Christopher M. Andolina

Assembly of Near‐Infrared Luminescent Lanthanide Host(Host–Guest) Complexes With a Metallacrown Sandwich Motif

Optical devices and biomedical imaging probes increasingly utilize the long lifetimes and narrow linewidths of luminescent lanthanide (Ln) ions. Near-infrared (NIR) emitting Ln ions draw particular interest because of the transparency of biological tissue in this spectral range and applications in telecommunications. Ln ions are typically sensitized through ligand absorptions by the antenna effect because the low extinction coefficients of the Laporte-forbidden f–f transitions preclude direct excitation. The major hindrance in realizing efficient Ln ion luminescence in the NIR region is non-radiative quenching by high energy X H (X = C, N, O) vibrations in the ligand. Vibrational quenching has limited luminescence lifetimes to less than 6 ms in protic solvents. While careful ligand design can exclude N H and O H oscillators, C H bonds are difficult to eliminate from organic substrates without relying on synthetically cumbersome deuterated or fluorinated ligands. Herein we present a self-assembly approach to realizing long-lived Ln luminescence in the NIR region by utilizing the unique metallacrown (MC) topology to eliminate high energy X H oscillators from within 6.7 of the lanthanide ion. We report the synthesis, solution stability, and remarkable luminescence properties of a unique host(host–guest) complex in which a Ln[12-MC4]2 3+ sandwich complex is a guest encapsulated by a [24-MC8] host (Ln-1, Figure 1). MCs are inorganic analogues of crown ethers. Much of the interest in MCs has focused on the exceptional solid-state architectures, magnetic properties, and molecular recognition capabilities that arise from their metal-rich topologies. Ln MCs have been prepared that display singlemolecule magnetism and selectively encapsulate anions in monomeric cavitands or dimeric compartments. Chiral Ln[15-MC-5] complexes can serve as building blocks for mesoporous solids, resolved helices, and noncentrosymmetric solids that display second-harmonic generation. To date, Ln MCs have been prepared only with ring metals that contain partially filled d orbitals, which could provide a quenching pathway for luminescence. For this work, the Zn ion was judiciously chosen as the ring metal because its d electronic configuration precludes quenching through a d–d transition. To the best of our knowledge, no Ln MCs with Zn ring metals have been reported. Picoline hydroxamic acid (picHA) was selected as the ligand because it contains no N H or O H oscillators when bound in a Ln MC. The reaction between picHA, sodium hydroxide, zinc(II) triflate, and terbium(III) nitrate in methanol provided the complex formulated as Tb[12-MCZnII,N,picHA-4]2 [24MCZnII, N,picHA-8]·(pyridine)8·(triflate)3 (Tb-1, Figure 1) upon crystallization from the reaction solution with added pyridine. Single crystal X-ray crystallographic analysis shows two concave [12-MCZnII, N, picHA-4] units that sandwich an eightcoordinate Tb central metal. This sandwich complex (Figure 2A, B) is encapsulated in the cavity of a [24-MCZnII, N,picHA8] unit (Figure 2C). The Tb[12-MC-4]2 [24-MC-8] comFigure 1. X-ray crystal structure of Tb-1 shown a) perpendicular to the C4 axis, b) down the C4 axis, and c) highlighting the MC macrocycle. Color scheme: bronze= [12-MC-4], purple= [24-MC-8], green= Tb. Pyridine ligands are displayed as thin purple lines.

Photoluminescent gold-copper nanoparticle alloys with composition-tunable near-infrared emission.

Discrete gold nanoparticles with diameters between 2 and 3 nm show remarkable properties including enhanced catalytic behavior and photoluminescence. However, tunability of these properties is limited by the tight size range within which they are observed. Here, we report the synthesis of discrete, bimetallic gold-copper nanoparticle alloys (diameter ≅ 2-3 nm) which display photoluminescent properties that can be tuned by changing the alloy composition. Electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and pulsed-field gradient stimulated echo (1)H NMR measurements show that the nanoparticles are homogeneous, discrete, and crystalline. Upon varying the composition of the nanoparticles from 0% to 100% molar ratio copper, the photoluminescence maxima shift from 947 to 1067 nm, with excitation at 360 nm. The resulting particles exhibit brightness values (molar extinction coefficient (ε) × quantum yield (Φ)) that are more than an order of magnitude larger than the brightest near-infrared-emitting lanthanide complexes and small-molecule probes evaluated under similar conditions.

Dynamics of Soft Nanomaterials Captured by Transmission Electron Microscopy in Liquid Water

In this paper we present in situ transmission electron microscopy of synthetic polymeric nanoparticles with emphasis on capturing motion in a solvated, aqueous state. The nanoparticles studied were obtained from the direct polymerization of a Pt(II)-containing monomer. The resulting structures provided sufficient contrast for facile imaging in situ. We contend that this technique will quickly become essential in the characterization of analogous systems, especially where dynamics are of interest in the solvated state. We describe the preparation of the synthetic micellar nanoparticles together with their characterization and motion in liquid water with comparison to conventional electron microscopy analyses.

Ligand-Mediated “Turn On,” High Quantum Yield Near-Infrared Emission in Small Gold Nanoparticles

Small gold nanoparticles (∼1.4-2.2 nm core diameters) exist at an exciting interface between molecular and metallic electronic structures. These particles have the potential to elucidate fundamental physical principles driving nanoscale phenomena and to be useful in a wide range of applications. Here, we study the optoelectronic properties of aqueous, phosphine-terminated gold nanoparticles (core diameter = 1.7 ± 0.4 nm) after ligand exchange with a variety of sulfur-containing molecules. No emission is observed from these particles prior to ligand exchange, however the introduction of sulfur-containing ligands initiates photoluminescence. Further, small changes in sulfur substituents produce significant changes in nanoparticle photoluminescence features including quantum yield, which ranges from 0.13 to 3.65% depending on substituent. Interestingly, smaller ligands produce the most intense, highest energy, narrowest, and longest-lived emissions. Radiative lifetime measurements for these gold nanoparticle conjugates range from 59 to 2590 μs, indicating that even minor changes to the ligand substituent fundamentally alter the electronic properties of the luminophore itself. These results isolate the critical role of surface chemistry in the photoluminescence of small metal nanoparticles and largely rule out other mechanisms such as discrete (Au(I)-S-R)n impurities, differences in ligand densities, and/or core diameters. Taken together, these experiments provide important mechanistic insight into the relationship between gold nanoparticle near-infrared emission and pendant ligand architectures, as well as demonstrate the pivotal role of metal nanoparticle surface chemistry in tuning and optimizing emergent optoelectronic features from these nanostructures.

Decoupling Mechanisms of Platinum Deposition on Colloidal Gold Nanoparticle Substrates

Nanoscale platinum materials are essential components in many technologies, including catalytic converters and fuel cells. Combining Pt with other metals can enhance its performance and/or decrease the cost of the technology, and a wide range of strategies have been developed to capitalize on these advantages. However, wet chemical synthesis of Pt-containing nanoparticles (NPs) is challenging due to the diverse metal segregation and metal-metal redox processes possible under closely related experimental conditions. Here, we elucidate the relationship between Pt(IV) speciation and the formation of well-known NP motifs, including frame-like and core-shell morphologies, in Au-Pt systems. We leverage insights gained from these studies to induce a controlled transition from redox- to surface chemistry-mediated growth pathways, resulting in the formation of Pt NPs in epitaxial contact and linear alignment along a gold nanoprism substrate. Mechanistic investigations using a combination of electron microscopy and (195)Pt NMR spectroscopy identify Pt(IV) speciation as a crucial parameter for understanding and controlling the formation of Pt-containing NPs. Combined, these findings point toward fully bottom-up methods for deposition and organization of NPs on colloidal plasmonic substrates.

Seedless initiation as an efficient, sustainable route to anisotropic gold nanoparticles.

Seedless initiation has been used as a simple and sustainable alternative to seed-mediated production of two canonical anisotropic gold nanoparticles: nanorods and nanoprisms. The concentration of reducing agent during the nucleation event was found to influence the resulting product morphology, producing nanorods with lengths from 30 to 630 nm and triangular or hexagonal prisms with vertex-to-vertex lengths ranging from 120 to over 700 nm. The seedless approach is then used to eliminate several chemical reagents and reactions steps from classic particle preparations while achieving almost identical nanoparticle products and product yields. Our results shed light on factors that influence (or do not influence) the evolution of gold nanoparticle shape and present a dramatically more efficient route to obtaining these architectures. Specifically, using these methods reduces the total amount of reagent needed to produce nanorods and nanoprisms by as much as 90 wt % and, to the best of our knowledge, has yielded the first report of spectroscopically discernible, colloidal gold nanoplates synthesized using a seedless methodology.

Description and Role of Bimetallic Prenucleation Species in the Formation of Small Nanoparticle Alloys

We report the identification, description, and role of multinuclear metal-thiolate complexes in aqueous Au-Cu nanoparticle syntheses. The structure of these species was characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy techniques. The observed structures were found to be in good agreement with thermodynamic growth trends predicted by first-principles calculations. The presence of metal-thiolate complexes is then shown to be critical for the formation of alloyed Au-Cu architectures in the small nanoparticle regime (diameter ∼2 nm). In the absence of mixed metal-thiolate precursors, nanoparticles form with a Cu-S shell and a Au-rich interior. Taken together, these results demonstrate that prenucleation species, which are discrete molecular precursors distinct from both initial reagents and final particle products, may provide an important new synthetic route to control final metal nanoparticle composition and composition architectures.

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1-Hydroxypyridin-2-one Complexes in Aqueous Solution

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu(III) and Sm(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φtot) were found to vary with the linker length, and the same trend was observed for the Eu(III) and Sm(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φtotxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (nLI) is increased in length.

Efficient Energy Transfer from Near-Infrared Emitting Gold Nanoparticles to Pendant Ytterbium(III)

Here, we demonstrate efficient energy transfer from near-infrared-emitting ortho-mercaptobenzoic acid-capped gold nanoparticles (AuNPs) to pendant ytterbium(III) cations. These functional materials combine the high molar absorptivity (1.21 × 106 M-1 cm-1) and broad excitation features (throughout the UV and visible regions) of AuNPs with the narrow emissive properties of lanthanides. Interaction between the AuNP ligand shell and ytterbium is determined using both nuclear magnetic resonance and electron microscopy measurements. In order to identify the mechanism of this energy transfer process, the distance of the ytterbium(III) from the surface of the AuNPs is systematically modulated by changing the size of the ligand appended to the AuNP. By studying the energy transfer efficiency from the various AuNP conjugates to pendant ytterbium(III) cations, a Dexter-type energy transfer mechanism is suggested, which is an important consideration for applications ranging from catalysis to energy harvesting. Taken together, these experiments lay a foundation for the incorporation of emissive AuNPs in energy transfer systems.

In Situ Observations of Early Stage Oxidation of Ni-Cr and Ni-Cr-Mo Alloys

Results of in situ transmission electron microscopy experiments on the early stage oxidation of Ni-Cr and Ni-Cr-Mo alloys are reported. An epitaxial rock-salt oxide with compositions outside the conventional solubility limits initiated at the surface of both alloys, progressing by a layer-by-layer mode. Kirkendall voids were found in Ni-Cr alloys near the metal/oxide interface, but were not seen in the Ni-Cr-Mo. The voids initiated in the oxide then diffused to the metal/oxide interface, driven by the misfit stresses in the oxide. A sequential oxide initiation was observed in NiCr alloys: rock-salt → spinel → corundum; however, for NiCrMo alloys, the metastable Ni2-xCrxO3 (corundum structure) phase formed shortly after the growth of the rock-salt phase. Chemical analysis shows that solute atoms were captured in the initial oxide before diffusing and transforming to more thermodynamically stable phases. The results indicate that Mo doping inhibits the formation of Kirkendall voids via an increase in the nu...