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Ultrasensitive and Selective Hydrazine Determination in Water Samples Using Ag-Cu Heterostructures Grown ITO Electrode by Environmentally Benign Method N.S.K. Gowthaman, Sekar Shankar, and S. Abraham John ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04777 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Ultrasensitive and Selective Hydrazine Determination in Water Samples Using Ag-Cu Heterostructures Grown ITO Electrode by Environmentally Benign Method

N.S.K. Gowthaman, S. Shankar and S. Abraham John* Centre for Nanoscience and Nanotechnology, Department of Chemistry The Gandhigram Rural Institute (Deemed to be University) Gandhigram-624 302, Dindigul, Tamilnadu, India *E-mail: [email protected], [email protected]

KEYWORDS: Dendritic nanostructures, Ag-Cu heterostructures, Electro-electroless deposition, Carcinogenic hydrazine, Amperometric determination

ABSTRACT: The present study describes the facile and fast growth of Ag-Cu dendritic nanostructures (D-AgCuNSs) by an environmentally benign electro-electroless deposition method and determination of hydrazine (HZ) in water samples using the resultant D-AgCuNSs grown ITO electrode. HZ was also successfully determined with the aid of Raman spectroscopy in which the Raman signal was enhanced to 15-fold at D-AgCuNSs in the presence of HZ in contrast to bare ITO. Initially, CuNSs were grown on the ITO substrates at different applied potentials and the resultant substrates were used in the galvanic displacement reaction with Ag+ ions and thereby grown D-AgCuNSs on the ITO substrate. The growth of D-AgCuNSs was followed by SEM with respect to time and Ag+ concentration. The D-AgCuNSs grown on ITO substrates were further characterized by XRD, XPS, EDS, CV and EIS techniques. XPS shows that the grown D-AgCuNSs contain zero valent Ag and Cu. Further, the D-AgCuNSs modified

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ITO electrode was utilized for carcinogenic HZ determination and excellent catalytic activity was observed at this electrode in contrast to bare and Cu modified ITO electrodes. The amperometric determination was accomplished with wide range of HZ concentration from 20 × 10-9 M to 50 × 10-6 M (R2 = 0.9934). Further, the ITO/D-AgCuNSs electrode detects HZ with the superior selectivity of 2500-fold in the presence of common interfering ions and biological compounds. The lowest limit of detection (0.12 nM (S/N = 3)) and superior sensitivity (3722 μA mM-1 cm-2) were achieved towards HZ. In addition, the present sensor was exploited for the determination of HZ in environmental samples and exhibits excellent recovery.

INTRODUCTION Hydrazine (HZ) and its methyl derivatives, known as universal volatile toxic compounds are utilized extensively in industries as emulsifiers, antioxidants, oxygen scavengers, explosives, rocket fuels, fuel cells, corrosion inhibitors, dyes and catalysts and in agriculture as pesticides, herbicides and insecticides.1-4 However, it is mutagenic, hepatotoxic and carcinogenic with adverse health effects such as temporary blindness, bronchitis, lethal damage to the central nervous system, kidney and liver, since it is dangerously unstable even at low concentration.3-5 According to the WHO and United States Environmental Protection Agency,6 HZ is a persuasive carcinogen with the optimum level of 0.1 ppm in industrial and in agricultural sewages. Hence, an extensive attention has been paid for the accurate determination of trace level HZ in various environmental samples in recent years. Even though spectrophotometry,7 chromatography8 and chemiluminescence9 methods were successfully employed for the determination of HZ electroanalytical methods have been frequently employed by plenty of researchers due to their fast response, low cost, better detection limit, sensitivity and selectivity, wide linear range detection and suitability in real sample analysis.10-14


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Electrochemical oxidation of HZ is a slow and sluggish process and requires high overpotential at conventional electrodes.14-18 Generally, it has been shown that the electrocatalytic activity of the noble metal nanoparticles modified electrodes (Ag, Au, Cu, Pt and Pd) towards analytes was enhanced either by increase in oxidation/reduction current or decrease in the onset potential.10-19 Fortunately, the better sensing performance can be succeeded by the synergism of binary composition interfaces, mostly metal composites.14-16 Recently, bimetallic nanostructured materials are widely studied due to their ease in synthesis and surface functionalization besides enhanced catalytic activity, selectivity and stability when compared to their counterparts.14-16,20-22 In particular, Cu-Ag bimetallic NPs are employed widely in electronic industry, catalysis, sensors and biological devices due to high electrical conductivity and catalytic activity.23-25 Moreover, Cu-AgNPs not only possess better electromigration resistance than pure Cu but also avoids the oxidation of Cu.26 In addition, the catalytic activity and stability of Cu-Ag NPs modified electrode are enhanced due to synergism of Ag-Cu while loading with Ag.23-27 In this perspective, it is expected that Cu-Ag combination would enhance the sensitivity towards the detection of HZ due to the synergism between individual Cu and Ag. Further, it is also expected that CuNPs can hinder the adsorption of poisoning species on the Ag surface and the presence of Ag may offer more scenarios for electron transfer due to its higher electrical conductivity. Hydrothermal,27 wet chemical,24 electrodeposition28 and electroless deposition29 methods have been extensively used for the preparation of bimetallic catalysts for electrochemical sensors. Even though these methods have been successfully used for the synthesis of bimetallic nanostructured catalysts, the direct growth of bimetallic catalysts by electro-electroless deposition is not deliberated extensively. Recently, Jin et al. fabricated the Ag-Cu bimetallic


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arrays on Ni-foam by two step galvanic displacement reduction (GDR) reaction for oxygen reduction reaction.30 Reyna-Gonzalez et al. followed the electro-co-deposition pathway for the deposition of Ag-Cu in ionic liquid phase.31 Kim and co-workers prepared the Ag-Cu electrocatalyst for the reduction of CO2 by two step electrodeposition method.32 Li et al. electrochemically synthesized the Cu-Ag nanostructures (NSs) on graphene paper using two step electrodeposition and utilized the substrate for surface enhanced Raman spectroscopy applications.28 In the current work, an effort is placed to develop the electro-electroless method for Ag-Cu bimetallic NSs preparation. It is well known that electrodeposition controls the shape and size of the nanomaterial without the aid of surfactants and templates.10 On the other hand, electroless deposition provides short-time nucleation without any shape directing and complicated growth agents.29 Among the various electroless deposition methods (autocatalytic, substrate catalyzed and GDR), the GDR method received considerable attention because of its high throughput and simple experimentation. The reduction of metal ions into nanoparticles is favored by GDR in the absence of external electric current as well as reducing agent.22 Moreover, the electro-electroless deposition method is an environmentally benign since it follows the principles of green chemistry which include environmentally clean synthesis, avoiding hazardous reagents, offering high recovery and reuse.33 Previously, the detailed study on the formation of dendritic-CuNSs on indium-tin-oxide (ITO) substrate by electrodeposition and its mechanism were reported from our research group.10 In this study, the short-term growth of Ag-Cu dendritic nanostructures (D-AgCuNSs) on ITO substrate is established by electro-electroless combo deposition strategy for the determination of one of the carcinogenic compounds, hydrazine. The CuNSs were electrodeposited initially on the ITO substrate and the resultant substrate was subjected to undergo GDR for the growth of Ag-


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NSs on Cu. The resulting AgCuNSs were characterized by SEM, XRD, EDS, XPS, EIS and CV. The Ag-Cu nanostructured ITO substrate was further utilized for the electrochemical determination of HZ. A careful investigation of Ag-Cu nanostructures properties revealed that a synergism between Ag and Cu exists in Ag-Cu catalyst, which may act as protagonist in costeffectiveness for practical utility. EXPERIMENTAL SECTION Chemicals Copper sulfate (CuSO4·5H2O), silver nitrate (AgNO3), hydrazine hydrochloride (N2H4·HCl) and sulfuric acid (H2SO4) were obtained from Merck, India and used as received. 0.2 M phosphate buffer (PB) solution (pH 7) was prepared with Na2HPO4 and NaH2PO4. All other chemicals were of analytical grade and used as received. Indium tin oxide (ITO) substrates were received from Asahi Beer Optical Ltd., Japan. All solutions used in this study were prepared using double distilled. Electro-electroless deposition of Ag-CuNSs on ITO substrates CuNSs were electrodeposited on ITO surface according to our previous report.10 The well cleaned ITO plate was used as the scaffold for the CuNSs deposition using a solution containing 10 mM CuSO4 and 0.1 M H2SO4 at an applied potential of -0.30 V for 400 s at room temperature. Prior to use the, electrolyte was bubbling with ultra-pure N2 gas for 20 min for deoxygenation. The resulting CuNSs grown ITO substrates were rinsed thoroughly with double distilled water and immersed in the solution of 0.5 mM AgNO3 for the electroless deposition of AgNSs on CuNSs grown ITO substrate (Scheme 1).


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Scheme 1. Schematic representation of electro-electroless deposition of Ag-CuNSs. Instrumentation The D-AgCuNSs grown ITO substrates were characterized by different spectral and microscopic techniques. CHI electrochemical workstation was utilized for all electrochemical measurements. A detailed instrumentation and characterization were provided in electronic supplementary information (ESI). Real sample analysis To prove the practicability of the present sensor, ground water and industrial effluent are collected in and around with a radius of one kilometer from the Gandhigram Rural Institute campus because it is surrounded by several dye and pulp industries. The collected samples were diluted to 20 times with 0.2 M PB solution. Then, the known concentration of HZ is spiked with the water samples and the recovery percentage was calculated. Further, the obtained results were validated with HPLC analysis (ESI).


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RESULTS AND DISCUSSION Electrodeposition of CuNSs and their morphology analysis The morphology of the CuNSs with respect to applied potential was analyzed by depositing Cu at different applied potentials since the applied potential drives the apparent electroactive species concentration via Nernstian equation.10,34 The i–t chronoamperometric curves of Cu deposition at various electrolysis potentials (+0.10 V to -0.50 V) are shown in Fig.S1A and it is obviously seen that the more negative applied potential resulted the higher reduction current. The adequate applied potentials (+0.10 to -0.50 V) carried the electrolysis process of Cu2+ + 2e− = Cu0 (E0 = 0.339 V vs. NHE) at the working electrode. Electrochemical deposition of Cu is the nucleation and crystal growth process which is electrochemically driven by the overpotential in the constant potential mode which in turn affects the growth kinetics and eventually the morphology of CuNSs. The color changes in the ITO substrate after the electrodeposition is one of the direct evidences of potential dependent growth of CuNSs (Fig. S1B). To illustrate the above, Cu attained at different applied potentials with the Cu2+ concentration of 10 mM at 400 s deposition are investigated by SEM and the corresponding images are shown in Fig.S2. As expected, the deposited Cu exhibited different morphology with the fluctuation of the applied potential. The as prepared CuNSs were found in the form of cubic, spherical, dendritic and prickly NSs at +0.10, -0.10, -0.30 and -0.50 V applied potentials, respectively. It should be noted that at less negative applied potentials (+0.10 and -0.10 V), only cubic and spherical shaped particles were obtained with controlled growth and no dendritic structure was observed. On the other hand, the dendritic and prickly (aggregated dendritic) NSs were obtained at more negative applied potentials. This can be attributed that at low applied potential, the equal oxidation and reduction rates generate less Cu2+ ions to be reduced and spread over to the surface


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by nucleation followed by growth. However, at more negative applied potentials (-0.30 and -0.50 V), large abundant branched structure of dendritic and prickly NSs with well-defined morphology were observed (Fig.S2b and c). The fast electron transfer at more negative applied potentials rapidly reduces more Cu2+ to Cu dendritic and prickly NSs by mass transfer. Moreover, it is expected that the more negative applied potential exhibit faster growth rate than the less negative one.10,34 Electroless deposition of Ag on CuNSs and their morphology analysis After the electrodeposition of CuNSs on ITO substrate, it was immersed into the aqueous AgNO3 solution. When CuNSs contact the AgNO3 (Ag+ ions) solution, the instantaneous GDR reaction succeeded to produce Ag0 and Cu ions (i.e. electron transfer occurs between Ag+ and Cu0). The reaction proceeds on the basis of standard reduction potentials (Cu2+/Cu: 0.33 V and Ag+/Ag: 0.79 vs SHE; Cu is more easily ionized than Ag). In this process, Cu nanoparticles on the surface serve as a reductant as well as the source of electron to reduce the Ag+ ions. When the Cu nanoparticle active site undergoes the galvanic displacement, Cu0 was decayed into Cu2+ by releasing electrons (anodic reaction) that subsequently travelled to reduce Ag+ to Ag0 (cathodic reaction).30,35 Besides the Ag+ reduction, Cu2+ was reduced and the Cu0 was regenerated along with Ag0 and there is a co-existence of Ag and Cu. Subsequently, the Ag and Cu heterostructures were grown with the continuous galvanic displacement. The morphological relationship between Cu and Ag-CuNSs was followed by SEM analysis. The SEM images obtained by immersing the cubic, spherical, dendritic and prickly CuNSs deposited at various applied potentials on ITO substrate into the aqueous AgNO3 solution for 10 min are shown in Fig.1. It is presumed that AgNSs were grown by GDR on CuNSs without affecting the parental Cu nano-pattern as evidenced in Fig.S2. But, it can be realized from Fig.1,


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a

b

c

d

Figure 1. SEM images obtained for AgCuNSs prepared by the immersion of CuNSs deposited (Eapp = (a) +0.10, (b) -0.10, (c) -0.30 and (d) -0.50 V) in 0.50 mM AgNO3 for 10 min. growth of Ag on Cu leads to elongation from the basic pattern (Fig.S2) during the GDR. The asprepared Ag-Cu nanomaterials not only exhibit the uniform and fine structures but also high surface coverage for the CuNSs prepared at higher applied potential. It has been already reported in the literature that the dendritic NSs exhibit faster electron transfer, higher electroactive area and superior electrochemical performance than the individual monometallic NSs.10,27,28,34,35 The growth of Ag on Cu dendritic NSs is examined by SEM with regard to reaction time and the corresponding images are shown in Fig.2. The D-AgCuNSs were grown by the reaction of dendritic CuNSs with the AgNO3 solution. The galvanic displacement


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reaction time was varied from 10 to 60 min. Interestingly, after 10 min contact with AgNO3 solution, uniformly grown Ag-Cu dendritic NSs were formed on ITO substrate. This indicates that it is possible to create a bimetallic Ag-Cu array in a short period of time through GDR. While the contact time was increased into 10, 20 and 30 min, the dendritic NSs were close in contact with each other and try to collapse their morphology by aggregation. Further increasing the time to 60 min, the Ag-Cu dendrites were aggregated. This is attributed that the Ag deposition can possible until Cu ions can tolerate as well as the transfer of electrons. The prolonged electron transition from Cu to Ag followed by Ag to Cu affects the morphology of the parental pattern since the reaction generates holes followed by core deposition.21,22 Thus, the morphology of the Ag-Cu heterostructures has been highly influenced by the reaction time. Even though each reaction time persists the dendritic nanostructure, their dimensions varied with prolonged reaction. Since the reaction time influenced the morphology, it is decided to investigate D-AgCuNSs morphology with respect to Ag+ ion concentration at the contact time of 10 min. Fig.S3 shows the SEM images of the Ag-Cu heterostructures when contact with Ag+ ion concentration from 0.25 to 2 mM for 10 min. The Ag-Cu dendrites were grown well initially at 0.25 mM Ag+ ion and the size of the dendrites was increased and becomes imperfect on increase in concentration from 0.5 to 1 mM and completely aggregated at 2 mM. This indicates that Ag+ ion concentration also impacts the Ag-Cu morphology. The host, Cu2+ ion concentration is not enough to accommodate high concentration of Ag+ ion in the GDR. Thus, aggregation takes place at higher concentration of Ag+ ion. In general, deposition of shell can sustain until the hosting ions can tolerate them and electron transfer over the shell.21,22 Once the process becomes stopped, the excess ions may be deposited on the surface of the heterostructures aggregate the NSs.


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a

b

c

d

Figure 2. SEM images obtained for the formation of D-AgCuNSs when D-CuNSs contact with 0.5 mM AgNO3 at different contact time: (a) 10, (b) 20, (c) 30 and (d) 60 min. Characteristics of D-AgCuNSs The D-AgCuNSs prepared through electro-electroless deposition were further characterized by different surface characterization techniques. Fig.3 exhibits the XRD patterns obtained for ITO substrates modified with Cu dendrites and D-AgCuNSs prepared at different reaction times. The dendritic Cu nanostructured ITO substrate (Fig.3a) shows three main peaks at 43.1°, 48.7° and 73.5°, corresponding to the (111), (200) and (220) Cu planes, respectively (JCPDS. 85-1326), indicating that the crystalline CuNSs were grown on the ITO substrate. The peaks obtained at 22.7°, 30.2°, 35.6°, 38.3°, 40.1°, 46.4°, 50.7° and 67.1° are characteristics to ITO (JCPDS. 89-4599). On the other hand, after the GDR, the succeeded growth of AgNSs on


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Figure 3. XRD patterns of (a) D-CuNSs grown ITO and D-AgCuNSs grown at different reaction time of (b) 10 and (c) 20 min in 0.5 mM AgNO3 solution. CuNSs were confirmed by the distinguished diffraction planes (111), (200), (220), (311), and (222) of Ag at 38.17°, 44.34°, 64.52°, 77.48° and 82.10°, respectively (JCPDS. 065-7244) (Fig.3b). The diffraction pattern of D-AgCuNSs shows the combination of Cu (JCPDS. 85-1326) and Ag (JCPDS. 04-0783) peaks. Further, there is no diffraction peak for alloy is detected in the DAgCuNSs pattern, suggesting that the as-prepared DAgCuNSs are bimetallic composites and not Cu-Ag alloys. Furthermore, the peak intensities were increased by increasing the reaction time from 10 to 30 min (Fig.3c) and the crystallite size was found to be 63.48, 87.36 and 102.73 nm (Debye Scherer equation), respectively. The increase in size could be due to the loss in the surface energy of the deposited seeded and condensed particles on nucleation. The D-AgCuNSs grown by (111) facets which are predominantly oriented parallel to the substrate since (111) plane intensity is higher than (200).


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A

Cu

Ag

2p(3/2) Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

Cu2p 2p(1/2)

960

3d(5/2)

940

Binding energy (eV)

C

3d(3/2) Intensity (a.u)

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920

Ag3d

380

370

360

Binding energy (eV)

Figure 4. XPS of D-AgCuNSs grown ITO substrate: (A) survey and deconvoluted spectra of (B) Cu2p and (c) Ag3d regions. Further, the chemical state of Ag and Cu in the D-AgCuNSs was confirmed by XPS (Fig.4). The XPS survey spectrum of D-AgCuNSs on ITO substrate exhibits two major notable peaks around 370 and 940 eV respective to Ag3d and Cu2p regions (Fig.4A) and the remaining are characteristics to ITO. To find the nature of the Ag and Cu species, the Ag3d and Cu2p regions were deconvoluted. The deconvoluted XPS Ag3d spectrum shows two peaks for Ag3d5/2 and Ag 3d3/2 binding energies at 368.3 and 374.3 eV, respectively (Fig.4B).30,35 The 6.0 eV 3d doublet splitting of Ag is indicating that the AgNSs formed from GDR reaction were zero valent. The Cu2p deconvoluted spectrum resolves the 2p3/2 and 2p1/2 signals of elemental Cu at 932.5 and 952.8 eV, respectively (Fig.4C).10,30,35,36 Moreover, it does not display any satellite peaks in the region of 939-945 eV, confirming the presence of zero valent Cu in the D-AgCuNSs.36 The EDS spectrum in Fig.5A is obtained for the dendritic D-AgCuNSs on ITO substrate. The peripheral area of the D-AgCuNSs exhibits Ag and Cu peaks, confirms the effective deposition of D-AgCuNSs on ITO. The Ag (2.98 and 2.123 keV) and Cu (0.92 and 8.0 keV) peaks were appeared along with oxygen, indium and tin from the ITO source (0.53, 3.29 and 3.44 eV). Further, the EDS mapping analysis show the appearance of Ag and Cu in the Ag-Cu dendritic


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assembly with yellow and red colors, respectively (Fig.5B). Furthermore, the red colored CuNSs completely covered the yellow colored AgNSs though the reaction proceeds in opposite way. This is ascribed that during the (GDR) reaction, the CuNSs were dissolved by electron transfer and generates holes on their surface and the reduced AgNSs were deposited inside the holes. The distribution of Ag and Cu metals in the Ag-Cu nanostructure was obtained by EDS line-scanning analysis (Fig.5C), which is the plot of X-ray counts vs. the latitudinal location over a line21,22 and the line spectra of Ag-Cu nanostructured ITO substrate and confirms again the presence of Ag and Cu on the substrate. It also displays the Cu intensity (Fig.5C; red line) was relatively higher than Ag (Fig.5C; green line), implies that AgNSs were sheltered with CuNSs. cps/eV 5

A

4

3

2

Ag

Cu

Ag

Cu

1

0 1

2

B

3

4

5

6

keV keV

C

7

8

9

10

Cu Ag

Figure 5. (A) EDS spectrum, (B) mapping image and (C) EDS line spectra obtained for D-AgCuNSs grown ITO substrate.


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Electrochemical features of D-AgCuNSs Electrochemical impedance spectroscopy technique is well suited to inspect the conducting feature and electron transfer kinetics of Cu and Ag-Cu dendritic nanostructured ITO substrates (Fig.S4). The Nyquist plots of bare ITO and Cu and Ag-Cu dendritic nanostructured ITO electrodes in 1 mM each of K4[Fe(CN)6] and K3[Fe(CN)6] containing 0.2 M PB solution (pH 7) at 0.01 Hz to 100 kHz scanning frequencies were best fitted with the (Rs[C-Rp]) circuit (Fig.S4 inset), where Rs, C and Rp refer the solution resistance, capacitance and polarization resistance, respectively. The semicircle value of fitted Nyquist plot is equal to the charge transfer resistance (RCT), which can control the electron-transfer rate of the electrode-electrolyte interface.10,21,22 The RCT values for bare ITO and Cu and Ag-Cu dendritic nanostructured ITO electrodes were found to be 5437, 843.2 and 610.8 Ω, respectively. It is very interesting that the Ag-Cu dendritic nanostructured ITO electrode exhibits a drastic decrease in interfacial RCT value when compared to the dendritic Cu nanostructured ITO electrode. The higher surface homogeneity and porous nature of the dendritic nanostructure of the bimetallic array could be attributed to the lower RCT value. The RCT can be connected with the I0 (equilibrium current) using,37

RT nFAI0

---- (1)

I0 = nFAket

---- (2)

RCT = I0 is equal to,

where, ket = heterogeneous electron-transfer rate constant. By combining eqn (1) and (2),

ket =

RT n2 F2 A RCT C0

---- (3)


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The ket between the redox probe and the dendritic nanostructured electrode can be calculated using eqn (3), where n = number of electrons transferred (n =1 for [Fe(CN)6]3-/4- probe), A = electrode area (1 cm2) and C° = concentration of the redox probe (1 mM) and R,T and F have their standard significance. The calculated ket values are 5.04 × 10-5, 3.25 × 10-4 and 5.49 × 10-4 cm s-1 for bare, Cu and D-AgCuNSs modified ITO electrodes, accordingly. The higher ket value of the ITO/D-AgCuNSs electrode obviously indicates the facile electron transfer reaction of the electrode and the RS, C, RP and ket values are tabulated (Table S1). Further, the electroactive surface area of the different electrodes was calculated with the aid of Randles-Sevcik equation (4)10 by ramping the cyclic voltammograms (CVs) in 1 mM K3[Fe(CN)6] containing 0.2 M PB solution (pH 7).

ip = 2.69105 A n3/2 D01/2 ν1/2 C0

--- (4)

here, ip – peak current; A – electroactive surface area (cm2); n – number of electrons transferred (n=1 for [Fe(CN)6]3-/4- probe); D0 – diffusion coefficient (6.7 × 10-6 cm2 s-1 for [Fe(CN)6]3-/4-); ν – scan rate; C0 – concentration of the probe (1.0 mM). The electroactive surface area was found to be 0.29, 0.55 and 0.81 cm2 for bare ITO and D-CuNSs and D-AgCuNSs grown ITO electrodes, respectively. Further, the CV of D-AgCuNSs modified ITO electrode in 0.2 M PB solution (pH = 7.0) at 50 mV s-1 (Fig.S5) exhibits two anodic peaks for the oxidation of Cu(0) to Cu(I) and Cu(I) to Cu(II) at -0.40 and +0.05 V, respectively and their subsequent reduction at -0.30 and -0.60 V, respectively along with Ag oxide formation at +0.45 V and its subsequent reduction and +0.01 V.27 The observed peaks characteristics to the oxidation and reduction of Ag and Cu confirmed once again the successful formation of D-AgCuNSs on ITO electrode. Electrochemical oxidation of HZ


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With the intent of examining the electrocatalytic performance of the D-AgCuNSs grown ITO electrode, the electrochemical oxidation of hydrazine was studied by CV (Fig.6). Even though the electrochemical behavior of HZ is dependent on the pH of the medium, we have maintained pH 7.0 throughout this study since the prime goal of this study is to determine the HZ in water samples. The potential scanning was ramped in the range of 0.0 to 1.6 V at a scan rate of 50 mV s-1. In the absence of HZ, neither oxidation nor the reduction peak was observed in the studied potential window. The appearance of an anodic peak at +1.45 V after the addition of 0.5 mM HZ at bare ITO reveals that oxidation of HZ occurs at this potential (Fig.6a). The dendritic CuNSs modified electrode shifted the anodic peak potential to less positive side by oxidizing HZ at +1.05 V and enhanced its oxidation current (Fig.6b). Conversely, the D-AgCuNSs grown ITO electrode oxidizes HZ at +1.07 V with 1.25 and 1.85-folds enhanced oxidation current in contrast to D-CuNSs modified and bare ITO electrodes, respectively (Fig. 6c). The obtained higher oxidation current was ascribed to the porous nature of D-AgCuNSs having huge electroactive area along with numerous active sites and sharp edges besides its high conductivity.10,27,34,36

c 200

b A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

a

0 0

1

E/V vs. Ag/AgCl (NaCl sat)


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Figure 6. CVs of 0.5 mM HZ at (a) bare ITO, (b) D-CuNSs and (c) D-AgCuNSs modified ITO electrodes in 0.2 M PB solution at a scan rate of 50 mV s-1. The oxidation of HZ at D-AgCuNSs modified ITO electrode was further examined at different sweep rates (Fig.S6A). The oxidation peak current of 0.5 mM HZ was increased with the increase of scan rate from 10-150 mV s-1 with a slight change in the peak potential and the plot of anodic peak current against square root of the sweep rate shows linearity (R2=0.9989) (Fig.S6 B), suggesting that the oxidation of HZ was controlled by diffusion. Further, a shift in the peak potential (Ep) was observed with the increased ν and a linearity was obtained by plotting Ep and log (ν) (Fig.S6C), indicating the oxidation of HZ at D-AgCuNSs grown ITO electrode is an irreversible process. The slope obtained from the plot of Ep against log(ν) for an irreversible process is equal to

K=

RT 2nF

---- (5)

where, α = electron transfer coefficient and n = apparent number of electrons transferred. It can be seen form Fig.S6B, the slope (K) of Ep vs log(ν) was found to be 148.7 mV and modifying the eqn.(5) gives the nα value of 0.86. Then, ip can be related with ν by the eqn.(6) as follows.

ip = 0.4958 × 10-3 n F3/2 (RT )-1/2 (αn)1/2 A C DHZ1/2 ---- (6) where, DHZ =ν1/2 diffusion coefficient of HZ (1.39 × 10-5 cm2 s-1). By rearranging the eqn.(6), number of electrons transferred (n) can be calculated and it was found to be 4. Thus, the overall HZ oxidation reaction could be

NH2NH3+

N2 + 5H+ + 4e- ---- (7)


18

Page 19 of 35

Further, the Tafel slope (b) of the irreversible diffusion-controlled system is calculated using eqn. (8) as follows.38

Ep = b log v + constant ---- (8) 2

By rearranging the above eqn (8), the Tafel slope (b) was calculated to be 0.297 V, which is consistent with the rising part of CV of HZ at 10 mV s-1, indicating that the rate determining step succeeds with one electron transfer followed by a fast three electron transfer as follows,

N2H3 + 2H3O+ + e- (slow) ---- (9)

NH2NH3+ + H2O

N2 + 3H3O+ + 3e-

N2H3 + 3H2O

(fast) ---- (10)

The highly stable N2 molecule could be the reason for the totally irreversible overall reaction. Electrochemical determination of HZ

20

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

t

a 0 0.5

1

E/V vs. Ag/AgCl (NaCl sat)


19


Figure 7. DPVs of HZ with each addition of 10 μM in the linear range of 10-200 μM (a-t) at DAgCuNSs modified ITO electrode in 0.2 M PB solution. Inset: Plot of HZ oxidation current vs. the concentration of HZ. Error bars are constructed for three measurements (RSD 0.30%). The sensitive detection of HZ was carried out at D-AgCuNSs modified ITO electrode by differential pulse voltammetry (DPV) for every 10 µM HZ addition (Fig.7). The addition of 10 µM HZ shows an oxidation peak at 0.95 V. Further increasing each 10 µM HZ from 20 to 160 µM, the HZ oxidation peak current was steadily increased without shift in the oxidation potential. The plot of oxidation current against HZ concentration is linear (R2 = 0.9991) (Fig.7 inset).

(i)

b

150

11

(ii)

j

100

a

9

A

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

d c

50

10

8

a

b

e

2400

2500

a-e

A 800

1000

1200

f

g h

i

k

B

0 2400

Time (s)

2600

Time (s)

2800

Figure 8. Amperometric i-t curves obtained for (A) each 50 nM addition of HZ at (a) D-CuNSs and (b) D-AgCuNSs modified ITO electrodes. (B) Addition of (a) 0.02, (b) 0.05, (c) 0.10, (d) 0.020, (e) 0.50, (f) 1, (g) 2, (h) 5, (i) 10, (j) 20 and (k) 50 μM HZ at D-AgCuNSs modified ITO electrode in 0.2 M PB solution with a time interval of 50 s (Eapp = +1.20 V). Insets: (i and iii)


20

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Plot of HZ oxidation current vs. the concentration of HZ. Error bars are constructed for three measurements (RSD = 0.59%). Based on the CV and DPV studies, the amperometric response towards the oxidation of HZ was recorded at Ag-Cu dendritic nanostructured ITO electrode for its sensitive determination. Fig.8A shows the amperometric i-t curves for HZ at the Cu and Ag-Cu dendritic nanostructured ITO in a uniformly stirred 0.2 M PB solution at the applied +1.20 V potential. Initially, the DAgCuNSs nanostructured ITO electrode responded for 50 nM HZ by increasing the current (Fig.8A, curve a) and the subsequent each 50 nM addition at 50 s time interval increases the current response and the steady state was attained within 3 s. A linear dependence of current response on HZ concentration from 50-500 µM was observed with R2 of 0.9958 (Fig.8A, inset I). We have also compared the present electrode with the dendritic Cu nanostructured ITO electrode (Fig.8A, curve b). It also exhibits the similar response with the addition of HZ with R2 of 0.9969 (Fig.8A, inset). When compared to the dendritic Cu nanostructured electrode, the D-AgCuNSs electrode showed the excellent response by enhancing the amperometric oxidation current of HZ, indicates that the D-AgCuNSs modified electrode is an accurate sensor for the sensitive HZ determination. Further, amperometric determination of

HZ at wide range concentration

authenticate the D-AgCuNSs modified ITO electrode. Fig.8B depicts the amperometric i-t curves of HZ at D-AgCuNSs electrode in a uniformly stirred 0.2 M PB solution at the applied +1.20 V potential. Initially, the modified electrode responded for 20 nM HZ by increasing the oxidation current and subsequent 50, 100 and 500 nM HZ addition at 50 s interval increases the oxidation current steadily. The gradual increase in current response was also observed for the further 1, 2, 5, 10, 20 and 50 µM HZ addition. The HZ oxidation current was increased from 20 × 10-9 M to 50 × 10-6 M HZ linearly with a correlation coefficient of 0.9954 and the limit of detection (LOD)


21


Page 22 of 35

was found to be 0.12 nM (S/N=3). Further, the sensitivity towards HZ was found to be 3722 μA mM-1 cm-2 at the present modified electrode. The obtained linearity range, sensitivity and LOD of HZ at D-AgCuNSs modified electrode are superior when compared to the reported modified electrodes (Table 1).11-14,16,17,19,20,38-43 Scheme S1 illustrates the electrooxidation of HZ at Ag- DAgCuNSs modified ITO electrode. Effect of interferences The proposed sensor was also investigated amperometrically in the presence of interfering common ions and biological interferents to authenticate its anti-interfering ability towards HZ. Since the intake of HZ polluted water may lead to adverse health effects, the sensor must be selectively sense HZ in human fluids such as blood serum and urine samples. Thus, the possible physiological interferences such as uric acid and glucose were added as interferents. The amperometric i-t curve of HZ at Ag-Cu dendritic nanostructured electrode with possible interferents in 0.2 M PB solution is shown in Fig.9. Initially, the modified electrode responded for the addition of 100 nM HZ by increasing the oxidation current (curve a) and attained steady state within 3 s. But, the addition of each 100 μM Na, Mg, Cl, K, Ca, Zn and Cu ions, (b-h) does not increase the current response. However, addition of 100 nM HZ to the solution increases the current response similar to the early steps. Further addition of 100 μM each NH4+, CO32-, NO3-, SO42-, glucose, sucrose and uric acid (i-o) at 50 s interval to the solution does not increase the current response. On the other hand, the addition of 40 nM HZ to same solution increases the oxidation current similar to the early steps. These results indicated that the determination of 40 nM HZ is possible even in the presence of 2500-fold excess of common and physiological interferences.


22

Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of the electrocatalytic oxidation of HZ at various electrode materials using amperometric method. aPolyaniline. bGraphene

oxide. cGlassy carbon electrode. dNanorods. eElectrochemically reduced graphene oxide. fNanoflowers. gZeolitic imidazolate framework. hCarbon paste S.No

Electrocatalyst

Medium

Linear range

Detection

Sensitivity

limit (M)

(μA mM-1 cm-2)

Ref

1

CuNPs-PANIa-GOb/GCEc

pH 7.0

40 – 480 nM

4.50  10-9

359.93

11

2

Flower-like CuS/GCE

pH 7.5

0.0005 – 4.75 mM

0.097  10-6

359.3

12

3

ZnO NRsd/Ag/Glass

pH 7.0

0.01 – 98.6 μM

5  10-9

105.5

13

4

Ag@Pt-ErGOe/GCE

pH 7.4

1 – 10 mM

60  10-6

NR

14

5

Au@Pt nFsf/GO/GCE

pH 7.5

0. 8 – 429 μM

0.43  10-6

1695.3

16

6

Ag/ZIF-8g/CPEh

NaOH

6 – 5000 μM

1.57  10-6

54.46

17

7

AuNPs/CNTsi-ErGO/GCE

pH 7.4

0.3 – 319 μM

0.07  10-6

9.73

19

8

Ag@Fe3O4/GCE

pH 7.4

0.25 – 3400 μM

0.06  10-6

270

20

9

CeO2–OMCj/GCE

pH 8.0

0.04 – 192 μM

12  10-9

0.147

38

10

Au-Pd NRCsk/GCE

pH 7.0

0.1 – 501 μM

0.02  10-6

NR

39

11

Graphene nanobelt/GCE

pH 7.0

0.01 - 1.3 mM

1.10  10-6

0.08

40

12

CuNPs/PANI/Grl/GCE

NaOH

0.001 – 3.7 mM

0.27  10-6

150

41

13

GO/CTSm/Pt/GCE

pH 8.0

0.02 – 10 mM

3.6  10-6

104.6

42

14

F-TiO2NTs-Au@Pd/GCE

pH 7.0

0.06 to 700 μM

1.2  10-8

13

43

15

D-AgCuNSs/ITO

pH 7.0

0.02 – 50 μM

0.12  10-9

3722

this work

electrode. iCarbon nanotubes. jOrdered mesoporous carbon. kNanorod chanis. lGraphene. mChitosan


23


Page 24 of 35

Figure 9. Amperometric i-t curve obtained for (a) each 100 nM addition of HZ in the presence of 250 μM (b) Na+, (c) Mg2+, (d) Cl-, (e) K+, (f) Ca2+, (g) Zn2+, (h) Cu2+, (i) NH4+, (j) CO32-, (k) NO3-, (l) SO42-, (m) glucose, (n) sucrose and (o) uric acid at D-AgCuNSs modified ITO electrode in 0.2 M PB solution with a time interval of 50 s (Eapp = +1.20 V).

Practical applications To establish the practicability of the present HZ sensor, we extend this work to determine trace level concentration of carcinogenic HZ in ground water samples and industrial effluents by standard addition procedure. Various HZ concentrations (10 × 10-6 - 2 × 10-3 M) were added into the 20 times diluted ground water samples and industrial effluents and analyzed by DPV and amperometric techniques and the corresponding results are shown in Fig.S7. DPVs of collected water samples in PB solution (pH 7.0) did not show any response (curve b), indicating that the collected ground water sample and industrial effluents are free from HZ. However, addition of 10 µM HZ to the water sample, the HZ oxidation peak was appeared at +0.95 V (curve c). Further addition of 20 µM HZ to the solution increased the oxidation current due to the oxidation


24

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of HZ (curve d) and the recovery results of HZ are summarized in the Table S2. We have also achieved the ultra-trace level determination of HZ in environmental samples using amperometry (Fig. S7B). The addition of ground water sample does not change the amperometric current. On the other hand, 50 nM addition of HZ into the sample increases the current response sharply, indicating that the trace level determination of HZ is also possible at the D-AgCuNSs modified ITO electrode. Further, the reliability of the present sensor is validated with the high performance liquid chromatography (HPLC) method. The results obtained from the present method and HPLC are summarized in Table S2. It can be seen from Table S2, the results obtained from the present method closely agree with the results obtained by HPLC method. Further, the results obtained for the real sample analysis is comparable and consistent with the HZ electrochemical determination by D-AgCuNSs modified ITO electrode. This indicates that the proposed sensor detects 0.005 ppm HZ by DPV with 20-times less than the safety limit (0.1 ppm) prescribed by FDA and EPA. Moreover, the obtained results are the authentication of the good practicability of the present sensor for monitoring HZ in environmental real-time applications. Raman spectral analysis of HZ at D-AgCuNSs grown ITO substrate Raman spectroscopy offers chemical identification as well as structural information of materials on the basis of their exclusive vibrational fingerprint.45 Even though it possesses limited sensing applications because of low signal intensities and small molecular cross sections, anisotropic NSs exhibit strong effects when molecules are adsorbed onto their surface due to the coupling of the plasmon band of the metal with the molecule’s electronic states.43,45-47 Here, it is believed that the D-AgCuNSs would favor an increase of the Raman signaling because they hold enhanced electric fields at their tips and corners. Hence, in this study we tried to study the


25


Page 26 of 35

enhanced Raman signaling property of the D-AgCuNSs using HZ as a probe molecule for the first time. Briefly, the D-AgCuNSs grown ITO substrate was immersed into 0.1 mM HZ solution for 12 h and the HZ molecules are assembled on the surface of D-AgCuNSs. Similarly, bare ITO and DCuNSs grown ITO substrates were also analyzed for comparison. Fig.S8 shows the Raman spectra of HZ on bare, D-CuNSs and D-AgCuNSs grown ITO substrates. The Raman spectrum of HZ exhibits five pronounced polarized lines at 3302, 3296, 1624, 1076 and 510 cm-1 when it was attached on the ITO substrate (Fig. S8, curve a). Being C2 symmetry molecule HZ possesses two N-H stretching vibrations at 3398 and 3329 cm-1 which give rise to polarized Raman bands. The N-N stretching band of HZ can be observed at 1076 cm-1 and the polarized lines at 3302 and 3266 cm-1 are corresponding to the overtones of HZ deformations. The appearance of a weak line at 1642 cm-1 is ascribed to the antisymmetric deformation of HZ and the intense bad at 510 cm-1 is due to the ν7(a1) vibration mode of HZ molecule.48 When the HZ was attached on the surfaces of D-CuNSs and D-AgCuNSs grown ITO substrates, a noticeable change in the frequency was observed with enhanced intensity (Fig.S8, curves b and c). The Raman spectra of HZ attached DAgCuNSs grown ITO substrate exhibits the N-H stretching at 3367 and 3312 cm-1 and the N-N stretching at 934 cm-1. It is observed that the peaks were not only shifted toward the less positive frequency region but also exhibit a 15-fold enhanced intensity when compared to the Raman spectrum of HZ on bare ITO substrate. This indicates the strong chemical interaction of HZ with the D-AgCuNSs. Moreover, the enhancement in Raman signals of ν7(a1) vibrational mode (510 cm−1) of HZ was also observed at the D-AgCuNSs. The enhancement of HZ Raman signaling at the D-AgCuNSs is ascribed to their population and porous nature besides their enhanced electric fields at tips and corners of D-AgCuNSs. The 15-fold enhanced Raman signaling of HZ


26

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molecule at D-AgCuNSs grown ITO substrate clearly demonstrates that it is a suitable candidate for signaling molecule detection, clinical diagnostics and bioassays in the future. CONCLUSIONS In this study, a facile and fast growth of Ag-Cu dendritic nanostructures by electro-electroless deposition and their application towards the sensitive and selective determination of hydrazine were demonstrated. The determination of HZ was also successfully achieved with Raman spectroscopy in which the Raman signal was enhanced to 15-fold at D-AgCuNSs in the presence of HZ in contrast to bare ITO. Different CuNSs grown ITO substrates prepared with various applied potentials and subjected to undergo GDR with Ag+ ions and thereby Ag-Cu heterostructures were grown on the ITO substrate. The growth of D-AgCuNSs was optimized with respect to reaction time and Ag+ concentration and it was found that 10 min and 0.5 mM are the optimized conditions. The D-AgCuNSs modified ITO electrode exhibits an excellent electrocatalytic activity towards the HZ when compared to bare and CuNSs modified ITO electrodes. Further, the present modified electrode is selective towards HZ sensing in the presence of 2500-fold higher physiological interferents and common ions with the lowest LOD of 0.12 nM (S/N = 3) and superior sensitivity of 3722 μA mM-1 cm-2. Furthermore, the present sensor is successfully exploited for the determination of HZ in environmental samples. Thus, the D-AgCuNSs modified ITO electrode offers an auspicious platform for the development of reusable and economically viable ultra-sensitive and selective HZ sensor in various biological and environmental samples. ASSOCIATED CONTENT Supporting Information. The Supporting Information files are available free of charge. Additional experimental and characterization details (PDF).


27


Page 28 of 35

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Phone: +91 451 245 2371. Fax: + 91 451 245 3031. ORCID S. Abraham John: 0000-0002-9358-9998 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from DST-SERB (EMR/2016/002898), New Delhi is gratefully acknowledged. REFERENCES 1. Troyan, J.E. Properties, Production and Uses of Hydrazine. Eng. Chem. Res. 1953, 45, 26082612, DOI 10.1021/ie50528a020. 2. Garrod, S.; Bollard, M.E.; Nicholls, A.W.; Connor, S.C.; Connelly, J.; Son, J.K.N.; Holmes, E. Integrated Metabonomic Analysis of the Multiorgan Effects of Hydrazine Toxicity in the Rat. Chem. Res. Toxicol. 2015, 18, 115-122, DOI 10.1021/tx0498915.


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Graphical Abstract

Environmentally benign dendritic Ag-Cu nanostructures electrocatalyst were deposited on ITO substrate by electro-electroless deposition method and utilized for the ultra-sensitive detection of carcinogenic hydrazine in water samples by amperometry and Raman signaling of hydrazine.


Ultrasensitive and Selective Hydrazine ... - ACS Publications

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