Semi-quantitative analysis of nickel: counting-based μPADs built via hand drawing and yellow oily double-sided...

Excess exposure to a high environmental level of nickel can cause serious harm to human health. Therefore, it is particularly necessary to develop a low-cost, fast and sensitive method for nickel assay. In this paper, novel counting-based microfluidic paper-based devices (μPADs) were prepared by hand drawing and yellow oily double-sided adhesive tape. The dissolved adhesive tape was used for the first time to make the hydrophobic "ink". The marker filled with the "ink" drew the desirable layout on paper followed by a drying process. The μPADs were constituted of one circular sample introduction zone (diameter 4.5 mm) and four circular detection zones (diameter 3 mm). The adjoining detection zones were connected by a strip channel (1.2 mm × 2 mm). The fabrication conditions were optimized and the barriers created with the marker revealed good reproducibility. The analytical performance of the developed devices was investigated for nickel assay. The Ni²⁺ standard was added to the sample introduction zone, and subsequently moved into the detection zones containing dimethylglyoxime (DMG), where it reacted with Ni²⁺ and formed a reddish pink Ni-DMG complex. Through counting the number of colored dots, the developed μPADs realized the semi-quantitative analysis of nickel. More importantly, the fabrication of the developed counting-based μPADs only used a marker and adhesive tape, possessing the advantages of instrument-free simplicity and low cost. Our method has great potential for enabling μPADs to be easily implemented in laboratories or research centers with limited resources.