A General Method to Develop Highly Environmentally Sensitive Fluorescent Probes and AIEgens

Labeling selectivity of lipid droplets fluorescent probes: Twisted intramolecular charge transfer (TICT) vs intramolecular charge transfer (ICT)

Fluorescence imaging technology is a versatile and essential tool in the field of biomedical research. To obtain excellent imaging results, the precise labeling of fluorescent probes is an important prerequisite. Nevertheless, the labeling selectivity of most fluorescent probes is not satisfactory, new design concepts are desperately needed. In this context, two isomeric lipid droplets (LDs) fluorescent probes Lipi-Cz-1 and Lipi-Cz-2 have been sophisticatedly developed with TICT and ICT-emitting characteristic, respectively. The more environmentally sensitive TICT-emitting Lipi-Cz-1 exhibits a significantly enhanced labeling selectivity in LDs imaging compared to the ICT-emitting Lipi-Cz-2, sufficiently illustrating the effectiveness of TICT-emitting characteristic in improving labeling selectivity. Additionally, Lipi-Cz-1 displays high photostability and biocompatibility. These advantages enable Lipi-Cz-1 to be finely applied in multimode fluorescence imaging, e.g. time-lapse 3D confocal imaging to monitor changes of the number and size of LDs during starvation, two-photon 3D imaging to compare the variations of LDs in various liver tissues, and STED super-resolution imaging to visualize the nanoscale LDs with the resolution of 65 nm. Overall, these imaging findings validate the effectiveness of the new strategy for improving the labeling selectivity.

Aggregation Enhanced Thermally Activated Delayed Fluorescence through Spin-Orbit Coupling Regulation

Integrating aggregation-induced emission (AIE) into thermally activated delayed fluorescence (TADF) emitters holds great promise for the advancement of highly efficient organic light emitting diodes (OLEDs). Despite recent advancements, a thorough comprehension of the underlying mechanisms remains imperative for the practical application of such materials. In this work, we introduce a novel approach aimed at modulating the TADF process by manipulating dynamic processes in excited states through aggregation effect. Our findings reveal that aggregation not only enhances both prompt and delayed fluorescence simultaneously but also imposes constraints on molecular reorientation. This constraint reinforces spin-orbit coupling and reduces the energy gap between singlets and triplets. These insights deepen our understanding of the fundamental mechanisms governing the aggregation effect on TADF materials and provide valuable guidance for the design of high-efficiency photoluminescent materials.

Turn-on mode probe based on the sustainable xanthohumol extract for the efficient viscosity response in a liquid system

Viscosity is a typical physical parameter and plays an important role in nutrient transferring, diffusion process regulating and safety warning. Aberrant mitochondrial viscosity is closely associated with an imbalance in a liquid system. Nevertheless, there is currently a lack of convenient and efficient tools for the mutation of viscosity detection at the molecular level. Herein, a natural product xanthohumol (XTH) was extracted from Humulus lupulus and used to measure the microenvironmental viscosity. Due to the existence of carbonyl and phenolic hydroxyl groups, a typical twisted intramolecular charge transfer (TICT) was formed. The conjugated single and double bonds can be employed as the rotatable site. Consequently, a turn-on method based on viscosity response is developed. High sensitivity (x = 0.56) with a remarkable enhancement (55-fold) toward viscosity and a visualized fluorescent signal can be found. In addition, it displays a single selectivity with excellent photostability and pH stability in the complex liquid system. Using the extracted XTH, a typical application toward the liquid spoilage process was performed and a positive correlation was noted. Given the comprehensive properties of XTH, liquid safety inspection at a molecular level with natural source-extracted products can be obtained.

Literature Review on Conjugated Polymers as Light-Sensitive Materials for Photovoltaic and Light-Emitting Devices in Photonic Biomaterial Applications

Due to their extended p-orbital delocalization, conjugated polymers absorb light in the range of visible-NIR frequencies. We attempt to exploit this property to create materials that compete with inorganic semiconductors in photovoltaic and light-emitting materials. Beyond competing for applications in photonic devices, organic conjugated compounds, polymers, and small molecules have also been extended to biomedical applications like phototherapy and biodetection. Recent research on conjugated polymers has focused on bioapplications based on the absorbed light energy conversions in electric impulses, chemical energy, heat, and light emission. In this review, we describe the working principles of those photonic devices that have been applied and researched in the field of biomaterials.

Fluorescent Solvatochromic Probes for Long-Term Imaging of Lipid Order in Living Cells

High-resolution spatio-temporal monitoring of the cell membrane lipid order provides visual insights into the complex and sophisticated systems that control cellular physiological functions. Solvatochromic fluorescent probes are highly promising noninvasive visualization tools for identifying the ordering of the microenvironment of plasma membrane microdomains. However, conventional probes, although capable of structural analysis, lack the necessary long-term photostability required for live imaging at the cellular level. Here, an ultra-high-light-resistant solvatochromic fluorescence probe, 2-N,N-diethylamino-7-(4-methoxycarbonylphenyl)-9,9-dimethylfluorene (FπCM) is reported, which enables live lipid order imaging of cell division. This probe and its derivatives exhibit sufficient fluorescence wavelengths, brightness, polarity responsiveness, low phototoxicity, and remarkable photostability under physiological conditions compared to conventional solvatochromic probes. Therefore, these probes have the potential to overcome the limitations of fluorescence microscopy, particularly those associated with photobleaching. FπCM probes can serve as valuable tools for elucidating mechanisms of cellular processes at the bio-membrane level.

Fluorogenic Rhodamine Probes with Pyrrole Substitution Enables STED and Lifetime Imaging of Lysosomes in Live Cells

Fluorogenic dyes with high brightness, large turn-on ratios, excellent photostability, favorable specificity, low cytotoxicity, and high membrane permeability are essential for high-resolution fluorescence imaging in live cells. In this study, we endowed these desirable properties to a rhodamine derivative by simply replacing the N, N-diethyl group with a pyrrole substituent. The resulting dye, Rh-NH, exhibited doubled Stokes shifts (54 nm) and a red-shift of more than 50 nm in fluorescence spectra compared to Rhodamine B. Rh-NH preferentially exists in a non-emissive but highly permeable spirolactone form. Upon binding to lysosomes, the collective effects of low pH, low polarity, and high viscosity endow Rh-NH with significant fluorescence turn-on, making it a suitable candidate for wash-free, high-contrast lysosome tracking. Consequently, Rh-NH enabled us to successfully explore stimulated emission depletion (STED) super-resolution imaging of lysosome dynamics, as well as fluorescence lifetime imaging of lysosomes in live cells.

Ketocyanine-Based Fluorescent Probe Revealing the Polarity Heterogeneity of Lipid Droplets and Enabling Accurate Diagnosis of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has gradually become a pronoun for terrifying death owing to its high mortality rate. With the progression of HCC, lipid droplets (LDs) in HCC cells exhibit specific variations such as increased LDs number and decreased polarity, which can serve as the diagnostic target. However, developing an effective method to achieve HCC diagnosis and reveal LDs polarity heterogeneity is still a crucial challenge. Herein, the first high-performance LDs-targeting probe (1) is reported based on ketocyanine strategy with ultrasensitive polarity-responding ability and near-infrared emission. Probe 1 shows excellent sensitivity to polarity parameter Δf (0.027-0.290) with 808-fold fluorescence enhancement and the emission wavelength red-shifts 91 nm. In HCC cells, probe 1 shows a 2.5- to 5.9-fold fluorescence enhancement compared with normal and other cancer cells which exceeds clinical threshold of 2.0, indicating probe 1 can distinguish HCC cells. The LDs polarity heterogeneity is revealed and it displays a sequence, HCC cells < other cancer cells < normal cells, which may provide useful insight to engineer LDs-targeting probes for HCC cell discrimination. Finally, probe 1 realizes accurate HCC diagnosis on the cellular, organ, and in vivo levels, providing a satisfying tool for clinical HCC diagnosis and surgical navigation.

Ultrabright Acrylic Polymers with Tunable Fluorescence Enabled by Imprisoning Single TICT Probe

Bright and colorful fluorescent polymers are ideal materials for a variety of applications. Although polymers could be made fluorescent by physical doping or chemical binding of fluorescent units, it is a great challenge to get colorful and highly emissive polymers with a single fluorophore. Here the development of a general and facile method to synthesize ultrabright and colorful polymers using a single twisted intramolecular charge transfer (TICT) probe is reported. By incorporating polymerizable, highly fluorescent, and environmental sensitive TICT probe, a series of colorful acrylic polymers (emission from 481 to 543 nm) with almost 100% fluorescence quantum yields are prepared. Like the solvatochromic effect, functional groups within side chains of acrylic polymers (including alkyl chain, tetrahydrofurfuryl group, and hydroxyl group) provide varied environmental polarity for the incorporated fluorophore, resulting in a series of colorful polymeric materials. Benefiting from the excellent photophysical properties, the polymers show great potential in encryption, cultural relics protection, white light-emitting diode bulb making, and fingerprint identification.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Deciphering the photophysical properties of naphthalimide derivatives using ultrafast spectroscopy

Naphthalimide derivatives composed of donor-acceptor type structures hold significant promise across a wide range of applications. Here, the solvent polarity and viscosity controlled excited-state dynamics of a naphthalimide derivative with a donor-acceptor structure were studied using multiple spectroscopic techniques. From the stationary spectroscopic investigations, large Stokes shift and low fluorescence quantum yield were observed with increasing the solvent polarity, suggesting a more polar excited state relative to the ground state, which is evidenced by the Lippert-Mataga relationship. We also observe an enhanced fluorescence with a prolonged lifetime in a more viscous solution due to the restriction of excited-state molecular rearrangement. These observations result from the emerged twisted intramolecular charge transfer (TICT) state. The ultrafast spectroscopy studies further unravel a solvent polarity dependent excited state evolution from the intramolecular charge transfer state to the TICT state, revealing that the TICT state can be populated only in strong polar solvents. Control experiments by tuning the solvent viscosity in ultrafast experiments were employed to verify the excited state molecular rearrangement subsequently. These observations collectively emphasize how fine-tuning the photophysical properties of naphthalimide derivatives can be achieved through strategic manipulation of solvent polarity and viscosity.

A selenium-based NIR-II photosensitizer for a highly effective and safe phototherapy plan

High efficiency, stability, long emission wavelength (NIR-II), and good biocompatibility are crucial for photosensitizers in phototherapy. However, current Food and Drug Administration (FDA)-approved organic fluorophores exhibit poor chemical stability and photostability as well as short emission wavelength, limiting their clinical usage. To address this, we developed Se-IR1100, a novel organic photosensitizer with a photostable and thermostable benzobisthiadiazole (BBTD) backbone. By incorporating selenium as a heavy atom and constructing a D-A-D structure, Se-IR1100 exhibits a maximum fluorescence emission wavelength of 1100 nm. Compared with FDA-approved indocyanine green (ICG), DSPE-PEGylated Se-IR1100 nanoparticles exhibit prominent photostability and long-lasting photothermal effects. Upon 808 nm laser irradiation, Se-IR1100 NPs efficiently convert light energy into heat and reactive oxygen species (ROS), inducing cancer cell death in cellular studies and living organisms while maintaining biocompatibility. With salient photostability and a photothermal conversion rate of 55.37%, Se-IR1100 NPs hold promise as a superior photosensitizer for diagnostic and therapeutic agents in oncology. Overall, we have designed and optimized a multifunctional photosensitizer Se-IR1100 with good biocompatibility that performs NIR-II fluorescence imaging and phototherapy. This dual-strategy method may offer novel approaches for the development of multifunctional probes using dual-strategy or even multi-strategy methods in bioimaging, disease diagnosis, and therapy.

Exploring the Depth-Dependent Microviscosity inside a Micelle Using Butterfly-Motion-Based Fluorescent Probes

The viscosity distribution of micellar interiors from the very center to the outer surface is dramatically varied, which has been distinguished in theoretical models, yet it remains highly challenging to quantify this issue experimentally. Herein, a series of fluorophore-substituted surfactants DPAC-Fn (n = 3, 5, 7, 9, 11, 13, and 15) are developed by functionalizing the different alkyl-trimethylammonium bromides with the butterfly motion-based viscosity sensor, N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC). The immersion depth of DPAC units of DPAC-Fn in cetrimonium bromide (C16TAB) micelles depends on the alkyl chain lengths n. From deep (n = 15) to shallow (n = 3), DPAC-Fn in C16TAB micelles exhibits efficient viscosity-sensitive dynamic multicolor emissions. With external standards for quantification, the viscosity distribution inside a C16TAB micelle with the size of ∼4 nm is changed seriously from high viscosity (∼190 Pa s) in the core center to low viscosity (∼1 Pa s) near the outer surface. This work provides a tailored approach for powerful micelle tools to explore the depth-dependent microviscosity of micellar interiors.

Uncovering the substituted-position effect on excited-state evolution of benzophenone-phenothiazine dyads

Photofunctional materials based on donor-acceptor molecules have drawn intense attention due to their unique optical properties. Importantly, Systematic investigation of substitution effects on excited-state charge transfer dynamics of donor-acceptor molecules is a powerful approach for identifying application-relevant design principles. Here, by coupling phenothiazine (PTZ) at the ortho-, meta-, and para-positions of the benzene ring of benzophenone (BP), three regioisomeric BP-PTZ dyads were designed to understand the relationship between substituted positions and excited-state evolution channels. Ultrafast transient absorption is used to detect and trace the transient species and related evolution channels of BP-PTZ dyads at excited state. In a non-polar solvent, BP-o-PTZ undergoes the through-space charge transfer process to produce a singlet charge-transfer (1CT) state, which subsequently proceeds the intersystem crossing process and transforms into a triplet charge-transfer (3CT) state; BP-m-PTZ experiences intramolecular charge transfer (ICT) process to generate the 1CT state, which subsequently transforms into the 3CT state by the intersystem crossing (ISC) and finally converts into the local-excited triplet (3LE) state; as for BP-p-PTZ, only 3LE states can be detected after the ISC process from the 1CT state. On the other hand, the twisted ICT states are generated via twisted motion between the donor and acceptor for all BP-PTZ dyads or planarization of the PTZ unit in high polar solvents. The excited-state theoretical calculations unveil that the features of ICT and intramolecular interaction between the three dyads play a decisive role in determining the through-bond charge transfer and through-space charge transfer processes. Also, these results demonstrate that the excited-state evolution channels of PTZ derivatives could be modified by tuning the substituted positions of the donor-acceptor dyads. This study provides a deep perspective for the substitute-position effect on donor-acceptor-type PTZ derivatives.

Azacrown-calixpyrrole isosteres: receptors and sensors for anions

Calix[4]pyrroles (CPs) and polyammonium azacrowns (ACs) are well-known receptors for anions. CPs bind anions by directional hydrogen bonds that do not always work well for aqueous analytes. The positive charge in polyammonium ACs allows for a stronger but non-directional anion-ammonium electrostatic attraction but lack selectivity. Bridging the gap between CPs and ACs could increase affinity and potentially preserve the selectivity of anion binding. We have synthesized a flexible calixpyrrole-azacrown near isosteric receptor and incorporated an environmentally sensitive dansyl fluorophore to enable fluorescence measurements. Anion binding was evaluated using NMR and fluorescence titrations. The isosteric receptor shows a strong affinity for aqueous phosphates and phosphonates (Na⁺ salts) in the order HAsO₄^2- > H₂PO₄^- > H₂P₂O₇^2- > glyphosate^2- > AMP^- > methylphosphonate^- ≫ ADP^2- or ATP^3- but does not bind halides. This is in stark contrast to CP which shows a strong preference for halides over oxyanions. The anion binding by the new receptor was accompanied by analyte-specific changes in fluorescence intensity and spectral width and by a wavelength shift. These parameters were used in qualitative and quantitative sensing of aqueous anions. By applying machine-learning algorithms, such as linear discriminant analysis and support vector machine linear regression, this one sensor can differentiate between 10 different analytes and accurately quantify herbicide glyphosate and methylphosphonate, a product of sarin, soman or cyclosarin hydrolysis. In fact, glyphosate can be quantified even in the presence of competing anions such as orthophosphate (LODs were ≤ 1 μM).

Mitochondria-Targeting Multifunctional Fluorescent Probe toward Polarity, Viscosity, and ONOO- and Cell Imaging

Abnormal changes occurring in the mitochondrial microenvironment are important markers indicating mitochondrial and cell dysfunction. Herein, we designed and synthesized a multifunctional fluorescent probe DPB that responds to polarity, viscosity, and peroxynitrite (ONOO^-). DPB is composed of an electron donor (diethylamine group) and electron acceptor (coumarin, pyridine cations, and phenylboronic acid esters), in which the pyridine group with a positive charge is responsible for targeting to mitochondria. D-π-A structure with strong intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) properties give rise to respond to polarity and viscosity. The introduction of cyanogroup and phenylboronic acid esters increases the electrophilicity of the probe, which is prone to oxidation triggered by ONOO^-. The integrated architecture satisfies the multiple response requirements. As the polarity increases, the fluorescence intensity of probe DPB at 470 nm is quenched by 97%. At 658 nm, the fluorescence intensity of DPB increases with viscosity and decreases with the concentration of ONOO^-. Furthermore, the probe is not only successfully used to monitor mitochondrial polarity, viscosity, and endogenous/exogenous ONOO^- level fluctuations but also to distinguish cancer cells from normal cells by multiple parameters. Therefore, as-prepared probe provides a reliable tool for better understanding of the mitochondrial microenvironment and also a potential approach for the diagnosis of disease.

A light-up fluorescence probe for wash-free analysis of Mu-opioid receptor and ligand-binding events

With the aggravated burden of opioid use disorder spreading worldwide, demands for new forms of opioid receptor agonist/antagonist constitute immense research interest. The Mu-opioid receptor (MOR) is currently in the spotlight on account of its general involvement in opioid-induced antinociception, tolerance and dependence. MOR binding assay, however, is often complicated by difficulty in MOR separation and purification, as well as the tedious procedure in standard biolayer interferometry and surface plasmon resonance measurements. To this end, we present TPE2N as a light-up fluorescent probe for MOR, which exhibits satisfactory performance in both live cells and lysates. TPE2N was elaborately designed based on the synergistic effect of twisted intramolecular charge-transfer and aggregation-induced emission by incorporating a tetraphenylethene unit to emit strong fluorescence in a restrained environment upon binding with MOR through the naloxone pharmacore. The developed assay enabled high-throughput screening of a compound library, and successfully identified three ligands as lead compounds for further development.

A TD-DFT study of a class of D-π-A fluorescent probes for detection of typical oxidants

A deep understanding of the fluorescence response mechanisms is the foundation for design-oriented strategies for D-π-A probes for trace hazardous chemicals. Here, from the perspective of electronegativity regulation of the π-bridge recognition site, an electron-donation modulation strategy involving various comprehensive evaluations of the optical and chemical properties is proposed through a series of theoretical analyses. Due to the preferential combined interaction between the π-bridge recognition site and MnO₄^-, high electrophilic reactivity and feasible chemical reaction energy barrier, a high-performance filter paper chip and hydrogel chip for the detection of aqueous and air-suspended environmental KMnO₄ was achieved. We expect the present modulation strategy will facilitate efficient fluorescent probe design and provide a universal methodology for the exploration of functional D-π-A molecules.

A General Twisted Intramolecular Charge Transfer Triggering Strategy by Protonation for Zero-Background Fluorescent Turn-On Sensing

The exploration of organic fluorescent sensing materials and mechanisms is of great significance, especially for the deep understanding of twisted intramolecular charge transfer (TICT). Here, the electron-donating ability of a chemically protonated amino group and the corresponding excitation primarily ensure the occurrence of excited-state intramolecular proton transfer. Due to the hybridization of the amino group from sp³ to sp², the steric hindrance effect and conjugative effect together boost the rotation efficiency of the TICT process and the complete elimination of the background fluorescent signal. Furthermore, a sharp turn-on fluorescent detection of trace nitrite particulate with a diameter of 0.44 μm was realized. In addition, this protonation-induced change in the amino group configuration was verified through around nine categories of compounds. We expect this modulation of the photochemical activity path of the TICT process would greatly facilitate the exploration of novel fluorescent sensing mechanisms.

Overcoming Spectral Dependence: A General Strategy for Developing Far-Red and Near-Infrared Ultra-Fluorogenic Tetrazine Bioorthogonal Probes

Bioorthogonal fluorogenic dyes are indispensable tools in wash-free bioimaging of specific biological targets. However, the fluorogenicity of existing tetrazine-based bioorthogonal probes deteriorates as the emission wavelength shifts towards the NIR window, greatly limiting their applications in live cells and tissues. Herein, we report a generalizable molecular design strategy to construct ultra-fluorogenic dyes via a simple substitution at the meso-positions of various far-red and NIR fluorophores. Our probes demonstrate significant fluorescence turn-on ratios (10² -10³ -fold) in the range 586-806 nm. These results will greatly expand the applications of bioorthogonal chemistry in NIR bioimaging and biosensing.

A dual-targeting fluorescent probe for simultaneous and discriminative visualization of lipid droplets and endoplasmic reticulum

A single fluorescent probe (SF-probe) that can simultaneously and discriminatively visualize two organelles is a powerful tool to investigate their interaction in cellular processes.

Viscosity-sensitive NIR probe for in vivo imaging of early-stage hepatic fibrosis

A viscosity-sensitive and liver-targeted NIR fluorescent probe has been developed for early diagnosis of hepatic fibrosis.