C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8

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Synthesis, characterization and electrochemical propertiesof functionalized graphene oxide

Murugan Veerapandian a,b, Min-Ho Lee b,*, Karthikeyan Krishnamoorthy c,Kyusik Yun a,*

a Department of Bionanotechnology, Gachon University, Gyeonggi-DO 461-701, Republic of Koreab Korea Electronics Technology Institute, Medical IT Technology, Gyeonggi-DO 463-816, Republic of Koreac Nanomaterials and System Lab, Department of Mechanical Engineering, Jeju National University, Jeju 690-756, Republic of Korea

A R T I C L E I N F O A B S T R A C T

Article history:

Received 17 April 2012

Accepted 5 May 2012

Available online 14 May 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.05.004

* Corresponding authors: Fax: +82 31 7508819E-mail address: ykyusik@gachon.ac.kr (K

Graphene oxide (GO) was functionalized by a simple reaction of its carboxylic acid groups

with a silanized-metalloid polymer, which gave the resulting hybrid GO the property of

efficient dispersion in a variety of solvents. Spectroscopic investigations show that the

covalent attachment is effectively accomplished through an amidation process. The com-

bination of a metalloid polymer and GO is unique and the composite material exhibits

interesting features not seen in the individual structures. The electrochemical properties

of this metalloid–polymer-GO were demonstrated by immobilizing the sample on a con-

ventional gold-printed circuit board (Au-PCB) electrode. Functionalized GO showed a per-

fect scaling of steady-state currents with correlation coefficients of 0.9600 (Ipc) and 0.9552

(Ipa), indicating the promise of this new GO hybrid as a transducer material for many sens-

ing applications.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The design and fabrication of novel nanostructures with im-

proved properties by using novel techniques is an alluring

prospect for nanotechnology. Graphene and graphene oxide

are novel nanomaterials that have recently attracted a great

deal of consideration due to their wide variety of applications

in nanoelectronics [1], sensors [2], nanocomposites [3], batter-

ies [4], supercapacitors and energy storage [5]. In particular,

the unique surface properties (oxygenated functional groups

on the basal planes and edges), large surface area, layered

structure, and easy exfoliation into monolayers under water

mean that graphene oxide (GO) is a suitable building block

for fabricating versatile functional materials via covalent or

non-covalent approaches [6,7]. In recent years, there has been

a rush of interest in functionalizing graphene oxide materials

for medical and biomedical applications. For instance,

er Ltd. All rights reserved

.. Yun).

polyethylene glycol (PEG)-functionalized GO nanosheets have

been used to load anti-cancer drugs, doxorubicin hydrochlo-

ride and camptothecin [8]. Studies have also revealed that

PEGylated nanographene sheets (NGS) exhibit an ultrahigh

in vivo uptake by tumors and a significant photothermal effect

in mice [9]. In addition, researchers have also successfully

functionalized a natural linear cationic polysaccharide chito-

san (CS) on GO as a nanocarrier for drug and gene delivery. It

has been demonstrated that due to p–p stacking and hydro-

phobic interactions, GO–CS possesses a superior efficiency of

binding with anti-cancer drugs [10]. Besides these studies, sev-

eral biomolecules such as bovine serum albumin [11], adenine,

cysteine, nicotamide and ovalbumin (OVA) have also been

covalently (via diimide-activated amidation) attached to GO

nanosheets for the development of several biocomposites [12].

To obtain enhanced mechanical, thermal and electro-

chemical properties, organic conducting polymers and/or

.

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8 4229

inorganic metal/semiconducting nanomaterials have been

functionalized on GO surfaces. Simple solution mixing,

ultrasonication, high-speed shearing, melt compounding,

in situ polymerization/electro polymerization and layer-

by-layer (LbL) assembly have been the commonly used

methods for the fabrication of graphene based hybrids

[13]. Such advanced functional materials based on GO are

applied in several interesting applications such as the

removal of pollutants [14], desalination [15], nanofluids

[16], and solid state chemistry [17], among which biosen-

sors whose functions are based on their electrochemical

properties are predominant. The ability to retain the native

structure of graphene oxide while enabling the bioactivity

of the functionalizing moiety through a surface-confined

process, as well as effective direct electron transfer reac-

tion properties, mean that GO is a suitable material for

the construction of electrochemical substrates. Recently,

many varieties of functional GO hybrids have been re-

ported, such as SnO2/rGO, PtRu/rGO, and ZnO–rGO sheets

[18,19] and polyaniline nanofibers-chemically converted

rGO [20] for electrochemical supercapacitors, GO-Prussian

blue hybrid film [21] and rGO/AuNPs-chitosan nanocom-

posite film [22] for glucose biosensing, and poly(vinyl

pyrrolidone)-GO for selective determination of ochratoxin

A [23]. However, studies on the functionalization of metal-

loid polymer hybrids on either graphene or graphene oxide

for biomedical or electrochemical biosensors have not been

reported until now [7,13,24].

Metalloid polymer hybrids (MPHs) are well known for

their applications as coatings, reinforcements, resistance

enhancers, and substrate additives in a variety of surface

and interface studies. Our group has constructed some heter-

ogeneous MPHs, such as PEG-POSS and PEG stabilized Ag@-

SiO2NPs blended with ABA triblock copolymer, which

exhibited potential applications in the construction of thin

films [25], solid laminates [26] and as an electrochemical sub-

strate for antigen–antibody interactions [27]. In this present

work, we have demonstrated the functionalization of Ag@-

SiO2-PEG (MPHs) on GO via silanization of the MPHs and their

subsequent covalent functionalization with GO. Three differ-

ent features such as metal silver core, non-metal silica shell

and PEG as polymer layer available in single nanoplatforms

with an average particle size distribution of 12.5 ± 2 nm have

attracted us to modify on GO. Upon modification, the charac-

teristic morphology, chemical structure elucidation and

optical properties were extensively studied to ensure

successful functionalization. Additionally, the primitive elec-

trochemical properties of the final MPHs-GO were studied to

extend its further application in electrochemical biosensors.

At different scan rates, MPHs-GO exhibits a more efficient

electrochemical response with a more significant redox sig-

nal in phosphate buffered saline than that of the commercial

Au-PCB electrode, revealing the viability of MPHs-GO playing

a role in electroanalysis and the construction of an electro-

chemical biosensor. Furthermore, the synergistic properties

of the final MPHs-GO comprised of a metalloid (Ag@SiO2NPs)

and a polymer (PEG) on novel GO nanosheets provide us with

a new class of hybrid materials for use in interdisciplinary

fields.

2. Experimental

2.1. Materials

Silver nitrate (AgNO3), tetraethoxysilane (TEOS) (Si(OC2H5)4),

sodium borohydride (NaBH4), ammonium hydroxide (NH4OH),

3-aminopropyltriethoxysilane (3-APTES), phosphate buffered

saline (PBS) and anhydrous ethanol were purchased from Sig-

ma. Poly(ethylene glycol) (Mn = 10,000 g/mol) (PEG) was ob-

tained from Aldrich. Expandable graphite powder was

purchased from Sigma–Aldrich, USA. Sulfuric acid (H2SO4),

potassium permanganate (KMNO4), hydrogen peroxide

(H2O2) and hydrochloric acid (HCl) were obtained from Dae-

jung Chemicals and Metal Ltd., South Korea. Milli-Q water

with a resistance greater than 18 MX was used in all our

experiments. All chemicals were of analytical grade and were

used as received without any further purification.

2.2. Instrumentation

Structural characterizations were carried out using a conven-

tional field-emission scanning electron microscope (FE-SEM,

JEOL JSM-7500F), a high resolution transmission electron

microscope (HR-TEM, FEI Titan 80-300), and a bioatomic force

microscope (AFM: Nanowizard II, JPK Instruments) operating

in the intermittent air mode. The samples used for the mea-

surements were prepared by casting 5–10 lL of GO (0.1 mg/

mL) or MPHs-GO (0.2 mg/mL) suspension onto the surface of

a silica substrate (1 · 1 cm2) for FE-SEM, a copper grid for

HR-TEM, or a freshly cleaved mica sheet for AFM. The solvent

was allowed to evaporate before each measurement. Ultravi-

olet–visible absorbance spectra were measured using a Varian

Cary 50 UV–vis spectrophotometer. Chemical structure and

functional group modifications were examined by using a

Fourier transform-infrared spectrophotometer (FT-IR, NICO-

LET 6700) using a KBr disk at a resolution of 4 cm�1 and a

600 MHz high resolution nuclear magnetic resonance spec-

trometer (1H-NMR, AVANCE 600, Bruker) (D2O; solvent). To

complement the FT-IR and 1H-NMR data, a Raman spectral

study (T64000, HORIABA Jobin Yvon equipped with an Argon

laser source for 514 nm excitation) was performed on neat

GO as well as the MPHs-GO. Both the GO and MPHs-GO disper-

sions were drop casted onto the cleaned silica wafer

(2 · 2 cm2) and the solvent was allowed to evaporate at ambi-

ent temperature before measurements were carried out. A

Cary Eclipse fluorescence spectrophotometer was used to

examine the photo-luminescence properties of MPHs and

MPHs-GO. The fundamental electrochemical properties of

neat GO and MPHs-GO were confirmed by cyclic voltammetry

(CV) measurements using a VersaSTAT 3 in a three-electrode

configuration containing Au-PCB as the working electrode,

and a Pt wire and an Ag/AgCl electrode as the counter and ref-

erence electrodes, respectively.

2.3. Fabrication of Ag@SiO2-PEG (MPHs)

Ag@SiO2-PEG hybrids (average size distribution of 12.5 ± 2 nm)

were prepared by a sonochemical approach, following the

procedure outlined in our previous report [26]. Briefly, an

4230 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8

aqueous solution of silver precursor (AgNO3) was first added

to a reaction vessel containing the stabilizing agent PEG (Mn

= 10,000 g/mol) and the reductant NaBH4. The reaction mix-

ture was ultrasonicated (with controlled parameters) for a

period of 15 min to form the Ag core. Then the desired

amounts of aqueous TEOS and NH4OH were simultaneously

added. The reaction vessel containing the metal, non-metal

precursor, PEG and reducing agents again underwent ultra-

sonication for a period of 30 min to ensure complete reduc-

tion and growth of the hybrid structure. The resulting

colloidal solution containing hybrid particles (Ag@SiO2-PEG:

MPHs) was separated by centrifugation. The separated parti-

cles were washed twice and then utilized for further

experimentation.

2.4. Functionalization of MPHs on GO

The brownish colloidal suspensions of graphene oxide nano-

sheets (GO) utilized in the current experiment were synthe-

sized according to the modified Hummer’s method [28,29].

As-prepared Ag@SiO2-PEG hybrids were first silanized using

3-APTES, then utilized for covalent reaction with GO. Briefly,

an accurate amount of Ag@SiO2-PEG hybrids (200 lL) and

40 lL of 3-APTES (3% in C2H5OH) were added to a vial contain-

ing 3 mL of anhydrous ethanol and kept under magnetic stir-

ring (800 rpm) at room temperature for 10 h. Later, an

aqueous solution of GO (200 lL) was added and allowed to stir

(at 800 rpm) for another 10 h to ensure the covalent reaction

between silanized MPHs and GO. After the reaction process,

the Ag@SiO2-PEG functionalized GO sheets were separated

by centrifugation, washed thrice with ethanol and utilized

for further characterization.

2.5. Fabrication of Au-PCB/MPHs-GO electrode

The gold-PCB working electrode (Au-PCB; area �1 mm in

diameter) was surface-cleaned by sequential washing with

Fig. 1 – A schematic showing the process of the functionalizatio

covalent reaction between the silanized-MPHs and GO sheets.

anhydrous ethanol and acetone for 5 min each and rinsing

thoroughly with DI water. Before the surface immobilization

procedures were carried out, oxygen plasma treatment was

performed for 2 min. Several experimental optimizations

were done to obtain the best voltammetric response. Typically

4 lL of the above-prepared aqueous suspension of MPHs-GO

(2 mg/mL) was drop coated onto the Au-PCB electrode and al-

lowed to evaporate at ambient temperature for 1 h, and the

resulting film was utilized for electrochemical characteriza-

tion. All the electrochemical measurements were recorded

in 10 mM PBS at pH 7.4 in the potential range from �0.15 to

+0.25 V. A reproducible voltammogram can be obtained under

steady-state conditions after about five cycles.

3. Results and discussion

3.1. Structural characterizations

As stated previously [7], there is a possibility of a multiple

reaction occurring between amine groups and carboxylic

acid at the edges of the GO and epoxy groups on the basal

planes of GO. A schematic representation of the covalent

functionalization of MPHs (silanized) on GO is shown in

Fig. 1. Fig. 2 shows typical (a, b) FE-SEM, (c, d) TEM, and

AFM images of prepared GO sheets and MPHs-GO structures.

The micrographs of GO at two stages, before (a, c and e) and

after functionalization with MPHs (b, d, and f), show distinct

surface structures in each case, revealing the successful

modification of the surface. From Fig. 2b it can be observed

that the MPHs are largely modified on the GO sheets and

intrinsically multilayered. Such multilayered or large flakes

with thicknesses of more than a few layers observed in the

current FE-SEM study of MPHs-GO are consistent with simi-

lar kinds of functionalized GO sheets [12,14]. Generally, GO

platelets have unique chemically reactive oxygen moieties,

such as carboxylic acid (at their edges), and epoxy and hy-

droxyl groups on the basal planes. As shown by the sche-

n of MPHs on GO sheets via silanization of the MPHs and a

Fig. 2 – (a and b) FE-SEM, (c and d) HR-TEM, and (e and f) AFM images of GO (a, c and e) and MPHs-functionalized GO (b, d and f).

AFM scale bar = 3 lm · 3 lm. (e and f) The inset shows the 3D views of GO and MPHs-GO, while (f) the marked arrows denote

the locations of MPHs on the GO surface.

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8 4231

matic in Fig. 1, silanized-MPHs were chemically modified via

a covalent reaction at the carboxylic group of GO, which re-

sults in an amidation process [30]. Furthermore, the presence

of chemically reactive epoxy groups on the basal planes of

GO can also mediate the ring opening reaction through

nucleophilic attack at the a-carbon by the amine groups

[31] of silanized-MPHs. This augments the functionalization

of MPHs on GO sheets. As a complement to the FE-SEM

study, the TEM (Fig. 2d) and AFM (Fig. 2f) micrographs also

support the successful modification of GO. The TEM image

at a higher magnification shows that the metalloid-polymer

hybrids have a well dispersed arrangement on the GO sheets.

The elemental composition of the final MPHs-GO sheets was

determined using a TEM associated energy dispersive X-ray

analysis (EDXA) system. The respective EDXA zones and ele-

mental mapping in relation to Ag, Si and O orbitals were

identified (see Supplementary Fig. S1). The 2D and 3D surface

topographies of the GO and MPHs-GO sheets are shown in

Fig. 2e and f. However, the AFM micrographs do not clearly

show the hybrid particles as in the case of the electron

microscope images. More or less spherically shaped particles

can be identified (see the marked arrows in Fig. 2f) on the flat

surfaces of MPHs-GO sheets, whereas such notable particle

localization is not observed on the plain GO sheets (Fig. 2e).

This implies compatibility between the PEG stabilized Ag@-

SiO2 hybrids and GO. Structural characterization for pure

MPHs, such as FE-SEM and HR-TEM images, was carried

out and included as Supplementary data (see Fig. S2). The

average particle size distribution of MPHs was studied by

using the electrophoretic light scattering method using ELS-

8000 (Otsuka Electronics Co., Ltd.). Sample solutions were

introduced into the cell after being filtered through a Milli-

pore membrane (pore size 0.2 lm) three times. From the

ELS study it was observed that the average particle diameter

of MPHs was 12.5 ± 2 nm (see Fig. S3). This result agrees well

with the morphological data obtained from electron

micrographs of MPHs (Fig. S2). Moreover, the nanoscale mod-

ification of the PEG layer on the Ag@SiO2 can be observed

from the HR-TEM image of the MPHs (Fig. S2). This nanoscale

surface modification likely results in an enhanced mechani-

Fig. 3 – UV–vis absorption spectra of MPHs (inset a), GO

(inset b) and MPHs-GO.

4232 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8

cal interlocking of the MPHs with the GO sheets and, conse-

quently, better adhesion. Such an effect has been proposed

by several molecular dynamics studies that show altered

polymer mobility due to geometric constraints at the

nanoparticle interfaces [32]. Second, although the surface

chemistry of PEG coated Ag@SiO2 is relatively inert, silanized

MPHs form chemical bonds with the reactive groups of GO

sheets. Together with the high surface area and nanoscale

surface roughness of MPHs, this surface chemistry leads to

stronger interfacial interactions with GO sheets and thus a

substantially larger influence on the properties of the final

MPHs-GO structures. It is expected that MPHs-GO structures

may hold considerable potential as new GO-based inorganic

nanofillers.

3.2. Spectroscopic Investigations

3.2.1. UV–visible absorbance spectral studyFig. 3 shows the UV–vis absorption spectra of aqueous solu-

tions of MPHs, GO and MPHs-GO. As observed in Fig. 3 (inset

a), the maximum absorbance of MPHs occurs at a wavelength

of 418 nm with a broad peak. This can be assigned to the

absorption of surface-modified silver nanoparticles. Factors

such as the surface modification of silica and the coating of

organic PEG resulted in a broad absorption band at 418 nm

for the current Ag-containing MPHs. Generally a metallic

nanoparticle such as silver exhibits a localized surface

plasmon resonance (SPR) in the region 380–420 nm [33]. A

nanoparticle’s size, shape, capping agent and solvent envi-

ronment are the key features in determining the nature of

its absorption bands [34]. An SPR is composed of the collective

oscillations of the conductive electrons present on the surface

of a metal nanoparticle. Based on the excitation of the local-

ized surface plasmon caused by strong light scattering at a

specific wavelength, strong SPR bands are generated. Pro-

nounced SPR and third-order optical nonlinearities mean that

1 For interpretation of color in Fig. 4, the reader is referred to the w

AgNPs are of particular interest for applications in optical

waveguides and optical switches [35]. Due to its well known

absorption bands, generally UV–vis spectral analysis was per-

formed to ensure the successful fabrication of AgNPs.

Although the absorbance peak in the current study is not as

sharp and intense as it is for pure AgNPs, it still verifies the

presence of Ag inside the hybrid structures. Moreover, the

broadness of the absorption band indicates that surface func-

tionalization has taken place on the Ag core. The UV–vis

absorption peak for GO (Fig. 3b, inset) can be found at

233 nm, which corresponds to the characteristic p–p* electron

transition in the polyaromatic system of GO layers and agrees

well with previous studies [12]. It is difficult to see a specific

absorbance for Ag when it has undergone multiple surface

chemical modifications. As expected, the peak for the final

MPHs-GO structure became broad in comparison with that

of pure MPHs and shifted to 420 nm, indicating higher absor-

bance or reflection from MPHs-GO. This is probably due to the

strong interaction between the reactive GO sheets and MPHs.

The introduction of more organic species from the GO

molecules affects the width of the peak, which was expected

due to the increase in the thickness of the metalloid surface

coating [26]. Thus the distinct UV–vis absorbance spectra

from MPHs, GO and MPHs-GO provide us with optical

evidence of the successful surface functionalization of the

final hybrid. Furthermore, to evaluate the structural integrity

of MPHs-GO, FT-IR and 1H-NMR spectral analyses were

performed.

3.2.2. FT-IR spectral studyThe FT-IR spectra of MPHs (a, black trace), GO (b, red1 trace)

and MPH-functionalized GO sheets (c, blue trace) are shown

in Fig. 4. The structural integrity characteristics, such as

stretching and bending vibrations for individual nanostruc-

tures and functionalized nanostructures, were assigned

separately as follows (cm�1). As identified in Fig. 4a, MPHs

(Ag@SiO2-PEG) have significant symmetric and asymmetric

Si–O–Si stretching modes at 795 and 1075 cm�1, respectively

[36]. The specific C–H rocking signal and the symmetric/

asymmetric stretch from the PEG coating can be identified

at 889 and 2901, 2982 cm�1, respectively [26,37]. It was demon-

strated that bands such as C–O and O–H sometimes tend to be

less pronounced, and may overlap with other fingerprint

absorptions of the molecule [38]. Similarly, in the present case

the shoulder peak located at 1220 cm�1 is ascribed to the C–O

stretch. The short but intense peak at 1397 cm�1 can be

attributed to O–H bending and the broad peak centered at

3444 cm�1 can be associated with the hydroxy group as an

H-bonded OH stretch. The FT-IR spectrum of GO (Fig. 4b) illus-

trates peaks corresponding to C–O (carbonyl) at 1050 cm�1,

the C–O–C epoxy group at 1250 cm�1, and carboxyl-associated

O–H at 1413 cm�1 [39]. The peak at 1600 cm�1 is identified as

arising from C–C vibrations of the graphitic domains [40]. A

C@O peak from carboxylic acid is observed at 1728 cm�1

[41]. The relatively broad peak at 3260 cm�1 could be due to

the adsorbed water on the surface of the GO [38]. On the other

hand, the FT-IR spectrum of MPHs functionalized GO (Fig. 4c)

eb version of this article.

Fig. 4 – FT-IR transmittance spectra of MPHs (a), GO (b), and

MPHs-GO (c).

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8 4233

exhibits significant alterations compared to those of the

individual structures. For instance, we observe C–H

rocking signals at 892 cm�1, organic Si–O–C or Si–O–Si

asymmetric stretches at 1080 cm�1, specific epoxy signals

at 1252 cm�1, O–H bending at 1392 cm�1, asymmetric/

symmetric C–H bending (1478 cm�1), and C–H stretching at

2899 and 2981 cm�1. The peak intensity for the hydroxy

group H-bonded OH stretch in MPHs-GO is observed at

3442 cm�1 which is slightly higher than that in MPHs. The

major difference between this spectrum and the others is

the appearance of a new group frequency at 1568 cm�1 which

is related to the secondary amine N–H bending [11,12,38]. This

provides supporting evidence for a successful covalent reac-

tion having taken place between GO and silanized-MPHs.

Such a peak does not appear at all in the spectra of simple

GO or MPHs.

3.2.3. 1H-NMR spectral studyTo gain a deeper understanding of the chemical modification,1H-NMR spectral analysis was carried out for MPHs, GO and

MPHs-GO (Fig. 5). Their characteristic resonances, corre-

sponding to the different units of the GO, MPHs and MPHs

functionalized GO, can be discussed as follows. The MPHs

(Fig. 5i) show two characteristic triplet peaks centered at dH

3.64 ppm (a) and at dH 1.18 ppm (b), which are attributed to

the 4H, –2HC–CH2–O protons and the HC-OH protons of the

PEO segment, respectively [26]. A sharp resonance peak at

4.80 ppm is obtained from the solvent (D2O) used in the anal-

ysis. The 1H-NMR spectrum of GO (Fig. 5(ii)) exhibits charac-

teristic resonance peaks of the hydroxy proton (C–OH) at

two specific regions, namely, a sharp singlet at dH 3.35 ppm

(d) and a doublet at dH 3.66, 3.65 (c). Another characteristic

alcoholic proton associated with CH can be identified at dH

2.82 ppm (very weak, HO–CH) (d). The hydrogen protons of

the carboxylic acid group (–COOH) and its associated C–H pro-

tons (–CH–COOH) produce peaks that are located at dH 2.72

and 2.37 ppm (e). The C–H bonded with the hydroxy group

can be identified from its sharp intense peak at dH 1.18 ppm

(f). Significant proton resonance signals were observed in

the case of MPHs-functionalized GO (Fig. 5(iii)). For instance,

the protons of the PEO segment (4H, –2HC–CH2–O–) gave rise

to a quartet of signals at dH 3.66, 3.65, 3.64 and 3.63 ppm (h).

This is due to their interactions with neighboring H protons

associated with the C–H of GO (n + 1 rule). The disappearance

of the sharp peak at 3.55 ppm (Fig. 5(iii)) related to C–OH in GO

(as observed in Fig. 5(ii)) and the occurrence of new very weak

peaks at dH 3.53 and 3.52 indicates the absence of free alcohol

groups in the basal planes (MPHs-GO) and the formation of

new functionalization groups in the place of HC–OH. Other

interesting peaks were located at shifts of dH 3.76–3.74 ppm

and there was a short but intense singlet peak at dH

3.70 ppm (g). It is well known that NHCOR amide protons

can be observed within the resonance region, >5.5 and

<8.5 ppm [42], but in some cases their signals can be very

weak and difficult to identify because of the presence of the

D2O solvent, which causes the hydrogens on non-carbon

atoms to exchange with the deuterium [43]. However,

according to previous work, the presence of amide NHCOR

protons can also be identified by observing the signal from

the neighboring CH protons at around dH 3.70 ppm [44];

similarly in the present study it is possible to see a short

but intense peak at dH 3.70 ppm (g). This result again supports

the presence of a secondary amine NH bending signal

obtained in the FT-IR analysis, confirming the covalent func-

tionalization of MPHs on GO. Characteristic signals such as

short triplet peaks at dH 2.98–2.96 ppm (i) from Ar–CH–, a mild

shift at dH 1.91–1.88 ppm and multiplet peaks at dH 1.77–

1.72 ppm from –HC–CO– (j) functional groups were identified.

The distinct peaks appearing at dH 1.32–1.27 ppm are assigned

as arising from the –HC–O (k) of the PEO segment. The sharp

triplet signal located at dH 1.19–1.16 ppm and the short triplet

signal located at dH 1.08–1.06 ppm are attributed to the –CH–

protons in GO associated with the OH group and the –CH– pro-

ton signal from the PEO segment (l), respectively. The specific

signals located at dH 0.67–0.64 ppm (m) were ascribed to the

proton attached to the carbon adjacent to the silicon group

[45]. From these FT-IR and 1H-NMR spectroscopic investiga-

tions, we were able to understand the structural variations

of individual nanostructures and functionalized GO func-

tional groups.

Fig. 6 – Raman spectra of GO (black trace) and MPHs-

functionalized GO (red trace). (For interpretation of the

references to color in this figure legend, the reader is

referred to the web version of this article.)

Fig. 5 – 1H-NMR spectra of MPHs (i), GO (ii), and MPHs-GO (iii).

4234 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8

3.2.4. Raman spectral studyRaman spectroscopy is a prominent tool that is used to

probe the structural characteristics and properties of

graphene-based materials. Shown in Fig. 6 is a comparison

of the typical Raman spectra of GO (black trace) and MPH-

functionalized GO (red trace). The Raman spectrum of GO

shows the characteristic G-band at 1594 cm�1 and D-band at

1353 cm�1 respectively. The former is due to the vibration of

sp2-bonded carbon atoms in a 2D hexagonal lattice [11,46]

and the latter is caused by the vibrations of carbon atoms

with dangling bonds in plane terminations of disordered

graphite [11]. The Raman spectrum of graphite is reported

to have a strong G peak at 1570 cm�1 [47]. The shift in the

G-band towards longer wavenumbers (in comparison with

the pristine graphite) confirms the oxygenation of graphite

resulting in the formation of graphene oxide with several oxy-

genated functional groups such as carbonyl, carboxyl, hydro-

xyl, and epoxy groups.

The silanized MPHs are functionalized into the GO lattice

through covalent reactions with the carboxylic acid groups

at the edges of the GO sheets (as confirmed by the FT-IR and1H-NMR analyses), resulting in significant changes in the Ra-

man spectrum of the final hybrid material. After functionali-

zation, the D- and G-bands are broadened in the MPHs-GO

spectrum. In addition, the D peak intensity (I(D)) is increased

in the MPHs-GO spectrum compared to that of GO, suggesting

that the functionalization process modified the lattice struc-

ture of GO. Moreover, the ratio of I(D)/I(G) for MPHs-GO is calcu-

lated to be slightly larger than that of GO, indicating that the

metalloid-polymer surface modification on GO has signifi-

cantly influenced the structure. These results are in good

agreement with those presented in previous reports by Shen

et al. and Jiang et al. [12,14].

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8 4235

3.2.5. Photoluminescence studyUnderstanding the optical properties of graphene based mate-

rials is an important aspect of extending their applications in

photonics [48]. Although the current study is not directly de-

signed to feature the photonics applications of functionalized

GO nanosheets, a primitive understanding of the photolumi-

nescent properties of GO before and after selective chemical

functionalization can enable new applications of 2-D materi-

als. Previous reports revealed that the oxidation of graphite re-

sults in the formation of graphitic islands (sp2 clusters) in GO,

which generates a disruption of the p-network and thus opens

up a band gap in the electronic structure [49,50]. Photolumi-

nescence (PL) spectra of GO, MPHs and MPHs-functionalized

GO nanosheets were recorded using an excitation wavelength

of 325 nm. The PL spectrum of GO exhibits a characteristic

emission peak in the near ultraviolet (UV) region at around

366 nm (Fig. 7a) corresponding to the band emission of GO

[28]. The UV emission peak observed in GO is due to the amor-

phous sp3 matrix that surrounds the various sizes of crystal-

line graphitic sp2 clusters (unoxidized regions) that act as a

high tunnel barrier resulting in the generation of a band gap

in GO. On the other hand, MPHs do not exhibit any character-

istic PL emission spectrum in the near UV region (Fig. 7b). The

presence of the metallic core (Ag) and the PEG layer in the

MPHs could be the possible reason for them not having the

characteristic PL emission. Upon the functionalization of

MPHs on GO, the near band emission is nearly quenched and

only a very weak and broadened signal is observed (Fig. 7c).

In addition, all the samples possess a strong intensity at

650 nm which does not originate from the sample, but instead

arises due to the overlap of the second-order emissions associ-

ated with the excitation wavelength [46]. For better clarifica-

tion, the second-order emission was verified by using a range

of excitation wavelengths, such as 325, 400, and 500 nm. The

quenching of PL emission from MPHs-GO is due to the fact that

the surface modification of GO with MPHs results in the forma-

tion of new clusters in GO, which could probably reduce the PL

Fig. 7 – PL spectra of GO (a), MPHs (b) and MPHs-

functionalized GO (c) with an inset showing the focused

wavelength region of 350–380 nm.

emission intensity. This is supported by the results of FT-IR

(Fig. 4), 1H-NMR (Fig. 5) and Raman spectral studies (Fig. 6). It

is significant to mention that the PL of GO both before and after

surface modification needs to be further investigated by

employing other forms of hybrid materials, which will surely

help us to exploit the nanophotonic applications of GO-based

materials.

3.3. Electrochemical properties

Chemically customized electrodes containing surface-en-

trapped catalytic species have shown prominent advantages

over conventional metallic electrodes. It is feasible to modify

commercially available metallic electrodes with an organic

polymer or an inert surface to provide a good physical

distribution of the additive, leading to highly efficient electro-

catalysis [51]. The electrochemical activity of the MPH-func-

tionalized GO nanostructure was evaluated to assess its

applicability as a (transducer material) substrate platform for

an electrochemical biosensor. Fig. 8A presents the CV curves

of bare Au-PCB (a), GO (b) and MPHs-GO (c) modified Au-PCB

electrodes in PBS solution (as electrolyte, 10 mM, pH 7.4) at a

constant scan rate of 50 mV/s and a potential range from

�0.15 to +0.25 V. It is observed that no redox peaks appeared

either on the bare Au-PCB (a) or on the GO modified Au-PCB

electrode (b). However, as shown in Fig. 8A, curve c, a pair of

well defined and peak-shaped redox wave in the CV profile

was observed for the MPHs-GO modified Au-PCB electrode.

The characteristic cathodic peak potential (Epc) and anodic

peak potential (Epa) were found to be 0.198 V and �0.030 V,

respectively. We suggest that these redox peak correspond to

the oxidation–reduction of the MPHs (Ag@SiO2-PEG)-GO mod-

ified on the Au-PCB electrode, which proves that MPHs were

significantly functionalized on the GO/Au-PCB electrode. Fur-

ther we speculate that the broad redox wave with smallest dif-

ference in oxidation and reduction potential near zero volt

may be due to the enhanced electron transfer reaction by

functionalized GO. In general carbon based materials such

as graphene oxide and graphene hybrid films significantly pro-

mote the electron transfer reaction at the electrochemical

interfaces [18–21]. In addition metal silver nanoparticles and

defects associated during the surface modification of silver

nanoparticles are other possible factors to govern the redox

wave generation. Redox waves generated from silver metal

nanoparticles or silver doped amino silica nanoparticles mod-

ified electrodes have been reported previously in the literature

[52,53]. Mechanism of redox reactions associated with Ag is re-

ported to Ag! Ag2O and Ag2O!Ag at 0.37 V and �0.03 V, [54–

56] respectively. Studies further revealed that when silver

nanoparticles are within the aminosilica film it improves the

electron transfer rate by its smallest difference of oxidation

and reduction potential [53]. Similarly in the present case

the utilization of hybrid structures such as Ag core, silica shell,

and PEG layer collectively in one single nanoplatform as MPHs

and its functionalization on GO nanosheets has successfully

provided the enhanced kinetics of electron transfer between

PBS electrolyte and working electrode. The chemical bonding

of metal or oxide nanoparticles to graphene materials is dem-

onstrated to play an exciting role in electrochemical energy

generation and storage, in applications such as fuel cells,

Fig. 8 – (A) Cyclic voltammograms of bare Au-PCB (a), GO/Au-PCB (b), and MPHs-GO/Au-PCB (c). (B) Cyclic voltammograms of

MPHs-GO/Au-PCB at different scan rates (40–90 mV) in 10 mM PBS (pH 7.4) and the corresponding plot of peak currents

against the square roots of scan rates (inset).

4236 C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8

batteries, and pseudocapacitors [57]. Generally the electron

transfer reactions of a redox system are sensitive to nano/

microstructure modification and the electronic density of

states near the Fermi level, providing significant changes to

the redox curve [58]. MPHs functionalized on the GO provide

a suitable electron density and cause the electron transfer

reaction to proceed faster for functionalized GO than for pure

GO, with a well defined redox wave. In order to evaluate the re-

dox peak dependency on the scan rate, CV curves for MPHs-GO

were recorded at different scan rates from 40 to 90 mV/s. It can

be observed that the enhancement of the cathodic and anodic

peak currents was in relation to the scan rate (see the inset

shown in Fig. 8B, which is the plot of peak current vs. the

square root of scan rate) and the correlation coefficients are

found to be 0.9600 (Ipc) and 0.9552 (Ipa), indicating that is a

surface-confined redox process. The most important result

emerging from the current study is that cyclic voltammo-

grams of MPHs-GO in a PBS electrolyte system are valuable

C A R B O N 5 0 ( 2 0 1 2 ) 4 2 2 8 – 4 2 3 8 4237

and convenient for monitoring the surface status and barrier

of the modified electrode. Furthermore, the GO in the final hy-

brid material not only acts as the substrate for functionalizing

the MPHs but also as an active interface layer for exhibiting

the electrochemical transfer reaction between the solution

species and the electrode. The development of such a func-

tionalized GO hybrid in a controllable manner offers a new ap-

proach to construct an electrochemically active functionalized

material that can be used as a transducer for several electro-

chemical biosensor devices.

4. Conclusion

We have demonstrated the covalent functionalization of a

silanized-metalloid polymer on graphene oxide. Structural

characterization by electron and atomic force microscopy

reveals that this has been achieved. It is found that polymer

stabilized metalloid nanoparticles are uniformly distributed

on the graphene oxide sheets. Structural observations were

confirmed by UV–vis absorbance, FT-IR and 1H-NMR spectro-

scopic studies. Raman spectroscopy further shows that the

lattice structure of the graphene oxide sheets is significantly

modified after chemical bonding with the metalloid-polymer.

The quenching of near band emission from graphene oxide

hybrids (as shown by the photo-luminescent study) also sup-

ports the process of surface modification. Surface immobili-

zation of functionalized graphene oxide on the Au-PCB

electrode exhibits a characteristic voltammetric response

which is superior to that of graphene oxide and the conven-

tional Au-PCB electrode. The simple chemical functionaliza-

tion, hetero nano-environment, excellent dispersion in

various solvents, film forming ability, electrochemical proper-

ties and other synergistic effects of the obtained hybrid mate-

rial will direct us to several applications and developments in

graphene-based materials, electrocatalysis, sensor materials

and nano-electronics. Investigations into the biosensor appli-

cations of this hybrid material are currently ongoing in the

laboratory.

Acknowledgments

This research was supported by the Ministry of Knowledge

and Economy Grant No. 10039863 to K.S.Y. and the Ministry

of Knowledge and Economy Grant No. 10035501 to M.-H.L.

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.carbon.

2012.05.004.

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