Liquid-Phase PFIR and Total Internal Reflection PFIR
Aim: Infrared Nanoscopy in the Aqueous Phase
One limitation of traditional infrared microscopy, aside from the restriction of spatial resolution from Abbe's diffraction limit, is its application in the aqueous phase. Water strongly absorbs mid-IR radiation, making the optical delivery difficult. Water absorption also strongly attenuates optical detection after light-matter interactions, reducing IR signals. Infrared nanoscopy in aqueous has been a challenge.
The action-based PFIR microscopy has several advantages in fluid operation: spatial resolution is not limited by the diffraction limit; mechanical detection of photothermal effect bypasses the transmission attenuation after light-matter interactions; the peak force tapping mode has been proven to operate well in the fluid phase; time-gated signal detection scheme is not drastically affected by the reduction of the cantilever quality factor. All these advantages make PFIR a promising method for aqueous phase nano-IR microscopy.
How to achieve liquid-phase PFIR? A necessary route is to implement a total internal reflection geometry for beam delivery through the evanescent field, to develop the PFIR in the total internal reflection configuration.
Total Internal Reflection PFIR Configuration
Total internal reflection geometry was widely used for attenuated total reflection Fourier transformation IR spectroscopy (ATR-FTIR). The total internal reflection geometry was also used in some of the early contact mode AFM-IR experiments. When light undergoes total internal reflection, an evanescent field is established at the interface between two regions of different refractive indices. The evanescent field is of the same frequency as the incident light field and is capable of exciting molecules within its range. Our group has implemented the total internal reflection (TIR) geometry with the photothermal PFIR detection mechanism, which results in the TIR-PFIR microscopy, shown below.
In the TIR-PFIR setup, IR laser pulses are guided and focused through an IR transmitting prism (e.g., Germanium prism). The geometry of the prism allows for the beam to undergo total internal reflection at the region underneath the AFM tip and establishes the non-propagating IR field. Metal coating of the AFM tip enhances the IR field through forming a gap, with nanometer size lateral confinement.
Molecules within the strong field confinement absorb the IR energy and convert it into heat for photothermal expansion, which is measured by the AFM cantilever with the PFIR-type signal extraction procedure. As the IR absorption is molecule-specific, chemical imaging can be performed with the TIR-PFIR geometry. The details of the construction of the TIR-PFIR microscope is described in our paper. Analytical Chemistry, 93, 7, 3567-3575 (2021)
Liquid-phase PFIR Microscopy
The liquid-phase PFIR microscopy is based on the TIR-PFIR configuration described above. A miniature fluid chamber or cell is used to enclose the AFM tip and sample to the TIR prism. The cell allows for changing the fluid type or control of the fluid temperature. The evanescent field from the total internal reflection excites the molecules underneath the metallic AFM tip and the photothermal signal is recovered using the PFIR-type signal detection mechanism through time-gated detection. The research on liquid-phase PFIR microscopy is sponsored by the Beckman Foundation.
Demonstrations and Applications of Liquid-phase PFIR microscopy
Demonstration of nano-imaging and spectroscopy with liquid phase PFIR on polymer blend of PS/PMMA in fluid phase
Liquid phase PFIR imaging of protein fibrils in water and heavy water. The spatial resolution is demonstrated to be ~ 10 nm.