Investigation of morphologies and characterization of rare earth metal samarium hexacyanoferrate and its composite with surfactant intercalated graphene oxide for sensor applications

Abstract

Herein, we report a facile electrochemical approach for the hierarchical growth of samarium hexacyanoferrate (SmHCF) on surfactant intercalated reduced graphene oxide (SRGO). The fabricated SRGO/SmHCF modified glassy carbon electrode (GCE) has excellent electrocatalytic activity towards catechol (CC) sensor applications. The sunflower-like SmHCF microparticles were achieved by controlling the number of cycles during the electrodeposition process. In addition, the electrolyte plays a key role in the morphology of SmHCF and was investigated using different electrochemical techniques. The as-prepared SmHCF microparticles were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Fourier transform infrared (FT-IR) spectroscopy. In addition, electrochemical impedance spectroscopy (EIS) was carried out to understand the mechanism of interfacial electrochemical reactions on the proposed SmHCF modified glassy carbon electrode (GCE). The obtained EIS data confirmed that the electron transfer rate at SmHCF/GCE was faster than bare GCE. The electrochemical detection of CC using the SRGO/SmHCF modified GCE was performed by cyclic voltammetry and difference pulse voltammetry. The fabricated modified GCE exhibits a good linear range from 50 μM to 250 μM, with a limit of detection (LOD) of 0.38 μM and a sensitivity of 0.430 μA μM⁻¹ cm² for the CC electrochemical sensor.

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1. Introduction

Metal hexacyanoferrates (MHCFs) preparation, characterization and electrochemical studies were pioneered by Neff and Itaya et al.^1,2 MHCFs are considered to be a class of poly-nuclear inorganic mixed valence compounds, zeolitic materials, showing redox and ionic conductivity properties.^3,4 Because of the high stability of the metal cyanide framework and cation exchanging properties, which are beneficial for widespread applications such as electro-chromism, electrocatalysis, electroanalytical applications,⁵ solid state batteries^6–8 and photoimage formation.^9,10 Several studies on the preparation of transition metal hexacyanoferrate and their applications towards electrocatalysis have been reported so far.^11–13 Nevertheless, rare-earth MHCFs have been studied in the past few decades¹⁴ because of their special properties, which have received significant attention for applications in batteries, sensor fields^15,16 and the electrocatalytic hydrogenation of alkenes.¹⁷

The novel synthesis of nanostructured materials with different morphologies has recently attracted attention of the scientific community;¹⁸ the finite size effects of the prepared nanomaterials have unusual properties and are utilized in electrical, optical and magnetic devices.¹⁹ On the other hand, in recent years, various rare earth MHCFs with different morphologies were reported by electrochemical methods, such as flower-like and christmas tree-like cerium hexacyanoferrate (CeHCF),²⁰ carambola-like and flower-like holmium hexacyanoferrate (HoHCF)²¹ and our previously reported work on microstar-like dysprosium hexacyanoferrate (DyHCF).²² In general, all the lanthanide elements have high charge density and high affinity²³ and form complexes with ferricyanide, which results in the formation of metal hexacyanoferrates (MHCFs). Herein, we successfully achieved a samarium hexacyanoferrate modified electrode by an electrodeposition method using a ferricyanide solution containing Sm³⁺ with different electrolytes. The SmHCF shows excellent electron transfer ability for application as an electrochemical sensor.²⁴ In addition, there are a few reports available for the preparation and characterization of SmHCF by Ping Wu et al.,^3,25 which demonstrated an electrodeposition approach towards SmHCF under different electrolyte conditions. However, studies on the specified morphology of SmHCF have not been reported to date. Hence, it is required to investigate the different morphologies and electrochemical behaviors of different rare earth MHCFs to develop novel kinds of electrochemical sensors.^26,27

Being a versatile dispersant, graphene oxide (GO) has unique properties such as an exceptional ability to form stable aqueous dispersions and superior electrocatalytic properties. Because of its unique properties, it was considered to be a gifted material for biological applications.²⁸ Hence, these fascinating and unique properties of GO have gained significant interest in the emerging electrochemical field. In our study, the organic surfactant, cetyl-trimethyl ammonium bromide (CTAB), was intercalated with GO (SGO) and its subsequent reduction was achieved (SRGO). SRGO exhibits good dispersion in aqueous solvents and stabilizes the wrinkled structure of the graphene sheets at the time of reduction, enhancing the specific capacitance.²⁹

Catechol (CC, 1,2-dihydroxybenzene) is a phenolic compound and an isomer of dihydroxybenzene,³⁰ which is widely used for pesticides, flavoring agents, medicines, dyes and antioxidant chemicals.³¹ According to the US Environmental Protection Agency (EPA) and the European Union (EU), CC is considered to be an environmental pollutant and is included in the priority pollutant list.^32,33 Moreover, an increase in the concentration of CC to a certain level in the environment could be very harmful to humans, animals, plant and aquatic life.³⁴ Especially, pesticides, tannery industrial effluents and sanitary wastewater contains high CC levels.³⁰ Therefore, it is important to develop a highly sensitive analytical method to monitor CC levels in samples. So far, numerous methods have been developed for the detection of CC such as spectrophotometry,³⁵ high-performance liquid chromatography techniques,³⁶ fluorescence,³⁷ chemiluminescence,³⁸ gas chromatography/mass spectrometry³⁹ and capillary electrophoresis.⁴⁰ In contrast to the aforementioned techniques, electrochemical methods have the advantages of low cost, good response and high sensitivity.

In the present work, a facile electrochemical route has been used for the preparation of SmHCF with sunflower like structure. The composite of SmHCF/SRGO modified GCE has been applied for CC electrochemical sensor. The SRGO/SmHCF composite, combining the individual properties of GO (large surface area and high conductivity) and SmHCF (high recognition, thermal stability, chemical inertness, and lack of toxicity) and which could be an excellent approach for catechol detection. The morphology of SmHCF was investigated by SEM and the fabricated SmHCF electrode was characterized by Fourier transform infra-red (FT-IR) and energy dispersive X-ray (EDX) spectroscopy. The electrochemical characteristics of the SmHCF modified electrode were studied by electrochemical impedance spectroscopy (EIS). To the best of our knowledge, the sunflower-like SmHCF morphology has not been reported to date. To the best of our knowledge, preparation, characterization and electrochemical studies of SmHCF are limited to a few preliminary reports in recent years.^3,25,27

2. Experimental

2.1 Materials and methods

Graphite powder and samarium(III) chloride hexahydrate [SmCl₃·6H₂O] (>99%) were purchased from Sigma Aldrich and were used as received. Potassium ferricyanide [K₃Fe(CN)₆], potassium chloride (KCl) and lithium chloride (LiCl) were purchased from Wako pure chemical industries, Ltd. Sodium chloride (NaCl) was obtained from J. T. Baker (Phillipsburg, USA). Catechol was procured from Sigma Aldrich and was used as without purification. Cetyl trimethyl-ammonium bromide (CTAB) was obtained from Janssen Chemicals. Doubly distilled (DD) water was utilized for preparing all the working solutions. Nitrogen gas was purged throughout the electrochemical cell for few minutes prior to the electrodeposition process.

The electrochemical deposition process was performed by a cyclic voltammogram (CV) technique using a CHI 1205 A electrochemical analyzer. A conventional electrochemical cell consists of a three-electrode system. A glassy carbon electrode (GCE) (surface area = 0.075 cm²) was used as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode and a platinum wire as the counter electrode. The potentials of all the experimental results were referred to the standard Ag/AgCl (saturated KCl) reference electrode. Surface characterization was studied by a scanning electron microscope (Hitachi, Japan). Electrochemical impedance studies (EIS) were performed by a ZAHNER impedance analyzer (ZAHNER Elektrik Gmbh & Co KG, Germany). X-ray diffraction, ultra violet spectroscopy and infra-red spectroscopy instruments were used for the characterization.

2.2 Preparation of the SRGO/SmHCF sunflower modified glassy carbon electrode (GCE)

The preparation procedure for graphene oxide (GO) was followed by Hummers' method. Briefly, raw graphite powder was converted into graphite oxide upon treatment with a mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate under the reaction conditions of Hummers' procedure. Finally, the obtained brown color graphite oxide was exfoliated through continuous ultrasonication in water for 2 h to obtain GO. Finally, GO was centrifuged for 15 min to remove the un-exfoliated graphite oxide. To prepare the surfactant intercalated graphene oxide (SGO), 10 mL of the as-prepared GO solution was added to 10 mL of 0.1 M CTAB in a flask. This reaction mixture was vigorously stirred for 72 h at ambient temperature. After three days, the SGO was filtered and dried at room temperature. The powder was re-dispersed in water (1 mg mL⁻¹) and sonicated for 2 h to achieve a uniform dispersion. Then, 5 μL of this dispersion was drop casted on a GCE and dried at room temperature. The SGO was electrochemically reduced to SRGO using 0.1 M KCl (pH 5) scanning between 0 and −1.5 V at a scan rate of 50 mV s⁻¹ for 10 cycles.

The SmHCF modified SRGO/GCE was prepared by an electrodeposition method using a CV technique. As fabricated, the SRGO/GCE was placed in an electrochemical cell containing freshly prepared 5 mM SmCl₃·6H₂O and K₃Fe (CN)₆ in a 0.2 M KCl electrolyte. 20 consecutive cyclic voltammogram scans were performed between a potential range of 0.8 and −0.2 V at a scan rate of 50 mV s⁻¹ (explained in detail in Section 3.1). The flower-like SmHCF particles were electrochemically deposited on the SRGO modified GCE. Then, the SmHCF modified SRGO/GCE was rinsed with water and dried at room temperature. Thus, the fabricated GCE was used for further electrochemical studies.

3. Results and discussion

3.1 Electrochemical deposition of SmHCF microflowers on the GCE and SRGO/GCE

The electrochemical deposition of metal hexacyanoferrate using a supporting electrolyte solution containing K⁺ cations shows a well-defined reversible cyclic voltammogram (CV). In this work, we prepared SmHCF using a KCl solution as the electrolyte. Fig. 1 depicts the CV of the electrodeposition of the SmHCF micro-sunflowers in a KCl solution with the electrolyte containing equal concentrations (5 mM) of SmCl₃·6H₂O and K₃Fe(CN)₆. Here, it can be seen that a cathodic peak appears at 0.255 V, indicating the reduction of [K₃Fe(CN)₆]³⁻ and a anodic peak at 0.3 V, which reveals the characteristic oxidation of [K₃Fe(CN)₆]⁴⁻. As expected, the redox couple progressively decreased with an increase in cycle scans. Likewise, the redox peaks were appeared for SmHCF deposition on SRGO/GCE. The two redox couples shown in Fig. 1B, correspond to the growth of SmHCF on the SRGO/GCE. While when the reduction reaction occurred, the Sm³⁺ ions reacted with K₃Fe(CN)₆⁴⁻ to form SmHCF microflower particles. SRGO with negatively charged functional groups (such as carboxyl, carbonyl and epoxy) and surfactant, as a supporting material, allow effective anchoring and adsorption of positively charged Sm³⁺ ions that consequently adsorb the negatively charged HCF particles and stabilize the as-formed SmHCF particles on the SRGO surface. As expected, relative to the deposition of SmHCF on the surface of a bare GCE, that on the SRGO surface occurred efficiently because of the large surface area of SRGO and improved anchoring of SmHCF particles on the SRGO surface (Fig. 1B). The petal-like structures of SmHCF allows them to anchor strongly on the SRGO, which minimized the dissolution of the SmHCF particles. The following reactions ((1) & (2)) express the mechanism of SmHCF formation.

[Fe(CN)₆]³⁻ + e⁻ → [Fe(CN)₆]⁴⁻ (1)

[Fe(CN)₆]⁴⁻ + Sm³⁺ + K⁺ → K[SmFe(CN)₆] (2)

Fig. 1 CV deposition of SmHCF on (A) GCE, (B) SRGO modified GCE from 0.1 M KCl containing 5 mM of SmCl₃ and K₃Fe(CN)₆ at a scan rate of 50 mV s⁻¹ (20 cycles).

Here, an equal amount (5 mM) of Sm³⁺ and hexacyanoferrate has been used for the balanced deposition of SmHCF microflowers. Moreover, a high concentration of Sm³⁺ and K₃Fe(CN)₆ was required for the effective formation of SmHCF microflowers. In the case of lower concentrations (1 mM to 4 mM), the SmHCF particles do not form effectively. On the other hand, the electrochemical deposition of SmHCF using different cation (Na⁺ and Li⁺) containing electrolyte solutions was compared. As expected, the redox peak corresponding to Na⁺ and Li⁺ appears at the same potential of the K⁺ electrolyte solution [figure not shown]. Moreover, the obtained redox peak currents and the analytical procedures were similar to that reported in the studies using a KCl electrolyte solution. According to the results obtained, electrolytes containing different cations do not affect the formation of SmHCF particles on the GCE. Nevertheless, morphological changes occurred because of the cationic properties of the electrolyte (explained in Section 3.2.2).

3.2 Scanning electron microscopy

3.2.1 Morphology of the as-prepared SRGO/SmHCF composite. The surface morphology of the as-prepared SGO and SRGO/SmHCF composite were characterized using scanning electron microscopy. Fig. 2A shows the surface of the as-prepared SGO. It can be seen that highly surfactant loaded GO is composed of flaky sheets. As shown in Fig. 2B, the SmHCF with a flower-like structure is formed on the surface of the SRGO. The flower-like SmHCF particles are evenly distributed on the SRGO surface and clearly shows that the SRGO/SmHCF composite was formed.

Fig. 2 SEM images of (A) SRGO (B) electrodeposited SmHCF on SRGO from 0.1 M KCl containing 5 mM of SmCl₃ and K₃Fe(CN)₆ at a scan rate of 50 mV s⁻¹ (20 cycles).

3.2.2 Morphological effects of SmHCF under different electrolytes. The morphology of SmHCF under different cation (K⁺, Na⁺ and Li⁺) containing electrolytes were investigated. As shown in Fig. 3A–C, K[SmHCF] exhibits a flower-like shape, Na[SmHCF] exhibits a butterfly-like structure and Li[SmHCF] shows a sheet-like morphology, respectively. These morphological changes are mainly attributed to monovalent cations. An increase in the charge and atomic number of the cation causes an increase in the affinity in the order of Li⁺ < H⁺ < Na⁺ < K⁺. Moreover, the formal potential for the metal hexacyanoferrate always depends on the monovalent cations used in the electrolyte. However, the ionic radii differ for each electrolyte and change the morphology of SmHCF.^41,42

Fig. 3 SEM images of the electrodeposited SmHCF with different supporting electrolytes at a concentration of 0.1 M. 20 cycles at scan rate of 50 mV s⁻¹ (A) KCl (B) NaCl (C) LiCl.

3.2.3 Morphology changes at different cycles. The growth of the SmHCF microflower structure was carefully monitored over the period of 5^th, 10^th, 15^th and 20^th cycles. Fig. 4 displays a clear insight into SmHCF microflower blooming under optimized conditions. As displayed in Fig. 4A, the apparent hexagonal flower bud shape with an average size of 3 ± 4 μM was formed at the end of the 5^th cycle. Further increase in the cycle number up to 10 and 15 cycles leads to the formation of an encapsulated flower bud-like structure, as shown in Fig. 4B and C. This results suggest that SmHCF micro particle was attained flower shape (similar with unfolding stalk from the flower bud). Moreover, the size of this structure became wider than the flower bud. At the end of the 20^th cycle, the flower grew to larger sizes, as displayed in Fig. 4D. Here, we indicate that the intrinsic smooth SmHCF sunflower-like morphology (inset Fig. 4D) was strongly anchored on the electrodes surface. This surface morphology illustrates the structure of the SmHCF complex relative to the balanced combination between Sm³⁺ and [Fe (CN)₆]³⁻.

Fig. 4 SEM images of electro deposited SmHCF at different cycles (A) 5 cycles (B) 10 cycles (C) 15 cycles (D) 20 cycles from 0.1 M KCl containing 5 mM of SmCl₃ and K₃Fe(CN)₆ at a scan rate of 50 mV s⁻¹.

3.3 Characterization

Thus, the prepared SmHCF microflowers were confirmed by characterization using EDX and FT-IR spectroscopy. Fig. 5A displays the EDX spectra of SmHCF. It can be seen that the SmHCF modified electrode contains the entire element with valid proportions. Fig. 5B shows the weight% of elements present in the SmHCF complex. The EDX profile of SmHCF exhibits 28 weight% of samarium, 9% of Fe and 7.6% of potassium. The oxygen content indicates moisture absorbed on the SmHCF complex. On the other hand, Fig. 5C shows the typical FT-IR spectrum of the SmHCF samples. A highly intensive peak appeared at around 2065.5 cm⁻¹, indicating the presence of a C≡N stretching vibration group corresponding to the hexacyanoferrate complex of SmHCF. There are two reasonable peaks at 1596.5 and 1654.7 cm⁻¹, which validate the binding vibration of crystal water present in SmHCF. The abovementioned results confirmed the SmHCF complex formation.

Fig. 5 (A) EDX elemental analysis of SmHCF (B) Chart of weight percentage (%) of the different elements in SmHCF (C) infra-red spectra of SmHCF.

3.4 Electrochemical impedance spectroscopy (EIS)

EIS is one of the promising techniques to elucidate the electrocatalytic behavior of the modified electrode. Fig. 6A shows the Nyquist plots of (a) K[SmHCF], (b) Na[SmHCF] and (c) Li[SmHCF]. Among the various Randles equivalent circuit parameters (inset), charge transfer resistance (R_ct) is the valuable parameter used to understand the redox reaction on the electrodes surface. As shown in Fig. 6A, the K[SmHCF] modified GCE has an extremely small semicircle relative to the other modified GCE such as Na[SmHCF] and Li[SmHCF]. The semicircle electron transfer R_ct values were calculated to be 210, 230 and 250 Ω, corresponding to K[SmHCF], Na[SmHCF] and Li[SmHCF], respectively. The lowest R_ct value of K[SmHCF] was attributed to the lower electron transfer resistance (high electron transfer ability) and good conductivity. Hence, we concluded that the K⁺ ion containing electrolyte enhanced the electron transfer rate at the MHCF complex and the surface of the electrode than the other two cationic electrolytes (Na⁺ and Li⁺). According to the R_ct value, the Na[SmHCF] electron transfer resistance lies in the middle of the two MHCF complexes. Therefore, the SmHCF modified GCE electrode reaction is an alkali metal cation dependent reaction.⁴² The electrocatalytic behavior of SmHCF can be sorted in the following order.

K[SmHCF] > Na[SmHCF] > Li[SmHCF]

Fig. 6 EIS of SmHCF deposition (A) with different electrolytes NaCl, KCl and LiCl under the deposition conditions of 20 CV cycles at a scan rate of 50 mV s⁻¹. (B) at different cycles, 5, 10 and 20 cycles, using 0.1 M KCl electrolyte containing K₃Fe(CN)₆ at the measured frequency from 100 mHz to 100 kHz.

The electrocatalytic activity of the SmHCF modified electrode was controlled by the number of deposition cycles. Fig. 6B shows the Nyquist plot of the SmHCF modified electrode at 5, 10 and 20 cycles. The largest semi-circle with a higher R_ct value was observed for SmHCF modified GCE after 5 cycles, indicating that the electron transferring process rate was leisurely and also shows an extremely sluggish electrocatalytic behavior. Interestingly, the semicircle portion of the SmHCF modified GCE decreased after 10 cycles. When compared with the SmHCF modified GCE after 5 cycles, the SmHCF after 10 cycles exhibits a smaller R_ct value of 350. This result reveals that a fast electron transfer process occurred on SmHCF/GCE after the 10^th cycle. The enormous difference in R_ct value was mainly attributed to the increasing size of SmHCF. However, the lowest semicircle value was observed at the SmHCF modified GCE after 15 and 20 cycles. The R_ct value of SmHCF/GCE in both the 15 and 20 cycles remains a similar value (figure not shown). Hence, we can conclude that the SmHCF/GCE exhibits rapid electron transfer process after 20 cycles. It is worth noting that the optimum cycle deposition of the SmHCF modified GCE was seen for the electrochemical oxidation reaction. As shown in Fig. S1,† the K[SmHCF]/GCE has a lower electron transfer value than the bare GCE. The aforementioned results validate that the SmHCF has a lower charge transfer resistance value at 20 CVs cycles in a KCl electrolyte.

3.5 Cyclic voltammogram of SRGO/SmHCF modified GCE

Fig. 7A shows the CV of (a) SRGO/SmHCF/GCE, (b) RGO/SmHCF/GCE and (c) SmHCF/GCE in deoxygenated 0.1 M KCl at a scan rate of 50 mV s⁻¹. The highly stable enhanced redox peak was observed on SRGO/SmHCF/GCE, suggesting that the SmHCF particles are strongly attached on the surface of the SRGO/GCE. However, without surfactant on GO, the deposition of SmHCF on RGO was not as stable as compared to GO with a surfactant. Though the SmHCF strongly deposited directly on GCE, it does not show any obvious redox peak relative to the SRGO/SmHCF modified electrode. This result was attributed to the intercalation of surfactant on graphene sheets, which improves the wettability of the electrode. Hence, SmHCF strongly interacts with the surfactant present in the graphene sheets and formed the SRGO/SmHCF modified GCE.

Fig. 7 (A) CVs of various films (a) SRGO/SmHCF/GCE (b) RGO/SmHCF/GCE and (c) SmHCF/GCE in 0.1 M KCl solution at a scan rate of 50 mV s⁻¹. (B) CVs of SRGO/SmHCF/GCE (a), RGO/SmHCF (b), SmHCF/GCE (c) and bare GCE (d) in 0.3 mM of catechol in deoxygenated 0.1 M KCl at a scan rate of 50 mV s⁻¹.

3.6 Electrochemical determination of catechol (CC)

For better and sensitive electrochemical determination of CC, the different modified electrode was employed in 0.1 M KCl solution containing 0.3 mM of CC using a cyclic voltammetric method at a scan rate of 50 mV s⁻¹. To discern the electrochemical behavior of the as-synthesized GO, the GO modified GCE (without SmHCF) was employed directly for CV measurement. Fig. S2† shows the CV response of GO/GCE in the presence of CC. As shown in Fig. S2,† an extremely poor CC oxidation response was found at +0.35 V. At the same time, the CC oxidation response was monitored with different modified electrodes. Fig. 7B depicts the CVs response of 0.3 mM CC at the different modified electrodes SRGO/SmHCF/GCE (a), RGO/SmHCF (b), SmHCF/GCE (c) and bare GCE (d) in deoxygenated 0.1 M KCl at a scan rate of 50 mV s⁻¹. SRGO/SmHCF/GCE exhibits a well-defined enhanced catechol oxidation peak at +0.350 V. While compared with the other modified electrodes such as RGO/SmHCF and bare GCE, the proposed modified SRGO/SmHCF/GCE showed a less positive potential for the oxidation of CC. Besides, when compared with GO/GCE, the fabricated electrode exhibits a high response for CC. Therefore, the surfactant intercalated GO and deposited SmHCF has highly efficient compared with GO modified GCE. Moreover, the oxidation peak current noted at SRGO/SmHCF/GCE was higher than that of RGO/SmHCF, SmHCF and bare GCE. Hence, the CC detection at SRGO/SmHCF/GCE shows an enhanced anodic current compared to other electrodes, which have been discussed above, and this method could be used to detect CC in industrial effluents and in environmental monitoring laboratories in near future.

3.7 Differential pulse voltammetry

Fig. 8 shows the DPV response of CC at SRGO/SmHCF/GCE in 0.1 M KCl at a scan rate of 50 mV s⁻¹. It can be seen that a linear increase in the oxidation peak was observed with each addition of CC. The inset figure shows the linear calibration plot for concentration vs. current. The linear regression equation can be expressed as I (μA) = 0.0343C (μM) + 0.54, R² = 0.9755. The linear range of CC was obtained ranging from 50 μM to 550 μM. The low limit of detection was found to be 0.38 μM. The sensitivity of the SRGO/SmHCF/GCE film towards CC was found to be 0.430 μA μM⁻¹ cm². These results suggest that the fabricated GCE has excellent catalytic activity towards CC and could be used as an electrochemical sensor for CC determination in the near future. We have compared the reported sensor results with other reported methods. As shown in Table 1, the fabricated electrode has a low limit of detection compared with other previously reported methods.

Fig. 8 DPV of SRGO/SmHCF/GCE with various concentrations of catechol (50 to 550 μM) in 0.1 M KCl solution.

Table 1 Comparison table of the catechol sensor at SRGO/SmHCF/GCE with other reported methods

S. no	Modified electrode^a	Linear range	LOD	Ref.
a CPE – carbon paste electrode, PPy – polypyrrole, CNT – carbon nanotube, SPE – screen printed electrode, MWCNT – multi walled carbon nanotube, G – graphene, HRP – horseradish peroxidase, PASA – poly-amidosulfonic acid, and PPABA – poly(p-aminobenzoic acid).
1	SRGO/SmHCF/GCE	50–250	0.38	This work
2	Palygorskite/CPE	5–100	0.57	43
3	Tyrosinase	60–800	6	44
4	PPy/CNT/Tyrosinase	3–50	0.671	45
5	Tyrosinase/SPE	—	1.7	46
6	GCE in micellar solution	0.5–900	0.6	47
7	MWCNT/GCE	20–1200	10	48
8	G/Chitosan/GCE	1–400	0.75	49
9	MWCNT/PPy/HRP/Au	1.6–8	0.93	50
10	PASA/MWCNT/GCE	6–180	1	51
11	PPABA/GCE	2–900	0.5	52

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4. Conclusions

A novel composite of SRGO/SmHCF modified GCE has been developed for CC determination using electrochemical methods. The change in the morphology for SmHCF was observed with different electrolytes and cycles using SEM. The electrochemical impedance spectroscopy of SmHCF with different electrolytes and cycles have been studied. In addition, the SRGO/SmHCF/GCE showed excellent electrocatalytic activity towards the determination of catechol.

Acknowledgements

This work was supported by the Ministry of Science and Technology, Taiwan (ROC).

References

1. V. D. Neff, J. Electrochem. Soc., 1978, 125, 886.
2. K. Itaya, T. Ataka and S. Toshima, J. Am. Chem. Soc., 1982, 104, 3751.
3. P. Wu, S. Lu and C. Cai, J. Electroanal. Chem., 2004, 569, 143–150.
4. S. S. Narayanan and F. Scholz, Electroanalysis, 1999, 11, 465–469.
5. K. Itaya, I. Uchida and V. D. Neff, Acc. Chem. Res., 1986, 19, 162.
6. M. Kaneko and T. Okada, J. Electroanal. Chem., 1988, 255, 45.
7. S. Kalwellis-Mohn and E. W. Grabner, Electrochim. Acta, 1989, 34, 1265.
8. M. Jayalakshmi and F. Scholz, J. Power Sources, 2000, 91, 217.
9. M. Nishizawa, S. Kuwabata and H. Yoneyama, J. Electrochem. Soc., 1996, 143, 3462.
10. Y. Wu, B. W. Pfenning and A. B. Bocarsly, Inorg. Chem., 1995, 34, 4262.
11. A. P. Baionia, M. Vidottia, P. A. Fiorito and S. I. Cordoba de Torresi, J. Electroanal. Chem., 2008, 622, 219–224.
12. X. Cui, L. Hong and X. Lin, J. Electroanal. Chem., 2002, 526, 115–124.
13. S. M. Chen, J. Electroanal. Chem., 2002, 521, 29–52.
14. D. F. Mullica, H. O. Perkins, E. L. Sappenfield and D. A. Grossie, J. Solid State Chem., 1988, 74, 9.
15. J. J. G. Willems, Philips J. Res., 1984, 39, 1.
16. S. Zhao, J. K. O. Sin, B. Xu, M. Zhao, Z. Peng and H. Cai, Sens. Actuators, B, 2000, 64, 83.
17. G. M. R. V. DrutenLabbe, V. P. Boncour, J. Perichon and G. A. Percheron, J. Electroanal. Chem., 2000, 487, 31.
18. Z. Wang, Adv. Mater., 2003, 15, 432–436.
19. Y. Shi, B. Zhou, P. Wu, K. Wang and C. Cai, J. Electroanal. Chem., 2007, 611, 1–9.
20. L. D. Feng, M. M. Gu, Y. L. Yang, G. X. Liang, J. R. Zhang and J. J. Zhu, J. Phys. Chem. C, 2009, 113, 8743–8749.
21. Y. Sun, W. Zhou, D. Zhao, J. Chen, X. Li and L. Feng, Int. J. Electrochem. Sci., 2012, 7, 7555–7566.
22. M. Rajkumar, B. Devadas and S. M. Chen, Electrochim. Acta, 2013, 105, 439–446.
23. M. R. Ganjali, Z. Memari, F. Faridbod and P. Norouzi, Int. J. Electrochem. Sci., 2008, 3, 1169–1179.
24. Y. Liu, Z. Yang, Y. Zhong and J. Yu, Appl. Surf. Sci., 2010, 256, 3148–3154.
25. P. Wu and C. X. Cai, J. Electroanal. Chem., 2005, 576, 49–56.
26. Q. L. Sheng, H. Yu and J. B. Zheng, J. Solid State Electrochem., 2008, 12, 1077–1084.
27. P. Wu and C. X. Cai, J. Solid State Electrochem., 2004, 8, 538–543.
28. C. Chung, Y. K. Kim, D. Shin, S. R. Ryoo, B. H. Hong and D. H. Min, Acc. Chem. Res., 2013, 46, 2211–2224.
29. K. Zhang, L. Mao, L. L. Zhang, H. S. O. Chan, X. S. Zhao and J. Wu, J. Mater. Chem., 2011, 21, 7302.
30. W. Liu, L. Wu, X. Zhang and J. Chen, Bull. Korean Chem. Soc., 2014, 35, 204–210.
31. A. J. S. Ahammad, M. M. Rahman, G. R. Xu, S. Kim and J. J. Lee, Electrochim. Acta, 2011, 56, 5266.
32. S. R. Nambiar, P. K. Aneesh and T. P. Rao, Analyst, 2013, 138, 5031–5038.
33. A. J. S. Ahammad, S. Sarker, M. A. Rahman and J. J. Lee, Electroanalysis, 2010, 22, 694.
34. C. Wang, R. Yuan, Y. Chai and F. Hu, Anal. Methods, 2012, 4, 1626–1628.
35. Y. C. Fiamegos, C. D. Stalikas, G. A. Pilidis and M. I. Karayannis, Anal. Chim. Acta, 2000, 403, 315–323.
36. H. Cui, C. He and G. Zhao, J. Chromatogr., A, 1999, 855, 171–179.
37. M. F. Pistonesi, M. S. D. Nezio, M. E. Centurion, M. E. Palomeque, A. G. Lista and B. S. F. Band, Talanta, 2006, 69, 1265.
38. L. Zhao, B. Lv, H. Yuan, Z. Zhou and D. Xiao, Sensors, 2007, 7, 578.
39. S. C. Moldoveanu and M. J. Kiser, J. Chromatogr. A, 2007, 1141, 90.
40. M. J. Schoning, M. Jacobs, A. Muck, D. T. Knobbe, J. Wang, M. Chatrathi and S. Spillmann, Sens. Actuators, B, 2005, 108, 688.
41. Application of ion exchange processes for the treatment of radioactive waste and management of spent ion exchangers – Vienna: International AtomicEnergy Agency, 2002.
42. I. Markovich and D. Mandler, J. Electroanal. Chem., 2000, 484, 194–202.
43. Y. Konga, X. Chena, W. Wanga and Z. Chen, Anal. Chim. Acta, 2011, 688, 203–207.
44. S. Tembe, S. Inamdar, S. Harama, M. Karve and S. F. D. Souza, J. Biotechnol., 2007, 128, 80–85.
45. S. K. Ozoner, M. Yalvac and E. Erhan, Curr. Appl. Phys., 2010, 10, 323–328.
46. R. Solna, S. Sapelnikova, P. Skladal, M. Winther-Nielsen, C. Carlsson, J. Emneus and T. Ruzgas, Talanta, 2005, 65, 349–357.
47. J. Peng and Z. N. Gao, Anal. Bioanal. Chem., 2006, 384, 1525–1532.
48. Z. Xu, X. Chen, X. Qu and S. Dong, Electroanalysis, 2004, 16, 685–687.
49. H. Yina, Q. Zhang, Y. Zhoud, Q. Maa, T. liu, L. Zhu and S. Ai, Electrochim. Acta, 2011, 56, 2748–2753.
50. S. Korkut, B. Keskinler and E. Erhan, Talanta, 2008, 76, 1147–1152.
51. D. M. Zhao, X. H. Zhang, L. J. Feng, L. Jia and S. F. Wang, Colloids Surf., B, 2009, 74, 317–321.
52. P. Yang, Q. Zhu, Y. Chen and F. Wang, J. Appl. Polym. Sci., 2009, 113, 2881–2886.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05867d

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