“The study highlights the utility of PTIR, especially O-PTIR, in overcoming prior analytical challenges of biological aerosol identification via vibrational microspectroscopic techniques.”
FULL PUBLICATION
Reporting in The Journal of Physical Chemistry A, researchers at the University of Michigan present the first application of optical-photothermal infrared spectroscopy (O-PTIR) and atomic force microscopy-PTIR (AFM-PTIR) to characterize individual primary biological aerosol particles (PBAPs) generated from a cyanobacterial harmful algal bloom.
The work addresses a critical analytical gap: while vibrational spectroscopy offers direct molecular information about biological aerosols, traditional IR and Raman methods cannot reliably characterize the submicron particles that dominate atmospheric transport and longest atmospheric residence times.
Bioaerosols—particles containing bacteria, fungi, pollen, and algae—impact public health through allergen and toxin exposure and influence climate by nucleating ice crystals and cloud droplets. Yet single-particle characterization has remained elusive due to technical barriers that few methods have overcome.
The researchers generated lake spray aerosol (LSA) from freshwater collected during a harmful algal bloom dominated by Planktothrix agardhii in Grand Lake St. Marys, Ohio. Using both O-PTIR and AFM-PTIR, they analyzed size-resolved particles collected across the atmospheric size range (0.18–3.2 micrometers).
O-PTIR operates by inducing photothermal expansion in IR-absorbing molecules and probing that expansion with elastic scattering from a visible laser—circumventing the IR diffraction limit while remaining completely insensitive to fluorescence that plagues conventional Raman.
AFM-PTIR achieves even higher spatial resolution (∼10 nanometers) by using the AFM cantilever tip to measure photothermal deflection. Both methods produced spectra with characteristic biological signatures: amide I vibrations (1630–1700 cm⁻¹) and amide II vibrations (1530–1560 cm⁻¹) consistent with established Fourier transform IR spectra of proteins.
Key findings demonstrate O-PTIR’s suitability for this application. Bioaerosol fractions increased with particle size: only 4% of smaller particles (0.56–1.0 micrometers) contained biological material when LSA was generated from raw water, compared to 19% of supermicron particles (1.8–3.2 micrometers). After cell lysis to release intracellular contents, the biological aerosol fraction on supermicron particles jumped to 59%, while microcystin (MC) toxin concentrations in the aerosol increased more than 10-fold—a quantitative relationship impossible to establish without molecular-level spectroscopic identification of individual particles. AFM-PTIR analysis of ultrafine particles (0.18–0.32 micrometers) revealed fewer particles with definitive biological signatures, supporting prior mass spectrometry findings that biological material concentrates in larger particles. The spectral diversity in the amide I region also allowed the team to infer protein secondary structure (α-helix, β-sheet, β-turns) across particles, demonstrating that O-PTIR provides compositional and structural information simultaneously.
This work validates O-PTIR and AFM-PTIR as complementary tools for atmospheric bioaerosol science. O-PTIR is particularly suited for characterizing ambient biological aerosol across the critical size range where particles remain airborne longest and pose direct health risks.
The elimination of fluorescence interference—the primary limitation that has confined Raman spectroscopy largely to non-biological aerosol—opens entirely new experimental possibilities. The approach also extends naturally to toxin detection, as demonstrated by the spectroscopic identification of microcystin signatures and the link between toxin aerosolization and cellular lysis. For researchers studying harmful algal bloom aerosols, lake spray emissions, sea spray bioaerosol, or any biological particle population where fluorescence has proven problematic, O-PTIR represents a decisive analytical advance.
Authors:
Jia H. Shi, Carlie J. Poworoznek, Rebecca L. Parham, Katherine R. Kolozsvari, Nicole E. Olson, Yao Xiao, Ziying Lei, Johnna A. Birbeck, Stephen J. Jacquemin, Judy A. Westrick, and Andrew P. Ault
