Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering

Abstract

The ligand shell (LS) determines a number of nanoparticles’ properties. Nanoparticles’ cores can be accurately characterized; yet the structure of the LS, when composed of mixture of molecules, can be described only qualitatively (e.g., patchy, Janus, and random). Here we show that quantitative description of the LS’ morphology of monodisperse nanoparticles can be obtained using small-angle neutron scattering (SANS), measured at multiple contrasts, achieved by either ligand or solvent deuteration. Three-dimensional models of the nanoparticles’ core and LS are generated using an ab initio reconstruction method. Characteristic length scales extracted from the models are compared with simulations. We also characterize the evolution of the LS upon thermal annealing, and investigate the LS morphology of mixed-ligand copper and silver nanoparticles as well as gold nanoparticles coated with ternary mixtures. Our results suggest that SANS combined with multiphase modeling is a versatile approach for the characterization of nanoparticles’ LS.

Mixed Fluorinated/Hydrogenated Self‐Assembled Monolayer‐Protected Gold Nanoparticles: In Silico and In Vitro Behavior

“…Matrix‐assisted laser desorption/ionization mass analysis (MALDI‐MS) could also distinguish different surface organizations of immiscible thiolate ligands, but only if they desorb with equal probability, which is not necessarily the case, particularly in presence of fluorinated ligands, as we have also observed . Finally, another alternative, small angle neutron scattering (SANS), is limited to nanoparticle samples with very narrow size dispersity, not achievable for C8TEG/F8PEG‐modified AuNPs with our current synthetic methodologies. This left an in silico approach as the only viable method currently available to directly characterize this type of monolayer self‐organization, given also it has already been successful in predicting phase segregation of several mixtures of immiscible thiols at molecular level …”

“…Finally, another alternative, small angle neutron scattering (SANS), is limited to nanoparticle samples with very narrow size dispersity, not achievable for C8TEG/F8PEG‐modified AuNPs with our current synthetic methodologies. This left an in silico approach as the only viable method currently available to directly characterize this type of monolayer self‐organization, given also it has already been successful in predicting phase segregation of several mixtures of immiscible thiols at molecular level …”

Mass spectrometry and Monte Carlo method mapping of nanoparticle ligand shell morphology

“…The method does not have a true feedback for symmetry and in fact often the symmetry retrieved is lower than the input. Interestingly, the SANS method developed recently has the opposite characteristic with a signal that mostly based on the overall symmetry of the particles and much less on the local LSM composition 33 . Meanwhile, the fitting procedure is based on random switch of bead assignments and does not lead to higher symmetry.…”

“…In order to further prove the validity of our method, we focus on the comparison between SANS and MALDI-TOF, as SANS is the only existing technique that can be used to quantitatively characterize mixed ligand shells with complicated morphologies. We chose to analyze with MALDI the same Ag NP that had been reported previously, i.e., Ag NPs protected with deuterated PET and DDT 33 . As shown in Fig.…”

“…Spectroscopic techniques need a full series of NPs with varying composition to reach a qualitative conclusion 26 – 28 . The only two methods that can quantitatively characterize the ligand shell structures are STM 29 – 32 and SANS 33 . While STM can be used to extract length scales of stripe-like domains, it falls short in describing any other morphology.…”