Fluorescent Nile blue‐functionalized poly (N‐isopropylacrylamide) microgels responsive to temperature and polyamines

Abstract

Fluorescent poly(N‐isopropylacrylamide‐co‐Nile blue) (pNIPAm‐co‐NB) microgels were synthesized that exhibited fluorescence intensity changes in a water temperature‐dependent fashion. NB is well known to exhibit fluorescence intensity that depends on the hydrophobicity of the environment, while pNIPAm‐based microgels are well known to transition from swollen (hydrophilic) to collapsed (relatively hydrophobic) at temperatures greater than 32 °C; hence, we attribute the above behavior to the hydrophobicity changes of the microgels with increasing temperature. This phenomenon is ultimately due to NB dimers (relatively quenched fluorescence) being broken in the hydrophobic environment of the microgels leading to relatively enhanced fluorescence. We went on to show that the introduction of cucurbit[7]uril (CB[7]) into the pNIPAm‐co‐NB microgels enhanced their fluorescence allowing them to be used for polyamine (e.g., spermine [SPM]) detection. Specifically, CB[7] forms a host–guest interaction with NB in the microgels, which prevents NB dimerization and enhances their fluorescence. When SPM is present, it forms a host–guest complex that is favored over the CB[7]‐NB host–guest interaction, which frees the NB for dimerization and leads to fluorescence quenching. As a result, we could generate an SPM sensor capable of SPM detection down to ~0.5 µmol/L in complicated matrixes such as serum and urine.