3D-printed rolling circle amplification chip for on-site colorimetric detection of inorganic mercury in drinking water.

A point-of-care testing chip was developed for the colorimetric detection of inorganic mercury ion (Hg). The disposable chip fabricated by three-dimensional printing technology contains DNAzymes produced by rolling circle amplification (RCA); a color change caused by the enzymatic reaction between DNAzymes and the peroxidase substrate 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) is measured using a portable spectrophotometer. In the "turn-off"-type RCA reaction, the annealing of the T(12) primer that initiates the RCA reaction is blocked by the interaction of thymine with Hg; thus, the amount of amplified DNAzymes causing a color change is decreased depending on Hg concentration. The colorimetric signal is enhanced by amplifying double-repeat DNAzymes from a circular DNA template. The chip detected Hg in tap drinking water samples with high sensitivity (lowest validated value: 3.6 μg/L) and showed better selectivity, precision, and reproducibility than conventional analysis instruments. This low-cost easy-to-use platform can reduce the risk of accidental Hg poisoning.