Resonating Platforms for Nanomaterial Analysis

Our goal is to develop quartz crystal microbalance (QCM) instrumentation and advanced oscillators with enhanced sensitivity to address characterization of nanoparticles, including their interactions with the environment. Examples include: nanoparticle-based water remediation; nanoparticle purity and quality control; and nanoparticle uptake in biological systems. Our focus is on smaller sample volumes, which will decrease the cost of materials analysis and enable more rapid collection of statistically significant data populations.

Quartz crystal microbalances (QCM) are highly sensitive acoustic devices capable of monitoring sub-picogram mass changes in rigid coatings and thin films. Devices are typically comprised of a thin, piezoelectric, AT-cut, quartz crystal sandwiched between two metal excitation electrodes. When an AC voltage is applied to the electrodes, the quartz crystal oscillates at a characteristic frequency based on the crystal geometry, referred to as the resonant frequency. Any perturbation of the crystal surface (e.g., adsorbed mass) alters this characteristic frequency. To determine mass changes in the material coating, we monitored the resonance frequency of the quartz crystal using an impedance analyzer. During coating deposition, the resonance frequency decreases, with the shift in frequency directly proportional to the change in mass. On heating, the coating mass decreases due to oxidation of the material, resulting in an increase in resonance frequency.

We have developed an elevated temperature QCM technique that interrogates samples on the order of 1 microgram or less. This elevated temperature platform allows for thermogravimetric measurements to be made on samples 1000 times smaller above than commercially available thermogravimetric analysis (TGA), resulting in cheaper, rapid collection of statistically significant populations of data for monitoring nanoparticle purity. QCM measurements on nanoparticles in aqueous environments are being used to determine the feasibility of nanoparticle use in water remediation. 

• Elevated temperature QCM is used to characterize materials and mixtures of materials to do microscale TGA-like measurements. Carbon nanotube samples have been characterized using 100 times less sample than conventional TGA measurements by heating the QCM externally. Gold nanoparticle coatings have also been characterized using elevated temperature QCM as an alternative to NMR-based experiments.

• With integrated heaters on the QCM surface, the sample can be heated while monitoring the resonance frequency. The first generation of these heaters can reach temperatures in excess of 250°C. Further development of the QCM heaters will allow for real time TGA-like monitoring of carbon nanotube samples using microgram quantities.

• Silver nanoparticles used in ceramic membranes are monitored on the QCM to determine the leaching rate of the nanoparticles from ceramic surfaces. Water conditions (e.g., pH, ionic strength) are investigated to understand the impact on nanoparticle leaching from membrane surfaces. 
• Metallic coatings on zero-valent iron particles increase degradation of water pollutants such as halogenated disinfection by-products. Zero-valent iron nanoparticles are synthesized and studied on the QCM for remediating halogenated acetamides.