Synthesis of cyclodextrin-silicate sol–gel composite embedded gold nanoparticles and its electrocatalytic application

Chemical Engineering Journal 210 (2012) 195–202

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Synthesis of cyclodextrin-silicate sol–gel composite embedded gold nanoparticles and its electrocatalytic application Shanmugam Manivannan, Ramasamy Ramaraj ⇑ Centre for Photoelectrochemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" Synthesis of Au NPs embedded in

The green and single step synthetic method for the preparation of gold nanoparticles (Au NPs) embedded in amine functionalized silicate (TPDT) and b-cyclodextrin (CD) composite in aqueous solution using biocompatible b-CD is presented. The electrocatalytic reduction and sensing of nitroaromatics were evaluated at the GC/b-CD–Au–TPDT modified electrode using the cyclic voltammetric and the square-wave voltammetric techniques.

TPDT sol–gel and b-CD nanocomposite materials. " Size controlled Au NPs of 4.9 nm embedded in b-CD–TPDT size obtained. " TPDT–CD–Au electrode was used for nitrobenzene electrocatalysis and sensing. " b-CD played a major role both in the synthesis and electrocatalysis.

a r t i c l e

i n f o

Article history: Received 28 April 2012 Received in revised form 16 August 2012 Accepted 23 August 2012 Available online 4 September 2012 Keywords: Gold nanoparticles Silicate sol–gel b-Cyclodextrin Nitroaromatics Electrocatalysis Electrochemical sensor

a b s t r a c t Single step synthesis of gold nanoparticles (Au NPs) in aqueous solution using amine functionalized silane and b-cyclodextrin (CD) composite is reported. High resolution transmission electron microscopy was used to characterize the 3 and 5 nm Au NPs embedded in b-CD-silicate sol–gel matrix (TPDT) composite. The electrocatalysis and the sensing of nitroaromatics were studied at the Au NPs embedded in the b-CD–TPDT composite modified electrode using the cyclic voltammetric and the square-wave voltammetric techniques. The square-wave voltammetric technique provides a facile, simple and fast quantitative method for the detection and determination of nitroaromatics at the Au NPs modified electrode. It is inferred that the b-CD plays a predominant role both in the green synthesis of Au NPs embedded in bCD–TPDT composite and its applications in the electrocatalysis and sensing of nitroaromatics. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of Gold nanoparticles (Au NPs) with various sizes and shapes is always the fundamental key point for various ⇑ Corresponding author. E-mail address: [email protected] (R. Ramaraj). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.08.085

applications [1,2]. Au NPs with the characteristic size dependent properties show a wide range of applications in catalysis, nanoelectronic and optical devices, and biosensor and other related areas [3–7]. Among the conventional synthetic methods, the most popular one is the reduction of HAuCl4 by citrate in water to obtain Au NPs with size range varying from 10 nm to 150 nm [8–10]. The Au NPs, which display unusual physical and chemical properties

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because of their unique size and shape, can provide a mild microenvironment similar to that of redox enzyme in native systems [11] to make possible conducting channels between the prosthetic groups and the electrode surface, and reduce the effective electron transfer distance thereby facilitating the charge transfer at the interface of the electrode [12]. The metal NPs with excellent physical and chemical properties do not often possess suitable surface properties for specific applications [13]. For this reason, surface-modification techniques are adopted which transforms these materials into valuable finished products. Another important issue in the preparation of nanomaterials is to develop greener synthetic approaches. The use of an aqueous medium for the surfactant directed growth technique is in accordance with the principles of green chemistry. A further step to a greener process will be the replacement of sodium borohydride and alkyltrimethylammonium halides with biocompatible agents. In this respect, the potential usefulness of silicate sol–gel matrix (SSG) and cyclodextrins (CDs) as the greener alternative to sodium borohydride and alkyltrimethylammonium halides was exploited to synthesize the Au NPs. The b-cyclodextrin (b-CD) is a water-soluble and nontoxic cyclic oligosaccharide having a hydrophilic exterior and a hydrophobic interior cavity. The synthesis and stabilization of metal NPs embedded in SSG matrix have the advantages of a single step preparation, uniform distribution of nanometer sized particles and the versatility of making the matrix in the form of sols, gels, films and monoliths. The sol–gel processing of materials coupled with the inherent advantages of the organically modified silicates (Ormosils) makes these matrices very attractive as supports and stabilizers for metal NPs in both liquid and solid phases [14–21]. Previously unmodified b-CD was used to control the size and distribution of Au, silver and palladium NPs [22,23]. The nature of interaction between the b-CD and the Au NPs was studied using the laser-induced ablation technique and the effect of medium pH for size control was also studied [24]. Owing to the poor film-forming capability of colloidal Au NPs, it is desirable to provide a matrix support in terms of the encapsulation of Au NPs to find various applications. In the present work, Au NPs in the presence of b-CD–TPDT composite (b-CD–Au–TPDT NPs) and in the absence of b-CD (Au–TPDT NPs) were prepared and characterized. Modified electrodes were prepared using these Au NPs and used for the electrocatalytic reduction and sensing of nitroaromatics. The reduction of nitroaromatics is of considerable interest since they are common pollutant in wastewater originating from numerous industrial and agricultural activities [25]. The present work demonstrated that the b-CD in combination with amine functionalized TPDT SSG played a predominant role both in the green synthesis of Au NPs and its electrocatalytic applications at the modified electrode.

Agilent Technologies 8453 spectrophotometer using a 1 cm quartz cell. The TEM images of the Au NPs were recorded with a FEI TECNAI 30 G2 S-TWIN instrument. A sample was prepared by placing a drop of fresh Au NPs on copper grid and then evaporating the solvent under vacuum.

2. Experimental section

3. Results and discussion

2.1. Materials and instruments

3.1. Spectral studies

Gold(III) chloride hydrate (HAuCl4
3H2O), b-cyclodextrin, N-[3(trimethoxysilyl)propyl] diethylenetriamine (TPDT), N-[3-(trimethoxysilyl) propyl] ethylenediamine (EDAS), (3-aminopropyl) triethoxysilalne (APS) and triethoxymethylsilane (ETMOS) were received from Sigma–Aldrich. Hydroquinone was received from Merck. Nitrobenzene, nitrotoluene, nitrobenzoic acid and nitroaniline were received from SRL. All glassware were thoroughly cleaned with aqua regia (1:3 HNO3/HCl (v/v)) (Precaution: Aqua regia is a powerful oxidizing agent, and it should be handled with extreme care) and rinsed extensively with doubly distilled water before use. Absorption spectra of Au NPs were recorded with an

Stepping-down the nanomaterial’s size by introducing different reagents and synthetic approaches is one of the current trends in contemporary research. The optical property of the Au NPs strongly depends on their size and surface modification and it can easily be monitored through absorption spectroscopy by means of analyzing their SPR band [28–32]. Fig. 1 shows the SPR absorption spectra obtained for the TPDT SSG embedded Au NPs in the absence (Fig. 1a) and presence of the b-CD (Fig. 1b solutions. The faster formation of the Au NPs and stabilization was observed only in the presence of the b-CD–TPDT composite. Previous reports [23,26,33,34] reveal that alkaline conditions facilitate the deproto-

2.2. Synthesis of core/shell b-CD–Au–TPDT NPs A homogeneous b-CD–TPDT composite was prepared by the addition of 25 lL of 1 M TPDT silane monomer into 5 mL of aqueous solution containing 7 mM b-CD under vigorous stirring and the stirring was continued for another 60 min. The Au NPs in b-CD– TPDT composite (b-CD–Au–TPDT NPs) was prepared by adding 50 lL of 0.1 M HAuCl4 to this composite and continued the stirring for 24 h. The color of the mixture quickly turned into dark yellow because of the ammine–chloride complex formation [14–17,26] between amine functionalized silane and AuCl4 . The mixture was stirred for 24 h till the color of the solution turned dark wine red (for TPDT silane) which confirmed the formation of b-CD–Au–TPDT NPs. Light wine red color or dark blue color was observed for EDAS, APS and ETMOS silanes, respectively due to the incomplete formation of Au NPs. The same procedure was followed to prepare the Au–TPDT NPs without b-CD. 2.3. Electrochemistry The glassy carbon (GC) electrode (dia = 3 mm) (CH Instruments, USA) was twice polished using alumina powder (0.05 lm) followed by sonication in doubly distilled water for 3 min. The cleaned GC electrode was dried for 5 min at room temperature. A known volume of TPDT silicate sol–gel or b-CD–TPDT composite or Au–TPDT or b-CD–Au–TPDT NPs solution was drop casted on the bare GC electrode surface and allowed to dry at room temperature for 1 h (represented as GC/TPDT, GC/b-CD–TPDT, GC/Au– TPDT and GC/b-CD–Au–TPDT, respectively). The dried electrode was then soaked in distilled water for 5 min and used for electrochemical experiments. The thickness of the film coated on the electrode was calculated as 1 lm [27]. The cyclic voltammograms (CVs) of the modified electrodes were recorded using CH Instruments Electrochemical Workstation (model-760D). The electrochemical experiments were performed using a single compartment three-electrode cell. The modified GC electrode was used as a working electrode and platinum wire as counter electrode. The reference electrode used was a saturated calomel electrode. The solution was deaerated by purging nitrogen gas for 25 min before each experiment. Square wave voltammetric measurements were carried out in the potential range 0.3 V to 1 V with a step potential of 4 mV, amplitude of 25 mV, and a frequency of 25 Hz (unless otherwise stated).

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was chosen for further studies of b-CD–Au–TPDT NPs. The SPR absorption spectrum of the b-CD stabilized Au NPs (b-CD–Au NPs) (Fig. S3(inset)) without TPDT silicate sol–gel matrix showed an absorption band at 520 nm due to the Au NPs [33,35].

b 3.2. Effect of amine group of the TPDT SSG on the Au NPs synthesis

1.0

a b

0.5

a 0.0 300

400

500

600

700

800

Wavelength/nm Fig. 1. Surface plasmon absorption spectra obtained for Au–TPDT (a) and b-CD–Au– TPDT NPs (b) solution after 24 h stirring. Inset show the photograph of the prepared samples (a and b).

nation the of primary –OH group in b-CD and promote the kinetic evolution and stabilization of NPs. In the present synthetic procedure, the b-CD (pH 4.86) and the TPDT silane (pH 9.72) were mixed together in an aqueous medium (pH of the mixture was 9.84) and the TPDT silane on hydrolysis formed silanol functionalities and bound (through hydrogen bond) to the –OH group of the b-CD. The pH (9.84) of the solution was sufficient to facilitate the deprotonation of the b-CD (exact role of pH and the b-CD discussed later) and induced the faster formation of the b-CD–Au–TPDT NPs from the HAuCl4 precursor [35]. In the absence of the b-CD, the tri-amine groups of the TPDT SSG contribute in the formation and stabilization of the Au NPs [14– 17]. The reduction of HAuCl4 by the amine groups of the TPDT was very slow during the formation of the Au–TPDT NPs (Fig. 1a. The inset in Fig. 1 represents the comparison of photograph of the Au–TPDT (a) and the b-CD–Au–TPDT NPs (b), respectively. The development of intense wine red color was observed in the presence of the b-CD–TPDT composite and light wine red color was observed in the absence of the b-CD even after 5 days (Fig. S1). The Au–TPDT NPs growth reached saturation even in the absence of the b-CD, and this confirmed the reducing capability of the TPDT SSG to reduce the Au metal precursor. This slower and faster formation of the Au–TPDT and the b-CD–Au–TPDT NPs from the metal precursors, respectively, was reflected in the absorption spectra recorded with time and their size and distribution of the Au NPs in the matrix. The SPR bands observed for the Au–TPDT and the b-CD–Au–TPDT NPs at 535 and 526 nm (Fig. 1) are different from the SPR band observed for the citrate stabilized Au NPs at 520 nm for an average particle size of 15 nm [36] due to the fact that the Au NPs was embedded in the TPDT SSG matrix and the b-CD–TPDT composite, respectively. In addition to the SPR band new absorption bands were observed at 352 and 348 nm (Fig. 1) for the Au–TPDT and the b-CD–Au–TPDT NPs, respectively. The absorption band observed at 362 nm was due to the TPDT SSG and upon the addition of the HAuCl4, the formation of ammine– chloride complex between the HAuCl4 and the TPDT [14–17] showed an absorption band at 352 nm (Fig. 1a). The mixing of bCD with the TPDT SSG enhanced the optical intensity at 352 nm (Fig. S2) with a small red shift of 10 nm due to the interaction of Au NPs with amine and hydroxyl groups of the b-CD–TPDT composite (Fig. 1b). This observation further supported the fact that the Au NPs were embedded inside the b-CD–TPDT composite. Faster formation of the Au NPs was observed in the presence of the b-CD when compared to a-CD and c-CD (Fig. S3) due to the smaller and larger cavity size, respectively [22,23] and the b-CD

To understand the role of amine groups present in the TPDT SSG (tri-amine), controlled experiments were carried out using EDAS SSG (di-amine), APS SSG (mono-amine), and ethyl functionalized ETMOS SSG (absence of amine group) in the presence of b-CD. Comparison of SPR absorption spectra observed for the four different b-CD–SSG composite matrices embedded Au NPs (Fig. 2) clearly revealed that the reduction of Au precursor did not take place in the presence of the ETMOS SSG and the SPR band due to the Au NPs around 520–540 nm was disappeared (Fig. 2a). The slow formation of unstable Au NPs with time dependant color change was observed for Au NPs in the presence of b-CD–APS and b-CD–EDAS composites. The inset in Fig. 2 depicts the comparison of photograph taken after 24 h for b-CD–Au–ETMOS mixture (a), b-CD–Au–APS (b) b-CD–Au–EDAS (c), and b-CD–Au–TPDT NPs (d) solutions. Among the four different b-CD–SSG composites, the stable bright wine red color was observed only for the b-CD–TPDT embedded Au NPs when compared to other composites. The Au NPs formation did not occur in the presence of the ETMOS SSG (Fig. 2a). 3.3. TEM studies Fig. 3 represents the comparison of the TEM images obtained for the Au–TPDT (A and B) and b-CD–Au–TPDT NPs (C and D) at different magnifications. The Au NPs embedded by TPDT SSG matrix were understood from their contrast image as dark and bright, respectively [26,35,36]. The observed red shift in the SPR band for Au–TPDT (535 nm) and b-CD–Au–TPDT NPs (526 nm) when compared to the citrate stabilized Au NPs (520 nm) was due to the TPDT and b-CD–TPDT composite, respectively. Further, less aggregation was observed for the b-CD–Au–TPDT NPs when compared to the Au–TPDT NPs due to the encapsulation by b-CD–TPDT composite around the Au NPs. In the absence of the b-CD (in the case of the Au–TPDT NPs), the Au NPs were very close to each other due to the TPDT SSG encapsulation. From the TEM images, the average particle size of the Au–TPDT and the b-CD–Au–TPDT NPs was found to be 9 and 4.95 nm (Fig. S4), respectively. Nearly a two-

2.0

a b c d

1.5

Absorbance

Absorbance

1.5

1.0

b

0.5

c d

a

0.0 300

400

500

600

700

800

900

Wavelength/nm Fig. 2. Surface plasmon absorption spectra obtained for b-CD–Au–ETMOS (a), bCD–Au–APS (b), b-CD–Au–EDAS (c), and b-CD–Au–TPDT NPs (d) solutions after 24 h stirring. Inset show the photograph of the prepared samples (a–d).

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Fig. 3. TEM of images of Au–TPDT (A and B) and b-CD–Au–TPDT NPs (C and D) at different magnifications.

fold decrease in the Au NPs size was observed when the b-CD– TPDT composite was used to prepare the Au NPs. The particle size was analyzed for the Au–TPDT (A) and b-CD–Au–TPDT NPs (C) considering Fig. S4A and B, respectively. The Au NPs size distribution and mean size of 9 and 4.95 nm with standard deviation 1.785 and 1.786 for Au–TPDT and b-CD–Au–TPDT NPs, respectively are estimated form the histogram (Fig. S4). This analysis clearly reveals that the b-CD–TPDT composite plays a predominant role in controlling the Au NPs size. A similar observation of decrease in particle size was reported for the Au NPs preparation in the presence of the b-CD using the laser ablation technique [11,22]. 3.4. Role of b-CD and pH on the formation of Au NPs Luong and co-workers [11,22,37] discussed in detail the interaction involved between the Au NPs and b-CD resulting in a reduction in the size of Au NPs. The –OH groups present on the Au NPs form a hydrogen bond with the primary hydroxyl groups of the b-CD moiety. The secondary –OH groups of the b-CD are not oxidized in the basic condition. If the secondary –OH groups are oxidized, this will lead to the formation of dioxirane and such formation is not observed as confirmed by the ESI mass spectral studies [38,39]. In basic condition, the primary –OH groups of the b-CD undergo deprotonation in the first step to become a nucleophile and this nucleophile reduces the Au3+ ions and gets oxidized to – COOH group [33–35]. This –COO is effectively involved in the stabilization of metal NPs. During the addition of Au3+ into the pre-stirred basic b-CD– TPDT composite solution (pH = 9.84), it is expected that the Au3+ ions get adsorbed at the b-CD–TPDT composite and get reduced

by the deprotonated primary –OH groups of the b-CD leading to the nucleation of the embryonic Au core. The b-CD would exhibit hydrophobic interaction with the embryonic Au core and hence the consecutive particle growth due to the mutual coalescence between the nanoclusters would severely be limited leading to the formation of smaller Au NPs. The reduction of the metal precursor leads to the quick agglomeration of the nucleation centers and results in the formation of larger particle size. In the presence of bCD, the mutual coalescence between the nucleation centers is restricted by the presence of the b-CD cavity as nanogages for metal NPs growth [37] resulting in the smaller Au NPs. Considering the Au NPs size and the b-CD cavity size, the inclusion of the Au NPs into the cavity of the b-CD is ruled out. Therefore, the formation of sub 5 nm Au NPs in the presence of b-CD is attributed to the stabilization of the Au NPs by b-CD–TPDT composite rather than the inclusion of smaller Au NPs into the b-CD cavity. The FT-IR spectra were recorded to understand the chemical interaction of TPDT and b-CD with Au NPs (Fig. S5). Lev and coworkers [40] have already reported the IR spectra of amine functionalized sol–gel matrix and confirmed the hydrolysis of silanes by observing very weak IR bands due to the methoxy groups when compared to unhydrolyzed silanes. A similar observation was also noticed in present work with very weak bands and a shift in the bands due to the TPDT (Fig. S5a) at 2937 and 2846 cm 1 and a band around 1019 cm 1 due to siloxane group [40,41]. The IR bands for b-CD in the region 1200–900 cm 1 [42] are noticed (Fig. S4b and c) (941 cm 1 was due to the R-1,4-bond skeleton vibration of CD and the peak at 1028 cm 1 due to the antisymmetric glycosidic m(C–O– C) vibration and with a small shift for the b-CD–Au NPs (Fig. S5d). As Lev and co-workers proposed, the disappearance of the IR band

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b

Z'' KΩ

-45

e a

-30

a b c d e

-15

0 0

300

600

900

1200

1500

Z' K Ω Fig. 4. Electrochemical impedance spectroscopic (EIS) responses observed for bare GC (a), GC/TPDT (b), GC/b-CD–TPDT (c), GC/Au–TPDT (d) and GC/b-CD–Au–TPDT (e) electrodes in the presence of 5 mM hydroquinone and 5 mM sodium nitrite in 0.2 M PBS (pH 6.8). The electrode potential was 0.25 V and the frequency range was 1 Hz to 100 kHz.

at 3290 cm 1 (N–H stretching) for b-CD–TPDT–Au NPs is noticed (Fig. S5e). The IR band at 1556 cm 1 (primary –NH2) in the TPDT showed a shift to 1536 cm 1 for the b-CD–TPDT–Au NPs (Fig. S5e). The comparison of the IR spectra showed the presence of hydrogen bonding between the -CD and Au NPs in b-CD–Au (Fig. S5d) and b-CD–Au–TPDT NPs (Fig. S5e) when compared to the b-CD alone (Fig. S5b). Considerable shift in the band position and reduction in the band broadening around 3400 cm 1 due to the –OH stretching mode were observed in the presence of Au NPs when compared to that of the b-CD alone. Such a shift is generally attributed to the increase in the strength of the intermolecular hydrogen bonding [43]. The pH of the reaction mixture plays a crucial role in the reduction of AuCl4 ions in the presence of b-CD–TPDT composite. It can be considered that the lack of deprotonation in the b-CD (pH = 2.69) is the reason for the absence of AuCl4 ions reduction and the formation of Au NPs [25]. The presence of TPDT alone (pH = 9.72) led to the slow reduction of AuCl4 ions due to the presence of tri-amine groups in the TPDT. When b-CD (pH = 4.86) was mixed with TPDT silane (pH = 9.72) deprotonation of b-CD occurred and effective the reduction of AuCl4 ions into Au NPs was observed at the pH of 9.84. The amine functionalized TPDT silane undergoes hydrolysis to form silicate sol–gel and then homogeneous b-CD–TPDT composite with pH 9.84 and at this basic pH the Au NPs formation occurred.

Modified electrodes

Rct (KO)

GC/TPDT GC/b-CD–TPDT GC/Au–TPDT GC/b-CD–Au–TPDT Bare GC

6238 2193 1068 872 609

is observed that the diffusion of hydroquinone towards the electrode surface via the TPDT SSG is hindered when compared to that of the bare GC electrode (Fig. 4b). In the presence of Au NPs the Rct values for Au–TPDT and b-CD–Au–TPDT NPs modified electrodes were 1068 and 872 KO, respectively. This confirms the fast electron transfer process taking place at the GC/b-CD–Au–TPDT NPs modified electrode (Fig. 4e). 3.6. Electrocatalysis of nitroaromatics at the Au NPs modified electrode The electrocatalytic reduction of nitroaromatics is of considerable interest because they are common pollutants in waste water

e

a b c d e f

-9

-6

Ι (μA)

-60

Table 1 Summary of Rct values of different modified electrodes and bare GC electrode.

c

d

c d b

-3 a f

0

0

-200

-400

-600

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-1000

E vs. SCE (mV) Fig. 5. CVs recorded for 100 lM of nitrobenzene at the bare GC (a), GC/TPDT (b), GC/b-CD–TPDT (c), GC/Au–TPDT (d) and GC/b-CD–Au–TPDT (e) electrodes in 0.1 M PBS (pH = 7.2) at a scan rate of 50 mV s 1. CV recorded at the GC/b-CD–Au–TPDT electrode (f) in the absence of nitrobenzene in 0.1 M PBS (pH = 7.2) at a scan rate of 50 mV s 1.

-9 st

3.5. Electrochemical impedance spectroscopy studies

-6

Ι (μA)

The electrochemical behavior of the Au NPs at the GC/Au–TPDT and the GC/b-CD–Au–TPDT modified electrodes was studied by recording the CVs of the modified electrodes in 0.1 M PBS (pH 7.2) (Fig. S6). The characteristic Au oxidation and reduction peaks were observed around 0.9 and 0.4 V, respectively. Fig. 4 shows the EIS responses observed for the GC/TPDT (Fig. 4b), GC/b-CD–TPDT (Fig. 4c), GC/Au–TPDT (Fig. 4d and GC/bCD–Au–TPDT NPs (Fig. 4e) electrodes in the presence of hydroquinone. Fits of the impedance curve were obtained using the suitable equivalent electrical circuit (Model-R(C(R(Q(R(C(RW)))))). Table 1 summarizes the Rct values obtained for the different modified electrodes and the bare GC electrode. The charge transfer resistance (Rct) values obtained from the fits of the EIS spectra for GC/TPDT and the bare GC electrode were 6238 and 609 KO, respectively. It

a (1 cycle) nd b (2 cycle)

-3

b a

0

400

200

0

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-400

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E vs. SCE (mV) Fig. 6. Cyclic voltammogram recorded for 100 lM of nitrobenzene at GC/b-CD–Au– TPDT electrode in 0.1 M PBS (pH = 7.2) at a scan rate of 50 mV s 1. (a) First cycle and (b) second cycle.

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a b c d

Ι (μA)

-9

a

d

c

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b -3 0 3 200

0

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E vs. SCE (mV) Fig. 7. Comparison of CVs recorded for 100 lM of nitroaniline (a), nitrobenzoic acid (b), nitrotoluene (c) and nitrobenzene (d) at GC/b-CD–Au–TPDT electrodes in 0.1 M PBS (pH = 7.2) at a scan rate of 50 mV s 1.

originating from industrial and agricultural activities. The electrocatalytic reduction of nitrobenzene was carried out in the phosphate buffer solution (PBS, pH 7.2) at the bare GC, GC/TPDT, GC/ b-CD–TPDT, GC/Au–TPDT and GC/b-CD–Au–TPDT NPs electrodes and the corresponding CVs are shown in Fig. 5. In the absence of nitrobenzene, the cathodic and anodic peaks were not observed at the GC/b-CD–Au–TPDT modified electrode (Fig. 5f). When nitrobenzene was introduced in the electrolyte solution, enhanced electrochemical responses were observed at the Au NPs modified electrodes due to the electrocatalytic reduction of nitrobenzene at the modified electrode (Fig. 5). The electrochemical response observed at the GC/TPDT modified electrode

Ι (μA)

-5 -4 -3

-6

A

80 μM 70 μM 60 μM 50 μM 40 μM 30 μM 20 μM 10 μM Blank

-2

B

80 μM 70 μM 60 μM 50 μM 40 μM 30 μM 20 μM 10 μM Blank

-5

Ι (μA)

-6

was slightly higher than of the bare GC electrode (Fig. 5b) due to the pre-concentration of nitrobenzene at the TPDT SSG film. The nitrobenzene reduction current observed at the GC/b-CD–Au–TPDT electrode was higher when compared to the GC/Au–TPDT electrode and this may be due to the encapsulation of Au NPs by b-CD–TPDT composite. Fig. 5d and e shows the CVs recorded for 100 lM of nitrobenzene at the GC/Au–TPDT and GC/b-CD–Au–TPDT electrodes. The electrochemical response observed at the GC/b-CD– Au–TPDT electrode (Fig. 5e) was around two fold higher with a small positive shift in the reduction potential when compared to that of the GC/Au–TPDT electrode (Fig. 5d). The electrochemical response observed at the modified electrode is due to the synergistic effect provided by the combination of the smaller size Au NPs and the b-CD–TPDT composite. Repetitive cycling was carried out at the GC/b-CD–Au–TPDT electrode for 100 lM nitrobenzene in PBS at a scan rate of 50 mV s 1 to understand the product formation as shown in Fig. 6. The cathodic peak due to nitrobenzene reduction was observed at 0.77 V in the first cycle. Two anodic peaks were obtained at 0.65 and 0.05 V in the reverse scan and one new cathodic peak appeared at 0.23 V during the second cycle (Fig. 6b). Further the CV was recorded for the GC/b-CD–Au–TPDT electrode in the potential window of 0.4 to 0.5 V (Fig. S7) and any of the peaks observed in Fig. 6 were not observed (Fig. S7). It can be understood that the new peak which appeared at 0.23 V in the second cycle (Fig. 6) is due to the reduction of the oxidative product of aniline formed at 0.05 V [44,45]. Fig. 7 displays the comparison of CVs recorded for 100 lM of nitroaniline, nitrobenzoic acid, nitrotoluene and nitrobenzene at the GC/b-CD–Au–TPDT modified electrode. The redox potentials of the nitroaniline, nitrobenzoic acid and nitrotoluene were shifted towards negative potential with respect to that of nitrobenzene, because of their different functionalities.

-4 -3 -2

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-300 -400 -500 -600 -700 -800 -900 -1000

E vs. SCE (mV)

E vs. SCE (mV)

-10.0

Ι (μA)

-7.5

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D 80 μM 70 μM 60 μM 50 μM 40 μM 30 μM 20 μM 10 μM Blank

C

-2.5 -300 -400 -500 -600 -700 -800 -900 -1000

E vs. SCE (mV)

7 6 5 4 3 2 1 0

80 [N 70 itr 60 ob 50 e

nz 40 0 en 3 e] 20 /μ M 10

Ι (μA)

-12

C A B

Fig. 8. Square wave voltammograms recorded for each addition of 10 lM of nitrobenzene at GC/b-CD–TPDT (A), GC/Au–TPDT (B) and GC/b-CD–Au–TPDT (C) electrodes in 0.1 M PBS (pH = 7.2). (D): Corresponding calibration plot.

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3.7. Square wave voltammetric sensing of nitroaromatics Fig. 8 displays the square wave voltammograms (SWV) recorded for each addition of 10 lM nitrobenzene at the GC/b-CD– TPDT (A), GC/Au–TPDT (B) and GC/b-CD–Au–TPDT (C) modified electrodes and their corresponding calibration plot (D). Similar to the observation of electrocatalytic reduction of nitroaromatics at the GC/b-CD–Au–TPDT (Fig. 5e) modified electrode, the SWVs (Fig. 8C) showed high sensitivity, and the response was very stable and offered a linear range. The GC/b-CD–Au–TPDT modified electrode was stored in water at room temperature after each measurement and tested for 5 days. An observable change of <1% peak current was noticed after 5 days for sensing of nitrobenzene and this observation revealed that the present electrode was stable and reproducible.

4. Conclusion The green and single step synthetic method for the preparation of Au NPs embedded by b-CD–TPDT composite in aqueous solution by using bio-compatible b-CD was reported. A two fold decrease in the Au NPs size was observed when b-CD–TPDT composite was used to prepare the Au NPs. The electrocatalytic reduction and sensing of nitroaromatics at the GC/b-CD–Au–TPDT modified electrode were studied using cyclic voltammetric and square-wave voltammetric analysis. These electrochemical techniques provide a simple, fast, and facile quantitative detection analysis for nitroaromatics. It is concluded that the b-CD–TPDT plays the predominant role both in the green synthesis of smaller size Au NPs and its electrocatalytic reduction and sensing of nitroaromatics. Acknowledgement RR acknowledges the financial support from the Department of Science and Technology (DST), New Delhi. SMV is a recipient of the CSIR-Senior Research Fellowship. The TEM images were recorded at the National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum.

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