A simple and facile method is proposed for treatment of the inner surface of micro-mesoporous spherical particles of amorphous silica with the solutions of heavy element salts, which allows direct visualization of the particles’ porous structure by the transmission electron microscopy (TEM) technique. The method is based on the adsorption of ions of heavy elements (on the example of Er3+ ions) by the surface of the pores, which makes it possible to enhance the contrast in the TEM image. The particles before and after erbium ions adsorption are characterized by means of TEM, nitrogen porosimetry and energy dispersive X-ray spectroscopy. It is demonstrated that the developed method for the particles’ surface functionalization allows implementing erbium adsorption without affecting the morphology and inner structure of particles. It is shown that erbium content after the adsorption procedure does not exceed 0.1 at.% and the pore structure of silica particles remains the same – the specific surface area, volume and pore size do not change.