Analytica Chimica Acta 764 (2013) 78–83
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Analytica Chimica Acta
journal homepage: www.elsevier.com/locate/aca
Visual detection of arginine based on the unique guanidino
group-induced aggregation of gold nanoparticles
Wendan Pu a , Huawen Zhao a,∗ , Chengzhi Huang b , Liping Wu a , Dan Xu a,b
a
Department of Chemistry, Third Military Medical University, Chongqing 400038, PR China
College of Chemistry and Chemical Engineering, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715,
PR China
b
h i g h l i g h t s
g r a p h i c a l
A simple, fast and novel colorimetric
method for arginine detection was
developed.
This method showed high sensitivity
and selectivity.
As low as 0.4 ␮M arginine could be
easily detected by the naked eye.
This method was successfully used to
detect arginine in real samples.
Schematic diagram of visual detection of arginine based on the citrate-capped AuNPs aggregation.
a r t i c l e
a b s t r a c t
i n f o
Article history:
Received 20 September 2012
Received in revised form 5 December 2012
Accepted 13 December 2012
Available online 22 December 2012
Keywords:
Visual detection
Arginine
Gold nanoparticles
Guanidino group
Isoelectric point
a b s t r a c t
A simple, cost-effective and rapid method for visual detection of arginine based on the citrate-capped
gold nanoparticles (AuNPs) aggregation has been developed in this paper. Arginine is the only amino
acid with guanidino group, and has the highest isoelectric point (pI) at about 10.8. At pH 9.62, negatively
charged citrate-capped AuNPs are well dispersed because of strong electrostatic repulsion. However,
positively charged arginine (pH < pI) easily induces negatively charged citrate-capped AuNPs aggregation
through electrostatic and hydrogen-bonding interactions, resulting in a red to blue color change of the
solution. Using a UV–vis spectrophotometer, the method enables the detection of arginine in the range
of 0.08–13.2 ␮M with a detection limit (3/slope) of 16 nM. Particularly, as low as 0.4 ␮M arginine can be
easily detected by the naked eye without using any complicated or expensive instruments. Furthermore,
this method can provide satisfactory results for the determination of arginine in arginine injection and
compound amino acid injection samples.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
As the most alkaline amino acid, arginine plays a critical role in
many biological functions, for example, cell division, wound healing, erectile dysfunction, and protein production [1–3]. In addition,
arginine is also the precursor of nitric oxide (NO) [4], a key mediator in vascular homeostasis which is implicated in the pathogenesis
of many cardiovascular diseases including hypertension, diabetes,
and atheroma [5]. Obviously, the detection of arginine is very
∗ Corresponding author. Tel.: +86 023 68752217; fax: +86 023 68752217.
E-mail address: sydzhw@yahoo.com.cn (H. Zhao).
0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.aca.2012.12.026
significant. Currently, varieties of strategies for detecting arginine have been reported, for example, high-performance liquid
chromatography (HPLC), liquid chromatography–tandem mass
spectrometry (LC–MS) and molecular recognition technology
[5–7]. Although these approaches make great contributions to the
arginine detection, most of them suffer from specialized equipments, complicated laboratory procedures and low selectivity,
which restrict their practical applications. Therefore, it is necessary
to develop a rapid, inexpensive, selective and sensitive method for
arginine determination.
With the developments in nanotechnology, gold nanoparticles
(AuNPs) have attracted much attention due to their several interesting physical and chemical properties [8]. The well-dispersed
W. Pu et al. / Analytica Chimica Acta 764 (2013) 78–83
AuNPs solution shows red color, whereas the aggregated AuNPs
solution appears purple or blue depending on the degree of aggregation [9]. By taking advantage of this distinct color change, AuNPs
modified with specific ligands have been successfully used for colorimetric detection of many analytes (e.g., DNA, proteins, small
molecules, and metal ions) [10]. Today, many AuNPs-based colorimetric methods for cysteine and homocysteine detection have
been established based on their unique thiol groups [11,12], however, the application of AuNPs in other amino acids assay is scarce
because of the similar structures. Yoo et al. have reported that
arginine, lysine and cysteine could be modified on AuNPs surfaces
and induced AuNPs aggregation [13]. Patel et al. have proposed a
AuNPs-based colorimetric assay for lysine, arginine and histidine
detection [14]. In their system (p-sulfonatocalix [4] arene) thiolmodified AuNPs were used as the colorimetric probes, the presence
of lysine, arginine and histidine simultaneously induced the AuNPs
aggregation through the electrostatic and host–guest interactions.
It is obvious that this method is rather complex and has poor selectivity. Therefore, it remains a challenge to develop a facile, cheap,
and selective method for arginine detection.
Among the 20 amino acids, arginine with guanidino group has
the highest pI at about 10.8 (Table S1) [15,16]. In this paper, we
found that the guanidino group of arginine easily interacted with
the carboxyl groups of citrate-capped AuNPs through electrostatic
and hydrogen-bonding interactions under appropriate acidity. As
a result, the AuNPs aggregated and the solution color changed
from red to blue. However, aggregation could not occur for other
amino acids. Based on the color and absorption spectra change of
citrate-capped AuNPs, we aim at developing a novel method for
the determination of arginine, in hopes that this method does not
involve complicated operational procedures.
79
2.3. Synthesis of citrate-capped AuNPs
AuNPs were synthesized by reducing HAuCl4 with trisodium
citrate [17]. All glassware was thoroughly cleaned with freshly prepared aqua regia solution (1:3 HNO3 /HCl) and rinsed with water
before to use. Briefly, 4 mL 1% (w/w) HAuCl4 solution was added
into the 96 mL distilled water so that the final concentration of
HAuCl4 is about 1 mM, and the solution heated to boiling with stirring. Then 2 mL 5% (w/w) Na3 C6 H5 O7 ·2H2 O was quickly added to
the boiled HAuCl4 solution with vigorous stirring, which result in a
color change of solution from pale yellow to wine red within about
3 min. The resulting solution was kept boiling for another 5 min,
then cooled to room temperature with stirring, and put in refrigerator (4 ◦ C) for reserve. The size of AuNPs was verified through
TEM image and analyzed with the Nano Measurer 1.2 software,
which showed about 13.5 ± 1.6 nm (Fig. S1). The concentration of
AuNPs (11.8 nM) was calculated according to Beer’s law using an
extinction coefficient of ca. 2.7 × 108 M−1 cm−1 at 520 nm for 13 nm
particles [18].
2.4. Detection of arginine
500 ␮L BR buffer, appropriate concentration of arginine, and
500 ␮L citrate-capped AuNPs (11.8 nM) were subsequently added
into the 4 mL centrifugal tube, vortex-mixed and incubated at room
temperature for 5 min. After that, the mixture was diluted with
distilled water to 2.5 mL with thoroughly vortexing. The UV–vis
absorption spectra were recorded over the wavelength range from
300 nm to 800 nm. The selectivity for arginine was confirmed by
adding other amino acids instead of arginine in the same way.
3. Results and discussion
2. Experimental
3.1. Arginine induced citrate-capped AuNPs aggregation
2.1. Materials and reagents
In this work, it was found that arginine easily induced citratecapped AuNPs aggregation under appropriate acidity, while it was
difficult for other amino acids. As shown in Fig. 1A, the citratecapped AuNPs were stable and showed a red color (Fig. 1A, bottle
a) in buffer, which displayed a maximal absorption peak at 520 nm
(Fig. 1A, Curve a). After addition of arginine, however, citratecapped AuNPs aggregated quickly. This aggregation results in red to
blue color change (Fig. 1A, bottle b), accompanying a new absorption peak at about 650 nm (Fig. 1A, Curve b) appeared. The new
absorption peak is attributed to electric dipole–dipole interaction and coupling between plasmons of neighboring particles in
the aggregates [19]. On the other hand, TEM images showed that
citrate-capped AuNPs without arginine exhibited a well monodispersed state (Fig. 1B), while aggregation occurred in the presence of
arginine (Fig. 1C). Based on the color and absorption spectra change
of citrate-capped AuNPs, thus, we can develop a novel method for
the detection of arginine.
Chloroauric acid tetrahydrate (HAuCl4 ·4H2 O) and amino acids
(Arg, Cys, Asp, Gly, Ala, Gln, Glu, His, Asn, Lys, Ile, Leu, Phe, Met,
Thr, Pro, Ser, Tyr, Val, Trp) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Trisodium citrate dihydrate
(Na3 C6 H5 O7 ·2H2 O) was ordered from Chengdu Kelong Chemical
Reagent Co., Ltd. (Chengdu, China). Arginine hydrochloride injection was purchased from Shanghai Xinyi Jinzhu Pharmaceutical Co.,
Ltd. (Shanghai, China). Compound amino acid injection (15-HBC)
was purchased from Hubei Halfsky Pharmacy Inc. (Hubei, China).
The stock solution (10 mM) of arginine was prepared by dissolving
the amount of arginine with distilled water, and the working solution was obtained via dilution. Milli-Q purified water (18.2 M cm)
was used throughout the experiments. The pH of the solution was
adjusted by Britton–Robinson (BR) buffer.
2.2. Apparatus
The absorption spectra of AuNPs were measured on a TU-1901
UV-vis spectrophotometer (Beijing Purkinje General Instrument
Co., Ltd., Beijing, China). Transmission electron microscopy (TEM)
images of the nanoparticles were carried on a H-7500 (Hitachi
Ltd., Tokyo, Japan). Transmission Infrared spectra in KBr pellets
were measured with FTIR-650 spectrometer (Tianjin Gangdong
Sci&Tech. Development Co., Ltd. Tianjin, China). A vortex mixer
XW-80A (Shanghai Jingke Industrial Co., Ltd. Shanghai, China) was
employed to blend the solution. All measurements were taken at
room temperature.
3.2. Mechanism of arginine induced citrate-capped AuNPs
aggregation
In order to identify the mechanism of arginine induced
citrate-capped AuNPs aggregation, a series of experiments were
investigated. Citrate-capped AuNPs were stable when pH ranged
from 6.59 to 11.12 (Figs. S2 and 2A). It is attributed to that the citric
acid is a tribasic acid with three pKa values of 3.13, 4.76 and 6.40
[20], respectively. When pH was over 6.40, the carboxyl groups
of citric acid were fully deprotonated and existed in the anionic,
so citrate-capped AuNPs dispersed well due to strong electrostatic
repulsion. But slight self-aggregation were observed when the pH
80
W. Pu et al. / Analytica Chimica Acta 764 (2013) 78–83
Fig. 1. Absorption spectra (A) and TEM images (B and C) of citrate-capped AuNPs in the absence and presence of arginine. Inset of (A): arginine induced color change of
citrate-capped AuNPs solution. Concentrations: citrate-capped AuNPs, 2.36 nM; arginine, 12 ␮M; pH, 9.62. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
was higher than 11.12 due to the fact that shielding effect of buffer
salts increased with increasing pH (Fig. S2) [21].
However, in the presence of arginine, significant color and spectra change of citrate-capped AuNPs were observed when pH ranged
from 6.59 to 11.12. As shown in Fig. 2B, when pH was much
lower than pI of arginine (e.g., pH 7.00), citrate- capped AuNPs
aggregated severely and produced precipitates because of strong
electrostatic attraction between positively charged arginine and
negatively charged citrate-capped AuNPs. When pH was close to
the pI of arginine (e.g., pH 9.91), aggregation extent of citratecapped AuNPs was suppressed because positive charges of arginine
gradually decreased. Furthermore, when pH was greater than pI
(e.g., pH 11.12), both arginine and citrate-capped AuNPs were
negatively charged, the citrate-capped AuNPs did not aggregate
anymore because of strong electrostatic repulsion.
In addition, if arginine induced citrate-capped AuNPs aggregation was only a simple electrostatic attraction like previously
reported articles [22,23], other amino acids should produce
the same aggregation phenomenon by turning pH. Here, lysine
(pI = 9.5) and histidine (pI = 7.6) were used as controls. As shown in
Fig. S3, 8 ␮M lysine made the citrate-capped AuNPs almost remain
dispersed even at relatively low pH (below its pI ca. 2). However,
Fig. S4 shows 4 ␮M arginine easily induced citrate-capped AuNPs
aggregation even at relatively high pH (below its pI ca. 1). Similar to
lysine, histidine also could not induce citrate-capped AuNPs aggregation when pH was lower than its pI (Fig. S5). Thus, it indicated
that arginine induced citrate-capped AuNPs aggregation was not a
simple electrostatic attraction and the guanidino group may play a
key role.
In order to validate the above hypothesis, FTIR spectra of arginine and citrate- capped AuNPs/arginine complex were measured.
As shown in Fig. S6, band at 1719 cm−1 assigned to the stretching vibrations of C O in arginine (Fig. S6, Curve a) shifted to
1636 cm−1 (Fig. S5, Curve b), and an absorption band at 1680 cm−1
assigned to the C N stretching vibrations of guanidino group
(Fig. S6, Curve a) almost disappeared in AuNPs/arginine complex
(Fig. S5, Curve b), thus suggesting that the involvement of the C N
in the formation of hydrogen bonds [24–26]. To our knowledge,
W. Pu et al. / Analytica Chimica Acta 764 (2013) 78–83
81
Fig. 3. Schematic diagram of visual detection of arginine based on the citrate-capped
AuNPs aggregation. (For interpretation of the references to color in text, the reader
is referred to the web version of this article.)
3.3. Principle of the detection method
Fig. 2. UV–vis absorption spectra of citrate-capped AuNPs in the absence (A) and
presence (B) of arginine at different pH values. Inseted pictures display the color
change of citrate-capped AuNPs corresponding to Curves a–j. pH (in the order of
Curves a–j): 6.59, 7.00, 7.96, 8.36, 8.95, 9.62, 9.91, 10.38, 10.88, 11.12. Concentrations: citrate-capped AuNPs, 2.36 nM, arginine, 20 ␮M. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
hydrogen-bonding recognition plays a key role in targets detection [27–29], for example, Ai et al. developed a novel strategy
for visual detection of melamine based on the hydrogen-bonding
recognition-induced color change of AuNPs [27]. It is well known
that the positively charged guanidino group of arginine can connect
to the surface of citrate-capped AuNPs through electrostatic and
hydrogen-bonding interactions [25,26,30,31]. Here, the connected
arginine can neutralize the negative charges of citrate-capped
AuNPs, and underwent inter-particle cross-linking to form aggregates through hydrogen-bonding interaction between carboxyl and
amido of arginine [32].
On the basis of above results, it is concluded that the guanidino group of arginine plays a key role in inducing citrate-capped
AuNPs aggregation. Although lysine and histidine are positively
charged when pH are lower than their pI, large amounts of lysine
and histidine also do not effectively induce citrate-capped AuNPs
aggregation (Fig. S7). This also demonstrates that the arginine
induced aggregation is not only caused by electrostatic attraction.
At last, pH 9.62, which is close to the pI of lysine but much lower
than the pI of arginine was used to control the experimental acidity.
According to the above analysis, Fig. 3 presents the design rationale. The citrate-capped AuNPs were synthesized by reduction
of HAuCl4 with sodium citrate according to previous literature
[17], and thus the surface of the AuNPs contained plenty of carboxyl groups. Initially, at pH 9.62, citrate-capped AuNPs were well
dispersed in water due to the strong electrostatic repulsion, and
the color of the solution was red. In the presence of arginine,
however, positively charged guanidino group of arginine easily
interact with the carboxyl groups of citrate-capped AuNPs through
electrostatic and hydrogen-bonding interactions (Fig. 3, Frame A),
leading arginine connected upon the surface of AuNPs, and farther
crosslinking of the neighboring nanoparticles through hydrogenbonding interaction of carboxyl and amido (Fig. 3, Frame B). As a
result, aggregation of citrate-capped AuNPs occurred, resulting in
a red to blue color change.
3.4. Sensitivity for arginine detection
The arginine induced aggregation of citrate-capped AuNPs could
result in a color change from red to blue. As shown in Fig. 4A,
the naked eye alone could judge the presence of 0.4 ␮M arginine
without the use of any advanced instruments. In addition, to quantitatively detect arginine by using the developed method, UV–vis
absorbance spectra of the citrate-capped AuNPs in the presence
of different concentrations of arginine were recorded. As shown
in Fig. 4B, with increasing of the concentrations of arginine, the
absorption peak at 520 nm gradually decreased (Fig. 4B, Curves
1–14), and a new peak located at about 650 nm gradually increased
(Fig. 4B, Curves 1–14). The absorption ratio between 650 nm and
520 nm (A650 /A520 ) was linearly related to the arginine concentration with the linear range from 0.08 to 13.20 ␮M (Fig. 4C, the
calibration data are shown in Table S2), which could be expressed as
A650 /A520 = 0.1062 + 0.06156 carginine (␮M) with a correlation coefficient of 0.9973. And the limit of the detection (defined as 3/slope)
was estimated to be 16 nM (where was the relative standard deviation of a blank solution, n = 11). As shown in Table S3, it can be
concluded that the proposed method shows good performance in
comparison with other methods.
3.5. Selectivity for arginine detection
To assess the selectivity of this method for detecting arginine,
the effect of other 19 essential amino acids were examined. As
82
W. Pu et al. / Analytica Chimica Acta 764 (2013) 78–83
Fig. 5. Values of A650 /A520 and a photographic images (inset) of citrate-capped
AuNPs in the presence of arginine or other amino acids. The error bars represent the
standard deviation from three independent measurements. Concentrations: citratecapped AuNPs, 2.36 nM; arginine, 8.5 ␮M; the other amino acids, 200 ␮M; pH 9.62.
(For interpretation of the references to color in text, the reader is referred to the
web version of this article.)
Table 1
Determination results of arginine in synthetic samples.
Sample
1
2
Added (␮M)
6.00
7.00
Main additives
+
His, Cys, Asp, Ag
Pro, Glu, His, Ag+
Found (␮M)a
Recovery (%)
5.98 ± 0.11
7.08 ± 0.19
99.67
101.14
a
Mean ± SD of six measurements.
Concentrations: citrate-capped AuNPs, 2.36 nM; His, Cys, Asp, Pro and Glu, 200 ␮M;
Ag+ , 300 ␮M; BR buffer, pH 9.62.
3.6. Detection of arginine sample
Fig. 4. (A) The photo shows the color change of citrate-capped AuNPs with the
increase of arginine concentrations. (B) UV–vis absorption spectra of citrate-capped
AuNPs in the presence of different concentrations of arginine. The arrows indicate
the signal changes with the increase of arginine concentrations. (C) The plot of
A650 /A520 vs. the concentrations of arginine from 0.08 to 13.2 ␮M. The error bars
represent the standard deviation from three independent measurements. Concentrations: citrate-capped AuNPs, 2.36 nM; arginine (in the order of Curves 1–14): 0,
0.08, 0.16, 0.4, 0.5, 1.2, 2.2, 3.2, 5.0, 7.6, 8.4, 9.6, 11.2, 13.2; pH 9.62. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
shown in Fig. 5, it was found that most of the other amino acids
(pI = 3.0–9.5) did not interfere with the detection of arginine except
cysteine. It can attribute to that these amino acids exist in the
anionic when pH of solution is 9.62, which do not induce citratecapped AuNPs aggregation because of the electrostatic repulsion
force between them. Although cysteine (pI = 5.02) was negatively
charged at pH 9.62 and could not interact with citrate-capped
AuNPs through electrostatic attraction, its SH preferred to bind
with citrate-capped AuNPs and induced AuNPs aggregation. But
the interference of cysteine could be masked upon the use of
Ag+ [33,34]. Here, cysteine reacted with Ag+ to form cysteine/Ag+
complex, which made the SH of cysteine unable to connect to
the surface of citrate-capped AuNPs anymore. As expressed in
Fig. 5, significant absorption ratio (A650 /A520 ) and color change were
observed in the presence of arginine, while other amino acids did
not cause any change though at large amount. After the addition
of Ag+ , cysteine did not interfere with the detection of arginine
too. These results clearly indicated that the method shows a highly
selective response to arginine.
In order to verify the performance of the method for the detection of arginine, artificial samples, in which a series of arginine
together with other common amino acids and Ag+ had been added
displayed in Table 1. The results for the artificial samples indicate
that the assay of arginine by this method is reliable.
On the other hand, in order to evaluate the real applicability
of this method, arginine hydrochloride injection and compound
amino acid injection (15-HBC) were used as real samples, and
the detection results are shown in Table 2. The results show this
method can be directly used for the detection of arginine in pure
arginine injection (sample a in Table 2). Because other amino acids
do not interfere with the detection of arginine or interferences can
be eliminated by addition of Ag+ , the method also can be used for
the detection of arginine in compound amino acids injection (sample b in Table 2). Furthermore, although some proteins could induce
citrate-capped AuNPs aggregation, the interferences can be eliminated by pre-treatment, therefore, this method could be used in
practical applications.
Table 2
Determination results of arginine in real samples with the proposed method.
Sample
Certified concentration (␮M)
Found (␮M)a
Recovery (%)
a
b
9.00
10.0
9.19 ± 0.30
10.03 ± 0.29
102.11
100.30
Sample a: arginine hydrochloride injection obtained form Shanghai Xinyi Jinzhu
Pharmaceutical Co., Ltd. (Shanghai, China).
Sample b: compound amino acid injection (15-HBC) obtained from Hubei Halfsky
Pharmacy Inc. (Hubei, China).
a
Mean ± SD of six measurements.
Concentrations: citrate-capped AuNPs, 2.36 nM; BR buffer, pH 9.62.
W. Pu et al. / Analytica Chimica Acta 764 (2013) 78–83
4. Conclusions
In summary, we have developed a novel method for simple
and rapid visual detection of arginine. When pH is lower than pI
of arginine, positively charged guanidino group of arginine connects to the surface of citrate-capped AuNPs through electrostatic
and hydrogen-bonding interactions leads to citrate-capped AuNPs
aggregation. Compared with other methods that employ specific
strategies, our present method possesses several advantages. First
of all, as low as 0.4 ␮M arginine can be simply visualized by the
naked eye without the requirement of any complicated or expensive instruments. Second, this method is simple in design and fast in
manipulation, and the detection can be completed within 15 min.
Third, the labo-intensive, and cumbersome AuNPs modification
steps are avoided, which is facile, low-cost and particularly useful for resource-limited conditions. More importantly, the method
has been successfully applied for the detection of arginine in arginine hydrochloride injection and compound amino acid injection
samples.
Acknowledgments
This work was supported by the Chongqing Natural Science
Foundation (CSTC, 2007BB0049) and the National Natural Science
Foundation Committee of China.
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.aca.2012.12.026.
References
[1] J.K. Stechmiller, B. Childress, L. Cowan, Nutr. Clin. Pract. 20 (2005) 52–61.
[2] H. Tapiero, G. Mathé, P. Couvreur, K.D. Tew, Biomed. Pharmacother. 56 (2002)
439–445.
83
[3] M.A. Vermeulen, M.C. van de Poll, G.C. Ligthart-Melis, C.H. Dejong, M.P. van den
Tol, P.G. Boelens, P.A. van Leeuwen, Crit. Care Med. 35 (2007) S568–S576.
[4] N. Guelzim, F. Mariotti, P.G.P. Martin, F. Lasserre, T. Pineau, D. Hermier, Amino
Acids 41 (2011) 969–979.
[5] V. Gopalakrishnan, P.J. Burton, T.F. Blaschke, Anal. Chem. 68 (1996) 3520–
3523.
[6] H. Chen, L. Gu, Y. Yin, K. Koh, J. Lee, Int. J. Mol. Sci. 12 (2011) 2315–2324.
[7] K. Vishwanathan, R.L. Tackett, J.T. Stewart, M.G. Bartlett, J. Chromatogr. B 748
(2000) 157–166.
[8] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293–346.
[9] H. Li, L.J. Rothberg, J. Am. Chem. Soc. 126 (2004) 10958–10961.
[10] Y. Du, B. Li, E. Wang, Bioanal. Rev. 1 (2010) 187–208.
[11] I.I.S. Lim, W. Ip, E. Crew, P.N. Njoki, D. Mott, C.J. Zhong, Y. Pan, S. Zhou, Langmuir
23 (2007) 826–833.
[12] F.X. Zhang, L. Han, L.B. Israel, J.G. Daras, M.M. Maye, N.K. Ly, C.J. Zhong, Analyst
127 (2002) 462–465.
[13] E.J. Yoo, T. Li, H.G. Park, Y.K. Chang, Ultramicroscopy 108 (2008) 1273–1277.
[14] G. Patel, S. Menon, Chem. Commun. (2009) 3563–3565.
[15] L. Pogliani, J. Pharm. Sci. 81 (1992) 334–336.
[16] H. Cohen, A. Gedanken, Z. Zhong, J. Phys. Chem. C 112 (2008) 15429–
15438.
[17] Y. Wang, Y.F. Li, J. Wang, Y. Sang, C.Z. Huang, Chem. Commun. 46 (2010)
1332–1334.
[18] R. Jin, G. Wu, Z. Li, C.A. Mirkin, G.C. Schatz, J. Am. Chem. Soc. 125 (2003)
1643–1654.
[19] S. Basu, S.K. Ghosh, S. Kundu, S. Panigrahi, S. Praharaj, S. Pande, S. Jana, T. Pal, J.
Colloid Interface Sci. 313 (2007) 724–734.
[20] Y.K. Leong, J. Am. Ceram. Soc. 93 (2010) 2598–2605.
[21] J. Wang, Y.F. Li, C.Z. Huang, T. Wu, Anal. Chim. Acta 626 (2008) 37–43.
[22] Y.M. Chen, C.J. Yu, T.L. Cheng, W.L. Tseng, Langmuir 24 (2008) 3654–3660.
[23] R. Cao, B. Li, Chem. Commun. 47 (2011) 2865–2867.
[24] Y. Ma, Y. Guo, J. Li, J. Guan, L. Xu, W. Yang, Chem. Eur. J. 15 (2009) 13135–
13140.
[25] N. Metanis, E. Keinan, P.E. Dawson, J. Am. Chem. Soc. 127 (2005) 5862–5868.
[26] R. Zhou, K.Y. Wei, J.S. Zhao, Y.B. Jiang, Chem. Commun. 47 (2011) 6362–6364.
[27] K. Ai, Y. Liu, L. Lu, J. Am. Chem. Soc. 131 (2009) 9496–9497.
[28] W.J. Qi, D. Wu, J. Ling, C.Z. Huang, Chem. Commun. 46 (2010) 4893–4895.
[29] M. Xue, X. Wang, H. Wang, D. Chen, B. Tang, Chem. Commun. 47 (2011)
4986–4988.
[30] S.V.P. Barreira, F. Silva, Langmuir 19 (2003) 10324–10331.
[31] G. Tomoaia1, P.T. Frangopol, O. Horovitz, L.D. Bobos, A. Mocanu, M. TomoaiaCotisel, J. Nanosci. Nanotechnol. 11 (2011) 7762–7770.
[32] S. Panigrahi, S. Kundu, S. Basu, S. Praharaj, S. Jana, S. Pande, S.K. Ghosh, A. Pal,
T. Pal, Nanotechnology 17 (2006) 5461–5468.
[33] Z. Lin, X. Li, H.B. Kraatz, Anal. Chem. 83 (2011) 6896–6901.
[34] T. Li, L. Shi, E. Wang, S. Dong, Chem. Eur. J. 15 (2009) 3347–3350.