Process for preparing substrates with porous surface

A process for preparing a coated core comprising the steps of:

• (a) providing a spherical silica core to be coated in a fluid;
• (b) treating said spherical core with polyelectrolyte by exposing said core to a solution or suspension of polyelectrolyte to form a first-treated surface;
• (c) removing excess non-bound polyelectrolyte without drying said first-treated surface;
• (d) further treating said first-treated surface by attaching a first multilayer comprising a plurality of charged silica nanoparticles that are opposite in charge to said polyelectrolyte by exposing said product from step (c) to a suspension comprising said charged nanoparticles;
• (e) removing excess non-bound charged nanoparticles;
• (f) repeating steps (b), (c), (d) and (e) by adding further layers of polyelectrolyte followed by multilayers of charged silica nanoparticles as many times as desired to produce a second and subsequent multilayers of charged silica nanoparticles on said core;
• (g) removing said polyelectrolyte layers by volatilization or extraction;
• (h) optionally fixing said silica nanoparticles to each other and to said core by thermal treatments;
• and
• (i) optionally adding a bonded phase to functionalize said core and said charged silica nanoparticles,
• wherein said polyelectrolyte has a weight average molecular weight of 100 kD or greater and the ionic strength of the fluid is less than 0.05M such that said first, second, and subsequent multilayers independently comprise a multiplicity of charged silica nanoparticle layers that are thicker than monolayers.