From common considerations, some (less polar?) solvents can provoke nanoparticle coalescence. If the particle size increases, the optical properties change.

Optical properties of the nanomaterials is a reflection of its absorption, transmission, reflection or refraction behaviour should be attributed to its structural characteristics. Solvents with varying polarity and dielectric constants may allow protonation or deprotonation of the functional groups at the surfaces of nanomaterials or may alter charge density or may augment nullification of charges carried by nanoparticles which may augment agglomeration of particles and thereby there behaviour when incident upon by light.

Dear colleague Shiksant Kulkarni

thank for this usful openion,

regards

Raad