“Vanishing mass” in the Sauerbrey world: Quartz Crystal Microbalance study of self-assembled monolayers based on tripod-branched structure with tuneable molecular flexibility
Complex loadings that appear on a surface with flexible spatial organisation can reveal anti-Sauerbrey behaviour due to their variable interfacial architecture even for an ultrathin monomolecular sensitive layer. Presented results demonstrate that compounds with branched and flexible structure may be organised in various spatial architectures with different viscoelastic characteristics. They act not only as a mass loading (Sauerbrey behaviour) but also as a viscous damping (anti-Sauerbrey one) at the same time. The concept is illustrated by SAM formed on QCM silver electrode surface with tripod-like hydroxamic thioligand (S3HX). The obtained data indicates that the interfacial layer was a combination of two S3HX surface structures - namely, the ordered α-(alfa)- and the disordered σ-(sigma)-conformation of SAM. Sigma-conformation is likely to belong to Sauerbrey-like materials with negative frequency shift: such a system is sufficiently rigid because of multiple interactions and spatial restrictions due to the so-called “tangle of tendrils”. Alfa-conformation with labile structure and highly-ordered flexible spatial organisation is likely to be predominated in anti-Sauerbrey-like samples. Sigma- and alfa-based interfacial ensembles demonstrate different behaviour under gaseous analyte exposure. The possibility of anti-Sauerbrey-like behaviour opens the way for the forming of a unique (oppositely shifted) signal in analytical applications, especially ones that require a false-free detection of highly dangerous xenobiotics.