Clay Composite Modified Electrodes II: Voltammetric Method for Simultaneous Detection of Dopamine and Serotonin
Authors: Agyeman Augustine Ofori , Obata Chinye Frances , Tran Minh Tri 
Published: February 2020

Dopamine (DA) and serotonin (SN) are important catecholamine neurotransmitters in the central nervous system that play key roles in muscle stimulation and transmission of signals.  DA release in the brain plays a key role in the regulations of pleasure and pain while SN helps to relay messages from one area of the brain to another.  Abnormal DA levels are associated with serious diseases such as Schizophrenia and Parkinson’s.  Imbalance in SN levels affects the muscles and cardiovascular system.  Thus developing methods for detection of DA and SN in the presence of interfering species is an important area of research.  Electrochemical methods for DA and SN detection have been developed since there is increasing demand for more reliable and rapid methods.

Excess levels of interfering species such as ascorbic acid (AA) and uric acid (UA) co-exist in biological samples.  These pose problems for DA and SN detection during electrochemical measurements since they all oxidize around the same potential region.  Several types of electrode modifications have been reported for selective detection of DA and SN, but each one has its own advantages and limitations.  The use of clay-composite modified electrodes have not been exploited.  

This work aims to employ clay composite modified glassy carbon electrodes for selective detection of DA and SN.  The goal is to use clay composites to exclude AA and UA while enhancing DA and SN detection.  Clay film serves as the polymeric membrane as clays are able to form membrane-like films.  Clays are naturally occurring and much more stable than synthetic membranes.  Clay film on an electrode surface provides important functions such as charge-exclusion and catalysis of electrochemical reactions.

The clay composite was prepared by incorporating methylene blue and xanthate into clay interlayers by simply mixing them with clay suspension and sonicating.  Several voltammetric experiments were undertaken to investigate the behavior of DA, SN, AA, and UA at the bare electrode (BE), clay modified electrode (CME), glycerol-clay modified electrode (GCME), glycerol-methylene blue-clay modified electrode (GMBCME), glycerol-xanthate-clay modified electrode (GXTCME), and glycerol-methylene blue-xanthate-clay modified electrode (GMBXTCME).  Voltammetric techniques used were Cyclic Voltammetry (CV), Square-Wave Voltammetry (SWV), and Differential-Pulse Voltammetry (DPV).  Experiments were performed at the physiological pH of 7.4.  Glycerol was added to prevent cracks in films.    

At GCME, calibration curves by CV show linear range of 0.0010 – 0.25 and 0.0010 – 0.10 mM for DA and SN, respectively.  The DA and SN oxidation peak potentials are around 0.26 and 0.27 V versus Ag/AgCl, respectively.  For a mixture of DA and SN, the CV shows an oxidation peak around 0.43 V (due to SN) with a small hump around 0.28 V (due to DA).  The hump is more pronounced at the GCME than at the BE, indicating catalytic effect by the clay on DA.  For a mixture of DA, SN, AA, and UA, one broad oxidation peak is observed.  Oxidation currents for both AA and UA at GCME are much lower than those at BE.  This is evidenced by the fact that at pH of 7.4, AA and UA exist in their anionic form while DA and SN exist in the cationic form.  Most clay layer surfaces are negatively charged so readily attract the cationic DA/SN into the interlayers and repel the anionic AA/UA.  This enhances DA/SN detection while diminishing AA/UA detection.  Only clay does not efficiently exclude AA/UA, hence the incorporation of methylene blue and xanthate.  

At GMBCME, both DA and SN exhibit oxidation potential at 0.20 V.  However, a mixture of DA and SN exhibits peak potentials at 0.22 (DA) and 0.32 V (SN).  Hence, the GMBCME film is able to separate and distinguish between the two peaks.  It is also observed that both DA and SN peak potentials shift slightly negative compared to the BE.  The DA peak currents increased at GMBCME compared to BE, while those of SN decreased.  This confirms the catalytic effect of clay on DA stated above, but little or insignificant catalytic effect on SN.  Again, GMBCME is unable to efficiently exclude AA and UA.

With the addition of xanthate to form GMBXTCME, no oxidation peaks for AA and UA are seen.  This is true for all three voltammetric techniques.  This implies that the glycerol-methylene blue-xanthate-clay composite film efficiently discriminates against AA and UA detection.

GMBXTCME film stability was also investigated and was observed that all old films exhibited great stability, reproducible results, and efficiently exclude both AA and UA. 

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