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Citrate stabilized gold nanoparticles on graphenic carbon spheres for the selective detection of hydrazine
Microchemical Journal 151 (2019) 104234
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Citrate stabilized gold nanoparticles on graphenic carbon spheres for the selective detection of hydrazine Annamalai Yamuna, Periyasamy Sundaresan, Shen-Ming Chen
T
⁎
Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No.1, Section 3, ChungHsiao East Road, Taipei 106, Taiwan, ROC
ARTICLE INFO
ABSTRACT
Keywords: Hydrothermal carbonization Citrate ligand Alkaline environment XPS spectra of gold CV of gold
We have studied the electrocatalytic properties of hydrazine in the alkaline environment by using the stable sensor matrix. Thereby, we have selected the citrate-capped gold nanoparticles (AuNPs) decorated graphenic carbon spheres (GCS). The hydrothermal carbonization method is utilized to prepare the GCS by using reduced graphene oxide (rGO) and glucose. In hydrothermal treatment, the glucose undergoes polycondensation and simultaneously reduced the rGO which results in spherical morphology. These spheres consist of a carbonized nucleus and a hydrophilic surface that facilitates the formation of citrate-capped AuNPs on its surface. This novel composite provides the stable platform to determine the hydrazine in pH 13. Various physicochemical techniques confirm the formation of GCS-AuNPs and also characterized by electrochemical techniques. This GCSAuNPs modified GCE has showed the good electrochemical performance to the hydrazine oxidation. Also, it revealed the acceptable analytical performance to the determination of hydrazine. The estimated linear concentration range, limit of detection and sensitivity are 0.02 to 530.1 μM, 6 nM and 4.45 μA μM−1 cm−2. Moreover, GCS-AuNPs/GCE shows good reproducibility and good recovery in real sample analysis.
1. Introduction Hydrazine is an essential inorganic chemical that broadly utilized in chemical industries as the reducing agent and also employed in fuel cell, corrosion inhibitors, pharmaceutical, agriculture, bioimaging fields [1–3], fuel propellant [4], military [5] and aerospace applications [6]. Due to colorless and water miscibility, hydrazine can cause harmful effects to the humans through the contaminated water bodies. The most considerable health issues of hydrazine are a mutagen, carcinogenic and hepatotoxic impacts [7–9]. Moreover, it can also affect the central nervous system, lungs, liver and kidneys [10]. Hence, the United States environmental protection agency (EPA) characterized hydrazine to be the hazardous environmental toxin and human carcinogen [11]. Also, the China National Standards limits the hydrazine in water sources (GB 18061-2000) about 0.02 mg/L (permissible limit) [12]. Therefore, it is essential to monitor the hydrazine level in water bodies, especially in drinking water. Accordingly, different methods have been developed to determine the hydrazine, e.g. spectrophotometry, fluorescence probe, electrochemical method, colorimetry and chromatography [13–17], in which the electrochemical technique provides the most promising platform to detect the hydrazine. However, the electrochemical method is lagging from its fabrication of the working electrode due to sensitivity ⁎
and selectivity. Till now, many metal nanoparticles decorated highly conductive carbon materials based working electrodes have been reported to enhance the sensing abilities though still a lack in its selectivity. Especially, the gold nanoparticles (AuNPs) have utilized to detect the hydrazine molecule because it has less overpotential and good sensitivity towards hydrazine oxidation. However, the shape and microstructures of AuNPs influence the electrocatalytic activity [18]. Thereby, many researchers have reported different synthesis protocol to prepare the AuNPs with high conductive support. Most of the reported hydrazine sensors have studied in almost pH 7, but the aqueous hydrazine has the alkaline pH (pKa ~ 8.0). Moreover, the effective oxidation potential of hydrazine at AuNPs in pH 7 is ~+0.2 V vs Ag/AgCl (Table 1), some of the active molecules such as dopamine, catechol, ascorbic acid, and hydroquinone can interfere with the detection of hydrazine in this potential [19]. Eliminating such interferences in pH 7 is debatable; thus, we have considered investigating the electrocatalytic properties of hydrazine in pH 13 by utilizing citrate stabilized AuNPs decorated graphenic carbon spheres. Herein, the graphenic carbon spheres are providing good conductivity and support to the AuNPs and citrate provides the negative surface charge to the AuNPs which certainly avoids the aggregation of AuNPs. Moreover, the citrate ligand
Corresponding author. E-mail address: [email protected] (S.-M. Chen).
https://doi.org/10.1016/j.microc.2019.104234 Received 22 April 2019; Received in revised form 31 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Comparison of the performance of GCS-AuNPs/GCE hydrazine sensor with other hydrazine sensors. Electrode
Method
pH
Ep (V)
Linear range (μM)
LOD (μM)
Ref
GNPS/Ch/GCE AuNG/CMG/GCE Au/ppy/GCE Au@Pd/CB-DHP/GCE Ac/Au/GCE AuPD NPst/GR/GCE AuCuNPs/GN/IL/GCE AuNPs/ERGO/Au AG/Au-NPs/SPCE AuNPs/Poly(BCP)/ CNTs/GCE Au-Cu/NPz/CPE Ni-DAP/Au-Pt NPs/NFs/GCE GCS-AuNPs/GCE
LSV Amperometry DPV Amperometry LSV Amperometry Amperometry DPV Amperometry LSV
7.4 7.4 7.0 10.0 7.0 6.0 6.8 8.0 7.0 10
0.15 0.17 0.135 0.15 0.2 0.2 0.15 0.087 0.15 −0.028
5–500 6–30 1–500 2.5–88.0 1–291 2–185 0.2–110 4–1000 0.002–936 0.5–1000
0.1 0.5 1.0 1.77 0.005 0.2 0.1 0.074 0.00057 0.1
[29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
Amperometry Amperometry DPV
7.4 13 13
0.22 0.35 −0.17
10–2000 0.2–85 0.02–530.1
0.043 0.1 0.006
[39] [40] This work
holds some of partially charged Au ions in the composite that could facilitate the better electrochemical activity.
2.5. Fabrication of electrode GCS-AuNPs/GCE Before to the electrode modification, the GC electrode was well polished with alumina slurry paste (0.05 μm) and rinsed thoroughly with DD water. Then the cleaned electrode was dried at room temperature. 5 mg of GCS-AuNPs re-dispersed in 1 mL of water and sonicated for an hour. From this dispersed solution, 7 μl was taken and drop casted on the surface of the GCE and allowed to dry at room temperature. This GCS-AuNPs modified electrode was used for the further electrochemical studies.
2. Experimental 2.1. Materials Graphite, hydrazine, auric chloride (HAuCl4), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4) and other chemicals obtained from Alfa Aesar, Sigma-Aldrich, Fluka and Wako chemicals and used as received.
3. Results and discussions
2.2. Methods
3.1. Material characterizations
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using CHI 1205 and CHI 900 electrochemical workstations. Raman spectra were recorded using a Raman spectrometer, Dong Woo 500i, Korea, equipped with a charge-coupled detector. Conventional three-electrode system used for the electrochemical experiments where the modified glassy carbon electrode (GCE; working area = 0.07 cm2) used as a working electrode, saturated Ag/AgCl used as a reference electrode and platinum electrode used as the counter electrode.
Previous reports clearly documented the hydrothermal carbonization of glucose and their mechanism. Briefly, the dehydration of glucose undergoes a hexamethyl furfural formation then it will convert to carbon spheres by the decarboxylation and polymerization [22]. Herein, we just introduced the reduced graphene oxide to prepare the graphenic carbon spheres. Under the hydrothermal condition, the glucose carbonization was simultaneously reduced the rGO and wrapped the rGO sheets. In this case, the glucose diffused into the rGO sheets and formed the sphere like morphology. Later, the small nanospheres of glucose carbon were aggregated on the rGO surface and grown as microspheres. These carbon spheres consist of a carbonized nucleus (reduced graphene oxide sheets with the aromatic carbon network (inside)) and a hydrophilic surface (hydroxyl groups contained amorphous network). Fig. 1a & b shows the HRTEM images of GCS-AuNPs which revealed the spherical carbon particles with the average diameter of 600 ± 50 nm and the AuNPs anchored on the surface of the GCS. The SAED pattern of GCS-AuNPs showed in Fig. 1c, which endorsed that the amorphous-like ring pattern for the GCS and polycrystalline pattern for the AuNPs. Fig. 1d shows the Raman spectra of GCS-AuNPs, GCS and rGO wherein two typical peaks observed for all compounds. Those two peaks are corresponding to the graphitic peak (G band) and disordered peak (D band) [23,24] which centered at 1591 cm−1 and 1340 cm−1 for rGO, 1597 cm−1 and 1368 cm−1 for GCS, 1596 cm−1 and 1332 cm−1 for GCS-AuNPs, respectively. The relative intensity ratios of D and G band (ID/IG) are 1.38, 0.55 and 0.76 for rGO, GCS and GCSAuNPs respectively. In general, the ID/IG ratio is used to identify the defects and disorders in carbon lattices. Obviously, the rGO shows higher ID/IG ratio because it has large defects and it considerably reduced when converted to GCS. That means the glucose carbonization reaction simultaneously reduced the oxygen functionalities of rGO which results in more graphitic band. However, ID/IG ratio of GCSAuNPs increased to 0.76 that is because of the incorporation of citrate
2.3. Preparation of GCS-AuNPs 4 g of glucose dissolved in 40 mL of water and 0.2 g of reduced graphene oxide was dispersed in the glucose solution by ultra-sonication for an hour. This dispersion is hydrothermally treated for 4 h at 180 °C, which yields the blackish brown precipitate. Then, the precipitate washed with water until the impurities removed then dried at 40 °C, which named as GCS [20]. Afterwards, 0.2 g of GCS dispersed in 40 mL of DD water then 0.02 g of HAuCl4 was added into the GCS solution with the continuous stirring for an hour and the adequate amount of sodium citrate added into that solution [21]. Next, this mixture was kept in the hydrothermal condition at 180 °C for 2 h, herein, the gold ions reduced into gold nanoparticles (AuNPs) as shown in Scheme 1. Finally, the formed GCS-AuNPs was collected by centrifugation and washed with DD water and ethanol. 2.4. Sampling procedure The tap water and pond water samples were collected randomly from the five different sites in Taipei, Taiwan. The samples are filtered through the filter membrane (pore size 0.45 μm) and directly used for the real sample analysis. The known concentrations of the hydrazine was added to the real samples and examined by standard addition method. 2
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Scheme 1. Schematic representation of the preparation of GCS-AuNPs.
ligands. The Raman spectrum of GCS-AuNPs shows the sharp peak centered at 1431 cm−1 for the symmetric stretching vibration of carboxylate ion and the weak peak at 1633 cm−1 for the asymmetric stretching vibration of carboxylate ion [25]. This citrate ligand intensity considerably increases the D band intensity of GCS thus the overall intensity is increased which produce high ID/IG ratio. This result confirmed that the citrate ligand is highly coordinated with the AuNPs with the carboxylate ion and also it is bonded with GCS by the nucleophilic addition. Fig. 2a shows the XPS survey spectrum of GCS-AuNPs, where the peaks at 85, 286, 499 and 533 eV indicated the presence of Au, C, Na and O. The strong auger lines of Na KLL inferred that the citrate ions are adsorbed on the surface of AuNPs. The high-resolution spectrum of Au is deconvoluted into four peaks wherein the sharp two peaks at 84.0 eV and 87.7 eV corresponds to the Au 4f7/2 and 4f5/2 respectively, which confirms the metallic formation of Au which shown in Fig. 2b. Additionally, the remaining two peaks appeared at 84.9 and 88.6 eV which corresponds to the partially charged Au ions. This behavior insisted that some of Au ions are capped by the citrate ligands on the surface of AuNPs [21]. Moreover, the C 1s high resolution spectrum was deconvoluted into four major peaks at 285.2, 286.1, 287.2 and 289.1 (Fig. 2c). The peaks at 285.2, 286.1 and 287.25 eV corresponds to the CeC, CeO and C]O respectively. These peaks mainly arise from the graphenic carbon spheres and the peak at and 289.01 eV attributed to the O-C=O. Mostly, the CeO and O-C=O peaks mentioned that the alcoholic and carboxylate groups of citrate ions. Fig. 2d shows the O 1s spectrum which further deconvoluted into three peaks centered at 532.8, 534.3 and 536.4 eV. The peaks at 534.3 and 536.4 eV corresponds to the CeO and O-C=O bond, which further confirmed the presence of citrate ions on AuNPs. These comprehensive studies confirmed the formation of citrate capped AuNPs on graphenic carbon spheres. This GCS-AuNPs modified GCE further applied to the electrochemical characterization towards hydrazine.
3.2. Electrochemical characterization of modified electrodes The electrochemical behavior of GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE and bare GCE were characterized by using [Fe(CN)6]3−/4− probe which shown in (Fig. S1.a–d). The GCS-AuNPs/GCE showed high catalytic activity when compared to the other electrodes. These electrocatalytic activities are described in terms of electron transfer rate, conductivity, peak to peak separation potential (ΔEp) and electrochemical active surface area. The calculated electrochemical active surface areas are 0.137, 0.07, 0.043, 0.035 cm2 for GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE, and bare GCE, respectively which was calculated from the scan rate studies in [Fe(CN)6]3−/4− (see the Supplementary information) [44]. However, all the modified GCEs are shown some electrochemical activity in Fe(CN)63−/4− probe. So, it is required to characterize the effectual heterogeneous rate constant (keffo) [24]. The calculated keffo values are 3.6 × 10−3, 1.3 × 10−3, 3.2 × 10−4, 1.2 × 10−3 cm s−1 for the GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE and bare GCE, respectively [23,24], (For the detailed information see the supplementary). 3.3. Electrochemical oxidation of hydrazine The electrochemical behaviors of hydrazine oxidation on modified GCEs were investigated by CV technique by sweeping the potential from −0.6 V to 0.6 V vs Ag/AgCl in 0.1 M KOH solution at a scan rate of 50 mV/s. Fig. 3a & b shows the CVs of GCS-AuNPs/GCE, AuNPs/GCE and GCS/GCE in the absence and presence of 200 μM hydrazine. In the absence of hydrazine, GCS-AuNPs/GCE shows two redox couples centered at −0.34 V for the GCS and +0.28 V for the citrated capped AuNPs, respectively. The first redox couple (O1/R1; −0.28/−0.41 V) associated with the carbonyl functional group in the carbon matrix (GCS) which mainly arises from the catechol or hydroquinone moiety [26]. The second redox couple (O1/R1; +0.36/+0.17 V) corresponds to the redox behavior of citrate capped AuNPs wherein the partially charged Au ions undergo the oxidation and reduction reaction [27]. In 3
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Fig. 1. Low magnification (a), high magnification (b) TEM images and SAED pattern (c) of GCS-AuNPs. (d) Raman spectra of rGO, GCS and GCS-AuNPs.
0.01 μA mV−1s + 41.28 ± 0.05 μA (R2 = 0.9943). This behavior suggested that the electrochemical oxidation of hydrazine on the GCSAuNPs/GCE followed the adsorption controlled process (Fig. 3c, inset). In general, the oxidation of hydrazine follows two-step reactions; the first step involves with the slow one electron transfer (rate determining step) and second step proceeds with fast three electrons transfer [28].
contrary, this redox peak does not appear for the AuNPs, which evinced that the citrate capped AuNPs have some partially charged Au ions (Auδ+) which facilitates the selective oxidation of hydrazine. On the other hand, the GCS/GCE shows the capacitance behavior with weak oxidation and reduction peaks for the surface bound oxygen functionalities such as carbonyl, ester or carboxylic acid. In the presence of hydrazine, GCS-AuNPs/GCE reveals the sharp well defined irreversible oxidation peak at −0.176 V for the hydrazine oxidation and weak redox peak at +0.34 V for the redox of citrate capped Auδ+. In contrast, the AuNPs/GCE and GCS/GCE is not showing appreciable oxidation peaks for the hydrazine. Besides, we compared the activity with bare GCE which not showing the appreciable electrocatalytic activity for the hydrazine oxidation (Fig. S3). Therefore, we claim this reaction as a synergistic effect of AuNPs and GCS. The rationale behind this high activity of GCS-AuNPs/GCE laid on the threephase boundary interaction with hydrazine. Herein, the citrate ions provide a negative charge to the AuNPs (through the COO−) and GCS provides the good electric and ionic conductivity. This platform acts as a good adsorbent and welcomes the adsorptive (N2H4) to the complex formation (COO–H–HN-NH2). Later, this complex gets oxidize at the surface of GCS-AuNPs/GCE which confirmed by the scan rate studies. Fig. 3c shows the CV responses of hydrazine oxidation at GCSAuNPs/GCE for the different scan rates from 10 to 300 mV/s. Herein, the oxidation peak current increased with increasing the scan rates from 10 to 300 mV/s which follow the linearity over the scan rates. The observed result showed the linear regression of Ipa (μA) = 0.265 ±
N2 H 4 + H2 O N2 H3 + 3H2 O
N2 H3 + H3 O+ + e (Slow) N2 + 3H3
O+
+ 3e (Fast)
(1) (2)
These reactions are the typical hydrazine oxidation mechanism in the alkaline solution. The GCS-AuNPs/GCE also following the similar mechanism however we presume that some surplus adduct formation also influences the reaction kinetics. In most cases, the oxidation peak potentials shifted to the positive direction due to the diffusion layer properties. In our case, the peak potentials are shifted to the negative direction for the scan rates 10 to 100 then it is stabilized (Fig. 3d). In such slow scan rates, most of the hydrazine molecules initially adsorbed on the electrode surface and formed as adduct with carboxylate ions. This complex formation slows down the electrochemical reaction and takes more overpotential to oxidize the hydrazine. To further investigate the complex formation, the pH studies evaluated for the hydrazine oxidation. Fig. 4a showed the CV responses of hydrazine oxidation at GCSAuNPs/GCE recorded in various pH solutions containing 200 μM hydrazine. The oxidation peak current of hydrazine increased with increasing the pH of the electrolytes and it reaches the maximum value at 4
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Fig. 2. XPS survey (a), high resolution XPS spectra of Au 4f (b), C 1s (c) and O1s (d) of GCS-AuNPs.
pH 13. Moreover, the oxidation peak potential of hydrazine is shifted to negative potential when increasing the pH of the solution. The hydrazine has the alkaline pH (pKa ~ 8.0) while dissolved in water thus it can be protonated in the pHs < 9. On the other hand, the isoelectric point of citrated stabilized AuNPs is shown about pH 4.5. Therefore, hydrazine oxidation at AuNPs surface has taken more activation energy to oxidize the hydrazine at its protonation form. As a result, the electrochemical behavior of hydrazine oxidation increased with the increase in pH. Moreover, the effective oxidation potential of hydrazine at AuNPs in pH 7 is ~+0.2 V vs Ag/AgCl, some of the active molecules such as dopamine, catechol, ascorbic acid, and hydroquinone can interfere with the detection of hydrazine in this potential. Eliminating such interferences in pH 7 is debatable; thus, we have considered investigating the electrocatalytic properties of hydrazine in pH 13 by utilizing citrate stabilized AuNPs decorated graphenic carbon spheres. Fig. 4b shows the plot of oxidation peak potential vs various pH which reveals the slope of −78.8 mV. This slope value discloses that the oxidation of hydrazine reveals more than one protons coupled one electron transfer in the hydrazine oxidation reaction. This more than one proton to one electron transfer evinced that some of the protons from the citrate ions inevitably participate in the reactions. Not only the citrate ions, but the GCS also participates through its hydroxyl ions however the possibility is high for citrate ligands. Therefore, this study concluded that the hydrazine forms a complex with citrate ions which could be sped up the reactions. In addition, the catalytic rate constant
(kcat) was scrutinized by the chronoamperometry study by utilizing the following equation [41–43]:
Ic /IL =
1/2
(k C t)1/2
(3)
where the Ic and IL are the catalytic oxidation currents of GCS-AuNPs/ GCE in presence and absence of hydrazine with the concentration of 200 μM. The calculated kcat was 2.137 × 103 M−1 s−1 which was obtained from the slope of the Ic/IL vs t1/2 (Fig. S2). Therefore, this overall electrochemical study states that the citrate stabilized GCS-AuNPs is the good electrode material for the selective detection of hydrazine in the alkaline environment. Hence, this electrode material is used to further investigating their sensitivity and selectivity towards the hydrazine detection. 3.4. Sensitivity and selectivity Differential pulse voltammetry (DPV) is used to determine the hydrazine in the 0.1 M KOH solution. Fig. 4c shows the DPV responses of hydrazine oxidation at GCS-AuNPs/GCE with varying the concentration of hydrazine from 0.02 μM to 1269 μM. The oxidation peak current increased with increasing the concentration of hydrazine. Conversely, the oxidation peak potential was slightly shifted towards positive potential when increasing the concentration which may cause by the more adsorption of high concentration of the analyte between the electrodeelectrolyte interfaces. From the DPV studies, we observed the 5
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Fig. 3. CV responses of all modified GCEs in absence (a) and presence (b) of 200 μM hydrazine, in 0.1 M KOH solution (pH 13) at 50 mV/s. (c) Scan rate studies of hydrazine oxidation on GCS-AuNPs/GCE ranging from 10 mV/s to 300 mV/s. The plot againts the Ipa vs ν (inset) and Epa vs ν (d).
determination of hydrazine regarding linear concentration and limit of detection (LOD). Fig. 4c (inset) shows the calibration plot of hydrazine determination which revealed the linear concentration range from 0.02 μM to 530.1 μM. From this data, the sensitivity and LOD of GCSAuNPs/GCE were calculated as 4.45 ± 0.1 μA μM−1 cm−2 and 6 nM towards the electrochemical detection of hydrazine. This analytical performance of the GCS-AuNPs/GCE was compared with the previously reported hydrazine sensors based on the gold and their composite materials. So far, numerous materials are reported for hydrazine sensor; however, we ensue only Au/Gr/GCE based compounds, shown in Table 1. The performance of the projected material was compared with the peak potential, LOD, linear range and sensitivity of other reported materials, which shows the comparable performance with the other reported sensor materials. Selectivity is quite essential in the GCS-AuNPs/GCE hydrazine sensor. Hence the interference study was evaluated by possible interfering ions. The DPV study was evaluated to find out the selectivity of hydrazine oxidation with other interferents which oxidation potential closer to that and some common ions. Most commonly hydroquinone (HQ), catechol (CC), dopamine (DA) and ascorbic acid (AA) almost has a similar oxidation potential. Fig. 4d displays the DPV response of the hydrazine oxidation with the 5 fold excess of HQ, CC, DA, AA and the 10 fold excess of Ni2+, Fe2+, NO2−, Cu2+, Br−, SO42− ions. These investigated interference signals were plotted against the response signal of hydrazine. Fig. 4d (inset) shows the interference signal of aforementioned compounds which revealed less than ± 5%
interference signals corresponding to the hydrazine response signal. From this study, we concluded that GCS-AuNPs/GCE was not exhibited significant interference effect for the interfering compounds. Therefore, the GCS-AuNPs/GCE can be more suitable for the determination of hydrazine in practical applications. 3.5. Practical applicability Considering practical applicability, GCS-AuNPs/GCE hydrazine sensor is investigated towards the reusability and reproducibility. The reproducibility is performed towards the oxidation of hydrazine in 0.1 M KOH solution containing 200 μM hydrazine by DPV for 5 different modified electrodes. This experiment revealed similar results with slight variations. For the reproducibility, these variations showed the relative standard deviations (RSD) of 3.25% (n = 5). However, the reusability of same GCS-AuNPs/GCE is not given a similar result for 20 consecutive measurements. The retention peak current is decreased by about 20% from its initial value. Therefore, the GCS-AuNPs can be used as a viable electrode for hydrazine sensor. For the real sample analysis, the hydrazine was determined in tap water (spiked) and pond water (spiked) [45]. The observed results showed that more than 98% of recoveries for the analyzed samples (Table 2). These overall results certified that GCS-AuNPs can be a promising electrode material for the hydrazine determination.
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Fig. 4. (a) CVs of hydrazine oxidation in pH 3 to pH 13 (b) Linear plot of Ep vs pH. (c) DPV responses of GCS-AuNPs/GCE in the presence of hydrazine in various concentrations (0.02 μM to 1269 μM) inset: the plot of hydrazine oxidation peak current vs different hydrazine concentrations. (d) DPV curves of hydrazine oxidation with the 5 fold excess of HQ, CC, DA, AA and the 10 fold excess of Ni2+, Fe2+, NO2−, Cu2+, Br−, SO42− ions, inset the corresponding signals of the interference.
Acknowledgements
Table. 2 Determination of hydrazine in real samples by GCS-AuNPs/GCE. Source
Samples
Added (μM)
Founda,b (μM)
Recovery (%)
Tap water
1 2 3 1 2 3
2.00 5.00 10.00 2.00 5.00 10.00
1.98 ± 0.01 4.97 ± 0.02 9.96 ± 0.02 1.97 ± 0.01 4.99 ± 0.02 10.01 ± 0.02
99.0 99.4 99.6 98.5 99.8 100.1
Pond water
a b
This project was supported by the Ministry of Science and Technology, Taiwan, ROC. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.104234.
Standard addition method. Standard deviation for five measurements.
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4. Conclusions We have prepared the GCS-AuNPs by hydrothermal carbonization assisted reduction method. The prepared materials have thoroughly studied by Raman, HRTEM, XPS and CV analysis. This GCS-AuNPs/GCE applied to the investigation of hydrazine oxidation in pH 13 and it revealed a selective determination of hydrazine in alkaline environment. Also, GCS-AuNPs/GCE has shown good stability in 0.1 M KOH solution. Hence, we found that the citrate ligand have done the crucial role in the stabilization of AuNPs and also in the determination of hydrazine. Moreover, we applied GCS-AuNPs/GCE to the real time applications and received acceptable performances.
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Citrate stabilized gold nanoparticles on graphenic carbon spheres for the selective detection of hydrazine Annamalai Yamuna, Periyasamy Sundaresan, Shen-Ming Chen
T
⁎
Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No.1, Section 3, ChungHsiao East Road, Taipei 106, Taiwan, ROC
ARTICLE INFO
ABSTRACT
Keywords: Hydrothermal carbonization Citrate ligand Alkaline environment XPS spectra of gold CV of gold
We have studied the electrocatalytic properties of hydrazine in the alkaline environment by using the stable sensor matrix. Thereby, we have selected the citrate-capped gold nanoparticles (AuNPs) decorated graphenic carbon spheres (GCS). The hydrothermal carbonization method is utilized to prepare the GCS by using reduced graphene oxide (rGO) and glucose. In hydrothermal treatment, the glucose undergoes polycondensation and simultaneously reduced the rGO which results in spherical morphology. These spheres consist of a carbonized nucleus and a hydrophilic surface that facilitates the formation of citrate-capped AuNPs on its surface. This novel composite provides the stable platform to determine the hydrazine in pH 13. Various physicochemical techniques confirm the formation of GCS-AuNPs and also characterized by electrochemical techniques. This GCSAuNPs modified GCE has showed the good electrochemical performance to the hydrazine oxidation. Also, it revealed the acceptable analytical performance to the determination of hydrazine. The estimated linear concentration range, limit of detection and sensitivity are 0.02 to 530.1 μM, 6 nM and 4.45 μA μM−1 cm−2. Moreover, GCS-AuNPs/GCE shows good reproducibility and good recovery in real sample analysis.
1. Introduction Hydrazine is an essential inorganic chemical that broadly utilized in chemical industries as the reducing agent and also employed in fuel cell, corrosion inhibitors, pharmaceutical, agriculture, bioimaging fields [1–3], fuel propellant [4], military [5] and aerospace applications [6]. Due to colorless and water miscibility, hydrazine can cause harmful effects to the humans through the contaminated water bodies. The most considerable health issues of hydrazine are a mutagen, carcinogenic and hepatotoxic impacts [7–9]. Moreover, it can also affect the central nervous system, lungs, liver and kidneys [10]. Hence, the United States environmental protection agency (EPA) characterized hydrazine to be the hazardous environmental toxin and human carcinogen [11]. Also, the China National Standards limits the hydrazine in water sources (GB 18061-2000) about 0.02 mg/L (permissible limit) [12]. Therefore, it is essential to monitor the hydrazine level in water bodies, especially in drinking water. Accordingly, different methods have been developed to determine the hydrazine, e.g. spectrophotometry, fluorescence probe, electrochemical method, colorimetry and chromatography [13–17], in which the electrochemical technique provides the most promising platform to detect the hydrazine. However, the electrochemical method is lagging from its fabrication of the working electrode due to sensitivity ⁎
and selectivity. Till now, many metal nanoparticles decorated highly conductive carbon materials based working electrodes have been reported to enhance the sensing abilities though still a lack in its selectivity. Especially, the gold nanoparticles (AuNPs) have utilized to detect the hydrazine molecule because it has less overpotential and good sensitivity towards hydrazine oxidation. However, the shape and microstructures of AuNPs influence the electrocatalytic activity [18]. Thereby, many researchers have reported different synthesis protocol to prepare the AuNPs with high conductive support. Most of the reported hydrazine sensors have studied in almost pH 7, but the aqueous hydrazine has the alkaline pH (pKa ~ 8.0). Moreover, the effective oxidation potential of hydrazine at AuNPs in pH 7 is ~+0.2 V vs Ag/AgCl (Table 1), some of the active molecules such as dopamine, catechol, ascorbic acid, and hydroquinone can interfere with the detection of hydrazine in this potential [19]. Eliminating such interferences in pH 7 is debatable; thus, we have considered investigating the electrocatalytic properties of hydrazine in pH 13 by utilizing citrate stabilized AuNPs decorated graphenic carbon spheres. Herein, the graphenic carbon spheres are providing good conductivity and support to the AuNPs and citrate provides the negative surface charge to the AuNPs which certainly avoids the aggregation of AuNPs. Moreover, the citrate ligand
Corresponding author. E-mail address: [email protected] (S.-M. Chen).
https://doi.org/10.1016/j.microc.2019.104234 Received 22 April 2019; Received in revised form 31 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Comparison of the performance of GCS-AuNPs/GCE hydrazine sensor with other hydrazine sensors. Electrode
Method
pH
Ep (V)
Linear range (μM)
LOD (μM)
Ref
GNPS/Ch/GCE AuNG/CMG/GCE Au/ppy/GCE Au@Pd/CB-DHP/GCE Ac/Au/GCE AuPD NPst/GR/GCE AuCuNPs/GN/IL/GCE AuNPs/ERGO/Au AG/Au-NPs/SPCE AuNPs/Poly(BCP)/ CNTs/GCE Au-Cu/NPz/CPE Ni-DAP/Au-Pt NPs/NFs/GCE GCS-AuNPs/GCE
LSV Amperometry DPV Amperometry LSV Amperometry Amperometry DPV Amperometry LSV
7.4 7.4 7.0 10.0 7.0 6.0 6.8 8.0 7.0 10
0.15 0.17 0.135 0.15 0.2 0.2 0.15 0.087 0.15 −0.028
5–500 6–30 1–500 2.5–88.0 1–291 2–185 0.2–110 4–1000 0.002–936 0.5–1000
0.1 0.5 1.0 1.77 0.005 0.2 0.1 0.074 0.00057 0.1
[29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
Amperometry Amperometry DPV
7.4 13 13
0.22 0.35 −0.17
10–2000 0.2–85 0.02–530.1
0.043 0.1 0.006
[39] [40] This work
holds some of partially charged Au ions in the composite that could facilitate the better electrochemical activity.
2.5. Fabrication of electrode GCS-AuNPs/GCE Before to the electrode modification, the GC electrode was well polished with alumina slurry paste (0.05 μm) and rinsed thoroughly with DD water. Then the cleaned electrode was dried at room temperature. 5 mg of GCS-AuNPs re-dispersed in 1 mL of water and sonicated for an hour. From this dispersed solution, 7 μl was taken and drop casted on the surface of the GCE and allowed to dry at room temperature. This GCS-AuNPs modified electrode was used for the further electrochemical studies.
2. Experimental 2.1. Materials Graphite, hydrazine, auric chloride (HAuCl4), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4) and other chemicals obtained from Alfa Aesar, Sigma-Aldrich, Fluka and Wako chemicals and used as received.
3. Results and discussions
2.2. Methods
3.1. Material characterizations
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using CHI 1205 and CHI 900 electrochemical workstations. Raman spectra were recorded using a Raman spectrometer, Dong Woo 500i, Korea, equipped with a charge-coupled detector. Conventional three-electrode system used for the electrochemical experiments where the modified glassy carbon electrode (GCE; working area = 0.07 cm2) used as a working electrode, saturated Ag/AgCl used as a reference electrode and platinum electrode used as the counter electrode.
Previous reports clearly documented the hydrothermal carbonization of glucose and their mechanism. Briefly, the dehydration of glucose undergoes a hexamethyl furfural formation then it will convert to carbon spheres by the decarboxylation and polymerization [22]. Herein, we just introduced the reduced graphene oxide to prepare the graphenic carbon spheres. Under the hydrothermal condition, the glucose carbonization was simultaneously reduced the rGO and wrapped the rGO sheets. In this case, the glucose diffused into the rGO sheets and formed the sphere like morphology. Later, the small nanospheres of glucose carbon were aggregated on the rGO surface and grown as microspheres. These carbon spheres consist of a carbonized nucleus (reduced graphene oxide sheets with the aromatic carbon network (inside)) and a hydrophilic surface (hydroxyl groups contained amorphous network). Fig. 1a & b shows the HRTEM images of GCS-AuNPs which revealed the spherical carbon particles with the average diameter of 600 ± 50 nm and the AuNPs anchored on the surface of the GCS. The SAED pattern of GCS-AuNPs showed in Fig. 1c, which endorsed that the amorphous-like ring pattern for the GCS and polycrystalline pattern for the AuNPs. Fig. 1d shows the Raman spectra of GCS-AuNPs, GCS and rGO wherein two typical peaks observed for all compounds. Those two peaks are corresponding to the graphitic peak (G band) and disordered peak (D band) [23,24] which centered at 1591 cm−1 and 1340 cm−1 for rGO, 1597 cm−1 and 1368 cm−1 for GCS, 1596 cm−1 and 1332 cm−1 for GCS-AuNPs, respectively. The relative intensity ratios of D and G band (ID/IG) are 1.38, 0.55 and 0.76 for rGO, GCS and GCSAuNPs respectively. In general, the ID/IG ratio is used to identify the defects and disorders in carbon lattices. Obviously, the rGO shows higher ID/IG ratio because it has large defects and it considerably reduced when converted to GCS. That means the glucose carbonization reaction simultaneously reduced the oxygen functionalities of rGO which results in more graphitic band. However, ID/IG ratio of GCSAuNPs increased to 0.76 that is because of the incorporation of citrate
2.3. Preparation of GCS-AuNPs 4 g of glucose dissolved in 40 mL of water and 0.2 g of reduced graphene oxide was dispersed in the glucose solution by ultra-sonication for an hour. This dispersion is hydrothermally treated for 4 h at 180 °C, which yields the blackish brown precipitate. Then, the precipitate washed with water until the impurities removed then dried at 40 °C, which named as GCS [20]. Afterwards, 0.2 g of GCS dispersed in 40 mL of DD water then 0.02 g of HAuCl4 was added into the GCS solution with the continuous stirring for an hour and the adequate amount of sodium citrate added into that solution [21]. Next, this mixture was kept in the hydrothermal condition at 180 °C for 2 h, herein, the gold ions reduced into gold nanoparticles (AuNPs) as shown in Scheme 1. Finally, the formed GCS-AuNPs was collected by centrifugation and washed with DD water and ethanol. 2.4. Sampling procedure The tap water and pond water samples were collected randomly from the five different sites in Taipei, Taiwan. The samples are filtered through the filter membrane (pore size 0.45 μm) and directly used for the real sample analysis. The known concentrations of the hydrazine was added to the real samples and examined by standard addition method. 2
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Scheme 1. Schematic representation of the preparation of GCS-AuNPs.
ligands. The Raman spectrum of GCS-AuNPs shows the sharp peak centered at 1431 cm−1 for the symmetric stretching vibration of carboxylate ion and the weak peak at 1633 cm−1 for the asymmetric stretching vibration of carboxylate ion [25]. This citrate ligand intensity considerably increases the D band intensity of GCS thus the overall intensity is increased which produce high ID/IG ratio. This result confirmed that the citrate ligand is highly coordinated with the AuNPs with the carboxylate ion and also it is bonded with GCS by the nucleophilic addition. Fig. 2a shows the XPS survey spectrum of GCS-AuNPs, where the peaks at 85, 286, 499 and 533 eV indicated the presence of Au, C, Na and O. The strong auger lines of Na KLL inferred that the citrate ions are adsorbed on the surface of AuNPs. The high-resolution spectrum of Au is deconvoluted into four peaks wherein the sharp two peaks at 84.0 eV and 87.7 eV corresponds to the Au 4f7/2 and 4f5/2 respectively, which confirms the metallic formation of Au which shown in Fig. 2b. Additionally, the remaining two peaks appeared at 84.9 and 88.6 eV which corresponds to the partially charged Au ions. This behavior insisted that some of Au ions are capped by the citrate ligands on the surface of AuNPs [21]. Moreover, the C 1s high resolution spectrum was deconvoluted into four major peaks at 285.2, 286.1, 287.2 and 289.1 (Fig. 2c). The peaks at 285.2, 286.1 and 287.25 eV corresponds to the CeC, CeO and C]O respectively. These peaks mainly arise from the graphenic carbon spheres and the peak at and 289.01 eV attributed to the O-C=O. Mostly, the CeO and O-C=O peaks mentioned that the alcoholic and carboxylate groups of citrate ions. Fig. 2d shows the O 1s spectrum which further deconvoluted into three peaks centered at 532.8, 534.3 and 536.4 eV. The peaks at 534.3 and 536.4 eV corresponds to the CeO and O-C=O bond, which further confirmed the presence of citrate ions on AuNPs. These comprehensive studies confirmed the formation of citrate capped AuNPs on graphenic carbon spheres. This GCS-AuNPs modified GCE further applied to the electrochemical characterization towards hydrazine.
3.2. Electrochemical characterization of modified electrodes The electrochemical behavior of GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE and bare GCE were characterized by using [Fe(CN)6]3−/4− probe which shown in (Fig. S1.a–d). The GCS-AuNPs/GCE showed high catalytic activity when compared to the other electrodes. These electrocatalytic activities are described in terms of electron transfer rate, conductivity, peak to peak separation potential (ΔEp) and electrochemical active surface area. The calculated electrochemical active surface areas are 0.137, 0.07, 0.043, 0.035 cm2 for GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE, and bare GCE, respectively which was calculated from the scan rate studies in [Fe(CN)6]3−/4− (see the Supplementary information) [44]. However, all the modified GCEs are shown some electrochemical activity in Fe(CN)63−/4− probe. So, it is required to characterize the effectual heterogeneous rate constant (keffo) [24]. The calculated keffo values are 3.6 × 10−3, 1.3 × 10−3, 3.2 × 10−4, 1.2 × 10−3 cm s−1 for the GCS-AuNPs/GCE, AuNPs/GCE, GCS/GCE and bare GCE, respectively [23,24], (For the detailed information see the supplementary). 3.3. Electrochemical oxidation of hydrazine The electrochemical behaviors of hydrazine oxidation on modified GCEs were investigated by CV technique by sweeping the potential from −0.6 V to 0.6 V vs Ag/AgCl in 0.1 M KOH solution at a scan rate of 50 mV/s. Fig. 3a & b shows the CVs of GCS-AuNPs/GCE, AuNPs/GCE and GCS/GCE in the absence and presence of 200 μM hydrazine. In the absence of hydrazine, GCS-AuNPs/GCE shows two redox couples centered at −0.34 V for the GCS and +0.28 V for the citrated capped AuNPs, respectively. The first redox couple (O1/R1; −0.28/−0.41 V) associated with the carbonyl functional group in the carbon matrix (GCS) which mainly arises from the catechol or hydroquinone moiety [26]. The second redox couple (O1/R1; +0.36/+0.17 V) corresponds to the redox behavior of citrate capped AuNPs wherein the partially charged Au ions undergo the oxidation and reduction reaction [27]. In 3
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Fig. 1. Low magnification (a), high magnification (b) TEM images and SAED pattern (c) of GCS-AuNPs. (d) Raman spectra of rGO, GCS and GCS-AuNPs.
0.01 μA mV−1s + 41.28 ± 0.05 μA (R2 = 0.9943). This behavior suggested that the electrochemical oxidation of hydrazine on the GCSAuNPs/GCE followed the adsorption controlled process (Fig. 3c, inset). In general, the oxidation of hydrazine follows two-step reactions; the first step involves with the slow one electron transfer (rate determining step) and second step proceeds with fast three electrons transfer [28].
contrary, this redox peak does not appear for the AuNPs, which evinced that the citrate capped AuNPs have some partially charged Au ions (Auδ+) which facilitates the selective oxidation of hydrazine. On the other hand, the GCS/GCE shows the capacitance behavior with weak oxidation and reduction peaks for the surface bound oxygen functionalities such as carbonyl, ester or carboxylic acid. In the presence of hydrazine, GCS-AuNPs/GCE reveals the sharp well defined irreversible oxidation peak at −0.176 V for the hydrazine oxidation and weak redox peak at +0.34 V for the redox of citrate capped Auδ+. In contrast, the AuNPs/GCE and GCS/GCE is not showing appreciable oxidation peaks for the hydrazine. Besides, we compared the activity with bare GCE which not showing the appreciable electrocatalytic activity for the hydrazine oxidation (Fig. S3). Therefore, we claim this reaction as a synergistic effect of AuNPs and GCS. The rationale behind this high activity of GCS-AuNPs/GCE laid on the threephase boundary interaction with hydrazine. Herein, the citrate ions provide a negative charge to the AuNPs (through the COO−) and GCS provides the good electric and ionic conductivity. This platform acts as a good adsorbent and welcomes the adsorptive (N2H4) to the complex formation (COO–H–HN-NH2). Later, this complex gets oxidize at the surface of GCS-AuNPs/GCE which confirmed by the scan rate studies. Fig. 3c shows the CV responses of hydrazine oxidation at GCSAuNPs/GCE for the different scan rates from 10 to 300 mV/s. Herein, the oxidation peak current increased with increasing the scan rates from 10 to 300 mV/s which follow the linearity over the scan rates. The observed result showed the linear regression of Ipa (μA) = 0.265 ±
N2 H 4 + H2 O N2 H3 + 3H2 O
N2 H3 + H3 O+ + e (Slow) N2 + 3H3
O+
+ 3e (Fast)
(1) (2)
These reactions are the typical hydrazine oxidation mechanism in the alkaline solution. The GCS-AuNPs/GCE also following the similar mechanism however we presume that some surplus adduct formation also influences the reaction kinetics. In most cases, the oxidation peak potentials shifted to the positive direction due to the diffusion layer properties. In our case, the peak potentials are shifted to the negative direction for the scan rates 10 to 100 then it is stabilized (Fig. 3d). In such slow scan rates, most of the hydrazine molecules initially adsorbed on the electrode surface and formed as adduct with carboxylate ions. This complex formation slows down the electrochemical reaction and takes more overpotential to oxidize the hydrazine. To further investigate the complex formation, the pH studies evaluated for the hydrazine oxidation. Fig. 4a showed the CV responses of hydrazine oxidation at GCSAuNPs/GCE recorded in various pH solutions containing 200 μM hydrazine. The oxidation peak current of hydrazine increased with increasing the pH of the electrolytes and it reaches the maximum value at 4
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Fig. 2. XPS survey (a), high resolution XPS spectra of Au 4f (b), C 1s (c) and O1s (d) of GCS-AuNPs.
pH 13. Moreover, the oxidation peak potential of hydrazine is shifted to negative potential when increasing the pH of the solution. The hydrazine has the alkaline pH (pKa ~ 8.0) while dissolved in water thus it can be protonated in the pHs < 9. On the other hand, the isoelectric point of citrated stabilized AuNPs is shown about pH 4.5. Therefore, hydrazine oxidation at AuNPs surface has taken more activation energy to oxidize the hydrazine at its protonation form. As a result, the electrochemical behavior of hydrazine oxidation increased with the increase in pH. Moreover, the effective oxidation potential of hydrazine at AuNPs in pH 7 is ~+0.2 V vs Ag/AgCl, some of the active molecules such as dopamine, catechol, ascorbic acid, and hydroquinone can interfere with the detection of hydrazine in this potential. Eliminating such interferences in pH 7 is debatable; thus, we have considered investigating the electrocatalytic properties of hydrazine in pH 13 by utilizing citrate stabilized AuNPs decorated graphenic carbon spheres. Fig. 4b shows the plot of oxidation peak potential vs various pH which reveals the slope of −78.8 mV. This slope value discloses that the oxidation of hydrazine reveals more than one protons coupled one electron transfer in the hydrazine oxidation reaction. This more than one proton to one electron transfer evinced that some of the protons from the citrate ions inevitably participate in the reactions. Not only the citrate ions, but the GCS also participates through its hydroxyl ions however the possibility is high for citrate ligands. Therefore, this study concluded that the hydrazine forms a complex with citrate ions which could be sped up the reactions. In addition, the catalytic rate constant
(kcat) was scrutinized by the chronoamperometry study by utilizing the following equation [41–43]:
Ic /IL =
1/2
(k C t)1/2
(3)
where the Ic and IL are the catalytic oxidation currents of GCS-AuNPs/ GCE in presence and absence of hydrazine with the concentration of 200 μM. The calculated kcat was 2.137 × 103 M−1 s−1 which was obtained from the slope of the Ic/IL vs t1/2 (Fig. S2). Therefore, this overall electrochemical study states that the citrate stabilized GCS-AuNPs is the good electrode material for the selective detection of hydrazine in the alkaline environment. Hence, this electrode material is used to further investigating their sensitivity and selectivity towards the hydrazine detection. 3.4. Sensitivity and selectivity Differential pulse voltammetry (DPV) is used to determine the hydrazine in the 0.1 M KOH solution. Fig. 4c shows the DPV responses of hydrazine oxidation at GCS-AuNPs/GCE with varying the concentration of hydrazine from 0.02 μM to 1269 μM. The oxidation peak current increased with increasing the concentration of hydrazine. Conversely, the oxidation peak potential was slightly shifted towards positive potential when increasing the concentration which may cause by the more adsorption of high concentration of the analyte between the electrodeelectrolyte interfaces. From the DPV studies, we observed the 5
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Fig. 3. CV responses of all modified GCEs in absence (a) and presence (b) of 200 μM hydrazine, in 0.1 M KOH solution (pH 13) at 50 mV/s. (c) Scan rate studies of hydrazine oxidation on GCS-AuNPs/GCE ranging from 10 mV/s to 300 mV/s. The plot againts the Ipa vs ν (inset) and Epa vs ν (d).
determination of hydrazine regarding linear concentration and limit of detection (LOD). Fig. 4c (inset) shows the calibration plot of hydrazine determination which revealed the linear concentration range from 0.02 μM to 530.1 μM. From this data, the sensitivity and LOD of GCSAuNPs/GCE were calculated as 4.45 ± 0.1 μA μM−1 cm−2 and 6 nM towards the electrochemical detection of hydrazine. This analytical performance of the GCS-AuNPs/GCE was compared with the previously reported hydrazine sensors based on the gold and their composite materials. So far, numerous materials are reported for hydrazine sensor; however, we ensue only Au/Gr/GCE based compounds, shown in Table 1. The performance of the projected material was compared with the peak potential, LOD, linear range and sensitivity of other reported materials, which shows the comparable performance with the other reported sensor materials. Selectivity is quite essential in the GCS-AuNPs/GCE hydrazine sensor. Hence the interference study was evaluated by possible interfering ions. The DPV study was evaluated to find out the selectivity of hydrazine oxidation with other interferents which oxidation potential closer to that and some common ions. Most commonly hydroquinone (HQ), catechol (CC), dopamine (DA) and ascorbic acid (AA) almost has a similar oxidation potential. Fig. 4d displays the DPV response of the hydrazine oxidation with the 5 fold excess of HQ, CC, DA, AA and the 10 fold excess of Ni2+, Fe2+, NO2−, Cu2+, Br−, SO42− ions. These investigated interference signals were plotted against the response signal of hydrazine. Fig. 4d (inset) shows the interference signal of aforementioned compounds which revealed less than ± 5%
interference signals corresponding to the hydrazine response signal. From this study, we concluded that GCS-AuNPs/GCE was not exhibited significant interference effect for the interfering compounds. Therefore, the GCS-AuNPs/GCE can be more suitable for the determination of hydrazine in practical applications. 3.5. Practical applicability Considering practical applicability, GCS-AuNPs/GCE hydrazine sensor is investigated towards the reusability and reproducibility. The reproducibility is performed towards the oxidation of hydrazine in 0.1 M KOH solution containing 200 μM hydrazine by DPV for 5 different modified electrodes. This experiment revealed similar results with slight variations. For the reproducibility, these variations showed the relative standard deviations (RSD) of 3.25% (n = 5). However, the reusability of same GCS-AuNPs/GCE is not given a similar result for 20 consecutive measurements. The retention peak current is decreased by about 20% from its initial value. Therefore, the GCS-AuNPs can be used as a viable electrode for hydrazine sensor. For the real sample analysis, the hydrazine was determined in tap water (spiked) and pond water (spiked) [45]. The observed results showed that more than 98% of recoveries for the analyzed samples (Table 2). These overall results certified that GCS-AuNPs can be a promising electrode material for the hydrazine determination.
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Fig. 4. (a) CVs of hydrazine oxidation in pH 3 to pH 13 (b) Linear plot of Ep vs pH. (c) DPV responses of GCS-AuNPs/GCE in the presence of hydrazine in various concentrations (0.02 μM to 1269 μM) inset: the plot of hydrazine oxidation peak current vs different hydrazine concentrations. (d) DPV curves of hydrazine oxidation with the 5 fold excess of HQ, CC, DA, AA and the 10 fold excess of Ni2+, Fe2+, NO2−, Cu2+, Br−, SO42− ions, inset the corresponding signals of the interference.
Acknowledgements
Table. 2 Determination of hydrazine in real samples by GCS-AuNPs/GCE. Source
Samples
Added (μM)
Founda,b (μM)
Recovery (%)
Tap water
1 2 3 1 2 3
2.00 5.00 10.00 2.00 5.00 10.00
1.98 ± 0.01 4.97 ± 0.02 9.96 ± 0.02 1.97 ± 0.01 4.99 ± 0.02 10.01 ± 0.02
99.0 99.4 99.6 98.5 99.8 100.1
Pond water
a b
This project was supported by the Ministry of Science and Technology, Taiwan, ROC. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.104234.
Standard addition method. Standard deviation for five measurements.
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4. Conclusions We have prepared the GCS-AuNPs by hydrothermal carbonization assisted reduction method. The prepared materials have thoroughly studied by Raman, HRTEM, XPS and CV analysis. This GCS-AuNPs/GCE applied to the investigation of hydrazine oxidation in pH 13 and it revealed a selective determination of hydrazine in alkaline environment. Also, GCS-AuNPs/GCE has shown good stability in 0.1 M KOH solution. Hence, we found that the citrate ligand have done the crucial role in the stabilization of AuNPs and also in the determination of hydrazine. Moreover, we applied GCS-AuNPs/GCE to the real time applications and received acceptable performances.
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