Microscopia a fluorescenza a riflessione interna totale

When an excitation laser hits a glass coverslip at a critical angle, total internal reflection occurs instead of light passing through. The reflected light creates a narrow electromagnetic field called an evanescent wave at the glass-sample interface. This wave only penetrates 100 to 200 nanometers into the sample before decaying, selectively exciting fluorophores near the cell surface while leaving background fluorophores unexcited.

Prism-based TIRF uses a prism placed on the coverslip surface to direct the evanescent wave into the sample. Objective-based TIRF eliminates the prism; instead, the objective lens itself acts as both the light source and creates the evanescent wave at the coverslip interface. Both approaches achieve the same selective surface imaging but differ in optical configuration and experimental setup.

TIRF produces sharp images because the evanescent wave only illuminates fluorophores within 100 to 200 nanometers of the cell surface. Background fluorophores deeper in the sample are not excited, eliminating out-of-focus blur and interference. This selective surface illumination combined with rapid evanescent wave decay results in high-contrast, clear images of proteins and structures near the solid surface.

TIRF offers multiple benefits for live-cell studies: it minimizes photobleaching because cells are not exposed to intense direct illumination, reducing phototoxicity and cell damage. The technique selectively images surface-bound proteins and structures without exciting background fluorophores. These advantages make TIRF ideal for studying protein dynamics in living cells, such as motor protein movement over microtubules attached to the coverslip.

TIRF depends on a refractive index difference between the coverslip and the sample. Light traveling from a lower refractive index medium (air or sample) toward a higher refractive index surface (glass coverslip) undergoes total internal reflection at the critical angle. This refractive index mismatch is essential for creating the evanescent wave that selectively illuminates only the cell surface region.

The evanescent wave rapidly decays as it penetrates deeper into the sample, with significant intensity loss beyond 100 to 200 nanometers. This decay ensures that only fluorophores and structures very close to the coverslip surface are excited. Consequently, TIRF is optimized for imaging cell-surface interactions, membrane proteins, and adhesion structures while excluding deeper cellular components.

TIRF works best with samples mounted on solid surfaces with higher refractive indices, such as glass coverslips. It is ideal for studying cells or structures in direct contact with the coverslip, particularly live cells adhered to glass. TIRF excels at visualizing membrane dynamics, focal adhesions, and protein interactions at the cell-substrate interface where fluorophore-tagged molecules are concentrated near the surface.