Miniaturized metal oxide pH sensors for bacteria detection

It is well known that the metabolic activity of some microorganisms results in changes of pH of the culture medium, a phenomenon that can be used for detection and quantification of bacteria. However, conventional glass electrodes that are commonly used for pH measurements are bulky, fragile and expensive, which hinders their application in miniaturized systems and encouraged to the search for alternatives. In this work, two types of metal oxide pH sensors have been tested to detect the metabolic activity of the bacterium Escherichia coli (E. coli). These pH sensors were produced on silicon chips with platinum metal contacts, onto which thin layers of IrOx or Ta2O5 were incorporated by two different methods (electrodeposition and e-beam sputtering, respectively). In order to facilitate measurement in small sample volumes, an Ag/AgCl pseudo-reference was also screen-printed in the chip and was assayed in parallel to an external Ag/AgCl reference electrode. As it is shown, the developed sensors generated results indistinguishable from those provided by a conventional glass pH-electrode but could be operated in significantly smaller sample volumes. After optimization of the detection conditions, the metal oxide sensors are successfully applied for detection of increasing concentrations of viable E. coli, with detection of less than 103 cfu mL−1 in undiluted culture medium in just 5 h.Authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (Project CTQ2014-54553-C3-1-R), and from European Union (Project ENIAC-ED-52 v161210). E.B. is supported by the Miguel Servet Program, funded by the Fondo de Investigaciones Sanitarias of the Instituto de Salud Carlos III (Grant CP13/00052) and co-financed by the European Regional Development Fund (ERDF). This work was partially financially supported by Government of Russian Federation, Grant 074-U01.Peer reviewe