Surface-Functionalized CdS Clusters with Recognition Sites near the Interface: Selective Luminescence Response to Lipophilic Phenols

A series of water-soluble cadmium sulfide clusters bearing an alkyl-chain layer between the inorganic core and the outer PEG layer were synthesized by the ligand-exchange reaction of Cd<sub>10</sub>S<sub>4</sub>(SPh)<sub>12</sub> with thiols functionalized by an <i>N</i>-(ω-PEGylated alkyl) amide moiety. The photoluminescence titration experiments in aqueous media revealed that clusters with a sufficiently hydrophobic inner environment exhibit definite emission enhancements upon the addition of bisphenol A or 4-nonylphenol. The dramatic effect of the alkyl chain length on the emission responses demonstrated that the hydrophobic layer around the inorganic surface serves as guest binding sites to facilitate the access of the lipophilic phenols near the organic−inorganic interface. A marked preference for the lipophilic phenols over related compounds, such as methylated bisphenol A, long-chain <i>n</i>-alkanol, and nonlipophilic phenols, was observed in the emission responses of the “hydrophobic” cluster, suggesting that not only the hydrophobic interaction but also the attractive force involving the phenolic OH group contributes to the positive responses. The results of control experiments and IR studies indicated that the hydrogen bonding interaction between the phenolic OH group and the amide group in the surface organic units is responsible for the positive emission responses. The present work shows that the precise tuning of the molecular recognition environments near the organic−inorganic interface is useful for developing guest-specific functions.