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Dendritic cells (DCs) can infiltrate tight junctions of the epithelium to collect remote antigens during immune surveillance. While elongated membrane structures represent a plausible structure to perform this task, their functional mechanisms remain elusive owing to the lack of high-resolution characterizations in live DCs. Here, we developed fluorescent artificial antigens (FAAs) based on quantum dots coated with polyacrylic acid. Single-particle tracking of FAAs enables us to superresolve the membrane fiber network responsible for the antigen uptake. Using the DC2.4 cell line as a model system, we discovered the extensive membrane network approaching 200 μm in length with tunnel-like cavities about 150 nm in width. The membrane fiber network also contained heterogeneous circular migrasomes. Disconnecting the membrane network from the cell body decreased the intracellular FAA density. Our study enables mechanistic investigations of DC membrane networks and nanocarriers that target this mechanism.

Characterization of semiconductor nanoparticle (NP) morphologies is demonstrated using the Warren–Averbach (WA) method of powder X-ray diffraction. WA analysis provides crystallographic direction-d...

Fungus-growing ants and their bacterial symbionts have emerged as a model animal-microbe symbiosis and an ideal system for understanding antibiotic deployment in an ecological context. We found that Pseudonocardia symbionts of the ant Trachymyrmex septentrionalis have strong antibiotic activity against their most likely competitors: other strains of ant-associated bacteria. Activity-guided fractionation revealed the defensive molecule produced by these bacteria to be the thiopeptide antibiotic GE37468. Here we assign an ecological role – host-associated niche defense – for this antibiotic, previously identified in a biochemical screen and known only for its in vitro activity against clinically-relevant pathogens. Genomic analysis uncovered a split biosynthetic gene cluster for this molecule and suggests that these symbionts acquired it from soil bacteria. Similar thiopeptide antibiotics have recently been ascribed host-associated niche defense roles, and the function of GE37468 in this insect niche intriguingly parallels thiopeptide defense in the human microbiome. Molecular defenses from animal-associated microbes may have particular promise as therapeutics, and indeed thiopeptide antibiotics with high structural similarity to GE37468 are already under clinical investigation.

ConspectusSemiconductor quantum dots (QDs), also known as nanocrystals, have unique photophysical properties that have allowed them to find utility in many applications, including television and display technologies. They also have significant potential as imaging agents in the biomedical field. To gain the most value from the use of QDs as health-related fluorescent probes, they must be biologically targetable and sensitive to metabolic analytes such as pH and O2, and the resulting signal must be quantifiable. To achieve these goals, QDs need to be conjugated to vectors such as antibodies or environmentally sensitive chromophores. Until recently, the functionalization of these nanomaterials required a complex fully “bottom-up” approach beginning with the synthesis of the QDs and subsequent manipulations. To simplify this process, our group set out to develop straightforward methods to prepare functionalized nanomaterials for biological imaging and sensing using low-cost, commercially available aqueous QD...

Understanding the electronic structure of doped semiconductors is essential to realize advancements in electronics and in the rational design of nanoscale devices. Reported here are the results of time-resolved X-ray absorption studies on copper-doped cadmium sulfide nanoparticles that provide an explicit description of the electronic dynamics of the dopants. The interaction of a dopant ion and an excess charge carrier is unambiguously observed via monitoring the oxidation state. The experimental data combined with DFT calculations demonstrate that dopant bonding to the host matrix is modulated by its interaction with charge carriers. Furthermore, the transient photoluminescence and the kinetics of dopant oxidation reveal the presence of two types of surface-bound ions that create midgap states.

Silver dimetal chalcogenides (Ag–V–VI2) are ternary semiconductors that have potential alternative energy applications due to their optimal band gaps and large extinction coefficients. The synthesis of these materials is challenging due to the lack of effective pnictide precursors. We report the use of tris[N,N-bis(trimethylsilyl)amido]antimony (Sb[N(SiMe3)2]3) and tris[N,N-bis(trimethylsilyl)amido]bismuth (Bi[(N(SiMe3)2]3) to synthesize nanocrystalline AgSbSe2 and AgBiSe2 quantum dots. The use of these reagents results in the creation of high quality nanomaterials with good crystallinity and narrow size distributions. Furthermore, electrical measurements on monolithic pellets of processed AgSbSe2 and AgBiSe2 nanomaterials demonstrate linear current–voltage behavior at room temperature, which indicates potential for use in electrical applications.

Molecular oxygen is known to be an efficient quencher of the excited states of organic molecules. This fact has been exploited to develop fluorescent O2 sensors, which is topical for cancer screening and many other applications. In this regard, our group and others have reported the development of robust, ratiometrically reporting chemosensors by conjugating O2-sensitive organic chromophores to inorganic quantum dots. Recently, an attempt was made to prepare a multifunctional sensor system by attaching the emissive poly(aromatic) hydrocarbons pyrene and perylene to magnetic nanomaterials, specifically CdSe/CdMnZnS and ZnSe/ZnMnS/ZnS quantum dots. However, the fluorescence of both dyes became invariant to oxygen levels even if the solutions were fully saturated. We ruled out any material dependency beyond the presence of Mn2+ ions by studying control samples, while molecular dynamics simulations negated any possibility of spatial sequestration of O2 by the magnetic fields. In the case of pyrene, the proxim...

A quantum-dot based ratiometric fluorescent oxygen probe for the detection of hypoxia in live cells is reported. The system is comprised of a water-soluble near-infrared emissive quantum dot conjugated to perylene dye. The response to the oxygen concentration is investigated using enzymatic oxygen scavenging in water, while in vitro studies were performed with HeLa cells incubated under varying O2 levels. In both cases a significant enhancement in dye/QD emission intensity ratio was observed in the deoxygenated environment, demonstrating the possible use of this probe for cancer research.

The synthesis of metal arsenide semiconductor nanocrystals is challenging compared to other conventional materials (e.g., II–VI family CdSe) due to a high degree of covalency in crystal lattice and a lack of a variety of effective pnictide precursors. We reported here the use of [(Me3Si)2N]2AsCl (“arsenic silylamide”) to synthesize a variety of binary and tertiary quantum confined species such as InAs (III–V), Cd3As2 (II–V), and Cu3AsS4 (enargite, I3–V–VI4) nanocrystals. The physical properties of this reagent resolve many issues, especially concerning safety, with other precursors such as pyrophoric (TMS)3As and highly toxic AsH3 gas. Furthermore, usage of this reagent has allowed for the realization of enargite quantum rods for the first time. The role of arsenic silylamide in the reaction mechanism to form InAs has been elucidated with a combination of NMR spectroscopy as well as quantum chemical modeling. Cd3As2 and Cu3AsS4 nanostructures form via a diffusion mechanism.

We report a platform for the ratiometric fluorescent sensing of the endogenously generated gaseous transmitter H2S in its aqueous form (bisulfide, or hydrogen sulfide anion) based on the alteration of Förster resonance energy transfer from an emissive semiconductor quantum dot (QD) donor to a dithiol-linked organic dye acceptor. The disulfide bridge between the two chromophores is cleaved upon exposure to bisulfide, resulting in termination of FRET as the dye diffuses away from the QD. This results in enhanced QD emission and dye quenching. The resulting ratiometric response can be correlated quantitatively to the concentration of bisulfide, and was found to have a detection limit as low as 1.36 ± 0.03 μM. The potential for use in biological applications was demonstrated by measuring the response of the QD-based FRET sensor microinjected into live HeLa cells upon extracellular exposure to bisulfide. The methodology used here is built upon a highly multifunctional platform that offers numerous advantages such as low detection limit, enhanced photochemical stability, and sensing ability within a biological milieu.

Tantalum oxynitride (TaON) is a visible photocatalyst (λ < 520 nm) that has been investigated for green energy applications, specifically, the solar production of hydrogen gas. Metal oxynitrides are often generated by the ammonolysis of metal oxides at very high temperatures. We have investigated an alternative synthetic procedure to produce gram-scale quantities of photocatalytically active TaOxNy materials using room-temperature sol–gel chemistry. X-ray photoelectron spectroscopy (XPS) results confirm that nitrogen was incorporated into materials with a Ta2O5 crystal structure. Optical characterization reveals that sol–gel-synthesized TaOxNy has a 2.3 eV bandgap, in agreement with bulk TaON. Furthermore, photoluminescence experiments reveal that two distinct material systems are being created during the initial sol phase of the synthesis: one species is primarily metal oxide, while the other is a N-doped species. This observation may lead to synthetic methods that improve the doping content of sol–gel-d...

Semiconductor nanocrystals (NCs or quantum dots) have significant potential for use in a variety of applications from renewable energy generation to biological imaging. Modern methods of colloidal synthesis can be used to create crystalline materials with tight size distributions; this assures high quantum yields and narrow emission profiles in the case of direct-bandgap semiconductors. The optical properties of NCs may also be tuned with size due to quantum confinement effects. Quantum confinement also creates problems when characterizing nanomaterials; specifically, the absorptivity of a sample is a function of the size and structure of the quantum dots. We demonstrate here a simple method for determining the molar absorptivity of aqueous CdSe/CdZnS NCs through Forster resonant energy transfer. Energy transfer from NC donors to dye acceptors was measured and modeled using standard Forster theory incorporating Poissonian statistics to calculate the acceptor/donor ratio leading to a direct determination o...

Developing simple synthetic methods to control the size and morphology of nanocrystals is an active area of research as these parameters control the material's electronic and optical properties. For a semiconductor with a symmetrical crystal structure such as lead selenide, anisotropic colloidal growth has been previously accomplished via the use of templates, seeds, or by block assembly of smaller, symmetrical subunits. Here, we present a simple method to create monodisperse lead selenide nanorods and multipods at low temperatures. The size distribution and the observed morphologies are consistent with a continuous, anisotropic growth of material. The syntheses of these anisotropic shapes are due to the nature of the nuclei that form upon injection of precursors into partially oxidized alkene solvents that may contain lactone and carbonate-functional derivatives.

We report a novel method for cap exchange of emissive semiconductor nanocrystals (NCs) using thiol functional ligands metalated with zinc. Utilizing this method, the NCs can be several times brighter and much more resistant to precipitation compared to control samples. This method has been applied using a variety of caps such as dihydrolipoic acid and cysteine. Our data suggest that the improved properties of the metalated cap exchanged NCs are due to a ligand metathesis process occurring at the NC surface where the zinc complex reacts with NC surface bound ligands, gently removing them and replacing them with another cap. Overall, the use of metalated ligands helps resolve many long-standing issues concerning the application of small cap exchanged NCs for biological imaging.

Emissive semiconductor nanocrystals can be efficient FRET donors to chemically responsive organic fluorophores; conjugating the two creates a ratiometric (or self-calibrating) sensor. Given that the organic component engenders chemical and/or biological sensitivity, it is natural to assume that the NC−dye response is reflective of the photochemical properties of the neat dye. In the case of a pH sensor like fluorescein, the dye emission can be analytically described by the Henderson−Hasselbalch equation. Our research demonstrates that the ratiometric sensing response of CdS/ZnS−fluorescein pH sensors, however, does not directly reflect the Henderson−Hasselbalch relationship and is difficult to predict. The reasons for this complexity are the alternations of the dye microenvironment within the nanoscale architecture and the nature of energy transfer from the NC donor to the acceptor dye.

Oleic acid capped lead sulfide (PbS) and lead selenide (PbSe) were synthesized, then subjected to a postsynthesis washing in a 1:1 ethanol/hexane solution. The relationship of third order nonlinear optical properties to nanocrystal surface chemistry as affected by washing was analyzed using nanosecond 532 nm Z-scan and measurement of near-IR radiative emission. The results indicated a significant change in optical nonlinearities which emerged only after the nanocrystals were washed in the ethanol/hexane mixture. Transmission electron microscopy of the oleic acid capped PbSe and PbS nanocrystals showed a cubic shape, narrow size distribution and an average size of ∼10 nm. Neither size nor shape of the nanocrystals were modified by the washing process, indicating that all optical differences were related to changes in surface chemistry and the formation of deep trap states. The “as grown” nanocrystals showed high emission efficiency, weak saturable absorption, and a self-defocusing refractive index, n2, det...

We have studied manganese doping of zinc selenide core/zinc sulfide shell nanocrystals (NCs) where the impurity phosphor resides primarily in the shell. We have found that a simple two-step synthesis can be used to create these nontoxic materials that display efficient energy transfer from the core to the Mn doped shell. These core/shell NCs retain ample quantum efficiency ( approximately 25%) when solubilized in water, which opens the possibility of using these materials as bioimaging agents. As recent work has shown that nanocrystals can be functionalized with organic dyes to operate as ratiometric chemical sensing agents, we have conjugated the doped NCs with an organic dye to showcase efficient Förster resonant energy transfer from the shell-doped phosphor to the surface-bound dye. This result indicates that doped NCs can be used to develop nontoxic ratiometric sensing/biological imaging agents.

The mechanism of intermolecular ligand exchange has been studied using transition path sampling (TPS) based molecular dynamics (MD) simulations. Specifically, the exchange of solvent molecules bound to unsaturated Cr(CO)5 in methanol solution has been investigated. The results of the TPS simulations have shown that there are multiple steps in the reaction mechanism. The first involves partial dissociation of the coordinated solvent from the Cr metal center followed by association with a new methanol molecule between the normally void first and second solvent layers. After diffusive motion of the exchanging ligands, the last step involves the originally bound methanol molecule moving into the bath continuum followed by solvation of the Cr metal fragment by the exchanging ligand. It has been found that the reaction center (defined as the organometallic fragment and two exchanging ligands only) and the solvent bath have favorable interactions. This is likely due to the adiabatic nature of the ligand exchange transition. The ability to understand the microscopic molecular dynamics of a chemical process based on a free energy analysis is also discussed.

The low-temperature equilibrium and nonequilibrium solvation dynamics of an excess electron in bulk and at a surface interface have been characterized using mixed quantum/classical simulation methods. A methanol bath was modeled using classical molecular dynamics, whereas the properties of an excess electron were calculated from a wave function based approach. The temperature dependence of the bulk response has been found to be minimal, which may indicate that large scale hydrogen bond breaking and diffusive motion may not play important roles in the solvation dynamics. The equilibrium dynamics have also been compared to nonequilibrium simulations of charge injection into the neat bath. An excess electron at a methanol/Pt(100) surface interface has been shown to become solvated by the bath yet is less bound by a factor of ∼2 compared to the bulk. The solvent response function also displays interesting differences when compared to the low temperature bulk glass. These results also reveal many of the micros...

The photosubstitution reactions of Fe(CO)5 with alcohols and triethylphosphine have been studied in room temperature solution using femtosecond UV pump IR probe techniques. The dynamics of Fe(CO)5 in alcohols show that the coordination of the hydroxyl group of the solvent with triplet Fe(CO) 4 is generally much faster than the coordination timescale observed with comparable singlet species. The physical reason for this is the fact that the metal C-H agostic bond is much weaker for triplets, thus allowing fast rearrangement to the hydroxyl group of the alcohol. In the case of triethylphosphine photosubstitution, the reaction is divided into parallel channels which include intersystem crossing and a spin conserving double carbonyl loss. The intermediates observed were studied using density functional theory (DFT) as well as ab initio quantum chemical calculations.

The reactivity of triplet 16-electron organometallic species has been studied in room-temperature solution using femtosecond UV pump IR probe spectroscopy. Specifically, the Si-H bond-activation reaction of photogenerated triplet Fe(CO)(4) and triplet CpCo(CO) with triethylsilane has been characterized and compared to the known singlet species CpRh(CO). The intermediates observed were studied using density functional theory (DFT) as well as ab initio quantum chemical calculations. The triplet organometallics have a greater overall reactivity than singlet species due to a change in the Si-H activation mechanism, which is due to the fact that triplet intermediates coordinate weakly at best with the ethyl groups of triethylsilane. Consequently, the triplet species do not become trapped in alkyl-solvated intermediate states. The experimental results are compared to the theoretical calculations, which qualitatively reproduce the trends in the data.

The photochemical Si−H bond activation reaction by the group VB, d4 organometallic compound η5-CpV(CO)4 (Cp = C5H5) has been studied in neat triethylsilane under ambient conditions. The spectral evolutions of the metal-bonded CO stretching bands were monitored from 300 fs to 800 ps following UV photolysis using femtosecond pump−probe spectroscopic methods. The reactive intermediates observed on the ultrafast time scale were also studied using density functional theory as well as ab initio quantum chemical modeling. It was found that photolysis of vanadium tetracarbonyl resulted in the formation of tricarbonyls in either the singlet or triplet electronic state following CO loss. The subsequent reaction was partitioned into two pathways by the initial solvation of the tricarbonyls through the Si−H bond or an ethyl group of the solvent molecule. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the chemical ...

The development of a reversible chemical sensor based on a CdSe/ZnS nanocrystal (NC) is described. Signal transduction is accomplished by fluorescence resonance energy transfer (FRET) between the NC and a fluorescent pH-sensitive squaraine dye attached to the surface of the NC. The efficiency of FRET, and consequently the relative intensity of NC and dye emissions, is modulated with the pH-dependent absorption cross section of the squaraine dye. The design of a NC sensor based on FRET results in a ratiometric sensor since the emission intensities of dye and NC may be referenced to the isosbestic point between NC and dye emissions. The ratiometric approach allows sensing to be performed, regardless of issues surrounding collection efficiency (scattering environment, light fluctuations, etc.) and dye:NC loadings.