Equipment-free quantitative measurement for microfluidic paper-based analytical devices fabricated using the principles of movable-type printing.

Microfluidic paper-based analytical devices (µPADs) are a growing class of low-cost chemo/biosensing technologies designed for point-of-use applications. In this article, we describe MTWP (movable-type wax printing), a facile method for the fabrication of µPADs. MTWP is inspired by the Chinese movable-type printing and requires only a hot plate and homemade small iron movable components. It is able to pattern various wax microstructures in paper via a simple adjustment of the number, patterning forms or types of the metal movable components. This inexpensive and versatile method may thus hold great potential for producing wax-patterned µPADs by untrained operators at minimized cost in developing countries. In addition, two novel equipment-free assay methods are further developed to render µPAD measurements straightforward and quantitative. They use the flow-through time of a detection reagent in a three-dimensional paper device and the number of colored detection microzones in a 24-zone paper device as the detection motifs. The timing method is based on the selective wettability change of paper from hydrophilic to hydrophobic that is mediated by enzymatic reactions. The counting method is carried out on the basis of oxidation-reduction reactions of a colored substance, namely iodine. Their utility is demonstrated with quantitative detection of hydrogen peroxide as a model analyte. These methods require only a timer or a cell phone with a timing function and the abilities of seeing color and of counting for quantitative µPAD measurement, thus making them simple, cost-efficient, and useful sensor technologies for a great diversity of point-of-need applications especially in resource-poor settings.

An electrochemical sensing platform based on local repression of electrolyte diffusion for single-step, reagentless, sensitive detection of a sequence-specific DNA-binding protein.

In this paper, we report for the first time an electrochemical biosensor for single-step, reagentless, and picomolar detection of a sequence-specific DNA-binding protein using a double-stranded, electrode-bound DNA probe terminally modified with a redox active label close to the electrode surface. This new methodology is based upon local repression of electrolyte diffusion associated with protein-DNA binding that leads to reduction of the electrochemical response of the label. In the proof-of-concept study, the resulting electrochemical biosensor was quantitatively sensitive to the concentrations of the TATA binding protein (TBP, a model analyte) ranging from 40 pM to 25.4 nM with an estimated detection limit of ∼10.6 pM (∼80 to 400-fold improvement on the detection limit over previous electrochemical analytical systems).

Low-cost fabrication of paper-based microfluidic devices by one-step plotting.

In this technical note, we describe a facile method for one-step fabrication of paper-based microfluidic devices, by simply using commercially available permanent markers and metal templates with specific patterns. The fabrication process involves only a single step of plotting pattern in paper; it can be typically finished within 1 min. The ink marks formed in the patterned paper will act as the hydrophobic barriers to define the hydrophilic flow paths or separate test zones. Various paper devices can be created by using different templates with corresponding patterns. Transparent adhesive tape-sandwiched devices could protect their assay surfaces from potential contamination. In the proof-of-concept experiments, circular paper test zones (~3 mm diameter) were fabricated for colorimetric and quantification detection of prostate-specific antigen (PSA) as a model target, based on dot-immunogold staining assays coupled with gold enhancement amplification. Several serum specimens were additionally evaluated with this new approach and the results were compared with the commercial chemiluminescence immunoassay, validating its feasibility of practical applications. Such a one-step plotting method for paper patterning does not require any specialized equipments and skills, is quite inexpensive and rapid, and thus holds great potential to find wide applications especially in remote regions and resource-limited environments such as small laboratories and private clinics.