Voltammetric determination of thiomersal with a new modified electrode based on a carbon paste electrode decorated with La2O3

Accepted Manuscript Voltammetric determination of thiomersal with a new modified electrode based on a carbon paste electrode decorated with La2O3

Johisner Penagos-Llanos, Jorge A. Calderón, Edgar Nagles, John Hurtado PII: DOI: Reference:

S1572-6657(18)30850-6 https://doi.org/10.1016/j.jelechem.2018.12.040 JEAC 12809

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

20 November 2018 20 December 2018 21 December 2018

Please cite this article as: Johisner Penagos-Llanos, Jorge A. Calderón, Edgar Nagles, John Hurtado , Voltammetric determination of thiomersal with a new modified electrode based on a carbon paste electrode decorated with La2O3. Jeac (2018), https://doi.org/10.1016/ j.jelechem.2018.12.040

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ACCEPTED MANUSCRIPT Voltammetric determination of thiomersal with a new modified electrode based on a carbon paste electrode decorated with La2O3 Johisner Penagos-Llanos1, Jorge A. Calderón2*, Edgar Nagles1*, John Hurtado3 Facultad de Ciencias Naturales y Matemáticas, Universidad de Ibagué, Carrera 22 Calle 67, 730001, Ibagué. 2Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad de Antioquia –UdeA, Calle 70 No. 52-21, Medellín, Colombia. 3Departamento de Química, Universidad de los Andes, Carrera 1 No. 18A-12, 111711, Bogotá, Colombia.

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Abstract: A carbon-paste modified electrode decorated with La2O3 (LaOx/CPE) is reported for the first time for the voltammetric determination of thiomersal (THM) content. The LaOx/CPE surface was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The presence of La2O3 on the surface of the carbon paste reduces the charge transfer resistance and improves the sensitivity toward the oxidation of THM, with an oxidation current increase of almost 60% and allowsits determination by square-wave stripping voltammetry (SWSV). The detection limit (3σ/b) was 0.09 µmol L-1 and the relative standard deviation was 2.5% (n=7) with three different electrodes. Furthermore, the new sensor was applied in the detection of THM in vaccines and pharmaceutical dosages.

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Keywords: Lanthanum (III) oxide; Carbon paste; Thiomersal; Vaccines; Square wave stripping voltammetry. [email protected](E.

Nagles);

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Authors correspondence: E-mail address: [email protected](J. A. Calderon)

Introduction

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The compound thiomerosal is very commonly used in topical medications and as an antiseptic preservative in cosmetics and vaccines. Thimerosal (THM), also known as merthiolate, sodium thiosalicyst of ethylmercury, and thiomersal, has two different molecules in its chemical structure. These compounds are thiosalicylate and organic mercury [1,2]. The World Health Organization (WHO) is responsible for prequalifying the thiomersal content in products, and these identifications are sent to the United Nations Procurement Agencies [3, 4]. THM is an adjuvant that is widely used in the medicinal market, including as a preservative in vaccines [5] to prevent their decomposition by some bacteria and microorganisms [6, 7] .Some vaccines that contain THM are used against hepatitis B, influenza, Haemophilusinfluenzae type b (Hib), diphtheria, tetanus and whooping cough (DTP) infections [2, 3]. THM is composed of 49.6% ethylmercury; for this reason, vaccines that contain 0.01% THM as a preservative present 50.0 μg of THM per 0.50 mL dose, or approximately 25.0 μg of ethylmercury. The most-used topical vaccines in pediatricpatients are those that contain the highest

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ACCEPTED MANUSCRIPT dose of ethylmercury[4, 8, 9]. Furthermore, THM is widely used in many developing countries as a fungicide as part of the green revolution for the transformation of agriculture [10]. Frequent contact with THM causes exposure to its possible toxic effects in some parts of the body, such as the neonate and uterus, and causes transformations in the neurological development of a person's behavior. In addition, thimerosal has a half-life of approximately 227 to 540 daysin the brain and approximately 40 to 70 days in blood. [11-13]. The uncontrolled release of products such as preservatives in clinical, hospital and pharmaceutical centers has increased THM pollution in the environment [14, 15].

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To eliminatethe THM that is present in some pharmaceutical residues that are discarded to the

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environment, some methods of adsorption with active carbon and photochemical degradation with TiO2 have been used.[16-18]. In addition, advanced oxidation techniques with heterogeneous

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photocatalysis have been used successfully in the degradation of organometallic substances with pharmaceutical products containing mercury, where ZnO and TiO2 are the most used catalysts [19,

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20].

For the determination of THM, atomic fluorescence spectrometry techniques coupled with UV-Vis radiation with cold vapor generation has been used (UV-CV-AFS) [21, 22], as well asinductively

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coupled plasma optical emission spectroscopy (ICP-OES) [23] and photochemical oxidative decomposition on-line assisted by microwaves (CV-AFS) [24, 25]. On the other hand, highperformance liquid chromatography (HPLC) is the most widely used technique in the detection of

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organomercury species [26]. Although these techniques are sensitive, selective and reproducible, they have high instrumental and operational costs. Alternatively, electroanalytical techniques have been

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widely used to detect mercury in natural samples and wastewater using mainly carbon electrodes modified with gold nanoparticles [27], although they have not been used much in the detection of organomercury species. A few reports have denoted its use as an electrochemical steam generator on

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a composite formed by cysteine and carbon paste with fluorescence detector [28], by liquid chromatography with electrochemical detection [29], polarography [30], mercury film [31] and chitosan on screen-printed carbon electrodes [32]. These last two studies are among the few reports

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to detect THM through the reduction and oxidation of sodium thiosalicylate in THM with a detection limit below 1.0 μmol L-1. In addition, these methods proved to be selective, easily produced and low-

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cost methodologies.

Oxides belonging to rare earths such as lanthanum have begun to generate interest in the modification of electrodes with electroanalytical applications due to their properties as catalysts [33]. They have been used as an alternative cathode for molten carbonate fuel cells [34] and in the modification of electrodes used in the detection of dopamine, ascorbic acid, uric acid and nitrite on glassy carbon [35, 36], and mefenamic acid and dopamine on carbon paste [37, 38],showing a great activity toward the oxidation of these analytes. In addition, other lanthanum complexes such as hexacyanoferrate lanthanum and lanthanum 2,6-dichlorophenolindophenol were used in the detection of ascorbic acid on glassy carbon and screen-printed carbon electrodes [39, 40]. These reports justify the development of new sensitive, selective and easy-to-manufacture methodologies for detecting THM in biological samples such as vaccines and pharmaceutical

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ACCEPTED MANUSCRIPT dosages. In addition, the use of lanthanide complexes in the modification of electrodes in different types of analytes justifies their use in this work. Furthermore, lanthanide oxide with carbon paste has not yet been used in the detection of THM. The aim of this work is the development of a new sensitive, selective and reproducible methodology to detect THM in different types of samples using carbon paste with addition of lanthanide oxide. Instruments and materials

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General information

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Water obtained from a purifier system (Wasselab ASTM D1193) was used for the preparation of solutions. THM standard solutions at concentrations of 1.7 and 0.017 mmol L-1were prepared from pure reagent. The phosphate buffer solution (PBS, electrolyte support) was prepared from H3PO4/H2PO4- (Merck). Dopamine (DP), uric acid (UA) and ascorbic acid (AA) were purchased from Merck. Voltammetry measurements were performed with a DropSens μStat400 Potentiostat (Oviedo, Spain).Electrochemical impedance spectroscopy (EIS) was carried using a Versa STAT 3 potentiostat/galvanostat from Princeton Applied Research (Oak Ridge, TN, USA) and scanning electron microscopy (SEM) was performed using a JEOL model JSM 6490-LV (Tokyo, Japan) with a secondary electron detector. A system with three electrodes was used: a carbon powder as the working electrode (Sigma-Aldrich) supported inside a PVC cylinder with a copper wire as the electrical contact, a reference electrode of Ag/AgCl saturated with KCl (3 mol L-1) and a platinum wire as the auxiliary electrode. pH measurements were made with an Orion–430 digital pH/mV meter equipped with a combined pH glass electrode. Preparation of carbon paste coated with La2O3microcomposite (LaOx/CPE)

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The CPE without La2O3 was prepared with 50.0 mg of graphite powder and 20 μL of paraffin oil. The LaOX/CPE was prepared according to the following procedure: between 2.0 and 6.0 mg of La 2O3 was mixed uniformly with 50.0 mg of graphite powder and 20 μL of paraffin oil. Then, the composite was pressed into a PVC cavity until making contact with a copper wire as the electrical contact. The surfaces of the electrodes were polished with filter paper until the surfaces were smooth. It was possible to obtain a new surface with each electrode by removing the excess material from the surface and repolishing. The optimal amount of La2O3 was 5.0 mg with 50.0 mg of carbon powder. Development of measurements

In the electrochemical cell, 9.5 mL of ultrapure water with 0.5 mL of PBS (0.01 mol L-1) and 0.10 mL of THM (1.7mmol L-1) were combined. After a 3 s equilibration time, cyclic voltammograms (CV) were recorded while the potential was scanned from 0.0 to 1.2 V at 0.50 V s-1. Each cyclic voltammogram was repeated three times. For stripping square wave measurements, 9.5 mL of ultrapure water with 0.20 mL of PBS (0.01 mol L-1) and between 10.0 and 100 μL of AM an (0.15mmol L-1) were added into the electrochemical cell. After 60 s of accumulation at 0.0 V, the square wave voltammograms were recorded from 0.40 to 1.20 V with a modulation frequency of 50 Hz, a step potential of 10 mV and an amplitude potential of 100 mV. Each square wave voltammograms was repeated three times. Detection limits (DL) were calculated from the slope and random error in x and y from the calibration curve and linear regression. The standard addition method was used to eliminate matrix effects in the analysis of real samples and validation study. Stability and

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ACCEPTED MANUSCRIPT reproducibility was studied as a function of time using the same electrode and several electrodes for six days. The electrochemical characterization of the surface LaOx/CPE was assessed by CV at a scan rate of 0.1 V s-1, and electrochemical impedance spectroscopy (EIS) was performed using 10 mmol L1Fe(CN)63-/4- in 10 mmol L-1KCl as prove electrolyte. EIS measurements were performed at the open circuit potential (OCP) with a perturbation amplitude of 10 mV and a frequency range between 10.0 kHz and 0.1 Hz.The topography of the surface was analyzed byscanning electron microscopy (SEM). Sample preparation

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The samples of thiomersal as pharmaceutical doses (Meltiolate) were obtained from a local supermarket, and 100.0-200.0 µL were added to the cell. Vaccines used for the prevention of clostridial diseases and Pasteurellosis in cattle were purchased at a veterinary store in the city of Ibagué, which contained the following ingredients: aluminum hydroxide, physiological serum and purified water. The vaccines were stored in arefrigerator at 4 °C.

Assessment of CPE decorated with La2O3 by CV, EIS and SEM

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3. Results and Discussion.

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With the objective of evaluating the changes produced in the surface of the CPE and in its electroactive properties by the addition of La2O3, the CPE and LaOx/CPE electrodes were tested by CV and EIS techniques, using the redox couple reaction (ferro/ferricyanide) of 10.0 mmol L -1 Fe(CN)63/4- in KCl. The changes in the surface topography of the CPE electrodes were analyzed by SEM. The results are shown in Fig. 1. CV curves (Fig. 1a) show that the anodic and cathodic peak currents for Fe(CN)63-/4- using the CPE with La2O3 (red curve) increased approximately 50.0% with respect to the pure CPE electrode. Peak currents increased from 142.8 µA to 307.7 µA for the anodic peak and from -90.5 µA to -270.4 μA for cathodic peak. Moreover, the ΔE peaks (the difference between anodic peak potential and cathodic peak potential) was reduced from 0.61 V to 0.35 V when the CPE electrode was modified with La2O3. These results indicate that the redox reaction in the modified CPE electrode is kinetic and thermodynamically more favorable. Additionally,the ratio of peak currents Iap/Icp was approximately 1.0 for LaOX/CPE, indicating a superior reversibility of the system when the electrode is modified with La2O3. A similar result was reported for electrodes of lanthanum hydroxide nanowires on glassy carbon for detection of mefenamic acid [38]. In that electrode type, the electrocatalytic effect was attributed to a greater surface area. Fig. 1b shows the Nyquist plot of the electrochemical impedance measurements of 10.0 mmol L Fe(CN)6-3/-4 in 10.0 mol L−1KCl for the CPE (black curve) and the LaOX/CPE (red curve). Additionally, the Randles equivalent circuits used to fit the impedance results are presented in the inset of Fig. 1b. Because of the heterogeneityof the electrode surfaces,constant phase element (CPE) was used instead of pure capacitance in the fitting. Rs is the electrolyte resistance, and CPEdl is the constant phase element that takes into account the effective capacitance of the electrical double layer (Cdleff.). Rct is the charge transfer resistance of the redox process, and ZD is the finite-length diffusion impedance. A similar physical representation of the electrochemical impedance response for the CPE is commonly reported in the literature [41-44]. −1

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Figure 1 (a) Cyclic voltammograms and (b) Nyquist plot of 10.0 mmol L−1Fe(CN)6-3/-4 in 10.0 mol L−1 KCl using the CPE (black curves) and LaOx/CPE (red curves), symbol (experimental values) lines (fitted values), (c, d) Images of the microstructures of the CPE and LaOX/CPE and (e) EDS image of the microstructure of the LaOX/CPE. Conditions: scan rate 0.1 V s-1.

The impedance of a simple Faradic reaction without diffusion can be calculated using equation (1):

𝑍() = 𝑅𝑠 +

𝑅𝑠 1+(𝑗)𝑎 𝑄 𝑅𝑐𝑡

(1)

Where Q and a are the CPE parameters,  is the angular frequency, and Q is the differential capacity of the surface in the case where a = 1. The equivalent capacitance of the electrical double layer (Cdleq.) was calculated from the CPEdl according to equation (2) [45], which considersa surface-timeconstant distribution of the global impedance of the electrode.

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𝑅𝑠 𝑅𝑐𝑡

𝑅𝑠 +𝑅𝑐𝑡

)(1−𝑎)/𝑎

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The effective capacitance (Cdleff.) of the double layer is obtained after dividing Cdleq. by the electroactive surface area (A). Table 1 shows the values of the parameters of the Randles electrical circuits (inset Fig. 1b) used to fit the experimental electrochemical impedance results.

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The impedance diagrams of the CPE and LaOX/CPE exhibited an open capacitive loop with appreciable distortion at high frequencies, which can be associated with the coupling of at least two time constants corresponding to the electric relaxation of the double layer capacitance (C dleff.) in parallel with the charge transfer resistance (Rct) and the open finite-length diffusion impedance (ZD). The latter is associated with the diffusion of the electroactive species into a definite diffusion layer [46, 47]. The Rct value of the LaOX/CPEis one order of magnitude lower than the Rct of the unmodified CPE. This result indicates that the modification of the CPE with the LaOX oxide was effective and increased both the electrical conductivity and the electrochemical response of the electrode. The open finite-length diffusion impedance (ZD) observed for the CPE and LaOX/CPE can be expressed by equation (3):

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ZD() = R D (B 2 j)−1/2

(3)

where RD is the diffusion resistance into the diffusion layer (ohm s -1/2), and B2 denotes the

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time constant of the diffusion process. B can be related to the characteristic diffusion length (L), which should be taken as half of the thickness of the diffusion layer [48]. L

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Considering the diffusivity of FeCN63-/4- to be 7.6x10-6 cm2 s-1 and the values of B from Table 1, the thicknesses of the diffusion layers (δ) of the LaOX/CPE and CPE are 141 µm and 131 µm, respectively. This means that the modification of the electrode with lanthanum oxide slightly equation (5):

Rct δIlim nFACD

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RD =

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increases the diffusion layer. The diffusion resistance of the porous film can be calculated by using

(5)

where n is the number of electrons exchanged in the redox process, F is the Faraday constant, A is the electroactive area of the electrode, D and C are the diffusivity and concentration of the electroactive species (FeCN63-/4-) respectively, and Ilim. is the limit current of the diffusion process, which at the open circuit potential can be replaced by the current exchange (I0) [49], where I0 = 7.14x106

mA [41]. As seen in Table 1, the diffusion resistance of the LaOX/CPEis almost equal for both

electrodes. The electroactive areas of the electrodes (A) can be calculated using equation (5). The calculated values of the A for LaOX/CPE and CPE were 1.6x10-5cm2 and 2.8x10-4 cm2, respectively. Table 1. Values of the parameters used to fit the impedance results using the electrical circuit shown in the inset of Fig. 1b

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Rs (ohm) Rct (ohm) CPEdl (S sa) α Cdleq. (µF) RD (ohm s-1/2) B (sec1/2) Goodness of fit

Electrode CPE LaOX/CPE 350 349 5.14x104 2.82x103 3.8x10-6 6.13x10-6 0.88 0.80 1.8 1.3 4 2.38x10 2.39x104 4.78 5.14 -4 5.51x10 5.28x10-4

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According to the EIS results, the modification of the CPE by the incorporation of lanthanum

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oxide improve the conductivity of the CPE. As a consequence of this, a superior electrochemical response and sensitivity of the LaOX/CPE can be expected. The EIS results are in accordance with the previously discussed cyclic voltammetry results, where the charge transfer resistance of the redox

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couple in the LaOX/CPE is significantly lower compared to that observed for the pure CPE, indicating that the faradaic processes are facilitated by the surface modification of the electrode.

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The morphologies of the CPE and LaOX/CPE surfaces were studied with SEM and EDS. The results are shown in Fig. 1c-e. For the CPE without La2O3, a porous surface was observed, which is typical of carbon surfaces (Fig. 1c) [44]. For the LaOX/CPE, the SEM image clearly shows that La2O3microparticles are evenly distributed in the form of sheets (Fig. 1d). The EDS image of the CPE (Fig 1S) and LaOX/CPE microstructure (Fig. 1e) shows the presence of C, La and O. Moreover, the percentage weight ratio of O/La given by the EDS analysis was 0.19. This value is close to 0.17, which is the percentage ratio of O/La in the La2O3. These results confirm the presence of La2O3 in the microstructure of the modified electrode.

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3.2 Electrochemical properties of THM by cyclic voltammetry andsquare-wave stripping voltammetry (SWSV)using CPEand LaOX/CPE

Figure 2 (a) Cyclic voltammograms of THM (16.0 µmol L-1) and (b) square wave voltammograms of THM (3.0 µmol L-1) using the CPE (black curves) and LaOx/CPE (red curves). Conditions: CV: pH 3.1 (PBS), scan rate 0.1 V s-1; SWV: pH 3.1 (PBS), Tacc 60 s, Eacc 0.0 V, frequency 10 Hz, pulse amplitude 0.05 V

The thiosalicylate group in the chemical structure of THM is responsible for electroactivity by the oxidation of a proton (H+) [32]. This redox activity was evaluated using CPE and LaOX/CPE by CV and SWSV. The results are summarized in Fig. 2. It was clearly observed that in both techniques,

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Effect of the pH and scan rate (ʋ) on anodic peak current of THM.

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anodic peak currentsfor THM were observed between 0.80-0.90 V, and an increase of 60.0% of the anodic peak current using LaOX/CPE was observed compared with CPE. Moreover, SWSV was at a concentration ten times lower and the anodic peak currents were higher compared with CV. Therefore, SWSV was chosen for further study. It is possible that the LaOX/CPE,with a considerable increase in the surface area increases the concentration of THM on the surface of the modified electrode. Using screen-printed carbon modified with chitosan, anodic peak currents for THM were observed at less positive potential values [32]. Possibly, the affinity of THM with La2O3 is stronger and requires more energy to oxidize compared to chitosan.

Figure3 (a) Effect of pH on the anodic peak currents, (b) effect of pH on the potential peak and (c) effect of

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scan rate on the anodic peak currents for THM (16.0 µmol L-1) using the LaOX/CPE by cyclic voltammetry.

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A pH study was performed on a solution of 16.0 µmol L-1 THM using PBS with pH values between 2.1 and 6.5 by CV between 0.0 - 1.2 V s-1. The results are shown in Fig. 3. It was clearly observed that at pH 3.2,the anodic peak current for THM was higher (Fig. 3a). In addition, the potentialvalues were displaced to values of less positive potential with the increase of pH (Fig. 3b) with a regression equations of ipa (µA)= 1.028±0.02 +0.033±0.03pH (correlation coefficient r = 0.919). This slope value is almost half the theoretical value 0.059 of the Nernst equation. This result indicates that H+ protons are involved in the oxidation of THM using LaO X/CPE. using a screen-printed electrode modified with chitosan [32] reported a slope value for the regression equations of Ep vs pH close to the value reported in this work, confirming a reaction between protons and electrons equal to 1H+:2e-. On the other hand, by varying the scan rate, anodic peak currents were proportionally increased between 0.02 and 0.12 V s-1 with a regression equation of ipa (µA) = -2.34±0.02 +117.67±0.05Vs-1 (correlation coefficient r = 0.988) and Log Ip (µA) = -1.88±0.01 + 1.13±0.02LogV s-1 This result indicates that the process is controlled by adsorption. There are very few studies reported that can be compared with these results. This one was with mercury film combined with silver by reduction, where the process is also controlled by adsorption [31]. 3.4

Square wave stripping voltammetry variables for THM detection The optimal parameters for square-wave stripping voltammetry were frequency (Hz)

between 10-60, step amplitude between 0.005-0.020 V and pulse amplitude between 0.02-0.10 V. The results showed that anodic peak currents for THM were increased when the frequency was 50 Hz,

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Analytical procedure Net anodic currents in the oxidation of THM using LaOX/CPE were developed by square

wave stripping voltammetry under optimal conditions: pH = 3.1 (0.20mL of 0.01 mol L–1 PBS), EACC (accumulation potential) 0.1 V by tACC (accumulation time) 60 s.At more and less positive potentials

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and for timeslonger than 60 s, thenet anodic currents decreased,possibly due to the saturation of the electrode surface. Fig. 4a shows the voltammograms and Fig. 4b shows the calibration plot with the

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regression equation (insert) for THM between 1.0 – 10.0 µmol L-1. The DLs obtained for this new

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method, based on the values of the slopes and the random errors in x and y, was 0.09 µmol L-1. The reproducibility, defined as the relative standard deviation (RSD), was 2.5% (n=7) with the same electrode and 3.5% with six different electrodes for THM 5.0μmol L–1. Use of screen-printed carbon

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modified with chitosan and mercury film electrodes for detection of THM reported detection limits between 0.009-0.038 μmol L-1 [31, 32]. The results reported with this new modified electrode are equal.

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Moreover, the new method isenvironmentally friendly.

Figure 4 (a) Square wave voltammograms and (b) calibration plot for THM (1.0 to 10.0 µmol L-1) using LaOx/CPE.

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Conditions: pH 3.0; tACC60 s; EACC 0.10 V; frequency 50 Hz; step amplitude 0.01 V; and potential amplitude 0.10 V.

The stability of LaOX/CPE was evaluated using cyclic voltammetry after 50 cycles at 0.10 V sbetween 0.0 – 1.2 V with THM 16.0 µmol L-1. The results showed that anodic peak currents for THM decayed only approximately 10.0%, corresponding to a decrease of 0.3% by each cycle. These results indicate that the performance of the modified electrode allows it to be used for a long period of time without an appreciable reduction of its activity.Moreover, the durability was evaluated with one LaOX/CPE over five days for THM 3.0 µmol L-1. Anodic peak currents for THM were measured every day and an average of 5.8 µA was observed,with a relative standard deviation (RSD) of 1.8%. These results indicate that the same LaOX/CPE can be used for a long period of time for detection of THM. In addition, LaOX/CPE can be easily manufactured. 1

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Urine chemistry control (Liquichek BIO-RAD) was spiked with acertain amount of THM and used to validate the accuracy of LaOx/CPE in the detection of THM. The results are summarized in Table 2. The relative errorwas less than 15.0%. These results showed good accuracy at low concentrations. On the other hand, the values of slope obtained based on the calibration plots were 0.32 and 0.40, indicating that the urine chemistry control matrix did not affect the stability of the LaOX/CPE. Fig. 5ashows the voltammograms and calibrate curve (insert) for sample 1 in Table 2. Table 2 Determination of THM in urine chemistry control spiked with THM

Found (µmol L-1)

Samples

THM

THM

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% Relative Error

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2 10.0 10.6 6.00 *Fig. 5b shows the square wave voltammograms and calibration plot (insert)

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The interference of some substances that may be present in biological and pharmaceutical samples that may cause interference with charge and interaction with the THM, such as metal ions, dopamine (DP), uric acid (UA), ascorbic acid (AA), caffeine (CF) and synthetic colors such as amaranth (AM), allura red (AR), sunset yellow (SY) and tartrazine (TZ) were evaluated in concentrations 100 times higher than THM. Metal ions were obtained from an ICP multielement standard solution IX (Merck) containing As, Be, Cd, Cr(VI), Hg, Ni, Pb, Se and Tl (100 mg L-1). The results showed that only DP, UA and AA presented activity with LaOX/CPE, but did not present a relevant interference because their activity was observed at less positive potential values. Fig. 5b shows the square wave voltammograms for THM in the presence of DP, UA, AA, CF, AM, SY, TZ, AR and metal ions. Therefore, this new method can be used in the presence of these substances.

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The detection and quantitation of THM in vaccines and pharmaceutical dosage samples was performed to confirm the utility, versatility and selectivity of the new method.The samples did not need a previous treatment and were analyzed three times by the standard addition method. In total, THM was detected in concentrations close to 1 mg L -1. The manufacturers do not indicate the amount of THM, but the detected values are similar to previous reports. The results are summarized in Table 3. The position of the signals for THM in the real samples was observed at almost the same potential value as when ultra-pure water was used, indicating that the matrix of the real samples did not affect the stability of the sensor.Fig. 5c shows the square wave voltammograms and calibration plot (insert) for the pharmaceutical dosage sample in Table 3. The values obtained are very close to published values for THM in vaccines using-modified electrodes [31, 32] and by photochemical vapor generation coupled to ICP OES [23]. Table 3 Results of the analysis of THM in real samples. Real THM data Sample (mg L-1) Vaccine Vaccine Pharmaceutical dosage

1.1±0.2

Found TMS (mg L-1) 1.50 1.33 1.28

Hg2+ (mg L-1) 0.73 0.66 0.64

RSD (%) 0.15 0.08 0.10

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*Pharmaceutical dosage 1.1±0.2 1.36 0.68 *Fig. 5c shows the square wave voltammograms and calibration plot (insert)

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metal ions (100 times higher) and (c) square wave voltammograms and calibration plot (insert) for sample 4* in Table 3 using the LaOx/CPE. Conditions: as in Fig. 4.

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In summary, we reported a new method for detection of THM in biological samples and pharmaceutical dosages. The new method was validated and applied in the analysis of real samples with results consistent with previously reported literature. We demonstrate that a simple device fabricated with La2O3 can be used for the electroanalytical sensing of submicromolar levels of thiomersal. The incorporation of La2O3 in carbon improves the sensing properties of the device by a simple method such as square wave voltammetry. In addition, lanthanum complexes are attractive for sensing devices due to their abundance, easy preparation, high thermal stability, and low cost.

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Acknowledgments:The authors thank the Universidad de Ibagué (18-541-INT) and the Department of Chemistry and the School of Science of the Universidad de los Andes for the financial support.

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ACCEPTED MANUSCRIPT TABLES CAPTIONS Table 1. Values of the parameters used to fit the impedance results using the electrical circuit shown in the inset of Fig. 1b Table 2 Determination of THM in urine chemistry control spiked with THM Table 3 Results of the analysis of THM in real samples.

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FIGURE CAPTIONS

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Figure 1 (a) Cyclic voltammograms and (b) Nyquist plot of 10.0 mmol L−1 Fe(CN)6-3/-4 in 10.0 mol L−1 KCl using the CPE (black curves) and LaOx/CPE (red curves), symbol (experimental values) lines (fitted values) (c, d). Images of the microstructures of the CPE and LaOX/CPE and (e) EDS image of the microstructure of the LaOX/CPE. Conditions: scan rate 0.1 V s-1.

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Figure 2 (a) Cyclic voltammograms of THM (16.0 µmol L-1) and (b) square wave voltammograms of THM (3.0 µmol L-1) using the CPE (black curves) and LaOx/CPE (red curves). Conditions: CV: pH 3.1 (PBS), scan rate 0.1 V s-1; SWV: pH 3.1 (PBS), Tacc 60 s, Eacc 0.0 V, frequency 10 Hz, pulse amplitude 0.05 V

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Figure 3 (a) Effect of pH on the anodic peak currents, (b) effect of pH on the potential peak and (c) effect of

scan rate on the anodic peak currents for THM (16.0 µmol L-1) using the LaOX/CPE by cyclic voltammetry.

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Figure 4 (a) Square wave voltammograms and (b) calibration plot for THM (1.0 to 10.0 µmol L-1) using LaOx/CPE. Conditions: pH 3.0; tACC60 s; EACC 0.10 V; frequency 50 Hz; step amplitude 0.01 V; and potential

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amplitude 0.10 V.

Figure 5 (a) Square wave voltammograms and calibration plot (insert) for sample 1 (table 2), (b) square wave voltammograms for THM at 10.0 µmol L-1 in the presence of DP, UA, AA, CF, AM, SY, TZ, AR and

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metal ions (100 times higher) and (c) square wave voltammograms and calibration plot (insert) for

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sample 4* in Table 3 using the LaOx/CPE. Conditions: as in Fig. 4.

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Graphical abstract

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Highlights

New application for lanthanum oxide

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A new selective, sensitive and stable sensor to detect thimerosal in biological and pharmaceutical samples.

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Easy to manufacture and reproducible.

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The samples do not need prior treatment before being analyzed

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