(i) they are considered as a suitable platform for tuning surface chemistry, (ii) they tend toself-organize spontaneously, (iii) they offer the possibility to study sintering with or without structural transition and (iv) the dissolution of silica (according to the equation SiO2 +2H2O ⇨ H4SiO4 [37]) is congruent, which guarantees to exclude selective precipitation phenomena and makes the system easier to control.