Sum Frequency Generation at Material Interfaces
Sum frequency generation (SFG) spectroscopy gives valuable about the molecules adsorbed to a surface or at an interface. Our heterodyne detected – vibrational SFG spectrometer gives us orientational information of adsorbed molecules while simultaneously tracking the phase of the signal.
Dye-sensitized solar cells are an inexpensive alternative to conventional solar cells on the market today. N3 dye is often used in these devices to adsorb incident sunlight and create usable electricity. We use vibrational SFG spectroscopy to study how the orientation of N3 dye impacts device efficiency and what conditions affect dye orientation. The figure shown here demonstrates the pH dependence of the binding orientation of N3 to a gold surface.
Visualizing structure and dynamics of room temperature ionic liquids via 2D IR microscopy
Microdroplet technology has been used for the acceleration of reactions in small, confined volumes. Understanding more about the impact of confinement on a solution is key to further developing this field. Room temperature ionic liquids are a candidate for these studies as they have advantages as solvents, with low vapor pressure, wide electrochemical windows, and structural microheterogeneity. We are studying the dynamics of room temperature ionic liquids (RTIL) at interfaces via two dimensional infrared (2D IR) spectroscopy and microscopy. Changes in 2D IR microscopy can track differences in structure and dynamics of molecules in bulk or interfacial environments.
a) Visible image of an RTIL microdroplet b) Single 2D IR spectrum corresponding to the circled dot in (a).
100 kHz Mid-IR OPCPA Laser Source for Use in 2D IR Spectroscopy and Imaging
To more efficiently image, we have moved away from the one to several kHz Ti:sapphire based laser systems and optical parametric amplification systems typically used in 2D IR spectroscopy. Instead we have moved to higher repetition rates, demonstrating for the first time 2D IR spectroscopy at 100 kHz. Achieving this higher repetition rate was accomplished by utilizing advances in diode pumped ytterbium (Yb) oscillators and amplifiers, and is based on an optical parametric chirped-pulse amplifier (OPCPA) utilizing magnesium doped periodically poled lithium niobate (MgO:PPLN) followed by difference frequency generation (DFG) in zinc germanium diphosphide (ZGP). We have successfully collected 2D IR spectra of KOCN in DMF using this new laser source. Shown below are a single 2D IR spectrum of KOCN in DMF, a 20 spectra average, and a 100 spectra average, from left to right. We are working on the development of a 2D microscopy set up at 100 kHz using this laser source to greatly increase acquisition rates, allowing for the more rapid imaging of samples.
Pseudo Halide Anions
Pseudo-halide anions are of interest due to their potential as a marker for proteins and the experimental observation of protein folding in solution. These anions have long vibrational lifetimes, absorb in a unique spectral region, and are highly sensitive to their local solvent environments conditions. Information can be obtained from 2D IR spectroscopy experiments about the effects of different environments on the relaxation rate of the CN stretch of the pseudo-halide anion. In particular, we are interested in observing the cyanate ion in the presence of mixed solvent environments.
Technology Development - Microfluidics
Two-dimensional infrared spectroscopy requires small sample amounts to be used and is a promising tool for kinetics experiments due the intrinsic time and structural resolution of 2D IR spectroscopy. In order to fully realize this potential, we have developed optically thin microfluidic devices that are nearly transparent to mid-IR light. Microfluidic devices afford precision control over sample handling and mixing, as well as a path to reduce the amounts of materials required in a kinetics experiment. We are developing new approaches to achieve fast-mixing in IR transparent polymeric materials. Shown below are examples of designs optimized for fast mixing. On the left is a design based on diffusive mixing which can be incorporated with 2D IR to decrease acquisition time of taking spectra in multiple mixed solvent environments. On the right is picolinic acid chelating with iron (II) chloride in the fast mixing, gradient generator device which can be used to extract reaction kinetics. We have successfully interfaced the precision control of microfluidic technology with our 100 kHz 2D IR spectrometer. Producing high-throughput 2D IR spectroscopy measurements allows us to observe the vibrational response of mixed solvent environments across a microfluidic device without having to create numerous samples, successfully minimizing chemical usage and experiment time. 2D IR spectra were captured laterally across microfluidic devices tailored to produce a tunable gradient to observe the OCN- vibrational response to mixed solvent environments, seen in figure on bottom.
Asphaltenes are molecules that are naturally produced in the earth’s subsurface. In general, these molecules contain polycyclic aromatic hydrocarbon cores with alkyl appendages of varying lengths and chemical moieties attached. Asphaltenes are known as the “cholesterol” of the petroleum industry, because the aggregation of asphaltenes contributes to costly flow assurance problems. We are exploring a variety of vibrational tags appropriate for reporting on solvent-solute interactions important in the asphaltene nanoaggregation process. In addition we are developing experimental approaches for describing the stacking interactions between asphaltene-like molecules. Asphaltenes are defined by their solubility characteristics in toluene and heptanes or hexanes. Thus, we are working towards probing the aggregated structure of asphaltenes, as well as, the solvent dynamics related to this system using 2D IR spectroscopy. Shown below is a 2D IR spectrum of violanthrone aggregates. The carbonyl and ring modes are vibrational probe for reporting the stacked structure of violanthrone.
Integration of Theoretical Calculations of 2D IR Spectra
To gain a more complete understanding of our spectroscopic results we are coupling theory with experiment by using Molecular Dynamics (MD) simulations and Quantum Mechanical calculations of spectra from structures in those simulations to elucidate the nature of our samples. MD simulations are used to generate a statistically weighted set of nanoaggregate configurations from which we can calculate relevant spectra. The potential mean force of two lumogen orange molecules dimerizing is shown below along with relevant structures for some of the more prominent features. We are working on developing a coupling model to describe asphaltene nanoaggregation which can later be generalized for various chemical systems.
Technology Development - Pulse Shaping
Two-dimensional infrared spectroscopy provides a wealth of knowledge regarding chemical systems. As such, it is important to apply 2D IR to new chemical systems, but it is equally important to continue the advancement of multidimensional infrared spectroscopy. We have added to efforts aimed at improving 2D IR spectroscopy measurements through the development of a mid-IR pulse shaper capable of shaping ultrafast mid-IR pulses with broad bandwidths. Our pulse-shaper is a 4f shaper based on utilizing a Germanium acousto-optic modulator (Ge-AOM); a drawing is shown below. Recently we developed a robust method to mitigate angular dispersion in our Germanium AOM-based pulse shaper. Our innovation affords transform-limited, Gaussian shaped, pulses and pulse-pairs to be produced with optical bandwidths greater than 1500 nm at the full width at half maximum of the mid-IR pulse.
Pore Forming Toxins
Pore forming toxins (PFTs) are the first line of offense for many organisms and are interesting molecules due to their conformational flexibility and their ability to self-assemble. We are interested in developing approaches to monitor the insertion and self-assembly of PFTs in lipid membranes. Currently, we are investigating the self-assembly of surfactin--a cyclic lipopeptide naturally secreted by Bacillus subtilis. A 3D rendering of surfactin is shown below (left). Surfactin is implicated in a variety of biological activities including cell signaling during biofilm formation and potency as an anti-viral, anti-microbial, and anti-tumor medication. However, the mechanisms by which surfactin interacts with lipid membranes is not fully understood. We use a tunable bicelle system as a model lipid bilayer whose characteristics can be controlled. We use linear IR and 2D IR spectroscopy to directly probe surfactin self-assembly in buffer and in bicelles. In buffer, surfactin self-assembles into micelles and higher order structures. In the presence of bicelles, surfactin interacts with the lipids in a non-destructive manner, but as the surfactin concentration is increased mixed micelles form. These interactions are depicted below (right).