On the other hand, during the introduction of coreactants into the solution, the coreactant-related physicochemical behaviors like diffusion, absorption, as well as its concentration, may complicate the ECL systems. Because ECL reaction involves both electrochemical and chemical reactions, it is possible to find new coreactants using chemically modified electrode. However, other electrochemical characters, such as electrocatalytic behavior of QDs, have not attracted much attention. Nucleic acids have long been considered as a kind of molecule with genetic information.
The development of sensitive and selective methods for the detection of trace amount of sequence specific DNA is of great importance in clinical diagnosis, food analysis, and environmental monitoring. To fulfill these requirements, numerous DNA detection systems combined with elegant signal transduction and some amplification strategies based on the hybridization between a DNA probe and its complementary target have been described 26 , 27 , 28 , 29 , 30 , 31 , However, effective solutions for the above studies are limited, and this has driven the search for developing new ECL-based techniques in DNA detection.
It was reported that DNA has the capacity to be intercalated with some small molecules into its grooves with high affinity. As a result, several DNA sensors have been developed by the use of the intercalation of small molecules probes into the DNA structures 34 , 35 , 36 , 37 , However, the limitation of TPA solution makes it necessary to explore new coreactants.
Previous work revealed that QDs had good catalytic properties besides their good luminescent properties, which had rarely been studied in ECL High resolution transmission electron microscopy image displayed the crystalline feature of CdSe QDs with average size of about 3.
One strong anodic ECL peak can be observed at 1. In order to further support the conclusion, several experiments were carried out as shown in Figure S4. At the bare GCE, a reversible redox peak observed at 1. This conclusion is also supported by the reported results 24 , The oxidation current increased with the increase of potential scan rate.
In Fig. The mechanism of ECL process can be as follows:. Therefore, the extremely weak ECL peak in Fig. As an effective tool for the characterization of the interface properties, electrochemical impedance spectroscopy EIS was used to monitor the biosensor fabrication process as shown in Fig. The impedance spectra of all modified process consisted of a semicircle at high AC modulation frequency and line at low AC modulation frequency, demonstrating that the electrode process was controlled by electron transfer at high frequency and by diffusion at low frequency.
The charge transfer resistance R ct which equals the diameter of semicircle reflects the restricted diffusion of the redox probe through the electrode surface. On the other hand, the insulating effect of DNA could perturb the interfacial charge transfer. As a result, the R ct was greatly increased again.
These results suggested a successful stepwise fabrication of the proposed ECL biosensor. The ECL responses of the modified electrodes in different stages were examined as shown in Fig. The ECL signal of the biosensor at 1. The detection limit for target DNA was estimated to be 1.
It can be found that the present biosensor is superior to most of other reported ECL biosensors. In order to investigate the specific response of the biosensor to DNA, control experiments were performed by incubating the biosensor in several aqueous solutions containing complementary target DNA, single-base mismatched DNA, three-base mismatched DNA, and noncomplementary random DNA, respectively. Figure 9 showed the comparison of the ECL intensity changes of the biosensors for the same concentration of different DNA.
The comparison indicated that this method had high sequence specificity, and this detection approach had potential application in single nucleotide polymorphism analysis. The possible application of the biosensor in real samples detection was evaluated by recovery experiments determined in human serum samples. Two human serum samples were spiked with known concentrations of target DNA and were determined using the calibration curves of Fig.
This work has several key meritorious novelties.
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Secondly, the immobilization of QDs coreactant on the electrode can avoid the use of coreactant in solution and simplify ECL system. The inherent selectivity of the probe endows the biosensor with high base discrimination ability. The proposed approach provides a promising detection platform for genetic analysis and clinic biomedical applications. Other chemicals were analytical grade and double distilled water was used throughout.
Tris-HCl buffer 0. Shanghai, China. Two hairpin DNA probes with different diameter loop were adopted.
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The loop of probe-1 is complementary to the target DNA. The target DNA can hybrid to the loop of the hairpin DNA, which will cause the hairpin probe to adopt the open form The DNA sequences used are as follows:. All experiments were carried out with a conventional three-electrode system, including a modified GCE as the working electrode, a platinum wire as the counter electrode and a saturated calomel electrode SCE as the reference electrode, respectively.
CdSe QDs were synthesized following the literature procedures The modified electrode was immersed in pH 7. The electrode was washed by tris-HCl buffer twice to remove the unbonded probe. How to cite this article : Dong, Y. Richter, M. Electrochemiluminescence ECL.
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Yuan, Y. In electrogenerated chemiluminescence, electrochemically generated intermediates undergo a highly exergonic reaction to produce an electronically excited state that then emits light upon relaxation to a lower-level state. This wavelength of the emitted photon of light corresponds to the energy gap between these two states.
ECL is usually observed during application of potential several volts to electrodes of electrochemical cell that contains solution of luminescent species polycyclic aromatic hydrocarbons , metal complexes, Quantum Dots or Nanoparticles  in aprotic organic solvent ECL composition.
In organic solvents both oxidized and reduced forms of luminescent species can be produced at different electrodes simultaneously or at a single one by sweeping its potential between oxidation and reduction. The excitation energy is obtained from recombination of oxidized and reduced species. In aqueous medium, which is mostly used for analytical applications, simultaneous oxidation and reduction of luminescent species is difficult to achieve due to electrochemical splitting of water itself so the ECL reaction with the coreactants is used. In the later case luminescent species are oxidized at the electrode together with the coreactant which gives a strong reducing agent after some chemical transformations the oxidative reduction mechanism.
ECL proved to be very useful in analytical applications as a highly sensitive and selective method.