Palladium Nanoparticles Decorated on Reduced Graphene Oxide

Nov 13, 2014 - Although metal nanoparticle/graphene composites have been widely used as the electrode in electrochemical sensors, two effects, consist...
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Palladium Nanoparticles Decorated on Reduced Graphene Oxide Rotating Disk Electrodes toward Ultrasensitive Hydrazine Detection: Effects of Particle Size and Hydrodynamic Diffusion Atiweena Krittayavathananon,† Pattarachai Srimuk,† Santamon Luanwuthi,† and Montree Sawangphruk*,†,‡ †

Department of Chemical Engineering, Faculty of Engineering, ‡Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: Although metal nanoparticle/graphene composites have been widely used as the electrode in electrochemical sensors, two effects, consisting of the particle size of the nanoparticles and the hydrodynamic diffusion of analytes to the electrodes, are not yet fully understood. In this work, palladium nanoparticles/reduced graphene oxide (PdNPs/rGO) composites were synthesized using an in situ polyol method. Palladium(II) ions and graphene oxide were reduced together with a reducing agent, ethylene glycol. By varying the concentration of palladium(II) nitrate, PdNPs with different sizes were decorated on the surface of rGO sheets. The as-fabricated PdNPs/rGO rotating disk electrodes (RDEs) were investigated toward hydrazine detection. Overall, a 3.7 ± 1.4 nm diameter PdNPs/ rGO RDE exhibits high performance with a rather low limit of detection of about 7 nM at a rotation speed of 6000 rpm and provides a wide linear range of 0.1−1000 μM with R2 = 0.995 at 2000 rpm. This electrode is highly selective to hydrazine without interference from uric acid, glucose, ammonia, caffeine, methylamine, ethylenediamine, hydroxylamine, n-butylamine, adenosine, cytosine, guanine, thymine, and L-arginine. The PdNPs/ rGO RDEs with larger sizes show lower detection performance. Interestingly, the detection performance of the electrodes is sensitive to the hydrodynamic diffusion of hydrazine. The as-fabricated electrode can detect trace hydrazine in wastewater with high stability, demonstrating its practical use as an electrochemical sensor. These findings may lead to an awareness of the effect of the hydrodynamic diffusion of analyte that has been previously ignored, and the 3.7 ± 1.4 nm PdNPs/rGO RDE may be useful toward trace hydrazine detection, especially in wastewater from related chemical industries.

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with respect to the Fermi level, leading to an enhancement of the rate of electrooxidation of molecules, e.g., formic acid on the surface of PdNPs.14 In addition, the adsorption energy of the intermediate, e.g., formate on the surface of PdNPs, is lower than that on the bulk Pd. As a result, it enhances the rate of molecule decomposition or electrooxidation on the surface of the PdNPs.14 However, there are two important issues related to the electrochemical detection of hydrazine. The first is the effect of the particle size of the nanoparticles being used as the electrocatalyst toward hydrazine oxidation. The second is the missing quantitative information on the stirring solution previously used for the electrochemical detection of hydrazine (see previous work in Table 2). Hydrazine was previously detected by means of chronoamperometry in a stirring solution without quantitative information. As a result, it is difficult to compare the performance of the electrode. In this work, PdNPs/reduced graphene oxide (rGO) with different particle sizes was produced by in situ polyol synthesis.

ydrazine is widely used in rocket fuel, photographic chemicals, insecticides, herbicides, emulsifiers, blowing agents, textile dyes, and corrosion inhibitors in various industries.1 However, hydrazine is rather toxic, colorless, and flammable and is classified as a human carcinogen in group B2 by the United States Environmental Protection Agency (US EPA).2 Hydrazine is mutagenic and carcinogenic, which can severely injury lungs, liver, kidneys, brain, and spinal cord.2 As a result, an accurate determination of trace hydrazine discharged to the environment is rather important. There are many methods being used for hydrazine determination such as high-performance liquid chromatography (HPLC), a tri-output optical signal (colorimetric, ratiometric, and chemiluminometric),1 spectrophotometry,3 gas chromatography−mass spectrometry (GC−MS),4 and electrochemical sensors.5,6 Among all techniques being used for detecting hydrazine, electroanalysis is of interest because it has high sensitivity and selectivity, simplicity, and low cost.7−9 However, an electrochemical sensor requires high-performance electrocatalysts. Palladium nanoparticles (PdNPs) are currently of interest as the electrocatalyst due to their excellent catalytic activity, sensitivity, and low overpotential toward hydrazine oxidation.10−13 This is because small PdNPs exhibit a high binding energy shift and high valence band center downshift © XXXX American Chemical Society

Received: September 13, 2014 Accepted: November 13, 2014

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where ILOD is the current change (step) observed at the LOD, Ib̅ is the mean of the blank current, k is a numerical factor, which is equal to 3 and was chosen according to the confidence limit (ca. 1%) desired for the LOD, and Sb is the standard deviation of the blank current.21 Note that k is equal to 10 for the limit of quantification (LOQ).20

The rGO, produced from the reduction process of graphene oxide (GO), is a graphene-like structure with carboxylic groups at the edge of the graphene sheets.15 rGO is more conductive than GO (ca. 225.3 S m−1) but less conductivity than graphene (1738 S m−1).16 A PdNPs/rGO rotating disk electrode (RDE) was, for the first time, fabricated and used to detect trace hydrazine in wastewater. The results showed that a 3.7 ± 1.4 nm diameter PdNPs/rGO RDE exhibits higher performance than that of electrodes with larger sizes. To the best of our knowledge, the as-fabricated electrode in this work also exhibits the lowest limit of detection (LOD) of ca. 7 nM when compared with those from previous reports (see Table 2) based on electrochemical detection. It was also found in this work that the hydrodynamic diffusion of hydrazine to the electrode plays a major role in determining the detection performance of the electrode.

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RESULTS AND DISCUSSION Physical Characterization. The TEM image in Figure 1a shows a few layers of rGO sheets that overlap each other

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EXPERIMENTAL SECTION In Situ Polyol Synthesis of PdNPs/rGO. In brief, 50 mg of GO, produced from Hummers’ method17 with our modification,18 was added to 50 mL of Milli-Q water. To form a homogeneous dispersion (brown solution), the mixture was ultrasonicated for 30 min. Palladium(II) nitrate with different concentrations (i.e., 10, 50, 200, 600, 1000 ppm) was added to the GO suspension. After that, 100 mL of ethylene glycol was added to the mixed suspension, which was then stirred at 100 rpm at 130 °C with nitrogen purging for 4 h. The color of the products turned from brown to black, indicating the complete reduction of both Pd2+ and GO with ethylene glycol. Finally, the PdNPs/rGO colloidal suspension was filtered and washed with Milli-Q water three times. The final products were dried using a vacuum oven at 80 °C overnight. Fabrication of PdNPs/rGO Electrodes. To fabricate an electrochemical sensor electrode, a 3 mm diameter glassy carbon (GC) rotating disk electrode (RDE) was polished with alumina slurry and washed with purified water five times. Then, 5 μL of PdNPs/rGO in acetone (0.63 wt %) was dropped on the GC RDE and left at ambient temperature (25 °C) for 1 h. Structural and Morphological Characterization. The morphology of the PdNPs/rGO composites was characterized using transmission electron microscopy (TEM; TECNAI G2 20, FEI Ltd.). For the preparation of the TEM specimen, the PdNPs/rGO samples were dispersed in methanol before dropping them on a copper grid (CF200-Cu). The particle size distribution was determined from the TEM images.19 The chemical structure and oxidation state of the PdNPs were evaluated from X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Electrochemical Evaluation of Hydrazine Detection. PdNPs/rGO RDEs were used as the electrochemical sensing electrode toward hydrazine oxidation in phosphate buffered saline (PBS) at pH 7.4, which is an auxiliary electrolyte, under ambient temperature (25 °C) using cyclic voltammetry (CV) and chronoamperometry with a computer-controlled μAUTOLAB II potentiostat (Eco-Chemie, Utrecht, The Netherlands) equipped with a FRA2 frequency response analyzer module running GPES/FRA software. By following the IUPAC method,20 the LOD (the smallest concentration of hydrazine that the PdNPs/rGO RDE can detect) was determined as follows (eq 1) ILOD = Ib̅ + kS b

Figure 1. TEM images of (a) rGO and (b−f) PdNPs/rGO with different sizes and inset histograms of the particle size distribution with the average particle sizes of (b) 3.7 ± 1.4 nm, (c) 5.0 ± 1.7 nm, (d) 7.7 ± 1.9 nm, (e) 8.9 ± 1.4 nm, and (f) 10.8 ± 1.8 nm.

because rGO is a metastable material. The van der Waals interaction attractive force among the adjacent rGO sheets leads to restacking of the rGO. The morphology of the PdNPs/ rGO composites in Figure 1b−f is highly dispersed because the coated PdNPs reduce the attractive force among neighboring rGO sheets. By varying the concentration of palladium(II) nitrate precursors, PdNPs with different sizes were decorated on the surface of the rGO sheets. At 10 ppm palladium(II) nitrate, the smallest nanoparticles produced in this work have an average size of 3.7 ± 1.4 nm, which are sitting on rGO’s surface (see the inset histogram of the particle size distribution in Figure 1b). The agglomeration of PdNPs starts to become visible at higher concentrations of palladium(II) nitrate used in the preparation process. PdNPs with sizes of 5.0 ± 1.7, 7.7 ± 1.9, 8.9 ± 1.4, and 10.8 ± 1.8 nm were produced using 50, 200,

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600, and 1000 ppm of palladium(II) nitrate, respectively (see inset histograms in Figure 1c−f). In order to investigate the structure of PdNPs/rGO, the electron diffraction and XRD patterns were determined and are shown in Figure S1 of the Supporting Information. The ring diameter in the electron diffraction pattern (Figure S1a−c) becomes wider when the particle size of the PdNPs increases. This is in agreement with a previous report.22 To further investigate the crystalline structure of PdNPs, the XRD patterns of PdNPs (Figure S1d) revealed that they exhibit a crystalline face centered cubic (fcc) structure with space group Fm3m(225). In addition, all peaks of the XRD patterns become shaper when the particle size increases. These results agree well with the Scherer equation.23 The FTIR spectra in Figure S2 of the Supporting Information also indicate that the functional groups, i.e., carboxylic, epoxide, and hydroxyl, of the GO were reduced after the reduction of GO, providing rGO sheets. Further details of the FTIR and RAMAN spectra of GO and rGO can be found in our previous reports.18,24 FTIR was also used to confirm the structure of the PdNPs/rGO composites (see Figure S2). Impurity of the Pd(NO)3 precursor was not observed in the powdered composite. The Pd L3-edge XANES spectra of the PdNPs/rGO composites (Figure 2) are in association with Pd metal because

Figure 3. (a) Cyclic voltammograms of PdNPs/rGO in 100 mM hydrazine in 0.2 M PBS (pH 7.4) at a scan rate of 0.1 V s−1; (b) chronoamperomograms of PdNPs/rGO in 0.2 M PBS (pH 7.4) with successive injections of hydrazine at different concentrations (0.1− 1000 μM) at a rotation speed of 2000 rpm.

displays an irreversible process. The oxidation reaction mechanism of hydrazinium is as follows27 N2H5+ → N2 + 5H+ + 4e−

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Note that a stable form of the hydrazine in a pH 7.4 solution is hydrazinium (N2H5+), having pKa of 8.1.28 The PdNPs/rGO electrodes with different sizes can detect hydrazinium at oxidation potentials from 0.32 to 0.4 V vs Ag/ AgCl, which is a potential range of hydrazine electrooxidation against PdNPs electrodes.29 Interestingly, it was found that the peak potential toward hydrazinium oxidation depends on the particle size of the PdNPs. When increasing the particle size, the peak potential is shifted to a more positive value or higher overpotentials.30,31 Also, the smaller nanoparticles exhibit a higher specific current density or sensitivity. Herein, the anodic current of the 3.7 ± 1.4 nm PdNPs/rGO at 0.32 V vs Ag/AgCl in 100 mM hydrazinium in PBS (pH 7.4) is about 24.4 A/mg cm2, which is 2.5-, 5.7-, 16.3-, and 30.5-fold higher than that of the electrodes with larger sizes, i.e., 5.0 ± 1.7, 7.7 ± 1.9, 8.9 ± 1.4, and 10.8 ± 1.8 nm PdNPs, respectively. Chronoamperomograms (current vs time) were subsequently used to quantitatively determine the concentration of hydrazine by applying a constant oxidation potential of hydrazine and are shown in Figure 3b. Successive injections with a wide range of hydrazine concentrations from 0 to 1 M in 0.2 M PBS (pH 7.4) were carried out against the 3.7 ± 1.4 nm PdNPs/rGO RDE at a rotation speed of 2000 rpm. It was found that the anodic current observed depends on the particle size of the PdNPs. Increasing the particle size from 3.7 ± 1.4 to

Figure 2. Pd L3-edge XANES spectra of PdNPs/rGO samples and standard bulk Pd foil.

they absorb proton energy near the L3 edge at ca. 3174 eV. Note that the energy was calibrated using data acquired on Pd foil. However, a slight photon energy shift is found when the particle sizes are decreased from 10.8 ± 1.8 to 3.7 ± 1.4 nm. The absorbed energies of 3.7 ± 1.4, 5.0 ± 1.7, 7.7 ± 1.9, 8.9 ± 1.4, and 10.8 ± 1.8 nm PdNPs are observed at 3174.4, 3174.6, 3174.8, and 3175.1 eV, respectively. This is because the lattice of tiny PdNPs is contracted, causing the energy to be raised near the edge.14,25,26 In other words, the bond distance of Pd− Pd is tighter, or shorter, for smaller PdNPs. Electrochemical Evaluation. Effect of Particle Size. Figure 3a shows CV curves of pure rGO and PdNPs/rGO composites coated on GC RDEs in 100 mM hydrazine in 0.2 M PBS (pH 7.4) at a scan rate of 0.1 V s−1. For the PdNPs/rGO electrodes, the result shows sharp peaks relating to the hydrazine oxidation reaction. Note that there is no oxidation peak found in the case of the rGO electrode.15 This indicates that PdNPs play a major role toward hydrazine oxidation. The electrochemical response against the as-fabricated electrodes C

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10.8 ± 1.8 nm reduces the detection performance of the PdNPs/rGO electrode. The 3.7 ± 1.4 nm PdNPs/rGO RDE provides a wider linear range (Figure 4 and Table 1) when

Figure 4. Linear calibration curves (R2 > 0.99) of the PdNPs/rGO RDE with different particle sizes.

Table 1. Summary of Linear Ranges and Limit of Detection (LOD) Values of PdNPs/rGO RDEs with Different Particle Sizes toward Hydrazine Detection at a Rotation Rate of 2000 rpm in 0.2 M PBS (pH 7.4) average particle size (nm)

linear range (μM)

limit of detection (μM)

± ± ± ± ±

0.1−1000 0.1−300 1−1000 10−4000 1−900

0.05 0.08 0.1 0.3 0.5

3.7 5.0 7.7 8.9 10.8

1.4 1.7 1.9 1.4 1.8

Figure 5. (a) Chronoamperomograms of the 3.7 ± 1.4 nm PdNPs/ rGO RDE with various rotation rates (0−6000 rpm) in 0.2 M PBS solution (pH 7.4) with successive injections of hydrazine and (b) linear ranges at different rotation rates.

compared with that of larger size PdNPs/rGO RDEs. Further observation of each injection at, for example, 100 μM shows the same current change, indicating good stability of the PdNPs/ rGO RDE. In addition, the LOD values are 0.05, 0.08, 0.1, 0.3, and 0.5 μM for the PdNPs/rGO electrodes with particle sizes of 3.7 ± 1.4, 5.0 ± 1.7, 7.7 ± 1.9, 8.9 ± 1.4, and 10.8 ± 1.8 nm, respectively. As a result, it can be concluded that reducing the particle size of the PdNPs leads to a higher sensitivity and wider linear range for hydrazine detection. This is likely due to the higher electrochemical active surface area-to-volume ratio as well as to quantum confinement, for which the valence band center of the PdNPs was shifted ca. +0.2 eV vs Pd bulk,32,33 leading to faster charge transfer or lower overpotentials toward the hydrazine oxidation reaction against smaller PdNPs.14 Hydrodynamic Diffusion Effect of Hydrazine. Figure 5a presents chronoamperometric curves of the 3.7 ± 1.4 nm PdNPs/rGO RDE at different concentrations and different rotation speeds (0−6000 rpm). The experiments were tested at an oxidation potential of hydrazinium (ca. 0.32 V vs Ag/AgCl) against the 3.7 ± 1.4 nm PdNPs/rGO RDE with successive injections of hydrazine in 0.2 M PBS (pH 7.4) at ambient temperature (25 °C). The LOD values and linear ranges (Figure 5b) were evaluated at different rotation speeds (0− 6000 rpm) to investigate the effect of the hydrodynamic diffusion of hydrazine. For a rotation rate of 0 rpm, the asfabricated electrode shows a poor response to hydrazine oxidation due to the solution being inhomogeneous. At faster rotation speeds of 500−6000 rpm, the as-fabricated electrode

can detect hydrazine with different linear ranges and LOD values, as listed in Table 2. The LOD values were found to be 0.2, 0.05, 0.01, and 0.007 μM at 500, 2000, 4000, and 6000 rpm, respectively. Note that the inset chronoamperomogram in Figure 5a shows the LOD determined at 6000 rpm. The LOD in this work is much lower than the value of 0.9 μM previously achieved by GO/GCE.15 This shows higher catalytic activity of small PdNPs toward hydrazine oxidation over that for a GO carbocatalyst. In addition to the very low LOD, the linear ranges are also sensitive to the rotation speed of the asfabricated RDE. The linear ranges with R2 > 0.99 are 10−300, 0.1−1000, 0.1−1000, and 0.04−200 μM at 500, 2000, 4000, and 6000 rpm, respectively. Note that, at high rotation rate, the hydrazine solution not only contacts the outer surface of PdNPs/rGO but also can diffuse deeper to react with the PdNPs located between layers of rGO sheets.34 The LOD values and linear ranges for hydrazine detection were compared with those from previous work performed with different detection methods (Table 2). Note that previous work based on chronoamperometry did not provide quantitative information on the stirring solution, leading to difficulty in comparing the performance of the electrode. In this work, the mass transport of hydrazine is a crucial point for determining the electrode’s performance. To the best of our knowledge, the as-fabricated 3.7 ± 1.4 nm PdNPs/rGO RDE in this work D

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Table 2. Comparison of the Detection Performance of Pd-Based Electrodes toward Hydrazine Detection electrodes

techniques

pH of buffer solution

stirring rate

LOD (μM)

linear range (μM)

Pd/CPEa 25 nm Pd-MCVb/GCE Pd-MWCNTs/GCE PEDOPc/MWCNT −Pd/GCE PdNPs-PANI/GCE Cu-Pd/SPEd Pd/PANI- PAMPSA/GCE Pd/carbon microsphere/CNT/GCE LDH-Pd/GCE PdNPs/BDDe PdNPs/GG-PAMf /GCE Pd/CB/GCE PdNPs/MWCNT 3.7 ± 1.4 nm PdNPs/rGO RDE 3.7 ± 1.4 nm PdNPs/rGO RDE 3.7 ± 1.4 nm PdNPs/rGO RDE 3.7 ± 1.4 nm PdNPs/rGO RDE

CAg CA CA CA CA CA CA LSVh CA LSV CV CA LSV CA CA CA CA

7.0 7.0 7.0 7.0 6.8 7.4 6.7 7.0 7.0 7.0 7.0 Na2HPO4 (pH 9.0) 7.0 7.4 7.4 7.4 7.4

FIA at 2.0 mL min−1 N/Ai N/A N/A N/A FIA at 500 μL min−1 N/A N/A N/A N/A N/A N/A N/A RDE (500 rpm) RDE (2000 rpm) RDE (4000 rpm) RDE (6000 rpm)

0.014 0.014 0.016 0.04 0.06 0.27 0.42 0.6 0.95 2.6 4.1 8.8 10 0.2 0.05 0.01 0.007

0.19−100 0.02−71 0.1−10 0.1−5000 10−300 2−100 40−1000 2−500 10−200 27.2−85 50−600, 600−180 000 5−50 000 56−157 10−300 0.1−1000 0.1−1000 0.04−200

ref. 35 36 37 38 39 40 41 42 43 44 13 45 46 this this this this

work work work work

a

CPE, carbon paste electrode. bMCV, mesoporous carbon vehicle. cPEDOP, polyethylenedioxy pyrrole. dScreen-printed electrode. eBoron-doped diamond. fGG-PAM, Guar gum grafted with poly(acrylamide). gCA, chronoamperometry (I−t). hLSV, linear sweep voltammetry. iN/A, not available.

of hydrazine.13 The anodic current density observed from the oxidation of hydrazine is about 20- and 30-fold higher than that of ascorbic acid. At a higher concentration, i.e., 1000 μM, interfering signals can be observed, but the interefering signals are much lower than that of hydrazine. Detection of Hydrazine in Wastewater. To demonstrate a practical use of the as-fabricated 3.7 ± 1.4 nm PdNPs/rGO RDE, an additional method was used to detect trace hydrazine in wastewater by adding different concentrations of hydrazine (40−200 nM) to wastewater samples. The chronoamperomograms of the 3.7 ± 1.4 nm PdNPs/rGO RDE at a rotation rate of 6000 rpm are shown in Figure 7. The current response after

exhibits the lowest LOD (7 nM) at a rotation speed of 6000 rpm when compared with that from previous work based on Pd electrocatalysts. Selectivity to Hydrazine Oxidation. Agents such as uric acid, ammonia, ascorbic acid, glucose, caffeine, methylamine, ethylenediamine, hydroxylamine, n-butylamine, adenosine, cytosine, guanine, thymine, and L-arginine at different concentrations, which typically interfere with hydrazine detection, were used to evaluate the selectivity of the 3.7 ± 1.4 nm PdNPs/rGO RDE at an applied oxidation potential of 0.32 V vs Ag/AgCl. Three concentrations of all chemicals (100, 500, and 1000 μM) were tested against the 3.7 ± 1.4 nm PdNPs/rGO RDE at 2000 rpm. The results in Figure 6 show that at diluted concentrations (100 and 500 μM) there is no interfering signal observed for uric acid, ammonia, glucose, and caffeine against the 3.7 ± 1.4 nm PdNPs/rGO RDE. However, a slight signal from ascorbic acid was still observed. This result agrees well with a previous report, as the oxidation potential of ascorbic acid is close to that

Figure 7. Chronoamperomograms of the 3.7 ± 1.4 nm PdNPs/rGO RDE at a rotation rate of 6000 rpm with consecutive sample injections.

adding the samples exhibits the same response as that of pure hydrazine. The average percent recovery, including RSD, was calculated from three samples, as listed in Table 3. The result shows that wastewater contains trace hydrazine. Note that Figure S4 and Table S1 of the Supporting Information also show the detection of hydrazine in wastewater with higher added hydrazine concentrations at a slower rotation speed (i.e., 2000 rpm).

Figure 6. Chronoamperomograms of the 3.7 ± 1.4 nm PdNPs/rGO RDE at a rotation rate of 2000 rpm by interfering with hydrazine (a) oxidation using uric acid (b), ammonia (c), ascorbic acid (d), glucose (e), caffeine (f), methylamine (g), ethylenediamine (h), hydroxylamine (i), n-butylamine (j), adenosine (k), cytosine (l), guanine (m), thymine (n), and L-arginine (o) at different concentrations. E

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Science and Technology Development Agency are also acknowledged.

Table 3. Determination of Hydrazine in Wastewater Samples Using the 3.7 ± 1.4 nm PdNPs/rGO RDE at a Rotation Rate of 6000 rpm added (nM)

found (nM)

recovery (%)

RSD (%) (n = 3)

40 80 120 200

40.50 81.33 120.75 200.73

101.24 101.66 100.63 100.37

1.76 0.86 1.65 1.47

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CONCLUSIONS PdNPs decorated on rGO sheets were produced by an in situ polyol technique by reducing GO and Pd2+ precursors to form PdNPs/rGO composites using ethylene glycol as the reducing agent. Different PdNPs particle sizes, from 3.7 ± 1.4 to 10.8 ± 1.8 nm, were achieved by varying the concentration of the palladium(II) nitrate precursor. The as-fabricated PdNPs/rGO RDE shows excellent electrochemical activity toward hydrazine oxidation. It was found that the electrode’s performance depends on both the particle size of the PdNPs and the hydrodynamic diffusion of hydrazine to the electrode. The 3.7 ± 1.4 nm PdNPs/rGO RDE exhibits the lowest LOD of about 7 nM at a rotation rate of 6000 rpm when compared with that of electrodes with larger sizes. The rotating rate plays an important role in the electrode’s performance. The LOD values against the 3.7 ± 1.4 nm PdNPs/rGO RDE are 0.2, 0.05, 0.01, and 0.007 μM at 500, 2000, 4000, and 6000 rpm, respectively. In addition to the LOD, linear ranges with R2 ≥ 0.99 are 10− 300, 0.1−1000, 0.1−1000, and 0.04−200 μM at 500, 2000, 4000, and 6000 rpm, respectively. These findings, based on the effects of Pd particle size and the mass transport of hydrazine, may lead to the practical use of the PdNPs/rGO RDE toward trace hydrazine detection, especially in wastewater.

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ASSOCIATED CONTENT

S Supporting Information *

Electron diffraction patterns, XRD, and FTIR of PdNPs/rGO of different sizes as well as chronoamperomograms of the 3.7 ± 1.4 nm PdNPs/rGO RDE at a rotation rate of 2000 rpm with consecutive sample injections. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

M.S. conceived and designed this work and wrote the paper; A.K. carried out the experiments (synthesis, electrochemical evaluation, TEM, SEM, and BET), and P.S. and S.L. performed Raman FTIR and XRD. All authors participated in the analysis and discussion of the results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Thailand Research Fund (TRF) and the Commission on Higher Education (TRG5680043). Support from the Kasetsart University Research and Development Institute (KURDI), National Research University Project of Thailand (NRU), and National Nanotechnology Center (NANOTEC) under the National F

dx.doi.org/10.1021/ac503446q | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

Article

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