Upconversion-based nanosystems for fluorescence sensing of pH and H(2)O(2)

Hydrogen peroxide (H(2)O(2)), a key reactive oxygen species, plays an important role in living organisms, industrial and environmental fields. Here, a non-contact upconversion nanosystem based on the excitation energy attenuation (EEA) effect and a conventional upconversion nanosystem based on the joint effect of EEA and fluorescence resonance energy transfer (FRET) are designed for the fluorescence sensing of H(2)O(2). We show that the upconversion luminescence (UCL) is quenched by MoO(3−x) nanosheets (NSs) in both systems due to the strong absorbance of MoO(3−x) NSs in the visible and near-infrared regions. The recovery in UCL emissions upon addition of H(2)O(2) enables quantitative monitoring of H(2)O(2). Benefiting from the non-contact method, hydrophobic OA-NaYF(4):Yb,Er can be used as the luminophore directly and ultrahigh quenching efficiency (99.8%) is obtained. Moreover, the non-contact method exhibits high sensitivity toward H(2)O(2) with a detection limit of 0.63 μM, which is lower than that determined by simple spectrophotometry (0.75 μM) and conventional upconversion-based nanocomposites (9.61 μM). As an added benefit, the same strategy can be applied to the sensing of pH, showing a broad pH-responsive property over a range of 2.6 to 8.2. The successful preparation of different upconversion-based nanosystems for H(2)O(2) sensing using the same material as the quencher provides a new design strategy for fluorescence sensing of other analytes.