Advanced Colorimetric Paper Sensors Using Color Focusing Effect Based on Asymmetric Flow of Fluid

Although paper-based colorimetric sensors utilizing enzymatic reactions are well suited for real-field diagnosis, their widespread use is hindered by signal blurring at the detection spot due to the action of capillary forces on the liquid and the corresponding membrane. In this study, we eliminated signal losses commonly observed during enzyme-mediated colorimetric sensing and achieved pattern-free quantitative analysis of glucose and uric acid by mixing enzymes and color-forming reagents with chitosan oligosaccharide lactate (COL), which resulted in perfectly focused colorimetric signals at the detection spot, using asymmetric flow induced by changing the flow rate of the COL-treated paper. The targets were calibrated with 0-500 mg/dL of glucose and 0-200 mg/dL of uric acid, and the limit of detection was calculated to be 0.6 and 0.03 mg/dL, respectively. In human urine, the correlation has a high response between the measured and spiked concentrations, and the stability of the enzyme mixture including COL increased by 41% for glucose oxidase mixture and 29% for uricase mixture, compared to the corresponding mixtures without COL. Thus, the color focusing and pattern-free sensor, which have the advantages of easy fabrication, easy handling, and high stability, should be applied to real-field diagnosis.