Transformation of bulk MnO2 to fluorescent quantum dots for selective and sensitive detection of ferric ions and ascorbic acid by turn-off-on strategy

MnO2 nanostructure has a unique light absorption ability and redox feature, due to which it is mainly used as a fluorescence quencher. However, its quantum dots can also act as a self-fluorescent nanomaterial with exclusive properties for sensing applications. Therefore, a cheap and straightforward route for fabricating citric acid-functionalized MnO2 quantum dots (CA-MnO2 QDs) was proposed through one-step ultrasonication of bulk MnO2 in the presence of citric acid as acidifying and stabilizing agent. The prepared CA-MnO2 QDs exhibited high photostability, good water solubility, high photoluminescence (PL) stability from pH range 5 to 9, and significant fluorescence quantum yield value, i.e., 13.3 %. Further, CA-MnO2 QDs was used for the detection of Fe3+ ions with a detection limit of 43 nM, based on dynamic quenching. The fluorescence quenching mechanism was confirmed by determining different photophysical parameters such as fluorescence quantum yield, average lifetime values, radiative constants (kr), non-radiative rate constants (knr), rate of electron transfer (kET), and electron transfer efficiency (phi EET). The CA-MnO2 QDs in the presence of Fe3+ was used for the detection of ascorbic acid with a detection limit of 90 nM based on the redox reaction between Fe3+ ions and AA. Further-more, the developed sensing probe was also used to detect Fe3+ ions and AA in real samples, i.e., iron and AA supplements, respectively.