Revealing the Viscosity Changes in Lipid Droplets during Ferroptosis by the Real-Time and In Situ Near-Infrared Imaging

Ferroptosis is characterized by the massive lipid peroxidation, and recently has been demonstrated to be closely associated with lipid droplets (LDs). However, the changes of LDs viscosity during ferroptosis are still unrevealed. Herein, we present the changes of the LDs viscosity during ferroptosis by a novel viscosity-sensitive near-infrared (NIR) fluorescent probe (BDHT). Probe BDHT (2-(benzo[d]thiazol-2-yl)-7-(4-(dimethylamino)phenyl)hepta-2,4,6-trienenitrile, C₂₂H₁₉N₃S) showed highly sensitive and selective response to viscosity, mainly distributed in cellular LDs. By means of the real-time and in situ NIR imaging, we discovered that the LDs viscosity showed an obvious increase in HeLa cells during the erastin-induced ferroptosis process, while it displayed nearly no change when the cells were simultaneously treated with ferrostatin-1, which is a common inhibitor of ferroptosis. It is also confirmed that the LDs viscosity increased in several types of the cancer cells of erastin-induced and RSL3-induced ferroptosis. We expect that this new NIR probe could provide an effective approach to rapidly monitor ferroptosis, and these findings could greatly promote the in-depth understanding of the biological effects of LDs during ferroptosis.

An Enumerated Outlook of Intracellular Micropolarity Using Solvatochromic Organic Fluorescent Probes

The spatiotemporal distribution of intracellular physical parameters of a live cell is heterogeneous and complex. Measuring physical properties inside given cellular compartments (organelles) is challenging and important for therapy and diagnostics. The tiny volume of a single cell and even tinier organelles are not accessible by classical measuring devices. The microenvironment inside an organelle vastly controls the outcome of any biochemical and biophysical processes taking place inside it, which is crucial for the overall cellular health. Therefore, it is very important to understand the microenvironmental physical properties inside cellular organelles. Moreover, specific alterations of such microenvironmental properties were observed in the disease condition, making them a diagnostic hallmark. With this high demand, small-molecule organic fluorophores are emerging as the most successful tool due to their small relative size, bioavailability, and ease of functionalization. In this mini-review, the development of micropolarity-sensitive small organic fluorophore with the capability of targeting a specific cellular organelle has been discussed. Here, we have highlighted the strategies of targeting a specific organelle, the micropolarity, and the challenges and prospects of the field.

A near-infrared fluorescent probe with large Stokes shift for visualizing and monitoring mitochondrial viscosity in live cells and inflammatory tissues

Mitochondria are cellular energy factory, having an essential role in cellular metabolism. Furthermore, abnormal changes in mitochondrial viscosity have been confirmed to be closely related to many diseases. Therefore, the development of probe that responsive to mitochondrial viscosity and its application in mitochondrial viscosity measurement is considered to be a new tool for understanding diseases. In this paper, a mitochondrial viscosity probe (DICB) with a large Stokes shift (214-253 nm) was designed and synthesized by modifying the structure of the carbazole fluorophore. The probe DICB has a favorable responsive to viscosity in the near-infrared (NIR) region (703 nm). In the water-glycerol system (0.893 cP -945 cP), the fluorescence intensity of DICB at 703 nm has a 74 times increase; in the range of 5.041 cP-856.0 cp, it has a well linear fitting relationship. Meantime, the probe has excellent sensitivity to viscosity. The probe (DICB) has been confirmed to be able to detect changes of mitochondrial viscosity in cell models induced by nystatin, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and lipopolysaccharide (LPS); it has also been validated that DICB can be used in the process of autophagy to monitor mitochondrial viscosity. More importantly, DICB can be applied to the detection of abnormal mitochondrial viscosity in inflammatory tissues at the biological level. The outstanding characteristics of DICB for mitochondrial viscosity detection are not only of great importance to the development of viscosity probes, but also provides a universal strategy to study the relationship between inflammatory and mitochondrial viscosity.

A sensitive fluorescent probe for β-galactosidase activity detection and application in ovarian tumor imaging

The development of non-invasive and sensitive optical probes for in vivo bioimaging of cancer-related enzymes is desirable for early diagnosis and effective cancer therapy. β-galactosidase (β-gal) is regarded as a key ovarian cancer biomarker, owing to its overexpression in primary ovarian cancer. Herein, we designed a sensitive near-infrared (NIR) probe (DCMCA-βgal) for the detection and real-time imaging of β-gal activity in ovarian tumors, thereby achieving the visualization of ovarian tumors by β-gal activity detection. DCMCA-β-gal could be triggered by β-gal, resulting in the release of a NIR chromophore, DCM-NH₂; the linear range of fluorescent response to β-gal concentration was 0-1.2 U with a low detection limit of 1.26 × 10^-3 U mL^-1. We used DCMCA-β-gal to detect and visualize β-gal activity in SKOV3 human ovarian cancer cells, as well as for real-time imaging of β-gal activity in ovarian cancer mouse models. DCMCA-β-gal possessed high sensitivity, "turn-on" NIR emission, a large spectral shift, and high photostability in a dynamic living system and thus could serve as a highly sensitive sensor for real-time tracking of β-gal activity in vivo and ovarian tumor imaging.

Two-photon fluorescent probes for detecting the viscosity of lipid droplets and its application in living cells

Lipid droplets (LDs) are storage organelles at the centre of lipid and energy homeostasis, which act as vital hubs of cellular metabolism and the key to maintaining lipid and energy homeostasis. We synthesized a new two-photon fluorescent probe (CIV) that could detect the viscosity of lipid droplets. The probe is constructed via the typical ICT system of D–π–A using carbazole as the donor and imidazole as the acceptor. With the increase in viscosity from PBS to 99% glycerol, the fluorescence intensity of CIV increased by 13-fold, showing sensitivity and specificity towards viscosity. In addition, CIV showed low toxicity and excellent biocompatibility in cytotoxicity tests, and was successfully used for living cell LD imaging. Taken together, the results widen the way for the development of novel fluorescent probe-based the visualization LDs and detection in solutions, physiology and pathology.

A novel two-photon fluorescence probe (CIV) can detect the viscosity and locate lipid droplets in living cells.

A fluorogenic probe for dynamic tracking of lipid droplets’ polarity during the evolution of cancer

Exploring the changes in the polarity of intracellular lipid droplets (LDs) during the evolution of cancer is important for cancer detection and treatment.

Rhenium carbonyl complexes bearing methylated triphenylphosphonium cations as antibody-free mitochondria trackers for X-ray fluorescence imaging

A convenient rhenium-based multimodal mitochondrial-targeted probe compatible with Synchrotron Radiation X-ray Fluorescence nano-imaging.

A quinoline–benzothiazole hybrid as the first near-infrared fluorescent probe for transthyretin

A quinoline-benzothiazole hybrid was rationally developed as the first NIR fluorescent probe for detecting transthyretin.

A bifunctional mitochondrial targeting AIE-active fluorescent probe with high sensitivity to hydrogen peroxide and viscosity for fatty liver diagnosis

A mitochondrial targeting AIE-active fluorescent probe with highly sensitive dual-detection of hydrogen peroxide and viscosity for fatty liver diagnosis.

A novel near-infrared fluorescent probe with large stokes shifts for sensing extreme acidity and its application in bioimaging

In this work, we reported a novel near-infrared (NIR) fluorescent probe RQNN with large Stokes shift (98 nm) for monitoring pH under extremely acidic conditions. For the preparation of this probe, a 1,4-diethylpiperazine moiety was introduced in rhodamine scaffold to tune the electron-donating character, and an o-phenylenediamine was introduced in spironolactone to provide larger steric hindrance. The deprotonated-protonated equilibrium between RQNN, RQNN-H⁺ and RQNN-H⁺⁺ were evaluated in different pH by absorption and emission spectra. As expected, RQNN exhibited lower pk_a values (pk_a1 = 4.83, pk_a2 = 2.99), indicating that the probe can be used in extremely acidic pH. Moreover, RQNN possessed highly selective response to H⁺ over essential metal ions and biologically related redox molecules, high photo-stability, rapid response time, and excellent reversibility. Importantly, the probe had excellent cell membrane permeability and was further applied successfully to monitor pH fluctuations in live cells.

Optimising molecular rotors to AIE fluorophores for mitochondria uptake and retention

Molecular rotors exhibit fluorescence enhancement in a confined environment and thus have been used extensively in biological imaging. However, many molecular rotors suffer from small Stokes shift and self-aggregation caused quenching. In this work, we have synthesised a series of red emissive molecular rotors based on cationic α-cyanostilbene. Profoundly enhanced aggregation-induced emission (AIE) properties and greatly widened Stokes shifts can be achieved by molecular engineering. With specificity to stain mitochondria, we demonstrate a simple approach to achieve cell uptake and retention upon tuning the pyridinium substituent of the dyes.

Building Accurate Intracellular Polarity Maps through Multiparametric Microscopy

The precise knowledge of intracellular polarity, a physiological parameter that involves complex and intertwined intracellular mechanisms, may be relevant in the study of important diseases like cancer or Alzheimer’s. In this technical note, we illustrate our recently developed, accurate method for obtaining intracellular polarity maps employing potent fluorescence microscopy techniques. Our method is based on the selection of appropriate luminescent probes, in which several emission properties vary with microenvironment polarity, specifically spectral shifts and luminescence lifetime. A multilinear calibration is performed, correlating polarity vs. spectral shift vs. luminescence lifetime, to generate a powerful and error-free 3D space for reliable interpolation of microscopy data. Multidimensional luminescence microscopy is then used to obtain simultaneously spectral shift and luminescence lifetime images, which are then interpolated in the 3D calibration space, resulting in accurate, quantitative polarity maps.

Array-Based Discriminative Optical Biosensors for Identifying Multiple Proteins in Aqueous Solution and Biofluids

Identification of proteins is an important issue both in medical research and in clinical practice as a large number of proteins are closely related to various diseases. Optical sensor arrays with recognition ability have been flourished to apply for distinguishing multiple chemically or structurally similar analytes and analyzing unknown or mixed samples. This review gives an overview of the recent development of array-based discriminative optical biosensors for recognizing proteins and their applications in real samples. Based on the number of sensor elements and the complexity of constructing array-based discriminative systems, these biosensors can be divided into three categories, which include multi-element-based sensor arrays, environment-sensitive sensor arrays and multi-wavelength-based single sensing systems. For each strategy, the construction of sensing platform and detection mechanism are particularly introduced. Meanwhile, the differences and connections between different strategies were discussed. An understanding of these aspects may help to facilitate the development of novel discriminative biosensors and expand their application prospects.

Visualizing Peroxynitrite in Microvessels of the Brain with Stroke Using an Engineered Highly Specific Fluorescent Probe

Stroke is one of the leading causes of death and disability in the world, which is associated with malfunction of reactive oxygen species and reactive nitrogen species (ROS/RNS) in cerebral microvessels. In vivo monitoring these species, such as ONOO^-, with high selectivity in stroke process is of great significance for early diagnoses and therapies of the disease. Herein, by engineering an indoline-2,3-dione moiety as the recognizing domain, we proposed a novel fluorescence probe Rd-PN2 with highly specific response toward ONOO^-, even in the coexistence of other ROS/RNS with high concentration. Rd-PN2 showed high sensitivity and reaction speed in response to ONOO^- and exhibited satisfying performances in tracking the endogenously generated ONOO^- in living cells and zebrafish. Accordingly, Rd-PN2 can furnish real-time and in vivo visualizing of ONOO^- in cerebral microvessels of mice with ischemic and hemorrhagic strokes under two-photon microscopy. This work presented a precisely modulated fluorescence probe for real-time visualizing of ONOO^- production in cerebral micovessels, which will also help to acquire more accurate information in the studies of ONOO^- functions in the future.

Characterization of Push-Pull-Type Benzo[X]quinoline Derivatives (X = g or f): Environmentally Responsive Fluorescent Dyes with Multiple Functions

Benzo[X]quinoline (X = g or f: BQX) derivatives bearing bis-trifluoromethyl and amine groups have been designed as push-pull-type fluorescent dyes. Through the synthesis of BQX derivatives from 2,7-diaminonaphthalene, linear-type (BQL) and angular-type (BQA) structural isomers were obtained. X-ray structures of single crystals from six given BQX derivatives revealed that the BQL and BQA series adopt planar- and bowl-shaped structures. In the fluorescence spectra, interestingly, the BQL series emitted in the near-infrared region over 700 nm in polar solvents. Based on the visible absorptions and base properties related to the amine moiety, the ammonia responsiveness was investigated using an ion-exchange reaction by the BQX-HCl salt. By exploiting the environmentally responsive fluorescence probe, cell imaging through confocal laser microscopy was conducted using HeLa and 3T3-L1 cells, emitting specific lipid droplets. The results indicate that BQX derivatives have multiple functions and may be applied in materials chemistry and biochemistry.

A novel dual functional pyrene-based turn-on fluorescent probe for hypochlorite and copper (II) ion detection and bioimaging applications

A novel dual-function fluorescent probe PPH was conveniently synthesized by linking pyrazinehydrazide unit and pyrenyl fluorophore. The present probe PPH could be used for simultaneous detection of ClO^- and Cu²⁺ through a fluorescence turn-on response. The probe PPH displayed excellent selectivity and sensitivity towards ClO^- and Cu²⁺ over other anions and metal ions. The detection limits of PPH for ClO^- and Cu²⁺ were calculated to be as low as 4.02 nM and 157 nM, respectively. The probe PPH had good pH stability and was highly responsive to ClO^- and Cu²⁺ in a wide pH range. Furthermore, this probe could be readily applied to visualize ClO^- and Cu²⁺ on the test paper. Additionally, this probe was successfully utilized to the fluorescence imaging of ClO^- and Cu²⁺ in HeLa cells.

Detection Sensitivity Enhancement of Naphthalimide PET Fluorescent Probes by 4-Methoxy-Substitution

Naphthalimide photoinduced electron transfer (PET) fluorescent probes are widely used in fluorescence imaging. Thereinto, detection sensitivity is the vital parameter of PET probes. However, the modulation of detection sensitivity is yet to be reported for naphthalimide PET probes. Herein, the detection sensitivity enhancement of naphthalimide PET fluorescent probes through 4-methoxy-substitution is proposed in this work. Taking Zn²⁺ detection an example, 4-methoxy-naphthalimide PET probe 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-6-methoxy-1H-benzo[de]isoquinoline-1,3(2H)-dione (BPNM) and control PET probe 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (BPN) are separately synthesized. The addition of 4-methoxy group with ability of strong electron donating to naphthalimide facilitates the construction of electronic push-pull system in the fluorophore resulting in the bathochromic shift of absorption and fluorescence emission spectra of BPNM and is further conducive to the enhancement of molar extinction coefficient ε and fluorescence quantum yield Φ_f of BPNM. Compared with BPN, BPNM shows lower Zn²⁺ detection limit in titration assays. Meanwhile, the fluorescence signal change (off-on) before and after Zn²⁺ addition of intracellular BPNM is more obvious and easier to control in confocal laser scanning imaging. Therefore, 4-methoxy-substitution improves the detection sensitivity of naphthalimide PET probe, which is favorable for the precise sensing of analyte, and further lays a good foundation for the synthesis of PET probe with high sensitivity.

Environment-Sensitive Probes for Illuminating Amyloid Aggregation In Vitro and in Zebrafish

The aberrant aggregation of certain peptides and proteins, forming extracellular plaques of fibrillar material, is one of the hallmarks of amyloid diseases, such as Alzheimer's and Parkinson's. Herein, we have designed a new family of solvatochromic dyes based on the 9-amino-quinolimide moiety capable of reporting during the early stages of amyloid fibrillization. We have rationally improved the photophysical properties of quinolimides by placing diverse amino groups at the 9-position of the quinolimide core, leading to higher solvatochromic and fluorogenic character and higher lifetime dependence on the hydrophobicity of the environment, which represent excellent properties for the sensitive detection of prefibrillar aggregates. Among the different probes prepared, the 9-azetidinyl-quinolimide derivative showed striking performance in the following β-amyloid peptide (Aβ) aggregation in solution in real time and identifying the formation of different types of early oligomers of Aβ, the most important species linked to cytotoxicity, using novel, multidimensional fluorescence microscopy, with one- or two-photon excitation. Interestingly, the new dye allowed the visualization of proteinaceous inclusion bodies in a zebrafish model with neuronal damage induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Our results support the potential of the novel fluorophores as powerful tools to follow amyloid aggregation using fluorescence microscopy in vivo, revealing heterogeneous populations of different types of aggregates and, more broadly, to study protein interactions.

Simultaneous Detection of Carbon Monoxide and Viscosity Changes in Cells

A new family of robust, non‐toxic, water‐compatible ruthenium(II) vinyl probes allows the rapid, selective and sensitive detection of endogenous carbon monoxide (CO) in live mammalian cells under normoxic and hypoxic conditions. Uniquely, these probes incorporate a viscosity‐sensitive BODIPY fluorophore that allows the measurement of microscopic viscosity in live cells via fluorescence lifetime imaging microscopy (FLIM) while also monitoring CO levels. This is the first example of a probe that can simultaneously detect CO alongside small viscosity changes in organelles of live cells.

A fluoran-based viscosity probe with high-performance for lysosome-targeted fluorescence imaging

A new fluorescent probe Lyso-Fl has been facilely prepared by an esterification reaction of spironolactone fluoran dye Rdi with ethanol, which shows viscosity-selective response by fluorescence. The new probe delivers obvious fluorescence signal enhancement when environmental viscosity changes from 1.01 cP (water) to 1256 cP (98% glycerol). And, both the emission intensity (575 nm) and fluorescence lifetime of Lyso-Fl exhibit individually good linear relationships with the solution viscosity. Besides, Lyso-Fl gives a selective response to viscosity among various biological species and exhibits pH-independent (1-10) fluorescent signals towards viscosity. More importantly, Lyso-Fl shows low cytotoxicity and can be utilized for monitoring of dexamethasone-stimulated viscosity enhancement by cell imaging with excellent lysosome-targeted performance, promoting it a promising fluorescent probe for lysosomal viscosity detection.

Boron Schiff bases derived from α-amino acids as nucleoli/cytoplasm cell-staining fluorescent probes in vitro

The size, shape, and number of nucleoli in a cell's nucleus might help to distinguish a malignant from a benign tumor. Cellular biology and histopathology often require better visualization to understand nucleoli-related processes, thus organelle-specific fluorescent markers are needed. Here, we report the design, synthesis, and fully chemo-photophysical characterization of fluorescent boron Schiff bases (BOSCHIBAs), derived from α-amino acids (i.e., phenylalanine, tyrosine and tryptophan), with nucleoli- and cytoplasm-specific staining in cells. It is the first time that Boron Schiff bases derived from α-amino acids act as notorious dual (nucleoli and cytoplasm) cell-staining fluorescent probes. The boron derivatives not only showed good photostability and acceptable quantum yields (∼5%) in solution, but also exhibited low cytotoxicity (>90% cell viability at 0.1 and 1 μg mL^-1), which make them good candidates to be used in medical diagnosis.

Self-assembly of an alkynylpyrene derivative for multi-responsive fluorescence behavior and photoswitching performance

Highly emissive fluorophores based on polyaromatic hydrocarbons with tunable emission properties and aggregated structures play a very important role in relevant functional studies. In this study, a novel alkynylpyrene derivative 1 was synthesized, which exhibits unimolecular to excimer emission in methanol with an increasing concentration accompanied by the formation of nanovesicles via the π-π stacking, hydrogen bond and hydrophobic interaction. The self-assembly behavior as well as emission properties of 1 in aprotic polar solvents (ACN, acetone, DMF and DMSO) can also be adjusted by the volume fraction of the poor solvent H2O, which can induce 1 self-assembly to excimer state and could be applied in information transfer. Moreover, upon visible light irradiation, photoswitchable performance of nanovesicles of 1 was observed in which the emission markedly changes from yellow to blue; this is attributed to the cycloaddition reaction of alkynyl groups and singlet oxygen, which can be generated without the addition of external photosensitizers. The multi-responsive and fluorescence behavior of the alkynylpyrene derivative show that the self-assembly can be used to expand the development of this type of fluorophores, and the novel photoinduced tunability of the fluorescence emission provides an effective strategy to obtain high-performance transmitting and sensing materials.

Microenvironment-sensitive iridium(iii) complexes for disease theranostics

Microenvironmental parameters, including hypoxia, pH, polarity, viscosity and temperature, play pivotal roles in controlling the biological, physical or chemical behaviors of local molecules. Abnormal changes in these parameters would cause cellular malfunction or become a hallmark of the occurrence of severe diseases. Recently, a number of phosphorescent Ir(iii) complexes have been designed to respond to such parameters due to their attractive properties such as high photostability, long emission lifetimes, and environment-sensitive emission profiles. This review aims to provide a summary of the progress achieved in developing iridium-based probes responding to microenvironmental parameters in biological systems in recent years for diagnosis and treatment of diseases such as cancer and diabetes.

Confined Microenvironments from Thermoresponsive Dendronized Polymers

Confined microenvironments in biomacromolecules arising from molecular crowding account for their well-defined biofunctions and bioactivities. To mimick this, synthetic polymers to form confined structures or microenvironments are of key scientific value, which have received significant attention recently. To create synthetic confined microenvironments, molecular crowding effects and topological cooperative effects have been applied successfully, and the key is balance between self-association of structural units and self-repulsion from crowding-induced steric hindrance. In this article, formation of confined microenvironments from stimuli-responsive dendronized polymers carrying densely dendritic oligoethylene glycols (OEGs) moieties in their pendants is presented. These wormlike thick macromolecules exhibit characteristic thermoresponsive properties, which can provide constrained microenvironments to encapsulate effectively guest molecules including dyes, proteins, or nucleic acids to prevent their protonation or biodegradation. This efficient shielding effect can also mediate chemical reactions in aqueous phase, and even enhance chirality transferring efficiency. All of these can be switched off simply through the thermally-induced dehydration and collapse of OEG dendrons due to the amphiphilicity of OEG chains. Furthermore, the switchable encapsulation and release of guests can be greatly enhanced when these dendronized polymers are used as major constituents for fabricating bulk hydrogels or nanogels, which provide a higher-level confinement.

Multimodal Imaging Iridium(III) Complex for Hypochlorous Acid in Living Systems

Biomolecule tracing with different imaging methods is of great significance for more accurately unravelling the fundamental processes in living systems. However, considering the different principles of each imaging method for probe design, it is still a great challenge to apply one molecular probe to achieve two or even more imaging analyses for biomarkers. In general, traditional oxime was reported as a recognition group for fluorescence imaging of HOCl. Herein, for the first time, we designed the oxime decorated iridium(III) complex, which can be directly used for chemiluminescence as well as two-photon luminescence and photoluminescence lifetime imaging of HOCl in living systems. Moreover, the novel chemiluminescence mechanism of Ir-CLFLPLIM for HOCl was also proposed and explored by continuously monitoring chemiluminescence peak shapes and mass spectra, inferring the reaction intermediate and calculating the chemical reaction energy range of the reaction process. This strategy could lead us to expand the chemiluminescence application of transition metal complexes and develop more multimodal imaging probes.

Articulated Structures of D-A Type Dipolar Dye with AIEgen: Synthesis, Photophysical Properties, and Applications

Articulated structures of naphthalene-based donor (D)-acceptor (A) type dipolar dye and aggregation-induced emission luminogen (AIEgen) based on tetraphenylethylene (TPE) were synthesized, and their photophysical properties were analyzed for the first time. There are many fluorophore backbones, which have dipolar structure and AIEgen. However, there has been neither property analysis nor research that closely articulates DA and AIE through non-conjugation linker. We have therefore prepared two representative fluorophores; DA-AIE series (DA-AIE-M and DA-AIE-D), and characterized their UV/vis absorption and emission properties with quantum chemical calculations. In addition, we utilized the unique photophysical properties of DA-AIE-D for monitoring a trace of dimethyl sulfoxide (DMSO) in aqueous media, including real water samples.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Perylene Bisimide and Naphthyl-Based Molecular Dyads: Hydrogen Bonds Driving Co-planarization and Anomalous Temperature-Response Fluorescence

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho-methoxy group (NM) at its bay positions (PBI-2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol^-1 ) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co-planarization of the different aromatic units, resulting in delocalization of the π-electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co-planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H-bond-driven co-planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.

Single-molecule displacement mapping unveils nanoscale heterogeneities in intracellular diffusivity

Intracellular diffusion underlies vital cellular processes. However, it remains difficult to elucidate how an unbound protein diffuses inside the cell with good spatial resolution and sensitivity. Here we introduce single-molecule displacement/diffusivity mapping (SMdM), a super-resolution strategy that enables the nanoscale mapping of intracellular diffusivity through local statistics of the instantaneous displacements of freely diffusing single molecules. We thus show that the diffusion of an average-sized protein in the mammalian cytoplasm and nucleus is spatially heterogeneous at the nanoscale, and that variations in local diffusivity correlate with the ultrastructure of the actin cytoskeleton and the organization of the genome, respectively. SMdM of differently charged proteins further unveils that the possession of positive, but not negative, net charges drastically impedes diffusion, and that the rate is determined by the specific subcellular environments. We thus unveil rich heterogeneities and charge effects in intracellular diffusion at the nanoscale.

A novel dual-function probe for recognition of Zn2+ and Al3+ and its application in real samples

A dual-function probe NAHH based on naphthalene was synthesized and characterized. Based on the combination effects derived from the inhabitation of photo-induced electron transfer (PET) and CN isomerization, probe NAHH achieved in the recognition of Zn2+ and Al3+ both through obvious fluorescence enhancement and color changes detected by naked eye, respectively. Probe NAHH showed high sensitivity with the limit of detection as low as 3.02 × 10-7 M for Zn2+ and 7.55 × 10-8 M for Al3+, indicated the capability of probe NAHH in trace detection for Zn2+ and Al3+. The binding ratio of NAHH with Zn2+ and Al3+ were all 1:1 determined by Job plot, and the corresponding association constant was calculated as 8.48 × 104 M-1 and 4.45 × 105 M-1, respectively. The mechanism was further confirmed by FT-IR, 1H NMR titration and ESI-MS analysis. Furthermore, probe NAHH was successfully applied in logic gate construction and the detection of Zn2+ and Al3+ in Songhua River and test stripe. Fluorescence imaging experiments confirmed that NAHH could be used to monitor Zn2+ in plant root.

Deciphering interior polarity of lysosome in live cancer and normal cells using spectral scanning microscopy

A lysosome specific, pH tolerant, and polarity-sensitive fluorescent probe (LyPol) is designed and synthesized for the determination of lysosomal polarity in live cells. LyPol possesses an intramolecular charge transfer (ICT) properties with high quantum yield in water and in other polar solvents such as methanol, ethanol, dimethyl sulfoxide, acetonitrile, etc. The fluorescence maxima and lifetime increase linearly with a non-specific manner with an increase in the polarity of its surrounding environment. A morpholine group connected with an alkyl linker acts as a lysosome directing moiety, which is attached to the fluorescent core of LyPol. The selective localization of LyPol inside the lysosome was confirmed with live-cell confocal imaging. Further, the spectral scanning confocal technique was utilized to determine the emission spectrum of LyPol inside lysosome, and the polarity turns out to be quite lower as compared to water. Moreover, the combined spectroscopic and live-cell microscopy confirms that the interior of the lysosome is significantly non-polar in cancer cells compared to normal cells. We believe that this report on the measuring polarity inside the biological system with a solvatofluorochromic probe will be of immense interest to researchers working in the multidisciplinary area of biophysics, microscopy, chemical biology, and organelle biology.

Exploring NIR Aza-BODIPY-Based Polarity Sensitive Probes with ON-and-OFF Fluorescence Switching in Pluronic Nanoparticles

Because of their deep penetration capability in tissue, red or near infrared (NIR) fluorophores attract much attention in bio-optical imaging. Among these fluorophores, the ones that respond to the immediate microenvironment (i.e., temperature, polarity, pH, viscosity, hypoxia, etc.) are highly desirable. We studied the response of six NIR aza-BODIPY-based and structurally similar fluorophores to polarity and viscosity for incorporation inside Pluronic nanoparticles as switchable fluorescent probes (SFPs). Based on our results, all of these fluorophores were moderately to strongly sensitive to the polarity of the microenvironment. We concluded that attaching amine groups to the fluorophore is not necessary for having strong polarity sensitive probes. We further studied the response of the fluorophores when embedded inside Pluronic nanoparticles and found that four of them qualified as SFPs. We also found that the switching ratio of the fluorophore-encapsulated Pluronic nanoparticles (I_ON-to-I_OFF) is related to the length of the hydrophobic chain of the Pluronic tri-block copolymers. As such, the highest switching ratio pertained to F-68 with the lowest hydrophobic block poly (propylene oxide) (PPO chain of only 30 units).

Trans-to-cis photoisomerization of cyclocurcumin in different environments rationalized by computational photochemistry

Cyclocurcumin is a turmeric component that has attracted much less attention compared to the well-known curcumin. In spite of the less deep characterization of its properties, cyclocurcumin has shown promising anticancer effects when used in combination with curcumin. Especially, due to its peculiar molecular structure, cyclocurcumin can be regarded as an almost ideal photoswitch, whose capabilities can also be exploited for relevant biological applications. Here, by means of state-of-the-art computational methods for electronic excited-state calculations (TD-DFT, MS-CASPT2, and XMS-CASPT2), we analyze in detail the absorption and photoisomerization pathways leading from the more stable trans isomer to the cis one. The different molecular surroundings, taken into account by means of the electrostatic solvent effect and compared with available experimental data, have been found to be critical in describing the fate of irradiated cyclocurcumin: when in non-polar environments, an excited state barrier prevents photoisomerization and favours fluorescence, whereas in polar solvents, an almost barrierless path results in a striking decrease of fluorescence, opening the way toward a crossing region with the ground state and thus funneling the photoproduction of the cis isomer.

Luminescent Carbazole-Based EuIII and YbIII Complexes with a High Two-Photon Absorption Cross-Section Enable Viscosity Sensing in the Visible and Near IR with One- and Two-Photon Excitation

The newly synthesized Eu^III and Yb^III complexes with the new carbazole-based ligands CPAD^2- and CPAP^4- display the characteristic long-lived metal-centered emission upon one- and two-photon excitation. The Eu^III complexes show the expected narrow emission bands in the red region, with emission lifetimes between 0.382 and 1.464 ms and quantum yields between 2.7% and 35.8%, while the Yb^III complexes show the expected emission in the NIR region, with emission lifetimes between 0.52 and 37.86 μs and quantum yields between 0.028% and 1.12%. Two-photon absorption cross sections (σ_2PA) as high as 857 GM were measured for the two ligands. The complexes showed a strong dependence of the one- and two-photon sensitized emission intensity on solvent viscosity in the range of 0.5-200 cP in the visible and NIR region.