Preparation, Properties And Analytical Applications Of CdTe Quantum Dots As PH-Sensitive Fluorescent Probes

Quantum dots (QDs) have been attracting much attention owing to their ideal optical properties in the past decade. In comparison to traditional organic dyes and fluorescent proteins, QDs offer advantages in many aspects such as a narrow, tunable, symmetric emission from visible to near infrared wavelengths, high level of brightness and photochemical stability. In principle, this novel feature of QDs can be extended for detecting specific analytes if appropriate conditions are established.In previous researches, it was noted that the pH value of some media could obviously affected the fluorescence intensity of QDs, indicating [H+] is also correlated with their fluorescence properties. In the present work, thioglycolic acid-capped CdTe QDs were prepared by hydrothermal synthesis method. It was found that the fluorescence intensity of CdTe QDs in PBS was pH sensitive and went through a well linear change in a range of 5.8～8.0. Under appropriate conditions, pH-sensitive CdTe QDs probes can be used for the detection of acidity analytes and acidic middle production in the process of reaction.A new method for the determination of NH4+ using pH-sensitive probes based on the fluorescence quenching of CdTe QDs caused by pH value changes due to the presence of NH4+ in aqueous medium was developed. Under optimized conditions, the fluorescence quenching of CdTe QDs was proportional to the concentration of NH4+ in the range of 5.0×10-5～6.0×10-3 mol/L with a correlation coefficient of 0.9997, a detection limit of 1.5×10-5 mol/L (3σ) and RSD of 3.2%(1×10-3 mol/L, n=11) were obtained. The method was applied to the determination of NH4+ in water samples. The results were in a good agreement with those obtained by the standard distillation-titration method.The pH-sensitive CdTe QDs fluorescence probes can also be used to determine glucose, and another new method for the determination of glucose in human saliva samples was developed. This method possesses many advantages, including simplicity, low cost, high flexibility, good sensitivity and high specificity mainly due to the fluorescence resonance energy transfer (FRET) between glucose oxidase (GOD) and QDs as well as to the high substrate specificity to glucose of GOD. The fluorescence quenching of CdTe QDs was directly proportional to the concentration of glucose in the range of 1.0×10-5～8.0×10-4 mol/L (R=0.9990). The detection limit of 3.3×10-6 mol/L glucose (3a) was obtained with the relative standard deviation (RSD) of 4.1%(5.0×10-5 mol/L, n=11). This method also provided a convenient route to detect the glucose concentration by visualizing the color change of QDs fluorescence, which offered a new way to determine the glucose concentration without using complicate instrumental application.