Electrochemical Immunosensing Based On Electrocatalytic Water-splitting Reaction

Electrochemical immunosensors that combine immunosensing and electrochemical techniques have attracted widespread attention due to their high sensitivity,short response time and good selectivity.Compared with natural enzymes,nanoelectrocatalysts are often used as signal labels for immunosensing because of their high stability and low cost.To further improve the sensitivity of this kind of immunosensors,it is important to develop highly active nanoelectrocatalysts.Electrocatalytic water split-ting,including oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）,is generally regarded as a promising and sustainable method for producing clean hydrogen fuel from aqueous solutions.H₂O is the catalytic substrate of water splitting reaction,and the concentration of H₂O is as high as 55.5 mol L^-1,so it is possible to construct a highly sen-sitive electrochemical immunosensors by using highly active water split-ting catalysts as electrocatalytic labels.Using Human Interleukin 6（IL-6）and Carcinoembryonic Antigen（CEA）as model targets,novel immuno-sensing platforms were constructed by using several water splitting cata-lysts as new electrocatalytic labels.The main contents are as follows:（1）NiCoO₂ nanoboxes（NBs）were prepared by Ni²⁺exchange and heat treatment of ZIF-67,followed by the growth of CeO₂ nanoparticles on Ni CoO₂ NBs.The Ni CoO₂@CeO₂ NBs offers superior OER activity to Ni CoO₂ NBs in a neutral solution due to the synergetic effect between Ni CoO₂ NBs and CeO₂.The formation of ester-like bridging between CeO₂ and carboxylic group of antibody enables direct immobilization of antibody on the Ni CoO₂@CeO₂ NBs surface.Under the optimal experi-mental conditions,a sandwich-type electrochemical immunosensor using Ni CoO₂@CeO₂ NBs as an electrocatalytic label shows the linear range for detecting IL-6 from 2.5×10^-5 to 10 ng mL^-1,with a detection limit as low as 7 fg mL^-1（S/N=3）.（2）MoS₂-MWCNTs nanocomposites were prepared by growth of Mo S₂ nanosheets on carboxylated multi-walled carbon nanotubes（MWCNTs）through a hydrothermal method.Electrochemical studies show that Mo S₂-MWCNTs have excellent HER activity in a neutral solu-tion.Under the optimal experimental conditions,a sandwich-type elec-trochemical immunosensor using Mo S₂-MWCNTs as an electrocatalytic label shows the linear range for detecting CEA from 10^-4 to 50 ng mL^-1,with a detection limit as low as 25 fg mL^-1（S/N=3）.（3）CoP-MWCNTs nanocomposites were prepared by hydrothermal growth of Co₃O₄ nanoparticles on MWCNTs and low-temperature phos-phatization of Co₃O₄ nanoparticles.The MWCNTs-CoP catalyst shows excellent electrocatalytic activities in a neutral medium toward both hy-drogen evolution reaction（HER）and oxygen evolution reaction（OER）,enabling MWCNTs-CoP as the electrocatalytic label for sensitive im-munosensing.Under the optimal experimental conditions,the linear range of the sandwich-type immunosensor for detecting CEA based on the HER signal is from 10^-4 to 100 ng mL^-1,with a detection limit of 12 fg mL^-1;whereas a linear range for detecting CEA based on the OER signal is achieved from 10^-4 to 10 ng mL^-1,with a detection limit from 10 fg mL^-1（S/N=3）.