# Fabric Sol-gel Phase Sorptive Extraction Technique: A Review
## 3 Preparation of FPSE Media
### Preparation of the Sol Solution for the Sol-gel Coating Process  
Designing the sol solution is the main step on the way to the development of a sol-gel sorbent due to the fact that its composition and the relative ratio of the constituents, define the porosity as well as the selectivity and specificity of the resulting sorbent [24]. For an effective sol-gel sorbent, the selection of the sol-gel active organic polymer, the inorganic or organically modified inorganic sol-gel precursor, the solvent/solvent system, the catalyst, the amount of water, as well as an appropriate relative molar ratio of the constituents must be considered. The composition of the sol solutions for the FPSE media preparation is given in details in Table 3.  
The coating procedure is integrated by inserting gently the treated fabric pieces into the vial containing the sol solution. As a result, a three dimensional network is formed throughout the porous substrate matrix (cellulose, polyester etc.). After a predetermined coating time the fabric is taken away from the sol solution, dried, and placed in a desiccator overnight for solvent evaporation and for aging of the sol-gel coating. The objective of this step is to complete the condensation reaction and remove solvents and unreacted residuals from the sol-gel matrix, ensuring a clean, surface bonded sol-gel sorbent free of structural deformation and internal stress. The coated FPSE media is then rinsed with the appropriate solvent system under sonication for a few minutes in order to remove residual sol solution ingredients from the coated surface. The FPSE media is then cut into 2.5 cm \(\times\) 2.0 cm pieces and stored in clean airtight containers, for future use. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Sol-gel Precursor**} & \multicolumn{2}{c}{**Operation**} \\ \hline \multirow{2}{*}{Unchanged:} & TEOS, Titanium isopropoxide, & Inorganic component to the hybrid polymeric \\  & Zirconium butoxide, & network. Offers active hydroxyl group to facilitate \\  & Tetramethoxygermane, Alumina & chemical bonding to the fiber/capillary surface. \\ \hline \multirow{2}{*}{Organically modified:} & MTMS, C\({}_{\text{IS}}\)-MTMS, C\({}_{\text{S}}\)-TMS, & Forms inorganic backbone of the hybrid material. \\  & TMSPA, VTEOS, PTMOS & Organic moieties provide intermolecular \\  & interactions between analytes and the sorbent. \\ \hline \multicolumn{2}{c}{**Catalyst**} & \multicolumn{2}{c}{**Operation**} \\ \hline \multirow{3}{*}{Acid/Fluoride/Base:} & CAA, HCl, HNO\({}_{3}\), Citric acid, & Catalyzes the hydrolysis and condensation \\  & TFA, HF, NH\({}_{4}\)OH, NaOH, & reactions and controls the network structure and \\ \cline{1-1}  & Basic amino acids & porosity. Acid catalyzed sol–gel materials possess \\ \cline{1-1} \cline{2-3}  & & weakly branched microporous structures. Base \\ \cline{1-1} \cline{2-3}  & & catalyzed sol–gel materials have highly branched \\ \cline{1-1} \cline{2-3}  & & particulate structures with large pore sizes. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Types of precursors and catalysts used in a sol-gel process. Images of the surface by scanning electron microscope (SEM) of (a) uncoated surface of cellulose fabric substrate at 100\(\times\) magnification; (b) sol-gel poly-PEG coated surface of FPSE media at 100\(\times\) magnification; (c) uncoated surface of cellulose fabric substrate surface at 500\(\times\) magnification; and (d) sol-gel poly-PEG coated surface of FPSE media at 500\(\times\) magnification are presented in Figure 3. Both low and high magnification SEM images of the sol-gel PEG coated FPSE media clearly demonstrate that the ultra-thin coating of the sol-gel sorbent is uniformly distributed throughout the substrate matrix without blocking the pores of the fabric. The easy access of the low viscosity sol solution into the fabric matrix helps in achieving this uniform ultrathin sol-gel PEG coating.