In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities

A modular design approach to polymer-coated ZnO nanocrystals

Hybrid materials based on inorganic nanocrystals with organic polymers feature peculiar and fascinating properties and various applications. However, there is still a need for simple synthesis procedures that provide precise control over the polymer/nanocrystal microstructure of these materials. Herein, a novel organometallic approach to polymer-coated ZnO nanocrystals was developed. The presented method merges the initial ring-opening polymerization of ϵ-caprolactone mediated by an organozinc alkoxide initiator and an air-promoted transformation of the resulting macromolecular organozinc species. This one-pot procedure results in quantum-sized ZnO crystals with a core diameter of ca 3 nm coated by poly(ϵ-caprolactone) covalently bonded to the surface. Overall, the ability to create well-defined hybrid composites should provide a unique ability to access various nanosystems.

Dynamics of the nanocrystal structure and composition in growth solutions monitored by in situ lab-scale X-ray diffraction

In situ characterization of nanoparticle (NP) growth has become the state-of-the-art approach for studying their growth mechanisms; there is broad consensus on the reliability and precision of in situ characterization techniques compared to more traditional ex situ ones. Nonetheless, most of the currently available methods require the use of sophisticated setups such as synchrotron-based X-ray sources or an environmental liquid transmission electron microscopy (TEM) cell, which are expensive and not readily accessible. Herein, we suggest a new approach to study NP growth mechanisms: using a commercially available heating chamber for time-resolved X-ray diffraction (TR-XRD) measurements of NP growth in solution. We demonstrate how this lab-scale in situ XRD can be used to study NP growth mechanisms when complemented by standard ex situ techniques such as TEM and UV-vis spectroscopy. TR-XRD reveals the crystallographic phase and real-time evolution of NP size, shape, and composition. A detailed analysis allows determining the growth mechanism and measuring the alloying kinetics of multinary nanocrystals, demonstrated herein for a colloidal Cd_xZn_1-xS system. This approach proves itself as a promising strategy for NP growth research and could be expanded to related fields that study dynamic changes as the formation and evolution of crystalline materials in solutions.

Combined Small-Angle Neutron Scattering/Small-Angle X-ray Scattering Analysis for the Characterization of Silver Nanoparticles Prepared via Photoreduction in Water-in-Oil Microemulsions

In this study, we used small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) to investigate the formation process of silver (Ag) nanoparticles (NPs) in water-in-oil (w/o) reverse microemulsions comprising sodium bis(2-ethylhexyl) sulfosuccinate (AOT), water, and organic solvents (such as benzene, octane, and decane) by the photoreduction of silver perchlorate (AgClO₄). Combining SANS and SAXS, the structural changes in the w/o microemulsions before and after the formation of Ag NPs via photoreduction were quantitatively evaluated. From the SANS experiments performed using the contrast-variation method, the size of water cores containing Ag NPs and the thickness of the AOT shells were calculated using the core-shell hard-sphere model. The size of the Ag NPs and their aggregates was calculated via SAXS analysis based on the polydisperse sphere model with a Schulz-Zimm distribution. We found that aggregates of three or four primary Ag NPs are formed by, first, the aggregation of water droplets through the entanglement of the tails of the AOT shell, followed by the self-assembly of Ag NPs into their aggregates because of particle-particle attractive interactions.

ZnO nanocrystals derived from organometallic approach: Delineating the role of organic ligand shell on physicochemical properties and nano-specific toxicity

The surface organic ligands have profound effect on modulation of different physicochemical parameters as well as toxicological profile of semiconductor nanocrystals (NCs). Zinc oxide (ZnO) is one of the most versatile semiconductor material with multifarious potential applications and systematic approach to in-depth understand the interplay between ZnO NCs surface chemistry along with physicochemical properties and their nano-specific toxicity is indispensable for development of ZnO NCs-based devices and biomedical applications. To this end, we have used recently developed the one-pot self-supporting organometallic (OSSOM) approach as a model platform to synthesize a series of ZnO NCs coated with three different alkoxyacetate ligands with varying the ether tail length which simultaneously act as miniPEG prototypes. The ligand coating influence on ZnO NCs physicochemical properties including the inorganic core size, the hydrodynamic diameter, surface charge, photoluminescence (quantum yield and decay time) and ZnO NCs biological activity toward lung cells was thoroughly investigated. The resulting ZnO NCs with average core diameter of 4-5 nm and the hydrodynamic diameter of 8-13 nm exhibit high photoluminescence quantum yield reaching 33% and a dramatic slowing down of charge recombination up to 2.4 µs, which is virtually unaffected by the ligand's character. Nano-specific ZnO NCs-induced cytotoxicity was tested using MTT assay with normal (MRC-5) and cancer (A549) human lung cell lines. Noticeably, no negative effect has been observed up to the NCs concentration of 10 µg/mL and essentially very low negative toxicological impact could be noticed at higher concentrations. In the latter case, the MTT data analysis indicate that there is a subtle interconnection between inorganic core-organic shell dimensions and toxicological profile of ZnO NCs (strikingly, the NCs coated by the carboxylate bearing a medium ether chain length exhibit the lowest toxicity level). The results demonstrate that, when fully optimized, our organometallic self-supporting approach can be a highly promising method to obtain high-quality and bio-stable ligand-coated ZnO NCs.

Disclosing Interfaces of ZnO Nanocrystals Using Dynamic Nuclear Polarization: Sol-Gel versus Organometallic Approach

The unambiguous characterization of the coordination chemistry of nanocrystal surfaces produced by wet-chemical synthesis presently remains highly challenging. Here, zinc oxide nanocrystals (ZnO NCs) coated by monoanionic diphenyl phosphate (DPP) ligands were derived by a sol-gel process and a one-pot self-supporting organometallic (OSSOM) procedure. Atomic-scale characterization through dynamic nuclear polarization (DNP-)enhanced solid-state NMR (ssNMR) spectroscopy has notably enabled resolving their vastly different surface-ligand interfaces. For the OSSOM-derived NCs, DPP moieties form stable and strongly-anchored μ₂ - and μ₃ -bridging-ligand pairs that are resistant to competitive ligand exchange. The sol-gel-derived NCs contain a wide variety of coordination modes of DPP ligands and a ligand exchange process takes place between DPP and glycerol molecules. This highlights the power of DNP-enhanced ssNMR for detailed NC surface analysis and of the OSSOM approach for the preparation of ZnO NCs.

Zonal rotor centrifugation revisited: new horizons in sorting nanoparticles

Density gradient centrifugation is an effective method for the isolation and purification of small particles. Hollow rotors capable of hosting density gradients replace the need for centrifuge tubes and therefore allow separations at large scales. So far, zonal rotors have been used for biological separations ranging from the purification of whole cells down to serum proteins. We demonstrate that the high-resolution separation method opens up exciting perspectives apart from biology, namely in sorting mixtures of synthetic nanoparticles. Loading and unloading, while the rotor is spinning, avoids perturbations during acceleration and deceleration periods, and thus makes a vital contribution to sorting accuracy. Nowadays one can synthesize nanoscale particles in a wide variety of compositions and shapes. A prominent example for this are “colloidal molecules” or, generally speaking, defined assemblies of nanoparticles that can appear in varying aggregation numbers. Fractionation of such multimodal colloids plays an essential role with regard to their organization into hierarchical organized superstructures such as films, mesocrystals and metamaterials. Zonal rotor centrifugation was found to be a scalable method of getting “colloidal molecules” properly sorted. It allows access to pure fractions of particle monomers, dimers, and trimers, just as well as to fractions that are essentially rich in particle tetramers. Separations were evaluated by differential centrifugal sedimentation, which provides high-resolution size distributions of the collected nanoparticle fractions. The performance achieved in relation to resolution, zone widths, sorting efficiencies and recovery rates clearly demonstrate that zonal rotor centrifugation provides an excellent solution to the fractionation of nanoparticle mixtures.

Hollow bowl-shaped rotors allow for efficient fractionation of nanoparticle mixtures at large scale.

In Situ Synchrotron X-ray Characterization Shining Light on the Nucleation and Growth Kinetics of Colloidal Nanoparticles

Rational synthesis of colloidal nanoparticles with desirable properties relies on precise control over the nucleation and growth kinetics, which is still not well understood. The recent development of in situ high energy synchrotron X-ray techniques offers an excellent opportunity to quantitatively monitor the growth trajectories of colloidal nanoparticles in real time under real reaction conditions. The time-resolved, quantitative data of the growing colloidal nanoparticles are unique to reveal the mechanism of nanoparticle formation and determine the corresponding intrinsic kinetic parameters. This review discusses the kinetics of major steps of forming colloidal nanoparticles and the capability of in situ synchrotron X-ray techniques in studying the corresponding kinetics.

Determination of the two-dimensional distributions of gold nanorods by multiwavelength analytical ultracentrifugation

Properties of nanoparticles are influenced by various parameters like size, shape or composition. Comprehensive high throughput characterization techniques are urgently needed to improve synthesis, scale up to production and make way for new applications of multidimensional particulate systems. In this study, we present a method for measuring two-dimensional size distributions of plasmonic nanorods in a single experiment. Analytical ultracentrifuge equipped with a multiwavelength extinction detector is used to record the optical and sedimentation properties of gold nanorods simultaneously. A combination of sedimentation and extinction properties, both depending on diameter and length of the dispersed nanorods, is used to measure two-dimensional distributions of gold nanorod samples. The length, diameter, aspect ratio, volume, surface and cross-sectional distributions can be readily obtained from these results. As the technique can be extended to other non-spherical plasmonic particles and can be used for determining relative amounts of particles of different shapes it provides complete and quantitative insights into particulate systems.

2D analysis of polydisperse core-shell nanoparticles using analytical ultracentrifugation

Accurate knowledge of the size, density and composition of nanoparticles (NPs) is of major importance for their applications. In this work the hydrodynamic characterization of polydisperse core-shell NPs by means of analytical ultracentrifugation (AUC) is addressed. AUC is one of the most accurate techniques for the characterization of NPs in the liquid phase because it can resolve particle size distributions (PSDs) with unrivaled resolution and detail. Small NPs have to be considered as core-shell systems when dispersed in a liquid since a solvation layer and a stabilizer shell will significantly contribute to the particle's hydrodynamic diameter and effective density. AUC measures the sedimentation and diffusion transport of the analytes, which are affected by the core-shell compositional properties. This work demonstrates that polydisperse and thus widely distributed NPs pose significant challenges for current state-of-the-art data evaluation methods. The existing methods either have insufficient resolution or do not correctly reproduce the core-shell properties. First, we investigate the performance of different data evaluation models by means of simulated data. Then, we propose a new methodology to address the core-shell properties of NPs. This method is based on the parametrically constrained spectrum analysis and offers complete access to the size and effective density of polydisperse NPs. Our study is complemented using experimental data derived for ZnO and CuInS₂ NPs, which do not have a monodisperse PSD. For the first time, the size and effective density of such structures could be resolved with high resolution by means of a two-dimensional AUC analysis approach.

Advanced Multiwavelength Detection in Analytical Ultracentrifugation

This work highlights significant advancements in detector hardware and software for multiwavelength analytical ultracentrifugation (MWL-AUC) experiments, demonstrating improvement in both the spectral performance and UV capabilities of the instrument. The hardware is an extension of the Open AUC MWL detector developed in academia and first introduced in 2006 by Bhattacharya et al. Additional modifications as well as new analytical methods available for MWL data have since been reported. The present work describes new and continuing improvements to the MWL detector, including mirror source and imaging optics, UV sensitive acquisition modes and revised data acquisition software. The marked improvement of experimental data promises to provide access to increasingly complex systems, especially semiconductor nanoparticles, synthetic polymers, biopolymers, and other chromophores absorbing in the UV. Details of the detection system and components are examined to reveal the influences on data quality and to guide further developments. The benchmark comparisons of data quality across platforms will also serve as a reference guide for evaluation of forthcoming commercial absorbance optics.

Influence of Tail Groups during Functionalization of ZnO Nanoparticles on Binding Enthalpies and Photoluminescence

We report on the tailoring of ZnO nanoparticle (NP) surfaces by catechol derivatives (CAT) with different functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The influence of electron-donating/-withdrawing properties on enthalpy of ligand binding (ΔH) was resolved and subsequently linked with optical properties. First, as confirmed by ultraviolet/visible (UV/vis) and Fourier transform infrared (FT-IR) spectroscopy results, all CAT molecules chemisorbed to ZnO NPs, independent of the distinct functionality. Interestingly, the ζ-potentials of ZnO after functionalization shifted to more negative values. Then, isothermal titration calorimetry (ITC) and a mass-based method were applied to resolve the heat release during ligand binding and the adsorption isotherm, respectively. However, both heat- and mass-based approaches alone did not fully resolve the binding enthalpy of each molecule adsorbing to the ZnO surface. This is mainly due to the fact that the Langmuir model oversimplifies the underlying adsorption mechanism, at least for some of the tested CAT molecules. Therefore, a new, fitting-free approach was developed to directly access the adsorption enthalpy per molecule during functionalization by dividing the heat release measured via ITC by the amount of bound molecules determined from the adsorption isotherm. Finally, the efficiency of quenching the visible emission caused by ligand binding was investigated by photoluminescence (PL) spectroscopy, which turned out to follow the same trend as the binding enthalpy. Thus, the functionality of ligand molecules governs the binding enthalpy to the particle surface, which in turn, at least in the current case of ZnO, is an important parameter for the quenching of visible emission. We believe that establishing such correlations is an important step toward a more general way of selecting and designing ligand molecules for surface functionalization. This allows developing strategies for tailored colloidal surfaces beyond empirically driven formulation on a case by case basis.

Changes within the stabilizing layer of ZnO nanoparticles upon washing

ZnO nanoparticles (NPs) are highly relevant for various industrial applications, however, after synthesis of the NPs residual chemicals need to be removed from the colloidal raw product by washing, as they may influence the performance of the final device. In the present study we focus on the effect of washing by antisolvent flocculation with subsequent redispersion of the NPs on the stabilizing acetate shell. Purification of the ZnO nanoparticles is reported to be optimal with respect to zeta potential that has a maximum after one washing cycle. In this work, we will shed light on this observation using small angle X-ray and neutron scattering (SAXS, SANS) by demonstrating that after the first washing cycle the content of acetate in the ligand shell around the ZnO NPs increases. In detail, it was observed that the diffuse acetate shell shrinks to the size of a monolayer upon washing but the acetate content of this monolayer is higher than within the diffuse shell of the particles of the native dispersion. A second washing cycle reduces the acetate concentration within the stabilizing shell and the stability of the dispersion drops accordingly. After another (third) washing cycle strong agglomeration was observed for all investigated samples.

Modelling and validation of particle size distributions of supported nanoparticles using the pair distribution function technique

The particle size of supported catalysts is a key characteristic for determining structure–property relationships. It is a challenge to obtain this information accurately andin situusing crystallographic methods owing to the small size of such particles (<5 nm) and the fact that they are supported. In this work, the pair distribution function (PDF) technique was used to obtain the particle size distribution of supported Pt catalysts as they grow under typical synthesis conditions. The PDF of Pt nanoparticles grown on zeolite X was isolated and refined using two models: a monodisperse spherical model (single particle size) and a lognormal size distribution. The results were compared and validated using scanning transmission electron microscopy (STEM) results. Both models describe the same trends in average particle size with temperature, but the results of the number-weighted lognormal size distributions can also accurately describe the mean size and the width of the size distributions obtained from STEM. Since the PDF yields crystallite sizes, these results suggest that the grown Pt nanoparticles are monocrystalline. This work shows that refinement of the PDF of small supported monocrystalline nanoparticles can yield accurate mean particle sizes and distributions.

From evaporation-induced self-assembly to shear-induced alignment

The functionality of compact nanostructured thin films depends critically on the degree of order and hence on the underlying ordering mechanisms during film formation. For dip coating of rigid nanorods the counteracting mechanisms, evaporation-induced self-assembly (EISA) and shear-induced alignment (SIA) have recently been identified as competing ordering mechanisms. Here, we show how to achieve highly ordered and homogeneous thin films by controlling EISA and SIA in dip coating. Therefore we identify the influences of the process parameters including temperature, initial volume fraction and nanorod aspect ratio on evaporation-induced convective flow and externally applied shear forces and evaluate the resulting films. The impact of evaporation and shear can be distinguished by analysing film thickness, surface order and bulk order by careful in situ SAXS, Raman and SEM-based image analysis. For the first time we derive processing guidelines for the controlled application of EISA and SIA towards highly ordered thin nematic films.

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution

We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture.

Quantum Dots as Nonagglomerated Nanofillers for Adhesive Resins

Nanoparticles used in adhesive resins are prone to agglomeration, turning the material susceptible to physical failure. Quantum dots are nonagglomerated inorganic nanoparticles (1 to 10 nm) when in equilibrium. The aim of the present study was to synthesize and characterize zinc oxide quantum dots (ZnOQDs) and to develop and evaluate an adhesive resin with the addition of ZnOQDs. ZnOQDs were formulated by self-organization in chemical reaction with isopropanol and added to 2-hydroxyethyl methacrylate (HEMA). HEMA containing ZnOQDs was used for the experimental group and neat HEMA for the control group. Mean ZnOQD diameter was evaluated in isopropanol and in HEMA by ultraviolet-visible spectroscopy. The adhesives were evaluated for degree of conversion ( n = 5), softening in solvent ( n = 5), ultimate tensile strength ( n = 5), microtensile bond strength ( n = 20) at 24 h and after 6 mo, SEM-EDS (scanning electron microscopy–energy-dispersive x-ray spectroscopy; n = 3), and superresolution confocal microscopy ( n = 3). Data of microtensile bond strength after 6 mo and Knoop hardness after solvent immersion were evaluated by paired t test with a 0.05 level of significance. The other data were evaluated by independent t test with a 0.05 level of significance. Ultraviolet-visible spectroscopy indicated that the mean ZnOQD diameter remained stable in isopropanol and in HEMA (1.19 to 1.24 nm). Fourier transform infrared spectroscopy analysis showed the peak corresponding to zinc and oxygen bond (440 cm-1). The experimental group achieved a higher degree of conversion as compared with the control group and presented dentin/adhesive interface stability after 6 mo without altering other properties tested. SEM-EDS indicated 1.54 ± 0.46 wt% of zinc, and the superresolution confocal microscopy indicated nonagglomerated nanoparticles with fluorescence blinking in the polymerized adhesive. The findings of this study showed a possible and reliable method to formulate composites with nonagglomerated nanoscale fillers, shedding light on the nanoparticle agglomeration concern.

‘Clickable’ ZnO nanocrystals: the superiority of a novel organometallic approach over the inorganic sol–gel procedure

We demonstrate for the first time a highly efficient Cu(i)-catalyzed alkyne–azide cycloaddition reaction on the surface of ZnO nanocrystals with retention of their photoluminescence properties.