Contrast-enhancing fluorescence detection of copper ions by functional fluorescent microgels

Highly sensitive and selective detection of trace Cu2+ over fourteen kinds of metal ions in aqueous solutions was achieved by the use of a novel contrast-enhancing fluorescent microgels. The microgels were prepared by the use of N-isopropylacrylamide and 1-vinylimidazole as monomers and fluorescent tetraphenylethene (TPE) derivatives as cross-linker, followed by post-functionalization with 4-(2-pyridylazo)-resorcinol (PAR). The obtained microgels were spherical in shape with narrow size distribution and could selectively fluorescent response to Cu2+ in aqueous solution or in real water samples at acidic condition, which could be even observed by naked eyes. The multi-color fluorescence emissions including white light were obtained by controlling the amount of Cu2+ added in the microgels. The detection limit for Cu2+ was ∼5.2 nM, which was nearly 1/4000 the maximum level in drinking water allowed by United States Environmental Protection Agency. The TPE-PAR probe could be renewed by adding excessive S2− to the TPE-PAR MG-Cu2+ complex. The discoloration of fluorescent detection was attributed to the coexistence of TPE and PAR in the microgels. PAR absorbed both excitation and emission fluorescence of TPE moieties, resulting in low fluorescence intensity of the microgels. After adding Cu2+, the interaction between PAR and Cu2+ changed the UV absorption intensity and position of PAR, hence leading to the change of fluorescence spectrum and color of the microgels.