Here we report the synthesis and size-dependent optical study of a series of bulky t-butyl thiolated monodisperse nanomolecules (NMs): Au23(S-tBu)16, Au30(S-tBu)18, Au46(S-tBu)24, and Au65(S-tBu)29. We further employ this series of NMs to address how the NM core size affects electrocatalytic oxygen reduction reactivity of these NMs in alkaline media. Though Au23(S-tBu)16 and Au30(S-tBu)18 have been reported before, the expansion of this series is brought about by the discovery, through the synthesis and characterization, of two new and larger species, in the bulky t-butyl thiolated NM series: namely Au46(S-tBu)24 and Au65(S-tBu)29. Furthermore, we take an in-depth look at the ligand effects in the bulky thiolated NM series and ligand dictation over the gold atoms to surface ligands. Assignments of molecular formulae are based on results obtained in high-resolution nanoelectrospray ionization mass spectrometry (nESI-MS). Purity was confirmed with matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Further analysis of the new bulky thiolated NMs, Au46(S-tBu)24 and Au65(S-tBu)29, was conducted using UV–vis-NIR spectroscopy to observe their unique optical properties. With the expansion of this series, we conclude that the bulky t-butyl thiolated series is unique from those observed with aliphatic and aromatic ligands; that is, how the bulky ligands dictate the size, composition, and structure of the NM is distinct from those of aliphatic and aromatic ligands. This provides insight into the tunability and core size manipulation of gold thiolate NM (i.e., we can make different sizes of NMs using specific types of ligands by varying the gold to thiol ratio). The largest NM Au65(S-tBu)29 was found to facilitate 80% OH– production, while the smallest size NM Au23(S-tBu)16 produced 53% OH–. From the optical measurements we determined that the excited state lifetimes of this series of t-butyl thiolate protected gold NMs follow the energy gap law except for Au30(S-tBu)18 which can be ascribed to its symmetry.

The influence of passivating ligand on electron-phonon relaxation dynamics of the smallest sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The electron dynamics in Au279, Au329, and Au329 passivated with 4-tert-butylbenzene thiol (TBBT), phenylethane thiol (SC2Ph) and hexane thiol (SC6), respectively, were investigated. These clusters were chosen as they are the smallest gold clusters reported till-date to show plasmonic behavior. Ultrafast transient absorption measurements were also carried out on Au~1400 (SC6) and Au~2000 (SC6) to understand the influence of the size on electron-phonon relaxation with the same passivating ligand. The study has revealed interesting aspects on the role of ligand on electron-phonon relaxation dynamics

Accurate measurements of intracellular pH are of crucial importance in understanding the cellular activities and in the development of intracellular drug delivery systems. Here we report a highly sensitive pH probe based on a fluorescein-conjugated Au nanocluster. Steady-state photoluminescence (PL) measurements have shown that, when conjugated to Au, fluorescein exhibits more than 160-fold pH-contrasting PL in the pH range of 4.3-7.8. Transient absorption measurements show that there are two competing ultrafast processes in the fluorescein-conjugated Au nanocluster: the intracore-state relaxation and the energy transfer from the nonthermalized states of Au to fluorescein. The latter becomes predominant at a higher pH, leading to dramatic PL enhancement of fluorescein. In addition to the intrinsically low toxicity, fluorescein-conjugated Au nanoclusters exhibit high pH sensitivity, wide dynamic range, and excellent photostability, providing a powerful tool for the study of intracellular processes.

No abstract

A copper storage protein is used to synthesize gold clusters with tunable emission that follow the Irving–Williams series for metal detection.