Enzyme-activatable charge transfer in gold nanoclusters.

Surface-protecting ligands, as a major component of metal nanoclusters (MNCs), can dominate molecular characteristics, performance behaviors, and biological properties of MNCs, which brings diversity and flexibility to the nanoclusters and largely promotes their applications in optics, electricity, magnetism, catalysis, biology, and other fields. We report herein the design of a new kind of water-soluble luminescent gold nanoclusters (AuNCs) for enzyme-activatable charge transfer (CT) based on the ligand engineering of AuNCs with 6-mercaptopurine ribonucleoside (MPR). This elaborately designed cluster, Au5(MPR)2, can form a stable intramolecular CT state after light excitation, and exhibits long-lived color-tunable phosphorescence. After the cleavage by purine nucleoside phosphorylase (PNP), the CT triplet state can be easily directed to a low-lying energy level, leading to a bathochromic shift of the emission band accompanied by weaker and shorter-lived luminescence. Remarkably, these ligand-engineered AuNCs show high affinity towards PNP as well as decent performance for analyzing and visualizing enzyme activity and related drugs. The work of this paper provides a good example for diversifying physicochemical properties and application scenarios of MNCs by rational ligand engineering, which will facilitate future interest and new strategies to precisely engineer solution-based nanocluster materials.

Highly Efficient Luminescence from Charge-Transfer Gold Nanoclusters Enabled by Lewis Acid.

Understanding the complicated intramolecular charge transfer (ICT) behaviors of nanomaterials is crucial to the development of high-quality nanoluminophores for various applications. However, the ICT process in molecule-like metal nanoclusters has been rarely explored. Herein, a proton binding-induced enhanced ICT state is discovered in 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). Such an excited-state electron transfer process gives rise to the weakened and red-shifted photoluminescence of these nanoclusters. By the joint use of this newfound ICT mechanism and a restriction of intramolecular motion (RIM) strategy, a red shift in the emission maxima of 30 nm with 27.5-fold higher fluorescence quantum efficiency is achieved after introducing rare-earth scandium ion (Sc3+) into ATT-AuNCs. Furthermore, it is found that upon the addition of Sc3+, the photoinduced electron transfer (PET) rate from ATT-AuNCs to minocycline is largely accelerated by forming a donor-bridge-acceptor structure. This paper offers a simple method to modulate the luminescent properties of metal nanoclusters for the rational design of next-generation sensing platforms.

Regulation of metal ion selectivity of fluorescent gold nanoclusters by metallophilic interactions.

Although numerous sensors have been successfully fabricated for the detection of various heavy metal ions by employing fluorescent gold nanoclusters (AuNCs) as nanoprobes, serious cross-interference often occurs when these ions coexist in samples, which results in glaring errors in quantification. In this study, glutathione-protected AuNCs (GSH-AuNCs) were synthesized and found to respond to both Cu2+ and Hg2+ via fluorescence suppression. Intriguingly, addition of Ag+ to GSH-AuNCs could completely inhibit the quenching effect of Hg2+ while not affecting the Cu2+-mediated quenching process. Ag+ can combine with Au+ on the surface of AuNCs to form a strong Ag+-Au+ metallic bond, which disrupts the interaction between Hg2+ and Au+ and thus eliminates the corresponding quenching effect. Based on this phenomenon, a simple sensing approach for highly selective and sensitive detection of Cu2+ in aqueous solution was developed using the GSH-AuNC/Ag+ complex as a fluorescent turn-off nanoprobe. The proposed method exhibited good linearity in the concentration range 0.02-10 µM with a limit of detection of 12 nM. This assay was demonstrated to be suitable for determination of Cu2+ in real water samples even in the presence of Hg2+, showing great promise as a tool for assessment of environmental security and drinking water quality.