I have been synthesising Ag nanoparticle films on Silica beads, I wished to show the limit of detection by making a concentration serial dilution of Rhodamine 6 G, however, there was strange results both times I performed the procedure.
At 1mM R6G I had achieved good intensity, at 1µM it was attenuated as expected, at 1nM the spectrum achieved was not exactly the same, with different peaks, at 1pM it was the same as 1nM.
However at 1 fM and aM there the spectrum returned and had a reasonable intensity.
The dye was diluted in a serial manner from the 1mM stock, all samples had the same reaction time for surface adsorption, they were all cleaned 3x in fresh solvent.
If it had been once I would have thought it was due to a mistake, but twice seems odd.
What I'd like to know is, is there any way that at really low concentrations of dye can there be any way that the signal would be more intense?
Thanks in advance for any that help.
Hi,
I do not know the answer but here are my thoughts:
Is it the absolute raman signal more intense or the signal to background is bigger?. Is it possible that upon dilution the fluorescence background progressively gets smaller and then you are left with raman signal that stands out against a smaller background.
Were the nanoparticles (that you used at the different rhodamine concentrantions ) made in the exact same way ?. SERS hot spots on the nanopaticles will depend on the particle size and shape and that may be different if the particles were made on different batches.
Another possibility is aggregation of the dye on the nanoparticles. These may have weaker raman signatures and upon dilution you may be getting the monomeric form of rhodamine dye with perhaps a stronger raman signal.
Hi Jose,
Firstly, thanks for taking the time.
The particles are films over 1.5µm Silica, and for the experiment all came from the same starting batch.
For SERS experiments with aromatic thiols the bead to bead variation is quite low, and all SEM data shows that the layers are quite uniform.
At higher concentrations I get a good deal of fluorescence background but the peaks show through quite well, I attributed that to the 532nm excitation that I was using.
At lower concentrations I don't see so much fluorescence background as you asked.
The last point regarding aggregation of the dye I hadn't considered, the dye was dissolved in ethanol, maybe there is a solvent effect at this concentration.
Strange results indeed....
Do you have an internal SERS Standard? Can you see the Ag-Cl band at ~230cm-1, which can be used to normalise the data; is their NaCl in your sample?. Idealy you could use isotopically enriched R6G in your samples to normalise the data.
Potential SERS standards, nitriles & CC triple bonds:
https://www.researchgate.net/post/What_would_be_the_best_resonance_label_eg_triple_CC_bond_yn_containing_to_give_the_most_intensive_SERRS_signal_for_532_or_785_nm_excitation
The method of addition of SERS subsrate and liquid sample can also be important. Are the silca beads in solution or on a surface? If in solution, the relative volume of SERS liquid substrtae added to volume of sample can alter resulting SERS ntensities: this may need optimised.
Hotspot generation can also generate intense spectra - the above relative volume ratio optimisation can optimise the generation of hotspots.
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