Paper has shown potential as a ubiquitous material for fabricating micro analytical devices for diagnostic and drinking water screening applications for resource-limited regions; paper-based sensing technology has become a hot research field since 2007. Intensive research in the past decade has accumulated a large number of scientific publications. However, commercialization of microfluidic paper-based analytical devices (μPADs) for real applications is noticeably lagging behind. The “ASSURED” criteria (i.e., Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free, Deliver to the users who need them), set by the World Health Organization, specified the whole spectrum of requirements for a low-cost sensor designed for use in developing countries; they define the technical capabilities (i.e., “ASSR”) and user acceptance (i.e., “UED”) of low-cost sensing technology. While ASSR should be taken as the basic requirements of any sensor, UED determines whether or not the sensor could potentially be commercialized and gain user acceptance. This Perspective presents these two critical aspects of paper-based diagnostics by revisiting the original motivation of the paper-based analytical platform. It is our opinion that UED are important requirements that deserve more research to increase the commercialization of paper-based analytical devices.

This study introduced a barcode-like design into a paper-based blood typing device by integrating with smartphone-based technology. The concept of presenting a paper-based blood typing assay in a barcode-like pattern significantly enhanced the adaptability of the assay to the smartphone technology. The fabrication of this device involved the use of a printing technique to define hydrophilic bar channels which were, respectively, treated with Anti-A, -B, and -D antibodies. These channels were then used to perform blood typing assays by introducing a blood sample. Blood type can be visually identified from eluting lengths in bar channels. A smartphone-based analytical application was designed to read the bar channels, analogous to scanning a barcode, interpret this information, and then report results to users. The proposed paper-based blood typing device is rapidly read by smartphones and easy for the user to operate. We envisage that the adaptation of paper-based devices to the widely accepted smartphone technology will increase the capability of paper-based diagnostics with rapid assay result interpretation, data storage, and transmission.

If a paper-based analytical device (μ-PAD) could be made by printing indicators for detection of heavy metals in chemical symbols of the metals in a style of the periodic table of elements, it could be possible for such μ-PAD to report the presence and the safety level of heavy metal ions in water simultaneously and by text message. This device would be able to provide easy solutions to field-based monitoring of heavy metals in industrial wastewater discharges and in irrigating and drinking water. Text-reporting could promptly inform even nonprofessional users of the water quality. This work presents a proof of concept study of this idea. Cu(II), Ni(II), and Cr(VI) were chosen to demonstrate the feasibility, specificity, and reliability of paper-based text-reporting devices for monitoring heavy metals in water.

Wetting behaviors on stretchable supports are very common in our daily lives, however, received limited attention even they show promising potentials in flexible electronics and other fields. In this study, stretchable wetting behaviors of conductive liquids deposited onto two horizontal rubber fibers are investigated. A firm liquid/solid interaction during the stretching process can contribute to a stable liquid bridge between the fibers even under extremely stretching, showing their proof‐to‐principle ability to monitor human movement toward early diagnosis of Parkinson's disease or sports injury prevention.

Here the distance dependence of metal-enhanced quantum dots (QDs) fluorescence in solution is studied systematically by capillary electrophoresis (CE). Complementary DNA oligonucleotides-modified CdSe/ZnS QDs and gold nanoparticles (Au NPs) were connected together in solution by the hybridization of complementary oligonucleotides, and a model system (QD-Au) for the study of metal-enhanced QDs fluorescence was constructed, in which the distance between the QDs and Au NPs was controlled by adjusting the base number of the oligonucleotide. In our CE experiments, the metal-enhanced fluorescence of the QDs solution was only observed when the distance between the QDs and Au NPs ranged from 6.8 to 18.7 nm, and the maximum enhancement by a factor of 2.3 was achieved at 11.9 nm. Furthermore, a minimum of 19.6 pg of target DNA was identified in CE based on its specific competition with the QD-DNA in the QD-Au system. This work provides an important reference for future study of metal-enhanced QDs fluorescence in solution and exhibits potential capability in nucleic acid hybridization analysis and high-sensitivity DNA detection.

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