Update on Gold Nanoparticles - From Cathedral Windows to Nanomedicine

166 Pages·2013·2.665 MB·166\166

by  Voliani, Valerio

Update on Gold
Nanoparticles
From Cathedral Windows to Nanomedicine
Valerio Voliani
A Smithers Group Company
Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom
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First Published in 2013 by
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© 2013, Smithers Rapra Technology Ltd
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ISBN: 978-1-84735-643-7 (hardback)
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P
reface
In the last few years the discoveries in the field of nanotechnology
have triggered a revolution in the field of medicine, delivering a
profusion of potential and actual applications of nanosized objects for
the diagnosis and treatment of several diseases (from cancer to gene
diseases). From this point of view, gold nanoparticles (AuNP) have
afforded a suitable platform for the development of efficient multi-
functional therapeutic, diagnostic, delivery and release systems. AuNP
can be easily synthesised, functionalised, and are biocompatible. The
possibility to tuning the size and geometry of nanoparticles by a wet
chemistry synthesis enables complete control of their optical and
physical properties, while through the coating of the nanoparticles’
surface it is possible to influence their functionality and stability
behaviour.
In this Update, readers are introduced to the intriguing world of gold
at the nanoscale. In particular there is a comprehensive overview of
the synthetic pathways, of the physical and biochemical features of
AuNP, and of their most recent applications to nanomedicine (with
a special focus on theranostics and release topics).
In this update, Chapter 1 covers the methods used mostly for the
wet chemistry synthesis of gold nanostructures (spheres, cages,
cubes, rods), followed by an exhaustive ‘guide to the synthesis’
personally tested by the author. In Chapter 2 the links between
optical behaviour and geometries of AuNP, their biological features,
and the common encapsulation processes is explained. Thus, the
link between synthesis and final applications will be stated. Finally,
Chapter 3 presents the most recent and promising applications in
nanomedicine of AuNP, according to their geometry and, thus, with
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Update on Gold Nanoparticles
their light-matter interactions. It is useful to emphasise that every
chapter is accomplished by a complete and up-to-date bibliography,
in order to give the readers the opportunity to further extend their
research of the topics addressed.
This update offers a state-of-the-art and comprehensive coverage from
the synthesis of the gold nanoparticles to their medical applications.
This is possible thanks to the focus on a single topic: gold, the most
promising metal in nanomedicine thanks to its biocompatibility and
simple synthetic pathways. In this way the general reader can have
a complete vision of nanostructured gold, physicians and biologists
can have an idea about the new nano-tools, and chemists can have a
general guide to the synthesis of AuNP. In conclusion this is a practical
guide ‘from the test tube to the organism’ for the production and use
of gold nanoparticles.
Acknowledgements
I would like to thank Giovanni Signore, Fernanda Ricci, Riccardo
Nifosì, Stefano Luin, Paolo Faraci, Julia Pérez-Prieto and Fabio
Beltram for their friendship, suggestions, and criticism, helping me in
the development of my knowledge in the ‘nano’-world. A thank you
also has to go to my family, and in particular to my sisters, Matilda
and Linda, and to my brother Vincent.
A special thank goes to the warm light of my life-journey, Camilla.
I also would like to thank a person that will see the world for the
first time in some months, and that stimulated me to write this book,
Sesamina.
Valerio Voliani
2013
vi
C
ontents
Preface ..........................................................................................v
1. Synthesis of Gold Nanostructures.........................................1
1.1 Gold Nanospheres ......................................................3
1.1.1 Turkevich Method............................................3
1.1.2 Zhong Method.................................................6
1.1.3 Brust Method...................................................8
1.2 Gold Nanoparticles......................................................9
1.2.1 Gold Nanorods................................................9
1.2.2 Gold Nanocubes and Polyhedral
Nanocrystals..................................................14
1.2.3 Gold Nanocages.............................................19
1.2.4 Gold Nanoshell..............................................21
1.3 Methods....................................................................26
1.3.1 Nanoparticle Characterisation.......................26
1.3.2 Turkevich Method..........................................28
1.3.3 Brust Method.................................................29
1.3.4 Xia Method...................................................29
1.3.5 Zhong Method...............................................30
1.3.6 Gold Nanorods..............................................30
1.3.7 Gold Nanocubes............................................31
1.3.8 Gold Nanocubes and Polyhedrons.................31
1.3.9 Gold Nanocages.............................................32
References..........................................................................33
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Update on Gold Nanoparticles
2. Behaviour of Gold Nanoparticles.......................................39
2.1 Optical Features.........................................................40
2.1.1 General Description.......................................40
2.1.2 Dielectric Function.........................................51
2.1.3 Plasmonic Properties of Small Spherical
Metal Nanoparticles......................................54
2.1.4 Plasmonic Properties of Large Spherical
Metal Nanoparticles......................................57
2.1.5 Surface Enhanced/Quenched Fluorescence.....59
2.1.6 Surface Enhanced Raman Scattering..............60
2.2 Coatings for Gold Nanoparticles ..............................62
2.3 Biological Features ....................................................68
References..........................................................................79
3. Gold Applied to Nanomedicine..........................................87
3.1 Diagnostics and Imaging ...........................................87
3.1.1 Colorimetric Essays........................................87
3.1.2 Surface Enhanced Raman Scattering .............90
3.1.3 Imaging..........................................................95
3.2 Therapeutics............................................................102
3.2.1 Photothermal Therapy.................................104
3.2.2 Releasing Systems........................................108
3.3 Summary.................................................................119
References........................................................................119
Abbreviations ...........................................................................129
Index ........................................................................................133
iv
1
Synthesis of Gold Nanostructures
Optical and biological properties of metal colloids [1] result from a
delicate balance between material, size, shape and dispersion of the
nanostructures (see Chapter 2). This prompted the exploration of
many synthetic processes in the last decade to achieve the desired
nano-materials [2]. In this Chapter the most commonly used methods
to produce nanoparticles will be discussed and a practical guide to
the synthetic processes is reported at the end of the chapter.
It is possible to synthesise nanostructures by two opposite
approaches [3]: bottom-up or top-down. In the first approach, the
synthesis starts from the interaction of metal ions, in order to build
a more complex structure, while in the latter the base material is
gradually eroded by physico-chemical mechanisms until the desired
size and shape is achieved. The production of nanostructures for
biological applications is generally bottom-up [4]. This is because
the synthesis techniques involved (wet chemistry, vapour deposition,
pyrolysis) allow: i) a tight control of the surface composition,
for example coating and functionalisation (see Chapter 2) and
consequently the (bio)toxicity and stability of the colloids, and ii)
to produce large quantities of nanoparticles. In general, the reaction
processes are based on the reduction of salts of the metal of interest
(the precursor) in the presence of reducing and surfactant agents
in aqueous or organic media. By changing some key variables such
as the reactants, their relative molar concentrations, the reaction
temperature or the stirring velocity, it is possible to control the
nucleation and growth processes, achieving colloids with the desired
properties (Figure 1.1).
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Update on Gold Nanoparticles
The control on the temporal separation of these two effects is critical
to obtain gold nanoparticles (AuNP) with a narrow size-dispersion
(Figure 1.1). In the following descriptions, the synthetic strategies are
called by the name of the first person who used the method.
Figure 1.1 Formation mechanism of gold nanoparticles (AuNP)
with various particle sizes and shapes by chemical reduction
method. Reproduced with permission from D.T. Nguyen, D.J. Kim
and K.S. Kim, Micron, 2011, 42, 207. ©2011, Elsevier [5]
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Synthesis of Gold Nanostructures
1.1 Gold Nanospheres
1.1.1 Turkevich Method
The method [6] proposed by Turkevich is based on the reduction
of tetrachloroauric acid (HAuCl ) with sodium citrate in water
4
at 90-100 °C. This is the most commonly used process to
synthesise gold nanospheres (AuNS) due to its fairly simple and
environmentally benign procedure and the possibility of tuning
the size of the nanospheres from 10 to 150 nm by varying the
molar ratio of citrate to HAuCl . The experimental protocols
4
are based on a rapid addition of sodium citrate solution to a hot
(90–100°C), aqueous solution of HAuCl . In this redox reaction,
4
sodium citrate acts both as a reducing/capping and buffering
agent. In particular, a study reported by Ji and co-workers [7]
highlighted the fact that citrate species buffer the pH of the
HAuCl solution from pH ≈ 2 to higher values (even neutral),
4
depending on the amount of citrate added. According to the pH
of the reaction solution, AuCl – ions [acid dissociation constant
4
(pK ) 3.3] are hydrolysed into different types of auric precursor
a
ions: AuCl (OH)– (pK 6.2), AuCl (OH) – (pK 7.1), AuCl(OH) –
3 a 2 2 a 3
(pK 8.1), and Au(OH) – (pK 12.9). Their reactivity decreases in
a 4 a
the following sequence [6,8]: AuCl – >AuCl (OH)– >AuCl (OH) –
4 3 2 2
>AuCl(OH) – >Au(OH) –. In the first step of the reaction
3 4
(Figure 1.2), sodium citrate is oxidised to sodium acetone
dicarboxylate (SADC) while any precursor of HAuCl is reduced
4
to AuCl by pH-dependent kinetics.
Taking into account that both the nucleation and crystal growth of
AuNS are very fast at high temperatures (less than 10 minutes at
100°C), the buffering effect of citrate plus a possible inhomogeneous
mixing of the reaction solution can cause inhomogeneous nucleation,
leading to a temporal overlap between nucleation and crystal
growth and broadening the size distribution of the final colloid.
The nucleation process (formation of gold clusters) is caused by
the formation of AuCl/SADC macromolecular complexes [10]
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Update on Gold Nanoparticles
and, subsequently, the growth step of AuNS is catalysed by the
presence of the formed gold clusters, which cause dismutation of
AuCl on their surfaces (Figure 1.2). It is important to remember
here, that a dismutation reaction occurs when the same ion acts as
oxidant and reducing agent. The coagulation of macromolecular
SADC/AuCl complexes is induced by concentration fluctuation
(in agreement with LaMer [11] theory). Matijevic and co-workers
have demonstrated that rapid coagulation favours the formation of
monodisperse spherical particles [12]. Thus, a rapid formation of a
large amount of SADS should favour the formation of nanoparticles
with a narrower size distribution. On the other hand, SADC can also
readily decompose to acetone [13] at high temperatures (>~90°C),
especially at neutral to basic pH. Also acetone can reduce the HAuCl
4
precursor ions to AuCl, thus leading to a secondary nucleation and
in turn broadening the size distribution of the final colloid. This
secondary nucleation can be minimised by fast oxidation of citrate
to SADC at a lower pH using silver (Ag)+ ions [14] under ultraviolet
(UV)-light irradiation. Silver ions could also lead to a re-shaping
of the nanostructures to become very spherical (Figure 1.3c). For
example, Xia and colleagues [9] improved the Turkevich method
by adding a catalytic quantity of Ag+ ions during the reaction,
obtaining quasi-spherical nanoparticles. Theoretical calculations on
the deposition potential for Ag on the gold (Au), (111), (100), and
(110) facets demonstrated values of 0.12, 0.17, and 0.28 eV [15].
This suggests that Ag atoms, when obtained by citrate reduction
of Ag+ ions, deposit preferentially on the (110) and (100) facets
of AuNS. The Ag layer is then oxidised and replaced by the gold
ions. In this way the deposition of silver may significantly slow
down the growth rate of AuNS on the (110) and (100) facets, thus
rendering the shape of AuNS more spherical. Citrate is a weaker
stabilising ligand enabling the deposition and decomposition of Ag+
on AuNS, guaranteeing reshaping of the polycrystalline colloid to
quasi-spherical [10].
4