From wikipedia on surface enhanced raman spectroscopy:

"The light incident on the surface can excite a variety of phenomena in the surface, yet the complexity of this situation can be minimized by surfaces with features much smaller than the wavelength of the light, as only the dipolar contribution will be recognized by the system. The dipolar term contributes to the plasmon oscillations, which leads to the enhancement. The SERS effect is so pronounced because the field enhancement occurs twice. First, the field enhancement magnifies the intensity of incident light which will excite the Raman modes of the molecule being studied, therefore increasing the signal of the Raman scattering. The Raman signal is then further magnified by the surface due to the same mechanism which excited the incident light, resulting in a greater increase in the total output. At each stage the electric field is enhanced as E2, for a total enhancement of E4.[10]"

If I understand this correctly, the excitation of the nanoparticles will enhance the raman scattering process, but not shift the frequencies (certainly worth double checking). You should look at the optical absorbance spectra of the gold particles and see how strongly they absorb at 532 compared to 785. This may give you a ball park of the loss due to your laser, but remember, the loss is quartic in E so it will be exaggerated.

I

Thank you Adam. I will definitely have a closer look to the absorption of these nanoparticules.

That stronly depend on the surface. We have tested a couple of surfaces with sharp resonances or more flat (see my papers).

The easiest way is: Try it.

In order to estimate (calculate) the drop of the SERS signal one needs to know the absorbance of SERS-active nanoparticles and their geometry. If the plasmonic nanoparticles have elongated shape they exhibit two Plasmon peaks in the visible region of light. In this case if the excitation wavelength is green the nanoparticles should give a strong EM enhancement because of the additional Plasmon peak in the visible region and the drop of the SERS signal should not be high. In addition, one might easily check the morphology of nanoparticles by TEM or SEM depending on the size of colloids. Are these nanoparticles aggregated or monodisperse? Monodisperse colloids with the plasmonic properties will give a strong narrow Plasmon peak(s), while their aggregates normally exhibit a broad band of low intensity. The width of the Plasmon band from the aggregates might serve as a open region (window) for the SERS enhancement, that is why aggregated or agglomerated particles with different geometries are preferred more for the SERS enhancement than monodisperse colloids.

In summary, one should always measure the absorbance of the nanoparticles and check ttheir size (DLS or TEM) and morphology (TEM, SEM) in order to estimate the SERS enhancement or vice versa.

There are two mechanisms which are important in enhancement of Raman signals: physical mechanisms and chemical mechanism.The chemical enhancement has been observed in off-plasmon resonance excitation condition. This enhancement,however, is different for various shapes. For example gold nanorods show more activity than gold nanosphere in same conditions.

Ref:

Surface-enhanced Raman scattering of molecules adsorbed on gold nanorods: off-surface plasmon resonance condition

Chemical Physics Letters,Volume 366, Issues 1–2, 25 November 2002, Pages 17–23

http://www.sciencedirect.com/science/article/pii/S0009261402014926

The SERS effect depends of tree mechanisms: the electromagnetic, chemical and raman resonant effect.

You can see this reference:

http://www.ncbi.nlm.nih.gov/pubmed/19361212

If you are using particles optimized for 785 nm which have some absorbance in 532 nm the elctromagnetic factor will be small. But you can enhance your signal making the other mechanisms bigger using dyes which absorve in 532 nm, or thiols ligands which make more covalent bounds with the particles surface.

First thing, it would be importante to take into account the composition of your nanoparticle. If you are working with gold nanoparticles, the drop will be huge, as the absorption (imaginary part of the dielectric funcion) of gold nanoparticles is large in the green, it is not expected you get any field enhancement out of the excitation with a 532 nm laser line. Similarly to copper nanoparticles. You may be able to get some field enhancement out of silver nanoparticles excited with 532 nm, although you may experience a drop due to decrease in the quality factor for field enhancement as well.

The decrease you may get using silver nanoparticles may be balanced or even surpassed by some additional Raman enhancement factor, like some resonance Raman effect of your probe molecule, if you get an green absorbing probe.

For details, you may refer to the book by Etchegoin and Le Ru, "Principles of Surface-enhanced Raman scattering and related plasmon effects", Elsevier, 2009, or a review by Fan, Andrade, and Brolo, Analytica Chimica Acta, 2011, 693, 7-25 on the recent developments of SERS substrates.

Dear Marc:

It will depend on the surface plasmon absorption width and on your analyte. We have prepared nanoparticle substrates that are nearly independent on excitation wavelength.

First of all - with 532 nm laser you will work with "normal" SERS but with 785 nm laser used on your platform it will be Surface Enhanced Resonance Raman Spectroscopy. That`s why you can get EF even few orders of magnitude bigger working with SERRS than with just SERS.

Best advice, as written above - try it and compare.

Enjoy :)

Thank you all for these precious informations.

I will try the experiment in a few days as you suggested but, I have revised my expectations to have a strong signal.

Thanks again,

Marc

For a plasmon resonance of 785 nm the nanoparticle material seems to be

Gold. If you measure the extinction spectrum of a sample with a

resonance at the specified wavelength,

you might observe a very low relative intensity (eventually barely

above the signal background.) compared to the maximum position. This means that a SERS signal should be

extremely weak, if detectable at all.

Frank,

I understand that silver may give me better results with 532nm excitation.

Marc

Marc,

That´s true, if you have the plasmon resonance at 532 nm. In the red part of the spectrum, i.e. at 785 nm, gold shows the better optical properties.

Frank

Don`t forget about harmonic wavelenghts, Marc. There also exists the resonance, only weaker.

Thank you Frank and Marta for the info.

Marc