Voltammetric detection of heparin at polarized blood plasma/1,2-dichloroethane interfaces

Heparin, a highly negatively charged polysaccharide, which has been used widely as an anticoagulant and antithrombotic, was detected by ion-transfer voltammetry at the interface between 1,2-dichloroethane and an aqueous buffer solution or undiluted blood plasma. Quaternary ammoniums with different numbers of methyl and long alkyl groups were examined as positively charged heparin ionophores using pipet electrodes filled with the organic electrolyte solutions of their tetrakis(4-chlorophenyl)borate salts. It was shown that octadecyltrimethylammonium most selectively facilitates interfacial heparin adsorption without interference from potential-dependent ionophore transfer into the aqueous phase. Water-filled pipet electrodes were also used to study the stoichiometry of the interfacial complex between a heparin molecule and multiple ionophore molecules, which is discussed as a counterion condensation effect. Stripping voltammetry based on facilitated heparin adsorption and desorption gives a detection limit of 0.012 unit/mL in 0.12 M NaCl buffered at pH 7.2, which is 1 order of magnitude lower than therapeutic heparin concentrations (>0.2 unit/mL) and is comparable to a detection limit of the most sensitive heparin sensor reported so far. The biomedical utility of ion-transfer voltammetry was demonstrated for the first time in an undiluted blood sample. Despite interferences by Na+, Cl-, and plasma proteins such as serum albumin, a detection limit of 0.13 unit/mL was obtained in sheep blood plasma with the stripping method.