Multi-emitting fluorescence sensor of MnO2-OPD-QD for the multiplex and visual detection of ascorbic acid and alkaline phosphatase

A blue-, yellow- and red-emitting fluorescence sensor was constructedviathe synergy of manganese dioxide (MnO2) nanosheets,o-phenylenediamine (OPD) and red-fluorescent quantum dots (r-QDs), namely MnO2-OPD-QD, for visualization of ascorbic acid (AA) and alkaline phosphatase (ALP). Initially, by virtue of the oxidase-like activity of MnO(2)nanosheets, OPD was oxidized into yellow-fluorescent OPDox (emission: 569 nm), which quenched the r-QDs (emission: 659 nm). After interaction with AA, one reaction product of ALP and 2-phospho-l-ascorbic acid (AA2P), the MnO(2)nanosheets were reduced and lost the oxidase-like activity, so that the generation of OPDox was lessened and the fluorescence of the r-QDs was thereupon recovered. Further increasing AA or ALP, the MnO(2)nanosheets were fully decomposed so that OPD was not oxidized but was reduced into blue-fluorescent OPDred (emission: 441 nm), while the r-QDs were not affected. Overall, AA or ALP triggered emission intensity changes at 441, 569 and 659 nm and led to profuse color variation over the yellow-orange-red-purple range, realizing multiplex and visual determination of AA and ALP in the ranges of 0.5-70 mu M and 0.1-100 mU mL(-1), respectively. The color variation range of the ternary-emission mode was greatly enlarged, especially compared with that of the single-/dual-emission mode. Accordingly, the MnO2-OPD-QD sensor provided detection limits for AA and ALP as low as 0.18 mu M and 0.06 mU mL(-1), respectively. Furthermore, the highly selective/sensitive measurement and accurate readout of AA and ALP were accomplished in complicated food and serum samples. The multi-emitting fluorescence sensor held great potential for onsite visualization of various targets in more fields.