Hi,

I'm not sure to really understand what you mean...You prepared your graphene quantum dot, you measured it by DLS but the size you obtained is not the same you expected? What do you mean as "appropriate" particle size?

If it is a problem related to DLS (i.e. you know that your dots have a specific size by other means and DLS gives wrong results) It would be nice to see the intensity correlation functions and to know more details about the DLS measurement.

Guessing, based on the way you have written your question, that your samples have been failing internal software checks on the validity of your obtained data. Am I right? While your attempts to screen out contamination are admirable, some DLS devices do not easily handle fluorescent particles like QDs, particularly when the emission spectra of the QD is close to the wavelength of the laser light used in the instrument. Have you checked to make sure your instrument can handle highly fluorescent samples? I once tried to measure QDs and QDs+surfactant by DLS to confirm their size, what I got out was nonsense, that strongly indicated that the instrument was being overloaded with signal. A sign that this is the problem is that the kcps and Z-average size increases almost geometrically with each repeat measurement of the sample. If you are outfitted for fluorescent samples, or your QDs are already known to not interfere with detection, then it is also possible that you have reduced the concentration of your QDs to the point of being undetectable. Filtering can do this, if your membrane has affinity for your particles. I realize at the scales you are dealing with the hydrophobic properties of graphene are mitigated, but it is still possible. If that cannot be it, it may also be that the shape of your QDs is skewed heavily towards flat sheets and oblate spheroids, which can potentially give you strange data, pay attention to PDI. I (like the others here) can't say more without seeing your raw data.

A potential issue with quantum dots can be fluorescence. This can possibly be overcome by addition of a narrow band filter to reduce the influence of (incoherent) fluorescent light at the DLS detector. Many DLS instruments automatically scale the input and thus scattering intensity and would not necessarily show an intensity overload, however the intercept of the correlation function will be significantly reduced compared to non-fluorescent samples. This only happens if there is fluorescent light emitted and if it reaches the detectors. Adding a narrow band filter (for example 5-10nm band width) will reduce the "extra" non-scattered light and yield better correlation functions.

However, the sample itself will have to be suitable, general sample preparation and data quality concerns remain. The intensity based size by DLS may be larger than a size obtained by other means (number based TEM for example.)

http://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#qdots

http://www.materials-talks.com/blog/2014/01/23/intensity-volume-number-which-size-is-correct/

I really appreciate you all, for your kind attention.

i must explain it with more details,

At the time I wanna know if the nano-particles are composed or not?

And can I use DLS method to earn the size and distribution of GQD?

There are much better ways to analyze but the method I just mentioned, is more accessible. I wanna be sure if it is ok for "Graphene Particles"? Because I couldnt reach an exact size for the GQD solution. I found too larg particles before and even after several times of filteration and sometimes I got zero size. Even in different levels of density.

Also When I reach to an exact size, the distribution is too improper that I cant relay on.

Regarding to these odd data Ive got, is DSL propriate for measuring the GQD? Was there any special conditions I should have follow?

I would appreciate if you allot your valuable time to help me figuring out the problem.

What sizes are you getting exactly? What does your correlation curve look like? What do your number, volume and intensity peaks look like? Thus far Ulf has given the best advice (as he often does, being from Malvern). DLS can handle non-spherical particles, but it requires that you carefully examine your data, and observe a bit of Perrin theory to get a better idea of what your data actually means. You should post your size distributions and if possible the correlation curves and settings you are using that will help those here identify your particular problem.  

Many thanks to you Dear Jordan  and Mr. Nobbmann, I'll write again with more accurate details

The major concern which I had during the analysis of my samples was the imaginary refractive index, which I had to provide during DLS analysis. Can anyone let me know how this imaginary refractive index for graphene quantum dots can be determined.