title: The importance of correcting for variable probe-sample interactions in AFM-IR spectroscopy: AFM-IR of dried bacteria on a polyurethane film.
abstract: AFM-IR is a combined atomic force microscopy-infrared spectroscopy method that shows promise for nanoscale chemical characterization of biological-materials interactions. In an effort to apply this method to quantitatively probe mechanisms of microbiologically induced polyurethane degradation, we have investigated monolayer clusters of ∼200 nm thick Pseudomonas protegens Pf-5 bacteria (Pf) on a 300 nm thick polyether-polyurethane (PU) film. Here, the impact of the different biological and polymer mechanical properties on the thermomechanical AFM-IR detection mechanism was first assessed without the additional complication of polymer degradation. AFM-IR spectra of Pf and PU were compared with FTIR and showed good agreement. Local AFM-IR spectra of Pf on PU (Pf-PU) exhibited bands from both constituents, showing that AFM-IR is sensitive to chemical composition both at and below the surface. One distinct difference in local AFM-IR spectra on Pf-PU was an anomalous ∼4× increase in IR peak intensities for the probe in contact with Pf versus PU. This was attributed to differences in probe-sample interactions. In particular, significantly higher cantilever damping was observed for probe contact with PU, with a ∼10× smaller Q factor. AFM-IR chemical mapping at single wavelengths was also affected. We demonstrate ratioing of mapping data for chemical analysis as a simple method to cancel the extreme effects of the variable probe-sample interactions.
authors.affiliation: Chemistry Division, Naval Research Lab, Washington, DC 20375, USA. daniel.barlow@nrl.navy.mil.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Barlow
authors.affiliation: Chemistry Division, Naval Research Lab, Washington, DC 20375, USA. daniel.barlow@nrl.navy.mil.
authors.forename: Justin C
authors.initials: JC
authors.lastname: Biffinger
authors.affiliation: National Research Council Post-doctoral Research Associate, Washington, DC 20001, USA.
authors.forename: Allison L
authors.initials: AL
authors.lastname: Cockrell-Zugell
authors.affiliation: Anasys Instruments, Inc., Santa Barbara, CA, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Lo
authors.affiliation: Anasys Instruments, Inc., Santa Barbara, CA, USA.
authors.forename: Kevin
authors.initials: K
authors.lastname: Kjoller
authors.affiliation: Anasys Instruments, Inc., Santa Barbara, CA, USA.
authors.forename: Debra
authors.initials: D
authors.lastname: Cook
authors.affiliation: Chemistry Division, Naval Research Lab, Washington, DC 20375, USA. daniel.barlow@nrl.navy.mil.
authors.forename: Woo Kyung
authors.initials: WK
authors.lastname: Lee
authors.affiliation: Chemistry Division, Naval Research Lab, Washington, DC 20375, USA. daniel.barlow@nrl.navy.mil.
authors.forename: Pehr E
authors.initials: PE
authors.lastname: Pehrsson
authors.affiliation: Soft Matter Materials Branch, Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA.
authors.forename: Wendy J
authors.initials: WJ
authors.lastname: Crookes-Goodson
authors.affiliation: Soft Matter Materials Branch, Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA.
authors.forename: Chia-Suei
authors.initials: CS
authors.lastname: Hung
authors.affiliation: Soft Matter Materials Branch, Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA.
authors.forename: Lloyd J
authors.initials: LJ
authors.lastname: Nadeau
authors.affiliation: Chemistry Division, Naval Research Lab, Washington, DC 20375, USA. daniel.barlow@nrl.navy.mil.
authors.forename: John N
authors.initials: JN
authors.lastname: Russell
mesh_terms: D018625:Microscopy, Atomic Force; D011108:Polymers; D011140:Polyurethanes; D011549:Pseudomonas; D013055:Spectrophotometry, Infrared
chemical_list: D011108:Polymers; D011140:Polyurethanes