RESEARCH PAPER

Sensing behavior study of silica-coated Ag nanoparticlesdeposited on glassy carbon toward nitrobenzene

Pooja Devi • Pramod Reddy • Swati Arora •

Suman Singh • C. Ghanshyam • M. L. Singla

Received: 27 June 2012 / Accepted: 28 August 2012 / Published online: 12 September 2012

� Springer Science+Business Media B.V. 2012

Abstract In this study, we report the synthesis and

characterization of silica-coated silver core/shell

nanostructures (NSs) and their sensing behavior when

deposited on glassy carbon (GC) electrode for nitro-

benzene (NB) detection. Synthesized silica-coated

silver core/shell NSs were characterized for their

chemical, structural and morphological properties.

TEM analysis confirmed that the silica-coated silver

nanoparticles (size *200 nm) are spherical in shape

and the core diameter is *38 nm. FT-IR spectra also

confirmed the coating of silica on the surface of silver

nanoparticles. Cyclic voltammetry studies of NB with

silica-coated silver core–shell nanoparticles-modified

GC electrodes revealed two cathodic peaks at

-0.74 V (C1) and -0.34 V (C2) along with two

anodic peaks at -0.64 V (A1) and -0.2 V (A2).

Enhanced cathodic peak current (C1, IP) of the core–

shell NSs-modified electrode is observed relative to

bare and silica-modified electrodes. Amperometric

studies revealed a very high current sensitivity

(114 nA/nM) and linearly dependent reduction current

with NB amount in the low concentration range and a

detection limit of 25 nM. Moreover, the core–shell

NSs-modified electrode showed good reproducibility

and selectivity toward NB in the presence of many

cationic, anionic, and organic interferents.

Keywords Core/shell nanostructures � Modified

electrode � Electrochemical sensor � Nitrobenzene

Introduction

Nitro aromatic compounds (NACs) are ubiquitous

pollutants and have detrimental toxicological effects

on the environment and human health (LD50 *20 to

50 mg/kg) (Leftwich et al. 1982; Finkel 1983). These

are widely used in pesticides (dinitrophenol deriva-

tives), explosives (TNT, RDX etc.), antibiotics (Chlor-

amphenicol), solvents, dyes, and other high-volume

chemicals (Hartter 1985; Rosenblatt et al. 1991). NB is

a high priority pollutant among the NACs as declared

by the Environment Protection Agency (EPA) on the

basis of its known carcinogenicity, mutagenicity, and

acute toxicity (Davies 2003; Zhang et al. 2007).

Analysis of NB is routinely carried out by various

conventional techniques including high performance

liquid chromatography (HPLC) (Thompson et al.

Pooja Devi and Pramod Reddy contributed equally to this

work.

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-012-1172-2) containssupplementary material, which is available to authorized users.

P. Devi � P. Reddy � S. Singh � C. Ghanshyam �M. L. Singla (&)

Central Scientific Instruments Organization, CSIR,

Sector-30C, Chandigarh 160030, India

e-mail: singla_min@yahoo.co.in

S. Arora

Shri Mata Vaishno Devi University, Katra 182121, India

123

J Nanopart Res (2012) 14:1172

DOI 10.1007/s11051-012-1172-2

1996; Galeano-Diaz et al. 2000), gas chromatography,

mass spectrometry (MS), GC/MS, LC/MS (Takahashi

et al. 2009), capillary zone electrophoresis (Guo et al.

2004), etc. However, these methods suffer from time-

consuming preliminary sample treatment approaches

such as separation, extraction, incubation, adsorption,

etc., (Zhang et al. 2007) and are difficult to implement

in field applications. New techniques based on spec-

troscopy involving absorption, fluorescence, and

luminescence, fiber optode (Yang et al. 2001), immu-

nochemistry, and electrochemistry (Zhang et al. 2006)

are being investigated extensively to provide porta-

bility along with selectivity and sensitivity.

In particular, electrochemical detection techniques

are receiving wide attention as they offer advantages

of real time analysis, sensitivity, selectivity, and cost

effectiveness along with portability. Chemical modi-

fication of working electrodes with nanostructures is

shown to improve sensitivity and/or selectivity over

conventional bare electrodes (Zhang et al. 2007).

Nanostructures comprising of various organic and

inorganic materials and their composites have been

used for electrode modification to detect NB (Luo

et al. (2010); Ma et al. (2011); Sanchez-Pedreno et al.

1986; Lorenzo et al. 1988; Liu et al. 2006; Qi et al.

2008). Silver being among the best conductors forms a

good candidate and the electrodes modified with silver

nanoparticles exhibit improved current response and

sensitivity due to increased electrode/catalytic surface

area. However, silver nanoparticles tend to aggregate

resulting in partial loss of surface area. Silica provides

surface functional groups required for nitro benzene

adsorption via acid–base chemistry and is among the

most efficient adsorbents used in the remediation of

nitro explosive contaminants (Weissmahr et al. 1996;

Zhang et al. 2006). Silica NPs exhibit uniform pore

sizes, surface areas in excess of *1,000 m2/g, and a

long-range ordering of the packing of pores making

them good adsorption agents for NB (Zhang et al.

2006). In addition, silica shell tends to provide

extraordinary stability against coagulation due to a

very low value of Hamaker constant defining the

Vander Waals forces of attraction among the particles

and the medium (Kalele et al. 2006). Hence, silica-

coated metallic cores have attracted much attention in

various applications. They exhibit easily controlled

dimensions, stability, monodispersity, reusability,

porosity, and good catalytic properties (Li et al.

2010; Kalele et al. 2006). In the present work, we have

explored the sensing behavior of silica-coated silver

core/shell NSs toward NB.

Materials and methods

All chemicals procured were of analytical grade and

used as received: silver nitrate and nitrobenzene

(Spectrochem. Pvt. Ltd.), polyvinylpyrollidione (Si-

sco Research Laboratory Pvt. Ltd.), ethylene glycol

(Loba Chemie), tetraethylorthosilicate (Merk Special-

ties Pvt. Ltd.), ammonium hydroxide (s.d.fine-Chem.

Ltd.), acetonitrile (Spectrochem Pvt. Ltd.), and etha-

nol (Changshu Yangyuan Chemical China).

A Quanta-200 FEG-SEM (FEI, The Netherlands)

equipped with X-Ray analyzer was employed for

morphological and chemical composition studies.

High Resolution Transmission Electron Microscope

(HR-TEM, FEI) was used to obtain TEM micro-

graphs. FT-IR analysis was done using Nicolet iS10

FT-IR spectrophotometer. Absorption spectra were

recorded using PerkinElmer� Lambda35 UV–visible

spectrophotometer. All cyclic voltammetric (CV)

measurements were performed using CHI 680 (CH

Instruments) electrochemical station.

Silver NPs were synthesized via reduction of silver

nitrate (Sun and Chiu 2004). In a typical experiment,

PVP (2 g) was added under constant stirring to AgNO3

(80 mg) in ethylene glycol (15 mL). The reaction

mixture was heated to 120 �C (1 �C/min) for about

1 h. The NPs were precipitated out by adding acetone

(*200 mL). The precipitate was collected, washed, and

re-dispersed in ethanol (100 mL). The synthesized Ag

NPs were coated by silica by Stober’s method (Stober

et al. 1968). Synthesized Ag NPs solution (1 mL) was

added to a mixture containing ethanol (25 mL), DI water

(4 mL), and varying amounts of TEOS (5–30 lL). To

this reaction mixture, ammonium hydroxide (2 mL) was

added to catalyze the hydrolysis and condensation of

TEOS on PVP stabilized Ag NPs. After 1 h of stirring,

the mixture was centrifuged, washed, and finally

dispersed in ethanol. A glassy carbon (GC) electrode

(*3 mm diameter) was polished by slow overnight

stirring in alumina slurry before each set of experiments.

It was then washed with distilled water and dried using

argon. Synthesized core/shell NSs were deposited on

GC electrode, air dried, and used for NB detection.

For electrochemical detection, 0.1 M NaCl (20 mL)

was used as an electrolyte to which different amounts

Page 2 of 8 J Nanopart Res (2012) 14:1172

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of NB were added during measurements. Interference

studies were conducted using solutions of various ions

(CO32-, NO3

-, Ni2?, and Zn2?) and organic pollutants

(4-nitrophenol, benzene, 1,2-dichloronitrobenzene,

chlorobenzene, and 4-nitrotoluene).

Results and discussion

Characterization of silica-coated silver core/shell

NSs

Silver and core/shell NSs were characterized as

synthesized for their optical behavior (Fig. 1). The

Ag NPs exhibited a sharp surface plasmon resonance

(SPR) band at *402 nm indicating formation of

uniform sized silver NPs (Smitha et al. 2008).

Figure 2 shows SPR peaks for core/shell NSs

synthesized with different amounts of TEOS. Red-

shifted SPR peaks at 411 and 418 nm were observed in

particles synthesized with 5 and 10 lL of TEOS,

respectively. Higher quantities of TEOS resulted in no

further red shift. The red shift is attributed to the local

increase of the refractive index of the surrounding

medium from 1.36 (ethanol) to 1.45 (silica) (Kobay-

ashi et al. 2005). Increase in absorbance with increase

in amount of TEOS may be ascribed to the increased

scattering due to increase in shell thickness as

predicted by Mie theory (Liz-Marzan et al. 1996;

Kobayashi et al. 2005).

Figure 3 shows TEM micrograph (inset) and EDX

analysis of the as-synthesized (10 lL TEOS) silica-

coated silver core/shell NSs (diameter of *200 nm)

confirming the formation of silica shell over silver

core (size *38 nm).

Figure 4 shows FTIR spectra of silver NPs and

silica-coated silver core/shell NSs. Silver NPs (curve a)

show peaks at 1,653; 1,042–1,085; 1,284–1,292 (Wang

et al. 2005); 1,423; and 1,374/cm corresponding to

C=O stretching, C–N vibration, N–OH vibration, C–H

(alkane) bending, and OH bending (in plane) vibra-

tional modes respectively present in the PVP moiety

stabilizing them (Wang et al. 2005). Upon coating with

silica, the appearance of new peaks (curve b) at 1,084

and 750–800/cm corresponding to asymmetric and

symmetric stretching modes of Si–O–Si, respectively,

and a peak at 1,633/cm due to protonation of pyridine

unit confirms the coating of Ag NPs with silica (Kirk

1988; Patra et al. 1999). The broad peak in the region of

3,400–3,740/cm is attributed to free silanol (Si–OH)

groups on silica surface (Brinker and Scherer 1990) as

well as O–H groups in adsorbed moisture. The

appearance of a shoulder at 2,900–2,976 can be

attributed to the alcohol (Brinker and Scherer 1990).

Electrochemical detection of NB using silica-

coated silver core–shell nanoparticles

An attempt to explore the capability of silica-coated

silver core/shell NSs to enhance the sensitivity of

electrode toward NB via surface absorption by silica

and catalytic reduction at silver cores (Zhu et al.

(2010); Grirrane et al. 2008; Turner et al. 2008) is

reported. CV studies were performed with 0.1 M NaCl

Fig. 1 Absorption spectra of a Ag NPs b silica-coated core–

shell NSs

Fig. 2 Absorption spectra of a Ag NPs and b, c, d, and e silica-

coated core–shell NSs synthesized with b 5 lL c 10 lL d 20 lL

and e 30 lL of TEOS

J Nanopart Res (2012) 14:1172 Page 3 of 8

123

solution as electrolyte. Figure 5 shows the response of

bare and modified electrodes at a scan rate of 50 mV/s

with Ag/AgCl reference electrode for 500 lL of NB

(4.76 mM). The reduction of NB at the bare electrode

produces two cathodic peaks at -0.74 V (C1) and

-0.34 V (C2) along with two anodic peaks at -0.64 V

(A1) and -0.2 V (A2). The peaks A1/C1 are a

consequence of a four electron process involving the

reduction of nitro group into hydroxyl amine group

and further reduction to amine group. Peaks A2/C2 are

due to a two electron process that oxidizes hydroxyl-

amine group to nitroso group (Chen et al. 2005, 2006).

R� NO2 þ 4e� þ 4Hþ ! R� NHOHþ H2O ð1Þ

R� NHOH$ R� NOþ 2Hþ þ 2e� ð2ÞCyclic voltammetric studies with core/shell NSs

(5 lL TEOS)-modified electrode in the presence of

different amounts of 0.1 M NB (2–500 lL) showed

significantly increased cathodic peak currents (C1) as

shown in Fig. 5.

The increase in current may be attributed to the

adsorption of nitro benzene on mesoporous silica (Li

et al. 2010; Won et al. 2010). Adsorption occurs via

the formation of EDA (electron donor–acceptor)

complex through acid–base interaction between elec-

tron-rich –OH group on silica and electron-deficient

aromatic ring of nitrobenzene (Weissmahr et al. 1996).

In addition, the pores in the shell provide access to

catalytic silver cores. A similar mechanism has been

reported for Pt/silica core/shell nanocatalyst-mediated

Fig. 3 EDXA analysis of silica-coated core–shell NSs (inset TEM image of single core–shell particle)

Fig. 4 FT-IR spectra of a Ag NPs and b silica-coated core–

shell NSs

Page 4 of 8 J Nanopart Res (2012) 14:1172

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ethylene hydrogenation and CO oxidation (Joo et al.

2009).

Figure 6 shows CV studies with electrodes modified

with different amounts of core/shell NSs and the inset

shows an increase in peak reduction current with

amount up to 15 lL. At higher amounts, the current

decreases and finally remains constant. The decreased

current response may be ascribed to the decreased mass

transfer (Zhang et al. 2007). Hence, for all further

studies, electrodes modified with 15 lL of core/shell

NSs solution were chosen for optimal current response.

Both CV and amperometric studies were performed

with optimally modified electrodes. CV studies (Fig. 7)

show a monotonic increase in C1 cathodic peak current

with increase in NB amount. During Amperometric

studies, 1 mM NB solution was added at regular

intervals in time in steps of 0.5 lL. Amperometric

(i–t) studies (Fig. 8) at the reduction potential and the

corresponding calibration plot (Fig. 9) reveal a linear

dependency (R2 = 0.997) of the current on amount of

NB with highly enhanced sensitivity (*114 nA/nM)

relative to bare (*60 nA/nM) and silica-modified

(*25 nA/nM) electrodes. Silver NPs-modified elec-

trodes showed larger background current in compari-

son to core–shell-modified electrodes, but with low

sensitivity (25 nA/nM). The limit of detection for

optimally modified electrode is found to be 25 nM.

Fig. 5 Cyclic voltammetric response of a bare and b silica-

coated core–shell NSs-modified GC electrode in 0.1 M NaCl

solution in the presence of NB

Fig. 6 Cyclic voltammetric response of a bare GC in 0.1 M

NaCl, b bare GC and GC modified with c 5 lL and d 10 lL of

silica-coated core–shell NSs in 0.1 M NaCl containing 0.01 M

NB (inset effect of amount of core–shell NPs on reduction peak

current)

Fig. 7 Cyclic voltammogram of silica-coated core–shell

NSs-modified electrode as a function of NB amount

Fig. 8 Amperometric (i–t) curve of silica (blue), bare (green)

and silica-coated silver NSs (SiO2/Ag, black)-modified elec-

trodes in NB (step addition of 0.5 lL/100 s). (Color figure

online)

J Nanopart Res (2012) 14:1172 Page 5 of 8

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Table 1 shows the comparative analytical performance

(sensitivity and limit of detection) of different modified

electrodes for NB detection as reported in literature.

At higher concentrations of NB ([100 lL), the

sensitivity is found to decrease. The electrodes

modified with silver/silica core–shell NSs of higher

shell thicknesses (10 lL TEOS and above) showed the

same behavior as that of bare and silica NPs-modified

electrode. This may be attributed to reduced catalytic

activity and surface area.

NB detection assay in real water samples

Water samples from two different sources (tap water

and bottled water) were taken and studied. No detect-

able NB was present in water under examination. These

samples were then spiked with known amount of NB

and the sensor response is reported in Table 2.

Under optimized conditions, no influence of ionic

species like NO3-, Ni2?, and Zn2? was observed on

the peak current (Ip) and peak potential (EP)

corresponding to C1 even at higher amounts relative

to that of NB. However, in the presence of CO32-, IP

decreased and this may be attributed to increase in

pH (10). Other organic species like 1,2-dicholoro-

4-nitrobenzene, 4-nitrotoluene, 4-nitrophenol, and

chlorobenzene showed no response with modified

electrode and confirms the selectivity of the electrode

toward nitrobenzene [ESI]. Influence of other param-

eters like pH, temperature, and ionic strength of the

solution on the analytical performance of the elec-

trodes modified with silica-coated silver core/shell

nanoparticles is under investigation.

Conclusions

Silica-coated silver core–shell NSs were investigated

as a highly sensitive electrochemical sensing platform

toward NB determination. The good adsorption capa-

bility of mesoporous silica shells along with the

catalytic properties of silver cores facilitated the

detection of NB resulting in a very high current

response (*114 nA/nM) and a good limit of detection

of 25 nM. The modified electrode is found to be

selective toward NB with the interference effects of

different ionic and organic species with the detection

being negligible.

Fig. 9 Calibration curve of silica/silver-modified electrode

Table 1 Comparison of analytical performance of the different modified electrodes toward nitrobenzene determination

Material used Detection limit Sensitivity References

Au NPs/MWCNTs 2.4 lM – Liu et al. (2006)

Macro-/mesoporous carbon materials 8 nM 2.36 nA/nM Ma et al. (2011)

Bismuth-film 0.83 lM 0.289 nA/nM Luo et al. (2010)

OMC/didodecyldimethylammonium bromide

(DDAB) composite

10 lM – Qi et al. (2008)

10 % C60 30 lM – Qian et al. (1997)

Silver nanoparticles embedded in functionalized

silicate shell

2.5 nM 5.41 nA/nM Maduraiveeran and

Ramaraj (2009)

m-Silica/Ag NPs 24 nM 132 nA/nM This work

Table 2 Determination and recovery of NB in real samples

Sample Found

(mol/L)

Standard

added

(mol/L)

Found after

addition

(mol/L)

Recovery

(%)

Tap water 0 5 9 10-6 4.9 9 10 6 98

Bottled water 0 5 9 10-6 4.8 9 10-6 96

Page 6 of 8 J Nanopart Res (2012) 14:1172

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Acknowledgments The authors are thankful to Dr. Pawan

Kapur, Director, CSIO-CSIR, for his valuable support.

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