Did you really have a peak at 150nm or do you possibly just have 2 overlapping ones with 150 average? 

There are different modes you can switch between and two peaks might be easier to differentiate in one or the other: You can do volume distributions (volume of particles at certain size) number distributions (number of particles at a certain size) and intensity distribution (intensity scattering from different particles sizes) for more infos on what is what see:

http://nanocomposix.com/sites/default/files/handbooks/introduction.pdf

What kind of nanoparticles are you looking at?

Also can you check how they actually look like with SEM that might be useful. 

Alternative solution: Also quinolone itself is a relatively small molecule so 200 nm is certainly an aggregate. It could be that the aggregate dissolves in the medium where the nanoparticles are in or that the aggregates disassemble and reassemble around the nanoparticles. 

This might be visible in SEM or TEM as well. I hope this helps. Romana

In additions to the good points that were mentioned earlier, DLS tends to always give a result, in a normalized fashion; so you can check for the number of photons collected by the detector, combined with the attenuation used in your system in order to get an idea of the amount of scaterring that actually was generated by your sample.

(for a ZetaSizer instrument, look for count rate (usually in the 200-300 kilo counts per second) and for attenuator (position 11 means no signal attenuation, 10 means that a third of the signal is collected, 9 means that a ninth of the signal is collected, etc.))

Best,

Hi everyone. thanks for the response

Schirhagl - thanks for the notes I will look into that. you may right there may be aggregates. I used high concentration 20ug/ml of antibiotic in the solution. what would be the right concentration to start with?

Allan - I agree with your fluorescent issues. i am working with nadifloxacin and it is fluoroscent compound. now I am not sure, if fluorescent if the issue then how I am suppose to get rid of that problem?

thanks

Probably it doesn't matter that you have aggregates in the beginning. Since DLS anyways suggests that you dont have them later on. But here is a paper about the solubility of you antibiotic. You can check up to which concentration it is soluble than you dont start with aggregates.

http://link.springer.com/article/10.1023/A%3A1018910431216

Hi, 

You may want to check the correlation function of the antibiotic aggregates.  If the lines were clear than you may be sure that you have antibiotic aggregates.  Also check how many peaks you have when using DLS and you may do time sweep as there might be additional peaks from released antibiotic. 

You may also try to freeze dry the nanoparticles together with the antibiotic.  Later you may resolubilize the dried particles, this way you may prevent aggregation of free antibiotic during initial DLS reading.

Hope this help.

Best Regards

Terima kasih Yusuf. Indeed very helpful suggestion.