Biomedical Engineering Reference
In-Depth Information
extracellular matrix produced by microorganisms
[18]
. Microscopic observations have indicated
that the interactions of E. coli biofilm cells and nano-Ag resulted in an increase in final average
aggregate size by a factor of 40
[18]
. The increase in the particle size may reduce the silver toxicity
and retard the particle diffusion within the biofilms, resulting in resistance to nano-Ag
[18]
.
The size of nanoparticle and the type of stabilizing agent are not crucial to their efficacy against
biofilms probably due to particle aggregation. Nano-Ag with different diameters (5, 10, and 60 nm)
formed through the reduction of silver nitrate with sodium citrate
[22]
and stabilized with ammonia
or polyvinyl pyrrolidone (PVP) (to control particle growth and prevent aggregation) showed no sig-
nificant differences in the effect against C. albicans and C. glabrata mature biofilms
[19]
. Thus,
the size of nano-Ag originally synthesized is not a good indication of the true nanoparticle size
when in contact with biofilms.
The uptake of nano-Ag by biofilms has also been studied. Interestingly, in a study by Fabrega
et al.
[23]
, only approximately 10% of the total mass of nano-Ag to which Pseudomonas putida
biofilms were exposed (during 24 h) remained trapped through the biofilms.
A study on the diffusion of nanoparticles in Pseudomonas fluorescens biofilms found that self-
diffusion coefficients decreased with the square of the radius of the nanoparticles
[24]
. In addition,
nano-Ag showed a greater tendency to accumulate in dense biofilms when compared to loose
flakes. These aspects may influence the susceptibility of biofilms to nano-Ag.
In conclusion, the interaction between biofilms and nano-Ag is complex and, possibly, several
factors may interfere with the efficacy of these particles against biofilms. Thus, these issues need to
be more explored in order to better understand the behavior of nanoparticles on biofilms.
9.2.2.2
Processing silver nanoparticles
To obtain small metallic colloidal nanoparticles, a high density of nuclei at the beginning of the
process is necessary. For this reason, the addition of a reducing agent, such as sodium citrate, into
a solution of silver nitrate should be carried out as fast as possible to form a large number of nano-
particles simultaneously. However, unlike other noble metals, only a fraction of silver ions are
reduced to metal, even when using an excess of sodium citrate or any different reducing reagent.
Due to this inevitable presence of silver ions, new nuclei will still be formed while the nanoparti-
cles initially formed will continue to grow, fed by Ag
1
that was not consumed during the formation
of the first nuclei. This process results in a system with a broad size distribution, consisting of large
particles formed initially and small particles that were formed subsequently. To overcome this
problem and obtain stable aqueous colloidal suspensions of spherical silver nanoparticles with sharp
size distribution, there is a way to prevent particle growth and the generation of new nuclei using
ammonia to trap all free silver ions. It is well known that when an excess of ammonia is added in
the presence of Ag
1
, soluble diamine silver (I) complexes are formed immediately (
Eq. (9.2)
),
removing the silver ions that have not yet been reduced. It prevents the formation of new
nuclei and the growth of already formed nanoparticles, resulting in virtually monodisperse silver
nanoparticles
[22]
.
NH
3
Þ
2
1
(9.2)
It is not easy to propose a kinetic model for the growth of silver nanoparticles since there are
many variables to consider in this case. However, the interdependence of density of nuclei and reac-
tion time may serve to shed light on the formation of a monodisperse system of nanoparticles, and
Ag
1
1
2NH
3
-
½
ð
Ag
