Use of piezoelectric-excited millimeter-sized cantilever sensors to measure albumin interaction with self-assembled monolayers of alkanethiols having different functional headgroups

In this paper, we describe a new modality of measuring human serum albumin (HSA) adsorption continuously on CH3-, COOH-, and OH-terminated self-assembled monolayers (SAMs) of C-11-alkanethiols and the direct quantification of the adsorbed amount. A gold-coated piezoelectric-excited millimeter-sized cantilever (PEMC) sensor of 6-mm(2) sensing area was fabricated, where resonant frequency decreases upon mass increase. The resonant frequency in air of the detection peak was 45.5 +/- 0.01 kHz. SAMs of C-11-thiols (in absolute ethanol) with different end groups was prepared on the PEMC sensor and then exposed to buffer solution containing HSA at 10 mu g/ mL. The resonant frequency decreased exponentially and reached a steady-state value within 30 min. The decrease in resonant frequency indicates that the mass of the sensor increased due to HSA adsorption onto the SAM layer. The frequency change obtained for the HSA adsorption on CH3-, COOH-, and OH-terminated SAM were 520.8 +/- 8.6 (n = 3), 290.4 +/- 6.1 (n = 2), and 210.6 +/- 8.1 Hz (n = 3), respectively. These results confirm prior conclusions that albumin adsorption decreased in the order, CH3 > COOH > OH. Observed binding rate constants were 0.163 +/- 0.003, 0.248 +/- 0.006, and 0.381 +/- 0.001 min(-1), for methyl, carboxylic, and hydroxyl end groups, respectively. The significance of the results reported here is that both the formation of self-assembled monolayers and adsorption of serum protein onto the formed layer can be measured continuously, and quantification of the adsorbed amount can be determined directly.