Achieving High Electroluminescence Efficiency and High Color Rendering Index for All-Fluorescent White OLEDs Based on an Out-of-Phase Sensitizing System

Sensitizing conventional fluorescence (CF) dopants with thermally activated delayed fluorescence (TADF) materials has achieved considerable progress, by which the advantages of TADF materials and CF dopants can be fully harnessed. However, the usually used co-phase configuration of CF dopant-engaged sensitizing systems often encounters exciton loss due to Dexter energy transfer (DET). Herein, an effective out-of-phase configuration is proposed to sensitize CF dopants in the fabrication of white organic light-emitting diodes (WOLEDs). Based on a new efficient sky-blue TADF luminogen DCP-BP-DPAC which has an electroluminescence (EL) peak at 486 nm and an EL efficiency of 26.6%, a green TADF material BDMAC-XT, and a red CF dopant DBP sensitized by BDMAC-XT through an out-of-phase configuration without interlayer, efficient WOLEDs are successfully fabricated. By further adopting orange TBRB or 4CzTPNBu as intermediate sensitizers, more efficient energy transfer to DBP is achieved via Förster energy transfer. Through step-by-step energy transfer and elimination of excess DET process, high-performance all-fluorescent WOLEDs are achieved, providing excellent EL efficiencies over 23.0%, and highly stable white light with a high color rendering index of 87. The outstanding EL performance and high-quality emission color demonstrate the great potential of the proposed out-of-phase design for sensitizing systems of WOLEDs.     

Deoxyribonucleic Acid Photonic Wires with Three Primary Color Emissions for Information Encryption

DNA photonic wires (PWs) are a new type of photon delivery nanodevice and have attracted wide attention due to their excellent photon delivery ability via Förster resonance energy transfer (FRET) but are dramatically challenged in real applications. In this study, 7-amino-4-methyl-3-coumarinylacetic acid is used as a donor, Texas Red is used as an acceptor, and acridine orange is used as a bridge to intercalate DNA to facilitate the homo-FRET process, which leads to DNA PWs with high-energy transfer efficiencies (≈0.9). Notably, the newly developed DNA PWs exhibit characteristic emissions in the three primary colors, which are successively adjusted by simply changing the extent of FRET to make over 36 subtypes of fluorescence emissions. This polychroism is further applied for information encryption with high efficiency, which is a new application for DNA PWs.     

Efficiency Breakthrough of Fluorescence OLEDs by the Strategic Management of “Hot Excitons” at Highly Lying Excitation Triplet Energy Levels

Aggregation-induced emission (AIE) and hybridized local and charge-transfer (HLCT) materials are two kinds of promising electroluminescence systems for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) by harnessing “hot excitons” at the high-lying triplet exciton states (T_n, n ≥ 2). Nonetheless, the efficiency of the resulting OLEDs did not meet expectations due to the possible loss of T_n→T_n−1. Herein, experimental results and theoretical calculations demonstrate the “hot exciton” process between the high-lying triplet state T₃ and the lowest excited singlet state S₁ in an AIE material 4⁗-(diphenylamino)-2″,5″-diphenyl-[1,1″:4′,1″:4″,1′″:4′″,1⁗-quinquephenyl]-4-carbonitrile (TPB-PAPC) and it is found that the Förster resonance energy transfer (FRET) between two molecules can facilitate the “hot exciton” process and inhibit the T₃→T₂ loss by doping a blue fluorescent emitter in TPB-PAPC. Finally, the doped TPB-PAPC blue OLEDs achieve a maximum external quantum efficiency (EQE_max) of 9.0% with a small efficiency roll-off. Furthermore, doping the blue fluorescent emitter in a HLCT material 2-(4-(10-(3-(9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d] imidazole (PAC) is used as the emission layer, and the resulting blue OLEDs exhibit an EQE_max of 17.4%, realizing the efficiency breakthrough of blue fluorescence OLEDs. This work establishes a physical insight in the design of high-performance “hot exciton” molecules and the fabrication of high-performance blue fluorescence OLEDs.     

Near-infrared distributed feedback laser emission based on cascade Förster resonance energy transfer to Nile Blue aggregates

We present the real time tunable distributed feedback lasing in the region of near-infrared light obtained in a dye doped polymeric layer via cascade energy transfer based on Förster resonance (FRET). The Rhodamine 6G and Rhodamine B laser dyes were used as a donor for FRET effect, while the Nile Blue dye served as an acceptor. The shift of the emission from visible to near-infrared region was possible due to the formation of the dyes aggregates of lower than for molecules energy levels. Therefore, through the sequence of energy transfers between molecules and aggregates, the shift between excitation and stimulated emission spectra of about 200 nm can be achieved. The use of the periodically modulated pumping beam from pulsed Nd:YAG laser allows to select and amplify particular wavelength from the gain profile according to Bragg conditions, what can be easily tuned by changing the period of the interference pattern. The reported tunability of distributed feedback emission from 725 nm to 745 nm confirmed that only Nile Blue aggregates were involved in the laser action, while donor molecules and aggregates, as well as acceptor in non-aggregated form, acted as a canal for energy transfer.     

Atomically Structured Metal-Organic Frameworks: A Powerful Chemical Path for Noble Metal-Based Electrocatalysts

Noble metal-based electrocatalysts (NMECs), particularly those with active sites at the nano or atomic level, are indispensable in heterogeneous catalysis, which has attracted considerable research interests, especially in the energy communities. Due to the enormous inherent merits, such as ultrahigh surface area, tunable atomic structure, and diverse chemical tailorability, metal-organic frameworks (MOFs) have been proposed as ideal candidates for creating efficient and programmable NMECs. In this review, from an interdisciplinary opinion,the recent progresses on the synthetic principles and catalytic site design protocols for the atomically structured MOFs and their derivatives are comprehensively discussed. Particularly, it is dedicated to summarizing the modulation strategies on creating the catalytic centers and bond microenvironments of MOFs-based NMECs, including the single-atom, dual-atom, cluster, and nanoparticle engineering. Furthermore, the critical mechanisms of how the structures of MOFs-based NMECs affect the corresponding electrochemical behaviors is outlined and disclose the critical essences for their future applications. Finally, the current developments, challenges, and perspectives for engineering the atomically structured MOFs-based NMECs are discussed to inspire the broad utilization of MOFs-based NMECs-equipped catalysts in energy conversion, which offers cutting-edge guidance for future prosperity in developing efficient NMECs.     

Tunable Fluorescent Artificial Receptor Biosensor Based on Programmable Lanthanide Metal-Organic Framework for Highly Selective Neurotransmitter Detection

The design and fabrication of artificial receptors suitable for highly selective and sensitive sensing of neurotransmitters in aqueous media remains a challenge, especially for the significant biomarker dopamine (DA) for neurological diseases. Herein, a novel modular fluorescent artificial receptor design strategy based on multiple parameters programming engineering is proposed. The optimal artificial receptor DA biosensor based on fluorescent lanthanide metal-organic frameworks (Ln-MOFs) is coded by adjusting the modular design parameters such as preparation solvents and ligands. It is prepared by Eu ion and ligand 1,2,4,5-benzenetetracarboxylic acid (BTEC) in DMF/water, which can selectively recognize DA even in complex biological fluids, without the interference of other structurally similar neurotransmitters such as levodopa, serotonin and noradrenaline, and the limit of detection for DA is as low as 10 nm . Ln-MOFs are believed to uniquely utilize their tunable host-guest interactions (specific recognition) and confinement catalysis capabilities to achieve highly selective and dual-response DA detection, similar to lock-and-key theory, which is supported by experiments and density functional theory calculations. Fluorescent artificial receptors based on Ln-MOFs are proved to have broad potential for label-free detection, imaging and diagnosis. They can overcome the long-standing limitations of complex synthetic biosensors, and bringing new solutions to personalized medicine.     

Sub-2 nm Ultrathin and Robust 2D FeNi Layered Double Hydroxide Nanosheets Packed with 1D FeNi-MOFs for Enhanced Oxygen Evolution Electrocatalysis

Water oxidation is a critical process for electrochemical water splitting due to its inherent sluggish kinetics. In spite of the high catalytic activities of noble metal-based electrocatalysts for water oxidation, their high cost, rare reserves, and low stabilities drive researchers to exploit efficient but low-cost electrocatalysts. Ultrathin 2D nanomaterials are considered efficient electrocatalysts for oxygen evolution reaction (OER) in water splitting. Herein, a facile strategy is proposed to fabricate 2D FeNi layered double hydroxide (FeNi-LDH) nanosheets packed with the in situ produced 1D sword-like FeNi-MOFs by using FeNi-LDH as a semi-sacrificial template. In the composite, the thickness of the formed nanosheets is only 1.34 nm, much thinner than that of most previously reported 2D materials. The 1D porous sword-like MOF nanorods have a long length of around 1.3 µm. Due to the unique 2D/1D combined structure, the as-prepared FeNi LDH/MOF is directly used as electrocatalyst for the OER displays enhanced OER electrocatalytic performance with a low overpotential of 272 mV@100 mA cm^–2, a small Tafel slope of 34.1 mV dec^–1, high long-term durability. This work provides a new way to fabricate integrated ultrathin 2D nanosheets and MOFs as advanced catalysts for electrochemical energy conversion.     

Disrupting Intracellular Iron Homeostasis by Engineered Metal-Organic Framework for Nanocatalytic Tumor Therapy in Synergy with Autophagy Amplification-Promoted Ferroptosis

Metal-organic frameworks (MOFs) featuring good biocompatibility and tunable microstructures are developed to generate reactive oxygen species (ROS) for nanocatalytic therapy. However, the relatively low catalytic activity of MOF and intracellular ion homeostasis, a self-protective mechanism to resist the intracellular accumulation of metal ions, results in the undesirable efficacy of tumor therapy. Herein, a therapeutic strategy is introduced of breaking intracellular iron homeostasis for nanocatalytic therapy in synergy with autophagy amplification-promoted ferroptosis, based on etched MOF nanocatalyst (denoted COS@MOF), which is self-etched by thiamine pyrophosphate (TPP) and further modified with autophagy agonist chitosan oligosaccharides (COS). Such self-etched MOF exhibit an open cavity structure that is more conducive to adsorbing reactive molecules and producing more active sites, and an enhanced Fe(II)/Fe(III) ratio, reinforcing catalytic activity for ROS generation. The catalytic process of COS@MOF can be accelerated by overexpressed endogenous hydrogen sulfide (H₂S) within colorectal tumors which reduces Fe³⁺ into more active Fe²⁺. In vitro and in vivo results demonstrate that COS@MOF amplifies autophagy to break iron homeostasis for facilitating ROS production to promote ferroptosis, achieving synergetic nanocatalytic/ferroptosis tumor therapy. This study provides a promising paradigm to elevate MOF-based catalytic performance in synergy with autophagy amplification-promoted ferroptosis for enhanced therapeutic efficacy.     

Bioinspired Metal-Organic Frameworks in Mixed Matrix Membranes for Efficient Static/Dynamic Removal of Mercury from Water

The mercury removal efficiency of a novel metal-organic framework (MOF) derived from the amino acid S-methyl-L-cysteine is presented and the process is characterized by single-crystal X-ray crystallography. A feasibility study is further presented on the performance of this MOF—and also that of another MOF derived from the amino acid L-methionine—when used as the sorbent in mixed matrix membranes (MMMs). These MOF-based MMMs exhibit high efficiency and selectivity—in both static and dynamic regimes—in the removal of Hg²⁺ from aqueous environments, due to the high density of thioalkyl groups decorating MOF channels. Both MMMs are capable to reduce different concentration of the pollutant to acceptable limits for drinking water (<2 parts per billion). In addition, a novel device, consisting of the recirculation and adsorption of contaminated solutions through the MOF–MMMs, is designed and successfully explored in the selective capture of Hg²⁺. Thus, filtration of Hg²⁺ solutions with multiple passes through the permeation cell shows a gradual decrease of the pollutant concentration. These results suggest that MOF-based MMMs can be implemented in water remediation, helping to reduce either contaminants from accidental unauthorized or deliberate metal industrial dumping and to ensure access for clean and potable freshwater.     

Phthalocyanine‐Based 2D Conjugated Metal‐Organic Framework Nanosheets for High‐Performance Micro‐Supercapacitors

2D conjugated metal‐organic frameworks (2D c‐MOFs) are emerging as a novel class of conductive redox‐active materials for electrochemical energy storage. However, developing 2D c‐MOFs as flexible thin‐film electrodes have been largely limited, due to the lack of capability of solution‐processing and integration into nanodevices arising from the rigid powder samples by solvothermal synthesis. Here, the synthesis of phthalocyanine‐based 2D c‐MOF (Ni₂[CuPc(NH)₈]) nanosheets through ball milling mechanical exfoliation method are reported. The nanosheets feature with average lateral size of ≈160 nm and mean thickness of ≈7 nm (≈10 layers), and exhibit high crystallinity and chemical stability as well as a p‐type semiconducting behavior with mobility of ≈1.5 cm² V^?1 s^?1 at room temperature. Benefiting from the ultrathin feature, the nanosheets allow high utilization of active sites and facile solution‐processability. Thus, micro‐supercapacitor (MSC) devices are fabricated mixing Ni₂[CuPc(NH)₈] nanosheets with exfoliated graphene, which display outstanding cycling stability and a high areal capacitance up to 18.9 mF cm^?2; the performance surpasses most of the reported conducting polymers‐based and 2D materials‐based MSCs.     

Tm3+/Ho3+共掺碲酸盐玻璃的中红外发光及能量传递机理

用高温熔融法制备了Tm3+/Ho3+共掺碲酸盐玻璃(TeO2-ZnO-Na2O),根据测量得到的吸收光谱,应用Judd-Ofelt理论计算分析了玻璃样品中Ho3+离子的强度参数Ωt(t=2,4,6)、自发辐射跃迁几率A、荧光分支比β和荧光辐射寿命τrad等各项光谱参数。同时,测量得到了不同Ho3+离子掺杂浓度下玻璃样品的荧光发射谱。结果显示,在808nm抽运光激励下Tm3+/Ho3+共掺碲酸盐玻璃样品发射出较强的2.0μm中红外荧光。分析表明,较强的Ho3+离子中红外荧光来自于Tm3+/Tm3+离子间共振的能量传递过程,以及Tm3+/Ho3+离子间基于零声子和单声子辅助非共振的两部分能量传递过程。由此进一步计算得到了Tm3+/Tm3+、Tm3+/Ho3+离子间的能量传递微观速率、临界半径和声子的贡献。最后,计算分析了Ho3+…5I7→5I8能级间跃迁的2.0μm波段吸收截面、受激发射截面和增益系数。研究表明,Tm3+/Ho3+共掺TeO2-ZnO-Na2O玻璃可以作为2.0μm波段中红外固体激光器的潜在增益基质。     

Manipulation of light harvesting for efficient dye‐sensitized solar cell by doping an ultraviolet light‐capturing fluorophore

An organic fluorophore is doped into a mesoporous TiO₂ photoelectrode to absorb ultraviolet light and convert it to green light for more efficient light harvesting of N719 dye. This fluorescence conversion enables the absorption of additional green light by dye molecules by means of Förster resonance energy transfer between fluorescent compound donor and N719 dye acceptor. Owing to close fit between the emission peak of fluorophore and the absorption peak of N719 dye, the Förster resonance energy transfer effect enhances the incident photon to current conversion efficiency of the dye‐sensitized solar cells based on fluorophore‐doped TiO₂ photoelectrodes. Improved power conversion efficiency (8.03–8.13%) is also achieved for the fluorophore‐doped (10⁻⁴ M) dye‐sensitized solar cells compared with a cell without the doping of fluorophore (7.63%). Copyright © 2013 John Wiley & Sons, Ltd.     

Manipulating Complementarity of Binary White Thermally Activated Delayed Fluorescence Systems for 100% Exciton Harvesting in OLEDs

Different to fluorescent and phosphorescent counterparts, white thermally activated delayed fluorescence (TADF) involves in multiple reverse intersystem crossing (RISC), leading to the correlation but competition between blue and other color components in both singlet and triplet allocations. Herein, three blue TADF emitters SSFAPO, DSFAPO, and TSFAPO, collectively named xSFAPO are developed, featuring a moderately electron-withdrawing phosphine oxide (PO) acceptor respectively linked 1-3 donors. Despite nearly identical blue emissions, photoluminescence quantum yields of xSFAPO are proportional to donor number. But, their RISC efficiencies are below 70%, markedly less than 85% of a conventional yellow TADF emitter 2,3,5,6-tetrakis(3,6-di-(tert-butyl)carbazol-9-yl)-1,4-dicyanobenzene (4CzTPNBu). Furthermore, sp³ hybrid configuration of PO enlarges steric hindrance of peripheral donor groups. So, Dexter energy transfer is impeded by increasing donor numbers. Among xSFAPO and 4CzTPNBu dually doped white-emitting films, yellow emission from SSFAPO-based film is the strongest, reflecting the predominance of fast Dexter energy transfer in triplet allocation. Therefore, SSFAPO endowed its warm-white organic light-emitting diodes (WOLEDs) with an external quantum efficiency of 25.1%, corresponding to 100% internal quantum efficiency, which are 1.25 and 1.60 folds of those of DSFAPO and TSFAPO-based WOLEDs. These results suggest advantage complementarity of different components is crucial for developing white-emitting systems with 100% exciton utilization.     

A Facile Strategy for Achieving Polymeric Afterglow Materials with Wide Color-Tunability and Persistent Near-Infrared Luminescence

Polymer-based room-temperature phosphorescence (RTP) materials show promising applications in anti-counterfeiting. To further realize multiscale and/or multimodal anti-counterfeiting, it is highly desirable to develop polymeric afterglow materials with multiple security features. Herein, a facile strategy is presented to endow polymeric afterglow materials with ultralong lifetime, wide color-tunability, persistent near-infrared (NIR) luminescence, and good water solubility via constructing non-traditional phosphorescence resonance energy transfer (PRET) and two-step sequential resonance energy transfer systems. Specifically, the 1-bromocarbazole derivatives with ultralong blue-color RTP property act as the energy donor while traditional dyes with red/NIR luminescence act as the energy acceptor. By simply regulating the doping composition and concentration of these non-traditional energy transfer systems, persistent and multicolor organic afterglow covering from the visible to NIR region is successfully realized. Notably, compared to the single-step PRET, the two-step sequential resonance energy transfer has the unique advantages of higher transfer efficiency of triplet excitons from the initial donor, a wider range of color-tunability mediated by the intermediary acceptor, and enhanced delayed fluorescence efficiency of the final acceptor. Finally, these water-soluble polymeric afterglow materials with ultralong lifetime, wide color-tunability, and persistent NIR luminescence show great potential applications in advanced anti-counterfeiting and information security technologies.     

Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes

Two novel spiro-based host materials, namely 3-(9,9′-spirobi[fluoren]-6-yl)-9-phenyl-9H-carbazole (SF3Cz1) and 9-(3-(9,9′-spirobi[fluoren]-6-yl)phenyl)-9H-carbazole (SF3Cz2) were designed and synthesized. Due to the meta-linkage of spirobifluorene backbone, both SF3Cz1 and SF3Cz2 possess triplet energies over 2.70 eV, indicating they could serve as suitable hosts for blue and even white phosphorescent organic light-emitting diodes (PHOLEDs). The fabricated bis(4,6-(difluorophenyl)-pyridinato -N,C′)picolinate (FIrpic) based PHOLEDs hosted by SF3Cz1 and SF3Cz2 exhibited excellent performance with maximum external quantum efficiencies (EQEs) of 18.1% and 19.7%, respectively. Two-color warm white PHOLEDs fabricated by utilizing SF3Cz1 and SF3Cz2 as hosts also achieved high EQEs and low efficiency roll-offs. The results demonstrate that SF3Cz1 and SF3Cz2 are promising hosts for blue and white PHOLEDs.     

Achieving Color-Tunable and Time-Dependent Organic Long Persistent Luminescence via Phosphorescence Energy Transfer for Advanced Anti-Counterfeiting

Organic ultralong room-temperature phosphorescence (RTP) materials have promising applications in anti-counterfeiting. To improve the encryption level, the exploration of organic materials with tunable solid-state long persistent luminescence is in urgent need. Herein, a series of organic ultralong RTP polymeric systems are prepared by doping versatile indolocarbazole isomers into the poly(vinyl alcohol) (PVA) matrix. Notably, the doping film 11,12-ICz@PVA exhibits excellent RTP property with an ultralong lifetime of 2.04 s and a high phosphorescence quantum yield of 44.1%. Theoretical calculations reveal that this excellent RTP property can be attributed to the strong electrostatic attraction resulting from the synergistic double hydrogen-bond between the isomer 11,12-ICz and PVA matrix. More impressively, color-tunable and time-dependent long persistent luminescence is successfully achieved through efficient phosphorescence energy transfer between the indolocarbazole isomers with ultralong blue RTP emissions and commercially available fluorescent dyes with emission colors ranging from green to red doped into the PVA matrix. Besides, diversified encryption patterns are fabricated to demonstrate the promising applications of these water-soluble doping PVA systems with tunable solid-state persistent luminescence in advanced anti-counterfeiting technology.     

Reversible Fluorescent On–Off Recording in a Highly Transparent Polymeric Material Utilizing Fluorescent Resonance Energy Transfer (FRET) Induced by Heat Treatment

One productive technique for ultrahigh resolution readout of tiny regions is the measurement of the fluorescence signal of materials. A transparent polymeric materials whose fluorescence quantum yield is changed and recorded by thermally controlling the aggregation of fluoran dyes and developers with long alkyl chains has been developed. The recording medium can be fabricated easily by casting or coating recording materials. Fluorescence is observed after annealing at 363 K for about twelve seconds and then cooling to room temperature (RT), and quenched by annealing at 423 K for a few seconds and then quenching to RT. Nondestructive readout by excitation light with a fluorescent contrast of above 10 is achieved using red, green, and blue fluorescent dyes. Fluorescence on–off switching is induced by fluorescent resonance energy transfer (FRET) from a fluorescent dye to a colored fluoran dye in the recording material. Fluorescence was uniformly quenched in the visible region after erasing. Since the recording materials allow the penetration of laser light due to the presence of crystals smaller than the wavelength range of visible light in both the emission and quenching states, nondestructive readout of the fluorescent signal by two‐photon absorption is accomplished. This work provides an important stepping‐stone for achieving rewritable‐type near‐field optical storage or multilayer recording.     

Multicomponent Organic Nanoparticles for Fluorescence Studies in Biological Systems

The formation of dual‐component organic nanoparticles by a modified emulsion‐templated freeze‐drying approach leads to aqueous nanosuspensions showing fluorescence (Förster) resonance energy transfer (FRET) from within a distribution of single nanoparticles. The combination of both FRET dyes within dual‐component nanoparticles (<200 nm) allows the spatial and physical monitoring of the particles, as the FRET signal is lost on dissolution and breakdown of the nanoparticles. The monitoring of accumulation by Caco‐2 cells and macrophages shows very limited internalization within the non‐phagocytic cells. Conservation of FRET within the macrophages confirms extensive whole‐particle internalization. The cellular permeability through Caco‐2 monolayers is also assessed and movement of intact dual‐component particles is observed, suggesting a mechanism for enhanced pharmacokinetics in vivo.     

pH‐Responsive Cyanine‐Grafted Graphene Oxide for Fluorescence Resonance Energy Transfer‐Enhanced Photothermal Therapy

Stimuli‐responsive anticancer agents are of particular interest in the field of cancer therapy. Nevertheless, so far stimuli‐responsive photothermal agents have been explored with limited success for cancer photothermal therapy (PTT). In this work, as a proof‐of‐concept, a pH‐responsive photothermal nanoconjugate for enhanced PTT efficacy, in which graphene oxide (GO) with broad NIR absorbance and effective photothermal conversion efficiency is selected as a typical model receptor of fluorescence resonance energy transfer (FRET), and grafted cyanine dye (e.g., Cypate) acts as the donor of near‐infrared fluorescence (NIRF), is reported for the first time. The conjugate of Cypate‐grafted GO exhibits different conformations in aqueous solutions at various pH, which can trigger pH‐dependent FRET effect between GO and Cypate and thus induce pH‐responsive photothermal effect of GO‐Cypate. GO‐Cypate exhibits severe cell damage owing to the enhanced photothermal effect in lysosomes, and thus generate synergistic PTT efficacy with tumor ablation upon photoirradiation after a single‐dose intravenous injection. The photothermal nanoconjugate with broad NIR absorbance as the effective receptor of FRET can smartly convert emitted NIRF energy from donor cyanine dye into additional photothermal effect for improving PTT. These results suggest that the smart nanoconjugate can act as a promising stimuli‐responsive photothermal nanoplatform for cancer therapy.