Compare the FWHM for both particles before and after. This gives an indication of particle stability. Broadening of your resonance relates to the size distribution within your sample. By degradation, you get a larger range in size (shape) particles, thus giving an overall broadening and associated decrease in the absolute absorbance (assuming both samples are in the identical conditions (solution). If the FWHM is the same, the shift (blue in your case) would more than likely be completely from the change in coating or molecules. Additionally look to find the dielectric constants of both coatings and compare. Then deduce if the difference in these values coincides with the expected resonance shift.

Blue shift may be an indicative that you are capping your nanoparticles with the stabilization molecule. The plasmon ressonace band for gold nanoparticle is dependent from the particle size but also from the surrounding medium dielectric constant. When you change the stabilization molecule probably the dielectric constant is also changing and therefore you can see some shift at the band.

You can download the MiePlot program that make some simulations in this case.

http://www.philiplaven.com/mieplot.htm

agree

The dielectric constant of the medium may have been changed, this results in a differen frequency of plasmon resonance. Particles formed by citrate reduction have size ~ 20nm and DSPR ~ 520nm.

Considering the method of Turkevich, UV-Visible spectroscopy can be used as a simple and reliable method for monitoring the stability of nanoparticle solutions. The destabilize the particles, the original peak extinction will Decrease in intensity (due to the depletion of stable nanoparticles), and will broaden Often the peak or a secondary peak at longer wavelengths will form (due to the formation of aggregates - red sift)

Simulations on NP coating shows a progressive increase in the peak intensity (to a limit) as a function of the coating concentration. The blue shift is more than likely as explained previously. The new coating has a refractive index lower than the original. But there is also a possibility of nanoparticle reshaping (gold atom migration) to a slightly different shape (aspect ratio), whose resonance is more to the blue. This effect is sometimes referred to as solvent annealing, more typically seen in lithographically prepared nanostructures.

thanks for your valuable input.....but can I claim the depletion in size of the nanoparticles...with blue shift and increased intensity .......let me know...and some papers to cite in my paper...

Compare the FWHM for both particles before and after. This gives an indication of particle stability. Broadening of your resonance relates to the size distribution within your sample. By degradation, you get a larger range in size (shape) particles, thus giving an overall broadening and associated decrease in the absolute absorbance (assuming both samples are in the identical conditions (solution). If the FWHM is the same, the shift (blue in your case) would more than likely be completely from the change in coating or molecules. Additionally look to find the dielectric constants of both coatings and compare. Then deduce if the difference in these values coincides with the expected resonance shift.

Some suggest that blue shift of the surface plasmon absorbance is due to decrease in size of the NP. It may also indicate the surface plasmon absorbance is also caused by the stabilizing polymers or molecules around the NP. More homogeneous shape (usually spherical) can also contribute to this phenomenon. Micro imaging should answer some of the questions.

Can u suggest me some papers to cite ? If you can ?

Check this out J. Chem. Phys. 116, 6755 (2002)

How can i calculate particles size using FWHM in case of UV-Vis spectroscopy? Please Help....

On an approach, you can use it here: http://www.ncbi.nlm.nih.gov/pubmed/17458937

Look at the pioneering paper of Nikoobakht & El Sayed where the plasmon position relates to the aspect ratio (Chem. Mater. 2003, 15, 1957-1962)

It was great learning from your valuable guidelines ......thanks a lot Especially Mark Kruzer and Robin...

In case of gold nanoparticles, size of the particles is actually indicated by the wavelength of the peak. FWHM qualitatively refers to the dispersity in size. No more information can be provided from it. Again the size cannot be exactly monitored from UV-Vis because of the non-linearity induced by multipolar oscillation in addition to dipolar one. You can get a vague idea only. Red coloured solution means small particles and towards blue are bigger ones or agglomerated ones. Only way to get size accurately is by TEM.

the minor blue shift is an indication of agglomeration and increase in size, herein u can use high resolution TEM to image samples before and after coating especially for you to determine the increase in size of Gold nanoparticle after coating, and perhaps you need determine the correlation of the increase in size and the contribution After attachment of your novel peptide.

Blue shift often refers to reverse ostwald ripening...but never agglomeration. Agglomeration would lead to plasmon coupling and hence red shift. TEM would be good only if the particles are uniform in size.

Blue shift is an indicator of decrease in size. In you case I think after coating particles become more dispersed in water. Although your coating has increased the size of the individual particles, but it become smaller than the agglomerated particles which you initially had. Thats why you also got a increase in pick hight. You can also expect a decrease of pick hight at the far red to near IR region (~650-800 nm). You need to take pictures of the particles to get an idea about the phenomenon. TEM, AFM, Hyperspectarl imaging might give you idea about the same. Measuring of zeta potential will give you some idea as well.