Electrochemical Determination of Morphine in Urine Samples by Tailoring FeWO4/CPE Sensor

Abstract

Morphine (MORPH) is natural alkaloid and mainly used as a pain reliever. Its monitoring in human body fluids is crucial for modern medicine. In this paper, we have developed an electrochemical sensor for submicromolar detection of MORPH. The sensor is based on modified carbon paste electrode (CPE) by investigating the Fe_xW_1-xO₄ ratio in iron tungstate (FeWO₄), as well as the ratio of this material in CPE. For the first time, the effect of the iron-tungsten ratio in terms of achieving the best possible electrochemical characteristics for the detection of an important molecule for humans was examined. Morphological and electrochemical characteristics of materials were studied. The best results were obtained using Fe₁W₃ and 7.5% of modifier in CPE. For MORPH detection, square wave voltammetry (SWV) was optimized. Under the optimized conditions, Fe₁W₃@CPE resulted in limit of detection (LOD) of the method of 0.58 µM and limit of quantification (LOQ) of 1.94 µM. The linear operating range between 5 and 85 µM of MORPH in the Britton-Robinson buffer solution (BRBS) at pH 8 as supporting electrolyte was obtained. The Fe₁W₃@CPE sensor resulted in good selectivity and excellent repeatability with relative standard deviation (RSD) and was applied in real-world samples of human urine. Application for direct MORPH detection, without tedious sample pretreatment procedures, suggests that developed electrochemical sensor has appeared to be a suitable competitor for efficient, precise, and accurate monitoring of the MORPH in biological fluids.

Keywords: carbon paste electrode; electroanalysis; iron tungstate; morphine; real-world sample; square wave voltammetry.

Figure 1

XRD patterns of Fe₁W₃ (blue line), Fe₁W₁ (green line), and Fe₃W₁ (red line). Corresponding pattern for FeWO₄ (JCPDS file no. 74-1100) is given as reference.

Figure 2

FE-SEM micrograph of (A) Fe₃W₁; (B) Fe₁W₁, and (C) Fe₃W₃. Corresponding elemental analysis is given in (D–F).

Figure 3

(A) CVs of 5 mM K₃[Fe(CN)]₆/K₄[Fe(CN)]₆ (1:1) at unmodified CPE, Fe₁W₁–, Fe₁W₃– and Fe₃W₁–modified CPE. Supporting electrolyte 0.1 M KCl (scan rate 50 mV/s); (B) EIS at CPE and modified-CPE of 5 mM K₃[Fe(CN)]₆/K₄[Fe(CN)]₆ (1:1) in 0.1 M KCl; (C) CV voltammograms of CPE, Fe₁W₁/CPE, Fe₁W₃/CPE, and Fe₃W₁/CPE 30 mM MORPH and (D) CVs with different content of the Fe₁W₃ in CPE at 0.1 mM MORPH in BRBS pH 8 (scan rate 50 mV/s).

Scheme 1

Process of oxidation of morphine to pseudomorphine.

Figure 4

(A) CV voltammograms for 0.1 mM MORPH solution in BRBS at pH 2–10 (scan rate 50 mV/s). (B) Evolution of peak current and peak potential with pH. (C) Linear fit of E_p vs. pH. (D) CV voltammograms for 0.1 mM solution of MORPH on BRBS at pH 8, scan rate 50–200 mV/s and (E) I_p vs. log v.

Figure 5

(A) SWV voltammograms of 0.1 mM MORPH in BRBS at pH 8, for pulse amplitude in range 10–100 mV and (B) SWV voltammograms for 0.1 mM MORPH in BRBS at pH 8, frequency range 10–100 mV.

Figure 6

(A) Sensor’s SWV voltammograms toward the addition of increasing concentrations of MORPH in the range 5–85 µM in BRBS at pH 8, under previously optimized parameters; (B) Calibration curve plotted using extracted data from the corresponding SWVs.

Figure 7

SWV voltammograms of MORPH at Fe₁W₃@CPE in absence (green line) and presence (red line) of (A) ascorbic acid (AA); (B) uric acid (UA); (C) citric acid (CA); (D) dopamine (DOP); (E) glucose (GLU); and (F) peak current signal (%) before and after addition of interferents.

Figure 8

HPLC chromatograms obtained for morphine standard solutions and tested samples.