Optical Properties of 2‑Methacryloyloxyethyl Phosphorylcholine-Protected Au₄ Nanoclusters and Their Fluorescence Sensing of C‑Reactive Protein

We present the solution synthesis of thiolated 2-methacryloyloxyethyl phosphorylcholine (MPC)-protected Au nanoclusters (NCs). This water-soluble lipid-mimetic MPC was first used for the size focusing synthesis of thiolate (SR)-protected Au_<i>n</i>(SR)_<i>m</i> NCs. Au₂₅(MPC)₁₈ and Au₄(MPC)₄ NCs are selectively synthesized, without the need for electrophoretic or chromatographic isolation of size mixed products, by including ethanol or not in the solvent. The Au₄(MPC)₄ NCs emit at yellow wavelengths (580–600 nm) with a quantum yield (3.6%) and an average lifetime of 1.5 μs. Also for the first time, we report C-reactive protein (CRP) sensing using Au NCs, with a detection limit (5 nM) low enough for the clinical diagnosis of inflammation. This is based on the quenching effect of specific CRP–MPC interactions on the fluorescence of the Au₄(MPC)₄ NCs

Generation of Singlet Oxygen by Photoexcited Au₂₅(SR)₁₈ Clusters

The generation of highly reactive singlet oxygen (¹O₂) is of major importance for a variety of applications such as photodynamic therapy (PDT) for cancer treatment, water treatment, catalytic oxidation, and others. Herein, we demonstrate that ¹O₂ can be efficiently produced through the direct photosensitization by Au₂₅(SR)₁₈^– clusters (H−SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532, 650, and 808 nm) irradiation. ¹O₂ was successfully detected by direct observation of the characteristic ¹O₂ emission around 1276 nm as well as three different ¹O₂-selective probes. Water-soluble Au₂₅(captopril)₁₈^– clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition, selective catalytic oxidation of organic sulfide to sulfoxide by ¹O₂ was demonstrated on the photoexcited Au₂₅(SC₂H₄Ph)₁₈^– clusters. It is suggested that the optical gap of Au₂₅(SR)₁₈ clusters (∼1.3 eV) being larger than the energy of ¹O₂ (0.97 eV) allows for the efficient energy transfer to ³O₂. In addition, the long lifetime of the electronic excited states of Au₂₅(SR)₁₈ and the well-defined O₂ adsorption sites are the key factors that promote energy transfer from Au₂₅(SR)₁₈^– to molecular oxygen, thus facilitating the formation of ¹O₂. Finally, neutral Au₂₅(SR)₁₈⁰ can also produce ¹O₂ as efficiently as does the anionic Au₂₅(SR)₁₈⁻