Environmental Engineering Reference
In-Depth Information
This phenomenon can also be explained from the hydrophilic/hydrophobic chemical nature of PNIPAM. At lower temperatures,
it is water-soluble and hydrophilic. When temperature increases, PNIPAM is more hydrophobic due to the breaking of the
hydrogen bonds between water and the NH and Co groups, and so there is a tendency for the polymer to associate with itself
rather than with the solvent. This leads to a phase separation, forming two pure phases. This translates as a decrease in entropy
and a positive increase in the gibbs energy.
The conformation of the polymer chains above the lCsT is a hydrated and expanded state, and below the lCsT, the chains
are dehydrated and aggregated. This behavior exhibited by PNIPAM has been used for several applications such as sensors,
externally triggered drug delivery systems, and biological separations [56, 57].
22.3
noble metal nanopartIcles
Metallic nanostructures have received a lot of attention since they exhibit very different optical, electrical, thermodynamic, and
chemical properties from the bulk material. These novel properties found in these materials have earned them a place in applica-
tions of sensors [58], molecular labels [59], and even as biocides and antimicrobials [60, 61]. such properties exhibited at the
nanoscale are exemplified in the way gold interacts with light at different scales. gold in bulk looks shiny yellow in reflected light,
but when very thin gold films are prepared, they look blue in transmitted light. Additionally, the colors change to orange and sev-
eral tones of purple and red as the film thickness decreases up to a few nanometers. This phenomenon is due to the collective
oscillation of conducting electrons at the interface between the metal film and the surrounding dielectric [62, 63]. The conducting
electrons at the interface of metallic nanostructures respond to electromagnetic fields with electronic resonant absorption in the
visible wavelength range, called localized surface plasmon resonance (lsPR), and give rise to very intense colors.
The lsPR is highly sensitive to size and shape of the nanostructures as well as to other parameters such as the refractive
index of the surrounding medium and the distance between neighboring particles. Methods of calculating theoretical extinction
spectra of metallic nanoparticles will be described in the next section. When the nanostructures are nonspherical and exhibit
higher growth in one dimension, to form wires or rods, the plasmon band splits into two bands corresponding to oscillation of
the conducting electrons along (longitudinal) and perpendicular to (transverse) the long axis [64]. The transverse mode reso-
nance resembles the observed peak for spheres, but the longitudinal mode is considerably shifted toward red and depends
strongly on the difference in length compared to width. The absorption spectrum is able to shift from the blue to the red by
forming elongated wires, tubes, belts, and other more sophisticated structures (stars, rings, cubes, nanoshells, and onion-like
structures) that incorporate different metals [65-69]. All these shapes are synthesized to move the absorption peaks along the
absorption spectrum and design sensors and molecular labels. The high sensitivity to size and shape that the absorption peaks
present need to be synthesized in order to produce highly monodisperse metallic structures.
22.4
calculus of extInctIon spectra In metallIc nanopartIcles usIng mIe theory
optical properties of metal nanostructures have very strong dependence on their specific size and shape as well as on several other
parameters, such as the refractive index of the surrounding medium and the distance between neighboring particles. specifically,
the optical properties of nanoparticles can be characterized and calculated theoretically with accuracy using Mie theory.
Mie theoretical calculations are based on the rigorous solutions of Maxwell's equations with spherical boundary conditions
at the sphere. When the particles have a surface stabilizing agent such as a polymer shell or thick film surrounding them, Mie
theory requires to take into account the dielectric function of both the metallic particle and the surrounding medium. Therefore,
in the case of metallic particles, the optical properties can be described by simply determining the nanoparticle radii and the
bulk frequency-dependent dielectric constant (in the case of aqueous solution, the water dielectric constant is used). When a
different medium is taken into account, such as a thin film on the surface of the nanoparticle or a dielectric shell, the dielectric
constant used has a dependence of position and varies along the radius [70, 71]. examples of these structures are illustrated
in Figure 22.3.
22.5
energy conversIon of metallIc nanopartIcles
Metallic nanoparticles interact with light at different wavelengths through scattering and absorption. The scattering compo-
nent of light is enhanced by the increase in particle size after approximately 100 nm; however, the absorption stays mostly
constant and slowly increases with size. The nanoparticles show an extinction spectrum, with a higher intensity at the surface
