Fig. 1(a) shows the EDS spectrum of the nS obtained at pH 7. A
strong intensity of Si and O peaks between 0 and 5 keV, confirming
the presence of silica, are predominant in this sample. Fig. 1(b)
shows the EDS spectrum of the CCS obtained at pH 10; the
spectrum clearly shows the impurities that are present in the silica
prepared at pH 10. The silica surface consists of two types of
functional groups, namely silanol (Si-OH) (SiO–H) and siloxane
(Si–O–Si), with silanol being more reactive and possessing more
adsorptive properties. Because more cations appear on the surface
of silica nanoparticles at a higher pH, greater mineral contamination
occurs.