How to produce sponges with tailored nano-holes:
The sol-gel process
The Sol-Gel process allows the elaboration of porous matrices with tailored nano-holes. The process starts with the preparation of a Sol, a solution composed of various ingredients such as ethanol (an alcohol), water and acid (Chloridric acid) and the main reactant, tetraethoxy silane (TEOS).
TEOS is a molecule which displays 4 functional arms (O-C2H5). In the presence of water, the arms (O-C2H5) are transformed into new and very reactive (O-H) arms, via a reaction called « hydrolysis ».
(C2H5O)3-Si-OC2H5 + H2O ® (C2H5O)3-Si-OH + C2H5OH
The resulting compounds Si(OH)4 are particularly friendly and sticky and as soon as they encounters a neighbour, they will interact together to form a Si-O-Si bridge, which will link them together. The reaction is called « polycondensation ».
(C2H5O)3-Si-OH + OH-Si- (OC2H5)3 ® (C2H5O)3-Si-O-Si-(OC2H5)3
The continuous formation of bridges among the population of Si(OH)4 induces the formation of a polymeric tri-dimensional network. At this stage and before the Sol turns into a Gel because of the increasing viscosity, the probe molecules are added in the solution together with surfactant molecules. The cationic surfactant molecules display a long hydrophobic chain and a hydrophilic head. Therefore, in the Sol, the molecules reorganize themselves into spherical micelles or aggregates, the size of which depends on the length of the hydrophobic chain. The probe molecules are incorporated within these micelles.
To obtain a thin film, a glass substrate is dipped into the Sol and withdrawn at a constant speed. During the withdrawal process, the Sol becomes a Gel and the micelles are entrapped in the gel network. After being dried, the film was washed with ethanol to remove the surfactant molecules. The cavities formed by the surfactants are then freed and are now ready to trap the targeted pollutants.