Biomedical Engineering Reference
In-Depth Information
how ammonia acts as a moderator of nanoparticle growth in the colloidal medium (
Figure 9.5
)
[22]
.
This apparently simple idea effectively results in stable colloidal aqueous suspensions of spheri-
cal silver nanoparticles with narrow size distributions. The addition of ammonia, or any agent to
trap the excess of silver ions, to the system immediately after the nucleation stage is an efficient
way to stabilize colloidal suspensions of monodisperse silver nanoparticles with controlled size,
since free silver ions, which are responsible for particle growth and the formation of new nuclei,
are trapped by the formation of soluble complexes, preserving the silver nanoparticles for long
storage periods without coalescing or precipitating. This procedure, which involves the use of
inexpensive and nontoxic reagents, also allows the particle size to be determined by choosing the
right moment at which ammonia. The earlier the ammonia is added the smaller the average parti-
clesizewillbe.
When silver nanoparticles are synthesized based on Turkevich
[25]
methods, it is fundamental
to use deionized water to prepare the solution, although the chemicals used are usually of analytical
grade, with no further purification. Silver nanoparticles can be prepared by the reduction of silver
nitrate with sodium citrate or any reducing agent. In a typical procedure, a volume of 100 mL of an
aqueous solution of silver nitrate (1.0 mmol/L) is heated and stirred gently. The silver nitrate solu-
tion should be heated to temperatures between 60
C and 90
C before the addition of 1.0 mL of a
solution of sodium citrate (0.3 mol/L) to accelerate the reaction. At room temperature, this reaction
is too slow that is not possible to follow it. This reaction can be monitored through the typical
yellow color the characteristic plasmon absorbance band at approximately 425 nm. The intensity is
proportional to the nanoparticle concentration; however concentrated colloidal dispersion is not
stable for long time and in this case, an extra stabilizing agent such as PVP is necessary.
Addition of ammonia
Nucleation stage
Without saturation
Saturation
C
∞
C
0
Nucleation and growth
Time
FIGURE 9.5
Reduction in the metallic silver concentration as a function of reaction time, showing the initial nucleation
stage after the addition of citrate ion and the addition of ammonia immediately after the system turned yellow,
reducing the amount of metallic silver to a quantity below the saturation limit
[22]
.
