Hello,
I have an experimental setup with which I am trying to get optical absorbance spectra of gold nanoparticles. From right to left, I have a light source (Ocean optics HL 2000), a collimating lens, an iris, a long glass tube through which the light passes via two flat faces at entry and exit sides (approx 1 meter long) and a spectrometer (FLAME VIS NIR). I am using a blank substrate of quartz as a reference and the sample substrate contains dried and drop-casted Au nanoparticles (prepared by simple NaBH4 reduction). I am unable to get smooth absorbance spectra and instead am only getting undulating curves, as shown in the attached image. I am unsure of what is going wrong with my acquisitions, and any help in this regard will be greatly appreciated.
obtaining hazy rough spectra can because of many reason;
#the concentration of sample of do matter in results, as make sure they follow the lambert beers law, you conc. seems to be ok in figure.
#your sample might be contaminated or aggregated, filter by 0.22mm filter tube. #check the buffer for the same, the pH if haven’t hampere.
#sometimes we can adjust the path lengths 5nm,10nm etc, so try wider range if you are getting rough graph.
#cuvetts also play the roles in getting foul results, for absorbable spectra quarts Black double sides Cuvette; make sure they are clean before use. Since you have very heavy metals in your sample; study the prior experimental graphs for references.
It is not so clear to me how your exp setup is built. If I have well understood, you want to measure absorbnce by Au NPs "deposited" onto a qurtz substrate (I suppose a very thin slub...).
If so, you are probably facing at least a couple of serious problems:
1) the drop casting of your original dispersion of NPs rarely provides an homogeneous film. Instead, very often you have a strongly uneven surface trace with very few nanoparticles in the body of the trace and a large concentration on the drop perimeter, where aggregation mechanisms may also have modified your deposited nanostructures. As a matter of facts, this calls for a precise positioning of the beam spot onto the slab surface, if possible.
2) The density of nanoparticles in your "film" is possibly low, too low to provide a significant Au absorption signal.
Instead of measuring in Transmission mode, why do not you try a reflection mode setup, also sensibly reducing the optical path lenghts and using a multi-reflection device (ATR)?
Thank you Giuseppe Curro and Sania Bashir and am extremely sorry for the late reply. Giuseppe Curro you are right in that using an ATR setup will reduce the light lost from the sample to the spectrometer, but I NEED to have it in the transmission mode as the design of my chamber for sensing experiments is like that. I also would really like some help on how to collimate the light from the light source I have, an ocean optics hl2000.
I have spun the films and not drop casted onto the quartz substrate. Although it is not going to give me a perfectly homogeneous distribution of gold on the substrate, I imagine it is better than say dip coating or drop casting. You could be right that I do not have sufficient density of Au nps to cause a stronger absorption, so I will try spin coating more layers.
I have also confirmed that the Au nps do give a good spectra but doing an independent measurement on a bench-top UV spectrometer, which gives me a nice symmetric peak for the same sample.
Sania Bashir I am not using cuvettes and only substrates so cuvette contamination issues are not present. The substrates have been cleaned by sonication with IPA as well as RCA cleaning to remove organics.
dear Gnanaprakash Dharmalingam
I understand your needs now. By the way, I mentioned drp casting because you wrote to have used such method in your original question.
I do not know what kind of focussing element you can use. In principle I'm even not sure that focussing would be a proper solution, because of possible complex refractive losses at the interfaces.
Have you collected also a blank spectrum (only the quartz substrate) to get a baseline?
There is another point to further account for.: when you carry on a reflectance measurement, the reflection spectrum would not involve any substrate (or better to say, interface) interference. The reflected light coming from the NPs is directly collected by the analyzer. This is obviously not true in a transmittance measurement, where backscattering can occur at the NPs/substrate interface affecting the output transmited signal ina complex way (which may cause aso the presence of undulating patterns).
I can not give you more help, I'm sorry.
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