Question

A) What phenomenon gives rise to the optical properties of these particles, and what information can be gained from a UV-visible spectrum?

B) List five factors that affect nanoparticle growth. Which one do you think the synthesis of silver and gold nanoparticles is most sensitive to and why (briefly)?

Answer 1

A) Optical properties are shown by particles as a result of the various phenomena reflection, refraction, diffraction and absorption. These phenomena give rise to various colours in particles depending upon the wavelength at which these phenomena takes place.

A UV-visible spectrum is obtained when a molecule undergoes a transition between its electronic energy levels absorbing characteristic frequency in the u=UV-visible region. A UV-visible spectrum can be used to get the following informations:

(i) Identification of compounds: By comparing the obtained spectrum of an unknown molecule with the spectral catalogue we could identify the compound since spectrum of any compound is a unique characteristic of its structure

(ii) Determination of functional group: Characteristic absorption maxima is shown by functional groups containing centres or hetero atom or both. hence chromophores in an unknown compound can be identified by comparing the spectrum with the spectrum of a known compound.

(iii) Identification of geometrical isomers: High _max and _max are shown by trans isomers compare to their cis counterparts. By recoeding electronic spectrum cis and trans isomers can be identified.

(iv) Study of conjugation: Presence of conjugation generally shifts _max and _max to higher values. Greater the extend of conjugation higher will be the _max. hence conjugated compounds and non -conjugated compounds can be distinguished by recording their electronic spectra.

Other application of UV-visible spectra in organic chemistry includes impurity detection, study of tautomeric equilibira, study of strains, etc.

B) Factors affecting nanoparticle growth are:

(i) pH

(ii) temperature

(iii) pressure

(iv) time

(v) concentration and ratio of particles used for preparation

Synthesis of silver nanaparticles are more sensitive to pH of the reaction medium. Formation of the silver nanoparticles is favourable in basic medium than in acidic medium. In acidic medium due to the repulsion of anions in the reaction mixture, slow rate of reduction of silver salt to silver nanoparticles is observed.

The ratio between the reducing agent and stabilising agent affects the size of the nanoparticles significantly. The use of various concentration of borohydide with metallic salt of choloroauric acid as a gold precursor,nanogold of various size and morphologies likenanorods, nanowires, spherical and triangular gold nanoparticles.