Paper-based analytical devices for point-of-need applications.

Paper-based analytical devices (PADs) are powerful platforms for point-of-need testing since they are inexpensive devices fabricated in different shapes and miniaturized sizes, ensuring better portability. Additionally, the readout and detection systems can be accomplished with portable devices, allying with the features of both systems. These devices have been introduced as promising analytical platforms to meet critical demands involving rapid, reliable, and simple testing. They have been applied to monitor species related to environmental, health, and food issues. Herein, an outline of chronological events involving PADs is first reported. This work also introduces insights into fundamental parameters to engineer new analytical platforms, including the paper type and device operation. The discussions involve the main analytical techniques used as detection systems, such as colorimetry, fluorescence, and electrochemistry. It also showed recent advances involving PADs, especially combining optical and electrochemical detection into a single device. Dual/combined detection systems can overcome individual barriers of the analytical techniques, making possible simultaneous determinations, or enhancing the devices' sensitivity and/or selectivity. In addition, this review reports on distance-based detection, which is also considered a trend in analytical chemistry. Distance-based detection offers instrument-free analyses and avoids user interpretation errors, which are outstanding features for analyses at the point of need, especially for resource-limited regions. Finally, this review provides a critical overview of the practical specifications of the recent analytical platforms involving PADs, demonstrating their challenges. Therefore, this work can be a highly useful reference for new research and innovation.

Enhancing of detection resolution via designing of a multi-functional 3D connector between sampling and detection zones in distance-based microfluidic paper-based analytical device: multi-channel design for multiplex analysis.

One of the problems in the distance-based microfluidic paper-based analytical device (DB-µPAD) that limits the detection resolution is the mixing of reagents from the detection to the sampling zone or vice versa due to spreading by capillary action. In the present paper, to overcome mixing of the reagents in the zones, a multi-functional connector using a three-dimensional (3D) design has been developed externally to connect the two zones. Using such a novel design, it is acertained that there is no any mixing due to the dispersion of the reagents in the two zones. Interestingly, the simple 3D connector has other functions, such as its potential to be used as a masking zone and/or reaction zone whenever is needed. Based on this proposed 3D connector-based DB-µPAD, three parallel microchannels were built as detection zones with one sampling zone for multiplex analysis for the detection of Fe2+, Ni2+, and hardness of water. In the Ni2+ channel, the connector piece worked as both masking part and connection part. In the Fe2+ line, the connector served as the connector and reducing pad (Fe3+ to Fe2+ ions). While in the third channel, the connector has connection function only. Satisfactory limit of detection (LoD), accuracy, and precision were obtained using this design. The LODs obtained with the proposed design were 1.13, 0.62, and 1.87 mg.L-1 for total hardness, Fe2+, and Ni2+, respectively.

A nanosilica-coated thread-based analytical device for nitrate and nitrite detection in food samples.

A new microfluidic thread-based analytical device (µTAD) for nitrate and nitrite determination in food samples was developed. The cotton thread substrate was coated with nanosilica to increase its hydrophilicity and stability, and polylactic acid was applied to one end of the nanosilica-coated thread to constrain the fluid flow along the thread in one direction. Quantification of nitrate and nitrite was based on the modified Griess reaction, using sulfanilamide and N-(1-naphthyl) ethylenediamine as chromogenic reagents, and utilizing a distance-based detection technique. Linear responses were observed in a range of 4-25 mg L-1 (R2 = 0.9991) for nitrite and a range of 8-50 mg L-1 (R2 = 0.9989) for nitrate. The limits of detection for nitrite and nitrate were 1.5 and 3.1 mg L-1, respectively. The detection time was 5 min for nitrite analysis, and 7 min for nitrate analysis. The new method demonstrated good precision, accuracy, selectivity, and stability. The performance of the proposed µTAD for nitrite and nitrate determination in real food samples was comparable to that of the conventional UV-Vis spectrophotometric method. The proposed µTAD could serve as a simple, low-cost, and portable method for nitrite and nitrate detection in food samples.

Distance-Based Biosensor for Ultrasensitive Detection of Uracil-DNA Glycosylase Using Membrane Filtration of DNA Hydrogel.

In the deoxyribonucleic acid (DNA) repair pathways, DNA repair enzymes have great significance for genomic integrity. As one important initiator of the base-excision repair pathway, the aberrant activity of uracil-DNA glycosylase (UDG) is closely associated with many diseases. Herein, we developed a simple distance-based device for visual detection of UDG activity using a load-free DNA hydrogel. The DNA hydrogel consists of polyacrylamide-DNA chains being bridged by a single-stranded DNA crosslinker containing a responsive uracil base site. UDG can recognize and remove the uracil, resulting in the cleavage effect of the DNA crosslinker strand with the assistance of endonuclease IV (Endo IV). Plugging one end of the capillary tube, the DNA hydrogel acting as a filter membrane separator would control molecules to flow into the tube. The integrity of the DNA hydrogel networks is affected by the excision of UDG. Therefore, taking full advantage of membrane filtration of the DNA hydrogel, the activity of UDG can be quantitatively detected via reading the distance of the red indicator solution in the capillary tube. Without any instruments and complicated procedures, this method realizes high sensitivity and specificity for the detection of UDG as low as 0.02 mU/mL and can even measure UDG in complex cell samples. Additionally, this method is simple, universal, and can be used to screen inhibitors, which shows great potential for point-of-care testing, clinical diagnosis, and drug discovery.

Read-by-eye quantification of aluminum (III) in distance-based microfluidic paper-based analytical devices.

Herein we report the first two distance-based microfluidic paper-based analytical devices (µPADs) using fluorescence to quantify aluminum. In addition to their read-by-eye quantification, the devices are simple to fabricate, require no sample pretreatment or preconcentration, and have a shelf life of >5 months. The first device is designed in a "chemometer" format where the length of a fluorescent band linearly responds to an Al(III) concentration. The second device uses a radial design where the fluorescent diameter also linearly responds to an Al(III) concentration. The chemometer device has a detection limit of 2.5 ppm (100 µM) and a linear range from 2 to 54 ppm Al(III) (100 µM-1 mM), R2 = 0.989). The radial device has a detection limit of 0.9 ppm (33 µM) and a linear range from 2 to 24 ppm Al(III) (100-900 µM, R2 = 0.968). The utility of the µPADs were successfully demonstrated by measuring Al(III) in two water effluent samples from the Gold King Mine near Silverton, CO.

High-throughput deposition of chemical reagents via pen-plotting technique for microfluidic paper-based analytical devices.

The deposition of chemical reagent inks on paper is a crucial step in the development and fabrication of microfluidic paper-based analytical devices (µPADs). A pen-plotting approach, delivering chemical ink deposition using technical pens filled with reagents and inserted into a desktop electronic plotter, is shown herein to be a versatile, low-cost, simple, rapid, reproducible, and high-throughput solution. The volume of the deposited ink was quantified gravimetrically, confirming that nanoliter volumes of reagents can be deposited reproducibly (e.g. 7.55 ±â€¯0.37 nL/mm for a plotting speed of 10 cm/s) in detection zones of µPADs, typically spatially defined using wax printing. This approach was further investigated with regard to deposition of reagents in different geometrical forms (circular and linear), so demonstrating its applicability for preparation of µPADs with flexible design and application. By adjusting the plotting speed for linear deposition, lines with a relatively large range of widths (≈628-1192 µm) were created. Circular deposition was also demonstrated via delivery of reagents within wax printed circular fluidic barriers of a range of diameters (inner diameter = 1.5-7 mm). These capabilities were practically demonstrated via the fabrication of µPADs, based upon differing detection principles for determination of aluminum in natural waters using Morin as the fluorescent reagent. Traditional µPADs based on digital image colorimetry (DIC) were produced using circular deposition, whilst distance-based µPADs exploited linear deposition. Both types of µPADs developed using this method showed excellent precision for determination of trace concentrations of aluminium (average RSDs = 3.38% and 6.45%, and LODs = 0.5 ng (0.25 ppm) and 2 ng (0.5 ppm), for traditional and distance-based detection, respectively).

Highly Sensitive Paper-based Analytical Devices with the Introduction of a Large-Volume Sample via Continuous Flow.

The implementation of continuous flow in paper-based analytical devices (PADs) was challenging because of the large-volume introduction that was created; but this allowed for the development of novel types of PADs for preconcentration, separation, and sensitive detection. In this study, pump-free continuous flow was applied to a distance-based PAD for the determination of iron ions. Continuous flow enabled the introduction of a volume that exceeded what was necessary to fill the hydrophilic channel of a PAD. Thus, this continuous-flow method significantly improved both the limits of detection (LOD) and the limits of quantification (LOQ) for a distance-based PAD by increasing the sample volume that could be introduced into the PAD. The values for LOD and LOQ were 20 and 26 ppb, respectively, which were more than 150-times lower than that obtained using a small sample volume (50 µL), and were comparable to those of inductively coupled plasma-atomic emission spectrometry. The continuous-flow technique was applicable to the determination of iron ions at levels of several tens of ppb in natural water without preconcentration.

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.

Paper-Based DNA Reader for Visualized Quantification of Soil-Transmitted Helminth Infections.

Soil-transmitted helminth (STH) infections are a global health issue affecting nearly one-third of the world's population. As most endemic areas of STH are impoverished countries or regions with limited healthcare resources, the accurate diagnosis of STH requires analytical tools that are not only quantitative, but also portable, inexpensive, and with no or minimal demand for external instrument. Herein, we introduce a novel paper-based diagnostic device, termed quantitative paper-based DNA reader (qPDR), capable of quantifying STH at the molecular level by measuring distance as readout, thus eliminating the need for external readers. On the basis of the unique interfacial interaction of a DNA intercalating dye, SYBR Green I, with native cellulose on a chromatographic paper, qPDR allows the distance-based quantification of minute amounts of double-stranded DNA as short as 6 min. By integrating qPDR with polymerase chain reactions that were performed using a smartphone-controlled portable thermal cycler, we were able to quantify minute amount of genetic markers from adult worms of an STH (Trichuris trichiura) that were expelled post-treatment by infected children living in the rural areas of Honduras.

Integrated Distance-Based Origami Paper Analytical Device for One-Step Visualized Analysis.

An integrated distance-based origami paper analytical device (ID-oPAD) is developed for simple, user friendly and visual detection of targets of interest. The platform enables complete integration of target recognition, signal amplification, and visual signal output based on aptamer/invertase-functionalized sepharose beads, cascaded enzymatic reactions, and a 3D microfluidic paper-based analytical device with distance-based readout, respectively. The invertase-DNA conjugate is released upon target addition, after which it permeates through the cellulose and flows down into the bottom detection zone, whereas sepharose beads with larger size are excluded and stay in the upper zone. Finally, the released conjugate initiates cascaded enzymatic reactions and translates the target signal into a brown bar chart reading. By simply closing the device, the ID-oPAD enables a sample-in-answer-out assay within 30 min with visual and quantitative readout. Importantly, bound/free probe separation is achieved by taking advantage of the size difference between sepharose beads and cellulose pores, and the downstream enzymatic amplification is realized based on the compatibility of multiple enzymes with corresponding substrates. Overall, with the advantages of low-cost, disposability, simple operation, and visual quantitative readout, the ID-oPAD offers an ideal platform for point-of-care testing, especially in resource-limited areas.

A selective distance-based paper analytical device for copper(II) determination using a porphyrin derivative.

Meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin sulfonate (TDMPzP), a water-soluble porphyrin derivative, was synthesized and used as a colorimetric reagent for Cu2+ detection on a microfluidic paper-based analytical device (µPAD) using distance-based quantification. TDMPzP showed a high selectivity for Cu2+ detection in aqueous solutions. When Cu2+ was added to the TDMPzP under acidic conditions, a color change from green to a pink was observed by the naked eye. Under optimized conditions, the application of this system to a distance-based µPAD exhibited good analytical response. The presence of common metal ions (Al3+, Fe3+, Mg2+, Co2+, Mn2+, Zn2+, Pb2+, Cd2+, Sn2+, and Ni2+) did not interfere with Cu2+ detection within reasonable tolerance ratios. The lowest concentration of copper that could be measured was 1mgL-1 (1ppm) which meets the requirements for drinking water contamination regulations from the US Environmental Protection Agency (EPA) and World Health Organization (WHO) guidelines for drinking water. Real drinking water samples were analyzed to confirm the practical application of this system and the results showed good agreement with ICP-MS data. This distance-based µPAD based on TDMPzP for Cu2+ detection is convenient and effective for real-time drinking water analysis.