A portable system for on-site quantification of formaldehyde in air based on G-quadruplex halves coupled with A smartphone reader.

A portable colorimetric determination system based on G-quadruplex DNAzyme integrated with a smartphone was developed to quantitatively detect formaldehyde (FA). The method is based on the oxidation of FA by H2O2, which prevents the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)–H2O2 reaction catalyzed by G-quadruplex halves. With the addition of FA, the amount of the blue-green-colored free-radical cation (ABTS+) was reduced. The concentration of FA can be determined by monitoring this competitive reaction with a UV-vis spectrometer. Response surface methodology (RSM) and Box-Behnken design (BBD) were applied for optimization of the colorimetric assay. A smartphone-based colorimetric reader was also developed, which could display FA responses and report the concentration in real-time. The system could detect FA as low as 0.01 µM with a linear range of 1-600 µM. Taking advantages of smartphone and DNAzyme, the assay provides great potential for its practical application as a home testing or on-site analysis with high sensitivity and selectivity.