J Mater Sci (2014) 49:7143–7150
DOI 10.1007/s10853-014-8422-x
Negatively charged gold nanoparticles as an intrinsic peroxidase
mimic and their applications in the oxidation of dopamine
Yanping Liu • Cunwen Wang • Ning Cai
Sihui Long • Faquan Yu
•
Received: 2 April 2014 / Accepted: 24 June 2014 / Published online: 10 July 2014
Ó Springer Science+Business Media New York 2014
Abstract Artificial inorganic peroxidase is of great
interest due to its intrinsic advantages over natural counterpart. Negatively charged gold nanoparticles (AuNPs)
were discovered to function like a peroxidase in the present
study. Two AuNPs in different size were prepared and
characterized by TEM, and assayed for peroxidase activity.
Its catalytic activity was found to follow Michaelis–Menten kinetics. The negative surface charge notably improves
the affinity toward a substrate TMB, proved by the determined kinetic parameters. The particles expressed optimal
catalytic activity under mildly acidic environment and
resistance to elevated temperature and increased concentration of sodium azide. The origin of the activity was
investigated tentatively. Hydrogen peroxide-treated AuNPs
exhibited an enhanced activity. EDTA temporarily blocked
the activity partially, while thiol groups permanently
blocked the activity completely. Tests imply that it is the
surface Au? that provides the activity. The successful
oxidation of dopamine, as an instance, under the action of
AuNPs as a peroxidase was conducted. These studies
would lead to a wide range of potential applications.
Introduction
In the past decades, gold nanoparticles (AuNPs) have
attracted a continuous interest due to their unusual properties in electronics, optics, especially in biotechnology [1–
3]. The unique color, stemming from surface plasma
Y. Liu C. Wang N. Cai S. Long F. Yu (&)
Key Laboratory for Green Chemical Process of Ministry of
Education, School of Chemical Engineering and Pharmacy,
Wuhan Institute of Technology, Wuhan 430073, Hubei, China
e-mail: fyuwucn@gmail.com; fyu@mail.wit.edu.cn
resonance (SPR), is a versatile signal for studying chemisorption, redox reactions, (bio)sensing, alloying, and electrochemical processes [3]. AuNPs can efficiently quench
the absorbed fluorophores, which will be turned on upon
disruption by an analyte. These complexes are thus quite
useful in biosensors by judicious design [4, 5]. Hyperthermia can be produced by near-infrared laser irradiation
of AuNPs present in tumors and thus kills tumor cells [6].
Moreover, AuNPs are the candidate carrier of drug or gene
for the therapy of disease [7, 8]. Its ability to catalyze the
selective oxidation of organic substances under mild conditions attracts a wide range of researches as well [9, 10].
These functions, plus the good biocompatibility and the
ease of preparation, size-tuning techniques, and surface
modifications, lead to the active studies of AuNPs.
Horseradish peroxidase (HRP) is a natural peroxidase,
often utilized in determination of the presence of a
molecular target such as a small amount of a specific
protein in a western blot, as well as in techniques such as
enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry [11]. A variety of studies in relation with
HRP-conjugated AuNPs have been reported. To note, the
conjugates demonstrated an enhanced activity over HRP
itself [12, 13].
Recently, there is an ever-increasing interest in artificial
enzymes because they demonstrate copious advantages
over naturally occurring protein-based enzymes. Yan’s
group reported that ferromagnetic nanoparticles possess
peroxidase-like activity [14]. The following researchers
conducted a series of modifications on ferromagnetic
nanoparticles and improved the catalytic activities [15, 16]
or subsequently put the activities in practice [17]. Specifically, this kind of nanoparticles possesses dual natures of
enzyme and inorganic nano-materials such as
superparamagnetism.
123
7144
J Mater Sci (2014) 49:7143–7150
Positively charged AuNPs were revealed to behave like a
peroxidase recently [18]. The discovery will surely widen
AuNP applications. In this study, negatively charged AuNPs
were discovered to possess the intrinsic peroxidase-like
activities. The catalysis mechanism was explored through the
investigation of Michaelis–Menten kinetic process. Furthermore, the peroxidase-like feature of AuNP was successfully
employed for the oxidation of dopamine, as an example.
sequentially loading 100 lL of 0.2 M acetate buffer (pH
4.0), 50 lL of 0.1 mM TMB in water, 50 lL of 0.1 mg/
mL s-AuNP in water or 50 lL of 0.2 lg/mL HRP in 0.1 M
PBS (pH 7.2), and 50 lL of 0.25 M peroxide hydrogen.
The blue color that developed as reactions proceeded was
monitored kinetically at a wavelength of 652 nm. Catalytic
parameters were determined by fitting the absorbance data
to the Michaelis–Menten kinetics model. HRP was tested
for the purpose of comparison.
Materials and methods
Oxidation of dopamine
Chemicals and reagents
All materials were purchased and used as received without
further treatment. Materials including 3,30 ,5,50 -tetramethylbenzidine dihydrochloride hydrate (TMB), HRP (type
VI, from horseradish), HAuCl4, hydrogen peroxide, sodium
borohydride, and sodium citrate were all from SigmaAldrich.
The oxidation process of dopamine is performed as follows: to the mixture of 50 lL 0.1 mg/mL s-AuNPs and
50 lL 0.125 M H2O2, 50 lL 1 mM dopamine in 0.2 M
acetate buffer (pH 4.0) was added. The system was scanned
spectrometrically from 200 to 800 nm wavelength every
5 min. Two controls were designed. In one H2O was used
in place of H2O2 and in the other H2O was used in place of
s-AuNPs.
Synthesis and characterization of gold nanoparticles
(AuNPs)
Results
Small-size AuNPs(s-AuNP) were synthesized according to
a literature procedure with minor modifications [19]. In
brief, to a tube with 18.4 ml of water, 0.5 mL of 10 mM
HAuCl4, and 0.5 mL of 10 mM sodium citrate were added.
Then 0.6 mL of 10 mM NaBH4 was introduced in one
portion under vigorous agitation. After 60 min, the reaction
mixture was ultrafiltrated via 500-MWCO membrane using
an ultrafiltration unit for removal of small molecules.
Without using NaBH4, the big-size AuNPs (b-AuNP) were
obtained following the literate procedure [13].
Particle size and zeta potential were estimated on a
Nano ZS90 dynamic light scattering (DLS) instrument
(Malvern, UK) equipped with a HeNe laser at 632 nm as
the incident light. Zeta potential measurements were carried out after nanoparticles were diluted and dispersed in
deionized water. Transmission electron microscopy (TEM)
was conducted using a JEM 3011-type electron microscope
(JEOL Tokyo, Japan) operated at an accelerating voltage of
300 kV. Gold content was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) on
an Optima 4300 DV instrument (Perkin-Elmer, Inc., Boston, MA, USA). Absorbance was monitored using a
Molecular Devices SpectraMax M2e spectrometer
(Molecular Devices, Sunnyvale, CA, USA).
Peroxidase-like activity
Unless otherwise stated, the assays of the peroxidase
activity were conducted at 25 °C in a 1.5 mL tube by
123
Verification of the peroxidase activity
The synthesis of AuNPs is an established technique. The size
can be easily controlled, dependent on synthesis parameters.
Herein, the AuNPs were synthesized following the literature
methods [13, 19]. AuNPs of two sizes were obtained, suggested by TEM observation in Fig. 1a, b. According to the
Image J software, the average size in diameter is
19.4 ± 0.4 nm, noted as b-AuNP, and 5.1 ± 0.2 nm, noted
as s-AuNP, respectively. Figure 1c exhibits the characteristic SPR spectrum of AuNPs. s-AuNP has a SPR band at
520 nm and b-AuNP’s SPR band locates at 528 nm, a small
shift toward higher wavelength. These observations are in
agreement with the previous report [20]. The zeta potential
for the s-AuNP and the b-AuNP was -26.7 and -22.3 mV,
respectively. The charge on the particles renders them well
suspended in the medium. The negative charge is believed to
stem from the stabilizing agent of citrate, which is located on
the surface of AuNPs. The structure was identified recently
in detail [21].
The peroxidase-like behavior of the synthesized negatively charged AuNPs was examined at room temperature
using TMB as a chromogenic substrate. Figure 2 demonstrates the color development of TMB under various conditions. As well known, TMB is a common chromogenic
substrate and displays blue when HRP, a conventional
peroxidase, is used. The blue color will be converted into
yellow when H2SO4 is applied to terminate the oxidation
process, a standard termination mode [22]. Both of the blue
J Mater Sci (2014) 49:7143–7150
7145
Fig. 1 TEM image of b-AuNPs
(a) and s-AuNPs (b), and their
surface plasma resonance
spectrum (c)
Fig. 2 a The visual observation of the peroxidase-like activity by the
color change (from left to right: TMB, AuNP, H2O2, TMB/AuNP,
TMB/H2O2, AuNP/H2O2, and AuNP/TMB/H2O2) and b spectra of
TMB under the catalysis of AuNP or HRP and of those subjected to
H2SO4-termination
and yellow colors are characteristic of the occurrence of the
reaction of peroxidase catalysis. Figure 2a shows the visual
observation of the reaction. Blue color appears under the
co-action of AuNPs and H2O2, identical with the color
under the action of HRP (photo not shown). All other tests
without H2O2 or AuNPs would not show such colors.
Figure 2b exhibits the consistent spectrum variation of
TMB substrate under the action of either AuNPs or HRP,
with the typical appearance of either peak at 652 nm (blue)
or at 450 nm after terminated by H2SO4 (yellow). This
experiment verifies the fact that AuNPs function just like
HRP as a peroxidase.
A peroxidase catalytic process is supposed to follow the
Michaelis–Menten (M–M model) kinetic behavior. The
close fit of the experimental data with the M–M model
allows the peroxidase activity to be further verified. Figure 3 is the result of the kinetic analysis, the points being
the experimental data and the curves being the fit of M–M
model. The absorbance of the system was monitored over
time. Apparent steady-state reaction rates at different
concentrations of substrate were obtained by calculating
the slopes of initial absorbance over time. Absorbance data
were used to determine the concentration of TMB-derived
oxidation products by the Beer–Lambert Law using a
molar absorption coefficient of 39000 M-1cm-1 [23]. Data
shown in Fig. 3 indicated a hyperbolic kinetics and were fit
to the Michaelis–Menten equation. A set of model
parameters (Vmax and KM) were, therefore, extracted. The
Fig. 3 Michaelis–Menten analysis of AuNPs peroxidase-like activity. The points represent real experimental data and the curves
correspond to theoretical fit of the Michaelis–Menten kinetics
catalytic constants were calculated out: KM, H2O2 = 33.0 mM,
Vmax = 6.1 9 10-8mol/(L s), KM, TMB = 11.2 lM, and
Vmax = 8.3 9 10-8mol/(L s).
Effect of pH
A natural peroxidase such as HRP is usually pH sensitive
and functions only around neutral pH. Yan reported an
acid-dependent feature of the peroxidase-like activity of
MION [14]. MION executes the maximum catalytic
activity at pH 4 [14]. Therefore, a series of pH media were
prepared in order to explore the influence of pH on peroxidase activity of AuNPs. Experimental results exhibit
that AuNPs have strong acid-dependent catalytic activity,
shown in Fig. 4a. Acidic environment, specifically at pH
B5.7, favors the activity. At pH 6.4 or above, AuNPs
123
7146
J Mater Sci (2014) 49:7143–7150
Fig. 4 The relative catalytic activity dependent on pH (a), temperature (b), and sodium azide (c)
drastically lose their catalytic activity. This observation is
very close to the results observed in the case of MION [14].
Effect of temperature
Enzyme activity is usually temperature-dependent. HRP
loses its activity once the temperature rises over 37 °C
(Fig. 4b). AuNPs, in sharp contrast, keep over 60 % activity
even when the temperature is elevated over 90 °C. It is
uncertain so far, whether it is because of the evaporation or
decomposition of hydrogen peroxide or the change of AuNP
itself at elevated temperature that results in the 40 % loss of
the catalytic activity. But it is certain that AuNPs still keep
noteworthy activity over 37 °C, at which HRP lose its
functions completely, as a comparison. The temperatureresistant feature will allow wide applications of AuNPs in
those fields, where HRP is not competent.
Effect of inhibitor
Sodium azide is employed as a conventional bacterium
inhibitor in many detection applications such as ELISA, in
which HRP is the mostly used peroxidase. However, the
challenge that HRP faces is the loss of its activity in
sodium azide environment. It is necessary to test the sensitivity effect of this inhibitor on AuNP peroxidase. This
effect in comparison with HRP is summarized in Fig. 4c. It
is shown that AuNPs exhibit 60 % original activity at a
concentration of sodium azide as high as 0.2 g/L, a conventional concentration applied in practice, while HRP is
completely deactivated at this concentration. In this sense,
AuNPs would be more resistant to the disturbance of
sodium azide than HRP if being utilized in such fields as
ELISA.
Oxidation of dopamine
Dopamine is a neurotransmitter in the catecholamine and
phenethylamine families that plays a number of important
123
Fig. 5 The oxidation of dopamine under the peroxidase-like catalysis
of s-AuNPs. a the spectrum of oxidized dopamine with oxidation
time, b the variation of absorbance at respective peak 295, 475, and
630 nm with oxidation time
roles in the brain and body of animals. Peroxidase/H2O2
couple is commonly chosen as a biomimetic oxidizing
agent to examine the early stages of dopamine oxidation
[24]. Here, we chose AuNPs in place of well-known peroxidases to conduct the process. Figure 5a is the spectra of
oxidized dopamine under the action of H2O2 and s-AuNP.
The same system at the zero-time point was taken as the
J Mater Sci (2014) 49:7143–7150
7147
Fig. 6 a the elevated activity after H2O2 treatment, b the retarded activity after EDTA chelating, c the blocked activity after thiglycolic acid
treatment, and d the size-dependent activity
background. In a strict sense, the background at a real zerotime point is hard to obtain as a single scan takes seconds.
However, the scan in as short a time as could was still
arbitrarily accepted as the background. In experiments, two
new peaks developed with time at 295 and 475 nm positions. Further, the absorbance at these two positions was
plotted with time, illustrated in Fig. 5b. It was discovered
that the absorbance linearly increases with time. On the
other hand, the peak at 630 nm position linearly fades with
time and finally disappears (Fig. 5b). As a comparison, not
any alteration of absorbance was observed with time
without H2O2 or AuNPs applied. As well known, dopamine
has a very complex oxidization process with a series of
intermediates, and melanin is the final oxidized product.
Though it is difficult to assign the peaks so far, the intensity
of these peaks varies linearly with time. This observation
implies that the couple of AuNPs and H2O2 certainly led to
the oxidation of dopamine and that the oxidation process
takes one-order kinetics. Neither s-AuNPs nor H2O2 can
individually oxidize dopamine.
Tentative explanation to the mechanism
To rule out the role of the leached Au? or Au3? ions in
solution in the peroxidase behavior, we tested the supernatant solution of AuNPs by ultrafilteration separation. No
catalytic activity was observed in the case of the supernatant solution. Therefore, this observed activity originated
from AuNPs itself.
Herschbach and Sandroff concluded that Au(0) and
Au(I) sites coexist on the colloidal surface, and they
speculated that the Au(I) sites form complexes with citrate
[25]. In order to explore the mechanism, we treated the
AuNPs in H2O2 (pH 4.0) at 75 °C for 7 h. It was found out
that the H2O2-treated AuNPs enhanced the activity by
21 %, compared with the intact AuNPs, or even 38 %
higher than the particles treated in H2O instead of H2O2 in
the same way above (Fig. 6a). The H2O2 treatment process
is believed to generate more surface gold ions Au?. This
experiment provides a hint that it is the surface Au? ions
that probably provide the catalytic activity.
In an opposite way, we tried to block the surface Au?
ions to test the decrease of the catalytic activity if the
presumption is correct. EDTA is a conventional chelating
agent, which is able to block the action of surface Au? ions
via the chelating effect and thus would perhaps suppress
the catalytic activity. As demonstrated in Fig. 6b, EDTA
reduced the activity, as anticipated. However, the inhibition effect was very limited. It faded with time and finally
the catalytic activity recovered over 96 % of the level
without EDTA applied in 9 min, in the range of experimental concentrations of EDTA. Likely, the chelating of
EDTA with surface Au? ions is not tight enough and thus
could be replaced gradually by the substrate TMB, which
may explain why the inhibition effect would fade gradually
with time. To clarify this idea, EDTA was replaced by thiol
groups to test again since the latter binds Au? much more
tightly than EDTA. As displayed in Fig. 6c, 92 % of
activity was lost when thioglycolic acid was applied in
place of EDTA. The loss is much higher than that in the
case of EDTA. Furthermore, the loss of activity was irreversible with time.
Interestingly, the thiol amount that completely inhibited
the activity is just the amount that can cover all the surface
area of AuNPs in terms of quantitative analysis. By virtue
of the equation reported previously [26], 2166 gold atoms
are contained in a 4.2 nm-in-diameterAuNP. As the gold
amount applied in the experiment is known, the number of
particle can thus be calculated out as 1.29 9 10-11 mol.
The surface area of each particle was 55.4 nm2, assuming
the particle was a sphere. The head area of thiol group
occupies 0.21 nm2 in light of the literature [27]. Put
together, the AuNPs could hold a total of 3.4 9 10-9mol
thiol groups theoretically if the surface was occupied
completely. On the other hand, the result in Fig. 6c provided a critical concentration of thiol group for just
123
7148
blocking all the activities. The concentration was estimated
to be 11 lmol/L. After calculating by virtue of the added
volume, the consumption amount at the critical point is
3.3 9 10-9 mol thiol groups experimentally. Clearly, both
the theoretical (3.4 9 10-9 mol) and the experimental
(3.3 9 10-9 mol) data are in good agreement. This analysis demonstrates that only after all the surface area is
occupied, will the activity be lost.
The effect of size on activity was investigated as well.
AuNPs with big size exhibit lower activity than small-size
AuNPs, shown in Fig. 6d, for the same amount of total
gold atoms. This is expected as AuNPs in bigger size
possess smaller surface area than those in smaller size.
Discussion
Some inorganic nanoparticles have been recently found to
have the intrinsic peroxidase-like activity [14–17]. A
couple of researches have exhibited the advantages of such
nanoparticles over HRP. The present work will obviously
widen and intensify the applications of AuNPs further.
Our previous work [15, 16] revealed that surface charge
imposed great effect on the activity. Whether the surface
charge favors or disfavors, the catalytic activity is dependent on the affinity between substrate and nanoparticles
being studied. In the present work, we investigated the
peroxidase-like activity of negatively charged AuNPs. Li
[18] reported a similar peroxidase activity originating from
positively charged AuNPs and addressed that negatively
charged AuNPs show very low activity of this kind. To be
noted, the experimental conditions varied to a great extent.
Here, a very low TMB concentration was applied. In
contrast, the concentration is dozens of times higher in
their experiments. Enhanced TMB concentration was
found to lead to a considerably reduced catalytic rate in our
experiments.
The analyses of Michaelis kinetics parameters (KM) are
beneficial for the understanding of the catalytic process
more. KM is an indicator of an enzyme’s affinity for its
substrate. A high KM represents a weak affinity, whereas a
low value suggests a high affinity. The KM, H2O2 is very
close to the previous results observed in the case of citratestabilized MION [15], verifying the reliability of our data.
On the other hand, the KM, TMB in the case of citratemodified AuNPs is only 11.2 lM, nearly 3 orders of
magnitude lower than KM, H2O2. TMB tends to attract more
sufficiently the negatively charged particles via electrostatic attraction. The affinity between TMB and AuNPs
promoted and hence lowered the KM, TMB. H2O2 lacks this
sort of interaction. Since no similar value has been reported
for the positively charged AuNPs [18], a comparison cannot be made.
123
J Mater Sci (2014) 49:7143–7150
Herein, it was investigated that the surface Au? would
be assumedly the principal contributor to the activity. The
H2O2-treated AuNPs showed elevated activity. EDTA was
tested to chelate with the Au? on the AuNPs and thus
retarded the activity. But the effect is not permanent and
the activity will recover with time. Thiol groups are prone
to bind Au? covalently and thus block the catalytic activity
permanently. Direct evidence is still needed to explore for
the activity mechanism.
HRP or other enzymes are often conjugated on AuNPs
to impart the peroxidase function before the discovery of
peroxidase activity of AuNPs [13]. More importantly, these
enzymes on AuNPs have been found to show enhanced
activity in comparison with them alone [13, 28]. The
enhanced activity was commonly attributed to the favorable conformational changes once they are conjugated on
nanoparticles in previous reports [28]. Since AuNPs were
discovered to possess peroxidase activity, AuNPs would
supply the extra activity to HRP conjugated on AuNPs.
This is perhaps another or substitute explanation to the
enhanced activity of HRP–AuNP conjugates. Theoretically, the conjugation perhaps becomes evitable due to the
fact that AuNPs themselves possess the catalytic feature of
HRP.
It is common to explore the behavior of enzymes at
different pH and temperature. AuNPs carry plenty of surface carboxylic groups, with negative zeta potential. TMB,
however, carries two amino groups each molecule. The
interaction between TMB and AuNPs is dependent on the
combination of amino groups and carboxylic groups. At a
specific pH, amino groups will combine with carboxylic
groups intimately. The pH point is just like the isoelectric
point (IEP) of an amino acid. At IEP, the positive charge
from amino groups counteracts the negative charge from
the carboxylic groups. Likely, TMB tightly binds onto the
surface of AuNPs at the pH corresponding to an IEP, and
subsequently leads to high affinity or low KM, TMB. In this
case, AuNPs show high catalytic activity. As most IEPs are
around five, this analysis tentatively explains the catalytic
activity of citrate-modified AuNPs in an acidic medium.
Higher pH will dissociate the binding between TMB and
AuNPs, and, therefore, suppress the activity. Normal cells
have a neutral pH, whereas the pH in cancerous cells is
usually lower or mildly acidic. This kind of pH off–on
feature of AuNPs will perhaps induce intracellular redox
imbalance and affect cell proliferation in cancerous cells.
This is probably one of the key reasons that AuNPs function like an inhibition agent for cancer cells, already
reported in literatures [29].
The previous work [28] reported that the enzymes
conjugated onto AuNPs have high resistance to temperature or have high thermal stability, but no explanation was
provided, to the best of our knowledge. Herein, AuNPs
J Mater Sci (2014) 49:7143–7150
were revealed to exhibit over 60 % activity even at 90 °C.
Unlike the natural enzymes such as HRP, whose structure
is vulnerable to heat treatment, AuNPs are not easy to
denature presumably because the inorganic structure is
hard to alter and specifically the surface structure almost
keeps consistent during the thermal treatment. It is reasonable to comprehend that the enzymes conjugated on
AuNPs express activity at elevated temperature because
AuNPs still function under this circumstance.
The study of dopamine oxidation is of interest for
understanding some physiological process and some diseases such as Parkinson etc., and thus has been quite
conducted [24]. Prota [24] studied the oxidation, specifically under the peroxidase (HRP)/H2O2 couple as a biomimetic oxidizing agent. Here, we substituted HRP for
AuNPs for the oxidation process. A series of UV absorbance was monitored. Specifically, the peak intensity linearly varied with time. These tests testified the oxidation
occurred under the action of AuNPs in place of HRP
though the products needed to be determined further. This
test implied AuNPs may have wide practical applications
as a peroxidase substitute.
Conclusion
Negatively charged AuNPs were unveiled to function like a
peroxidase. It has good catalytic activity under mildly
acidic conditions and the activity expressed resistance to
elevated temperature. Hydrogen peroxide-treated AuNPs
exhibited an enhanced activity. EDTA temporarily blocked
the activity partially, while thiol groups blocked activity
permanently. These tests imply that it is the surface Au?
that provides the activity. Michaelis–Menten kinetics
parameters were figured out, and these parameters were
compared with previous studies. Dopamine, as an instance,
was oxidized with AuNPs as a peroxidase substitute. These
studies elucidated a bunch of previous results and would
lead to some new application areas of AuNPs.
Acknowledgements This research was supported by the National
Natural Science Foundation of China (Grant No. 21071114), as well
as by the Excellent Program of Activity of Science and Technology
for Overseas-Returned Scientists founded by the Ministry of Human
Resources and Social Security of the People’s Republic of China, by
the Key Natural Science Foundation of Hubei Province
(2012FFA100), the Program for Innovative Research Team of Outstanding Youth of Universities in Hubei Province, and by Hubei
Collaborative Innovation Center for Catalysis Materials.
References
1. Saha K, Agasti SS, Kim C et al (2012) Gold nanoparticles in
chemical and biological sensing. Chem Rev 112:2379–2739
7149
2. Louis C, Pluchery O (2012) Gold nanoparticles for physics,
chemistry, and biology. Imperial College Press, London
3. Sardar R, Funston AM, Mulvaney P et al (2009) Gold nanoparticles: past, present, and future. Langmuir 25:13840–13851
4. Bunz UHF, Rotello VM (2010) Gold Nanoparticle–fluorophore
complexes: sensitive and siscerning ‘‘noses’’ for biosystems
sensing. Angew Chem Int Ed 49:3268–3279
5. Liu J, Guan Z, Lv Z et al (2014) Improving sensitivity of gold
nanoparticle based fluorescence quenching and colorimetric aptasensor by using water resuspended gold nanoparticles. Biosens
Bioelectron 52:265–270
6. Jain KK (2012) The Handbook of Nanomedicine. Springer, New
York Heidelberg Dordrecht London
7. Jia F, Liu X, Li L et al (2013) Multifunctional nanoparticles for
targeted delivery of immune activating and cancer therapeutic
agents. J Control Release 172:1020–1034
8. Mieszawska AJ, Mulder WJM, Fayad ZA et al (2013) Multifunctional gold nanoparticles for diagnosis and therapy of disease. Mol Pharm 10:831–847
9. Ansar SM, Perera GS, Ameer FS et al (2013) Desulfurization of
mercaptobenzimidazole and thioguanine on gold nanoparticles
using sodium borohydride in water at room temperature. J Phys
Chem C 137:13722–13729
10. Liu X, He L, Liu Y-M et al (2014) Supported gold catalysis: from
small molecule activation to green chemical synthesis. Acc Chem
Res 47:793–804
11. Biswas S, Sengupta S, Roy Chowdhury S et al (2014) CXCL13CXCR5 co-expression regulates epithelial to mesenchymal transition of breast cancer cells during lymph node metastasis. Breast
Cancer Res Treat 143:265–276
12. Li Y, Schluesener HJ, Xu S (2010) Gold nanoparticle-based
biosensors. Gold Bulletin 43:29–41
13. Lan D, Li B, Zhang Z (2008) Chemiluminescence flow biosensor
for glucose based on gold nanoparticle-enhanced activities of
glucose oxidase and horseradish peroxidase. Biosen Bioelectron
24:940–944
14. Gao L, Zhuang J, Nie L et al (2007) Intrinsic peroxidase-like
activity of ferromagnetic nanoparticles. Nat Nanotech 2:577–583
15. Yu F, Huang Y, Cole AJ et al (2009) The artificial peroxidase
activity of magnetic iron oxide nanoparticles and its application
to glucose detection. Biomaterials 30:4716–4722
16. Liu Y, Yu F (2011) Substrate-specific modifications on magnetic
iron oxide nanoparticles as an artificial peroxidase for improving
sensitivity in glucose detection. Nanotechnology 22:145704–
145711
17. Zuo X, Peng C, Huang Q et al (2009) Design of a carbon
nanotube/magnetic nanoparticle-based peroxidase-like nanocomplex and its application for highly efficient catalytic oxidation of phenols. Nano Res 2:617–623
18. Jv Y, Li B, Cao R (2010) Positively-charged gold nanoparticles
as peroxidiase mimic and their application in hydrogen peroxide
and glucose detection. Chem Commun 46:8017–8019
19. Brown KR, Fox AP, Natan MJ (1996) Morphology-dependent
electrochemistry of cytochrome c at Au Colloid-Modified SnO2
Electrodes. J Am Chem Soc 118:1154–1157
20. Storhoff JJ, Lazarides AA, Mucic RC et al (2000) What controls
the optical properties of DNA-linked gold nanoparticle assemblies? J Am Chem Soc 122:4640–4650
21. Park JW, Shumaker-Parry JS (2014) Structural study of citrate
layers on gold nanoparticles: role of intermolecular interactions
in stabilizing nanoparticles. J Am Chem Soc 136:1907–1921
22. Bally RW, Gribnau TCJ (1989) Some aspects of the chromogen
3,30 ,5,50 -tetramethylbenzidine as hydrogen donor in a horseradish
peroxidase assay. J Clin Chem Clin Biochem 27:791–796
23. Karasyova EI, Losev YuP, Metelitza DI (2002) Peroxidase-catalyzed oxidation of 3,30 ,5,50 -tetramethylbenzidine in the presence
123
7150
of 2,4-dinitrosoresorcinol and polydisulfide derivatives of resorcinol and 2,4-dinitrosoresorcinol. Russ J Bioorg Chem
28:128–135
24. Napolitano A, Crescenzi O, Pezzella A et al (1995) Generation of
the neurotoxin 6-hydroxydopamine by peroxidase/H2O2 oxidation of dopamine. J Med Chem 38:917–922
25. Sandroff CJ, Herschbach DR (1985) Kinetics of displacement and
charge transfer reactions probed by SERS: evidence for distinct
donor and acceptor sites on colloidal gold surfaces. Langmuir
1:131–135
26. Cortie MB, Lingen EVD (2002) Catalytic gold nano-particles.
Mater Forum 26:1–14
123
J Mater Sci (2014) 49:7143–7150
27. Badia A, Cuccia L, Demers L et al (1997) Structure and dynamics
in alkanethiolate monolayers self-assembled on gold nanoparticles: a DSC, FT-IR, and deuterium NMR study. J Am Chem Soc
119:2682–2692
28. Pandey P, Singh SP, Arya SK et al (2007) Application of thiolated gold nanoparticles for the enhancement of glucose oxidase
activity. Langmuir 23:3333–3337
29. Mackey MA, Saira F, Mahmoud MA et al (2013) Inducing cancer
cell death by targeting its nucleus: solid gold nanospheres versus
hollow gold nanocages. Bioconjug Chem 24:897–906