Dopamine is a neurotransmitter found in several deep structures of the brain. For example, in the ventral tegmental area (VTA) of the midbrain, in the substantia nigra pars compacta, and in the arcuate nucleus of the hypothalamus of the human brain . Dopamine plays a key role during addiction. One hypothesis  is that the control of dopamine in the brain may either advance or delay addiction behavior in adolescents exposed to drugs. We therefore hypothesized that a long-term sensor that can reliably monitor dopamine would allow us to investigate basic mechanisms of addiction in cell cultures and rodents. Since dopamine can be detected by oxidation at a potential of 0.3 V at the surface of a conductive working electrode,  we designed a catechol-chitosan modified sensor. A catechol-chitosan film was deposited on gold-coated surfaces. The electrodes were immersed in a preparation of chitosan solution followed by electrodeposition. Chitosan was prepared by dissolving 15 g/L of chitosan in deionized (DI) water. Using a 3-electrode setup, with platinum foil as a counter electrode, gold as a working electrode and Ag/AgCl as a reference electrode, a cathodic current of 6 A/m2
was applied for 45 seconds using a potentiostat (CH1660DD). To conclude the electrodeposition, an anodic potential of 0.6 V was applied on the chitosan electrodes immersed into 5 mM catechol solution for 180 seconds. The experiment was conducted on the modified gold electrodes (n=3) and bare gold electrodes (n=3), in phosphate-buffer saline (PBS), and in 5 concentrations of dopamine solution to determine basic characteristics of the sensor. Cyclic voltammetry was also performed with a three electrode setup, with voltages from -0.8 V to 0.7 V at a scan rate of 100 mV/s for a total of 5 cycles. The average LOD was measured, based on a blank phosphate-buffered solution, to be 18 mM. Dopamine solution was tested in 5 concentrations (10-5
M). The one-way ANOVA test revealed a statistical significance between the mean of the oxidative peaks for each concentration between the modified and the unmodified electrode (p< 0.05). The bare gold electrodes detected minimal current for dopamine in each concentration while the modified electrodes showed an average slope of 1.2 uA/M for the dopamine range tested. Interestingly, we observed that the lowest dopamine concentration tested still yields a reasonable signal, which is lower than the calculated LOD. The implementation of chitosan catechol modification on the gold electrodes amplifies the dopamine signal and the modified gold electrodes were able to detect dopamine. The principle of utilizing chitosan and catechol as coating agents in the detection of dopamine on gold electrodes improved the capacity of bare electrodes. For future research, a study determining the lifespan and sensibility in clinical samples of the modified sensor will be explored. It will allow for a better characterization of the sensor for its use in the brain.
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