An efficient ratiometric fluorescence and colorimetric dual-mode probe for convenient determination of nitrite in real samples and E. coli

Developing an effective and convenient nitrite detection method is indispensable in food safety, environmental monitoring, clinical diagnosis of diseases, and many other areas. Herein, a dicyanoisophorone derivative, TMN-NH2 with large Stokes shift and near-infrared (NIR) emission, was proposed as a ratiometric fluorescence and colorimetric dual-mode probe for the rapid determination of NO2- in acidic media, showing excellent selectivity and high sensitivity. The sensing mechanism is based on the diazotization of TMN-NH2 with NO2- and subsequent diazonium salt hydrolysis to form a hydroxyl-substituted product (TMN-OH). Under the optimized conditions of reaction and detection, a new quantitative analysis method based on TMN-NH2 was established for NO2- detection, exhibiting good linear relationships to NO2- in the range of 0.5 to 15 μM with practical detection limits of 26.6 nM and 17.6 nM for the colorimetric and fluorescent readout, respectively. The quantitative detection of NO2- in real samples demonstrated satisfactory recoveries and repeatability. Moreover, TMN-NH2 was successfully applied for monitoring NO2- in Escherichia coli by confocal fluorescence imaging.

Plasmonic surface-enhanced Raman scattering nano-substrates for detection of anionic environmental contaminants: Current progress and future perspectives

Surface-enhanced Raman scattering spectroscopy (SERS) is a powerful technique of vibrational spectroscopy based on the inelastic scattering of incident photons by molecular species. It has unique properties such as ultra-sensitivity, selectivity, non-destructivity, speed, and fingerprinting properties for analytical and sensing applications. This enables SERS to be widely used in real-world sample analysis and basic plasmonic mechanistic studies. However, the desirable properties of SERS are compromised by the high cost and low reproducibility of the signals. The development of multifunctional, stable and reusable nano-engineered SERS substrates is a viable solution to circumvent these drawbacks. Recently, plasmonic SERS active nano-substrates with various morphologies have attracted the attention of researchers due to promising properties such as the formation of dense hot spots, additional stability, tunable and controlled morphology, and surface functionalization. This comprehensive review focused on the current advances in the field of SERS active nanosubstrates suitable for the detection and quantification of anionic environmental pollutants. The common fabrication methods, including the techniques for morphological adjustments and surface modification, substrate categories, and the design of nanotechnologically fabricated plasmonic SERS substrates for anion detection are systematically presented. Here, the need for the design, synthesis, and functionalization of SERS nano-substrates for anions of great environmental importance is explained in detail. In addition, the broad categories of SERS nano-substrates, namely colloid-based SERS substrates and solid-support SERS substrates are discussed. Moreover, a brief discussion of SERS detection of certain anionic pollutants in the environment is presented. Finally, the prospects in the fabrication and commercialization of pilot-scale handheld SERS sensors and the construction of smart nanosubstrates integrated with novel amplifying materials for the detection of anions of environmental and health concern are proposed.

Current advances on g-C3N4-based fluorescence detection for environmental contaminants

The development of highly-sensitive fluorescence detection systems for environmental contaminants has become high priority research in the past years. Special attention has been paid to graphitic carbon nitride (g-C₃N₄)-based nanomaterials, whose unique and superior optical property makes them promising and attractive candidates for this purpose. It is necessary to enhance the current understanding of the various classes of g-C₃N₄-based fluorescence detection systems and their mechanisms, as well as find suitable approaches to improve detection performance for environmental monitoring, protection, and management. In this review, the recent progresses on g-C₃N₄-based fluorescence detections for environmental contaminants, mainly including their basic principles, mechanisms, applications, modification strategies, and conclusions, are summarized. A particular emphasis is placed on the design and development of modification strategies for g-C₃N₄ with the objective of improving detection performance. High photoluminescence quantum yield, tunable fluorescence emission characteristics, and strong adsorption capacity of g-C₃N₄ could ensure the ultrasensitivity and selectivity of fluorescence detection of environmental contaminants. Concluding perspectives on the challenges and opportunities to design highly efficient g-C₃N₄-based fluorescence detection system are intensively put forward as well.

A Simple and Rapid Spectrophotometric Method for Nitrite Detection in Small Sample Volumes

A simple, rapid, and environmentally-friendly spectrophotometric method for nitrite detection was developed. Detection was based on a redox reaction with iodide ions in an acidic condition. The reaction was evaluated by detecting the increase in absorbance of the colored product of iodine at 362 nm wavelength. To obtain a good spectrophotometric performance, the iodide ions concentration, hydrochloric acid concentration, and reaction time were optimized. In the optimal condition, the developed spectrophotometric method provided a linear range of 0.0625 to 4.00 mg L−1 (r = 0.9985), reaction time for 10 min, a limit of detection of 25 µg L−1, and a limit of quantitation of 85 µg L−1. This method showed good repeatability (RSD < 9.21%), high sample throughput (9 samples min−1), and good accuracy (recovery = 88 ± 2 to 99.5 ± 0.4%). The method has the potential to be used in crime scene investigations as a rapid screening test for gunshot residue detection via nitrite detection.

An Improved Algorithm for Measuring Nitrate Concentrations in Seawater Based on Deep-Ultraviolet Spectrophotometry: A Case Study of the Aoshan Bay Seawater and Western Pacific Seawater

Nowadays, it is still a challenge for commercial nitrate sensors to meet the requirement of high accuracy in a complex water. Based on deep-ultraviolet spectral analysis and a regression algorithm, a different measuring method for obtaining the concentration of nitrate in seawater is proposed in this paper. The system consists of a deuterium lamp, an optical fiber splitter module, a reflection probe, temperature and salinity sensors, and a deep-ultraviolet spectrometer. The regression model based on weighted average kernel partial least squares (WA-KPLS) algorithm together with corrections for temperature and salinity (TSC) is established. After that, the seawater samples from Western Pacific and Aoshan Bay in Qingdao, China with the addition of various nitrate concentrations are studied to verify the reliability and accuracy of the method. The results show that the TSC-WA-KPLS algorithm shows the best results when compared against the multiple linear regression (MLR) and ISUS (in situ ultraviolet spectrophotometer) algorithms in the temperatures range of 4–25 °C, with RMSEP of 0.67 µmol/L for Aoshan Bay seawater and 1.08 µmol/L for Western Pacific seawater. The method proposed in this paper is suitable for measuring the nitrate concentration in seawater with higher accuracy, which could find application in the development of in-situ and real-time nitrate sensors.

A novel near-infrared fluorescent sensor for zero background nitrite detection via the "covalent-assembly" principle

Different chemical states of nitrogen are present in many freshwater and marine ecosystems, and nitrite ions are one of the most toxic water-soluble nitrogen species. Developing an effective and convenient sensing method to constantly detect the concentration of nitrite has become a wide concern. Here, a novel near-infrared fluorescent probe (AAC) was designed and synthesized via the "covalent assembly" principle, showing excellent selectivity and high sensitivity for nitrite. A new nitrite-quantitative method was established with the help of AAC, and the detection limit of nitrite using the new method was as low as 6.7 nM. AAC was successfully applied for the quantitative detection of nitrite in real-world environmental and food samples (including river water and Chinese sauerkraut), and the detection results were essentially identical to the results obtained from the traditional Griess assay. Moreover, AAC was successfully applied for tracking nitrite in Escherichia coli by fluorescence imaging. Since nitrite can have devastating effects, the method established with AAC allowed us to "see" effectively about the water quality, food quality, etc.

Synthesis by partial oxygenation of graphite-like carbon nitride (OCN) decorated with oleic diethanolamide borate (ODAB) for oil-based lubricant additives and its tribological properties

ODAB-OCN retaining long-term dispersion stability in an oil environment exhibits excellent tribological properties.

Boron-doped graphitic carbon nitride as a novel fluorescent probe for mercury(ii) and iron(iii): a circuit logic gate mimic

Boron-g-C₃N₄ was applied as a novel practical fluorescent probe to detect mercury and ferric ions in real samples.

Detection of nitrite with a surface-enhanced Raman scattering sensor based on silver nanopyramid array

Nutrient pollution is of worldwide environmental and health concerns due to extensive use of nitrogen fertilizers and release of livestock waste, which induces nitrite compounds in aquatic systems. Herein a surface-enhanced Raman scattering (SERS) sensor is developed for nitrite detection based on coupling between the plasmonic gold nanostars and the silver nanopyramid array. When nitrite is present in the assay, an azo group is formed between the 1-naphthylamine-functionalized silver nanopyramids and the 4-aminothiophenol-functionalized gold nanostars. This not only generates the SERS spectral fingerprint for selective detection, but also creates "hot spots" at the gap between the Au nanostars and the Ag nanopyramids where the azo group is located, amplifying SERS signals remarkably. Finite-difference time-domain (FDTD) simulation shows a SERS enhancement factor of 4 × 10¹⁰ at the "hot spots". As a result, the SERS sensor achieves a limit of detection of 0.6 pg/mL toward nitrite in water, and enables nitrite detection in real-world river water samples. In addition, this sensor eliminates the use of any Raman reporter and any expensive molecular recognition probe such as antibody and aptamer. This highly sensitive, selective and inexpensive SERS sensor has unique advantages over colorimetric, electrochemical and fluorescent devices for small molecule detection.