Manipulating the emission intensity and lifetime of NaYF4:Yb3+,Er3+ simultaneously by embedding it into CdS photonic crystals

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

Mechanochromic and Solvomechanochromic Fluorescent Photonic Crystals for Dual-Mode Modulating Fluorescence and Multilevel Anticounterfeiting

Fluorescent photonic crystals (FPCs) are ideal candidates for regulating dyes' fluorescence through their unique photonic band gaps (PBGs). However, challenges, including the lack of dynamic regulation of fluorescence, dye release in solvents, and instability, dramatically limit their practical applications. Here, we report mechanochromic and solvomechanochromic rhodamine B (RhB)-based FPCs with dynamic regulation of photoluminescence (PL) by stretching and swelling, brilliant fluorescent and structural colors, and no release of the RhB in solvents. The FPCs with force/solvent-responsive nonclose-packing structures were fabricated by (1) preparing RhB-silica particles by combining click chemistry and cohydrolysis processes and (2) self-assembling these particles in poly(ethylene glycol) phenyl ether acrylate followed by a photopolymerization. Maximal PL inhibition (37%, stretching strain of 6.8%) and enhancement (150%, swelling time of 8 min) were gained when PBGs and their blue edges are precisely adjusted to the PL peak position, respectively. Compared with stretching, PL regulation is more efficient by swelling. These characteristics benefit from the rational design and combination of unique compositions, chemical bonds, nonclosely packed micro/nanostructures, and solvents for swelling. Moreover, these FPCs have been used to encrypt photonic patterns, which display background/strain/angle/UV-dependent color contrasts, showing their potential applications in multilevel anticounterfeiting, optical devices, wireless sensors, etc.

Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications

Organisms in nature have evolved unique structural colors and stimuli-responsive functions for camouflage, warning, and communication over millions of years, which are essential to their survival in harsh conditions. Inspired by these characteristics, colloidal photonic composites (CPCs) composed of colloidal photonic crystals embedded in the polymeric matrix are artificially prepared and show great promise in applications. This review focuses on the summary of building blocks, i.e., colloidal particles and polymeric matrices, and constructive strategies from the perspective of designing CPCs with robust performance and specific functionality. Furthermore, their state-of-the-art applications are also discussed, including colorful coatings, anti-counterfeiting, and regulation of photoluminescence, especially in the field of visualized sensing. Finally, current challenges and potential for future developments in this field are discussed. The purpose of this review is not only to clarify the design principle for artificial CPCs but also to serve as a roadmap for the exploration of next-generation photonic materials.

Color-Tuning Mechanism of Electrically Stretchable Photonic Organogels

In contrast to nano-processed rigid photonic crystals with fixed structures, soft photonic organic hydrogel beads with dielectric nanostructures possess advanced capabilities, such as stimuli-responsive deformation and photonic wavelength color changes. Recenlty, advanced from well-investigated mechanochromic method, an electromechanical stress approach is used to demonstrate electrically induced mechanical color shifts in soft organic photonic hydrogel beads. To better understand the electrically stretchable color change functionality in such soft organic photonic hydrogel systems, the electromechanical wavelength-tuning mechanism is comprehensively investigated in this study. By employing controllable electroactive dielectric elastomeric actuators, the discoloration wavelength-tuning process of an electrically stretchable photonic organogel is carefully examined. Based on the experimental in-situ response of electrically stretchable nano-spherical polystyrene hydrogel beads, the color change mechanism is meticulously analyzed. Further, changes in the nanostructure of the symmetrically and electrically stretchable organogel are analytically investigated through simulations of its hexagonal close-packed (HCP) lattice model. Detailed photonic wavelength control factors, such as the refractive index of dielectric materials, lattice diffraction, and bead distance in an organogel lattice, are theoretically studied. Herein, the switcing mechanism of electrically stretchable mechanochromic photonic organogels with photonic stopband-tuning features are suggested for the first time.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

Band-Edge Effect-Induced Electrochemiluminescence Signal Amplification Based on Inverse Opal Photonic Crystals for Ultrasensitive Detection of Carcinoembryonic Antigen

Photonic crystals (PCs) have emerged as a promising electrochemiluminescence (ECL) matrix in the domain of immunoassay. Making maximum use of light manipulation properties of PCs is highly desired for improving the sensitivity. In this work, we proposed a band-edge effect-induced ECL enhancement strategy based on silica inverse opal PCs (SIOPCs). By fine-tuning the lattice constant and carefully calibrating the stopband position, we found that the band edge of the stopband exerted significant influences on the ECL intensity and spectral distribution. The high density of states at the blue edge of the photonic band gap increased the radiative transition probability of ECL emitters and enhanced the photon extraction during propagation, giving rise to ∼20-fold ECL signal amplification accompanied by a redistributed ECL spectrum for the Ru(bpy)₃²⁺-TPrA system. In combination with the intrinsic structural superiority, like large specific surface area and interconnected macropores, the developed SIOPC electrode was successfully applied in constructing a sandwich-type immunosensor. The fabricated immunosensor displayed a very low detection limit of 0.032 pg/mL and a wide linear range of 0.1 pg/mL-150 ng/mL for a carcinoembryonic antigen assay, showing its potential application in disease diagnosis.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Super-Resolution Imaging With Lanthanide Luminescent Nanocrystals: Progress and Prospect

Stimulated emission depletion (STED) nanoscopy has overcome a serious diffraction barrier on the optical resolution and facilitated new discoveries on detailed nanostructures in cell biology. Traditional fluorescence probes employed in the super-resolution imaging approach include organic dyes and fluorescent proteins. However, some limitations of these probes, such as photobleaching, short emission wavelengths, and high saturation intensity, still hamper the promotion of optical resolution and bio-applications. Recently, lanthanide luminescent probes with unique optical properties of non-photobleaching and sharp emissions have been applied in super-resolution imaging. In this mini-review, we will introduce several different mechanisms for lanthanide ions to achieve super-resolution imaging based on an STED-like setup. Then, several lanthanide ions used in super-resolution imaging will be described in detail and discussed. Last but not least, we will emphasize the future challenges and outlooks in hope of advancing the next-generation lanthanide fluorescent probes for super-resolution optical imaging.

Metasurface Enhanced Sensitized Photon Upconversion: Toward Highly Efficient Low Power Upconversion Applications and Nanoscale E-Field Sensors

Large-scale nanoimprinted metasurfaces based on silicon photonic crystal slabs were produced and coated with a NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticle (UCNP) layer. UCNPs on these metasurfaces yield a more than 500-fold enhanced upconversion emission compared to UCNPs on planar surfaces. It is also demonstrated how the optical response of the UCNPs can be used to estimate the local field energy in the coating layer. Optical simulations using the finite element method validate the experimental results and the calculated spatial three-dimensional field energy distribution helps us to understand the emission enhancement mechanism of the UCNPs closely attached to the metasurface. In addition, we analyzed the spectral shifts of the resonances for uncoated and coated metasurfaces and metasurfaces submerged in water to enable a prediction of the optimum layer thicknesses for different excitation wavelengths, paving the way to applications such as electromagnetic field sensors or bioassays.

Multi-mode optical coded patterns enabled by upconversion nanoparticles and photonic crystals

In this study, we designed the luminescent enhanced composites by combining upconversion nanoparticles (UCNPs) and photonic crystals (PCs), and prepared multi-mode coding patterns through utilizing the optical properties of both materials, respectively. We prepared UCNPs with different luminescence and the composite materials can be quickly obtained by spin-coating the UCNPs on the surface of the PCs. We discover that the composite materials have significant luminescent enhancement and the enhancement of upconversion is attributed to the Bragg reflection of the photonic band gap. We fabricated upconversion luminescent lattices based on the surface of the PCs by dropping and endowed with the meaning of the Morse code so that it can have more information in different modes. In addition, we further obtained the cipher table pattern according to the structural color of the PCs and the luminescence of the UCNPs. The results reveal great potential applications in the field of multi-mode optical imaging and anti-counterfeiting.

Energy transfer from Er to Nd ions by the thermal effect and promotion of the photocatalysis of the NaYF4:Yb,Er,Nd/W18O49 heterostructure

The NaYF4:Yb,Er/W18O49 heterostructure is an excellent photocatalyst that can promote H2 evolution by hydrolyzing BH3NH3 under near-infrared (NIR) light irradiation. At the same time, the photothermal effect can be produced in photocatalytic reactions, which will cause the luminescence efficiency and photocatalytic activity to decrease. Determining how to take advantage of that photothermal effect becomes a major problem. Moreover, the energy transfer (ET) process from Er ions to Nd ions in NaYF4 co-doped with Yb/Er/Nd ions (NaYF4:Yb,Er,Nd) occurred at high temperature. Herein, the NaYF4:Yb,Er,Nd/W18O49 quasi-core-shell heterostructure was designed to achieve better H2 production capacity; this heterostructure exhibits a 1.5-fold enhancement of photocatalytic activity for H2 evolution as compared with the NaYF4:Yb,Er/W18O49 heterostructure. This study provides a new way to explore the catalytic activities in the NIR field for application in the development of a sustainable energy source.

Fe3+-sensing by 3,3',5,5'-tetramethylbenzidine-functionalized upconversion nanoparticles

Detection of Fe³⁺ ion is essential for human health because it is an important element of hemoglobin, which carries oxygen to cells in the body. Here, a 3,3',5,5'-tetramethylbenzidine (TMB) functionalized NaYF₄: Yb³⁺, Er³⁺@NaYF₄ composite upconversion probe was developed, and demonstrated Fe³⁺ sensing ability with high sensitivity and selectivity. The red emission of upconversion nanoparticles (UCNPs) has a higher penetration depth in tissue than green light and works within the biological window. The obtained hydrophobic NaYF₄: Yb³⁺, Er³⁺@NaYF₄ nanoparticles were treated with HCl to achieve hydrophilic ligand-free nanoparticles with non-saturated metal ions on their surface. Then, a Fe³⁺ responsive TMB-UNCPs composite luminescence probe was formed through linking TMB onto the ligand-free UCNPs by a coordination bond between the NH₂ groups in TMB and the metal ions on the UCNPs. Due to the efficient fluorescence resonance energy transfer from UCNPs to Fe³⁺-TMB, the obtained probe shows high sensitivity for detecting Fe³⁺ in the range of 0-100 μM with a detection limit of 0.217 μM. And the color change of the detection system can also be easily recognized by the naked eye. The 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) experiments and the bioimaging experiments show promising prospects in tissue imaging.

Ramachandran A, Mohamed AP, Pillai S. Photonic band gap effect and dye-encapsulated cucurbituril-triggered enhanced fluorescence using monolithic colloidal photonic crystals

Enhanced fluorescence was achieved by tuning the photonic band gaps in colloidal photonic crystals and host–guest chemistry.

Separating and enhancing the green and red emissions of NaYF4:Yb3+/Er3+ by sandwiching them into photonic crystals with different bandgaps

Rational control of the multiple emission outputs and achieving single-band and strong luminescence of Ln³⁺ doped upconversion nanoparticles is highly desirable for their applications in sensor and display fields. Here, we designed a sandwich structure to separate and enhance the green and red emission of NaYF₄:Yb³⁺/Er³⁺ simultaneously and realized pure strong green and red emissions. NaYF₄:Yb³⁺/Er³⁺ nanocrystals were sandwiched between two layers of photonic crystals, which have bandgaps at 660 nm and 530 nm, respectively. The photonic crystal with a bandgap at 530 nm on top of the NaYF₄:Yb³⁺/Er³⁺ layer can filter the green emission of NaYF₄:Yb³⁺/Er³⁺, prohibiting its emission upward, and at the same time, enhancing its emission downward. Similarly, the photonic crystal with a bandgap at 660 nm can prohibit the transmission of the red emission, and at the same time enhance its reflection in the opposite direction. Consequently, enhanced green emission was observed from the bottom of the sandwich structure and enhanced red emission was observed from the top of the sandwich structure. Thus, the green and red emissions of NaYF₄:Yb³⁺/Er³⁺ were separated and both of them were enhanced. On the other hand, when using a photonic crystal with a bandgap that overlapped with the excitation light of NaYF₄:Yb³⁺/Er³⁺ nanoparticles, their emissions were all greatly enhanced. Our results suggest that photonic crystals are good candidates to separate and enhance the emissions of Ln³⁺ doped luminescent materials.

Manipulating Luminescence of Light Emitters by Photonic Crystals

The modulation of luminescence is essential because unwanted spontaneous-emission modes have a negative effect on the performance of luminescence-based photonic devices. Photonic crystals are promising materials for the control of light emission because of the variation in the local density of optical modes within them. They have been widely investigated for the manipulation of the emission intensity and lifetime of light emitters. Several groups have achieved greatly enhanced emission by depositing emitters on the surface of photonic crystals. Herein, the different modulating effects of photonic crystal dimensions, light-emitter positions, photonic crystal structure type, and the refractive index of photonic crystal building blocks are highlighted, with the aim of evaluating the fundamental principles that determine light propagation. The applications of using photonic crystals to manipulate spontaneous emission in light-emitting diodes and sensors are also reviewed. In addition, potential future challenges and improvements in this field are presented.

Luminescence property tuning of Yb3+-Er3+ doped oxysulfide using multiple-band co-excitation

Lanthanide ions have abundant excited-state channels which result in a radiation relaxation process generally accompanied by a non-radiation relaxation process. However, non-radiation relaxation processes will consume the activation energy and reduce the luminescence efficiency of the phosphor. Two lasers with an excitation energy which matched the ground state absorption and excited state absorption of ions were used to excite the phosphors to avoid the non-radiation relaxation process. This approach can achieve the purpose of populating specific states of the lanthanide ions, and furthermore effectively tunes the luminescence intensity and color output of the sample. Results show that the red emission intensity of the sample is significantly improved and this is caused by populating the ⁴F_9/2 level under simultaneous 1510 nm and 980 nm excitation. Then when the 1510 nm and 808 nm co-operate to excite the sample, the green emission obtained increased sharply because the ²H_11/2/⁴S_3/2 states were efficiently populated. As a proof-of-concept experiment, this new approach has potential in the applications of solar cells.

The emission intensity of the sample excited by dual-wavelength (980 and 1510 nm) is 3.2 times than the sum of UCL emission under the two single wavelength excitation.

Fabrication of tough photonic crystal patterns with vivid structural colors by direct handwriting

Patterned photonic crystals (PCs) have attracted considerable attention due to their great potential in practical applications. Direct writing is an important and convenient method to fabricate patterned PCs. However, due to the limited interaction among spheres and the evaporation of ink, the obtained patterns usually suffer from poor structure strength, and non-uniform and unstable structural colors. In this work, an in situ embedding and locking strategy for fabricating tough PC patterns in one step was demonstrated. With properly dried polymer films as "paper" and dispersions of CdS spheres as "inks" to write on the "paper", the self-assembly of CdS spheres and locking of the PC structure can be achieved simultaneously, which gives rise to tough composite patterned PCs with uniform, stable and permanent structural colors. Based on this simple method, tough PC patterns can be easily and quickly created by direct hand writing or drawing without special treatment, equipment, masks or templates. The vivid structural colors of the tough PC patterns and this simple method show great potential for practical applications.