Studying viscoelastic materials using quartz crystal resonator

Viscoelastic materials are often encountered in nature from biological matter, such as microbes and mammalian cells to non-biological matter such as polymers and even metals. Studying these materials is often challenging due to their complex physical properties and many widely used methods are limited in their portability, simplicity, and the possibility for system integration. Quartz Crystal Microbalance (QCM) is a tool which overcomes these limitations and can be used to gain insights into complex physical phenomena. Using piezoelectricity, QCM gives us information about the environment it has been submerged in through parameters such as resonance frequency and dissipation. This immensely versatile tool has been chosen to tackle the problems explored in this project. Two unsolved problems in the area of viscoelastic materials were chosen as subjects of this investigation.

The first explored subject is bacterial detection, which is challenging at Point-Of-Care (POC) with traditional methods due to the requirement of, slow detection times, high cost, and a need for laboratory infrustructure. QCM gives the potential to overcome these issues and in addition allow for portable online integration. POC bacterial diagnostics is necessary due to the increasing of Antimicrobial Resistance (AMR) with non-specific use of antibiotics. QCM has shown potential for clinical bacterial detection in recent studies. Recently, the Anharmonic Detection Technique (ADT) was presented as promoising but is yet to be validated for diagnosis of infections in complex biological samples and Antibiotic Susceptibility Testing (AST). In this study the ADT methodology was investigated and refined, leading to improved bacterial binding to QCM surface and enhanced transduction. For the first time ADT was used in a bacterial infection setting, by detecting the infectious E. coli O111 strain in biological urine, mimicking a urinary tract infection. Detection was achieved at a sensitivity of 106 cells/ml within a 50-minute test time. Additionally, initial AST measurements were demonstrated with penicillin antibiotic.

The second problem is from the area of charge storage which key for renewable energy. QCM was used to study novel amine based electrolytes for aluminium batteries. Aluminium as a battery material overcomes several issues faced with lithium, most importantly availability and recyclability. The development of suitable electrolytes is a key hindrance for aluminium ion-batteries. The electrolytes studied in this project, although exhibiting promising electrochemical properties, are experimentally challenging due to their viscoelastic properties and complex viscosity behaviour during redox cycles. It was shown, how through QCM study, it was possible to extract crucial insight about the nature of these electrolytes and assess their potential for battery application. While Electrochemical QCM (EQCM) is an established method for electrolyte studies, the unsolved problem exists of in-situ mass transport measurement for electrolytes exhibiting significant viscosity changes throughout the redox cycle, such as the electrolytes encountered in this research. In this work a new methodology is presented to overcome this issue, by using a model combining resonance frequency and dissipation data. It was shown that it can be used for in-situ measurements and has the potential to be applied for different electrolytes and electroplating applications.