I am doing some absorption measurements on nanoparticles, for which I am getting very low signal. I am thinking of increasing the concentration of the nanoparticles which should increase the signal. But I am not sure how exactly I should do that while keeping the size of the particles intact. Is there any suggestion as to how I can go about it? My measurement is in the transmittance mode.
In general there are 3 things that increase the signal - larger size, greater concentration, higher optical contrast (relative refractive index). If your size and optical properties are fixed, then you only have concentration to play with, as I suspect you've correctly identified.
You could centrifuge (then decant the clear upper liquid) or filter (removing liquid and then recombining the solids into a smaller volume) to increase the concentration. Both have dangers but the key is preserving the same ionic background as your (I assume) stable starting suspension. Re-suspension will always be a problem. Zeta potential is key. See also:
Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge
November 3rd, 2015 Adhesion and cohesion http://tinyurl.com/zwb2wlh
If the nanoparticles are in water, a roto-evaporator would probably work well for you.
you can try keeping the solution of NPs in oven (temperature about 40 C) for few hours. Then re-measure the absorption of NPs- solution, if the maximum absorption wavelength not changed with increasing the intensity, means NPs still OK.
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