Research

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TECHNIQUES & INSTRUMENTATION

CHEMISTRY AT THE INTERFACE

BIOMIMETIC AND NANOPOROUS INTERFACES

Blanchard group research is focused on three problems of interest to both the analytical chemistry and materials science communities:

Design and characterization of molecular monolayer and multilayer assemblies
Understanding energy exchange processes between dissimilar molecules
Achieving a molecular understanding of how confinement and immobilization affect the ability for a catalytic species to function

We use picosecond nonlinear laser spectroscopies in conjunction with more traditional methods to address these problems. Learn more about our picosecond laser instruments here or by clicking on the "Techniques & Instrumentation" page above.

CONTROLLING INTERFACIAL FLUIDITY

Covalently bound interfacial adlayers are not fluid, and fluid adlayers are not physically or chemically robust. These limiting cases have frustrated advances in fields such as molecular-scale lubrication, chemical separations and cellular adhesion.

We are developing a novel family of interfaces that can be bound to a surface and at the same time retain the properties of a fluid. Both the thermodynamic driving force for complexation and the kinetics of surface diffusion can be controlled through metal ion complexation, system pH, surface complexing moieties, and the amphiphile head group.

Example amphiphilic monolayer with metal ion complexation on a solid support.

Diffusional Motion as a Gauge of Interfacial Adhesion and Fluidity

Combining two well known relationships for the translational and rotational diffusion behavior of a molecular in a homogeneous environment, we have derived an equation which allows for the comparison between theoretically derived diffusional behavior to experimentally measure rotational and translational diffusion values. The utilization of this relationship allows for quantitative characterization of interfacial adhesion strengths and a measure of heterogeneity depending upon derivation from the expected theoretical values.

Comparing the measured translational diffusion to the effective translational diffusion, derivded from the combination of the Debeye-Stokes-Einstein-Sutherland relationships and the measured rotational diffusion, allows for the determination of the equilibrium binding constant of the film to the supporting surface. The equilibrium binding constant ultimately allows for the quantitative determination of the interfacial adhesion of the supported film.

Controlling Interface Organization Using Metal-Ion Oxidation State Toggling

Several well-established means of binding monolayers to surfaces in a robust manner (e.g., tiol-gold interactions, ionic complexation chemsitry) involve surface-monolayer interactions that are fixed, and in many of these instances the means of attachment determines the organization of the resulting monolayer. Tthe inability to change the binding properties of these films post-deposition can limit their applications.

We aim to address this problem by investigating the possibility of controlling interfaces and interfacial properties after a monolayer or film has been deposited onto a solid substrate, specifically, controlling in situ and reversibly the manner in which a layer or film binds to a surface. We plan to electrochemically toggle the oxidation state of the metal ions in a metal ion-complexed monlayer and explore the consequences of this process on the organization of that layer on the surface.

The ability to change the oxidation state and create monolayers with controllable properties after deposition is advantageous for applications such as stationary phases in chemical separations, films with selective and controllable permeability, or surfaces with controllable optical properties, for example.

Copper-complexed monolayer with an initial oxidation state (2+, top) toggled to a different oxidation state (1+, bottom).

INTERFACIAL CHARGE GRADIENTS

Room temperature ionic liquids (RTILs) are particularly intriguing due to the wide temperature range in which they exist in the liquid phase. Additionally, they exhibit a wide redox window and are able to dissolve both polar and non-polar species. We are investigating the fundamental properties of ionic liquids to understand the dynamics, organization, and response of these materials to external forces. Once these properties are more fully understood, the systems can be applied to various uses, such as solvents for organic synthesis, electrolytes in supercapacitors, and electro-optical materials.

Modulating Induced Charge Density Gradients

We have observed direct evidence for charge-induced long-range (ca. 100 μm) order in the RTIL 1-butyl-3-methylimidazolium tetrafluoroborate, supported on a silica surface. We have measured the rotational diffusion dynamics of anionic, cationic, and neutral chromophores as a function of distance from a silica surface. The results reflect the excess charge density gradient induced in the RTIL by the (negative) charge present on the silic asurface. Identical measurements in ethylene glycol reveal spatially invariant reorientation dynamics for all chomophores. Capping the silica support with dimethyldichlorosilane (silanization) results in spatially invariant dynamics in the RTIL.

We have further demonstrated experimental control over the sign and magnitude of an induced charge density gradient, ρ_f, in the RTIL BMIM^₊BF₄^-. The spatial extent of ρ_fwas characterized through the rotational diffusiontime constant gradient of a cationic chromophore in the RTIL. The sign and magnitude of ρ_f in BMIM^₊BF₄^-is linked directly to the surface chargedensity of the electrode, which can be controlled. ITO and FTO are used as solid supports and transparent conductive electrodes during these experiments to demonstrate this control.

BIOMIMETIC INTERFACES

The creation of interfacial structures that can function biomimetically is a gateway to the design of biosensors. We have been actively involved in the creation of such structures where we deposit a lipid bilayer structure, with fluorescent and electrochemical probes embedded at specific locations within the interface. Our efforts are aimed at probing local organization and fluidity in the bilayer, so that we will be able to incorporate transmembrane proteins into these interfaces for use as sensing elements.

Idealized structure of a supported lipid bilayer showing the surface binding layer o substrate (pink), hydrophilic polymer layer (yellow), lipid connecting layer (blue), and the outer and inner lipid leaflets (green).

PHOTO-ACTIVATED PRECISION CHEMISTRY

Super-photobase molecules can be utilized in various applications such as precision chemistry and high-speed chemical sensing. Upon excitation with electromagnetic radiation, reactivity of such compounds is activated (pKa > 20). Precursor molecules of these photoactive compounds also exhibit distinct properties, such as unique solvent-dependent excited-state behavior. Current work focuses on characterizing the excited-state behaviorFR0 of these molecules using time-correlated single photon counting (TCSPC). Results have indicated that certain percursor molecules exhibit spectral shift dynamics and multiple excited-state populations resulting from interactions with surrounding solvent molecules. Specific factors influencing this unique behavior are currently being investigated, including proton exchange, solvent viscosity, and solvent hydroxyl group concentration. Using TCSPC, the spectral shift dynamics across a particular wavelength range can be determined in protic vs aprotic solvents, as well as the population densities of the various excited states of such precursor molecules (see figure).