Application of iridium(III) complex in label-free and non-enzymatic electrochemical detection of hydrogen peroxide based on a novel "on-off-on" switch platform

Abstract

We herein report a label-free and non-enzymatic electrochemical sensor for the highly sensitive detection of hydrogen peroxide (H2O2) based on a novel "on-off-on" switch system. In our design, MB was used as an electron mediator to accelerate the electron transfer while AuNPs was used to amplify the electrochemical signal due to its excellent biocompatibility and good conductivity. The "switch-off" state was achieved by introducing the guanine-rich capture probe (CP) and an iridium complex onto the electrode surface to form a hydrophobic layer, which then hinders electron transfer. Upon addition of H2O2, fenton reaction occurs and produces OH• in the presence of Fe(2+). The OH• cleaves the CP into DNA fragments, thus resulting in the release of CP and iridium complex from the sensing interface, recovering the electrochemical signal to generate a "switch-on" state. Based on this novel switch system, a detection limit as low as 3.2 pM can be achieved for H2O2 detection. Moreover, satisfactory results were obtained by using this method for the detection of H2O2 in sterilized milk. To the best of our knowledge, this is the first G-quadruplex-based electrochemical sensor using an iridium(III) complex.

Figure 1. Scheme diagram for the preparation of H₂O₂ sensor.

Figure 2

(A) Conductivity investigation of the iridium(III) complex and other organic dyes that specific to G-quadruplex in 5 mM of Fe(CN)^3−/4−: (a) Gold electrode; (b) CP/gold electrode; (c) iridium(III) complex/CP/gold electrode; (d) CV/CP/gold electrode. (e) Rhodamine/CP/gold electrode. (B) DPV of the different modified electrodes in 10 mM tris-buffer solution (pH 7.4): (a) Gold electrode; (b) Nf@MB/gold electrode; (c) (+)AuNPs/Nf@MB/gold electrode; (d) iridium(III) complex/CP/(+)AuNPs/Nf@MB/gold electrode; (e) (d) after the incubation with 1.5 nM of H₂O₂. (C) Circular dichroism of CP before (a) and after (b) the incubation of it with tris-buffer that contained 100 mM of K⁺. (D) Circular dichroism of G-quadruplex structure before (a) and after (b) the selective interaction of it with iridium(III) complex, and after the cleavage with H₂O₂ (c).

Figure 3

Effect of the self-assembly time of (+)AuNPs on the sensor properties (A) Effect of the concentration of Fe²⁺ (B), the pH value (C) and the incubation time for CP cleavage in the presence of 1.2 nM H₂O₂ (D).

Figure 4

(A) DPV experiments for H₂O₂ detection in 10 mM of tris-buffer solution (pH 7.4); (B) Calibration curve for the sensor.

Figure 5. Selectivity of the sensor for H₂O₂ (1.5 nM) detection against interferences including ascorbic acid (150 nM), purine trione (150 nM), glucose (150 nM) and metal ions (150 nM).