Label-free analysis of water-polluting parasite by electrochemical impedance spectroscopy.

A new, label-free, real time and non-invasive method is presented to detect the presence of infectious parasites in water and determine accurately their concentration by electrochemical impedance spectroscopy (E.I.S.) using interdigitated microelectrode array. Cryptosporidium parvum was taken as model. Buffer influence on parasite detection was investigated by comparing parasites suspended in purified water versus phosphate buffer saline. It was shown that a low conductive buffer is required for parasite detection. Different suspensions of C. parvum oocysts were measured in purified water. By fitting resulting electrochemical impedance spectrums with an equivalent electrical circuit, solution conductance was extracted. Conductance increased linearly with C. parvum oocyst concentration. The reasons of conductance modification induced by parasite presence are discussed. Cell constant was calculated for circular interdigitated electrode arrays. Thus sample conductivity can be obtained from raw impedance spectrums and it was established that water conductivity was proportional to C. parvum oocyst amount. This relationship can be expressed by: sigma (Sm(-1))=2.88228x10(-6)xC (oocysts/microl)+1.64565x10(-4) with R(2)=0.99. In this way, E.I.S. can be used as a rapid alternative to current parasite counting procedures which consists in fluorescent staining and microscopic observation. The distinction between dead and living parasites by E.I.S. was also approached. Between 10 kHz and 100 kHz, electrochemical impedance showed a difference of 15% between dead and living oocysts.