A highly selective chromogenic probe for the detection of nitrite in food samples

Rapid, portable and visualizing nitrite detection enabled by a rationally designed meso-aminoindole substituted pyronine-based fluorescent probe

Nitrite (NO2-) widely exists in our daily diet, and its excessive consumption can lead to detrimental effects on the human central nervous system and an elevated risk of cancer. The fluorescence probe method for the determination of nitrite has developed rapidly due to its simplicity, rapidity and sensitivity. Despite establishing various nitrite sensing platforms to ensure the safety of foods and drinking water, the simultaneous achievement of rapid, specific, affordable, visualizing, and on-site nitrite detection remains challenging. Here, we designed a novel fluorescent probe by using Rhodamine 800 as the fluorescent skeleton and 5-aminoindole as the specific reaction group to solve this problem. The probe shows a maximal fluorescence emission at 602 nm, thereby avoiding background emission interference when applied to food samples. Moreover, this unique probe exhibited excellent sensing capabilities for detecting nitrite. These included: a rapid response time within 3 min, a noticeable color change that the naked eye can observe, a low detection limit of 13.8 nM, and a remarkable selectivity and specificity to nitrite. Besides that, the probe can detect nitrite quantitatively in barreled drinking water, ham sausage, and pickles samples, with good recoveries ranging from 89.0 % to 105.8 %. More importantly, based on the probe fixation and signal processing technology, a portable and smart sensing platform was fabricated and made convenient and rapid analysis the content of NO2- in real samples possible. The results obtained in this work provide a new strategy for the design of high-performance nitrite probes and feasible technology for portable, rapid and visual detection of nitrite, and this probe holds the potential as a practical tool for alleviating concern regarding nitrite levels.

Sialic Acid-Functionalized Gold Nanoparticles for Sensitive and Selective Colorimetric Determination of Serotonin

We present a novel colorimetric method inspired by nature's complex mechanisms, capable of selectively determining serotonin with high sensitivity. This method exploits the inherent binding affinity of serotonin with sialic acid (SA) molecules anchored to gold nanoparticles (SA-AuNPs). Upon serotonin binding, SA-AuNPs aggregate, and a characteristic red shift in the absorbance of SA-AuNPs accompanied by a dramatic color change (red to blue) occurs, readily observable even without instrumentation. The proposed method effectively eliminates interventions from potential interfering species such as dopamine, epinephrine, l-tyrosine, glucosamine, galactose, mannose, and oxalic acid. The absence of a color change with l-tryptophan, a structurally related precursor of serotonin, further confirms the high selectivity of this approach for serotonin detection. The colorimetric method has a wide linear dynamic range (0.05-1.0 μM), low limit of detection (0.02 μM), and fast response time (5 min). The limit of detection of the method is lower than other colorimetric serotonin sensors reported so far. The possible use of the proposed method in biological sample analysis was evaluated by employing a serotonin recovery assay in processed human plasma. The recoveries ranged from 90.5 to 104.2%, showing promising potential for clinical applications.

Nonmetal catalyst boosting amplification of both colorimetric and electrochemical signal for multi-mode nitrite sensing

Recently, nanozymes as an outstanding alternative to natural enzymes has attracted wide attention because of its high stability performance. In this study, PNC nonmetal nanozymes with high oxidase-like activity was synthesized can specifically catalyze colorless 3,3,5,5-tetramethyl-benzidine(TMB) to form blue oxidized TMB (TMBox). In the presence of nitrite, it further oxidizes TMBox to obtain yellow derivative products attributed to nitrite inducing diazotization reaction in TMBox. Based on this principle, a colorimetric and electrochemical sensing system was developed, and the ultra-sensitive multi-mode detection of nitrite was realized by combining RGB mode of smart phone, UV-Vis spectrum and electrochemical method. Compared with single signal detection, the multi-mode sensing system can realize self-validation to achieve more reliable detection results. What's more, the developed multi-mode sensing could quickly and sensitively detect nitrite in real sample, especially RGB mode of smart phone meeting the equipment limited areas, suggesting a broad application prospects in food safety.

Ti3C2 Nanosheets Functionalized with Ferritin-Biomimetic Platinum Nanoparticles for Electrochemical Biosensors of Nitrite

Nitrites widely exist in human life and the natural environment, but excessive contents of nitrites will result in adverse effects on the environment and human health; hence, sensitive and stable nitrite detection systems are needed. In this study, we report the synthesis of Ti3C2 nanosheets functionalized with apoferritin (ApoF)-biomimetic platinum (Pt) nanoparticle (Pt@ApoF/Ti3C2) composite materials, which were formed by using ApoF as a template and protein-inspired biomineralization. The formed nanohybrid exhibits excellent electrochemical sensing performance towards nitrite (NaNO2). Specifically, the Pt@ApoF catalyzes the conversion of nitrites into nitrates, converting the chemical signal into an electrical signal. The prepared Pt@ApoF/Ti3C2-based electrochemical NaNO2 biosensors demonstrate a wide detection range of 0.001-9 mM with a low detection limit of 0.425 μM. Additionally, the biosensors possess high selectivity and sensitivity while maintaining a relatively stable electrochemical sensing performance within 7 days, enabling the monitoring of NaNO2 in complex environments. The successful preparation of the Pt@ApoF/Ti3C2 nanohybrid materials provides a new approach for constructing efficient electrochemical biosensors, offering a simple and rapid method for detecting NaNO2 in complex environments.

Metal-organic framework-based ratiometric point-of-care testing for quantitative visual detection of nitrite

Nitrite (NO2-) is categorized as a carcinogenic substance and is subjected to severe limitations in water and food. To safeguard the public's health, developing fast and convenient methods for determination of NO2- is of significance. Point-of-care testing (POCT) affords demotic measurement of NO2- and shows huge potential in future technology beyond those possible with traditional methods. Here, a novel ratiometric fluorescent nanoprobe (Ru@MOF-NH2) is developed by integrating UiO-66-NH2 with tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) through a one-pot approach. The special diazo-reaction between the amino group of UiO-66-NH2 and NO2- is responsible for the report signal (blue emission) with high selectivity and the red emission from [Ru(bpy)3]2+ offers the reference signal. The proposed probe shows obviously distinguishable color change from blue to red towards NO2- via naked-eye. Moreover, using a smartphone as the detection device to read color hue, ultra-sensitive quantitative detection of NO2- is achieved with a low limit of detection at 0.6 μΜ. The accuracy and repeatability determined in spiked samples through quantitative visualization is in the range of 105 to 117% with a coefficient of variation below 4.3%. This POCT sensing platform presents a promising strategy for detecting NO2- and expands the potential applications for on-site monitoring in food and environment safety assessment.

Recent Studies and Applications of Hydrogel-Based Biosensors in Food Safety

Food safety has increasingly become a human health issue that concerns all countries in the world. Some substances in food that can pose a significant threat to human health include, but are not limited to, pesticides, biotoxins, antibiotics, pathogenic bacteria, food quality indicators, heavy metals, and illegal additives. The traditional methods of food contaminant detection have practical limitations or analytical defects, restricting their on-site application. Hydrogels with the merits of a large surface area, highly porous structure, good shape-adaptability, excellent biocompatibility, and mechanical stability have been widely studied in the field of food safety sensing. The classification, response mechanism, and recent application of hydrogel-based biosensors in food safety are reviewed in this paper. Furthermore, the challenges and future trends of hydrogel biosensors are also discussed.

A Dual-mode Ratiometric Fluorometric and Colorimetric Platform Based on Nitrogen-doped Carbon Dots and o-phenylenediamine for the Detection of Nitrite

In this study, a dual-mode ratiometric fluorometric and colorimetric platform for the determination of nitrite in pickles was proposed by exquisitely employing the fact that non-fluorescent o-Phenylenediamine (OPD) was oxidized by nitrite under acidic conditions to form fluorescent 2,3-diaminophenazine (DAP) (Em = 575), which meanwhile quench the fluorescent nitrogen-doped carbon dots (N-CDs) at 455 nm, the ratio of fluorescence intensity of DAP to N-CDs (F575/F455) changed with the increase of nitrite accompanied by visible color changes. Thus, nitrite can be quantitatively detected within a wide linear range (10-500 µM) with a low detection limit of 0.45 µM due to the high quantum yield of 39.7% of N-CDs. In addition, the colour of the N-CDs/OPD system changed from transparent to yellow when the nitrite was introduced, enabling colorimetric and on-site visual detection. The detection limit of the colorimetric method was 3.03 µM with a linear range of 10-500 µM. The proposed ratiometric fluorometric method has pleasant selectivity and good immunity to interference.

A naphthalimide-based fluorescent probe for rapid detection of nitrite and its application in food quality monitoring

Nitrite (NO₂^-) is a widely used food additive and long-term aging of cooked leftovers may also contribute to the formation of NO₂^-, excessive consumption of NO₂^- is harmful to human health. Developing an effective sensing strategy for on-site monitoring of NO₂^- has attracted considerable attention. Herein, a novel colorimetric and fluorometric probe ND-1 based on photoinduced electron transfer effect (PET) was designed for highly selective and sensitive detection of nitrite (NO₂^-) in foods. The probe ND-1 was strategically constructed by employing naphthalimide as the fluorophore and o-phenylendiamine as the specific recognition site for NO₂^-. The triazole derivative ND-1-NO₂^- could be produced exclusively by reacting with NO₂^-, leading to a visible colorimetric change from yellow to colorless accompanied by a significantly enhanced fluorescence intensity at 440 nm. The probe ND-1 exhibited promising sensing performances towards NO₂^- including high selectivity, rapid response time (within 7 min), low detection limit (47.15 nM) and wide quantitative detection range (0-35 μM). In addition, probe ND-1 was capable of quantitative detecting of NO₂^- in real food samples (including pickled vegetables and cured meat products) with satisfactory recovery rates (97.61%-103.08%). Moreover, the paper device loaded by probe ND-1 could be utilized for visual monitoring of NO₂^- levels variation of stir-fried greens. This study provided a feasible method for the accurate, traceable and rapid on-site monitoring NO₂^- in foods.

Highly sensitive and ratiometric detection of nitrite in food based on upconversion-carbon dots nanosensor

Nitrite (NO₂^-) is extensively found in the daily dietary environment. However, consuming too much NO₂^- can pose serious health risks. Thus, we designed a NO₂^--activated ratiometric upconversion luminescence (UCL) nanosensor which could realize NO₂^- detection via the inner filter effect (IFE) between NO₂^--sensitive carbon dots (CDs) and upconversion nanoparticles (UCNPs). Due to the exceptional optical properties of UCNPs and the remarkable selectivity of CDs, the UCL nanosensor exhibited a good response to NO₂^-. By taking advantage of NIR excitation and ratiometric detection signal, the UCL nanosensor could eliminate the autofluorescence thereby increasing the detection accuracy effectively. Additionally, the UCL nanosensor proved successful in detecting NO₂^- quantitatively in actual samples. The UCL nanosensor provides a simple as well as sensitive sensing strategy for NO₂^- detection and analysis, which is anticipated to extend the utilization of upconversion detection in food safety.

The target-induced redox and diazotized reaction for colorimetric ratio detection of nitrite using CoOOH nanosheets as mimetic oxidase

Nitrite is a commonly used food additive and water contaminant that has received widespread attention due to its harmful effects on humans. Here, a colorimetric ratio sensing platform for the detection of nitrite in foods as well as aquatic systems was developed via the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by CoOOH nanosheets (CoOOH NSs). Interestingly, in the presence of nitrite, TMB complexes in acidic environments can be oxidized and diazotized to produce yellow oxidized TMB (oxTMB) and diazotized TMB, resulting in the nitrite concentration-dependent ratio variation for the absorbance peaks at 655 and 450 nm (A₆₅₅/A₄₅₀). The colorimetric ratio sensing offers higher sensitivity and better selectivity compared to conventional detection methods because of the specific target-induced reduction-oxidation and diazotized reaction, as well as the excellent mimetic oxidase activity of CoOOH NSs. Based on this strategy, a smartphone-assisted portable approach was designed for the in-situ/visual detection of nitrite, which has good application prospects.

Highly Selective and Sensitive Detection of Nitrite Ion by an Unusual Nitration of a Fluorescent Benzimidazole

Nitrite (NO₂^-) is a physiologically significant anion having implications for cellular signaling. Here we report our serendipitous discovery of highly selective fluorescence-based nitrite sensing using a benzimidazole which stems from hitherto-unknown direct nitration of a benzimidazole using sodium nitrite. Using one- and two-dimensional NMR techniques, we elucidate the chemical structures of the new nitrated benzimidazoles and show differences in the nitration products using conventional nitration with nitric acid. We also show its utility in robust sensing of nitrite-containing samples.

Accurate quantification, naked eyes detection and bioimaging of nitrite using a colorimetric and near-infrared fluorescent probe in food samples and Escherichia coli

Nitrite (NO₂^-) is an inorganic contaminant that exists widely in the environment including water and food products, excessive amounts of NO₂^- would threaten humans and aquatic life. Developing a rapid and convenient sensing method for NO₂^- remains a great challenge. Herein, a colorimetric and near-infrared fluorescent probe (TBM) was synthesized and applied for sensitively and selectively detecting NO₂^- in water, food samples and Escherichia coli (E. coli). With the addition of NO₂^-, the probe TBM solution has a distinct visual color changed from red to colorless and fluorescence intensity at 620 nm quickly decreased. The probe TBM could detect NO₂^- quantitatively with a detection limit of 85 nM based on a 3σ/slope. Under optimum conditions, TBM has been successfully used to detect NO₂^- in real-world environmental and dietary samples, with positive results. Besides, paper strips loaded with TBM have been used to visually determine NO₂^- levels. Most importantly, TBM has also been proven to be able to discriminate from different concentrations of NO₂^- in E. coli by fluorescence imaging. In summary, the probe TBM was successfully developed for the accurate quantification, naked eyes detection and bioimaging of NO₂^- in water, food samples and E. coli, which provides a useful tool to better guarantee the quality and safety of daily life and food industry.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB⁺. When nitrite was introduced, the TMB⁺ species generated would specifically react with nitrite to produce diazotized TMB⁺, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB⁺ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

Recent progress on quality improvement and detection technologies of special foods used for activities in space and aviation: a review

This paper focuses on the development and evolution, quality improvement and research progress in the rapidly emerging area of new detection technologies of special foods for use in space and to some extent aviation. The quality improvement aspects covered in this review ranged from the special food processing technology, sterilization treatment and product packaging to new detection technologies for quality assurance based on DNA microarray technology, sensor, imaging technology, carbon nanotubes and novel probe technology.

Determination of nitrite in food based on its sensitizing effect on cathodic electrochemiluminescence of conductive PTH-DPP films

Herein, a novel strategy for electrochemiluminescence (ECL) detection of nitrite based on its sensitization effect on cathode ECL emission of 3,6-di(2-thienyl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (TH-DPP) polymeric films (PTH-DPP) was formulated, by means of a one-step electropolymerization of TH-DPP with a short time on the glassy carbon electrode (GCE). It was shown that the PTH-DPP film-modified GCE exhibited a strong ECL response when S₂O₈^2- was used as a co-reactant. The ECL emission could be greatly enhanced by PTH-DPP with nitrite in a K₂S₂O₈/PBS solution system and occurred at a relatively lower potential in comparison with traditional cathode ECL emitter, leading to high sensitivity and good selectivity. The ECL sensor exhibits excellent linear relationship in the ranges of 0.3 to 100 μM and 100 to 1000 μM for nitrite detection, with an outstanding detection limit of 0.08 μM (S/N = 3). The ECL sensor provides an impressive outcome for the detection of practical samples.

A novel highly specific colorimetric fluorescent probe for the detection of nitrite in aqueous solution

Developing an effective method for the detection of nitrite (NO₂ ^- ions) in natural environment especially environmental waters and soils is very necessary, because it will cause serious damage to human health once excess NO₂ ^- ions enters the human body. Herein, a new colorimetric fluorescent probe NB-NO₂ ^- for determining NO₂ ^- ions was designed, and it possesses good water-solubility and pleasurable selectivity over others common ions for NO₂ ^- ions. The addition of NO₂ ^- ions changed the color of solution from blue to colorless by naked-eye. And through the test and calculation, the detection limit of the probe NB-NO₂ ^- is 129 nM. Based on the above excellent characteristics, the probe NB-NO₂ ^- was successfully used for monitoring NO₂ ^- ions in environmental waters and soils.

A Novel Highly Sensitive Electrochemical Nitrite Sensor Based on a AuNPs/CS/Ti3C2 Nanocomposite

Nitrite is common inorganic poison, which widely exists in various water bodies and seriously endangers human health. Therefore, it is very necessary to develop a fast and online method for the detection of nitrite. In this paper, we prepared an electrochemical sensor for highly sensitive and selective detection of nitrite, based on AuNPs/CS/MXene nanocomposite. The characterization of the nanocomposite was demonstrated by scanning electron microscopy (SEM), a transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Under the optimized conditions, the fabricated electrode showed good performance with the linear range of 0.5–335.5 μM and 335.5–3355 μM, the limit of detection is 69 nM, and the sensitivity is 517.8 and 403.2 μA mM⁻¹ cm⁻². The fabricated sensors also show good anti-interference ability, repeatability, and stability, and have the potential for application in real samples.

Four Anderson-type [TeMo6O24]6−-based metal–organic complexes with a new bis(pyrimidine)-bis(amide): multifunctional electrochemical and adsorption performances

Four isostructural Anderson-type POM-based metal–organic complexes derived from a new bis(pyrimidine)-bis(amide) ligand were synthesized, showing multifunctional electrochemical sensing activities and good adsorption performances for organic dyes.

Ratiometric Colorimetric Detection of Nitrite Realized by Stringing Nanozyme Catalysis and Diazotization Together

Due to the great threat posed by excessive nitrite in food and drinking water to human health, it calls for developing reliable, convenient, and low-cost methods for nitrite detection. Herein, we string nanozyme catalysis and diazotization together and develop a ratiometric colorimetric approach for sensing nitrite in food. First, hollow MnFeO (a mixture of Mn and Fe oxides with different oxidation states) derived from a Mn-Fe Prussian blue analogue is explored as an oxidase mimic with high efficiency in catalyzing the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to blue TMBox, presenting a notable signal at 652 nm. Then, nitrite is able to trigger the diazotization of the product TMBox, not only decreasing the signal at 652 nm but also producing a new signal at 445 nm. Thus, the analyte-induced reverse changes of the two signals enable us to establish a ratiometric colorimetric assay for nitrite analysis. According to the above strategy, facile determination of nitrite in the range of 3.3–133.3 μM with good specificity was realized, providing a detection limit down to 0.2 μM. Compared with conventional single-signal analysis, our dual-signal ratiometric colorimetric mode was demonstrated to offer higher sensitivity, a lower detection limit, and better anti-interference ability against external detection environments. Practical applications of the approach in examining nitrite in food matrices were also verified.

Quantitative image analysis method for detection of nitrite with cyanine dye-NaYF4:Yb,Tm@NaYF4 upconversion nanoparticles composite luminescent probe

In this work, a quantitative image analysis method based on cyanine dye-upconversion nanoparticles composite luminescent nanoprobe for the detection of nitrite was developed. The nanoprobe was constructed by combining the NaYF₄:Yb,Tm@NaYF₄ upconversion nanoparticles (UCNPs) and the new cyanine dye IR-790. The upconversion nanoparticles transferred energy to IR-790, resulting in the luminescence quenching, while the luminescence of UCNPs was recovered after adding NO₂^-. The increase in photons was related to the concentration of NO₂^-. Under the optimal experimental conditions, the detection range was 0.20-140 μM and the limit of detection was 0.030 μM. The measurement for NO₂^- can be completed in 29 min. The method has the characteristics of fast response (~0.1 s), low sample consumption (10 μL) and powerful data support (550 frame time series images). Furthermore, the quantitative image analysis method was successfully applied for the analysis of nitrite in environmental water and food samples.

Reusable 3D silver superposed silica SERS substrate based on the Griess reaction for the ratiometric detection of nitrite

When nitrite is ingested and absorbed by the body, it can be converted into highly toxic nitrosamines (carcinogens, teratogens, and mutagens), posing health risks to the general population. Therefore, it calls for establishing a method for determination of nitrite. In this paper, the glass-SiO₂-Ag surface-enhanced Raman scattering (SERS) substrate with a large number of "hot spots" were prepared by two kinds of silane coupling agents. The SERS substrate had high sensitivity and repeatability. Silicon dioxide supported the silver nanoparticles (Ag NPs), which increased surface roughness of the substrate, generated a great quantity of hot spots and enhanced the SERS signal. In the SERS spectrum, the intensity ratio of the two characteristic peaks (1287 cm^-1 and 1076 cm^-1) had a good linear correlation with the logarithm of the concentration of nitrite, R² = 0.9652. The recoveries of 50 μM and 100 μM nitrite in three kinds of foods, three kinds of cosmetics and tap water were 90.9-105.3%.

Mesoporous CoOx/C Nanocomposites Functionalized Electrochemical Sensor for Rapid and Continuous Detection of Nitrite

Nitrite is widespread in the environment, and is frequently used as an additive to extend the shelf life of meat products. However, the excess intake of nitrite can be harmful to human health. Hence, it is very important to know and control the content of nitrite in foodstuffs. In this work, by the means of self-assembly induced by solvent evaporation, we used the amphiphilic PEO-b-PS diblock copolymers resol and cobalt nitrate as a template to synthesize ordered mesoporous CoOx/C nanocomposites. Then, the CoOx/C nanocomposites were modified on a glassy carbon electrode (GCE), which showed excellent sensitivity, good selectivity, and a wide detection range for nitrite. Through cyclic voltammetry and current–time techniques, the electrochemical performance of the GCE modified with CoOx/C nanocomposites was analyzed. Under the optimized conditions, we found that anodic currents were linearly related to nitrite concentrations with a regression equation of lp (µA) = 0.36388 + 0.01616C (R2 = 0.9987) from 0.2 µM to 2500 µM, and the detection limit was 0.05 µM. Furthermore, the electrochemical sensor behaved with high reproducibility and anti-interference ability towards various organic and inorganic ions, such as NO3−, SO42−, Cl−, COOH− (Ac−), Na+, K+, Mg2+, and NH4+. Our results indicated that these CoOx/C nanocomposites could be applied in electrochemical sensors for the rapid and sensitive detection of the food preservative nitrite.

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.