We have been working with some organic solvents lately and noticed a sharp decrease in surface-enhanced Raman spectroscopy (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:
- Why will the nanoparticles over aggregate in organic solvents? Is this specific to my vendor's nanoparticles?
- Why would that even matter? How does over aggregation lead to poor SERS performance?
- Since methanol is immiscible in water, shouldn't the analyte molecules (Malachite Green) on average have the same average distance to the SERS gold nanoparticles?