Multispectral and Circular Polarization-Sensitive Carbon Dot-Polydiacetylene Capacitive Photodetector.

Multispectral photodetectors (MSPs) and circularly polarized light (CPL) sensors are important in opto-electronics, photonics, and imaging. A capacitive photodetector consisting of an interdigitated electrode coated with carbon dot/anthraquinone-polydiacetylene is constructed. Photoexcitation of the carbon dots induces transient electron transfer to the anthraquinone moieties, and concomitant change in the film dielectric constant and recorded capacitance. This unique photodetection mechanism furnishes wavelength selectivity that is solely determined by the absorbance of the carbon dots incorporated in the anthraquinone-polydiacetylene matrix. Accordingly, employing an array of polymerized-anthraquinone photodetector films comprising carbon dots (C-dots) exhibiting different excitation wavelengths yielded optical "capacitive fingerprints" in a broad spectral range (350-650 nm). Furthermore, circular light polarization selectivity is achieved through chiral polymerization of the polydiacetylene framework. The carbon dot/anthraquinone-polydiacetylene capacitive photodetector features rapid photo-response, high fidelity, and recyclability as the redox reactions of anthraquinone are fully reversible. The carbon dot/anthraquinone-polydiacetylene platform is inexpensive, easy to fabricate, and consists of environmentally friendly materials.

Size-Selective Detection of Nanoparticles in Solution and Air by Imprinting.

Monitoring of nanoparticles (NPs) in air and aquatic environments is an unmet challenge accentuated by the rising exposure to anthropogenic or engineered NPs. The inherent heterogeneity in size, shape, and the stabilizing shell of NPs makes their selective recognition a daunting task. Thus far, only a few technologies have shown promise in detecting NPs; however, they are cumbersome, costly, and insensitive to the NPs morphology or composition. Herein, we apply an approach termed nanoparticle-imprinted matrices (NAIM), which is based on creating voids in a thin layer by imprinting NPs followed by their removal. The NAIM was formed on an interdigitated electrode (IDE) and used for the size-selective detection of silica NPs. Three- and 5-fold increases in capacitance were observed for the reuptake of NPs with similar diameter, compared to smaller or larger NPs, in air and liquid phase, respectively. En masse, the proposed approach lays the foundation for the emergence of field-effective, inexpensive, real-life applicable sensors that will allow online monitoring of NPs in air and liquids.

Red Microalgal Sulfated Polysaccharide-Cu2O Complexes: Characterization and Bioactivity.

The anion-exchange capacity of the cell-wall sulfated polysaccharide of the red microalga Porphyridium sp. can be exploited for the complexation of metal ions (e.g., Cu, Zn, Ag) to produce novel materials with new bioactivities. In this study, we investigated this algal polysaccharide as a platform for the incorporation of copper as Cu2O. Chemical and rheological characterization of the Cu2O-polysaccharide complex showed that the copper is covalently bound to the polysaccharide and that the complex exhibits higher viscosity and conductivity than the native polysaccharide. Examination of the complex's inhibitory activity against the bacteria Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Bacillus subtilis and the fungus Candida albicans revealed a relatively high antimicrobial activity, especially against C. albicans (92% growth inhibition) as compared to the polysaccharide and to Cu2O alone. The antibiofilm activity was also found against P. aeruginosa PA14 and C. albicans biofilms. An atomic force microscopy examination of the surface morphology of the complex revealed needle-like structures (spikes), approximately 10 nm thick, protruding from the complex surface to a maximum height of 1000 nm, at a density of about 5000/µm2, which were not detected in the native polysaccharide. It seems that the spikes on the surface of the Cu2O-polysaccharide complex are responsible for the antimicrobial activities of the complex, that is, for disruption of microbial membrane permeability, leading to cell death. The study thus indicates that the superior qualities of the novel material formed by complexion of Cu2O to the polysaccharide should be studied further for various biotechnological applications.

Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self-assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.