Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality.

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

Active control of polariton-enabled long-range energy transfer.

Optical control is achieved on the excited state energy transfer between spatially separated donor and acceptor molecules, both coupled to the same optical mode of a cavity. The energy transfer occurs through the formed hybrid polaritons and can be switched on and off by means of ultraviolet and visible light. The control mechanism relies on a photochromic component used as donor, whose absorption and emission properties can be varied reversibly through light irradiation, whereas in-cavity hybridization with acceptors through polariton states enables a 6-fold enhancement of acceptor/donor contribution to the emission intensity with respect to a reference multilayer. These results pave the way for synthesizing effective gating systems for the transport of energy by light, relevant for light-harvesting and light-emitting devices, and for photovoltaic cells.

Light-Emitting Microfibers from Lotus Root for Eco-Friendly Optical Waveguides and Biosensing.

Optical biosensors based on micro/nanofibers are highly valuable for probing and monitoring liquid environments and bioactivity. Most current optical biosensors, however, are still based on glass, semiconductors, or metallic materials, which might not be fully suitable for biologically relevant environments. Here, we introduce biocompatible and flexible microfibers from lotus silk as microenvironmental monitors that exhibit waveguiding of intrinsic fluorescence as well as of coupled light. These features make single-filament monitors excellent building blocks for a variety of sensing functions, including pH probing and detection of bacterial activity. These results pave the way for the development of new and entirely eco-friendly, potentially multiplexed biosensing platforms.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers: erratum.

The beam shape coefficients for cylindrical vector modes are of great importance for other researchers to reproduce our results, however they were accidentally reported incorrectly in our recently published manuscript [Opt. Express30(14), 24407 (2022)10.1364/OE.458674]. This erratum reports the correct form for the two expressions. Two typographical errors in auxiliary equations are also reported and two labels in particle time of flight probability density function plots are fixed.

Benchmarking YOLOv5 and YOLOv7 models with DeepSORT for droplet tracking applications.

Tracking droplets in microfluidics is a challenging task. The difficulty arises in choosing a tool to analyze general microfluidic videos to infer physical quantities. The state-of-the-art object detector algorithm You Only Look Once (YOLO) and the object tracking algorithm Simple Online and Realtime Tracking with a Deep Association Metric (DeepSORT) are customizable for droplet identification and tracking. The customization includes training YOLO and DeepSORT networks to identify and track the objects of interest. We trained several YOLOv5 and YOLOv7 models and the DeepSORT network for droplet identification and tracking from microfluidic experimental videos. We compare the performance of the droplet tracking applications with YOLOv5 and YOLOv7 in terms of training time and time to analyze a given video across various hardware configurations. Despite the latest YOLOv7 being 10% faster, the real-time tracking is only achieved by lighter YOLO models on RTX 3070 Ti GPU machine due to additional significant droplet tracking costs arising from the DeepSORT algorithm. This work is a benchmark study for the YOLOv5 and YOLOv7 networks with DeepSORT in terms of the training time and inference time for a custom dataset of microfluidic droplets.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers.

Optical forces on microspheres inside hollow core photonic crystal fibers (HC-PCFs) are often predicted using a ray optics model, which constrains its validity based on wavelength and microsphere sizes. Here, we introduce a rigorous treatment of the electromagnetic forces based on the Lorenz-Mie theory, which involves analytical determination of beam shape coefficients for the optical modes of a HC-PCF. The method is more practicable than numerical approaches and, in contrast with ray optics models, it is not limited by system size parameters. Time of flight measurements of microspheres flying through the HC-PCF lead to results consistent with the Lorenz-Mie predictions.

Sensitivity and spectral control of network lasers.

Recently, random lasing in complex networks has shown efficient lasing over more than 50 localised modes, promoted by multiple scattering over the underlying graph. If controlled, these network lasers can lead to fast-switching multifunctional light sources with synthesised spectrum. Here, we observe both in experiment and theory high sensitivity of the network laser spectrum to the spatial shape of the pump profile, with some modes for example increasing in intensity by 280% when switching off 7% of the pump beam. We solve the nonlinear equations within the steady state ab-initio laser theory (SALT) approximation over a graph and we show selective lasing of around 90% of the strongest intensity modes, effectively programming the spectrum of the lasing networks. In our experiments with polymer networks, this high sensitivity enables control of the lasing spectrum through non-uniform pump patterns. We propose the underlying complexity of the network modes as the key element behind efficient spectral control opening the way for the development of optical devices with wide impact for on-chip photonics for communication, sensing, and computation.

Fluorescence lifetime microscopy unveils the supramolecular organization of liposomal Doxorubicin.

The supramolecular organization of Doxorubicin (DOX) within the standard Doxoves® liposomal formulation (DOX®) is investigated using visible light and phasor approach to fluorescence lifetime imaging (phasor-FLIM). First, the phasor-FLIM signature of DOX® is resolved into the contribution of three co-existing fluorescent species, each with its characteristic mono-exponential lifetime, namely: crystallized DOX (DOXc, 0.2 ns), free DOX (DOXf, 1.0 ns), and DOX bound to the liposomal membrane (DOXb, 4.5 ns). Then, the exact molar fractions of the three species are determined by combining phasor-FLIM with quantitative absorption/fluorescence spectroscopy on DOXc, DOXf, and DOXb pure standards. The final picture on DOX® comprises most of the drug in the crystallized form (∼98%), with the remaining fractions divided between free (∼1.4%) and membrane-bound drug (∼0.7%). Finally, phasor-FLIM in the presence of a DOX dynamic quencher allows us to suggest that DOXf is both encapsulated and non-encapsulated, and that DOXb is present on both liposome-membrane leaflets. We argue that the present experimental protocol can be applied to the investigation of the supramolecular organization of encapsulated luminescent drugs/molecules all the way from the production phase to their state within living matter.

WO3 Nanowires Enhance Molecular Alignment and Optical Anisotropy in Electrospun Nanocomposite Fibers: Implications for Hybrid Light-Emitting Systems.

The molecular orientation in polymer fibers is investigated for the purpose of enhancing their optical properties through nanoscale control by nanowires mixed in electrospun solutions. A prototypical system, consisting of a conjugated polymer blended with polyvinylpyrrolidone, mixed with WO3 nanowires, is analyzed. A critical strain rate of the electrospinning jet is determined by theoretical modeling at which point the polymer network undergoes a stretch transition in the fiber direction, resulting in a high molecular orientation that is partially retained after solidification. Nearing a nanowire boundary, local adsorption of the polymer and hydrodynamic drag further enhance the molecular orientation. These theoretical predictions are supported by polarized scanning near-field optical microscopy experiments, where the dichroic ratio of the light transmitted by the fiber provides evidence of increased orientation nearby nanowires. The addition of nanowires to enhance molecular alignment in polymer fibers might consequently enhance properties such as photoluminescence quantum yield, polarized emission, and tailored energy migration, exploitable in light-emitting photonic and optoelectronic devices and for sensing applications.

Energy Dissipation and Asymmetric Excitation in Hybrid Waveguides for Routing and Coloring.

The delivery of optical signals from an external light source to a nanoscale waveguide is highly important for the development of nanophotonic circuits. However, the efficient coupling of external light energy into nanophotonic components is difficult and still remains a challenge. Herein, we use an external silica nanofiber to light up an organic-inorganic hybrid nanowaveguide, namely, a system composed of a polymer filament doped with MoS2 quantum dots. Nanofiber-excited nanowaveguides in a crossed geometry are found to asymmetrically couple excitation signals along two opposite directions, with different energy dissipation resulting in different colors of the light emitted by MoS2 quantum dots and collected from the waveguide terminals. Interestingly, rainbow-like light in the hybrid waveguide is achieved by three-in-one mixing of red, green, and blue components. This heterodimensional system of dots in waveguide represents a significant advance toward all-optical routing and full-color display in integrated nanophotonic devices.

Three-Dimensional Printable Conductive Semi-Interpenetrating Polymer Network Hydrogel for Neural Tissue Applications.

Intrinsically conducting polymers (ICPs) are widely used to fabricate biomaterials; their application in neural tissue engineering, however, is severely limited because of their hydrophobicity and insufficient mechanical properties. For these reasons, soft conductive polymer hydrogels (CPHs) are recently developed, resulting in a water-based system with tissue-like mechanical, biological, and electrical properties. The strategy of incorporating ICPs as a conductive component into CPHs is recently explored by synthesizing the hydrogel around ICP chains, thus forming a semi-interpenetrating polymer network (semi-IPN). In this work, a novel conductive semi-IPN hydrogel is designed and synthesized. The hybrid hydrogel is based on a poly(N-isopropylacrylamide-co-N-isopropylmethacrylamide) hydrogel where polythiophene is introduced as an ICP to provide the system with good electrical properties. The fabrication of the hybrid hydrogel in an aqueous medium is made possible by modifying and synthesizing the monomers of polythiophene to ensure water solubility. The morphological, chemical, thermal, electrical, electrochemical, and mechanical properties of semi-IPNs were fully investigated. Additionally, the biological response of neural progenitor cells and mesenchymal stem cells in contact with the conductive semi-IPN was evaluated in terms of neural differentiation and proliferation. Lastly, the potential of the hydrogel solution as a 3D printing ink was evaluated through the 3D laser printing method. The presented results revealed that the proposed 3D printable conductive semi-IPN system is a good candidate as a scaffold for neural tissue applications.

Circularly Polarized Laser with Chiral Nematic Cellulose Nanocrystal Cavity.

Circularly polarized (CP) lasers derived from low-cost and renewable raw sources are attracting increasing attention in photonics and material science. Here, we present a facile and effective approach to fabricate CP lasers by the evaporation-induced assembly of cellulose nanocrystals (CNCs) and a laser dye. The obtained laser exhibits a controlled chiral nematic structure, which acts as a chiral optical cavity, and varied chiral coupling interactions. It is shown that the CNC-based laser can modify the polarization state of the laser into left-handed polarization, leading to strong CP laser emission (CPLE) with a dissymmetry factor up to 0.35. The chiral nematic CNC structure proves to be a versatile yet straightforward strategy to generate strong and tailored CPLE.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives.

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor-acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440-460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.

Dye Stabilization and Wavelength Tunability in Lasing Fibers Based on DNA.

Lasers based on biological materials are attracting an increasing interest in view of their use in integrated and transient photonics. Deoxyribonucleic acid (DNA) as optical biopolymer in combination with highly emissive dyes has been reported to have excellent potential in this respect. However, achieving miniaturized lasing systems based on solid-state DNA shaped in different geometries to confine and enhance emission is still a challenge, and the physicochemical mechanisms originating fluorescence enhancement are not fully understood. Herein, a class of wavelength-tunable lasers based on DNA nanofibers is demonstrated, for which optical properties are highly controlled through the system morphology. A synergistic effect is highlighted at the basis of lasing action. Through a quantum chemical investigation, it is shown that the interaction of DNA with the encapsulated dye leads to hindered twisting and suppressed channels for the nonradiative decay. This is combined with effective waveguiding, optical gain, and tailored mode confinement to promote morphologically controlled lasing in DNA-based nanofibers. The results establish design rules for the development of bright and tunable nanolasers and optical networks based on DNA nanostructures.

Assembly of Pt Nanoparticles on Graphitized Carbon Nanofibers as Hierarchically Structured Electrodes.

Carbon-based nanofibers decorated with metallic nanoparticles (NPs) as hierarchically structured electrodes offer significant opportunities for use in low-temperature fuel cells, electrolyzers, flow and air batteries, and electrochemical sensors. We present a facile and scalable method for preparing nanostructured electrodes composed of Pt NPs on graphitized carbon nanofibers. Electrospinning directly addresses the issues related to large-scale production of Pt-based fuel cell electrocatalysts. Through precursors containing polyacrylonitrile and Pt salt electrospinning along with an annealing protocol, we obtain approximately 180 nm thick graphitized nanofibers decorated with approximately 5 nm Pt NPs. By in situ annealing scanning transmission electron microscopy, we qualitatively resolve and quantitatively analyze the unique dynamics of Pt NP formation and movement. Interestingly, by very efficient thermal-induced segregation of all Pt from the inside to the surface of the nanofibers, we increase overall Pt utilization as electrocatalysis is a surface phenomenon. The obtained nanomaterials are also investigated by spatially resolved Raman spectroscopy, highlighting the higher structural order in nanofibers upon doping with Pt precursors. The rationalization of the observed phenomena of segregation and ordering mechanisms in complex carbon-based nanostructured systems is critically important for the effective utilization of all metal-containing catalysts, such as electrochemical oxygen reduction reactions, among many other applications.

Intelligent non-colorimetric indicators for the perishable supply chain by non-wovens with photo-programmed thermal response.

Spoiled perishable products, such as food and drugs exposed to inappropriate temperature, cause million illnesses every year. Risks range from intoxication due to pathogen-contaminated edibles, to suboptimal potency of temperature-sensitive vaccines. High-performance and low-cost indicators are needed, based on conformable materials whose properties change continuously and irreversibly depending on the experienced time-temperature profile. However, these systems can be limited by unclear reading, especially for colour-blind people, and are often difficult to be encoded with a tailored response to detect excess temperature over varying temporal profiles. Here we report on optically-programmed, non-colorimetric indicators based on nano-textured non-wovens encoded by their cross-linking degree. This combination allows a desired time-temperature response to be achieved, to address different perishable products. The devices operate by visual contrast with ambient light, which is explained by backscattering calculations for the complex fibrous material. Optical nanomaterials with photo-encoded thermal properties might establish new design rules for intelligent labels.

Naturally Degradable Photonic Devices with Transient Function by Heterostructured Waxy-Sublimating and Water-Soluble Materials.

Combined dry-wet transient materials and devices are introduced, which are based on water-dissolvable dye-doped polymers layered onto nonpolar cyclic hydrocarbon sublimating substrates. Light-emitting heterostructures showing amplified spontaneous emission are obtained on transient elements and used as illumination sources for speckle-free, full-field imaging, and transient optical labels are realized that incorporate QR-codes with stably encoded information. The transient behavior is also studied at the microscopic scale, highlighting the real-time evolution of material domains in the sublimating compound. Finally, the exhausted components are fully soluble in water thus being naturally degradable. This technology opens new and versatile routes for environmental sensing, storage conditions monitoring, and organic photonics.

Transforming colloidal Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) into stable CsPbBr3 perovskite emitters through intermediate heterostructures.

The preparation of strongly emissive CsPbBr3 perovskite nanocrystals with robust surface passivation is a challenge in the field of lead halide perovskite nanomaterials. We report an approach to prepare polymer-capped CsPbBr3 perovskite nanocrystals by reacting oleylammonium/oleate-capped Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) (PMAO). PMAO contains succinic anhydride units that are reactive towards the oleylamine species present on the Cs4PbBr6 nanocrystals' surface and produces polysuccinamic acid, which, in turn, triggers the Cs4PbBr6 to CsPbBr3 conversion. The transformation occurs through the formation of Cs4PbBr6-CsPbBr3 heterostructures as intermediates, which are captured because of the mild reactivity of PMAO and are investigated by high-resolution electron microscopy. The Cs4PbBr6-CsPbBr3 heterostructures demonstrate a dual emission at cryogenic temperature with an indication of the energy transfer from Cs4PbBr6 to CsPbBr3. The fully-transformed CsPbBr3 NCs have high photoluminescence quantum yield and enhanced colloidal stability, which we attribute to the adhesion of polysuccinamic acid to the NC surface through its multiple functional groups in place of oleate and alkylammonium ligands. The PMAO-induced transformation of Cs4PbBr6 NCs opens up a strategy for the chemical modification of metal halide NCs initially passivated with nucleophilic amines.

Synthesis, crystal structure, polymorphism and microscopic luminescence properties of anthracene derivative compounds.

Anthracene derivative compounds are currently investigated because of their unique physical properties (e.g. bright luminescence and emission tunability), which make them ideal candidates for advanced optoelectronic devices. Intermolecular interactions are the basis of the tunability of the optical and electronic properties of these compounds, whose prediction and exploitation benefit from knowledge of the crystal structure and the packing architecture. Polymorphism can occur due to the weak intermolecular interactions, requiring detailed structural analysis to clarify the origin of observed material property modifications. Here, two silylethyne-substituted anthracene compounds are characterized by single-crystal synchrotron X-ray diffraction, identifying a new polymorph in the process. Additionally, laser confocal microscopy and fluorescence lifetime imaging microscopy confirm the results obtained by the X-ray diffraction characterization, i.e. shifting the substituents towards the external benzene rings of the anthracene unit favours π-π interactions, impacting on both the morphology and the microscopic optical properties of the crystals. The compounds with more isolated anthracene units feature shorter lifetime and emission spectra, more similar to those of isolated molecules. The crystallographic study, supported by the optical investigation, sheds light on the influence of non-covalent interactions on the crystal packing and luminescence properties of anthracene derivatives, providing a further step towards their efficient use as building blocks in active components of light sources and photonic networks.

When nanocellulose meets diffraction grating: freestanding photonic paper with programmable optical coupling.

Photonic crystals based on plasmonic or dielectric periodic structures have attracted considerable interest owing to their capabilities to control light-matter interactions with tailored precision. By using a nanocellulose derived chiral liquid crystal as a building block, here we demonstrate a bio-inspired dual photonic structure that contains the combination of microscopic periodic 1D surface grating and nanoscopic helical organization, giving rise to programmable colour mixing and polarization rotation. We show that a variation in the photonic band-gap in the bulk matrix leads to simultaneous control over the reflection and diffraction of light with controllable iridescence.