An Electrochemiluminescence Sensor Based on CdSe@CdS and God Signal Amplification for Sensitive Detection of Hg2+
Authors: Kang Tian-Fang , He Zhang-Jin 
Published: February 2020

Mercury is highly toxic, so it is important to develop a rapid and sensitive method for the detection of mercury. In this paper, mercaptopropionic acid-capped core-shell CdSe@CdS quantum dots (QDs) were synthesized by one-step hydrothermal method[1]. Molybdenum disulfide (MoS2) nanosheets were prepared by the method which we previously reported[2]. 5'-amino-modified oligonucleotide capture probe DNA1 (5'-NH2-ss-DNA) and 5'-thiol-modified oligonucleotide signal probe DNA2 (5'-SH-ss-DNA) was synthesized by Shanghai Bioengineering Technology Co., Ltd. (China). The sequence (5' to 3') is as follows[3]:
DNA1：5'-NH2-AAAATTTTGCTTTGGTTT-3'
DNA2：5'-SH-AAAAATTTCCTTTGCTTTT-3'
Polycationic poly(diallyldimethylammonium chloride) (PDDA) and negatively charged QDs were adsorbed on the surface of MoS2, in sequence, to form the QDs- PDDA-MoS2 composites, which were employed as a matrix for the immobilization of DNA1 strand[1]. Gold nanoparticles (AuNPs) were prepared and were combined with the 5'-thiol-modified DNA2 probe and glucose oxidase (GOD) to prepare GOD-AuNPs-DNA2 conjugates[2]. First, the QDs-PDDA-MoS2 nanocomposites were modified onto the surface of a glassy carbon electrode (GCE) as a carrier for preparing an electrochemiluminescence (ECL) sensor.
The QDs-PDDA-MoS2/GCE electrode was immersed in an aqueous solution consists of 10.0 mM ethyl-3-(dimethylaminopropyl) carbodiimide (EDC) and 5.0 mM N-hydroxysuccinimide (NHS) for 30 min to activate the terminal carboxylic acid group of QDs. DNA1 could immobilize onto the surface of the CdSe@CdS QDs via the interaction between the activated carboxylic acid group and the amino group of DNA1. The modified electrode was denoted as DNA1/QDs-PDDA-MoS2/GCE. The schematic diagram of fabrication of the sensor was shown in Fig. 1.

Fig. 1 Frabrication process and the detection machanisms of the ECL sensor.
Insert: curve (a) absence and (b) presence of Hg2+.
Electrochemical measurement for ECL was carried out on an MPI-A detection system (Xi'an Rimax Electronics Co., Ltd. China) with a three-electrode system and a photomultiplier tube. The voltage was set at 800 V. A glassy carbon electrode (diameter, 3 mm) was used as a working electrode; a platinum wire and an Ag/AgCl (saturated KCl solution) were used as a counter electrode and a reference electrode, respectively.
The DNA1/QDs-PDDA-MoS2/GCE electrodes were incubated with GOD-AuNPs-DNA2 suspension and different concentrations of Hg2+ solution in turns. Then the electrodes were immersed in 0.1 M phosphate buffer solution containing 1% glucose to measure ECL signal, respectively. The potential window was from 0 to 1.5 V with a scan rate of 100 mV/ s. Hg2+ ions can specifically bind to the T-T mismatch base of DNA1 and DNA2 to form the T-Hg2+-T complex. Therefore, GOD-AuNPs-DNA2 can be modified to the electrode surface. GOD can catalyze the oxidation reaction of dissolved oxygen molecules towards glucose in the solution to produce H2O2 which is a co-reactant and can enhance the ECL of the QDs. The ECL signal increased linearly with Hg2+ concentration from 1.0×10-12 to 1.0×10-6 M. The detection limit of 0.1 pM can be estimated at a signal-to-noise ratio of 3. The ECL sensor has the advantages of simple preparation method, high sensitivity, good specificity and wide linear range for the determination of Hg2+ ions.

References

[1] X.-L. Du, T.-F. Kang, L.-P. Lu, S.-Y. Cheng, Analytical Methods, 2018, 10(1): 51-58.
[2] H.-Y. Song, T.-F. Kang, L.-P. Lu, S.-Y. Cheng, Talanta, 2017, 164: 27-33.
[3] Z. Qiu, J. Shu, G. Jin, M. Xu, Q. Wei, G. Chen, D. Tang, Biosensors and Bioelectronics. 2016, 77: 681-686.

Figure 1