Pandia: Gold Nanorods and Their Applications in Cancer Therapy and In Vivo Imaging in Companion Animals and Their Potential Application to Humans - Nanotechnology for Biomedical Imaging and Diagnostics - page 346

If reliable and inexpensive techniques to make AuNRs become available, it will
significantly increase the scope and number of their real-life applications [44]. Both
experimental and theoretical studies indicate that long-range periodic assemblies of
anisotropic nanostructures may possess vectorial optical and electrical properties [45].
These are key elements for making macroscopically large functional structures.
Many reports in the literature highlight examples of the NR assemblies, which are
based on noncovalent interactions [46], polymer-induced self-corralling [47],
external electric fields [48], tip-selective functionalization [49], and breath figures
technique [50]. One critically important feature required for the assembly of rod-
shaped particles into large colloidal superstructures is their narrow size distribution
both in terms of their diameter and length. While the diameter of AuNRs is normally
very uniform, the length distribution is very difficult to control. This is the main
reason why superlattices made of inorganic NRs are often quite small and rarely
exceed several microns in lateral dimensions.
The main goal in this effort was to develop a method to scale up nanorod batch
manufacturing. The working hypothesis for this goal is that the yield of conversion in
a seed-mediated synthesis can be increased from conventional approximately 10 up to
100% if the unreacted gold ions are slowly reduced on the surface of preformed NRs.
This is based on preliminary data, which shows that it is possible to suppress random
nucleation as long as the rate of reduction is very low. In order to keep this rate low,
reducing agent is added to the growth solution in small aliquots. each addition should
be followed by several hours period in order to allow for a full consumption of the
added amount of reducing agent. Under these conditions, the reduction happens only
on the surface of metallic gold, which catalyzes the disproportionation of Au + ions. As
a result, the initial NRs would undergo a uniform amplification until all gold ions are
consumed. Once the yield is improved to near-quantitative level, an increase in volume
of the reaction should allow for the synthesis of gram quantities of well-defined NRs.
Wet chemistry synthesis of inorganic nanostructures has a distinct advantage over the
alternative techniques because it is more suitable for the large-scale preparation and
chemical functionalization of nanostructures in solution. The research conducted by
many groups clearly suggests that the seed-mediated synthesis of AuNRs is the most
efficient method [38]. There are many variables and chemical species that may affect
the outcome of the synthesis. detailed investigations of the role of seed, CTAB, silver
ions, and different reducing agents have been reported in the literature [37, 51].
While these findings are extremely valuable, nearly all of them pertain to the
original small-scale synthesis (10 mL of 5 × 10 −4 M solution), which cannot produce
more than 0.1 mg of AuNRs even if the yield of gold ion conversion were 100%. In
reality, that yield is only approximately 10%, which generates approximately 0.01 mg
of NRs. Because this synthesis is a kinetically controlled process, it strongly depends
on the actual scale, which makes the existing findings not amenable to a multiliter scale.
This may also explain why a synthesis that could produce more than a few milligrams
of isolated and pure AuNRs has not been reported to date, nearly one decade after the
introduction of this method. The seed solution is normally made by reducing gold
(III) ions with excess sodium borohydride (NaBh 4 ) in 0.1 M aqueous solution of
CTAB. The fast reduction leads to small spherical particles of gold (seeds) that are