Colorimetric Nanoparticle-Embedded Hydrogels for a Biosensing Platform

Glucose Oxidase Activity Colorimetric Assay Using Redox-Sensitive Electrochromic Nanoparticle-Functionalized Paper Sensors

Glucose oxidase (GOx) activity assays are vital for various applications, including glucose metabolism estimation and fungal testing. However, conventional methods involve time-consuming and complex procedures. In this study, we present a colorimetric platform for in situ GOx activity measurement utilizing redox-sensitive electrochromic nanoparticles based on polyaniline (PAni). The glucose-adsorbed colorimetric paper sensor, herein termed Glu@CPS, is created by immobilizing ferrocene and glucose onto paper substrates that have been functionalized with PAni nanoparticles. Glu@CPS not only demonstrated rapid detection (within 5 min) but also exhibited remarkable selectivity for GOx and a limit of detection as low as 1.25 μM. Moreover, Glu@CPS demonstrated consistent accuracy in the measurement of GOx activity, exhibiting no deviations even after being stored at ambient temperature for a duration of one month. To further corroborate the effectiveness of this method, we applied Glu@CPS in the detection of GOx activity in a moldy red wine. The results highlight the promising potential of Glu@CPS as a convenient and precise platform for GOx activity measurement in diverse applications including food quality control, environmental monitoring, and early detection of fungal contamination.

Colorimetric sweat analysis using wearable hydrogel patch sensors for detection of chloride and glucose

Sweat is a promising non-invasive biofluid that can provide valuable insights into the physiological state of the human body. However, a major obstacle to analyzing sweat in real-time is the fabrication of simple, fast-acting, accurate, and low-cost sensing constructs. To address this challenge, we introduced easily-prepared wearable hydrogel sensors that can be placed on the skin and used colorimetric techniques to assess sweat analytes without invasive procedures. Two typical sweat sensors, chloride ion (Cl-) responsive patches for cystic fibrosis (CF) analysis and glucose response patches for diabetic monitoring, were demonstrated for real sample analysis. The Cl- colorimetric sensor, with a detection limit down to 100 μM, shows a good linear response from 1.56 mM to 200 mM Cl-, and the glucose colorimetric sensor, with a detection limit down to 1 μM, exhibits an adequate linear response from 10 μM to 1 mM glucose. These colorimetric hydrogel sensors are also incorporated into a medical dressing to create wearable sensor devices for real-time sweat analysis. The acquired readings closely match the results obtained from the benchmark analyzing instrument, with a small deviation of less than 10%. Therefore, our simple colorimetric hydrogel sensing patches hold great potential to advance real-time sweat testing and contribute to the transitional development of wearable medical devices.

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

A regression-based machine learning approach for pH and glucose detection with redox-sensitive colorimetric paper sensors

Colorimetric paper sensors are used in various fields due to their convenience and intuitive manner. However, these sensors present low accuracy in practical use because it is difficult to distinguish color changes for a minute amount of analyte with the naked eye. Herein, we demonstrate that a machine learning (ML)-based paper sensor platform accurately determines the color changes. We fabricated a colorimetric paper sensor by adsorbing polyaniline nanoparticles (PAni-NPs), whose color changes from blue to green when the ambient pH decreases. Adding glucose oxidase (GOx) to the paper sensor enables colorimetric glucose detection. Target analytes (10 μL) were aliquoted onto the paper sensors, and their images were taken with a smartphone under the same conditions in a darkroom. The red-green-blue (RGB) data from the images were extracted and used to train and test three regression models: support vector regression (SVR), decision tree regression (DTR), and random forest regression (RFR). Of the three regression models, RFR performed the best at estimating pH levels (R² = 0.957) ranging from pH 2 to 10 and glucose concentrations (R² = 0.922) ranging from 0 to 10 mg mL^-1.

Fast Responsive, Reversible Colorimetric Nanoparticle-Hydrogel Complexes for pH Monitoring

Hydrogels containing redox-sensitive colorimetric nanoparticles (NPs) have been used to sense ambient pH in many fields owing to their simple and fast visualization capabilities. However, real-time pH monitoring still has limitations due to its poor response rate and irreversibility. Herein, we developed a fast responsive colorimetric hydrogel called ferrocene adsorption colorimetric hydrogel (FACH). Ferrocene, an organometallic compound, plays a vital role as an electron transfer mediator (i.e., redox catalyst) within the hydrogel network. FACH shows fast color change performance with high reactivity and penetrability to ambient pH changes. In detail, FACH shows distinct color change within 2 min under various pH conditions from four to eight, with good reliability. The speed for color change of FACH is approximately six times faster than that of previously developed colorimetric hydrogels, suggesting the fastest hydrogel-based colorimetric pH sensor. Furthermore, FACH shows reversibility and repeatability of the redox process, indicating scalable utility as a sustainable pH monitoring platform.

Colorimetric Sensing of Lactate in Human Sweat Using Polyaniline Nanoparticles-Based Sensor Platform and Colorimeter

In emergency medicine, the lactate level is commonly used as an indicator of the severity and response to the treatment of hypoperfusion-related diseases. Clinical lactate measurements generally require 3 h for clinical determination. To improve the current gold standard methods, the development of sensor devices that can reduce detection time while maintaining sensitivity and providing portability is gaining great attention. This study aimed to develop a polyaniline (PAni)-based single-sensor platform for sensing lactate in human sweat using a CIELAB color system-based colorimetric device. To establish a lactate sensing platform, PAni nanoparticles were synthesized and adsorbed on the filter paper surface using solvent shift and dip-coating methods, respectively. PAni is characterized by a chemical change accompanied by a color change according to the surrounding environment. To quantify the color change of PAni, a CIELAB color system-based colorimetric device was fabricated. The color change of PAni was measured according to the chemical state using a combination of a PAni-based filter paper sensor platform and a colorimetric device, based on the lactate concentration in deionized water. Finally, human sweat was spiked with lactate to measure the color change of the PAni-based filter paper sensor platform. Under these conditions, the combination of polyaniline-based sensor platforms and colorimetric systems has a limit of detection (LOD) and limit of quantitation (LOQ) of 1 mM, linearity of 0.9684, and stability of 14%. Tbe confirmed that the color of the substrate changes after about 30 s, and through this, the physical fatigue of the individual can be determined. In conclusion, it was confirmed through this study that a combination of the PAni paper sensor platform and colorimeter can detect clinically meaningful lactate concentration.