Synthesis of 1.4 nm α-Cyclodextrin-Protected Gold Nanoparticles for Luminescence Sensing of Mercury(II) with Picomolar Detection Limit

Abstract

The synthesis of gold nanoparticles (core size less than 2.0 nm) capped by thiolate α-cyclodextrin (α-CD-SH) has been studied and characterized by infrared spectroscopy, UV−visible absorption spectroscopy, high-resolution transmission electron microscopy, and photoluminescence spectroscopy. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4•3H2O) is reduced by NaBH4 in the presence of α-CD-SH to produce thiolate α-cyclodextrin-stabilized gold nanoparticles (α-CD-S-AuNPs). The particle size of the as-synthesized α-CD-S-AuNPs is highly dependent on the initial molar ratio of α-CD-SH to AuCl4− (α-CD-SH/Au) precursors. When the α-CD-SH/Au is kept greater than or equal to 1, α-CD-S-AuNPs (core size 2.5 nm) with typical surface plasmon bands are obtained, and the particle size increases with the decrease in α-CD-SH/Au. The average chemical compositions of such AuNPs in the empirical formula Aux(α-CD-S)y are further determined by thermogravimetric analysis, mass spectrometry, and atomic absorption spectroscopy. These α-CD-S-AuNPs (core size < 2.0 nm) display remarkably strong blue emissions at 478 nm when excited at 400 nm. The 1.4 nm-sized α-CD-S-AuNP shows photoluminescence enhancement in the presence of tetraalkylammonium ions but is strongly quenched by Hg(II). The α-CD-S-AuNP possesses ultrahigh sensitivity and good selectivity for the determination of Hg(II) with the limit of detection at 49 pM (9.7 ppt)