Study On Novel Electrochemiluminescence Systems And Analytical Methods Based On Quantum Dots

Nanotechnology has become an integrative subject by combining with chemistry, biology, iatrology and material science. A kind of functional nanocrystals, quantum dots (QDs), due to their advantages in optical and electric aspects, has been widely used for bioanalysis in cell image, in vivo observation and multiplexed immunoassay or analysis of DNA hybridization processes. Today, electrochemiluminescence (ECL) as a widely used analytical technology has associated with various QDs as ECL emitters for development of novel analytical methods. However, the reported QDs-ECL systems always suffer from high application potential and the presence of strong oxidant as coreactant, which suppress the development of QDs based ECL bioanalysis. In this dissertation, a series of novel QDs-ECL systems were developed, both in anodic and cathodic processes, and their ECL mechanisms were proposed. By coupling nanotechnology, surface science and chemical biology, new bioanalytical and biosensing strategies were constructed with good performance. This dissertation includes the following six parts:1. Anodic electrochemiluminescence of CdTe quantum dots and its energy transfer for detection of catechol derivativesThis work reported for the first time the anodic ECL of CdTe QDs in aqueous system and its analytical application based on the ECL energy transfer to analytes. The CdTe QDs were modified with mercaptopropionic acid to obtain water soluble QDs and stable and intensive anodic ECL emission with peak value at +1.17 V (vs. Ag/AgCl) in pH 9.3 PBS at an indium tin oxide (ITO) electrode. The ECL emission was demonstrated to involve the participation of superoxide ion produced at the ITO surface, which could inject an electron into the 1Se quantum-confined orbital of CdTe to form QDs anions. The collision between these anions and the oxidation products of QDs led to the formation of excited state of QDs and ECL emission. The ECL energy transfer from the excited CdTe QDs to quencher produced a novel methodology for detection of catechol derivatives. Using dopamine and L-adrenalin as model analytes this ECL method showed wide linear ranges from 50 nM to 5μM and 80 nM to 30μM for these species. Both ascorbic acid and uric acid, which are common interferences, did not interfere with the detection of catechol derivatives in practical biological samples.2. Coreactant enhanced anodic electrochemiluminescence of CdTe quantum dots at low potential for highly sensitive biosensing amplified by enzymatic cycleThis work used sulfite as a coreactant to enhance the anodic ECL of mercaptopropionic acid modified CdTe QDs. This strategy proposed the first coreactant anodic ECL of QDs and led to a sensitive ECL emission of QDs in aqueous solution at relatively low potential. In presence of dissolved oxygen, the stable ECL emission resulted from the excited QDs. Thus an ECL detection method was proposed at +0.90 V (vs Ag/AgCl) based on the quenching of excited QDs by the analyte. Using tyrosine as a model compound, whose electro-oxidized product could quench the excited QDs and thus the ECL emission, an analytical method for detection of tyrosine in a wide concentration range was developed. Furthermore, by combining an enzymatic cycle of trace tyrosinase to produce the oxidized product with an'energy transfer process an extremely sensitive method for ECL detection of tyrosine with a sub-pM limit of detection was developed. The sulfite-enhanced anodic ECL emission provided an alternative for traditional ECL light-emitters and a new methodology for extremely sensitive ECL detection of mono- and dihydroxy benzenes at relatively low anodic potential. This strategy could be easily realized and opened new avenues for the applications of QDs in ECL biosensing.3. Dopamine detection based on its quenching effect to anodic electrochemiluminescence of CdSe quantum dots The first anodic ECL emission of CdSe QDs in aqueous solution enhanced by sulfite as a coreactant is observed at an ITO electrode. In air-saturated Tris-HCl buffer the reaction of the electro-oxidation product of sulfite with dissolved oxygen produces O2- for formation of electrons injected QDs and then the excited QDs, leading to the strong anodic ECL emission at +0.926 V (vs. Ag/AgCl). The maximum ECL emission occurs at a relatively low potential compared to the traditional ECL system. Based on the quenching effect, a method for detection of dopamine with acceptable accuracy is developed with a linear range of 0.5～70μM. This report provides a new coreactant ECL system for anodic ECL analysis of QDs. Furthermore, the optimal pH value near to physiological condition is suitable for multiple biomolecules detection.4. Determination of nitrite based on its quenching effect on anodic electrochemiluminescence of CdSe quantum dotsA novel method for ECL detection of nitrite was proposed based on its quenching effect on anodic ECL emission of CdSe QDs. The ECL emission could be greatly enhanced by sulfite and dissolved oxygen in a neutral system and occurred at a relatively low potential in comparison with traditional anodic ECL emitter, leading to high sensitivity and good selectivity. The quenching mechanism followed an "electrochemical oxidation inhibition" process, which was completely different from those of some analytes on the ECL emission of QDs. The coincidence of photoluminescence and ECL spectra of the QDs indicated that the ECL emission resulted from the redox process of QDs core and the sulfite acted as a coreactant. The nitrite quenched ECL emission could be analyzed according to the treatment of Stern-Volmer equation with a linear range from 1μM to 0.5 mM for detection of nitrite. This work presented a new efficient ECL methodology for quencher-related detection.5. Surface trap of quantum dots by bidentate chelation for low-potential electrochemiluminescent biosensingBidentate chelation, meso-2,3-dimercaptosuccinic acid, was used as a stabilizer for the synthesis of CdTe QDs. The bidentate chelate QDs, characterized with FT-IR, photoluminescent and UV-vis spectroscopy, element analysis and high-resolution transmission electron microscope, exhibited surface traps due to the large surface/volume ratio of QD particle and the steric hindrance of the DMSA molecule. The unpassivated surface of QDs could produce narrower band gap than the core and ECL emission at relatively low cathodic potential. In air-saturated pH 7.0 buffer, the QDs immobilized on electrode surface showed an intensive ECL emission peaked at-0.85 V (vs. Ag/AgCl). H2O2 produced from electrochemical reduction of dissolved oxygen was demonstrated to be the coreactant, which avoided the need of strong oxidant as the coreactant and produced a sensitive analytical method for peroxidase-related analytes. Using hydroquinone-horseradish peroxidase- H2O2 as a model system, a novel reagentless phenolic ECL biosensor for hydroquinone was thus constructed based on the quenching effect of ECL emission of QDs by consumption of coreactant H2O2. The biosensor showed a linear range of 0.2～10μM with acceptable stability and reproducibility. This work opened new avenues to search new ECL emitters for excellent analytical performance and made QDs more alternative in biosensing.6. Quantum qots based electrochemiluminescent immunosensor by coupling enzymatic amplification with self-produced coreactant from oxygen reductionA highly sensitive competitive immunosensor based on the ECL of quantum dots QDs was proposed by coupling with an enzymatic amplification. The fabrication process of the immunosensor was traced with atomic force microscopic images and electrochemical impedance spectra. The strong cathodic ECL emission of the immobilized QDs could be detected at a relatively low emission potential. The reduction of dissolved oxygen during the cathodic process provided a self-produced coreactant, H2O2, for the ECL emission. Using human IgG (HIgG) as a model protein, upon the immuno-recognition of the immobilized HIgG to its antibody labeled simply with horseradish peroxidase, the ECL intensity decreased due to the steric hindrance of the proteins to electron transfer. The decrease could be greatly amplified by an enzymatic cycle to consume the self-produced coreactant, leading to a wide calibration range of 0.05 ng mL-1～5μg mL-1 and a low limit of detection for the competitive immunoassay of HIgG. This immunosensor showed good stability and fabrication reproducibility. The immunoassays of practical samples showed acceptable results. This facile immunosensing strategy opened a new avenue for detection of proteins and application of QDs in ECL biosensing.