Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 105

and the shift of the SPR angle (Liedberg, Nylander et al . 1995; Homola, Yee et al .
1999). SPR response values are usually expressed in resonance or refractive index
units. Detection of adsorption of small molecules (less than 200 Dalton) is diffi-
cult in SPR. On the other hand, much larger molecular masses are also difficult to
sense due to limitations in the penetration depth of the evanescent wave. However,
both situations are not relevant in most experimental cases and the linear relation-
ships hold. The reader is referred to a number of excellent review papers that discuss
SPR and its principles of operation (Liedberg, Nylander et al . 1995; Homola, Yee

et al . 1999).

4.9 Quartz Crystal Microbalance with Dissipation (QCM)

A QCM crystal consists of a thin quartz disc sandwiched between a pair of (gold)
electrodes. Due to the piezoelectric properties of quartz, it is possible to excite the
crystal to oscillation by applying an AC voltage across its electrodes.
The resonant frequency ( f) of the quartz crystal depends on the total oscillating mass,
including water coupled to the resonator. When a thin film is attached to the crystal
its frequency decreases. If the film is thin and rigid, negligible or minimum energy
dissipation occurs and the decrease in frequency is proportional to the mass of the film.
In this case the Sauerbrey relation can be applied (Sauerbrey 1959):

ρ q t q f
nf 0

ρ q v q f
2 nf 0

cf
n

m =− =− =− (4.5)

typically 17.7 ng Hz − 1 cm − 2 C = for a 5 MHz quartz crystal.

n = 1, 3, 5, 7 is the overtone number.

Because the change in frequency can be detected very accurately the QCM operates as
a very sensitive balance. The quartz crystal microbalance was first used to monitor thin
film deposition in vacuum or gas atmospheres. Later on, it was shown that QCM may
be used in the liquid phase thus dramatically increasing the number of applications. The
Sauerbrey relation was initially developed for adsorption from the gas phase but it is
now extended to liquid media where it holds in most cases. In order to describe soft
adlayers of polymer adsorbing from liquid media, the dissipation value D was introduced.
Rodahl et al . (Rodahl, Hook et al . 1995) extended the use of the QCM technique and
introduced the measurement of the dissipation factor simultaneously with the resonance
frequency by switching on and off the voltage applied onto the quartz. The measured
change in dissipation is originated by changes in the coupling between the oscillating
sensor and its surroundings and it is influenced by the layer's viscoelasticity and slip
of the adsorbed layer on the surface.

The dissipation factor D , is the inverse of the

so-called Q factor and it is defined by:

1
Q =

E disspated
2 πE stored

D = (4.6)