Use of pyrolyzed paper as disposable substrates for voltammetric determination of trace metals

Author’s Accepted Manuscript Use of pyrolyzed paper as disposable substrates for voltammetric determination of trace metals Luiz André Juvêncio Silva, Weberson Pereira da Silva, Jason G. Giuliani, Sheila Cristina Canobre, Carlos D. Garcia, Rodrigo Alejandro Abarza Munoz, Eduardo Mathias Richter www.elsevier.com/locate/talanta

PII: DOI: Reference:

S0039-9140(16)30945-6 http://dx.doi.org/10.1016/j.talanta.2016.12.009 TAL17099

To appear in: Talanta Received date: 9 October 2016 Revised date: 30 November 2016 Accepted date: 3 December 2016 Cite this article as: Luiz André Juvêncio Silva, Weberson Pereira da Silva, Jason G. Giuliani, Sheila Cristina Canobre, Carlos D. Garcia, Rodrigo Alejandro Abarza Munoz and Eduardo Mathias Richter, Use of pyrolyzed paper as disposable substrates for voltammetric determination of trace metals, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.12.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Use of pyrolyzed paper as disposable substrates for voltammetric determination of trace metals. Luiz André Juvêncio Silva1, Weberson Pereira da Silva, Jason G. Giuliani2, Sheila Cristina Canobre1, Carlos D Garcia3, Rodrigo Alejandro Abarza Munoz1, Eduardo Mathias Richter1* 1

Universidade Federal de Uberlândia, Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2121, Uberlândia, MG, 38408100, Brazil. 2

Department of Chemistry, UT San Antonio, Texas, U.S.A.

3

Department of Chemistry, Clemson University, South Carolina, U.S.A.

*Av. João Naves de Ávila, 2121-Bairro Santa Mônica-Uberlândia-MG-Brazil. CEP 38400-902.

Tel.:

55-34-3239-4143

–

r

206;

Fax:

55-34-3239-4208.

[email protected]

ABSTRACT The possibility of using pyrolyzed paper as disposable working electrodes for trace metals determination is reported for the first time. A small piece of pyrolyzed paper (0.7 x 0.7 cm) was positioned at the bottom side of the electrochemical cell using a rubber O-ring, which defined the electrode area (0.48 cm; 0.18 cm2). A large number of electrodes can be obtained from a single piece of standard dimensions (2.5 cm × 7.5 cm) of paper, therefore minimizing the cost per 1

unit. The electrochemical performance of the pyrolyzed paper was demonstrated by cyclic voltammetry, electrochemical impedance spectroscopy and by the determination of Zn, Cd, and Pb by square-wave anodic stripping voltammetry. The unmodified pyrolyzed paper showed excellent performance for Pb and Cd detection (LOD = 0.19 and 0.16 ppb, respectively). In the presence of Bi3+ (in-situ film formation), the simultaneous determination of Zn, Cd and Pb was also possible (LOD = 0.26, 0.25, and 0.39 ppb, respectively).

Keywords: Paper electrode; Carbon substrate; Stripping voltammetry; Cadmium; Lead; Zinc; Simultaneous determination.

1. Introduction The search to identify new substrates for use in electrochemical sensing systems is constant. Among the substrates that have seen the most important advances in recent decades, it’s worth pointing out carbon-based substrates [1-3]. These materials exhibit significant advantages in electroanalytical chemistry, such as mechanical and chemical stability, low cost, wide potential range (especially in the anodic region), and low background currents [4-7]. Among the different types of carbon-based substrates used for analytical purposes, glassy carbon (GC), graphite, and carbon paste have been extensively used for electrochemical applications. While nanostructured carbon-based materials (nanotubes, graphene, dots, etc) have also experienced a tremendous growth, their chemical variability,

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cost, and limited availability have impaired their widespread use in disposable electrodes [8-10] and portable electrochemical systems [11]. Addressing some of these shortcomings, a new type of carbon based electrodes was recently proposed by Giuliani et al. [12]. In this work, the authors demonstrated the possibility of producing of carbon substrates from various commercially available paper sources. After a simple pyrolysis step (typically 1000 °C, 5% H2, during 30 min), the paper is converted into a conductive material which can be used as carbon substrate for the construction of working electrodes. This material has already been used as an electrochemical biosensor for uric acid (after modification with uricase) [12] and for the non-enzymatic amperometric determination of glucose (after modification with copper nanoparticles) [13]. The production of other kind of carbon substrates from pyrolysis steps have also been reported in the literature [14-17]. Heavy metals are natural constituents of the environment and their spatial distribution has been changing rapidly due to the increase in human activities [18]. Therefore, the monitoring of heavy metals in different samples needs to be continuous. Several techniques have frequently been used for determination of metals, such as inductively coupled plasma emission spectroscopy, atomic absorption spectroscopy [19-20] and electroanalytical stripping methods [21-23]. The latter techniques can provide portability and impose relatively low instrumental demands, making them attractive alternatives to spectroscopic techniques [24]. In addition, electroanalytical methods are also simple, rapid, sensitive (limit of detection in ppb range), and have the capacity for multi-element determination [25-

3

26]. Further supporting these advantages, a number of traditional carbon-based materials have been used as substrates for the preparation of modified electrodes with high sensitivity for the determination of metals at trace levels [6, 27-29]. With the objective to bridge the advantages of both electroanalytical techniques with those from paper-derived carbon electrodes, pyrolyzed paper was applied for the first time as conductive substrate for the preparation of disposable working electrodes for the detection of trace metals. The performance of the disposable paper electrodes was investigated using zinc, cadmium, and lead as model analytes.

2. Experimental

2.1. Reagents, stock solutions and samples. All solutions were prepared with ultrapure water (R ≥ 18 MΩ.cm) obtained from a Millipore Direct-Q3 water purification system (Bedford, MA, USA). Nitric (64.0 %), sulfuric (95.0-99.0 %) and acetic (99.7%) acids were obtained from Vetec (Rio de Janeiro, Brazil), sodium acetate (98.0% m/v), potassium ferricyanide (99.0%), potassium hexacyanoferrate(II) trihydrate (98.5%) and ethanol (99.5% v/v) from Synth (Diadema, Brazil) and sodium hydroxide and sodium acetate (97.0% m/m) from Dinâmica (Diadema, Brazil). Stock solutions (1000 mg L-1) of zinc, cadmium, lead and bismuth were purchased from Quimlab (Jacareí, Brazil). All reagents were used without further purification. 4

The river water sample was collected from Uberabinha River located in Uberlândia City, Minas Gerais State, Brazil. The rainwater sample was collected in a pre-cleaned plastic tray in the courtyard of Federal University of Uberlândia. After collection, both samples were immediately acidified using nitric acid (0.1 % v/v) and stored in a plastic bottle in a refrigerator (< 4 oC). Prior to the analysis, the pH of the samples was adjusted to 4.7 by the addition of sodium acetate.

2.2 Paper electrode fabrication, electrochemical cell and electrodes The conductive substrates (used as disposable working electrodes) were obtained by pyrolysis of commercial paper pieces (7.5 x 2.5 cm; Whatman 3MM chromatography paper; GE Health Care; Pittsburgh, PA), according to a previously-described procedure [12]. The method is based on the pyrolysis of pieces of paper in the presence of a controlled atmosphere (95% Ar / 5% H2, at 1000 oC for 30 min). In previous works [12-13], the layout and geometric area of the electrodes constructed from pyrolyzed paper pieces were defined using a commercial CO2 laser engraver. In addition, paraffin paper was melted on one side of the substrate (increase in mechanical strength and reduction of water absorption) and silver paint was applied at the upper end of the stem (contact region with the potentiostat). In the present work, we propose a new way to use the substrate as working electrode that significantly simplifies its integration in the electrochemical cell. Figure 1 shows the procedure herein proposed for positioning of the conductive paper substrate in the electrochemical cell. FIGURE 1 5

As can be observed in Fig. 1, a small piece of pyrolyzed paper (0.7 x 0.7 cm) can be easily positioned in the electrochemical cell and used as working electrode. The substrate was placed on a metallic base (stainless steel) and then screwed against a rubber O-ring, immobilized near an orifice at the outside of the cell background (prevents leaks and defines an electrode area of 0.18 cm2). With this procedure, it is possible to get approximately 20 electrodes from a single piece of paper (7.5 x 2.5 cm). In addition, this configuration minimizes the resistance of the electrical contact between the paper electrode and the potentiostat. All voltammetric measurements (using cyclic voltammetry, electrochemical impedance spectroscopy and square wave voltammetry) were performed using an Autolab PGSTAT 128N potenciostat/galvonostat with FRA2 module (Metrohm Autolab B. V., Utrecht, The Netherlands) interfaced to a microcomputer and controlled by Nova 1.11.0 software. A small Ag/AgCl (saturated KCl) electrode [30] and a platinum wire were employed as the reference and auxiliary electrodes, respectively. For comparison, a glassy-carbon disk (geometric area =0.07 cm2, MF-2012 – BASi, West Lafayette, USA) was also used as working electrode. Electrochemical impedance spectroscopy experiments were performed using an Autolab PGSTAT 128N potenciostat with FRA2 module. The AC signal, with an amplitude of 10 mV (rms), was varied in the 0.1 Hz to 50 kHz frequency range. The impedance spectra were then analyzed with the simulation software Nova 1.11.0 by fitting the spectra with a Randles-type equivalent circuit. All electrochemical measurements were performed at room temperature and in the presence of dissolved oxygen.

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3. Results and discussion

3.1. Characterization of the pyrolyzed paper electrode positioned in the electrochemical cell Initially, the electrochemical behavior of the pyrolyzed paper as working electrode (positioned in the electrochemical cell as described in Fig. 1) was examined by cyclic voltammetry using 1 mmol L-1ferro/ferricyanide redox couple as a probe. For comparison, the experiment was also carried out under the same conditions using glassy carbon as working electrode. Representative results obtained are presented in Fig. 2. FIGURE 2 As can be observed, both electrodes exhibit similar electrochemical behavior. A slightly smaller difference between the anodic and cathodic peak potentials was observed for the pyrolyzed paper (435 – 366 = 69 mV) electrode compared to the glassy carbon (440 - 366 = 74 mV) electrode. However, the current density (current/geometric area) detected with the pyrolyzed paper electrode (457 µA cm-2) was 30 % higher than the current density detected with the glassy carbon electrode (349 µA cm-2). This superior current density observed at the pyrolyzed paper electrode can be attributed to a combination of the composition and 3D morphology (cellulose fibers with around 8 μm of diameter) [12]. In order to gain further insights related to the interfacial characteristics of the 7

proposed electrodes (in comparison to those obtained with a traditional glassy carbon electrode), EIS experiments were performed. These measurements were performed in 0.1 mol L−1 H2SO4 containing 1 mmol L-1 ferro/ferricyanide and applying the respective half-wave potentials (Fig. 3). FIGURE 3 The Nyquist spectra obtained for both electrodes (pyrolyzed paper and glassy carbon) present well-defined semicircles from which the values of the ohmic resistance (R) and the charge transfer resistance (Rct) were obtained by extrapolation of the higher-frequency to the real impedance axis. This semicircle at higher frequency is related to the charge-transfer process, which was electrically described as a resistance in parallel with a capacitor related to the solution resistance and charge transfer for electrode/film double layer, respectively. In addition, these spectra evidence contributions from electrolytes and a line with an inclination of approximately 45◦ (Bode plot; data not shown) in the complex-plane impedance plot defining a Warburg region of semi-infinite diffusion of species [31]. By comparing the diameter of these semicircles, it can be concluded that the proposed paper-derived electrodes feature lower impedance values and lower charge transfer resistances (Rct = 55 ). These results are consistent with the reduced thickness of the glassy carbon electrode (Rct = 257 ), which does not seem to be significant for the proposed arrangement. It is also known that the overpotential gradually decreases along the electrode thickness, and the penetration depth of the current may be limited due to ohmic drop in the electrolyte when the electrode thickness is not optimized [32-34]. In addition, these results

8

show that the interfacial charge transfer resistance across the pyrolyzed paper electrode is smaller than for the glassy carbon electrode, which is highly promising for electrochemical sensors. This result also is consistent with the cyclic voltammetric responses (Fig. 2), where the current density detected for the pyrolyzed paper electrode in 0.1 mol L-1 H2SO4 containing 1 mmol L-1 ferro/ferricyanide was 30% higher than that obtained for glassy carbon electrode, indicating that the reduced thickness and porous morphology favors the swelling. Probably, the swelling process may be associated with the accumulation of charge at the electrode-solution interface, which improves the electrochemical sensing parameters [35]. Thus, the process of swelling resulted in the expansion of the active surface area of the pyrolyzed paper electrode, which increased the rate of charge transfer. The performance of the pyrolyzed paper electrode positioned in the electrochemical cell (Fig. 1) was also superior if compared to the results obtained with the same material in a previous work (lower charge transfer resistance) [12]. In that work, the pyrolyzed paper electrode was used as stripshaped base with a length of approximately 3 cm. The electrical contact between the paper electrode and the potentiostat was performed by placing the connector on one side of the paper strip and the working electrode on the other side. In this condition, the charge transfer resistance obtained by EIS was much higher (63 kΩ). The better performance of the pyrolyzed paper electrode in the proposed work is probably due to the improvement in the electrical contact with the potentiostat (see Fig. 1). In the configuration here proposed, much lower resistivity values should be achieved in the electrical contact between the paper electrode and the potentiostat. The inter-electrode reproducibility (pyrolyzed electrodes derived from the same

9

paper

sheet)

was

measured

by

cyclic

voltammetry

using

1 mmol L-1

ferro/ferricyanide and 0.1 mol L−1 H2SO4 as analyte and supporting electrolyte, respectively. The relative standard deviation values (n = 5) for the anodic and cathodic peaks were 3.8 and 3.7 %, respectively.

3.2. Stripping analysis of zinc, cadmium and lead After the superior performance observed for the pyrolyzed paper by cyclic voltammetry

and

electrochemical

impedance

spectroscopy,

studies

were

performed to determine the three metals (Zn, Cd, and Pb) at trace levels by square wave anodic stripping voltammetry (SWASV). Figure 4 shows the typical stripping voltammograms obtained for a solution containing 100 µg L-1 of Zn2+, Cd2+, and Pb2+ in 0.1 mol L-1acetic acid/acetate buffer (pH 4.7) without and with modification of the pyrolyzed paper with a bismuth film (in situ film formation) [36]. FIGURE 4 As can be observed, well defined and separate peaks were obtained for Cd and Pb using pyrolyzed paper as working electrode. However, stripping peaks for the three target metals (Zn, Cd, and Pb) were only obtained after the addition of Bi3+ 1 mg L-1 to the solution (in situ deposited bismuth film). These results demonstrate that, similar to other carbon substrates, the pyrolyzed paper substrate displays limited cathodic range (hydrogen evolution at potentials more negative than -1.0 V) and the determination of Zn was only possible after in situ formation of bismuth film (increase in its cathodic potential range) [37]. The Bi3+ is reduced on

10

the carbon substrates forming a rather uniform and non-porous film [37]. According to our studies, the detection mechanisms for the target metals on the bismuth modified paper is similar to that reported for bismuth film on glassy carbon [38, 39]. The electrode mechanisms of Cd and Pb at bismuth-modified paper are affected by adsorption, diffusion, and attractive interactions. Specifically for Zn, the mechanism is affected by adsorption, diffusion, and repulsive interactions [38, 39]. These results demonstrate that the pyrolyzed paper has great potential as carbon substrate for the determination of metals at trace levels. In subsequent studies, a solution containing 100 µg L -1 of Zn2+, Cd2+, and Pb2+ in 0.1 mol L-1 acetate buffer (pH 4.7) was used to identify the best operated parameters for the determination of the three metals by SWASV. The parameters were selected in order to achieve the best sensitivity and selectivity (half-height peak width method) in accordance with the procedures used in previous works [21, 39, 40]. The optimized SWASV parameters are shown in Table 1. TABLE 1 Figure 5 shows the SWASV recorded for solutions containing increasing concentrations of Zn2+, Cd2+, and Pb2+ using pyrolyzed paper as working electrode without (A) and with (B) use of in situ bismuth film. FIGURE 5

As shown in Fig. 5A, two well-separated stripping peaks were obtained for Cd and Pb in the concentration range between 10 and 400 µg L-1 (r ≥ 0.994) using the unmodified pyrolyzed paper as working electrode. Under these conditions, Zn 11

was not detected. When Bi3+ (1 mg L-1) was added for the in situ bismuth-film formation (Fig. 5B), three well-resolved stripping peaks and linear relationships with concentration were obtained for Zn (10 to 200 µg L-1; r = 0.998) and both Cd and Pb (5 to 300 µg L-1; r ≥ 0.998). Furthermore, a repeatability study was performed by consecutive analysis (n=10) of a solution containing 50 µg L−1 of Zn, Cd, and Pb (data not shown). Low relative standard deviation values were obtained for stripping peaks of Cd and Pb (3.3% and 1.8%, respectively) using unmodified pyrolyzed paper and for Zn, Cd, and Pb (6.2%, 3.7% and 3.3%, respectively) when Bi3+ 1 mg L-1 was added to the standard solutions (in situ bismuth film formation). The results presented here suggest that the pyrolyzed paper material is promising as a disposable carbon substrate for metals analysis at trace levels. The analytical characteristics of the disposable pyrolyzed paper for the determination of the target metals are summarized in Table 2. TABLE 2 The possibility of using commercially available paper (easy access and widely available) combined with a simple pyrolysis procedure and the use of very small pieces (0.7cm x 0.7 cm) of pyrolyzed paper to obtain dozens of individual electrodes allow the use of this material as disposable sensors. Tedious and laborintensive electrode cleaning procedures may no longer be needed. Moreover, it is important to emphasize that a single piece of pyrolyzed paper (0.7 cm x 0.7 cm) positioned in electrochemical cell was used as working electrode for metals analysis during a period of around one week (good robustness). Inter-day studies 12

have demonstrated a gradual loss of sensitivity in this period (around 30 %), but the intra-day precision results (repeatability study) remained relatively constant (RSD < 6 % for all metals). Probably, the gradual loss of sensitivity may be related to the contamination or passivation of the electrode surface (mechanical cleaning cannot be used). As the standard addition method is commonly used in the stripping determination of metals, a single piece of paper could be used for several days.

3.3. Analysis of metals in real samples In order to evaluate the performance of the pyrolyzed paper, the target metals were determined in river and rain water samples. Table 3 lists the concentration of the three metals found in these samples before and after addition of known concentrations of the three metals (recovery values for the spiked samples). TABLE 3 As can be observed in Table 1, the proposed method had good accuracy and provided sufficiently high levels of recovery (99 – 109 %). Moreover, the accuracy of the proposed method was also evaluated by the determination of cadmium and lead in a water sample obtained from Alpha Resources – Stevensville – USA (APS 1071 - LOT 918224) containing known concentrations of both Cd and Pb. The certified and the obtained values were 50.0 ± 0.3 and 51 ± 3

13

µg mL-1 for Cd and 100 ± 0.5 and 101 ± 3 µg mL-1 for Pb. These results reinforce the good accuracy of the method proposed in the present study.

4. Conclusions

The present study shows that conductive substrates obtained from commercially available paper (pyrolyzed paper) offer competitive advantages with respect to traditional glassy carbon electrodes. According to the results herein presented, incorporating the electrodes in the described cell allows taking advantage of the reduced thickness of the carbon material, decreasing the resistance, and increasing the charge transfer rate. These results provide additional evidence regarding the advantages of this material as electrochemical sensor. In addition, it was also demonstrated for the first time that the pyrolyzed paper material can be successfully used to the determination of Zn, Cd, and Pb at trace levels. Under optimized conditions and using in-situ bismuth film formation, the anodic peak currents for Zn, Cd and Pb were linearly dependent to their concentrations in the ranges from 10 to 200 µg L-1 for Zn and from 5 to 300 µg L-1 for both Cd and Pb. The limits of detection were 0.26, 0.25, and 0.39 µg L-1 for Zn, Cd, and Pb, respectively. These results point to future application of the conductive substrate as disposable sensors, for on-site analysis, and studies involving modified electrodes.

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Acknowledgments The authors acknowledge financial support from FAPEMIG (CEX - APQ-0211815), and CAPES (PRO FORENSES–Process number: 23038.007073/2014-12). EMR

(Process

number:

307333/2014-0)

and

RAAM

(Process

number:

308174/2013-5) thank CNPQ for the fellowship.

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Fig. 1. (A) Pyrolyzed paper substrate (2.5 x 7.5 cm); (B) A piece of pyrolyzed paper substrate (0.7 x 0.7 cm) was positioned on a metallic base and under a rubber O-ring; (C) Side view of the electrochemical cell (paper electrode was placed over the metallic base and under the rubber O-ring); (D) Top view of the electrochemical cell with the working electrode positioned in the cell background.

Fig. 2. Cyclic voltammograms recorded using pyrolysed paper (green line) and glassy carbon (red line) as working electrodes in 0.1 mol L-1 H2SO4 without and with the presence of 1 mmol L-1ferro/ferricyanide as probe molecules; scan rate: 50 mV s-1; potential step: 5.0 mV.

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Fig. 3. Nyquist plots obtained for 1 mmol L-1ferro/ferricyanide in 0.1 mol L-1H2SO4 using pyrolyzed paper (■) and glassy carbon () as working electrodes.

Fig. 4. SWASVs of 100 µg L-1 of Zn2+, Cd2+, and Pb2+ at pyrolyzed paper electrode without (─) and with (─) the presence of Bi3+ 1 mg L-1. Supporting electrolyte: 0.1 mol L-1 acetate buffer (pH 4.7); Measurement conditions: frequency 20 Hz; amplitude 25 mV; step potential 2.5 mV; preconcentration potential: -1.5 V, preconcentration time 180 s; equilibration time 15 s.

Figure 5. SWASV recordings for the detection of Zn, Cd, and Pb in solutions containing increasing concentrations of the three metals. Working electrode: pyrolyzed paper without (A) and with (B) the use of Bi3 (1 mg L-1). Linear concentration ranges: 10 to 400 µg L-1 for both Cd and Pb (A); 10 to 200 µg L-1 for Zn and 5 to 300 µg L-1 for both Cd and Pb (B). Experimental conditions as in Table 1.

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Table 1. SWASV parameters for the determination of Zn2+, Cd2+, and Pb2+using pyrolyzed paper as working electrode.

Parameters

Conditioning potential

Conditioning time

Optimized value

+0.6 V

60 s

Deposition potential

-1.5 V

Deposition time

180 s

Initial potential

-1.35 V

End potential

-0.35 V

Step potential

2 mV

Amplitude

20 mV

Frequency

20 Hz

Bi3+ concentration

1 – 2 mg L-1

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Table 2. Analytical characteristics of the pyrolyzed paper electrode for the determination of Zn, Cd, and Pb (confidence interval = 95%).

Characteristics

Pyrolyzed paper

Pyrolyzed paper + Bi film

Cd

Pb

Zn

Cd

Pb

R

0.994

0.998

0.999

0.998

0.999

Linear range (µg L-1)

10-400

10-400

10-200

5-300

5-300

LOD (µg L-1)

0.19

0.16

0.26

0.25

0.39

LOQ (µg L-1)

0.64

0.53

0.87

0.82

1.33

1.8 %

6.2 %

3.7 %

3.3 %

Intra-day RSD (n=10) 3.3 %

r: correlation coefficient; LOD: limit of detection; LOQ: limit of quantification; RSD: relative standard deviation.

Table 3. Zinc, cadmium, and lead concentrations found in river and rain water samples and respective recovery values (n = 3).

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River water sample

Rain water sample

Analyze

Spyke

Foun

Recover

Analyze

Spyke

Foun

Recover

d

d

d

y

d

d

d

y

(µg L-1)

(µg L-1) (µg L-

(%)

(µg L-1)

(µg L-1) (µg L-

1

1

)

Zn < LOD

20

20.3 ±

)

102 ± 3

< LOD

20

0.6

C

< LOD

20

d

P b

21.8 ±

20

19.7 ± 0.7

19.8 ±

99 ± 1

0.2

109 ± 2

< LOD

20

0.3

< LOD

(%)

19.9 ±

100 ± 5

1.0

99 ± 4

< LOD

20

20.4 ±

102 ± 1

0.1

25

26

27

Highlights 

Disposable electrodes were produced from pyrolized paper.



Pyrolized paper is used for the first time for the determination of trace metals.



Pyrolized paper has similar performance to glassy carbon as working electrode.

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