General Areas of Research:
- Nonlinear optical sum frequency, second harmonic generation
- Linear vibrational spectroscopies – Raman, IR reflection
- Surface Potential measurement and methodology
- Liquid surface imaging
- Interfacial structure, dynamics, and chemistry
- Water Surfaces
- Ion pairing and hydration at surfaces
- Surfactant science
- Lipid – binding mechanisms and ion recognition
- 2D molecular organization
- Electric fields at liquid surfaces
- Physical chemistry of liquid interfaces
- Atmospheric aerosol and ocean surface proxy systems
- Biomembranes, hydration, and organization
- Instrument development – US Patent App. 16/085,21 ; US Patent App. 15/664,425; Pending Patent App. 61/857,511
1. Water
Water plays a key role in many chemical, biological, and environmental (atmospheric and geochemical) processes. We are interested in the surface propensity of ions and ion pairing at the aqueous surface, how water mediates this propensity, as well as lipid organization and the intermolecular interactions that drive this organization. To obtain molecular-level organizational surface information, nonlinear vibrational sum frequency generation (VSFG), reflection absorption infrared spectroscopy (IRRAS), Raman, second harmonic generation (SHG), and surface potential measurements are utilized and developed. Water research permeates all of the research areas in the Allen lab.
2. N2O4 and N2O5 Charge Stabilization at Liquid Surfaces to Understand NOx Reactivity on Marine and Atmospheric Aerosol Surfaces – Climate Change and Atmospheric Chemistry Relevance
3. Microplastics and Climate Change (Hoover Foundation)
4. Lipid and Fatty Acid Binding to Understand Sea Surface Microlayer Enrichment Factors
Sea surface microlayer and sea spray proxy system research is conducted for understanding enrichment of ions and molecules transported from sea to air through aerosol production. This project is about chemical complexity and key issues for climate change, and is a high impact project that extends the state of current knowledge. We currently collaborate with many universities worldwide.
5. Controlling Aqueous Interfacial Phenomena of Redox Active Ions
Redox ions are an important class of ions in which their multivalent states provide the potential for significant interfacial impact, with and without an externally applied field. There is a critical need to develop a thorough understanding of interfacial hydration of redox ions and solvent organization in externally applied fields, including the effect of ion charge, ion speciation, and the effect of co-solutes. In the absence of such knowledge, opportunities to improve new energy platforms are stunted and narrowly restricted thereby reducing the speed of such developments that might improve our energy infrastructure. Furthering the understanding of geochemical phenomena including mineral dissolution is also a potential outcome of this work.
The organization of, and induced by, redox ions Fe(II) and Fe(III) and externally applied electric fields at the hydrophobic air /water interface is being explored in our laboratory. Surface acidity, co-anion perturbation, ionic strength, electric field and electrode polarity effects, as well as the associated perturbations to hydration and organization to these redox ion interfacial systems are investigated. For these studies, we employ vibrational sum frequency generation spectroscopy, glancing angle Raman spectroscopy, surface tension and surface potential instrumentation. Expected outcomes of this project include instrumentation development and advances in surface-sensitive spectroscopy.
6. Surfactant Science and Biological Membrane Mimics
We study a myriad of fundamental phenomena relevant to surfactants, organization, binding and intermolecular interactions to better understand the driving forces behind organization and binding at aqueous and organic interfaces. Studies span applications such as surfactants, aerosols, biological membrane mimics to ocean and fresh water lakes and water bodies. an example of the biomembrane mimic focuses on pulmonary lung surfactant (PS). PS is a surface active lipoprotein formed by alveolar type II cells. PS adsorbs to the air-water interface of the alveoli with the head group (hydrophilic) facing the water, and the tail (hydrophobic) facing the air; hence the reason why some lipids have the ability to decrease the surface tension of water. The main component of PS is the lipid dipalmitoylphosphatidylcholine (DPPC), which can reduce the surface tension of water to near zero values. Deficiencies or inactivation of these lipids at the air-water interface can lead to clinically significant diseases such as Respiratory Distress Syndrome (RDS) and Acute Respiratory Distress Syndrome (ARDS). Our lab interrogates lipid monolayers, submonolayers, and in complex subphases to understand the permeability and electric fields at such intefaces to shed light on driving factors of organization and function.
7. Molecular Recognition in Aqueous Environments
We now have advanced to molecular and, in general, ion recognition with collaborations with Amar Flood at Indiana University and more recently, collaboration with Jovica Badjic at Ohio State. Proteins for example exploit microenvironments of low dielectric constant and partial dehydration. Yet, such biologically inspired environments have rarely been examined in artificial receptors. There is a critical need to understand and discover new recognition modes at bioinspired receptor-water interfaces.
8. Machine Learning applied to Problems in Identifying Molecules in Complex Environments such as in the Sea Surface Microlayer
Dr. Allen’s lab through graduate students Abbie Enders and Nicole North on machine learning methods to identify molecules or molecular classes using machine learning tools. This work focused on the application of machine learning to ocean samples and the sea surface micro and nanolayer. (updated 1/2023). Dr. Allen is not recruiting at this time for machine learning projects.
9. Cancer Detection
Through IR Medtek, LLC, Dr. Allen continues to work with cancer-related projects for developing methods of cancer diagnostics using mid-IR technology (see IR Medtek LLC). However, Dr. Allen is not recruiting through OSU at this time for the cancer projects.