Volatile Memristive Devices with Analog Resistance Switching Based on Self-Assembled Squaraine Microtubes as Synaptic Emulators.

In this work, the discovery of volatile memristive devices that exhibit analog resistive switching (RS) and synaptic emulation based on squaraine materials is presented. Specifically, organic microtubes (MTs) based on 2,4-bis[(4-(N,N-diisobutyl)-2-6-hydroxyphenyl]squaraine (SQ) are prepared by evaporation-induced self-assembly (EISA). The MTs are ca. 2 µm in diameter (aspect ratio: 10-130). While powder X-ray diffraction data for MTs identify monoclinic and orthorhombic polymorphs, optical data report the monoclinic phase with energetic disorder. By favorable energetic alignment of the Au work function with the SQ HOMO energy, unipolar (hole-only) symmetric metal-insulator-metal devices are formed by EISA of MT meshes on interdigitated electrodes. The DC I-V characteristics acquired exhibit pinched hysteretic I-V loops, indicative of memristive behavior. Analysis indicates Ohmic transport at low bias with carrier extraction by thermionic emission. At high bias, space-charge-limited conduction in the presence of traps distributed in energy, enhanced by a Poole-Frenkel effect and with carrier extraction by Fowler-Nordheim tunneling, is observed. These data indicate purely electronic conduction. I-V hysteresis attenuates at smaller voltage windows, suggesting that carrier trapping/detrapping underpins the hysteresis. By applying triangular voltage waveforms, device conductance gradually increases sweep-on-sweep, with wait-time-erase or voltage-erase options. Using square waveforms, repeated erase-write-read of multiple distinct conductance states is achieved. Such analog RS behavior is consistent with trap filling/emptying effects. By waveform design, volatile conductance states may also be written so that successive conductance states exhibit identical current levels, indicating forgetting of previously written states and mimicking the forgetting curve. Finally, advanced synaptic functions, i.e., excitatory postsynaptic current, paired-pulse facilitation, pulse-dependent plasticity, and a transition from short- to long-term memory driven by post-tetanic potentiation, are demonstrated.

Template-Assisted Synthesis of Luminescent Carbon Nanofibers from Beverage-Related Precursors by Microwave Heating.

Luminescent carbon nanomaterials are important materials for sensing, imaging, and display technologies. This work describes the use of microwave heating for the template-assisted preparation of luminescent carbon nanofibers (CNFs) from the reaction of a range of beverage-related precursors with the nitrogen-rich polyethyleneimine. Highly luminescent robust carbon fibers that were 10 to 30 m in length and had a diameter of 200 nm were obtained under moderate conditions of temperature (250-260 °C) and a short reaction time (6 min). The high aspect ratio fibers showed wavelength-dependent emission that can be readily imaged using epifluorescence. The development of these multi-emissive one-dimensional (1D) carbon nanomaterials offers potential for a range of applications.

Templated microwave synthesis of luminescent carbon nanofibers.

Carbon based nanomaterials offer the potential to provide solutions to key technological challenges. This work describes the preparation of luminescent carbon nanofibers by template-assisted microwave pyrolysis of environmentally friendly precursors, citric acid and polyethyleneimine, in aqueous solution. SEM reveals a dense forest of vertically aligned cylindrical carbon nanofibers with an average diameter of ca. 200 nm, which are shown by TEM to be amorphous. Compositional analysis indicated the incorporation of amino and pyrrolic nitrogen, and carbon-oxygen moieties. These species contribute to UV light absorption with an absorption shoulder and tail towards visible wavelengths. UV excitation gave visible (blue) emission at ca. 450 nm with a quantum yield of ca. 5%; emission decay under pulsed excitation was predominantly mono-exponential with a lifetime of ca. 1 ns. The emission maximum is largely excitation wavelength independent suggesting the involvement of citrazinic acid-type functionalities in the fiber photophysics. Reversible pH-dependent excitation and emission behaviour was observed, with maximum emission at ca. pH 7. Nanofiber emission was also quenched in aqueous solutions of metal cations, in a concentration-dependent manner. Single nanofiber emission intensity was quite stable under continuous excitation permitting single fiber quenching-based metal ion detection whereby a significant (>90%) and prompt (sub-10 s) quenching was observed upon exposure to sub-millimolar Fe(iii) solutions. The introduction of these new 1D luminescent carbon nanofibers offers the potential for exciting developments across a range of applications.

Functionalization of emissive conjugated polymer nanoparticles by coprecipitation: consequences for particle photophysics and colloidal properties.

The functionalization of polyfluorene (PFO) nanoparticles by coprecipitation of the conjugated polymer with an amphiphilic comb polymer, consisting of a hydrophobic polystyrene backbone with hydrophilic, carboxylic acid-terminated polyethylene oxide side-chains (PS-PEG-COOH), is investigated. The comb polymer affects the properties of the formed hybrid nanoparticles. Non-functionalized particles are typically larger (28 nm) than functionalized ones (20 nm); peak molar extinction coefficients are found to differ in a similar trend. Zeta potentials are negative, consistent with negative surface charge on PFO particles due to chemical defect formation, with additional charge on functionalized particles due to the pendant carboxylic acid groups. Emission quantum yields of functionalized particles are typically larger, consistent with lower efficiency of energy transfer to quenchers in smaller particles and weaker PFO interchain interactions due to chain dilution. The trend in per-particle fluorescence brightness values, as confirmed by single particle fluorescence imaging, reflects the nanoparticle extinction coefficients. Photostability studies on aqueous dispersions of hybrid particles indicate mild photobrightening under continuous illumination while PFO particles exhibit slow exponential emission decay. Functionalized particles are also resistant to aggregation during exposure to adenocarcinoma cells. Generally, the hybrid particles exhibit more favorable time-, pH- and medium-dependent stabilities, likely due to steric and electrostatic stabilization by PEG-carboxylic acid functionalities. Overall, the functionalized particles exhibit attractive properties: Reasonably small size, tight size distribution, high absorption cross section, radiative rate and emission quantum yield, excellent brightness and photostability, and good colloidal stability.

Surfactant-free, low band gap conjugated polymer nanoparticles and polymer:fullerene nanohybrids with potential for organic photovoltaics.

Stable, aqueous dispersions of nanoparticles based on the low band gap polymers poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) and poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) were prepared, using a flexible, surfactant-free reprecipitation method, and characterized by a variety of optical techniques. Light scattering measurements indicated average nanoparticle hydrodynamic diameters of approximately 40 nm. The particles presented wide-bandwidth absorption and photoluminescence excitation spectra with high absorption cross-sections on the order of 10(-12) cm(2). Nanoparticle emission spectra were significantly red-shifted, with decreased emission quantum yields and lifetimes, consistent with increased inter-polymer chain interactions in the condensed phase. Single particle photoluminescence studies highlighted the multi-chromophoric nature of the polymer nanoparticles and confirmed their favorable photostabilities. When the nanoparticles were doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the correspondence of photoluminescence emission quenching, quantum yield decreases, emission lifetime shortening, and increased non-radiative rates with increasing PCBM concentration suggested efficient photo-induced donor-to-acceptor charge transfer between the conjugated polymers and the fullerene dopants co-localized in the nanoparticle cores. Taken together, the data suggest that these surfactant-free hybrid nanomaterials may be useful for integration with future nanostructured organic photovoltaics technologies.

A sustained release formulation of novel quininib-hyaluronan microneedles inhibits angiogenesis and retinal vascular permeability in vivo.

Pathologic neovascularisation and ocular permeability are hallmarks of proliferative diabetic retinopathy and age-related macular degeneration. Current pharmacologic interventions targeting VEGF are effective in only 30-60% of patients and require multiple intraocular injections associated with iatrogenic infection. Thus, our goal is to develop novel small molecule drugs that are VEGF-independent are amenable to sustained ocular-release, and which reduce retinal angiogenesis and retinal vascular permeability. Here, the anti-angiogenic drug quininib was formulated into hyaluronan (HA) microneedles whose safety and efficacy was evaluated in vivo. Quininib-HA microneedles were formulated via desolvation from quininib-HA solution and subsequent cross-linking with 4-arm-PEG-amine prior to freeze-drying. Scanning electron microscopy revealed hollow needle-shaped particle ultrastructure, with a zeta potential of -35.5mV determined by electrophoretic light scattering. The incorporation efficiency and pharmacokinetic profile of quininib released in vitro from the microneedles was quantified by HPLC. Quininib incorporation into these microneedles was 90%. In vitro, 20% quininib was released over 4months; or in the presence of increasing concentrations of hyaluronidase, 60% incorporated quininib was released over 4months. Zebrafish hyaloid vasculature assays demonstrated quininib released from these microneedles significantly (p<0.0001) inhibited ocular developmental angiogenesis compared to control. Sustained amelioration of retinal vascular permeability (RVP) was demonstrated using a bespoke cysteinyl leukotriene induced rodent model. Quininib-HA microparticles significantly inhibited RVP in Brown Norway rats one month after administration compared to neat quininib control (p=0.0071). In summary, quininib-HA microneedles allow for sustained release of quininib; are safe in vivo and quininib released from these microneedles effectively inhibits angiogenesis and RVP in vivo.

Labeling the Structural Integrity of Nanoparticles for Advanced In Situ Tracking in Bionanotechnology.

Observing structural integrity of nanoparticles is essential in bionanotechnology but not always straightforward to measure in situ and in real-time. Fluorescent labels used for tracking intrinsically nonfluorescent nanomaterials generally do not allow simultaneous observation of integrity. Consequently, structural changes like degradation and disassembly cannot easily be followed in situ using fluorescence signals. We show that thioflavin T (ThT), a fluorophore and molecular rotor known to tag specific fibril structures in amyloids, can "label" the structural integrity of widely used and intrinsically nonfluorescent, silica nanoparticles (SiNPs). Entrapment of ThT in SiNPs controls the fluorohphore's relaxation pathway and leads to a red-shifted fluorescence spectrum providing real time information on SiNP integrity. The dynamic change of ThT fluorescence during degradation of doped SiNPs is found much higher than that of common labels fluorescein and rhodamine. Degradation kinetics of core-shell structures recorded by ThT fluorescence and light scattering prove the capability to clearly distinguish structural features during SiNPs degradation and allow obtaining degradation kinetics in vitro, in biological media, in serum, and in cells. The effect is transferable to different types of materials, here shown for ThT incorporated SiNPs with tightly tailorable sizes (9-100 nm), poly(lactic-co-glycolic acid) (PLGA) nanoparticles, poly(9-vinylcarbazole) (PVK) nanoparticles, and iron-doped-SiNPs (FeSiNPs). We thus suggest molecular rotors such as ThT as additional labels to effectively and easily sense nanoparticle structural status in situ and to enhance understanding and development of programmed nanoparticle disassembly in bionanotechnology.

Encapsulation of MEH-PPV:PCBM Hybrids in the Cores of Block Copolymer Micellar Assemblies: Photoinduced Electron Transfer in a Nanoscale Donor-Acceptor System.

The objective of this work is to demonstrate that conjugated polymer:fullerene hybrid nanoparticles encapsulated in the hydrophobic cores of triblock copolymer micelles may successfully act as spatially confined donor-acceptor systems capable of facilitating photoinduced charge carrier separation. To this end, aqueous dispersions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) nanoparticles were first prepared by solubilization of the polymer in the cores of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymer, Pluronic F-127 micelles. A number of significant optical spectroscopic changes were observed on transfer of the conjugated polymer from a nonaqueous solvent to the aqueous micellar environment. These were primarily attributed to increased interchain interactions due to conjugated polymer chain collapse during encapsulation in the micellar cores. When prepared in buffer solution, the micelles exhibited good long-term collodial stability. When MEH-PPV micelles were blended by the addition of controlled amounts of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the observed correspondence of photoluminescence emission quenching, quantum yield decreases, and emission lifetime shortening with increasing PCBM concentration indicated efficient photoinduced donor-to-acceptor charge transfer between MEH-PPV and the fullerenes in the cores of the micelles, an assignment that was confirmed by transient absorption spectroscopic monitoring of carrier photogeneration and recombination.

Nanoscale Piezoelectric Properties of Self-Assembled Fmoc-FF Peptide Fibrous Networks.

Fibrous peptide networks, such as the structural framework of self-assembled fluorenylmethyloxycarbonyl diphenylalanine (Fmoc-FF) nanofibrils, have mechanical properties that could successfully mimic natural tissues, making them promising materials for tissue engineering scaffolds. These nanomaterials have been determined to exhibit shear piezoelectricity using piezoresponse force microscopy, as previously reported for FF nanotubes. Structural analyses of Fmoc-FF nanofibrils suggest that the observed piezoelectric response may result from the noncentrosymmetric nature of an underlying ß-sheet topology. The observed piezoelectricity of Fmoc-FF fibrous networks is advantageous for a range of biomedical applications where electrical or mechanical stimuli are required.

Luminescent optical detection of volatile electron deficient compounds by conjugated polymer nanofibers.

Optical detection of volatile electron deficient analytes via fluorescence quenching is demonstrated using ca. 200 nm diameter template-synthesized polyfluorene nanofibers as nanoscale detection elements. Observed trends in analyte quenching effectiveness suggest that, in addition to energetic factors, analyte vapor pressure and polymer/analyte solubility play an important role in the emission quenching process. Individual nanofibers successfully act as luminescent reporters of volatile nitroaromatics at sub-parts per million levels. Geometric factors, relating to the nanocylindrical geometry of the fibers and to low nanofiber substrate coverage, providing a less crowded environment around fibers, appear to play a role in providing access by electron deficient quencher molecules to the excited states within the fibers, thereby facilitating the pronounced fluorescence quenching response.

Synthesis, optical properties and alignment of poly(9,9-dioctylfuorene) nanofibers.

Poly(9,9-dioctlylfluorene) (PFO) nanofibers were fabricated by solution template wetting of anodic alumina membranes. Nanofibers with controlled thickness of 23 nm and length between 0.8 and 10 µm, were obtained, regulated by the dimensions of the used template. Nanofibers displayed spectroscopic characteristics associated with the formation of significant percentages of planar and elongated ß phase within the amorphous PFO glassy-phase. Optical polarized microscopy displayed high birefringence resulting from the high degree of internal order induced by ß phase generation within the fibers. The structural intra-chain reorganization associated with formation of ß phase was promoted by the strong geometrical confinement imposed on the material by the porous template during polymer wetting and solvent evaporation. Flow and shear force alignment techniques were used to control the orientation of fabricated PFO nanofibers, yielding to formation of large oriented nanofiber arrays on transparent substrates.

Highly polarized luminescence from ß-phase-rich poly(9,9-dioctylfluorene) nanofibers.

Novel poly(9,9-dioctlylfluorene) (PFO) nanofibers were fabricated by solution template wetting of anodic aluminum oxide (AAO) templates with a pore diameter of 25 nm. Individual nanofibers displayed a pronounced axially polarized luminescence with a typical emission dichroic ratio of 15 and low spread of the emissive species angular distribution. The strong optical characteristics were ascribed to intrachain reorientation of amorphous PFO to a more planar and elongated ß-phase conformation induced by mechanical strain during polymer template pore infiltration. Absorption optical spectroscopy on nanofiber mats confirmed formation of 24% ß-phase emissive segments, which dominated the nanofiber luminescence characteristics. X-ray diffraction measurements were used to confirm and quantify the extent of nanofiber internal molecular alignment.

Colour-coded photoluminescence and chemiluminescence of fluorene polymer-based organic nanowires in random and organised arrangements.

Organic nanowires based on a fluorene homopolymer and a copolymer, i.e., poly(9,9-dioctylfluorene), F8, and poly(9,9-dioctylfluorene-co-benzothiadiazole), F8BT, respectively, were synthesised by solution assisted wetting of porous anodic alumina templates. Nanowires ranged between 3 microm and 50 microm in length, and were ca. 200 nm in diameter. Absorption and photoluminescence studies of F8BT nanowires yielded spectra characteristic of the parent material. By contrast, the well resolved spectra obtained for F8 nanowires indicated that, during synthesis, a fraction of the molecules within the wires underwent intra-chain re-orientation from the more random molecular conformations of the glassy phase to the more planar extended molecular conformation of the beta-phase. Importantly, both F8 and F8BT nanowires exhibited a distinct emission anisotropy, consistent with internal alignment of the emissive polymer chains along the long axes of the wires. This property was exploited by forming nanowire crossbar structures in which, by selecting either luminescence wavelength or polarisation state, spatial confinement and colour tuning of polarised light emission could be readily achieved. Finally, nanowire chemiluminescence was demonstrated. Characteristic blue and green-yellow luminescence was observed for F8 and F8BT wires, respectively, confirming that these novel nanostructures may act as nanoscale chemiluminescent light sources.

Reversible modulation of photoluminescence from conjugated polymer nanotubes by incorporation of photochromic spirooxazine molecules.

A novel photo-switching system based on fluorescent polymer nanotubes incorporating spirooxazine molecules has been designed. Reversible photoluminescence modulation of up to 60% has been shown in individual nanotubes subjected to repeated photo-conversion cycles.

Oxidative stress and toxicity of gold nanoparticles in Mytilus edulis.

Gold nanoparticles (AuNP) have potential applications in drug delivery, cancer diagnosis and therapy, food industry and environment remediation. However, little is known about their potential toxicity or fate in the environment. Mytilus edulis was exposed in tanks to 750 ppb AuNP (average diameter 5.3 ± 1 nm) for 24h to study in vivo biological effects of nanoparticles. Traditional biomarkers and an affinity procedure selective for thiol-containing proteins followed by two-dimensional electrophoresis (2DE) separations were used to study toxicity and oxidative stress responses. Results were compared to those obtained for treatment with cadmium chloride, a well known pro-oxidant. M. edulis mainly accumulated AuNP in digestive gland which also showed higher lipid peroxidation. One-dimensional SDS/PAGE (1DE) and 2DE analysis of digestive gland samples revealed decreased thiol-containing proteins for AuNP. Lysosomal membrane stability measured in haemolymph gave lower values for neutral red retention time (NRRT) in both treatments but was greater in AuNP. Oxidative stress occurred within 24h of AuNP exposure in M. edulis. Previously we showed that larger diameter AuNP caused modest effects, indicating that nanoparticle size is a key factor in biological responses to nanoparticles. This study suggests that M. edulis is a suitable model animal for environmental toxicology studies of nanoparticles.

Exposure of the blue mussel, Mytilus edulis, to gold nanoparticles and the pro-oxidant menadione.

Relatively little is known about how gold nanoparticles (GNP) might interact in vivo with marine organisms. Mytilus edulis was exposed (24h) to approximately 15 nm GNP, menadione and both compounds simultaneously (GNP/menadione). GNP was detected by inductively coupled plasma-optical emission spectroscopy mainly in digestive gland of samples exposed to GNP though not GNP/menadione, perhaps due to impaired feeding. Thioredoxin reductase activity and malondialdehyde levels were determined in all tissues. Thioredoxin reductase inhibition was detected only in digestive gland exposed to menadione whilst malondialdehyde levels did not vary in response to treatment in all tissues. GNP caused a decrease in the reduced/oxidized glutathione ratio in digestive gland, but no difference was found in other tissues or for other treatments. One dimensional electrophoresis of proteins containing thiol groups was performed in all tissues and revealed a reduction in protein thiols for all treatments in digestive gland. Two dimensional electrophoresis of digestive gland extracts, from GNP and control groups, showed decreased levels of thiol proteins in response to GNP which we attribute to oxidation. Our results suggest that GNP causes a modest level of oxidative stress sufficient to oxidize thiols in glutathione and proteins but without causing lipid peroxidation or induction of thioredoxin reductase activity.

Fabrication of nanopore array electrodes by focused ion beam milling.

Single nanopore electrodes and nanopore electrode arrays have been fabricated using a focused ion beam (FIB) method. High aspect ratio pores (approximately 150-400-nm diameter and 500-nm depth) were fabricated using direct-write local ion milling of a silicon nitride layer over a buried platinum electrode. This local milling results in formation of a recessed platinum electrode at the base of each nanopore. The electrochemical properties of these nanopore metal electrodes have been characterized by voltammetry. Steady-state voltammograms were obtained for a range of array sizes as well as for single nanopore electrodes. High-resolution scanning electron microscopy imaging of the arrays showed that the pores had truncated cone, rather than cylindrical, conformations. A mathematical model describing diffusion to an electrode located at the base of a truncated conical pore was developed and applied to the analysis of the electrode geometries. The results imply that diffusion to the pore mouth is the dominant mass transport process rather than diffusion to the electrode surface at the base of the truncated cone. FIB milling thus represents a simple and convenient method for fabrication of prototype nanopore electrode arrays, with scope for applications in sensing and fundamental electrochemical studies.