Our group applies several state-of-the-art single particle micro-spectroscopic and chemical imaging analyses of aerosol particles to infer chemical composition, morphology, and mixing state:

SoMAS houses the Nano Raman Molecular Imaging Laboratory, NARMIL. The Daniel Knopf is co-director of the nano-Raman facility founded in 2014 by an NSF MRI grant (PI, G. Taylor, co-PI D. Knopf). This facility houses a Renishaw inVia High Resolution Confocal Raman Microspectrometer (CRM) coupled to a Bruker Innova atomic force microscope allowing for tip enhanced Raman scattering measurements (TERS). The CRM is equipped with 3 lasers (457/514, 633, and 785 nm) and 600, 1200, and 1800 l/mm spectrographic gratings and appropriate filters with > 30% optical throughput achieving a spectral resolution of 0.5 cm^-1 @ 680 nm using the 1800 l/mm grating. The main sampling platform is a Leica DM2500 upright microscope with computer-controlled motorized stage for XYZ mapping with <0.1 um step-size; 10x, 20x, 40x, 50x, 50xLWD, 63x and 100x objective lenses. The instrument possesses a streamline spectrograph upgrade for rapid 2D and 3D Raman scans. Raman spectra in mapping mode can be recorded with <0.1 μm step-size. CRM can be operated in transmitted, reflected, bright field, dark field, epifluorescent, and Raman chemometric modes including DAPI, FITC, CY3, and CY5 fluorescent filter and 150 mW Hg lamp for epifluorescence. Additonally, it is equipped with Luminera Infinity 3 TE cooled camera for high-res video images and Renishaw camera for laser focusing. A Linkam THMS600 Hot/Cold stage (-196 to +600°C) with T95 controller, enabling programmable local and remote control is attached to the CRM. The AFM features stacked piezo XY and Z scanners with resolutions as high as <0.02 and <0.01 nm, respectively, an optical AFM head for contact, tapping, lateral force, and phase imaging AFM and scanning tunneling microscopy (STM). Furthermore, it is equipped with an optical AFM head for co-localization and tip-enhanced Raman scattering (TERS), improving Raman spatial resolution to <20 nm in XY and Raman scattering efficiency by >1,000-fold. Lastly, it is equipped with a special AFM-TERS package, i.e. tuning fork cartridge and Au-wire probes for non-conducting samples (biologicals and organic polymers). This AFM features co-location with confocal Raman micro-spectrometer. The facility allows Raman spectroscopy (composition analysis) in parallel to atomic force microscopy (physical state/morphology) measurements.

Confocal Raman spectroscopy allows monitoring the change in chemical composition in aqueous droplets as a function of temperature. Left hand side shows the spectroscopic changes in an aqueous sulfuric acid water droplet as it is cooled until ice nucleates homogeneously at 228 K (Knopf et al. 2003).

On SBU Campus several facilities are available to our research including scanning electron microscopy (SEM) with energy-dispersive analysis of X-rays (EDX) in the Department of Materials Science and Chemical Engineering and the Nuclear Magnetic Resonance Facility in the Department of Chemistry.

Off campus, the Knopf group are long term users at several national user facilities such as the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL) and the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL). One of our main investigation tools at EMSL is the computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) that allows us to probe the elemental composition and morphology  of thousands of individual particles on the nanoscale to assess the nature of the ambient particle population (see, e.g., Wang et al. 2012, Knopf et al. 2014, Laskin et al. 2016). Subsequently, cluster analysis determines independent particle-type classes.

Figure above gives an example of the application of CCSEM/EDX to identify particle-type classes and identification of ice-nucleating particles (taken from Knopf et al. 2014).

The ALS serves as a source of energy defined X-rays necessary to conduct scanning transmission X-ray microscopy with near-edge X-ray absorption
fine structure spectroscopy (STXM/NEXAFS). STXM measures the transmission of soft X-ray beams generated from the synchrotron light source across a
raster-scanned sample at a given photon energy to obtain an image. Spatially resolved X-ray spectra yield chemical composition and mixing states of individual particles. The spatial resolution is about 25 nm. Exploiting the carbon K-edge spectra allows the identification of organic carbon (OC), elemental carbon (EC, i.e., soot), potassium (K), and other overall contribution of inorganic components (INO) within individual particles. This technique allows us to speciate the organic matter and infer the inorganic-organic mixing state.

Figure above shows an example of STXM/NEXAFS application. The composition of an ambient ice-nucleating particle is shown (Knopf et al. 2014). In addition, a high resolution NEXAFS spectrum of the organic matter associated with fresh sea spray aerosol is depicted. The false color image to the right displays the cubic shape of solid NaCl crystals embedded in the surrounding organic matrix (unpublished data).