Solvent effect on gold nanoparticle and silver nanoparticle in UV spectroscopy?

Can anyone explain to me that how solvent factor affect the wavelength and intensity of gold nanoparticle and silver nanoparticle in UV spectroscopy? 




Attenuation of light related to the properties of material is governed by beer-lambert law Many molecules absorb ultraviolet or visible light. The absorbance of a solution increases as attenuation of the beam increases. Absorbance is directly proportional to the path length, b, and the concentration, c, of the absorbing species. Beer's Law states that
A = ebc, where e is a constant of proportionality, called the absorbtivity.


Different molecules absorb radiation of different wavelengths. An absorption spectrum will show a number of absorption bands corresponding to structural groups within the molecule. For example, the absorption that is observed in the UV region for the carbonyl group in acetone is of the same wavelength as the absorption from the carbonyl group in diethyl ketone.


here is beer law description:<http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm>


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Mentioned in previous answer (beer law) explains differences in intensity because, apart from nanoparticles, each solvent absorbs differently. But in the most of the measurments of the nanoparticles suspended in solvent, spectrum originating from solvent itself is treated as baseline. Therefore final measured spectrum originates only from nanoparticles.


But medium in vincinity of the nanoparticle alters the spectrum of the nanoparticle itself.


UV-Vis absorption spectrum of the Au/Ag nanoparticles consists of more than absorption, because they manifest presence of LSPR (Localized Surface Plasmon Resonance).


First thing is interband absorption, in which nanoparticles absorbs light causing excitation of charges between bands in nanoparticle electronic band structure. Molecules and quantum dots (called artificial atoms because of their small size and similiar electronic structure) have discrete absorption spectrum and discrete energy levels. When we deal with larger structures e.g. nanoparticles, we must consider their band structure causing continous absorption spectrum). Nanoparticles are treated as 3D objects (which have parabolic energy-momentum electronic dispersion: Wikipedia - density of states). Density of states illustrates shape of absorption spectrum profile originating only from interband excitation and look like this: <http://britneyspears.ac/physics/dos/images/Image441.gif>.


Second thing is LSPR (<https://en.wikipedia.org/wiki/Localized_surface_plasmon>). It originates from oscillation of excited electrons in nanoparticles and it causes additional absorption and scattering events in nanoparticles solution. LSPR causes Lorentzian-type absorption profile with maximum peak placed by the wavelength corresponding to resonant oscillations of the electrons. This phenomenon occurs in the vincinity of surface of the nanoparticle and surrounding medium. Wavelength where LSPR peak is placed, depends strongly on the electrical permittivity of both the nanoparticle and the surrounding medium. So when You change between solvents, which have different permittivity, You can change placement of LSPR peak. That also counts for diffrent surface countings of the nanoparticles (because change of the permittivity surrounding metallic core), which also changes LSPR absorption.


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