Figure 5.

Harmonic measurements with the impedance QCM up to 120 MHz. (a) A broadband impedance sweep is done between 1 MHz and 120 MHz (1601 points). Based on the fundamental resonance frequency at 10 MHz, subsequent odd harmonic resonances are present up to 110 MHz corresponding to the 11th harmonics; (b) modeled impedance sweep using harmonic resonators. The individual resonances correspond to the multiple LCR circuits (sketches of LCR₁, LCR₃, LCR₅, and LCR₇ as inset) with the overall slope of the impedance curve mainly defined by the electrode capacitance C₀; and (c) experimental results of the higher harmonics impedance QCM. The two step bio-chemical reaction (first step is NeutrAvidin adsorption to gold; second step is biotin-IgG binding to the avidin layer) is measured at the fundamental frequency and at higher harmonics (third, fifth, and seventh harmonics). For the harmonic frequencies, both the absolute frequency and the frequency shifts are multiple integer values of the corresponding fundamental values. The normalized frequency shifts are shown in the inset.