Biomedical Engineering Reference
In-Depth Information
nanoparticles. However, other studies demonstrate silver to have superior activity to copper against
a wide range of different species and strains
[40]
.
The antimicrobial properties of both silver and copper nanoparticles were also investigated by
Ruparelia et al.
[54]
using strains of E. coli, B. subtilis, and S. aureus. The bactericidal effect of the
nanoparticles was compared using disc diffusion tests and MIC and MBC determinations. Bacterial
sensitivity was found to differ according to the species tested and the test system employed. For all
strains of S. aureus and E. coli, the action of silver nanoparticles was found to be superior. Strain-
specific variation for S. aureus was negligible, while some strain-specific variation was observed
for E. coli. A higher sensitivity, as shown with B. subtilis, may be attributed to more amine and car-
boxyl groups (in comparison to other species) on the cell surface; these groups having a greater affin-
ity for copper
[55]
. Released copper ions within the cell may then disrupt nucleic acid and key
enzymes
[56]
. In theory, a combination of silver and copper nanoparticles may give rise to a more
complete bactericidal effect, especially against a mixed population of bacteria. Indeed, the studies of
Ren et al.
[40]
demonstrated that populations of gram-positive and gram-negative bacteria could be
reduced by 68% and 65%, respectively, in the presence of 1.0 mg/mL nanocopper oxide within 2 h.
This was significantly increased to 88% and 100%, respectively, with the addition of a relatively
low concentration (0.05 mg/mL) of nanosilver.
10.3.1.3
Gold (Au)
Gold shows a weak antimicrobial effect in comparison to silver and copper. However, gold nano-
particles are employed in multiple applications involving biological systems. The binding properties
of gold are exceptional, and this makes it particularly suitable for attaching ligands to enhance bio-
molecular interactions. Gold nanoparticles also exhibit an intense color in the visible range and
contrast strongly for imaging by electron microscopy
[57]
. Despite all the current and potential
applications for gold nanoparticles, there remains little information as to how these particles affect
microorganisms. Growth inhibition studies, to measure the effect of gold nanoparticles (polyethyl-
ene glycol (PEG) coated to allow dispersion) on E. coli at various concentrations, demonstrated no
significant activity
[58]
. Studies with PEG-coated gold nanoparticles also showed no activity
against E. coli. However, the growth of the gram-negative Proteus species and P. aeruginosa was
inhibited at a concentration of 1.0 mg/mL (R.P. Allaker, unpublished observations).
10.3.2
Nanoparticulate metal oxides as antimicrobial agents
Nanoparticulate metal oxides have been of particular interest as antimicrobial agents as they can be
prepared with extremely high surface areas and unusual crystal morphologies that have a high num-
ber of edges, corners, and other potentially reactive sites
[59]
. However, certain metal oxides are
now coming under close scrutiny because of their potential toxic effects
[60]
. Oxides under consid-
eration as antimicrobial agents include those of copper, zinc oxide, titanium dioxide (titania), and
tungsten trioxide (WO
3
).
10.3.2.1
Copper oxide (CuO and Cu
2
O)
Copper oxide (CuO) is a semi-conducting compound with a monoclinic structure. CuO has attracted
particular attention because it is the simplest member of the family of copper compounds and exhi-
bits a range of potentially useful physical properties, such as high temperature superconductivity,
