Acrylonitrile-butadiene-styrene (ABS) composite

Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-018-3897-z

ORIGINAL PAPER

Acrylonitrile-butadiene-styrene (ABS) composite electrode for the simultaneous determination of vitamins B2 and B6 in pharmaceutical samples Grasielli C. de Oliveira 1 & Lucas C. Pereira 1 & Ana L. Silva 1 & Felipe S. Semaan 1 & Marilza Castilho 2 & Eduardo A. Ponzio 1 Received: 13 July 2017 / Revised: 6 January 2018 / Accepted: 7 January 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract A lab-made affordable composite electrode based on acrylonitrile-butadiene-styrene (ABS) and graphite was developed and applied for the simultaneous determination of vitamins B2 (riboflavin, VB2) and B6 (pyridoxine, VB6) in pharmaceutical samples. Different ABS-graphite composite electrodes (AGCE) were prepared in proportions ranging from 40 to 80% (graphite, m/m) and characterized by a many complimentary techniques such as thermogravimetry (TG), Raman spectroscopy, Fourier transform-infrared (FTIR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Differential pulse voltammetry (DPV) was employed for analytical purposes, being several parameters investigated to determine the optimum experimental conditions. Best performance was obtained using as electrolyte 0.1 mol L−1 acetate buffer solution (pH 4.0), with a pulse amplitude of 100 mV, a scan increment of 5 mV, a modulation of time of 0.05 s, and a time interval of 0.5 s, resulting in a scanning rate of 10 mV s−1. The use of a 70% AGCE electrode under optimized conditions provided as linear responses for VB2 and VB6 intervals from 0.25 to 1.2 μmol L−1 (r = 0.997), and from 25 to 454 μmol L−1 (r = 0.989), respectively, with limits of detection of 0.15 μmol L−1 for VB2 and 10 μmol L−1 for VB6. The AGCE presented satisfactory results for the simultaneous determination of VB2 and VB6 in commercially available tablets, with recoveries between 99.5 and 98.0%, being those statistically compatible to those found by a reference spectrophotometric procedure. Keywords Composite electrode . Graphite . ABS . Pyridoxine . Riboflavin

Introduction

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-018-3897-z) contains supplementary material, which is available to authorized users. * Grasielli C. de Oliveira [email protected] 1

Grupo de Eletroquímica e Eletroanalítica (G E), Instituto de Química, Universidade Federal Fluminense, Campus Valonguinho, Niterói, RJ CEP 24020-141, Brazil

2

Grupo de Eletroanalítica e Novos Materiais (GENMAT), Departamento de Química, Universidade Federal de Mato Grosso, Cuiabá, MT CEP 78060-900, Brazil

The term Bvitamin^ comes from vital amine, a class of essential nutrients which acts, even in small amounts, in many different biochemical processes in the human body, and since they are not synthesized by the own organism, people must have a suitable uptake to obtain them from food or dietary supplements. Such category of nutrients can be classified as either water soluble (vitamin C and the B group) or fat soluble (A, D, E, K). Vitamin B2 (VB2) or riboflavin, is a required compound for the normal metabolic activity of the human body by mediating the conversion of other vital nutrients like carbohydrates, fats, and proteins into ATP. Inadequacy of VB2 uptake results in various problems such as itching and burning eyes, sensitivity to light, sore tongue, and peeling skin. In the case of vitamin B6 (VB6), it must be considered that it has different chemical forms (pyridoxine, pyridoxal, pyridoxamine, and their

J Solid State Electrochem

phosphorylated derivatives), but the most stable and, thus, commercially available, is pyridoxine (especially under hydrochloride form). This vitamin is a vital cofactor involved in the metabolism of amino acids and lipids, synthesis of neurotransmitters, maintenance of body cells, supports DNA repair against free radical damage, and other key biologic processes. Lack of these vitamins may cause several diseases such as anemia, nerve damage, depression, cognitive problems, seizures, skin problems, and sores in the mouth [1–3]. As a response to the wide myriad of complex samples, several techniques have been developed for the determination of VB2 and VB6 that include chromatography [4, 5], fluorescence and chemiluminescence [6, 7], and capillary electrophoresis [8]. In general, photometric procedures, especially when coupled to separation techniques, are sensitive and versatile, but, on the other hand, they are equally time consuming, often requiring sample pretreatments, expensive instruments, and reagents. Compared to these methods, electroanalytical techniques have received a great attention due to its simplicity, miniaturization capabilities, relative low cost of implementation and operation, suitable sensibility, and precision. Focusing on electroanalytical strategies, many articles describe the individual determinations of VB6 [9–16] and VB2 [17–22] using different electrodes (bare or its coated form with modification) and electroanalytical techniques. Söderhjelm and Lindquist [9] were the first to study the voltammetric determination of VB6 using a carbon paste electrode (CPE) in ammonia buffer by cyclic voltammetry. Teixeira et al. [10] exploited the properties of a chemically modified carbon paste using vanadium compounds as mediators while Desai and collaborators [11] used a CPE-modified with oxa crown ether for determination of pyridoxine hydrochloride in multivitamin pharmaceutical preparations by differential pulse voltammetry. Fonseca et al. [12] used ionic pair formation in order to enhance electrode performance and Liu et al. [13] assessed the improvement generated by single-strain DNA chains onto electrodes surface as catalysts for VB6 oxidation. Different nanostructures where assessed for electroanalytical purposes [14–16]. In general, such approach evolved gold substrates in order to reach an amplification of signals. Another different carbon-based used substrate was the boron-doped diamond electrode, as described by Kuzmanović and coworkers [17]. Kubota and Gorton [18] conducted electrochemical investigations of the reaction mechanism and kinetics between VB2 immobilized on zirconium phosphate in carbon paste and NADH. Bai and collaborators [19] used an ordered mesoporous carbon-modified glassy carbon electrode for determination of VB2 in vitamin tablets. The use of Cr-doped SnO2 nanoparticles for biosensing is also described [20]. Other reports proposed the electrocatalytic effects of zeolites [21], bismuth films [22], and methylene blue-incorporated mesoporous silica microsphere-based platforms. [23].

The goals and proposed options are wide; however, few papers deal with the simultaneous electroanalytical determination of VB2 and VB6 in different samples, often using complex systems and/or surface modified electrodes [24–29]. Electrochemical pretreatment is an important step described by Gu and coworkers [24]. Shaidarova et al. [25] and Nie [26, 27] et al. proposed the use of inorganic and organic polymeric conducting films in order to exploit their electrocatalytic effects while Kaur and colleagues [28] proposed the use of nanocrystalline metallosilicates as electrode modifiers. Dos Santos [29] presented a graphite-paraffin electrode for such simultaneous determination, reaching good analytical parameters, but with some restrictions regarding temperature and mechanical resistance. All those approaches seemed to be quite promising although they are cost affordable and sometimes show low reproducibility. Among the solid electrodes, composite electrodes consist in an attractive proposal since they stand for a cheap product obtained from an easy and reproductive fabrication process, with adequate surface renewal, useful potential interval, and applicability in diverse pH media when compared to commercial glassy carbon electrode. Handcrafted composite electrodes consist of mixtures between, at least two phases, one insulating and one conductor, generating, after mixing and incorporating processes, a conductive material with physical and chemical properties different to those from the original phases [30–32]. Considering the conducting phases, carbon-derived materials are widely used because of their well-established properties [33, 34]. Among the most popular examples are glassy carbon [35], graphite [33, 34], nanotubes [36], graphene sheets [37, 38], or even boron-doped diamond [35]. With respect to the insulating phases, more recently, we can mention many polymeric materials such as silicone [39, 40], polyurethanes [12, 40], and epoxy [41]. Thermoplastic polymers have gained attention in recent decades due to the almost infinite possibilities of rapid prototyping with 3D printing. Electrochemistry is a branch of science that has benefited from 3D printing technologies, with the fabrication of cheaper and higher performing electrochemical devices. Some composites have been used in 3D printing [42–45]. Among such compounds, one takes special place, Acrylonitrile-butadiene-styrene (ABS), which is a thermoplastic polymer formed by condensation polymerization of different proportions of acrylonitrile, butadiene, and styrene. The choice of using ABS as an insulating phase raised from the possibility of combination of physical and chemical resistance of the acrylonitrile and styrene polymers with the toughness and impact strength of the polybutadiene [32]. In addition, other interesting characteristics, such as moldability (which allows the obtainment of specific formats and shapes) and flexibility (since it is a thermoplastic) must be taken into account. In this work, a lab-made composite based


on ABS and graphite was used in the construction of solid electrodes for the simultaneous electroanalytical determination of B-complex vitamins in pharmaceutical samples using differential pulse voltammetry (DPV). Composite electrodes were prepared and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetry (TG), differential scanning calorimetry (DSC), Raman spectroscopy, Fourier transforminfrared (FTIR), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), being then applied for the simultaneous electroanalytical determination of both analytes in commercial samples.

Reagents and chemicals Stock solutions for Britton-Robinson (BR) buffers were prepared by suitable direct dissolution of boric acid, sodium hydroxide, and phosphoric acid (all analytical grade reagents, purchased from Sigma-Aldrich® and employed without further purification) in ultrapure water (18.2 MΩ cm) obtained from a Millipore (Millipore®, Germany) Milli-Q Gradient purification system. Buffers were prepared in usual way by mixing of 40 mmol L−1 of all necessary components (phosphoric, acetic, and boric acids), being the final pH value adjusted by slightly adding sodium hydroxide (pellets or few drops of 0.2 mol L−1 solution). As electrochemical probe solutions containing 5.0 mmol L −1 potassium ferricyanide (Mallinckrodt®, Ireland) in 0.50 mol L−1 KCl (Vetec®, Brazil) were used, being prepared prior the use. Acetate buffer (0.1 mol L−1, pH 4.0) also used as supporting electrolyte was prepared by mixing glacial acetic acid and sodium acetate (Vetec®, Brazil). Working solutions of VB2 and VB6 were freshly prepared on the day of the experiment by appropriate dilution with the supporting electrolyte (PharmaNostra®, Italy). Graphite powder (< 20 μm, Sigma-Aldrich®) and ABS polymer (Movtech®) were used in the fabrication of the working electrode composites.

rate 10 °C min−1) under a dynamic atmosphere of synthetic air flowing at 50 mL min−1. Solids were also characterized by Raman spectroscopy using an Alpha 300 system (Witec®, Germany). The experiments were performed at 25 °C using a ×100 objective lens and a Nd:YAG green laser at 532 nm, with a small entrance (726 cts) to avoid the decomposition of possible organic compounds in the samples during the analysis. SEM analysis were performed on a Zeiss DSM 940-A microscope (Zeiss®, Germany) operated at accelerating voltage of 15 kV, at a working distance of 8 mm at ×2000 magnitude. AFM images were obtained on a Nanosurf FlexAFM C3000 Controller (Nanosurf®, Switzerland). Experiments were conducted using a 2.8 N/m Pt-Ir probe in intermittent contact (topography and phase contrast), with scans covering an area of about 30 μm × 30 μm, containing 256 × 256 pixels at maximum resolution. The Gwyddion 2.40 software package was used to analyze the AFM images. Voltammetric experiments were performed, in triplicate, using an Autolab PGSTAT128N potentiostat (Metrohm®, Switzerland), controlled by the Nova 1.1.1 software (Metrohm®, Switzerland). The electrochemical system consisted of a three-electrode cell. An Ag|AgCl (3 mol L−1 KCl) electrode was used as the reference electrode in combination with a platinum wire as counter electrode and ABS-graphite composite electrodes (AGCE) as the working electrode. Electrochemical activation of the electrode was performed by ten successive cycles, from − 1.0 to 1.0 V, under a scan rate of 100 mV s−1, with a potential step of 5 mV. Graphical data presentation was plotted using the Origin Pro® 8.0 software. All the experiments were carried out in room temperature and without removing diluted oxygen. The spectrophotometric analyses were performed using a Varian Cary® 50 UV-Vis spectrophotometer with a quartz cell (optical path of 1.0 cm). A Hettich® centrifuge, model Rotanta 460 R, was used in the preparation of the samples.

Preparation of composite electrodes Instrumentation Thermogravimetric analysis (TGA) were carried out on a Shimadzu TG/DTA 60 (Shimadzu®, Japan), by placing approximately 7 mg of each sample into alumina crucibles and heating them from 25 to 800 °C (heating rate 10 °C min−1) under an inert dynamic atmosphere of N 2 flowing at 50 mL min−1. DSC were carried out on a Shimadzu DSC 60 (Shimadzu®, Japan), by placing of each sample into aluminum crucibles and heating them from 25 to 600 °C (heating

Composite electrodes were prepared by mixing graphite (Sigma Aldrich®, < 20 μm, USA) and ABS polymer (Movtech®) previously solubilized in acetone, getting different graphite ratios (m/m) in the final material (40, 50, 60, 70, and 80%), from now on named as AG1, AG2, AG3, AG4, and AG5, respectively, for the sake of simplicity. Mixtures were mechanically homogenized during 10 min, and subsequently added into the syringe body (1.0-mm internal diameter) under pressure, being then inserted a copper wire to establish the external electrical contact. After preparation, electrodes were stored at room temperature without major care.


Electrochemical characterization An electrochemical probe of well-known behavior (K3Fe(CN)6 in KCl media) was used to evaluate the performance of the prepared materials as electronic conductor in a wide range of potentials; such probe was chosen due to many predictable and useful electroanalytical parameters [46]. The electrochemical characterization of the AGCE prepared in different proportions of graphite was obtained with respect to their voltammetric curves in 5 mmol L−1 K3Fe(CN)6 solution supported by 0.5 mol L−1 KCl, using cyclic scans at 25 mV s−1 between − 0.3 and 0.8 V (vs. Ag|AgCl). Nyquist curves of the AGCE were also obtained by electrochemical impedance spectroscopy (EIS) in the frequency range between 0.1 and 104 Hz, using the redox system described above.

Analysis of real samples The applicability of the electrode for the simultaneous determination of VB2 and VB6 vitamins was evaluated in both standard solutions and pharmaceutical samples. Commercial samples of Complex B®, tablets from EMS® (Brazil), were purchased in pharmacies (Cuiabá, MT, Brazil). Samples were prepared by direct dilution of 0.5 g of the tablet powder (previously triturated in mortar with a suitable pistil) in 10.0 mL of acetate buffer 0.1 mol L−1 (pH 4.0). The solution was then vortexed (3000 rpm) for 10 min and submitted to an ultrasonic bath for 10 min. Subsequently, the solution was centrifuged at 2000 rpm (27 °C) for 10 min. An aliquot of the supernatant was added to the supporting electrolyte for the simultaneous determination. Successive standard additions of 5 × 10−3 mol L−1 VB2 and VB6 stock solution were performed being the measurements carried out by differential pulse voltammetry (DPV). The optimized values obtained for DPV were 100 mV of amplitude and 5 mV of step potential, with modulation of time of 0.05 s and time interval of 0.5 s, resulting in a scanning speed of 10 mV s−1. In order to assess the proposed sensor, a comparative method was applied to the same samples, being performed according to the Brazilian Pharmacopeia [47]. Ten tablets were weighed and macerated in mortar with a suitable pistil. For determination of VB2, 0.250 mg was transferred to a falcon tube containing 50 mL of 0.1 mol L−1 acetate buffer (pH 4.0) and submitted to ultrasound for 10 min. The volume was completed to 100 mL and posteriorly the solution was filtered. The absorbance was monitored at 444 nm and the zero adjustment was done with acetate buffer. For the calculation of the VB2 content in the sample, an analytical curve was constructed from the dilutions in acetate buffer of a stock solution of 1 × 10−4 mol L−1 of VB2. In the case of VB6, 0.100 mg was transferred to a falcon tube containing 50 mL of HCl 0.1 mol L−1 and submitted in water

bath for 15 min. The volume was completed to 100 mL and posteriorly the solution was filtered. The absorbance was measured at 290 nm and the zero adjustment was done with HCl 0.1 mol L−1. For the calculation of the VB6 content in the sample, an analytical curve was constructed from the dilutions in HCl 0.1 mol L−1 of a stock solution of 5 × 10−4 mol L−1 of VB6.

Results and discussion Characterization of the composite composition Thermal characterization of the obtained composites Thermal behavior and stability of the AGCE at high temperatures can be verified using thermal methods such as thermogravimetric analysis (TGA), whether under air or inert atmosphere. The results obtained for both ABS and composites ABS graphite (AG1, AG2, AG3, AG4, and AG5) led to sufficiently different temperatures of decomposition, as can be seen in Fig. 1. From Fig. 1, it can be observed that events related to pure ABS decomposition are also present in the assessment of prepared composites; these events have their intensities reduced according to the composite’s proportion, as easily noted in AG1 and AG5. ABS degrades in two clear steps. The first step occurs between 300 and 450 °C, and the second step from 450 to 600 °C, with a large loss of weight occurring in the first step, probably related to butadiene- and styrene-rich domains as they are most vulnerable sites for degradation [48]. As for the thermal degradation of ABS, some researchers observed one step [49–51] and others observed two steps [52, 53]. Suzuki and collaborators [50] applied thermogravimetry/ Fourier transform-infrared (TGA/FTIR) to investigate the

Fig. 1 TGA curves for ABS and composites prepared with proportions different of graphite (heating rate 10 °C min−1, under atmosphere of N2 flowing at 50 mL min−1)


degradation of ABS, according to their findings evolution of butadiene starts at 340 °C and styrene at 350 °C, while the evolution of monomeric acrylonitrile begins at 400 °C. The discrepancy comes from the sample itself (molecular weight, acrylonitrile, butadiene, and styrene component ratios); sample weight; sample volume; heating rate; purging gases; purging gas rate; etc. All these factors affect the actual degradation of ABS. For a while, a single exothermic loss above 600 °C is observed, facilitating thus the development of a quantitative study in terms of graphite concentrations, results are shown in Table S1. Commercial ABS presents thermal stability until close 300 °C, after this, the complete decomposition occurs in two successive steps between 300–450 °C and 450–600 °C (being a great amount of energy delivered in the first step). Besides this, the ABS also was evaluated by differential scanning calorimetry (DSC) (Fig. 2). Although the DSC was performed in synthetic air atmosphere, a behavior close to that observed in the TGA analyses was verified, with the difference in the decomposition temperature of the ABS. Oxidative decomposition has been shown to occur at a lower temperature than non-oxidative decomposition. This has been attributed to the easier occurrence, presumably on energetic grounds, of oxidative reactions than other reaction types. Oxygen reacts with the polymer molecules and is known to penetrate polymer films [54]. In Fig. 2, the ABS presents a great endothermic peak around 420 °C, which is characteristic of styrene containing polymers [55] and an exothermic peak above 530 °C. Spectroscopic characterization of the obtained composites by Raman Raman spectra of the different materials (pure components and their composites) are displayed in Fig. 3. The second spectrum refers to pure graphite, where three characteristic D

Fig. 2 DSC curve for ABS (heating rate 10 °C min−1, under synthetic air flowing at 50 mL min−1)

Fig. 3 Raman spectra obtained for the assessed samples

and G bands were observed [56, 57]. ABS has multiple Raman active modes from 1000 to 3200 cm−1. The peaks displayed in the Raman spectra indicate the presence of acrylonitrile (2242 cm −1 ) due to C≡N stretch, butadiene (1670 cm−1) due to C=C stretch, and styrene (1608 cm−1) due to C–H aromatic bending [58, 59]. ABS-graphite composites electrodes presented first-order characteristic D and G bands, located at 1341 and 1568 cm−1, attributed to the breathing modes of the carbon atoms (D) and stretching of all C–C sp2 bonds (G), as well as a second 7-order G′ band observed at 2669 cm−1. The AG samples presented characteristic bands that, as expected, showed a decrease in intensity while increasing graphite contents. Another study to verify the chemical interaction between graphite and ABS was done using the technique of IV spectroscopy. The FTIR spectra of ABS pure (a) and AG4 (b) are shown in Fig. 4.

Fig. 4 Infrared spectra of (a) ABS pure and (b) AG4


As can be seen in Fig. 4, there is a small peak present at about 2230 to 2240 cm−1 indicating the presence of C≡N bonds typical of nitrile, which may be related to the acrylonitrile group of the ABS copolymer [60]. The most important bands that provide more information on the structure of the compounds are found in the low-frequency region, between 900 and 675 cm−1, which are derived from the angular deformation of the C–H bonds of the aromatic ring. Polymer chain skeleton vibrations are also observed at approximately 1600 to 1350 cm−1, related to the axial deformation of the ring carbon-carbon bonds. Peaks near 910 and 960 cm−1 indicate butadiene phases (−CH=CH− bonds) which may be related to the butadiene group of the ABS copolymer [61]. In addition, peaks close to 700 and 760 cm−1 indicate the presence of aromatic ring related to the aromatic ring of styrene in the ABS copolymer. Both in the IR and in the Raman spectra, a decrease in the intensities of the characteristic bands of the ABS is observed as the proportion of graphite increases, corroborating with the Raman results. All these results suggest that no chemical modification or degradation occurs while preparing the composite materials. Electrochemical characterization of the proposed electrodes The electrochemical behavior of the different compositions for the AGCE was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Figure S1a illustrates a typical cyclic voltammogram for each composite, with the voltammetric scan rate of 25 mV s−1 using a 5.0 mmol L−1 K3Fe(CN)6 in 0.50 mol L−1 KCl solution as supporting electrolyte, from − 0.3 to 0.8 V. In all cases, welldefined responses were observed. A couple of resolved peaks were observed for all the electrodes, especially for the electrodes AG3, AG4, and AG5. Compositions below 40% (graphite, m/m) showed no voltammetric response since they did not contain enough graphite for adequate conductivity, resulting in high electrical resistance composites (Fig. S1b and Table S2). However, differences were observed for peak to peak potential separation (ΔEp) and peak current (Ip), as shown in Table S2. As it can be seen, the ΔEp value was decreased and the enhanced peak current as the amount of graphite increased in the composites electrodes, favoring the kinetics of electron transfer. This reflects the decrease of the resistance of the graphite electrode surface. Increasing the graphite content results in increasing sensitivity of composites electrodes. EIS is a technique very effective for the characterizations of the surface of the electrodes by electron transfer resistance, contact resistance, resistance of the solution, and capacitance since it allows differentiation between the capacitive component and the faradaic current by applying a small potential difference to measure the next system to equilibrium

conditions. The electroanalytical properties required by an electrode are high electron transfer rate, the lowest doublelayer capacitance, and ohmic resistance in order to guarantee a high signal/noise ratio high sensitivity, and low-detection limits. By this technique, it is possible to determine the composite composition that exhibits these electroanalytical properties. These results can also be contrasted with voltammetric measurements. In this job, EIS was used to examine the interfacial behavior of the electrodes with different conducting phase proportion. The obtained EIS curves of constructed electrodes are shown in Fig. S1b. As depicted in this figure, the obtained curves include a semicircular part and a linear part. The semicircle observed at high frequencies values in the Nyquist plots is related to the charge transfer limiting processes (faradaic process), and its diameter corresponds to the charge transfer resistance (Rct) of the redox probe at the interfacial surface, while the linear portion represents the diffusion process. As depicted in this figure, the diameter of semicircle part is decreased as the graphite ratio was increased. The quantitative values of parameter Rct is calculated by fitting the impedance spectra to the well-known Randles circuit (inset of Fig. S1b) and the obtained results are summarized in Table S2. It can be observed that Rct decreases as the graphite ratio increases, complementing the CV results, where better electrochemical responses and reversibility behaviors were observed in AGCE with proportions above 50% (graphite, m/m). These results are in accordance with the percolation theory, which predicts the existence of unconnected particles on the electrodic surface that are electroactive sites isolated from each other, forming independent microelectrodes or microarray electrodes. Although AG5 presented lower resistance to load transfer and higher peak currents, it was not used for further studies because it tends to break under mechanical stress and at high scanning rates, graphite grains can fall. Therefore, in this work, the graphite/ABS composite electrode was used in the proportion of 70% (graphite, m/m) or AG4 and characterization analyses focused on this electrode. Such optimal graphite loading value allow us to fabricate attractive and robust composite electrodes with very interesting application as sensors at low electroanalyte concentration. Distribution of graphite in ABS by morphological and topographic characterization To verify if the addition of ABS polymer to the graphite causes agglomerations of particles during the construction of the electrodes, morphological studies of pure graphite and AG4 by SEM are presented in Fig. S2. SEM images showed that pure graphite grains have the appearance of flakes with irregular sizes ranging between 2 and 30 μm, showing that the


size of some graphite particles are slightly larger than that declared by the manufacturer (< 20 μm, Sigma-Aldrich®). By analyzing SEM images of AG4, it was noticed that the graphite grains are not agglomerated, presenting a homogeneous distribution throughout the electrode. In addition, it is possible to observe have a coverage that resembles a Bfilm.^ The dielectric nature of the polymer on the material surface is suggested by the white stains observed (Fig. S2b white zones). Such behavior is also described by Beaunier and collaborators [62] and Silva and collaborators [40], who observed in SEM images with the secondary electron detector, white stains on the material surface in the electrodes of polyethylene/graphite and silicone/graphite, respectively. Atomic force microscopy (AFM) is used to acquire physical information (topography, elasticity, hardness, tribological behavior) and chemical properties needed with submicrometer resolution. Phase contrast microscopy is used to detect and quantify changes in composition across polymer nanocomposites, taking advantage of the contrast in viscoelastic (viscous energy dissipation) properties of the different materials across the surface. The tip is excited into a sinusoidal oscillation at a particular frequency using tapping (intermittent contact) mode. As the sample is scanned, not only will the cantilever deflection amplitude change as it forces the tip encounters change, but the cantilever will generally oscillate at a frequency that lags that of the excitation force (the force from the piezo stack to which the cantilever/tip assembly is attached). The phase lag will be a function, at least partially, of the viscoelastic properties of the sample surface that the tip encounters. A lock-in amplifier is used to extract phase information. The phase lag measured during tapping mode may be related to the attractive and repulsive, or adhesive, forces at the interface and the sample’s viscoelastic properties. Figure S3 shows topographic image and phase contrast characterization image using AFM technic of the AG4. It was also possible to analyze how the graphite grains are spread on the electrode surface (topography) and in relation to its viscoelastic properties (phase contrast) shown in Fig. S3a, b, respectively. Topography images (Fig. S3a) shows undefined grain boundaries. Whereas the phase contrast image (Fig. S3b), reveals a distinctly different image with clear and more defined details of the grains and their boundaries. Furthermore, it displays qualitatively the viscoelastic properties of the AG4, as this composite has several areas differing from each other, suggesting that lighter areas are softer (ABS), and darker areas are directly related to the graphite grains, since these are harder than the ABS. However, the lighter areas are larger in relation to the darker areas (Fig. S3b), that being related to what was seen in the SEM images for the AG4, since it is observed that the graphite grains can either be fully or partially covered by ABS. Clearer shades represent the higher voltage reading at the exit of the lock-in, indicating

that there was a delay in the oscillation of the probe, remaining constant while under the polymer. The delay is due to the greater viscous dissipation capacity of the sample, relative to its elastic storage capacity.

Electroanalytical developments and applications Optimization and electrochemical behavior of the electrode for VB2 and VB6 In the optimization studies of the procedure, some of the experimental parameters such as type and pH of the electrolyte support, pulse amplitude and scan increment were investigated to obtain the best experimental working conditions. The type, concentration of the supporting electrolyte, and pH used, affects the voltammetric response of VB6 and VB2. The VB2 is unstable and easily decomposed in basic solution, it must be also consider its values for pKa (pKa1 ~ 1.3, pKa2 ~ 6.7, pKa3 ~ 8.5, and pKa4 ~ 10.4 [63]). The VB6 and its oxidation products are involved in complex acid base and hydration equilibria. By considering the acidity of the media and pKa value for VB6 (pKa1 ~ 5.0, pKa2 ~ 8.9 [12]), it is possible to conclude that the majority of its specie is positively charged in acid solution and in an anionic form in strongly basic solution because of the deprotonation of the OH group bonded to the pyridinic ring. As the objective of this work is to make the simultaneous determination of vitamins B2 and B6 and knowing that these vitamins are unstable and easily decomposed in basic solutions, the influence of pH of the electrolyte support on the sensor response was at first assessed in a range of 2.5–7.0 in BR buffer 0.04 mol L−1. Based on this information and as shown in Fig. 5, pH value that showed best performance for the determination of VB2 was 4.0 and VB6 was 3.5. These pH values showed better resolution and sensibility; however, for the simultaneous determination of these vitamins, pH of 4.0 seemed to be the most suitable since the loss were negligible. The type and concentration of the supporting electrolyte (not shown) was studied in acetate (0.1 mol L−1) and BR buffer (0.04 mol L−1), both in pH 4.0, as previously selected. The results found by choosing pH 4 acetate buffer as the medium in which the best compromise between sensitivity, reproducibility of the signal and linearity of the calibration curve was achieved, in agreement to the media used during the comparison method. Therefore, all further experiments were then carried out with the optimized voltammetric conditions obtained: acetate buffer (0.1 mol L−1, pH 4.0). Possible electro-oxidative mechanisms for riboflavin and pyridoxine, in agreement with those previously described are illustrated in Fig. 6, in a simplified way. The oxidation of pyridoxine to pyridoxal and the subsequent oxidation to

J Solid State Electrochem 30

(a)

4.5

(b)

30

pH

7.0

15

2.5

25 0

20

Ip / µA

Ip / µA

Fig. 5 Cyclic voltammograms obtained com AG4 in a solution of 2.5 × 10−4 mol L−1 of VB2 (a) and 2.5 × 10−4 mol L−1 of VB6 (b) in BR buffer at different pH values, v = 100 mV s−1. Dependencies of the anode peak current (filled squares) and the peak potential (empty squares) with the pH of VB2 (c) and VB6 (d)

-15

3.5

15 10

-30

5 -45

pH

7.0 -60 -0.8

-0.6

-0.4

-0.2

2.5

0

0.0

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E / V vs (AgIAgCl)

E / V vs (AgIAgCl) 2.6

(c)

(d)

-0.4

21

-0.2

15 12

-0.1

2.2

Ip / µA

18

E / V vs (AgIAgCl)

1.1 -0.3

Ip / µA

1.2

2.4

9

1.0

2.0

0.9

1.8 1.6

0.8

1.4

0.7

E / V vs (AgIAgCl)

24

0.0 2

3

4

5

6

7

2

3

4

pH

pH

5

6

7

pyridoxic acid occurred irreversibly at E p = 0.9 V in 0.1 mol L−1 acetate buffer pH 4.0. For the riboflavin, under the same conditions, a reversible couple of anodic-cathodic peaks were observed, a reduction peak located at − 0.30 V and an oxidation peak at − 0.20 V. At the starting potential of − 0.1 V, VB2 is in the oxidized form (quinone). Hence, the peak at − 0.30 V corresponds to the reaction to the reduced form (hydroquinone), while the peak at − 0.20 V corresponds to the reaction back to the oxidized form. This interpretation involves the transfer of two electrons and two protons during the electrochemical reactions. However, the riboflavin also has the ability to participate in chemical reactions with transference of an electron. In this

case, the riboflavin accepts a proton and an electron to its unsaturated nitrogen to form a semiquinoid free radical, then another proton and another electron can approach another unsaturated nitrogen to form the reduced form of riboflavin. Hence, the redox peak potential is pH sensitive, shifting negatively with the increase of the pH value, as show in Fig. 5. The following ranges were studied for the optimization of the instrumental parameters of the differential pulse voltammetry: pulse amplitude (10–100 mV) and scan increment (1.0– 10.0 mV). This study was conducted in 1.22 × 10−4 mol L−1 of VB2 and VB6 in 0.1 mol L−1 acetate buffer (pH 4.0). The best analytical signals were obtained for the sensor employing a pulse amplitude of 100 mVand a scan increment of 5 mV (not

Fig. 6 Possible electro-oxidative processes for riboflavin and pyridoxine

OH HO CH CH2OH HO CH2OH HO + H2O H2O H3C H3C N

HO H3C

CH2OH CH2OH HO -2H+, -2e-

CHO

N

N

pyridoxine

H3C

OH CH O HO -2H+, -2eN

H3C

N

pyridoxic acid

pyridoxal

ribityl chain CH2(CHOH)3CH2OH N N H3C O +2H+, +2eNH -2H+, -2eH3C N O isoalloxazine ring group riboflavin

O C OH CH2OH

H3C H3C

CH2(CHOH)3CH2OH H N O N N H

O leucoriboflavin

NH


shown). Therefore, these experimental conditions were applied in the following experiments. Effect of scan rate Further, the effect of scan rate on the oxidations of VB2 and VB6 were studied individually at AG4. The CVs obtained for VB2 and VB6 at AG4 in acetate (pH 4.0) at scan rates from 10 to 500 mV s−1 are exhibited in Fig. 7a, c. The peak current of VB2 increased linearly with square root of the scan rate indicating that the oxidation and reduction process on the electrode surface is controlled by diffusion (Fig. 7b) and the peak current of VB6 increased linearly with the scan rate indicating that the oxidation process is controlled by adsorption (Fig. 7d). In Fig. 7b, it was verified that the VB2 presents equivalent diffusion coefficients, evidenced by the magnitudes of the angular coefficients. The electrode was able to accomplish the proposed test, since, according to the coefficients of correlation found, the variation is linear for both vitamins. These results show that the electrode behaves in much the same way in anode and cathodic scans, showing that it is a suitable material for applications in wide ranges of working potential.

(0.1 mol L−1; pH 4.0) containing 2.38 × 10−5 mol L−1 VB2 and 2.38 × 10−4 mol L−1 VB6, taking several separate measurements using the same sensor. The relative standard deviation was 6.5% for VB2 and 8.2% for VB6, for nine successive assays. The reproducibility was also investigated by utilizing three separate sensors prepared and used independently under the optimized conditions described previously. The sensor showed an acceptable reproducibility with a relative standard deviation (DPR) of 10.8% for VB2 and 3.2% for VB6. The AG4 presented acceptable repeatability and reproducibility, indicating good precision of the proposed electrode. The long-term stability of the composite electrode was evaluated by measuring the voltammetric current response in triplicate over a period of 4 weeks. The current response was recorded in 4.54 × 10−5 mol L−1 VB2 and 4.54 × 10−4 mol L−1 VB6 in a 0.1 mol L−1 acetate buffer solution (pH 4.0). The electrode was dry-stored and maintained at room temperature (25 °C). The control chart shows that the mean value of the measures for each week remained within the limits of statistical control. This result may be due to the efficiency in the construction of the proposed electrode. Electroanalytical evaluation for VB2 and VB6

Repeatability, reproducibility, and stability The repeatability of the proposed sensor was examined by measuring the current response in acetate buffer Fig. 7 Cyclic voltammograms of 4.76 × 10−5 mol L−1 of VB2 (a) and 4.76 × 10−4 mol L−1 of VB6 (c) at AG4, in 0.1 mol L−1 acetate (pH 4.0) at different scan rates. The scan rate (from bottom to top): 10, 25, 50, 75, 100, 150, 200, 250, 300, 350, 400, and 500 mV s−1 for VB2 and 10, 25, 50, 75, 100, 150, 200, 250, and 300 mV s−1 for VB6. b Calibration plots of VB2 oxidation and reduction peak currents vs. square root of the scan rate and d VB6 oxidation peak current vs. scan rate

The main objective of the present work is to simultaneously determination VB2 and VB6 in acetate buffer at pH 4.0. As shown in Fig. S4 and S5, clear DPV signals for the oxidation


of different concentrations of VB2 and VB6 mixture at AG4 were obtained. The well-defined anodic peaks are observed and increase in the peak currents of VB2 and VB6 are linear with increasing VB2 and VB6 concentrations. DPVs also clearly show that the plot of peak current vs. VB2 and VB6 concentrations are constituted of various slopes. These results indicate that the oxidation processes of VB2 and VB6 are independent and, therefore, simultaneous measurements of the two analytes are possible without any interference. Figure S4 shows the DPV (a) and analytical curve (b) for VB2. The calibration curve was linear from 2.5 × 10−7 to 1.2 × 10−6 mol L−1 (Ip = (1.04 ± 0.17) × 10−5 + (96.2 ± 2.4) [VB2], r = 0.997), where Ip is the peak resultant current (μA), and [VB2] is the concentration of VB2 (μmol L−1). Figure S5 shows the DPV (a) and analytical curve (b) for VB6. The calibration curve was linear from 2.5 × 10−5 to 4.54 × 10−4 mol L−1 (Ip = (2.7 ± 0.1) × 10−6 + (0.04 ± 0.001) [VB6], r = 0.989), where Ip is the peak resultant current (μA), and [VB6] is the concentration of VB6 (μmol L−1). The detection limit (DL) and quantification limit (QL) were calculated using the standard deviation of the lower level of concentration (s) and the slope of analytical curve (S) as follows: DL = 3.3 s/S and QL = 10 s/S. The DL and QL were of 0.15 and 0.5 μmol L−1 for VB2 and 10 and 32 μmol L−1 for VB6. The results of the present method are not very different from other works devoted to determination of VB2 and VB6 (Table 1), but with the advantages of a fast response, acceptable detection and quantification limit, low cost, and simple development and application.

Analysis of real samples The proposed method was applied to quantify the VB2 and VB6 in commercial tablets, using previously optimized Table 1

Fig. 8 Example differential pulse voltammograms obtained with AG4 in a acetate buffer (0.1 mol L−1, pH 4.0), b 500 μL of sample and additions of 100 to 300 μL (c–e) of standard solution at 5 × 10−3 mol L−1 VB2 and VB6

experimental conditions. The standard addition method was used to prevent any matrix influence. The determination of vitamins was carried out by DPV, by adding 500 μL of the diluted samples over 20.0 mL of support electrolyte, followed by three standard solution additions of a mixture of VB2 and VB6. Figure 8 shows DPV results for a sample studied. A comparative method based on UV-Vis spectroscopy was also used in the quantification of the samples. The results obtained by both methods are presented in Table S3. Table S3 displays the average results for the triplicates of each sample studied, comparing DPV and UV-Vis. The results of the recovery rates in both methods were compared with the concentrations of VB2 and VB6 to the tablets samples. Table S3 shows recovery values of 99.5% of VB2 and 98.0% of VB6 for the proposed electrode and from 107.8% of VB2 and 114.5% of VB6 to for UV-Vis. According to Student’s t test, at a 95% confidence level, there are no

Analytical performances of the methods for determining vitamins B2 and B6

Vitamin

Electrode

Technique

Linear range (μM)

LOD (μM)

Sample

Reference

B6

[VO(Salen)]/CPE

LSV

450–3300

37

[10]

ssDNA-GC AGCE OMC/GCE Co2+/zeolite-CP Bi film-Cu AGCE

CV DPV CV CV SWAdSV DPV

100–6000 25–454 0.4–1 1.7–34 0.3–0.8 0.25–1.2

40 10 0.02 0.71 0.1 0.15

Pharmaceutical formulations Tablets Tablets Tablets Tablets Pharmaceuticals Tablets

B2

[13] In this work [19] [21] [22] In this work

OMC/GCE ordered mesoporous carbono modified glassy carbon electrode, Co2+ /Zeolite-CP carbon-paste electrode modified with Co2+ -Y zeolite, Bi film-Cu bismuth-film electrode, [VO(Salen)]/CPE carbon paste electrode modified with the N,N-ethylene-bis(salicylideneiminato)oxovanadium(IV) complex, ssDNA-GC glassy carbono disk electrode modified with salmon deoxyribonucleic acid, CV cyclic voltammetry, SWAdSV square-wave adsorptive stripping voltammetry, DPV differential pulse voltammetry, LSV linear scan voltammetry


significant differences between the recoveries obtained using the electrode and the concentrations of the vitamins in tablets samples. The average recoveries demonstrate the accuracy of the proposed procedure and endorsed the use the developed electrode.

Final remarks The present study described the preparation of a composite electrode with a 70% graphite/ABS ratio, relatively easy to prepare in a low cost and its application to biomedical analysis. A deeper look inside this composite is given in order to provide complementary information and allow its use as 3D printer supplies when in small amounts. Electrochemical studies by cyclic voltammetry were carried out for VB2 and VB6 in different values of pH, providing a better understanding about electrochemical behavior, reversibility, and pH dependence. From the analytical point of view, the differential pulse voltammetry was chosen in acetate buffer 0.1 mol L−1 (pH 4.0) which leaded to good linearity for both vitamins, being the peak potentials sufficiently resolved. The limits of detection for the vitamins were of 0.15 μmol L−1 for VB2 and 10 μmol L−1 for VB6. The electrode presented satisfactory results for VB2 and VB6 in pharmaceutical samples, with good recoveries, repeatability, reproducibility, and long-term stability. Acknowledgments The authors are indebted to the Carlos Chagas Filho Foundation for Research Support in the State of Rio de Janeiro (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro– FAPERJ, E–26/102.971/2012 and E–26/111.407/2013) and to the Commission for the Improvement of Higher Education Personnel (Comissão de Aperfeiçoamento de Pessoal do Nível Superior-CAPES) for the research support; to Prof. Marcelo Camargo Severo de Macedo (UFES); to the Laboratory for NMR and Petrophysics Applications (UFF–LAR) for the use of the SEM; to the Reactors, Kinetics and Catalysis research laboratory (Laborátorio de Reatores, Cinética e Catálise–RECAT) for the use of the TG; to the Group of Electrochemistry and New Materials (Grupo de Eletroquímica e Novos Materiais–GENMAT) located at the Federal Mato Grosso University (Universidade Federal de Mato Grosso–UFMT) for the use of the potentiostat and to the Solid State Experimental Physics research laboratory (LEFES) for the use of the Raman spectrometer, both located at the Federal Fluminense University (Universidade Federal Fluminense–UFF).

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

Conflict of interest The authors declare that they have no competing interests. 16.

References 1.

2.

Drummond JC (1920) The nomenclature of the so-called accessory food factors (vitamins). Biochem J 14(5):660. https://doi.org/10. 1042/bj0140660 de Souza ACS, Ferreira CV, Jucá MB, Aoyama H, Cavagis ADM, Peppelenbosch MP (2005) Riboflavina: uma vitamina

17.

multifuncional. Química Nova 28(5):887–891. https://doi.org/10. 1590/S0100-40422005000500028 Brunetti B (2016) Recent advances in electroanalysis of vitamins. Electroanalysis 28(9):1930–1942. https://doi.org/10.1002/elan. 201600097 Wirkus D, Jakubus A, Owczuk R, Stepnowski P, Paszkiewicz M (2017) Development and application of novelty pretreatment method for the concurrent quantitation of eleven water-soluble B vitamins in ultrafiltrates after renal replacement therapy. J Chromatogr B 1043:228–234 Moreno P, Salvadó V (2000) Determination of eight water-and fatsoluble vitamins in multi-vitamin pharmaceutical formulations by high-performance liquid chromatography. J Chromatogr A 870(1– 2):207–215. https://doi.org/10.1016/S0021-9673(99)01021-3 Mohamed A-MI, Mohamed HA, Abdel-Latif NM, Mohamed MR (2011) Spectrofluorimetric determination of some water-soluble vitamins. J AOAC Int 94(6):1758–1769. https://doi.org/10.5740/ jaoacint.9-493 Llorent-Martínez EJ, Ortega-Barrales P, Molina-Díaz A (2008) Sequential injection multi-optosensor based on a dualluminescence system using two sensing zones: application to multivitamin determination. Microchim Acta 162(1):199–204 da Silva DC, Visentainer JV, de Souza NE, Oliveira CC (2013) Micellar electrokinetic chromatography method for determination of the ten water-soluble vitamins in food supplements. Food Anal Methods 6(6):1592–1606. https://doi.org/10.1007/s12161-013-9576x Soderhjelm P, Lindquist J (1975) Voltammetric determination of pyridoxine by use of a carbon paste electrode. Analyst 100(1190): 349–354. https://doi.org/10.1039/an9750000349 Teixeira MFS, Marino G, Dockal ER, Cavalheiro ÉTG (2004) Voltammetric determination of pyridoxine (vitamin B6) at a carbon paste electrode modified with vanadyl(IV)—Salen complex. Anal Chim Acta 508(1):79–85. https://doi.org/10.1016/j.aca.2003.11. 046 Desai PB, Kotkar RM, Srivastava AK (2008) Electrochemical behaviour of pyridoxine hydrochloride (vitamin B6) at carbon paste electrode modified with crown ethers. J Solid State Electrochem 12(9):1067–1075. https://doi.org/10.1007/s10008-007-0435-9 Fonseca CA, Vaz GCS, Azevedo JPA, Semaan FS (2011) Exploiting ion-pair formation for the enhancement of electroanalytical determination of pyridoxine (B6) onto polyurethane-graphite electrodes. Microchem J 99(2):186–192. https://doi.org/10.1016/j. microc.2011.05.003 Liu S-Q, Sun W-H, Li L-C, Li H, Wang X-L (2012) Electrocatalytic oxidation and voltammetric determination of vitamin B6 by a ssDNA-modified electrode. Int J Electrochem Sci 7:324–337 Patra S, Roy E, Das R, Karfa P, Kumar S, Madhuri R, Sharma PK (2015) Bimetallic magnetic nanoparticle as a new platform for fabrication of pyridoxine and pyridoxal-5′-phosphate imprinted polymer modified high throughput electrochemical sensor. Biosens Bioelectron 73:234–244 Motaghedifard M, Behpour M, Ghoreishi SM, Honarmand E (2016) Electro-deposition of gold nanostructures on carbon paste electrode: a platform with signal amplification for voltammetric study and determination of pyridoxine (vitamin B6). Russ J Electrochem 52(5):477– 487. https://doi.org/10.1134/S1023193516050098 Kassem MA, Hazazi OA, Ohsaka T, Awad MI (2016) Electroanalysis of pyridoxine at copper nanoparticles modified polycrystalline gold electrode. Electroanalysis 28(3):539–545. https://doi.org/10.1002/elan.201500209 Kuzmanović D, Khan M, Mehmeti E, Nazir R, Amaizah NRR, Stanković DM (2016) Determination of pyridoxine (vitamin B6) in pharmaceuticals and urine samples using unmodified borondoped diamond electrode. Diam Relat Mater 64:184–189. https:// doi.org/10.1016/j.diamond.2016.02.018

J Solid State Electrochem 18.

Kubota LT, Gorton L (1999) Electrochemical investigations of the reaction mechanism and kinetics between NADH and riboflavin immobilised on amorphous zirconium phosphate. J Solid State Electrochem 3(7-8):370–379. https://doi.org/10.1007/ s100080050169 19. Bai J, Ndamanisha JC, Liu L, Yang L, Guo L (2010) Voltammetric detection of riboflavin based on ordered mesoporous carbon modified electrode. J Solid State Electrochem 14(12):2251–2256. https://doi.org/10.1007/s10008-010-1065-1 20. Lavanya N, Radhakrishnan S, Sekar C, Navaneethan M, Hayakawa Y (2013) Fabrication of Cr doped SnO2 nanoparticles based biosensor for the selective determination of riboflavin in pharmaceuticals. Analyst 138(7):2061–2067 21. Nezamzadeh-Ejhieh A, Pouladsaz P (2014) Voltammetric determination of riboflavin based on electrocatalytic oxidation at zeolitemodified carbon paste electrodes. J Ind Eng Chem 20(4):2146– 2152. https://doi.org/10.1016/j.jiec.2013.09.044 22. Sá ES, da Silva PS, Jost CL, Spinelli A (2015) Electrochemical sensor based on bismuth-film electrode for voltammetric studies on vitamin B2 (riboflavin). Sensors Actuators B Chem 209:423– 430. https://doi.org/10.1016/j.snb.2014.11.136 23. Gupta R, Rastogi PK, Srivastava U, Ganesan V, Sonkar PK, Yadav DK (2016) Methylene blue incorporated mesoporous silica microsphere based sensing scaffold for the selective voltammetric determination of riboflavin. RSC Adv 6(70):65779–65788. https://doi. org/10.1039/C6RA12336H 24. Gu HY, Yu AM, Chen HY (2001) Electrochemical behavior and simultaneous determination of vitamin B2, B6, and C at electrochemically pretreated glassy carbon electrode. Anal Lett 34(13): 2361–2374. https://doi.org/10.1081/AL-100107301 25. Shaidarova LG, Davletshina LN, Budnikov GK (2006) Flowinjection determination of water-soluble vitamins B1, B2, and B6 from the electrocatalytic response of a graphite electrode modified with a ruthenium (III) hexacyanoruthenate (II) film. J Anal C hem 61(5):502 –50 9. htt ps: //d oi.o rg /1 0.113 4/ S1061934806050133 26. Nie T, Xu J-K, Lu L-M, Zhang K-X, Bai L, Wen Y-P (2013) Electroactive species-doped poly(3,4-ethylenedioxythiophene) films: enhanced sensitivity for electrochemical simultaneous determination of vitamins B2, B6 and C. Biosens Bioelectron 50:244– 250. https://doi.org/10.1016/j.bios.2013.06.057 27. Nie T, Zhang K, Xu J, Lu L, Bai L (2014) A facile one-pot strategy for the electrochemical synthesis of poly(3,4ethylenedioxythiophene)/zirconia nanocomposite as an effective sensing platform for vitamins B2, B6 and C. J Electroanal Chem 717-718:1–9. https://doi.org/10.1016/j.jelechem.2014.01.006 28. Kaur B, Srivastava R (2014) Nanocrystalline metallosilicate modified electrodes for the simultaneous, sensitive, and selective determination of riboflavin, rutin, and pyridoxine. Electroanalysis 26(5): 1078–1089. https://doi.org/10.1002/elan.201400064 29. Dos Santos TAD, Barreto LM, Ritta AGSL, de Meneses WS, Nunes RS, Semaan FS (2013) Cost-effective composite electrode for the fast voltammetric screening and determination of riboflavin (B2) and pyridoxine (B6) in pharmaceuticals. Rev. Virtual Quim. doi: https://doi.org/10.5935/1984-6835.20130042 30. Adams RN (1958) Carbon paste electrodes. Anal Chem 30(9): 1576–1576. https://doi.org/10.1021/ac60141a600 31. Tallman DE, Petersen SL (1990) Composite electrodes for electroanalysis: principles and applications. Electroanalysis 2(7):499–510. https://doi.org/10.1002/elan.1140020702 32. Buzzetti PHM, de Oliveira GC, Azevedo ALM, Michel RC, Ponzio EA, Semaan FS (2015) Conductive composites as versatile substrates for modeling of interfaces and sensors. In: Wythers MC (ed) Advances in materials science research, vol 21. Nova Science Publishers, New York, pp 1–23

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44.

45.

46. 47.

48.

49.

Mc Creery RL (2008) Advanced carbon electrode materials for molecular electrochemistry, chemical reviews, ACS publications. Washington 108(7):2646–2687 Vilalba MM, Davis J (2008) New directions for carbon-based detectors: exploiting the versatility of of carbon substrates in electroanalysis. J Solid State Electrochem 12(10):1245–1254. https://doi. org/10.1007/s10008-008-0535-1 Scholz, F, Bond, AM (2010) Electroanalytical methods: guide to experiments and applications, 2a. ed., Springer, New York, DOI: https://doi.org/10.1007/978-3-642-02915-8 Yogesvaran U, Chen SM (2008) Recent trends in the application of carbon nanotubes-polymer composite modified electrodes for biosensors: a review. Anal Lett 41(2):210–243. https://doi.org/10. 1080/00032710701792638 Allen MJ, Tung VC, Kaner RB (2010) Honeycomb carbon: a review of graphene. Chem Rev, ACS Publications, Washington 110(1):132–145 Potts JR, Dreyer DR, Bielawsky CW, Ruoff RS (2011) Graphenebased polymer nanocomposites. Polymer 52(1):5–25. https://doi. org/10.1016/j.polymer.2010.11.042 Sousa RA, Dos-Santos SX, Cavalheiro ETG, Brett CMA (2013) Graphite-polyurethane and graphite-silicone rubber composite electrodes for electrochemical characterization and determination of minoxidil. Electroanalysis 25(3):706–715. https://doi.org/10.1002/ elan.201200606 Silva AL, Corrêa MM, de Oliveira GC, Florez-Rodriguez PP, Rodrigues Costa CA, Semaan FS, Ponzio EA (2017) Development of graphite/silicone composites for use as flexible electrode materials. J Alloys Compd 691:220–229. https://doi.org/ 10.1016/j.jallcom.2016.08.232 Azevedo ALM, Oliveira RO, Ponzio EA, Semaan FS (2015) Sensor development exploiting graphite-epoxy composite as electrode material. IOP Conference Series: Materials Science and Engineering 97(1):012008. https://doi.org/10.1088/1757-899X/ 97/1/012008 Czyżewski J, Burzyński P, Gaweł K, Meisner J (2009) Rapid prototyping of electrically conductive components using 3D printing technology. J Mater Process Technol 209(12–13):5281–5285. https://doi.org/10.1016/j.jmatprotec.2009.03.015 Zhu C, Han TYJ, Duoss EB, Golobic AM, Kuntz JD, Spadaccini CM, Worsley MA (2015) Highly compressible 3D periodic graphene aerogel microlattices. Nat Commun 6(1):6962. https:// doi.org/10.1038/ncomms7962 Wei X, Li D, Jiang W, Gu Z, Wang X, Zhang Z, Sun Z (2015) 3D printable graphene composite. Sci Rep 5(1):11181. https://doi.org/ 10.1038/srep11181 Wang H, Qian D, Xiao X, Gao S, Cheng J, He B, Liao L, Deng J (2017) A highly sensitive and selective sensor based on a graphenecoated carbon paste electrode modified with a computationally designed boron-embedded duplex molecularly imprinted hybrid membrane for the sensing of lamotrigine. Biosens Bioelectron 94: 663–670. https://doi.org/10.1016/j.bios.2017.03.055 Trasatti S, Petri OA (1991) Real surface area measurements in electrochemistry. Pure & Applied Chemistry 63(5):711–734 Agência Nacional de Vigilância Sanitária – ANVISA (2010) Farmacopéia Brasileira, Brasília http://wwwanvisagovbr/hotsite/ cd_farmacopeia/indexhtm Access 06 Feb 2017 Motyakin MV, Schlick S (2002) Thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) containing a hindered amine stabilizer: a study by 1D and 2D electron spin resonance imaging (ESRI) and ATR–FTIR. Polym Degrad Stab 76(1):25– 36. https://doi.org/10.1016/S0141-3910(01)00262-2 Yang M-H (2000) The thermal degradation of acrylonitrilebutadiene-styrene terpolymer under various gas conditions. Polym Test 19(1):105–110. https://doi.org/10.1016/S01429418(98)00067-1

J Solid State Electrochem 50.

Suzuki M, Wilkie CA (1995) The thermal degradation of acrylonitrile-butadiene-styrene terpolymei as studied by TGA/ FTIR. Polym Degrad Stab 47(2):217–221. https://doi.org/10. 1016/0141-3910(94)00122-O 51. Dong D, Tasaka S, Aikawa S, Kamiya S, Inagaki N, Inoue Y (2001) Thermal degradation of acrylonitrile–butadiene–styrene terpolymer in bean oil. Polym Degrad Stab 73(2):319–326. https://doi.org/10. 1016/S0141-3910(01)00093-3 52. Yang S, Rafael Castilleja J, Barrera EV, Lozano K (2004) Thermal analysis of an acrylonitrile–butadiene–styrene/SWNT composite. Polym Degrad Stab 83(3):383–388. https://doi.org/10.1016/j. polymdegradstab.2003.08.002 53. Fǎtu D, Geambaş G, Segal E, Budrugeac P, Ciutacu S (1989) On the thermal decomposition of the copolymer ABS and of nylon polyamide. Thermochim Acta 149:181–187 54. Shapi MM (1991) TG and DSC studies of some thermal properties and stability aspects of poly (acrylonitrile butadiene styrene), polystyrene and poly (acrylonitrile styrene) plastics. Thermochim Acta 175(1):25–34. https://doi.org/10.1016/0040-6031(91)80242-B 55. Di Cortemiglia MPL, Camino G, Costa L, Guaita M (1985) Thermal degradation of ABS. Thermochim Acta 93:187–190. https://doi.org/10.1016/0040-6031(85)85048-6 56. Mohammadi N, Adeh NB, Najafi M (2016) Synthesis and characterization of highly defective mesoporous carbon and its potential use in electrochemical sensors. RSC Adv 6(40):33419–33425. https://doi.org/10.1039/C6RA03429B 57. Dervisevic M, Çevik E, Durmuş Z, Şenel M (2016) Electrochemical sensing platforms based on the different carbon

derivative incorporated interface. Mater Sci Eng C 58:790–798. https://doi.org/10.1016/j.msec.2015.09.052 58. Cole DP, Riddick JC, Iftekhar Jaim HM, Strawhecker KE, Zander NE (2016) Interfacial mechanical behavior of 3D printed ABS. J Appl Polym Sci 133(30). https://doi.org/10.1002/app.43671 59. Marissen R, Schudy D, Kemp AVJM, Coolen SMH, Duijzings WG, Van Der Pol A, Van Gulick AJ (2001) The effect of material defects on the fatigue behaviour and the fracture strain of ABS. J Mater Sci 36(17):4167–4180. https://doi.org/10.1023/A: 1017960704248 60. Balart R, Sánchez L, López J, Jiménez A (2006) Kinetic analysis of thermal degradation of recycled polycarbonate/acrylonitrile–butadiene–styrene mixtures from waste electric and electronic equipment. Polym Degrad Stab 91(3):527–534. https://doi.org/10.1016/ j.polymdegradstab.2005.01.055 61. Ma H, Wang J, Fang Z (2012) Cross-linking of a novel reactive polymeric intumescent flame retardant to ABS copolymer and its flame retardancy properties. Polym Degrad Stab 97(9):1596–1605 62. Beaunier L, Keddam M, Garcı́a-Jareño JJ, Vicente F, NavarroLaboulais J (2004) Surface structure determination by SEM image processing and electrochemical impedance of graphite+polyethylene composite electrodes. J Electroanal Chem 566(1):159–167. https://doi.org/10.1016/j.jelechem.2003.11.022 63. Bailey MR, Schultz ZD (2016) SERS speciation of the electrochemical oxidation-reduction of riboflavin. Analyst 41(17):5078– 5087