I have done dialysis to remove CTAB on gold nanorods and I have done absorption test by using spectrophotometer pre and post dialysis. I noticed that peak absorbance of nanorod after dialysis is lower compare with nanorod before dialysis. Do you know the possible reason for this?
Fall in absorption peak usually represents aggregation of nanorods. Possibly, lot of your CTAB got washed away from the nanorods causing the aggregation.(CTAB is weakly bound to the nanorods!). The best way to remove excess CTAB is centrifugation but still you have to optimize the speed and the duration of centrifugation as excess speed and time may again cause aggregation.
I'd also expect seeing a red-shift in the plasmon resonance when aggregation takes place.
What about the simplest case, a change in the concentration of NRs after dialysis? If you observed no shift in the plasmon resonance but only the lowered absorbance then you may get fewer NRs in the final product. All other effects like aggregation, a change in the dielectric environment would be manifested as a broadening and/or a shift of the resonance.
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