Surfactant-Induced Modification of Dopants Reactivity in Sol−Gel Matrixes

Abstract

Organically and bio-organically doped sol−gel materials have attracted much attention due to their ability to reproduce solution molecular activities within the ceramic environment. We take now this methodology one step forward and explore conditions under which the dopant properties can be modified by the matrix. Specifically we report that the co-entrapment of the surfactant cetyltrimethylammonium bromide (CTAB, the modifier) at low concentrations, with an extensive series of pH indicators representing several key molecular families (the primary dopant) within tetramethoxysilane (TMOS)-derived silica sol−gel matrixes, greatly modifies the indicating performance of the primary dopant. Thus, very large pK i shifts of up to 3−4 orders of magnitude obtained upon the co-entrapment cause methyl orange (MO) to become an indicator for higher acidities and phenolphthalein for higher basicities, compared to their solution behavior. In another example, the ΔpK i between the two indicating transitions of alizarin increased from ∼4.5 pH units in solution to ∼10.5 pH units (!) in the glass, transforming it into an indicator for both the high acidic and high basic pH ranges. In yet another example, the two indicating transitions of phenol red were shifted to a more acidic pH range, pushing the tail of the more acidic titration branch into the negative pH values range. These and other effects were found to be more pronounced by the co-entrapment than by the use of CTAB solutions or of sol−gel matrixes without CTAB, pointing to a synergetic effect between the surfactant and the silica cage. The indicators also proved highly sensitive in revealing the properties of the local environment created by the surfactant. Thus, the indicator molecules were shown to migrate and reorient within the hydrophobic and the hydrophilic regions of micellar environment, according to their acquired charge upon pH changes. The concentration-dependent and humidity-dependent surfactant aggregation processes within the silica cage were probed with MO, and the results were compared with the behavior of entrapped MO in sol−gel matrixes of varying hydrophobicities, obtained by the copolymerization of CH3Si(OCH3)3 with TMOS at various ratios, with and without CTAB. Of practical importance have been the observations that the dopant/surfactant co-entrapment greatly improves the leaching profiles: half-lives, ranging from several months to several years, were found for MO and methyl red; and using SAXS, surface area and porosity measurements, it was shown that CTAB can be used to stabilize the microscopic structure of the material upon heat-drying. This provides a potential solution to the problem of continuing structural changes, which take place with sol−gel materials long after the completion of their synthesis.

Fluorescence-lifetime-based pH sensing using resorufin

“…Addition of surfactants to solgels has been shown to shift the ground state pKa of entrapped indicators with a resulting shift in the absorption based pH response curves. 11,12 A similar effect would be expected for the excited state pKa (pKa*), allowing the tuning of the fluorescence lifetime pH response curves.…”

“…This again is probably due to changes in the polarity of the microenvironment of the dye, shifting the pKa to higher pH, a similar effect was also observed with aminofluorescein-doped sol-gels. 12 R − in S2 also absorbs over a wider wavelength range than R − in solution, and this coupled with the fact that the fluorescence decay is bi-exponential (Section 3.2) indicates the presence of RH in S2 at pH 9. The surfactant modified sol-gel (S3) has a very similar absorption spectrum to that of R − in solution indicating that the surfactant has counteracted part of the pKa change induced by incorporation into sol-gel.…”

“…This has been shown to be a facile method of adjusting the apparent pKa of immobilised indicators and should help to produce sensors with the optimum pH range for biological and clinical applications. 11,12 Our goal was to determine whether or not a suitable pH sensing scheme could be developed using 460 nm LED excitation and sol-gel matrices.…”

Thin film optical BTB pH sensors using sol–gel method in presence of surfactants

“…More recently, organic functional groups were introduced in inorganic precursors, and their copolymerization with tetraethylorthosilicate (TEOS) results in inorganic/organic hybrid materials [8,9]. In addition to using different organically modified precursors, film properties can be tuned with surfactants to have an effect on film morphology and porosity of encapsulated reagents [10,11]. Optimization of the glass composition and the right choice of processing parameters are vital for obtaining high quality films that will retain the required sensing performance characteristics.…”

An inorganic–organic hybrid material with a novel oxometallic framework: Hydrothermal synthesis and characterization of [Zn3O3(C13H14N2)3]V6O15

“…Hybrid materials have shown excellent laser efficiencies and photostabilities [6], fast photochromic responses [7], as well as high and stable optical responses [8]. These materials can also be used as pH sensors [9] and many of them have shown attractive gas storage capacity [10][11][12][13][14].…”