Fabrication of paper-based analytical devices using stencil-printed glass varnish barriers for colorimetric detection of salivary α-amylase.

BACKGROUND: Developing disposable paper-based devices has positively impacted analytical science, particularly in developing countries. Some benefits of those devices include their versatility, affordability, environmentally friendly, and the possibility of being integrated with portable electrochemical or colorimetric detectors. Paper-based analytical devices (PADs) comprising circular zones and microfluidic networks have been successfully employed in the analytical chemistry reign. However, the combination of the stencil-printing method and alternative binder has not been satisfactorily explored for fabricating colorimetric paper devices. RESULTS: We developed PADs exploring the stencil printing approach and glass varnish as the hydrophobic chemical agent. As a proof-of-concept, the colorimetric assay of salivary α-amylase (sAA) was performed in saliva samples. Through the scanning electron microscopy measurements, it was possible to indicate satisfactory definitions between native fibers and barrier, and that the measured values for the channel width revealed suitable fidelity (R2 = 0.99) with the nominal widths (ranging from 400 to 5000 µm). The proposed hydrophobic barrier exhibited excellent chemical resistance. The analytical applicability for detecting sAA revealed linear behavior in the range from 2 to 12 U mL-1 (R2 = 0.99), limit of detection of 0.75 U mL-1, reproducibility (RSD ≤2.4%), recovery experiments ranged from 89 to 108% and AGREE response (0.86). In addition, the colorimetric analysis of sAA in four different saliva samples demonstrated levels ranging from 202 to 2080 U mL-1, which enabled monitoring the absence and presence of periodontitis. SIGNIFICANCE: This report has presented the first use of a self-adhesive mask and glass varnish for creating circular zones and microfluidic architectures on paper without using thermic or UV curing treatments. Also, the proposed analytical methodology for detecting sAA exhibited suitable ecological impact considering the AGREE tool. We believe the proposed fabrication of paper devices emerges as a novel, simple, high-fidelity microfluidic channel and portable analytical approach for colorimetric sensing.

Polyester resin and graphite flakes: turning conductive ink to a voltammetric sensor for paracetamol sensing.

The development of a disposable electrochemical paper-based analytical device (ePAD) is described using a novel formulation of conductive ink that combines graphite powder, polyester resin, and acetone. As a proof of concept, the proposed sensor was utilized for paracetamol (PAR) sensing. The introduced ink was characterized via morphological, structural, and electrochemical analysis, and the results demonstrated appreciable analytical performance. The proposed ePAD provided linear behavior (R2 = 0.99) in the concentration range between 1 and 60 µmol L-1, a limit of detection of 0.2 µmol L-1, and satisfactory reproducibility (RSD ~ 7.7%, n = 5) applying a potential of + 0.81 V vs Ag at the working electrode. The quantification of PAR was demonstrated in different pharmaceutical formulations. The achieved concentrations revealed good agreement with the labeled values, acceptable accuracy (101% and 106%), and no statistical difference from the data obtained by HPLC at the 95% confidence level. The environmental impact of the new device was assessed using AGREE software, which determined a score of 0.85, indicating that it is eco-friendly. During the pharmacokinetic study of PAR, it was found that the drug has a maximum concentration of 23.58 ± 0.01 µmol L-1, a maximum time of 30 min, and a half-life of 2.15 h. These results are comparable to other studies that utilized HPLC. This suggests that the combination of graphite powder and polyester resin can transform conductive ink into an effective ePAD that can potentially be used in various pharmaceutical applications.

Digital microfluidic platform assembled into a home-made studio for sample preparation and colorimetric sensing of S-nitrosocysteine.

Digital microfluidics (DMF) is a versatile lab-on-a-chip platform that allows integration with several types of sensors and detection techniques, including colorimetric sensors. Here, we propose, for the first time, the integration of DMF chips into a mini studio containing a 3D-printed holder with previously fixed UV-LEDs to promote sample degradation on the chip surface before a complete analytical procedure involving reagent mixture, colorimetric reaction, and detection through a webcam integrated on the equipment. As a proof-of-concept, the feasibility of the integrated system was successfully through the indirect analysis of S-nitrosocysteine (CySNO) in biological samples. For this purpose, UV-LEDs were explored to perform the photolytic cleavage of CySNO, thus generating nitrite and subproducts directly on DMF chip. Nitrite was then colorimetrically detected based on a modified Griess reaction, in which reagents were prepared through a programable movement of droplets on DMF devices. The assembling and the experimental parameters were optimized, and the proposed integration exhibited a satisfactory correlation with the results acquired using a desktop scanner. Under the optimal experimental conditions, the obtained CySNO degradation to nitrite was 96%. Considering the analytical parameters, the proposed approach revealed linear behavior in the CySNO concentration range between 12.5 and 400 µmol L-1 and a limit of detection equal to 2.8 µmol L-1. Synthetic serum and human plasma samples were successfully analyzed, and the achieved results did not statistically differ from the data recorded by spectrophotometry at the confidence level of 95%, thus indicating the huge potential of the integration between DMF and mini studio to promote complete analysis of lowmolecular weight compounds.

Recycling 3D Printed Residues for the Development of Disposable Paper-Based Electrochemical Sensors.

Here, we propose a recyclable approach using acrylonitrile-butadiene-styrene (ABS) residues from additive manufacturing in combination with low-cost and accessible graphite flakes as a novel and potential mixture for creating a conductive paste. The graphite particles were successfully incorporated in the recycled thermoplastic composite when solubilized with acetone and the mixture demonstrated greater adherence to different substrates, among which cellulose-based material made possible the construction of a paper-based electrochemical sensor (PES). The morphological, structural, and electrochemical characterizations of the recycled electrode material were demonstrated to be similar to those of the traditional carbon-based surfaces. Faradaic responses based on redox probe activity ([Fe(CN)6]3-/4-) exhibited well-defined peak currents and diffusional mass transfer as a quasi-reversible system (96 ± 5 mV) with a fast heterogeneous rate constant value of 2 × 10-3 cm s-1. To improve the electrode electrochemical properties, both the PES and the classical 3D-printed electrode surfaces were modified with a combination of multiwalled carbon nanotubes (MWCNTs), graphene oxide (GO), and copper. Both electrode surfaces demonstrated the suitable oxidation of nitrite at 0.6 and 0.5 V vs Ag, respectively. The calculated analytical sensitivities for PES and 3D-printed electrodes were 0.005 and 0.002 µA/(µmol L-1), respectively. The proposed PES was applied for the indirect amperometric analysis of S-nitroso-cysteine (CysNO) in serum samples via nitrite quantitation, demonstrating a limit of detection of 4.1 µmol L-1, with statistically similar values when compared to quantitative analysis of the same samples by spectrophotometry (paired t test, 95% confidence limit). The evaluated electroanalytical approach exhibited linear behavior for nitrite in the concentration range between 10 and 125 µmol L-1, which is suitable for realizing clinical diagnosis involving Parkinson's disease, for example. This proof of concept shows the great promise of this recyclable strategy combining ABS residues and conductive particles in the context of green chemical protocols for constructing disposable sensors.

Sandpaper-based electrochemical devices assembled on a reusable 3D-printed holder to detect date rape drug in beverages.

This study describes the development of a new electrochemical paper-based analytical device (ePAD) on alumina sandpaper substrate through a pencil-drawing process for square wave voltammetry measurements of midazolam maleate used as a "date rape drug" in beverages. The proposed ePAD was assembled on a reusable 3D printed holder to delimit its geometric area and ensure better robustness. The ePAD was characterized by scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and Raman spectroscopy. The direct drawing of ePADs on sandpaper platforms through a graphite pencil has offered suitable repeatability (RSD = 1.0%) and reproducibility (RSD = 4.0%) using [Fe(CN)6]4- as redox probe. The proposed ePAD provided linear behaviour in the midazolam maleate concentration range between 2.5 and 150 mg L-1 and a limit of detection of 2.0 mg L-1. The feasibility of the ePAD for forensic application was successfully demonstrated through the detection of midazolam in different beverages (water, beer, liquor, and vodka). The intended application revealed low interference of other compounds present in beverages. Based on the achieved results, the proposed ePAD has offered great accuracy with no statistical difference at 95% confidence level from the data recorded by high performance liquid chromatography. The operational simplicity and the robustness ensured by the assembling on a reusable 3D printed holder make the ePAD drawn on sandpaper platform a powerful and promising analytical tool for the analysis of "date rape drugs" opening new possibilities for on-site forensic investigations.