Peroxynitrous-acid-induced chemiluminescence detection of nitrite based on Microfluidic chip

Hydrogen peroxide enhanced glow-type chemiluminescence of hydrazine hydrate modified carbon quantum dots-potassium persulfate system

Most known chemiluminescence (CL) systems are flash-type that generate weak luminescence and decline quickly after dozens of seconds, while the glow-type CL systems have stable emission for an extended period to achieve accurate quantitation. In this work, a long-term CL system based on hydrazine-hydrate (N2H4·H2O) modified carbon quantum dots (N-CQDs) as a luminescent probe, with K2S2O8 and H2O2 as co-reactants, was proposed. The CL emission enhanced by H2O2 increased 18-fold more than that of N-CQDs and K2S2O8 direct reaction, and decayed by 5% of the maximum intensity over 700 s. In the reaction system, K2S2O8 and H2O2 co-reactants can promote each other to continuously generate corresponding radicals (•OH, O2•-, 1O2), which in turn trigger the CL emission of N-CQDs. This phenomenon was identified as the primary cause for the production of persistent CL. In addition, a stable and selective CL sensor based on the N-CQDs-K2S2O8-H2O2 CL enhancing system was developed for ascorbic acid quantitation in the linear range from 0.1 to 10.0 mM with a detection limit of 0.036 mM. The method has been applied to the analysis of tablet samples and holds potential in pharmaceutical analysis field.

Technical Evaluation of a Paper-Based Electrochemical Strip to Measure Nitrite Ions in the Forensic Field

Nitrite is a compound used as a food additive for its preservative action and coloring capability, as well as an industrial agent for its antifreezing action and for preventing corrosion, and it is also used as a pharmaceutical in cyanide detoxification therapy. However, even recently, because of its high toxicity, it has been used as a murder and suicidal agent due to its affordability and ready availability. In this technical report, we describe an electrochemical paper-based device for selectively determining nitrite in complex biofluids, such as blood, cadaveric blood, vitreous humor, serum, plasma, and urine. The approach was validated in terms of the linearity of response, selectivity, and sensitivity, and the accuracy of the determination was verified by comparing the results with a chromatographic instrumental method. A linear response was observed in the micromolar range; the sensitivity of the method expressed as the limit of detection was 0.4 μM in buffer measurements. The simplicity of use, the portability of the device, and the performance shown make the approach suitable for detecting nitrite in complex biofluids, including contexts of forensic interest, such as murders or suicides in which nitrite is used as a toxic agent. Limits of detection of ca. 1, 2, 4, 5, 3, and 4 μM were obtained in vitreous humor, urine, serum and plasma, blood, and cadaveric blood, also highlighting a satisfactory accuracy comprised between 91 and 112%.

Rapid colorimetric assay based on the oxidation of 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid-diammonium salt for nitrite detection in meat products

This work developed a rapid colorimetric method for nitrite detection in meat products. The detection was based on the reaction of nitrite with 60 mM HCl to produce radicals which further oxidized ABTS (50 µM) to form a water-soluble blue-green product (ABTS•+). The absorbance was measured at a maximum absorption wavelength of 412.5 nm. Parameters such as concentration of HCl, concentration of ABTS and reaction time were evaluated. The absorbance was linearly proportional to the concentration of nitrite (0.1-20 µM) with the limit of detection of 0.34 µM. The proposed method was a time-saving assay since it required only 2 min to complete one measurement. There was no effect of the interference produced by other ions. The assay was robust with 2.5%RSD (n = 50). In meat product samples, high accuracy was observed with the recoveries between 100 ± 2.2% and 105 ± 3.7%. The amount of nitrite in meat products detected by the ABTS method was found in the range of 5.41 - 7.62 mg/kg. The conventional Griess method was applied to determine nitrite in the same meat products. There was no statistically significant difference between the two methods (P = 0.05).

A chemiluminescence method induced by microplasma jet for nitrites detection and the miniature detection system using smartphone

A method of luminol-diazonium chemiluminescence (CL) induced by microplasma for hazardous substance detection is proposed. The luminol-diazonium CL is caused by microplasma jet, rather than hydrogen peroxide reagent or other oxidizing agents. The CL intensity is increased by the concentration of nitrites. Based on the process of microplasma generation and CL mechanism, the optimal work conditions of the method are obtained. The linear range for nitrites detection is 0.03-1 mmol L^-1 with the limit of detection (LOD) of 0.01 mmol L^-1. Furthermore, a miniature system using test paper and smartphone is designed for nitrites detection in emergency. The detection system is confined in the custom-tailored shell which is only 28 cm in length, 18 cm in width and 10 cm in height. After microplasma jet treatment, the color of the test paper changes with the NO₂^- concentration. The photographs of the test paper are taken by the built-in camera of smartphone and analyzed by visiting the website via smartphone. The LOD is 1 mmol L^-1 obtained by the CL miniature detection system based on test paper and smartphone. The accuracy, reliability and practicability of the proposed method is verified in this paper.

Electrochemical sensors based on platinum-coated MOF-derived nickel-/N-doped carbon nanotubes (Pt/Ni/NCNTs) for sensitive nitrite detection

As excess nitrite has a serious threat to the human health and environment, constructing novel electrochemical sensors for sensitive nitrite detection is of great importance. In this report, platinum nanoparticles were deposited on nickel-/N-doped carbon nanotubes, which were obtained through a self-catalytically grown process with Ni-MOF as precursors. The as-prepared Pt/Ni/NCNTs were applied as amperometric sensors and presented superior sensing properties for nitrite detection. Benefiting from the synergy of Pt and Ni/NCNTs, Pt/Ni/NCNTs displayed much wider detection ranges (0.5-40 mM and 40-110 mM) for nitrite sensing. The sensitivity is 276.92 μA mM^-1 cm^-2 and 224.39 μA mM^-1 cm^-2, respectively. The detection limit is 0.17 μM. The Pt/Ni/NCNTs sensors also showed good feasibility for nitrite sensing in real samples (milk and peach juice) analysis. The active Pt/Ni/NCNTs composites and facile fabrication technique may provide useful strategies to develop other sensitive nitrite sensors.

A fluorescent carbon dots synthesized at room temperature for automatic determination of nitrite in Sichuan pickles

In this paper, highly fluorescent carbon dots were synthesized from sodium ascorbate and polyethyleneimine at room temperature (R-CDs). The proposed green synthesis method was energy-saving, environmentally friendly and easy online. R-CDs exhibit an optimal emission peak of 490 nm under excitation at 380 nm with a quantum yield of 32 %. R-CDs morphology, composition, and properties were characterized using TEM, FTIR, XPS, UV-vis and fluorescence spectroscopy. The study revealed that nitrite quenched the fluorescence of R-CDs under acidic conditions. Subsequently, this discovered reaction of R-CDs and nitrite was combined with flow-injection technology, and a simple, precise and automatic fluorescence strategy for nitrite determination was accomplished. The response to nitrite was linear in 5-300 μg·L^-1 concentration range and the limit of detection was 2.85 μg·L^-1 (3.3 S/k). This method was applied to nitrite determination in Sichuan pickles during the pickling process and results were consistent with the standard method, demonstrating its feasibility in practical applications.

A turn-on fluorescent nano-probe base on methanobactin-AuNPs for simple and efficient detection of nitrite

Nitrite ions are important markers threatening humans and environmental security. A highly selective method for rapid detection of nitrite needs to be developed. Herein, a novel and rapid fluorescence method for nitrite determination is established on the basis of diazotization-coupling reaction of methanobactin (Mb) extracted by Methylosinus trichosporium OB3b with nitrite on the fluorescence. In the presence of gold nanoparticles (AuNPs), the fluorescence of AuNPs was strongly quenched by the Mb because the sulfhydryl or amino structures on the surface of Mb could be bound to the surface of AuNPs by forming Au-S or Au-N bonds. Upon addition of nitrite, the Mb easily reacts with nitrite to form azo products in the acidic medium. Then, with the increase of nitrite concentration, the Mb-AuNPs fluorescence was gradually recovered, realizing the turn-on fluorescence sensing of nitrite. Under optimal conditions, the proposed method has a good linear relationship with nitrite concentration in the range of 0-8.0 μM and 8.0-50.0 μM, and the detection limit is 16.21 nM. In addition, satisfactory results were obtained for nitrite analysis using milk, ham sausage and leaf mustard as real samples, which demonstrated that the method as-developed would have great practical application prospects.

Nitrite Determination in Environmental Water Samples Using Microchip Electrophoresis Coupled with Amperometric Detection

Nitrite is considered an important target analyte for environmental monitoring. In water resources, nitrite is the result of the nitrogen cycle and the leaching processes of pesticides based on nitrogenous compounds. A high concentration of nitrite can be associated with intoxication processes and metabolic disorders in humans. The present study describes the development of a portable analytical methodology based on microchip electrophoresis coupled with amperometric detection for the determination of nitrite in environmental water samples. Electrophoretic and detection conditions were optimized, and the best separations were achieved within 60 s by employing a mixture of 30 mmol L^-1 lactic acid and 15 mmol L^-1 histidine (pH = 3.8) as a running buffer applying 0.7 V to the working electrode (versus Pt) for amperometric measurements. The developed methodology revealed a satisfactory linear behavior in the concentration range between 20 and 80 μmolL^-1 (R² = 0.999) with a limit of detection of 1.3 μmolL^-1. The nitrite concentration was determined in five water samples and the achieved values ranged from (28.7 ± 1.6) to (67.1 ± 0.5) µmol L^-1. The data showed that using the proposed methodology revealed satisfactory recovery values (83.5-103.8%) and is in good agreement with the reference technique. Due to its low sample consumption, portability potential, high analytical frequency, and instrumental simplicity, the developed methodology may be considered a promising strategy to monitor and quantitatively determine nitrite in environmental samples.

Partnered Excited-State Intermolecular Proton Transfer Fluorescence (P-ESIPT) Signaling for Nitrate Sensing and High-Resolution Cell-Imaging

Nitrite (NO₂⁻) is a common pollutant and is widely present in the environment and in human bodies. The development of a rapid and accurate method for NO₂⁻ detection is always a very important task. Herein, we synthesized a partnered excited-state intermolecular proton transfer (ESIPT) fluorophore using the “multi-component one pot” method, and used this as a probe (ESIPT-F) for sensing NO₂⁻. ESIPT-F exhibited bimodal emission in different solvents because of the solvent-mediated ESIPT reaction. The addition of NO₂⁻ caused an obvious change in colors and tautomeric fluorescence due to the graft of NO₂⁻ into the ESIPT-F molecules. From this basis, highly sensitive and selective analysis of NO₂⁻ was developed using tautomeric emission signaling, achieving sensitive detection of NO₂⁻ in the concentration range of 0~45 mM with a detection limit of 12.5 nM. More importantly, ESIPT-F showed the ability to anchor proteins and resulted in a recognition-driven “on-off” ESIPT process, enabling it to become a powerful tool for fluorescence imaging of proteins or protein-based subcellular organelles. MTT experimental results revealed that ESIPT-F is low cytotoxic and has good membrane permeability to cells. Thus, ESIPT-F was further employed to image the tunneling nanotube in vitro HEC-1A cells, displaying high-resolution performance.

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.

Microfluidic technology and its application in the point-of-care testing field

Since the outbreak of the coronavirus disease 2019 (COVID-19), countries around the world have suffered heavy losses of life and property. The global pandemic poses a challenge to the global public health system, and public health organizations around the world are actively looking for ways to quickly and efficiently screen for viruses. Point-of-care testing (POCT), as a fast, portable, and instant detection method, is of great significance in infectious disease detection, disease screening, pre-disease prevention, postoperative treatment, and other fields. Microfluidic technology is a comprehensive technology that involves various interdisciplinary disciplines. It is also known as a lab-on-a-chip (LOC), and can concentrate biological and chemical experiments in traditional laboratories on a chip of several square centimeters with high integration. Therefore, microfluidic devices have become the primary implementation platform of POCT technology. POCT devices based on microfluidic technology combine the advantages of both POCT and microfluids, and are expected to shine in the biomedical field. This review introduces microfluidic technology and its applications in combination with other technologies.

Enhanced Nitrite Detection by a Carbon Screen Printed Electrode Modified with Photochemically-Made AuNPs

Excessive nitrite amounts harm the environment and put public health at high risk. Therefore, accurate and sensitive detection of nitrite in surface and groundwater is mandatory for mitigating its adverse effects. Herein, a highly sensitive electrochemical sensor based on carbon screen-printed electrodes (CSPE) surface-modified with photochemically-made gold nanoparticles (AuNPs, ~12 nm) is proposed for nitrite detection. Scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy show that AuNPs uniformly coat the CSPE, increase its surface area, and contribute to oxidizing nitrite to much lower potential (+0.5 V vs. Ag/AgCl) and faster rate. Under optimized differential pulse voltammetry conditions, the CSPE/AuNPs-PEI electrode responds linearly (R2 > 0.99) to nitrite within a wide concentration range (0.01–4.0 µM), showing a sensitivity of 0.85 µA·µM−1·cm−2 and limit of detection as low as 2.5 nM. The CSPE/AuNPs-PEI electrode successfully detects nitrite in tap water and canned water of olives, showing no influence of those matrices. In addition, the electrode’s response is highly reproducible since a relative standard deviation lower than 10% is observed when the same electrode is operated in five consecutive measurements or when electrodes of different fabrication batches are evaluated.

Dual determination of nitrite and iron by a single greener sequential injection spectrophotometric system employing a simple single aqueous extract from Areca catechu Linn. serving as a natural reagent

Dual determination of nitrite and iron was proposed by using a single greener sequential injection (SI) spectrophotometric system employing a simple single aqueous extract from Areca catechu Linn. The extract served as a natural reagent to replace N-(1-naphthyl)ethylenediamine (NED) of the Griess reagent with nitrite and 1,10-phenanthroline with iron. The color products possessed analytical wavelengths at 430 and 560 nm, respectively. Conditions for the SI procedure were optimized using a univariate experimental design. Calibration ranges were up to 5.0 mg L⁻¹ and 10.0 mg L⁻¹ with limits of detection (LODs) of 0.04 mg L⁻¹ and 0.05 mg L⁻¹ for nitrite and iron(iii), respectively, and relative standard deviations (RSDs) being less than 3%. Recoveries of spiked standard nitrite and iron(iii) at 0.3 mg L⁻¹ and 0.5 mg L⁻¹ in water samples were 88 to 104% and 84 to 109%, respectively. The developed method successfully achieved dual determination of nitrite and total iron agreeing at a 95% confidence level with the reference methods of the conventional Griess assay and flame atomic absorption spectrometry (FAAS), respectively. The proposed method utilized locally available material from plants and serves the UN-SDGs.

Dual determination of nitrite and iron was proposed by using a single greener sequential injection (SI) spectrophotometric system employing a simple single aqueous extract from Areca catechu Linn.

Construction of polythiophene-derivative films as a novel electrochemical sensor for highly sensitive detection of nitrite

Herein, a novel, convenient, and highly selective electrochemical sensor for determination of nitrite based on a polythiophene-derivative film-modified glassy carbon electrode (GCE) was established. In this work, 2,5-di-thiophen-3-yl-thiazolo[5,4-d]thiazole (DTT), a novel thiophene derivative, was synthesized and used to form an original and excellent polymer film (PolyDTTF) on GCE through one-step electropolymerization for the first time. The modified electrodes were characterized by electron microscopy (SEM), Fourier transform infra-red spectroscopy (FT-IR), UV-visible spectra, Raman spectroscopy, and electrochemical technologies, in which the electrochemical sensor based on PolyDTTF was successfully constructed and demonstrated a significant electrocatalytic effect on nitrite. The influence of pH value, electrodeposition scanning times, scanning speed, and potential on the electrochemical behavior of nitrite were investigated in detail. Furthermore, the nitrite sensor exhibits excellent responses proportional to nitrite concentrations (R² = 0.9972) over a concentration range of 5.5 × 10^-9 ~ 3.5 × 10^-5 M with a detection limit (LOD) of 2 nM, and has extremely good anti-interference ability for nitrite detection. This proposed sensor can be used to detect nitrite in actual samples, opening the possibility for applications in the food industry and environmental analysis.

Integrating electrochemical sensor based on MoO3/Co3O4 heterostructure for highly sensitive sensing of nitrite in sausages and water

Considering excess nitrites are detrimental to the human body and environment, designing a rapid, sensitive, and real-time quantitative determination for nitrite is of great significance for environmental preservation and public health. In this paper, Co₃O₄ nanoflowers coupled with ultrafine MoO₃ nanoparticles (MoO₃/Co₃O₄) are obtained via a hybrid electrochemical deposition strategy (HED). The as-designed MoO₃/Co₃O₄/CC integrating electrode exhibits superior electrocatalytic properties towards nitrite oxidation, owing to the synergistic effect between MoO₃ and Co₃O₄ caused by the heterostructure of MoO₃/Co₃O₄. The electrode achieved a low response time of 2 s, an excellent sensitivity of 1704.1 μA mM^-1 cm^-2, and a low limit of detection of 0.075 μM (S/N = 3). Furthermore, the electrode displays promise for nitrite detection in complex food such as water and sausages samples. Our study will provide a significant strategy for the application of bimetallic heterostructure to explore the design of sensing interfaces.

Flow-based System: A Highly Efficient Tool Speeds Up Data Production and Improves Analytical Performance

In this review, we cite references from the period between 2015 and 2020 related to the use of a flow-based system as a tool to obtain a modern analytical system for speeding up data production and improving performance. Based on a great deal of concepts for automatic systems, there are several research groups introduced in the development of flow-based systems to increase sample throughput while retaining the reproducibility and repeatability as well as to propose new platforms of flow-based systems, such as microfluidic chip and paper-based devices. Additionally, to apply a developed system for on-site analysis is one of the key features for development. We believe that this review will be very interested and useful for readers because of its impact on developing novel analytical systems. The content of the review is categorized following their applications including quality control and food safety, clinical diagnostics, environmental monitoring and miscellaneous.

Nanomaterial-enhanced chemiluminescence reactions and their applications

Chemiluminescence (CL) analysis is a trace analytical method that possesses advantages including high sensitivity, wide linear range, easy operation, and simple instruments. With the development of nanotechnology, many nanomaterial (NM)-enhanced CL systems have been established in recent years and applied for the CL detection of metal ions, anions, small molecules, tumor markers, sequence-specific DNA, and RNA. This review summarizes the research progress of the nanomaterial-enhanced CL systems the past five years. These CL reactions include luminol, peroxyoxalate, lucigenin, ultraweak CL reactions, and so on. The CL mechanisms of the nanomaterial-enhanced CL systems are discussed in the first section. Nanomaterials take part in the CL reactions as the catalyst, CL emitter, energy acceptor, and reductant. Their applications are summarized in the second section. Finally, the challenges and opportunities are discussed.

Gold nanoparticles decorated bimetallic CuNi-based hollow nanoarchitecture for the enhancement of electrochemical sensing performance of nitrite

Gold nanoparticles (AuNPs) decorated bimetallic CuNi-based hollow nanoarchitecture (CNHN) are reported for the first time as a nonenzymatic sensor for the quantification of nitrite in neutral solution . The CNHN was prepared via a convenient calcining routine using the bimetallic CuNi-MOFs as a coprecursor. The unique chemical structure of hollow CNHN with high specific surface area and abundant terminal amino groups effectively avoid the aggregation of AuNPs and facilitate the subsequent adsorption of nitrite. The Au/CNHN exhibited high electrocatalytic activity towards nitrite oxidation due to the synergetic catalytic effect of AuNPs and CNHN. Chronoamperometric detection of nitrite at the Au/CNHN/GCE achieved a lower linear calibration range of 0.05 to 1.15 mM, with an LOD of 0.017 μM compared with previous reports. The proposed method obtained satisfactory recoveries for nitrite determination in practical applications, which was verified by UV-Vis spectrophotometry. The prepared sensor based on Au/CNHN featured favorable selectivity and stability, which provides a promising approach for real sample analysis. Graphical abstract.

Ultra-Weak Chemiluminescence Enhanced by Cerium-Doped LaF3 Nanoparticles: A Potential Nitrite Analysis Method

In this work, cerium-doped LaF₃ nanoparticles (LaF₃:Ce NPs) were successfully synthesized and characterized. Its chemiluminescence (CL) property was studied, and it was amazingly found that it intensely enhanced the ultra-weak CL of the NaNO₂-H₂O₂ system. The CL mechanism was systematically investigated and suggested to be the recombination of electron-injected and hole-injected LaF₃:Ce NPs. The new CL system was developed to be a facile, original, and direct method for nitrite analysis. Experimental conditions were optimized and then a satisfactory linear relationship between CL intensity and nitrite concentration was obtained. This work introduced a new pathway for the research and application of traditional fluoride NPs doped with RE³⁺.

An Electrochemical Sensor Based on Gold-Nanocluster-Modified Graphene Screen-Printed Electrodes for the Detection of β-Lactoglobulin in Milk

A simple and low-cost electrochemical sensor based on multimodified screen-printed electrodes (SPEs) was successfully synthesized for the sensitive detection of β-lactoglobulin (β-Lg). The surface treatment of SPEs was accomplished by a simple drip coating method using polyethyleneimine (PEI), reduced graphene oxide (rGO), and gold nanoclusters (AuNCs), and the treated SPEs showed excellent electrical conductivity. The modified SPEs were then characterized with UV-Vis, SEM, TEM, and FTIR to analyze the morphology and composition of the AuNCs and the rGO. An anti-β-Lg antibody was then immobilized on the composite material obtained by modifying rGO with PEI and AuNCs (PEI-rGO-AuNCs), leading to the remarkable reduction in conductivity of the SPEs due to the reaction between antigen and antibody. The sensor obtained using this novel approach enabled a limit of detection (LOD) of 0.08 ng/mL and a detection range from 0.01 to 100 ng/mL for β-Lg. Furthermore, pure milk samples from four milk brands were measured using electrochemical sensors, and the results were in excellent agreement with those from commercial enzyme-linked immunosorbent assay (ELISA) methods.

Application of Microfluidic Chip Technology in Food Safety Sensing

Food safety analysis is an important procedure to control food contamination and supervision. It is urgently needed to construct effective methods for on-site, fast, accurate and popular food safety sensing. Among them, microfluidic chip technology exhibits distinguish advantages in detection, including less sample consumption, fast detection, simple operation, multi-functional integration, small size, multiplex detection and portability. In this review, we introduce the classification, material, processing and application of the microfluidic chip in food safety sensing, in order to provide a good guide for food safety monitoring.

Redox-derivatization reaction-based rapid and sensitive determination of nitrite using resonance Rayleigh scattering method

It remains a problem for direct detection of small inorganic nitrite ions using resonance Rayleigh scattering (RRS) method based on the direct dye-binding reaction. In the present study, a redox-derivatization reaction taking only 5 min was introduced prior to nitrite detection. In the redox-derivatization reaction, nitrite ions were reduced by excess iodine ions to generate triiodide ions (I₃^-), which were further derivatized with a cationic dye (basic violet 1, BV1) to form the ion associates of I₃^--BV1. Therefore, the RRS signal was significantly enhanced, resulting from the increase of particle size and resonance-enhanced scattering effect. The analytical procedure was performed by just mixing nitrite, oxidant, acid, and dye all-in-one, avoiding the tediousness of a multi-step process or the preparation of nanoparticles. The whole detection process including the redox-derivatization reaction was less than 6 min. The reaction conditions such as concentration of hydrochloric acid, potassium iodide, and BV1, reaction time, and temperature were investigated. Under optimum conditions, the concentration of nitrite was linear with an RRS signal of I₃^--BV1 ion associates at 320 nm in the range of 0.015-1.2 mg/L. The limit of detection (LOD) was calculated to be 3.0 μg/L. The RRS method was applied to the determination of nitrite in real samples such as pork sausage, milk powder, and water with recovery of 95.2-112%. With advantages of rapidness, high sensitivity, and high selectivity, the method indicates potential applicability for detection of nitrite in complex samples. The method also provides an instructive protocol for detection of analytes that generate no/weak RRS enhancement after the direct dye-binding reaction. Graphical abstract.

A colorimetric fluorescent probe for rapid and specific detection of nitrite

The method of fluorescent probes has been an important technique for detection of nitrite (NO₂ ^- ). As an important inorganic salt, excessive nitrite would threaten humans and the environment. In this paper, a colorimetric fluorescent probe P-N (1,2-diaminoanthraquinone) with rapid response and high selectivity, which could detect NO₂ ^- by visual colour changes and fluorescence spectroscopy is presented. The probe P-N solution (pH 1) changed from pink to colourless with the addition of NO₂ ^- and fluorescence intensity at 639 nm clearly decreased. Good linear exists between fluorescence intensities and NO₂ ^- concentrations for the range 0-16 μM, and the detection limit was 54 nM (based on a 3σ/slope). Moreover, probe P-N could also detect NO₂ ^- in real water samples, and results were all satisfactory. Probe P-N shows great practical application value for detecting NO₂ ^- in the environment.

Facile synthesis of highly porous CuO nanoplates (NPs) for ultrasensitive and highly selective nitrogen dioxide/nitrite sensing

Copper oxide (CuO) nanoplates (NPs of ∼100 nm width) were successfully synthesized via a chemical method (emulsion method). Superior catalytic activities towards both chemical and electrochemical sensing of nitrite were achieved.

Copper oxide (CuO) nanoplates (NPs of ∼100 nm width) have been successfully synthesized by using a chemical method (emulsion method). Superior catalytic activities towards both chemical and electrochemical sensing of nitrite were achieved.

Biogreen Synthesis of Carbon Dots for Biotechnology and Nanomedicine Applications

Over the past decade, carbon dots have ignited a burst of interest in many different fields, including nanomedicine, solar energy, optoelectronics, energy storage, and sensing applications, owing to their excellent photoluminescence properties and the easiness to modify their optical properties through doping and functionalization. In this review, the synthesis, structural and optical properties, as well as photoluminescence mechanisms of carbon dots are first reviewed and summarized. Then, we describe a series of designs for carbon dot-based sensors and the different sensing mechanisms associated with them. Thereafter, we elaborate on recent research advances on carbon dot-based sensors for the selective and sensitive detection of a wide range of analytes, including heavy metals, cations, anions, biomolecules, biomarkers, nitroaromatic explosives, pollutants, vitamins, and drugs. Lastly, we provide a concluding perspective on the overall status, challenges, and future directions for the use of carbon dots in real-life sensing.

Electrostatically in situ binding of zwitterionic glycine on the surface of MGO for determination of nitrite in various real samples

Zwitterionic dispersive magnetic solid phase extraction (ZI-DMSPE) was developed through in situ binding of glycine on the magnetic graphene oxide, electrostatically. This highly selective sorbent was applied for the determination and preconcentration of trace levels of nitrite in soil, sausage, water samples (tap, mineral, and rain), and vegetables (potato, onion, spinach, radish, and lettuce) prior to its determination by UV-Vis spectrophotometry. The major advantage of the method is the analyte adsorption in both acidic and basic media. The sorbent was characterized by SEM, XRD, EDS, and FT-IR. Several parameters affecting ZI-DMSPE were optimized. Under the optimal conditions, LOD and RSD were obtained 17 ng L^-1 and 1.3% respectively. Preconcentration factor and sorption capacity of the proposed method were 666 and 238 mg g^-1 respectively. Accuracy was assessed by comparing results with those obtained by direct determination using ion chromatography and spiked real samples.

Local surface modification at precise position using a chemical pen

Push-pull cannula system, which was first proposed by Gaddum, has grown to be an important method for the perfusion of brain and region-selective surface treatment. However, reported push-pull cannula systems only concerned on single reagent applications. Microfluidic system was then an exciting tool for multi-reagent treatment on substrate in closed microchannels. Nowadays, it is still a challenge to apply online mixing and reaction for surface pattern in an open environment. Here, we present a novel method using a chemical pen that enables region-selective online chemical reactions for the micro-surface modification/patterning. We utilized this method to fabricate labeling protein array using an online labeling strategy. Moreover, the device was applied for local modification of biomaterials surface by using a three-component reaction at precise position. This tool was the first demonstration of design to perform online reaction of two different reagents on a real solid sample in an open environment. It was demonstrated a useful method for protein array fabrication with online labeled protein.

Sensitive paper-based analytical device for fast colorimetric detection of nitrite with smartphone

On-site rapid monitoring of nitrite as an assessment indicator of the environment, food, and physiological systems has drawn extensive attention. Here, electrokinetic stacking (ES) was combined with colorimetric reaction on a paper-based device (PAD) to achieve colorless nitrite detection with smartphone. In this paper, nitrite was stacked on the paper fluidic channel as a narrow band by electrokinetic stacking. Then, Griess reagent was introduced to visualize the stacking band. Under optimal conditions, the sensitivity of nitrite was 160-fold increased within 5 min. A linear response in the range of 0.075 to 1.0 μg mL^-1 (R² = 0.99) and a limit of detection (LOD) of 73 ng mL^-1 (0.86 μM) were obtained. The LOD was 10 times lower than the reported PAD, and close to that achieved by a desktop spectrophotometer. The applicability was demonstrated by nitrite detection from saliva and water with good selectivity, adding 100 times more concentrated co-ions. High recovery (91.0~108.7%) and reasonable intra-day and inter-day reproducibility (RSD < 9%) were obtained. This work shows that the sensitivity of colorless analyte detection-based colorimetric reaction can be effectively enhanced by integration of ES on a PAD. Graphical abstract Schematic of the experimental setups (left) and the corresponding images (right) of the actual portable device.

Microfluidic channels with renewable and switchable biological functionalities based on host–guest interactions

Microfluidic channels with renewable and switchable biological functionalities were prepared using host–guest interactions.

Non-enzymatic sensors based on a glassy carbon electrode modified with Au nanoparticles/polyaniline/SnO2 fibrous nanocomposites for nitrite sensing

A non-enzymatic sensor based on Au nanoparticles (AuNPs)/polyaniline (PANI)/tin oxide (SnO₂) nanocomposites (Au/PANI/SnO₂) was prepared for enhanced electrochemical sensing of nitrite (NO₂⁻).

Synthesis of Au nanoparticles dispersed on halloysite nanotubes–reduced graphene oxide nanosheets and their application for electrochemical sensing of nitrites

Electrochemical sensing of nitrite based on a novel Au–HNTs–GO nanocomposite.