Until now analytical techniques for
Until now, analytical techniques for HE4 were mainly focused on photoelectrochemical sensing method , chemiluminescence im-munoassay , electrochemical method  and molecularly im-printed photoelectrochemical sensing strategy . Nevertheless, these Resolvin E 1 methods were limited in linear range and sensitivity for the HE4 ana-lysis. Currently, electrochemiluminescence (ECL), a hotspot in analy-tical chemistry field for its intrinsic characteristics of low-light back-ground, high sensitivity, controllability and simplicity, providing a desired option for sensitive HE4 detection.  However, single signal ECL detection usually suffers from positive or negative errors due to some environmental factors [13,14]. Recently, ratiometric mode is
Corresponding authors at: Fujian Provincial Maternity and Children`s Hospital, Fuzhou, Fujian, 350108, China.
1 These author equally and should be regarded as co-first author.
Available online 22 February 2019
regarded as an ideal approach to eliminate the interferences and ensure the precision of measurement results through its self-calibration and normalized variation in environmental changes. The primary require-ment for ratiometric assay was finding a pair of potential-resolved ECL luminophores. Despite many ECL emitter pairs, such as CdSe/ZnS QDs/ Luminol,  Luminol/C3N4,  Ru(bpy)32+/CdTe QDs,  and PTC-NH2/1(O2)2*  were developed for ratiometric strategies. Un-fortunately, 2-(Dibutylamino)ethanol (DBAE) has never been con-sidered into ratiometric ECL mode because the recent research about DBAE is mainly focused on acting as coreactants of other luminophors, which requires the additional ECL emitters such as Ru(bpy)32+ and quantum dots (QDs). [19,20] Hence, directly pouring DBAE into the detection solution without additional ECL emitters to initiate a strong anodic ECL was simple and promising in the ratiometric strategy. In-spired by the following facts: (i) obvious potential-resolution was rea-lized between DBAE and lucigenin; (ii) DBAE was often used as co-reactants in ECL systems rather than directly pouring into detection solution to trigger ECL; (iii) the cathodic ECL of lucigenin has never been explored for ratiometric mode, the development of a ratiometric ECL strategy based on DBAE/lucigenin ECL pair holds great promise. Previous studies proven that the ECL of DBAE was in fact derived from dissolved oxygen , thus oxygen evolution reaction (OER) , the process of water splitting to produce oxygen, was considered as an ef-fectively enhanced channel for DBAE ECL.
Recently, great efforts have been made to explore efficient electro-catalysts for OER. To the best of our knowledge, the transition metal oxides with multiple valence state, such as Co3O4, NiO, Fe3O4, CeO2 have been demonstrated to possess catalytic activities for the OER. [23–28] Among them, the bimetallic oxide nanomaterials have drawn considerable attention for their higher electrical conductivity resulted from the hopping process between metal ions of variable oxidation states at oxygen sites . Nickel ferrite (NiFe2O4), an inverse spinel with equal number of Ni2+ and Fe3+ locating on octahedral sites and remaining Fe3+ lying on tetrahedral sites has excited our interests.  Here, NiFe2O4 nanotubes (NiFe2O4 NTs) were initially prepared via a facile method, and the superior electrical conductivity and higher electrochemical activity makes it promising in improving OER process.  Additionally, many other advantages such as large specific area, chemical stability, low toxicity, high adsorption ability enable NiFe2O4 NTs to be a desirable matrix for biosensor.
To realize higher analytical performance of the ratiometric ECL mode, a proper nanolabels was considerable for ensuring the intense and stable ECL signal of lucigenin in alkaline condition. Hexagonal boron nitride nanosheets (h-BN), has aroused extensive interests for its special sheet structure, which is formed via the strong sp2 covalent bonds between boron and adjacent nitrogen atoms in the layer and the weak van der Waals forces between adjacent layers. The unique struc-ture endows h-BN a great deal of charming properties, including ex-cellent mechanical properties, thermal and chemical stability, espe-cially the excellent corrosion resistance quality.  However, to our best knowledge, there is no report of h-BN nanosheets on the design of ratiometric ECL biosensor. In this work, the h-BN nanosheets with large surface area were synthesized though a simple approach and served as a scaffold for immobilization of lucigenin. Moreover, to improve the sensitivity of the ratiometric ECL biosensor, the Envision complex consisting of a large number of horseradish peroxidase (HRP) was in-troduced into the design proposal, which has never been reported in a ratiometric ECL strategy.