## Wednesday, April 22, 2015

### gold nanoparticles. Surface stress

Among nanoparticles, gold nanoparticles (Au NPs) have been extensively studied;
this is partly because gold is the subject of one of the most ancient themes of
investigation in science. Au NPs are among the most stable metal nanoparticles [5]
since Au is relatively chemically inert. They present fascinating aspects such as
low-symmetry structures at some geometric magic numbers [9] and size-related
electronic, magnetic [5, 7] and optical properties (due to quantum size effect).
Moreover, their applications to catalysis, owing to the high surface-to-volume ratio,
and biology are also significant. For example, conjugates of Au NPs-oligonucleotides
are of great interest owing to the potential use of the programmability of DNA
base-pairing to organize nanocrystals in space. This gives multiple ways of providing
a signature for the detection of precise DNA sequences to develop biosensors, disease
diagnosis, and gene expression, etc [5]. Besides, research is expanding on Au NP
catalytic effects associated with CO oxidation, NO reduction, and the water-gas shift
reaction, i.e. the chemical reaction in which carbon monoxide reacts with water vapor
to form carbon dioxide and hydrogen; new Au NP based catalytic systems are now
being explored. [4][5]
Applications exploiting the optical properties of Au NPs utilize functionalization
of NPs with chromophores. Chromophore means the part of a molecule responsible for its color. These have diverse applications due to a range of options for the
chromophore. In 2003, K. Thomas et al. reported that gold nanoparticles associated
with fluorophores were utilized in photocurrent generation and fluorescent display
devices [10]. Furthermore, they also showed that gold nanoparticles can bind and
release amino acids when linked with appropriate chromophores. All in all, it appears
safe to assume that Au NPs will be a key building block for nano-science and
-technology in the 21st century.
Given the many applications of Au NPs, as well as the extensive literature on
them, this thesis will focus on Au NPs as a canonical metal NP system.

The structure of gold nanoparticles has been perhaps the most investigated aspect
about them.[4] It has been found, theoretically and experimentally that gold
nanoparticles have crystallographic structures different from the bulk material.[4,11]
In general, gold nanoparticles have icosahedral or decahedral motifs depending on
their size while bulk gold has face-centered cubic (fcc) crystal structure [4].
For Au NPs larger than 2 nm, the truncated-octahedral structure (fcc) is the dominant shape [4, 12].
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Because NPs in general - and Au NPs specifically - possess significant surface
area to volume ratio, their surface thermodynamic properties can be very important in
determining a NP‘s properties. An important surface thermodynamic quantity is the
surface stress. Surface stress, f, is a thermodynamic quantity that describes the amount
of energy or reversible work per unit area required to elastically deform a solid
surface. It differs from another fundamental thermodynamic parameter, i.e. surface
free energy γ, which represents the energy needed to form a new surface by a process
like cleavage.[24] In liquid, these two values are identical, as the configuration of
fluid surface remains constant owing to bulk atoms or molecules moving exteriorly to
the surface when a fluid surface is stretched. Put differently, a liquid cannot support a
shear stress so the only way to create/eliminate new surface is by adding/removing
atoms from the surface, rather than elastically deforming existing atoms at the surface.
In contrast, when a solid surface is put in tension (within an elastic limit), the total
number of surface atoms is conserved; therefore, the number of atoms per unit area
changes and consequently, f ≠ γ.

Ref: http://preserve.lehigh.edu/cgi/viewcontent.cgi?article=2257&context=etd
Molecular Dynamics Simulation of Gold Nanoparticles and Surface Stress Effect
Siming Zhang
Lehigh University
2011