Versatile Three-Dimensional-Printed Platform for Nitrite Ion Analyses Using a Smartphone with Real-Time Location

Exploration of a new approach for detection of nitrite with hydroxyl radical fluorescence probe in aqueous solutions

Nitrite (NO2-) has been widely recognized by the international community as an important substance affecting water quality safety and human health, and the detection of NO2- has always been a hot topic for researchers. Fluorescent probe method is an emerging and ideal way for detecting NO2-. Due to the high dependence of the reported reactive NO2- fluorescent probes on strong acidic systems, using the idea of photochemistry, a fluorescence analysis method for detecting NO2- was proposed in this work to change the necessity of strong acidic solutions in probe detection process. A 365 nm UV-LED lamp was used to irradiate NO2- in aqueous solution to convert it into hydroxyl radicals (HO·), and capture the photodegradation product of NO2- using coumarin-3-carboxylic acid as probe 3-CCA that can react with HO· to generate only one type of strong fluorescent substance. This probe has excellent photostability, selectivity, and anti-interference ability, and can realize the quantitative detection of NO2- (0-15 μM) in pure aqueous solution with pH of 7.4. In addition, its application in actual water samples is also satisfactory, with a recovery rate of (85.91 %-107.30 %). Importantly, we hope that this photolysis strategy can open up the novel thinking to develop suitable fluorescent probes for the analysis and detection of some hardly detected analytes.

Development of an accurate hand-held sensing platform for nitrite detection based on nitrogen-doped carbon dots

As is well known, excessive nitrite can seriously pollute the environment and can harm human health. Although existing methods can be used to determine nitrite content, they still have some drawbacks, such as relatively complicated operation and expensive equipment. Herein, a hand-held sensing platform (HSP) for NO2- determination was developed. First, ammonia-rich nitrogen-doped carbon dots with orange-yellow emission were designed and synthesised, which were suitable as fluorescent probes because of their good optical properties and stability. Then, the HSP based on fluorescence using photoelectric conversion technology was designed and manufactured using three-dimensional printing technology. Under optimum conditions, the voltage (V/V0) of the proposed HSP showed good linearity for NO2- detection in the range of 10-500 μM, with a detection limit of 1.95 μM. This portable sensor showed good stability, accuracy and reliability in detecting actual water and meat samples, which may ensure food safety in practical applications. Moreover, the HSP is compact, portable and easily assembled and is suitable for on-site real-time detection, which shows great application potential and prospects.

A microfluidic concentration gradient colorimetric system for rapid detection of nitrite in surface water

A microfluidic concentration gradient colorimetric detection system consisting of a microfluidic concentration gradient colorimetric detection chip, a self-built colorimetric signal acquisition box and a self-written smartphone APP was constructed for the rapid, in-field and visual quantitative detection of nitrite. Specifically, nitrite with initial concentration of C0 can be automatically diluted into 8 concentration gradients characterized by arithmetic series, and the concentrations are 0, 0.20 C0, 0.33 C0, 0.46 C0, 0.59 C0, 0.72 C0, 0.86 C0 and C0. The colorimetric signal acquisition box avoided the interference of light spots on data acquisition. Under the optimal experimental conditions, the quantitative detection of nitrite was achieved by the proposed two-step colorimetric method based on the inhibition of AuNPs signal amplification, and the limit of detection (LOD) was 0.14 mg/L. The microfluidic concentration gradient colorimetric detection system was able to detect nitrite as low as 0.43 mg/L and showed a good specificity. The practical application was investigated by analyzing 10 actual samples of river and lake water, pure water and tap water. The recoveries of the microfluidic concentration gradient colorimetric detection system ranged from 94.92% to 105.60%, which indicates that the method had a good application prospect in the detection of practical samples.

Application of three-dimensional printing technology in environmental analysis: A review

The development of environmental analysis devices with high performance is essential to assess the potential risks of environmental pollutants. However, it is still challenging to develop environmental analysis equipment with miniaturization, portability, and high sensitivity based on traditional processing techniques. In recent years, the popularity of 3D printing technology (3DP) with high precision, low cost, and unlimited design freedom has provided opportunities to solve the existing challenges of environmental analysis. 3D printing has brought solutions to promote the high performance and versatility of environmental analysis equipment by optimizing printing materials, enhancing equipment structure, and integrating multidisciplinary technology. In this paper, we comprehensively review the latest progress in 3D printing in various aspects of environmental analysis procedures, including but not limited to sample collection, pretreatment, separation, and detection. We highlight their advantages and challenges in determining various environmental contaminants through passive sampling, solid-phase extraction, chromatographic separation, and mass spectrometry detection. The manufacturing of 3D-printed environmental analysis devices is also discussed. Finally, we look forward to their development prospects and challenges.

Toward Citizen Science-Based Ocean Acidification Observations Using Smartphone Devices

pH is a key parameter in many chemical, biological, and biogeochemical processes, making it a fundamental aspect of environmental monitoring. Rapid and accurate seawater pH measurements are essential for effective ocean observation and acidification investigations, resulting in the need for novel solutions that allow robust, precise, and affordable pH monitoring. In this study, a versatile smartphone-based environmental analyzer (vSEA) was used for the rapid measurement of seawater pH in a field study. The feasibility of the use of the vSEA algorithm for pH quantification was explored and verified. When used in conjunction with a three-dimensional (3D)-printed light-proof shell, the quality of captured images is guaranteed. The quantitative accuracy of vSEA pH measurements reached 0.018 units with an uncertainty of <0.01, meeting the requirements of the Global Ocean Acidification Observing Network (GOA-ON) for "weather" goals (permitting a maximum pH uncertainty of 0.02). The vSEA-pH system was successfully applied for on-site pH measurements in coastal seawater and coral systems. The performance of the vSEA-pH system was validated using different real-world samples, and t-test results showed that the vSEA-pH system was consistent with pH measurements obtained using a state-of-the-art benchtop spectrophotometer (t = 1.986, p = 0.7949). The vSEA-pH system is applicable to different types of smartphone devices, making it possible for vSEA-pH to be widely promoted for public citizen use. The vSEA-pH system offers a simple, accurate, and applicable method for the on-site measurement of seawater pH, assisting the large-scale monitoring of ocean acidification by allowing the contribution of citizen science-based data collection.

Cobalt-Nitrogen Co-Doped Carbon as Highly Efficient Oxidase Mimics for Colorimetric Assay of Nitrite

Transition metal-N-doped carbon has been demonstrated to mimic natural enzyme activity; in this study, cobalt-nitrogen co-doped carbon (Co-N-C) nanomaterial was developed, and it could be an oxidase mimic. Firstly, Co-N-C with oxidase-like activity boosts the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to produce the oxidized TMB (oxTMB). And the aromatic primary amino group of oxTMB reacts with nitrite (NO₂^-) to form diazo groups. Based on this background, we developed a cascade system of a Co-N-C-catalyzed oxidation reaction and a diazotization reaction for nitrite determination. The low detection limit (0.039 μM) indicates that Co-N-C is superior compared with the vast majority of previously reported nitrite assays. This study not only provides a novel nanozyme with sufficiently dispersed active sites, but it also further applies it to the determination of nitrite, which is expected to expand the application of nanozymes in colorimetric analysis.

Development of a versatile smartphone-based environmental analyzer (vSEA) and its application in on-site nutrient detection

The citizen-science-based environmental survey can benefit from the smartphone technology used in chemical and biological sensing of a wide range of analytes. Quantification by smartphone-based colorimetric assays is being increasingly reported, however, most of the quantification uses empirical formula or complex exhaustive methods. In this study, a versatile and robust algorithm is proposed to overcome these limitations. A model is established to simulate and analyze the conversion process from the camera's spectral information into RGB (Red, Green, Blue) color information. Moreover, the feasibility of the algorithm for the quantification of different analytes is also explored. Based on this algorithm, a versatile smartphone-based environmental analyzer (vSEA) is built and its reliability, versatility, and analytical performance are comprehensively optimized. The good linearity (R² ≥ 0.9954) and precision (relative standard deviations < 5.3%) indicates that the vSEA is accurate enough to quantify the nutrients in most natural waters. Furthermore, the vSEA is used for the field measurement of five important nutrients, and the results show no significant difference compared to conventional methods. The vSEA offers a simpler and easier method for the on-site measurement of nutrients in natural water bodies, which can aid in the emergency monitoring of aqueous ecosystems and the performance of citizen-science-based research.

Intelligently design primary aromatic amines derived carbon dots for optical dual-mode and smartphone imaging detection of nitrite based on specific diazo coupling

Primary aromatic amines derived carbon dots (PAA-CDs) with the protonated amino groups and high quantum yield of 46% were favorably obtained by one-step solvothermal treatment of m-phenylenediamine (m-PDA) in acidic environment. The interaction between the PAA-CDs and nitrite (NO₂^-) was inherited the characteristic reaction of m-PDA (a primary aromatic amine) and NO₂^-, resulting in strong fluorescence quenching and obvious absorption variation of the PAA-CDs. Meanwhile, a chromogenic reaction of diazo coupling can cause significant color changes. Hence, the PAA-CDs were developed for an optical dual-mode and smartphone imaging sensor for NO₂^- detection in the range of 3.0 ~ 40.0 μM with high selectivity, good sensitivity, and excellent anti-interference capability. A limit of detection (LOD) of 0.024 μM and 0.16 μM was implemented by fluorometry and colorimetry, respectively. For smartphone imaging colorimetry, the LODs of 0.46 μM (visible color) and 0.99 μM (fluorescence color) were acquired. More importantly, the established sensor has been successfully applied for the dynamic detection of NO₂^- in various food samples with the satisfying results. A smartphone imaging colorimetry method based on the CDs was firstly proposed to visually and quantitatively detect NO₂^-, which will broaden the application range of the CDs in food safety inspection.

Towards citizen science. On-site detection of nitrite and ammonium using a smartphone and social media software

Citizen scientists-based water quality surveys are becoming popular because of their wide applications in environmental monitoring and public education. At present, many similar studies are reported on collecting samples for later laboratory analysis. For environmentally toxic analytes such as ammonium and nitrite, on-site detection is a promising choice. However, this approach is limited by the availability of suitable methods and instruments. Here, a simple on-site detection method for ammonium and nitrite is reported. The chemistry of this method is based on the classic Griess reaction and modified indophenol blue reaction. Digital image colorimetry is carried out using a smartphone with a custom-made WeChat mini-program or free built-in applications (APPs). Using a simple and low-cost analytical kit, the detection limit of 0.27 μmol/L and 0.84 μmol/L is achieved for nitrite and ammonium, respectively, which are comparable to those achieved with a benchtop spectrophotometer. Relative standard deviations (n = 7) for low and high concentrations of nitrite are 3.6% and 4.3% and for ammonium are 5.6% and 2.6%, respectively. Identical results with a relative error of less than 10% are obtained using different smartphones (n = 3), color extracting software (n = 6), and with multiple individual users (n = 5). These results show the robustness and applicability of the proposed method. The on-site application is carried out in an in-campus wastewater treatment plant and at a local river. A total of 40 samples are analyzed and the analytical results are compared with that obtained by a standard method and a spectrophotometer, followed by a paired t-test at a 95% confidence level. This proposed on-site analytical kit has the advantages of simplicity and portability and has the potential to be popular and useful for citizen science-based environmental monitoring.

Headspace Solid-Phase Microextraction Following Chemical Vapor Generation for Ultrasensitive, Matrix Effect-Free Detection of Nitrite by Microplasma Optical Emission Spectrometry

A new chemical vapor generation method coupled with headspace solid-phase microextraction miniaturized point discharge optical emission spectrometry (HS-SPME-μPD-OES) for the sensitive and matrix effect-free detection of nitrite in complex samples is described. In an acidic medium, the volatile cyclohexene was generated from cyclamate in the presence of nitrite, which was volatilized to the headspace of the container, efficiently separated, and preconcentrated by HS-SPME. Consequently, the SPME fiber was transferred to a laboratory-constructed thermal desorption chamber wherein the cyclohexene was thermally desorbed and swept into μPD-OES for its sensitive quantification via monitoring the carbon atomic emission line at 193.0 nm. As a result, the quantification of nitrite was accomplished through the determination of cyclohexene. The application of HS-SPME as a sampling technique not only simplifies the experimental setup of μPD-OES but it also preconcentrates and separates cyclohexene from N₂ and sample matrices, thus eliminating the interference from water vapor and N₂ and significantly improving the analytical performance on the determination of nitrite. Under the optimum experimental conditions, a limit of detection of 0.1 μg L^-1 was obtained, which is much better than that obtained by conventional methods. The precision, expressed as relative standard deviation, was better than 3.0% at a concentration of 10 μg L^-1. The proposed method provides several advantages of portability, simplicity, high sensitivity, and low energy consumption and eliminates expensive instruments and matrix interference, thus retaining a promising potential for the rapid, sensitive, and field analysis of nitrite in various samples.

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.