MALDI mass-spectrometry measurements are combined with self-consistent mean-field (SCF) calculations to detect and predict ligand phase separation on Ag nanoparticles. The experimental and theoretical techniques complement each other by enabling quantification of the nearest-neighbor distribution of a ligand mixture in a monolayer shell. By tracking a characteristic metallic fragment family, analysis of a MALDI spectrum produces a frequency distribution corresponding to specific ligand patterning. Inherent to the SCF calculation is the enumeration of local interactions that dictate ligand assembly. Interweaving MALDI and SCF facilitates a comparison between the experimentally and theoretically derived frequency distributions as well as their deviation from a well-mixed state. Thus, we combine these techniques to detect and predict phase separation in monolayers that mix uniformly or experience varying degrees of de-mixing, including microphase separation and stripe formation. Definition of MALDI removed as this is a commonly recognized technique.

Silver nanoparticles with mixed ligand self-assembled monolayers were synthesized from dodecanethiol and another ligand from a homologous series of alkanethiols (butanethiol, pentanethiol, heptanethiol, octanethiol, nonanethiol, decanethiol, undecanethiol, or dodecanethiol[D25]). These were hypothesized to exhibit ligand phase separation that increases with degree of physical mismatch between the ligands based on the difference in the number of carbons in the two ligands. Dodecanethiol/dodecanethiol[D25] was expected to exhibit minimal phase separation as the ligands have only isotopic differences, while dodecanethiol/butanethiol was hypothesized to exhibit the most phase separation due to the difference in chain length. Phase separation of all other ligand mixtures was expected to fall between these two extremes. Matrix-assisted laser desorption ionization (MALDI) mass spectroscopy provided a value for ligand phase separation by comparison with a binomial (random) model and subsequent calculation of the sum-of-squares error (SSR). These nanoparticle systems were also modeled using the Scheutjens and Fleer self-consistent mean-field theory (SCFT), which determined the most thermodynamically favorable arrangement of ligands on the surface. From MALDI, it was found that dodecanethiol/dodecanethiol[D25] formed a well-mixed monolayer with SSR = 0.002, and dodecanethiol/butanethiol formed a microphase separated monolayer with SSR = 0.164; in intermediate dodecanethiol/alkanethiol mixtures, SSR increased with increasing ligand length difference as expected. For comparison with experiment, an effective SSR value was calculated from SCFT simulations. The SSR values obtained by experiment and theory show good agreement and provide strong support for the validity of SCFT predictions of monolayer structure. These approaches represent robust methods of characterization for ligand phase separation on silver nanoparticles.

Classical nucleation theory and Derjaguin, Landau, Verwey, Overbeek (DLVO) theory for colloidal stability were applied to gain insight into the synthesis of dodecanethiol (DDT) functionalized silver nanoparticles (NPs) by reduction of silver nitrate with sodium borohydride in ethanol. This analysis indicated the importance of quickly establishing a dense DDT ligand brush on inherently unstable primary particles to achieve colloidal stability. The DLVO calculations also indicated that the electrostatic potential was a minor contributor to repulsive interactions, signifying that it would be possible to control NP size and uniformity in solutions with high ionic strength, as long as sufficient DDT was available to form a densely packed ligand layer on the NPs. These insights were applied to design a new straightforward, one-step, one-phase synthesis for the production of alkanethiol-functionalized silver NPs. To test the insights from DLVO theory, 16 samples were synthesized in the parameter space R = 3-12, S = 1-12 where R = [NaBH4]/[AgNO3], S = [DDT]/[AgNO3], and [AgNO3] = 10 mM. In general, samples with R = 3 or S = 1 were polydisperse; however, samples in the R = 6-12 and S = 3-12 range had uniform particle sizes with average diameters between 3.5 and 4.7 nm. Additionally, samples with R = 72-108 and S = 12 were synthesized to test particle stability at high ionic strength; again, uniform NPs with average diameters from 3.5 to 3.8 nm were produced. Ultimately, the insights gained from DLVO theory successfully guided the development of a one-step, one-phase technique for the synthesis of uniform, spherical alkanethiol-functionalized silver NPs.

MALDI mass-spectrometry measurements are combined with self-consistent mean-field (SCF) calculations to detect and predict ligand phase separation on Ag nanoparticles. The experimental and theoretical techniques complement each other by enabling quantification of the nearest-neighbor distribution of al igand mixture in am onolayer shell. By tracking ac haracteristic metallic fragment family,a nalysis of aM ALDI spectrum produces af requency distribution corresponding to specific ligand patterning. Inherent to the SCF calculation is the enumeration of local interactions that dictate ligand assembly.I nterweaving MALDI and SCF facilitates ac omparison between the experimentally and theoretically derived frequency distributions as well as their deviation from awell-mixed state.Thus,wecombine these techniques to detect and predict phase separation in monolayers that mix uniformly or experience varying degrees of de-mixing,i ncluding microphase separation and stripe formation. Definition of MALDI removed as this is ac ommonly recognized technique.The interfacial engineering of nanoparticles is an emergent area of research that is garnering significant interest for application in areas such as optics, [1] electronics, [2] and drug delivery. [3] Thus,i ti si mportant to exert control over the interface of nanoparticles,w hich dictates their degree of compatibility with and assembly in soft materials, [4] provides reactive sites for attachment of molecules,s uch as drug payloads, [3] and tunes the surface plasmon to awavelength of interest. [1] Thestarting point for interfacial modification is the self-assembled monolayer (SAM), or the layer of molecules (i.e., ligands) that form as hell around the nanoparticle.O ne strategy for controlling SAM properties involves the use of two or more ligands,r esulting in am ixed-ligand monolayer. On interfaces where ligands can attach and detach through adsorption/desorption equilibrium or move through surface diffusion, molecular simulations indicate that the phase separation of ligands occur for those with significant physical and/or chemical differences,s uch as alkane thiols with as ufficientc hain-lengthm ismatch. [5] Experimentally,e nsemble-basedm ethods,s ucha sF TIRs pectroscopy, or singlenanoparticle methods, such as scanning tunnelingm icroscopy (STM), have been used to interrogatel igandp hase separation. [6] However, such ensemble-based methodstypically yield semiquantitative resultso nm ixed ligand SAMs,a nd STM measurements of ligand phases eparationh aver ecentlyb een called into question. [7] Towardst he advancemento fs inglenanoparticle methods,w eh ave developed ac omplementary set of experimental and theoretical tools to probe the effects of nearest-neighbor interactions on ligand phase separation.Thee xperimental technique employed is MALDI mass spectrometry,a ne nsemble-based method which produces mass spectra of solid analytes through ionization with aU Vlaser, thereby accelerating analyte fragments towards ad etector that typically resolves species by time-of...