Analytical preconcentration systems based on nanostructured materials

by Kijak, Anna M.

Analytical Preconcentration Systems Based on Nanostructured Materials by Anna M. Kijak Sol-gel materials have properties which suggest their use in various analytical devices. However, they have also an important limitation: they often fracture during immersion in solvents or upon rapid drying. We hypothesized that crosslinking the backbone will provide a more robust material. We used polyamidoamine (PAMAM) dendrimers as a test. ASTM methods demonstrated that generation zero (G0) PAMAM increased the strength of silica from 17 to 30 mJ. The stability during wetting/drying also was improved. G0-PAMAM apparently acts as a crosslinking agent, thereby forming a composite material. In contrast, macromolecular G4-PAMAM templated the sol-gel into the mesoporous domain, but the silica was not strengthened significantly. In support of this model, the surface area decreased from 314 m 2g-1 to 204 m 2g-1 with inclusion of G0-PAMAM but increased to 494 m2g-1 when G4-PAMAM was the dopant. Various applications of the resulting nanostructured composite were demonstrated. Included were uses as platforms for electrochemical and thermochromic devices and as supports for solid phase extraction (SPE). The SPE studies were performed by doping the composites with a crown ether, 18-crown-6-2,3,11,12-tetracarboxylic acid. The capacities for microporous and mesoporous silica were 5.0 ± 1.4 µmole g-1 and 10.8 ± 0.3 µmole g-1, respectively, when Pb2+ was the analyte. A limitation was poor accessibility of the capture agent by the analyte. Therefore, we extended the study to functionalized self-assembled monolayers (SAMs). An analytical strategy whereby 2-[(6-mercaptohexyl) oxy]methyl-15-crown-5-SAM (CESAM) on gold was released by electrochemical oxidation of gold thiolate into a flow stream after capture of the analyte was developed and evaluated. Detection of 0.2 nmol PbII was observed. In addition, a procedure for rapid, reproducible renewal of the CESAM was developed. With 30 - 90 s modifications, a surface coverage of 7.13 x 10-10 ± 2 x 10-12 mol cm -2 (fraction covered, 0.97) was obtained. Finally, a trap-release concept using SAMs of dendrimers was tested. Ionic strengthcontrolled conformation changes of G4-PAMAM provided the entrapment and release of a neutral molecule, ferrocene. The method was demonstrated using voltammetry to prove uptake and release. The ferrocene was quantified with LC-MS. Analytical Preconcentration Systems Based on Nanostructured Materials A Dissertation Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemistry and Biochemistry by Anna M. Kijak Miami University Oxford, Ohio 2003 Dissertation Director: Dr. James A. Cox