Reversible detection of heparin and other polyanions by pulsed chronopotentiometric polymer membrane electrode

Abstract

The first fully reversible polymeric membrane-based sensor for the anticoagulant heparin and other polyanions using a pulsed chronopotentiometry (pulstrode) measurement mode is reported. Polymeric membranes containing a lipophilic inert salt of the form R(+)R(-) (where R(+) and R(-) are tridodecylmethylammonium (TDMA(+)) and dinonylnaphthalene sulfonate (DNNS(-)), respectively) are used to suppress unwanted spontaneous ion extractions under zero-current equilibrium conditions. An anodic galvanostatic current pulse applied across the membrane perturbs the equilibrium lipophilic ion distribution within the membrane phase in such a way that anions/polyanions are extracted into the membrane from the sample. The membrane is then subjected to an open-circuit zero current state for a short period, and finally a 0 V vs reference electrode potentiostatic pulse is applied to restore the membrane to its initial full equilibrium condition. Potentials are sampled as average values during the last 10% of the 0.5 s open circuit phase of the measurement cycle. Fully reversible and reproducible electromotive force (emf) responses are observed for heparin, pentosan polysulfate (PPS), chondroitin sulfate (CS), and oversulfated chondroitin sulfate (OSCS), with the magnitude of the potentiometric response proportional to charge density of the polyanions. The sensor provides an emf response related to heparin concentrations in the range of 1-20 U/mL. The responses to variations in heparin levels and toward other polyanions of the pulstrode configuration are analogous to the already established single-use, nonreversible potentiometric polyion sensors based on membranes doped only with the lipophilic anion exchanger TDMA(+).

Figure 1

(A) Calibration curve toward heparin in 10 mM NaCl buffered at pH 7.4 with 10 mM PBS; each data point in the curve is an average of 16 potentials sampled at intervals of 16.5 s (i.e., full measurement cycle) when the sensor was exposed to the given buffer solution with given level of polyanion and (B) corresponding potential-time trace for the calibration curve shown in (A).

Figure 2

Potential-time behavior of polyanion sensor EMF response obtained by alternating measurements in a background solution (without heparin) and in the indicated heparin solutions. Each discrete section in the curve is composed of 11 voltage data points sampled at intervals of 16.5 s.

Figure 3

Potential-time trace of pulstrode polyanion sensor EMF response upon alternate measurements in a background solution and in 400 μg/mL solutions of the indicated polyanions possessing different charge densities. Each discrete section (background solution and sample solution) in the dynamic response plot is composed of 16 voltage data points sampled at intervals of 16.5 s. Concentration used is at level where rapid full equilibrium potentiometric response at phase-boundary of membrane/sample is expected.