3 With the above, C(x,t) can be solved for planar diffusion 1 or spherical diffusion if transformed to the spherical coordinates, which is detailed in Equation S3 in the Supporting Information (SI). In either case, when t << L 2 /D, where L is the diffusion length or particle radius in the spherical case, semi-infinite diffusion can be assumed and the concentration at the surface can be expressed as: