Improved sensitivity in paper-based microfluidic analytical devices using a pH-responsive valve for nitrate analysis.

This paper presents an innovative application of chitosan material to be used as pH-responsive valves for the precise control of lateral flow in microfluidic paper-based analytical devices (µPADs). The fabrication of µPADs involved wax printing, while pH-responsive valves were created using a solution of chitosan in acetic acid. The valve-forming solution was applied, and ready when dry; by exposure to acidic solutions, the valve opens. Remarkably, the valves exhibited excellent compatibility with alkaline, neutral, and acidic solutions with a pH higher than 4. The valve opening process had no impact on the flow rate and colorimetric analysis. The potential of chitosan valves used for flow control was demonstrated for µPADs employed for nitrate determination. Valves were used to increase the conversion time of nitrate to nitrite, which was further analyzed using the Griess reaction. The µPAD showed a linear response in the concentration range of 10-100 µmol L-1, with a detection limit of 5.4 µmol L-1. As a proof of concept, the assay was successfully applied to detect nitrate levels in water samples from artificial lakes of recreational parks. For analyses that require controlled kinetics and involve multiple sequential steps, the use of chitosan pH-responsive valves in µPADs is extremely valuable. This breakthrough holds great potential for the development of simple and high-impact microfluidic platforms that can cater to a wide range of analytical chemistry applications.

Direct immunoassay on a polyester microwell plate for colorimetric detection of the spike protein in swab and saliva samples.

This study presents the development of a polyester microplate for detecting the S-protein of the SARS-CoV-2 virus in saliva and nasopharyngeal swab samples using direct enzyme-linked immunosorbent assay (ELISA) technology. The polyester microplate was designed to contain 96 zones with a 3 mm diameter each, and a volume of 2-3 µL. The experimental conditions including reagent concentration and reaction time were optimized. The microplate image was digitized and analyzed using graphical software. The linear range obtained between protein S concentrations and pixel intensity was 0-10 µg mL-1, with a correlation coefficient of 0.99 and a limit of detection of 0.44 µg mL-1. The developed methodology showed satisfactory intraplate and interplate repeatability with RSD values lower than 7.8%. The results achieved through immunoassay performed on polyester microplates were consistent with those of the RT-PCR method and showed a sensitivity of 100% and 90% and specificity of 85.71% and 100% for saliva and nasopharyngeal samples, respectively. The proposed direct immunoassay on polyester microplates emerges as an alternative to conventional immunoassays performed on commercial polystyrene plates, given the low cost of the device, low consumption of samples and reagents, lower waste generation, and shorter analysis time. Moreover, the immunoassay has shown great potential for diagnosing COVID-19 with precision and accuracy.

Distance-based detection of paracetamol in microfluidic paper-based analytical devices for forensic application.

Whisky adulteration is a prevalent practice driven by the high cost of these beverages. Counterfeiters commonly dilute whisky with less expensive alcoholic beverages, water, food additives, drugs or pharmaceuticals. Paracetamol (PAR), an analgesic drug that mitigates hangovers and headaches, is commonly used to adulterate whisky. Currently, the primary method for quantifying PAR levels is high-performance liquid chromatography, but this technique is both time consuming and usually generates more residues. In this context, the utilization of miniaturized and portable analytical devices becomes imperative for conducting point-of-care/need analyses. These devices offer several advantages, including portability, user-friendliness, low cost, and minimal material wastage. This study proposes the selective distance-based PAR quantification on whisky samples using a paper-based microfluidic analytical device (µPAD). Colorimetric detection on paper-based platforms offers great benefits such as affordability, portability, and the ability to detect PAR without complicated instrumentation. The optimal detection conditions were achieved by introducing 5 µL of a mixture containing 7.5 mmol L-1 of Fe(III) and K3[Fe(CN)6] into the detection zone, along with 12 µL of whisky samples into the sample zone. The method exhibited linear behavior within the concentration range from 15 to 120 mg L-1, with a determination coefficient of 0.998. PAR was quantified in adulterated samples. The results obtained with the paper-based devices were compared with a referenced method, and no significant differences were observed at a confidence level of 95%. The µPAD allowed to determine ca. 1 drop of pharmaceutical medicine PAR of 200 mg mL-1 in 1 L of solution, demonstrating excellent sensitivity. This method offers cost-effective and rapid analysis, reducing the consumption of samples, reagents, and wastes. Consequently, it could be considered a viable and portable alternative for analyzing beverages at criminal scenes, customs, and police operations, thereby enhancing the field of forensics.

Fabrication of microwell plates and microfluidic devices in polyester films using a cutting printer.

This study reports, for the first time, the possibility to manufacture analytical devices on polyester substrates using a cutting printer. The protocol involves the design of a layout in a graphical software, the cut into polyester films and the lamination against one or multiple polyester films coated with a thermosensitive layer. The feasibility of the proposed approach was demonstrated through the fabrication of 96-microwell plates, 3D microfluidic mixing and distance-based microfluidic devices. The printer has enabled cutting microchannels wider than 300 µm on polyester films and a thickness of 250 µm. Urea and glucose assays were performed on microwell plates aiming for their quantification in artificial urine and serum samples. The presented results revealed good agreement with the expected values. The complexation reaction between Fe2+ and o-phenanthroline was selected as model to investigate the feasibility of the 3D mixing device. Absorbance measurements were recorded for the reaction product performed in both on and off-chip modes. Considering the achieved data, the on-chip mixing exhibited similar behavior when compared to off-chip reaction, thus demonstrating to be efficient to perform mixtures due to the turbulence generated inside three-dimensional channels. Lastly, a distance-based device was designed to detect H2O2 based on the displacement of a dye plug promoted by the oxygen generation using a copper-modified paper sheet. The distance-based peroxymeter revealed a linear behavior in the concentration range between 1 and 5% (v/v) and a LOD equal to 0.5% (65.2 mM). Based on the results herein reported, the proposed method represents a simple and alternative protocol to produce microdevices, using affordable and inexpensive raw materials, within 10 min, and at a cost lower than US$ 0.10 per unit.