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This paper describes the modification of microfluidic paper-based analytical devices (μPADs) with chitosan to improve the analytical performance of colorimetric measurements associated with enzymatic bioassays. Chitosan is a natural biopolymer extensively used to modify biosensing surfaces due to its capability of providing a suitable microenvironment for the direct electron transfer between an enzyme and a reactive surface. This hypothesis was investigated using glucose and uric acid (UA) colorimetric assays as model systems. The best colorimetric sensitivity for glucose and UA was achieved using a chromogenic solution composed of 4-aminoantipyrine and sodium 3,5-dichloro-2-hydroxy-benzenesulfonate (4-AAP/DHBS), which provided a linear response for a concentration range between 0.1 and 1.0 mM. Glucose and UA were successfully determined in artificial serum samples with accuracies between 87 and 114%. The limits of detection (LODs) found for glucose and UA assays were 23 and 37 μM, respectively. The enhanced analytical performance of chitosan-modified μPADs allowed the colorimetric detection of glucose in tear samples from four nondiabetic patients. The achieved concentration levels ranged from 130 to 380 μM. The modified μPADs offered analytical reliability and accuracy as well as no statistical difference from the values achieved through a reference method. Based on the presented results, the proposed μPAD can be a powerful alternative tool for non-invasive glucose analysis.

This paper describes the fast stamping of microfluidic paper-based analytical devices with chemically modified surface for improved colorimetric measurements.

This study reports the use of microfluidic paper-based analytical devices (µPADs) associat e d with colorimetric detection for the determination of nitrite in clinical, food and environment a l samples. µPADs were fabricat ed by simple and fast stamping process in a geomet ry containin g eight circular detection zones and one central zone to sample inlet interconnected by microfluid i c channels . The colorimet ric determination of nitrite was performed through the modified Gries s reaction. Detection zones were spotted with a 0.75 µL aliquot of a solution containing 50 mmol L -1 sulfanilamide, 1.2 mol L -1 hydrochloric acid and 4 mmol L -1 N-(1-napthyl)ethy lenediamin e . The monitoring of the background colorimetric response revealed good stability over 12h for devices stored in the absence of light. After the addition of standard or real samples, the result in g images were captured with a scanner, converted to a color scale and analyzed in the magen t a channel. The analytical sensitivity and the limit of detection achieved after a preconcentrat io n stage were 0.56 (AU/µM ) and 5.6 µM , respectively. The preconcentrat ion provided an enrichment factor of ca. 3.2 times. The concentration levels of nitrite were successful ly determined in saliva, preservative water, ham, sausage and river water samples. The concentrat ion levels attained for each sample using µPADs were compared to the values found by spectrophotomet ry and there was no significative difference from one another at a confiden c e level of 95%.

The adulteration of whiskey with analgesics and sedation drugs has been a common practice to prevent hangover the following day and promote loss of consciousness. In both situations, the portable and low cost detection platforms are of paramount importance for forensic investigations. This report describes the use of electrochemical paper‐based analytical devices (ePADs) fabricated by pencil drawing for detecting metamizole, paracetamol and midazolam maleate in whiskey. Different types of paper substrates and graphite pencils were initially characterized with ferrocyanide. The best results were achieved using vegetal paper and Aquarelle/6B pencils. ePADs revealed a decrease in current signal indicating a short lifetime, thus limiting their use to disposable sensors. Despite the short lifetime, the graphite pencil ePADs revealed good electrochemical reproducibility (RSD=3.3 %). The forensic feasibility of the proposed ePADs was demonstrated through the analysis of metamizole and paracetamol in whiskey. The limit of detection (LOD) achieved for paracetamol and metamizole were 45 and 20 mg L−1, respectively. ePADs were also tested to detect midazolam maleate in whiskey. The signal recorded exhibited linear correlation in a wide concentration range (25–1000 mg L−1) and a LOD of ca. 5 mg L−1. Considering the disposability and operational simplicity, ePADs offer a good strategy for detecting adulterations in alcoholic beverages at the point‐of‐need.

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