Study On A Novel Principle And Method For The Detection Of Cadmium And Silver Based On A Multifunctional Aptamer Probe

Cadmium and silver are the most common toxic heavy metals,which are widely distributed in the environment. They have received great attention due to their serious threat to human health. The reported methods for the detection of cadmium and silver either require expensive equipment and skilled operators, or involve tedious and time-consuming procedure for sample pre-treatment. So, the development of a facile,sensitive and selective method for cadmium and silver assays is highly desirable for protecting human health and ecological environment. The biosensing methods for the detection of heavy metals based on aptamer probe have become an emerging research field. Cadmium aptamer?Cd–4?has been reported to be able to combine with cadmium ion specificity. We found that this aptamer can be applied for the sensitive detection of Cd²⁺.Meanwhile, we have demonstrated that this novel multifunctional aptamer probe?CSAP? can act as a recognition element for Ag+ and a reporter. Thereby, two fluorescence methods for detecting Cd²⁺ and Ag+have been established based on a specific multifunctional aptamer probe.In the first part, the potential hazards caused by cadmium and silver are outlined. Then, the main methods for detecting cadmium and silver and their recent developments are summarized. Furthermore, the application of nucleic acid aptamer to detect metal ion are introduced.A novel fluorescence strategy for detecting Cd²⁺ based on the a cadmium specific aptamer probe?CSAP? has been established in chapter2. In HEPES buffer solution of p H 7.3, this multifunctional probe acts as a recognition element for Cd²⁺ and a reporter. The presence of Cd²⁺ can induce the conformational switching of CSAP, whih caused the G4 close to the 6–FAM, accompanied by a change in fluorescence intensity of the sensing system. Thereby, a novel method for the quantitative detection of Cd²⁺ was developed. The fluorescence change value was directly proportional to the concentrations ranging from 7.19×10–9 mol·L^-1 to5.00×10–6 mol·L^-1 for cadmium. The proposed method gives the detection limits down to 2.15 ×10–9 mol·L^-1 for Cd²⁺, which are much lower than those reported by related literatures. This strategy has been apllied for monitoring Cd²⁺ in envioronmental water. Furthermore, the mechanisms of the interaction of CSAP with Cd²⁺ were also demonstrated by CD,fluorescence and UV–vis spectra, which provided the new thought,technology, and method for biomedical science, molecular biology,toxicology and environmental science.In the chapter 3, based on the conformational switching of CSAP induced by Ag+, we proposed a new method for the fluorescent quenching determination of Ag+. In the buffer of Hepes, an intramolecular fold of CSAP is able to occur by the inducing of Ag+, bringing the four modified guanine?G4? at 5' –end close to 6–carboxyfluorescein to form a stable stem–loop structure. This results in a light-induced electron transfer along with the fluorescence quenching of the system. The linear range of this method was from 2.13×10^-9 mol·L^-1 to 1.00×10^-7 mol·L^-1 with a correlation coefficient?r? of 0.9979. The linear regression equation was?F = 28.46 + 5.960c(×10–9 mol·L^-1), and the detection limit was6.40×10^-10 mol·L^-1. This sensitive strategy has been used for the determination of silver ion in real samples. In addition, this strategy avoids the label steps of a quencher in common molecular beaconstrategies, without expensive equipment. It should be more practical,convenient and economical in its operation.