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A urea quantification method integrated with a pipette-tip-based biosensor and a smartphone was developed for the on-site diagnosis of blood urea nitrogen. When a sample solution containing phenol red is drawn into a pipette tip with urease immobilized on its inner wall, the color of the solution changes owing to an enzymatic reaction. The urea concentration can be quantified by photographing the pipette tip with a smartphone and quantifying the color intensity of the solution. In actual urea measurements, the detection limit (3σ) was estimated to be 1.3 mg/dL, indicating that the method has sufficient performance to quantify urea in serum. Furthermore, the method was found to reduce sample consumption and analysis time to 1/10 and 1/3, respectively, compared with conventional methods using microwell plates. This method requires only a micropipette and a smartphone to carry out measurements and does not require bench-top precision equipment or special training. Therefore, the method is expected to develop into a practical analytical platform for biomarker diagnostics that can be used in the field. [ABSTRACT FROM AUTHOR]

In this study, we developed an instrument-free visual pH determination method using a wedge-shaped microspace with pH-dependent swelling hydrogel particles. This method of pH measurement is achieved by utilizing the property of the hydrogel to swell with increasing pH and the wedge-shaped microspace in a 3D structure. Hydrogel particle formation using a microdroplet-forming chip and an ultraviolet LED enabled the fabrication of spherical pH-responsive hydrogel particles. Microchips with a wedge-shaped microspace of 500–3500 µm could be fabricated using the structures created by a photo-engineered 3D printer as templates. The evaluation of the method using a pH standard demonstrated that the settling distance of the particles decreased with increasing pH, and that the sensitivity and pH resolution were estimated to be 2.94 mm/pH and 0.2 pH, respectively. These results indicated that the method has the potential to visually measure pH. Furthermore, the developed method was applied to the determination of creatinine using creatinine deiminase. The method is readily applicable to biosensing technologies based on pH transduction, making it useful for on-site analysis in a wide range of fields, including medicine, environment, and food. Moreover, it can be adapted to microfluidic analysis for visual quantification using device-free micrototal analysis systems or lab-on-a-chip. [ABSTRACT FROM AUTHOR]