Near-infrared electrogenerated chemiluminescence of Au22(glutathione)18 nanoclusters in aqueous solution and its analytical application

© 2020Gold nanoclusters (AuNCs) have attracted great attention in analytical sciences because they have unique optical and electrochemical properties with low biological toxicity. However, the analytical application of Au NCs-based electrogenerated chemiluminescence (ECL) still remains relatively unexplored largely due to their low ECL efficiency in aqueous solution. Moreover, the precise information of molecular formula of Au NCs have not been determined in large number of reports. In the present study, the ECL behavior of atomically precise Au22(SG)18 NCs, where SG is glutathione, were characterized in the aqueous solution. The oxidation of Au22(SG)18 NCs together with coreactant tripropylamine (TPA) at the working electrode potential of 1.25 V vs Ag/AgCl (3 M NaCl) has led to strong ECL emission at ~767 nm in the near-infrared region. The ECL intensity of the Au22(SG)18 NCs was about 14-fold higher than the ECL intensity obtained with the Au@BSA NCs, which is most commonly used Au NCs in aqueous solution. The ECL intensity is strongly dependent upon the concentrations of Au22(SG)18 NCs and TPA. However, the ECL intensity of Au22(SG)18 NCs with TPA is diminished in the presence of phenolic compounds depending upon their concentrations. Using catechol as a model analyte, the present ECL system can detect it in the linear range from 1.0 × 10−7 to 1.0 × 10−4 M with a detection limit of 3.3 × 10−9 M (S/N = 3). The present study has demonstrated the potential application of the Au NCs-based ECL in real analytical problems.11Nsciescopu

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

Atomistic View of the Energy Transfer in a Fluorophore-Functionalized Gold Nanocluster

Understanding the dynamics of Förster resonance energy transfer (FRET) in fluorophore-functionalized nanomaterials is critical for developing and utilizing such materials in biomedical imaging and optical sensing applications. However, structural dynamics of noncovalently bound systems have a significant effect on the FRET properties affecting their applications in solutions. Here, we study the dynamics of the FRET in atomistic detail by disclosing the structural dynamics of the noncovalently bound azadioxotriangulenium dye (KU) and atomically precise gold nanocluster (Au25(p-MBA)18, p-MBA = para-mercaptobenzoic acid) with a combination of experimental and computational methods. Two distinct subpopulations involved in the energy transfer process between the KU dye and the Au25(p-MBA)18 nanoclusters were resolved by time-resolved fluorescence experiments. Molecular dynamics simulations revealed that KU is bound to the surface of Au25(p-MBA)18 by interacting with the p-MBA ligands as a monomer and as a π–π stacked dimer where the center-to-center distance of the monomers to Au25(p-MBA)18 is separated by ∼0.2 nm, thus explaining the experimental observations. The ratio of the observed energy transfer rates was in reasonably good agreement with the well-known 1/R6 distance dependence for FRET. This work discloses the structural dynamics of the noncovalently bound nanocluster-based system in water solution, providing new insight into the dynamics and energy transfer mechanism of the fluorophore-functionalized gold nanocluster at an atomistic level.peerReviewe

Tailoring the interaction between a gold nanocluster and a fluorescent dye by cluster size : creating a toolbox of range-adjustable pH sensors

We present a novel strategy for tailoring the fluorescent azadioxatriangulenium (KU) dye-based pH sensor to the target pH range by regulating the pKa value of the gold nanoclusters. Based on the correlation between the pKa and surface curvature of ligand-protected nanoparticles, the pKa value of the gold nanoclusters was controlled by size. In particular, three different-sized para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters, Au25(p-MBA)18, Au102(p-MBA)44, and Au210–230(p-MBA)70–80 were used as the regulator for the pH range of the KU response. The negatively charged gold nanoclusters enabled the positively charged KU to bind to the surface, forming a complex and quenching the fluorescence of the KU by the energy transfer process. The fluorescence was restored after adjusting the surface charge of the gold nanocluster by controlling the solution pH. In addition, the KU exhibited a significantly different pH response behaviour for each gold nanocluster. Au210–230(p-MBA)70–80 showed a higher pH response range than Au102(p-MBA)44, which was intuitive. However, Au25(p-MBA)18 showed an unexpectedly high pH response behaviour. pKa titration measurement, molecular dynamics simulations, and essential dynamics analysis showed that small nanoclusters do not follow the scaling between the curvature and the pKa value. Instead, the behaviour is governed by the distribution and interaction of p-MBA ligands on the nanocluster surface. This work presents an effective design strategy for fabricating a range adjustable pH sensor by understanding the protonation behaviour of the ultrasmall gold nanoclusters in an atomic range.peerReviewe

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 <i>n</i>-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