NaYF4:Yb3+–Er3+ nanocrystals/P(NIPAM-co-RhBHA) core–shell nanogels: Preparation, structure, multi stimuli-responsive behaviors and application as detector for Hg2+ ions

• Novel core–shell nanogels of NaYF4:Yb3+–Er3+ nanocrystals/P(NIPAM-co-RhBHA).
• Probes based on NIR excitation and fluorescence resonance energy transfer.
• Fluorescent sensors constructed with up-conversion nanocrystals and organic dyes.
• Nanogels can response to multi-stimuli, such as pH, temperature, metal ions.

In the present study, we report a novel core–shell nanogels of NaYF4:Yb3+–Er3+ nanocrystals/P(NIPAM-co-RhBHA) based on up-conversion excitation and fluorescence resonance energy transfer (FRET). Firstly, up-conversion NaYF4:Yb3+–Er3+ nanocrystals were synthesized by hydrothermal method with EDTA, Er(NO3)3, Yb(NO3)3, Y(NO3)3 and NaF as raw materials. Then, the nanocrystals were decorated with MPS. Finally, core–shell nanogels with thermo-responsive shells were fabricated via free radical emulsion polymerization of N-isopropylacrylamide (NIPAM) and N-acrylyl-N′-rhodamine B acylhydrazine thiourea (RhBHA) dye monomers. Their structure, stimuli-responsive behaviors were investigated via TEM, FTIR, XRD, DSC, UV–vis and PL in detail. The results suggest that the nanogels can response to multi-stimuli, such as pH value, temperature, metal ions, and they can be used as sensitive detector for Hg2+ ions.

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