br Furthermore the ECL behaviors of electrode
Furthermore, the ECL behaviors of electrode at different decorated stage was investigated in 0.1 M PBS solution containing 0.5% DBAE, 10 mM H2O2 and 10 mM NaOH to further confirm the successful con-struction of the ratiometric mode. As presented in Fig. 2B, an anodic ECL signal (+1.0 V) from DBAE was obtained at the bare GCE (curve a). Interestingly, when NiFe2O4 NTs was spread on the electrode, a sig-nificantly enhanced ECL signal was observed (curve b) owing to the NiFe2O4 NTs could catalyze the OER process and bring O2, which played crucial role in the ECL emission of DBAE. After the modification of Envision complex, an further amplified ECL signal was realized (curve c), this phenomenon was ascribed to the massive HRP molecules embedded on the Envision complex could catalyze the decomposition of H2O2 to generate reactive oxygen species, such as O2•−, which parti-cipated in the DBAE ECL process and induced an intensive ECL signal. With the incubation of Ab1 (curve d) and HE4 (curve e), the ECL signal gradually descended because of the insulation layer of protein hindered the electron-transfer. However, when [email protected]@Ab2 bioconju-gates was packed on the surface of electrode, the cathodic ECL signal from lucigenin was observed at about -0.5 V (curve f) and anodic ECL from DBAE declined sharply, which on the one hand was caused by the hindrance from [email protected]@Ab2 bioconjugates for electron-
Scheme 1. The fabrication process of the ratiometric ECL immunosensor.
Fig. 1. (A) TEM image, (B) XRD pattern and (C) N2 adsorption–desorption isotherms analysis of NiFe2O4 NTs.
transfer, on the other hand was explained by the 6-NBDG for O2%− between lucigenin and DBAE, indicating the effective preparation of ratiometric ECL strategy. Moreover, from the table in the inset of Fig. 2B, the output occurs only if both inputs are present in the logic gate system, revealing two ECL signals induced at the same time was the necessary condition of ratiometic ECL strategy.
3.3. High-efficiency NiFe2O4 NTs electrocatalytic sensing platform
To demonstrate the catalytic performance of the NiFe2 O4 NTs for the OER process, the linear sweep voltammetry (LSV) and cyclic vol-tammetry (CV) were performed. As displayed in Fig. 3A, the NiFe2O4 NTs modified electrode presented lower onset potential and stronger current density (curve b) compared with the bare GCE (curve a), re-vealing the good conductivity of NiFe2 O4 NTs could facilitated the electron transport and improve the OER activity due to the composite effect of nickel and iron oxides. Besides, to further clarify the NiFe2O4 NTs could catalyze the OER process to produce O2, the CV behaviors was investigated after an OER process in 0.1 M KOH after bubbling nitrogen for 10 min. Seen from Fig. 3B, compared with the bare GCE (curve a), an obvious oxygen reduction peak was observed after NiFe2O4 NTs modified electrode going through an OER process (curve b). These results validated that the NiFe2O4 NTs could promote the OER process and yield O2.
3.4. The ECL mechanisms of two luminophors
To illustrate the ECL mechanisms of two luminophors, several key factors taking part in the ECL emission process were investigated. According to Fig. 4A, a weak ECL signal was obtained at the bare GCE (curve a), while an intensive ECL signal was realized at the NiFe2O4 NTs decorated electrode (curve b), which was no doubt that NiFe2O4 NTs
promoted the OER process and generated O2, indicating O2 was indis-pensable for the DBAE ECL. After Envision complex was assembled on the electrode, a slightly declined ECL signal was occurred owing to the impediment of protein for electron transport (curve c). However, when H2O2 was added into the testing solution, a highest ECL emission was induced (curve d), which might be ascribed to the fact that HRP ad-hering on Envision complex catalyzed the decomposition of H2O2 to release O2%− and further enhanced the ECL. These results disclosed that O2 and DBAE in one system could trigger a strong ECL signal and O2%− also played crucial role in the reaction. According to previous reports , the main ECL mechanism of DBAE in this work was as follows: H2 O NiFe 2 O4 O 2 + 4 H+ + 4e
Additionally, the ECL behaviors of lucigenin was discussed via adding different substances, as described in Fig. 4B. It’s evident that [email protected] modified electrode exhibited a higher ECL signal (curve
b) than that of sole lucigenin (curve a), which was benefited from the high surface area of h-BN, leading to a large quantity loading of luci-genin. When H2O2 was added into the detection solution, an slightly increased ECL signal was observed (curve c), which was in agreement with previous report . Nevertheless, a remarkably enhanced ECL
Fig. 5. (A) ECL responses of ratiometric biosensor and (B) Calibration plot of the ECL intensity with different concentrations of HE4 from 10 fg/ml to 10 ng/ml in 0.1 M PBS containing 0.5% DBAE, 10 mM H2O2 and 10 mM NaOH. (C) The selective of the proposed ratiometric ECL biosensor and (D) the stability of the proposed ratiometric ECL biosensor under consecutive cyclic potential scans for 10 cycles.