A quartz crystal microbalance (QCM) measures mass per unit area by determining the change in frequency of a quartz crystal resonator caused by the addition of a small mass. The nominal resonance frequency of the shear mode of the quartz sensor is 5 MHz. A frequency change of 0.1 Hz corresponds to an addition of about 2 ng/cm 2 of mass to the quartz surface. Sauerbrey was the first to recognize the potential usefulness of the QCM technology and demonstrate its extreme sensitivity to mass changes . He developed the fundamental equation describing the QCM: Δ f o 2/(ρ qμ q 1/2 Δm=-56.6 Δm (1) Here Δf = change in frequency (Hz), f 0 = fundamental frequency of resonator (MHz), ρ q = quartz density (2.648 g cm -3), μ q = shear modulus of AT-cut quartz (2.947 x 10 11 g cm -1 s -2), and Δm = mass change/unit area (μg/cm 2). The AT-cut is used because its resonant frequency is almost temperature-independent.