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graphene nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor
Accepted Manuscript Title: A novel rapid synthesis of Fe2 O3 /grapheme nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor Author: Mohammad Ali Karimi Fatemeh Banifatemeh Abdolhamid Hatefi-Mehrjardi Hossein Tavallali Gohar Deilamy-Rad PII: DOI: Reference:
S0025-5408(15)00380-3 http://dx.doi.org/doi:10.1016/j.materresbull.2015.06.010 MRB 8267
To appear in:
MRB
Received date: Revised date: Accepted date:
18-12-2014 3-6-2015 4-6-2015
Please cite this article as: Mohammad Ali Karimi, Fatemeh Banifatemeh, Abdolhamid Hatefi-Mehrjardi, Hossein Tavallali, Gohar Deilamy-Rad, A novel rapid synthesis of Fe2O3/grapheme nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.06.010 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 proof before it is published in its final 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.
A novel rapid synthesis of Fe2O3/grapheme nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor
Mohammad Ali Karimia,b*[email protected] [email protected], Fatemeh Banifatemeha,c, Abdolhamid Hatefi-Mehrjardia,b, Hossein Tavallalia,d, Gohar Deilamy-Radd
a
Department of Chemistry, Payame Noor University, 19395-4697, Tehran, I.R. of IRAN
b
Department of Chemistry &Nanoscience and Nanotechnology Research Laboratory (NNRL), Payame Noor
University, Sirjan, Iran c
Department of Chemistry, Payame Noor University, Mashhad, Iran
d
Department of Chemistry, Payame Noor University, Shiraz, Iran
*
Corresponding author. Tel: +982122440041; Fax: +982122442038.
Graphical abstract
Highlights
A novel rapid synthesis of rGO-Fe2O3 nanocomposite was developed using Fe(VI).
Fe(VI) as an environmentally friendly oxidant was introduced for GO synthesis.
Synthesized rGO-Fe2O3 nanocomposite was applied as electrochemical sensor.
A non-enzymatic sensor was developed for H2O2.
1
Abstract In this study, a novel, simple and sensitive non-enzymatic hydrogen peroxide electrochemical sensor was developed using reduced graphene oxide/Fe2O3 nanocomposite modified glassy carbon electrode. This nanocomposite was synthesized by reaction of sodium ferrate with graphene in alkaline media, this reaction completed in 5 min and the products were stable and its deposition on the surface of electrode is investigated. It has been found the apparent charge transfer rate constant, ks = 0.52 and transfer coefficient, α = 0.61 for electron transfer between the modifier and glassy carbon electrode. Electrochemical behavior of this electrode and its ability to catalyze the electro-reduction of H2O2 has been studied by cyclic voltammetry and choronoamperometry at different experimental conditions. The analytical parameters showed the good ability of electrode as a sensor for H2O2 amperometric reduction.
Keywords A.
Composites;
A.
Nanostructures;
B.
Chemical
synthesis;
C.
Electrochemical
measurements; D. Electrochemical properties
1. Introduction Hydrogen peroxide is a simple molecule but plays a critical role in diversified biological systems, clinical, food, and environmental chemistry [1]. Due to intrinsic simplicity, high sensitivity and selectivity, electrochemical methods have been extensively employed in H2O2 sensor designs [2].
2
Most of the employed electrodes in the fabrication of electrochemical H2O2 sensors are based on enzymes [3,4]. By the way, these electrodes have some practical restrictions related to the use of enzyme; enzymes are relatively expensive and unstable [5,6]. Moreover, enzyme-based electrodes are the electrochemical sensors that generate anodic current during electro oxidation of H2O2. The redox reaction of H2O2 has a relatively high potential at these electrodes. However, electrochemically active interfering species, which are usually present in real samples, are easily oxidized at that potential and dramatically influence the biosensor sensitivity by producing an interfering current [7–9]. Consequently, the main problem of these analytical devices is their sensitivity to the interferences, present in analyte solution [10]. Graphene is a planer monolayer of carbon atoms ordered into a honeycomb lattice [11]. The good conductivity and macroscopic tunnel effect can make graphene an effective electronic interface between the immobilized enzyme and electrode and consequently, it can improve the sensitivity of the sensor and the current response and also shorten the response time. One of the most promising applications of graphene emerged so far is its utilization as a new way of designing novel electrochemical sensors and biosensors [12, 13]. As a result, graphene modified electrodes were successfully applied to study and determine some biological and organic molecules, including enzyme [14], DNA [15], small biomolecules [12,16], heavy metal ions [17, 18], gas [19] etc. Non-enzyme electrochemical sensors have been fabricated which are based on composite graphene with metal nanoparticles such Au [20] and Ag [21]. Also Hybridization of the graphene and Fe in the form of Fe2O3 or Fe3O4hasbeen extensively studied for various applications such as H2O2 detection because of abundant, low cost, and nontoxic material. Shen et al. [22] synthesized graphene/Fe3O4 nanocomposite and analyzed it by differential scanning calorimetry (DSC). Moreover, Deng et al. [23] utilized magnetic graphene oxide
3
nanocomposite as an adsorbent for simultaneous removal of Cd(II) and ionic dyes from aqueous solutions. Zhang et al. [24], synthesized Fe2O3/graphene composite and used it as electrode materials for Li-ion batteries. Wang et al. [25], prepared α-Fe2O3/rGO modified glassy carbon electrode and used it for H2O2 detection. They synthesized α-Fe2O3/rGO with reduction of GO in the presence of FeCl2 with urea in ammonia media in 150 °C. Yuan et al. [26] prepared Fe2O3/graphene nanocomposite and concluded that graphene obviously improved the catalytic activity of Fe2O3 on the thermal decomposition of ammonium perchlorate due to its high specific area. They also prepared Fe2O3/graphene with reduction of GO in the presence of iron chloride in 160 °C for 48 h. Yang et al. [27] fabricated reduced grphene sheet (GNS)/Fe2O3nanorods composite for supercapacitor electrode. For synthesis of GNS/Fe2O3, they added NaOH to suspension of Fe(OH)3 and GNS in 160 °C for 20 h. In this paper, we used ferrate (FeO42-) for oxidation of graphene in alkaline media for the first time. Ferrate is a synthesized salt of iron with high oxidation state that can be used as apowerful oxidant in natural waters at the wide pH range [28]. The reduction of Fe(VI) results in a relatively non-toxic by-product iron(III), which suggests that Ferrate is an environmentally friendly oxidant [29]. The effect of Ferrate on microorganisms in waste water and some of viruses and bacteria is also investigated [30,31].
One of the main
approaches to ferrate synthesis is wet chemical synthesis. In this approach an iron(III) salt is oxidized by alkaline hypochlorite [32]. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) showed the main product of reaction of ferrate with reduced graphene oxide (rGO) were Fe2O3 (hematite) molecules which located on rGO. With incorporation of the nonmagnetic Fe2O3 between layers of rGO, after washing, the agglomeration of dark brown residual is reduced. Unlike the previous methods, synthesis of rGO-Fe2O3 with ferrate was very fast and the reaction completed in 5 min and the products were stable. Also the
4
adsorbed hematite on the surface of rGO enhance its electrochemical properties. For study of this effect rGO-Fe2O3 deposit on glassy carbon electrode (GCE), it stability to catalyze the electro-reduction of H2O2 has been studied. Electrochemical behavior of (rGO-Fe2O3) modified GCE and the analytical performance of the rGO-Fe2O3 modified GCE was then evaluated with respect to detection limit, linearity, stability and reproducibility. This study provides a new kind of nanocomposite modified electrode for electrochemical sensors. This sensor exhibits many advantages such as good reproducibility, a facile preparation procedure, anti-interference ability, wide linear range and long-term stability of the response signal, which indicate that rGO-Fe2O3-GCE is a promising candidate for H2O2 sensing in clinical and environmental samples. Also in this research we investigate antibacterial properties of rGO-Fe2O3 and Ferrate on two kinds of gram negative and positive bacteria. As results will be showed, rGO-Fe2O3 opposite of ferrate didn't show antibacterial properties.
2. Experimental
2.1 Materials and methods Graphite was purchased from Fluka, H2SO4 (98%) (d = 1.84 Kg L-1), KMnO4, H2O2 (30%), NaNO3, Fe(NO3)3.9H2O, NaBH4 and sodium hydroxide were purchased from Merck. Sodium hypochlorite (NaOCl) was purchased from Mojallali Chemical Laboratory (Iran). All solutions were prepared with distilled water. Phosphate buffer solutions (PBS, 0.01 mol L-1) were prepared from 0.01 mol L-1 H3PO4-NaH2PO4, and the pH was adjusted with 0.01 mol L1
H3PO4 or 0.1 mol L-1 NaOH. The pH measurements were carried out with a Jenway model
6103 pH/mV meter. Ammonia buffer solution (0.04 mol L-1) was prepared from the mixture of 8.0 mL ammonium chloride (2.0 mol L-1) and 2.0 mL NH3 (2.0 mol L-1) in 100 mL flask. UV–Vis absorption spectra were recorded using Optizen 2120 UV- PC spectrophotometer.
5
The powder X-ray diffraction (XRD) analyses of the samples were carried out with a X-Ray Tube: Cu (Kα =1.54 Å) X'Pert Pro MPD (PANalytical) with a scanning speed of 4°/min and a step of 0.04° (2θ) in the range from 5° to 80°. The morphologies of the samples were investigated by scanning electron microscopy (SEM) Model Phenom Pro X (Netherlands). The compositions of the powders were characterized at room temperature by Fourier transform infrared spectrometer (FTIR, Prestige-21, Shimadzu) Electrochemical experiments were performed with an IVIUM electrochemical workstation by using a three-electrode electrolytic cell. Glassy carbon electrode (GCE, 2 mm in diameter) acted as the working electrode. A KCl saturated calomel electrode (SCE) served as the reference electrode and a platinum electrode (2 mm in diameter) was used as the counter electrode. The bactericidal experiments were carried out with gram negative bacteria Escherichia coli PTCC1394 and gram positive bacteria Staphylococcus aureus PTCC1431 in the same nutrient media. These bacteria were obtained from the culture collection of I.R. Department (Tehran, Iran). Moreover, stock cultures were stored in the -80 °C freezer. The strains were propagated on Tryptic Soy Agar (TSA; Merck, Darmstadt, Germany) at 37 °C and maintained at 0-2 °C before use.
2.2 Synthesis of sodium ferrate by wet oxidation method
The wet oxidation method involves the oxidation of a Ferric containing solution to form Ferrate solution under high alkaline conditions. 7.0 g of sodium hydroxide was dissolved to 10 mLcold water and 10 mL NaOCl was added. Then 2.5 g of well powdered Fe(NO3)3.9H2O was added slowly while the solution was being stirred at 20 °C. Then the temperature of solution was raised to 45-55 °C. The color of solution changed to purple. 6
After removal of the residual Fe(OH)3 by centrifuge, the resulting Ferrate was measured using an established spectroscopy method where the absorbance of the Ferrate solution was measured at 510 nm and the absorbance was converted to the concentration using the following equation: C = A/ε × b
(1)
Where A, ε, b and C are absorbance (at 510 nm), extinction coefficient (1150 L/mol cm) [33,34], cell path length (1 cm) and concentration (mol L-1), respectively.
2.3 Synthesis of graphene oxide
GO was prepared by a modified Hummer’s method [35]. Briefly, 23 mL of 98% H2SO4 and 100 mg of NaNO3 were added to 1 g of graphite, followed by stirring at room temperature over a 24-h period. Subsequently, the mixture was kept below 5 °C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35–40 °C, the mixture was stirred for another 30 min. Then 10 mL of 30% H2O2 was added into the mixture to stop the reaction. Finally, the unexploited graphite in the resulting mixture was removed by centrifugation,
2.4 Synthesis of rGO-Fe2O3
0.25 g of the synthesized GO was dispersed into individual sheets in 100 mL distilled water with the aid of vigorous ultrasonic waves. After centrifugation, to 5 mL of GO dispersion 10 mL water and 0.05 g NaBH4 were added and the mixture was heated to 80-90 °C for 10 min. After cooling, 5 mL sodium ferrate (0.005 mol L-1) was added. As it could be observed, Ferrate rapidly was adsorbed. After washing the aggregate products to natural pH,
7
a dark brown dispersion was obtained. rGO was also prepared through a similar method except for the absence of ferrate.
2.5 Preparation of the modified electrode and measurement procedures
Prior to modification, GCE was polished with 400, 1000 and 3000 mesh polishing papers until a mirror shiny surface appeared, and was cleaned in ethanol, HNO3(1:1, V/V), NaOH (1.0 mol L-1) and double distilled water for 5 min. The treated electrode was swept between -1.0 and 1.0 V versus SCE in ammonia buffer solution (pH=9.0) for sufficient cycles to obtain reproducible cyclic voltammograms.
2.6 Antimicrobial activity studies
The antimicrobial properties of sodium ferrate and composites of rGO-Fe2O3 were investigated using Escherichia coli, and Staphylococcus aureus by the Kirby-Bauer diffusion method [36,37]. The bacterial suspension was applied uniformly on the surface of a Tryptic Soy Agar (TSA) plate at a concentration of 105 CFU/mL before placing ferrate or composite simpregnated disks. For antibacterial study of Ferrate and composites, laden disks have been prepared by keeping 6 disks in 2 mL Ferrate solution and colloidal solution of composites for 1-h. These disks absorbed the Ferrate in 5 min and the purple solution became colorless. After drying, disks fixed on plates. The plates with the disks were incubated at 35 °C for 24 h, after which the average diameter of the inhibition zone surrounding the disk was measured with a ruler. Fig. 1 shows plates which S. aureus and E. coli bacterial suspensions were applied with samples laden disk and antibiotic impregnated disks. The diameters of inhibition zones around the disk containing Ferrate in S. aureus, and E. coli bacterial suspension are 12 and 14 mm, respectively. This test shows that Ferrate is nearly 43 and 56 % effective
8
compare to tetracycline disk for S. aureus, and E. coli bacterial, respectively. It was observed that in the presence of Ferrate, the bacterial growth was inhibited but composites rGO-Fe2O3 didn't show any antibacterial properties. In this research the effect of Ferrate concentration on bacteria growth inhibition was not studied and subjected to the future research.
3. Results and discussion
3.1 Characterization of reaction of Ferrate with rGO Ferrate ion which has the molecular formula of FeVIO42- is a powerful oxidant and its aqueous solution has a characteristic of black-purple colour. A number of alkali and alkaline earth salts of Ferrate(VI) have been synthesized. The basic wet chemical reaction is:
(2)
2FeCl3 + 3NaOCl+ 10NaOH→2Na2FeO4 + 9NaCl+ 5H2O
Ferrate oxidation is known to be active over a wide pH range. Na2 FeO4 has relatively high solubility in a saturated NaOH solution. However its decomposition is faster under acidic conditions [34].
For acidic solutions:
FeO42− + 8H+ +3e−[Maths Symbol
(3)
here]Fe3+ + 4H2O FeO42− + 2H2O +3e−[Maths Symbol (4)
For alkaline solutions:
here]FeO2− + 4OH− FeO42− + 4H+ + 3e−[Maths Symbol
For weak acid and neutral solutions:
here]Fe(OH)3+ OH−
9
(5)
The reaction of rGO with Ferrate is very fast and any spontaneous decomposition of Ferrate in water does not happen because of the formation of molecular oxygen according to the following reaction was not observed. 2FeO42- + 5H2O [Maths Symbol here]2Fe(OH)3 + 3/2O2 +
(6)
4OHBecause of the fast decomposition of Ferrate in acidic media, it is impossible to use for oxidation of graphite. In contact of Ferrate with graphite in dry environment, it was observed that Ferrate is adsorbed on graphite surface as well as paper disks in antibacterial test. As is shown in Fig. 2 Ferrate has a λmax≈ 510 nm in UV-Vis spectrum. In this paper we used Ferrate for oxidation of graphene in alkaline media. Moreover, the UV–Vis spectra of GO, rGO and rGO-Fe2O3 exhibit maximum peaks at 235, 260 and 275 nm, respectively corresponding to π→π* transitions of aromatic C―C bonds. It is obvious that the UV–Vis spectrum of rGO usually red-shifts from 235 to 275 nm, compared with that of GO. The band at 275 nm in the spectrum of rGO is assigned to π→π* transitions of aromatic C═C bonds. The FTIR spectrum (Fig. 3) of the GO illustrates the presence of C―O, C═C, C═O and C―OH bonds from the peaks at 1048, 1455, 1625 and 3425 cm−1, respectively. Moreover, approximately the same peaks were observed in spectrum of rGO-Fe2O3 (1041, 1581, 1726, 3416 cm-1), respectively. In addition to this, the peak in 576 cm-1 is attributed to Fe–O, and the enhanced intensity for Fe–O is indicative of the iron loading in rGO-iron oxides [38]. The XRD patterns of GO, rGO and rGO-Ferrate are shown in Fig 4. GO exhibits a typical sharp diffraction peak centered at 2θ = 9.8° corresponding to the interlayer distance of ∼0.86 nm [39] and typical diffraction peak of GO nanosheets, was attributed to the (002) plane [40]. In comparison, rGO and rGO-Fe show broad and low intensity peaks under 2θ = 10°. The broad diffraction peaks are indicative of nanoparticles with very small size. The main peaks at about 2θ = 34.5° (104), 45.5° (200), 56.5° (116), 66.3° (030) and 83.8° (312) show the characteristics of Fe2O3 (hematite) on rGO [38]. The presence of hematite reduces the
10
aggregation of graphene sheets, which results in more monolayer of graphene, leading to weaker peaks from carbon being observed. The SEM micrograph and Energy dispersive X-ray (EDX) spectrum of rGO-Fe2O3 are shown in Fig. 5. The EDX spectrum confirmed the incorporation of Fe on the rGO surfaces. Also, the percentages elements from the points of 1, 2 and 3 in SEM graph (Fig. 5A) and the average of three points are plotted in Fig. 5C.
3.2 Electrochemical behavior of the rGO-Fe2O3 modified glassy carbon electrode
Electrochemical behavior of rGO-Fe2O3 on GCE was investigated in ammonia buffered solution (pH 9.0) using cyclic voltammetry (CV). Fig. 6 shows the cyclic voltammograms obtained from the bare GCE and modified GCE with GO, rGO and rGO-Fe2O3, at a scan rate of 100 mV s-1. As expected, no redox process occurs at bare GCE,GO- and rGO modified GCE while the rGO-Fe2O3 modified GCE exhibit anodic and cathodic peaks at the potentials of -0.26, -0.52 V and the ratio between the peak currents Ipa/Ipc=1.41. The peak separation potential, |∆Ep|=|Epc-Epa|=0.26 V, was more than (0.059/n)V excepted for a reversible system[41]. This result indicates that the redox couple in rGO-Fe2O3-GCE shows quasireversible behavior in an aqueous medium. The E0' value found for rGO-Fe2O3-GCE (-0.39 V vs. SCE) is smaller than -0.210 V (vs. Ag/AgCl) reported for carbon paste electrode modified with Fe doped mesoporous carbon aerogel [46]. The effect of pH and buffer solution on the electrochemical behavior of rGO-Fe2O3 on GCE was investigated over the range of pH 2-9 with PBS and the results are illustrated in Fig. 7. As the results show the peak currents did not change with increasing the solution pH in PBS 0.01 mol L-1, the ammonia buffer (pH=9.0) shows an enhancement in redox peak currents clearly. Therefore, the ammonia buffer solution (pH 9.0) was chosen as the optimum working buffer solution for subsequent measurements. The anodic and cathodic currents were 11
constant in 1 to 20th measuring sweep cycles and also solution of rGO-Fe2O3 was steady in the period of research for 4 months (Fig. 8). Figure 9 shows the effect of the potential scan rate on electrochemical response of the rGO-Fe2O3-GCE in 0.04 mol L-1 ammonia buffer solution (pH 9.0). As can be seen in Fig. 9B, the plots of the anodic and cathodic peak currents (Ip) were linearly dependent on scan rate (ν) from 30 to 300 mV/s, indicating a surface-confined-redox process. According to Laviron theory [42], the apparent charge transfer rate constant, ks, and the charge transfer coefficient, α, of a surface-confined redox couple were evaluated based on the variation of the anodic and cathodic peak potentials with the logarithm of the scan rate (Fig. 9C) obtained from the cyclic voltammograms of a quasi-reversible system [41]. We found that the Ep values are proportional to the logarithm of the potential scan rate, for scan rates higher than 120 mV/s (Fig. 9D). The slopes of the plots can be used to extract the kinetic parameter α (anodic charge transfer coefficient). The slope of the linear segment (0.151) is equal to 2.303RT/(1-α)nF for the anodic peaks. When n=1 (for Fe(III)→Fe(II)), the evaluated value for α is 0.61. The following equation can be used to determine the electron transfer rate constant between the modifier rGO-Fe2O3 and GCE: log ks = αlog(1-α) + (1-α)logα- log(RT/nFυ) -α(1-α)nF∆Ep/2.3RT
(7)
Where υ is the potential scan rate, (n=1, Fe(III)→Fe(II)) is the number of electrons involved in the overall redox reaction of the modifier rGO-Fe2O3-GCE and all the other symbols have their conventional meanings. The value of ks was evaluated to be 0.52 s−1 using Eq. (7). Because of the electron transfer rate constant for the rGO-Fe2O3 is about 0.52 s-1, it can be used as an excellent electron transfer mediator for electrocatalytic processes.
3.3 Electrocatalytic redox properties of the rGO-Fe2O3 modified GCE toward H2O2 cyclic voltammetric studies
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Comparative electrochemical behavior of the bare GCE, GO modified GCE and rGOFe2O3 modified GCE in ammonia buffer (pH 9) in the presence of H2O2 by CV method is shown in Fig. 10A. It can be seen the currents response on the rGO-Fe2O3 modified electrode is more than bare GCE and GO-GCE. Also, the effect of pH on rGO-Fe2O3-GCE in the presence of H2O2 (1 mmol L-1) in PBS and ammonia buffer is shown in Fig 10B. It is obvious that, ammonia buffer with pH 9 is the best media for such electrocatalytic studies. In order to examine the abilities of the sensor in the determination of H2O2, the CVs were recorded by addition of different amounts of H2O2 at the optimized solution conditions (Fig. 11A). The results showed that by addition of the concentration ranges of H2O2 from 0.5 to 25 mmol L-1 both anodic and cathodic peaks were grown linearly but the anodic peak currents calibration curve was more sensitive (Fig. 11B). The linear increase of the anodic and cathodic peak currents illustrates the excellent electrocatalytic ability of the modified electrode towards the oxidation/reduction of H2O2.
3.4 Chronoamperometric studies
Chronoamperometry as well as the other electrochemical methods may be used for investigation of the electrode processes. Chronoamperometric measurements of H2O2 at rGOFe2O3 modified GCE were carried out by setting the working electrode potential at -0.6 V vs. SCE for the various concentration of H2O2 in buffered aqueous solutions (pH 9.0) (Fig. 12). For an electroactive material (H2O2 in this case) diffusion coefficient (D) is obtained from the Cottrell equation [41]: I = nFAD1/2Cb(πt)-1/2
(8)
13
Where D and Cb are the diffusion coefficient (cm2/s) and the bulk concentration (mol cm-3) of H2O2, respectively. A is the electrode surface area (0.031 cm2), I is a current controlled by diffusion of H2O2 from the bulk of the solution to the electrode/solution interface. Experimental plots of I vs. t-1/2 were employed, with the best fits for different concentrations of H2O2 (Fig. 12A). The slopes of the resulting straight lines were then plotted vs. H2O2 concentrations (Fig. 12B). Based on resulting slope (slope (I vs. t-1/2) = -0.095 [H2O2] (mmol L-1) - 0.035, r = 0.994) the mean value of the D was found to be 3.1× 10−2 cm2/s. Chronoamperometry can also be employed to evaluate the catalytic rate constant, k, for the reaction between H2O2 and the rGO-Fe2O3-GCE according to the method of Galus [43]. IC/IL = γ1/2[π1/2erf(γ1/2) + exp(-γ)/γ1/2]
(9)
Where IC is the catalytic current of H2O2 at the rGO-Fe2O3-GCE, IL is the limited current in the absence of H2O2 and γ = kCbt is the argument of the error function. In cases where γ exceeds the value of 2, the error function is almost equal to 1 and therefore, the above equation can be reduced to: IC/IL = π1/2γ1/2 = π1/2 (kCbt)1/2
(10)
Where k, Cb and t are the catalytic reaction rate constant (L/mol.s), the bulk concentration (mol L-1) of H2O2 and time elapsed (s), respectively. The above equation can be used to calculate the k, of the catalytic process from the slope of IC/IL vs. t1/2 (Fig. 12C) at a given H2O2 concentration. From the values of the slopes, the average value of k was found to be 1.5×104 L mol-1.
3.5 Amperometric determination of H2O2 at rGO-Fe2O3- GCE
14
The current-time response of rGO-Fe2O3-GCE in the optimized solution conditions was recorded to evaluate the calibration curve and the limit of detection for hydrogen peroxide determination. The sensor response at an applied potential of -0.6 V vs. SCE toward increasing concentration of H2O2 was recorded in ammonia buffer solution. Fig. 13 specifies the sensor response for successive addition of hydrogen peroxide from stock solution of 0.1 mol L-1 by 0.1 mL increments to electrochemical cell containing 20 mL ammonia buffer solution. At the optimal pH 9, the sensor exhibited a detection limit of 6.0 µmol L-1, broad linearity from 0.05 to 9.0 mmol L-1, good reproducibility (RSD of 1.9%), and long-term stability. Table 1 compare some of the analytical characteristics from this work and those previously enzymic and nonenzymic reported in the literature [21, 25, 44-53] for H2O2 detection. The detection limits and linear calibration range of the proposed sensor are comparable with those provided by other modified carbon electrodes. On the other hand, in comparison with enzyme-based sensors, proposed nonenzymatic sensor in addition to its low cost exhibit a high stability, free from the influence of temperature, pH and oxygen restricts. The interference study was conducted by placing the modified GCE into a solution containing target analyte and potentially interfering species at optimum conditions. It was -
found that a excess of Na+, K+, Ca2+, Mg2+, NO3 , Ni2+, Cd2+, Co2+, ascorbic acid, vitamin B1 have no influence on the signal of 1.0 mmol L-1 of H2O2.
4. Conclusion A simple and very fast method is introduced to fabricater GO-Fe2O3 nanocomposite based on reaction of ferrate on graphene. The XRD analysis shows that Fe2O3 (hematite) is adsorb on graphene sheet and incorporating iron oxide molecules between the layer of rGO prevent agglomeration of dark brown residual reduced. In addition, Fe2O3 (hematite) on surface of rGO enhance its electrochemical properties. Thus, the GCE modified with rGOFe2O3 have been investigated by cyclic voltammetry. A quasi-reversible, surface confined redox process was put in evidence and was attributed to presumably Fe3 +/Fe2+redox system. It
15
was demonstrated that rGO-Fe2O3 can be a promising material for preparing highly active electrode materials with practical applications, e.g. sensor for H2O2 detection. The kinetic and analytical parameters of the new modified electrode obtained with chronoamperometry measurements performed at a low applied potential (-0.6 V vs. SCE), recommend it as a stable, sensitive and reproducible sensor for H2O2amperometric reduction. The proposed sensor exhibits many advantages such as good reproducibility, a facile preparation procedure, anti-interference ability, wide linear range and long-term stability of the response signal, which indicate that rGO-Fe2O3-GCE is a promising candidate for H2O2 sensing in clinical and environmental samples.
Acknowledgements
The authors would like to express their appreciations to Professor Afsaneh Safavi for her valuable discussion and useful suggestions. This research was supported by the Nano Structured Coatings Institute of Payame Noor University of Yazd and Nanoscience and Nanotechnology Research Laboratory (NNRL) of Payame Noor University of Sirjan.
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polytetrakis(2-
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Caption of Figures Fig. 1 Representative image of agar plates containing Ferrate (VI) and rGO-Fe2O3 impregnated discs for E. coli and S. aureus Fig. 2 The optical adsorption spectra of sodium Ferrate (VI) before and after addition to graphite, GO, rGO and rGO- Fe2O3 Fig. 3 FT-IR spectra of GO, GO and rGO- Fe2O3 Fig. 4 XRD pattern of GO (a), rGO (b) and rGO- Fe2O3 (c). Fig. 5 (A) SEM image, (B) EDX spectrum of rGO-Fe2O3 and (C) EDX elements analysis from the points of 1, 2, 3 and average in SEM image. Fig. 6 CVs of bare GCE, GO, rGO and rGO- Fe2 O3 modified GCE in ammonia buffer (0.04 mol L-1, pH = 9) at scan rate of 100 mV/s. Fig. 7 CVs of rGO- Fe2O3 modified GCE in 0.01 mol L-1 PBS at different pH values (2-9) and ammonia buffer (pH = 9) on the top.
24
Fig. 8 (A) CVs of rGO-Fe2O3-GCE in ammonia buffer scan 1-20, (B) compare of CVs at first (a) and after 4 months (b). Fig. 9 (A) CVs of rGO-Fe2O3-GCE in ammonia buffer (pH 9.0) at various scan rate from 30 to 300 mV/s, insets: (B) variations of Ipa and Ipc versus scan rate, (C) variation of Epa and Epc versus the logarithm of scan rate, and (D) same as (C) for scan rates higher than 120 mV/s. Fig. 10 (A) The second scan of CVs from bare GCE, GO-GCE and rGO- Fe2O3-GCE in ammonia buffer (pH 9) in the presence of H2O2 (2.0 mmol L-1) and (B) rGO- Fe2O3-GCE in the presence of H2O2 and different pHs. Fig. 11 (A) CVs of rGO- Fe2O3-GCE in ammonia buffer (pH 9) solution after addition the different concentration of H2O2 at scan rate of 100 mV/s, (B) calibration plots and related equations extracted from anodic and cathodic peaks as a function of H2O2 concentration. Fig. 12 Short time chronoamperograms obtained at rGO-Fe2O3-GCE in ammonia buffer solution (pH 9.0) for H2O2 concentrations of 0.0, 0.5, 1.0, 1.5, and 2.0 mmol L-1. (A) plots of I vs. t-1/2 obtained from the chronoamperogram data, (B) plot of the slope of the straight lines against the H2O2 concentration and(C) dependence of IC/IL on t1/2 derived from the data of chronoamperograms Fig. 13 Amperometric response obtained at rGO- Fe2O3-GCE in ammonia buffer solution (pH 9.0) at a potential of -0.600 V (vs. SCE) by addition of the different concentration of H2O2. Inset shows the plot of extracted currents vs.H2O2 concentration and the related calibration equation.
25
Table 1. Comparison of analytical characteristics of different enzymic and non- enzymic modified carbon electrodes for electrochemical detection of H2O2 Modified electrode Eapp (V) Electrolyte LDR (mM) LOD (mM) Ref. AgNPs/rGO/GCE -0.43 0.2 M PBS (pH 6.9) 0.05-5.0 0.01 [21] [25] α-Fe2O3/rGO/GCE -0.22 0.1 M KOH 0.005-4.495 0.001 Peroxidase (type VI-A)//GCE 0.05 0.05 M PBS (pH 7.2) 0.00005-0.005 0.00004 [44] Horseradish peroxidase (type 0.089 0.05 M PBS (pH 6.5) 0.00005-0.006 0.00005 [45] VI)/graphite electrode Fe/CA/CPE -0.3 0.1 MPBS (pH 7) 1-50 0.5 [46] Ni(OH)2/SiNWs 0.2 0.5 M NaOH 0-5.5 0.0032 [47] Sn/[(Fe(CN)6]3-/CCE -0.1 0.1 M acetate buffer(pH 4) 0.004-0.050 0.0014 [48] CuO/SPCE -0.1 0.1 M PBS (pH 7) 0.003-15.0 0.0015 [49] NNBP/GE -0.1 0.1 M PBS (pH 7) 0.0005-0.0015 0.0003 [50] PTAPPNW/SWCNTs/NGCE 0.0 PBS (pH 7.4) 0.1-1.0 0.001 [51] Ni/[(Fe(CN)6]3-/CPE 0.6 0.1 M NaOH 0.6-6 , 17-54 0.34 [52] PdNPs/GCE -0.2 0.1 M PBS (pH 7.4) 0.001-14 0.0003 [53] rGO/Fe2O3/GCE -0.6 0.04 M ammonia buffer (pH 9.0) 0.05-9.0 0.006 This work CA: carbonaerogel; SiNWs: silicon nanowires; CCE: carbon ceramic electrode; SPCE: screen-printed carbon electrode; NNBP/GE: nafion-nile blue peroxidase graphite electrode; PTAPPNW/SWCNTs/NGCE: nanowires of polytetrakis(o-aminophenyl)porphyrin/single-walled carbon nanotubes/nafion coated glassy carbon electrode; CPE: carbon paste electrode.
Figure 1
Tetracyclin
Ferrat rGO-Fe2O3 E. coli
S. aureus
Figure 2
26
3.0 Fe (VI) Fe (VI) + graphite GO rGO rGO-Fe2 O3
2.5
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Figure 4
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Figure 5
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Figure 6
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Figure 6
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Figure 8
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Figure 10
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Figure 11
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0.5
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-0.4 0.0
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34
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Figure 12 -0.5 -0.20
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S0025-5408(15)00380-3 http://dx.doi.org/doi:10.1016/j.materresbull.2015.06.010 MRB 8267
To appear in:
MRB
Received date: Revised date: Accepted date:
18-12-2014 3-6-2015 4-6-2015
Please cite this article as: Mohammad Ali Karimi, Fatemeh Banifatemeh, Abdolhamid Hatefi-Mehrjardi, Hossein Tavallali, Gohar Deilamy-Rad, A novel rapid synthesis of Fe2O3/grapheme nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.06.010 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 proof before it is published in its final 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.
A novel rapid synthesis of Fe2O3/grapheme nanocomposite using ferrate(VI) and its application as a new kind of nanocomposite modified electrode as electrochemical sensor
Mohammad Ali Karimia,b*[email protected] [email protected], Fatemeh Banifatemeha,c, Abdolhamid Hatefi-Mehrjardia,b, Hossein Tavallalia,d, Gohar Deilamy-Radd
a
Department of Chemistry, Payame Noor University, 19395-4697, Tehran, I.R. of IRAN
b
Department of Chemistry &Nanoscience and Nanotechnology Research Laboratory (NNRL), Payame Noor
University, Sirjan, Iran c
Department of Chemistry, Payame Noor University, Mashhad, Iran
d
Department of Chemistry, Payame Noor University, Shiraz, Iran
*
Corresponding author. Tel: +982122440041; Fax: +982122442038.
Graphical abstract
Highlights
A novel rapid synthesis of rGO-Fe2O3 nanocomposite was developed using Fe(VI).
Fe(VI) as an environmentally friendly oxidant was introduced for GO synthesis.
Synthesized rGO-Fe2O3 nanocomposite was applied as electrochemical sensor.
A non-enzymatic sensor was developed for H2O2.
1
Abstract In this study, a novel, simple and sensitive non-enzymatic hydrogen peroxide electrochemical sensor was developed using reduced graphene oxide/Fe2O3 nanocomposite modified glassy carbon electrode. This nanocomposite was synthesized by reaction of sodium ferrate with graphene in alkaline media, this reaction completed in 5 min and the products were stable and its deposition on the surface of electrode is investigated. It has been found the apparent charge transfer rate constant, ks = 0.52 and transfer coefficient, α = 0.61 for electron transfer between the modifier and glassy carbon electrode. Electrochemical behavior of this electrode and its ability to catalyze the electro-reduction of H2O2 has been studied by cyclic voltammetry and choronoamperometry at different experimental conditions. The analytical parameters showed the good ability of electrode as a sensor for H2O2 amperometric reduction.
Keywords A.
Composites;
A.
Nanostructures;
B.
Chemical
synthesis;
C.
Electrochemical
measurements; D. Electrochemical properties
1. Introduction Hydrogen peroxide is a simple molecule but plays a critical role in diversified biological systems, clinical, food, and environmental chemistry [1]. Due to intrinsic simplicity, high sensitivity and selectivity, electrochemical methods have been extensively employed in H2O2 sensor designs [2].
2
Most of the employed electrodes in the fabrication of electrochemical H2O2 sensors are based on enzymes [3,4]. By the way, these electrodes have some practical restrictions related to the use of enzyme; enzymes are relatively expensive and unstable [5,6]. Moreover, enzyme-based electrodes are the electrochemical sensors that generate anodic current during electro oxidation of H2O2. The redox reaction of H2O2 has a relatively high potential at these electrodes. However, electrochemically active interfering species, which are usually present in real samples, are easily oxidized at that potential and dramatically influence the biosensor sensitivity by producing an interfering current [7–9]. Consequently, the main problem of these analytical devices is their sensitivity to the interferences, present in analyte solution [10]. Graphene is a planer monolayer of carbon atoms ordered into a honeycomb lattice [11]. The good conductivity and macroscopic tunnel effect can make graphene an effective electronic interface between the immobilized enzyme and electrode and consequently, it can improve the sensitivity of the sensor and the current response and also shorten the response time. One of the most promising applications of graphene emerged so far is its utilization as a new way of designing novel electrochemical sensors and biosensors [12, 13]. As a result, graphene modified electrodes were successfully applied to study and determine some biological and organic molecules, including enzyme [14], DNA [15], small biomolecules [12,16], heavy metal ions [17, 18], gas [19] etc. Non-enzyme electrochemical sensors have been fabricated which are based on composite graphene with metal nanoparticles such Au [20] and Ag [21]. Also Hybridization of the graphene and Fe in the form of Fe2O3 or Fe3O4hasbeen extensively studied for various applications such as H2O2 detection because of abundant, low cost, and nontoxic material. Shen et al. [22] synthesized graphene/Fe3O4 nanocomposite and analyzed it by differential scanning calorimetry (DSC). Moreover, Deng et al. [23] utilized magnetic graphene oxide
3
nanocomposite as an adsorbent for simultaneous removal of Cd(II) and ionic dyes from aqueous solutions. Zhang et al. [24], synthesized Fe2O3/graphene composite and used it as electrode materials for Li-ion batteries. Wang et al. [25], prepared α-Fe2O3/rGO modified glassy carbon electrode and used it for H2O2 detection. They synthesized α-Fe2O3/rGO with reduction of GO in the presence of FeCl2 with urea in ammonia media in 150 °C. Yuan et al. [26] prepared Fe2O3/graphene nanocomposite and concluded that graphene obviously improved the catalytic activity of Fe2O3 on the thermal decomposition of ammonium perchlorate due to its high specific area. They also prepared Fe2O3/graphene with reduction of GO in the presence of iron chloride in 160 °C for 48 h. Yang et al. [27] fabricated reduced grphene sheet (GNS)/Fe2O3nanorods composite for supercapacitor electrode. For synthesis of GNS/Fe2O3, they added NaOH to suspension of Fe(OH)3 and GNS in 160 °C for 20 h. In this paper, we used ferrate (FeO42-) for oxidation of graphene in alkaline media for the first time. Ferrate is a synthesized salt of iron with high oxidation state that can be used as apowerful oxidant in natural waters at the wide pH range [28]. The reduction of Fe(VI) results in a relatively non-toxic by-product iron(III), which suggests that Ferrate is an environmentally friendly oxidant [29]. The effect of Ferrate on microorganisms in waste water and some of viruses and bacteria is also investigated [30,31].
One of the main
approaches to ferrate synthesis is wet chemical synthesis. In this approach an iron(III) salt is oxidized by alkaline hypochlorite [32]. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) showed the main product of reaction of ferrate with reduced graphene oxide (rGO) were Fe2O3 (hematite) molecules which located on rGO. With incorporation of the nonmagnetic Fe2O3 between layers of rGO, after washing, the agglomeration of dark brown residual is reduced. Unlike the previous methods, synthesis of rGO-Fe2O3 with ferrate was very fast and the reaction completed in 5 min and the products were stable. Also the
4
adsorbed hematite on the surface of rGO enhance its electrochemical properties. For study of this effect rGO-Fe2O3 deposit on glassy carbon electrode (GCE), it stability to catalyze the electro-reduction of H2O2 has been studied. Electrochemical behavior of (rGO-Fe2O3) modified GCE and the analytical performance of the rGO-Fe2O3 modified GCE was then evaluated with respect to detection limit, linearity, stability and reproducibility. This study provides a new kind of nanocomposite modified electrode for electrochemical sensors. This sensor exhibits many advantages such as good reproducibility, a facile preparation procedure, anti-interference ability, wide linear range and long-term stability of the response signal, which indicate that rGO-Fe2O3-GCE is a promising candidate for H2O2 sensing in clinical and environmental samples. Also in this research we investigate antibacterial properties of rGO-Fe2O3 and Ferrate on two kinds of gram negative and positive bacteria. As results will be showed, rGO-Fe2O3 opposite of ferrate didn't show antibacterial properties.
2. Experimental
2.1 Materials and methods Graphite was purchased from Fluka, H2SO4 (98%) (d = 1.84 Kg L-1), KMnO4, H2O2 (30%), NaNO3, Fe(NO3)3.9H2O, NaBH4 and sodium hydroxide were purchased from Merck. Sodium hypochlorite (NaOCl) was purchased from Mojallali Chemical Laboratory (Iran). All solutions were prepared with distilled water. Phosphate buffer solutions (PBS, 0.01 mol L-1) were prepared from 0.01 mol L-1 H3PO4-NaH2PO4, and the pH was adjusted with 0.01 mol L1
H3PO4 or 0.1 mol L-1 NaOH. The pH measurements were carried out with a Jenway model
6103 pH/mV meter. Ammonia buffer solution (0.04 mol L-1) was prepared from the mixture of 8.0 mL ammonium chloride (2.0 mol L-1) and 2.0 mL NH3 (2.0 mol L-1) in 100 mL flask. UV–Vis absorption spectra were recorded using Optizen 2120 UV- PC spectrophotometer.
5
The powder X-ray diffraction (XRD) analyses of the samples were carried out with a X-Ray Tube: Cu (Kα =1.54 Å) X'Pert Pro MPD (PANalytical) with a scanning speed of 4°/min and a step of 0.04° (2θ) in the range from 5° to 80°. The morphologies of the samples were investigated by scanning electron microscopy (SEM) Model Phenom Pro X (Netherlands). The compositions of the powders were characterized at room temperature by Fourier transform infrared spectrometer (FTIR, Prestige-21, Shimadzu) Electrochemical experiments were performed with an IVIUM electrochemical workstation by using a three-electrode electrolytic cell. Glassy carbon electrode (GCE, 2 mm in diameter) acted as the working electrode. A KCl saturated calomel electrode (SCE) served as the reference electrode and a platinum electrode (2 mm in diameter) was used as the counter electrode. The bactericidal experiments were carried out with gram negative bacteria Escherichia coli PTCC1394 and gram positive bacteria Staphylococcus aureus PTCC1431 in the same nutrient media. These bacteria were obtained from the culture collection of I.R. Department (Tehran, Iran). Moreover, stock cultures were stored in the -80 °C freezer. The strains were propagated on Tryptic Soy Agar (TSA; Merck, Darmstadt, Germany) at 37 °C and maintained at 0-2 °C before use.
2.2 Synthesis of sodium ferrate by wet oxidation method
The wet oxidation method involves the oxidation of a Ferric containing solution to form Ferrate solution under high alkaline conditions. 7.0 g of sodium hydroxide was dissolved to 10 mLcold water and 10 mL NaOCl was added. Then 2.5 g of well powdered Fe(NO3)3.9H2O was added slowly while the solution was being stirred at 20 °C. Then the temperature of solution was raised to 45-55 °C. The color of solution changed to purple. 6
After removal of the residual Fe(OH)3 by centrifuge, the resulting Ferrate was measured using an established spectroscopy method where the absorbance of the Ferrate solution was measured at 510 nm and the absorbance was converted to the concentration using the following equation: C = A/ε × b
(1)
Where A, ε, b and C are absorbance (at 510 nm), extinction coefficient (1150 L/mol cm) [33,34], cell path length (1 cm) and concentration (mol L-1), respectively.
2.3 Synthesis of graphene oxide
GO was prepared by a modified Hummer’s method [35]. Briefly, 23 mL of 98% H2SO4 and 100 mg of NaNO3 were added to 1 g of graphite, followed by stirring at room temperature over a 24-h period. Subsequently, the mixture was kept below 5 °C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35–40 °C, the mixture was stirred for another 30 min. Then 10 mL of 30% H2O2 was added into the mixture to stop the reaction. Finally, the unexploited graphite in the resulting mixture was removed by centrifugation,
2.4 Synthesis of rGO-Fe2O3
0.25 g of the synthesized GO was dispersed into individual sheets in 100 mL distilled water with the aid of vigorous ultrasonic waves. After centrifugation, to 5 mL of GO dispersion 10 mL water and 0.05 g NaBH4 were added and the mixture was heated to 80-90 °C for 10 min. After cooling, 5 mL sodium ferrate (0.005 mol L-1) was added. As it could be observed, Ferrate rapidly was adsorbed. After washing the aggregate products to natural pH,
7
a dark brown dispersion was obtained. rGO was also prepared through a similar method except for the absence of ferrate.
2.5 Preparation of the modified electrode and measurement procedures
Prior to modification, GCE was polished with 400, 1000 and 3000 mesh polishing papers until a mirror shiny surface appeared, and was cleaned in ethanol, HNO3(1:1, V/V), NaOH (1.0 mol L-1) and double distilled water for 5 min. The treated electrode was swept between -1.0 and 1.0 V versus SCE in ammonia buffer solution (pH=9.0) for sufficient cycles to obtain reproducible cyclic voltammograms.
2.6 Antimicrobial activity studies
The antimicrobial properties of sodium ferrate and composites of rGO-Fe2O3 were investigated using Escherichia coli, and Staphylococcus aureus by the Kirby-Bauer diffusion method [36,37]. The bacterial suspension was applied uniformly on the surface of a Tryptic Soy Agar (TSA) plate at a concentration of 105 CFU/mL before placing ferrate or composite simpregnated disks. For antibacterial study of Ferrate and composites, laden disks have been prepared by keeping 6 disks in 2 mL Ferrate solution and colloidal solution of composites for 1-h. These disks absorbed the Ferrate in 5 min and the purple solution became colorless. After drying, disks fixed on plates. The plates with the disks were incubated at 35 °C for 24 h, after which the average diameter of the inhibition zone surrounding the disk was measured with a ruler. Fig. 1 shows plates which S. aureus and E. coli bacterial suspensions were applied with samples laden disk and antibiotic impregnated disks. The diameters of inhibition zones around the disk containing Ferrate in S. aureus, and E. coli bacterial suspension are 12 and 14 mm, respectively. This test shows that Ferrate is nearly 43 and 56 % effective
8
compare to tetracycline disk for S. aureus, and E. coli bacterial, respectively. It was observed that in the presence of Ferrate, the bacterial growth was inhibited but composites rGO-Fe2O3 didn't show any antibacterial properties. In this research the effect of Ferrate concentration on bacteria growth inhibition was not studied and subjected to the future research.
3. Results and discussion
3.1 Characterization of reaction of Ferrate with rGO Ferrate ion which has the molecular formula of FeVIO42- is a powerful oxidant and its aqueous solution has a characteristic of black-purple colour. A number of alkali and alkaline earth salts of Ferrate(VI) have been synthesized. The basic wet chemical reaction is:
(2)
2FeCl3 + 3NaOCl+ 10NaOH→2Na2FeO4 + 9NaCl+ 5H2O
Ferrate oxidation is known to be active over a wide pH range. Na2 FeO4 has relatively high solubility in a saturated NaOH solution. However its decomposition is faster under acidic conditions [34].
For acidic solutions:
FeO42− + 8H+ +3e−[Maths Symbol
(3)
here]Fe3+ + 4H2O FeO42− + 2H2O +3e−[Maths Symbol (4)
For alkaline solutions:
here]FeO2− + 4OH− FeO42− + 4H+ + 3e−[Maths Symbol
For weak acid and neutral solutions:
here]Fe(OH)3+ OH−
9
(5)
The reaction of rGO with Ferrate is very fast and any spontaneous decomposition of Ferrate in water does not happen because of the formation of molecular oxygen according to the following reaction was not observed. 2FeO42- + 5H2O [Maths Symbol here]2Fe(OH)3 + 3/2O2 +
(6)
4OHBecause of the fast decomposition of Ferrate in acidic media, it is impossible to use for oxidation of graphite. In contact of Ferrate with graphite in dry environment, it was observed that Ferrate is adsorbed on graphite surface as well as paper disks in antibacterial test. As is shown in Fig. 2 Ferrate has a λmax≈ 510 nm in UV-Vis spectrum. In this paper we used Ferrate for oxidation of graphene in alkaline media. Moreover, the UV–Vis spectra of GO, rGO and rGO-Fe2O3 exhibit maximum peaks at 235, 260 and 275 nm, respectively corresponding to π→π* transitions of aromatic C―C bonds. It is obvious that the UV–Vis spectrum of rGO usually red-shifts from 235 to 275 nm, compared with that of GO. The band at 275 nm in the spectrum of rGO is assigned to π→π* transitions of aromatic C═C bonds. The FTIR spectrum (Fig. 3) of the GO illustrates the presence of C―O, C═C, C═O and C―OH bonds from the peaks at 1048, 1455, 1625 and 3425 cm−1, respectively. Moreover, approximately the same peaks were observed in spectrum of rGO-Fe2O3 (1041, 1581, 1726, 3416 cm-1), respectively. In addition to this, the peak in 576 cm-1 is attributed to Fe–O, and the enhanced intensity for Fe–O is indicative of the iron loading in rGO-iron oxides [38]. The XRD patterns of GO, rGO and rGO-Ferrate are shown in Fig 4. GO exhibits a typical sharp diffraction peak centered at 2θ = 9.8° corresponding to the interlayer distance of ∼0.86 nm [39] and typical diffraction peak of GO nanosheets, was attributed to the (002) plane [40]. In comparison, rGO and rGO-Fe show broad and low intensity peaks under 2θ = 10°. The broad diffraction peaks are indicative of nanoparticles with very small size. The main peaks at about 2θ = 34.5° (104), 45.5° (200), 56.5° (116), 66.3° (030) and 83.8° (312) show the characteristics of Fe2O3 (hematite) on rGO [38]. The presence of hematite reduces the
10
aggregation of graphene sheets, which results in more monolayer of graphene, leading to weaker peaks from carbon being observed. The SEM micrograph and Energy dispersive X-ray (EDX) spectrum of rGO-Fe2O3 are shown in Fig. 5. The EDX spectrum confirmed the incorporation of Fe on the rGO surfaces. Also, the percentages elements from the points of 1, 2 and 3 in SEM graph (Fig. 5A) and the average of three points are plotted in Fig. 5C.
3.2 Electrochemical behavior of the rGO-Fe2O3 modified glassy carbon electrode
Electrochemical behavior of rGO-Fe2O3 on GCE was investigated in ammonia buffered solution (pH 9.0) using cyclic voltammetry (CV). Fig. 6 shows the cyclic voltammograms obtained from the bare GCE and modified GCE with GO, rGO and rGO-Fe2O3, at a scan rate of 100 mV s-1. As expected, no redox process occurs at bare GCE,GO- and rGO modified GCE while the rGO-Fe2O3 modified GCE exhibit anodic and cathodic peaks at the potentials of -0.26, -0.52 V and the ratio between the peak currents Ipa/Ipc=1.41. The peak separation potential, |∆Ep|=|Epc-Epa|=0.26 V, was more than (0.059/n)V excepted for a reversible system[41]. This result indicates that the redox couple in rGO-Fe2O3-GCE shows quasireversible behavior in an aqueous medium. The E0' value found for rGO-Fe2O3-GCE (-0.39 V vs. SCE) is smaller than -0.210 V (vs. Ag/AgCl) reported for carbon paste electrode modified with Fe doped mesoporous carbon aerogel [46]. The effect of pH and buffer solution on the electrochemical behavior of rGO-Fe2O3 on GCE was investigated over the range of pH 2-9 with PBS and the results are illustrated in Fig. 7. As the results show the peak currents did not change with increasing the solution pH in PBS 0.01 mol L-1, the ammonia buffer (pH=9.0) shows an enhancement in redox peak currents clearly. Therefore, the ammonia buffer solution (pH 9.0) was chosen as the optimum working buffer solution for subsequent measurements. The anodic and cathodic currents were 11
constant in 1 to 20th measuring sweep cycles and also solution of rGO-Fe2O3 was steady in the period of research for 4 months (Fig. 8). Figure 9 shows the effect of the potential scan rate on electrochemical response of the rGO-Fe2O3-GCE in 0.04 mol L-1 ammonia buffer solution (pH 9.0). As can be seen in Fig. 9B, the plots of the anodic and cathodic peak currents (Ip) were linearly dependent on scan rate (ν) from 30 to 300 mV/s, indicating a surface-confined-redox process. According to Laviron theory [42], the apparent charge transfer rate constant, ks, and the charge transfer coefficient, α, of a surface-confined redox couple were evaluated based on the variation of the anodic and cathodic peak potentials with the logarithm of the scan rate (Fig. 9C) obtained from the cyclic voltammograms of a quasi-reversible system [41]. We found that the Ep values are proportional to the logarithm of the potential scan rate, for scan rates higher than 120 mV/s (Fig. 9D). The slopes of the plots can be used to extract the kinetic parameter α (anodic charge transfer coefficient). The slope of the linear segment (0.151) is equal to 2.303RT/(1-α)nF for the anodic peaks. When n=1 (for Fe(III)→Fe(II)), the evaluated value for α is 0.61. The following equation can be used to determine the electron transfer rate constant between the modifier rGO-Fe2O3 and GCE: log ks = αlog(1-α) + (1-α)logα- log(RT/nFυ) -α(1-α)nF∆Ep/2.3RT
(7)
Where υ is the potential scan rate, (n=1, Fe(III)→Fe(II)) is the number of electrons involved in the overall redox reaction of the modifier rGO-Fe2O3-GCE and all the other symbols have their conventional meanings. The value of ks was evaluated to be 0.52 s−1 using Eq. (7). Because of the electron transfer rate constant for the rGO-Fe2O3 is about 0.52 s-1, it can be used as an excellent electron transfer mediator for electrocatalytic processes.
3.3 Electrocatalytic redox properties of the rGO-Fe2O3 modified GCE toward H2O2 cyclic voltammetric studies
12
Comparative electrochemical behavior of the bare GCE, GO modified GCE and rGOFe2O3 modified GCE in ammonia buffer (pH 9) in the presence of H2O2 by CV method is shown in Fig. 10A. It can be seen the currents response on the rGO-Fe2O3 modified electrode is more than bare GCE and GO-GCE. Also, the effect of pH on rGO-Fe2O3-GCE in the presence of H2O2 (1 mmol L-1) in PBS and ammonia buffer is shown in Fig 10B. It is obvious that, ammonia buffer with pH 9 is the best media for such electrocatalytic studies. In order to examine the abilities of the sensor in the determination of H2O2, the CVs were recorded by addition of different amounts of H2O2 at the optimized solution conditions (Fig. 11A). The results showed that by addition of the concentration ranges of H2O2 from 0.5 to 25 mmol L-1 both anodic and cathodic peaks were grown linearly but the anodic peak currents calibration curve was more sensitive (Fig. 11B). The linear increase of the anodic and cathodic peak currents illustrates the excellent electrocatalytic ability of the modified electrode towards the oxidation/reduction of H2O2.
3.4 Chronoamperometric studies
Chronoamperometry as well as the other electrochemical methods may be used for investigation of the electrode processes. Chronoamperometric measurements of H2O2 at rGOFe2O3 modified GCE were carried out by setting the working electrode potential at -0.6 V vs. SCE for the various concentration of H2O2 in buffered aqueous solutions (pH 9.0) (Fig. 12). For an electroactive material (H2O2 in this case) diffusion coefficient (D) is obtained from the Cottrell equation [41]: I = nFAD1/2Cb(πt)-1/2
(8)
13
Where D and Cb are the diffusion coefficient (cm2/s) and the bulk concentration (mol cm-3) of H2O2, respectively. A is the electrode surface area (0.031 cm2), I is a current controlled by diffusion of H2O2 from the bulk of the solution to the electrode/solution interface. Experimental plots of I vs. t-1/2 were employed, with the best fits for different concentrations of H2O2 (Fig. 12A). The slopes of the resulting straight lines were then plotted vs. H2O2 concentrations (Fig. 12B). Based on resulting slope (slope (I vs. t-1/2) = -0.095 [H2O2] (mmol L-1) - 0.035, r = 0.994) the mean value of the D was found to be 3.1× 10−2 cm2/s. Chronoamperometry can also be employed to evaluate the catalytic rate constant, k, for the reaction between H2O2 and the rGO-Fe2O3-GCE according to the method of Galus [43]. IC/IL = γ1/2[π1/2erf(γ1/2) + exp(-γ)/γ1/2]
(9)
Where IC is the catalytic current of H2O2 at the rGO-Fe2O3-GCE, IL is the limited current in the absence of H2O2 and γ = kCbt is the argument of the error function. In cases where γ exceeds the value of 2, the error function is almost equal to 1 and therefore, the above equation can be reduced to: IC/IL = π1/2γ1/2 = π1/2 (kCbt)1/2
(10)
Where k, Cb and t are the catalytic reaction rate constant (L/mol.s), the bulk concentration (mol L-1) of H2O2 and time elapsed (s), respectively. The above equation can be used to calculate the k, of the catalytic process from the slope of IC/IL vs. t1/2 (Fig. 12C) at a given H2O2 concentration. From the values of the slopes, the average value of k was found to be 1.5×104 L mol-1.
3.5 Amperometric determination of H2O2 at rGO-Fe2O3- GCE
14
The current-time response of rGO-Fe2O3-GCE in the optimized solution conditions was recorded to evaluate the calibration curve and the limit of detection for hydrogen peroxide determination. The sensor response at an applied potential of -0.6 V vs. SCE toward increasing concentration of H2O2 was recorded in ammonia buffer solution. Fig. 13 specifies the sensor response for successive addition of hydrogen peroxide from stock solution of 0.1 mol L-1 by 0.1 mL increments to electrochemical cell containing 20 mL ammonia buffer solution. At the optimal pH 9, the sensor exhibited a detection limit of 6.0 µmol L-1, broad linearity from 0.05 to 9.0 mmol L-1, good reproducibility (RSD of 1.9%), and long-term stability. Table 1 compare some of the analytical characteristics from this work and those previously enzymic and nonenzymic reported in the literature [21, 25, 44-53] for H2O2 detection. The detection limits and linear calibration range of the proposed sensor are comparable with those provided by other modified carbon electrodes. On the other hand, in comparison with enzyme-based sensors, proposed nonenzymatic sensor in addition to its low cost exhibit a high stability, free from the influence of temperature, pH and oxygen restricts. The interference study was conducted by placing the modified GCE into a solution containing target analyte and potentially interfering species at optimum conditions. It was -
found that a excess of Na+, K+, Ca2+, Mg2+, NO3 , Ni2+, Cd2+, Co2+, ascorbic acid, vitamin B1 have no influence on the signal of 1.0 mmol L-1 of H2O2.
4. Conclusion A simple and very fast method is introduced to fabricater GO-Fe2O3 nanocomposite based on reaction of ferrate on graphene. The XRD analysis shows that Fe2O3 (hematite) is adsorb on graphene sheet and incorporating iron oxide molecules between the layer of rGO prevent agglomeration of dark brown residual reduced. In addition, Fe2O3 (hematite) on surface of rGO enhance its electrochemical properties. Thus, the GCE modified with rGOFe2O3 have been investigated by cyclic voltammetry. A quasi-reversible, surface confined redox process was put in evidence and was attributed to presumably Fe3 +/Fe2+redox system. It
15
was demonstrated that rGO-Fe2O3 can be a promising material for preparing highly active electrode materials with practical applications, e.g. sensor for H2O2 detection. The kinetic and analytical parameters of the new modified electrode obtained with chronoamperometry measurements performed at a low applied potential (-0.6 V vs. SCE), recommend it as a stable, sensitive and reproducible sensor for H2O2amperometric reduction. The proposed sensor exhibits many advantages such as good reproducibility, a facile preparation procedure, anti-interference ability, wide linear range and long-term stability of the response signal, which indicate that rGO-Fe2O3-GCE is a promising candidate for H2O2 sensing in clinical and environmental samples.
Acknowledgements
The authors would like to express their appreciations to Professor Afsaneh Safavi for her valuable discussion and useful suggestions. This research was supported by the Nano Structured Coatings Institute of Payame Noor University of Yazd and Nanoscience and Nanotechnology Research Laboratory (NNRL) of Payame Noor University of Sirjan.
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polytetrakis(2-
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Caption of Figures Fig. 1 Representative image of agar plates containing Ferrate (VI) and rGO-Fe2O3 impregnated discs for E. coli and S. aureus Fig. 2 The optical adsorption spectra of sodium Ferrate (VI) before and after addition to graphite, GO, rGO and rGO- Fe2O3 Fig. 3 FT-IR spectra of GO, GO and rGO- Fe2O3 Fig. 4 XRD pattern of GO (a), rGO (b) and rGO- Fe2O3 (c). Fig. 5 (A) SEM image, (B) EDX spectrum of rGO-Fe2O3 and (C) EDX elements analysis from the points of 1, 2, 3 and average in SEM image. Fig. 6 CVs of bare GCE, GO, rGO and rGO- Fe2 O3 modified GCE in ammonia buffer (0.04 mol L-1, pH = 9) at scan rate of 100 mV/s. Fig. 7 CVs of rGO- Fe2O3 modified GCE in 0.01 mol L-1 PBS at different pH values (2-9) and ammonia buffer (pH = 9) on the top.
24
Fig. 8 (A) CVs of rGO-Fe2O3-GCE in ammonia buffer scan 1-20, (B) compare of CVs at first (a) and after 4 months (b). Fig. 9 (A) CVs of rGO-Fe2O3-GCE in ammonia buffer (pH 9.0) at various scan rate from 30 to 300 mV/s, insets: (B) variations of Ipa and Ipc versus scan rate, (C) variation of Epa and Epc versus the logarithm of scan rate, and (D) same as (C) for scan rates higher than 120 mV/s. Fig. 10 (A) The second scan of CVs from bare GCE, GO-GCE and rGO- Fe2O3-GCE in ammonia buffer (pH 9) in the presence of H2O2 (2.0 mmol L-1) and (B) rGO- Fe2O3-GCE in the presence of H2O2 and different pHs. Fig. 11 (A) CVs of rGO- Fe2O3-GCE in ammonia buffer (pH 9) solution after addition the different concentration of H2O2 at scan rate of 100 mV/s, (B) calibration plots and related equations extracted from anodic and cathodic peaks as a function of H2O2 concentration. Fig. 12 Short time chronoamperograms obtained at rGO-Fe2O3-GCE in ammonia buffer solution (pH 9.0) for H2O2 concentrations of 0.0, 0.5, 1.0, 1.5, and 2.0 mmol L-1. (A) plots of I vs. t-1/2 obtained from the chronoamperogram data, (B) plot of the slope of the straight lines against the H2O2 concentration and(C) dependence of IC/IL on t1/2 derived from the data of chronoamperograms Fig. 13 Amperometric response obtained at rGO- Fe2O3-GCE in ammonia buffer solution (pH 9.0) at a potential of -0.600 V (vs. SCE) by addition of the different concentration of H2O2. Inset shows the plot of extracted currents vs.H2O2 concentration and the related calibration equation.
25
Table 1. Comparison of analytical characteristics of different enzymic and non- enzymic modified carbon electrodes for electrochemical detection of H2O2 Modified electrode Eapp (V) Electrolyte LDR (mM) LOD (mM) Ref. AgNPs/rGO/GCE -0.43 0.2 M PBS (pH 6.9) 0.05-5.0 0.01 [21] [25] α-Fe2O3/rGO/GCE -0.22 0.1 M KOH 0.005-4.495 0.001 Peroxidase (type VI-A)//GCE 0.05 0.05 M PBS (pH 7.2) 0.00005-0.005 0.00004 [44] Horseradish peroxidase (type 0.089 0.05 M PBS (pH 6.5) 0.00005-0.006 0.00005 [45] VI)/graphite electrode Fe/CA/CPE -0.3 0.1 MPBS (pH 7) 1-50 0.5 [46] Ni(OH)2/SiNWs 0.2 0.5 M NaOH 0-5.5 0.0032 [47] Sn/[(Fe(CN)6]3-/CCE -0.1 0.1 M acetate buffer(pH 4) 0.004-0.050 0.0014 [48] CuO/SPCE -0.1 0.1 M PBS (pH 7) 0.003-15.0 0.0015 [49] NNBP/GE -0.1 0.1 M PBS (pH 7) 0.0005-0.0015 0.0003 [50] PTAPPNW/SWCNTs/NGCE 0.0 PBS (pH 7.4) 0.1-1.0 0.001 [51] Ni/[(Fe(CN)6]3-/CPE 0.6 0.1 M NaOH 0.6-6 , 17-54 0.34 [52] PdNPs/GCE -0.2 0.1 M PBS (pH 7.4) 0.001-14 0.0003 [53] rGO/Fe2O3/GCE -0.6 0.04 M ammonia buffer (pH 9.0) 0.05-9.0 0.006 This work CA: carbonaerogel; SiNWs: silicon nanowires; CCE: carbon ceramic electrode; SPCE: screen-printed carbon electrode; NNBP/GE: nafion-nile blue peroxidase graphite electrode; PTAPPNW/SWCNTs/NGCE: nanowires of polytetrakis(o-aminophenyl)porphyrin/single-walled carbon nanotubes/nafion coated glassy carbon electrode; CPE: carbon paste electrode.
Figure 1
Tetracyclin
Ferrat rGO-Fe2O3 E. coli
S. aureus
Figure 2
26
3.0 Fe (VI) Fe (VI) + graphite GO rGO rGO-Fe2 O3
2.5
Absorbance
2.0
1.5
1.0
0.5
0.0 200
300
400
500
600
700
800
Wavelenght / nm
Figure 3 100
Transmitance (%)
80
60
40
20 4000
rGO GO rGO-Fe2O3
3000
2000
1000 -1
Wavenumbers / cm
Figure 4
27
10000
8000 rGO-Fe2O 3 GO rGO
Intensity
a 6000
4000
c
b
2000
10
20
30
40 2 Theta
Figure 5
A
28
50
60
70
80
Figure 6
I/mA
0.2
GCE GCE-rGO GCE-GO GCE-rGO-Fe2O3
0.1
0.0
-0.1 -1.0
-0.5
0.0 E/V (vs. SCE)
Figure 6
29
0.5
1.0
I/mA
0.2
GCE GCE-rGO GCE-GO GCE-rGO-Fe2O3
0.1
0.0
-0.1 -1.0
-0.5
0.0 E/V (vs. SCE)
Figure 8
30
0.5
1.0
0.1
A
0.0
I/mA
0.2
B -0.1
a b
0.1 0.0
-0.2
-0.1 -0.2 -1.0
-0.5
0.0
0.5
1.0
-0.3 -1.0
-0.5
0.0
0.5
E/V (vs. SCE)
Figure 9
31
1.0
1.5
0.15
0.2
0.10
Ip /mA
0.1
A
B
0.0 -0.1 -0.2 50 100 150 200 250 300
0.05
0.0
v/(mVs-1)
C
I/mA
Ep /V
-0.2 -0.4 -0.6
0.00
-0.8
1.5
-0.2 Ep /V
2.1
2.4
Log(v/mV s -1)
0.0
-0.05
1.8
D
-0.4 -0.6 -0.8
-0.10
2.1
2.2
2.3
2.4
Log(v/mV s-1)
-1.0
-0.5
0.0
0.5 E/V (vs. SCE)
Figure 10
32
1.0
1.5
2.0
0.4
A
0.2
I/mA
0.0
-0.2 rGO-Fe2O 3-GCE GO-GCE bare-GCE
-0.4
-0.6 -1.0
-0.5
0.0
0.5
1.0
E/V (vs. SCE)
0.4
B
I/mA
0.0
-0.4
pH 2 pH 4 pH 7 pH 9
-0.8
-1.2 -1.0
-0.5
0.0 E/V (vs. SCE)
Figure 11
33
0.5
1.0
A
0.0
I/mA = 10.6 C(mmol/L) + 0.0241 R2 = 0.996
-0.4 0.0
B
I/mM
I/mA
0.4
-0.4
-0.8 I/mA = -21.0 C(mmol/L) - 0.0395 2 R = 0.992 -0.8 0
5
10
15
20
25
C (mmol/L)
-1.0
-0.5
0.0 E/V (vs. SCE)
34
0.5
1.0
Figure 12 -0.5 -0.20
-0.24
A
I/mA
-0.4
B slope(mAs 1/2)
-0.16 -0.12 -0.08
-0.18
-0.12
-0.04
-0.3 0.4
0.5 0.6 -1/2 -1/2 t (s )
I/mA
0.3
-0.06
0.7
0.2
0.6
0.8
1.0
1.2
C (mmol/L)
0 mmol/L 0.5 mmol/L 1.0 mmol/L 1.5 mmol/L 2.0 mmol/L
60
C
45 IC/IL
-0.2
0.4
-0.1
30 15 0 1.5
2.0 2.5 1/2 1/2 t (s )
3.0
0.0 0
5
10
15 t/s
Figure 13
35
20
25
30
0.0
0.5 mmol/L
-1.0 -1
-0.4
-0.8 I/mA
I/mA
-0.2
-0.6
I/mA = -0.085 C (mmolL ) - 0.040 2 R = 0.9958
-0.6 -0.4
0.9 mmol/L
-0.2 0.0 0
2
-0.8 0
100
4 6 8 [H2O2]/mmol/L
10
200
300 t/s
36
1.5 mmol/L
400