Synthesis of Cit-NaYF4: Yb, Tm@phenolic formaldehyde resin (PFR)@Au composites as an optical sensor for the detection of Cu(Ⅱ) ions

Abstract A facile hydrothermal method was employed to synthesize the core-shell structure up-conversion Cit-NaYF4:Yb, Tm (UC)@phenolic formaldehyde resin (PFR) nanoparticles (NPs). UC NPs can enhance the emission intensity of the PFR shell, which contributes to improving the fluorescence detection sensitivity of UC@PFR NPs. Au NPs were loaded onto the surface of PFR shell in situ through the good reduction of the hydroxyl group in PFR. The emission intensity of UC@PFR decreased greatly in the presence of Au nanoparticles due to fluorescence resonance energy transfer (FRET) mechanism between UC@PFR and Au NPs. However, the mixture of H2O2 and SCN− could prevent the decrease of emission intensity of UC@PFR@Au composites because of the oxidation reaction between H2O2 and Au nanoparticles in the presence of SCN−. Also, Cu2+ can prevent the oxidation reaction to occur, which results in the slight change of emission intensity of UC@PFR@Au in the presence of H2O2 and SCN−. Thus, the as-synthesized UC@PFR@Au particles were used to detect Cu2+ sensitivity. The good linear correlation equation between the fluorescence intensity of UC@PFR@Au particles and the concentration of Cu2+ in the range of 4–90 nM (R2 = 0.99) was established with the low detection limit (LOD) of 0.54 nM. Furthermore, the as-synthesized particles were employed to detect Cu2+ in the actual water sample with satisfying results.