Sol-gel method

It doesn't matter which method you use for the synthesis of nanoparticles to be characterized by UV spectroscopy.

Adding more to the above suggestions, if your nanoparticles can be made as a thin film form as the suggested sol gel method, you could measure UV spectrum to extract some useful information. What kind of nanoparticles you are doing if you don't mind to say?

If possible, transfer the powders of your nanoparticles into a thin-pass cuvette and perform diffuse reflectance measurements instead.