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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

enter image description here

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: 

  1. Why will the nanoparticles over aggregate in organic solvents? Is this specific to my vendor's nanoparticles? 
  2. Why would that even matter? How does over aggregation lead to poor SERS performance? 
  3. 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?
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