Multi-feature round silicon membrane filters enable fractionation and analysis of small micro- and nanoplastics with Raman spectroscopy and nano-FTIR.

Visualization of small micro-(20-1 µm) and nanoplastics (<1 µm) combined with chemical identification is still a challenge. To address this, we designed and manufactured easy-to-handle silicon membrane filters with a standard round filter geometry of 25 mm in diameter and a 10 mm diameter filtration area, holding hexagonal sections with periodically arranged pores of either 250 nm or 1 µm. Due to their flat and reflective surface, the filters serve as a versatile substrate for spectroscopic identification of particles. Optical markers at different levels of magnification, including the bare eye, allow for an easy transfer and repositioning of samples between instruments and methods as well as for a re-measurement of nanoscale particles. We demonstrate how nanoscale particles of weakly absorbing polymers such as polyethylene and polystyrene are analyzed on these filters by nano-FTIR, a combination of atomic force microscopy and Fourier transform infrared spectroscopy. By sequential filtration we separated the fractions of small micro and nanoplastics from a degraded polylactic acid coffee cup lid and achieved subsequent identification by Raman and nano-FTIR spectroscopy. The applications presented in this study will enable future research regarding the identification of small polymer particles difficult to access by other methods.

Critical Assessment of Analytical Methods for the Harmonized and Cost-Efficient Analysis of Microplastics.

Microplastics are of major concerns for society and is currently in the focus of legislators and administrations. A small number of measures to reduce or remove primary sources of microplastics to the environment are currently coming into effect. At the moment, they have not yet tackled important topics such as food safety. However, recent developments such as the 2018 bill in California are requesting the analysis of microplastics in drinking water by standardized operational protocols. Administrations and analytical labs are facing an emerging field of methods for sampling, extraction, and analysis of microplastics, which complicate the establishment of standardized operational protocols. In this review, the state of the currently applied identification and quantification tools for microplastics are evaluated providing a harmonized guideline for future standardized operational protocols to cover these types of bills. The main focus is on the naked eye detection, general optical microscopy, the application of dye staining, flow cytometry, Fourier transform infrared spectroscopy (FT-Ir) and microscopy, Raman spectroscopy and microscopy, thermal degradation by pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) as well as thermo-extraction and desorption gas chromatography-mass spectrometry (TED-GC-MS). Additional techniques are highlighted as well as the combined application of the analytical techniques suggested. An outlook is given on the emerging aspect of nanoplastic analysis. In all cases, the methods were screened for limitations, field work abilities and, if possible, estimated costs and summarized into a recommendation for a workflow covering the demands of society, legislation, and administration in cost efficient but still detailed manner.

Electrostatic-Driven Gelation of Colloidal Nanocrystals.

The assembly of colloidal nanocrystals (NCs) is a unique strategy to produce porous materials with high crystallinity and unmatched control over structural and chemical parameters. This strategy has been demonstrated mostly for single-component nanomaterials. In the present work, we report the gelation of colloidal NC solutions driven by the electrostatic interaction of oppositely charged NCs. A key step for leading this strategy to success is to produce a stable colloidal solution of the positively charged component. We achieved this goal by functionalizing the NCs with inexpensive and nontoxic amino acids such as glutamine. We demonstrate the combination of positively and negatively charged NCs in proper concentrations to result in gels with a homogeneous distribution of the two compounds. In this way, porous nanocomposites with virtually any combination can be produced. We illustrate this approach by combining positively charged ceria NCs with negatively charged gold NCs to form Au-CeO2 gels. These gels were dried from supercritical CO2 to produce highly porous Au-CeO2 aerogels with specific surface areas of 120 m2 g-1. The formation of a proper interface is confirmed through the evaluation of nanocomposite catalytic activity toward CO oxidation. We further demonstrate the versatility of this strategy to produce porous metal chalcogenide-metal oxide and metal-metal chalcogenide nanocomposites by the examples of PbS-CeO2 and Au-PbS.

Colloidal Synthesis of CsX Nanocrystals (X = Cl, Br, I).

A facile colloidal synthesis of highly ionic cesium halide nanocrystals is reported. Colloidal nanocrystals of CsI, CsCl and CsBr with unprecedentedly small dimensions are obtained using oleylammonium halides and cesium oleate as precursors. The ease and adaptability of our method enables its universalization for the formation of other highly ionic nanocrystals.

Polymer-Enhanced Stability of Inorganic Perovskite Nanocrystals and Their Application in Color Conversion LEDs.

Cesium lead halide (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) offer exceptional optical properties for several potential applications but their implementation is hindered by a low chemical and structural stability and limited processability. In the present work, we developed a new method to efficiently coat CsPbX3 NCs, which resulted in their increased chemical and optical stability as well as processability. The method is based on the incorporation of poly(maleic anhydride-alt-1-octadecene) (PMA) into the synthesis of the perovskite NCs. The presence of PMA in the ligand shell stabilizes the NCs by tightening the ligand binding, limiting in this way the NC surface interaction with the surrounding media. We further show that these NCs can be embedded in self-standing silicone/glass plates as down-conversion filters for the fabrication of monochromatic green and white light emitting diodes (LEDs) with narrow bandwidths and appealing color characteristics.

Metal Domain Size Dependent Electrical Transport in Pt-CdSe Hybrid Nanoparticle Monolayers.

Thin films prepared of semiconductor nanoparticles are promising for low-cost electronic applications such as transistors and solar cells. One hurdle for their breakthrough is their notoriously low conductivity. To address this, we precisely decorate CdSe nanoparticles with platinum domains of one to three nanometers in diameter by a facile and robust seeded growth method. We demonstrate the transition from semiconductor to metal dominated conduction in monolayered films. By adjusting the platinum content in such solution-processable hybrid, oligomeric nanoparticles the dark currents through deposited arrays become tunable while maintaining electronic confinement and photoconductivity. Comprehensive electrical measurements allow determining the reigning charge transport mechanisms.

Cl-capped CdSe nanocrystals via in situ generation of chloride anions.

Halide ions cap and stabilize colloidal semiconductor nanocrystal (NC) surfaces allowing for NCs surface interactions that may improve the performance of NC thin film devices such as photo-detectors and/or solar cells. Current ways to introduce halide anions as ligands on surfaces of NCs produced by the hot injection method are based on post-synthetic treatments. In this work we explore the possibility to introduce Cl in the NC ligand shell in situ during the NCs synthesis. With this aim, the effect of 1,2-dichloroethane (DCE) in the synthesis of CdSe rod-like NCs produced under different Cd/Se precursor molar ratios has been studied. We report a double role of DCE depending on the Cd/Se precursor molar ratio (either under excess of cadmium or selenium precursor). According to mass spectrometry (ESI-TOF) and nuclear magnetic resonance ((1)H NMR), under excess of Se precursor (Se dissolved in trioctylphosphine, TOP) conditions at 265 °C ethane-1,2-diylbis(trioctylphosphonium)dichloride is released as a product of the reaction between DCE and TOP. According to XPS studies chlorine gets incorporated into the CdSe ligand shell, promoting re-shaping of rod-like NCs into pyramidal ones. In contrast, under excess Cd precursor (CdO) conditions, DCE reacts with the Cd complex releasing chlorine-containing non-active species which do not trigger NCs re-shaping. The amount of chlorine incorporated into the ligand shell can thus be controlled by properly tuning the Cd/Se precursor molar ratio.

Interfacing quantum dots and graphitic surfaces with chlorine atomic ligands.

The performance of devices based on semiconductor nanocrystals (NCs) improves both with stronger interface interactions among NCs and between NCs and solid electrode surfaces. The combination of X-ray photoelectron spectroscopy (XPS) and solid (31)P CP/MAS NMR (cross-polarization/magic angle spinning nuclear magnetic resonance) shows that the selective substitution of long organic chains by chlorine atomic ligands during the colloidal synthesis by the hot injection method promotes the adsorption of CdSe NCs to carbon sp(2) surfaces, leading to the formation of well-ordered NC monolayers on graphitic materials.

Carbon supported CdSe nanocrystals.

Insights to the mechanism of CdSe nanoparticle attachment to carbon nanotubes following the hot injection method are discussed. It was observed that the presence of water improves the nanotube coverage while Cl containing media are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well-defined crystallographic facets, and interaction with an organics-free sp2 carbon lattice. Furthermore, this procedure can be extended to cover graphene by quantum dots.