Drop Coating Deposition Raman Spectroscopy of Human Tear Fluid

Recently there has been much interest in methods of enhancing the sensitivity of Raman spectroscopy. The most common method of enhancement is called surface-enhanced Raman spectroscopy (SERS) and can be achieved by having the analyte in close proximity to a roughened noble metal surface. SERS permits the detection of molecules in concentrations as low as parts per billion, but its effectiveness is very dependent on the active surface and can suffer from poor reliability and reproducibility, although the use of photonic crystal substrates is improving this area.

An alternative method of increasing the Raman signal from an analyte is by pre-concentration. One emerging area in analytical chemistry is to use the coffee-ring drying pattern to pre-concentrate weak protein solutions prior to Raman analysis, the so-called drop coating deposition Raman spectroscopy (DCDRS) technique. The coffee-ring pattern arises from a form of capillary flow in which pinning of the contact line of the drying drop ensures that solvent evaporating from the drop edge is replenished by liquid from the interior of the drop. This outward flow of solvent carries the solute material towards the edge of the drop. In DCD Raman spectroscopy, spectra are obtained from this ring of excess of material deposited after evaporation has completed.

Human tear fluid composition is strongly related to the continued health and proper functioning of the eye. A quantitative or qualitative change in tear composition can lead to dysfunction of the ocular surface, which may ultimately affect the transparency of the cornea. Variation in the biochemical constitution of tear fluid can be a symptom of disease as well as a cause.

Here at the Biophotonics Research Unit in Gloucester Royal Hospital we are investigating the use of DCDRS of tear fluid as an aid to the diagnosis of ocular infection. DCDRS allows us to obtain information about the tear protein composition very rapidly using only minute volumes of fluid. It is hoped that in the future we may be able to relate these protein (or spectral) changes to specific diseases of the eye. The references to our published articles on the subject of tear fluid can be found on the group publications pages.