Functionalization is the process of adding new functions, features, capabilities, or properties to a material by changing the surface chemistry of the material. It is a fundamental technique used throughout chemistry, materials science, biological engineering, textile engineering, and nanotechnology. Functionalization is performed by attaching molecules or nanoparticles to the surface of a material, sometimes with a chemical bond but sometimes just through adsorption (that is, the thing you’re trying to attach sticks to the surface without forming a covalent or ionic bond).
To paraphrase aerogel scientist Dr. Debra Rolison, here’s a way to think about functionalization: