An acoustic wave biosensor for human low-density lipoprotein particles: construction of selective coatings

Cholesterol is found in four major classes of blood particles including chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). The most studied fraction is LDL as it is most closely associated with heart disease. The challenge in current methods of analysis is the determination of the cholesterol in the individual lipoprotein fractions. Accordingly, the critical step in any analysis is the complete separation of the lipoprotein fractions. In this work, enhanced selectivity for the LDL fraction was achieved by the covalent binding of dextran sulfate (DS) to the gold surface of a thickness shear-mode acoustic wave sensor. The thickness and surface concentration of the DS layer was estimated by in situ ellipsometry to be 219 A and 0.8 ng/mm(2), respectively, but it was difficult to construct the sensing layer reproducibly. The DS coated sensor was ten times more responsive to LDL than the other lipoprotein (LP) fractions. The sensor was a main component in a flow injection analysis system that exposed LDL, VLDL and HDL to not only the DS layer, but also to the underlayers used in the construction of the DS layer. A possible regeneration solution was found which would rinse the LDL from the layer, restoring the sensor for repeated use. Frequency shifts from LP absorption into the DS layer were corrected for dissipative losses through the DS layer using an oscillator circuit equipped with an automatic gain control feature.