Simultaneous Detection of Morphine and Codeine in the Presence of Ascorbic Acid and Uric Acid and in Human Plasma at Nafion Single-Walled Carbon Nanotube Thin-Film Electrode

Abstract

In clinical settings, the dosing and differential diagnosis of the poisoning of morphine (MO) and codeine (CO) is challenging due to interindividual variations in metabolism. However, direct electrochemical detection of these analytes from biological matrices is inherently challenging due to interference from large concentrations of anions, such as ascorbic acid (AA) and uric acid (UA), as well as fouling of the electrode by proteins. In this work, a disposable Nafion-coated single-walled carbon nanotube network (SWCNT) electrode was developed. We show facile electron transfer and efficient charge separation between the interfering anions and positively charged MO and CO, as well as significantly reduced matrix effect in human plasma. The Nafion coating alters the voltammetric response of MO and CO, enabling simultaneous detection. With this SWCNT/Nafion electrode, two linear ranges of 0.05-1 and 1-10 μM were found for MO and one linear range of 0.1-50 μM for CO. Moreover, the selective and simultaneous detection of MO and CO was achieved in large excess of AA and UA, as well as, for the first time, in unprocessed human plasma. The favorable properties of this electrode enabled measurements in plasma with only mild dilution and without the precipitation of proteins.

Figure 1

Characterization of the Nafion/SWCNT electrode. (A) SEM cross-sectional image of Nafion/SWCNT electrode, (B) HRTEM image of typical iron nanoparticles, (C) fast Fourier transform of the HRTEM image of typical iron nanoparticles, (D) TEM image of the SWCNT network, and (E) Raman spectra of the SWCNT, Nafion/SWCNT, and Nafion-coated glass slide. The inset in (E) shows the radial breathing mode (RBM) region for all samples.

Figure 2

Cyclic voltammograms for SWCNT and SWCNT + Nafion electrodes in (A) 1 mM Ru(NH₃)₆^2+/3+, (B) 1 mM Fe(CN)₆^4–/3–, and (C) 1 mM IrCl₆^2–/3– in 1 M KCl, as well as (D) differential pulse voltammogram of 0.5 mM AA and UA in phosphate-buffered saline (PBS). Cyclic voltammetry (CV) scan rate of 100 mV/s for all measurements.

Figure 3

Simultaneous detection of MO and CO in PBS. Differential pulse voltammograms for (A) SWCNT and (B) SWCNT + Nafion electrodes and assignment of the oxidation peaks in 50 μM MO and CO, (C) individual and simultaneous detection of 10 μM MO and CO in the presence and absence of 500 μM AA and UA with SWCNT + Nafion electrodes, and (D) 10 μM MO and CO with accumulation times of (a) 0 s, (b) 150 s, and (c) 300 s.

Figure 4

Linear ranges of MO and CO in PBS. Differential pulse voltammograms of (A) 10 μM CO and 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 10, 25, and 50 μM MO, and (C) 10 μM MO and 0, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 10, 25, and 50 μM CO. Both (A) and (C) are in the presence of 0.5 mM AA and UA. The average currents with standard deviations (n = 3) with linear fits for (B) MO and (D) CO.

Figure 5

Differential pulse voltammetry measurements in human plasma. (A) DPV of increasing concentrations of MO and CO in plasma and (B) linear ranges of the measurements with standard deviations as error bars (n = 5). Accumulation time 5 min.