Aykutlu Dana

Towards Unimolecular Luminescent Solar Concentrators: Bodipy‐Based Dendritic Energy‐Transfer Cascade with Panchromatic Absorption and Monochromatized Emission

Today, efficient and effective utilization of solar energy is a high-priority target and is expected to be even more so in the near future. For the large-scale exploitation of the stellar energy source, cost is always the major prohibitive item. The use of polycrystalline silicon, amorphous thin films of silicon, or alternative semiconducting materials such as Cu(In,Ga)Se2 (CIGS), [4] together with dye-sensitized solar cells already have or are expected to have big impacts on the production costs, but more effort in all aspects of the solar energy transduction is needed. One approach is to break down this massive problem into relatively easily addressable components, such as absorption of solar photons and conversion of absorbed solar energy into electricity. Installation and transmission of the produced electrical energy are two other components, which are essentially engineering problems. For the efficient absorption of the solar radiation component, it has been known for some time that even without major changes in solar cell design, it should be possible to obtain substantial enhancements by making use of solar concentrators. Optical solar concentrators have been around for the last four or five decades, however, overheating is always a troublesome issue, with an additional need for solar tracking with most optical concentrators. Luminescent solar concentrators on the other hand seem to be more promising. Conversion of the incident solar radiation into monochromatized light is expected to lead to a large enhancement in the efficiency of solar cells. Key features of the luminescent solar concentrators are the dispersed dye or dyes in a transparent waveguide. Through total internal reflection, reemitted light is trapped within a plastic or glass matrix, and photovoltaic units are fixed to the sides through which the light is channelled out. The advantages are striking: no tracking or cooling is needed and much smaller areas have to be covered by expensive solar-cell components. However, such concentrators are not free from problems; self absorption of the emitted light is a major problem. Recently a different luminescent concentrator design that made use of a mixture of dyes in amorphous thin films placed in a tandem design with one terminal absorber was reported. The other two dyes absorb light at different wavelengths and are expected to transfer the excitation energy to the terminal absorber. The intermolecular Fçrster energy transfer (FRET) was invoked as the operational mechanism of the energy transfer. With the assumption of efficient intermolecular energy transfer in the solid (gel) phase, the only emission will be at the longer wavelength region with large pseudo-Stokes shifts, thus minimizing self-absorption. The intermolecular energy-transfer efficiency is an important limiting factor that requires high concentrations of the dyes for optimal results, but higher concentrations will lead to larger losses caused by self-absorption. Herein, we propose that this apparent dilemma can be addressed at least in principle, by replacing a cocktail of dyes with a dendritic lightharvesting energy gradient with a core molecule as the terminal absorber and emitter. In the dendritic system, energy-transfer efficiency will remain high, regardless of its concentration within the matrix. Unimolecular energy gradients have been reported previously with a number of peripheral antenna molecules and a core chromophore absorbing at a longer wavelength. Typically, they are characterized in solution. In this work however, we explicitly targeted an energy cascade system SC composed of bodipy dyes (see below) with varying degrees of substitution with styryl groups. This approach will ensure strong absorption in most parts of the visible spectrum, however, through efficient energy-transfer processes, emission is expected to originate only from the terminal absorber. An optimal solar cell placed on the sides of the matrix is expected to produce efficient and cost-effective conversion. In addition, we wanted to demonstrate the efficiency of every single step of cascading energy transfers; to that end we synthesized energy-transfer modules of ET-1, ET-2, and ET-3. Bodipy dyes are highly versatile chromophores and can be conveniently derivatized to span the entire visible spectrum and beyond, showing exceptional photochemical and photophysical qualities. These properties of Bodipy dyes, including sharp absorption and emission maxima, were previously exploited in energy-transfer modules. In our design, the goals were to optimize the absorption in a large part of the visible spectrum and also the conversion to emission centered at 672 nm, which is ideally suited for [*] Dr. O. Altan Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, Dr. A. Dana, Prof. Dr. E. U. Akkaya UNAM-Institute of Materials Science and Nanotechnology Bilkent University, Ankara 06800 (Turkey) E-mail: eua@fen.bilkent.edu.tr

One-step synthesis of size-tunable Ag nanoparticles incorporated in electrospun PVA/cyclodextrin nanofibers

One-step synthesis of size-tunable silver nanoparticles (Ag-NP) incorporated into electrospun nanofibers was achieved. Initially, in situ reduction of silver salt (AgNO3) to Ag-NP was carried out in aqueous solution of polyvinyl alcohol (PVA). Here, PVA was used as reducing agent and stabilizing polymer as well as electrospinning polymeric matrix for the fabrication of PVA/Ag-NP nanofibers. Afterwards, hydroxypropyl-beta-cyclodextrin (HPβCD) was used as an additional reducing and stabilizing agent in order to control size and uniform dispersion of Ag-NP. The size of Ag-NP was ∼8 nm and some Ag-NP aggregates were observed for PVA/Ag-NP nanofibers, conversely, the size of Ag-NP decreased from ∼8 nm down to ∼2 nm within the fiber matrix without aggregation were attained for PVA/HPβCD nanofibers. The PVA/Ag-NP and PVA/HPβCD/Ag-NP nanofibers exhibited surface enhanced Raman scattering (SERS) effect. Moreover, antibacterial properties of PVA/Ag-NP and PVA/HPβCD/Ag-NP nanofibrous mats were tested against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria.

Portable Microfluidic Integrated Plasmonic Platform for Pathogen Detection

Timely detection of infectious agents is critical in early diagnosis and treatment of infectious diseases. Conventional pathogen detection methods, such as enzyme linked immunosorbent assay (ELISA), culturing or polymerase chain reaction (PCR) require long assay times, and complex and expensive instruments, which are not adaptable to point-of-care (POC) needs at resource-constrained as well as primary care settings. Therefore, there is an unmet need to develop simple, rapid, and accurate methods for detection of pathogens at the POC. Here, we present a portable, multiplex, inexpensive microfluidic-integrated surface plasmon resonance (SPR) platform that detects and quantifies bacteria, i.e., Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) rapidly. The platform presented reliable capture and detection of E. coli at concentrations ranging from ~105 to 3.2 × 107 CFUs/mL in phosphate buffered saline (PBS) and peritoneal dialysis (PD) fluid. The multiplexing and specificity capability of the platform was also tested with S. aureus samples. The presented platform technology could potentially be applicable to capture and detect other pathogens at the POC and primary care settings.

Self-assembled peptide nanofiber templated one-dimensional gold nanostructures exhibiting resistive switching.

An amyloid-like peptide molecule self-assembling into one-dimensional nanofiber structure in ethanol was designed and synthesized with functional groups that can bind to gold ions. The peptide nanofibers were used as templates for nucleation and growth of one-dimensional gold nanostructures in the presence of ascorbic acid as reducing agent. We performed multistep seed-mediated synthesis of gold nanoparticles by changing peptide/gold precursor and peptide/reducing agent ratios. Gold nanostructures with a wide range of morphologies such as smooth nanowires, noodle-like one-dimensional nanostructures, and uniform aggregates of spherical nanoparticles were synthesized by use of an environmentally friendly synthesis method. Nanoscale electrical properties of gold-peptide nanofibers were investigated using atomic force microscopy. Bias dependent current (IV) measurements on thin films of gold-peptide nanofiber hybrid revealed tunneling dominated transport and resistive switching. Gold-peptide nanofiber composite nanostructures can provide insight into electrical conduction in biomolecular/inorganic composites, highlighting their potential applications in electronics and optics.

Label-Free Nanometer-Resolution Imaging of Biological Architectures through Surface Enhanced Raman Scattering

Label free imaging of the chemical environment of biological specimens would readily bridge the supramolecular and the cellular scales, if a chemical fingerprint technique such as Raman scattering can be coupled with super resolution imaging. We demonstrate the possibility of label-free super-resolution Raman imaging, by applying stochastic reconstruction to temporal fluctuations of the surface enhanced Raman scattering (SERS) signal which originate from biomolecular layers on large-area plasmonic surfaces with a high and uniform hot-spot density (>1011/cm2, 20 to 35 nm spacing). A resolution of 20 nm is demonstrated in reconstructed images of self-assembled peptide network and fibrilated lamellipodia of cardiomyocytes. Blink rate density is observed to be proportional to the excitation intensity and at high excitation densities (>10 kW/cm2) blinking is accompanied by molecular breakdown. However, at low powers, simultaneous Raman measurements show that SERS can provide sufficient blink rates required for image reconstruction without completely damaging the chemical structure.

CDs Have Fingerprints Too

We introduce a new technique for extracting unique fingerprints from identical CDs. The proposed technique takes advantage of manufacturing variability found in the length of the CD lands and pits. Although the variability measured is on the order of 20 nm, the technique does not require the use of microscopes or any advanced equipment. Instead, we show that the electrical signal produced by the photodetector inside the CD reader is sufficient to measure the desired variability. We investigate the new technique by analyzing data collected from 100 identical CDs and show how to extract a unique fingerprint for each CD. Furthermore, we introduce a technique for utilizing fuzzy extractors over the Lee metric without much change to the standard code offset construction. Finally, we identify specific parameters and a code construction to realize the proposed fuzzy extractor and convert the derived fingerprints into 128-bit cryptographic keys.

Interfiber interactions alter the stiffness of gels formed by supramolecular self-assembled nanofibers

Molecular self-assembly is a powerful technique for developing novel nanostructures by using non-covalent interactions such as hydrogen bonding, hydrophobic, electrostatic, metal–ligand, π–π and van der Waals interactions. These interactions are highly dynamic and are often delicate due to their relatively weak nature. However, a sufficient number of these weak interactions can yield a stable assembly. In this work, we studied the mechanical properties of self-assembled peptide amphiphile nanostructures in the nanometre and micrometre scale. Hydrogen bonding, hydrophobic and electrostatic interactions promote self-assembly of peptide amphiphile molecules into nanofibers. Bundles of nanofibers form a three-dimensional network resulting in gel formation. The effect of the nanofiber network on the mechanical properties of the gels was analyzed by AFM, rheology and CD. Concentration and temperature dependent measurements of gel stiffness suggest that the mechanical properties of the gels are determined by a number of factors including the interfiber interactions and mechanical properties of individual nanofibers. We point out that the divergence in gel stiffness may arise from the difference in strength of interfiber bonds based on an energetic model of elastic rod networks, along with continuum mechanical models of bundles of rods. This finding differs from the results observed with traditional polymeric materials. Understanding the mechanisms behind the viscoelastic properties of the gels formed by self-assembling molecules can lead to development of new materials with controlled stiffness. Tissue engineering applications can especially benefit from these materials, where the mechanical properties of the extracellular matrix are crucial for cell fate determination.

Electro-optic and electro-absorption characterization of InAs quantum dot waveguides

Optical properties of multilayer InAs quantum dot waveguides, grown by molecular beam epitaxy, have been studied under applied electric field. Fabry-Perot measurements at 1515 nm on InAs/GaAs quantum dot structures yield a significantly enhanced linear electro-optic efficiency compared to bulk GaAs. Electro-absorption measurements at 1300 nm showed increased absorption with applied field accompanied with red shift of the spectra. Spectral shifts of up to 21% under 18 Volt bias was observed at 1320 nm.

Microscopic characterization of peptide nanostructures

Peptide-based nanomaterials have been utilized for various applications from regenerative medicine to electronics since they provide several advantages including easy synthesis methods, numerous routes for functionalization and biomimicry of secondary structures of proteins which leads to design of self-assembling peptide molecules to form nanostructures. Microscopic characterization at nanoscale is critical to understand processes directing peptide molecules to self-assemble and identify structure-function relationship of the nanostructures. Here, fundamental studies in microscopic characterization of peptide nanostructures are discussed to provide insights in widely used microscopy tools. In this review, we will encompass characterization studies of peptide nanostructures with modern microscopes, such as TEM, SEM, AFM, and advanced optical microscopy techniques. We will also mention specimen preparation methods and describe interpretation of the images.

All-chalcogenide glass omnidirectional photonic band gap variable infrared filters

We report on the design, fabrication, and characterization of spatially variable infrared photonic band gap filter that consists of thermally evaporated, high refractive index contrast, amorphous chalcogenide glass multilayers. Due to graded thickness structure, the filter exhibits a position dependent stop band and a cavity mode ranging from 1.8 to 3.4 μm wavelengths. Reflection measurements on the variable filter agree well with theoretical calculations. These results pave the way to low-loss infrared mirrors, filters, spectral imaging, and miniaturized spectrometers at infrared region.