Abstract

A dip-and-read microfluidic paper-based analytical device (μPAD) was developed for the qualitative and quantitative detection of the total hardness of water. To create well-defined hydrophobic barriers on filter paper, a regular office printer and a commercially available permanent marker pen were utilized as a quick and simple technique with easily accessible equipment/materials to fabricate μPAD in new or resource-limited laboratories without sophisticated equipment. After a wettability and barrier efficiency analysis on the permanent marker colors, the blue and green ink markers exhibited favorable hydrophobic properties and were utilized in the fabrication of the developed test devices. The device had five reaction and detection zones modeled after the classification given by the World Health Organization (WHO), so qualitatively it determined whether the water was ‘soft’, ‘moderately hard’, ‘hard’, or ‘very hard’ by changing color from blue to pink in about 3 min. The device was also used to introduce an alternative colorimetric reaction for quantitative analysis of the water hardness without the need for ethylenediaminetetraacetic acid (EDTA) and without compromising the simplicity and low cost of the device. The developed μPAD showed a calculated limit of detection (LOD) of 0.02 mM, which is at least 80% less than those of commercially available test strips and other reported μPADs, and the results of the real-world samples were consistent with those of the standard titration (with EDTA). In addition, the device exhibited stability for 2 months at room and frigid condition (4 °C) and at varying harsh temperatures from 25 to 100 °C. The results demonstrate that the developed paper-based device can be used for rapid, on-site analysis of water with no interferences and no need for a pipette for sample introduction during testing.
A mathematical estimation of the flow of liquid water and blood serum on the fabricated paper device was computed using a geometrically modified version of the Lucas-Washburn equation to predict the signal time of the paper sensor during each test. The estimation correlated excellently with experimental data and observation, hence making the modification of the Lucas-Washburn equation valid specifically for the fabricated μPAD.
Finally, all-inclusive pullulan tablets were fabricated as an alternative analytical platform to detect the total hardness of water. The assay was used to compute a calibration curve which can be used to quantify the total hardness of water in about five minutes by just dropping the required number of tablets in the water sample, and a limit of detection of 0.0140 mM was achieved.