Synthesis,Characterization And Properties Of Mercaptobenzoic Acid Protected Metal Nanoclusters

Metal nanoclusters（e.g.,gold and silver;NMCs for short）are a class of ultrasmall sub-genre nanoparticles containing few to several hundred metal atoms,which serves as a bridge between single atoms and large sized metal nanoparticles.NMCs have a"core-shell"structure:a metal core of≤2 nm,and an organic ligand shell（e.g.,thiolate,phosphine,and alkyne ligands）.Among these,thiolate-protected NMCs can be represented by the general formula[M_n（SR）_m]^q,where n and m respectively denote the number of metal atoms and thiolate ligands,while q represents the charge carried by the clusters.Owing to the quantum confinement effect,ultrasmall NMCs exhibit some interesting molecular-like properties that are different from those of metal nanoparticles,such as discrete electronic states,unique atomic packing,HOMO-LOMO transition,strong fluorescence,magnetism,and optical chirality.These interesting properties are directly correlated to the size of NMCs,which are of great value for both fundamental and applied studies,and also the driving force for the efficient synthesis of monodisperse NMCs.In this thesis,the sodium hydroxide-assisted sodium borohydride reduction strategy is applied for the synthesis of a series of thermodynamically stable monodisperse mercaptobenzoic acid-protected NMCs with atomic precision.The size information at atomic level of such NMCs is analyzed by electrospray ionization mass spectrometry（ESI-MS）,the optical properties are studied by UV-visible spectrophotometer,and their electrocatalytic activity towards hydrogen peroxide detection is examined by an electrochemical workstation.Please see more contents as follows:（1）The subtle structural change of hydrophilic ligands on the size control of metal nanoclusters（NCs）is unclear but critically important for fundamental understanding.Herein,we report our findings that subtle changes of isomeric ligands lead to a dramatic difference in the size of water-soluble Au NCs.By using isomeric para-mercaptobenzoic acid（p-MBA,m-MBA,and o-MBA）as model ligands,it is found that both the steric hindrance and the electronic effect of isomeric ligands significantly influences the size of Au NCs,resulting in the formation of different sized Au₄₄（p-MBA）₂₆ NCs,Au₂₅（m-MBA）₁₈ NCs,and Au₃₇/₄₃（o-MBA）₂₂/₂₆ NCs.In addition,the growth process of Au₄₄NCs from Au（I）-MBA complexes in the NaBH₄ reduction system is also monitored by real-time UV-vis absorption spectroscopy and ESI mass spectrometry,which complies with the 2e^-hopping growth principle,indicating the universal applicability of this principle in the synthesis of thiolated metal NCs.（2）While the transformation of NMCs from one size to another size has been widely studied in the oil-phase system,the same study in the water-phase system is rarely reported,and the underlying transformation mechanism of water-soluble NMCs is still undiscovered.To address this issue,we employ p-MBA and o-MBA as bi-ligand for the synthesis of brownish Ag NCs（b-Ag NCs for short）.Subsequently,the b-Ag NCs could be successfully converted into green Ag NCs（g-Ag NCs for short）via the NaBH₄-induction strategy,which demonstrates the facile conversion between Ag NCs based on the reduction induction of NaBH₄ in water phase.（3）Hydrogen peroxide（H₂O₂）detection is an important application in biomedicine and other fields,and it is urgently needed to develop a low-cost,high-sensitivity,and facile detection method for H₂O₂ detection.Herein we report a facile detection probe for H₂O₂ based on the usage of Au₄₄（p-MBA）₂₆ NCs as electrocatalyst model.Firstly,the successfully synthesized Au₄₄（p-MBA）₂₆ NCs are grafted to the surface of glassy carbon electrode（GCE）for constructing the electrochemical sensor.Electrochemical results show that the Au₄₄（p-MBA）₂₆ NCs exhibit excellent detection sensitivity for H₂O₂.