Multi-Stimuli Responsive FRET Processes of Bifluorophoric AIEgens in an Amphiphilic Copolymer and Its Application to Cyanide Detection in Aqueous Media

Photochemically and Thermally Programmed Optical Multi-States from a Single Diacetylene-Functionalized Cyanostilbene Luminogen

To develop advanced optical systems, many scientists have endeavored to create smart optical materials which can tune their photophysical properties by changing molecular states. However, optical multi-states are obtained usually by mixing many dyes or stacking multi-layered structures. Here, multiple molecular states are tried to be generated with a single dye. In order to achieve the goal, a diacetylene-functionalized cyanostilbene luminogen (DACSM) is newly synthesized by covalently connecting diacetylene and cyanostilbene molecular functions. Photochemical reaction of cyanostilbene and topochemical polymerization of diacetylene can change the molecular state of DACSM. By thermal stimulations and the photochemical reaction, the conformation of polymerized DACSM is further tuned. The synergetic molecular cooperation of cyanostilbene and diacetylene generates multiple molecular states of DACSM. Utilizing the optical multi-states achieved from the newly developed DACSM, switchable optical patterns and smart secret codes are successfully demonstrated.

Fluorescent Carbon Dots: Aggregation-Induced Emission Enhancement, Application as Probe for CN- and Cr2O7-2, Sensing Strips and Bio-imaging Agent

Fluorescent carbon dots (Trp-CDs) were prepared using tryptophan as precursor and were characterized on the basis of elemental analysis, powder-XRD, IR, Raman spectroscopy, 13C-NMR, UV-Vis, fluorescence and TEM. Trp-CDs exhibit poor fluorescence in 100% water but showed strong Aggregation Induced Emission (AIE) in ethanol and higher alcohols. The anion sensing study of Trp-CD revealed that it selectively detects CN- and Cr2O7-2 and from fluorescence quenching titration study, quenching constant, LOD and range of detection were evaluated. The emission life-time of Trp-CD before and after addition of CN- and Cr2O7-2 were measured, the decay curve before addition of anion was best fitted with a bi-exponential function with life-time of τ1 2.79 ns (10.74%) and τ2 18.93 ns (89.26%). The mechanistic study revealed that for CN-, the fluorescence quenching is due to its interaction with protons attached to surface functional groups and for Cr2O7-2, it is due to inner filter effect (IFE). Sensing strips were prepared by coating Trp-CDs onto various solid surfaces including agarose films and were used for detection of CN- and Cr2O7-. Trp-CD was found to be nontoxic and biocompatible and used as staining agent for Artemia and Bacteria (Bacillus Subtilis, Pseudomonas) and detection of CN- and Cr2O7-.

Preparation and dynamic color-changing study of fluorescent polymer nanoparticles for individualized and customized anti-counterfeiting application

Preparing new fluorescent materials for individualized and customized anti-counterfeiting applications to meet needs from the rapid development of e-commerce is of great significance. This paper reports the preparation of dynamic color-changing fluorescent polymer nanoparticles (PNPs) by constructing a fluorescence resonance energy transfer (FRET) pair between aggregation-induced emission (AIE) structures and photochromic structures. At first, methyl methacrylate (MMA) was used as the main monomer and tetraphenylethylene (TPE, a typical AIE structure) modified methacrylate (TPE-MA) and photochromic spiropyran (SP) modified methacrylate (SP-MA) as minor monomers were copolymerized to obtain the ternary copolymer PMMA-TPE-SP. Then, two types of PNPs based on this terpolymer was prepared via the reprecipitation method, with and without the addition of an amphiphilic polymer as the surfactant. The photophysical study shows that the fluorescence color of PNPs dynamically changes from blue to light violet and finally to red under UV light irradiation, a process that can be reversed under visible light. The PNPs were alternately irradiated with UV light and visible light for 10 cycles, which proved their good photoswitching reproducibility. The PNPs prepared with addition of surfactant were found to have stronger fluorescence and better stability. Finally, the photochromic fluorescent inks were prepared based on these PNPs. Several anti-counterfeiting scenarios and modes were designed, exhibiting excellent photochromic behavior on cellulose paper, even after 120 days of long-term storage. With simple equipment, desirable anti-counterfeiting effects with dynamic fluorescence color changing was achieved. This study demonstrated a promising hard-to-imitate anti-counterfeiting encryption strategy, which can achieve multiple outputs with simple operation and can be personalized and customized as needed.

Structural Determinants of Stimuli-Responsiveness in Amphiphilic Macromolecular Nano-assemblies

Stimuli-responsive nano-assemblies from amphiphilic macromolecules could undergo controlled structural transformations and generate diverse macroscopic phenomenon under stimuli. Due to the controllable responsiveness, they have been applied for broad material and biomedical applications, such as biologics delivery, sensing, imaging, and catalysis. Understanding the mechanisms of the assembly-disassembly processes and structural determinants behind the responsive properties is fundamentally important for designing the next generation of nano-assemblies with programmable responsiveness. In this review, we focus on structural determinants of assemblies from amphiphilic macromolecules and their macromolecular level alterations under stimuli, such as the disruption of hydrophilic-lipophilic balance (HLB), depolymerization, decrosslinking, and changes of molecular packing in assemblies, which eventually lead to a series of macroscopic phenomenon for practical purposes. Applications of stimuli-responsive nano-assemblies in delivery, sensing and imaging were also summarized based on their structural features. We expect this review could provide readers an overview of the structural considerations in the design and applications of nanoassemblies and incentivize more explorations in stimuli-responsive soft matters.

A Photoactivatable Luminescent Motif through Ring-Flipping Isomerization for Multiple Photopatterning

Photoactivatable luminescent materials have garnered enormous attention in the field of intelligent responsive materials, yet their design and applications remain challenging due to the limited variety of photoactivatable motifs. In the work described herein, we discovered a new photoactivatable luminescent motif that underwent ring-flipping isomerization under UV irradiation. The emission of this motif exhibited a rapid transformation from dark yellow to bright green, accompanied by a significant enhancement of quantum yield from 1.9% to 34.2%. Experimental and theoretical studies revealed that the effective intramolecular motion (EIM) was crucial to the distinct luminescence performance between two isomers. In addition, polymers containing this motif were achieved through a one-pot alkyne polymerization, exhibiting both photofluorochromic and photo-cross-linking properties. Furthermore, multiple types of photopatterning, including luminescent encryption, fluorescent grayscale imaging, and high-resolution photolithographic patterns, were realized. This work developed a new photoactivatable luminescent motif and demonstrated its potential applications in both small molecules and macromolecules, which will help in the future design of photoactivatable luminescent materials.

Illuminating the path: aggregation-induced emission for food contaminants detection

Food safety is a global concern that deeply affects human health. To ensure the profitability of the food industry and consumer safety, there is an urgent need to develop rapid, sensitive, accurate, and cost-effective detection methods for food contaminants. Recently, the Aggregation-Induced Emission (AIE) has been successfully used to detect food contaminants. AIEgens, fluorescent dyes that cause AIE, have several valuable properties including high quantum yields, photostability, and large Stokes shifts. This review provides a detailed introduction to the principles and advantages of AIE-triggered detection, followed by a focus on the past five years' applications of AIE in detecting various food contaminants including pesticides, veterinary drugs, mycotoxins, food additives, ions, pathogens, and biogenic amines. Each detection principle and component is comprehensively covered and explained. Moreover, the similarities and differences among different types of food contaminants are summarized, aiming to inspire future researchers. Finally, this review concludes with a discussion of the prospects for incorporating AIEgens more effectively into the detection of food contaminants.

Kinetic Analysis of the Photochemical Paths in Asymmetric Diarylethene Dimer

An asymmetric diarylethene dimer composed of 2- and 3-thienylethene units linked by m-phenylene developed various colors upon UV irradiation via an independent photochromic reaction on each unit. The change in contents and the other photoresponses of the photogenerated four isomers were analyzed using quantum yield for all the possible photochemical paths, i. e., photoisomerization, fluorescence, energy transfer, and the other non-radiative paths. Almost all the rate constants of photochemical paths were calculated using measurable quantum yields and lifetimes. It was found that a significant contribution on photoresponse was the competition between photoisomerization and intramolecular energy transfer. The clear difference was observed in the photoresponses of the dimer and the 1 : 1 mixture solution of the model compounds. The m-phenylene spacer appropriately regulated the rate of energy transfer in the asymmetric dimer, and the spacer enabled isolation of the excited state of the dimer, making the above quantitative analysis possible.

Light, pH, and Temperature Triple-Responsive Magnetic Composites for Highly Efficient Phosphopeptide Enrichment

Smart materials can dynamically and reversibly change their structures and functions in response to external stimuli. In this study, we designed a smart magnetic composite (MNP-pSPA-b-pNIPAm) with a triple response to ultraviolet (UV) light, pH, and temperature. Relying on the response of spiropyranyl acrylate (SPA) and N-isopropylacrylamide (NIPAm) to external stimuli (light, pH, and temperature), MNP-pSPA-b-pNIPAm was used for the controlled capture and release of phosphopeptides. The established phosphopeptide enrichment platform exhibits high sensitivity (detection limit of 0.04 fmol), high selectivity (BSA/β-casein, 1000:1), and good reusability (6 cycles). In addition, the method was also applied to the enrichment of phosphopeptides in real samples (skim milk, human saliva, and serum), demonstrating the feasibility of this method for phosphoproteomic analysis. After enriching from human nonsmall cell lung cancer cell (A549) lysates with MNP-pSPA-b-pNIPAm, 2595 phosphopeptides corresponding to 2281 phosphoproteins were identified. The novel responsive enrichment probe is highly specific for phosphoproteomic analysis and provides an effective method for studying the significance of protein phosphorylation in complex biological samples.

Glutathione-Responsive Tannic Acid-Assisted FRET Nanomedicine for Cancer Therapy

In cancer combination therapy, a multimodal delivery vector is used to improve the bioavailability of multiple anti-cancer hydrophobic drugs. Further, targeted delivery of therapeutics along with simultaneous monitoring of the drug release at the tumor site without normal organ toxicity is an emerging and effective strategy for cancer treatment. However, the lack of a smart nano-delivery system limits the application of this therapeutic strategy. To overcome this issue, a PEGylated dual drug, conjugated amphiphilic polymer (CPT-S-S-PEG-CUR), has been successfully synthesized by conjugating two hydrophobic fluorescent anti-cancer drugs, curcumin (CUR) and camptothecin (CPT), through an ester and a redox-sensitive disulfide (-S-S-) linkage, respectively, with a PEG chain via in situ two-step reactions. CPT-S-S-PEG-CUR is spontaneously self-assembled in the presence of tannic acid (TA, a physical crosslinker) into anionic, comparatively smaller-sized (~100 nm), stable nano-assemblies in water in comparison to only polymer due to stronger H-bond formation between polymer and TA. Further, due to the spectral overlap between CPT and CUR and a stable, smaller nano-assembly formation by the pro-drug polymer in water in presence of TA, a successful Fluorescence Resonance Energy Transfer (FRET) signal was generated between the conjugated CPT (FRET donor) and conjugated CUR (FRET acceptor). Interestingly, these stable nano-assemblies showed a preferential breakdown and release of CPT in a tumor-relevant redox environment (in the presence of 50 mM glutathione), leading to the disappearance of the FRET signal. These nano-assemblies exhibited a successful cellular uptake by the cancer cells and an enhanced antiproliferative effect in comparison to the individual drugs in cancer cells (AsPC1 and SW480). Such promising in vitro results with a novel redox-responsive, dual-drug conjugated, FRET pair-based nanosized multimodal delivery vector can be highly useful as an advanced theranostic system towards effective cancer treatment.

Heat-Resistant and Color-Changing Luminescent Polysulfone for Information Encryption and Fire Alarming

An intrinsic difficulty with thermally responsive photoluminescent materials is that high temperatures usually destroy luminance due to the notorious thermal quenching effect. Limited by the vulnerable chemical structure and soft skeleton, most of the existing photoluminescent responsive materials fail to indicate or work at a surging temperature over 100 °C, thus limiting application in display and alarming in harsh conditions. Herein, enlightened by chameleon's adaptive nature to external stimulus, we introduce a topologically optimized electron donor-acceptor (DA) structure and supramolecular interactions of lanthanide ions into the polymer backbone. The emission color determined by the DA structure is stable at high temperatures, and metal-ligand interaction phosphorescence is temperature-adjustable. Owing to the excellent reproducibility and heat resistance of composite films, the sensors can be bent into different three-dimensional structures and adhered to metal surfaces as flexible thermometers with superior display resolution. The polymer composite film could be directly applied as a photoluminescent quick response (QR) code, with patterns simultaneously variable to a temperature from 30 to 150 °C free of manual operation. More importantly, the polymeric composite could be in-situ-oxidized to a "sulfone" structure with an enhanced glass transition temperature of 297-304 °C. The heat- and flame-resistant characteristics of the oxidized films give rise to the application of fire alarming devices since it can locate the fire source and respond exactly depending on the distance from the fire. The unique display, encryption, and alarming functions of the polymeric composite studied in this work bring forward a new concept of developing a great information security and disaster monitoring system with the application of temperature-responsive materials.

Water-Soluble Cationic Eu3+-Metallopolymer with High Quantum Yield and Sensitivity for Intracellular Temperature Sensing

Lanthanide-based (Ln³⁺) luminescent materials are ideal candidates for use in fluorescence intracellular temperature sensing. However, it remains a great challenge to obtain a Ln³⁺-ratiometric fluorescence thermometer with high sensitivity and quantum yield in an aqueous environment. Herein, a cationic Eu³⁺-metallopolymer was synthesized via the coordination of Eu(TTA)₃·2H₂O with an AIE active amphipathic polymer backbone that contains APTMA ((3-acrylamidopropyl) trimethylammonium) and NIPAM (N-isopropylacrylamide) units, which can self-assemble into nanoparticles in water solution with APTMA and NIPAM as the hydrophilic shell. This polymer exhibited highly efficient dual-emissive white-light emission (Φ = 34.3%). Particularly, when the temperature rises, the NIPAM units will transform from hydrophilic to hydrophobic in the spherical core of the nanoparticle, while the VTPE units are moved from inside the nanoparticle to the shell, activating its nonradiative transition channel and thereby decreasing its energy transfer to Eu³⁺ centers, endowing the Eu³⁺-metallopolymer with an extremely high temperature sensing sensitivity within the physiological temperature range. Finally, the real-time monitoring of the intracellular temperature variation is further conducted.

Controllable Aggregation-Induced Emission and Förster Resonance Energy Transfer Behaviors of Bistable [c2] Daisy Chain Rotaxanes for White-Light Emission and Temperature-Sensing Applications

Bistable [c2] daisy chain rotaxanes with respective extended and contracted forms of [c2]A and [c2]B containing a blue-emissive anthracene (AN) donor and orange-emissive indandione-carbazole (IC) acceptor were successfully synthesized via click reaction. Tunable-emission bistable [c2] daisy chain rotaxanes with fluorescence changes from blue to orange, including bright-white-light emissions, could be modulated by the aggregation-induced emission (AIE) characteristics and Förster resonance energy transfer (FRET) processes through altering water fractions and shuttling processes (i.e., acid/base controls). Accordingly, as a result of excellent fine-tuning AIE (at 60% water content of H₂O/THF) and FRET (with a compatible energy transfer of E_FRET = 33.2%) behaviors after the shuttling process (by adding base), the brightest white-light emission at CIE (0.31, 0.37) with a quantum yield of Φ = 15.64% was obtained in contracted [c2]B with good control of molecular shuttling to possess higher photoluminescence (PL) quantum yields and better energy transfer efficiencies (i.e., the manipulation of reduced PET and enhanced FRET processes) due to their intramolecular aggregations of blue AN donors and orange IC acceptors with a proper water content of 60% H₂O. Furthermore, dynamic light-scattering (DLS) and time-resolved photoluminescence (TRPL) measurements, along with theoretical calculations, were utilized to investigate and confirm AIE and FRET phenomena of bistable [c2] daisy chain rotaxanes. Especially, both bistable [c2] daisy chain rotaxanes [c2]A and [c2]B and noninterlocked monomer M could be exploited for the applications of ratiometric fluorescence temperature sensing due to the temperature effects on the AIE and FRET features. Based on these desirable bistable [c2] daisy chain rotaxane structures, this work provides a potential strategy for the future applications of tunable multicolor emission and ratiometric fluorescence temperature-sensing materials.

Dual and Sequential Locked/Unlocked Photochromic Effects on FRET Controlled Singlet Oxygen Processes by Contracted/Extended Forms of Diarylethene-Based [1]Rotaxane Nanoparticles

Manipulations of singlet oxygen (¹ O₂ ) generations by the integration of both aggregation-induced emission luminogen (AIEgen) photosensitizer and photochromic moieties have diversified features in photodynamic therapy applications. Through Förster resonance energy transfer (FRET) pathway to induce red PL emissions (at 595 nm) for ¹ O₂ productions, [1]rotaxane containing photosensitive tetraphenylethylene (TPE) donor and photochromic diarylethene (DAE) acceptor is introduced to achieve dual and sequential locked/unlocked photoswitching effects by pH-controlled shuttling of its contracted/extended forms. Interestingly, the UV-enabled DAE ring closure speeds follow the reversed trend of DAE self-constraint degree as: contracted < extended < noninterlocked forms in [1]rotaxane analogues, thus FRET processes can be adjusted in contracted/extended forms of [1]rotaxane upon UV irradiations. Accordingly, the contracted form of [1]rotaxane is FRET-OFF locked and inert to UV exposure due to the larger bending conformation of DAE parallel (p-)conformer, compared with its extended and noninterlocked analogues possessing switchable FRET-OFF/ON behaviors activated by dual and sequential pH- and photoswitching. Owing to the advantages of ¹ O₂ productions tuned by multistimuli inputs (pH, UV, and blue light), an useful logic circuit for toxicity outputs of the surface modified [1]rotaxane nanoparticles (NPs) has been demonstrated to offer promising ¹ O₂ productions and managements based on mechanically interlocked molecules for future bioapplications.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.

Delicate and Independent Manipulation of Dynamic Fluorescence Behavior of Polymer Nanoparticles Based on a Core-Shell Strategy

Fluorescent polymer nanomaterials with dynamic fluorescence properties hold great potential in many advanced applications, including but not limited to information encryption, adaptive camouflage, and biosensors. The key to improving the application value of materials is to establish an accurate control strategy for dynamic fluorescence behavior. Herein, we develop a core-shell engineering strategy to precisely and independently manipulate the dynamic fluorescence behavior through the shell polymeric matrix. The core-shell fluorescent polymer nanoparticles (CS-FPNPs) are constructed through a sequential process of miniemulsion polymerization and seeded emulsion polymerization. Taking advantage of the core-shell structure, the rigid core matrix ensures the strong initial emission of AIE units, while the photoisomerization behavior of spiropyrane (SP) units is delicately and independently regulated by the rigidness of the shell matrix. Thereby, CS-FPNPs exhibit bright time-dependent reversible dynamic fluorescence behavior under alternating UV/vis irradiation. Benefited from the excellent processability and film formation ability, we have successfully applied CS-FPNPs to dynamic decorative painting, warning labels, and dynamic QR code security. Impressively, the fluorescence manipulation strategy based on core-shell engineering allows the independent regulation of specific luminescent units in complicated emission systems to accurately embody designed emission behavior.

Reversible Three-Color Fluorescence Switching of an Organic Molecule in the Solid State via "Pump-Trigger" Optical Manipulation

In photoswitches that undergo fluorescence switching upon ultraviolet irradiation, photoluminescence and photoisomerization often occur simultaneously, leading to unstable fluorescence properties. Here, we successfully demonstrated reversible solid-state triple fluorescence switching through "Pump-Trigger" multiphoton manipulation. A novel fluorescence photoswitch, BOSA-SP, achieved green, yellow, and red fluorescence under excitation by pump light and isomerization induced by trigger light. The energy ranges of photoexcitation and photoisomerization did not overlap, enabling appropriate selection of the multiphoton light for "pump" and "trigger" photoswitching, respectively. Additionally, the large free volume of the spiropyran (SP) moiety in the solid state promoted reversible photoisomerization. Switching between "pump" and "trigger" light is useful for three-color tunable switching cell imaging, which can be exploited in programmable fluorescence switching. Furthermore, we exploited reversible dual-fluorescence switching in a single molecular system to successfully achieve two-color super-resolution imaging.

D-π-A structure fluorophore: NIR emission, response to viscosity, detection cyanide and bioimaging of lipid droplets

Intracellular viscosity, an important microenvironment factor, is closely involved in various cell processes as well as diseases. On the other hand, cyanide is one of the most hazardous chemicals for human health and environments. However, a NIR fluorescent probe for both response to viscosity and detection of cyanide remains vacant. Herein, we reported a D-π-A structure fluorophore (named CTR) which exhibited NIR emission and fluorescent enhancement response to viscosity via the molecular rotor strategy. Furthermore, CTR displayed fluorescent and colorimetric response to cyanide. Notably, test strips stained with CTR were fabricated, which could serve as an efficient and suitable cyanide test kit. Moreover, CTR could selectively accumulate in lipid droplets and visualize the metabolism of lipid droplets in live cells. These findings would provide new avenue to design fluorescent probe for effective response to viscosity, detection of cyanide, and bioimaging of lipid droplets in live cells.

AIE-active macromolecules: designs, performances, and applications

Aggregation-induced emission (AIE) macromolecules as emerging luminescent materials gained increasing attention owing to their good processability, high brightness, wide functionality, and smart responsiveness, with great potential in many fields.

Recent Advances in Aggregation-Induced Emission Active Materials for Sensing of Biologically Important Molecules and Drug Delivery System

The emergence and development of aggregation induced emission (AIE) have attracted worldwide attention due to its unique photophysical phenomenon and for removing the obstacle of aggregation-caused quenching (ACQ) which is the most detrimental process thereby making AIE an important and promising aspect in various fields of fluorescent material, sensing, bioimaging, optoelectronics, drug delivery system, and theranostics. In this review, we have discussed insights and explored recent advances that are being made in AIE active materials and their application in sensing, biological cell imaging, and drug delivery systems, and, furthermore, we explored AIE active fluorescent material as a building block in supramolecular chemistry. Herein, we focus on various AIE active molecules such as tetraphenylethylene, AIE-active polymer, quantum dots, AIE active metal-organic framework and triphenylamine, not only in terms of their synthetic routes but also we outline their applications. Finally, we summarize our view of the construction and application of AIE-active molecules, which thus inspiring young researchers to explore new ideas, innovations, and develop the field of supramolecular chemistry in years to come.

Multi-stimuli programmable FRET based RGB absorbing antennae towards ratiometric temperature, pH and multiple metal ion sensing

A red-green-blue (RGB) multichromophoric antenna 1 consisting of energy donors naphthalimides and perylenediimides and a central aza-BODIPY energy acceptor along with two subchromophoric red-blue (RB 6) and green-blue (GB 12) antennae was designed that showed efficient cascade Förster resonance energy transfer (FRET). RGB antenna 1 showed pronounced temperature-dependent emission behaviour where emission intensities in green and red channels could be tuned in opposite directions by temperature giving rise to unique ratiometric sensing with a temperature sensitivity of 0.4% °C. RGB antenna 1 showed reversible absorption modulation selectively in the blue region (RGB ↔ RG) upon acid/base addition giving rise to pH sensing behaviour. Furthermore, RGB antenna 1 was utilized to selectively sense metal ions such as Co2+ and Fe3+ through a FRET turn-off mechanism induced by a redox process at the aza-BODIPY site that resulted in the selective spectral modulation of the red band (i.e., RGB → GB). Model antenna RB 6 showed white light emission with chromaticity coordinates (0.32, 0.33) on acid addition. Antennae 1, 6 and 12 also exhibited solution state electrochromic switching characterized by distinct colour changes upon changing the potential. Finally, antennae 1, 6 and 12 served as reversible fluorescent inks in PMMA/antenna blends whereby the emission colours could be switched or tuned using different stimuli such as acid vapour, temperature and metal ions.

Aggregation-Induced Emission Luminogens with Photoresponsive Behaviors for Biomedical Applications

Fluorescent biomedical materials can visualize subcellular structures and therapy processes in vivo. The aggregation-induced emission (AIE) phenomenon helps suppress the quenching effect in the aggregated state suffered by conventional fluorescent materials, thereby contributing to design strategies for fluorescent biomedical materials. Photoresponsive biomedical materials have attracted attention because of the inherent advantages of light; i.e., remote control, high spatial and temporal resolution, and environmentally friendly characteristics, and their combination with AIE facilitates development of fluorescent molecules with efficient photochemical reactions upon light irradiation. In this review, organic compounds with AIE features for biomedical applications and design strategies for photoresponsive AIE luminogens (AIEgens) are first summarized briefly. Applications are then reviewed, with the employment of photoresponsive and AIE-active molecules for photoactivation imaging, super-resolution imaging, light-induced drug delivery, photodynamic therapy with photochromic behavior, and bacterial targeting and killing being discussed at length. Finally, the future outlook for AIEgens is considered with the aim of stimulating innovative work for further development of this field.

Core-Shell Fluorescent Polymeric Particles with Tunable White Light Emission Based on Aggregation Microenvironment Manipulation

White-light emitting polymers (WLEPs) based on aggregation microenvironment-sensitive aggregation-induced emission (AIE) and Förster resonance energy transfer (FRET) have aroused great interest in lighting and optoelectronic devices. Herein, we developed a novel strategy to construct WLEP particles via a stepwise self-stabilized precipitation polymerization of two emission-complementary AIEgens under core-shell engineering, where the AIE characteristics and FRET process of core-shell fluorescent polymeric particles (CS-FPPs) could be modulated by altering aggregation microenvironment under swelling and shrinking of polymers, facilitating the tunable white light emission of CS-FPPs. Furthermore, such tuning could be fast realized in the solid state, thus demonstrating the potential in anti-counterfeiting. This work proved the significance of aggregation microenvironment on emission of luminogens, guiding the development of high-efficiency emission-tunable materials.

Preparation of highly colloidal stable Yolk-Shell nanocomposite and its multi-stimuli responsive based on surface aggregation-induced emission (S-AIE)

Multi-stimuli responsive fluorescence probe could pave the way for monitoring more complex environmental changes. Here we prepared multifunctional nanoparticle Fe₃O₄@SiO₂@P(DMAEMA-co-TPEE), which displayed yolk-shell morphology with well-defined polymer brush. With superparamagnetic Fe₃O₄ component and pH/temperature dual sensitive PDMAEMA polymer brush, the as prepared nanoparticles (YS-NPs) exhibited as multi-stimuli responsive fluorescence probe for real-time visual monitoring of environmental changes such as magnetic field, temperature and pH. Such YS-NPs could also be applied as a sensitive detector for CO₂ in aqueous solution. Notably, the solution of YS-NPs showed high colloidal stability during the environmental changes, and surface aggregation-induced emission (S-AIE) was proposed for the aggregation of TPE residue on the surface of YS-NPs.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Dynamic Photochromic Polymer Nanoparticles Based on Matrix-Dependent Förster Resonance Energy Transfer and Aggregation-Induced Emission Properties

Dynamic color-tunable fluorescent materials are sought-after materials in many applications. Here, we report a polymeric matrix-regulated fluorescence strategy via synergistically modulating aggregation-induced emission (AIE) properties and the Förster resonance energy transfer (FRET) process, which leads to tunable dynamic variation of color and photoluminescence (PL) intensity of fluorescent polymeric nanoparticles (FRET-PNPs) driven by photoirradiation. The FRET-PNPs were prepared via a facile one-pot miniemulsion copolymerization with the tetraphenyletheyl (TPE) and spiropyran (SP) units chemically bonded to the polymer matrix. The FRET-PNPs exhibited dynamic variation of fluorescence properties (colors and PL intensity) under photoirradiation on the timescale of minutes. The variation of the polymer matrix composition could deliberately influence the AIE property of TPE units and the isomerization process of SP to merocyanine units, which further affect the FRET efficiency of FRET-PNPs and, eventually, lead to versatile dynamic fluorescence variation. The dynamic fluorescence property as well as the excellent processability and film formation ability of FRET-PNPs allowed for diverse applications, such as warning labels, dynamic decorative painting, and multiple information encryption. Without sophisticated molecular design or tedious preparation processes, a new perspective for the design, fabrication, and performance optimization of fluorescent nanomaterials for innovative applications was proposed.

Two cyanoethylene-based fluorescence probes for highly efficient cyanide detection and practical applications in drinking water and living cells

Cyanide detection methods are urgently needed due to the highly lethal to human beings. Herein, we report two fluorescence probes (Probe 1 and Probe 2) based on cyanoethylene group for cyanide anion (CN^-) detection. The selective recognition for CN^- was confirmed by the completely opposite green fluorescence of Probe 1 and red fluorescence of Probe 2 observed by fluorescence spectra and naked eyes. The probes take advantages of the large Stokes shift (~160 nm), rapid response (30 s), anti-interference performance and low detection limit (Probe 1: 12.4 nM, Probe 2: 101 nM). The sensing mechanism is certificated to the nucleophilic attack of CN^- to electron-deficient cyanoethylene group of probes, which was demonstrated by ¹H NMR titration, HR-MS, Job's plot and IR spectroscopy. Density functional theory (DFT) calculations were carried out to analyze the mechanism in theory. Further, practical applications were studied. Easy-to-use test strips treated with Probe 1 or Probe 2 are capable of CN^- detection in pure drinking water. The good biocompatibility and membrane penetrability have achieved the bioimaging capability of Probe 1 and Probe 2 in living HepG-2 cells, making the probes promising for use in real lives.

Rational design of a HA-AuNPs@AIED nanoassembly for activatable fluorescence detection of HAase and imaging in tumor cells

Aggregation induced emission (AIE) dots have gained broad attention in fluorescence bioimaging and biosensors in virtue of their distinctive optical properties of splendid biocompatibility, high brightness and good photostability. However, the application of AIE dots in sensing and imaging of enzymes in cells remains at an early stage and needs to be further explored. In this report, we proposed a novel AIE-dot-based nanoprobe for hyaluronidase (HAase) detection using a simple electrostatic self-assembly of AIE dots with gold nanoparticles functionalized using hyaluronic acid (HA-AuNPs), named HA-AuNPs@AIEDs. The fluorescence of AIE dots can be obviously quenched by HA-AuNPs via fluorescence resonance energy transfer (FRET). HAase could degrade HA into small pieces and thus induce disassembly of AuNPs and AIEDs, accompanied by fluorescence recovery of AIEDs. The as-prepared nanoprobe exhibited high sensitivity, excellent selectivity, wide response range and desirable anti-interference for quantitative sensing of HAase in vitro. The detection limit was down to 0.0072 U mL^-1. Moreover, the nanoprobe displayed good biocompatibility and excellent photostability, and thus offered a practicable "turn-on" strategy for specific, high-contrast fluorescence imaging of HAase in live tumor cells. The AIE-based nanoprobe may provide a novel universal platform for recognition and imaging of HAase in tumors, and may be beneficial for related biological research.

FRET processes of bi-fluorophoric sensor material containing tetraphenylethylene donor and optical-switchable merocyanine acceptor for lead ion (Pb2+) detection in semi-aqueous media

A novel aggregation-induced emission (AIE) structure containing a tetraphenylethene (TPE) unit covalently linked with a merocyanine (MC) unit was synthesized and investigated in semi-aqueous solutions with 90% water fraction. The open-form structure of red-emissive MC unit combined with TPE unit was utilized as a bi-fluorophoric sensor to detect lead(II) ion, which could be transformed from the close-form structure of non-emissive SP unit upon UV exposure. Moreover, the TPE unit as an energy donor with the blue-green photoluminescence (PL) emission at 480 nm was combined with the MC unit as an energy acceptor with the red PL emission at 635 nm. Due to the Förster resonance energy transfer (FRET) processes, the bi-fluorophoric sensor produced more efficient ratiometric PL behavior to induce a stronger red PL emission than that of the mono-fluorophoric MC unit. Hence, the PL sensor responses of the AIE bi-fluorophoric structure toward lead(II) ion could be further amplified via the FRET-OFF processes to turn off red PL emission of the coordinated MC acceptor and to recover blue-green PL emission of the TPE donor. Accordingly, the best LOD value for the AIE sensor detection toward Pb2+ was 0.27 μM. The highest red MC emission with the optimum FRET process of AIE sensor could be utilized in cell viability tests to prove the non-toxic and remarkable bio-marker of AIE sensor to detect lead(II) ion in live cells. The developed FRET-OFF processes with ratiometric PL behavior of the bi-fluorophoric AIE sensor can be utilized for future chemo- and bio-sensor applications.

Spiropyran based recognitions of amines: UV-Vis spectra and mechanisms

As one of the important photochromic molecules, spiropyran (SP) compounds are widely used as detectors and fluorescence probes in the environment and bio-imaging field. Although great achievements have been attained for various sophisticated spiropyrans in metal ion sensing, less success is achieved in sensing organic molecules due to the weak interaction between the spiropyran and the target of the organic molecule. In this study, a spiropyran derivative containing a hydroxyl group (SPOH) was employed for the recognition of four kinds of amines via ultraviolet-visible (UV-Vis) spectra. The aliphatic primary amines, aromatic primary amines, aliphatic secondary and tertiary amines, aromatic secondary and tertiary amines were successfully distinguished according to the shapes and trends of their UV-Vis absorption spectra. The chemical reaction between aliphatic, aromatic primary amines and SPOH as well as alkalinity are two vital interaction mechanisms for the recognition process which are testified by Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR). Although SP is generally water-insoluble, it is easy to achieve soluble by fixing SPOH inside micelle or vesicle and thus the results in this study is meaningful for amine recognition utility in environments and biological systems.

AIE-based fluorescent sensors for low concentration toxic ion detection in water

Ions, including anions and heavy metals, are extremely toxic and easily accumulate in the human body, threatening the health of humans and even causing human death at low concentrations. It is therefore necessary to detect these toxic ions in low concentrations in water. Fluorescent sensing is a good method for detecting these ions, but some conventional dyes often exhibit an aggregation caused quench (ACQ) effect in their solid state, limiting their large-scale application. Fluorescent probes based on aggregation-induced emission (AIE) properties have received significant attention due to their high fluorescence quantum yields in their nano aggragated states, easy fabrication, use of moderate conditions, and selevtive recognization of organic/inorganic compounds in water with obvious changes in fluorescence. We surmarize the recent advances of AIE-based sensors for low concentration toxic ion detection in water. The detection probes can be divided into three categories: chemical reaction types, chemical interaction types and physical interaction types. Chemical reaction types utilize nucleophilic addition and coordination reaction, while chemical interaction types rely on hydrogen bonding and anion-π interactions. The physical interaction types are composed of electrostatic attractions. We finally comment on the challenges and outlook of AIE-active sensors.

Supramolecular AIE polymer-based rare earth metallogels for the selective detection and high efficiency removal of cyanide and perchlorate

Two novel supramolecular AIE polymer-based rare earth metallogels (PT-GEu and PT-GTb) have been rationally designed and synthesized for the efficient detection and removal of cyanide (CN⁻) and perchlorate (ClO₄⁻).

An AIE-active dual fluorescent switch with negative photochromism for information display and encryption

A negative photochromic molecular switch with AIE and two-color fluorescence conversion properties was synthesized.

One-step construction of a novel AIE probe based on diaminomaleonitrile and its application in double-detection of hypochlorites and formaldehyde gas

A novel and efficient probe with AIE property was designed and synthesized for application in double-detection of hypochlorites and formaldehyde gas.

Efficient FRET Approaches toward Copper(II) and Cyanide Detections via Host-Guest Interactions of Photo-Switchable [2]Pseudo-Rotaxane Polymers Containing Naphthalimide and Merocyanine Moieties

A supramolecular [2]pseudo-rotaxane containing a naphthalimide-based pillararene host and a spiropyran-based imidazole guest was synthesized and investigated in a semiaqueous solution with 90% water fraction. Upon UV exposure, the close-form structure of nonemissive spiropyran guest could be transformed into the open-form structure of red-emissive merocyanine guest reversibly, which was utilized as a monofluorophoric sensor to detect copper(II) and cyanide ions. Moreover, the naphthalimide host as an energy donor with green photoluminescence (PL) emission at 505 nm was complexed with the merocyanine guest as an energy acceptor with red PL emission at 650 nm in 1:1 molar ratio to generate a [2]pseudo-rotaxane polymer, which was further verified by the diffusion coefficients of DOSY nuclear magnetic resonance (NMR) measurements. Due to the Förster resonance energy transfer (FRET) processes, the bifluorophoric [2]pseudo-rotaxane produced more efficient ratiometric PL behavior to induce a stronger red PL emission than that of the monofluorophoric guest; therefore, the PL sensor responses of the supramolecular [2]pseudo-rotaxane toward copper(II) and cyanide ions could be further amplified via the FRET-OFF processes to turn off red PL emission of the reacted merocyanine acceptor and to recover green PL emission of the naphthalimide donor. Accordingly, the best and prominent values of the limit of detection (LOD) for the host-guest detections toward Cu²⁺ and CN^- were 0.53 and 1.34 μM, respectively. The highest red MC emission with the optimum FRET processes of [2]pseudo-rotaxane was maintained around room temperature (20-40 °C) in wide pH conditions (pH = 3-13), which can be utilized in the cell viability tests to prove the nontoxic and remarkable biomarker of [2]pseudo-rotaxane to detect Cu²⁺ and CN^- in living cells. The developed FRET-OFF processes with ratiometric PL behavior of the bifluorophoric supramolecular [2]pseudo-rotaxane polymer will open a new avenue to the future applications of chemo- and biosensors.

Highly Efficient Förster Resonance Energy Transfer Modulations of Dual-AIEgens between a Tetraphenylethylene Donor and a Merocyanine Acceptor in Photo-Switchable [2]Rotaxanes and Reversible Photo-Patterning Applications

A series of novel photo-switchable [2]rotaxanes (i.e., Rot-A-SP and Rot-B-SP before and after shuttling controlled by acid-base, respectively) containing one spiropyran (SP) unit (as a photochromic stopper) on the axle and two tetraphenylethylene (TPE) units on the macrocycle were synthesized via click reaction. Upon UV/visible light exposure, both mono-fluorophoric rotaxanes Rot-A-SP and Rot-B-SP with the closed form (i.e., non-emissive SP unit) could be transformed into the open form (i.e., red-emissive merocyanine (MC) unit) to acquire their respective bi-fluorophoric Rot-A-MC and Rot-B-MC reversibly. The aggregation-induced emission (AIE) properties of bi-fluorophoric TPE combined with MC AIEgens of these designed rotaxanes and mixtures in semi-aqueous solutions induced interesting ratiometric photoluminescence (PL) and Förster resonance energy transfer (FRET) behaviors, which were further investigated and verified by dynamic light scattering (DLS), X-ray diffraction (XRD), and time-resolved photoluminescence (TRPL) measurements along with theoretical studies. Accordingly, in contrast to the model axle (Axle-MC) and the analogous mixture (Mixture-MC, containing the axle and macrocycle components in a 1:1 molar ratio), more efficient FRET behaviors and stronger red PL emissions were obtained from dual-AIEgens between a blue-emissive TPE donor (PL emission at 468 nm) and a red-emissive MC acceptor (PL emission at 668 nm) in both novel photo-switchable [2]rotaxanes Rot-A-MC and Rot-B-MC under various external modulations, including water content, UV/Vis irradiation, pH value, and temperature. Furthermore, the reversible fluorescent photo-patterning applications of Rot-A-SP in a powder form and a solid film with excellent photochromic and fluorescent behaviors are first investigated in this report.

Synthesis of poly(N-isopropylacrylamide) polymer crosslinked with an AIE-active azonaphthol for thermoreversible fluorescence

A fluorescent polymer was synthesized using N-isopropylacrylamide (NIPAM) crosslinked with a divinylazonaphthol monomer via radical emulsion polymerization. Because the crosslinked polymer contained an aggregation-induced emissive (AIE) azonaphthol-based crosslinker, a thermoreversible sol-gel transformation and gelation-induced reversible fluorescence alteration were successfully attained in an aqueous medium. Like typical PNIPAM, the size and transmittance of the polymer dramatically decreased near the lower critical solution temperature (LCST, 36 °C). Such gelation facilitated aggregation of the polymer chains, resulting in the close contact between azonaphthol groups producing fluorescence. The crosslinked polymer exhibited changes in dual properties: one is related to PNIPAM structural alteration, which corresponds to conventional swelling/shrinkage behavior; and the other is involved in the reversible fluorescence change in response to the swelling/shrinkage. Because the major backbone of the polymer was composed of NIPAM with an LCST at 36 °C, the resultant polymer is expected to have potential applications in biologically related fields.