Bile acid measurements using a cholestyramine-coated TSM acoustic wave sensor

A novel chemical sensor for bile acids is described. A 10-MHz piezoelectric crystal operating in the thickness-shear mode (TSM) is coated on one side with cholestyramine resin and mounted in a batch-mode sensor block which exposes the coating to solution. After sample injection, the binding process is observed in real time as a drop in frequency as the bile salt binds to the coating, reaching > 90% completion within 10 min with most of the binding occurring within the first minute. Linear calibration curves are generated with sensitivity increasing in the order cholate approximately glycocholate < taurocholate < < taurodeoxycholate. Detection limits in water are in the range 0.2-9 nmol and are better than those observed in phosphate buffer. A multistep regeneration protocol allows the coating to be reused more than 400 times over a period of several months. Precision for replicate injections is approximately 10% RSD and depends on the reproducibility of the regeneration and injection steps. In terms of the binding process, hydrophobic interactions are observed to be of importance in the ability of bile salts to displace other counterions. However, anions with greater charge density also appear to compete effectively for binding sites on the resin. In particular, at equimolar concentrations of citrate and bile salt, the trivalent citrate anion reduces the amount of bile salt binding by approximately 40%. This suggests that the efficiency of cholestyramine-based bile salt sequestering drugs used in the reduction of hypercholesterolemia may be improved by eliminating citric acid as an excipient and avoiding the use of fruit juices during ingestion.