Charged Nanocrystals Research Articles

Nonradiative Auger recombination in semiconductor nanocrystals.

We calculate the rate of nonradiative Auger recombination in negatively charged CdSe nanocrystals (NCs). The rate is nonmonotonic, strongly oscillating with NC size, and sensitive to the NC surface. The oscillations result in nonexponential decay of carriers in NC ensembles. Using a standard single-exponential approximation of the decay dynamics, we determine the apparent size dependence of the Auger rate in an ensemble and derive CdSe surface parameters consistent with the experimental dependence on size.

Preparation of zwitterionically charged nanocrystals by surface TEMPO-mediated oxidation and partial deacetylation of α-chitin

Zwitterionic nanocrystals were prepared by TEMPO-mediated oxidation, partial deacetylation, and subsequent mechanical disintegration of α-chitin. The pH dependence of the morphology, transparency, and viscosity of the nanocrystals were evaluated. After those reactions, the carboxylate and amino group contents of the chitin derivative were 0.45 and 1.26mmol/g, respectively. After mechanical treatment, the water dispersion consisted of nanocrystals approximately 250nm long and 10nm thick. Under acidic and basic conditions, the water dispersions were highly transparent. On the other hand, under neutral conditions, the dispersion was turbid due to the ionic interaction between the cationic and anionic groups on the nanocrystal surface. Although the surface zwitterionic nanocrystals collected from acidic and basic dispersion were randomly oriented due to electrostatic repulsions, nanocrystals formed aggregates in neutral water due to the cationic and anionic interaction between them. Nanocrystals in neutral water had higher viscosity than those in acidic and basic water, since ionic interaction caused nanocrystal networks to form in water.

Facile deposition of YSZ-inverse photonic glass films.

An alternative all-colloidal and single-step deposition method of yttrium-stabilized zirconia (YSZ)-infiltrated polymeric photonic glass films is presented. Heterocoagulation of oppositely charged polystyrene (PS) microspheres and YSZ nanocrystals in aqueous dispersions created PS/YSZ core-shell spheres. These composite particles were deposited on glass substrates by a simple drop-coating process. Heterocoagulation impaired self-assembly of the particles, resulting in a disordered structure. Burn-out of the polymer yielded a random array of YSZ shells. The effect of the filling fraction of YSZ between these shells was explored. YSZ-inverse photonic glass films with a thickness below 40 μm achieved 70% reflectance of the incident radiation over a broad wavelength range between 0.4 and 2.2 μm. The YSZ structures demonstrated structural stability up to 1000 °C and maintained high reflectance up to 1200 °C for several hours, thus enabling applications as broadband reflectors at elevated temperatures.

Oral delivery of paclitaxel nanocrystal (PNC) with a dual Pgp-CYP3A4 inhibitor: Preparation, characterization and antitumor activity

Several molecular inheritances have severely restrained the peroral delivery of taxanes. The main objective of the present investigation was to develop a paclitaxel (PTX) formulation which can circumvent the hurdles of its extremely poor solubility and permeability, Pgp efflux and high pre-systemic metabolism. Positively charged PTX nanocrystals of 209nm were prepared by sonoprecipitation with high pressure homogenization technique, wherein an arginine based surfactant was explored as a stabilizer. The BET surface area analysis revealed that the surface area of PNC was 8.53m2/gm, reflecting significant rise in surface area with nanonization of PTX. The DSC and XRD pattern suggested that the PTX is in the form of the most stable dihydrate crystal. The PNC showed very rapid dissolution profile compared to plain PTX in both sinks and non-sink conditions. Clarithromycin (CLM) was evaluated as a better alternative to cyclosporin A in improving PTX permeability. The PNC-CLM showed remarkable enhancement of 453% in relative bioavailability along with maintaining the therapeutic concentration of PTX for 8h. Efficacy data in B16 F10 melanoma tumor bearing mice showed substantial reduction in tumor volume and improvement in percentage survival compared to the control group.

The Structural Evolution of V2O5 Nanocystals during Electrochemical Cycling Studied Using In operando Synchrotron Techniques

The structural evolution of vanadium oxide (V2O5) nanocrystals was studied during Li+ ion intercalation and deintercalation (i.e. electrochemical discharge/charge) processes using in operando high-energy x-ray diffraction (HEXRD) and in operando x-ray adsorption near edge spectroscopy (XANES). The HEXRD results clearly show that V2O5 undergoes phase transformations during the first Li+ ion intercalation (i.e. discharge) process. The analysis of the XANES data suggests that the average oxidation state of vanadium in fully charged V2O5 nanocrystals decreases to less than +5 after the first four cycles. The combined results of the unchanged crystal structure (HEXRD) and the decreased oxidation state (XANES) lead to the conclusion that some of the Li+ ions are trapped within the V2O5 framework and the V2O5 exists as Li0.18V2O5 instead of pure V2O5 after the first four cycles, while the trapped Li+ ion may increase the stability of V2O5 framework.

Preparation, in vitro and in vivo evaluation of bexarotene nanocrystals with surface modification by folate-chitosan conjugates

Bexarotene (Bex) is a synthetic retinoid that exhibits anti-tumor activities. However, the low solubility of the compound hinders its development. In this study, bexarotene nanocrystal was developed and its surface was modified to improve the dissolution and absorption of the drug. The nanocrystals were prepared via precipitation, high-pressure homogenization method, and modified by folate-chitosan (FA-CS), which relied on the charge interaction between the negatively charged nanocrystals and the positively charged FA-CS. The physical–chemical properties in terms of particle size, size distribution, zeta potential, morphology and crystallinity were evaluated. The results showed that bexarotene nanocrystals with surface modification by folate–chitosan conjugates (FC–NC–Bex) with a mean particle size of 631.3 ± 2.7 nm, a polydispersity index of 0.33 ± 0.06 and a zeta potential of 24.6 ± 1.9 mV was obtained. The result of differential scanning calorimetry (DSC) showed that the nanocrystals were still in crystalline state after the preparation procedure. In the in vitro dissolution test, FC-NC-Bex showed significant increase in dissolution rate compared to raw bexarotene (nearly 6.5-fold). Compared to bexarotene suspension, FC-NC-Bex exhibited significant increase in AUC0–∞ (approximately 3-fold) and Cmax (about 1.5-fold). Taken together, the results suggested that FC-NC-Bex may provide a potential opportunity in enhancing the dissolution rate of bexarotene and its gastrointestinal absorption.

ChemInform Abstract: Spectroscopy of Single Nanocrystals

AbstractReview: 208 refs.

Dark and Photo-Conductivity in Ordered Array of Nanocrystals

A theory of photo- and dark-band conductivities in semiconductor supercrystals consisting of nanocrystals is developed by assuming scattering by structural defects in the supercrystals. A new proposed mechanism of photoexcitation, which is triggered by an efficient Auger ionization of charged nanocrystals, provides explanation for the measured photocurrent being 2-3 orders of magnitude larger than the dark current. For dark conductivity, the metal-insulator transitions and temperature dependence of mobility in the metal phase are considered.

Spectroscopy of single nanocrystals

As colloidal semiconductor nanocrystals are developed for a wider range of diverse applications, it becomes more important to gain a deeper understanding of their properties in order to direct synthetic efforts. While most synthetic developments are guided by changes in ensemble properties, certain applications such as those in nano-electronics and nano-photonics rely on properties of nanocrystals at the individual level. For such applications and even for a more detailed understanding of the ensemble behavior, single nanocrystal spectroscopy becomes a vital tool. This review looks at how single nanocrystal spectroscopy has been applied to materials based on modern synthetic techniques and how these studies are elucidating properties that remain hidden at the ensemble level. First, recent theoretical models that are important for understanding many observed phenomena are explored. Then the review highlights new insights into many of the photophysical properties that are of interest in semiconductor nanocrystal materials, such as the ubiquitous spectral instability, magneto-optical identification of the band-edge exciton fine structure, emission from multi-excitons, and the spectroscopic properties of charged nanocrystals that challenge long standing theories on photoluminescence blinking behavior. To date most of the research has been conducted on materials based on cadmium selenide primarily due to its many years of development as a prototypical nanocrystal system. The review ends with a discussion of new materials that would also benefit from a detailed photophysical understanding afforded by single nanocrystal spectroscopy.

Composite for smarter windows

Glass has been prepared that selectively absorbs visible and near-infrared light when an electrochemical voltage is applied. This opens the way to 'smart' windows that block heat on demand, with or without optical transparency. See Letter p.323 Glass windows let in light and keep out extreme heat and cold, but with the advance of the field of electrochromism — the reversible change of optical transmittance in response to electrochemical charge and discharge — they could do much more. Delia Milliron and co-workers have made a step towards producing windows that can greatly reduce lighting and heating costs with the demonstration of a composite film composed of tin-doped indium oxide nanocrystals embedded in a niobium oxide glass matrix. By varying an applied voltage over a 2.5-volt range, the charged nanocrystals selectively block near-infrared light, while the glass strongly modulates visible light owing to its reconstructed bonding near the nanocrystal–glass interface. Practical 'smart windows' and other devices are still some way off, but this solution-processed nanocrystal-in-glass approach is modular and permits extensive tuning of structure and composition, and so provides a solid platform for further refinement.

Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals

Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that their fluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, because non-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesize colloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes at low temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy in the temperature range 30-300K shows that the non-radiative Auger recombination is thermally activated around 200K. Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trion electrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to a route for the design of core/shell structures with 100% quantum yield at room temperature.

One-step solvothermal synthesis of highly water-soluble, negatively charged superparamagnetic Fe3O4 colloidal nanocrystal clusters

Highly charged hydrophilic superparamagnetic Fe3O4 colloidal nanocrystal clusters with an average diameter of 195 nm have been successfully synthesized using a modified one-step solvothermal method. Anionic polyelectrolyte poly(4-styrenesulfonic acid-co-maleic acid) sodium salt containing both sulfonate and carboxylate groups was used as the stabilizer. The clusters synthesized under different experimental conditions were characterized with transmission electron microscopy and dynamic light scattering; it was found that the size distribution and water dispersity were significantly affected by the concentration of the polyelectrolyte stabilizer and iron sources in the reaction mixtures. A possible mechanism involving novel gel-like large molecular networks that confined the nucleation and aggregation process was proposed and discussed. The colloidal nanocrystal clusters remained negatively charged in the experimental pH ranges from 2 to 11, and also showed high colloidal stability in phosphate buffered saline (PBS) and ethanol. These highly colloidal stable superparamagnetic Fe3O4 clusters could find potential applications in bioseparation, targeted drug delivery, and photonics.

Open-tubular Electrochromatographic Chiral Separation of Amino Acids Using an Organic Nanocrystals Immobilized Capillary

The preparation of cinchonidine (CCND) nanocrystals and their immobilization onto the inner surface of a fused silica capillary was investigated for use in the enantioseparation by capillary electrochromatography. The CCND nanocrystals were prepared by an emulsion method that utilizes crystal growth in an oil-in-water emulsion without a stabilizer. As a result, white-colored aqueous dispersions of organic nanoparticles were obtained. SEM images showed that the prepared CCND nanocrystals were from 300 to 700 nm in size. When the obtained dispersion was introduced into a poly(diallyldimethylammonium chloride) (PDDAC) coated capillary, the nanocrystals were strongly adsorbed onto the inner surface due to an electrostatic interaction between cationic PDDAC and the negatively charged organic nanocrystals. In CCND nanocrystals coated capillary, CEC enantioseparations of racemic amino acids were successfully achieved. The reproducibilities of the detection times were acceptable with a relative standard deviation of less than 7%, indicating that stable nanocrystal coating could be obtained by our proposed method.

Spin coherence generation and detection in spherical nanocrystals

A theoretical description of electron spin orientation and detection by short optical pulses is proposed for ensembles of singly charged semiconductor nanocrystals. The complex structure of the valence band in spherical nanocrystals is taken into account. We demonstrate that the direction of electron spin injected by the pump pulse depends on both the pump pulse helicity and the pump pulse power. It is shown that a train of optical pulses can lead to the complete orientation of the resident electron spin. The microscopic theory of the spin Faraday, Kerr and ellipticity effects is developed and the spectral sensitivity of these signals is discussed. We show that under periodic pumping pronounced mode-locking of electron spins takes place and manifests itself as significant spin signals at negative delays between pump and probe pulses.

Local Cathode Luminescence Resonant Peak in Hybrid Organic Nanocrystal Systems

Nanostructures are likely to become primary components of future electronic devices. Self-assembled hybrid organic nanocrystal devices are a route to achieve this goal. One central issue in using molecules in such devices deals with applying measurement methods which locally probe the molecular device. In this work we investigated hybrid molecular semiconductor quantum dot devices using cathodoluminescence measurements. A narrow cathodoluminescence peak was detected at proper conditions. This narrow resonant-like peak was found to be sensitive to the substrate material and its doping, to the nanoparticle sizes, and to the organic molecule linkers. The peak is assigned to resonant charge transfer from the charged nanocrystals to the deep surface states of the substrate. This suggests that cathodoluminescence measurement may probe the charge transfer in the hybrid device.

Modeling of the electrostatic coupling between nanocrystals of a disordered nanocrystal floating gate memory

This paper presents a realistic model that explicitly takes into account the electrostatic coupling between the nanocrystals of a disordered layer constituting the floating gate of a non-volatile memory. A statistical study of the neighborhood of a given nanocrystal is carried out, leading to the mean number of neighboring nanocrystals as a function of the radius of the central nanocrystal. We show that the empty neighborhood of every nanocrystal can be represented by an equivalent torus ring in the previous model of a single nanocrystal. Then the effects of charged nanocrystals are taken into account by an appropriate rigid shift of the energy levels of the central nanocrystal. The proposed model is validated by statistical comparisons with exact 3D computations, and the influence of the electrostatic coupling is analyzed and discussed.

General one-pot strategy to prepare multifunctional nanocomposites with hydrophilic colloidal nanoparticles core/mesoporous silica shell structure

A general and facile strategy was developed to coat hydrophilic inorganic nanoparticles directly with mesoporous silica nanoparticles (MSNs). The cationic surfactant of cetyltrimethylammonium bromide (CTAB) was adsorbed to various negatively charged CdTe quantum dots, Fe3O4 nanocrystals or Au nanoparticles, introducing the bilayer of CTAB overcoating with positive charge. The subsequent sol–gel reaction of TEOS with the basic catalyst resulted in uniform nanocomposites. The concentration of CTAB and NH4OH in the recipe strongly influenced the number of inorganic nanoparticles in the nanocomposites and the homogeneity of MSNs shell. One dimensional Au nanorods and larger size of solid SiO2 nanoparticles were also able to coat with MSNs using a similar synthetic procedure. The proposed method was greatly simplified without the help of any mediators or silane coupling agents and excellent mesostructural performance was readily achieved. Compared to the methods known from the literatures for the coating of hydrophobic nanoparticles, this efficient way is especially useful for trapping different hydrophilic nanoparticles with arbitrary sizes and shapes into MSNs. These highly versatile multifunctional nanocomposites, together with the pH-responsible drug release behaviors, non-toxicity to normal cells and ease of uptake into cancer cells, are expected to be utilized as drug delivery system for simultaneous imaging and therapeutic applications.

Electron transfer between colloidal ZnO nanocrystals.

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e(-)(CB):ZnO-S) with a solution of uncharged large nanocrystals (ZnO-L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e(-)(CB):ZnO-L + ZnO-S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV(2+)). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.

Self-assembled Fe3O4-layered double hydroxide colloidal nanohybrids with excellent performance for treatment of organic dyes in water

We report a remarkably simple and efficient method for the preparation of colloidal nanohybrids consisting of Fe3O4 nanoparticles and nitrate-containing MgAl-layered double hydroxide (LDH) nanocrystals. The electrostatic interaction between the negatively charged magnetite nanoparticles and the positively charged LDH nanocrystals is sufficient to induce the formation of stable self-assembly of the two components. At a mass ratio of 1 : 0.3 (LDH:Fe3O4), all Fe3O4 nanoparticles are attached on the LDH nanocrystals. The resulting nanohybrids maintained an overall positive zeta potential of +42 mV and can form a stable colloidal suspension in water. Separation/re-dispersion of the nanohybrids in the aqueous phase can be easily achieved in the presence/absence of an external magnetic field. The combination of these unique properties allows the nanohybrids to exhibit superior performance in the treatment of organic dyes in wastewater. The colloidal suspension of the nanohybrids leads to fast adsorption kinetics and high adsorption capacity of the acid organic dyesvia quick surface adsorption. In addition, the spent nanohybrids can be easily recycled after catalytic regeneration by advanced oxidation technology.

Tailored coordination nanoparticles: assessing the magnetic single-domain critical size

Negatively charged nanocrystals of the magnetic coordination network CsNiCr(CN)(6) were prepared in water through a seed-mediated growth with a few atomic layers accuracy and final sizes tailored from 6 to 30 nm. A lower limit of the magnetic single-domain critical size was determined to be around 15 nm possessing a blocking temperature above 20 K.