A novel colorimetric method for H2O2 sensing and its application: Fe2+-catalyzed H2O2 prevents aggregation of AuNPs by oxidizing cysteine (FeHOAuC)

A paper-based label-free plasmonic nanosensor for portable pre-diagnosis of multiple metabolic diseases

Early diagnosis is crucial for improving the prognosis of patients with metabolic diseases. In this study, we developed an innovative, multiplexed, and user-friendly paper-based plasmonic nanosensor by integrating previously established FeHOAuC (Fe2+-catalyzed H2O2 prevents the aggregation of AuNPs by oxidizing cysteine) label-free plasmonic nanosensor. Initially, we prepared a paper art with designated sampling and colorimetric sections by applying polydimethylsiloxane onto cellulose and nitrocellulose papers. Subsequently, we fabricated and optimized the oxidase-coupled FeHOAuC system on the paper platform. The proposed nanosensor's sensitivity, specificity, and feasibility were evaluated using a quantitative color algorithm. In this sensor, pre-loaded oxidases convert target analytes into H2O2, which subsequently induces a color change in AuNPs by oxidizing cysteine under the catalytic action of Fe2+. This paper-based sensor can quantitatively measure glucose, cholesterol, uric acid, and lactate within 40 min. The limit of detection of 5-10 μM, combined with its demonstrated specificity, makes it highly suitable for the early diagnosis of related metabolic diseases. Importantly, through a straightforward dropping procedure and a smartphone camera, the plasmonic nanosensor can distinguish disease-related small molecules in real serum samples. In conclusion, the proposed paper-based plasmonic nanosensor device exhibited favorable performance with simple operation, presenting significant potential for domiciliary early diagnosis of multiple metabolic diseases.

Optical Bioassays Based on the Signal Amplification of Redox Cycling

Optical bioassays are challenged by the growing requirements of sensitivity and simplicity. Recent developments in the combination of redox cycling with different optical methods for signal amplification have proven to have tremendous potential for improving analytical performances. In this review, we summarized the advances in optical bioassays based on the signal amplification of redox cycling, including colorimetry, fluorescence, surface-enhanced Raman scattering, chemiluminescence, and electrochemiluminescence. Furthermore, this review highlighted the general principles to effectively couple redox cycling with optical bioassays, and particular attention was focused on current challenges and future opportunities.

Advances in Non-Electrochemical Sensing of Human Sweat Biomarkers: From Sweat Sampling to Signal Reading

Sweat, commonly referred to as the ultrafiltrate of blood plasma, is an essential physiological fluid in the human body. It contains a wide range of metabolites, electrolytes, and other biologically significant markers that are closely linked to human health. Compared to other bodily fluids, such as blood, sweat offers distinct advantages in terms of ease of collection and non-invasive detection. In recent years, considerable attention has been focused on wearable sweat sensors due to their potential for continuous monitoring of biomarkers. Electrochemical methods have been extensively used for in situ sweat biomarker analysis, as thoroughly reviewed by various researchers. This comprehensive review aims to provide an overview of recent advances in non-electrochemical methods for analyzing sweat, including colorimetric methods, fluorescence techniques, surface-enhanced Raman spectroscopy, and more. The review covers multiple aspects of non-electrochemical sweat analysis, encompassing sweat sampling methodologies, detection techniques, signal processing, and diverse applications. Furthermore, it highlights the current bottlenecks and challenges faced by non-electrochemical sensors, such as limitations and interference issues. Finally, the review concludes by offering insights into the prospects for non-electrochemical sensing technologies. By providing a valuable reference and inspiring researchers engaged in the field of sweat sensor development, this paper aspires to foster the creation of innovative and practical advancements in this domain.

Preparation of conductive polyaniline hydrogels co‐doped with hydrochloric acid/phytic acid and their application in Ag NPs@PA/GCE biosensor for H2O2 detection

Accurate measurement of hydrogen peroxide is of great significance in monitoring human diseases and environmental safety. At present, there are a variety of testing methods, among which the use of electrochemical sensors to accurately measure hydrogen peroxide has gradually become a research hotspot. In this article, phytic acid (PA) with dual functions of doping and cross‐linking and HCl with strong ionization capacity was introduced as dopants to co‐doping polyaniline, and polyaniline hydrogel with a three‐dimensional network structure was prepared. The influence of the HCl/PA co‐doping ratio on conductive polyaniline hydrogel (CPAniH) was explored. The synthesis process and mechanism of HCl/PA‐CPAniH were analyzed and explained. In addition, the conductivity and mechanism of HCl/PA‐CPAniH, as well as the hydrogel characteristics were characterized and studied. A new electrochemical sensor based on polyaniline hydrogel and silver nanoparticles was constructed to detect hydrogen peroxide. The sensor has a good linear relationship, a wide linear range, and a high‐detection limit for H2O2. In addition, it also shows that HCl/PA‐CPAniH is a good interface material for electrochemical sensors and has a certain application potential.

Analyte-triggered in situ “off–on” of Tyndall effect for smartphone-based quantitative nanosensing of Ag+ ions

This work describes two new colorimetric methods for smartphone-based point-of-care nanosensing of toxic Ag⁺ ions. They were based on the analyte-triggered in situ "off-on" of Tyndall effect (TE) of non-plasmonic colloid or plasmonic metal nanoprobes. The first TE-inspired assay (TEA) focused on the initial analytical application of precipitation reactions where a non-plasmonic AgCl colloid could be formed once mixing the analyte with a NaCl solution. Such AgCl colloid displayed strong visual TE signals after their irradiation by a laser pointer pen, which unexpectedly achieved a detection limit of ~ 400 nM. The second TEA was further designed to reduce the limit down to ~ 78 nM using the analyte's oxidizability towards 3,3',5,5'-tetramethylbenzidine molecules. The redox reaction could create positively charged products that could make negatively charged plasmonic gold nanoparticles aggregate through electrostatic interactions to remarkably amplify their TE responses. Both limits were lower than the minimum allowable Ag⁺ level (~ 460 nM) in drinking water issued by the World Health Organization. The satisfactory recovery results for detecting Ag⁺ ions in river, pond, tap, and drinking water additionally demonstrated good selectivity, accuracy and practicality of the proposed methods for potential point-of-need uses in environmental analysis, public health, water safety, etc.

Decorating Zirconium on Graphene Oxide to Design a Multifunctional Nanozyme for Eco-Friendly Detection of Hydrogen Peroxide

Peroxidase enzymes are crucial in analytical chemistry owing to significant peroxide analytes and their key role in hydrogen peroxide (H2O2) detection. Therefore, exploiting appropriate catalysts for the peroxidase like reactions has become crucial for achieving desired analytical performance. Zirconium (Zr) has attracted growing interest, as a safe and stable potential eco-friendly catalyst for various organic transformations that address increasing environmental challenges. Hence, aiming at fast, sensitive and selective optical detection of H2O2, a colorimetric platform is presented here, based on the excellent peroxidase enzyme-like activity of Zr decorated on graphene oxide (GO). The synergistic effect achieved due to intimate contact between an enzyme like Zr and the high surface area 0f GO ensures efficient electron transfer that increases the chemical and catalytic activity of the composite and advances the decomposition of H2O2 into hydroxyl radicals. The designed probe, thus, efficiently catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), via hydroxyl radicals, thereby transforming the colorless TMB into blue oxidized TMB within 2 min. The catalytic mechanism of the Zr-GO enzyme mimic is proposed herein and verified using a fluorescent probe terephthalic acid (TA) and other scavenger experiments. The multifunctional optical probe allows sensitive and highly selective recognition of H2O2 in a linear range from 100 to 1000 µM with a low detection limit of 0.57 µM. Essentially, the direct accessibility of Zr prevents having to use the complicated preparation and purification procedures mostly practiced for conventional biozymes and nanozymes. The devised method offers several gains, including being green and an inexpensive catalyst, having lower LOD, being fast, cost-effective and sensitive, and having selective work-up procedures.