Impedance Analysis of Colloidal Gold Nanoparticles in Chromatography Paper for Quantitation of an Immunochromatographic Assay

Microfluidic Paper-based Analytical Devices (μPADs): Miniaturization and Enzyme Storage Studies

This paper describes the design and development of miniaturized microfluidic paper-based analytical devices (μPADs) for biological assays and enzyme storage instruments. Here, a glucose assay utilizing glucose oxidase (GOx), horseradish peroxidase (HRP), and potassium iodide (KI) is used as the model system. The efficacy of the miniaturized devices is further examined by assessing the activity of acetylcholinesterase (AChE). Two types of μPADs were developed: one, "strip" chips of detection zones of area 0.5, 0.1 cm² and, two, "grid" chips of detection zone 0.05 cm². The devices are easily fabricated via a wax printing process whereby lines of wax are deposited onto chromatographic paper and heated to create rows of hydrophobic barriers. The "strip" chips were subjected to three different temperature environments (-20, 0, and 20°C) over 30 days and glucose assays conducted at intermittent times yielding a correlation between corrected average inverse yellow intensity, days, and glucose concentration. Calculated and experimentally derived color intensity values for 1, 4, and 9 mM glucose concentrations after a 7-day storage study showed a good correlation (0.89 - 15.76% error). Both types of μPADs are effective platforms as potential point-of-care (POC) diagnostic devices and display minimal enzyme denaturation. μPADs of this size show promise as alternative devices for resource-limited regions and especially those areas where materials and instrumentation are not always available.

An Instrument-free Detection of Antioxidant Activity Using Paper-based Analytical Devices Coated with Nanoceria

This work reports a portable distance-based detection paper device that has a thermometer-like shape for rapid, instrument-free determination of antioxidant activity using a nanoceria assay. The assay is based on partial reduction of cerium ion from Ce⁴⁺ to Ce³⁺ on nanoceria deposited along the detection channel by antioxidants present in food, giving highly reactive oxidation products. Either these products or the parent antioxidant compounds could then bind to the OH-rich ceria nanoparticles and generate charge transfer ceria-antioxidant complexes resulting in a yellow to brown color change. The distance of the brown color on the detection channel is directly proportional to antioxidant activity, and can be easily measured using an integrated ruler without the need of any external sophisticated instrument for detection. The paper sensor has been studied for the analysis of common antioxidants and its performance was validated against traditional antioxidant assays for 11 tea sample analyses. Using the Spearman rank correlation coefficient method, the antioxidant activity of tea samples obtained from the paper device correlated with the traditional assay at the 95% confidence level. The developed sensor provided a high recovery and tolerance limit and was stable for 50 days both when stored at ambient and low temperature (6 and -20°C). The results demonstrated that the developed paper device is an alternative to allow for fast, simple, instrument-free, cheap, portable and high-throughput screening of antioxidant activity analysis in real samples.

Microfluidic Paper-based Analytical Device for the Determination of Hexavalent Chromium by Photolithographic Fabrication Using a Photomask Printed with 3D Printer

This article describes a simple and inexpensive microfluidic paper-based analytical device (μPAD) for the determination of hexavalent chromium (Cr^VI) in water samples. The μPADs were fabricated on paper by photolithography using a photomask printed with a 3D printer and functionalized with reagents for a colorimetric assay. In the μPAD, Cr^VI reacts with 1,5-diphenylcarbazide to form a violet-colored complex. Images of μPADs were captured with a digital camera; then the red, green, and blue color intensity of each detection zone were measured using images processing software. The green intensity analysis was the best sensitive among the RGB color. A linear working range (40 - 400 ppm; R² = 0.981) between the Cr^VI and green intensity was obtained with a detection limit of 30 ppm. All of the recoveries were between 94 and 109% in recovery studies on water samples, and good results were obtained.