Fast electrochemical treatment of graphite sheet flexible electrodes towards improved morphine detection in biological samples

Abstract

Flexible graphite paper is a promising material to construct low-cost and portable electrochemical devices. Herein, we propose a fast (10 s) electrochemical treatment (+5.0 V vs Ag|AgCl|KCl_(sat.)) in a basic medium (0.5 mol L⁻¹ NaOH solution) to enhance the electrochemical performance of graphite paper. The cyclic voltammetric response of a 1:1 mmol L⁻¹ [Fe(CN)₆]^3-/4− solution in 0.1 mol L⁻¹ KCl at 50 mV s⁻¹ demonstrated improved electrochemical reversibility (from ΔEp = 1032 ± 6 mV to 176 ± 10 mV), enhanced ratio of anodic (I_pa) and cathodic (I_pc) current intensities (from I_pa/I_pc = 0.30 ± 0.00 and to 0.99 ± 0.03), and a significant increase in current intensity (from 10.1 ± 0.1 µA and to 28.1 ± 0.6 μA) for untreated and treated surfaces, respectively. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of treated graphite sheet electrodes showed a high number of flakes and Raman spectroscopy revealed more structural defects (measured by D/G bands ratios), which can explain the better voltammetric profile of the redox probe. These results agreed with the estimation of electroactive area which increased 2.2-times after the treatment. Cyclic voltammetric experiments reveal that the surface treatment improved the current responses towards the oxidation of different molecules, such as morphine, ascorbic acid, dopamine, hydroxychloroquine, paracetamol and ciprofloxacin. To demonstrate the potential applicability of treated graphite sheets, morphine, an important opioid used as a medical agent and often reported as overdose cases, was determined in pharmaceutical (tablet), spiked synthetic urine and saliva samples, using differential-pulse voltammetry. Linear ranges of 0.1-20.0 and 20.0-80.0 µmol L⁻¹, with a limit of detection value of 0.08 µmol L⁻¹, were achieved for morphine. Appropriate apparent recovery values (∼92 %) were also obtained for sample analysis.