H
2
Gold surfaces were cleaned in a piranha solution (1:3 30%
O
2
/concentrated H
2
SO
4
) for 2 min before coating. The cleaned
gold surfaces were immersed into 2-propene-1-thiol (0.30 mM in
ethanol) for 24 h, in order to introduce thiol groups onto gold surface
of QCMelectrode. The electrode was then washed with ethanol
and deionized water for 10 min to remove the excess of thiols. Thus,
a stable self-assembled monolayer of thiol was formed on the gold
surface. Fig. 1a and b shows the Atomic Force Microscopy (AFM)
images of the gold and thiol-modified gold surface, in micrometer
scale, respectively. Standard silicon cantilevers (Quesant) with
a force constant ∼40 N/m, resonant frequency ∼137 kHz, and radius
of curvature