Near‐Infrared Excitation/Emission and Multiphoton‐Induced Fluorescence of Carbon Dots

Carbon dots (CDs) have significant potential for use in various fields including biomedicine, bioimaging, and optoelectronics. However, inefficient excitation and emission of CDs in both near‐infrared (NIR‐I and NIR‐II) windows remains an issue. Solving this problem would yield significant improvement in the tissue‐penetration depth for in vivo bioimaging with CDs. Here, an NIR absorption band and enhanced NIR fluorescence are both realized through the surface engineering of CDs, exploiting electron‐acceptor groups, namely molecules or polymers rich in sulfoxide/carbonyl groups. These groups, which are bound to the outer layers and the edges of the CDs, influence the optical bandgap and promote electron transitions under NIR excitation. NIR‐imaging information encryption and in vivo NIR fluorescence imaging of the stomach of a living mouse using CDs modified with poly(vinylpyrrolidone) in aqueous solution are demonstrated. In addition, excitation by a 1400 nm femtosecond laser yields simultaneous two‐photon‐induced NIR emission and three‐photon‐induced red emission of CDs in dimethyl sulfoxide. This study represents the realization of both NIR‐I excitation and emission as well as two‐photon‐ and three‐photon‐induced fluorescence of CDs excited in an NIR‐II window, and provides a rational design approach for construction and clinical applications of CD‐based NIR imaging agents.     

Nanowire‐Intensified Metal‐Enhanced Fluorescence in Hybrid Polymer‐Plasmonic Electrospun Filaments

Hybrid polymer‐plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long‐range transport in subwavelength dielectric structures. Here, the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal‐enhanced fluorescence (MEF) with unique characteristics compared to conventional structures, is reported. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire‐related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also indicated, with MEF significantly increasing upon reduction of the wire diameter. These findings are rationalized by finite element simulations, predicting a position‐dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble‐averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, and the general nanowire‐enhanced MEF effects associated to them, are highly relevant for developing nanoscale light‐emitting devices with high efficiency and intercoupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.     

Understanding and Manipulating the Interplay of Wide‐Energy‐Gap Host and TADF Sensitizer in High‐Performance Fluorescence OLEDs

Comprising an emitting layer (EML) constituting a wide‐energy‐gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF‐sensitizing‐fluorescence organic light‐emitting diodes (TSF‐OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under‐researched. Taking the host type (TADF or non‐TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide‐energy‐gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses. Based on those findings, a maximum external quantum efficiency (EQE)/power efficiency (PE) of 23.2%/76.9 lm W^?1 is realized with a newly developed TADF host, significantly outperforming the reference devices. Further device optimization leads to unprecedently high EQE/PE of 24.2%/89.5 lm W^?1 and a half‐lifetime of over 400 h at an initial luminance of 2000 cd m^?2, with the peak PE being the highest value among the reported TSF‐OLEDs. This work reveals the importance of manipulating the component interplay in EMLs, opening a new avenue toward highly efficient TSF‐OLEDs.     

One‐Step Synthesis of Silica‐Coated Carbon Dots with Controllable Solid‐State Fluorescence for White Light‐Emitting Diodes

Carbon dots (CDots)‐based solid‐state luminescent materials have important applications in light‐emitting devices owing to their outstanding optical properties. However, it still remains a challenge to develop multiple‐color‐emissive solid‐state CDots, due to the serious self‐quenching of the CDots in the aggregation or solid state. Herein, a one‐step synthesis of multiple‐color‐emissive solid‐state silica‐coated CDots (silica/CDots) composites by controlling CDots loading fraction and composite morphology to realize the adjustment of emitting color is reported. The emission of resultant silica/CDots composites shifts from blue to orange with the photoluminescence quantum yields of 57.9%, 34.3%, and 32.7% for blue, yellow, and orange emitting, respectively. Furthermore, the yellow emitting silica/CDots composites exhibit an excellent fluorescence thermal stability, and further have been applied to fabricate white‐light‐emitting devices with a high color rendering index of above 80.     

Nanoscale Ultrasound‐Switchable FRET‐Based Liposomes for Near‐Infrared Fluorescence Imaging in Optically Turbid Media

A new approach for fluorescence imaging in optically turbid media centered on the use of nanoscale ultrasound‐switchable FRET‐based liposome contrast agents is reported. Liposomes containing lipophilic carbocyanine dyes as FRET pairs with emission wavelengths located in the near‐infrared window are prepared. The efficacy of FRET and self‐quenching for liposomes with a range of fluorophore concentrations is first calculated from measurement of the liposome emission spectra. Exposure of the liposomes to ultrasound results in changes in the detected fluorescent signal, the nature of which depends on the fluorophores used, detection wavelength, and the fluorophore concentration. Line scanning of a tube containing the contrast agents with 1 mm inner diameter buried at a depth of 1 cm in a heavily scattering tissue phantom demonstrates an improvement in image spatial resolution by a factor of 6.3 as compared with images obtained in the absence of ultrasound. Improvements are also seen in image contrast with the highest obtained being 9% for a liposome system containing FRET pairs. Overall the results obtained provide evidence of the potential the nanoscale ultrasound‐switchable FRET‐based liposomes studied here have for in vivo fluorescence imaging.     

Bright Aggregation‐Induced‐Emission Dots for Targeted Synergetic NIR‐II Fluorescence and NIR‐I Photoacoustic Imaging of Orthotopic Brain Tumors

Precise diagnostics are of significant importance to the optimal treatment outcomes of patients bearing brain tumors. NIR‐II fluorescence imaging holds great promise for brain‐tumor diagnostics with deep penetration and high sensitivity. This requires the development of organic NIR‐II fluorescent agents with high quantum yield (QY), which is difficult to achieve. Herein, the design and synthesis of a new NIR‐II fluorescent molecule with aggregation‐induced‐emission (AIE) characteristics is reported for orthotopic brain‐tumor imaging. Encapsulation of the molecule in a polymer matrix yields AIE dots showing a very high QY of 6.2% with a large absorptivity of 10.2 L g^?1 cm^?1 at 740 nm and an emission maximum near 1000 nm. Further decoration of the AIE dots with c‐RGD yields targeted AIE dots, which afford specific and selective tumor uptake, with a high signal/background ratio of 4.4 and resolution up to 38 µm. The large NIR absorptivity of the AIE dots facilitates NIR‐I photoacoustic imaging with intrinsically deeper penetration than NIR‐II fluorescence imaging and, more importantly, precise tumor‐depth detection through intact scalp and skull. This research demonstrates the promise of NIR‐II AIE molecules and their dots in dual NIR‐II fluorescence and NIR‐I photoacoustic imaging for precise brain cancer diagnostics.     

High‐Contrast Fluorescence Detection of Metastatic Breast Cancer Including Bone and Liver Micrometastases via Size‐Controlled pH‐Activatable Water‐Soluble Probes

Breast cancer metastasis is the major cause of cancer death in women worldwide. Early detection would save many lives, but current fluorescence imaging probes are limited in their detection ability, particularly of bone and liver micrometastases. Herein, probes that are capable of imaging tiny (<1 mm) micrometastases in the liver, lung, pancreas, kidneys, and bone, that have disseminated from the primary site, are reported. The influence of the poly(ethylene glycol) (PEG) chain length on the performance of water‐soluble, pH‐responsive, near‐infrared 4,4′‐di?uoro‐4‐bora‐3a ,4a ‐diaza‐s ‐indacene (BODIPY) probes is systematically investigated to demonstrate that PEG tuning can provide control over micrometastasis tracking with high tumor‐to‐background contrast (up to 12/1). Optimized probes can effectively visualize tumor boundaries and successfully detect micrometastases with diameters <1 mm. The bone‐metastasis‐targeting ability of these probes is further enhanced by covalent functionalization with bisphosphonate. This improved detection of both bone and liver micrometastases (<2 mm) with excellent tumor‐to‐normal contrast (5.2/1). A versatile method is thus introduced to directly synthesize modular water‐soluble probes with broad potential utility. Through a single intravenous injection, these materials can image micrometastases in multiple organs with spatiotemporal resolution. They thus hold promise for metastasis diagnosis, image‐guided surgery, and theranostic PEGylated drug therapies.     

Adamantyl Substitution Strategy for Realizing Solution‐Processable Thermally Stable Deep‐Blue Thermally Activated Delayed Fluorescence Materials

Highly efficient solution‐processable emitters, especially deep‐blue emitters, are greatly desired to develop low‐cost and low‐energy‐consumption organic light‐emitting diodes (OLEDs). A recently developed class of potentially metal‐free emitters, thermally activated delayed fluorescence (TADF) materials, are promising candidates, but solution‐processable TADF materials with efficient blue emissions are not well investigated. In this study, first the requirements for the design of efficient deep‐blue TADF materials are clarified, on the basis of which, adamantyl‐substituted TADF molecules are developed. The substitution not only endows high solubility and excellent thermal stability but also has a critical impact on the molecular orbitals, by pushing up the lowest unoccupied molecular orbital energy and triplet energy of the molecules. In the application to OLEDs, an external quantum efficiency (EQE) of 22.1% with blue emission having Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.19) is realized. A much deeper blue emission with CIE (0.15, 0.13) is also achieved, with an EQE of 11.2%. These efficiencies are the best yet among solution‐processed TADF OLEDs of CIE y < 0.20 and y < 0.15, as far as known. This work demonstrates the validity of adamantyl substitution and paves a pathway for straightforward realization of solution‐processable efficient deep‐blue TADF emitters.