PNIPAM-immobilized gold-nanoparticles with colorimetric temperature-sensing and reusable temperature-switchable catalysis properties

Multifunctional hybrid particles have been attracting tremendous interest over decades. Herein we demonstrated the functionality of poly(N-isopropylacrylamide) immobilized gold nanoparticles (PNIPAMs-AuNP) as both colorimetric temperature-sensors and reusable temperature-switchable catalysts. The hybrid nanoparticles composed of a gold core (diameter = 14.8 +/- 0.2 nm) and a PNIPAM shell (thickness = 2.6 +/- 0.2 nm) exhibit reversible color changes and size adjustment in aqueous dispersion upon salt and environment temperature variation. Visual observation or colorimetric monitoring of the PNIPAMs-AuNP dispersion through a UV-vis spectrometer, we were able to detect and calibrate the changes in color between red-purple-red as the environmental temperature changes. The synergetic effect of different types of salt has been systematically investigated. It is observed that the sensitivity of the colorimetric temperature-sensor could be significantly improved by adding salts (NaCl < KCl < MgCl2) or controlling salt concentrations. Moreover, the temperature-switchable PNIPAMs-AuNP hybrid nanoparticles were applied to catalyze the reduction reaction of 4-nitrophenol to 4-aminophenol. The reduction process can be sped-up or slowed-down by controlling the system temperature, which switches the catalytic AuNPs surface on and off as the reversible PNIPAM chains collapse (hydrodynamic size = 41 +/- 1 nm) and expand (49 +/- 1 nm). Moreover, the switchable catalysts can be recycled through centrifugation and reused. The polymer-engineered core-shell PNIPAMs-AuNP hybrid nanoparticles are promising to enrich the development of temperature-responsive systems such as ambient temperature alarms and smart catalysts.