The peak potentials difference is around 60 mV. As discussed in Chapter 2, this shows
that, at this potential scan rate (500 mV/s), ferrocene methanol and ferrocene dimethanol
conversions remain reversible. The currents therefore remain basically diffusion controlled, and
the faradaic current (Ip, difference in intensity between the oxidation peak and the residual
current) is given by the following Randles-Sevcik equation (6-1) previously introduced in
Chapter 2,
I, = (2.69*"10' )n3/2ACD1/2 1/2 (6-1)
where C is the concentration of the electroactive species, A is the surface of the working
electrode in cm2, D is the diffusion coefficient of the electroactive species in cm2/S, v is the
potential scan rate in V/s, and n is the number of electrons transferred in the redox process. A, n,
and v are kept constant during the cyclic voltammetry scans, and therefore Ip is directly
proportional to D1/2C. As mentioned in the introduction of this chapter, when the nanocapsules
are added, the encapsulated molecules become either electrochemically inactive (electron
transfer cannot occur within distances larger than 5-10 nm and the particle shell is always larger)
or have an apparent diffusion coefficient sufficiently low so that the signal intensity arising from
the encapsulated probes can be neglected. The diffusion coefficient D for spherical particles is
given by the following Stokes-Einstein equation (6-2) previously introduced in Chapter 2,
k,T
D = (6-2)
67rqyR
where kB is the Boltzmann constant (kB =1.38 x 10-23 m2 kg s-2 K- ), Tis the temperature in K, rl
is the solution viscosity in P, and R is the particle radius in m. When this equation is applied to
our nanocapsules of 40-100 nm radius, the diffusion coefficient is calculated in the range (2-
6)x10-s cm2/S, which is a factor 102-103 Smaller than a typical diffusion coefficient for a single
molecule like ferrocene methanol or dimethanol (in the range 10- -10-6 CM2/S). The signal from