Responsive Photonic Crystals

Lamellar hydrogels with high toughness and ternary tunable photonic stop-band

A lamellar hydrogel with high toughness, exhibiting ternary stimuli-responsive structural color changes has been synthesized. The gel consists of alternating hard layers of a polymeric surfactant (PDGI) and soft layers of interpenetrating networks of poly(acrylamide)-poly(acrylic acid). Reversible, wide range switching of the stop-band position was achieved using different external stimuli of temperature, pH, and stress/strain.

In vivo imaging of the morphology and changes in pH along the gastrointestinal tract of Japanese medaka by photonic band-gap hydrogel microspheres

Colloidal crystalline microspheres with photonic band-gap properties responsive to media pH have been developed for in vivo imaging purposes. These colloidal crystalline microspheres were constructed from monodispersed core-shell nano-size particles with poly(styrene-co-acrylic acid) (PS-co-PAA) cores and poly(acrylic acid-co-N-isopropylacrylamide) (PAA-co-PNIPAM) hydrogel shells cross-linked by N,N'-methylenebisacrylamide. A significant shift in the photonic band-gap properties of these colloidal crystalline microspheres was observed in the pH range of 4-5. This was caused by the discontinuous volume phase transition of the hydrogel coating, due to the protonation/deprotonation of its acrylic acid moieties, on the core-shell nano-sized particles within the microspheres. The in vivo imaging capability of these pH-responsive photonic microspheres was demonstrated on a test organism - Japanese medaka, Oryzia latipes - in which the morphology and change in pH along their gastrointestinal (GI) tracts were revealed under an ordinary optical microscope. This work illustrates the potential of stimuli-responsive photonic band-gap materials in tissue-/organ-level in vivo bio-imaging.

Photonic labyrinths: two-dimensional dynamic magnetic assembly and in situ solidification

Creating novel structures by self-assembly processes and fixing the resultant assemblies are both critical to the design and fabrication of functional materials through bottom-up approaches. We demonstrate magnetically induced self-assembly of 2D photonic labyrinth structures and their solidification through a sol-gel method. The photonic labyrinth structures can be patterned into more regular arrangements using nonmagnetic substrates. This work may provide a platform for fabricating novel materials and devices with complex morphologies and spatial configurations.

Photonic crystal structures with tunable structure color as colorimetric sensors

Colorimetric sensing, which transduces environmental changes into visible color changes, provides a simple yet powerful detection mechanism that is well-suited to the development of low-cost and low-power sensors. A new approach in colorimetric sensing exploits the structural color of photonic crystals (PCs) to create environmentally-influenced color-changeable materials. PCs are composed of periodic dielectrics or metallo-dielectric nanostructures that affect the propagation of electromagnetic waves (EM) by defining the allowed and forbidden photonic bands. Simultaneously, an amazing variety of naturally occurring biological systems exhibit iridescent color due to the presence of PC structures throughout multi-dimensional space. In particular, some kinds of the structural colors in living organisms can be reversibly changed in reaction to external stimuli. Based on the lessons learned from natural photonic structures, some specific examples of PCs-based colorimetric sensors are presented in detail to demonstrate their unprecedented potential in practical applications, such as the detections of temperature, pH, ionic species, solvents, vapor, humidity, pressure and biomolecules. The combination of the nanofabrication technique, useful design methodologies inspired by biological systems and colorimetric sensing will lead to substantial developments in low-cost, miniaturized and widely deployable optical sensors.

Centrifugation-induced water-tunable photonic colloidal crystals with narrow diffraction bandwidth and highly sensitive detection of SCN-

Novel opal hydrogels with water-tunable photonic bandgap (PBG) exhibiting responses to external stimuli were self-assembled from polystyrene-co-poly(N,N-dimethylacrylamide) (PS-co-PDMAA) microspheres. The polymeric microspheres with narrow size distribution were successfully prepared in water, consisting of two regions. The inner region is rich in PS which is hard and hydrophobic; the outer region is rich in PDMAA which is soft and hydrophilic. The self-assembly of the PS-co-PDMAA hydrogel microspheres is ready induced by centrifugation and resulted in highly ordered three-dimensional (3D) photonic colloidal crystals (PCCs). With an increase of the amount of water, the PBG of the opal hydrogels shifted from the visible to near-infrared region of the electromagnetic spectrum. The maximum shift of diffraction peak positions could be larger than 500 nm with narrow full width at half maximum (FWHM) in the range of 20 to 40 nm only. The change in color was visible to the naked eye. The remarkable sensitivity to water of the lattice spacing of the opal hydrogels was repeatable after centrifugation. These observations are attributed to a reproducible degree of hydration of the hydrophilic outer region of the polymeric microspheres. Furthermore, the diffraction of the opal hydrogels was particularly sensitive to the presence of thiocyanate (SCN(-)) ions. The interaction between SCN(-) ions and DMAA repeat units is argued to block hydrogen bonds between DMAA and water molecules. Our PS-co-PDMAA opal hydrogels could be a practical system for diffraction-based detections.

One-step packing of anti-voltage photonic crystals into microfluidic channels for ultra-fast separation of amino acids and peptides

Packing of stable and crack-free photonic crystals (PCs) into micro channels is a prerequisite for ideal separation, but often takes several days and many steps, including assembly and immobilization. This work was dedicated to finding a fast, one-step solution. Simply by heating and blowing away the vapor, the packing of silica PCs into micro channels by classic evaporation-induced assembly was greatly accelerated and could unite the immobilization into one step. An apt method was thus established, which was able to pack 2 cm PCs into microfluidic channels in 15 min, saving a lot of time. The packed PCs showed no evident cracks along the borders of their continuous domain, therefore they are capable of withstanding an anti-electrical field at 2000 V cm(-1) for 5 h and storage in water for 2 months. This enables ultra-fast separation of amino acids along a 2.5 mm PC in 4 s, and peptides along a 10 mm PC in 12 s. The separation was highly efficient and reproducible, with a 300 nm plate height and 0.24%-0.35% relative standard deviation of migration time. This one-step approach is extendable to other gelling particles, and the resulted stable, crack-free PCs would have large potential in ultra-fast separation of other analytes.

Magnetic assembly and patterning of general nanoscale materials through nonmagnetic templates

Applied magnetic field represents an effective tool to rapidly assemble micro- and nanoscale magnetic objects into defined structures. Ordered assembly is typically achieved by using magnetic micropatterns, for which the downside is that they require advanced microfabrication techniques to produce. In addition, most conventional magnetic assembly strategies are restricted to target objects that possess magnetic properties. Herein we present a general strategy that allows convenient magnetically driven assembly of nonmagnetic objects in defined locations with high spatial resolution. The process involves immersing a polymer relief pattern in a uniformly magnetized ferrofluid, which modulates the local magnetic fields around the pattern. Nonmagnetic target objects dispersed in the same ferrofluid can then be magnetically assembled at positions defined by the polymer pattern. As the nonmagnetic polymer patterns can be conveniently fabricated at low cost through photolithography and soft-lithography processes, our method provides a general yet very effective means to assemble a wide range of nonmagnetic objects with controlled spatial distribution, paving the way toward patterning functional microstructures.

Optical sensing of the ionic strength using photonic crystals in a hydrogel matrix

Monodisperse, highly negatively charged, cross-linked polystyrene nanoparticles with diameters between 80 and 120 nm have been incorporated into a polyacrylamide hydrogel, where they display an iridescent color that conventionally is attributed to the so-called photonic crystal effect. The film is of red color if placed in plain water but turns to green in the presence of a 1 mM solution of an electrolyte such as sodium chloride and to purple in 100 mM solutions of electrolytes. Quantitative reflection spectroscopy was performed at various wavelengths and resulted in plots of reflected light wavelength versus ionic strength (IS) that are almost linear in the logarithmic concentration range from 5 × 10(-5) to 10(-2) mol·L(-1). We show that such films are capable of monitoring the IS of aqueous solutions in the pH range from 5 to 9. We also show that, in addition to visual and instrumental readout, the sensor films can be analyzed with a digital camera at fixed angle. The digital images were separated into their red, green, and blue channels and analyzed. The red channel was found to be best suited for determination of the IS and resulted in calibration plots that are comparable if not better than those obtained by reflectometry.

Optical Properties of the Self-Assembling Polymeric Colloidal Systems

In the last decade, optical materials have gained much interest due to the high number of possible applications involving path or intensity control and filtering of light. The continuous emerging technology in the field of electrooptical devices or medical applications allowed the development of new innovative cost effective processes to obtain optical materials suited for future applications such as hybrid/polymeric solar cells, lasers, polymeric optical fibers, and chemo- and biosensing devices. Considering the above, the aim of this review is to present recent studies in the field of photonic crystals involving the use of polymeric materials.

Photonic crystal based sensor for organic solvents and for solvent-water mixtures

Monodisperse polystyrene nanoparticles with a diameter of 173 nm were incorporated into a polydimethylsiloxane matrix where they display an iridescent color that can be attributed to the photonic crystal effect. The film is of violet color if placed in plain water, but turns to red in the presence of the non-polar solvent n-hexane. Several solvents were studied in some detail. We show that such films are capable of monitoring the water content of ethanol/water mixtures, where only 1% (v/v) of water leads to a shift of the peak wavelength of reflected light by 5 nm. The method also can be applied to determine, both visually and instrumentally, the fraction of methanol in ethanol/methanol mixtures. Here, a fraction of 1% of methanol (v/v) results in a wavelength shift of 2 nm. The reflected wavelength is not influenced by temperature changes nor impeded by photobleaching. The signal changes are fully reversible and response times are <1 s.

Nanomorphology tuning of the thermal response of TiO2/SiO2 Bragg stacks

Herein, we present a comparative study of thermo- and environmentally responsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated by different deposition methods and fabrication schemes, featuring various multilayer nanomorphologies. These include dense multilayer systems processed by physical vapor deposition and wet-chemistry protocols, as well as porous systems, namely, nanoparticle-based optical filters exhibiting textural porosity, and evaporation-induced self-assembled mesoporous Bragg stacks exhibiting predominantly structural porosity, as well as hybrid structures comprising both dense and porous layers. We investigate the spectral shift of the photonic stop band for the different Bragg stack nanomorphologies induced by the humidity-enhanced thermo-optic effect in a temperature range from 15 to 60 °C. We also demonstrate the response and recovery kinetics of the multilayer systems during external changes in ambient humidity. Notably, the choice of fabrication method plays a significant role in the thermal and humidity response of the system. Taking advantage of different material nanomorphologies we can tune the thermal shift of the photonic stop band in the range 0.2–32.9 nm for the Bragg stacks at ambient relative humidity. In addition, we can design dense multilayer systems nonresponsive to humidity and achieve time responses of the porous systems to external changes in humidity ranging from about 1 to 3 s.

Photonic multilayer sensors from photo-crosslinkable polymer films

Colorimetric temperature sensors are prepared from photo-crosslinkable polymers by sequentially spin-coating and crosslinking alternating layers of poly(N-isopropylacrylamide) and poly(para-methyl styrene). Layer thicknesses and copolymer chemistries are chosen to provide robust colorimetric temperature sensors that cover nearly the full visible spectrum.

Recent advances in polymer colloidal crystal lasers

Colloids with a size in the nanometres to micrometres range are frequently used in both fundamental research and industrial applications. In this context, colloidal crystals (CCs)-3D ordered arrays of monodispersed colloidal microparticles with a diameter of several hundred nanometres-have garnered a great deal of attention in the intriguing research realm of photonic crystals (PCs) due to the feasible and high-throughput 3D-PC fabrication with CCs. For optoelectronic applications, it is of prime importance to construct 3D-PCs with photonic band-gaps (PBGs) in the visible wavelength range. With regard to photonic device applications, many reports have been made on a wide variety of optical reflection sensors and displays using CCs that shift the visible PBG wavelength in response to external stimuli. This Minireview describes the research progress in the investigation of CCs and their laser applications. We highlight not only the research background of CCs as 3D-PCs, but also new potential applications of CCs as flexible and widely tunable lasers by low-threshold optical excitation.

Magnetic assembly route to colloidal responsive photonic nanostructures

Responsive photonic structures can respond to external stimuli by transmitting optical signals. Because of their important technological applications such as color signage and displays, biological and chemical sensors, security devices, ink and paints, military camouflage, and various optoelectronic devices, researchers have focused on developing these functional materials. Conventionally, self-assembled colloidal crystals containing periodically arranged dielectric materials have served as the predominant starting frameworks. Stimulus-responsive materials are incorporated into the periodic structures either as the initial building blocks or as the surrounding matrix so that the photonic properties can be tuned. Although researchers have proposed various versions of responsive photonic structures, the low efficiency of fabrication through self-assembly, narrow tunability, slow responses to the external stimuli, incomplete reversibility, and the challenge of integrating them into existing photonic devices have limited their practical application. In this Account, we describe how magnetic fields can guide the assembly of superparamagnetic colloidal building blocks into periodically arranged particle arrays and how the photonic properties of the resulting structures can be reversibly tuned by manipulating the external magnetic fields. The application of the external magnetic field instantly induces a strong magnetic dipole-dipole interparticle attraction within the dispersion of superparamagnetic particles, which creates one-dimensional chains that each contains a string of particles. The balance between the magnetic attraction and the interparticle repulsions, such as the electrostatic force, defines the interparticle separation. By employing uniform superparamagnetic particles of appropriate sizes and surface charges, we can create one-dimensional periodicity, which leads to strong optical diffraction. Acting remotely over a large distance, magnetic forces drove the rapid formation of colloidal photonic arrays with a wide range of interparticle spacing. They also allowed instant tuning of the photonic properties because they manipulated the interparticle force balance, which changed the orientation of the colloidal assemblies or their periodicity. This magnetically responsive photonic system provides a new platform for chromatic applications: these colloidal particles assemble instantly into ordered arrays with widely, rapidly, and reversibly tunable structural colors, which can be easily and rapidly fixed in a curable polymer matrix. Based on these unique features, we demonstrated many applications of this system, such as structural color printing, the fabrication of anticounterfeiting devices, switchable signage, and field-responsive color displays. We also extended this idea to rapidly organize uniform nonmagnetic building blocks into photonic structures. Using a stable ferrofluid of highly charged magnetic nanoparticles, we created virtual magnetic moments inside the nonmagnetic particles. This "magnetic hole" strategy greatly broadens the scope of the magnetic assembly approach to the fabrication of tunable photonic structures from various dielectric materials.

Rapid self-assembly of brush block copolymers to photonic crystals

The reduced chain entanglement of brush polymers over their linear analogs drastically lowers the energetic barriers to reorganization. In this report, we demonstrate the rapid self-assembly of brush block copolymers to nanostructures with photonic bandgaps spanning the entire visible spectrum, from ultraviolet (UV) to near infrared (NIR). Linear relationships were observed between the peak wavelengths of reflection and polymer molecular weights. This work enables "bottom-up" fabrication of photonic crystals with application-tailored bandgaps, through synthetic control of the polymer molecular weight and the method of self-assembly. These polymers could be developed into NIR-reflective paints, to combat the "urban heat island effect" due to NIR photon thermalization.

Self-assembly and magnetically induced phase transition of three-dimensional colloidal photonic crystals

Charged superparamagnetic colloidal Fe(3)O(4)@SiO(2) core-shell particles were chosen as model dipolar soft spheres to study their crystallization and magnetically induced phase transition in suspensions. The 3D colloidal crystals feature excellent magnetically responsive photonic properties with strong diffraction, fast response and wide tunability.

Centimeter-scale colloidal crystal belts via robust self-assembly strategy

Centimeter-scale poly(acrylic acid-co-DVB80) (PAA) 3D colloidal crystal belts were prepared via a novel robust vertical deposition technique based on negative pressure and curvature substrate of the glass vial. The formation of PAA colloidal crystal belts was investigated. The results indicated that curvature could control the dimension of PAA colloidal crystal belts. Well-controlled negative pressure resulted in rapid fabrication of well-defined PAA colloidal crystal belts. Curvature substrate of glass vial could distribute shrinking stress in the process of drying of colloidal films. Strong hydrogen bonding interactions among carboxyl groups on the surface of PAA colloidal particles was responsible for PAA colloidal crystal belts with closed-packing characteristics.

Determination of solvation layer thickness by a magnetophotonic approach

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory fails in explaining the superior stability of colloid particles in aqueous suspensions under conditions of high ionic strengths where electrostatic forces are effectively screened. Accumulating evidence shows that the formation of a thin rigid layer of solvent molecules in the vicinity of a colloidal particle surface provides an additional repulsive interaction when the interparticle distance is reduced to several nanometers. The effective determination of the thickness of the solvation layer however remains a challenge. Here, we demonstrate a simple yet powerful magnetophotonic technique that can be used to study the thickness of the solvation layers formed on the colloidal silica surface in various polar solvents. A relationship between the hydrogen-bonding ability of the solvents and the thickness of solvation layer on colloidal silica surfaces has been identified; this observation is found to be consistent with the previously proposed hydrogen-bonding origin of the solvation force.

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.