Millifluidic ion-selective optodes with integrated optical detection for colorimetric quantification of urinary calcium.

Measurement of urinary calcium concentration is essential for the diagnosis and management of various disorders related to calcium metabolism, including kidney stones, hypoparathyroidism, and hyperparathyroidism. Current testing protocols require patients to visit clinical laboratories or hospitals, and no at-home devices are currently approved for quantitative self-testing of urinary calcium. In this work, we report a low-cost and user-friendly colorimetric sensor for urinary calcium measurement. The sensor consists of a segment of sensing oil contained within a millifluidic channel, such as a pipette tip. The oil incorporates hydrophobic sensing chemicals typically used in ion-selective optodes, including a chromoionophore, an ion exchanger, and an ionophore. It is specially formulated to fully extract calcium ions from urine under exhaustive conditions, providing a highly sensitive response. The sensor exhibits a linear response for 0-10 mM calcium ions, enabling accurate diagnosis of both hypercalciuria and hypocalciuria. A custom-built absorbance detector, consisting of a paired light-emitting diode and photodetector, is attached to the pipette tip to measure the absorbance of the sensing oil and quantify calcium ions in urine. The colorimetric sensor is applied to five urine samples before and after calcium spiking. The calcium concentrations of the unspiked samples determined by the current method overall match those obtained by the reference methods although there are biases. The recovery rate ranges from 94 % to 103 % for the spiked samples, suggesting high reliability of the biphasic colorimetric sensor for high concentrations of urinary calcium. Given the cost of <20¢ for the chemicals and pipette tip for each test and the cost of < $50 for the custom-built optical detector, this sensing method holds promise for urinary calcium testing in resource-limited settings such as at patients' homes and in clinics.