Lab on a tip: atomic force microscopy - photothermal infrared spectroscopy of atmospherically relevant organic/inorganic aerosol particles in the nanometer to micrometer size range.

New developments in nanoscale analytical techniques have paved the way for detailed spectroscopic and microscopic measurements of substrate-deposited aerosol particles on a single particle basis. Atomic force microscopy based photothermal infrared (AFM-PTIR) spectroscopy is a technique that combines the nanometer spatial resolution of AFM with the chemical analysis capabilities of vibrational IR spectroscopy. Herein we demonstrate the capability of AFM-PTIR to investigate single and multi-component systems comprised of inorganic salts and organic compounds relevant to the atmosphere. Chemical and microscopic characterization of individual particles as small as 50 nm in diameter is shown. Moreover, single particle spectro-microscopic characterization as a function of relative humidity using this technique is shown for the first time. These new measurements as a function of relative humidity allow for the simultaneous and independent acquisition of photothermal IR spectra, contact resonance frequency shifts, and water uptake growth factors, providing insight on changes in the composition, stiffness, and size of the particles, respectively. These results lay the foundation for more detailed AFM-PTIR studies of multicomponent aerosol particles under a range of environmental conditions.