More About Electrochemical SPR
In electrochemical SPR, a change in the electrode
potential (DV) can shift the resonance angle (DqR), via different effects. Typically these
effects include changes in the molecular coverage
(Dc), index of refraction (Dn) and average thickness of the adsorbed layer.
An additional effect is the potential-induced change
in the surface charge density (Ds) of the
electrode. These effects can be written as
(1)
where a1, a1, a3 and a4 are constants
for a small potential change. The first term in
the above equation is due to potential-induced adsorption
or desorption. The second term describes changes
in the electronic states of the adsorbed layer,
which can arise from the Stark effect, structural
changes or chemical reactions of the adsorbates.
The third term is due to the change in the thickness
of the molecular layer due to structural change
and piezoelectric effect. The piezoelectric effect
is usually a small constant.
The last term is due to potential-induced
changes in the dielectric constant of the metal
film, em, via the electron density,
ne, according to the simple free electron model
of metals,
(2)
where e and me are the charge and mass of electron, respectively.
The change of em due to ne is
(3)
where Dne is related to the potential-induced excess surface charge,
Ds = CDV (C interfacial capacitance)
and given by
(4)
where d
m
is the thickness of the metal film. For a 50 nm silver film,
d
m
= 50 nm,
n
e
= 5.85 x 10
22
/cm
2
,
e
m
= -11.8 and
C
= 20
m
F/cm
2
, it leads to
De
m
~ -0.002 per volt, corresponding to an angular shift of ~ 0.02º per volt. Since this contribution is proportional to the interfacial capacitance, it decreases when the metal film is covered with molecular adsorbates.
BI-SPR Technology
•
Flow
Injection SPR •
Electrochemical
SPR •
Gas
Phase SPR
