Tailoring plasmonic sensing strategies for the rapid and sensitive detection of hypochlorite in swimming water samples

A tunable plasmonic sensor has been developed by varying the dextran content in the initially synthesized dextran-gold nanoparticle (dAuNPs) solution. A colloidal nanogold solution (dAuNPs-Sol) was initially prepared using dextran and gold salt in alkaline media by a one-pot green synthetic route. The dAuNPs-Sol was combined with varying amounts of dextran (ranging from 0.01 to 30.01%) to create a tunable probe, along with different solid formats, including tablet (dAuNPs-Tab), powder (dAuNPs-Powder), and composite (dAuNPs-Comp). Both the liquid and solid phase plasmonic probes were characterized using UV-vis spectroscopy, transmission electron microscopy (TEM) dynamic light scattering (DLS), and zeta potential analysis. The impact of dextran content in the dAuNP solution is studied in terms of surface charge and hydrodynamic size. The influence of operational treatments used to achieve solid dAuNPs probes is also explored. All plasmonic probes were employed to detect a broad range of OCl¯ concentrations (ranging from µM to mM) in water through aggregation followed by calculating a lower and upper limit of detection (LLoD, ULoD) of the proposed colorimetric sensors. Results indicate that the most sensitive detection is achieved with a lower dextran content (0.01%), which exhibits an LLoD of 50 µM. The dAuNPs-Sol sensor is selective and demonstrates real-world applicability, as confirmed by interference analysis and successful testing with various water samples. Additionally, it is found that a 20 × concentration of dextran-coated gold nanoparticles could be attained without any changes in the particle morphology. This concentration is achieved through a straightforward process that does not require the use of a centrifuge machine. This finding highlights the practicality and simplicity of the method, indicating its potential for scalable and cost-effective production of concentrated dAuNPs without compromising their structural integrity.

Tablet-Based Sensor: A Stable and User-Friendly Tool for Point-of-Care Detection of Glucose in Urine

The colorimetric detection of glucose in urine through enzymatic reactions offers a low-cost and non-invasive method to aid in diabetes management. Nonetheless, the vulnerability of enzymes to environmental conditions, particularly elevated temperatures, and their activity loss pose significant challenges for transportation and storage. In this work, we developed a stable and portable tablet sensor as a user-friendly platform for glucose monitoring. This innovative device encapsulates glucose oxidase and horseradish peroxidase enzymes with dextran, transforming them into solid tablets and ensuring enhanced stability and practicality. The enzymatic tablet-based sensor detected glucose in urine samples within 5 min, using 3,3',5,5'-tetramethylbenzidine (TMB) as the indicator. The tablet sensor exhibited responsive performance within the clinically relevant range of 0-6 mM glucose, with a limit of detection of 0.013 mM. Furthermore, the tablets detected glucose in spiked real human urine samples, without pre-processing, with high precision. Additionally, with regard to thermal stability, the enzyme tablets better maintained their activity at an elevated temperature as high as 60 °C compared to the solution-phase enzymes, demonstrating the enhanced stability of the enzymes under harsh conditions. The availability of these stable and portable tablet sensors will greatly ease the transportation and application of glucose sensors, enhancing the accessibility of glucose monitoring, particularly in resource-limited settings.

User-friendly and ultra-stable all-inclusive gold tablets for cysteamine detection

To date, a range of nanozymes has been reported for their enzyme-mimicking catalytic activity such as solution-based sensors. However, in remote areas, the need for portable, cost-effective, and one-pot prepared sensors is obvious. In this study, we report the development of a highly stable and sensitive gold tablet-based sensor for cysteamine quantification in human serum samples. The sensor is produced in two steps: synthesis of a pullulan-stabilized gold nanoparticle solution (pAuNP-Solution) using a pullulan polymer as a reducing, stabilizing, and encapsulating agent and then, casting the pAuNP-Solution into a pullulan gold nanoparticle tablet (pAuNP-Tablet) by a pipetting method. The tablet was characterized by UV-vis, DLS, FTIR, TEM, and AFM analyses. The pAuNP-tablet exhibited a high peroxidase-mimetic activity via a TMB-H₂O₂ system. The presence of cysteamine in the system introduced two types of inhibition which were dependent on the cysteamine concentration. By determining Michaelis-Menten's kinetic parameters, we gained mechanistic insights into the catalytic inhibition process. Based on the catalytic inhibition capability of cysteamine, the limit of detection (LoD) was calculated to be 69.04 and 82.9 μM in buffer and human serum samples, respectively. Finally, real human serum samples were tested, demonstrating the applicability of the pAuNP-Tablet for real-world applications. The % R values in human serum samples were in the range of 91-105% with % RSD less than 2% for all replicas. The stability tests over 16 months revealed the ultra-stable properties of the pAuNP-Tablet. Overall, with a simple fabrication method and a novel employed technique, this study contributes to the advancement of tablet-based sensors and helps in cysteamine detection in clinical settings.

Gold Nanoparticles-Based Colorimetric Assays for Environmental Monitoring and Food Safety Evaluation

Recent years have witnessed an exponential increase in the research on gold nanoparticles (AuNPs)-based colorimetric sensors to revolutionize point-of-use sensing devices. Hence, this review is compiled focused on current progress in the design and performance parameters of AuNPs-based sensors. The review begins with the characteristics of AuNPs, followed by a brief explanation of synthesis and functionalization methods. Then, the mechanisms of AuNPs-based sensors are comprehensively explained in two broad categories based on the surface plasmon resonance (SPR) characteristics of AuNPs and their peroxidase-like catalytic properties (nanozyme). SPR-based colorimetric sensors further categorize into aggregation, anti-aggregation, etching, growth-mediated, and accumulation-based methods depending on their sensing mechanisms. On the other hand, peroxidase activity-based colorimetric sensors are divided into two methods based on the expression or inhibition of peroxidase-like activity. Next, the analytes in environmental and food samples are classified as inorganic, organic, and biological pollutants, and recent progress in detection of these analytes are reviewed in detail. Finally, conclusions are provided, and future directions are highlighted. Improving the sensitivity, reproducibility, multiplexing capabilities, and cost-effectiveness for colorimetric detection of various analytes in environment and food matrices will have significant impact on fast testing of hazardous substances, hence reducing the pollution load in environment as well as rendering food contamination to ensure food safety.