Enhanced passive mixing for paper microfluidics

Office paper and laser printing: a versatile and affordable approach for fabricating paper-based analytical devices with multimodal detection capabilities

Multiple protocols have been reported to fabricate paper-based analytical devices (PADs). However, some of these techniques must be revised because of the instrumentation required. This paper describes a versatile and globally affordable method to fabricate PADs using office paper as a substrate and a laser printing technique to define hydrophobic barriers on paper surfaces. To demonstrate the feasibility of the alternatives proposed in this study, the fabrication of devices for three types of detection commonly associated with using PADs was demonstrated: colorimetric detection, electrochemical detection, and mass spectrometry associated with a paper-spray ionization (PSI-MS) technique. Besides that, an evaluation of the type of paper used and chemical modifications required on the substrate surface are also presented in this report. Overall, the developed protocol was suitable for using office paper as a substrate, and the laser printing technique as an efficient fabrication method when using this substrate is accessible at a resource-limited point-of-need. Target analytes were used as a proof of concept for these detection techniques. Colorimetric detection was carried out for acetaminophen, iron, nitrate, and nitrite with limits of detection of 0.04 μg, 4.5 mg mL-1, 2.7 μmol L-1, and 6.8 μmol L-1, respectively. A limit of detection of 0.048 fg mL-1 was obtained for the electrochemical analysis of prostate-specific antigen. Colorimetric and electrochemical devices revealed satisfactory performance when office paper with a grammage of 90 g m-2 was employed. Methyldopa analysis was also carried out using PSI-MS, which showed a good response in the same paper weight and behavior compared to chromatographic paper.

Capillary flow-driven immunoassay platform for COVID-19 antigen diagnostics

Over the last few years, the SARS-CoV-2 pandemic has made the need for rapid, affordable diagnostics more compelling than ever. While traditional laboratory diagnostics like PCR and well-plate ELISA are sensitive and specific, they can be costly and take hours to complete. Diagnostic tests that can be used at the point-of-care or at home, like lateral flow assays (LFAs) are a simple, rapid alternative, but many commercially available LFAs have been criticized for their lack of sensitivity compared to laboratory methods like well-plate ELISAs. The Capillary-Driven Immunoassay (CaDI) device described in this work uses microfluidic channels and capillary action to passively automate the steps of a traditional well-plate ELISA for visual read out. This work builds on prior capillary-flow devices by further simplifying operation and use of colorimetric detection. Upon adding sample, an enzyme-conjugated secondary antibody, wash steps, and substrate are sequentially delivered to test and control lines on a nitrocellulose strip generating a colorimetric response. The end user can visually detect SARS-CoV-2 antigen in 15-20 min by naked eye, or results can be quantified using a smartphone and software such as ImageJ. An analytical detection limit of 83 PFU/mL for SARS-CoV-2 was determined for virus in buffer, and 222 PFU/mL for virus spiked into nasal swabs using image analysis, similar to the LODs determined by traditional well-plate ELISA. Additionally, a visual detection limit of 100 PFU/mL was determined in contrived nasal swab samples by polling 20 untrained end-users. While the CaDI device was used for detecting clinically relevant levels of SARS-CoV-2 in this study, the CaDI device can be easily adapted to other immunoassay applications by changing the reagents and antibodies.

Clarity improvement of the discoloration boundary and detection of Hg2+ ions by using a polystyrene nanoparticle-modified paper-based microdevice

Distance-based microfluidic paper-based analytical devices (μPADs) can be used to calculate the analyte content by reading the length of the discolored area in the channel. A blurred discoloration boundary is difficult to distinguish, resulting in reading errors. In this study, we constructed a μPAD modified with carboxyl-containing polystyrene nanoparticles (PS-μPAD) to improve the discoloration-boundary clarity. The filling of the pores of the fibers with the deposited polystyrene nanoparticles (PS NPs) caused a decrease in the paper porosity, resulting in a flow delay. Meanwhile, the carboxyl groups carried by PS NPs were able to form hydrogen bonds with hydroxyl-containing compounds FLPI, a Hg²⁺ probe, and the two factors acted synergistically to fix the FLPI to react in situ, raising the discoloration-boundary clarity. Compared with the unmodified μPAD, the detection of Hg²⁺ ions using the PS-μPAD still had a good linear relationship. Importantly, the color-depth difference inside and outside the discoloration boundary improved by about four times and showed excellent reproducibility in different populations. The method was simple and easy to expand, thereby providing an idea for more widespread application of distance-based μPADs.

Dip-and-Read, Organic Solvent-Compatible, Paper-Based Analytical Devices Equipped with Chromatographic Separation for Indole Analysis in Shrimp

We developed an organic solvent-compatible paper-based analytical device (PAD) for the quantitative analysis of indole, which is an indicator of shrimp freshness. Although indole is insoluble in water, ethyl acetate is a suitable solvent to dissolve and extract indole from shrimp. The PADs are fabricated using a cutting method that allows the use of an organic solvent because no hydrophobic barrier is needed to form fluidic channels. Ehrlich's reagent consists of 4-(dimethylamino)benzaldehyde and p-dimethylaminobenzaldehyde and was deposited onto the reaction zone of the PAD followed by lamination to prevent evaporation of the ethyl acetate. Samples are introduced into the PAD via immersion in organic sample solutions. When the PAD is immersed into an indole solution of ethyl acetate in a closed bottle, the sample solution penetrates the channel of the PAD and successively flows into the detection zone to form a hydrophilic colored product. The PADs provide a linear relationship between the logarithm of the indole concentration and the color intensity within a range of 1.0-20 ppm with correlation coefficients of r² > 0.99. The limits of detection and quantification are 0.36 and 0.71 ppm, respectively. Relative standard deviations for both the intraday (n = 2) and interday (n = 3) precision were less than 2.5%. In the indole analysis of shrimp, the PADs separated the interfering orange-colored astaxanthin in the extract from the colored product of indole via the paper chromatographic principle. We used the PADs to investigate the degradation of shrimp, and the results showed a rapid increase in the indole level after 7 days. High-performance liquid chromatography verified the accuracy of the PADs by showing good agreement with the obtained indole levels.