Smartphone-based quartz crystal microbalance for detection of Salmonella Typhimurium

Quartz crystal microbalances (QCM) have been demonstrated as highly sensitive technologies for biosensing applications over the past few decades, making them valuable tools for detecting foodborne pathogens and mycotoxins in food safety. In recent years, biosensing trends have emphasized portability and simplicity, while also maintaining high sensitivity, thanks to advancements in measurement technology. Smartphone-based biosensors have especially garnered attention from researchers in various fields. In this study, a mass-based QCM and the potential for a smartphone-based QCM system for the detection of Salmonella Typhimurium are explored. To establish a fundamental understanding of a quartz crystal’s behavior, a theoretical framework was initially examined by comparing frequency shifts in response to changes in viscosity. The Butterworth Van-Dyke model of the quartz crystal and the experiments showed an average frequency shift difference of 3.7 %. In contrast, the variance between the Kanazawa equation and the experimental results was 4.4 %. Second, a benchtop QCM biosensor system equipped with a peristaltic pump was employed as a reference method for Salmonella Typhimurium detection. Polyclonal antibodies specific to Salmonella were immobilized on a 5 MHz quartz crystal using a self-assembled method, allowing for Salmonella detection on the crystal surface. Specificity tests demonstrated minimal cross-reactivity with non-Salmonella strains such as Escherichia coli O157, Listeria monocytogenes, and Staphylococcus aureus. Additionally, gold nanoparticles were used for mass enhancement to amplify the frequency signal via a biotin-avidin reaction, resulting in a signal improvement. The benchtop system achieved a detection limit ranging from 10³-10⁵CFU/mL. Lastly, a smartphone-based QCM device, integrated with a fluorescence imaging system, was developed for Salmonella Typhimurium detection. Fluorescein isothiocyanate-labeled antibodies were employed as secondary antibodies to amplify the signal and visualize the cells. The frequency shifts after introducing fluorescein isothiocyanate-labeled antibodies indicated the limit of detection of 10³-10⁴ CFU/mL. Similarly, specificity tests suggested no or low cross-reactivity with non-Salmonella strains such as E. coli O157, L. monocytogenes, and S. aureus. Food tests showed the real-world potential for detecting Salmonella Typhimurium. Reusability tests conducted on the single quartz crystal, using regeneration buffers, demonstrated its capability to be reused three times.