Optical biosensors for cell adhesion

Abstract

Planar optical waveguides offer an ideal substratum for cells on which to reside. The materials from which the waveguides are made—high refractive index transparent dielectrics—correspond to the coatings of medical implants (e.g., the oxides of niobium, tantalum, and titanium) or the high molecular weight polymers used for culture flasks (e.g., polystyrene). The waveguides can furthermore be modified both chemically and morphologically while retaining their full capability for generating an evanescent optical field that has its greatest strength at the interface between the solid substratum and the liquid phase with which it is invariably in contact (i.e., the culture medium bathing the cells), decaying exponentially perpendicular to the interface at a rate controllable by varying the material parameters of the waveguide. Analysis of the perturbation of the evanescent field by the presence of living cells within it enables their size, number density, shape, refractive index (linked to their constitution) and so forth to be determined, the number of parameters depending on the number of waveguide lightmodes analyzed. No labeling of any kind is necessary, and convenient measurement setups are fully compatible with maintaining the cells in their usual environment. If the temporal evolution of the perturbation is analyzed, even more information can be obtained, such as the amount of material (microexudate) secreted by the cell while residing on the surface. Separation of parallel effects simultaneously contributing to the perturbation of the evanescent field can be accomplished by analysis of coupling peak shape when a grating coupler is used to measure the propagation constants of the waveguide lightmodes.

Keywords::

• Evanescent field
• grating coupling
• microexudate
• peak shape
• waveguide

Acknowledgements

Declaration of interest: The authors alone are responsible for the content and writing of the paper.

Notes

¹ The actual situation is a little more complicated. Ref. (5) should be consulted for details.

² Similarly, in a hydrodynamically stagnant situation, a cell will sediment and touch the floor of the vessel in which it is initially suspended regardless of any repulsive molecular interaction between cell and floor.

³ Total internal reflexion has also been exploited in novel ways of microscopically imaging cell-substratum interactions (e.g., (17)).

⁴ The phenomenon is in some ways similar to that of surface plasmon resonance, in which collective excitations of the conduction electrons (a cold electron plasma) in a thin noble metal film are excited at optical frequencies, typically using an experimental configuration similar to that used with the optical waveguides. However, the high imaginary part of the complex dielectric constant of the metal causes the excitations to be rapidly damped (within a distance of the order of 1 μm) and, hence, the signal/noise ratio is at least in order of magnitude worse than that achievable using a regular optical waveguide. Furthermore, the need to have a noble metal substratum severely limits the range of applicability. Moreover, as discussed later, optical waveguides can support several modes (in contrast to the single surface plasmon mode), greatly increasing the amount of information obtainable from the experiments.

⁵ Called “reverse symmetry” waveguides in some earlier literature.

6 See (22) for original references. At the time of writing this review, to our knowledge no measurements with living cells using interferometry have been reported. Often the waveguide geometry is unfavorable; e.g., too thin to underlie the entire cell, and excessive scattering engendered by the long waveguides used to increase sensitivity.

⁷ Under these measurement conditions the actual arrival of the cells at the substratum surface by sedimentation takes a few minutes and causes a negligible increase in effective refractive index, as was verified by adding dead cells.