Understanding Molecular Aggregation of Ligand-Protected Atomically-Precise Metal Nanoclusters

Abstract

Monolayer-protected atomically precise nanoclusters (MPCs) are an important class of molecules due to their unique structural features and diverse applications, including bioimaging, sensors, and drug carriers. Understanding the atomistic and dynamical details of their self-assembly process is crucial for designing system-specific applications. Here, we applied molecular dynamics and on-the-fly probability-based enhanced sampling simulations to study the aggregation of Au25(pMBA)18 MPCs in aqueous and methanol solutions. The MPCs interact via both hydrogen bonds and π-stacks between the aromatic ligands to form stable dimers, oligomers, and crystals. The dimerization free energy profiles reveal a pivotal role of the ligand charged state and solvent mediating the molecular aggregation. Furthermore, MPCs’ ligands exhibit suppressed conformational flexibility in the solid phase due to facile intercluster hydrogen bonds and π-stacks. Our work provides unprecedented molecular-level dynamical details of the aggregation process and conformational dynamics of MPCs ligands in solution and crystalline phases.

Noncovalent Interaction Guided Precise Photoluminescence Regulation of Gold Nanoclusters in Both Isolate Species and Aggregate States

“…As pH values shifted from 8.0 to 6.2, the Au 25 (pMBA) 18 showed monodispersed single‐cluster distribution with HDs stably controlled at approximately 2.4 nm, indicating that the enhanced emission of Au 25 (pMBA) 18 was originated from the intra‐cluster interaction after partial protonation of carboxyl group at pH 6.2 with decreased electrostatic repulsion. However, with the continued pH value decrease from 6.2 to 5.8, 5.0 and 4.0, the HDs of Au 25 (pMBA) 18 significantly increased from 2.5±0.4 nm to 7.5±1.5 nm, 46.8±9.9 nm, and 1833±366 nm, respectively, while the emission of Au 25 (pMBA) 18 quenched significantly at low pH values (e.g., 5.8 and 4.0), demonstrating that the inter‐cluster interactions of Au 25 (pMBA) 18 occurred to form large aggregates through both π–π stacking and hydrogen bonding interaction [18] . While the emission increase was highly corresponding to the intra‐cluster interactions within one single cluster with well‐maintained low HDs (approximately 2.4 nm) at the pH range from 8.0 to 6.2 (Figure 1E), the emission quenching was resulting from the inter‐cluster interactions among the Au 25 (pMBA) 18 with extremely increased HDs (from 2.5±0.4 nm to 1833±366 nm) at low pH values from 6.2 to 4.0.…”

“…After analysis of the abnormal pH-dependent behaviours of Au 25 (pMBA) 18 with ultrafast electron dynamics and photothermal investigation, we discovered that the intra-cluster interaction prolonged the emission lifetime and inhibited the nonradiative relaxation to enhance the emission, while the inter-cluster interaction significantly shortened the excited state lifetime from ns-level to ps-level and accelerated the nonradiative relaxation to quench the emission. Inspired by this discovery, we developed two strategies to regulate the NIR-II emission of Au 25 (pMBA) 18 . The introduction of exogenous substances (e.g., BS-12) to block the inter-cluster interaction could enhance the…”

Noncovalent Interaction Guided Precise Photoluminescence Regulation of Gold Nanoclusters in Both Isolate Species and Aggregate States

“…The Au 25 (pMBA) 18 with a structure of an icosahedral Au 13 core and six Au 2 (pMBA) 3 staple motifs showed NIR-II emission at 1100 nm from the core based orbitals [16] and narrow size distributions of 1.0 � 0.1 nm (Figure 1B and Figure S1A). The HOMO and LUMO of Au 25 (pMBA) 18 were originated from the Au 13 core atoms, which showed well-observed characteristic UV/Vis absorption at 700 nm (Figure 1B). [17] The cluster formula and molecular-level purity were confirmed by an electrospray ionization mass spectrometry (ESI-MS, Figure S1B and S1C).…”

“…As for SDBS, the lack of hydrogen bonding interaction with Au 25 (pMBA) 18 caused 13.4 times emission increase at pH 4.0. The interaction of TBAB and Au 25 -(pMBA) 18 showed an emission increase of 7.7 times due to the lack of hydrogen bonding interaction and weakened hydrophobic interaction with the shortened alkyl chains. However, the negligible interaction between TMAB and Au 25 (pMBA) 18 showed undetectable emission changes at pH 4.0.…”

Electrostatic-Driven Self-Assembly of Janus-like Monolayer-Protected Metal Nanoclusters

“…A rational switching function of the following form is employed to calculate the number of water molecules present at the interface: s 2 = prefix∑ i = 1 N 1 − true( r i r o true) 12 1 − true( r i r o true) 24 where r i is the distance between the center of COMs of MPCs from the oxygen of water molecules, r o is the radial cutoff distance, and N is the total number of water molecules. From our previous experience, we realized that the solvent CV, s 2 , is beneficial for efficient sampling of the monomer–dimer transitions. Using these two CVs, an OPES e simulation was performed for ∼2 μs, and multiple back-and-forth transitions between the two metastable states were observed (Figure S1a).…”