Biomedical Engineering Reference
In-Depth Information
heavy metals but there is still a need to develop sensors which are able to
trace heavy metal ions as well as organic or microbial pathogens [11, 12].
Within the category of sensing and detection of particular interest, is the
development of new and enhanced sensors to detect biological, chemical
and emerging contaminants, present at very low concentration levels in the
environment, including water.
Here, we will concentrate on developing sensor materials for heavy
metal ions detection which are fast, simple and usable by non-experts. h is
will enable the actual use of sensors in an af ordable way.
10.2 Nanoparticles
Nanoparticles are discrete particles with at least one characteristic dimen-
sion: the size in nanometers, typically in the range of 1-100 nm [13]. Metal
nanoparticles have received particular interest in diverse i elds ranging
from material science to biotechnology. Although widespread interest
in nanoparticles is recent, the concept was introduced over 40 years ago.
Nanoparticles have actually been produced and used by humans for hun-
dreds of years; for example, the beautiful ruby red color of some glass is
due to gold nanoparticles (Au NPs) trapped in the glass matrix. In the dec-
orative glaze known as lustre, found on some medieval pottery, the special
optical properties of the glaze arose from metallic spherical nanoparticles
which were dispersed in the glaze in a random fashion.
In his well-known pioneering work in 1857, “Experimental relations of
gold (and other metals) to light” [14], Michael Faraday reported the for-
mation of deep red solutions of colloidal gold by reduction of an aqueous
solution of chloroaurate (Au) using phosphorus in CS2 (a two-phase sys-
tem). He investigated the optical properties of thin i lms prepared from
dried colloidal solutions and observed reversible color changes of the i lms
upon mechanical compression [15]. h e properties strongly depend on the
particle size, interparticle distance, nature of the protecting organic shell,
and shape of the nanoparticles. Quantum coni nement ef ect is observed
for very small sizes (below 2 nm) of metal nanoparticles [16]. Whereas in
the case of semiconductors, the Fermi level lies between two bands and
the edges of the bands dominate the optical and electrical properties. In
nanoparticles, there is a gap between the valence band and the conduction
band, unlike in bulk metals. h e optical properties of these nanomaterials
are due to the surface plasmon resonance, which is a collective oscillation
of conduction electrons of noble metals. Gold and silver colloids display
dif erent optical properties. Whereas Ag colloids display an absorption
