We have been working with some organic solvents lately and noticed a sharp decrease in SERS performance from our gold nanoparticles deposited in an organic solvent such as methanol.
With reference to the graph attached: the top line is 5 ppm Malachite Green Oxalate dissolved in water, the bottom line is 5 ppm Malachite Green Oxalate dissolved in 50/50 water/methanol
As you can see, the SERS performance of the gold nanoparticles decrease significantly when methanol is added.
We consulted our vendor and this is the explanation given: "Since the nanoparticles are stabilized in an aqueous solutions, the addition of methanol will destabilize it. I think in this case there is too much methanol, so the nanoparticles are over-aggregating, hence the decrease in SERS performance."
I have the following questions:
One is the surface plasmon resonance caused by the interaction of light with a metal nanoparticle or more importantly a cluster of nanoparticles. This resonance has the effect of greatly increasing the electric field near the particle surface where it can interact strongly with an adsorbed molecule. The second is a charge-transfer resonance between an adsorbed molecule and the metal conduction band. The third resonance occurs when one of the molecule resonances lies near the excitation frequency, and can greatly increase the observed enhancement factor. This is usually termed surface-enhanced resonance Raman scattering (SERS). SERS depends on the shape and size of the nanoparticles, adsorption, aggregation, solvent, adsorbed molecule structure. For the exact answer to your questions is necessary to do additional experiments with your objects.. Simple answers.
1. The aggregation of nanoparticles is calculated DLVO theory.
2. Increased aggregation, adsorption decreases, decreases the intensity SERS.
3. Malachite green has a positive charge. It interacts with the electrons of the surface of the gold nanoparticles. Electrostatic contributions inversely proportional to the dielectric constant of the solvent.
You should also test the UV/vis spectra of the Malachite Green in methanol/water compared to water to ensure there is no aggregration or large solvatochromatic effects
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