Optimized Substrates for Analysis of an Individual Atmospheric Particle Across Multiple Microspectroscopy Techniques: Fabrication and Characterization

Measurements of atmospheric particles are analytically challenging as the most important sizes (10-10,000 nm) contain between attograms and nanograms of material. However, measurements of individual particles are critical as particle-to-particle heterogeneity is a major factor in the uncertainties associated with aerosol impacts on global climate and health. Applying microspectroscopy techniques to atmospheric aerosols has provided unique insights into single-particle chemical composition and physical properties, including elemental composition from scanning electron microscopy coupled to energy-dispersive X-ray (SEM-EDX) spectroscopy, functional group information from Raman and photothermal infrared (PTIR) microspectroscopies, and viscosity and morphology information from atomic force microscopy (AFM). However, optimal substrates for one method often lead to substantial interferences when used with other methods, meaning that an individual sample cannot be effectively run on multiple instruments. Herein, we demonstrate the capabilities of a new substrate that overcomes these interferences to enable the analysis of an individual particle with SEM-EDX, Raman, optical-PTIR (O-PTIR), and AFM-PTIR. We fabricated this substrate with a gridded, 500 nm nickel coating on a silicon wafer, eliminating the Raman mode of silicon ∼1000 cm^-1, shifting EDX signal into a quiet region of the X-ray energy spectrum, while not adding PTIR signal in the fingerprint or C-H stretching regions. Laboratory-generated sodium sulfate, ammonium sulfate, ammonium nitrate, and sucrose particles, as well as ambient atmospheric particles were deposited on the newly fabricated substrates and analyzed to demonstrate colocalization capabilities across microspectroscopic methods. This approach also establishes a template for optimizing substrates for use in other fields requiring micro- and nanoscale analyses.