A hybrid material composed of guanine-rich single stranded DNA and cobalt(III) oxyhydroxide (CoOOH) nanosheets as a fluorescent probe for ascorbic acid via formation of a complex between G-quadruplex and thioflavin T

A hybrid material composed of guanine-rich single stranded DNA (G-rich ssDNA) and cobalt oxyhydroxide (CoOOH) nanosheets is used as a nanoprobe for fluorometric turn-on detection of ascorbic acid (AA). The CoOOH nanosheets function as a recognition component for AA. The G-rich ssDNA is used to produce a G-quadruplex, and the G-quadruplex/thioflavin T (ThT) complex acts as a fluorescent reporter. In the absence of AA, p-phenylenediamine (PPD) is oxidized to form oxPPD which has a dark red color. It causes the fluorescence of the G-quadruplex/ThT complex to be quenched. However, in the presence of AA, the CoOOH nanosheets of the nanoprobe are preferentially reduced by AA. Hence, PPD is not oxidized, and fluorescence is not quenched. A fluorometric turn-on method was developed based on these findings. It has a detection limit of 94 nM and works in the concentration range from 1 to 10 and 20 to 80 μM. This method was applied to the determination of AA in (spiked) fruit juice samples. Schematic presentation of a fluorescent assay of ascorbic acid (AA) is established using a nanoprobe composed of guanine-rich single stranded DNA (G-rich ssDNA) and cobalt oxyhydroxide (CoOOH) nanosheets. It is based on competitive reduction of CoOOH by p-phenylenediamine (PPD) and AA. Thioflavine T (ThT) induces the formation of fluorescent G-quadruplex/ThT complex. The oxidized form of PPD (oxPPD) can quench the fluorescence via fluorescence resonance energy transfer (FRET), but AA suppresses quenching.