New sensitive architectures built on soft surface architectures or nano-sized blocks also require a rethinking of the principles of operation of traditional physical recording methods. One of these types includes sensors based on Quartz Crystal Microbalance (QCM). Today, there is no doubt that in many scenarios of molecular adsorption, both frequency change and the dissipation is not primarily determined by the adsorbate itself, but rather by the link by which it is bound to the transducer.
In this abstract, we report an experimental study of tuning interfacial friction for latex nanoparticles (NP, LB1 (Sigma-Aldrich),100 nm in diameter, the surface is very hydrophobic in character) adsorbed on planar silver (hydrophilic)electrodes of QCM transducers by gaseous analytes (saturated water vapor as a case in point).Measurements were performed for both stock LB1 NP and those applied mixed with 0.2% TWEEN-20. Due to its surfactant nature, TWEEN-20 enhances the attraction of nanoparticles to the surface but prevent their aggregation. For such coatings, the classical Sauerbrey behavior was observed, when the adsorption of the analyte leads to a decrease in the resonant frequency of the sensor.
A qualitatively different response behavior was observed for native LB1 NP without the addition of a surfactant. The initial decrease in frequency due to water adsorption was replaced by a sharp increase in frequency up to values greater than the initial ones; with further exposure, the decrease and increase in frequency replace each other. The observed anti- Sauerbrey behavior is considered as a possibility of NP rolling, sliding (the NP purely skids on a surface) and slipping (the object also has some angular motion) as well as change of surface coating viscosity.
The possibility of a gaseous analyte not only to change the QCM loading, but also the features of the mechanical behavior of the mass associated with the surface through a local change in friction in the contact area, opens the way to the creation of highly selective sensors, the response of which to a given analyte has a large value of the opposite sign.
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