Molecular Features for Probing Small Amphiphilic Molecules with Self-Assembled Monolayer-Protected Nanoparticles

Spotting Local Environments in Self-Assembled Monolayer-Protected Gold Nanoparticles

Organic-inorganic (O-I) nanomaterials are versatile platforms for an incredible high number of applications, ranging from heterogeneous catalysis to molecular sensing, cell targeting, imaging, and cancer diagnosis and therapy, just to name a few. Much of their potential stems from the unique control of organic environments around inorganic sites within a single O-I nanomaterial, which allows for new properties that were inaccessible using purely organic or inorganic materials. Structural and mechanistic characterization plays a key role in understanding and rationally designing such hybrid nanoconstructs. Here, we introduce a general methodology to identify and classify local (supra)molecular environments in an archetypal class of O-I nanomaterials, i.e., self-assembled monolayer-protected gold nanoparticles (SAM-AuNPs). By using an atomistic machine-learning guided workflow based on the Smooth Overlap of Atomic Positions (SOAP) descriptor, we analyze a collection of chemically different SAM-AuNPs and detect and compare local environments in a way that is agnostic and automated, i.e., with no need of a priori information and minimal user intervention. In addition, the computational results coupled with experimental electron spin resonance measurements prove that is possible to have more than one local environment inside SAMs, being the thickness of the organic shell and solvation primary factors in the determining number and nature of multiple coexisting environments. These indications are extended to complex mixed hydrophilic-hydrophobic SAMs. This work demonstrates that it is possible to spot and compare local molecular environments in SAM-AuNPs exploiting atomistic machine-learning approaches, establishes ground rules to control them, and holds the potential for the rational design of O-I nanomaterials instructed from data.

Spontaneous desorption of protein from self-assembled monolayer (SAM)-coated gold nanoparticles induced by high temperature

The nonspecific binding of proteins with nanomaterials (NMs) is a dynamic reversible process including both protein adsorption and desorption parts, which is crucial for controlled release of protein drug loaded by nanocarriers. The nonspecific binding of proteins is susceptible to high temperature, whereas its underlying mechanism still remains elusive. Here, the binding behavior of human serum albumin (HSA) with an amino-terminated self-assembled monolayer (SAM)-coated gold (111) surface was investigated by using molecular dynamics (MD) simulations. HSA binds to the SAM surface through salt bridges at 300 K. As the temperature increases to 350 K, HSA maintains its native structure, while the salt bridges largely diminish owing to the considerable lateral diffusion of HSA on the SAM. Moreover, the interfacial water located between HSA and the SAM gets increased and prevents the reformation of the salt bridges of HSA with the SAM, which reduces the binding affinity of HSA. And HSA eventually desorbs from the SAM. The depiction of thermally induced spontaneous protein desorption enriches our understanding of reversible binding behavior of protein with NMs, and may provide new insights into the controlled release of protein drugs delivered by using nanocarriers under the regulation of high temperature.

Thiolate end-group regulates ligand arrangement, hydration and affinity for small compounds in monolayer-protected gold nanoparticles

The ability to control the properties of monolayer protected gold nanoparticles (MPNPs) discloses unrevealed features stemming from collective properties of the ligands forming the monolayer and presents opportunities to design new materials. To date, the influence of ligand end-group size and capacity to form hydrogen bonds on structure and hydration of small MPNPs (<5 nm) has been poorly studied. Here, we show that both features determine ligands order, solvent accessibility, capacity to host hydrophobic compounds and interfacial properties of MPNPs. The polarity perceived by a radical probe and its binding constant with the monolayer investigated by electron spin resonance is rationalized by molecular dynamics simulations, which suggest that larger space-filling groups - trimethylammonium, zwitterionic and short polyethylene glycol - favor a radial organization of the thiolates, whereas smaller groups - as sulfonate - promote the formation of bundles. Zwitterionic ligands create a surface network of hydrogen bonds, which affects nanoparticle hydrophobicity and maximize the partition equilibrium constant of the probe. This study discloses the role of the chemistry of the end-group on monolayer features with effects that span from molecular- to nano-scale and opens the door to a shift in the conception of new MPNPs exploiting the end-group as a novel design motif.