A quartz-crystal microbalance with dissipation is used to study molecular interactions and molecular adsorption at surfaces by measuring the change in resonant frequency of a quartz crystal upon addition of mass to a surface.120 It has the advantages of sub-nanogram detection capabilities, being label free and readily modifiable for a diverse range of surface chemistries. The mass of very thin surface bound layers and the binding events can be monitored in real time, allowing interface characterisation and evaluation of kinetics. The applications include detection of carbohydrates, nucleic acids, antibodies and cells and characterisation of enzyme activity.121 The QCM-D monitors both the frequency and energy dissipation response for the crystal and can therefore provide information about the kinetics of mass changes and the structural changes simultaneously. The quartz crystal sensors utilised in these experiments are in the form of a thin quartz disc sandwiched between two electrodes and they are excited to oscillation by applying A.C. voltage. This oscillation then decays exponentially once the driving A.C. voltage is switched off. The QCM-D instrument monitors this decay and extracts two parameters resonance frequency (f) and dissipation (D). The dissipation parameter gives information about how long the crystal continues to oscillate after the driving voltage of the system is turned off, i.e. how quickly the energy is dissipated from the system. For soft and viscous films, the dissipation is high due to frictional losses within the
44 films causing dampening of the sensor oscillation. Conversely, for rigid films the dissipation is low.
The data can be displayed as a raw data plot (ΔD/Δf against time) or modelling can be carried out in order to obtain additional quantitative information. The Sauerbrey equation, (Figure 1.25) can be used to model thin, rigid films (where the frequency and dissipation harmonic do not appear to spread during the course of the experiment).122 For soft and laterally homogenous films, viscoelastic modelling of ΔD and Δf must instead be performed. This is because the viscoelastic films will not fully couple to the oscillation of the crystal and will instead cause dampening. The Sauerbrey relation will therefore underestimate the mass at the surface and the Voigt or Maxwell model must be used instead.
∆𝑚 =−𝐶∆𝑓
𝑛
Figure 1.25: The Sauerbrey equation for calculating the change in mass (Δm) on the surface
C is ((crystal thickness x crystal density) / fundamental frequency), which is equal to 17.7 ng
Hz-1 cm-2 for a 5 MHz quartz crystal. n is the overtone number (i.e 1, 3, 5 etc.). Δf is the change
in frequency in Hz
The origins of QCM-D were not in polymer chemistry, however it has become a useful tool for studying polymer binding and surface ordering. The ability to monitor the binding of polymers to surfaces in real time and obtain data regarding the kinetics of these grafting processes is very desirable.123 Through such studies, it has been seen that at a low grafting density polymer chains can exhibit a pancake- like formation (if the chains interact attractively with the surface) or a mushroom- like formation (if the chain-surface interaction is non-attractive). Once a high grafting density is reached the chains stretch away from the surface and form brushes due to the combination of chain-chain repulsion and elasticity.124,125 A polymer chain
45 grafted to a surface can undergo conformational changes, which influence its interfacial properties such as hydrophobicity, hydrophilicity, adsorption and adhesion.126 The dehydration and hydration of the chains can be observed through monitoring of the frequency shift (Δf) of the QCM-D plot whereas the collapse and swelling are related to the dissipation change (ΔD). A plot of Δf vs. ΔD describes the relationship between dehydration and collapse. If ΔD increases linearly with -Δf it indicates that the conformational change involves only one kinetic process and that the dehydration and collapse must occur simultaneously.125
The thermoresponsive behaviour of pNIPAM brushes has been investigated using QCM-D. It is known that below its LCST (~32°C), free pNIPAM chains are swollen into a random coil, whereas above the LCST they collapse into globules.127 The pNIPAM chains were grafted to a gold surface and therefore a low density was observed, producing mushroom type structures. The mass layer on the surface decreases with temperature, with a transition occurring at ~34°C, due to the loss of water molecules once the polymer has reached its LCST.125 With grafting from
polymerisation a much higher grafting density, and pancake-like structure, is achieved yet the LCST transition is still observed at ~34°C.128 The pancake-to-brush conformational change can be observed over time.129 Initially there is a slow decrease in Δf showing slow grafting due to steric hindrance, this then dramatically speeds up once the grafted chains move to a brush conformation to accommodate the incoming chains.125
Work on thio-functional oligo(ethylene glycol) (OEG) polymers has shown that using disulfide di(ethylene glycol) (DEG) polymers results in more mass adsorbed onto the gold, when compared to thiols with only one sulfur atom available
46 for adsorption. The disulfide DEG polymers also showed greater adsorption than di- thio and tri-thio species, despite having the same or fewer available sulfur atoms. This has been rationalised as being due to the steric effects on the sulfur atoms and the resulting differences in adsorbing bond angle, which results in a weaker dissociation of charge. It was also found that when longer OEG polymers were used, the binding was less than that for the shorter counterparts.130 A variety of QCM chip coatings are commercially available allowing investigation of systems other than sulfur-gold binding. For example, silicon dioxide coated QCM chips have been functionalised with thiol-containing silanes to allow real-time monitoring of the addition of macromonomers of poly (ethylene glycol methyl ether acrylate) (pEGA454) and poly (ethylene glycol methyl ether methacrylate) (pEGMA1100).131
In addition to monitoring the adsorption of polymers onto the surfaces, QCM-D can also be used to monitor the interactions of a ready functionalised surface with a solution. Layer-by-layer assemblies of lectins onto glycopolymers have been produced, using the mannose/galactose glycopolymers as selective Con A/PNA lectin binders, respectively.132 Beyond the scope of polymer systems, QCM- D can be used to monitor the protein resistance of a functionalised chip and therefore select surface-coatings that exhibit desirable bio-inert properties.37
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