The hybrid solutions were prepared via sol-gel reactions of vinyl triethoxysilane (VTES) and poly(4-vinylphenol) (PVP, average molecular weight = 25 kDa), which were purchased from Sigma-Aldrich Co. (St Louis, Mo, USA), in the presence of deionized (DI) water and acetic acid (Sigma-Aldrich). The composition of PVP to VTES was 0, 5, and 10 mol. %, while the molar ratio of DI water and acetic acid was fixed at 1:1:6. The mixture solutions, which contain VTES, PVP, DI water and acetic acid, were subject to vigorous stirring initially, followed by sol-gel reactions at room temperature for 24 h.