INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 74)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
APPLICATIONS 6-2
Q
= integrated charge during the reduction in Coulombs,
n
= number of electrons transferred to induce deposition (i.e. n =1 for Ag deposition).
A
r
= active deposition area of the working (liquid contact) electrode in cm 2 ,
F
= Faraday’s constant = 9.648×10 4 Coulomb/mole,
A plot of ∆f vs Q will deliver the apparent mass per electron of the deposited species, when n is
taken into account. This is often used to elucidate the mass changes that accompany redox
processes, and hence is very useful for characterizing the mechanisms of electron-transfer
reactions.
However, before any calculations can be performed based on Equation 11, the EQCM must be
calibrated in order to properly derive (1) the proportionality constant, C
f
, of the Sauerbrey
equation in solution and (2) to account for the effective area of the working electrode.
This is generally done using a well-behaved electrochemical reaction – typically
electrodeposition of silver, copper or lead on a gold or platinum electrode. Several calibration
procedures are described in the electrochemistry literature 46 47 48 .
6.1.2 POLYMER MODIFIED ELECTRODES
The EQCM has been extensively used to study polymer-modified electrodes, particularly as a
gravimetric tool to follow redox processes 49 50 . However, for the linear frequency-to-mass
relationship (described by Equation 11) to hold true, the polymer over layer must exhibit no
changes in rigidity during the electrochemical process. Otherwise, the viscoelastic changes will
also contribute to the frequency change, leading to an erroneous interpretation of the mass
changes 51 52 . As a consequence, it is important to determine if viscoelastic properties of the
polymer film influence the frequency measurement during polymer film experiments.
A straightforward method to detect changes in film viscoelastic properties of redox films is to
simultaneously monitor the series resonance resistance, R, of the quartz oscillator during the
electrochemical experiment 53 54 . Some theoretical models 55 56 , based on the simultaneous
measurement of ∆f and ∆R, have been discussed in the literature for the extension of EQCM
gravimetric measurements to lossy films. The viscoelastic analysis of polymeric thin films in
EQCM systems, is complex because the shear wave exists simultaneously in the quartz crystal,
the viscoelastic film and the adjacent solution, so reflection of the shear wave must be taken into
account. However, solution of this problem would be worthwhile, especially if the material
properties of the film could be derived. This would allow correlation of the electrochemical
behavior of the film with its material properties 57 .
The unique property of the QCM technique is its ability to determine the mass of very thin layers
while simultaneously giving information about their viscoelastic properties. The ability to
measure both mass and structural changes means it is possible to detect phase-transitions, cross-
linking and swelling in polymeric thin films 58 .
6.2 CHEMICAL AND BIOLOGICAL SENSORS
A QCM will response to anything that has mass. Thus, it is imperative for the QCM user to
develop a “condition” where the QCM will only response to the analyte of interest, i.e build a
unique sensitivity into the sensor crystal. This usually involves a chemically or biologically
sensitive layer applied to the surface of the crystal 13 .
In recent years, QCM applications have seen a dramatic increase in field of biochemical analysis.
QCM devices are routinely used as biochemical and immunological probes 59 , as well as for the