Polyelectrolyte assisted synthesis and enhanced catalysis of silver nanoparticles: Electrocatalytic reduction of hydrogen peroxide and catalytic reduction of 4-nitroaniline

Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Polyelectrolyte assisted synthesis and enhanced catalysis of silver nanoparticles: Electrocatalytic reduction of hydrogen peroxide and catalytic reduction of 4-nitroaniline Perumal Viswanathan, Ramasamy Ramaraj ∗ School of Chemistry, Centre for Photoelectrochemistry, Madurai Kamaraj University, Madurai, 625 021, India

a r t i c l e

i n f o

Article history: Received 12 May 2016 Received in revised form 28 July 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Polyelectrolyte Silicate sol-gel Silver nanoparticle Hydreogen peroxide sensing Catalytic reduction 4-Nitroaniline

a b s t r a c t A facile method was developed for the one-step synthesis of silver nanoparticles (AgNPs), co-stabilized by the polyelectrolyte poly(acrylamide-co-diallyldimethylammoniumchloride) (PADA) and the silicate matrix N1-(3-trimethoxysilylpropyl)diethylenetriamine (TPDT). Here, TPDT acted as reducing agent and both PADA and TPDT served as capping agent for AgNPs. As PADA improved the catalytic properties of AgNPs, its concentration was optimized and the optimum concentration of PADA was found to be 1 wt.% among the concentrations 0.25, 0.5, 1 and 2 wt.%. The AgNPs were characterized using UV–vis absorption spectroscopy, HRTEM, EDX, SAED and FTIR analyses. The PADA(1)-Ag-TPDT NPs showed the better catalytic activity towards electrochemical reduction of H2 O2 and catalytic conversion of 4-nitroaniline (4NA) to p-phenylenediamine (PPD). The PADA(1)-Ag-TPDT NPs was used to construct the non-enzymatic electrochemical sensor for the detection of H2 O2 . Using the linear sweep voltammetry (LSV) and square wave voltammetry (SWV) techniques, the lowest experimental detection limits attained for H2 O2 sensing were 5 and 0.2 ␮M, respectively. Moreover, remarkably a fast conversion of 4-NA to PPD was observed over the PADA(1)-Ag-TPDT NPs catalyst with a rate constant of 0.096 s−1 and the product (PPD) formation was confirmed by 1 H NMR spectroscopy. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nanomaterials with unique physical and chemical properties find extensive applications when compared to their corresponding bulk materials [1]. Challenging issues in the fields of solar energy conversion, catalysis, medicine, and water treatment have been addressed greatly by means of nanoscience and nanotechnology [2,3]. Hence, the novel properties of nanoparticles (NPs) have been exploited in a wide range of potential applications. Among the metal NPs, AgNPs are the interesting materials due to their unique physical, chemical and biological properties [4]. Preparation of promising and cost effective catalyst in a simple and environmentally benign method is a challenging task in the field of catalysis science and technology. Nanoscience and technology showed the way to prepare such catalyst, which finds more than one application for a single material [5–7]. Synthesis of AgNPs with different size and shape in solution phase has been reported earlier [8,9].

∗ Corresponding author. E-mail address: [email protected] (R. Ramaraj). http://dx.doi.org/10.1016/j.molcata.2016.08.001 1381-1169/© 2016 Elsevier B.V. All rights reserved.

Polyelectrolytes capped AgNPs [10,11] and silicate sol-gel (SSG) stabilized AgNPs [5,12] have been reported and widely used. Effective quantification of hydrogen peroxide (H2 O2 ) is very important in the fields of food industry, pharmaceutical, clinical, industrial and environmental analysis and also H2 O2 is a reactive oxygen species and a by-product in many oxidative metabolic pathways [13]. The quantification of H2 O2 can be done by various analytical techniques such as titrimetry, fluorescence spectroscopy, UV–vis spectrophotometry, chemiluminescence, chromatography and electrochemical methods. Among them, electrochemical methods are the well-recognized techniques because of its simple instrumentation, high selectivity and high sensitivity [14]. Though, enzyme immobilized electrodes show good selectivity and sensitivity towards H2 O2 , they have several disadvantages such as instability, high cost of enzymes and complicated immobilization procedure [15]. Hence, the development of non-enzymatic electrochemical sensor for the detection of H2 O2 has great significance. Nitroaromatic compounds attract much attention over the decades in relation to pollutions, toxicity, mutagenesis, carcinogenesis, therapeutic action and as intermediates in the synthesis of important organic compounds. As far as the nitroaromatic compounds are concerned, the environmental contamination has been

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

the major problem [16]. The reduced products of nitroaromatics i.e., aromatic amines are less toxic in nature and are important starting materials for the preparation of many biologically active and pharmaceutical chemicals [17]. Hence, the development of simple, cost effective and efficient catalyst for the conversion of nitroaromatics into corresponding amines being the important filed of research. In the present work, AgNPs were prepared without employing any other external reducing agent in the presence of PADA and TPDT, and were well characterized. The prepared AgNPs were found to be catalytically active for the electrochemical reduction of H2 O2 and catalytic conversion of 4-NA to PPD in the presence of NaBH4 . As PADA improved the catalytic behaviour of AgNPs, its concentration was optimized and 1 wt.% PADA was found to produce catalytically more active AgNPs in combination with TPDT. 2. Experimental section 2.1. Materials and methods Silver nitrate (AgNO3 ), poly(acrylamide-co-diallyldimethylammonium chloride) (PADA) and N–[3–(trimethoxysilyl) propyl] diethylenetriamine (TPDT) were purchased from Sigma–Aldrich. All other chemicals are analytical grade and were received from Merck. All glassware was thoroughly cleaned with aqua regia (1:3 HNO3 /HCl v/v) (caution: Aqua regia is a powerful oxidizing agent and it should be handled with extreme care.) and rinsed extensively with distilled water before use. UV–vis absorption spectra were recorded using Agilent Technologies 8453 spectrophotometer with a 1 cm quartz cell. High resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) analyses were conducted in a JEOL JEM 2100 and Technai T20 instrument operated at 200 kV. The specimen for the HRTEM analysis was prepared by dropping the colloidal solution onto a carbon coated copper grid and dried at room temperature. Energy dispersive Xray (EDX) analysis was carried out using JEOL Model JSM-6390LV. Electrochemical characterization of so prepared AgNPs was performed by using a CH Instruments electrochemical workstation (Model–760D). FT-IR analyses were carried out using Shimadzu (8400S). NMR was recorded on Burker 300 MHz instrument using CDCl3 as solvent and the chemical shift values are reported as ␦ values (ppm) with reference to tetramethylsilane (TMS). Catalytic reduction of 4-NA was carried out by taking 0.1 mL of 2 mM of 4NA solution in 1.15 mL of water followed by 0.75 mL of 0.056 M NaBH4. To this mixture 10 ␮L of prepared AgNPs was added and the reaction progress was monitored by UV–vis spectroscopy.

129

tion was added and stirred for 48 h. The color of the solution was changed from colorless to yellow confirming the formation of AgNPs. 2.4. Electrochemistry All the electrochemical experiments were conducted in a single compartment three electrode cell using a CHI760D Electrochemical Workstation, CH Instruments, USA. A GC electrode (3 mm dia) and a platinum wire were used as working and counter electrodes, respectively. Saturated calomel electrode was used as reference electrode. A 5 ␮L of the AgNPs solution was drop-casted onto the cleaned GC surface and allowing it to dry for 2 h at room temperature and used for the electrochemical experiments. Phosphate buffer solution (PBS) (pH = 7.2) was used as electrolyte for electrocatalysis and sensor studies. Nitrogen gas was bubbled into the cell solution for 25 min prior to each experiment unless otherwise mentioned. 3. Results and discussion 3.1. Absorption spectral, HRTEM and FTIR studies of AgNPs The formation of AgNPs in the presence of PADA and TPDT matrix was initially confirmed by noticing the appearance of yellow color of the reaction solution. The slow growth of AgNPs was monitored by recording the absorption spectra at different reaction times (Fig. S1). The intensity of the SPR absorption band of the AgNPs at 413 nm increased gradually over the period of time without any change in the band position, which confirmed the uniform growth of AgNPs in the presence of PADA and TPDT. The absorption spectrum of the PADA(1)-Ag-TPDT NPs (Fig. 1a) showed a sharp SPR band at 413 nm, which suggested the formation of mono-dispersed spherical AgNPs and the Ag-TPDT NPs showed the SPR band at 420 nm (Fig. 1e). The small blue shift observed in the SPR band of PADA(1)-Ag-TPDT NPs in comparison with Ag-TPDT NPs is due to reduced particle size [18], which was evidenced form the HRTEM images (Fig. 2) and also might be due to interaction with PADA. The PADA(0.25)-Ag-TPDT (c), PADA(0.5)-Ag-TPDT (b) and PADA(2)-AgTPDT (d) NPs also showed the SPR absorption features of AgNPs at 416, 416 and 421 nm, respectively (Fig. 1). Hence, the absorption band around 410 nm confirmed the formation of AgNPs either in the presence of PADA-TPDT mixture or in the presence of only TPDT. In the present synthesis, no other external reducing agent

2.2. Synthesis of PADA(1)-Ag-TPDT NPs 1.5

Absorbance

A 5 mL of 1 wt.% PADA solution was mixed with 25 ␮L of 1 M TPDT solution and stirred for 15 min. To this solution, 0.1 mL of 0.25 M AgNO3 solution was added and stirred for 48 h. The appearance of yellow color solution confirmed the formation of silver nanoparticles (represented as PADA(1)-Ag-TPDT). Moreover, the prepared AgNPs were stable for more than a month. By following the same procedure, the PADA(0.25)-Ag-TPDT, PADA(0.5)-Ag-TPDT and PADA(2)-Ag-TPDT NPs were prepared with different concentrations of PADA. In the above representation, value in the parenthesis after PADA represents the wt.% of PADA solution used in the preparation. A control experiment was performed to check the formation AgNPs in the presence of PADA alone by adopting the same procedure but the formation of AgNPs was not observed.

a b

1.0

c e d

0.5

0.0 400

2.3. Synthesis of Ag-TPDT NPs A 25 ␮L of 1 M TPDT solution was added to 5 mL of water and stirred for 15 min. To this solution, 0.1 mL of 0.25 M AgNO3 solu-

600

800

wavelength (nm) Fig. 1. Absorption spectra of PADA(1)-Ag-TPDT (a), PADA(0.5)-Ag-TPDT (b), PADA(0.25)-Ag-TPDT (c), PADA(2)-Ag-TPDT (d) and Ag-TPDT (e) NPs solutions.

130

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

Fig. 2. HRTEM images of PADA(1)-Ag-TPDT (A and B) and Ag-TPDT NPs (D and E). C and F are the corresponding SEAD patterns of PADA(1)-Ag-TPDT and Ag-TPDT NPs, respectively.

was used for the reduction of Ag+ ions. The TPDT acted as a reducing agent and both TPDT and PADA acted as the stabilizing agents for AgNPs. The reduction of Ag+ ions by TPDT could be attributed to the presence of amine groups in the TPDT [12,19] and thereby, produced the AgNPs. The AgNPs formation was not observed in the presence of only PADA. HRTEM studies of the PADA(1)-Ag-TPDT and Ag-TPDT NPs (Fig. 2) confirmed the formation of AgNPs in the presence of TPDT and in the presence of both PADA and TPDT matrix. The EDX analysis further confirmed the persence of Ag in the PADA(1)-Ag-TPDT nanocomposite (Fig. S2). Fig. S3 shows the HRTEM images obtained for PADA(0.25)-Ag-TPDT, PADA(0.5)-Ag-TPDT and PADA(2)-AgTPDT NPs. Fig. S4 shows the particle size distribution of prepared AgNPs. When the particle size distribution is considered, majority of the AgNPs particles formed in the presence of PADA and TPDT are in the size range of 8–12 nm, whereas, in the presence of TPDT alone, majority of the AgNPs formed are in the size range of 16–20 nm. These HRTEM results clearly show the effect of PADA in combination with TPDT on the formation of smaller AgNPs. The d spacing values measured from the SAED pattern for PADA(1)-AgTPDT corresponds to the (111), (200) and (311) crystal planes of Ag. The d spacing values obtained from the SAED pattern for Ag-TPDT corresponds to the (111) crystal plane of Ag. This reveals the crystalline nature of the AgNPs. More bright spots in the SEAD pattern of PADA(1)-Ag-TPDT than in the Ag-TPDT showed the higher crystalline nature of PADA(1)-Ag-TPDT NPs. These results confirmed that better formation of AgNPs was observed in the presence of both PADA and TPDT than in the presence of only TPDT. Further, to understand the existence of TPDT and PADA with AgNPs, FTIR studies were carried out (Fig. S5). FTIR spectrum of PADA showed bands at 1666 and 2920 cm−1 due to the stretching vibrations of C O and CH2 groups, respectively. The hump observed around 1607 cm−1 represents the bending vibration of N H bond for PADA. The bands observed for TPDT at 1116 and 2920 cm−1 corresponds to the stretching vibrations of Si-O-Si bond and CH2 group, respectively and the band at 1627 cm−1 due to the bending vibrations of

Fig. 3. CVs recorded for GC/(PADA(1)-Ag-TPDT) (a) and GC/(Ag-TPDT) (b) in 0.1 M PBS (pH 7.2) at a scan rate of 50 mV s−1 .

N H bond. The FTIR spectrum of PADA(1)-Ag-TPDT contains the characteristic bands observed for both PADA and TPDT and this confirms that AgNPs were successfully capped by both TPDT and PADA in PADA(1)-Ag-TPDT sample. 3.2. Electrochemical behavior of the AgNPs modified electrodes In order to understand the electrochemical properties of the AgNPs, cyclic voltammograms (CVs) were recorded for the PADA (1)-Ag-TPDT and Ag-TPDT modified GC electrodes in 0.1 M PBS (Fig. 3). The modified GC electrodes were represented as GC/(PADA(1)-Ag-TPDT) and GC/(Ag-TPDT). The characteristic Ag oxidation and the corresponding reduction were observed in the CV for both GC/(PADA(1)-Ag-TPDT) and GC/(Ag-TPDT) electrodes. The anodic peaks observed at +0.198 and +0.260 V for the PADA(1)Ag-TPDT and Ag-TPDT electrodes, respectively, corresponds to the formation of Ag2 O at the surface of AgNPs and the correspond-

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

Fig. 4. CVs recorded for 1 mM H2 O2 at GC/(PADA(1)-Ag-TPDT) (f), GC/(PADA(0.5)Ag-TPDT) (e), GC/(PADA(0.25)-Ag-TPDT) (d), GC/(PADA(2)-Ag-TPDT) (g), GC/(AgTPDT) (c) NPs and bare GCE (b) electrodes in 0.1 M PBS. Scan rate is 50 mV s−1 . a: In the absence of H2 O2 at GC/(PADA(1)-Ag-TPDT).

131

Fig. 5. CVs recorded for 1 mM H2 O2 at GC/(PADA(1)-Ag-TPDT) in 0.1 M PBS at different scan rates (10–300 mV s−1 ). Inset shows the plot of cathodic peak current against square root of scan rate.

ing Ag2 O reduction peaks were observed at 0 V and −0.25 V for PADA(1)-Ag-TPDT and Ag-TPDT, respectively. These observations suggest that the AgNPs and GC electrode were in good electrical contact with each other. The higher oxidation and reduction peak currents observed for PADA(1)-Ag-TPDT NPs was attributed to the more amount of silver oxidized to Ag2 O compared to the Ag-TPDT NPs. 3.3. Non-enzymatic electrocatalytic reduction and detection of H2 O2 The AgNPs modified GC electrodes were successfully used for the enzymeless electrocatalytic reduction of H2 O2 in 0.1 M PBS (pH 7.2). Fig. 4 showed the CVs recorded for 1 mM H2 O2 at the bare GC (b), GC/(Ag-TPDT) (c), GC/(PADA(0.25)-Ag-TPDT) (d), GC/(PADA(0.5)-Ag-TPDT) (e), GC/(PADA(1)-Ag-TPDT) (f) and GC/(PADA(2)-Ag-TPDT) (g) electrodes. All the AgNPs modified GC electrodes showed the catalytic current responses for the electrocatalytic reduction of H2 O2 and however, the bare GC electrode did not show any observable catalytic current in the potential window. The GC/(Ag-TPDT) and GC/(PADA(0.25)-Ag-TPDT) electrodes showed the observable catalytic current at more negative potentials towards the reduction of H2 O2 . The GC/(PADA(0.5)-Ag-TPDT) and GC/(PADA(2)-Ag-TPDT) electrodes displayed the catalytic peak currents for the reduction of H2 O2 at −0.74 V and −0.64 V, respectively. Interestingly the GC/(PADA(1)-Ag-TPDT) electrode showed the higher catalytic current of 34 ␮A at −0.62 V in comparison with the other modified electrodes. No current response was observed at the GC/(PADA(1)-Ag-TPDT) electrode in the absence of H2 O2 (Fig. 4a). The observations from Fig. 4. clearly revealed the improved electrocatalytic effect of AgNPs formed in the presence of both PADA and TPDT than the Ag-TPDT NPs. The enhanced electocatalytic activity of PADA-Ag-TPDT NPs is attributed to decreased particles size distribution (Fig. S4) and thereby produced more electroactive silver on NPs surface when compared to Ag-TPDT NPs. Among the various PADA concentrations (0.25, 0.5, 1, 2%) employed to produce AgNPs in association with TPDT, PADA(1)-Ag-TPDT NPs showed the better performance towards electrocatalytic reduction of H2 O2 , this synergistic catalytic effect is attributed to the optimum concentration of PADA around the AgNPs. The effect of scan rate on the reduction of H2 O2 was studied at GC/(PADA(1)-Ag-TPDT) to understand the reaction process at the modified electrode. The peak current for the reduction of H2 O2 was increased with increasing the scan rate (Fig. 5). The plot of peak currents against the square

Fig. 6. LSVs recorded for 5–400 ␮M concentration range of H2 O2 at GC/(PADA(1)Ag-TPDT) electrode in 0.1 M PBS. Scan rate is 50 mV s−1 . Inset shows the calibration plot of current vs. concentration of H2 O2 .

root of scan rate showed a linear relationship with a correlation coefficient of 0.999. Hence, the electrocatalytic reduction of H2 O2 at the GC/(PADA(1)-Ag-TPDT) electrode was a diffusion controlled process. For the irreversible reduction of H2 O2 at the GC/(PADA(1)Ag-TPDT) electrode, the calculated heterogeneous rate constant was 1.80 × 10−3 cm2 s−1 using Eq. (1). ko = 1.11D0 1/2 (E p − E p /2 )−1/2 1/2

(1)

where, ko is heterogeneous rate constant, D0 is apparent diffusion coefficient of H2 O2 (1 × 10−5 cm2 s−1 ), Ep is peak potential (−0.630 V), Ep/2 is half wave potential of H2 O2 reduction (−0.440 V) and  is scan rate (0.05 V s−1 ). Owing to the excellent electrocatalytic behaviour of the GC/(PADA(1)-Ag-TPDT) electrode towards the reduction of H2 O2 , this electrode was chosen for sensing of H2 O2 at lower concentration levels. The LSV responses obtained at the GC/(PADA(1)Ag-TPDT) electrode for H2 O2 in the concentration range of 5 ␮M–0.4 mM showed an observable increase in the cathodic current for the reduction of H2 O2 (Fig. 6). The calibration plot (inset of Fig. 6) of peak current against concentration of H2 O2 showed the linear response for the addition of different concentration of H2 O2 . Moreover, the sensitivity of the modified electrode towards H2 O2 was found to be 0.034 ␮A/␮M. Fig. 7 shows the SWV responses

132

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

Table 1 The comparison of some reported H2 O2 sensors with the present sensor. Electrode

Technique

Detection limit (␮M)

Linear range

Sensitivity

Reference

GC/TPDT-Ag NPs GC/TPDT-SiO2/Au NPs GC/MTMOSb -Au NPs GC/MTMOS-Au/Ag NPs GC/Ag microspheres GC/PEId -Ag NCs Pt/PVAe -Ag Ag NPs/3DGf GCE/GOg -Ag nanocomposite GC/(PADA(1)-Ag-TPDT) GC/(PADA(1)-Ag-TPDT)

DPV LSV Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry SWV LSV

0.1 (Exp.a ) 5 (Exp.) 2.5(Exp.) 10 (Exp.) 1.2 (Cal.c ) 1.8 (Cal.) 1(Cal.) 14.9 (Cal.) 28.3 (Cal.) 0.2 (Exp.) 5 (Exp.)

– – 2.5–45 ␮M 10–70 ␮M 0.2–2 mM 0.01–1.44 mM 0.005–1 mM 0.03–16.21 mM 0.1–11 mM 0.2–2 ␮M 5–400 ␮M

1.144 ␮A/␮M 0.0118 ␮A/␮M – – – – 128 nA/␮M 1.094 mA/mM cm−2 0.1218 ␮A/mM 1.471 ␮A/␮M 0.034 ␮A/␮M

[5] [20] [21] [22] [23] [24] [25] [26] [27] Present work Present work

a b c d e f g

Experimental. Calculated. Methyltrimethoxysilane. Polyethyleneimine. Polyvinyl alcohol. Three-dimensional grapheme. Graphene oxide.

1.2

Absorbance

1.0

0s

0.8 0.6

24 s

0.4 0.2 0.0 300

400

500

Wavelength / nm Fig. 7. SWVs recorded for each addition of 0.2 ␮M H2 O2 at GC/(PADA(1)-Ag-TPDT) electrode in 0.1 M PBS. Scan rate is 50 mV s−1 . Inset shows the calibration plot of current vs. concentration of H2 O2 .

observed at the GC/(PADA(1)-Ag-TPDT) electrode for each addition of 0.2 ␮M H2 O2 in 0.1 M PBS. A linear increment in current was observed for the successive addition of H2 O2 and the sensitivity was found to be 1.471 ␮A/␮M. Table 1 shows the comparison of previously reported electrochemical H2 O2 sensors with the present sensor. 4.1. Catalytic reduction of 4-nitroaniline To further evaluate the catalytic activity of the AgNPs embedded in polyelectrolyte and silicate matrix, the catalytic reduction of 4-NA was chosen as a model system. The AgNPs were used as the catalyst for the reduction of 4-NA in the presence of NaBH4 and the spectral changes associated with the reduction of 4-NA were monitored using UV–vis spectroscopy. It is well known that NaBH4 could not reduce 4-NA in the absence of any catalyst. After the addition of aqueous suspension of AgNPs catalyst, there was a rapid decrease in the absorption of 4-NA at 380 nm along with simultaneous appearance of two bands at 238 and 305 nm. The bands observed at 238 and 305 nm were the characteristic absorption bands of PPD [17]. This observation confirmed the catalytic reduction of 4-NA to PPD by AgNPs catalyst. The mechanistic pathway for the reduction of aromatic nitrocompounds to the aromatic amine compounds has been reported to follow either a direct route or a condensation route

Fig. 8. Time-dependent UV–vis absorption spectra recorded for the borohydride reduction of 4-NA in the presence of PADA(1)-Ag-TPDT NPs catalyst.

[28,29]. In the most probable direct pathway, both BH4 − ion and 4NA would get adsorbed on to the catalyst surface first [28]. BH4 − ion acts as the hydrogen source, which was necessary to accomplish the hydrogenation of 4-NA on the surface of the AgNPs. Because of the great potential of NaBH4 in the hydrogen production fuel cells [30], the interaction of NaBH4 with the metal surfaces and its decomposition in the presence of metallic NPs have become the intense research topic. Hence, BH4 − ions could charge the metal NPs surface and the charged metal NPs could act as the electron source [31]. The aromatic nitrocompound was first reduced to the nitroso compound and then quickly to the corresponding hydroxylamine compound. The hydroxylamine compound was finally reduced to the aromatic amine [32]. And the reduced product (PPD) formation was also confirmed by recording 1 H NMR spectrum (Fig. S7). Figs. 8 and 9 show the UV–vis absorption spectral changes recorded for the catalytic reduction 4-NA to PPD as a function of time on PADA(1)-Ag-TPDT and Ag-TPDT NPs, respectively, upon the addition of NaBH4 . A 90% of 4-NA was converted into product within 24 s, when PADA(1)-Ag-TPDT NPs was used as catalyst, whereas the Ag-TPDT NPs showed the conversion of 4-NA with the reaction time of 110 s. This observation clearly revealed the improved catalytic effect of PADA(1)-Ag-TPDT NPs over Ag-TPDT NPs towards the reduction of 4-NA. The PADA(0.25)-Ag-TPDT, PADA(0.5)-Ag-TPDT and PADA(2)-Ag-TPDT NPs (Fig. S6) are also catalytically reduced the 4-NA with the time period of 60, 50 and 40 s, respectively. This observation showed the superior catalytic

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

133

1.2 0.0

0s -0.5

In (A)

Absorbance

1.0 0.8 0.6

e

-1.5

110 s

0.4

-1.0

a

-2.0

b

0.2

c d 300

400

0

500

20

40

60

80

100

120

Time / s

Wavelength / nm Fig. 9. Time-dependent UV–vis absorption spectra recorded for the borohydride reduction of 4-NA in the presence of Ag-TPDT NPs catalyst. Table 2 Comparison of the catalytic performances of prepared Ag catalyst with already reported catalysts. Catalyst

Time

Rate constant (k)

Reference

Au Rhombic Dodecahedra NPs Ag0.6 Ni0.4 alloy NPs Ag-p(NIPAM-co-AAAc)a hybrid microgels Au Nanowire networks Ag NPs PADA(1)-Ag-TPDT

60 min 80 s 10 min

7.575 × 10−2 min−1 23.5 × 10−3 s−1 0.215 min−1

[32] [35] [36]

a

17 min 27 min 24 s

0.182 min – 0.096 s−1

−1

[37] [38] Present work

poly (N-isopropylacrylamide-co-allylacetic acid).

effect of PADA(1)-Ag-TPDT NPs over the other AgNPs formed with different concentrations of PADA. Hence, 1% PADA was found to be the optimum concentration for the effective catalytic activity of AgNPs. Table 2 compares the catalytic performance of PADA(1)-AgTPDT NPs with other reported catalysts towards the reduction of 4NA, and this table shows the superior catalytic ability of present Ag catalyst over other catalysts. NMR analysis was carried out to further confirm the product formed in the catalytic reduction of 4NA. After completion of the reaction (0.2 mmol batch), the product was extracted with ethyl acetate, dried over sodium sulphate and combined organic layer was concentrated under reduced pressure at 40 ◦ C, which yielded PPD as a solid product. From the 1 H NMR spectrum of PPD (Fig. S7), ␦ values at 6.56(s, 4H-Aromatic) and 3.18 (s, 4H-amine) ppm confirms the successful formation of PPD over PADA(1)-Ag-TPDT NPs catalyst. In the catalytic conversion of 4-NA, the concentration of NaBH4 used was higher when compared to the 4-NA concentration. As the concentration of NaBH4 was very high, it would not alter the rate of the reaction. Therefore, it is reasonable to consider that the reduction of 4-NA reaction followed the pseudo-first order kinetics. Fig. 10 shows the kinetic plots of ln (A) versus time obtained for the catalytic conversion of 4-NA using different AgNPs as catalysts. The rate constants (k) estimated from the plots of ln (A) versus time are 0.040, 0.044, 0.096, 0.063 and 0.016 s−1 for PADA(0.25)-Ag-TPDT, PADA(0.5)-Ag-TPDT, PADA(1)-Ag-TPDT, PADA(2)-Ag-TPDT and Ag-TPDT NPs, respectively. Hence, in the present study, PADA(1)-Ag-TPDT NPs outforms the other AgNPs prepared here and the possible reason for the best catalytic effect of PADA(1)-Ag-TPDT NPs is as follows: As PADA is positively charged polyelectrolyte, they can attract electron rich analytes (H2 O2 and 4-NA) towards the AgNPs through electrostatic interaction [33,34]. At lower concentrations of PADA, it attracts lower concentration of analytes, whereas at higher concentration (2%) more amounts

Fig. 10. Plots of ln(A) vs. time obtained for the reduction of 4-NA to PPD at PADA(1)Ag-TPDT (a), PADA(2)-Ag-TPDT (b), PADA(0.5)-Ag-TPDT (c), PADA(0.25)-Ag-TPDT (d) and Ag-TPDT (e), NPs as catalyst in the presence of NaBH4 .

of analytes can be attracted by PADA. But at higher concentration, the higher coverage of PADA around AgNPs may hinder the diffusion of analytes towards the AgNPs. Hence, optimum concentration of PADA (1%) may favour both the analyte attraction and diffusion towards the catalytic sites and exhibits better catalytic activity. 5. Conclusion Polyelectrolyte and silicate matrix stabilized AgNPs were prepared without using any other external reducing agent to reduce the Ag+ ions and the prepared AgNPs were characterized using UV–vis absorption spectra, HRTEM, SAED, EDX and FTIR analyses. Moreover, polyelectrolyte (PADA) concentration was optimized to find the best AgNPs catalyst and 1% PADA was found to be the optimum concentration of PADA to produce best AgNPs catalyst. The prepared AgNPs catalyst was successfully used for the electrochemical reduction of H2 O2 and catalytic reduction 4-NA in the presence of NaBH4. Interestingly, the PADA(1)-Ag-TPDT NPs showed the best catalytic effect on both electrochemical reduction of H2 O2 and the chemical reduction of 4-NA. The PADA(1)-Ag-TPDT used to construct electrochemical sensor for H2 O2 with the detection limits of 5 ␮M (LSV) and 0.2 ␮M (SWV). Remarkably fast conversion of 4-NA to PPD was observed at the PADA(1)-Ag-TPDT NPs catalyst with a rate constant of 0.096 s−1 . Hence, it is quite reasonable to conclude that the combination of 1% PADA with functionalized silicate matrix assisted formation of AgNPs showed the best catalytic activity. Acknowledgments RR acknowledges the financial support received from the Science and Engineering Research Board (SERB), New Delhi. PV is the recipient of Senior Research Fellowship under UGC-BSR scheme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata.2016.08. 001. References [1] E.C. Dreaden, A.M. Alkilany, X. Huang, C.J. Murphy, M.A. El-Sayed, Chem. Soc. Rev. 41 (2012) 2740–2779. [2] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Chem. Rev. 107 (2007) 2228–2269. [3] J.E. Hutchison, ACS Nano. 2 (2008) 395–402. [4] Q.H. Tran1, V.Q. Nguyen, A.-T. Le, Adv. Nat. Sci. Nanosci. Nanotechnol. 4 (2013) 033001 (20pp).

134

P. Viswanathan, R. Ramaraj / Journal of Molecular Catalysis A: Chemical 424 (2016) 128–134

[5] P. Rameshkumar, P. Viswanathan, R. Ramaraj, Sens. Actuators B 202 (2014) 1070–1077. [6] S. Manivannan, R. Ramaraj, Analyst 138 (2013) 1733–1739. [7] M. Pal, V. Ganesan, Catal. Sci. Technol. 2 (2012) 2383–2388. [8] B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. Eur. J. 11 (2005) 454–463. [9] A. Tao, P. Sinsermsuksakul, P. Yang, Angew. Chem. Int. Ed. 45 (2006) 4597–4601. [10] H. Chen, Y. Wang, S. Dong, Inorg. Chem. 46 (2007) 10587–10593. [11] P. Viswanathan, S. Manivannan, R. Ramaraj, RSC Adv. 5 (2015) 54735–54741. [12] G. Maduraiveeran, R. Ramaraj, Anal. Chem. 81 (2009) 7552–7560. [13] P. Pang, Z. Yang, S. Xiao, J. Xie, Y. Zhang, Y. Gao, J. Electroanal. Chem. 727 (2014) 27–33. [14] Z. Meng, M. Zhang, H.Z.J. Zheng, Meas. Sci. Technol. 25 (2014) 025301 (5pp). [15] L. Li, Z. Du, S. Liu, Q. Hao, Y. Wang, Q. Li, T. Wang, Talanta 82 (2010) 1637–1641. [16] P. Kovacic, R. Somanathan, J. Appl. Toxicol. 34 (2014) 810–8241. [17] W. Wu, G. Liu, Q. Xie, S. Liang, H. Zheng, R. Yuan, W. Su, L. Wu, Green Chem. 14 (2012) 1705–1709. [18] E. Saion, E. Gharibshahi, K. Naghavi, Int. J. Mol. Sci. 14 (2013) 7880–7896. [19] A. Frattini, N. Pellegri, D. Nicastro, O.de Sanctis, Mater. Chem. Phys. 94 (2005) 148–152. [20] P. Rameshkumar, R. Ramaraj, J. Appl. Electrochem. 43 (2013) 1005–1010. [21] G. Maduraiveeran, R. Ramaraj, J. Electroanal. Chem. 608 (2007) 52–58. [22] S. Manivannan, R. Ramaraj, J. Chem. Sci. 121 (2009) 735–743. [23] B. Zhao, Z. Liu, G. Liu, Z. Li, J. Wang, X. Dong, Electrochem. Commun. 11 (2009) 1707–1710.

[24] B.L. Li, J.R. Chen, H.Q. Luo, N.B. Li, J. Electroanal. Chem. 706 (2013) 64–68. [25] M.R. Guascito, E. Filippo, C. Malitesta, D. Manno, A. Serra, A. Turco, Biosens. Bioelectron. 24 (2008) 1057–1063. [26] B. Zhan, C. Liu, H. Shi, C. Li, L. Wang, W. Huang, X. Dong, Appl. Phys. Lett. 104 (2014) 243704–243709. [27] A.M. Noor, M.M. Shahid, P. Rameshkumar, N.M. Huang, Microchim. Acta 183 (2016) 911–916. [28] K. Layek, M.L. Kantam, M. Shirai, D. N-Hamane, T. Sasaki, H. Maheswaran, Green Chem. 14 (2012) 3164–3174. [29 X.-B. Lou, L. He, Y. Qian, Y.-M. Liu, Y. Cao, K.-N. Fan, Adv. Synth. Catal. 353 (2011) 281–286. [30] B.H. Liu, Z.B. Li, J. Power Sources 187 (2009) 527–534. [31] A. Henglein, J. Lilie, J. Am. Chem. Soc. 103 (1981) 1059–1066. [32] C.-Y. Chiu, P.-J. Chung, K.-U. Lao, C.-W. Liao, M.H. Huang, J. Phys. Chem. C 116 (2012) 23757–23763. [33] C. Gu, H. Xu, M. Park, C. Shannon, Langmuir 25 (2009) 410–414. [34] C. Gu, H. Xu, M. Park, C. Shannon, ECS Trans. 16 (4) (2008) 181–190. [35] M. Kumar, S. Deka, ACS Appl. Mater. Interfaces 6 (2014) 16071–16081. [36] Z.H. Farooqi, N. Tariq, R. Begum, S.R. Khan, Z. Iqbal, A. Khan, Turk. J. Chem. 39 (2015) 576–588. [37] M. Chirea, A. Freitas, B.S. Vasile, C. Ghitulica, C.M. Pereira, F. Silva, Langmuir 27 (2011) 3906–3913. [38] Q. Zhou, G. Qian, Y. Li, G. Zhao, Y. Chao, J. Zheng, Thin Solid Films 516 (2008) 953–956.