Surface interactions of gold nanorods and polysaccharides: From clusters to individual nanoparticles

Carbohydrate Polymers 152 (2016) 479–486

Contents lists available at ScienceDirect

Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Surface interactions of gold nanorods and polysaccharides: From
clusters to individual nanoparticles
Heloise Ribeiro de Barros a , Leandro Piovan a , Guilherme L. Sassaki b ,
Diego de Araujo Sabry b , Ney Mattoso c , Ábner Magalhães Nunes d , Mario R. Meneghetti d ,
Izabel C. Riegel-Vidotti a,∗
a
Grupo de Pesquisa em Macromoléculas e Interfaces, Departamento de Química, Universidade Federal do Paraná—UFPR, CxP 19081, CEP 81531-980,
Curitiba, PR, Brazil
b
Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Paraná—UFPR, CxP 19046, CEP 81531-980, Curitiba, PR, Brazil
c
Departamento de Física, Universidade Federal do Paraná—UFPR, CxP 19044, CEP 81531-980, Curitiba, PR, Brazil
d
Grupo de Catálise e Reatividade Química, Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Av. Lourival de Melo Mota s/n, CEP
57072-970, Maceió, AL, Brazil

a r t i c l e

i n f o

Article history:
Received 14 April 2016
Received in revised form 28 June 2016
Accepted 5 July 2016

Available online 5 July 2016
Keywords:
Gold nanorods
Sulfated chitosan
Surface interactions
Self-assembling

a b s t r a c t
Gold nanorods (AuNRs) are suitable for constructing self-assembled structures for the development of
biosensing devices and are usually obtained in the presence of cetyltrimethylammonium bromide (CTAB).
Here, a sulfated chitosan (ChiS) and gum arabic (GA) were employed to encapsulate CTAB/AuNRs with
the purpose of studying the interactions of the polysaccharides with CTAB, which is cytotoxic and is
responsible for the instability of nanoparticles in buffer solutions. The presence of a variety of functional
groups such as the sulfate groups in ChiS and the carboxylic groups in GA, led to efficient interactions
with CTAB/AuNRs as evidenced through UV–vis and FTIR spectroscopies. Electron microscopies (HR-SEM
and TEM) revealed that nanoparticle clusters were formed in the GA-AuNRs sample, whereas individual AuNRs, surrounded by a dense layer of polysaccharides, were observed in the ChiS-AuNRs sample.
Therefore, the presented work contributes to the understanding of the driving forces that control the
surface interactions of the studied materials, providing useful information in the building-up of gold
self-assembled nanostructures.
© 2016 Elsevier Ltd. All rights reserved.

1. Introduction
Gold nanoparticles (AuNPs) have increasingly been given extensive attention due to their unique properties making those
materials useful in catalysis, nanoelectronics and, more interestingly, in optical sensing and diagnostics in the biomedical field
(Garabagiu & Bratu, 2013; Kopwitthaya et al., 2010; Mitamura,
Imae, Saito, & Takai, 2007; Pierrat, Zins, Breivogel, & Sonnichsen,
2007).
Among the AuNPs, considerable attention has been dedicated
to gold nanorods (AuNRs). The coherent oscillation of the electrons
along the short axis (transversal SPR) and the long axis (longitudinal

SPR) of the nanorods causes two surface plasmon resonance (SPR)
bands. At least one of these bands can be found in the visible spectra. The transversal SPR band has a maximum absorption around

∗ Corresponding author.
E-mail addresses: , (I.C. Riegel-Vidotti).
/>0144-8617/© 2016 Elsevier Ltd. All rights reserved.

520 nm (Rayavarapu et al., 2010), whereas the longitudinal SPR
band is observed in the range from 650 nm (shorter rods) to 950 nm
(longer rods) (Eutis & El-Sayed, 2006; Rayavarapu et al., 2010).
As transversal and longitudinal SPR are shape and size dependent
(Murphy & Jana, 2002; Xie et al., 2011), the AuNRs are particularly
suitable for building up self-assembled structures for the development of biosensors (Yu et al., 2014), nanodevices (Xie et al., 2011),
and non-invasive probes (Charan et al., 2012).
Regarding the applications of AuNPs in biological environments, some important issues arise concerning the maintenance
of their morphological stability, cytotoxicity, and interactions with
different organisms or their components. Therefore, the choice
for appropriate stabilizing agents is of the utmost importance in
obtaining AuNRs that are stable in different environmental conditions (pH and ionic strength) and that exhibit low toxicity.
The seed mediated method in the presence of the surfactant
cetyltrimethylammonium bromide (CTAB) is the most commonly
employed procedure to obtain AuNRs, although some other methods have been recently proposed (da Silva, Nunes, Meneghetti,


480

H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

& Meneghetti, 2013; Pérez-Juste, Pastoriza-Santos, Liz-Marzán, &
Mulvaney, 2005). In general, the method initially proposed by Murphy (Gole, Orendorff & Murphy, 2004; Jana, Gearheart & Murphy,

2001a; Jana, Gearheart, & Murphy, 2001b; Jana, Gearheart, &
Murphy, 2001c; Johnson, Dujardin, Davis, Murphy & Mann, 2002;
Murphy & Jana, 2002) and El-Sayed groups (Nikoobakht & El-Sayed,
2001) consists of the formation of the AuNRs from small sized
spherical AuNPs (seed solution), which then acts as nucleation centers in the AuNRs synthesis. In the presence of CTAB, the AuNRs
growth is mainly unidirectional since the interactions between the
polar head groups of the surfactant, i.e. the quaternary ammonium bromide moiety, and the crystallographic {110} facet of the
growing particle is preferential, causing the growth in the longitudinal direction, parallel to the {001} planes (da Silva, Meneghetti,
Denicourt-Nowicki, & Roucoux, 2014; Meena & Sulpizi, 2013;
Nikoobakht & El-Sayed, 2003). Therefore, the different growth rates
of the facets are the factors that determine the final shape of the
nanoparticle (short versus long NRs). In addition, CTAB is responsible for maintaining the colloidal stability since the bilayer structure,
which is formed by the self-interaction of the alkyl groups of CTAB,
promotes the suitable protection against particle agglomeration in
aqueous media through electrostatic and steric interactions (Boca
& Astilean, 2010). However, CTAB is cytotoxic and causes AuNRs
instability in buffer solutions, which then restricts their use in biological applications (Boca & Astilean, 2010; Hamon, Bizien, Artzner,
Even-Hernandez, & Marchi, 2014; Rayavarapu et al., 2010).
Therefore, when focusing on the obtention of AuNRs that
safely can be used in the biomedical field, it is fundamental to
replace part of CTAB or encapsulate the CTAB/AuNRs to obtain
particles with reduced toxicity that are also stable under different environments. Kopwitthaya et al. (2010) and Rayavarapu
et al. (2010) synthetized CTAB/AuNRs and showed that the
replacement of CTAB by thiolated poly(ethylene glycol) (PEGSH) produced particles with lower cytotoxicity compared to the
as-prepared AuNRs (Kopwitthaya et al., 2010; Rayavarapu et al.,
2010). In addition, other molecules have been used to replace
CTAB intending to reduce the cytotoxicity, such as polystyrene
sulfonate, polyethylene glycol (Rayavarapu et al., 2010), 1Mercaptoundec-11-yl)hexa(ethylene glycol) (EG6 OH) (Xie et al.,
2011), thio-polyethylene glycols (Bogliotti et al., 2011), polyacrylic acid, poly(allylamine) hydrochloride (PAH) (Huang, Jackson
& Murphy, 2012), and 3-mercaptopropionic acid (MPA) (Garabagiu

& Bratu, 2013).
Polysaccharides are part of a very promising family of naturally
occurring molecules that have also been described to interact with
gold nanoparticles. The presence of a variety of functional groups
in their structure assists the favorable interactions between the
AuNRs and the surrounding media, which is responsible for the
AuNRs stabilization and also provides sites for further chemical
modifications (Erathodiyil & Ying, 2011; Liu et al., 2013). In addition
to being natural products, polysaccharides have inherent properties such as biodegradability, biocompatibility, and low toxicity (Liu
et al., 2013). Surprisingly, there is a relatively low number of scientific works reporting the use of polysaccharides as stabilizing agents
of AuNRs (Wang, Chang & Peng, 2011; Yu et al., 2014), although
many works have reported the efficient stabilization of spherical
gold nanoparticles by polysaccharides.
Chitosan (Chi) is a linear polysaccharide extracted from the
exoskeleton of crustaceans, obtained by deacetylation of chitin.
Medium and high molar mass chitosan is only soluble in water at pH
lower than 6.0 (Williams & Phillips, 2000, Chp. 21). Chitosan (Boca
et al., 2011) and its derivatives have been successfully employed to
cap AuNPs for photothermal therapy (Wang, Chang & Peng, 2011;
Yang et al., 2015), for optoacoustic tomography (Wang et al., 2015),
among other applications. Gum arabic (GA) is a highly branched
natural polysaccharide exuded from the trunks and barks of acacia

trees. This polysaccharide has been extensively used for the stabilization of spherical AuNPs (Chanda et al., 2010; Kattumuri et al.,
2007; Wu & Chen, 2010), displaying optimal performance in a wide
pH range (Barros et al., 2016).
In order to reduce the cytotoxicity inherent to CTAB stabilized AuNRs and simultaneously improve the AuNRs stability in
physiological media, we used a sulfated chitosan (ChiS) or GA to
encapsulate CTAB/AuNRs. The non-toxicity and biocompatibility
of Chi and GA were evaluated in previous works (Bicho, Roque,

Cardoso, Domingos, & Batalha, 2009; Boca et al., 2011). The chemical modification of Chi to obtain ChiS is of interest because it
does not only keep the Chi main chain backbone intact, but it also
improves its solubility in aqueous media (Jayakumar, Nwe, Tokura,
& Tamura, 2007). Moreover, it can potentially infer new functionalities to the modified Chi since sulfated polysaccharides, like heparin
for example, are known to present important biological functions
such as anticoagulant and/or antithrombotic actions (Asif et al.,
2016; Jayakumar et al., 2007; Maas et al., 2012).
Herein we demonstrate by UV–vis (Ultraviolet–visible) and FTIR
(Fourier Transform Infrared) spectroscopies, and also by transmission and scanning electron microscopies that ChiS and GA interact
differently with the AuNRs. The differences are discussed in terms
of the different functional groups present in each polysaccharide
that leads to distinct polysaccharide/AuNRs structures. Therefore,
this study contributes to understanding and controlling the selfassembling behavior of AuNRs, mediated by the capping agent.

2. Materials and methods
2.1. Materials
Tetrachloroauric acid (HAuCl4 ·3H2 0, 30% in dilute HCl, 99,9%),
CTAB (≥98%), GA (Mw = 9.3 × 105 g mol−1 ; uronic acid content of
17%) (Grein et al., 2013), chitosan (≥75% deacetylated), and silver nitrate (AgNO3 , > 99%) were purchased from Sigma-Aldrich.
Sodium borohydride (NaBH4 , ≥ 98%) was purchased from Nuclear
(São Paulo, Brasil) and ascorbic acid (AA, >99%) was purchased from
Dinâmica (São Paulo, Brasil). Milli-Q grade water (18.2 M cm, Millipore, USA) was used in the preparation of all solutions. Prior to
use, GA powder was solubilized in water, left overnight at 4 ◦ C and
subsequently dialyzed for 48 h against distilled water through a
dialysis membrane (12–14 kDa cut-off) and freeze-dried.

2.2. Sulfation of a commercial chitosan
Commercial chitosan (Chi) underwent the sulfation reaction
according to Terbojevich, Carraro, and Cosani (1989)’s sulfuric
acid:chlorosulfonic acid method. Briefly, 1.00 g of commercial chitosan was added to the pre-cooled (4 ◦ C) reaction mixture (40 mL of

sulfuric acid (H2 SO4 95%) and 20 mL of chlorosulfonic acid (HClSO3
98%)). Then, the reaction was carried out at room temperature
under stirring for 1 h. The sulfation was stopped by pouring 250 mL
of cold diethyl ether (Et2 O) into the reaction mixture. The precipitate formed was washed with cold Et2 O, then suspended in distilled
water, neutralized with saturated NaHCO3 , dialyzed against tap
water through a 3500 kDa cut-off membrane, and freeze-dried. The
final product was fully characterized (ChiS, Mw = 1.4 × 104 g mol−1 ;
SO4 = 48%; SO3 O–3 = 6.8% and SO3 O–6 = 41.2%) (Supplementary
material—Fig. S1 and Table S1) and resulted in a pale yellow powder
that was stored in a moisture free environment.
The surface charge was obtained using a Zetasizer Nano ZS
instrument by solubilizing the polysaccharides (ChiS and GA) using
Milli-Q water.


H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

2.3. Synthesis of the gold nanorods (AuNRs)

2.4. Preparation of GA-AuNRs and ChiS-AuNRs
The AuNRs stabilized by the polysaccharides, either GA-AuNRs
or ChiS-AuNRs, were prepared by a simple method. First, the asprepared AuNRs were centrifuged (10.000 rpm, 15 min) and the
supernatant was discarded to remove the excess CTAB. The precipitate was dispersed in 0.5 mL of water. Then, it was added 4.5 mL of
GA or ChiS solution 0.1 wt%. The final solution was kept under gentle
magnetic stirring at room temperature (∼25 ◦ C) for 24 h. Afterwards, the sample was again centrifuged and the supernatant was
discarded to remove the unbound GA or ChiS from the solution. The
precipitate was dispersed in 2.0 mL of Milli-Q water (18.2 M cm
at 25 ◦ C) and used thereafter. UV–vis spectroscopy (Agilent, model
8453) was used to verify any changes in the SPR bands as a consequence of the interactions of the AuNRs with the polysaccharides.
2.5. Microscopic and spectroscopic investigation of AuNRs,

GA-AuNRs and ChiS-AuNRs
Transmission electron microscopy (TEM) was performed using
a JEOL 1200EX-II microscope working at an acceleration voltage
of 80 kV. A drop (∼10 ␮L) of the colloidal solution was deposited
onto 400 mesh carbon-coated grids and air-dried. High resolution
scanning electron microscopy (HR-SEM) was performed using a FEI
Quanta 450 FEG microscope working at an acceleration voltage of
10 kV. An aliquot of 80 ␮L of the colloidal solution was deposited
onto a sample support and air-dried. FTIR measurements were
performed using a BIORAD FTS-3500 GX FTIR spectrometer. The
measurements were made in the transmission mode in a spectral
domain ranging from 400 to 4000 cm−1 , using KBr pellets.
3. Results and discussion
In order to ensure the full surface coverage of the AuNRs, the
concentration of the polysaccharides (GA or ChiS) was kept much
higher than the concentration of the AuNRs in the preparation
of GA-AuNRs and ChiS-AuNRs. Both, GA and ChiS are negatively
charged in aqueous solution with pH ∼5. The zeta potential values
(␨) of GA and ChiS are −36.3 mV and −28.6 mV, respectively. The
negative charge of GA is due to the presence of the COO− groups,
whereas the negative charge of ChiS corresponds to the presence
of OSO3 − . However, due to the cationic quaternary ammonium
headgroup of CTAB, the as-prepared AuNRs bear positive surface

(c)
0,3

Absorbance

The AuNRs were synthetized by the seed-mediated method

according to da Silva et al. (2013) and previously described by
Sau and Murphy (2004). In a typical procedure, the seed solution was prepared by 5.0 mL of aqueous solution 0.5 × 10−3 mol L−1
HAuCl4 added to 2.5 mL of 0.20 mol L−1 CTAB solution. Then, 0.6 mL
of ice-cold 0.01 mol L−1 NaBH4 solution was added. The color of
the solution immediately changed from dark to brownish yellow.
Next, the solution was kept under gentle mixing for 2 min and left
to rest for at least 2 h prior to use. Afterwards, the growth solution was prepared by gentle mixing of a 2.5 mL of 0.20 mol L−1
CTAB solution, 5.0 mL of aqueous solution 1 × 10−3 mol L−1 HAuCl4
and 150 ␮L of 4.0 × 10−3 mol L−1 AgNO3 solution. Then, 70 ␮L of
80 × 10−3 mol L−1 ascorbic acid solution was added and the color
changed immediately from dark yellow to colorless. Lastly, 12 ␮L
of seed solution was added by mixing gently for 10 s. The color
changed slowly from colorless to purple. The final solution was
kept undisturbed for at least 4 h. UV–vis spectroscopy (Agilent,
model 8453) was used to monitor the AuNRs formation through
the observation of the SPR bands.

481

0,2

0,1

(b)
(a)

0,0
400

500

600

700

800

900

1000

Wavelength (nm)
Fig. 1. UV–vis absorption spectra of (a) as-prepared AuNRs, (b) GA-AuNR and (c)
ChiS-AuNRs.

charge (Rayavarapu et al., 2010). Therefore, the choice of using negatively charged polysaccharides aids the interactions between the
AuNRs and the polysaccharides through electrostatic attraction.
The as-synthesized AuNRs present typical SPR bands centered
around ␭1 = 515 nm and ␭2 = 740 nm (Fig. 1) that allow the determination of the particle concentration using the extinction coefficient
(Garabagiu & Bratu, 2013; Orendorff & Murphy, 2006). The estimated concentration of particles of the as-prepared AuNRs is
5 × 10−10 mol L−1 .
The profiles of the SPR bands of GA-AuNRs and ChiS-AuNRs
are also shown in Fig. 1. Slight shifts at the maximum wavelengths are observed since the presence of GA and ChiS changes
the dielectric constant of the surrounding environment. In the case
of ChiS-AuNRs, it is evident the appearance of a strong absorption
at longer wavelengths, possibly caused by the changes in the surrounding environment, which will be clarified in the TEM images.
Furthermore, the maintenance of the position of the SPR bands after
the polysaccharides adsorption indicates that aggregation has not
taken place, preserving the morphology of the particles.
Through TEM images the AuNRs were observed to have an

average size of 45 nm × 15 nm (aspect ratio = 3). Although some
spherical particles are seen, most of the objects are rod-like structures, characterizing a high yield synthesis (Fig. 2a). Also, the shape
and size of the AuNRs were preserved in the presence of GA or ChiS
(Fig. 2b–e). The GA-AuNRs and ChiS-AuNRs exhibited average sizes
of 47 nm × 15 nm and 43 nm × 14 nm, respectively, corroborating
what was observed by UV–vis spectroscopy.
It is noticeable from the TEM images that the GA-AuNRs sample
resulted in nanoparticle clusters (Fig. 2b). The high magnification
image revealed that the GA adsorbed molecules are not distinguishable from the substrate. However, the nanoparticles in the
ChiS-AuNR sample are separated from each other by a dense structure that is suggested to be composed of the ChiS molecules. This
behavior could be associated with the differences observed in the
shape of the UV–vis absorption spectra. For a better understanding of the organization of the polysaccharides around the AuNRs,
HR-SEM images were obtained using secondary and backscattered
electrons. The comparative analysis of the images provides a deeper
insight into the interactions between the polysaccharides and the
AuNRs.
Fig. 3a and b clearly shows the differences in the AuNRs surrounding medium due to the presence of GA or ChiS, respectively.
Both images were obtained using secondary electron signal that
provides topographic contrast. As observed by TEM, nanoparticle clusters were seen in Fig. 3a (GA-AuNRs), whereas in Fig. 3b
(ChiS-AuNRs) the AuNRs were individually surrounded by ChiS in


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H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

Fig. 2. TEM images of (a) as-prepared AuNR, (b-c) after the adsorption of GA (GA-AuNRs) and (d-e) of ChiS (ChiS-AuNRs) on AuNR surface.

a cloud-like arrangement. As a result, when GA is used as the stabilizing agent, an increased particle density (number or particles
per unit volume) is attained when compared with ChiS. Fig. 3c and

e corresponds to low magnification images that were collected at
the same region and obtained by secondary and backscattered electrons, respectively. The contrast obtained by backscattered electron
signal is related to the electron density of the material. As can be
seen, no important differences were observed supposedly because
GA is arranged as a thin layer on the SEM support. Sample ChiSAuNRs was analyzed similarly (Fig. 3d and f) and in this case,
striking differences were observed. In the case of Fig. 3d, which
was obtained by secondary electrons, the edges of the structure
are highlighted, providing a volume perspective (topographic contrast). In Fig. 3f, obtained by backscattered electrons, the electron
density and contrast of the organic matrix of ChiS with the metallic
support are strongly evidencing that the particles are surrounded
by ChiS molecules, confirming that ChiS acts as an efficient encapsulating/wrapping agent for individual AuNRs. At low magnification,
the sharp difference of contrast in this type of sample is very useful
to quickly find the area to be studied at greater magnifications.
The differences observed by comparing the images of GA-AuNRs
and ChiS-AuNRs can be ascribed to the differences between the
electron densities of GA and ChiS and to the different arrange-

ment of the polysaccharides around the AuNRs. ChiS has higher
electron density when compared with GA due to the presence
of sulfate groups. Additionally, the molecules are densely packed
around the nanoparticles due to their lower molar mass and chain
linearity, favoring the formation of a three-dimensional structure.
On the other hand, GA, which is a highly branched, high molar mass
polysaccharide, bears atoms with low electron density (mainly C,
O, H), forming a thin layer on the support.
Some other aspects can be discussed to clarify the HR-SEM
observations, as follows. It is widely known that GA presents
surfactant-like properties and is highly soluble in water (Grein
et al., 2013). Therefore, when GA-AuNRs were washed to remove
the excess of GA, the molecules that were weakly bounded on the

AuNRs surface most likely were removed, lowering the resulting
final concentration of GA around the gold surface. Conversely, it is
known that Chi exhibits agglutinative properties (Lehr, Bouwstra,
Schacht, & Junginger, 1992) and since Chi is soluble only in acidic
media, the sulfation process improves its solubility in water. However, GA is more water soluble than ChiS, resulting in a higher
concentration of ChiS than GA molecules around the AuNRs.
Thus, the adequate selection of the stabilizing agent can
efficiently tune the self- aggregation of AuNRs. Gold nanoparticle clusters have applications in photothermal therapy (Zharov,


H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

483

Fig. 3. HR-SEM images of GA-AuNRs (a, c, e) and ChiS-AuNRs (b, d, f) obtained by secondary electrons (a, b, c, d) and backscattered electrons (e, f).

AuNR

960

669

962

719
731

1487
1431
1487

1431

2918
2850

CTAB

2918
2850

Mercer, Galitovskaya, & Smeltzer, 2006), whereas individual functionalized gold nanoparticles can perform important biological
functions via specific signaling pathways (Li, Kawazoe & Chen,
2015; Nethi et al., 2014).
FTIR spectroscopy was chosen to evaluate the nature of the
interactions between the polysaccharides and the AuNRs. The displacement, appearance or disappearance of bands in the FTIR
spectra may be attributed to the interactions that occur in these
assemblies. First, the spectra of the as-prepared AuNRs and neat
CTAB is presented (Fig. 4). The assignments of the main bands are
in Table 1. It was observed that the characteristic bands present in
CTAB are preserved in the AuNRs (Tang, Huang, & Man, 2013). The
maintenance of the bands, relative to symmetric and asymmetric
stretching vibration of CH2 of CTAB chain (2918 and 2850 cm−1 ),
indicates that the hydrophobic tails of CTAB are not interacting
with the AuNRs surface. It is suggested that the alkyl tails are selfinteracting, forming a bilayer on the gold surface that does not
restrain the stretching vibrational modes. According to this proposition, unbound and bound surfactant headgroups are found in the
gold surroundings (Nikoobakht & El-Sayed, 2001), thus rendering
some C N+ groups free for further interactions (Gole et al., 2004).
However, the bands corresponding to symmetric and asymmetric C H scissoring vibrations of H3 C N+ moiety (1487, 1473, 1462
and 1431 cm−1 ), and the band corresponding to C N+ stretching

(960 cm−1 ) are relatively less intense and slightly shifted in AuNRs
when compared to pure CTAB. This decrease in intensity indicates

4000

3500

3000

1500

1000

500

-1

Wavenumber (cm )
Fig. 4. FTIR spectra of as-prepared AuNRs and neat CTAB.

that the hydrophilic portion of CTAB bound to the AuNRs surface.
Additionally, the bands corresponding to the CH2 chain rocking
mode demonstrate important differences. Pure CTAB shows two
bands at 719 and 731 cm−1 whereas AuNRs show only one band at
669 cm−1 . This fact is clear evidence of the constrainments that the
CTAB alkyl chains are subjected to due to the interactions with the
particles, resulting in the formation of a compact layered structure


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H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

ChiS-AuNRs

1004

1062

AuNRs

1647
1542

2918
2850

AuNRs

1060

2918
2850

1153

GA-AuNRs

3500

3000

1500

960

1153
1062
1004

1647
1541

1487
1431

2918
2850

960
1253

1143
1064
1031

1608

4000

1419

GA

1487
1431

2918
2850

ChiS

1000

500

4000

3500

3000

1500

1000

500

-1

-1

Wavenumber (cm )

Wavenumber (cm )

Fig. 5. FTIR spectra of GA-AuNR, as-prepared AuNR and GA.

Fig. 6. FTIR spectra of ChiS-AuNR, as-prepared AuNR and ChiS.

around the particles. Therefore, it was confirmed by FTIR spectroscopy that CTAB remained on the AuNRs surface even after the
washing procedure.
The FTIR spectrum of GA-AuNRs is presented in Fig. 5. Important differences can be seen when comparing with the as-prepared
AuNRs spectrum. The assignment of the main bands for both samples is shown in Table 2. The characteristic bands of AuNRs, relative
to the symmetric and asymmetric stretching vibrations of −CH2 −
of CTAB (2918 and 2850 cm−1 ) were maintained in the GA-AuNRs
spectrum. Thus, it was evidenced that CTAB is still present on the
AuNRs surface. Additionally, the absence of some bands attributed
to C N+ moiety (1473, 1462, 1433 and 960 cm−1 ) and the absence
of the two strong bands in the GA spectrum attributed to the asymmetric and symmetric stretching vibration of the COO− group
(1608 and 1419 cm−1 ). The profile change of the bands corresponding to the stretching of the C O (1253, 1143, 1064 and 1031 cm−1 )
in the GA-AuNRs spectrum is an indication of the interaction
between the C N+ headgroup of CTAB with the negatively charged
carboxylate groups on GA structure. Furthermore, the observation
of new bands around 1060 and 700–400 cm−1 (finger print region)
in the GA-AuNR spectrum could be associated to the interactions
that occur through GA adsorbed on the AuNR surface.
The presence of CTAB in ChiS-AuNRs samples was likewise
observed, as seen by the FTIR spectra in Fig. 6. The bands at 2918
and 2850 cm−1 are present, whereas the bands at 1487–1433 and in

960 cm−1 disappeared in the ChiS-AuNRs spectrum in comparison
to the AuNRs spectrum. Furthermore, the main bands observed in
the ChiS spectrum do not disappear when ChiS is associated with
the AuNRs, but instead exhibit an intensity decrease in the ChiSAuNRs spectrum (1647, 1542, 1153, 1072, 1062 and 1004 cm−1 ).
The assignment of the main bands is shown in Table 2. Therefore, the interaction between the C N+ from CTAB and ChiS occurs
through a mutual interaction of the diverse negative charge functional groups present in its structure.

It is widely known that alkanethiols exhibit a preferential
binding on the surface of gold nanoparticles, promoted by thermodynamically favored covalent bonds (Karpovich, & Blanchard,
1994; Leff, Brandt, & Heath, 1996; Templeton, Pietron, Murray, &
Mulvaney, 2000; Zakaria et al., 2013). Yet, the sulfur present in sulfate functional groups does not present the same characteristics as
alkanethiols since the stabilization and the absence of free electron pairs promoted by the electron delocalization between the
oxygen atoms hinders the occurrence of new bindings. However,
sulfate groups may stabilize nanoparticles by electrostatic interaction like hydroxyl, carbonyl and amino groups that are intrinsically
inhibitory to particle aggregation.
It was confirmed through FTIR analyses that CTAB was not
entirely removed from the AuNRs surface since sulfate and carboxyl groups from ChiS and GA, respectively, exhibit preferential
interactions with the positively charged headgroups of CTAB. The
interactions of the polysaccharides and the AuNRs surface, therefore, occur via the C N+ of CTAB and carboxylate or sulfate groups,
of GA and ChiS, respectively, as depicted in Fig. 7.
4. Conclusions
With a simple, straightforward methodology we demonstrated
that GA and ChiS efficiently interact with the surface of CTAB coated
AuNRs. The resulting self-assembled structures were fully characterized. Microscopy images showed that GA produced AuNRs
irregular clusters and ChiS acted as an efficient encapsulating/wrapping agent resulting in individual AuNRs, which were well
separated by the ChiS molecules. FTIR analyses clearly showed that
GA and ChiS interact with the AuNRs via charged groups of CTAB
by electrostatic interactions, leaving the CH2 groups intact. Combining our results with data already available in the literature, the
toxicity of ChiS-AuNRs and GA-AuNRs is expected to decrease in
relation to CTAB-AuNRs. Therefore, by using a new polysaccharide

we present an interesting strategy to produce individually wrapped

Fig. 7. Schematic representation of the interaction of CTAB/AuNRs with (a) ChiS (CTA+ ROSO3 − /CTA+ /CTAB/AuNR) and (b) GA (CTA+ RCOO− /CTA+ /CTAB/AuNR).


H.R. de Barros et al. / Carbohydrate Polymers 152 (2016) 479–486

485

Table 1
FTIR band assignments of CTAB and as-prepared AuNRs.
Assignmenta

Wavenumber (cm−1 )
CTAB

AuNR

Symmetric and assymetric stretching of CH2 of CTAB chain
Asymmetric and symmetric C H scissoring of H3 C N+ moiety
C N+ stretching
Rocking mode of the CH2 chain ((CH2 )n , n > 4)

2918 and 2850
1487, 1473, 1462 and 1431
962
719 and 731

2918 and 2850
1487, 1473, 1462 and 1431

960
669

a

Based on Nikoobakht & El-Sayed, 2001; Sui et al., 2006; Tang et al., 2013; Campbell et al., 2004; Innocenzi, Falcaro, Grosso & Babonneau 2006.

Table 2
FTIR band assignments of as-prepared AuNRs, GA, GA-AuNR, ChiS and ChiS-AuNRs.
Assignmenta

AuNR

GA

GA-AuNR

ChiS

ChiS-AuNR

Symmetric and asymmetric stretching of
C CH2 of CTAB chain
Assymetric and symetric stretching of the
carboxilic acid salt COO
Asymmetric and symmetric C H scissoring
vibrations of CH3 N+ moiety
C O stretching
Could be attributed to the interactions that
take place by GA-AuNRs interactions

C N+ stretching
C O Amide band
Band associated to the NH3 +
Symmetric stretching of C O C bands

2918 and 2850

–

2918 and 2850

–

2918 and 2850

–

1608 and 1419

–

–

–

1487, 1473, 1462 and 1431

–

1487

–

–

–
–

1253, 1143, 1064 and 1031
–

–
1060, 700–400

1062 and 1004
–

1062 and 1004
–

962
–
–
–

–
–
–
–

–
–
–
–

–
1647
1541
1153

–
1647
1542
1153

a

Based on Davidovich-Pinhas et al., 2014; Espinosa-Andrews et al., 2010; Tang et al., 2013.

AuNRs, applicable when well dispersed nanoparticles are required.
In addition, the observation of clusters or individual AuNRs adds
information to the proper manipulation and usage of these polysaccharide functionalized nanoparticles.
Acknowledgements
The authors acknowledge the support given by the Brazilian National Counsel of Technological and Scientific Development
(CNPq) mainly through the grants 577232/2008-8, 477467/20105 and 564741/2010-8. H. R. Barros, D. A. Sabry and A. M. Nunes
express their gratitude to CAPES for their fellowships. The authors
are very grateful to the Electron Microscopy Center of UFPR (CMEUFPR) for the TEM images and to SENAI PR- Institute of Innovation
in Electrochemistry for the zeta potential measurements.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in

the online version, at />018.
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