence enhancement upon the addition of dopamine and norepinephrine. E) Fluorescence spectra of (GT)₆‐ssDNA‐stabilized SWCNTs were presented before and after the addition of dopamine. Reproduced with permission from ref. [96] Copyright 2018, American Chemical Society.

In the realm of one‐time lipid sensing, an innovative optical reporter was unveiled by the Heller research group, utilizing a photoluminescent, single‐chirality carbon nanotube to detect lipid accumulation specifically within the lumen of endolysosomal vesicles.^{[ 101 ]} This advanced nanomaterial, composed of short, single‐stranded DNA and a single nanotube chirality, was shown to selectively localize within the endolysosomal lumen while preserving cell viability and organelle function. The reporter enabled quantitative mapping of lipid content in live cells, revealing lipid accumulation associated with various pathologies, including drug‐induced phospholipidosis and Niemann‐Pick type C disease. Furthermore, significant intracellular heterogeneity and varying rates of cholesterol accumulation in macrophages were demonstrated through single‐cell measurements, highlighting its potential to enhance drug development and deepen the understanding of lipid‐linked diseases.^{[ 101 ]}

For one‐time sensing of endolysosomal pH, the Heller research group reported the development of defect‐induced SWCNTs for the detection of autophagy‐mediated endolysosomal hyperacidification in live cells and in vivo.^{[ 100 ]} These nanosensors were localized to lysosomes, allowing emission band shifts in response to local pH changes, which facilitated spatial, dynamic, and quantitative mapping of lysosomal pH variations. Cellular and intratumoral hyperacidification was demonstrated following the administration of mTORC1 and V‐ATPase modulators, indicating that lysosomal acidification mirrored the dynamics of S6K dephosphorylation and LC3B lipidation. This innovative approach was shown to provide a valuable tool for transient and in vivo monitoring of the autophagy‐lysosomal pathway.

A fundamental challenge in chemistry is achieving precise control over regio‐ and stereoselective molecular interactions. In this regard, the Zheng research group has demonstrated a significant breakthrough by utilizing (+) and (−) (6,5) SWCNTs, as well as (−) (8,3) SWCNTs, for the stereoselective detection of amino acid enantiomers.^{[ 112 ]} This study represents an important advancement toward the accurate determination of stereochemical configurations, paving the way for highly selective molecular recognition in complex biological and chemical environments.

The Shiraki and Fujigaya research groups have demonstrated that sensing can be enhanced by incorporating reactive molecular defects into SWCNTs. For instance, palmitic acid derivatives—known for their strong affinity toward serum albumin—were integrated into SWCNTs to improve their selective sensitivity to albumin, even in complex biological environments.^{[ 43 ]} By functionalizing defect sites in SWCNTs with palmitic acid, the sensors exhibited sensitive and selective photoluminescence red‐shifts (up to 2.5 nm) upon serum albumin binding, enabling differentiation between human, bovine, and mouse serum albumin. The nanosensors successfully detected serum albumin in serum and albuminuria in diabetic mice, demonstrating their potential as NIR‐based diagnostic tools for kidney and liver‐related diseases and expanding the bioapplications of the SWCNTs. This study represents a significant advancement, as it introduces a strategy to impart otherwise unreactive SWCNTs with both sensitivity and selectivity.

Furthermore, SWCNTs enabled the one‐time sensing of various molecules, including riboflavin, quaternary ammonium compounds, cytokines, nitric oxide, reactive oxygen species, adenosine 5′‐triphosphate, nitroaromatics, L‐thyroxine, and doxorubicin.^{[ 19} , ⁷⁶ , ⁸³ , ⁹⁷ , ¹⁰³ , ¹⁰⁵ , ¹¹³ , ¹¹⁴ , ^{115 ]}

In conclusion, the diverse applications of SWCNTs‐based sensors for single‐event sensing demonstrate their significant potential in various fields, from environmental monitoring to biomedical diagnostics, paving the way for innovative solutions to complex challenges.

3.2. Real‐Time Monitoring

SWCNTs‐based sensors also enable real‐time detection of changes in analyte levels, providing immediate insights into physiological or environmental processes. Their high temporal resolution and sensitivity allow for accurate measurement of both the presence and concentration of specific analytes, making them well‐suited for applications that require prompt and precise tracking of biological responses, environmental pollutants, or metabolic changes. Simultaneously, their spatial monitoring capabilities enable localized detection, mapping analyte distribution across different regions within a system. This dual ability to resolve both temporal and spatial variations makes SWCNT‐based sensors highly effective for numerous applications. With non‐invasive, highly specific detection capabilities, SWCNT‐based sensors thus have significant potential for advancing medical diagnostics.

A key example of real‐time biosensing with SWCNTs is their application in the monitoring of enzyme activity. This application has been particularly effective for tracking cholinesterase activity in blood plasma.^{[ 66 ]} In this approach, myristoylcholine‐stabilized SWCNTs were used as a responsive platform, with myristoylcholine serving as the substrate for cholinesterase. Upon interaction with cholinesterase in blood plasma, myristoylcholine was hydrolyzed to myristic acid and choline, which led to a measurable decrease in SWCNTs’ fluorescence. This decrease corresponded to the extent of hydrolysis, offering a real‐time dynamic measure of cholinesterase activity. Furthermore, SWCNTs suspended with polymer dendron‐hybrids containing ester and amide groups were used to monitor the activity of esterase and amidase in real‐time.^{[ 63 ]} The hydrolysis of esters and amides by their respective enzymes featured a decrease in SWCNT fluorescence, directly reflecting product release and enabling continuous enzyme activity assessment.^{[ 63 ]}

In an allied vein, real‐time monitoring of insulin secretion from beta cells was pursued using SWCNTs (Figure 9A).^{[ 116 ]} The approach employed a synthetic lipid‐poly(ethylene glycol) (C16‐PEG(2000 Da)‐Ceramide) for functionalizing the SWCNTs. The lipid‐PEG‐SWCNTs sensors effectively detected insulin secreted by β‐cells in conditioned media, with quantification achieved by comparing the fluorescence response to a standard calibration curve, showing consistency with enzyme‐linked immunosorbent assays (ELISA). This innovative approach enables rapid assessment of pancreatic β‐cell function, advancing diabetes research.

Figure 9.

A) Schematic representation of SWCNTs‐based insulin sensor. The functionalized SWCNTs were shown to emit fluorescence in the NIR range, with insulin binding resulting in modulation of the emitted signal. In the insulin secretion assay, glucose was added to insulin‐secreting β‐cells and incubated for 2 h. Following this, the conditioned media was collected and combined with the SWCNT sensor solution. The fluorescence response recorded after a brief 5‐min incubation was used to determine the concentration of the secreted insulin. Reproduced with permission from ref. [116] Copyright 2021, John Wiley & Sons. B) Fluorescence spectra of glutathione‐DNA‐carbon nanotube construct showing variation in intensity and C) wavelength upon the addition of 1 µm GST. Reproduced with permission from ref. [117] Copyright 2020, American Chemical Society.

In the context of protein detection, the Heller research group demonstrated the quantification of glutathione‐S‐transferase (GST) fusion proteins using a novel nanosensor platform.^{[ 117 ]} This system, based on a glutathione‐DNA‐carbon nanotube construct, enabled real‐time monitoring of GST interactions via the fluorescence of SWCNTs. The sensor's fluorescence response was modulated specifically by GST and GST‐fusion proteins, influenced by the structure of the SWCNTs (Figure 9B,C). This work represented the first label‐free optical sensor for GST, providing significant potential for in situ assessment of protein expression in imaging and industrial bioreactor settings.

Targeting pollutants and pathogens, the Strano research group demonstrated real‐time detection of arsenic by engineering SWCNTs‐based nanobionic sensors embedded in wild‐type plants, leveraging their natural ability to hyperaccumulate arsenic for sensitive and selective monitoring of arsenite levels as low as 0.2 ppb.^{[ 118 ]} Furthermore, successful detection of pathogens was achieved using SWCNTs for real‐time diagnostics. The development of a NIR optical sniffer was detailed, utilizing peptide‐encapsulated (6,5) SWCNTs for detecting and classifying bacteria.^{[ 119 ]} Sixteen peptides were synthesized to optimize SWCNT dispersion, with longer peptides shown to enhance signal intensity. The sensors effectively distinguished between the odors of the sterile growth medium, Escherichia coli, and Klebsiella pneumonia, enabling real‐time results and demonstrating potential for antibiotic susceptibility testing at room temperature.

In the context of real‐time monitoring of microRNAs, the Heller research group developed a carbon‐nanotube‐based sensor for the optical quantification of hybridization events involving microRNAs and small oligonucleotides.^{[ 120 ]} This sensor leveraged the unique properties of SWCNTs to enable single‐molecule detection and multiplexing, demonstrating effective performance in whole urine and serum. It was successfully shown to non‐invasively measure DNA and microRNA after implantation in live mice.

Additionally, real‐time imaging of biologically relevant molecules, such as serotonin release from human blood platelets, was achieved using serotonin‐binding aptamer‐stabilized SWCNTs.^{[ 121 ]} These serotonin sensors enabled spatial mapping and localized visualization of release events at the subcellular level, revealing hotspots of serotonin secretion from individual platelets otherwise inaccessible with traditional electrode‐based methods. SWCNTs were also employed for real‐time detection of protein efflux from bacteria and yeasts.^{[ 58 ]} Furthermore, real‐time, reversible optical imaging of oxytocin, with micron‐scale spatial resolution and selectivity over vasopressin, was successfully performed in acute mouse brain slices using SWCNTs.^{[ 122 ]} Moreover, real‐time monitoring of a diverse range of small molecules was conducted using SWCNTs.^{[ 123} , ¹²⁴ , ¹²⁵ , ¹²⁶ , ¹²⁷ , ¹²⁸ , ^{129 ]} Another example by the Iverson research group is the real‐time quantification of nitric oxide diffusion gradients generated by human (THP‐1) and murine (RAW 264.7) macrophages using SWCNT sensors.^{[ 130 ]} Additionally, the uniform fluorescence distribution of the SWCNT array enabled precise analysis of NO efflux directionality, both beneath and surrounding the cell.

In summary, SWCNTs were demonstrated to have significant potential for continuous sensing, enabling real‐time monitoring of analytes. Their unique optical properties allowed for sensitive detection in complex biological environments, providing immediate feedback on analyte dynamics. This capability enhanced understanding of biochemical processes and improved diagnostic capabilities in real‐time applications, marking SWCNTs as promising tools for advancing continuous monitoring technologies.

3.3. Localization‐Based Sensing: Spatiotemporal Tracking of Particle Dynamics

Building on the advantages of continuous monitoring, an innovative SWCNT‐based sensing approach involves tracking the precise localization of the nanoparticles. This method leverages the positional dynamics of SWCNTs as indicators of biological processes, providing a sensitive and spatially resolved technique for real‐time detection. By observing the movement and spatial arrangement of functionalized SWCNTs within a sample, specific biochemical information can be inferred. This strategy not only reveals the spatial distribution of target molecules but also enables high‐resolution tracking of biological processes as they unfold within complex environments.

To facilitate this, SWCNTs have been deployed as real‐time probes for continuous monitoring of biological processes, spanning from the molecular level to cellular, tissue, and organismal scales.^{[ 39 ]}

At the molecular level, single‐particle tracking with SWCNTs has enabled real‐time observation of peptide self‐assembly into hydrogels, mimicking the fibrous structure of extracellular matrix proteins. SWCNTs suspended with Fmoc‐diphenylalanine (Fmoc‐FF) were embedded within hydrogels, utilizing their near‐infrared fluorescence to provide critical insights into the gelation process (Figure 10A).^{[ 131 ]} The formation of hydrogels was further tracked in real‐time with SWCNTs functionalized by various Fmoc‐aromatic amino acids, such as tyrosine, tryptophan, and phenylalanine.^{[ 132 ]} In hydrogels composed of Fmoc‐tyrosine and Fmoc‐phenylalanine, the displacement of SWCNTs remained stable, correlating with electron microscopy images that revealed pre‐gelation fibrillary structures. In contrast, the displacement of SWCNTs in Fmoc‐tryptophan hydrogels initially decreased, suggesting a constrained microenvironment with smaller pore sizes (Figure 10B).^{[ 132 ]} This single‐particle tracking approach offered a non‐invasive method for monitoring real‐time structural changes in peptide hydrogels, indicating potential applications in studying diseases characterized by peptide and protein assemblies. In a related study, SWCNTs functionalized with PEG and fibrinogen were employed to monitor fibrin clot formation in real‐time.^{[ 64 ]} The addition of thrombin significantly slowed the diffusion rates of SWCNTs as fibrinogen converted to insoluble fibrin, providing a direct, quantitative measure of clot progression dependent on both fibrinogen and thrombin concentrations (Figure 10C).

Figure 10.

A): NIR fluorescence imaging of SWCNTs embedded in Fmoc‐FF and Fmoc‐FF/polymer hydrogels one hour after gelation, revealing the hydrogels’ morphology. The left column shows images captured in the NIR channel, the middle column presents bright field images, and the right column displays the merged images. A scale bar of 20 µm is included for reference. Reproduced with permission from ref. [131] Copyright 2022, American Chemical Society. B) The left column illustrates the time‐dependent mean displacement of single SWCNTs within Fmoc‐phenylalanine (blue), Fmoc‐tryptophan (red), and Fmoc‐tyrosine (gray) hydrogels. The right column displays the corresponding morphology of these hydrogels as observed using scanning electron microscopy (SEM). Reproduced with permission from ref. [132] Copyright 2024, Elsevier. C) Time‐resolved NIR single‐particle tracking of DPPE‐PEG‐SWCNTs (highlighted by white arrows) on a microscope slide, showing their movement paths before and after thrombin addition. When thrombin was added to fibrinogen‐modified DPPE‐PEG SWCNTs, it triggered fibrin clot formation, leading to a subsequent reduction in SWCNTs diffusion. Reproduced with permission from ref. [64] Copyright 2023, American Chemical Society.

At the cellular level, 2D single‐particle tracking of SWCNTs provided real‐time insights into endocytosis, intracellular trafficking, and exocytosis.^{[ 133 ]} 3D tracking was also achieved using an orbital tracking microscope, offering valuable perspectives on intracellular dynamics.^{[ 134 ]} In another investigation, cytoskeletal dynamics were explored by binding SWCNTs to the kinesin‐1 motor protein, a critical cargo transporter in COS‐7 cells.^{[ 135 ]} This binding was accomplished through covalent attachment using DNA oligonucleotides and a Halo Tag. Individual SWCNTs were tracked for 90 min with a temporal resolution of 5 ms per frame. The movement of SWCNTs exhibited long, directed runs characteristic of kinesin activity, with an average velocity of 300 ± 210 nm s⁻¹. This research revealed various motion types, including thermally driven movements and active “stirring” induced by myosin, underscoring the potential of SWCNTs for real‐time cellular monitoring. Furthermore, DNA‐suspended SWCNTs conjugated with a green fluorescent protein (GFP) targeting nanobody were used to trace the movement of Kinesin‐5‐GFP motor proteins in Drosophila melanogaster embryos.^{[ 136 ]} SWCNTs also served as probes to investigate flow patterns in model actomyosin cortices with varying cross‐link densities.^{[ 137 ]} Additionally, SWCNTs integrated into Xenopus egg extracts were employed to explore nonequilibrium dynamics within complex cytoskeletal networks.^{[ 137 ]}

At the tissue level, the Cognet research group successfully conducted single‐particle tracking of SWCNTs within the extracellular space (ECS) of live brain tissue, yielding valuable insights into the dimensions and viscosity of the ECS (Figure 11A1–A3).^{[ 138 ]} They also mapped the local environment surrounding synapses using this tracking approach (Figure 11B).^{[ 139 ]} In another application, the same group utilized single‐particle tracking of SWCNTs in the hepatic extracellular matrix to detect the onset of liver fibrosis (Figure 11C).^{[ 140 ]}

Figure 11.

A1): Diagram illustrating SWCNTs introduced into the lateral ventricles of young rats, followed by their diffusion into the neocortex. A2) Imaging of SWCNTs within brain slices. A3) Color‐coded paths indicating single SWCNT diffusion in the extracellular space (ECS). Scale bar: 1 µm. Reproduced with permission from ref. [138] Copyright 2017, Nature. B) Median values and cumulative distribution (inset) of SWCNT local diffusivities measured across various coronal regions around synapses. Reproduced with permission from ref. [139] Copyright 2022, American Chemical Society. C) Schematic of SWCNTs injected into both healthy and fibrotic liver environments. Tracking SWCNTs in the short‐wave infrared (SWIR) range offered real‐time insights into liver fibrosis progression and prognosis. Reproduced with permission from ref. [140] Copyright 2024, American Chemical Society.

Real‐time monitoring of biological processes at the macroscale has been pioneered by the Weisman research group.^{[ 141} , ^{142 ]} At the organismal level, the biodistribution of conventional nanomaterials across various tissues and organs was mapped using SWCNTs as real‐time probes, contributing to a foundational understanding necessary for developing safe and advanced nanomedicine.^{[ 143 ]} Additionally, SWCNTs were effectively used to track the contraction and relaxation of the pharyngeal valve area in the anterior terminal bulb of C. elegans worms.^{[ 45 ]} Spatiotemporal tracking of SWCNTs within confined C. elegans in a microfluidics setup further provided real‐time insights into the worms’ feeding mechanisms.^{[ 144 ]}

In summary, single‐particle tracking of SWCNTs plays a transformative role in advancing real‐time sensing applications across various biological scales. By serving as sensitive probes, SWCNTs facilitate the monitoring of dynamic biochemical interactions and structural changes in real‐time, offering valuable insights into molecular, cellular, tissue, and organismal processes.^{[ 145 ]} Notably, super‐resolution microscopy in the near‐infrared holds promise for advancing real‐time tracking of biological processes with SWCNTs, offering enhanced spatiotemporal resolution.^{[ 146} , ¹⁴⁷ , ¹⁴⁸ , ¹⁴⁹ , ^{150 ]} Such high resolution would allow for precise visualization and tracking of SWCNTs within complex biological environments, revealing finer details that enrich our understanding of intricate biological processes. This versatility underscores the vast potential of SWNCT‐based optical probes for advancing our understanding of complex biological phenomena in real‐time.

3.4. Ratiometric Sensing

The previously discussed sensing techniques predominantly focus on monitoring changes in intensity and wavelength. Alternatively, ratiometric sensing offers a compelling approach by using an internal calibration system.^{[ 151} , ¹⁵² , ¹⁵³ , ^{154 ]} Instead of relying on absolute or single‐wavelength measurements, this method employs fluorescence emission from the system itself as a reference, effectively addressing challenges related to intensity fluctuations caused by the detector, light source, local probe concentration, and variations in optical path length.^{[ 154 ]} Notably, both single‐event sensing and real‐time monitoring, which have been discussed above, can be achieved using this approach, with the key distinction being that ratiometric sensing involves tracking emission at two wavelengths, one of which serves as a stable reference. This dual‐wavelength strategy ensures that observed signal variations arise specifically from analyte interactions rather than external influences, thereby improving measurement reliability. By providing a built‐in correction mechanism, ratiometric sensing thus offers a more reliable and selective platform for measurements, enhancing the accuracy and robustness of sensing applications.

A typical ratiometric probe features at least one emission peak that remains unaffected by the analyte, serving as an internal reference. Therefore, SWCNTs need to be composed of multiple or at least dual components to facilitate this process. Two primary strategies have been successfully employed to achieve this: i) using chirality‐separated SWCNTs^{[ 155} , ¹⁵⁶ , ¹⁵⁷ , ¹⁵⁸ , ^{159 ]} where one SWCNT chirality is stabilized by a corona that is responsive to the target, whereas a second SWCNT chirality is functionalized with a different corona unresponsive to the analyte, and ii) introducing defects in SWCNTs such that the original emission peak and the defects‐related peak can be monitored independently and compared.^{[ 160 ]} Both strategies enhance the functionality of the SWCNTs, resulting in multiple (or at least dual) emission peaks from different sources. This setup ensures that the interaction with the analyte does not alter the characteristics of at least one of the peaks, making SWCNTs highly suitable for ratiometric monitoring of analytes.

The Strano research group achieved significant advancements using chirality‐separated SWCNTs for ratiometric sensing. Ratiometric fluorescent SWCNT sensors were developed by functionally isolating single‐chirality SWCNTs to detect specific analytes, such as nitric oxide (NO) and hydrogen peroxide (H₂O₂), while maintaining an invariant emission peak.^{[ 161 ]} This approach enabled precise measurements of analyte concentrations based on the ratio of distinct emission peaks, making it resistant to variations in intensity and external noise. Two distinct sensors were demonstrated, each utilizing (7,6) emissions with a stable (6,5) reference wavelength. This ratiometric optical sensing platform showed potential for detecting trace analytes in complex environments, including biological tissues. Further, The Roxbury research group demonstrated that ratiometric, real‐time optical monitoring of hydrogen peroxide concentrations within in vitro wound models could be achieved using SWCNT‐incorporated textiles.^{[ 126 ]}

In the context of ratiometric sensing through the introduction of defects, notable advancements were made by the Zaumseil research group with sp³‐functionalized (6,5) SWCNTs featuring red‐shifted defect emissions (Figure 12A1) for the optical detection of inorganic pyrophosphate.^{[ 162 ]} The sensing mechanism was based on immobilizing Cu²⁺ ions on the SWCNT surface via coordination with covalently attached aryl alkyne groups and triazole complexes. The presence of Cu²⁺ ions resulted in fluorescence quenching through photoinduced electron transfer (Figure 12A2), which was restored by adding copper‐complexing analytes like pyrophosphate (Figure 12A3). These defect‐induced fluorescence changes enabled ratiometric measurements across a broad concentration range. Biocompatible phospholipid‐polyethylene glycol‐coated SWCNTs with sp³ defects were employed to detect pyrophosphate in cell lysates and to monitor DNA synthesis progress in polymerase chain reactions.

Figure 12.

Fluorescence spectra of defect‐engineered Dz‐alkyne SWCNTs: A1) initial state, A2 after copper complex addition, and A3) after subsequent pyrophosphate addition. The fluorescence of Dz‐alkyne SWCNTs was notably quenched upon copper complex addition and restored following pyrophosphate addition. Reproduced with permission from ref. [162]. Copyright 2024, Nature. B) Fluorescence spectra of PBA‐SWCNTs before and after dopamine addition. Notably, the defect‐related peak decreased while the SWCNT peak remained stable, enabling ratiometric sensing. C) Ratiometric fluorescence response of PBA‐SWCNTs as a function of dopamine concentration. Reproduced with permission from ref. [163] Copyright 2024, American Chemical Society. D) Fluorescence spectra of oxygen‐defect‐induced SWCNTs before and after the addition of varying cholesterol concentrations, showing ratiometric variation in emission peaks. E) Ratiometric fluorescence response of oxygen‐defect‐induced SWCNTs across different cholesterol concentrations. Reproduced with permission from ref. [164] Copyright 2024, American Chemical Society.

Furthermore, the Kruss research group developed a ratiometric probe for catecholamine neurotransmitters based on SWCNTs.^{[ 163 ]} Defects from phenylboronic acid (PBA) were incorporated to selectively interact with catechol moieties. These PBA‐SWCNTs were modified with poly(ethylene glycol) phospholipids (PEG‐PL) to enhance biocompatibility. While catecholamines did not affect the intrinsic fluorescence of the SWCNTs, the defect‐related emission decreased significantly (Figure 12B,C). This dual functionalization enabled tunable selectivity and established a ratiometric biosensor for dopamine function in brain slices.

Finally, the Bisker research group developed a ratiometric probe for cholesterol detection by introducing oxygen defects in surfactant‐stabilized (6,5) enriched SWCNTs.^{[ 164 ]} Notably, both the SWCNTs’ peaks and the defect peaks exhibited substantial changes in response to cholesterol, resulting in variations in the ratio of these two peaks (Figure 11D,E). This change effectively enabled cholesterol detection in both aqueous solutions and complex serum environments.

In conclusion, significant advancements in ratiometric sensing using SWCNTs have demonstrated their versatility and potential across various applications. By employing strategies such as chirality separation and defect introduction, robust fluorescent sensors were developed for accurately detecting analytes like nitric oxide, hydrogen peroxide, inorganic pyrophosphate, catecholamines, and cholesterol. The implementation of internal calibration through multiple emission peaks enhanced measurement reliability and specificity, effectively addressing challenges posed by environmental noise. These advancements paved the way for sensitive biosensors capable of functioning in complex biological settings, facilitating improved monitoring of critical biochemical processes.

4. Concluding Remarks

SWCNTs have established themselves as versatile optical probes for biosensing, offering a unique combination of tunable photophysical properties and surface adaptability. Beyond their superior optical properties, SWCNT‐based sensors offer exceptional stability in diverse biological environments, ensuring reliable long‐term performance with minimal signal degradation. Their biocompatibility, coupled with robust functionalization strategies, further reinforces their potential as next‐generation optical probes for real‐time biosensing and biomedical applications. In this review, we explored key concepts underlying SWCNTs‐based sensing, covering diverse strategies for sensor selection, including screening, directed evolution, and rational design of corona phases. Each of these approaches provides distinct advantages in optimizing sensor performance for specific analytes and environments. Additionally, we highlighted various sensing mechanisms, ranging from single‐event detection and real‐time monitoring to localization‐based tracking and ratiometric sensing. These methodologies enable precise and dynamic detection, expanding the applicability of SWCNTs to complex biological systems. To offer a comparative perspective, key examples of SWCNT‐based biosensors detailing the analytes detected, surface functionalization strategies, limits of detection (LOD), and dynamic ranges are summarized in Table 1 . Our aim is to equip readers with a thorough grounding in SWCNTs‐based biosensing, fostering curiosity and encouraging further exploration of these remarkable nanomaterials in diagnostics and biological monitoring. As research continues, the insights given here lay a foundation for SWCNTs to drive important advancements in understanding and addressing complex biological processes, offering promising pathways for future innovations in biosensing and beyond.

Table 1.

Details of SWCNTs‐based biosensors, summarizing the analyte, surface functionalization (corona), limit of detection (LOD), and dynamic range.

Analyte	Corona	LoD	Dynamic range	Refs.
Glucose	Apo‐GOx	42 µm	0.1 mm–3 mm	[54]
	GOx	–	3‐30 mm	[55]
HE4 (marker of high‐grade serous ovarian carcinoma)	HE4‐specific antibodies	2.5 nm	10–500 nm	[56]
Interleukin‐6	DNA aptamer	105 ng mL−1	105–8400 ng mL−1	[59]
miRNA biomarkers	Complementary DNA recognition unit	10 nm	0.1–1.8 µm	[60]
	(GT)15 + miRNA capture sequence	10 pm	10 pm–100 µm	[120]
Cholinesterase (in blood plasma)	Myristoylcholine	0.0168 ± 0.00005 U L−1	10−1–101 U L−1	[66]
acetylcholinesterase (in buffer)	(GT)15 DNA	0.38 U L−1	10−1–101 U L−1	[82]
	T30 DNA	0.02 U L−1	10−1–101 U L−1
Butyrylcholinesterase (in buffer)	GT)15 DNA	0.06 U L−1	10−1–101 U L−1
	T30 DNA	0.003 U L−1	10−1–101 U L−1
SARS‐CoV‐2 spike protein	Guanine defects rich DNA + complementary DNA strand	0.75 nm	1–10 nm	[70]
Dopamine	(GT)15 DNA	11 nm	10−7–10−4 m	[77]
	Oxygen defects + sodium cholate	3.7 ± 0.68 µm	50 µm–2.5 mm	[110]
	DNA + Al3+	–	1 nm–10 µm	[111]
	(GT)6 DNA	–	100 nm–2 µm	[96]
	(GT)10 DNA	1.9 × 10−8 m	10−8 –10−5 m	[165]
Nitric oxide	(AT)30 DNA	1.24 µm	–	[81]
	(AT)15 DNA	0.1 µm	0.1–10 µm	[166]
serotonin	ssDNA	–	100 nm–100 µm	[89]
	Oxygen defects + sodium cholate	0.25 ± 0.027 µm	10 µm–10 mm	[110]
	Serotonin binding DNA aptamer	–	100 nm–1 µm	[121]
Mg2+	Oxidized fluorenylmethyloxycarbonyl‐tyrosine (FmocY)	188 nm	100 nm–100 µm	[91]
Ca2+		194 nm
Cu2+		12 nm
Fe2+		413 nm
Mn2+		536 nm
Pb2+		315 nm
Cr3+		106 nm
Zn2+		19 nm
Hg2+ ions		397 nm
	DNA	33 nm	–	[93]
D‐2‐hydroxyglutarate (D2HG)	(ATTT)7	0.256 mm	0.256–432 mm	[95]
glutathione‐S‐transferase (GST) fusion proteins	glutathione‐DNA	1 nm	1–100 nm	[117]
pyrophosphate	SDS + aryl alkyne defects + Cu2+	4.4 µm	–	[162]
adenosine 5′‐triphosphate	PLPEG‐COOH + luciferase enzyme	240 nm	–	[103]
Cholesterol	SC + oxygen defects	0.28 ± 0.01 µm	80–1600 µm	[164]

Conflict of Interest

The authors declare no conflict of interest

Biographies

Srestha Basu received her Ph.D. from the Indian Institute of Technology Guwahati, followed by postdoctoral research at CNRS in France and the Technion–Israel Institute of Technology. In 2023, she joined Prof. Gili Bisker's group at Tel Aviv University, where she worked on biosensing applications of single‐walled carbon nanotubes (SWCNTs). Since August 2024, she is an Associate Professor of chemistry at the Saha Institute of Nuclear Physics, Kolkata. Her independent research focuses on engineering covalent defects in SWCNTs to enhance sensing capabilities and studying protein misfolding dynamics using SWCNTs for developing background‐free diagnostic platforms.

Gili Bisker is an Associate Professor in the Department of Biomedical Engineering at Tel Aviv University. She obtained her Ph.D. in Nanoscience and Nanotechnology from the Technion – Israel Institute of Technology, followed by postdoctoral research at MIT's Department of Chemical Engineering and the MIT Physics of Living Systems Group. Prof. Bisker's research focuses on developing optical nanosensors using near‐infrared fluorescent single‐walled carbon nanotubes (SWCNTs) for various biological targets, including enzymes, hormones, proteins, microRNA, and neurotransmitters. By leveraging the unique optical properties of SWCNTs, Prof. Bisker's work enables sensitive, selective, and minimally invasive monitoring of biological processes in real time.

Basu S., Bisker G., Near‐Infrared Fluorescent Single‐Walled Carbon Nanotubes for Biosensing. Small 2025, 21, 2502542. 10.1002/smll.202502542