Voltammetric Determination of Cocaine in Confiscated Samples Using a Carbon Paste Electrode Modified with Different [UO2(X-MeOsalen)(H2O)] · H2O Complexes

A fast and non-destructive voltammetric method to detect cocaine in confiscated samples based on carbon paste electrode modified with methoxy-substituted N,N'-ethylene-bis(salcylideneiminato)uranyl(VI)complexes, [UO2(X-MeOSalen)(H2O)].H2O, where X corresponds to the positions 3, 4 or 5 of the methoxy group on the aromatic ring, is described. The electrochemical behavior of the modified electrode and the electrochemical detection of cocaine were investigated using cyclic voltammetry. Using 0.1 mol·L−1 KCl as supporting-electrolyte, a concentration-dependent, well-defined peak current for cocaine at 0.62 V, with an amperometric sensitivity of 6.25 × 104 μA·mol·L−1 for cocaine concentrations ranging between 1.0 × 10−7 and 1.3 × 10−6 mol·L−1 was obtained. Chemical interference studies using lidocaine and procaine were performed. The position of the methoxy group affects the results, with the 3-methoxy derivative being the most sensitive.


Introduction
Cocaine (Figure 1) is the main alkaloid extracted from Erythroxylum coca. In the early twentieth century, it was used as a component of tonics and beverages. Today, however, it is almost exclusively associated with its misuse, which poses great health risks and can even lead to death. [1]. Cocaine acts as a local anesthetic and stimulates the central nervous system, leading to increased alertness and euphoria [2]. These effects stem from the ability of cocaine to block synaptic dopamine reuptake. However, this alkaloid also blocks norepinephrine and serotonin reuptake, so chronic cocaine use modifies these neurotransmitter systems [3]. Cocaine increases heart rate and blood pressure, culminating in heightened arousal, improved performance in tasks requiring attention and caution, and feelings of confidence and well-being [3]. Cocaine abuse damages the cardiovascular and neurological systems, as well as the liver [4]. High doses produce euphoria and chronic use leads psychological disorders such as paranoia, irritability, and violent behavior [3].
In forensic analysis, cocaine is detected by chromatographic techniques such as GC-MS, HPLC, and LC-MS, for which the literature reports good accuracy, scientific robustness, and low sensitivity [5]. Electrochemical techniques constitute an important analytical tool in chemical analyses and are often used in the pharmaceutical, industrial, and clinical fields, as well as forensic studies [6][7][8]. Oye et al. developed a new detection method based on the electrochemical behavior of cocaine in a non-aqueous medium. These authors employed a platinum disk electrode chemically modified with a cobalt hexacyanoferrate film to determine cocaine in confiscated samples [9]. Thus, this work presents better results because there is a significant increase in amperometric sensitivity.
Electrochemical methods have also been used to analyze other illicit drugs, such as hemp. Balbino et al. investigated the electrochemical behavior of delta 9 -tetrahydrocannabinol (Δ 9 -THC), the psychoactive substance in hemp, by cyclic and linear sweep voltammetry using a glassy carbon working electrode [10]. Electrochemical techniques offer several advantages: (i) they are sensitive, accurate, and precise; (ii) they enable one to work with a large range, relative instrumentation and materials; (iii) they can be applied to colored materials and samples containing dispersed solid particles [7,8]; (iv) they provide rapid response; (v) and they allow for analysis of the irreversible electrooxidation of the tertiary amine present in the cocaine molecule [11].
This process involves abstraction of a lone electron pair from the amine nitrogen, followed by rapid proton loss, to form a neutral radical that loses an electron and undergoes hydrolysis to a secondary amine and a ketone [12].
The choice of electrode modifier for a chemically modified electrode is crucial. Gold, platinum, glassy carbon and carbon paste film are some of the conventional electrode materials [13]. Chemically modified carbon paste electrodes have received considerable attention and have been increasingly used to measure a variety of organic compounds of biological and pharmaceutical interest [14]. These modified electrodes are inexpensive, easy to manufacture, display easily renewable surfaces, and low background current, are compatible with various types of modifiers, and allow for work in a wide potential range [14][15][16][17][18][19][20].
In  At this time, the interaction between the Schiff base complexes and cocaine has not been reported in the literature. There is evidence that the uranyl oxygen present in the Schiff base complex interacts with the carboxyl group of the cocaine molecule. Considering the Schiff base complexes can be used as electrode modifiers for the analysis of pharmaceutical substances, there are no reports on their application for forensic purposes. Thus, this research aimed to develop carbon paste electrodes chemically modified with Schiff base complexes to determine cocaine in samples seized by the police using voltammetric techniques.

Preparation of the Schiff Bases
The Schiff bases were prepared in a similar manner [42,43] using the following procedure: to a stirred solution of 10 mmol of the appropriate substituted salicylaldehyde (Sigma-Aldrich, St. Louis, MO, USA) in ethanol was added dropwise an ethanolic solution of 5 mmol ethylenediamine (Sigma-Aldrich). This mixture was then heated for 2 h. Afterwards the mixture was cooled to room temperature, then for 24 h at 5 °C. The solid was filtered and washed with cold ethanol (5 mL at 5 °C) and dried over silica. The ligands were used without further purification.

Preparation of [UO 2 (L)(H 2 O)]· H 2 O
To a boiling ethanol solution containing the appropriate ligand was added a solution of uranyl acetate dihydrate, UO 2 (C 2 H 3 O 2 ) 2 · 2H 2 O (0.706 g, 1.7 mmol, Merck, Darmstadt, Germany) in distilled water (40 mL) and four drops of glacial acetic acid. The orange-red solution was refluxed with stirring for 2 h. The resulting precipitate was collected by filtration, washed twice with distilled water (20 mL) and twice with ethanol (15 mL) and dried in a desiccator over silica at room temperature.

Cocaine Purification
All the cocaine samples and standard solutions were obtained via a partnership with the criminal experts of the laboratory of toxicological analysis-Institute of Criminalistics, Ribeirã o Preto, Sã o Paulo, Brazil. Solutions of cocaine from confiscated samples (seized cocaine samples) were prepared according with literature [9]. Aliquots of the water-soluble cocaine hydrochloride samples were pretreated with sodium bicarbonate solution, to remove HCl. [9]. Hydrochloride-free cocaine is insoluble in water, so this product was removed by filtration, rinsed with deionized water, dried, and dissolved in KCl 0.1 mol· L −1 supporting-electrolyte solution.

Preparation of the Supporting-Electrolyte and Analyte (Cocaine)
An aliquot of 1.8 g of KCl (PA Acros Organics, Geel, Belgium) was added to a 250 mL flask containing distilled water, to give the supporting electrolyte KCl 0.1 mol· L −1 . Procaine (Sigma-Aldrich) and lidocaine (Sigma-Aldrich) solutions (10 mL, 1 × 10 −2 mol· L −1 ) were prepared and acidified until pH 2 with of HCl (PA, Merck), to ensure the solubility.

Preparation of Chemically Modified Electrodes
The chemically modified electrodes were prepared using three different Schiff base complexes:

[UO 2 (3-MeOsalen)(H 2 O)]· H 2 O, [UO 2 (4-MeOsalen)(H 2 O)]· H 2 O, and [UO 2 (5-MeO-salen)(H 2 O)]· H 2 O.
The electrodes contained the following graphite mass/complex mass ratio: 75:25, 80:20, 85:15, 90:10, 95:5. First, 50 µL of nujol mineral oil was added to each composition, for agglutination. The mixture was homogenized under stirring with hexane and the solvent was removed in a rotary evaporator. Transducers were prepared using hollow cylindrical glass tubes with internal diameter of 1 mm containing a gold electric contact between the paste and the copper connection. The space filled by the copper wire acted as the external electrical contact. The paste was filled with about 1 cm of each investigated mixture. After the working electrode was ready, it was set up the in electrochemical cell. 0.1 mol· L −1 KCl supporting electrolyte (4 mL) was added to the cell and a nitrogen gas flow was applied for 15 minutes, to remove electroactive oxygen. The electrochemical cell consisted of carbon paste working electrode, an auxiliary platinum wire electrode, and the reference Ag/AgCl electrode.

Voltammetric Measurements
Cyclic voltammetry was conducted on an AutoLabIII potentiostat (Metrohm, Utrecht, Netherlands) coupled to a microcomputer. The potential scans were performed between −0.4 V and 1.2 V, at a speed of 100 mV· S −1 . Voltammograms at different concentrations of cocaine were recorded by the standard addition method. The same procedure was followed for procaine and lidocaine analysis during the study of interferents. The films have showed good stability during the first 20 cycles at 100 mV· s −1 . A decrease of current was not observed in these measurements.

Electrochemical Behavior of [UO 2 MeoSalen(H 2 O)]· H 2 O Carbon Paste
The uranyl(VI) complexes undergo reduction on electrode surface [44]. A redox reaction corresponds to the oxidation of the metal species chelated with the Schiff base, as follows:

Influence of the Composition of the Carbon Paste Containing [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O
Modifiers should have good sensitivity for the analyte. However, their proportion in the electrode may increase or decrease their affinity for the analyte. In this work we carried out voltammetry in KCl 0.1 mol· L −1 aqueous solution. As supporting-electrolyte, using as electrode modified with [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O, 75% graphite; 25% modifier, which was the ratio that gave the best electroanalytical response.

Influence of Cocaine Concentration
The working carbon paste electrode modified with [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O indicated a significantly increase of the amperometric sensitivity. Thus, we were able determine cocaine in µmol· L −1 using seized samples of cocaine. We obtained linear sweep (LSV) and cyclic (CV) by successively adding aliquots to the electrochemical cell, which analytical curve furnished the linearity over the studied concentration range from 1 × 10 −7 to 1.3 × 10 −6 mol· L −1 was good. The correlation coefficient (r) was 0.97 with a standard deviation (SD) of 0.005 μA. The equation for cocaine determination was: ipa = 0.24 μA + 4.6 × 10 4 μA/mol·L −1 [cocaine] Using the relation 3 SD/m and 10 SD/m (where m is the amperometric sensitivity of the curve), we obtained a limit of detection (LOD) of 0.326 µmol· L −1 and a limit of quantification (LOQ) of respectively, using cocaine concentration ratio remained as 1.1 × 10 −6 mol· L −1 . Therefore, the carbon paste electrode modified with [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O has high analytical sensitivity for cocaine. We determined the parameters Epa, Epc, ipa and ipc for the peaks observed between 0.5 and 0.6 V ( Table 1). The values confirm that increases linearly. Epa rose by 0.63 V for scan rates between 35 and 100 mV· s −1 . A scan rate of 100 mV· s −1 gave better ipa. The Epa/Epc and ip/v 1/2 ratios indicate the reaction mechanism. The lower the Epa/Epc ratio values suggested that a reversible process occurred, because Ep (Epa-Epc) was 56 mV.  (Figure 4) ratio revealed reduced peak current for oxidation upon successive addition of the standard cocaine solution (Table 2). For procaine, and lidocaine analyses, we employed a CV technique ( Figure 5).   We tested the proposed transducer in the presence of cocaine, lidocaine, and procaine. Using cyclic voltammetry, results indicated that lidocaine and procaine samples showed lower current peaks than cocaine in the same potential range. Therefore, the proposed system provides unequivocal cocaine identification in seized samples containing these interferents.

Conclusions
Voltammetric analysis of cocaine using carbon paste electrode modified with the Schiff base complex, [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O indicated the specific electrochemical activity of this electrode, without interference of lidocaine or procaine. The peak current at 0.62 V varies linearly with the cocaine concentration. This device has an amperometric sensitivity of 4.6 × 10 4 μA/mol·L −1 in the working range of 1.0 × 10 −7 to 1.3 × 10 −6 mol· L −1 cocaine, indicating that the transducer can be applied for quantitative analysis of cocaine. The results point to the importance of the position of the ethoxy group in the molecule of the modifier. To summarize, Schiff base complexes can be used to develop chemically modified electrodes for the detection of organic substances of forensic interest. The electrode modified with [UO 2 (3-MeOsalen)(H 2 O)]· H 2 O is potentially useful in the forensic field and can be employed in a more specific methodology for the preliminary testing of cocaine in drug samples.