Overview
The vast majority of both natural and industrial interfaces possess substantial heterogeneity in their chemical structure, with variations in density, composition, conformational packing, and orientational ordering, etc. These variations can range from the microscopic to macroscopic scale, forming a complex hierarchical chemical structure which often defines the functional behaviour of the interface. The objective of our research in the Nonlinear Chemical Imaging Group is to elucidate the complex connection between this heterogeneous chemical structure at interfaces and their macroscopic behaviour and properties.
Projects
Heterogeneous Structure of Phospholipid Membranes
Phospholipids, as the major constituent of cell membranes, are at the heart of many physiological functions. The complex mixture of these molecules with different chemical structures can often result in their spontaneous phase separation to form 2D liquid crystal domains that differ in their physicochemical properties. In fact, the formation of these ‘lipid rafts’ are thought to be critically important in many cell signalling processes. Nevertheless, as these membranes represent mono- or bilayer films, characterising their complex hierarchical structure is a monumental challenge.
In this project, model phospholipid membranes are investigated using a recently developed azimuthal-scanning Sum-Frequency Generation (SFG) microscope to exploit its unique sensitivity to the different aspects of their structural heterogeneity, namely their chemical compositions, 3D molecular orientations and conformations, and their molecular-to-mesoscopic packing structures. Tuning the different makeup of these model membranes leads to vastly different phase structures that gives crucial insight into the thermodynamics underlying their formation and resulting macroscopic properties.
Imprinted Chirality and Enantioselective Packing in Cell Membranes
As an extension to the above project looking at the heterogeneity in molecular packing structure in model phospholipid monolayers, this project focusses on a particularly important aspect of this structure, namely chirality. By using Sum-Frequency Generation (SFG) microscopy to characterise the packing structure in these lipid films, any impact of the molecular chirality of the lipids can be investigated. For example, in many cases this molecular chirality is directly imprinted on the mesoscopic packing structure of the condensed lipid domains, and the interactions between different lipids in the membrane can be strongly enantioselective, altering the macroscopic phase structure.
Surface Phonon Polaritons (SPPs) in Hexagonal Boron Nitride Monolayers and Heterostructures
Surface Phonon Polaritons (SPPs) in polar dielectric materials are extremely promising for nanophotonic applications due to the potential tuneability in controlling light at the nanoscale. Hexagonal Boron Nitride (hBN) is a particularly important example of such materials due to its relatively light constituent atoms that mean it has one of the most energetic phonons that is easily accessible through table-top light sources. Furthermore, due to the intrinsic lack of centrosymmetry in monolayer hBN, it yields strong nonlinear responses. This allows the phonons to be easily spatially mapped at sub-diffractional resolution via upconversion to visible wavelengths.
In collaboration with the Paarmann and Thämer groups, this project utilises the strong nonlinear activity of monolayer hBN to spatially map its phonons using phase-resolved and azimuthally dependent Sum-Frequency Generation (SFG) microscopy. Through the rotation-dependent imaging, the precise crystal structure of the hBN crystals can be determined along with any change in their local symmetries. Furthermore, the hyperspectral images show any spatial variations in the localised phonon modes arising from the changing symmetries or crystal strain.
The 3D Structure of Water at Organo-Aqueous Interfaces
Organo-aqueous interfaces, consisting of a thin film of organic molecules atop an electrolyte solution, are the most common type of interface in the world, describing not only the vast oceanic surface, but also atmospheric aerosol droplets and physiological membranes. At these interfaces, it is not only the heterogeneous structure of the 2D organic film that determines their macroscopic properties, but also the sub-phase water which solvates the surface molecules. The sheer presence of this interface causes structural perturbations to the sub-phase water, altering the molecular orientations and H-bond connectivity. These perturbations gradually decay moving away from the interface to recover the bulk-like water structure, hence yielding a defined depth-dependent heterogeneity.
In this project, collaborating with the Thämer group, we deviate from the investigation of lateral structural heterogeneity that is common to many chemical imaging methods to focus on depth-dependent ‘imaging’ of the interfacial water structure. Specifically, we use the recently developed technique which combines phase-resolved Sum- and Difference-Frequency Generation (SFG and DFG) spectroscopies to achieve nanoscale depth resolution of the molecular structure at interfaces, thus generating a 3D picture of interfacial water. By varying the composition of the organic film to represent different interfaces, this project aims to probe the impact of water solvation on the interfacial behaviour.
Techniques and Development
In order to characterise the heterogeneous chemical structure in the above systems, we use a range of different nonlinear optical techniques.
Vibrational Sum-Frequency Generation (SFG) Microscopy
One of the main imaging techniques we employ is vibrational Sum-Frequency Generation (SFG) microscopy due to its unparalleled capabilities. By using a mid-IR beam, molecular vibrations and crystal phonons can be resonantly probed. Upconverting this resonant information to visible wavelengths enables sub-diffractional vibrational imaging. Additionally, as the output signals are dependent on the unique symmetry properties of the second-order susceptibility, the technique not only achieves molecular recognition by accessing the vibrational spectrum, but is also sensitive to absolute molecular directions. This gives it the unique possibility to determine 3D molecular orientations, the amount of orientational order, and specific conformational structures, in addition to compositional information.
The main challenges with SFG microscopy, however, are its poor conversion efficiency of the second-order optical processes and the highly complicated nature of the obtained signals. Through recent technical advances, however, SFG microscopy has been shown to achieve sufficient sensitivity to spatially characterise variations in all of the above structural properties in monomolecular films. Furthermore, by implementing an azimuthal-scanning methodology into such an SFG microscope, the highly complex signals can be deconvoluted via Fourier decomposition, yielding high-level insights into the molecular structure.
Technical Developments
While many of our projects aim to utilise the significant elucidative potential that is already possible with the above-mentioned techniques, we are also always looking to further our capabilities in chemical imaging and extract more insights from the obtained structural information. Therefore, a significant focus of our group is also the continued advancement of these existing methods and the introduction of new techniques that can complement our projects and investigations.