Random lasing and amplified spontaneous emission from silk inverse opals: Optical gain enhancement via protein scatterers.

Gain amplification and coherent lasing lines through random lasing (RL) can be produced by a random distribution of scatterers in a gain medium. If these amplified light sources can be seamlessly integrated into biological systems, they can have useful bio-optical applications, such as highly accurate sensing and high-resolution imaging. In this paper, a fully biocompatible light source showing RL and amplified spontaneous emission (ASE) with a reduced threshold is reported. Random cavities were induced in a biocompatible silk protein film by incorporating an inverse opal with an inherent disorder and a biocompatible dye for optical gain into the film. By choosing the appropriate air-sphere diameters, clear RL spikes in the emission spectra that were clearly distinguished from those of the ASE were observed in the silk inverse opal (SIO) with optical gain. Additionally, the RL output exhibited spatial coherence; however, the ASE did not. The high surface-to-volume ratio and amplification of the SIO led to highly efficient chemosensing in the detection of hydrogen chloride vapor. Moreover, SIO could be miniaturized to be made suitable for injection into biological tissues and obtain RL signals. Our results, which open the way for the development of a new generation of miniaturized bio-lasers, may be considered as the first example of engineered RL with biocompatible materials.

Silk protein nanofibers for highly efficient, eco-friendly, optically translucent, and multifunctional air filters.

New types of air filter technologies are being called because air pollution by particulate matters (PMs) and volatile organic compounds has raised serious concerns for public health. Conventional air filters have limited application and poor degradability and they become non-disposable wastes after use. Here, we report a highly efficient, eco-friendly, translucent, and multifunctional air purification filter that is highly effective for reducing air pollution, protecting the environment, and detecting hazardous chemical vapors encountered in everyday life. Uniform silk protein nanofibers were directly generated on a window screen by an electrospinning process. Optical properties (translucence and scattering) of the silk nanofibrous air filters (SNAFs) are advantageous for achieving viewability and controlling the room temperature. Air filtration efficiencies of the fabricated SNAFs could reach up to 90% and 97% for PMs with sizes under 2.5 and 10 µm, respectively, exceeding the performances of commercial semi-high-efficiency particulate air (semi-HEPA) filters. After use, the SNAFs could be naturally degraded. Furthermore, we demonstrate the ability of SNAFs impregnated with organic dyes to sense hazardous and volatile vapors encountered in everyday life.

Protein-Based Electronic Skin Akin to Biological Tissues.

Human skin provides an interface that transduces external stimuli into electrical signals for communication with the brain. There has been considerable effort to produce soft, flexible, and stretchable electronic skin (E-skin) devices. However, common polymers cannot imitate human skin perfectly due to their poor biocompatibility, biofunctionality, and permeability to many chemicals and biomolecules. Herein, we report on highly flexible, stretchable, conformal, molecule-permeable, and skin-adhering E-skins that combine a metallic nanowire (NW) network and silk protein hydrogel. The silk protein hydrogels offer high stretchability and stability under hydration through the addition of Ca2+ ions and glycerol. The NW electrodes exhibit stable operation when subjected to large deformations and hydration. Meanwhile, the hydrogel window provides water and biomolecules to the electrodes (communication between the environment and the electrode). These favorable characteristics allow the E-skin to be capable of sensing strain, electrochemical, and electrophysiological signals.

The Investigation of the Waveguiding Properties of Silk Fibroin from the Visible to Near-Infrared Spectrum.

Silk fibroin protein has been reinvented as a new optical material for biophotonic applications because of its optical transparency, biocompatibility, and easy fabrication process. It is used in various silk-based optical devices, which makes it desirable to investigate the optical properties of silk from diverse perspectives. This paper presents our investigation of the optical properties of silk fibroin, extracted from Bombyx mori cocoons. We have measured transmission spectra from the visible to near-infrared region and investigated waveguiding properties by the prism-coupling technique for five wavelengths (473.0, 632.8, 964.0, 1311, and 1552 nm). From the measurements, we determined the values of refractive indices. The measurements also proved waveguiding properties for all of the wavelengths. Optical scattering losses were measured by the fiber probe technique at 632.8 nm and were estimated to be 0.22 dB·cm-1.

Photonic Crystal Phosphors Integrated on a Blue LED Chip for Efficient White Light Generation.

Following the proof-of-concept experiment in the unit structure level, photonic crystal (PhC) phosphors-structurally engineered phosphor materials based on the nanophotonics principles-are integrated with a blue light-emitting diode (LED) chip to demonstrate a compact and efficient white light source. Red- or green-emitting CdSe-based colloidal quantum dots (CQDs) are coated on a Si3 N4 thin-film grating to fabricate PhC phosphors. The underlying PhC structure is designed such that the photonic band-edge modes at the zone center (k∣∣ = 0) are tuned to the energy of the blue excitation photons. By progressively stacking the PhC phosphor plates on a blue LED chip, the blue, green, and red emission intensities can be tightly controlled to obtain white light with the desired properties. The chromaticity coordinates, (0.332, 0.341), and correlated color temperature, 5500 K, are obtained from a stack of 3 red and 11 green PhC phosphor plates; in contrast, a stack of 5 red and 16 green reference phosphor plates are required to generate a similar white light. Overall, the PhC phosphors produce 8% higher total emission intensity out of 33% less amount of CQDs than the reference phosphors.

Colored and fluorescent nanofibrous silk as a physically transient chemosensor and vitamin deliverer.

Biodegradable and physically transient optics represent an emerging paradigm in healthcare devices by harnessing optically active system and obviating issues with chronic uses. Light emitting components that can efficiently interact with their environments have advantages of high sensitivity, visibility, and wireless operation. Here, we report a novel combination of silk biopolymer and optically active organic dyes resulting in versatile fluorescent silk nanofibers (FSNs). FSNs generated by the electrospinning method exhibit attractive functions of the doped organic dyes along with programming the system that physically disappear at prescribed time. Red-green-blue (RGB) fluorescent nanofibrous mats, eco-friendly and transient fluorescent chemosensors for acid vapor detection, and disposable membranes for nutrition delivery were successfully demonstrated using FSNs. These functions introduced using four water soluble dyes: rhodamine B, sodium fluorescein, stilbene 420, and riboflavin. The FSN with sodium fluorescein especially, showed a sensing capability for hazardous and volatile hydrochloric acid vapors. Delivering riboflavin (vitamin B2, an important nutrient for skin care) in the FSN to a biological tissue could be observed by tracing the fluorescence of riboflavin.

A colloidal quantum dot photonic crystal phosphor: nanostructural engineering of the phosphor for enhanced color conversion.

Phosphors, long-known color-converting photonic agents, are gaining increasing attention owing to the interest in white LEDs and related applications. Conventional material-based approaches to phosphors focus on obtaining the desired absorption/emission wavelengths and/or improving quantum efficiency. Here, we report a novel approach for enhancing the performance of phosphors: structural modification of phosphors. We incorporated inorganic colloidal quantum dots (CQDs) into a lateral one-dimensional (1D) photonic crystal (PhC) thin-film structure, with its photonic band-edge (PBE) modes matching the energy of 'excitation photons' (rather than 'emitted photons', as in most other PBE application devices). At resonance, we observed an approximately 4-fold enhancement of fluorescence over the reference bulk phosphor, which reflects an improved absorption of the excitation photons. This nano-structural engineering approach is a paradigm shift in the phosphor research area and may help to develop next-generation higher efficiency phosphors with novel characteristics.

Deformable and conformal silk hydrogel inverse opal.

Photonic crystals (PhCs) efficiently manipulate photons at the nanoscale. Applying these crystals to biological tissue that has been subjected to large deformation and humid environments can lead to fascinating bioapplications such as in vivo biosensors and artificial ocular prostheses. These applications require that these PhCs have mechanical durability, deformability, and biocompatibility. Herein, we introduce a deformable and conformal silk hydrogel inverse opal (SHIO); the photonic lattice of this 3D PhC can be deformed by mechanical strain. This SHIO is prepared by the UV cross-linking of a liquid stilbene/silk solution, to give a transparent and elastic hydrogel. The pseudophotonic band gap (pseudo-PBG) of this material can be stably tuned by deformation of the photonic lattice (stretching, bending, and compressing). Proof-of-concept experiments demonstrate that the SHIO can be applied as an ocular prosthesis for better vision, such as that provided by the tapeta lucida of nocturnal or deep-sea animals.

Silk protein as a new optically transparent adhesion layer for an ultra-smooth sub-10 nm gold layer.

Ultra-thin and ultra-smooth gold (Au) films are appealing for photonic applications including surface plasmon resonances and transparent contacts. However, poor adhesion at the Au-dielectric interface prohibits the formation of a mechanically stable, ultra-thin, and ultra-smooth Au film. A conventional solution is to use a metallic adhesion layer, such as titanium and chromium, however such layers cause the optical properties of pure Au to deteriorate. Here we report the use of silk protein to enhance the adhesion at the Au-dielectric interface, thus obtaining ultra-smooth sub-10 nm Au films. The Au films that were deposited onto the silk layer exhibited superior surface roughness to those deposited on SiO2, Si, and poly(methyl methacrylate), along with improved adhesion, electrical conductivity, and optical transparency. Additionally, we confirm that a metal-insulator-metal optical resonator can be successfully generated using a silk insulating layer without the use of a metallic adhesion layer.

Colloidal quantum dot lasers built on a passive two-dimensional photonic crystal backbone.

We report the room-temperature lasing action from two-dimensional photonic crystal (PC) structures composed of a passive Si3N4 backbone with an over-coat of CdSe/CdS/ZnS colloidal quantum dots (CQDs) for optical gain. When optically excited, devices lased in dual PC band-edge modes, with the modal dominance governed by the thickness of the CQD over-layer. The demonstrated laser platform should have an impact on future photonic integrated circuits as the on-chip coupling between active and passive components is readily achievable.

Enhanced fluorescence from CdSe/ZnS quantum dot nanophosphors embedded in a one-dimensional photonic crystal backbone structure.

A nano-engineered phosphor structure that produces enhanced fluorescence is reported. Two kinds of polymer materials with different refractive indices are spin-coated alternately to realize a one-dimensional (1D) photonic crystal (PC) phosphor platform, in which CdSe/ZnS core-shell quantum dots (QDs) were embedded as a fluorescence agent. The 1D PC phosphor structure is designed to match the pump photon energy with one of the photonic band-edges (PBEs), where the photon group velocity becomes zero, and thus the interaction between pump photons and fluorescent centres strengthened. A reference phosphor structure is also designed and fabricated; however, it has no PBE and exhibited bulk-like photonic properties. The fluorescence intensity from the 1D PC phosphors is examined during the pump photon energy scanning across the PBE. It is found that fluorescence from the 1D PC phosphor reaches its maximum when the pump photon energy coincides with the PBE, which is consistent with the theoretical prediction. In comparison with the reference phosphor, the fluorescence from the 1D PC phosphor is measured to be enhanced by a factor of 1.36.

Model calculations for enhanced fluorescence in photonic crystal phosphor.

We propose a novel photonic structure, based on the photonic crystal (PC) effect, which simulations show results in an improved fluorescence efficiency from embedded phosphor. To be specific, the phosphor pumping efficiency can be significantly improved by tuning the pump photon energy to a photonic band-edge (PBE) of the PC phosphor. We have confirmed this theoretically by calculating optical properties of one-dimensional PC phosphor structures using the transfer-matrix method and plane-wave expansion method. For a particular model structure based on a quantum dot phosphor, the fluorescence enhancement factor was estimated to be as high as 6.9 for a monochromatic pump source and 2.2 for a broad bandwidth (20 nm) pump source.