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

The synthesis of gold nanoparticles (core size less than 2.0 nm) capped by thiolate a-cyclodextrin (alpha-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 center dot 3H(2)O) is reduced by NaBH4 in the presence of alpha-CD-SH to produce thiolate a-cyclodextrin-stabilized gold nanoparticles (alpha-CD-S-AuNPs). The particle size of the as-synthesized alpha-CD-S-AuNPs is highly dependent on the initial molar ratio of alpha-CD-SH to AuCl4- (alpha-CD-SH/Au) precursors. When the alpha-CD-SH/Au is kept greater than or equal to 1, alpha-CD-S-AuNPs (core size < 2.0 nm) are acquired, and their size increases with increasing alpha-CD-SH/Au. It is postulated that the increase in particle size is attributed to the interhydrogen bond between the alpha-CD-SH molecules at higher concentrations with a concomitant decrease in the availability of free alpha-CD-SH to stabilize the AuNP surface. By contrast, when the alpha-CD-SH/Au is controlled at <1, larger alpha-CD-S-AuNPs (core size >2.5 nm) with typical surface plasmon bands are obtained, and the particle size increases with the decrease in alpha-CD-SH/Au. The average chemical compositions of such AuNPs in the empirical formula Au-x(alpha-CD-S)(y) are further determined by thermogravimetric analysis, mass spectrometry, and atomic absorption spectroscopy. These alpha-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 alpha-CD-S-AuNP shows photoluminescence enhancement in the presence of tetraalkylammonium ions but is strongly quenched by Hg(II). The alpha-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).