- Email: [email protected]
Co3O4 nanospindle composite for sensitive detection of nitrite
Sensors and Actuators B 227 (2016) 92–99
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An enzyme-free electrochemical sensor based on reduced graphene oxide/Co3 O4 nanospindle composite for sensitive detection of nitrite Yuvaraj Haldorai a , Jun Yeong Kim b , A.T. Ezhil Vilian a , Nam Su Heo b , Yun Suk Huh b,∗ , Young-Kyu Han a,∗ a b
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2015 Received in revised form 20 November 2015 Accepted 11 December 2015 Available online 14 December 2015 Keywords: Graphene Cobalt oxide Nanocomposite Nitrite Non-enzymatic
a b s t r a c t Cobalt oxide (Co3 O4 ) nanospindles-decorated reduced graphene oxide (RGO) composite is prepared via thermal decomposition of a three-dimensional coordination complex precursor, cobalt benzoate dihydrazinate, at 200 ◦ C. Transmission electron microscopy reveals that Co3 O4 nanospindles with an average particle size of <25 nm are decorated on the RGO surface. The low decomposition temperature and lack of residual impurities are significant aspects of this simple and facile method. The electrochemical performance of the proposed sensor is investigated using cyclic voltammetry and chronoamperometry. Under optimum conditions, anodic peak currents are linearly proportional to their concentrations, in the range of 1–380 M for nitrite with a regression equation of I (A) = 2.0660 C + 6.7869 (R2 = 0.9992). The sensor exhibits a high sensitivity of 29.5 A M−1 cm−2 , a rapid response time of 5 s, and a low detection of limit of 0.14 M. The proposed electrode shows good reproducibility and long-term stability. The sensor is used to determine the nitrite level in tap water with acceptable recovery, implying its feasibility for practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a two-dimensional sp2 carbon nanomaterial, has attracted increasing interest because of its unique physical and chemical properties, including high surface area, good chemical resistance, excellent electrical conductivity, extraordinary mechanical properties, high charge mobility, and thermal conductivity [1–3]. Transition metal oxides have attracted increasing attention due to their unique properties. Among the metal oxides most studied of late, cobalt oxide (Co3 O4 ), a p-type semiconductor with a direct band gap of 1.5 eV and 2.19 eV [4]. Moreover, cobalt oxide has three well-known polymorphic structures: cobaltous oxide (CoO), cobaltic oxide (Co2 O3 ), and Co3 O4 [5]. The one-dimensional (1D) nanostructures of Co3 O4 are inexpensive, environmentally friendly, and have good conductivity [6]. Due to its high surface area to volume ratio, high ratio of surface atoms, and good chemical stability, it should meet the requirements of future energy applications [7]. Considering the excellent properties, Co3 O4 nanostructures have been used for a variety of applications
∗ Corresponding authors. E-mail addresses: [email protected] (Y.S. Huh), [email protected] (Y.-K. Han). http://dx.doi.org/10.1016/j.snb.2015.12.032 0925-4005/© 2015 Elsevier B.V. All rights reserved.
[8–11]. Taking into account the excellent individual properties of graphene and Co3 O4 , a combination might yield enhanced performance. Several studies have reported different approaches to synthesizing graphene/Co3 O4 nanocomposites. Song et al. [12] fabricated graphene/Co3 O4 nanosheet composites via a hydrothermal method. Choi et al. [13] synthesized three-dimensional (3D), porous graphene/Co3 O4 composite films using replication and deposition processes for lithium ion batteries. Li et al. [14] reported a simple approach for preparing graphene/Co3 O4 composites via a chemical deposition of Co3 O4 nanoparticles onto GO followed by reduction of GO to graphene using NaBH4 for high-performance lithium ion batteries. Ye et al. [15] fabricated graphene/Co3 O4 composite using electrodeposition to determine tryptophan. Park and Kim [16] reported the synthesis of graphene/Co3 O4 nanosheets using microwave-assisted method as supercapacitor electrodes. Dong et al. [17] prepared 3D graphene/Co3 O4 nanowire composites for supercapacitors and non-enzymatic glucose detection. Kong et al. [18] fabricated interconnected 1D Co3 O4 nanowires on graphene for non-enzymatic hydrogen peroxide detection. Shahid et al. [19] synthetized graphene/Co3 O4 nanocube and graphene/Co3 O4 nanocube@Pt composites for nitric oxide detection. Although graphene/Co3 O4 composites have been used for various applications [12–19], few studies have investigated
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
electrochemical sensors. Until now, no attempt has been made to use a graphene/Co3 O4 composite for electrochemical nonenzymatic detection of nitrite. In relation to such reported composite materials, it should be noted that the obtained Co3 O4 is generally in the form of particle. It has been reported that the electrochemical performance of Co3 O4 can be enhanced by processing 1D or 2D nanostructures such as nanowires, nanocubes, nanosheets [16–19], etc. It is therefore of interest to synthesize new1D or 2D Co3 O4 /graphene composites with a synergistic advantage between Co3 O4 and graphene for electrochemical sensing. In this regard, a simple, reliable synthetic method needs to be developed because the size and morphology of Co3 O4 have a significant effect on its chemical and physical properties as well as its applications. Nitrite ions are important toxic inorganic pollutants found in food, soil, water, and physiological systems. High concentrations of nitrite ions can originate from caustic radioactive waste [20]. Excess nitrite ions are detrimental to the human body, and a high concentration in drinking water is a serious health risk for infants. In the human body, nitrites can combine with hemoglobin to form methemoglobin, which leads to a condition commonly called “blue baby syndrome.” Nitrite can also be converted into nitrosamine, which causes cancer and hypertension [21]. Hence, it is essential to monitor toxic nitrite ions in the environment. We report a facile synthesis of a reduced graphene oxide (RGO)/Co3 O4 nanospindle composite using a 3D coordination complex as a precursor, and its application toward the electrochemical detection of nitrite. Electrochemical studies indicated that the composite exhibited a fast current response to nitrite, with a wide linear range and a low detection limit. In addition, the composite electrode showed excellent reproducibility and long-term stability. The fabricated electrochemical sensor exhibited significant sensitivity, selectivity and it was also applied to real sample analysis. 2. Experimental 2.1. Materials Graphite, cobalt (II) nitrate hexahydrate (Co(NO3 )2 ·6H2 O), hydrazine (N2 H4 ), benzoic acid (C6 H5 COOH), sodium nitrate (NaNO3 ), sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), potassium permanganate (KMnO4 ), hydrogen peroxide (H2 O2 ), and sodium nitrite (NaNO2 ) were purchased from Aldrich. All other reagents were analytical grade and were used as received. 2.2. Synthesis of Co3 O4 /RGO nanocomposite Graphene oxide (GO) was synthesized from natural graphite using a modified Hummers’ method [22]. The precursor complex was prepared by mixing an aqueous solutions of Co(NO3 )2 ·6H2 O (0.01 mol) and of hydrazinium benzoate (0.02 mol) under constant stirring. The resulting mixture was concentrated to one-third of its total volume; the contents were cooled, filtered, washed with alcohol, and dried. 20 mg of GO was dispersed in 50 ml of ethanol under sonication for 1 h, then 100 mg of cobalt benzoate dihydrazinate complex precursor was added and the mixture was sonicated for another 1 h. The resulting mixture was transferred into a Teflonlined reactor and the reaction was carried out at 200 ◦ C for 8 h. The obtained product was separated, then washed with ethanol and distilled water by centrifugation. Finally, it was dried at 65 ◦ C for 24 h in a vacuum oven. 2.3. Preparation of the modified electrode The nitrite sensor was fabricated on a 3 mm diameter glassy carbon electrode (GCE) that was polished to a mirror finish using an
93
alumina slurry (0.05 m) sequentially, then washed ultrasonically in ethanol and deionized water and dried at room temperature. The electrode material was prepared by mixing 1.0 mg/ml of the Co3 O4 /RGO nanocomposite in ethanol under ultrasonication for 1 h to achieve a homogeneous dispersion. Then, 6 l of the suspension was drop-casted on a GCE and dried at room temperature. The resulting electrode was denoted as Co3 O4 /RGO/GCE. The RGO/GCE electrode was also prepared similarly for comparison. 2.4. Characterization Raman spectra of the samples were obtained using a HR800 UV Raman microscope (Horiba Jobin–Yvon, France). X-ray diffraction (XRD) patterns were collected using an X’Pert PRO MRD (Philips) diffractometer with Cu K␣ radiation. Morphology of the composite was examined by a HITACHI SU8010 high resolution scanning electron microscope. Transmission electron microscopy (TEM, JEOL-2010F) was performed at an accelerating voltage of 200 kV. The TEM observation sample was prepared by depositing an ethanolic dispersion on a 200 mesh copper grid, followed by air drying. High-resolution TEM (HRTEM) recorded the selected area electron diffraction (SAED) image. X-ray photoelectron spectroscopy (XPS) measurements were obtained using a Thermo Scientific K-Alpha electron spectrometer with an Al X-ray source. 2.5. Electrochemical analysis Electrochemical performance was evaluated using a threeelectrode cell system. Cyclic voltammetry (CV) and chronoamperometry analyses were conducted on a CHI 660D electrochemical workstation at room temperature. A 1 M KOH solution was served as the electrolyte, while Co3 O4 /RGO nanocomposite represented the working electrode. A platinum wire was employed as a counter electrode, with calomel as the reference. Electrochemical impedance spectroscopy (EIS) was performed using an WonATech electrochemical workstation in 0.1 M KCl containing 5 mM Fe(CN)6 3−/4− at room temperature. 3. Results and discussion The Co3 O4 /RGO nanocomposite was synthesized from GO and a 3D coordination complex precursor, cobalt benzoate dihydrazinate [Co(C6 H5 COO)2 (N2 H4 )2 ], which was prepared according to the following reaction: Co(NO3 )2 ·6H2 O + 2C6 H5 COON2 H5 → Co(C6 H5 COO)2 (N2 H4 )2 + 2HNO3 + 6H2 O
(1)
Co(C6 H5 COO)2 (N2 H4 )2 + 15O2 → CoO + 2N2 H4 + 14CO2 + 5H2 O (2)
6CoO + O2 → 2Co3 O4
(3)
Fig. 1 outlines the fabrication of Co3 O4 nanospindles decorated RGO nanocomposite for nitrite sensing. Decomposition pathway of the precursor and the deposition of Co3 O4 nanospindles on the RGO surface started at a relatively low temperature of ca. 200 ◦ C. The precursor decomposed completely, losing N2 H4 , CO2 , and H2 O. The complex was stable in air and insoluble in water. The importance of using such a complex is the distribution of metal ions in a 3D coordination sphere so that the decomposition would results in very fine particles. The benzoate and hydrazine ligands enhance
94
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 1. Schematic diagram of the synthesis of Co3 O4 /RGO nanocomposite for nitrite sensing.
the decomposition easier due to the exothermic decomposition of hydrazine. Fig. 2(a) shows the Raman spectrum of the Co3 O4 /RGO nanocomposite. The composite exhibited two bands: D and G at 1358 and 1589 cm−1 , respectively. The D band represented disordered carbon arising from structural defects, while the G band corresponded to the E2g phonon of sp2 carbon atoms [3,23]. The bands at 194, 473, 513, and 679 cm−1 were assigned to the F2g , Eg , F2g , and A1g vibrational modes of the Co3 O4 , respectively [14]. No vibrational modes resulted from impurities. Fig. 2(b) illustrates the XRD pattern of the Co3 O4 /RGO nanocomposite. The RGO exhibited two significant diffraction peaks, at 2 = 26.5 and 56.5, attributed to the (0 0 2) and (0 0 4) reflections of graphitic carbon, respectively (JCPDS no. 75-1621). In addition, the composite exhibited nine obvious diffraction peaks, which were indexed to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 1) planes of the pure, face-centered cubic phase of Co3 O4 , which was in agreement with the literature values (JCPDS 42-1467) [13]. These results demonstrated the existence of both RGO and Co3 O4 in the as-synthesized composite. Fig. 3 represents SEM, TEM, HRTEM, and SAED pattern of the Co3 O4 /RGO nanocomposite. SEM image (Fig. 3(a)) of the composite showed that almost uniform size of spindle-like Co3 O4 nanoparticles were dispersed on the RGO surface. To ascertain the physical nature of Co3 O4 in the composite more clearly, the TEM image of composite is presented in Fig. 3(b). The TEM image displayed Co3 O4 nanospindles with a mean diameter of approximately <25 nm and a length of <45 nm were homogeneously dispersed on the RGO surface; a few nanospindles with different sizes were also observed. The HRTEM image (Fig. 3(c)) illustrated clear lattice fringes, highlighting the 0.26 nm inter-planar spacing, which represents the (311) plane of Co3 O4 . The SAED pattern of the composite (Fig. 3(d)) showed well-defined rings, which could be attributed to the crystalline nature of Co3 O4 . Fig. 4 demonstrates survey and core-level spectra of the Co3 O4 /RGO nanocomposite. The survey spectrum (Fig. 4(a)) exhibited C 1s (284.7 eV), O 1s (529.9 eV), Co 2p3/2 (780.6 eV), and Co 2p1/2 (797.2 eV) peaks, which confirmed the successful decoration of Co3 O4 on the RGO surface [14]. The core-level spectrum of Co 2p (Fig. 4(b)) had two peaks: a main peak (2p3/2 ) and a satellite peak (2p1/2 ) with binding energies of 779.8 eV and 795.8 eV, respectively. XPS spectra were in good agreement with the reported data of
Co3 O4 spin–orbit peaks [13]. The Gaussian fit of the C 1s core-level spectrum of GO (Fig. 4(c)) showed three peaks: non-oxygenated C C (284.8 eV), C O (epoxy and hydroxyl; 285.6 eV), and C O (carboxyl; 287.1 eV). Compared to GO, the peak intensities of C O and C O in the C 1s spectrum of the composite (Fig. 4(d)) decreased dramatically, indicating the successful reduction of GO to RGO. The electrochemical properties of the bare GCE, RGO/GCE, and Co3 O4 /RGO/GCE were studied by EIS. Fig. 5(a) represents impedance spectra of the electrodes recorded in a 5 mM Fe (CN)6 3−/4− solution containing 0.1 M KCl. The semicircular portion of the higher frequency region represents the confined electron transfer process (in this case, the electron transfer resistance (Rct ) is equal to the semicircle’s diameter). The Rct value for bare GCE was estimated to be 954 . In contrast, the Rct value for RGO/GCE decreased to 597 , suggesting that the introduction of RGO plays a vital role in increasing electroactive surface area, while also providing a conductive network for the electron-transfer of Fe(CN)6 3−/4− . However, the Rct value for Co3 O4 /RGO/GCE decreased to 210 , indicating the excellent electron transfer [15] properties of the Co3 O4 /RGO/GCE compared to RGO/GCE. Fig. 5(b) shows CVs of the bare GCE, RGO/GCE, and Co3 O4 /RGO/GCE in the absence of nitrite in N2 -saturated KOH (1 M) at a scan rate of 50 mV s−1 . The background current of the Co3 O4 /RGO/GCE was much greater than for the RGO/GCE and bare GCE, indicating the former has a more highly accessible surface area and better conductivity, which was concordant with previous studies [17,24] on electrochemical reactions of Co3 O4 in alkaline solutions. The mechanism involved hydroxyl ions (OH− ) in the redox reactions of Co3 O4 . As shown in Fig. 5(b), the redox peaks were ascribed to the transition between Co3 O4 and CoOOH. The reaction is expressed by Eq. (1) [17,24] Co3 O4 + OH− ↔ CoOOH + e−
(1)
Fig. 5(c) illustrates the CVs of the different electrodes in the presence of 2 mM nitrite in 1 M KOH at a scan rate of 100 mV s−1 . The bare GCE and RGO/GCE showed no obvious oxidation peaks in the given potential range of 0–0.7 V, whereas the Co3 O4 /GCE showed an oxidation peak at around 0.5 V. Interestingly, the Co3 O4 /RGO/GCE exhibited a sharper oxidation peak at around 0.54 V with higher background current than that of Co3 O4 /GCE. These results can be ascribed to the high electrocatalytic activity of the Co3 O4 and large active surface area of the RGO with regard to the oxidation of nitrite.
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
95
Fig. 2. (a) Raman spectrum and (b) XRD pattern of the nanocomposite.
Fig. 5(d) illustrates the CVs of Co3 O4 /RGO/GCE with different nitrite concentrations at a scan rate of 100 mV s−1 in 1 M KOH. The bare GCE showed no obvious redox peaks in the potential range of 0–0.8 V, whereas Co3 O4 /RGO/GCE exhibited oxidation and reduction peaks at around 0.54 and 0.36 V, respectively. The oxidation peak current increased linearly according to the nitrite concentration, indicating that the electrode had excellent sensitivity to nitrite. A positive shift in the oxidation peak could be attributed to the adsorption of nitrite and its oxidized products on the electrode surface, which may control the kinetics of the electrochemical reactions [17,24]. The mechanism of nitrite electrooxidation is explained by the following equations: In the first step, Co3 O4 was electrochemically oxidized to CoOOH in a basic solution (Eq. (1)) where an electron was released. In the second step, nitrite was oxidized using CoOOH, which was simultaneously reduced to Co3 O4 . This reaction is represented by Eq. (2) CoOOH + NO2 − → Co3 O4 + NO3 −
(2)
Fig. 5(e) shows a typical graph of the anodic peak current (Ipa ) versus nitrite concentration. The anodic peak current gradually increased as the nitrite concentration increased (0.5–6 mM). The regression coefficient (R2 ) value of 0.9947 indicated the excellent nitrite detection. Fig. 5(f) demonstrates the CVs of Co3 O4 /RGO/GCE at different scan rates in the presence of 2 mM of nitrite. The anodic peak current increased significantly when the scan rate increased from 10 to 100 mV s−1 , demonstrating a linear relationship to the square root of the scan rate, with an R2 value of 0.9974 (Fig. 5(g)). These results suggested that the electrochemical reaction was confined to the surface. Fig. 6(a) represents the amperometric response of the Co3 O4 /RGO/GCE to successive step-wise additions of nitrite in 1 M KOH. A significant increase in peak current was observed when nitrite was added to the stirred KOH. The electrode responded rapidly to the nitrite, reaching a plateau within 5 s. Fig. 6(b) shows the calibration curve for the nitrite. The electrode showed a linear
Fig. 3. (a) SEM, (b) TEM, (c) HRTEM, and (d) SAED pattern of the nanocomposite.
96
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 4. (a) Survey XPS of the Co3 O4 /RGO nanocomposite, (b) Co 2p spectrum of nanocomposite, (c) C 1s spectrum of GO, and (d) C 1s spectrum of nanocomposite. Table 1 Results of the present study compared with previous reports of various electrodes for nitrite sensing. Electrode
Detection limit (M)
Au/PEDOT Pd/RGO Au/SG Fe2 O3 /RGO Au/ZnO/MWCNT RGO/MWCNT/Pt/Mb Heteroatom enriched porous carbon Au/MWCNT Cu-NDs/RGO Cu/MWCNT/CS Co3 O4 /RGO
0.1 15.64 nM 0.2 0.015 0.4 0.93 0.13 0.01 0.4 0.024 0.14
Linear range (M) 3–300 0.04–108 10–3960 0.05–780 0.78–400 1–12,000 3–90 0.05–250 1.25–13,000 0.1–2500 1–380
Sensitivity (A M−1 cm−2 )
Ref.
– 7.672 45.44 A mM−1 cm−2 0.204 – 0.1651 9.0 2.5815 A L mol−1 cm−2 214 A mM−1 cm−2 −48.92 A mM−1 cm−2 29.5
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] This work
PEDOT, poly(ethylenedioxythiophene); SG, sulfonated graphene; Mb, myoglobin; MWCNT, multi-walled carbon nanotube; Cu-NDs, copper nanodendrites; CS, chitosan.
relationship to the nitrite concentration (1–380 M), with a regression equation of: I (A) = 2.0660 C (M) + 6.7869 (R2 = 0.9992). The low detection limit can be calculated using Eq. (3): LOD =
3Sb , S
(3)
where Sb is the standard deviation of five blank measurements and S is the sensitivity. The low detection limit of nitrite sensing was identified at 0.14 M. Sensitivity of the fabricated sensor can be calculated from slope of the calibration curve divided by working area of the electrode (0.07 cm2 ). The sensitivity was calculated as 29.5 A M−1 cm−2 . Together these results indicated that the electrode had excellent non-enzymatic nitrate detection. The linear range, detection limit, and sensitivity of other electrodes were compared to those of the Co3 O4 /RGO/GCE (listed in Table 1). Even though previously reported enzymatic and non-enzymatic nitrite sensors based on other nanomaterials showed good performances [25–34], the superior performance of Co3 O4 /RGO/GCE may be attributed to the large surface area of the RGO and the good
electron transfer between the Co3 O4 and RGO. Because of this high surface area, the composite can adsorb and catalyze more reactive substances, resulting in an instantaneous oxidation process. Selectivity is an important parameter of the nitrite sensor. Various common ions, such as Na+ , Mg2+ , NH4 + , SO4 2− , CH3 COO− , CO3 2− , and NO3− , did not interfere with the detection of nitrite. Fig. 6(c) showed that no significant amperometric responses were observed when 2 mM each of Na2 SO4 , CH3 COONa, NaNO3 , Na2 CO3 , MgSO4 , and NH4 NO3 were injected at regular intervals. However, upon the addition of 20 M nitrite, a notable amperometric response was immediately observed, revealing the excellent selectivity of the Co3 O4 /RGO/GCE toward nitrite determination. To demonstrate the stability of Co3 O4 /RGO/GCE, 100 cycles of CV (Fig. 6(d)) were carried out using 2 mM nitrite in 1 M KOH solution at a scan rate of 50 mV s−1 . The anodic peak current showed an 8% decrease in the peak current after 100 cycles, illustrating the good stability of the electrode. We also demonstrated the reproducibility of sensor by measuring changes in the CV peak currents (Fig. 6(e)) of five different electrodes during nitrite (2 mM)
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
97
Fig. 5. (a) Nyquist plots of the electrodes in 0.1 M KCl containing 5 mM Fe(CN)6 3−/4− , (b) CVs of electrodes in the absence of nitrite in1 M KOH at a scan rate of 50 mV s−1 , (c) CVs of electrodes in the presence of 2 mM nitrite in1 M KOH at a scan rate of 100 mV s−1 , (d) CVs of the Co3 O4 /RGO/GCE in 1 M KOH containing 0.5–6 mM of nitrite at a scan rate of 100 mV s−1 ), (e) plot of anodic peak current versus concentration, (f) CVs of the Co3 O4 /RGO/GCE at different scan rates (10–100 mV s−1 ) in the presence of 2 mM nitrite, and (g) plot of anodic peak current versus square root of the scan rate.
98
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 6. (a) Chronoamperometric data of the Co3 O4 /RGO/GCE with successive additions of nitrite in 1 M KOH, (b) plot of peak current versus concentration, (c) interference test of the electrode with 20 M nitrite in the presence of various common ions, (d) CV of the Co3 O4 /RGO/GCE for 100 multiple cycles in the presence of 2 mM nitrite at a scan rate of 50 mV s−1 , (e) peak current of five different composite electrodes for the detection of 2 mM nitrite at a scan rate of 50 mV s−1 , and (f) shelf-life of the composite electrode after 14 days.
detection. The relative standard deviation was calculated as 1.93%, which indicated good reproducibility of the electrode. The shelflife of the nanocomposite electrode was checked after 2 weeks (Fig. 6(f)). The electrode was stored in a vacuum at room temperature (24 ◦ C), and the detection of 2 mM nitrite was monitored at regular 2 day intervals. The high stability of the electrode may be attributed to the good chemical stability of Co3 O4 in the electrolyte. For real sample analysis, the fabricated Co3 O4 /RGO/GCE sensor was used to measure nitrite levels in tap water. Tap water
samples consisting of different concentrations of spiked nitrite were quantitatively analyzed using a standard addition method. Before analysis, the samples were filtered using a 0.2 m filter to remove micron-sized particles. The results showed that the electrode was highly selective and sensitive to nitrite. The recoveries (Table 2) were observed as 99.9%, 101.5%, 98.6%, and 99.3% for the 10, 20, 30, and 40 M nitrite spiked samples, respectively. These results indicated that the sensor showed excellent detection of nitrites in tap water.
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99 Table 2 Determination of nitrites in tap water. Real samples
Tap water
Nitrite spiked (M)
Nitrite found (M)
RSD (%)
Recovery (%)
10 20 30 40
9.99 20.3 29.6 39.7
2.47 2.93 1.51 3.04
99.9 101.5 98.6 99.3
4. Conclusions We have successfully demonstrated a simple and facile method to deposit Co3 O4 nanospindles on the RGO surface. The assynthesized nanospindles had a mean diameter and length of approximately <25 nm and <45 nm, respectively. The composite electrode exhibited high sensitivity, detecting a wide range of nitrite concentrations (from 1 to 380 M). The lowest detection limit was observed at 0.14 M. This high nitrite sensitivity could be attributed to the excellent electrocatalytic activity of Co3 O4 and large specific surface area of the RGO. In addition, the composite electrode displayed good selectivity toward nitrite, even in the presence of 100-fold higher concentrations of various common ions. The real sample analysis result indicated that the electrode was highly selective and sensitive to nitrite. The high stability, reproducibility, and shelf-life make this composite electrode a suitable device for the detection of nitrite in industrial samples. Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Science, ICT & Future Planning (2010-C1AAA001-0029018) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014R1A5A1009799). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A. Firsov, Nature 438 (2005) 197–200. [2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [3] A. Reina, X.T. Jia, J. Ho, D. Nezich, H.B. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Nano Lett. 9 (2009) 30–35. [4] A. Gulino, G. Fiorito, I. Fragal, J. Mater. Chem. 13 (2003) 861–865. [5] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Appl. Surf. Sci. 252 (2006) 7487–7492. [6] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885–888. [7] Y. Lu, Y. Wang, Y. Zou, Z. Jiao, B. Zhao, Y. He, M. Wu, Electrochem. Commun. 12 (2010) 101–105. [8] C.-H. Chen, Y.-C. Chen, M.-S. Lin, Biosens. Bioelectron. 42 (2013) 379–384. [9] J. Mu, L. Zhang, M. Zhao, Y. Wang, J. Mol. Catal. A: Chem. 378 (2013) 30–37. [10] S. Farhadi, K. Pourzare, S. Bazgir, J. Alloys Compd. 587 (2014) 632–637. [11] S. Farhadi, J. Safabakhsh, P. Zaringhadam, J. Nanostruct. Chem. 3 (2013) 1–9. [12] Z. Song, Y. Zhang, W. Liu, S. Zhang, G. Liu, H. Chen, J. Qiu, Electrochim. Acta 112 (2013) 120–126.
99
[13] B.G. Choi, S.-J. Chang, Y.B. Lee, J.S. Bae, H.J. Kim, Y.S. Huh, Nanoscale 4 (2012) 5924–5930. [14] B. Li, H. Cao, J. Shao, G. Li, M. Qu, G. Yin, Inorg. Chem. 50 (2011) 1628–1632. [15] D. Ye, L. Luo, Y. Ding, B. Liu, X. Liu, Analyst 137 (2012) 2840–2845. [16] S. Park, S. Kim, Electrochim. Acta 89 (2013) 516–522. [17] X.-C. Dong, H. Xu, X.-W. Wang, Y.-X. Huang, M.B. Chan-Park, H. Zhang, L.-H. Wang, W. Huang, P. Chen, ACS Nano 4 (2012) 3206–3213. [18] L. Kong, Z. Ren, N. Zheng, S. Du, J. Wu, J. Tang, H. Fu, Nano Res. 8 (2015) 469–480. [19] M.M. Shahid, P. Rameshkumar, A. Pandikumar, H.N. Lim, Y.H. Ng, N.M. Huang, J. Mater. Chem. A 3 (2015) 14458–14468. [20] D.H. Coleman, R.E. White, D.T. Hobbs, J. Electrochem. Soc. 142 (1995) 1152–1161. [21] A. Pandikumar, S. Manonmani, R. Ramaraj, Catal. Sci. Technol. 2 (2014) 345–353. [22] Y. Haldorai, A. Rengaraj, C.H. Kwak, Y.S. Huh, Y.-K. Han, Synth. Met. 198 (2014) 10–18. [23] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [24] X. Wang, X. Dong, Y. Wen, C. Li, Q. Xiong, P. Chen, Chem. Commun. 48 (2012) 6490–6492. [25] O. Zhang, Y. Wen, J. Xu, L. Lu, X. Duan, H. Yu, Synth. Met. 164 (2013) 47–51. [26] Y. Zhang, Y. Zhao, S. Yuan, H. Wang, C. He, Sens. Actuators B 185 (2013) 602–607. [27] S.J. Li, G.Y. Zhao, R.X. Zhang, Y.L. Hou, L. Liu, H.A. Pang, Microchim. Acta 180 (2013) 821–827. [28] S. Radhakrishnan, K. Krishnamoorthy, C. Sekar, J. Wilson, S.J. Kim, Appl. Catal. B: Environ. 148–149 (2014) 22–28. [29] A.J. Lin, Y. Wen, L.J. Zhang, B. Lu, Y. Li, Y.Z. Jiao, H.F. Yang, Electrochim. Acta 56 (2011) 1030–1036. [30] V. Mani, B. Dinesh, S.M. Chen, R. Saraswathi, Biosens. Bioelectron. 53 (2014) 420–427. [31] M. Rajesh, V. Veeramani, S.-M. Chen, Sci. Rep. 4 (2014) 4679. [32] A. Afkhami, F.S. Felehgari, T. Madrakian, H. Ghaedi, Biosens. Bioelectron. 51 (2014) 379–385. [33] D. Zhang, Y. Fang, Z. Miao, M. Ma, X. Du, S. Takahashi, J. Anzai, Q. Chen, Electrochim. Acta 107 (2013) 656–663. [34] S. Yang, X. Liu, X. Zeng, B. Xia, J. Gu, S. Luo, N. Mai, W. Wei, Sens. Actuators B 145 (2010) 762–768.
Biographies Yuvaraj Haldorai received his PhD degree in 2008 at Pukyong National University, South Korea. He is currently working as an Assistant Professor at the Department of Energy and Materials Engineering, Dongguk University, South Korea. His research interests include energy storage materials, sensors, and photocatalysts. A.T. Ezhil Vilian received his PhD degrees in Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan in 2014. He is currently working as an Assistant Professor at the Department of Energy and Materials Engineering, Dongguk University, South Korea. His current research interests are in the development of new design electrochemical sensors platforms and new types of biosensors for determination of clinical and hazardous content in natural samples. Jun Yeong Kim received his BS degree from Department of Biological Engineering, Inha University, South Korea in 2014. He is currently a master degree student at Inha University. His research interests are in the areas of surface plasmonic resonance and biosensor. Yun Suk Huh received his PhD degree in 2007 at Korea Advanced Institute of Science and Technology, South Korea. He is currently working as an Assistant Professor at Inha University, South Korea. His research interests include nano-imaging, energy storage materials, and nanocomposite materials. Young-Kyu Han received his PhD degree in 1999 at Korea Advanced Institute of Science and Technology, South Korea. He is currently an Associate Professor at Dongguk University, South Korea. His research interests are relativistic chemistry, materials computational chemistry, and cluster chemistry.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An enzyme-free electrochemical sensor based on reduced graphene oxide/Co3 O4 nanospindle composite for sensitive detection of nitrite Yuvaraj Haldorai a , Jun Yeong Kim b , A.T. Ezhil Vilian a , Nam Su Heo b , Yun Suk Huh b,∗ , Young-Kyu Han a,∗ a b
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2015 Received in revised form 20 November 2015 Accepted 11 December 2015 Available online 14 December 2015 Keywords: Graphene Cobalt oxide Nanocomposite Nitrite Non-enzymatic
a b s t r a c t Cobalt oxide (Co3 O4 ) nanospindles-decorated reduced graphene oxide (RGO) composite is prepared via thermal decomposition of a three-dimensional coordination complex precursor, cobalt benzoate dihydrazinate, at 200 ◦ C. Transmission electron microscopy reveals that Co3 O4 nanospindles with an average particle size of <25 nm are decorated on the RGO surface. The low decomposition temperature and lack of residual impurities are significant aspects of this simple and facile method. The electrochemical performance of the proposed sensor is investigated using cyclic voltammetry and chronoamperometry. Under optimum conditions, anodic peak currents are linearly proportional to their concentrations, in the range of 1–380 M for nitrite with a regression equation of I (A) = 2.0660 C + 6.7869 (R2 = 0.9992). The sensor exhibits a high sensitivity of 29.5 A M−1 cm−2 , a rapid response time of 5 s, and a low detection of limit of 0.14 M. The proposed electrode shows good reproducibility and long-term stability. The sensor is used to determine the nitrite level in tap water with acceptable recovery, implying its feasibility for practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a two-dimensional sp2 carbon nanomaterial, has attracted increasing interest because of its unique physical and chemical properties, including high surface area, good chemical resistance, excellent electrical conductivity, extraordinary mechanical properties, high charge mobility, and thermal conductivity [1–3]. Transition metal oxides have attracted increasing attention due to their unique properties. Among the metal oxides most studied of late, cobalt oxide (Co3 O4 ), a p-type semiconductor with a direct band gap of 1.5 eV and 2.19 eV [4]. Moreover, cobalt oxide has three well-known polymorphic structures: cobaltous oxide (CoO), cobaltic oxide (Co2 O3 ), and Co3 O4 [5]. The one-dimensional (1D) nanostructures of Co3 O4 are inexpensive, environmentally friendly, and have good conductivity [6]. Due to its high surface area to volume ratio, high ratio of surface atoms, and good chemical stability, it should meet the requirements of future energy applications [7]. Considering the excellent properties, Co3 O4 nanostructures have been used for a variety of applications
∗ Corresponding authors. E-mail addresses: [email protected] (Y.S. Huh), [email protected] (Y.-K. Han). http://dx.doi.org/10.1016/j.snb.2015.12.032 0925-4005/© 2015 Elsevier B.V. All rights reserved.
[8–11]. Taking into account the excellent individual properties of graphene and Co3 O4 , a combination might yield enhanced performance. Several studies have reported different approaches to synthesizing graphene/Co3 O4 nanocomposites. Song et al. [12] fabricated graphene/Co3 O4 nanosheet composites via a hydrothermal method. Choi et al. [13] synthesized three-dimensional (3D), porous graphene/Co3 O4 composite films using replication and deposition processes for lithium ion batteries. Li et al. [14] reported a simple approach for preparing graphene/Co3 O4 composites via a chemical deposition of Co3 O4 nanoparticles onto GO followed by reduction of GO to graphene using NaBH4 for high-performance lithium ion batteries. Ye et al. [15] fabricated graphene/Co3 O4 composite using electrodeposition to determine tryptophan. Park and Kim [16] reported the synthesis of graphene/Co3 O4 nanosheets using microwave-assisted method as supercapacitor electrodes. Dong et al. [17] prepared 3D graphene/Co3 O4 nanowire composites for supercapacitors and non-enzymatic glucose detection. Kong et al. [18] fabricated interconnected 1D Co3 O4 nanowires on graphene for non-enzymatic hydrogen peroxide detection. Shahid et al. [19] synthetized graphene/Co3 O4 nanocube and graphene/Co3 O4 nanocube@Pt composites for nitric oxide detection. Although graphene/Co3 O4 composites have been used for various applications [12–19], few studies have investigated
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
electrochemical sensors. Until now, no attempt has been made to use a graphene/Co3 O4 composite for electrochemical nonenzymatic detection of nitrite. In relation to such reported composite materials, it should be noted that the obtained Co3 O4 is generally in the form of particle. It has been reported that the electrochemical performance of Co3 O4 can be enhanced by processing 1D or 2D nanostructures such as nanowires, nanocubes, nanosheets [16–19], etc. It is therefore of interest to synthesize new1D or 2D Co3 O4 /graphene composites with a synergistic advantage between Co3 O4 and graphene for electrochemical sensing. In this regard, a simple, reliable synthetic method needs to be developed because the size and morphology of Co3 O4 have a significant effect on its chemical and physical properties as well as its applications. Nitrite ions are important toxic inorganic pollutants found in food, soil, water, and physiological systems. High concentrations of nitrite ions can originate from caustic radioactive waste [20]. Excess nitrite ions are detrimental to the human body, and a high concentration in drinking water is a serious health risk for infants. In the human body, nitrites can combine with hemoglobin to form methemoglobin, which leads to a condition commonly called “blue baby syndrome.” Nitrite can also be converted into nitrosamine, which causes cancer and hypertension [21]. Hence, it is essential to monitor toxic nitrite ions in the environment. We report a facile synthesis of a reduced graphene oxide (RGO)/Co3 O4 nanospindle composite using a 3D coordination complex as a precursor, and its application toward the electrochemical detection of nitrite. Electrochemical studies indicated that the composite exhibited a fast current response to nitrite, with a wide linear range and a low detection limit. In addition, the composite electrode showed excellent reproducibility and long-term stability. The fabricated electrochemical sensor exhibited significant sensitivity, selectivity and it was also applied to real sample analysis. 2. Experimental 2.1. Materials Graphite, cobalt (II) nitrate hexahydrate (Co(NO3 )2 ·6H2 O), hydrazine (N2 H4 ), benzoic acid (C6 H5 COOH), sodium nitrate (NaNO3 ), sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), potassium permanganate (KMnO4 ), hydrogen peroxide (H2 O2 ), and sodium nitrite (NaNO2 ) were purchased from Aldrich. All other reagents were analytical grade and were used as received. 2.2. Synthesis of Co3 O4 /RGO nanocomposite Graphene oxide (GO) was synthesized from natural graphite using a modified Hummers’ method [22]. The precursor complex was prepared by mixing an aqueous solutions of Co(NO3 )2 ·6H2 O (0.01 mol) and of hydrazinium benzoate (0.02 mol) under constant stirring. The resulting mixture was concentrated to one-third of its total volume; the contents were cooled, filtered, washed with alcohol, and dried. 20 mg of GO was dispersed in 50 ml of ethanol under sonication for 1 h, then 100 mg of cobalt benzoate dihydrazinate complex precursor was added and the mixture was sonicated for another 1 h. The resulting mixture was transferred into a Teflonlined reactor and the reaction was carried out at 200 ◦ C for 8 h. The obtained product was separated, then washed with ethanol and distilled water by centrifugation. Finally, it was dried at 65 ◦ C for 24 h in a vacuum oven. 2.3. Preparation of the modified electrode The nitrite sensor was fabricated on a 3 mm diameter glassy carbon electrode (GCE) that was polished to a mirror finish using an
93
alumina slurry (0.05 m) sequentially, then washed ultrasonically in ethanol and deionized water and dried at room temperature. The electrode material was prepared by mixing 1.0 mg/ml of the Co3 O4 /RGO nanocomposite in ethanol under ultrasonication for 1 h to achieve a homogeneous dispersion. Then, 6 l of the suspension was drop-casted on a GCE and dried at room temperature. The resulting electrode was denoted as Co3 O4 /RGO/GCE. The RGO/GCE electrode was also prepared similarly for comparison. 2.4. Characterization Raman spectra of the samples were obtained using a HR800 UV Raman microscope (Horiba Jobin–Yvon, France). X-ray diffraction (XRD) patterns were collected using an X’Pert PRO MRD (Philips) diffractometer with Cu K␣ radiation. Morphology of the composite was examined by a HITACHI SU8010 high resolution scanning electron microscope. Transmission electron microscopy (TEM, JEOL-2010F) was performed at an accelerating voltage of 200 kV. The TEM observation sample was prepared by depositing an ethanolic dispersion on a 200 mesh copper grid, followed by air drying. High-resolution TEM (HRTEM) recorded the selected area electron diffraction (SAED) image. X-ray photoelectron spectroscopy (XPS) measurements were obtained using a Thermo Scientific K-Alpha electron spectrometer with an Al X-ray source. 2.5. Electrochemical analysis Electrochemical performance was evaluated using a threeelectrode cell system. Cyclic voltammetry (CV) and chronoamperometry analyses were conducted on a CHI 660D electrochemical workstation at room temperature. A 1 M KOH solution was served as the electrolyte, while Co3 O4 /RGO nanocomposite represented the working electrode. A platinum wire was employed as a counter electrode, with calomel as the reference. Electrochemical impedance spectroscopy (EIS) was performed using an WonATech electrochemical workstation in 0.1 M KCl containing 5 mM Fe(CN)6 3−/4− at room temperature. 3. Results and discussion The Co3 O4 /RGO nanocomposite was synthesized from GO and a 3D coordination complex precursor, cobalt benzoate dihydrazinate [Co(C6 H5 COO)2 (N2 H4 )2 ], which was prepared according to the following reaction: Co(NO3 )2 ·6H2 O + 2C6 H5 COON2 H5 → Co(C6 H5 COO)2 (N2 H4 )2 + 2HNO3 + 6H2 O
(1)
Co(C6 H5 COO)2 (N2 H4 )2 + 15O2 → CoO + 2N2 H4 + 14CO2 + 5H2 O (2)
6CoO + O2 → 2Co3 O4
(3)
Fig. 1 outlines the fabrication of Co3 O4 nanospindles decorated RGO nanocomposite for nitrite sensing. Decomposition pathway of the precursor and the deposition of Co3 O4 nanospindles on the RGO surface started at a relatively low temperature of ca. 200 ◦ C. The precursor decomposed completely, losing N2 H4 , CO2 , and H2 O. The complex was stable in air and insoluble in water. The importance of using such a complex is the distribution of metal ions in a 3D coordination sphere so that the decomposition would results in very fine particles. The benzoate and hydrazine ligands enhance
94
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 1. Schematic diagram of the synthesis of Co3 O4 /RGO nanocomposite for nitrite sensing.
the decomposition easier due to the exothermic decomposition of hydrazine. Fig. 2(a) shows the Raman spectrum of the Co3 O4 /RGO nanocomposite. The composite exhibited two bands: D and G at 1358 and 1589 cm−1 , respectively. The D band represented disordered carbon arising from structural defects, while the G band corresponded to the E2g phonon of sp2 carbon atoms [3,23]. The bands at 194, 473, 513, and 679 cm−1 were assigned to the F2g , Eg , F2g , and A1g vibrational modes of the Co3 O4 , respectively [14]. No vibrational modes resulted from impurities. Fig. 2(b) illustrates the XRD pattern of the Co3 O4 /RGO nanocomposite. The RGO exhibited two significant diffraction peaks, at 2 = 26.5 and 56.5, attributed to the (0 0 2) and (0 0 4) reflections of graphitic carbon, respectively (JCPDS no. 75-1621). In addition, the composite exhibited nine obvious diffraction peaks, which were indexed to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 1) planes of the pure, face-centered cubic phase of Co3 O4 , which was in agreement with the literature values (JCPDS 42-1467) [13]. These results demonstrated the existence of both RGO and Co3 O4 in the as-synthesized composite. Fig. 3 represents SEM, TEM, HRTEM, and SAED pattern of the Co3 O4 /RGO nanocomposite. SEM image (Fig. 3(a)) of the composite showed that almost uniform size of spindle-like Co3 O4 nanoparticles were dispersed on the RGO surface. To ascertain the physical nature of Co3 O4 in the composite more clearly, the TEM image of composite is presented in Fig. 3(b). The TEM image displayed Co3 O4 nanospindles with a mean diameter of approximately <25 nm and a length of <45 nm were homogeneously dispersed on the RGO surface; a few nanospindles with different sizes were also observed. The HRTEM image (Fig. 3(c)) illustrated clear lattice fringes, highlighting the 0.26 nm inter-planar spacing, which represents the (311) plane of Co3 O4 . The SAED pattern of the composite (Fig. 3(d)) showed well-defined rings, which could be attributed to the crystalline nature of Co3 O4 . Fig. 4 demonstrates survey and core-level spectra of the Co3 O4 /RGO nanocomposite. The survey spectrum (Fig. 4(a)) exhibited C 1s (284.7 eV), O 1s (529.9 eV), Co 2p3/2 (780.6 eV), and Co 2p1/2 (797.2 eV) peaks, which confirmed the successful decoration of Co3 O4 on the RGO surface [14]. The core-level spectrum of Co 2p (Fig. 4(b)) had two peaks: a main peak (2p3/2 ) and a satellite peak (2p1/2 ) with binding energies of 779.8 eV and 795.8 eV, respectively. XPS spectra were in good agreement with the reported data of
Co3 O4 spin–orbit peaks [13]. The Gaussian fit of the C 1s core-level spectrum of GO (Fig. 4(c)) showed three peaks: non-oxygenated C C (284.8 eV), C O (epoxy and hydroxyl; 285.6 eV), and C O (carboxyl; 287.1 eV). Compared to GO, the peak intensities of C O and C O in the C 1s spectrum of the composite (Fig. 4(d)) decreased dramatically, indicating the successful reduction of GO to RGO. The electrochemical properties of the bare GCE, RGO/GCE, and Co3 O4 /RGO/GCE were studied by EIS. Fig. 5(a) represents impedance spectra of the electrodes recorded in a 5 mM Fe (CN)6 3−/4− solution containing 0.1 M KCl. The semicircular portion of the higher frequency region represents the confined electron transfer process (in this case, the electron transfer resistance (Rct ) is equal to the semicircle’s diameter). The Rct value for bare GCE was estimated to be 954 . In contrast, the Rct value for RGO/GCE decreased to 597 , suggesting that the introduction of RGO plays a vital role in increasing electroactive surface area, while also providing a conductive network for the electron-transfer of Fe(CN)6 3−/4− . However, the Rct value for Co3 O4 /RGO/GCE decreased to 210 , indicating the excellent electron transfer [15] properties of the Co3 O4 /RGO/GCE compared to RGO/GCE. Fig. 5(b) shows CVs of the bare GCE, RGO/GCE, and Co3 O4 /RGO/GCE in the absence of nitrite in N2 -saturated KOH (1 M) at a scan rate of 50 mV s−1 . The background current of the Co3 O4 /RGO/GCE was much greater than for the RGO/GCE and bare GCE, indicating the former has a more highly accessible surface area and better conductivity, which was concordant with previous studies [17,24] on electrochemical reactions of Co3 O4 in alkaline solutions. The mechanism involved hydroxyl ions (OH− ) in the redox reactions of Co3 O4 . As shown in Fig. 5(b), the redox peaks were ascribed to the transition between Co3 O4 and CoOOH. The reaction is expressed by Eq. (1) [17,24] Co3 O4 + OH− ↔ CoOOH + e−
(1)
Fig. 5(c) illustrates the CVs of the different electrodes in the presence of 2 mM nitrite in 1 M KOH at a scan rate of 100 mV s−1 . The bare GCE and RGO/GCE showed no obvious oxidation peaks in the given potential range of 0–0.7 V, whereas the Co3 O4 /GCE showed an oxidation peak at around 0.5 V. Interestingly, the Co3 O4 /RGO/GCE exhibited a sharper oxidation peak at around 0.54 V with higher background current than that of Co3 O4 /GCE. These results can be ascribed to the high electrocatalytic activity of the Co3 O4 and large active surface area of the RGO with regard to the oxidation of nitrite.
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
95
Fig. 2. (a) Raman spectrum and (b) XRD pattern of the nanocomposite.
Fig. 5(d) illustrates the CVs of Co3 O4 /RGO/GCE with different nitrite concentrations at a scan rate of 100 mV s−1 in 1 M KOH. The bare GCE showed no obvious redox peaks in the potential range of 0–0.8 V, whereas Co3 O4 /RGO/GCE exhibited oxidation and reduction peaks at around 0.54 and 0.36 V, respectively. The oxidation peak current increased linearly according to the nitrite concentration, indicating that the electrode had excellent sensitivity to nitrite. A positive shift in the oxidation peak could be attributed to the adsorption of nitrite and its oxidized products on the electrode surface, which may control the kinetics of the electrochemical reactions [17,24]. The mechanism of nitrite electrooxidation is explained by the following equations: In the first step, Co3 O4 was electrochemically oxidized to CoOOH in a basic solution (Eq. (1)) where an electron was released. In the second step, nitrite was oxidized using CoOOH, which was simultaneously reduced to Co3 O4 . This reaction is represented by Eq. (2) CoOOH + NO2 − → Co3 O4 + NO3 −
(2)
Fig. 5(e) shows a typical graph of the anodic peak current (Ipa ) versus nitrite concentration. The anodic peak current gradually increased as the nitrite concentration increased (0.5–6 mM). The regression coefficient (R2 ) value of 0.9947 indicated the excellent nitrite detection. Fig. 5(f) demonstrates the CVs of Co3 O4 /RGO/GCE at different scan rates in the presence of 2 mM of nitrite. The anodic peak current increased significantly when the scan rate increased from 10 to 100 mV s−1 , demonstrating a linear relationship to the square root of the scan rate, with an R2 value of 0.9974 (Fig. 5(g)). These results suggested that the electrochemical reaction was confined to the surface. Fig. 6(a) represents the amperometric response of the Co3 O4 /RGO/GCE to successive step-wise additions of nitrite in 1 M KOH. A significant increase in peak current was observed when nitrite was added to the stirred KOH. The electrode responded rapidly to the nitrite, reaching a plateau within 5 s. Fig. 6(b) shows the calibration curve for the nitrite. The electrode showed a linear
Fig. 3. (a) SEM, (b) TEM, (c) HRTEM, and (d) SAED pattern of the nanocomposite.
96
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 4. (a) Survey XPS of the Co3 O4 /RGO nanocomposite, (b) Co 2p spectrum of nanocomposite, (c) C 1s spectrum of GO, and (d) C 1s spectrum of nanocomposite. Table 1 Results of the present study compared with previous reports of various electrodes for nitrite sensing. Electrode
Detection limit (M)
Au/PEDOT Pd/RGO Au/SG Fe2 O3 /RGO Au/ZnO/MWCNT RGO/MWCNT/Pt/Mb Heteroatom enriched porous carbon Au/MWCNT Cu-NDs/RGO Cu/MWCNT/CS Co3 O4 /RGO
0.1 15.64 nM 0.2 0.015 0.4 0.93 0.13 0.01 0.4 0.024 0.14
Linear range (M) 3–300 0.04–108 10–3960 0.05–780 0.78–400 1–12,000 3–90 0.05–250 1.25–13,000 0.1–2500 1–380
Sensitivity (A M−1 cm−2 )
Ref.
– 7.672 45.44 A mM−1 cm−2 0.204 – 0.1651 9.0 2.5815 A L mol−1 cm−2 214 A mM−1 cm−2 −48.92 A mM−1 cm−2 29.5
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] This work
PEDOT, poly(ethylenedioxythiophene); SG, sulfonated graphene; Mb, myoglobin; MWCNT, multi-walled carbon nanotube; Cu-NDs, copper nanodendrites; CS, chitosan.
relationship to the nitrite concentration (1–380 M), with a regression equation of: I (A) = 2.0660 C (M) + 6.7869 (R2 = 0.9992). The low detection limit can be calculated using Eq. (3): LOD =
3Sb , S
(3)
where Sb is the standard deviation of five blank measurements and S is the sensitivity. The low detection limit of nitrite sensing was identified at 0.14 M. Sensitivity of the fabricated sensor can be calculated from slope of the calibration curve divided by working area of the electrode (0.07 cm2 ). The sensitivity was calculated as 29.5 A M−1 cm−2 . Together these results indicated that the electrode had excellent non-enzymatic nitrate detection. The linear range, detection limit, and sensitivity of other electrodes were compared to those of the Co3 O4 /RGO/GCE (listed in Table 1). Even though previously reported enzymatic and non-enzymatic nitrite sensors based on other nanomaterials showed good performances [25–34], the superior performance of Co3 O4 /RGO/GCE may be attributed to the large surface area of the RGO and the good
electron transfer between the Co3 O4 and RGO. Because of this high surface area, the composite can adsorb and catalyze more reactive substances, resulting in an instantaneous oxidation process. Selectivity is an important parameter of the nitrite sensor. Various common ions, such as Na+ , Mg2+ , NH4 + , SO4 2− , CH3 COO− , CO3 2− , and NO3− , did not interfere with the detection of nitrite. Fig. 6(c) showed that no significant amperometric responses were observed when 2 mM each of Na2 SO4 , CH3 COONa, NaNO3 , Na2 CO3 , MgSO4 , and NH4 NO3 were injected at regular intervals. However, upon the addition of 20 M nitrite, a notable amperometric response was immediately observed, revealing the excellent selectivity of the Co3 O4 /RGO/GCE toward nitrite determination. To demonstrate the stability of Co3 O4 /RGO/GCE, 100 cycles of CV (Fig. 6(d)) were carried out using 2 mM nitrite in 1 M KOH solution at a scan rate of 50 mV s−1 . The anodic peak current showed an 8% decrease in the peak current after 100 cycles, illustrating the good stability of the electrode. We also demonstrated the reproducibility of sensor by measuring changes in the CV peak currents (Fig. 6(e)) of five different electrodes during nitrite (2 mM)
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
97
Fig. 5. (a) Nyquist plots of the electrodes in 0.1 M KCl containing 5 mM Fe(CN)6 3−/4− , (b) CVs of electrodes in the absence of nitrite in1 M KOH at a scan rate of 50 mV s−1 , (c) CVs of electrodes in the presence of 2 mM nitrite in1 M KOH at a scan rate of 100 mV s−1 , (d) CVs of the Co3 O4 /RGO/GCE in 1 M KOH containing 0.5–6 mM of nitrite at a scan rate of 100 mV s−1 ), (e) plot of anodic peak current versus concentration, (f) CVs of the Co3 O4 /RGO/GCE at different scan rates (10–100 mV s−1 ) in the presence of 2 mM nitrite, and (g) plot of anodic peak current versus square root of the scan rate.
98
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99
Fig. 6. (a) Chronoamperometric data of the Co3 O4 /RGO/GCE with successive additions of nitrite in 1 M KOH, (b) plot of peak current versus concentration, (c) interference test of the electrode with 20 M nitrite in the presence of various common ions, (d) CV of the Co3 O4 /RGO/GCE for 100 multiple cycles in the presence of 2 mM nitrite at a scan rate of 50 mV s−1 , (e) peak current of five different composite electrodes for the detection of 2 mM nitrite at a scan rate of 50 mV s−1 , and (f) shelf-life of the composite electrode after 14 days.
detection. The relative standard deviation was calculated as 1.93%, which indicated good reproducibility of the electrode. The shelflife of the nanocomposite electrode was checked after 2 weeks (Fig. 6(f)). The electrode was stored in a vacuum at room temperature (24 ◦ C), and the detection of 2 mM nitrite was monitored at regular 2 day intervals. The high stability of the electrode may be attributed to the good chemical stability of Co3 O4 in the electrolyte. For real sample analysis, the fabricated Co3 O4 /RGO/GCE sensor was used to measure nitrite levels in tap water. Tap water
samples consisting of different concentrations of spiked nitrite were quantitatively analyzed using a standard addition method. Before analysis, the samples were filtered using a 0.2 m filter to remove micron-sized particles. The results showed that the electrode was highly selective and sensitive to nitrite. The recoveries (Table 2) were observed as 99.9%, 101.5%, 98.6%, and 99.3% for the 10, 20, 30, and 40 M nitrite spiked samples, respectively. These results indicated that the sensor showed excellent detection of nitrites in tap water.
Y. Haldorai et al. / Sensors and Actuators B 227 (2016) 92–99 Table 2 Determination of nitrites in tap water. Real samples
Tap water
Nitrite spiked (M)
Nitrite found (M)
RSD (%)
Recovery (%)
10 20 30 40
9.99 20.3 29.6 39.7
2.47 2.93 1.51 3.04
99.9 101.5 98.6 99.3
4. Conclusions We have successfully demonstrated a simple and facile method to deposit Co3 O4 nanospindles on the RGO surface. The assynthesized nanospindles had a mean diameter and length of approximately <25 nm and <45 nm, respectively. The composite electrode exhibited high sensitivity, detecting a wide range of nitrite concentrations (from 1 to 380 M). The lowest detection limit was observed at 0.14 M. This high nitrite sensitivity could be attributed to the excellent electrocatalytic activity of Co3 O4 and large specific surface area of the RGO. In addition, the composite electrode displayed good selectivity toward nitrite, even in the presence of 100-fold higher concentrations of various common ions. The real sample analysis result indicated that the electrode was highly selective and sensitive to nitrite. The high stability, reproducibility, and shelf-life make this composite electrode a suitable device for the detection of nitrite in industrial samples. Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Science, ICT & Future Planning (2010-C1AAA001-0029018) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014R1A5A1009799). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A. Firsov, Nature 438 (2005) 197–200. [2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [3] A. Reina, X.T. Jia, J. Ho, D. Nezich, H.B. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Nano Lett. 9 (2009) 30–35. [4] A. Gulino, G. Fiorito, I. Fragal, J. Mater. Chem. 13 (2003) 861–865. [5] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Appl. Surf. Sci. 252 (2006) 7487–7492. [6] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885–888. [7] Y. Lu, Y. Wang, Y. Zou, Z. Jiao, B. Zhao, Y. He, M. Wu, Electrochem. Commun. 12 (2010) 101–105. [8] C.-H. Chen, Y.-C. Chen, M.-S. Lin, Biosens. Bioelectron. 42 (2013) 379–384. [9] J. Mu, L. Zhang, M. Zhao, Y. Wang, J. Mol. Catal. A: Chem. 378 (2013) 30–37. [10] S. Farhadi, K. Pourzare, S. Bazgir, J. Alloys Compd. 587 (2014) 632–637. [11] S. Farhadi, J. Safabakhsh, P. Zaringhadam, J. Nanostruct. Chem. 3 (2013) 1–9. [12] Z. Song, Y. Zhang, W. Liu, S. Zhang, G. Liu, H. Chen, J. Qiu, Electrochim. Acta 112 (2013) 120–126.
99
[13] B.G. Choi, S.-J. Chang, Y.B. Lee, J.S. Bae, H.J. Kim, Y.S. Huh, Nanoscale 4 (2012) 5924–5930. [14] B. Li, H. Cao, J. Shao, G. Li, M. Qu, G. Yin, Inorg. Chem. 50 (2011) 1628–1632. [15] D. Ye, L. Luo, Y. Ding, B. Liu, X. Liu, Analyst 137 (2012) 2840–2845. [16] S. Park, S. Kim, Electrochim. Acta 89 (2013) 516–522. [17] X.-C. Dong, H. Xu, X.-W. Wang, Y.-X. Huang, M.B. Chan-Park, H. Zhang, L.-H. Wang, W. Huang, P. Chen, ACS Nano 4 (2012) 3206–3213. [18] L. Kong, Z. Ren, N. Zheng, S. Du, J. Wu, J. Tang, H. Fu, Nano Res. 8 (2015) 469–480. [19] M.M. Shahid, P. Rameshkumar, A. Pandikumar, H.N. Lim, Y.H. Ng, N.M. Huang, J. Mater. Chem. A 3 (2015) 14458–14468. [20] D.H. Coleman, R.E. White, D.T. Hobbs, J. Electrochem. Soc. 142 (1995) 1152–1161. [21] A. Pandikumar, S. Manonmani, R. Ramaraj, Catal. Sci. Technol. 2 (2014) 345–353. [22] Y. Haldorai, A. Rengaraj, C.H. Kwak, Y.S. Huh, Y.-K. Han, Synth. Met. 198 (2014) 10–18. [23] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [24] X. Wang, X. Dong, Y. Wen, C. Li, Q. Xiong, P. Chen, Chem. Commun. 48 (2012) 6490–6492. [25] O. Zhang, Y. Wen, J. Xu, L. Lu, X. Duan, H. Yu, Synth. Met. 164 (2013) 47–51. [26] Y. Zhang, Y. Zhao, S. Yuan, H. Wang, C. He, Sens. Actuators B 185 (2013) 602–607. [27] S.J. Li, G.Y. Zhao, R.X. Zhang, Y.L. Hou, L. Liu, H.A. Pang, Microchim. Acta 180 (2013) 821–827. [28] S. Radhakrishnan, K. Krishnamoorthy, C. Sekar, J. Wilson, S.J. Kim, Appl. Catal. B: Environ. 148–149 (2014) 22–28. [29] A.J. Lin, Y. Wen, L.J. Zhang, B. Lu, Y. Li, Y.Z. Jiao, H.F. Yang, Electrochim. Acta 56 (2011) 1030–1036. [30] V. Mani, B. Dinesh, S.M. Chen, R. Saraswathi, Biosens. Bioelectron. 53 (2014) 420–427. [31] M. Rajesh, V. Veeramani, S.-M. Chen, Sci. Rep. 4 (2014) 4679. [32] A. Afkhami, F.S. Felehgari, T. Madrakian, H. Ghaedi, Biosens. Bioelectron. 51 (2014) 379–385. [33] D. Zhang, Y. Fang, Z. Miao, M. Ma, X. Du, S. Takahashi, J. Anzai, Q. Chen, Electrochim. Acta 107 (2013) 656–663. [34] S. Yang, X. Liu, X. Zeng, B. Xia, J. Gu, S. Luo, N. Mai, W. Wei, Sens. Actuators B 145 (2010) 762–768.
Biographies Yuvaraj Haldorai received his PhD degree in 2008 at Pukyong National University, South Korea. He is currently working as an Assistant Professor at the Department of Energy and Materials Engineering, Dongguk University, South Korea. His research interests include energy storage materials, sensors, and photocatalysts. A.T. Ezhil Vilian received his PhD degrees in Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan in 2014. He is currently working as an Assistant Professor at the Department of Energy and Materials Engineering, Dongguk University, South Korea. His current research interests are in the development of new design electrochemical sensors platforms and new types of biosensors for determination of clinical and hazardous content in natural samples. Jun Yeong Kim received his BS degree from Department of Biological Engineering, Inha University, South Korea in 2014. He is currently a master degree student at Inha University. His research interests are in the areas of surface plasmonic resonance and biosensor. Yun Suk Huh received his PhD degree in 2007 at Korea Advanced Institute of Science and Technology, South Korea. He is currently working as an Assistant Professor at Inha University, South Korea. His research interests include nano-imaging, energy storage materials, and nanocomposite materials. Young-Kyu Han received his PhD degree in 1999 at Korea Advanced Institute of Science and Technology, South Korea. He is currently an Associate Professor at Dongguk University, South Korea. His research interests are relativistic chemistry, materials computational chemistry, and cluster chemistry.