This study presents a novel approach to probe in situ, real-time changes in mixed-species biofilms via quartz crystal microbalance with dissipation (QCM-D). The viscoelasticity of a model wastewater biofilm was monitored by QCM-D, and quantitative changes in both measured frequency, Delta f, and dissipation, Delta D, reflected biofilm structural changes in the presence of silver nanoparticles (Ag-NPs). Complimentary results from confocal laser scanning microscopy confirmed biofilm attachment and the presence of live cells. A step-input experiment with increasing concentrations of Ag-NPs (127, 666, 1160, 1632 ppb) was performed. A Voigt model analysis of QCM-D results shows differing biofilm viscoelastic behavior as a function of Ag-NP concentration. The step input response of Delta D/Delta f ratio showed that Ag-NPs cause a structural pattern of firmness at low concentrations (up to 666 ppb), and less rigidity of attached mass at higher concentrations (up to 1632 ppb) for the mixed species biofilm studied.
