Cause of incomplete QPI cell-border contrast

Determine the underlying cause of the incomplete and inconsistent cell-border contrast observed in quantitative-phase-imaging-based mid-infrared photothermal images of SYTO 9-stained A549 human lung adenocarcinoma cells suspended in aqueous buffer.

Background

The paper compares scattering-, fluorescence-, and quantitative-phase-imaging (QPI)-based readout mechanisms in a switchable wide-field mid-infrared photothermal microscope. In imaging fixed A549 cells suspended in buffer at 1240 cm⁻¹, scattering and QPI generally revealed photothermal contrast across the cell body and at the cell border, whereas fluorescence contrast was concentrated mainly in the nucleus because SYTO 9 labels nucleic acids.

Although QPI detected both nuclear and cell-border contrast, portions of the cell border were not consistently visible. The authors explicitly state that the cause of this incomplete border contrast is not yet clear, leaving its origin unresolved.