Poly(l-lactide) stabilized gold nanoparticles based QCM sensor for low humidity detection

A quartz crystal (QC) microbalance coated with poly(l-lactide) stabilized gold nanoparticles (PLLA-AuNP) was reported as a low humidity sensor. Enhancement of humidity sensitivity due to the high surface area of gold nanoparticles was demonstrated by comparison with those of a bulk gold electrode (blank) and a PLLA-coated film. These electrodes were investigated to measure the humidity between 275 and 2408 ppmv. The adsorption/desorption behavior was also examined at a humidity of 1163 ppmv. The sensitivities of the blank, PLLA and PLLA-AuNP films were 0.090, 0.052 and 0.886 Hz/ppmv, respectively, at a humidity level of 319 ppmv indicating the highest sensitivity of the PLLA-AuNP film. In the plot between sensor response (frequency shift) of the PLLA-AuNP film and humidity in the range of 275–847 ppmv, a nearly linear slope (sensitivity) of 0.480 Hz/ppmv was obtained with a correlation coefficient (R2) of 0.9769. Based on the Langmuir adsorption isotherm assumption, the association constants of water vapor molecules with the blank, PLLA and PLLA-AuNP films were estimated to be 11263.42, 7870.98 and 13354.68, respectively, thus explaining the relatively high sensitivity of PLLA-AuNP film with the larger association constant.