Silver nanoparticles stabilized by various organic coatings for gas sensors: comparative analysis by surface plasmon resonance and quartz crystal microbalance methods

Composite nanostructures stabilized by branched polymers are of undoubted interest for chemical sensors. The combination of a heavy metal with the polymer’s functionality creates smart nanobot, where the nanoparticle’s inertial mass enhances the initial adsorption effect. In this report, we discuss an advanced QCM sensors in which the informative signal is due to a change in the structural organization, the triggering factor for activation of which is the adsorption of the analyte.

Silver nanoparticles with a diameter of 60 nm with coatings based on polymeric PEG, BPEI, PVP and citric acid (CIT) were applied by dropping followed by drying at room temperature. Quartz resonators (10 MHz) and surface plasmon resonance chips (SPR) were used as physical transducers. The response of the SPR and quartz microbalance (QCM) transducers to vapors of water and ethyl alcohol was measured in a carrier gas flow at room temperature.

The SPR spectroscopy demonstrate typical shift to the large angles; the saturation level depends on the type of coating. QCM measurements confirm the possibility of analytes detection: the responses are specific for the analyte-sensitive layer pairs. However, in contrast to the SPR results, the responses to ethanol vapors for PEG and BPEI-based coatings have the opposite sign compared to sensitive layers based on CIT and PVP.

According to the Sauerbrey model, an increase in the adsorbed mass should lead to a decrease in frequency, which is observed for water and ethanol (CIT and PVP) vapors. For PEG and BPEI, anti-Sauerbrey behavior is dominated, in which the adsorption leads to a change in the viscoelastic characteristics of the coatings: heavy metal nanoparticles cease to be rigidly fixed on the surface. Such a mechanism allows the creation of highly selective sensors, the response of which differs not only in magnitude, but also in the sign of the response.