Infrared chemical imaging beyond the diffraction limit using fluorescence to transduce absorbance

Fluorescence-detected photothermal infrared (F-PTIR) microscopy is shown to support infrared absorption imaging with a resolution dictated by fluorescence. Rapid vibrational relaxation following IR absorption produces local heating. This photothermal response can be sensitively detected from the change in fluorescence intensity with temperature, encoding IR absorption within modulation in fluorescence. Increases in temperature generally result in reductions in fluorescence intensity, as additional nonradiative relaxation pathways become more energetically accessible.

Following assessment of key figures of merit for F-PTIR, applications will be presented using F-PTIR to probe local composition within pharmaceutically relevant materials. Intrinsic fluorescence native to many active pharmaceutical ingredients (APIs) supports label-free imaging. Single-particle spectroscopy is shown to allow detection of trace API crystal polymorphs at ppm levels, with direct relevance for production and manufacturing. Chemical imaging is also shown to enable characterization of liquid/liquid (or amorphous/amorphous) phase separation in model amorphous solid dispersions, providing physical and chemical insights into critical phenomena in dissolution studies. A critical discussion of strengths and limitations of F-PTIR will be presented along with possible future directions.