Highly Sensitive Portable Test Strip Based on Carboxylate Cellulose‐Derived Carbon Dots for On‐Site Visual Detection of Nitrite in Food Samples

The accurate determination of nitrite in food samples is of great significance for ensuring people’s health and safety. Herein, a rapid and low-cost detection method was developed for highly sensitive and selective detection of nitrite based on a surface-enhanced Raman scattering (SERS) sensor combined with electrochemical technology and diazo reaction. In this work, a gold nanoparticle (AuNP)/indium tin oxide (ITO) chip as a superior SERS substrate was obtained by electrochemical self-assembled AuNPs on ITO with the advantages of good uniformity, high reproducibility, and long-time stability. The azo compounds generated from the diazotization-coupling reaction between nitrite, 4-aminothiophenol (4-ATP), and N-(1-naphthyl) ethylenediamine dihydrochloride (NED) in acid condition were further assembled on the surface of AuNP/ITO. The detection of nitrite was realized using a portable Raman spectrometer based on the significant SERS enhancement of azo compounds assembled on the AuNP/ITO chip. Many experimental conditions were optimized such as the time of electrochemical self-assembly and the concentration of HAuCl4. Under the optimal conditions, the designed SERS sensor could detect nitride in a large linear range from 1.0 × 10−6 to 1.0 × 10−3 mol L−1 with a low limit of detection of 0.33 μmol L−1. Additionally, nitrite in real samples was further analyzed with a recovery of 95.1−109.7%. Therefore, the proposed SERS method has shown potential application in the detection of nitrite in complex food samples.

A derivative of 3,3′,5,5′-tetramethylbenzidine for highly robust colorimetric sensing of nitrite in food samples

Amperometry detection of nitrite in food samples using tetrasulfonated copper phthalocyanine modified glassy carbon electrode

A novel colourimetric method for the detection of nitrite in food samples using digital image colourimetry (DIC) by smartphone is developed. Nitrite directly oxidise 3,3′,5,5′‐tetramethylbenzidine (TMB) to form a yellow TMB diimine (oxTMB). A smartphone was used to capture the image of an inherent colour variation and analyse the image with the RGB (red, green, and blue) model, achieving the quantitative detection of nitrite. Linear response for the detection of nitrite was obtained from 10 μmol L −1 to 440 μmol L −1 and the limit of quantification was 2.34 μmol L −1 . The applicability of the method was confirmed by the detection of nitrite in cabbage, pickle, and ham. The recovery was varied in the range from 96.2 % to 108.2 % with a relative standard deviation of less than 5.0 %. The simple, sensitive, and inexpensive method was an alternative for the detection of nitrite in food samples.

A fluorescent carbon dots probe for the detection of aqueous nitrite was fabricated by a one-pot hydrothermal method, and the transmission electron microscope, X-ray diffractometer, UV–Vis absorption spectrometer and fluorescence spectrophotometer were used to study the property of carbon dots. The fluorescent property of carbon dots influenced by the concentration of aqueous nitrite was studied. The interaction between the electron-donating functional groups and the electron-accepting nitrous acid could account for the quenching effect on carbon dots by adding aqueous nitrite. The products of the hydrolysis of aqueous nitrite performed a stronger quenching effect at lower pH. The relationship between the relative fluorescence intensity of carbon dots and the concentration of nitrite was described by the Stern–Volmer equation (I0/I − 1 = 0.046[Q]) with a fine linearity (R2 = 0.99). The carbon dots-based probe provides a convenient method for the detection of nitrite concentration.

A facile dual-function fluorescent probe for detection of phosgene and nitrite and its applications in portable chemosensor analysis and food analysis

Paper-based test strips with on-site visual detection have become a hot spot in the field of target detection. Yet, low specific surface area and uneven deposition limit the further application of test strips. Herein, a novel “turn-on” ratio of molecularly imprinted membranes (Eu@CDs-MIMs) was successfully prepared based on a Eu complex-doped polyvinylidene fluoride membrane for the selective, rapid and on-site visual detection of norfloxacin (NOR). The formation of surface-imprinted polymer-containing carbon dots (CDs) improves the roughness and hydrophilicity of Eu@CDs-MIMs. Fluorescence lifetimes and UV absorption spectra verified that the fluorescence enhancement of CDs is based on the synergistic effect of charge transfer and hydrogen bonding between CDs and NOR. The fluorescent test strip showed a linear fluorescent response within the concentration range of 5–50 nM with a limit of detection of 1.35 nM and a short response time of 1 min. In comparison with filter paper-based test strips, Eu@CDs-MIMs exhibit a brighter and more uniform fluorescent color change from red to blue that is visible to the naked eye. Additionally, the applied ratio fluorescent test strip was combined with a smartphone to translate RGB values into concentrations for the visual and quantitative detection of NOR and verified the detection results using high-performance liquid chromatography. The portable fluorescent test strip provides a reliable approach for the rapid, visual, and on-site detection of NOR and quinolones.

A [5,10,15,20-tetrakis (4-methoxyphenyl) porphyrinato] manganese (III)chloride (TMOPPMn(III)Cl)-modified gold electrode sensor was developed for the determination of nitrite in food samples. The developed sensor showed an excellent catalytic activity and stability for nitrite oxidation. Under optimized conditions, nitrite concentration as low as 2.9 × 10−9 M can be determined in various food samples using the developed sensor. Effect of common interfering ions have been investigated in simulated mixtures containing high levels of interfering ions and the sensor was found to be tolerant against these ions. The determination of nitrite in food samples such as chicken ham, sausage and pickled vegetables with the proposed sensor was in good agreement with those obtained by standard spectrophotometric method.

A simple and sensitive solid‐phase fluorescence quenching method for the determination of trace amounts of nitrite in food samples has been developed. The method is based on that rhodamine B (RhB) which is used as an emission reagent and is included by β‐cyclodextrin polymer(β‐CDP), reacts with nitrite in the presence of iodide to form a nonfluorescence compound in acidic medium. The fluorescence intensity of the RhB‐included β‐CDP was measured in solid phase with excitation and emission wavelengths of 353 and 592 nm, respectively. The fluorescence quenching degree is good linear with the concentration of nitrite over the ranges of 1.0–3.0 µg with a detection limit of 0.04 µg and RSD is 1.2%. The general coexisting ions do not interfere to the reaction of RhB with nitrite. The proposed method has been applied to the determination of trace amount of nitrite in food samples with the recoveries of 102.8% (ham) and 99.0% (sausage), respectively.

A new test strip method for field screening of nitrite in aqueous samples based on the diazo‐coupling reaction between the nitrite and the Griess reagents has been developed. The test strip has a circular sensing zone that contains two layers: the Griess reagents act as the sensing reagent and is immobilized in the bottom layer; the top layer is a cellulose acetate membrane that can be used as a dialysis membrane to remove the matrix from the sample, which can enhance the selectivity of this method. When the test strip was directly dipped into the samples, a color change of the test strip was observed, and the intensity of color that appears on the test strip is proportional to the concentration of nitrite in the range from 0.50 to 25 µg mL−1 in food samples. Under the experimental conditions, as low as 0.20 µg mL−1 nitrite can be observed; most of anionic and cationic species as well as other sample matrixes basically do not interfere with the nitrite measurement.

·ATP-enhanced PB@MIL-100(Fe) nanozyme enables dual-mode ratiometric colorimetric and smartphone-based hydrogel detection of nitrite in food samples.

A simple and sensitive solid-phase fluorescent quenching method for the determination of trace amounts of nitrite in food samples has been developed. The method is based on the reaction between nitrite and neutral red, which is used as an emission reagent, to form a non-fluorescent compound in 0.1 mol/L hydrochloric acid. The fluorescent intensity was measured in 5-mm quartz cells with excitation and emission wavelengths of 548 and 609 nm, respectively. The degree of fluorescent quenching showed good linearity at concentrations of nitrite between the ranges of 40-200 μg/L. The detection limit is 8.1 μg/L and the RSD is 1.7%. The general coexisting ions do not interfere with the reaction of neutral red with nitrite. The proposed method was applied to the determination of trace amounts of nitrite in food samples with satisfactory results. In addition, the mechanism involved in the reaction was discussed.

In this work, a novel and very interesting analytical methodology based on coupling of digital image processing and three-way calibration has been developed for determination of nitrite in food samples. Nitrite in contact with Griess reagent is able to produce a red-colored azo dye whose color intensity is correlated with nitrite concentration and here, a piece of Whatman filter paper impregnated with Griess reagent was used as the platform of the sensor and a SONY Xperia Z5 cell phone was used for image capturing from the sensor surface. To generate second-order data, the F-number of the camera's sensor was changed as an instrumental parameter. Two calibration models were constructed by unfolded partial least squares-residual bilinearization (U-PLS/RBL) and multiway-PLS/RBL (N-PLS/RBL) and then, their performance for prediction of nitrite concentration in test samples was evaluated and the results confirmed a good performance for U-PLS/RBL (REP = 3.25 ppm, RMSEP = 8.82 ppm, RMSEC = 4.62 ppm, Q2 = 0.99, γ−1 = 0.05 and LOD = 0.1 ppm) which was better than that for N-PLS/RBL (REP = 13.98 ppm, RMSEP = 37.86 ppm, RMSEC = 6.46 ppm, Q2 = 0.98, γ−1 = 0.07 and LOD = 0.15 ppm) in predicting concentration of nitrite in test samples which motivated us to choose it for the analysis of cabbage, carrot, lettuce, watermelon, onion, potato, kielbasa and sausage as real samples.

Nano-palladium-decorated bismuth sulfide microspheres on a disposable electrode integrated with smartphone-based electrochemical detection of nitrite in food samples

Carbon dot (CD) nanozymes with excellent biocompatibility, optical properties, and catalytic activity show great promise for microbial detection and drug sensitivity testing. This study reports the synthesis of metal-doped green-emitting CDs with good peroxidase-like activity, which were synthesized via a one-step hydrothermal route using thiourea, N-[3-(trimethoxysilyl)propyl]ethylenediamine, and catechol as the starting materials and FeCl3 as the doping agent. In the presence of H2O2, CDs catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), producing a blue product; however, in the presence of bacteria and H2O2, the bacterial catalase enzyme decomposes H2O2 and inhibits the catalytic activity of CDs, preventing the color change. The bacterial catalase enzyme neutralizes H2O2, which prevents the CDs from producing the color-changing reaction with TMB. Based on the CDs-TMB-H2O2 cascade system of bioenzymes and nanozymes, we developed a rapid, sensitive, and direct colorimetric detection method for Staphylococcus aureus (S. aureus) with a detection limit of 2 × 103 CFU/mL and a linear range of 2 × 103-2 × 106 CFU/mL. This visual detection method was successfully applied to the detection of S. aureus in food samples. Antibiotics have different effects on the proliferation of sensitive and resistant bacterial strains, leading to different levels of hydrolysis of H2O2 in the bacterial solution and resulting in varying intensities of the solution color; therefore, we developed a simple and visual antibiotic susceptibility test. The applications of CD nanozymes provide a powerful tool for detecting pathogenic bacteria in food, clinical, and environmental samples and infections.