Graphene oxide-based electrochemical activation of ethionamide towards enhanced biological activity

Abstract

The electrochemical behavior of ethionamide (ETO) was investigated on GO (∼500 nm) using the linear sweep voltammetric (LSV) technique at the sweep rate of 10 mV s^-1 in 1 M PBS buffer solution, and the characteristic anodic signal was examined at 0.240 V over the potential range of -0.4 to 1 V vs. SCE. However, linearity was observed with the increase in scan rate (2-300 mV s^-1) and concentration of ETO (1 μM to 100 mM), suggesting that the process involved diffusion-controlled electron transfer. The results also exhibited excellent current and potential stability, limit of detection (LOD 1.33) and limit of quantification (LOQ 4.4) at optimized experimental conditions. This electrochemical oxidation method was successfully applied in the complete oxidation of ETO to its oxidized form, which was further confirmed by high resolution mass spectroscopy (HRMS) and Fourier transform infrared (FTIR) spectroscopic measurements. Interestingly, the comparative biological evaluation of ETO and ETO-O (oxidised form) showed good enhancement in the activity of oxidised ETO against some Gram-negative pathogens, such as E. aerogenes, S. abony, S. boydii, and E. coli.

Fig. 1. (A) FTIR spectra of GO, (B) XRD pattern of GO, (C) Raman spectrum and (D) TEM image of GO.

Fig. 2. Superimposed high resolution XPS spectra of the as-synthesized GO (A) C 1s spectra related to the three kinds of carbon (C–C, C–O–C and CO), (B) O 1s ascribed to (O–H and C–O) bonding interactions.

Fig. 3. (A) Electrochemical behaviour of 5 mM ETO on (i) bare GC and (ii) GO/GC working electrode at a scan rate of 10 mV s⁻¹ in PBS buffer. (B) Electrochemical impedance data of (i) GO modified GC and (ii) bare GC in 0.1 M buffer at pH 9.2 containing 5 mM ETO in 0.01 to 500 Hz frequency range.

Fig. 4. Effect of accumulation time on current density.

Fig. 5. Influence of pH on the shape of the anodic peak at pH: (A) 11.2, (B) 9.0, (C) 7.0, (D) 5.0 and (E) 3.2.

Fig. 6. Superimposed LSV curves showing the effect of scan rate (i–ix): 2, 5, 10, 50, 100, 150, 200, 250 and 300 mV s⁻¹ on the anodic peak current for ETO (5 mM) oxidation in pH 9.2 buffer as the supporting electrolyte.

Fig. 7. Superimposed LSV curves of the GO electrode with various concentrations of ETO (i–xi): bare GC, 0.001, 0.01, 0.1, 0.5, 1, 5, 10, 25, 50 and 100 mM in phosphate buffer as the supporting electrolyte at 10 mV s⁻¹.

Fig. 8. Superimposed LSV curves recorded for the complete oxidation of (5 mM) ETO on GO electrode vs. SCE in 100 mL of pH 9.2 buffer at the scan rate of 10 mV s⁻¹.

Fig. 9. Structures of (1) the pristine drug ETO and the (2) ETO-oxidised form, 4-amino-5-hydroxymethyl-2-methylpyrimidine.