Electroanalytical chemistry, as the name implies, involves the analysis of chemical
species through the use of electrochemical methods. Generally, we
monitor alterations in the concentration of a chemical species by
measuring changes in current in response to an applied voltage with
respect to time. According to Faraday's law, the charge is directly
proportional to the amount of species undergoing a loss (oxidation) or
gain (reduction) of electrons.
Q = n F e
Q is the total charge generated (coulombs)
n is the number of moles of a species undergoing oxidation or reduction
F is Faraday’s constant (96,487 C/mol)
e is the number of electrons per molecule lost or gained
Current is the change in charge as a function of time.
I = dQ / dt
Thus, the current response with respect to time (voltammogram) gives information about
changes in the concentration of the species of interest.
Constant potential amperometry,
high-speed chronoamperometry, fast cyclic voltammetry
(FCV) and differential pulse voltammetry (DPV) are the most common
voltammetric techniques used to detect monoamine neurotransmitters
(i.e., serotonin, dopamine, norepinephrine). Each method has its pros
In constant potential amperometry,
a uniform potential is applied and the change in current is monitored
as a function of time. The advantage of this technique is that the time
resolution is limited only by the data collection frequency of the
instrument. On the other hand, the primary disadvantage is the low
chemical selectivity. For example, all species with oxidation
potentials below the applied voltage will be oxidized and contribute to
the current (Figure 1).
a square wave pulsed voltammetric technique. Limited information about
the identity of the electrolyzed species can be obtained from the ratio
of the peak oxidation current versus the peak reduction current.
However, as with all pulsed techniques, chronoamperometry generates
high charging currents, which in this case, decay exponentially with
time. To measure the faradic current (the current that is proportional
to the concentration of the analyte), current in the last 70-80% of
each scan is integrated (when charging current has dissipated). In
chronoamperometry, it takes approximately one second to complete a scan
in the delayed pulse mode, the latter of which is necessary to prevent
fouling of the electrode by serotonin and its oxidation products. Since
the current is integrated over relatively longer time intervals,
chronoamperometry gives a good signal to noise ratio (Figure 2).
Fast cyclic voltammetry
is a linear sweep voltammetry technique in which the background
subtracted voltammogram gives additional information about the
electrolyzed species. The current response over a range of potentials
is measured, making it a better technique to discern additional current
contributions from other electroactive species. FCV is a relatively
fast technique with single scans typically recorded every 100 ms,
however, the fast scan rates decrease the signal to noise ratio (Figure
3 & Figure 4).
pulse voltammetry is a hybrid form of linear sweep and
pulsed voltammetries. It has found excellent usage in the
identification of electrolyzed species. However, multiple pulses in the
waveform make it a relatively slower technique with individual scans
taking minutes to complete (Figure
5 & Figure 6).
A summary of these is shown in Figure 7.
Suggested Further Reading
Electrochemical Methods: Fundamental and
Applications. Bard AJ and Faulkner LR, John Wiley and Sons, Inc. 2nd Ed
Principles of voltammetry and microelectrode surface states. Kawagoe
KT, Zimmerman JB, Wightman RM. J Neurosci Methods. 1993
Jul;48(3):225-40. Review (ABSTRACT).
Fast cyclic voltammetry: measuring transmitter release in 'real
time'. Stamford JA. J Neurosci Methods. 1990 Sep;34(1-3):67-72. Review
Andrews Group Related Papers
Chronoamperometry detects differential changes in synaptosomal
uptake in serotonin transporter knockout mice. X. A. Perez and A. M.
Andrews, Analytical Chemistry 77:818-826 (2005) (ABSTRACT or PDF).
Filtration compromises synaptosomal membranes during radiochemical
assay of serotonin uptake: Comparison with chronoamperometry is SERT
knockout mice. X. A. Perez, L. E. Bianco and A. M. Andrews, Journal of
Neuroscience Methods, 154:245-255 (2006) (ABSTRACT or PDF).
Locomotor hyperactivity and alterations in dopamine neurotransmission are
associated with overexpression of A53T mutant human
in mice. E. L. Unger, D. Eve, X. A. Perez, D. K. Reichenbach, Y. Xu, M.
K. Lee and A. M. Andrews, Neurobiology of Disease, 21:431-443 (2006) (ABSTRACT or PDF).
and dopamine uptake rates using high-speed chronoamperometry. X. A.
Perez, A. J. Bressler and A. M. Andrews, in Electrochemical Methods for
Neuroscience. A. C. Michael and L. A. Borland, eds., CRC Press LLC,
Boca Raton, FL: 103-124(2007). (LINK)
Part of figure 3 and 4 are taken from "http://www.qmw.ac.uk/~physiol/aboutFCV.html"
and part of Figure 6 is taken from Dr. Carew's The Joural of
Neuroscience (2002) paper (http://www.jneurosci.org/cgi/content/full/22/6/2299).
All the other text and figures are written and constructed by Yogesh
Singh with the help of Dr. Anne Andrews.