“…Compared with other techniques such as nuclear magnetic resonance (Guerrini et al, 2008), high performance liquid chromatography (Keire et al, 2010), colorimetry (Guarise et al, 2006;Fu et al, 2012) and fluorometry (Pu and Liu., 2008;Li et al, 2011;Hu et al, 2012;Fan et al, 2012) for protease or heparin assay, the potentiometric method has advantages of portability, ease of use, simple and reliable instrument, and resistance to color and turbid interference. However, unlike classical ion-selective electrodes working under fully equilibrium conditions, the response of a polyion-sensitive electrode is a quasi-steady-state one highly dependent on the polyion flux from the sample bulk into the sample-membrane interface (Fu et al, 1994).…”