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The bi-band silver microelectrode: Fabrication, characterization and analytical study
Sensors & Actuators: B. Chemical 302 (2020) 127152
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
The bi-band silver microelectrode: Fabrication, characterization and analytical study
T
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Katarzyna Jedlińska , Radosław Porada, Justyna Lipińska, Bogusław Baś AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Analytical Chemistry, Mickiewicza 30, 30-059, Cracow, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Mercury-free sensors Silver electrodes Microelectrodes Microband electrode Microelectrode effect Voltammetry
In this work a new type of working electrode for voltammetry – the bi-band silver microelectrode (b-BAgmE) – is presented for the first time. The sensor consists of two Ag bands 25 μm thick and 4.5 mm long. The bands are arranged symmetrically and contrariwise, along the lateral, cylindrical surface of the Ag guide, coated with a thin layer of epoxy resin. Simple and inexpensive microelectrode construction ensures its reliability of use as well as wide possibilities of application in electroanalysis. Electrochemical properties of the sensor were tested by means of the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) in the [Ru(NH3)6]2+/3+ redox system. Comparative experiments, carried out simultaneously on a homemade silver annular band electrode (AgABE), commercially available silver microelectrode (AgmE) and b-BAgmE, confirmed occurrence of the microelectrode effect in case of the AgmE and b-BAgmE. Moreover, the b-BAgmE is characterized by significantly lower value of charge-transfer resistance Rct = 179 ± 6 Ω in comparison to AgmE Rct = 6.0 ± 0.4 MΩ. The practical usefulness of the b-BAgmE was confirmed by determination of Pb2+, Tl+, In3+ and vitamin B2 using the differential pulse (DP) voltammetric technique.
1. Introduction
In 2009, the theoretical idea of a new type of working electrode – the annular microband electrode (AMB) was presented [17]. In contrast to conventional constructions, cylindrical surface of the AMB was located on lateral surface of the sensor. Numerical simulations demonstrated highly efficient diffusion properties of such kind of the electrode shape. Beside the theoretical model, there are several examples of applications of annular band electrodes made of silver [18], gold [19] and bismuth [20,21]. The unusual vertical position of the electrode surface causes significant facilitation of gas bubbles removal, therefore gaseous electroreaction products and deareation gases do not disturb recorded signals. It also hastens and facilitates polishing of the electrode surface at the same time preventing from making it uneven or slanted. Moreover, this unique shape allows improvement of this construction by use of additional items, such as outer cylindrical sensor body as an activation cell [18], or applying thin mercury film in the renewable electrodes [22]. Simple and inexpensive fabrications procedures as well as numerous advantages of the annular band electrodes induced creation of numerous constructions of this type [23,24]. In this communication, fabrication of a new voltammetric sensor – the bi-band silver microelectrode (b-BAgmE) is presented. This uncomplicated, low-cost and environmentally friendly construction is proposed for the first time. Thanks to its unique shape, this construction
Microelectrodes are used in various applications in contemporary electrochemical trace analysis [1–9] due to numerous advantageous properties, including low ohmic drop, fast mass-transport rate and low interfacial capacitance [9–12]. These unique electrochemical characteristics result from their micro-dimensions, much smaller (≤25 μm) than thickness of the diffusion layer, therefore very efficient hemispherical diffusion of the depolarizer to the microelectrode surface takes place. In such conditions there is no dependence of the maximal reaction current on the measurement time (the steady state is reached), higher current density is achieved and the signal-to-noise ratio is improved [9–15]. Moreover, the microelectrodes allow conducting measurements in highly resistive media or even without adding the supporting electrolyte [5–8]. The band type microelectrodes take the shape of an elongated band of width less than 25 μm and length even in the centimeter range [9–14,16]. They share advantageous features of the microelectrodes; however, much larger surface area causes registration of significantly higher current values. Therefore, in case of the band microelectrodes, use of additional equipment such as Faraday cage or special and sensitive measuring apparatus, is not necessary. ⁎
Corresponding author. E-mail address: [email protected] (K. Jedlińska).
https://doi.org/10.1016/j.snb.2019.127152 Received 25 July 2019; Received in revised form 12 September 2019; Accepted 15 September 2019 Available online 20 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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combines advantages of all types of electrodes mentioned above: microelectrodes, band and annular band electrodes. To investigate the favorable features of the b-BAgmE, cyclic voltammetry and electrochemical impedance spectroscopy were used. The proposed electrode was successfully applied in DP voltammetric determination of Pb2+, Tl+, In3+ and vitamin B2. 2. Experimental section 2.1. Measuring apparatus The voltammetric measurements were carried out using the M161 multipurpose electrochemical analyzer with the M164 electrode stand made by mtm-anko, Poland. The electrochemical impedance spectroscopic (EIS) measurements were performed using the μAutolab III (EcoChemie, the Netherlands) with FRA2 (Frequency Response Analyzer) module. A classical 3-electrode cell with the homemade biband silver microelectrode (b-BAgmE) as a working electrode, Pt wire as an auxiliary electrode and a silver chloride double-junction Ag|AgCl|3 M KCl|2.5 M KNO3 as a reference electrode, were applied. The signals registered at the b-BAgmE were compared with the signals obtained using the homemade silver annular band electrode (AgABE) [18] and the commercially available microelectrode (AgmE) (25 μm, Metrohm-Autolab, The Netherlands). A magnetic stirrer (ca. 200 rpm) was used during the accumulation period. All experiments were conducted in ambient temperature 22 °C.
Fig. 1. Construction of the b-BAgmE: the inner silver core scheme (A), the electrode construction scheme (B), photography of the sensor (C). Ag foil piece (a), Ag wire (b), flattened Ag wire end (c), epoxy resin (d).
high reliability and failure-free use of the electrode. Similar to the other types of annular band electrodes, vertical position of the electrode surfaces facilitates gases removal from the measuring system.
2.2. Chemicals and glassware All reagents were of analytical grade. Unless otherwise noted, chemicals were purchased from Sigma-Aldrich. Standard solutions of Pb2+, Tl+, In3+ were purchased from Merck (Certipur®, 1000 mg L-1) and diluted as required. The vitamin B2 (VB2, riboflavin) and [Ru(NH3)6]3+ standard stock solutions were prepared shortly before measurements. 1 mol·L-1 acetate buffer solution (pH 4.5) was prepared by mixing the appropriate amounts of acetic acid and crystalline sodium acetate. All solutions were prepared using 4 times distilled water (quartz). The b-BAgmE was prepared from a silver foil (Alfa Aesar, Germany, 99.95%, 25 μm), a silver wire (The Mint of Poland, Poland, 93%, ϕ = 0.7 mm) and TRANSLUX D180 resin (AKSON, France). For polishing the electrode surface, MicroPolish™ Alumina suspensions (1.0, 0.3 and 0.05 μm, Buehler Micropolish II, Lake Bluff, IL USA) and MasterTex polishing cloth (Buehler), were used.
2.4. Standard procedures of measurements Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical properties of the b-BAgmE in terms of its use in voltammetry. The measurements were performed in the presence of 5 mmol·L−1 [Ru(NH3)6]3+ redox probe in 0.1 mol·L−1 KCl solution. Cyclic voltammograms were recorded in the potential range from -0.45 to +0.10 V with scan rates varying between 1 and 200 mV·s−1. Impedance measurements were conducted at the formal potential of the Ru3+/Ru2+ redox couple, on which the sinusoidal signals of amplitude 10 mV and frequency ranging from 100 kHz to 0.1 Hz were subsequently superimposed. An electrical circuit model was fitted to the recorded data, providing an insight into the electron transfer kinetics (modelled by charge-transfer resistance Rct), type of diffusive movement of the electroactive species towards the b-BAgmE surface (Warburg impedance) and structure of the electrical double layer at the electrode – solution interface (capacitance Cdl). Differential pulse voltammetry (DPV) was employed for determination of Pb2+, Tl+, In3+ and vitamin B2 (VB2). Heavy metal ions were analyzed in the supporting electrolyte containing 10 mmol·L−1 HNO3 and 10 mmol·L−1 KCl, whereas VB2 was analyzed in 0.04 mol·L−1 acetate buffer solution. During measurements the following techniques and parameters were applied:
2.3. Preparation of the b-BAgmE A schematic diagram of the b-BAgmE fabrication steps and its photography are illustrated in Fig. 1. A 3D animation of the electrode construction is available in the electronic version of this article (Animation 1 ). The metallic core of the sensor (Fig. 1A) was prepared with a silver wire (ϕ = 0.7 mm) and a silver foil (25 μm). One end of the Ag wire was flattened with a vise over a length of ca. 20 mm. At a distance of ca. 10 mm from the end of the wire it was bent into the shape of the extended letter “U”. In the slit formed in this manner, a properly cut piece of Ag foil (4.50 x 12 mm) was symmetrically placed. Afterwards, sides of the Ag wire were mechanically tightened to stabilize the construction and to ensure electric contact. Prepared in this way main element of the electrode was cleaned with acetone and covered with epoxy resin. When the process of resin polymerization was finished the surplus of Ag foil and the excess of resin were removed with emery papers (up to 3000 grit size) and polished with 1.0, 0.3 and 0.05 μm alumina slurry. Finally, the electrode body was rinsed with water and cleaned for 5 min in an ultrasonic bath with distilled water. This extremely simple design enables easy reconstruction of the bBAgmE using low-cost, environmentally friendly materials and uncomplicated equipment. Furthermore, the plain structure guarantees
a) Pb2+ - differential pulse anodic stripping voltammetry (DP ASV): accumulation potential (Eacc), -0.5 V; accumulation time (tacc), 60 s; initial potential (Ei), -0.5 V; end potential (Ee), -0.2 V; step potential (Es) 3 mV; pulse amplitude (dE), 30 mV; pulse period (timp = (tw + ts), 20 ms; in each case: tw = ts (waiting time = current sampling time). b) Tl+ - DP ASV: Eacc = -0.8 V; tacc = 60 s; Ei = -0.8 V; Ee = -0.2 V; Es = 3 mV; dE = 30 mV; timp =20 ms. c) In3+ - DP ASV: Eacc = -1.3 V; tacc = 60 s; Ei = -1.3 V; Ee = -0.2 V; Es = 5 mV; dE = 30 mV; timp =20 ms. 2
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shape, characteristic for a reversible process under linear diffusion conditions. Clearly outlined peaks for both, cathodic (Ipc) and anodic (Ipa) directions are visible for all scan rate values (υ). In case of the AgmE (Fig. 2B), the same effect is noticeable only for υ > 50 mV·s−1. For lower υ values the microelectrode effect is visible, i.e. the CV curves have the sigmoidal shape characteristic for voltammograms registered under the spherical diffusion conditions. Plateau of the current is observed, and the curves of the oxidation processes are almost superimposed on that of the reduction processes. Comparing the voltammograms recorded on the b-BAgmE (Fig. 2C) with the AgABE and the AgmE signals, similarity to the curves registered on the AgmE can be clearly seen. It confirms, that the studied redox process very quickly reaches the constant maximum current value. Thus, the concentration gradient of the depolarizer at the microelectrode surface does not depend on the time of electrolysis, and its transport takes place almost exclusively through spherical diffusion. Consequently, it has been proven that for relatively wide range of scans it is possible to achieve the microelectrode effect also for the b-BAgmE. The results of comparative calculations made on the basis of the data obtained during CV measurements are summarized in Table 1. For all types of electrodes anodic and cathodic peaks’ currents increased proportionally with increasing of the scan rate, and linear dependences between those peak currents and square root of υ were, obtained. Moreover, in each case the peak current ratio Ipa/Ipc was close to 1. Therefore, it can be stated that the electrode reaction of Ru2+/Ru3+ on all tested electrodes is reversible and diffusion controlled. The effective surface areas (Aeff) of the tested electrodes were calculated basing on the Randles-Sevčic equation [24,25] for reversible processes (for Ru2+/ Ru3+ the electron transfer number n = 1, the diffusion coefficient D = 8.45·10−6 cm2·s-1). The Aeff value of the b-BAgmE is perfectly consistent with its geometrical surface (A). Furthermore, current density registered on the b-BAgmE is almost 3-times higher than on the macroelectrode (AgABE), and only 2-times lower than on the AgmE, which proves high efficiency of depolarizer ions transport to the bBAgmE surface. 3.2. Electrochemical impedance spectroscopic measurements Further comparative tests between the AgABE, AgmE and b-BAgmE electrodes were performed with EIS in accordance with the procedure set out in Section 2.4. The Nyquist plot for the AgABE (Fig. 2D) was composed only of the straight line, which indicates the fast charge-transfer kinetics and slower mass transfer. In case of the AgmE, the Nyquist plot (Fig. 2E) shows a semi-circle caused by limitation in charge-transfer resulting from small surface of the AgmE relative to the AgABE. Fitting the spectra gave the charge-transfer resistance value (Rct) equal to (6.0 ± 0.4) MΩ. For the b-BAgmE (Fig. 2F) the Nyquist plot is composed of semicircle in the range of high frequencies and straight line in the lower range. Basing on the fitted impedance spectra the value of Rct was estimated to be equal to (179 ± 6) Ω. The observed difference in the estimated parameters results from geometry and radius of the studied electrodes. The corresponding Bode plots (Fig. 2 G–I) provided better insight into that observation. The RC-time constant is directly proportional to the electrode radius, meaning that for the smaller electrodes, charging and discharging of the double layer occurs much faster [26]. For the AgABE the impedance magnitude was frequently independent in the range from 10 Hz to 100 kHz. Below that range, the impedance magnitude started to increase, which was caused by capacitive charging step at the electrode-solution interface. This frequency range is called the capacitive charging region [26]. For the AgABE the phase angle value at the highest frequencies was close to zero; due to the big time constant the double layer was not fully charged. Therefore, the impedance in this region was ascribed to be only the solution resistance. Because of small radius of the AgmE and the b-BAgmE, and thereby also small value of
Animation 1. Construction an preparation of the b-BAgmE.
d) VB2 – DPV: Ei = -0.1 V; Ee = -0.6 V; Es = 3 mV; dE = 50 mV; timp =40 ms. Before each series of experiments the b-BAgmE sensors surface was polished with 0.05 μm alumina slurry, and rinsed with distilled water. 3. Results and discussion 3.1. Cyclic voltammetry measurements Detailed CV study was performed on the b-BAgmE according to the procedure described in Section 2.4. The signals obtained for 5 mmol·L−1 [Ru(NH3)6]3+ redox probe which were registered on the silver annular band electrode (AgABE, 9.61 mm2), the micro-electrode (AgmE, 25 μm) and the self-made b-BAgmE are presented in Fig. 2A–C. The CV signals recorded for the AgABE (Fig. 2A) have the typical 3
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Fig. 2. Comparative electrochemical study of the tested electrodes in 5 mmol·L−1 [Ru(NH3)6]3+ and 0.1 mol·L−1 KCl. CV curves of AgABE (A), AgmE (B) and bBAgmE (C). Scan rates values: a) 200, b) 100, c) 50, d) 25, e) 12.5, f) 6.3, g) 4.2, h) 2.1 and i) 1 mV·s−1. EIS experiments results: Nyquist plots for AgABE (D), AgmE (E) and b-BAgmE (F) and Bode plots for AgABE (G), AgmE (H) and b-BAgmE (I). Red line in (E) and (F) represents charge-transfer capacitive semi-circle. Other parameters as in Section 2.4 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
and In3+ were selected as an example of substances which are often voltammetrically tested and vitamin B2 was chosen as an example of organic compound. Based on literature reports standard measurement procedures (described in details in Section 2.4) were applied. For each calibration curve, the limit of detection (LOD) and the limit of quantification (LOQ) were calculated according to the following equations: LOD = 3SD/b and LOQ = 10SD/b, where SD stands for standard deviation of blank and b is the slope of the calibration curve. The regression data of the obtained calibration curves are summarized in Table 2. For all determined analytes, the registered voltammograms show well-defined peaks current of relatively small half-width. In case of heavy metal ions the appearance of the peak is associated with simple anodic oxidation and in case of vitamin B2 redox reactions undergo by accepting and donating 2 electrons in the isoalloxazine ring [27]. Moreover, for Tl+ and In3+ measurements, at −0.85 V, a high, wellshaped peak coming from electroreduction of nitrate ions (NO3−) from the supporting electrolyte is visible [28]. With the increasing concentrations of the studied Tl+ and In3+ ions their signals increase linearly, while the peak from the supporting electrolyte decreases regularly. This additional peak of decreasing height, however, did not affect the quality of the obtained calibration curve. High background current caused by the supporting electrolyte (acetate buffer) is visible also in VB2 voltammograms. Therefore, in order to interpret voltammograms correctly, the baseline (background current) correction was necessary for all tested analytes.
Table 1 Comparison of properties of the different Ag electrodes on the basis of CV measurements. Parameter
symbol
unit
AgABE
AgmE
b-BAgmE
the peak current ratio geometric surface area effective surface areas anodic peak current density cathodic peak current density
Ipa/Ipc A Aeff ja jc
– mm2 mm2 μA·mm−2 μA·mm−2
0.95 9.61 9.47 6.39 6.75
1.03 0.00049 0.00038 37.6 36.6
1.03 0.225 0.227 18.2 17.4
their RC-time constant, the capacitive charging region shifted toward higher frequencies. The phase angle at high frequencies, close to 21° and 36° for the b-BAgmE and the AgmE respectively, confirmed nearly capacitive behavior of these electrodes. This proves that the developed in our team b-BAgmE poses the traits which are unique for microelectrodes [26].
3.3. Analytical performance In order to verify the usability of the b-BAgmE in voltammetric analysis a series of DP voltammograms for increasing concentrations of different analytes were recorded (Fig. 3). For this purpose typical, well-studied and easily determinable on silver electrodes substances were chosen. Heavy metal ions: Pb2+, Tl+ 4
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Fig. 3. DP voltammograms obtained at the b-BAgmE in oxygen-free supporting electrolyte containing increasing concentrations of Pb2+ (A), Tl+ (B), In3+ (C) and VB2 (D); the concentrations ranged from 0 (blank - dashed line) to 250, 2720, 560 and 3870 nmol·L−1, respectively. The instrumental parameters are described in Section 2.4.
this construction. The B-BAgmE does not require any surface modification, hence it is ready for use very quickly. The clear design of the sensor facilitates reproduction of this construction and makes it extremely reliable and versatile. Presented electrode has the properties that are needed to develop an automatic and maintenance-free sensor for environmental measurements. Moreover, it enables measurements under sterile conditions, which is necessary for tests carried out e.g. on biological samples. Its simplicity and all the advantages of microelectrodes (i.a. microelectrode effect, negligible residual current, possibility of working in a two-electrode system) enable a wide range of applications for this electrode. The EIS and CV experiments confirmed occurrence of the microelectrode effect. It has been proven that the depolarizer is transported to the b-BAgmE by spherical diffusion, therefore the maximum reaction current does not depend on the measurement time. A very favorable signal-to-noise ratio (S/N), high current density and negligible electrical capacity of the double layer were achieved. The b-BAgmE is characterized by short and long-term stability and excellent repeatability and reproducibility. This new type of electrode was successfully applied in quantitative determination of Pb2+, Tl+, In3+ ions and vitamin B2, what proves versatility of the b-BAgmE in both, organic and inorganic electroanalysis. Further investigations should focus on real samples analysis, such as environmental samples (water, soil) or pharmaceuticals.
3.4. Stability of the b-BAgmE Stability study was performed using DPV technique in the oxygenfree supporting electrolyte (10 mmol·L−1 KCl and HNO3) spiked with 100 nmol·L−1 Pb2+. After registering 20 curves, voltammetric response of the electrode in pure supporting electrolyte remained unaltered. Likewise, subsequent 20 signals obtained for 100 nmol·L−1 Pb2+ remained stable, and the calculated RSD was equal to 4.1%. In order to examine a long-term stability, the analogous experiment was repeated after 17 days. The registered voltammograms were characterized by very good repeatability (RSD = 5.7%). The obtained results show high quality of the b-BAgmE in respect to its stability. However, it should be mentioned that this stability was maintained only when the electrode was stored in the supporting electrolyte. Exposure to atmospheric oxygen caused deterioration of the signal quality (RSD ≈ 15%). Therefore, between the individual measuring series, when the electrode is in long contact with air, its surface should be polished with alumina slurry.
4. Conclusion In this work, for the first time, the design and fabrication of the original b-BAgmE sensor was described. This unusual and very simple construction is made of inexpensive and environmentally friendly materials, what in connection with uncomplicated use makes it a highly desirable tool, consistent with contemporary trends in electroanalysis. The unique shape of the b-BAgmE combines the advantages of microband and annular band electrodes. It supports removal of unwanted gas bubbles from the electrode body and enables further development of
Acknowledgements This work was supported by the National Science Centre, Poland (Project No. 2018/31/B/NZ6/02472).
Table 2 Regression data of the calibration curves for the DPV qualitative determination of Pb2+, Tl+, In3+ and VB2 on the b-BAgmE. Parameter Linear range Slope (b) Intercept (a) Correlation coefficient (R) LOD LOQ RSD (n = 5)
Unit −1
nmol·L μA·L·μmol−1 μA – nmol·L−1 nmol·L−1 %
Pb2+
Tl+
In3+
VB2
2.8 – 250 0.76 ± 0.02 −0.008 ± 0.002 0.9988 0.4 1.2 4.1
190 – 2720 0.81 ± 0.02 0.09 ± 0.04 0.9958 60 180 5.3
20 – 560 20.3 ± 0.4 0.3 ± 0.1 0.9981 5 15 4.7
660 – 3870 0.026 ± 0.002 0.016 ± 0.004 0.9951 70 210 5.9
5
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JL and RP gratefully acknowledge financial support from the National Center for Research and Development, Project No. POWR.03.02.00-00-1004/16. The authors would like to express their deepest gratitude to Dr. Witold Reczyński for his help, support and providing language help.
1016/j.jelechem.2008.10.002. [18] B. Baś, K. Jedlińska, K. Węgiel, New electrochemical sensor with the renewable silver annular band working electrode: fabrication and application for determination of selenium (IV) by cathodic stripping voltammetry, Electrochem. Commun. 49 (2014) 79–82, https://doi.org/10.1016/j.elecom.2014.10.004. [19] B. Baś, M. Jakubowska, W. Reczyński, F. Ciepiela, W.W. Kubiak, Rapidly renewable silver and gold annular band electrodes, Electrochim. Acta 73 (2012) 98–104, https://doi.org/10.1016/j.electacta.2011.12.125. [20] B. Baś, K. Węgiel, K. Jedlińska, The renewable bismuth bulk annular band working electrode: fabrication and application in the adsorptive stripping voltammetric determination of nickel(II) and cobalt(II), Anal. Chim. Acta 881 (2015) 44–53, https://doi.org/10.1016/j.aca.2015.05.005. [21] B. Baś, K. Węgiel, K. Jedlińska, New voltammetric sensor based on the renewable bismuth bulk annular band electrode and its application for the determination of palladium(II), Electrochim. Acta 178 (2015) 665–672, https://doi.org/10.1016/j. electacta.2015.08.047. [22] B. Baś, S. Baś, Rapidly renewable silver amalgam annular band electrode for voltammetry and polarography, Electrochem. Commun. 12 (2010) 816–819, https:// doi.org/10.1016/j.elecom.2010.03.041. [23] K. Jedlińska, M. Strus, B. Baś, A new electrochemical sensor with the Refreshable Silver Liquid Amalgam Film multi-Electrode for sensitive voltammetric determination of vitamin K2 (menaquinone), Electrochim. Acta 265 (2018) 355–363, https://doi.org/10.1016/j.electacta.2018.01.204. [24] K. Jedlińska, J. Lipińska, S. Smarzewska, B. Baś, The Bi-Disc glassy carbon electrode for determination of vitamin K2 (Menaquinone) using stripping voltammetry, J. Electrochem. Soc. 166 (2019) B360–B366, https://doi.org/10.1149/2.0331906jes. [25] F. Scholz, Electroanalytical Methods: Guide to Experiments and Applications, (2010), https://doi.org/10.1007/978-3-642-02915-8_4. [26] A.K. Ahuja, M.R. Behrend, J.J. Whalen, M.S. Humayun, J.D. Weiland, The dependence of spectral impedance on disc microelectrode radius, IEEE Trans. Biomed. Eng. 55 (2012) 1457–1460, https://doi.org/10.1109/TBME.2007.912430.The. [27] M.D. Lovander, J.D. Lyon, D.L. Parr, J. Wang, B. Parke, J. Leddy, Review-electrochemical properties of 13 vitamins: a critical review and assessment, J. Electrochem. Soc. 165 (2018) G18–G49, https://doi.org/10.1149/2.1471714jes. [28] S.M. Shariar, T. Hinoue, Simultaneous voltammetric determination of nitrate and nitrite ions using a copper electrode pretreated by Dissolution/Redeposition, Anal. Sci. 26 (2010) 1173–1179.
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Katarzyna Jedlińska (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Her research interests include designing and fabrication of new type of working electrodes for electrochemical analysis. The field of her interests involves also development and improvement of measurement procedures using electrochemical methods such as cyclic voltammetry, differential pulse voltammetry and chronopotentiometry. Radosław Porada (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Analysis of biologically active compounds, such as vitamins and drugs are the main goals of his research. He is also interested in study mechanisms and kinetics of the electrode processes using electrochemical impedance spectroscopy and cyclic voltammetry. Justyna Lipińska (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Her research is focused on electrochemical analysis of drugs, vitamins and other biological active compounds using methods such as cyclic voltammetry and differential pulse voltammetry. She is also interested in fabrication of new type of sensors based on co-called black glass. Bogusław Baś (Prof.) is a head of the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. He is the author of over 300 scientific publications and 10 patents. He has a long-standing interest in design and development of electroanalytical instruments, such as multipurpose electrochemical analyzers, potentiostats, double layer capacitance meter etc. which are commercially available. His interests involve also construction and improvement of different types of working electrodes and their application in electrochemical trace analysis.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
The bi-band silver microelectrode: Fabrication, characterization and analytical study
T
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Katarzyna Jedlińska , Radosław Porada, Justyna Lipińska, Bogusław Baś AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Analytical Chemistry, Mickiewicza 30, 30-059, Cracow, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Mercury-free sensors Silver electrodes Microelectrodes Microband electrode Microelectrode effect Voltammetry
In this work a new type of working electrode for voltammetry – the bi-band silver microelectrode (b-BAgmE) – is presented for the first time. The sensor consists of two Ag bands 25 μm thick and 4.5 mm long. The bands are arranged symmetrically and contrariwise, along the lateral, cylindrical surface of the Ag guide, coated with a thin layer of epoxy resin. Simple and inexpensive microelectrode construction ensures its reliability of use as well as wide possibilities of application in electroanalysis. Electrochemical properties of the sensor were tested by means of the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) in the [Ru(NH3)6]2+/3+ redox system. Comparative experiments, carried out simultaneously on a homemade silver annular band electrode (AgABE), commercially available silver microelectrode (AgmE) and b-BAgmE, confirmed occurrence of the microelectrode effect in case of the AgmE and b-BAgmE. Moreover, the b-BAgmE is characterized by significantly lower value of charge-transfer resistance Rct = 179 ± 6 Ω in comparison to AgmE Rct = 6.0 ± 0.4 MΩ. The practical usefulness of the b-BAgmE was confirmed by determination of Pb2+, Tl+, In3+ and vitamin B2 using the differential pulse (DP) voltammetric technique.
1. Introduction
In 2009, the theoretical idea of a new type of working electrode – the annular microband electrode (AMB) was presented [17]. In contrast to conventional constructions, cylindrical surface of the AMB was located on lateral surface of the sensor. Numerical simulations demonstrated highly efficient diffusion properties of such kind of the electrode shape. Beside the theoretical model, there are several examples of applications of annular band electrodes made of silver [18], gold [19] and bismuth [20,21]. The unusual vertical position of the electrode surface causes significant facilitation of gas bubbles removal, therefore gaseous electroreaction products and deareation gases do not disturb recorded signals. It also hastens and facilitates polishing of the electrode surface at the same time preventing from making it uneven or slanted. Moreover, this unique shape allows improvement of this construction by use of additional items, such as outer cylindrical sensor body as an activation cell [18], or applying thin mercury film in the renewable electrodes [22]. Simple and inexpensive fabrications procedures as well as numerous advantages of the annular band electrodes induced creation of numerous constructions of this type [23,24]. In this communication, fabrication of a new voltammetric sensor – the bi-band silver microelectrode (b-BAgmE) is presented. This uncomplicated, low-cost and environmentally friendly construction is proposed for the first time. Thanks to its unique shape, this construction
Microelectrodes are used in various applications in contemporary electrochemical trace analysis [1–9] due to numerous advantageous properties, including low ohmic drop, fast mass-transport rate and low interfacial capacitance [9–12]. These unique electrochemical characteristics result from their micro-dimensions, much smaller (≤25 μm) than thickness of the diffusion layer, therefore very efficient hemispherical diffusion of the depolarizer to the microelectrode surface takes place. In such conditions there is no dependence of the maximal reaction current on the measurement time (the steady state is reached), higher current density is achieved and the signal-to-noise ratio is improved [9–15]. Moreover, the microelectrodes allow conducting measurements in highly resistive media or even without adding the supporting electrolyte [5–8]. The band type microelectrodes take the shape of an elongated band of width less than 25 μm and length even in the centimeter range [9–14,16]. They share advantageous features of the microelectrodes; however, much larger surface area causes registration of significantly higher current values. Therefore, in case of the band microelectrodes, use of additional equipment such as Faraday cage or special and sensitive measuring apparatus, is not necessary. ⁎
Corresponding author. E-mail address: [email protected] (K. Jedlińska).
https://doi.org/10.1016/j.snb.2019.127152 Received 25 July 2019; Received in revised form 12 September 2019; Accepted 15 September 2019 Available online 20 September 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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combines advantages of all types of electrodes mentioned above: microelectrodes, band and annular band electrodes. To investigate the favorable features of the b-BAgmE, cyclic voltammetry and electrochemical impedance spectroscopy were used. The proposed electrode was successfully applied in DP voltammetric determination of Pb2+, Tl+, In3+ and vitamin B2. 2. Experimental section 2.1. Measuring apparatus The voltammetric measurements were carried out using the M161 multipurpose electrochemical analyzer with the M164 electrode stand made by mtm-anko, Poland. The electrochemical impedance spectroscopic (EIS) measurements were performed using the μAutolab III (EcoChemie, the Netherlands) with FRA2 (Frequency Response Analyzer) module. A classical 3-electrode cell with the homemade biband silver microelectrode (b-BAgmE) as a working electrode, Pt wire as an auxiliary electrode and a silver chloride double-junction Ag|AgCl|3 M KCl|2.5 M KNO3 as a reference electrode, were applied. The signals registered at the b-BAgmE were compared with the signals obtained using the homemade silver annular band electrode (AgABE) [18] and the commercially available microelectrode (AgmE) (25 μm, Metrohm-Autolab, The Netherlands). A magnetic stirrer (ca. 200 rpm) was used during the accumulation period. All experiments were conducted in ambient temperature 22 °C.
Fig. 1. Construction of the b-BAgmE: the inner silver core scheme (A), the electrode construction scheme (B), photography of the sensor (C). Ag foil piece (a), Ag wire (b), flattened Ag wire end (c), epoxy resin (d).
high reliability and failure-free use of the electrode. Similar to the other types of annular band electrodes, vertical position of the electrode surfaces facilitates gases removal from the measuring system.
2.2. Chemicals and glassware All reagents were of analytical grade. Unless otherwise noted, chemicals were purchased from Sigma-Aldrich. Standard solutions of Pb2+, Tl+, In3+ were purchased from Merck (Certipur®, 1000 mg L-1) and diluted as required. The vitamin B2 (VB2, riboflavin) and [Ru(NH3)6]3+ standard stock solutions were prepared shortly before measurements. 1 mol·L-1 acetate buffer solution (pH 4.5) was prepared by mixing the appropriate amounts of acetic acid and crystalline sodium acetate. All solutions were prepared using 4 times distilled water (quartz). The b-BAgmE was prepared from a silver foil (Alfa Aesar, Germany, 99.95%, 25 μm), a silver wire (The Mint of Poland, Poland, 93%, ϕ = 0.7 mm) and TRANSLUX D180 resin (AKSON, France). For polishing the electrode surface, MicroPolish™ Alumina suspensions (1.0, 0.3 and 0.05 μm, Buehler Micropolish II, Lake Bluff, IL USA) and MasterTex polishing cloth (Buehler), were used.
2.4. Standard procedures of measurements Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical properties of the b-BAgmE in terms of its use in voltammetry. The measurements were performed in the presence of 5 mmol·L−1 [Ru(NH3)6]3+ redox probe in 0.1 mol·L−1 KCl solution. Cyclic voltammograms were recorded in the potential range from -0.45 to +0.10 V with scan rates varying between 1 and 200 mV·s−1. Impedance measurements were conducted at the formal potential of the Ru3+/Ru2+ redox couple, on which the sinusoidal signals of amplitude 10 mV and frequency ranging from 100 kHz to 0.1 Hz were subsequently superimposed. An electrical circuit model was fitted to the recorded data, providing an insight into the electron transfer kinetics (modelled by charge-transfer resistance Rct), type of diffusive movement of the electroactive species towards the b-BAgmE surface (Warburg impedance) and structure of the electrical double layer at the electrode – solution interface (capacitance Cdl). Differential pulse voltammetry (DPV) was employed for determination of Pb2+, Tl+, In3+ and vitamin B2 (VB2). Heavy metal ions were analyzed in the supporting electrolyte containing 10 mmol·L−1 HNO3 and 10 mmol·L−1 KCl, whereas VB2 was analyzed in 0.04 mol·L−1 acetate buffer solution. During measurements the following techniques and parameters were applied:
2.3. Preparation of the b-BAgmE A schematic diagram of the b-BAgmE fabrication steps and its photography are illustrated in Fig. 1. A 3D animation of the electrode construction is available in the electronic version of this article (Animation 1 ). The metallic core of the sensor (Fig. 1A) was prepared with a silver wire (ϕ = 0.7 mm) and a silver foil (25 μm). One end of the Ag wire was flattened with a vise over a length of ca. 20 mm. At a distance of ca. 10 mm from the end of the wire it was bent into the shape of the extended letter “U”. In the slit formed in this manner, a properly cut piece of Ag foil (4.50 x 12 mm) was symmetrically placed. Afterwards, sides of the Ag wire were mechanically tightened to stabilize the construction and to ensure electric contact. Prepared in this way main element of the electrode was cleaned with acetone and covered with epoxy resin. When the process of resin polymerization was finished the surplus of Ag foil and the excess of resin were removed with emery papers (up to 3000 grit size) and polished with 1.0, 0.3 and 0.05 μm alumina slurry. Finally, the electrode body was rinsed with water and cleaned for 5 min in an ultrasonic bath with distilled water. This extremely simple design enables easy reconstruction of the bBAgmE using low-cost, environmentally friendly materials and uncomplicated equipment. Furthermore, the plain structure guarantees
a) Pb2+ - differential pulse anodic stripping voltammetry (DP ASV): accumulation potential (Eacc), -0.5 V; accumulation time (tacc), 60 s; initial potential (Ei), -0.5 V; end potential (Ee), -0.2 V; step potential (Es) 3 mV; pulse amplitude (dE), 30 mV; pulse period (timp = (tw + ts), 20 ms; in each case: tw = ts (waiting time = current sampling time). b) Tl+ - DP ASV: Eacc = -0.8 V; tacc = 60 s; Ei = -0.8 V; Ee = -0.2 V; Es = 3 mV; dE = 30 mV; timp =20 ms. c) In3+ - DP ASV: Eacc = -1.3 V; tacc = 60 s; Ei = -1.3 V; Ee = -0.2 V; Es = 5 mV; dE = 30 mV; timp =20 ms. 2
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shape, characteristic for a reversible process under linear diffusion conditions. Clearly outlined peaks for both, cathodic (Ipc) and anodic (Ipa) directions are visible for all scan rate values (υ). In case of the AgmE (Fig. 2B), the same effect is noticeable only for υ > 50 mV·s−1. For lower υ values the microelectrode effect is visible, i.e. the CV curves have the sigmoidal shape characteristic for voltammograms registered under the spherical diffusion conditions. Plateau of the current is observed, and the curves of the oxidation processes are almost superimposed on that of the reduction processes. Comparing the voltammograms recorded on the b-BAgmE (Fig. 2C) with the AgABE and the AgmE signals, similarity to the curves registered on the AgmE can be clearly seen. It confirms, that the studied redox process very quickly reaches the constant maximum current value. Thus, the concentration gradient of the depolarizer at the microelectrode surface does not depend on the time of electrolysis, and its transport takes place almost exclusively through spherical diffusion. Consequently, it has been proven that for relatively wide range of scans it is possible to achieve the microelectrode effect also for the b-BAgmE. The results of comparative calculations made on the basis of the data obtained during CV measurements are summarized in Table 1. For all types of electrodes anodic and cathodic peaks’ currents increased proportionally with increasing of the scan rate, and linear dependences between those peak currents and square root of υ were, obtained. Moreover, in each case the peak current ratio Ipa/Ipc was close to 1. Therefore, it can be stated that the electrode reaction of Ru2+/Ru3+ on all tested electrodes is reversible and diffusion controlled. The effective surface areas (Aeff) of the tested electrodes were calculated basing on the Randles-Sevčic equation [24,25] for reversible processes (for Ru2+/ Ru3+ the electron transfer number n = 1, the diffusion coefficient D = 8.45·10−6 cm2·s-1). The Aeff value of the b-BAgmE is perfectly consistent with its geometrical surface (A). Furthermore, current density registered on the b-BAgmE is almost 3-times higher than on the macroelectrode (AgABE), and only 2-times lower than on the AgmE, which proves high efficiency of depolarizer ions transport to the bBAgmE surface. 3.2. Electrochemical impedance spectroscopic measurements Further comparative tests between the AgABE, AgmE and b-BAgmE electrodes were performed with EIS in accordance with the procedure set out in Section 2.4. The Nyquist plot for the AgABE (Fig. 2D) was composed only of the straight line, which indicates the fast charge-transfer kinetics and slower mass transfer. In case of the AgmE, the Nyquist plot (Fig. 2E) shows a semi-circle caused by limitation in charge-transfer resulting from small surface of the AgmE relative to the AgABE. Fitting the spectra gave the charge-transfer resistance value (Rct) equal to (6.0 ± 0.4) MΩ. For the b-BAgmE (Fig. 2F) the Nyquist plot is composed of semicircle in the range of high frequencies and straight line in the lower range. Basing on the fitted impedance spectra the value of Rct was estimated to be equal to (179 ± 6) Ω. The observed difference in the estimated parameters results from geometry and radius of the studied electrodes. The corresponding Bode plots (Fig. 2 G–I) provided better insight into that observation. The RC-time constant is directly proportional to the electrode radius, meaning that for the smaller electrodes, charging and discharging of the double layer occurs much faster [26]. For the AgABE the impedance magnitude was frequently independent in the range from 10 Hz to 100 kHz. Below that range, the impedance magnitude started to increase, which was caused by capacitive charging step at the electrode-solution interface. This frequency range is called the capacitive charging region [26]. For the AgABE the phase angle value at the highest frequencies was close to zero; due to the big time constant the double layer was not fully charged. Therefore, the impedance in this region was ascribed to be only the solution resistance. Because of small radius of the AgmE and the b-BAgmE, and thereby also small value of
Animation 1. Construction an preparation of the b-BAgmE.
d) VB2 – DPV: Ei = -0.1 V; Ee = -0.6 V; Es = 3 mV; dE = 50 mV; timp =40 ms. Before each series of experiments the b-BAgmE sensors surface was polished with 0.05 μm alumina slurry, and rinsed with distilled water. 3. Results and discussion 3.1. Cyclic voltammetry measurements Detailed CV study was performed on the b-BAgmE according to the procedure described in Section 2.4. The signals obtained for 5 mmol·L−1 [Ru(NH3)6]3+ redox probe which were registered on the silver annular band electrode (AgABE, 9.61 mm2), the micro-electrode (AgmE, 25 μm) and the self-made b-BAgmE are presented in Fig. 2A–C. The CV signals recorded for the AgABE (Fig. 2A) have the typical 3
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Fig. 2. Comparative electrochemical study of the tested electrodes in 5 mmol·L−1 [Ru(NH3)6]3+ and 0.1 mol·L−1 KCl. CV curves of AgABE (A), AgmE (B) and bBAgmE (C). Scan rates values: a) 200, b) 100, c) 50, d) 25, e) 12.5, f) 6.3, g) 4.2, h) 2.1 and i) 1 mV·s−1. EIS experiments results: Nyquist plots for AgABE (D), AgmE (E) and b-BAgmE (F) and Bode plots for AgABE (G), AgmE (H) and b-BAgmE (I). Red line in (E) and (F) represents charge-transfer capacitive semi-circle. Other parameters as in Section 2.4 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
and In3+ were selected as an example of substances which are often voltammetrically tested and vitamin B2 was chosen as an example of organic compound. Based on literature reports standard measurement procedures (described in details in Section 2.4) were applied. For each calibration curve, the limit of detection (LOD) and the limit of quantification (LOQ) were calculated according to the following equations: LOD = 3SD/b and LOQ = 10SD/b, where SD stands for standard deviation of blank and b is the slope of the calibration curve. The regression data of the obtained calibration curves are summarized in Table 2. For all determined analytes, the registered voltammograms show well-defined peaks current of relatively small half-width. In case of heavy metal ions the appearance of the peak is associated with simple anodic oxidation and in case of vitamin B2 redox reactions undergo by accepting and donating 2 electrons in the isoalloxazine ring [27]. Moreover, for Tl+ and In3+ measurements, at −0.85 V, a high, wellshaped peak coming from electroreduction of nitrate ions (NO3−) from the supporting electrolyte is visible [28]. With the increasing concentrations of the studied Tl+ and In3+ ions their signals increase linearly, while the peak from the supporting electrolyte decreases regularly. This additional peak of decreasing height, however, did not affect the quality of the obtained calibration curve. High background current caused by the supporting electrolyte (acetate buffer) is visible also in VB2 voltammograms. Therefore, in order to interpret voltammograms correctly, the baseline (background current) correction was necessary for all tested analytes.
Table 1 Comparison of properties of the different Ag electrodes on the basis of CV measurements. Parameter
symbol
unit
AgABE
AgmE
b-BAgmE
the peak current ratio geometric surface area effective surface areas anodic peak current density cathodic peak current density
Ipa/Ipc A Aeff ja jc
– mm2 mm2 μA·mm−2 μA·mm−2
0.95 9.61 9.47 6.39 6.75
1.03 0.00049 0.00038 37.6 36.6
1.03 0.225 0.227 18.2 17.4
their RC-time constant, the capacitive charging region shifted toward higher frequencies. The phase angle at high frequencies, close to 21° and 36° for the b-BAgmE and the AgmE respectively, confirmed nearly capacitive behavior of these electrodes. This proves that the developed in our team b-BAgmE poses the traits which are unique for microelectrodes [26].
3.3. Analytical performance In order to verify the usability of the b-BAgmE in voltammetric analysis a series of DP voltammograms for increasing concentrations of different analytes were recorded (Fig. 3). For this purpose typical, well-studied and easily determinable on silver electrodes substances were chosen. Heavy metal ions: Pb2+, Tl+ 4
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Fig. 3. DP voltammograms obtained at the b-BAgmE in oxygen-free supporting electrolyte containing increasing concentrations of Pb2+ (A), Tl+ (B), In3+ (C) and VB2 (D); the concentrations ranged from 0 (blank - dashed line) to 250, 2720, 560 and 3870 nmol·L−1, respectively. The instrumental parameters are described in Section 2.4.
this construction. The B-BAgmE does not require any surface modification, hence it is ready for use very quickly. The clear design of the sensor facilitates reproduction of this construction and makes it extremely reliable and versatile. Presented electrode has the properties that are needed to develop an automatic and maintenance-free sensor for environmental measurements. Moreover, it enables measurements under sterile conditions, which is necessary for tests carried out e.g. on biological samples. Its simplicity and all the advantages of microelectrodes (i.a. microelectrode effect, negligible residual current, possibility of working in a two-electrode system) enable a wide range of applications for this electrode. The EIS and CV experiments confirmed occurrence of the microelectrode effect. It has been proven that the depolarizer is transported to the b-BAgmE by spherical diffusion, therefore the maximum reaction current does not depend on the measurement time. A very favorable signal-to-noise ratio (S/N), high current density and negligible electrical capacity of the double layer were achieved. The b-BAgmE is characterized by short and long-term stability and excellent repeatability and reproducibility. This new type of electrode was successfully applied in quantitative determination of Pb2+, Tl+, In3+ ions and vitamin B2, what proves versatility of the b-BAgmE in both, organic and inorganic electroanalysis. Further investigations should focus on real samples analysis, such as environmental samples (water, soil) or pharmaceuticals.
3.4. Stability of the b-BAgmE Stability study was performed using DPV technique in the oxygenfree supporting electrolyte (10 mmol·L−1 KCl and HNO3) spiked with 100 nmol·L−1 Pb2+. After registering 20 curves, voltammetric response of the electrode in pure supporting electrolyte remained unaltered. Likewise, subsequent 20 signals obtained for 100 nmol·L−1 Pb2+ remained stable, and the calculated RSD was equal to 4.1%. In order to examine a long-term stability, the analogous experiment was repeated after 17 days. The registered voltammograms were characterized by very good repeatability (RSD = 5.7%). The obtained results show high quality of the b-BAgmE in respect to its stability. However, it should be mentioned that this stability was maintained only when the electrode was stored in the supporting electrolyte. Exposure to atmospheric oxygen caused deterioration of the signal quality (RSD ≈ 15%). Therefore, between the individual measuring series, when the electrode is in long contact with air, its surface should be polished with alumina slurry.
4. Conclusion In this work, for the first time, the design and fabrication of the original b-BAgmE sensor was described. This unusual and very simple construction is made of inexpensive and environmentally friendly materials, what in connection with uncomplicated use makes it a highly desirable tool, consistent with contemporary trends in electroanalysis. The unique shape of the b-BAgmE combines the advantages of microband and annular band electrodes. It supports removal of unwanted gas bubbles from the electrode body and enables further development of
Acknowledgements This work was supported by the National Science Centre, Poland (Project No. 2018/31/B/NZ6/02472).
Table 2 Regression data of the calibration curves for the DPV qualitative determination of Pb2+, Tl+, In3+ and VB2 on the b-BAgmE. Parameter Linear range Slope (b) Intercept (a) Correlation coefficient (R) LOD LOQ RSD (n = 5)
Unit −1
nmol·L μA·L·μmol−1 μA – nmol·L−1 nmol·L−1 %
Pb2+
Tl+
In3+
VB2
2.8 – 250 0.76 ± 0.02 −0.008 ± 0.002 0.9988 0.4 1.2 4.1
190 – 2720 0.81 ± 0.02 0.09 ± 0.04 0.9958 60 180 5.3
20 – 560 20.3 ± 0.4 0.3 ± 0.1 0.9981 5 15 4.7
660 – 3870 0.026 ± 0.002 0.016 ± 0.004 0.9951 70 210 5.9
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K. Jedlińska, et al.
JL and RP gratefully acknowledge financial support from the National Center for Research and Development, Project No. POWR.03.02.00-00-1004/16. The authors would like to express their deepest gratitude to Dr. Witold Reczyński for his help, support and providing language help.
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Katarzyna Jedlińska (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Her research interests include designing and fabrication of new type of working electrodes for electrochemical analysis. The field of her interests involves also development and improvement of measurement procedures using electrochemical methods such as cyclic voltammetry, differential pulse voltammetry and chronopotentiometry. Radosław Porada (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Analysis of biologically active compounds, such as vitamins and drugs are the main goals of his research. He is also interested in study mechanisms and kinetics of the electrode processes using electrochemical impedance spectroscopy and cyclic voltammetry. Justyna Lipińska (M.Sc.) is a PhD student at the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. Her research is focused on electrochemical analysis of drugs, vitamins and other biological active compounds using methods such as cyclic voltammetry and differential pulse voltammetry. She is also interested in fabrication of new type of sensors based on co-called black glass. Bogusław Baś (Prof.) is a head of the Department of Analytical Chemistry at the AGH University of Science and Technology in Krakow. He is the author of over 300 scientific publications and 10 patents. He has a long-standing interest in design and development of electroanalytical instruments, such as multipurpose electrochemical analyzers, potentiostats, double layer capacitance meter etc. which are commercially available. His interests involve also construction and improvement of different types of working electrodes and their application in electrochemical trace analysis.
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