Welcome to the Schwartz Research Group!

 Kiersten Johnson, Greg Morrin, Raphael Safarti, James Weltz, Andres Chaparro-Sosa, Connor Thompson, Daniel Schwartz, Daniel Kienle, Jeremiah Traeger, Benjamin Greydanus, Haichao Wu, Evan Bisirri, Katherine Macri
The Schwartz Research Group at CU-Boulder is broadly interested in interfacial science. We are leading the way in analyzing interfacial behavior using single-molecule techniques as well as investigating chemically modified catalysts. This includes leveraging single- molecule imaging techniques to determine molecular behavior at solid-liquid interfaces, dynamics in confined environments, and interactions between biomolecules and surface modifications.

General Interests

  • Dynamics of (bio)molecules at interfaces
  • Molecular targeting (biosensing)
  • Diffusion in confined environments
  • (Catalytic) surface modification

Instruments and Methods

  • Single-molecule imaging
    • Total Internal Reflection Fluorescence Microscopy (TIRFM)
    • Förster Resonance Energy Transfer (FRET)
    • Double-helix point spread function
    • Dual excitation
  • Polarized light microscopy
  • Surface modification (thin films and self-assembled monolayers)
  • Surface characterization (contact angle goniometry, FTIR, ellipsometry)
  • Atomic force microscopy
  • Spectroscopy (circular dichroism, UV-vis, fluorescence)
  • Numerical simulations and machine-learning
  • Langmuir trough
  • Brewster angle microscopy

Research Highlights

Physical Review Letters: Three-Dimensional Tracking of Interfacial Hopping Diffusion

A microscope uses a phase mask to track molecules in 3 dimensions

(a) A standard wide-field microscope body was equipped with a Double-Helix SPINDLE module to implement DH PSF imaging. (b) Two-dimensional projection of a trajectory. (c) Actual 3D motion for the trajectory shown in (b). To guide the eye, the red spheres indicate steps for the polymer in solution while the black ones denote steps at the surface. The inset shows the corresponding trace of z position vs time.

DOI: https://doi.org/10.1103/PhysRevLett.119.268001 Theoretical predictions have suggested that molecular motion at interfaces—which influences processes including heterogeneous catalysis, (bio)chemical sensing, lubrication and adhesion, and nanomaterial self-assembly—may be dominated by hypothetical “hops” through the adjacent liquid phase, where a diffusing molecule readsorbs after a given hop according to a probabilistic “sticking coefficient.” Here, we use three-dimensional (3D) single-molecule tracking to explicitly visualize this process for human serum albumin at solid-liquid interfaces that exert varying electrostatic interactions on the biomacromolecule. Following desorption from the interface, a molecule experiences multiple unproductive surface encounters before readsorption. An average of approximately seven surface collisions is required for the repulsive surfaces, decreasing to approximately two and a half for surfaces that are more attractive. The hops themselves are also influenced by long-range interactions, with increased electrostatic repulsion causing hops of longer duration and distance. These findings explicitly demonstrate that interfacial diffusion is dominated by biased 3D Brownian motion involving bulk-surface coupling and that it can be controlled by influencing short- and long-range adsorbate-surface interactions.

ACS Catalysis: Controlling the Surface Reactivity of Titania via Electronic Tuning of Self-Assembled Monolayers

Catalytic Efficiency can be tuned through electronegativity

DOI: 10.1021/acscatal.7b02789 Reactivity of molecular catalysts can be controlled by organic ligands that regulate the steric and electronic properties of catalyst sites. This level of control has generally been unavailable for heterogeneous catalysts. We show that self-assembled monolayers (SAMs) on titania with tunable electronic properties provided fine control over surface reactivity. Controlling the identity of substituents on benzylphosphonic acid SAMs modulated the near-surface electrostatics, enabling regulation of the dehydration activity of 1-propanol and 1-butanol over a wide range, with activities and selectivities of the optimal catalyst far exceeding those of uncoated TiO2. The dipole moment of the adsorbed phosphonate was strongly correlated to the dehydration activity; kinetic measurements and computational modeling indicated that the interfacial electric field altered the transition-state structure and energy. Coating catalysts with SAMs having controllable charge distributions may provide a general approach to heterogeneous catalyst design analogous to the variation of ligands in molecular catalysts.

Biomacromolecules: Dense Poly(ethylene glycol) Brushes Reduce Adsorption and Stabilize the Unfolded Conformation of Fibronectin

Dense polymer brushes reduce adsorption to surfaces

DOI: 10.1021/acs.biomac.5b01657 Polymer brushes, in which polymers are end-tethered densely to a grafting surface, are commonly proposed for use as stealth coatings for various biomaterials. However, although their use has received considerable attention, a mechanistic understanding of the impact of brush properties on protein adsorption and unfolding remains elusive. We investigated the effect of the grafting density of poly(ethylene glycol) (PEG) brushes on the interactions of the brush with fibronectin (FN) using high-throughput single-molecule tracking methods, which directly measure protein adsorption and unfolding within the brush. We observed that, as grafting density increased, the rate of FN adsorption decreased; however, surface-adsorbed FN unfolded more readily, and unfolded molecules were retained on the surface for longer residence times relative to those of folded molecules. These results, which are critical for the rational design of PEG brushes, suggest that there is a critical balance between protein adsorption and conformation that underlies the utility of such brushes in physiological environments.