Real-time luminescence-based colorimetric determination of double-strand DNA in droplet on demand.

We have developed a new luminescence-based colorimetric droplet platform for the determination of double-stranded DNAs (dsDNA). This colorimetric sensor was realized via choosing a fluorescent ensemble probe comprising water-soluble N-acetylcysteine-capped CdTe quantum dots (QDs) and Ru(bpy)(2)(dppx)(2+) (Ru). To provide a convenient and low cost droplet platform for colorimetry, the microvalve technique was adapted to adjust droplet size precisely, achieve the desired fusion of multiple droplets and trap droplets on demand, as well as implement concentration gradients of DNA on a single chip. In the colorimetric sensor, Ru served as both an effective quencher for QDs and a reporter for dsDNA. With increasing concentration of dsDNA, a gradually enhanced color response was observed because of the competition of dsDNA with QDs for Ru. Under the optimum conditions, this biosensing system exhibited not only good sensitivity and specificity for calf thymus DNA with the detection limit of 1.0 pg, but also coincident performances in diluted human serum with the detection limit of 0.9 pg. The droplet biosensor provides a highly efficient, rapid and visual method for dsDNA analysis. The colorimetric droplet platform could be useful as a simple research tool for the study of limited and precious regents such as protein and virus samples, etc.