Responsive nanogel-based dual fluorescent sensors for temperature and Hg2+ ions with enhanced detection sensitivity

Abstract

We report on the fabrication of thermoresponsive poly(N-isopropylacrylamide) nanogel-based dual fluorescent sensors for temperature and Hg²⁺ ions, and the effects of thermo-induced nanogel collapse on the detection sensitivity of Hg²⁺ ions. Near-monodisperse thermoresponsive nanogels were prepared via emulsion polymerization of N-isopropylacrylamide (NIPAM) and a novel 1,8-naphthalimide-based polarity-sensitive and Hg²⁺-reactive fluorescent monomer (NPTUA, 3). At room temperature, PNIPAM nanogels labeled with a single type of naphthalimide-based dye (NPTUA) can act as ratiometric Hg²⁺ probes at the nanomolar level. Upon heating above the phase transition temperature, the fluorescence intensity of NPTUA-labeled nanogels in the absence of Hg²⁺ exhibit ∼3.4-fold increase due to that NPTUA moieties are now located in a more hydrophobic microenvironment. Moreover, it was observed that the detection sensitivity to Hg²⁺ can be further improved above the nanogel phase transition temperature. At a nanogel concentration of 0.05 g L⁻¹ and in the same Hg²⁺ concentration range (0–3.0 equiv.), ∼10 fold and ∼57 fold increase in fluorescence emission intensity ratio changes can be achieved at 25 and 40 °C, respectively.