Self assembled monolayer and protein adsorption studies on micromachined quartz crystal balances

We report the design and fabrication of a micromachined quartz crystal balance (QCM) array for self assembled monolayers (SAMs) and protein adsorption studies. The microQCM was fabricated using recently developed inductively coupled plasma etching process for quartz to realize resonators with 60 μm thickness and electrode diameters of 0.5 mm. The reduction in the thickness and lateral pixel size has resulted in a sensitivity improvement by factor of 1700 over a commercially available macro-sized QCM. Adsorption of hexadecanethiol on the gold electrode of the QCM in ethanol at a concentration of 1 mM was recorded in real time and a frequency shift of 3650 Hz was obtained. Modeling the SAMs layer as an ideal, rigid mass layer the expected frequency shift was calculated to be 1031 Hz. This was followed by a study of the adsorption of human serum albumin (HSA) protein on the SAMs layer. For 1.5×10^-10 moles/ml concentration of protein solution in phosphate buffer solution (PBS) we obtained a frequency change of 13.28 kHz. Modeling the protein layer as a viscoelastic layer in a viscous Newtonian fluid, for saturation protein surface coverage, the frequency change was calculated to be 17.27 kHz whereas the experimentally obtained frequency change was 51.82 kHz. In both rigid and viscoelastic film adsorption experiments, we find the microQCM to exhibit three times greater sensitivity than the predicted value when operated at the third overtone. These results show that the micromachined QCM in array format is a very sensitive gravimetric sensor capable of mass resolutions into the femtograms range.