Adsorption and Selective Electrochemical Analysis of Epinephrine Using Clay-Modified Glassy Carbon Electrode
Authors: Agyeman Augustine Ofori 
Published: February 2020

Epinephrine (EP), also known as adrenaline, is one of the important neurotransmitters in the central nervous system (CNS) of mammals. EP is involved in a number of mental disorders, and is used as a drug for heart surgeries and serious allergic reactions. Analysis of EP is, therefore, essential for nerve physiology and for the development of EP medications. Most of the methods developed for EP analysis have low sensitivity and are time consuming. Current methods focus on electrochemical detection because of low cost, simplicity, and high sensitivity. None of these methods, however, focused on the interfacial behavior and adsorption properties of EP, which is of great importance to biochemical and pharmacological research.
The Standard Wyoming montmorillonite (SWy-2) clay was used for this project. SWy-2 clay demonstrates good electrochemical properties and high thermal stability. Strong interaction between EP and the clay is expected as SWy-2 is an Fe-bearing smectite clay mineral that contains about 2.3 mass percent lattice Fe. The clay suspension was pre-mixed with glycerol (5% by volume) to prevent formation of cracks in the films. Glycerol is known to be very stable, inert, and nonvolatile. Glycerol intercalates into clay resulting in extremely strong interactions. Hence, glycerol-clay modified glassy carbon electrode (GCME) would increase sensitivity, selectivity, adsorption, and electron transfer rate.
This project explored the various factors that affect EP adsorption at electrodes, using the GCME. The GCME films were first characterized using an FTIR microscope system, which confirmed high quality films.
The interfacial behavior of EP at the bare glassy carbon electrode (BE) and the GCME were then evaluated and compared. The oxidation of EP, as well as the accumulation, adsorption, and reduction of adrenochrome (EP’s oxidation product) were monitored through continuous repetitive cyclic voltammetric scanning. The potential was scanned between -0.6 V and 0.6 V (vs Ag/AgCl) at a scan rate of 50 mV/s, using 50.0 μM EP in 0.10 M PBS (pH 7.4). Three major peaks were monitored; an oxidation peak corresponding to the oxidation of EP to adrenochrome (peak “a” around 0.28 V), a reduction peak corresponding to the reduction of adrenochrome to leucichrome (peak “b” around -0.19 V), and another oxidation peak corresponding to the oxidation of leucochrome to EP (peak “c” around -0.14 V).
At the BE, peak “a” decreased with subsequent cycles while peak “b” increased. Peak “c” was initially absent, appeared after the second cycle, and increased with subsequent cycles. This indicates production, adsorption, and accumulation of adrenochrome at the electrode surface. All peaks levelled off at the seventh cycle, when system equilibrium and maximum adsorptive accumulation (MAA) were achieved.
At the GCME, on the other hand, peak “c” was present during the first scan. Also, peak “b” was initially higher and decreased with repetitive cycles. This indicates that the GCME effectively catalyzed the oxidation of EP and facilitated the accumulation and adsorption of adrenochrome. Progressive accumulation was also observed during repetitive scanning as peak “c” increased. System equilibrium and MAA were achieved at the sixth cycle.
System conditions, such as glycerol-clay composition, concentration range, scan rate range, and pH were optimized. System equilibration and MAA were achieved only at the physiological pH of 7.4. A linear response of EP was obtained in the range of 0.2 μM to 75.0 μM, with detection limit of 0.10 μM, which is lower than most published reports that use conventional electrodes. The system obeyed the Langmuir isotherm, with an adsorption coefficient of 41.3 L/g.
Further, the enhanced adsorption of adrenochrome at GCME, and hence increased reduction peak, was utilized for the selective determination of EP in the presence of serotonin (5-HT), excess ascorbic acid (AA), and excess uric acid (UA). All these substances coexist in biological fluids and have similar oxidation potentials. Various materials have been used to modify electrodes for selective detection or co-detection of EP, 5-HT, AA, and UA. However, these modified electrodes only make use of the oxidation potential while suppressing the adsorption process, which is usually challenging. This method is unique in that the GCME rather enhances preferential adsorption. This makes it easier to detect EP using the enhanced reduction peak, which unequivocally belongs to EP. Urine samples were also spiked with EP, 5-HT, AA, and UA, and then analyzed. The EP reduction peak remained unaltered (within an error of 5%) irrespective of the presence of urine or the spiked interfering species.
This work offers different conditions under which adsorption can be enhanced, prevented, or minimized, depending on the intended application. The system offers high sensitivity and reproducibility, with long-term stability.