A simple paper-based optical biosensor for glucose monitoring was developed. As a glucose biosensing principle, a colorimetric glucose assay, using glucose oxidase (GOx) and horseradish peroxidase (HRP), was chosen. The enzymatic glucose assay was implanted on the analytical paper-based device, which is fabricated by the wax printing method. The fabricated device consists of two paper layers. The top layer has a sample loading zone and a detection zone, which are modified with enzymes and chromogens. The bottom layer contains a fluidic channel to convey the solution from the loading zone to the detection zone. Double-sided adhesive tape is used to attach these two layers. In this system, when a glucose solution is dropped onto the loading zone, the solution is transferred to the detection zone, which is modified with GOx, HRP, and chromogenic compounds through the connected fluidic channel. In the presence of GOx-generated H 2 O 2 , HRP converts chromogenic compounds into the final product exhibiting a blue color, inducing color change in the detection zone. To confirm the changes in signal intensity in the detection zone, the resulting image was registered by a digital camera from a smartphone. To minimize signal interference from external light, the experiment was performed in a specifically designed light-tight box, which was suited to the smartphone. By using the developed biosensing system, various concentrations of glucose samples (0-20 mM) and human serum (5-17 mM) were precisely analyzed within a few minutes. With the developed system, we could expand the applicability of a smartphone to bioanalytical health care.

Recently, much effort has been focused on developing three-dimensional, paper-based microfluidic analytical devices (3D-μPADs) targeting in vitro diagnostics. However, 3D-μPAD fabrication typically requires tedious assembly that hinders mass production. Here, we report on a fabrication method for 3D-μPADs made of plastics without the need for additional assembly. Both sides of the paper were printed via liquid resin photopolymerization using a digital light processing (DLP) printer. The sample reservoir and detection zones are located on the top of the 3D-μPADs, and three microchannels are located on the bottom. The detection limits for glucose, cholesterol, and triglyceride (TG) in phosphate-buffered saline (PBS) were 0.3 mM, 0.2 mM, and 0.3 mM, respectively. The detectable ranges of glucose, cholesterol, and TG in human serum were 5-11 mM, 2.6-6.7 mM, and 1-2.3 mM. These results suggest that our fabrication method is suitable to mass produce 3D-μPADs with relative ease using simple fabrication processes.

We developed retroreflective Janus microparticles (RJPs) as a novel optical immunosensing probe for use in a nonspectroscopic retroreflection-based immunoassay. By coating the metals on the hemispherical surface of silica particles, highly reflective RJPs were fabricated. On the basis of the retroreflection principle, the RJPs responded to polychromatic white light sources, in contrast to conventional optical probes, which require specific monochromatic light. The retroreflection signals from RJPs were distinctively recognized as shining dots, which can be intuitively counted using a digital camera setup. Using the developed retroreflective immunosensing system, cardiac troponin I, a specific biomarker of acute myocardial infarction, was detected with high sensitivity. On the basis of the demonstrated features of the retroreflective immunosensing platform, we expect that our approach may be applied for various point-of-care-testing applications.

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