CHARACTERISATION TECHNIQUES

(UV-Visible Spectroscopy)

UV/Visible spectroscopy is a technique used to quantify the light that is absorbed and scattered by a sample (a quantity known as the extinction, which is defined as the sum of absorbed and scattered light). In its simplest form, a sample is placed between a light source and a photodetector, and the intensity of a beam of UV/visible light is measured before and after passing through the sample. These measurements are compared at each wavelength to quantify the sample’s wavelength dependent extinction spectrum. The data is typically plotted as extinction as a function of wavelength . Each spectrum is background corrected using a buffer blank to guarantee that spectral features from the buffer are not included in the sample extinction spectrum.

Schematic of UV-Visible extinction spectrometer.

Importance with Nanoparticles

UV-visible spectroscopy is one of the most widely used techniques for structural characterization of silver nanoparticles. The absorption spectrum of the pale yellow-brown silver colloids prepared by hydrazine reduction showed a surface Plasmon absorption band with a maximum of 418 nm indicating the presence of spherical or roughly spherical Ag nanoparticles, and TEM imaging confirmed this. This image show agglomerates of small grains and some dispersed nanoparticles. The particle size histograms of silver particles (right-hand illustration in Figure) show that the particles range in size from 8 to 50 nm with mean diameter 24 nm. 

UV–Vis absorption spectrum of silver nanoparticles obtained.

TEM image of spherical silver nanoparticles and its particlesize distributions. Ag particles prepared from 1.1 mM AgNO3solution.

The dispersions of silver nanoparticles display intense colors due to the plasmon resonance absorption. The surface of a metal is like a plasma, having free electrons in the conduction band and positively charged nuclei. Surface Plasmon resonance is a collective excitation of the electrons in the conduction band; near the surface of the nanoparticles. Electrons are limited to specific vibrations modes by the particle’s size and shape. Therefore, metallic nanoparticles have characteristic optical absorption spectrums in the UV-Vis region.

SURFACE PLASMON RESONANCE

(Left) Surface plasmon resonance where the free electrons in the metal nanoparticle are driven into oscillation due to a strong coupling with a specific wavelength of incident light. (Right) Dark field microscopy image of 60 nm silver nanoparticles

UV-Visible spectroscopy provides a mechanism to monitor how the nanoparticles change over time. When silver nanoparticles aggregate, the metal particles become electronically coupled and this coupled system has a different SPR than the individual particles. For the case of a multi-nanoparticle aggregate, the plasmon resonance will be red-shifted to a longer wavelength than the resonance of an individual nanoparticle, and aggregation is observable as an intensity increase in the red/infrared region of the spectrum. This effect can be observed in Figure, which displays the optical response of a silver nanoparticle solution destabilized by the addition of saline. Carefully monitoring the UV-Visible spectrum of the silver nanoparticles with time is a sensitive technique used in determining if any nanoparticle aggregation has occurred.

(Left) Extinction (scattering + absorption) spectra of silver nanoparticles with diameters ranging from 10-100 nm at mass concentrations of 0.02 mg/mL. (Right) Extinction spectra of silver nanoparticles after the addition of a destabilizing salt solution.

For silver nanoparticle solutions that have not agglomerated and have a spectral shape that is identical to the as-received suspension, the UV/Visible extinction spectra can be used to quantify the nanoparticle concentration. The concentration of silver nanoparticle solutions is calculated using the Beer-Lambert law, which correlates the optical density (OD, a measure of the amount of light transmitted through a solution) with concentration. Due to the linear relationship between OD and concentration, these values can be used to quantify the concentration of nanoparticle solutions.

QUANTITATIVE TREATMENT

Beer’s LawAl1 = el1bc

e is molar absorptivity (unique for a given compound at l1)

b is path lengthc concentration

BEER’S LAWcuvette

sourceslit

detector

A = -logT = log(P0/P) = ebc

T = Psolution/Psolvent = P/P0

Works for monochromatic lightCompound x has a unique e at different wavelengths

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