Hydrogen Bonding Properties Of Water Molecules At The Interface Of The Monolayer Protected Au Nanoparticles From Molecular Dynamics Simulation

Recently, monolayer protected Au nanoparticles （MPANs） have extensivepotential applications in biosensing, imaging, diagnosis of diseases, and drug deliverydue to their superior properties. The properties of MPANs are significantly related tothe length and the terminal of surfactants, composition, as well as the surroundingsolvents. Therefore, understanding of intrinsic relationship among them is of greatimportance for the design and synthesis of various MPANs with tailored properties.Owing to the similar sizes and surface properties with the special proteins, aminoacid–terminated MPANs, including carboxylic acids and amino groups, are excellentcandidates for "artificial proteins", which have attracted more and more attention byboth experimental and computational researchers. With the rapid development ofcomputer technology and theoretical method, molecular simulation has beenconsidered as one of the promising approaches for studying nanoscale systems, sinceit can not only provide adequate and accurate microscopic details for experimentalresearchers, but also arbitrarily choose "artificial proteins" with desired sizes andstructures.In this work, we have applied molecular dynamics （MD） simulation and densityfunction theory （DFT） calculations to systematically investigate the structure,dynamics properties and hydrogen bonds （HBs） of H₂O molecules at the interface ofthe charged MPANs capped by the NH₃⁺–terminated pentanethiols[Au₁₄₀（SH（CH₂）₅NH₃⁺）₆₂]. Based on the simulation results, we found that awell–defined multi–layer structure hydration layer is formed around the cationicMPANs. Meanwhile, a stable "ion wall" consisting of NH₃⁺groups and Cl–counterions was also found at the outmost region of self–assembled monolayers （SAMs） owing to the combination of both strong electrostatic and HB interactions,where the translational and rotational motions of H₂O molecules slow considerablydown compared to those in the bulk owing to the presence of SAMs and ion wall.Furthermore, we found that the translational motions of interfacial H₂O moleculesdisplay a sub–diffusive behavior while their rotational motions exhibit anonexponential feature. The unique behavior of interfacial H₂O molecules around theMPANs can be attributed to the interfacial hydrogen bond （HB） dynamics. Bycomparison, the lifetime of NH₃⁺–Cl–HBs was found to be the strongest and longest,favoring the stability of ion wall. Meanwhile, the lifetime of H₂O–H₂O HBs shows anobvious increase when the H₂O molecules approach the Au core, suggesting theenhanced H₂O–H₂O HBs around the charged MPANs, which is contrary to theweaken H₂O–H₂O HBs around the neutral MPANs. Moreover, the HB lifetimesbetween H₂O molecules and the ion wall （i.e., the Cl––H₂O and NH₃⁺–H₂O HBs） aremuch longer than that of interfacial H₂O–H₂O HBs, which leads to the increasingrotational relaxation time and residence time of H₂O molecules surrounding the ionwall. In addition, the corresponding binding energies for different HB types obtainedfrom the precise DFT are in excellent accordance with above simulation results. Thedetailed HB dynamics studied in this work provides insights into the unique behaviorof H₂O molecules at the interface of charged self–assemblies of nanoparticles as wellas proteins, which is also valuable to the development experimental andcomputational of MPANs.