The phenomenon you're observing, where functionalized mesoporous silicon microparticles turn white when placed in water or PBS (phosphate-buffered saline), is likely due to the scattering of light by the microparticles, and it may not significantly affect their adjuvant potential. Let me explain in detail.

Scattering of Light: When mesoporous silicon microparticles are dispersed in a liquid like water or PBS, they can scatter light. This scattering occurs because of differences in refractive indices between the particles and the surrounding medium (water or PBS). The extent of scattering depends on various factors, including the size, shape, and surface properties of the microparticles. When light is scattered, it can give the appearance of whiteness because it disrupts the passage of light through the medium.

Size and Surface Chemistry: The properties of the microparticles play a significant role in the scattering of light. The mesoporous silicon microparticles likely have a structure that scatters light effectively. Furthermore, the functionalization of the microparticles can also impact their interaction with light. Surface functionalization can lead to changes in the refractive index or surface properties, which can increase or alter the scattering effect.

Adjuvant Potential: The change in color or appearance (from transparent or translucent to white) when these microparticles are placed in water or PBS is primarily a physical phenomenon related to light scattering. It does not necessarily indicate a loss of adjuvant potential. The adjuvant potential of such microparticles depends more on their chemical properties, structure, and ability to interact with the immune system when used in vaccine formulations.

In summary, the white appearance of functionalized mesoporous silicon microparticles in water or PBS is most likely due to light scattering effects. While this phenomenon may be interesting from a visual perspective, it is unlikely to have a significant impact on their adjuvant potential. To evaluate their adjuvant potential, you should focus on more relevant properties, such as their ability to interact with immune cells and enhance the immune response in the context of vaccine formulations.

The particles are oxidizing to SiO2 which is white. A good background article is: Kim, Thermally Induced Silane Dehydrocoupling on Silicon Nanostructures DOI:10.1002/anie.201601010