The authors have declared that no competing interests exist.
In 1924 Heyrovsky found that current at a mercury electrode was not directly proportional to the applied voltage, but there was presence of an extra-current determined by the oxidisable chemicals present in the solution. Such extra current, that is proportional to the concentration of the compound(s) oxidized and/or reduced, is called polarographic current when obtained at a mercury electrode, is called voltammetric current when obtained at all other types of electrodes
Different types of voltammetric techniques are available the most common of which are chrono-amperometry linear voltammetry, cyclic voltammetry, and pulse voltammetry
These methodologies are mainly based on the application of a “dynamic” oxidation or oxido-reduction (ox – red) potential and the resulting analysis of electrons “freed” by the chemical(s) under analysis (see
Voltammetric measurements are taken with a three-electrode potentiostat system made of a silver/silver chloride (Ag/AgCl) reference electrode, a copper or silver wire auxiliary (counter) electrode both approximately 100 μm in diameter and a working electrode (see
Different types of voltammetric electrodes have been developed since 1969, the most performing type appear to be the carbon based electrodes and in particular the carbon fiber - micro electrode (µCFE) (see
The association of voltammetry with these electrodes become an electrochemical methodology allowing continuous, in real time and in situ detection of oxidizable chemicals.
The turning point of the use of these micro-sensors has been the application of a variety of electrical pre-treatments that are applied to the sensors before use. This has indeed improved drastically sensitivity and selectivity for analysis of electro-active chemicals and this in particular when the electrochemical methods of normal pulse as well as differential pulse voltammetry are employed
DPV combines aspects of chronoamperometry and linear sweep voltammetry and exhibits high selectivity and sensitivity. Small voltage pulses of a constant amplitude (20-50 mV) are superimposed 3-5 times per second upon a linear voltage ramp (see Figure 4). The current is sampled immediately before (iA) a pulse and subtracted from the current at the end of the pulse (iB), then the difference iB - iA is expressed in terms of potential. This consents to DPV to combines the main advantage of chronoamperometry (suppression of charging current) with the resolution of sweep voltammetry, as it performs a local differentiation of the voltammogram obtained by linear voltammetry. The overlap between two oxidisable compounds is eliminated providing that they oxidize at sufficiently distinct potentials (at least 50-100 mV between both). Thus, the oxidation of a compound produces a sharp peak rather than the broad peak or plateau of linear sweep voltammetry, resulting in higher resolution
The association of DPV with pre-treated µCFE appears to be the best methodology for rapid in situ analysis of electro-active compounds. No other combination of electrode and voltammetric method allows the same sensitivity with high resolution between oxidizable chemicals and in particular:
The undersized probe allows sampling a region of approximately 10-6 mm3 volume i.e. there is high anatomical resolution of the site of measurement within discrete brain areas of rodents, with minimal damage to the nervous tissue.
The method allows rapid, repeated measurements with accurate time resolution in vivo, in situ in real time without requiring perfusion, sample preparation, chromatographic separation or radio-labeled transmitter supplies. This is the fundamental difference between voltammetry and the perfusion techniques such as micro-dialysis
The association DPV - μCFE can be performed in vivo in conscious freely moving animals. This solves the problems associated with anesthetics and allows correlations between neuronal activity and behavior
Pharmacological experiments performed with DPV - μCFE have indeed demonstrated that the following chemicals can be selectively monitored in vivo in brain areas:
Ascorbate, noradrenaline and/or dopamine and the metabolites DOPAC, homovanillic acid, 3-methoxytyramine
Uric acid,
5-OH-indoles (i.e.serotonin and its metabolite 5-OH-indolacetic acid)
In addition to the detection of monoamine release and their metabolism, in particular those of dopamine and serotonin, other electro-active chemicals have been successively detected with the association DPV - μCFE in vitro as well as in vivo as shown in
Variations of the pulse polarography technique have also been proposed. In particularDifferential Square Pulse Conditioning Voltammetry has been introduced since it is permitting longer “life” to the micro sensor when used in vivo
Finally, a very recent achievement of the association DPV - μCFE is the evidence of the feasibility of monitoring Lactic Acid both in vitro and in vivo in the frontal cortex of rodents at the selective oxidation potential +1.5 Volts
It appears therefore evident that this electrochemical methodology is still evolving in detecting a variety of chemicals, at the same time as presenting a range of advantages over methods based on the preparation of samples and/or separation steps. Indeed, it allows rapid, direct, concomitant finding of different chemicals based upon specific oxidative (or red-ox) potentials either in vitro, ex vivo and in vivo conditions
Such a flexibility of application is illustrated by the feasibility to couple this methodology with behavioral observations
A particular example of such flexibility of utilization is the feasibility to apply the methodology in physiologic as well as pathological conditions, thus proposing selective mechanisms of actions of the neurotransmitters that can be monitored in vivo, in situ and in real time. This, taken together with the recent improvement in the methodology permitting DPVoltammetric analysis in telemetric – wireless conditions, thus allowing electrochemical studies in absolutely freely moving conditions