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 347

2-3 nm in diameter. A small amount of the seed solution is then added to the growth
solution containing Au (I) ions to initiate the formation of NRs. however, the role of
seed has been a subject of debate [37], and it is not clear if all resulting NRs originate
from the preformed seed particles. In fact, several reports have shown that the
formation of rods can be initiated by direct addition of NaBh 4 [52] to the growth solu-
tion (seedless method).
Knowing the amount of metallic gold and the average size of the seed particles,
one is able to find the actual number of seeds that are introduced in the beginning
of the synthesis. The number of rods that subsequently form in solution is deter-
mined by the same combination of TeM and ICP and compared with that of seeds.
These experiments revealed the percentage of NRs that formed from seeds and
those originated from in situ nucleation. Under the conventional conditions, the
final number of rods is much lower than it could be if all ions were reduced [53].
however, attempts to increase the number of rods by introducing more seeds
generally lead to poor results [54]. Instead of more rods, the reaction produces a
mixture of spheres and rods. As mentioned before, the conventional seed-mediated
synthesis generates a very limited number of NRs, and their growth completely
stops even when there are still up to 90% of residual Au (I) ions present in solution
[53]. Preliminary studies show that all these ions can be reduced on the surface of
initial rods if the additional amount of reducing agent is introduced at a very low
rate. This process of amplification does not change the number of rods, but it
increases their mass more than 10 times. This allows one to reach a quantitative
yield of conversion while preserving the purity and narrow size distribution of
rods. In our procedure, the time interval between the consecutive additions
of reducing agent was kept at 2-3 h in order to ensure that all reducing agent is
completely consumed before the next portion is introduced. The number of initial
rods is larger, and therefore, the concentration of residual Au (I) ions is lower. This
eases the amplification process and decreases the probability of random nucle-
ation, especially in the early stages. It also produces NRs with a higher aspect ratio,
which is desirable for our proposed studies on colloidal crystallization. The most
reliable synthesis of AuNRs is based on the seed-mediated method in the presence
of silver ions [35].
however, this synthesis produces only submilligram quantities of NRs with a low
yield (10-15% with respect to the amount of gold chloride used in the synthesis). In
addition, the NRs made by this method do not undergo colloidal crystallization
because their size distribution is not sufficiently narrow. In this work we identified
the conditions for the uniform and “safe” amplification, and were able to increase the
reaction mixture from 10 [43] to 7000 mL and doubled the concentration of gold
ions. By doing so, we were able to amplify all NRs and isolate > 1 g of AuNRs,
thereby achieving nearly 100% in the isolated yield. The resulting NRs measured on
average anywhere from 10 to 25 nm in diameter and 40 to 60 nm in length. This
amplification resulted in a narrower size distribution and a significant narrowing
of the longitudinal peak during the amplification process. The ability to make NRs of
different sizes using this technique is shown in Figures 12.5 and 12.6.