Fully inkjet-printed distance-based paper microfluidic devices for colorimetric calcium determination using ion-selective optodes

Although the determination of calcium ions (Ca²⁺) is of high importance to monitor water hardness, currently available devices for on-site analysis suffer from a lack of user-friendliness and sensitivity. This work demonstrates fully inkjet-printed and low-cost microfluidic paper-based analytical devices (μPADs) for the simple naked-eye colorimetric determination of calcium ions (Ca²⁺) in drinking and tap water samples. The quantification of Ca²⁺ relies on visual readout of the length of a colour-changed detection channel modified with ionophore-doped ion-selective optode nanospheres (nano-optodes), eliminating the requirement of a scanner or a camera. All fabrication steps for deposition of assay reagents have been performed by means of a simple desktop thermal inkjet printer, which is expected to contribute to highly batch-to-batch reproducible device preparation. The detectable Ca²⁺ concentrations between 0.05 mmol L^-1 and 5 mmol L^-1 cover the range recommended by the International Organization for Standardization (0.05-2.5 mmol L^-1) and the World Health Organization (WHO) guideline for Ca²⁺ quantification in drinking water (less than 5 mmol L^-1). The lowest concentration of Ca²⁺ detectable by the naked eye was found to be 0.05 mmol L^-1, which is below the value achieved with previously reported paper-based devices. μPAD quantified Ca²⁺ concentrations in tap or drinking waters were within 15% error of the results obtained with a classical complexometric titration. Hence, distance-based μPADs relying on nano-optodes are sensitive and reproducible tools for equipment-free on-site assaying of Ca²⁺ in real samples.