1. Department of Analytical Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden, HeptaHelix AB, Jägershillsgatan 15, SE-23175 Malmö, Sweden, Department of Physiological Sciences, Division of Molecular Neurobiology, Lund University, BMC A12 Tornavägen 10, SE-22184 Lund, Sweden, and Department of Electrical Measurements, Lund Institute of Technology, Lund University, P.O. Box 118, SE-22100 Lund, Sweden