Synthesis And Fluorescence Of CdX Quantum Dots Capped With Mercapto Compounds

Quantum dots (QDs), also known as colloidal fluorescent nanocrystals or semiconductornanocrystals, were widely used as biological fluorescent probe and cell labelling for detection,analysis and tracking. The fluorescent properties of QDs were dependent on the capping agenton their surface. Guo group in Peking University focused on the synthesis of CdX（X=S,Se，Te）nanocrystals directly in the aqueous solution. They have reported the fuction of the ratioof hydrophobic segment of capping agents to hydrophilic segment on the optical properties ofCdSe QDs. And they also proposed a series of possible mechanisms and dynamics of thiols onthe nucleation and ripening of QDs and the rearrangement of capping agents on the surface ofQDs.In this paper, hydrophilic molecules with mercapto groups were adopted as cappingagents (L) to synthesize CdTe, CdS and ZnS QDs in aqueous solution. Among them, mercaptoacids or alcohols with oligo(ethylene glycol) were used for CdTe QDs. And2-mercaptopropionic acid (2MPA),3-mercaptopropionic acid (MPA),3-mercaptobutyric acid(3MBA) and3-mercapto-2-methylbutyric acid (MMBA) were used for ZnS and CdS QDs.Possible mechanism of these molecules on the growth of QDs was proposed. As-synthesizedQDs were characterized by UV-vis spectra and fluorescent emission spectra and theirquantum yield was obtained from the normalized area of fluorescent peaks. And theexperiment condition for the synthesis of QDs was optimized.The fact that capping agents with stronger hydrophilicity was favorable for rapidlyachieving QDs with relatively larger size and better optical properties was confirmed bycomparing QDs capped with mercapto acid and thiol with the same carbon length. Theexperiment using thiols with different length of oligo(ethylene glycol) segment alsoconfirmed this fact. Meanwhile, we also revealed that mercapto with many methyl side chainsled to QDs with larger size and lower fluorescent efficiency from the effect of the structure ofmercapto acids on the size and optical properties of QDs. CdS and ZnS QDs capped withmercapto acid with one methyl side group and suitable chain length (3MBA) exhibited thehighest fluorescent properties. The effect of experiment condition (pH, Cd/L, temperature, etc.) on the aqueoussynthesis of QDs was also studied. For CdS QDs, longer reaction time and higher reactiontemperature led to QDs with more prominent red shift of absorption peaks and faster growthof QDs. The faster growth of QDs led to aggregation of non-fluorescent QDs with certain size.With pH increasing, ZnS or CdS QDs capped with mercapto acids showed graduallyred-shifted absorption peaks, increase of their size, parabola-like fluorescent quantum yield.At optimum condition (pH=12，L:Zn:S=8:4:1，[S]=1.6mML-1), ZnS QDs capped with3MBAshowed the best optial properties. At optimized condition (L/Cd/S=5:2.5:1， pH=9.5T=115℃), CdS QDs capped with3MBA showed the highest quantum yield (11%) after100minof heating.Some radical initiators and electron acceptor for polymerization were synthesized. Theoptimum condition for the synthesis of3-((2-hydroxyethoxy)methyl)dibenziodolium tosylate(HDBI) with high yield was obtained. And ion(II) p-toluenesulfonate was also successfullysynthesized.