Optical control of the interface between gold surface and blood cell samples

Abstract

The optical properties of blood (spectra of the extinction coefficient, k, refractive index, n, etc.) carry important diagnostic information and are usually monitored using bulk samples. In this work, attention is drawn to the interface between the blood volume and the surface of glass or thin gold films on it, where the refractive index may differ from the bulk one. We draw attention to the relationship between two effects – SPR and TIR. It is shown that if the named effects are measured for two different external media 0 and 1 with different refractive indices, then the values of the angles SPR and TIR will be linearly related by the empirical formula SPR1=SPR0+TIR1- TIR0)*K, where the coefficient K depends on the thickness of the transition layer di between the surface and the volume of the liquid medium (suspension). Numerical calculation of K (di) for gold films shows that K = 1.6 at di = 0 and monotonically decreases to 0.01 with an increase in di to 300 nm (and further to 0). Measurement of the angular dependences of reflection, R(), on (1) 100% hematocrit blood samples, (2) hemolyzed samples and (3) washed erythrocytes with dilutions with a buffer solution. It was shown that all samples exhibit a minimum SPR, but the TIR angle can be measured only for blood samples with destroyed membranes (hemolyzed), buffer solution and plasma. The n-value for hemolyzed blood is 1.3505, which is indicative of a low hemoglobin content in the sample. At the same time, di for a sample of 100% hematocrit was 60-105 nm, which indicates a strong deformation of erythrocytes in the form of polyhedrocytes and their dense packing after centrifugation. Washing the cells with a buffer increases di to 280 nm and more and practically eliminates blood cells from the SPR sensitivity region. The reason for this may be that in the blood of 100% hematocrit, erythrocytes are in the form of polyhedrocytes tightly adhering to the gold surface, while as a result of washing and diluting with a buffer solution, the cells relax back into discocytes. As a result, the containing hemoglobin erythrocyte cytoplasm moves away from the surface at a distance di> 300 nm into the suspension volume and leaves the area of the enhanced plasmon-polariton field.