Development of Atomic Force Microscope-based Photothermal Two-dimensional Infrared Spectroscopy

Research being performed at SUNY at Stony Brook is developing a chemical measurement tool that combines the high spatial resolution of atomic force microscopy (AFM) with the information-rich capability of two-dimensional infrared (2D-IR) spectroscopy. Infrared light identifies chemicals by probing molecular vibrations; 2D-IR spectroscopy further reveals how those vibrations couple and how energy moves among them. Conventional infrared methods, however, cannot focus light to spots smaller than several micrometers, blurring the spatial details of materials and biological structures that are far smaller. This project instead confines infrared light beneath a sharp metal-coated tip to a region only ten to twenty nanometers across, letting scientists study molecules at surfaces and interfaces previously inaccessible. Because the method has the potential to examine individual protein assemblies, it will offer capabilities relevant to biotechnology, including the study of protein misfolding that underlies diseases such as Alzheimer’s. The project also trains students for careers in the scientific-instrument industry, and the principal investigator will also carry the research into graduate courses and perform outreach to local K-12 schools. The technique being developed, AFM-based two-dimensional infrared spectroscopy (AFM-2DIR), will detect a sample’s photothermal response to a sequence of femtosecond mid-infrared pulses under the field enhancement of an AFM tip, extending nanoscale infrared microscopy, currently limited to linear absorption, into time-domain multidimensional spectroscopy. The work will pursue three aims: developing a numerical framework for photothermal 2D-IR signals based on propagation of the density matrix through the Liouville equation in Lindblad form, which would guide spectral interpretation and phase-cycling design; integrating femtosecond IR pulse shaping into AFM-2DIR, which would suppress timing jitter and shorten acquisition time to tens of minutes; and benchmarking on individual amyloid fibrils and on molecules adsorbed on nanostructures. By yielding sub-ensemble spectra free of inhomogeneous broadening from spatial averaging, AFM-2DIR will deliver molecular structure, coupling, and energy-transfer information with nanometer spatial precision, resolving structural dynamics without the constraint of the diffraction limit. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.