Simultaneous Detection of Neurotransmitters Using Carbon Nanomaterials
Authors: Mazurkiewicz Wojciech , Małolepszy Artur , Witkowska Nery Emilia 
Published: July 2022

Glassy carbon electrodes cleaned and electrochemically pretreated in PBS were compared to electrodes modified with carbon black, graphene oxide, and carbon dots in terms of possibility of detection of neurotransmiters. We provide systematic studies regarding the detection of five neurobiologically relevant species: dopamine, serotonin, epinephrine, norepinephrine, and 3,4-dihydroxyphenylacetic acid on those four types of electrodes. We also evaluated the possibility of simultaneous detection of dopamine and serotonin in the presence of high levels of uric and ascorbic acids.
Dopamine and serotonin are easily detected at carbon surfaces but unfortunately at a similar potential. Polymerization of dopamine, resulting in sensitivity loss is another problem, when measurements are carried out in cell culture medium or even in model solutions at physiological pH. Norepinephrine and epinephrine are both products of dopamine metabolism. Their structure includes an o-hydroquinone, which upon oxidation and the transfer of two electrons transforms into o-quinone. Additional chemical reactions may occur resulting in e.g. cyclization of the chain containing the epinephrine’s amino group and subsequent additional quinone-hydroquinone reactions of this product. DOPAC is the product of dopamine deamination. In neutral pH, its oxidation is partially irreversible. As in case of dopamine, side reactions can lead to film formation on the electrode surface. It is considered an interferent in determining dopamine levels.
Detection in the presence of acids has proven possible on all three types of electrode modifications, although with different resolution. We evaluated sensitivity in terms of current density per active area of the sensor. In this way graphene oxide exhibited the highest sensitivity towards all tested neurotransmitters, whereas chemically functionalized carbon nanodots significantly increased oxidation peaks’ resolution. A considerable shift of signals towards more negative potentials for epinephrine, norepinephrine, and 3,4-dihydroxyphenylacetic acid was observed for all modifications as compared with glassy carbon electrodes. Analytical parameters obtained for each type of modification are resumed in Tab. 1 (dopamine), Tab. 2 (serotonin) and Tab.3 (epinephrine and norepinephrine).
As shown in this and other works electrode pretreatment has a huge impact on the recorded signal. Thus it is sometimes hard to compare materials tested by different research groups. The same procedure of electrochemical pretreatment was in this work applied to coated and uncoated glassy carbon electrodes. The pretreatment allowed for partial cleaning of polymerized dopamine.
Authors would like to thank The National Centre for Research and Development, Poland, for funding under grants LIDER/38/0138/L-9/17/NCBR/2018 and LIDER/33/0117/L-9/17/NCBR/2018. EWN would like to also thank the Foundation for Polish Science (FNP), from which she was supported through START programme and National Science Centre Poland for the MINIATURA grant NCN 2017/01/X/ST4/00463

Figure 1