Energy Transfer from a Cationic Conjugated Polyelectrolyte to a DNA Photonic Wire: Toward Label-Free, Sequence-Specific DNA Sensing

We demonstrate a label-free, sequence specific DNA sensor based on fluorescence resonant energy transfer (FRET) occurring between a cationic conjugated polyelectrolyte and a small intercalating dye, malachite green chloride. The sensor combines (1) conjugated polymer chain conformation changes induced by the binding with DNA, with the conjugated polymer wrapping/twisting around the DNA helical duplex and experiencing a 3-fold increase in its photoluminescence quantum yield and (2) FRET from the conjugated polymer to the intercalated DNA. Owing to its small size, the dye intercalates at maximal, one-to-one dye-to-base pair load, making the intercalated DNA a molecular photonic wire with dyes excitonically coupled and chiroptically active. Any sequence mismatch between probe and target DNA degrades the intercalated DNA photonic wire by decreasing its brightness, excitonic coupling, and chiroptical properties, and this provides a signal transduction method for the DNA sensor. Coupling of intercalated DNA with the conjugated polymer via FRET provides target signal amplification and increased sensitivity toward sequence mismatch, with the FRET efficiency decreasing with added DNA sequence mismatch

Transformation and Growth of Polymorphic Nuclei through Evaporative Deposition of Thin Films

Rapidly dip-coating a silicon substrate in an acetaminophen solution creates a thin film of polymorphic nuclei, and the relative amounts of each polymorph vary with the type of solvent. Polarized light microscopy (PLM) revealed that all films were initially amorphous and gradually crystallized over time scales of minutes to hours. Fourier transform infrared spectroscopy (FTIR) was used to identify the polymorphic form during crystallization and weeks after apparent stabilization of growth. Crystallites that initially nucleated from the amorphous films were found to be the metastable orthorhombic form. Over time, the orthorhombic crystallites stopped growing and the remaining amorphous regions transformed to the stable monoclinic form. The choice of solvent determined how fast the orthorhombic crystallites grew and thus controlled the polymorphic character of the film. For example, dip-coating from an ethanol solution produced a largely orthorhombic film, while water yielded a film with mixed character. Kinetic arguments are made to discuss these results in terms of relative nucleation rates, supersaturation, and evaporation rate of the solvent. We demonstrate that PLM and FTIR are suitable tools for exploring phase space with these thin films. This methodology might be applied broadly to polymorph screening and selection in evaluating pharmaceutical materials

Electrospinning of Biodegradable, Monolithic Membrane with Distinct Bimodal Micron-Sized Fibers and Nanofibers for High Efficiency PMs Removal

Atmospheric particulate matter (PMs) pollution has raised increasing public concerns, especially with the outbreak of COVID-19. The preparation of high-performance membranes for air filtration is of great significance. Herein, the biosynthetic polymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was adopted to create a hierarchical structure and biodegradable nonwoven membrane for PMs filtration through a facile directly electrospinning method. The as-prepared membranes with hierarchical structure contain abundant nanowires (5–100 nm) and microfibers (2–5 μm) with different diameter (1000–5000 nm). We have achieved realization of formation mechanisms of such bimodal micro- and nanofibers, which stem from the branching of microfiber at early stage of electrospinning. The PHBV membranes exhibit a very high PM0.3 removal efficiency of 99.999% and PM2.5 removal efficiency of 100% with 0.077% standard atmospheric pressure in the air flow speed of 5.3 cm/s. More importantly, the PHBV membranes can be completely disintegrated within 1 week under composted conditions, indicating the great biodegradability of PHBV membranes. Our work provides insights for the development of biodegradable, high performance air filters for pollutants, molds, bacteria, and viruses

Structure-Dependent Electrocatalytic Properties of Cu₂O Nanocrystals for Oxygen Reduction Reaction

Cu₂O nanocrystals with different morphologies are synthesized via a reductive solution route by controlling the reaction time and using different capping agents. Introducing poly­(ethylene glycol) (PEG) leads to nearly monodispersed Cu₂O nanocubes with 40 nm size and dominant {100} crystal planes. With prolonged reaction time, the nanocubes are truncated and transformed into sphere-like nanocrystals with more {111} planes exposed. In the presence of poly­(vinyl pyrrolidone) (PVP), porous Cu₂O nanocrystals with both {100} and {111} planes present are produced. The structure-dependent electrocatalytic activity of Cu₂O nanocrystals toward oxygen reduction reaction (ORR) has been studied in alkaline electrolyte. The electrocatalytic activity measured on Cu₂O {100} is higher than that on Cu₂O {111}. In addition, the Cu₂O nanocubes with dominant {100} crystal planes show the highest four-electron selectivity (<i>n</i> = 3.7) and lowest peroxide yield (15%) during the ORR. Kinetics analysis indicates that the ORR mechanism on Cu₂O nanocrystals is controlled simultaneously by charge transfer and intermediate migration. The Cu₂O nanocrystals also show better methanol tolerance and durability for ORR than the commercial Pt/C materials

Low-Temperature Synthesis of Au/Polyaniline Nanocomposites: Toward Controlled Size, Morphology, and Size Dispersity

Varying the concentration and molar ratio between aniline and AuCl₃ leads to a series of Au/polyaniline (PANI) nanocomposites with a wide range of morphologies ranges from nanosphere, nanorod, to complex nanosheet assemblies. These nanocomposites consist of an ensemble of very small Au nanoparticles held together by a PANI matrix, which arise from the oxidation of aniline by metal ions. Using a steric stabilizer such as poly­(vinyl pyrrolidone), we have demonstrated control over size dispersity and morphology to achieve monodispersed nanocomposites. These nanoparticles embedded in the PANI matrix are coupled electronically and thus lead to a shift of plasmonic absorption from 500 nm down to 900 nm depending on the Au nanoparticle size and morphology. PANI that is bound to the nanoparticles can be washed off with <i>N</i>-methyl-2-pyrrolidone (NMP) to release individual nanoparticles to form a stable Au nanoparticle solution. On the basis of the above results, we propose a possible formation mechanism of hybrid nanocomposites encompassing Au nanoparticles in a PANI matrix

Large Grained Perovskite Solar Cells Derived from Single-Crystal Perovskite Powders with Enhanced Ambient Stability

In this study, we demonstrate the large grained perovskite solar cells prepared from precursor solution comprising single-crystal perovskite powders for the first time. The resultant large grained perovskite thin film possesses a negligible physical (structural) gap between each large grain and is highly crystalline as evidenced by its fan-shaped birefringence observed under polarized light, which is very different from the thin film prepared from the typical precursor route (MAI + PbI₂)

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon–Multiexciton Interactions at Low Temperature

Using thick-shell or “giant” CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-<i>T</i> PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence

Stimuli-Responsive Poly‑<i>N</i>‑isopropylacrylamide: Phenylene Vinylene Oligomer Conjugate

Phenylene vinylene trimer (OPV) and PNIPAM conjugate with stimuli-responsive optical properties has been synthesized through the formation of amide linkage between PNIPAM and carboxylic-acid-terminated OPV. This material exhibits thermoresponsive optical properties as temperature exceeds the lower critical solution temperature (LCST), which is 32 °C for PNIPAM and the conjugate. This PNIPAM-trimer conjugate is fully characterized by using NMR, FT-IR, temperature-dependent UV–vis, and fluorescence spectroscopy. We have found that the polymer conjugate solution turns opaque as temperature exceeds lower critical solution temperature and a five-fold increase in fluorescence intensity as temperature increases from 20 to 70 °C. Such distinct increase in fluorescence intensity is likely due to the rigidchromism, that is, the change in optical properties due to confinement of the chromophores resulting from restriction of polymer conformational structures. The PNIPAM-trimer conjugate also shows a decrease in decay lifetime with increasing temperature, whereas OPV trimer alone shows no change in decay lifetime as a function of temperature. These unique optical properties are not observed in the trimer and PNIPAM mixture, suggesting that the stimuli-responsive optical properties can occur only in PNIPAM–trimer conjugate linked through covalent bond

Amino Acid-Assisted Synthesis of Hierarchical Silver Microspheres for Single Particle Surface-Enhanced Raman Spectroscopy

We demonstrate the use of amino acids as directing agents to synthesize hierarchical silver microspheres assembled by nanosheets with well-defined morphologies, in the absence of any other surfactants or capping agents. This fabrication method avoids the absorption of macromolecules and enables clean surface on the Ag microspheres. The chemical nature of the amino acids plays a vital role in the hierarchical structure of the Ag microspheres. As found, amino acids with simple structures and 2–3 carbon atoms like alanine and glycine lead to more loosely packed Ag microspheres, and those with more complicated structures and more carbon atoms, e.g. glycine, glutamine, and asparagine, result in close-packed Ag particles assembled by thinner nanosheets. By adjusting the concentration of AgNO₃ solution, size as well as the surface roughness of the Ag microspheres can be well controlled. Individual particles of the constructed hierarchical Ag microspheres with highly roughened surface can act as sensitive SERS platforms. Detection of chemical molecules and monitoring of the plasmon-driven chemical reactions have been carried out through a single particle SERS technique

Fabrication of Thorny Au Nanostructures on Polyaniline Surfaces for Sensitive Surface-Enhanced Raman Spectroscopy

Here we demonstrate, for the first time, the fabrication of Au nanostructures on polyaniline (PANI) membrane surfaces for surface enhanced Raman spectroscopy (SERS) applications, through a direct chemical reduction by PANI. Introduction of acids into the HAuCl₄ solution leads to homogeneous Au structures on the PANI surfaces, which show only sub-ppm detection levels toward the target analyte, 4-mercaptobenzoic acid (4-MBA), because of limited surface area and lack of surface roughness. Thorny Au nanostructures can be obtained through controlled reaction conditions and the addition of a capping agent poly (vinyl pyrrolidone) (PVP) in the HAuCl₄ solution and the temperature kept at 80 °C in an oven. Those thorny Au nanostructures, with higher surface areas and unique geometric feature, show a SERS detection sensitivity of 1 × 10^–9 M (sub-ppb level) toward two different analyte molecules, 4-MBA and Rhodamine B, demonstrating their generality for SERS applications. These highly sensitive SERS-active substrates offer novel robust structures for trace detection of chemical and biological analytes