Rationally Developed Molecular Recognition Systems for Electrophoretic Separations, Raman Spectrometry, and Microcantilever-Based Sensing
(contact Mike Sepaniak)

The underpinning theme of our work is the rational design and fundamental development of molecular recognition systems for chemical separations, spectroscopic detection, and trace-level sensing. Molecular mechanics (MM) and molecular dynamics (MD) are used to target macrocyle reagents that are expected to provide desirable molecular recognition properties and to help in understanding reagent performance in chemical analysis. Interaction energies involving the inclusion of organic analytes with commercial and new synthetic cyclodextrins (CDs), obtained using commercial MM/MD force fields, are shown to be sufficiently accurate to predict elution orders in CD-mediated capillary electrophoresis (CE) separations and correctly identify potentially effective new CDs. New classes of single isomer CDs have been synthesized using multi-step synthetic procedures. The reaction schemes create carboxymethyl-, thio-, or Si-tertbutyl-functionalities at the C6 positions of the CD to provide, respectively, mobility for electrophoresis, a means to chemically attach to metal surfaces, and increased volatility for vapor deposition. Substitution at the C2 and C3 positions is aimed at influencing inclusion selectivity. These reagents are shown to possess utility as running buffer additives for achiral and chiral CE separations. Although laser induce fluorescence is the staple high-sensitivity detection technique in CE, we have recently demonstrated surface enhanced Raman scattering SERS as an alternative that exhibits good sensitivity. Both on-column and off-column SERS approaches are possible and both provide information-rich spectra for analyte identification and/or high selectivity. In all cases nanoparticles of silver or gold are employed with inexpensive instrumentation to achieve enhancements sufficient for effective utilization in CE. In some cases distinctive spectra are obtained for injection amounts in the attomole range. The SERS process is very complex and several fundamental factors contribute, sometimes dramatically, to observed enhancements. Evaluation of thiolated CDs and other reagents as sequestering reagents on the surfaces of the nanoparticles are underway and represent a means to adjust analyte-metal surface geometry and chemical interactions. The aforementioned macrocycles are also useful molecular recognition and signal enhancement reagents when immobilized onto the surfaces of microcantilevers in micro-electro-mechanical systems. Chemi-mechanical responses of the cantilevers provide for both sensing and actuation. Responses in both gas and liquid phase systems are found to increase dramatically when new modes of analyte-induced surface stress are generated on specially prepared nanostructured cantilevers. These advances may lead to interactive devices that are powered solely by chemi-mechanical interactions.

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