Scatter-Free IR Absorption Spectra of Individual, 3–5 μm, Airborne Dust Particles Using Plasmonic Metal Microarrays: A Library of 63 Spectra

Abstract

While the concentration of airborne particulate matter is well-known to be correlated with people's health, a chemical evaluation must also be important. Noting that airborne dust particles in the 3À5 μm size regime are among the largest that get into people's lungs, such particles were collected by pumping air through plasmonic metal films with a 12.6 μm square lattice of 5 μm square holes. Capture of a dust particle in a metallic hole enables the recording of "scatter-free" infrared absorption spectra whose peaks reveal the infrared active components. The study of the spectra of individual particles allows minority components to be characterized in a way that is quite incisive which is difficult with bulk samples. A library of 63 spectra of individual particles captured from our laboratory air is presented along with a preliminary analysis of the contributing components.

Dust Library of Plasmonically Enhanced Infrared Spectra of Individual Respirable Particles

“…These were found to be the dominant components of the lab air dust. 12 The complex index of refraction of each calibrant is the square root of the complex permittivity where the index j is over the vibrations of the material, A j is the strength of a vibrational transition, ν~0,j is the position of a transition in wavenumbers, and Γj is the half-width at half the maximum of the vibrational feature. Given the vibrationally mediated index of refraction parameters and a particle radius of a calibrant, Mie theory can predict the IR spectrum (extinction or absorption) of a spherically averaged calibrant particle with standard formulas.…”

“…The experimental setup for recording single particle spectra of wavelength scale particles has been described in detail elsewhere. [10][11][12]15,34 The particles are trapped in a metal film with an array of microholes, plasmonic Ni mesh, 10,12,15 having a 12.6 mm square lattice parameter, 5 mm square hole size, and $2 mm thickness as available from Precision Eforming (Cortland, NY; http://www.preci sioneforming.com). Briefly, dust sample powder is placed over the mesh and air is sucked through it using a pump that traps globular particles having widths that are slightly smaller than the mesh holes.…”

“…Given the known symptoms of inhaling high levels of particulate matter and the risks associated with long-term exposure to elevated levels, there is a need to chemically evaluate such particles. 10 Our previous single particle work [10][11][12][13][14][15] includes the study of a spectral library of 63 individual dust particles extracted from our laboratory air 12 demonstrating chemical variability including minerals, sulfates, nitrates, and organics, as well as a model for analysis of volume fractions. 10 In this work, the study is extended to samples obtained from a home air filter and the 11 September 2001 World Trade Center (WTC) event.…”

“…Fourier transform infrared (FTIR) spectroscopy offers molecular characterization of chemical functional groups in individual dust particle samples, 12,[16][17][18][19][20] including quantitative analysis, 10 which compliments the more numerous studies characterizing atoms. 2,[21][22][23][24][25] But to quantitatively study subwavelength scale dust particles, infrared (IR) spectroscopy has challenges: 14 (1) vibrational peaks of wavelength-sized particles are dominated by Mie scattering effects that can be ten times larger; (2) vibrational lineshapes are distorted by dispersive scattering contributions, the Christiansen effect; 26,27 and (3) insignificant intensity of light is transmitted to the detector through apertures as the particle falls in the micrometer-scale size range, i.e., it is smaller than the probing wavelength and detector element.…”

Plasmonics under the Infrared Microscope: Sensing and Spectra of Single Particles Using Metal Film Hole Arrays

“…We think of the mesh as an inside out fiber optic [19]. As a result, the light is made to sample subwavelength spaces like the holes in the mesh, which turns out to be useful for taking scatter-free spectra of individual, subwavelength-sized particles [20][21][22]. We will also show how the addition of an aperture to an IR microscope system allows the observation of narrow plasmonic resonances in spite of the large range of angles employed by a microscope.…”