Microscopic Investigation of Chemoselectivity in Ag-Pt-Fe3O4 Heterotrimer Formation: Mechanistic Insights and Implications for Controlling High-Order Hybrid Nanoparticle Morphology.

Three-component hybrid nanoparticle heterotrimers, which are important multifunctional constructs that underpin diverse applications, are commonly synthesized by growing a third domain off of a two-component heterodimer seed. However, because heterodimer seeds expose two distinct surfaces that often can both support nucleation and growth, selectively targeting one particular surface is critical for exclusively accessing a desired configuration. Understanding and controlling nucleation and growth therefore enables the rational formation of high-order hybrid nanoparticles. Here, we report an in-depth microscopic investigation that probes the chemoselective addition of Ag to Pt-Fe3O4 heterodimer seeds to form Ag-Pt-Fe3O4 heterotrimers. We find that the formation of the Ag-Pt-Fe3O4 heterotrimers initiates with indiscriminate Ag nucleation onto both the Pt and Fe3O4 surfaces of Pt-Fe3O4, followed by surface diffusion and coalescence of Ag onto the Pt surface to form the Ag-Pt-Fe3O4 product. Control experiments reveal that the size of the Ag domain of Ag-Pt-Fe3O4 correlates with the overall surface area of the Pt-Fe3O4 seeds, which is consistent with the coalescence of Ag through a surface-mediated process and can also be exploited to tune the size of the Ag domain. Additionally, we observe that small iron oxide islands on the Pt surface of the Pt-Fe3O4 seeds, deposited during the formation of Pt-Fe3O4, define the morphology of the Ag domain, which in turn influences its optical properties. These results provide unprecedented microscopic insights into the pathway by which Ag-Pt-Fe3O4 heterotrimer nanoparticles form and uncover new design guidelines for the synthesis of high-order hybrid nanoparticles with precisely targeted morphologies and properties.

Cooperative assembly of Zn cross-linked artificial tripeptides with pendant hydroxyquinoline ligands.

An artificial peptide with three pendant hydroxyquinoline (hq) ligands on a palindromic backbone was designed and used to form multimetallic assemblies. Reaction of the tripeptide with zinc acetate led to a highly fluorescent tripeptide duplex with three Zn(II) coordinative cross-links. The binding process was monitored using spectrophotometric absorbance and emission titrations; NMR spectroscopy and mass spectrometry confirmed the identity and stoichiometry of the product structure. Titrations monitoring duplex formation of the zinc-tripeptide structure had a sigmoidal shape, equilibrium constant larger than the monomeric analogue, and a Hill coefficient >1, all of which indicate positive cooperativity. Photophysical characterization of the quantum yield, excited state lifetime, and polarization anisotropy are compared with the monometallic zinc-hq analogue. A higher than expected quantum yield for the trimetallic complex suggests a structure in which the central chromophore is shielded from solvent by π-stacking with neighboring Zn(II) complexes.

Analytical methods for separating and isolating magnetic nanoparticles.

Despite the large body of literature describing the synthesis of magnetic nanoparticles, few analytical tools are commonly used for their purification and analysis. Due to their unique physical and chemical properties, magnetic nanoparticles are appealing candidates for biomedical applications and analytical separations. Yet in the absence of methods for assessing and assuring their purity, the ultimate use of magnetic particles and heterostructures is likely to be limited. In this review, we summarize the separation techniques that have been initially used for this purpose. For magnetic nanoparticles, it is the use of an applied magnetic flux or field gradient that enables separations. Flow based techniques are combined with applied magnetic fields to give methods such as magnetic field flow fractionation and high gradient magnetic separation. Additional techniques have been explored for manipulating particles in microfluidic channels and in mesoporous membranes. Further development of these and new analytical tools for separation and analysis of colloidal particles is critically important to enable the practical use of these, particularly for medicinal purposes.

Purification and magnetic interrogation of hybrid Au-Fe3O4 and FePt-Fe3O4 nanoparticles.

Purifying heterodimers: differential magnetic catch and release separation is used to purify two important hybrid nanocrystal systems, Au-Fe(3)O(4) and FePt-Fe(3)O(4). The purified samples have substantially different magnetic properties compared to the as-synthesized materials: the magnetization values are more accurate and magnetic polydispersity is identified in morphologically similar hybrid nanoparticles.

Diffusive flux of nanoparticles through chemically modified alumina membranes.

Surface chemistry plays an important role in determining flux through porous media such as in the environment. In this paper diffusive flux of nanoparticles through alkylsilane modified porous alumina is measured as a model for understanding transport in porous media of differing surface chemistries. Experiments are performed as a function of particle size, pore diameter, attached hydrocarbon chain length and chain terminus, and solvent. Particle fluxes are monitored by the change in absorbance of the solution in the receiving side of a diffusion cell. In general, flux increases when the membranes are modified with alkylsilanes compared to untreated membranes, which is attributed to the hydrophobic nature of the porous membranes and differences in wettability. We find that flux decreases, in both hexane and aqueous solutions, when the hydrocarbon chain lining the interior pore wall increases in length. The rate and selectivity of transport across these membranes is related to the partition coefficient (K(p)) and the diffusion coefficient (D) of the permeating species. By conducting experiments as a function of initial particle concentration, we find that K(p)D increases with increasing particle size, is greater in alkylsilane-modified pores, and larger in hexane solution than water. The impact of the alkylsilane terminus (-CH(3), -Br, -NH(2), -COOH) on permeation in water is also examined. In water, the highest K(p)D is observed when the membranes are modified with carboxylic acid terminated silanes and lowest with amine terminated silanes as a result of electrostatic effects during translocation.

Differential magnetic catch and release: experimental parameters for controlled separation of magnetic nanoparticles.

Differential magnetic catch and release (DMCR) has been used as a method for the purification and separation of magnetic nanoparticles. DMCR separates nanoparticles in the mobile phase by magnetic trapping of magnetic nanoparticles against the wall of an open tubular capillary wrapped between two narrowly spaced electromagnetic poles. Using Au and CoFe(2)O(4) nanoparticles as model systems, the loading capacity of the 250 µm diameter capillary is determined to be ∼130 µg, and is scalable to higher quantities with larger bore capillary. Peak resolution in DMCR is externally controlled by selection of the release time (R(t)) at which the magnetic flux density is removed, however, longer capture times are shown to reduce the capture yield. In addition, the magnetic nanoparticle capture yields are observed to depend on the nanoparticle diameter, mobile phase viscosity and velocity, and applied magnetic flux. Using these optimized parameters, three samples of CoFe(2)O(4) nanoparticles whose diameters are different by less than 10 nm are separated with excellent resolution and capture yield, demonstrating the capability of DMCR for separation and purification of magnetic nanoparticles.

The use of magnetic nanoparticles in analytical chemistry.

Magnetic nanoparticles uniquely combine superparamagnetic behavior with dimensions that are smaller than or the same size as molecular analytes. The integration of magnetic nanoparticles with analytical methods has opened new avenues for sensing, purification, and quantitative analysis. Applied magnetic fields can be used to control the motion and properties of magnetic nanoparticles; in analytical chemistry, use of magnetic fields provides methods for manipulating and analyzing species at the molecular level. In this review, we describe applications of magnetic nanoparticles to analyte handling, chemical sensors, and imaging techniques.

Assembly of a trifunctional artificial peptide into an anti-parallel duplex with three Cu(II) cross-links.

Analogous to self-assembly in natural DNA or proteins, we describe the synthesis of a heterofunctional artificial tripeptide that self-assembles into an antiparallel duplex by coordination of three Cu(II) ions. The tripeptide contains three pendant ligands, pyridine (py), methyl bipyridine (bpy), and terpyridine (tpy), in series on an aminoethylglycine (aeg) backbone. These ligands chelate three Cu(II) ions, forming two [Cu(tpy)(py)](2+) and one [Cu(bpy)(2)](2+) complexes, that cross-link two tripeptide strands to give a trimetallic supramolecular structure. The tripeptide and metal-linked tripeptide duplex are characterized with NMR spectroscopy, mass spectrometry, and analytical high performance liquid chromatography (HPLC). Spectrophotometric titrations are used to quantitatively examine the stoichiometry of binding. Together with electron paramagnetic resonance (EPR) spectroscopy, the identities of the Cu(II) complexes and their environments are examined. The EPR spectrum reveals a significant amount of coupling between metal centers compared to a dimetallic dipeptide analogue. EPR and UV-vis absorbance spectroscopy, together with molecular modeling, provide evidence that the tripeptide acts as a scaffold to hold the metal centers in close proximity.

Cu(II) cross-linked antiparallel dipeptide duplexes using heterofunctional ligand-substituted aminoethylglycine.

Two artificial dipeptides containing both a pendant monodentate (pyridine (py)) and tridentate (terpyridine (tpy) or phenyl terpyridine (varphi-tpy)) ligand on an aminoethylglycine (aeg) backbone have been synthesized. These oligopeptides are fully characterized by one and two-dimensional NMR spectroscopy, mass spectrometry, and elemental analysis. The ligands were chosen because they coordinate Cu(2+) to form [Cu(py)(tpy)](2+) complexes; when bound to the dipeptide scaffold, Cu(2+) chelation cross-links the strands to form double-stranded duplex structures with an antiparallel arrangement. Using spectrophotometric titrations, we observe coordination of one Cu(2+) metal per dipeptide strand. Mass spectrometry, NMR spectroscopy, vapor pressure osmometry, and HPLC confirm that the resulting structures are the dipeptide duplex cross-linked by two metal centers.

Diffusive flux and magnetic manipulation of nanoparticles through porous membranes.

Measurement of transport of nanometer scale particles through porous media is important to begin to understand the potential environmental impacts of nanomaterials. Using a diffusion cell with two compartments separated by either a porous alumina or polycarbonate membrane as a model system, diffusive flux through mesoporous materials is examined. Experiments are performed as a function of particle size, pore diameter, and solvent, and the particle fluxes are monitored by the change in absorbance of the solution in the receiving cell. Using the measured extinction coefficient and change in absorbance of the solution as a function of time, the fluxes of 3, 8, and 14 nm diameter CoFe(2)O(4) particles are determined as they are translocated across pores with diameters 30, 50, 100, and 200 nm in hexane and aqueous solutions. In general, flux decreases with increasing particle size and increases with pore diameter. We find that fluxes are faster in aqueous solutions than in hexane, which is attributed to the hydrophilic nature of the porous membranes and differences in wettability. The impact of an applied magnetic flux gradient, which induces magnetization and motion, on permeation is also examined. For larger membrane pore diameters, applied magnetic fluxes increase the rate of transport of 14 nm CoFe(2)O(4) particles more than that of 3 or 8 nm diameter particles, reflecting their differences in susceptibility. However, larger particles are excluded from membranes with small diameter pores, consistent with magnetic interparticle attractions that reversibly induce magnetic aggregation.

Differential magnetic catch and release: analysis and separation of magnetic nanoparticles.

This article reports the purification and separation of magnetic nanoparticle mixtures using differential magnetic catch and release (DMCR). This method applies a variable magnetic flux orthogonal to the flow direction in an open tubular capillary to trap and controllably release magnetic nanoparticles. Magnetic moments of 8, 12, and 17 nm diameter CoFe2O4 nanoparticles are calculated using the applied magnetic flux and experimentally determined force required to trap 50% of the particle sample. Balancing the relative strengths of the drag and magnetic forces enables separation and purification of magnetic CoFe2O4 nanoparticle samples with <20 nm diameters. Samples were characterized by transmission electron microscopy to determine the average size and size dispersity of the sample population. DMCR is further demonstrated to be useful for separation of a magnetic nanoparticle mixture, resulting in samples with narrowed size distributions.

Inorganic biomimetic nanostructures.

Supramolecular structures modeled after biological systems (DNA and enzymes) are being developed to simultaneously mimic natural biological functions including catalysis, information storage, and self-assembly and to engineer novel electronic and magnetic properties. Structural mimics of nucleic acids containing multiple metal-coordinating ligands, and comprising natural and artificial bases or completely synthetic systems, create stable double-stranded structures with new electronic, spectroscopic, and magnetic properties. Supramolecular inorganic mimics of enzymatic function, including metallonucleases and metalloproteases, have begun to be constructed. Alternatively, metal-organic-frameworks have potential as artificial catalysts with substrate-specificity and size-selectivity analogous to biological processes. This review describes some of the recent themes in inorganic supramolecular systems that aim to mimic and exploit nature's ability to self-assemble polyfunctional architectures for new materials and biological applications.

Impacts of the location and number of [Cu(bpy)(2)](2+) cross-links on the emission photodynamics of [Ru(bpy)(3)](2+) with pendant oligo(aminoethylglycine) chains.

Multifunctional aminoethylglycine (aeg) derivatized [Ru(bpy)(3)](2+) complexes with pendant bipyridine (bpy) ligands coordinate Cu(2+) to form coordinative chain cross-links in a "hairpin loop" motif. In this paper, we report the synthesis and characterization of a series of Ru aeg hairpins in which the relative aeg chain length and number of pendant bpy ligands is varied. Reaction of each of these with Cu(2+) is monitored using spectrophotometric emission titrations to determine the binding stoichiometry. Coordination of Cu(2+) causes quenching of the emissive excited state Ru species; the degree of quenching efficiency depends on the location and number of coordinated Cu ions. The heterometallic structures are fully characterized, and using the quantum yields and time-resolved emission following excitation of the Ru complexes in deoxygenated solutions, the radiative (k(r)) and nonradiative (k(nr)) relaxation rates are compared. These data reveal only a shallow decrease in k(nr) with increasing distance between the Ru and Cu complexes. Activation energies, determined from temperature dependent studies of the time-resolved emission, also increase as the Ru-Cu separation increases, resulting in the smaller nonradiative rates. Together, these data are suggestive of excited state electron transfer as the quenching mechanism and demonstrate that metal coordination self-assembles structures made from modular artificial amino acids can provide controlled arrangements of chromophores, electron donors, and electron acceptors to shuttle electrons in a new approach for mimicking photosynthesis.

Aminoethylglycine-functionalized Ru(bpy)3(2+) with pendant bipyridines self-assemble multimetallic complexes by copper and zinc coordination.

Directed self-assembly using inorganic coordination chemistry is an attractive approach for making functional supramolecular structures. In this article, the synthesis and characterization of Ru(bpy) 3 (2+) compounds derivatized with aminoethylglycine (aeg) substituents containing pendant bipyridine (bpy) ligands is presented. The free bpy ligands in these complexes are available for metal chelation to form coordinative cross-links; addition of Cu (2+) or Zn (2+) assembles heterometallic structures containing two or three transition-metal complexes. Control over relative placement of metal complexes is accomplished using two strategies: two bipyridine-containing aeg strands tethered to Ru(bpy) 3 (2+) allow intramolecular coordination and result in a dimetallic hairpin motif. Ru(bpy) 3 (2+) modified with a single strand forms intermolecular cross-links forming the trimetallic complex. Each of these is characterized by a range of methods, and their photophysical properties are compared. These data, and comparison to an acetyl aeg- modified Ru(bpy) 3 (2+) complex, confirm that the metal ions cross-link bpy-containing aeg strands. Heterometallic complexes containing bound Cu (2+) cause a dramatic reduction in the Ru(bpy) 3 (2+) quantum yields and lifetimes. In contrast, the Ru(bpy) 3 (2+) hairpin with coordinated Zn (2+) has only a slight decrease in quantum yield but no change in lifetime, which could be a result of steric impacts on structure in the dimetallic species. Analogous effects are not observed in the trimetallic Ru-Zn-Ru structures in which this constraint is absent. Each of these heterometallic structures represents a facile and reconfigurable means to construct multimetallic structures by metal-coordination-based self-assembly of modular artificial peptide units.

Grignard functionalization of Weinreb amide-modified Au nanoparticles.

Reactions of Grignard and organolithium reagents are staple transformations in organic chemistry. However, their use in the chemical functionalization of monolayer-protected metallic nanoparticles is unprecedented. In this letter, we report the reaction of Au nanoparticles bearing a mixed monolayer of alkanethiol ligands that are methyl- and N-methoxy- N-methyl amide-terminated. The latter of these rapidly undergoes reaction with organometallic reagents, achieving high yields (in some cases, nearly quantitative) in only a few hours without the need for high pressure, temperature or catalysts. We assess the feasibility of this reaction with a range of organometallic reagents on the basis of both surface reaction yield and also the stability of the particles (defined as the mass % Au particles recovered vs a control). Demonstrating the utility of these strong organometallic reagents opens the door to a large class of reactions that are underutilized within the field of nanomaterials.

Controlling transport and chemical functionality of magnetic nanoparticles.

A wide range of metal, magnetic, semiconductor, and polymer nanoparticles with tunable sizes and properties can be synthesized by wet-chemical techniques. Magnetic nanoparticles are particularly attractive because their inherent superparamagnetic properties make them desirable for medical imaging, magnetic field assisted transport, and separations and analyses. With such applications on the horizon, synthetic routes for quickly and reliably rendering magnetic nanoparticle surfaces chemically functional have become an increasingly important focus. This Account describes common synthetic routes for making and functionalizing magnetic nanoparticles and discusses initial applications in magnetic field induced separations. The most widely studied magnetic nanoparticles are iron oxide (Fe2O3 and Fe3O4), cobalt ferrite (CoFe 2O4), iron platinum (FePt), and manganese ferrite (MnFe 2O4), although others have been investigated. Magnetic nanoparticles are typically prepared under either high-temperature organic phase or aqueous conditions, producing particles with surfaces that are stabilized by attached surfactants or associated ions. Although it requires more specialized glassware, high-temperature routes are generally preferred when a high degree of stability and low particle size dispersity is desired. Particles can be further modified with a secondary metal or polymer to create core-shell structures. The outer shells function as protective layers for the inner metal cores and alter the surface chemistry to enable postsynthetic modification of the surfactant chemistry. Efforts by our group as well as others have centered on pathways to yield nanoparticles with surfaces that are both easily functionalized and tunable in terms of the number and variety of attached species. Ligand place-exchange reactions have been shown quite successful for exchanging silanes, acids, thiols, and dopamine ligands onto the surfaces of some magnetic particles. Poly(ethylene oxide)-modified phospholipids can be inserted into nonpolar surface monolayers of as-prepared nanoparticles as a method for modifying the surface chemistry that induces water solubility. In general, reactive termini can subsequently be used to append a range of chemical groups. For example, surfactants with trifluoroethylester or azide termini have been used to attach a range of amine- or alkyne-containing species, respectively. Chemically functionalized magnetic nanoparticles are promising as advanced materials for analytical separations and analysis. Magnetic field flow fractionation leverages the size-dependent magnetic moments to purify and separate the components of a complex mixture of particles. Similarly, magnetic field gradients are useful for manipulating transport and separation in simple microfluidic devices. By either approach, magnet-induced transport of the particles is a simple method in which an attached reagent, catalyst, or bioanalytical tag can be moved between flow streams within a lab on a chip device.

Kinetics of 1,3-dipolar cycloaddition on the surfaces of Au nanoparticles.

Triazole formation via 1,3-dipolar cycloaddition, or "click" chemistry, is a powerful synthetic method for incorporating chemical functionality onto the surfaces of Au nanoparticles. To investigate the factors that govern azide/alkyne reactivity at particle surfaces, we measured the general kinetic trends for the uncatalyzed reaction using FTIR spectroscopy. This study examines the roles of ligand length, electronic substitution of the alkyne species, and solvent on the reaction under pseudo-first-order conditions. The conversion of azide to triazole is found to depend more strongly on the relative surface coverage of azide terminated alkanethiol than on the ligand length and solvent.

Transmission electron microscope-induced structural evolution in amorphous Fe, Co, and Ni oxide nanoparticles.

The high-energy electron beams in transmission electron microscopes (TEM) are known to cause structural changes and damage in some materials. In this paper, we describe unique and reproducible TEM-induced changes to the morphology of amorphous metal oxide (Fe, Co, and Ni) nanoparticles. The studied particles were synthesized via literature methods and fully characterized by X-ray powder diffraction and time-resolved, low-dose TEM. As a result of electron beam irradiation, we observe these particles to transform from initially solid spheres to core/void/shell structures and eventually to hollow nanoparticles. The rate of these transformations depends on the size and composition of the particles but is not unique to the Fe oxide we previously reported. These data suggest that structural analysis of nanoparticles by TEM must consider the impact of the high-energy electron beam and use low-dose imaging.

Magnetic field switching of nanoparticles between orthogonal microfluidic channels.

This paper reports on the manipulation of magnetic nanoparticles between microfluidic channels by the application of an external magnet. Two orthogonal channels were prepared using standard PDMS techniques with pressure-driven flow used to deliver the mobile phase. To study the ability to control magnetic nanoparticles within micrometer-sized channels, Fe2O3, MnFe2O4, and Au nanoparticle samples were compared. For the magnetic particles, transfer between flow streams is greatly increased by placing a permanent magnet beneath the intersection of the channels, but no change is observed for the nonmagnetic Au particles. More nanoparticles are magnetically transferred into the orthogonal channel as the solvent flow rate decreases. We demonstrate the ability to use this technique to perform multiple injections of plugs of magnetic particles by periodic application of a magnetic field.