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 constructed via the 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 MnO2 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 MnO2 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 MnO2 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 μM 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 μM 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.