Kyunglim Pyo

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)–Thiolate Shell

Luminescent nanomaterials have captured the imagination of scientists for a long time and offer great promise for applications in organic/inorganic light-emitting displays, optoelectronics, optical sensors, biomedical imaging, and diagnostics. Atomically precise gold clusters with well-defined core-shell structures present bright prospects to achieve high photoluminescence efficiencies. In this study, gold clusters with a luminescence quantum yield greater than 60% were synthesized based on the Au22(SG)18 cluster, where SG is glutathione, by rigidifying its gold shell with tetraoctylammonium (TOA) cations. Time-resolved and temperature-dependent optical measurements on Au22(SG)18 have shown the presence of high quantum yield visible luminescence below freezing, indicating that shell rigidity enhances the luminescence quantum efficiency. To achieve high rigidity of the gold shell, Au22(SG)18 was bound to bulky TOA that resulted in greater than 60% quantum yield luminescence at room temperature. Optical measurements have confirmed that the rigidity of gold shell was responsible for the luminescence enhancement. This work presents an effective strategy to enhance the photoluminescence efficiencies of gold clusters by rigidifying the Au(I)-thiolate shell.

Ionic liquid of a gold nanocluster: a versatile matrix for electrochemical biosensors.

Ionic liquids are room-temperature molten salts that are increasingly used in electrochemical devices, such as batteries, fuel cells, and sensors, where their intrinsic ionic conductivity is exploited. Here we demonstrate that combining anionic, redox-active Au25 clusters with imidazolium cations leads to a stable ionic liquid possessing both ionic and electronic conductivity. The Au25 ionic liquid was found to act as a versatile matrix for amperometric enzyme biosensors toward the detection of glucose. Enzyme electrodes prepared by incorporating glucose oxidase in the Au25 ionic liquid show high electrocatalytic activity and substrate affinity. Au25 clusters in the electrode were found to act as effective redox mediators as well as electronic conductors determining the detection sensitivity. With the unique electrochemical properties and almost unlimited structural tunability, the ionic liquids of quantum-sized gold clusters may serve as versatile matrices for a variety of electrochemical biosensors.

Highly Luminescent Folate-Functionalized Au22 Nanoclusters for Bioimaging

Gold nanoclusters are emerging as new materials for biomedical applications because of promises offered by their ultrasmall size and excellent biocompatibility. Here, the synthesis and optical and biological characterizations of a highly luminescent folate-functionalized Au22 cluster (Au22 -FA) are reported. The Au22 -FA clusters are synthesized by functionalizing the surface of Au22 (SG)18 clusters, where SG is glutathione, with benzyl chloroformate and folate. The functionalized clusters are highly water-soluble and exhibit remarkably bright luminescence with a quantum yield of 42%, significantly higher than any other water-soluble gold clusters protected with thiolate ligands. The folate groups conjugated to the gold cluster give rise to additional luminescence enhancement by energy transfer sensitization. The brightness of Au22 -FA is found to be 4.77 mM-1 cm-1 , nearly 8-fold brighter than that of Au22 (SG)18 . Further biological characterizations have revealed that the Au22 -FA clusters are well-suited for bioimaging. The Au22 -FA clusters exhibit excellent photostability and low toxicity; nearly 80% cell viability at 1000 ppm of the cluster. Additionally, the Au22 -FA clusters show target specificity to folate-receptor positive cells. Finally, the time-course in vivo luminescence images of intravenous-injected mice show that the Au22 -FA clusters are renal-clearable, leaving only 8% of them remained in the body after 24 h post-injection.

Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules

The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of n-hexanethiolate-protected gold clusters ranging from Au25 to Au333. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO-LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules.

Unique Energy Transfer in Fluorescein-Conjugated Au22 Nanoclusters Leading to 160-Fold pH-Contrasting Photoluminescence

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 Au22 nanocluster. Steady-state photoluminescence (PL) measurements have shown that, when conjugated to Au22, 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 Au22 nanocluster: the intracore-state relaxation and the energy transfer from the nonthermalized states of Au22 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 Au22 nanoclusters exhibit high pH sensitivity, wide dynamic range, and excellent photostability, providing a powerful tool for the study of intracellular processes.