Large Stokes shift benzothiazolium cyanine dyes with improved intramolecular charge transfer (ICT) for cell imaging applications

A highly sensitive ratiometric fluorescence probe for sensing and imaging sulfite in food samples and living cells

In this work, a ratiometric and chromogenic fluorescent probe 1 was synthesized for the detection of SO32-. The probe 1 at PBS (10 mM, pH = 7.4) presented a marked emission band at 661 nm. Upon addition of SO32- ions, a highly emissive adduct with a marked fluorescence at 471 nm were obtained through a Michael addition. The probe 1 displayed a noticeable fluorescence ratiometric response with a large shift (190 nm) in emission wavelength. The probe can quantitatively detect SO32- with high specificity, fast response (within 130 s) as well as low detection limit (13 nM), and a large Stokes shift (139 nm). Fluorescence imaging of HeLa cells indicated that 1 could be used for monitoring the intrinsically generated intracellular SO32- in living cells by ratiometric fluorescence imaging. Furthermore, 1 could be application in real water and sugar samples with high sensitivity and good recoveries.

Design and synthesis of large Stokes shift DNA dyes with reduced genotoxicity

Despite intensive search over the past decades, only a few small-molecule DNA fluorescent dyes were found with large Stokes shifts. These molecules, however, are often too toxic for widespread usage. Here, we designed DNA-specific fluorescent dyes rooted in benzimidazole architectures with a hitherto unexplored molecular framework based on thiazole-benzimidazole scaffolding. We further incorporated a pyrazole ring with an extended sidechain to prevent cell penetration. These novel benzimidazole derivatives were predicted by quantum calculations and subsequently validated to have large Stokes shifts ranging from 135 to 143 nm, with their emission colors changed from capri blue for the Hoechst reference compound to iguana green. These readily-synthesized compounds, which displayed improved DNA staining intensity and detection limits along with a complete loss of capability for cellular membrane permeation and negligible mutagenic effects as designed, offer a safer alternative to the existing high-performance small-molecule DNA fluorescent dyes.

A hemicyanine-based near-infrared fluorescent probe with large Stokes shift for non-invasive bioimaging of brown adipose tissue

Brown adipose tissue (BAT), characterized by the presence of numerous mitochondria, plays a key role in metabolism and energy expenditure. Accurately reporting the presence and activation of BAT is beneficial to study obesity, diabetes, and other metabolic disorders. Near-infrared (NIR) fluorescence imaging has the advantages of high sensitivity, non-radioactivity, and simple operation. However, most NIR probes for BAT imaging exhibit small Stokes shifts, which may lead to self-quenching, reducing the signal-to-noise ratio, and introducing cross-talk. Herein, we rationally designed and synthesized a series of hemicyanine-based NIR fluorescent probes HCYBAT-1-3. Among them, HCYBAT-1 demonstrated favorable properties such as near-infrared emission (776 nm), large Stokes shift (139 nm), good biocompatibility and specific mitochondrial targeting (Pearson's colocalization coefficient of 0.87). Meanwhile, HCYBAT-1 was successfully employed to differentiate BAT from white adipose tissue (WAT). Quantitative analysis of NIR fluorescent images showed that HCYBAT-1 could be used for real-time monitoring of BAT activation in mice stimulated by norepinephrine (NE) and cold exposure. Overall, probe HCYBAT-1 showcased its efficacy in non-invasive evaluation of BAT metabolism in vivo with high selectivity and sensitivity.

Dual-targeted fluorescent probe for tracking polarity and phase transition processes during lipophagy

Eukaryotic cells regulate various cellular processes through membrane-bound and membrane-less organelles, enabling active signal communication and material exchange. Lysosomes and lipid droplets are representative organelles, contributing to cell lipophagy when their interaction and metabolism are disrupted. Our limited understanding of the interacting behaviours and physicochemical properties of different organelles during lipophagy hinders accurate diagnosis and treatment of related diseases. In this contribution, we report a fluorescent probe, PTZ, engineered for dual-targeting of lipid droplets and lysosomes. PTZ can track liquid-liquid phase separation and respond to polarity shifts through ratiometric fluorescence emission, elucidating the lipophagy process from the perspective of organelle behavior and physicochemical properties. Leveraging on the multifunctionality of PTZ, we have successfully tracked the polarity and dynamic changes of lysosomes and lipid droplets during lipophagy. Furthermore, an unknown homogeneous transition of lipid droplets and lysosomes was discovered, which provided a new perspective for understanding lipophagy processes. And this work is expected to serve as a reference for diagnosis and treatment of lipophagy-related diseases.

An Asymmetric NIR-II Organic Fluorophore with an Ultra-Large Stokes Shift for Imaging-Guided and Targeted Phototherapy

NIR-II imaging-guided phototherapy is an attractive, yet challenging, tumor treatment strategy. By monitoring the accumulation of phototherapy reagents at the tumor site through imaging and determining the appropriate therapy window, the therapeutic effect could be significantly improved. Probes with NIR-II (1000-1700 nm) fluorescence emission and a large Stokes shift hold great promise for fluorescence imaging with deep penetration, minimized self-quenching, and high spatiotemporal resolution. However, due to the lack of a suitable molecular framework, the design of a simple small-molecule dye with a large Stokes shift and NIR-II fluorescence emission has rarely been reported. Herein, we prepare an asymmetric D-π-A type NIR-II fluorescence probe (TBy). The probe is incapsulated in an amphiphilic polymer and modified with a fibronectin targeting peptide CREKA, which could recognize the fibrin-fibronectin complex overexpressed in multiple malignant tumors. The nanoparticles thus constructed (TByC-NPs) have maximum fluorescence emission at 1037 nm with a large Stokes shift of 426 nm, which is the largest Stokes shift among organic NIR-II fluorescent dyes reported in the literature. The TByC-NPs exhibit a good NIR-II imaging performance, active tumor targeting, and good photothermal and photodynamic capabilities. In vitro and in vivo studies verify that the TByC nanoplatform shows outstanding biocompatibility for NIR-II imaging-guided phototherapy and provides an excellent antitumor effect.

Systematic Structural Modification of Squaraine Dye for Near-Infrared Window One and Two Multiplexed In Vivo Imaging and Photothermal Therapy

Due to the wide application of reporter gene-related visible/NIR-I bioluminescent imaging, multiplexed fluorescence imaging across visible/NIR-I/NIR-II has excellent potential in biomedical research. However, in vivo multiplexed imaging applications across those regions have rarely been reported due to the lack of proper fluorophores. Herein, nine squaraine dyes, which exhibit diverse adsorption and emission wavelengths, were synthesized. Among them, water-soluble SQ 710-5k and SQ 905 were found to have significant absorption differences, which allowed the tumor and lymph nodes to be identified. Then, for the first time, six-channel multiplexed fluorescence imaging across visible/NIR-I/II was achieved by coordination with reporter gene-related bioluminescent phosphors. Additional research revealed that SQ 710-5k exhibited higher-quality blood vessels and tumor imaging in NIR-II. H-aggregates SQ 905 demonstrated a high photothermal conversion efficiency for photothermal therapy. This study proposed an approach to creating small molecular dyes that coordinate with reporter gene-related bioluminescent phosphors for six-color fluorescence imaging.

Lighting Up Nucleolus To Report Mitochondria Damage Using a Mitochondria-to-Nucleolus Migration Probe

Visualization of the mitochondrial state is crucial for tracking cell life processes and diagnosing disease, while fluorescent probes that can accurately assess mitochondrial status are currently scarce. Herein, a fluorescent probe named "SYN" was designed and prepared, which can target mitochondria via the mitochondrial membrane potential. Upon pathology or external stimulation, SYN can be released from the mitochondria and accumulate in the nucleolus to monitor the status of mitochondria. During this process, the brightness of the nucleolus can then serve as an indicator of mitochondrial damage. SYN has demonstrated excellent photostability in live cells as well as an extremely inert fluorescence response to bioactive molecules and the physiological pH environment of live cells. Spectroscopic titration and molecular docking studies have revealed that SYN can be lit up in nucleoli due to the high viscosity of the nucleus and the strong electrostatic interaction with the phosphate backbone of RNA. This probe is expected to be an exceptional tool based on its excellent imaging properties for tracking mitochondrial state in live cells.

Developing NIR xanthene-chalcone fluorophores with large Stokes shifts for fluorescence imaging

A series of novel near-infrared (NIR) xanthene-chalcone fluorophores were constructed through a modular synthesis with the electron-donating xanthene moiety and the electron-withdrawing chalcone moiety. These fluorophores are convenient for fluorescence imaging in living cells, benefiting from their NIR emissions (650-710 nm), large Stokes shifts (>100 nm), moderate quantum yields and low cytotoxicity. The substituted hydroxyl group of the xanthene-chalcone fluorophore HCA-E facilitates the development of multifunctional fluorescent probes. As an example, a highly sensitive and selective probe N-HCA-E for glutathione (GSH) detection was developed based on the fluorophore HCA-E. A 4-nitrobenzenesulfonyl (4-Ns) group was introduced to cage the hydroxyl group of HCA-E, which was used as a selective recognition site for the thiol of GSH and an effective fluorescence quencher. Probe N-HCA-E revealed NIR "turn-on" fluorescence (709 nm) for endogenous and exogenous GSH detection in lysosomes with a large Stokes shift (129 nm) and high anti-interference ability.

Detecting labile heme and ferroptosis through 'turn-on' fluorescence and lipid droplet localization post Fe2+ sensing

Iron, a crucial biologically active ion essential for metabolic processes in living organisms, plays a vital role in biological functions, and imbalances in iron levels can lead to various diseases. In this study, we have developed two simple "turn-on" fluorescent probes, NOPy and NOCN, for the quick and selective detection of Fe2+ at nanomolar levels (LOD of 35 nM), accompanied by significant absorption and emission shifts, along with colorimetric demarcation. Both fluorophores exhibit an excellent "turn-on" emission response upon encountering Fe2+ in the cells. Flow cytometry and confocal fluorescence imaging studies demonstrate enhanced fluorescence signals in response to labile iron, efficiently detecting heme during erastin-induced ferroptosis. Interestingly, we also observed that the product formed after Fe2+ sensing localizes within the lipid droplets. These water-soluble and highly sensitive reactive probes, NOPy and NOCN, enable investigations of iron-dependent physiological and pathological conditions. The development of these probes represents an advancement in the field, offering a rapid and selective means for detecting Fe2+ with minimal cytotoxicity.

Design, synthesis, and biological application of A-D-A-type boranil fluorescent dyes

For the first time, three acceptor-donor-acceptor (A-D-A)-type boranil fluorescent dyes, CSU-BF-R (R = H, CH3, and OCH3), featuring phenothiazine as the donor, were designed and synthesized. CSU-BF-R exhibited remarkable photophysical characteristics, including large Stokes shifts (>150 nm), high fluorescence quantum yields (up to 40%), long-wavelength emissions, and strong red solid-state fluorescence. Moreover, these CSU-BF-R fluorescent dyes were demonstrated to function as highly selective and sensitive ratiometric fluorescent probes for detecting hypochlorous acid (HClO). The preliminary biological applications of CSU-BF-OCH3 for sensing intracellular HClO in living cells and zebrafish were demonstrated. Therefore, CSU-BF-R possess the potential to further explore the physiological and pathological functions associated with HClO and provide valuable insights into the design of high-performance A-D-A-type fluorescent dyes.

Nanoassemblies of heptamethine cyanine dye-initiated poly(amino acid) enhance ROS generation for effective antitumour phototherapy

Phototherapy shows great potential for pinpoint tumour treatment. Heptamethine cyanine dyes like IR783 have high potential as agents for antitumour phototherapy due to their inherent tumour targeting ability, though their effectiveness in vivo is unsatisfactory for clinical translation. To overcome this limitation, we present an innovative strategy involving IR783-based polymeric nanoassemblies that improve the dye's performance as an antitumoural photosensitizer. In the formulation, IR783 is modified with cysteamine and used to initiate the ring-opening polymerization (ROP) of the N-carboxyanhydride of benzyl-L-aspartate (BLA), resulting in IR783-installed poly(BLA). Compared to free IR783, the IR783 dye in the polymer adopts a twisted molecular conformation and tuned electron orbital distribution, remarkably enhancing its optical properties. In aqueous environments, the polymers spontaneously assemble into nanostructures with 60 nm diameter, showcasing surface-exposed IR783 dyes that function as ligands for cancer cell and mitochondria targeting. Moreover, the nanoassemblies stabilized the dyes and enhanced the generation of reactive oxygen species (ROS) upon laser irradiation. Thus, in murine tumor models, a single injection of the nanoassemblies with laser irradiation significantly inhibits tumour growth with no detectable off-target toxicity. These findings highlight the potential for improving the performance of heptamethine cyanine dyes in antitumor phototherapy through nano-enabled strategies.

Solvatochromic and Proton-Responsive characteristics of Bi-1,3,4-Oxadiazole derivatives with symmetric dimethylamino substitution

D-A molecules find extensive use in intelligent stimulus-response systems due to their exceptional attributes, including high sensitivity, rapid response, wide compatibility, and structural adaptability. The strength of Intramolecular Charge Transfer (ICT) plays a pivotal role in determining the performance of these devices. To enhance the ICT strength and explore new applications for D-A molecules, we meticulously designed a pair of symmetric dimethylamino-substituted bi-1,3,4-oxadiazole derivatives (DMAOXD and DMAOXDBEN). These symmetric D-A-A-D molecules, with strong electron donor terminals, displayed a modest redshift of less than 25 nm in the UV-vis absorption spectra. However, there was a significant redshift in the emission spectra (140 nm for DMAOXD and 170 nm for DMAOXDBEN) when transitioning from cyclohexane to dimethyl sulfoxide, indicating a pronounced ICT characteristic. Theoretical calculations support the idea that the dimethylaminophenyl unit serves as an electron donor in both DMAOXD and DMAOXDBEN, while the 1,3,4-oxadiazole and central benzene ring act as acceptors. The pronounced ICT characteristic observed in DMAOXD and DMAOXDBEN can be attributed to long-distance electron transfer. Additionally, it's noteworthy that the emission of DMAOXD and DMAOXDBEN solution samples can be quenched by adding trifluoroacetic acid (TFA) and restored by the addition of triethylamine (TEA). Inspired by this, a pattern created with ink samples containing DMAOXD and DMAOXDBEN can be concealed through fumigation with TFA and subsequently revealed by treating them with TEA, suggesting their potential use in data encryption.

Construction of a New Probe Based on Copper Chaperone Protein for Detecting Cu2+ in Cells

Biomacromolecular probes have been extensively employed in the detection of metal ions for their prominent biocompatibility, water solubility, high selectivity, and easy modification of fluorescent groups. In this study, a fluorescent probe FP was constructed. The probe FP exhibited high specificity recognition for Cu2+. With the combination of Cu2+, the probe was subjected to fluorescence quenching. The research suggested that the probe FP carried out the highly sensitive detection of Cu2+ with detection limits of 1.7 nM. The fluorescence quenching of fluorescamine was induced by Cu2+ perhaps due to the PET (photoinduced electron transfer) mechanism. The FP-Cu2+ complex shows weak fluorescence, which is likely due to the PET quenching effect from Cu2+ to fluorescamine fluorophore. Moreover, the probe FP can be employed for imaging Cu2+ in living cells. The new fluorescent probe developed in this study shows the advantages of good biocompatibility and low cytotoxicity. It can be adopted for the targeted detection of Cu2+ in cells, and it has promising applications in the mechanism research and diagnosis of Cu2+-associated diseases.

Construction of a Cu2+-Responsive NIR Fluorescent Probe and the Preliminary Evaluation of its Multifunctional Application

Cu2+ was deemed as toxic and the most common heavy metal pollution in the water and food. Meanwhile, endogenous Cu2+ was deeply involved in plenty of physiological and pathological processes of human. Cu2+ imbalance was related to multiple diseases. Here we developed a Cu2+-responsive NIR probe HX, which not only demonstrated obvious color change when subjected to Cu2+, but also showed linear-dependent NIR fluorescence emission to Cu2+ concentration for Cu2+ detection and quantification both in vitro and in vivo. When HX was applied to imaging Cu2+ in the cell or living animals, intracellular Cu2+ fluctuation and Cu2+ accumulation in the liver could be visualized to indicate the copper level in the cell or organs with low background signals. Meanwhile, by applying HX to monitor Cu2+ uptake in the tumor, copper transporter function could be evaluated to screen the patient who are sensitivity to platinum drug.

Cyclopentene ring effects in cyanine dyes: a handle to fine-tune photophysical properties

The aim of this study is to investigate the photophysical properties of a cyanine dye analogue by performing first-principles calculations based on density functional theory (DFT) and time dependent-DFT. Cationic cyanine dyes are the subject of great importance due to their versatile applications and the tunability of their photophysical properties, such as by modifying their end groups and chain length. An example of this is the vinylene shift, which is experimentally known for these molecules, and it consists of a bathochromic (red) shift of approximately 100 nm of the 0-0 vibronic transition when a vinyl group is added to the polymethine chain. Our study shows that when the saturated moiety C2H4 of the cyclopentene ring is added to the chain, it interacts with the conjugated π-system, resulting in a smaller HOMO-LUMO gap. Here, we demonstrate the origin of this interaction and how it can be used to fine tune the absorption energies of this class of dyes.

Malachite green: a long-buried water-soluble AIEgen with near-infrared fluorescence for living cell nucleus staining

Fluorescent imaging probes are crucial for exploring nucleus-related cellular events in live cells. Ideal probes should be photostable, small-sized, highly contrasted, and low in background. Here, we discovered that malachite green is a water-soluble near-infrared luminogen with aggregation-induced emission properties. Importantly, it can be used for living cell nucleus staining in a wash-free manner.

Design strategies and biological applications of β-galactosidase fluorescent sensor in ovarian cancer research and beyond

Beta-galactosidase (β-galactosidase), a lysosomal hydrolytic enzyme, plays a critical role in the catalytic hydrolysis of glycosidic bonds, leading to the conversion of lactose into galactose. This hydrolytic enzyme is used as a biomarker in various applications, including enzyme-linked immunosorbent assays (ELISAs), gene expression studies, tuberculosis classification, and in situ hybridization. β-Galactosidase abnormalities are linked to various diseases, such as ganglioside deposition, primary ovarian cancer, and cell senescence. Thus, effective detection of β-galactosidase activity may aid disease diagnoses and treatment. Activatable optical probes with high sensitivity, specificity, and spatiotemporal resolution imaging capabilities have become powerful tools for visualization and real time tracking in vivo in the past decade. This manuscript reviews the sensing mechanism, molecular design strategies, and advances of fluorescence probes in the biological application of β-galactosidase, particularly in the field of ovarian cancer research. Current challenges in tracking β-galactosidase and future directions are also discussed.

Design of 2-Pyridone Fluorophores for Brighter Emissions at Longer Wavelengths

The recent discovery of blue fluorophores with high quantum yields based on pyridone structures inspired the development of new low-molecular-weight fluorophores with bright emissions at tunable wavelengths, which are highly attractive for various applications. In this study, we propose a rational design strategy for 2-pyridone-based fluorophores with bright emissions at long wavelengths. With a detailed understanding of the positional substitution effects on each carbon atom of the 2-pyridone core, we developed a bright blue fluorophore (λabs =377 nm; λem =433 nm; ϵ=13,200 M-1  cm-1 ; ϕF =88 %) through C3 -aryl and C4 -ester substitutions followed by cyclization. Furthermore, by applying the intramolecular charge transfer (ICT) principle, we invented a bright green fluorophore through C3 - and C4 -diester and C6 -aryl substitutions. The ICT fluorophore based on the pyridone structure shows large molar absorptivity (ϵ=20,100 M-1  cm-1 ), longer emission wavelength (λem =539 nm), high emission quantum yield (ϕF =74 %), and large Stokes shift (Δv=5720 cm-1 ), which are comparable to those of practical fluorescent probes.

A new highly sensitive fluorescent probe for visualization of phosgene in liquid and gas phases

Phosgene is an important and widely used highly toxic chemical that poses a serious potential threat to public health and property if leaked or abused. Therefore, developing an efficient and convenient detection method for phosgene is of great significance. In this work, we synthesized a novel fluorescent probe, BCyP, based on benzohemicyanine for highly selective and sensitive detection of phosgene in both liquid and gas phases. The probe uses amino alcohol as a specific recognition group for phosgene and does not fluoresce due to the strong intramolecular charge transfer effect (ICT). However, in the presence of phosgene, the amino alcohol part in the probe can form oxazolidinone in situ with phosgene, reducing the ICT effect in the probe molecule and lighting fluorescence, thus realizing the selective phosgene detection. The probe exhibits good specificity towards phosgene, with significant fluorescence enhancement (approximately 400-fold), a remarkable Stokes shift (139 nm), a fast response speed (less than 17 s), and a low detection limit (0.12 ppm). Additionally, we prepared a phosgene detection paper strip loaded with the probe on filter paper and combined it with color recognition software on a smartphone to achieve visual detection of phosgene in the gas phase.

Increasing Chemical Diversity of B2 N2 Anthracene Derivatives by Introducing Continuous Multiple Boron-Nitrogen Units

Increasing the chemical diversity of organic semiconductors is essential to develop efficient electronic devices. In particular, the replacement of carbon-carbon (C-C) bonds with isoelectronic boron-nitrogen (B-N) bonds allows precise modulation of the electronic properties of semiconductors without significant structural changes. Although some researchers have reported the preparation of B2 N2 anthracene derivatives with two B-N bonds, no compounds with continuous multiple BN units have been prepared yet. Herein, we report the synthesis and characterization of a B2 N2 anthracene derivative with a BNBN unit formed by converting the BOBN unit at the zigzag edge. Compared to the all-carbon analogue 2-phenylanthracene, BNBN anthracene exhibits significant variations in the C-C bond length and a larger highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap. The experimentally determined bond lengths and electronic properties of BNBN anthracene are confirmed through theoretical calculations. The BOBN anthracene organic light-emitting diode, used as a blue host, exhibits a low driving voltage. The findings of this study may facilitate the development of larger acenes with multiple BN units and potential applications in organic electronics.

Combination of quinoxaline with pentamethinium system: Mitochondrial staining and targeting

Pentamethinium indolium salts are promising fluorescence probes and anticancer agents with high mitochondrial selectivity. We synthesized two indolium pentamethinium salts: a cyclic form with quinoxaline directly incorporated in the pentamethinium chain (cPMS) and an open form with quinoxaline substitution in the γ-position (oPMS). To better understand their properties, we studied their interaction with mitochondrial phospholipids (cardiolipin and phosphatidylcholine) by spectroscopic methods (UV-Vis, fluorescence, and NMR spectroscopy). Both compounds displayed significant affinity for cardiolipin and phosphatidylcholine, which was associated with a strong change in their UV-Vis spectra. Nevertheless, we surprisingly observed that fluorescence properties of cPMS changed in complex with both cardiolipin and phosphatidylcholine, whereas those of oPMS only changed in complex with cardiolipin. Both salts, especially cPMS, display high usability in mitochondrial imaging and are cytotoxic for cancer cells. The above clearly indicates that conjugates of pentamethinium and quinoxaline group, especially cPMS, represent promising structural motifs for designing mitochondrial-specific agents.

A wash-free fluorescent probe with a large Stokes shift for the identification of NAFL through tracing the change of lipid droplets

As one of the important organelles in cells, lipid droplets (LDs) are involved in various physiological processes, especially affecting the occurrence and progression of non-alcoholic fatty liver (NAFL). Therefore, it is of great significance to develop LD-specific probes with excellent biocompatibility, deep penetration and bright fluorescence. Herein, a fluorescent probe LD-HWZ was designed and synthesized based on triphenylamine and the dicyanoisophorone group. It is found that probe LD-HWZ has a large Stokes shift (Δλ = 160 nm in DMSO) and exhibits bright fluorescence in a lipid environment. In addition, biological experiments showed that LD-HWZ can localize in lipid droplets, which can be used to detect the dynamic changes of LDs. Importantly, LD-HWZ has been successfully used to discriminate NAFL tissues from normal livers. The excellent properties of probe LD-HWZ in this work are expected to shed new light on the design of lipid droplet probes for the study of fatty liver diagnosis.

Polydopamine-Based Nanoprobes Application in Optical Biosensing

Polydopamine (PDA), the synthetic counterpart of melanin, is a widely investigated bio-inspired material for its chemical and photophysical properties, and in the last few years, bio-application of PDA and PDA-based materials have had a dramatic increase. In this review, we described PDA application in optical biosensing, exploring its multiple roles as a nanomaterial. In optical sensing, PDA can not only be used for its intrinsic fluorescent and photoacoustic properties as a probe: in some cases, a sample optical signal can be derived by melanin generation in situ or it can be enhanced in another material thanks to PDA modification. The various possibilities of PDA use coupled with its biocompatibility will indeed widen even more its application in optical bioimaging.

A new class of teraryl-based AIEgen for highly selective imaging of intracellular lipid droplets and its detection in advanced-stage human cervical cancer tissues

Lipid droplets (LDs) have drawn much attention in recent years. They serve as the energy reservoir of cells and also play an important role in numerous physiological processes. Furthermore, LDs are found to be associated with several pathological conditions, including cancer and diabetes mellitus. Herein, we report a new class of teraryl-based donor-acceptor-appended aggregation-induced emission luminogen (AIEgen), 6a, for selective staining of intracellular LDs in in vitro live 3T3-L1 preadipocytes and the HeLa cancer cell line. In addition, AIEgen 6a was found to be capable of staining and quantifying the LD accumulation in the tissue sections of advanced-stage human cervical cancer patients. Unlike commercial LD staining dyes Nile Red, BODIPY and LipidTOX, AIEgen 6a showed a high Stokes shift (195 nm), a good fluorescence lifetime decay of 12.7 ns, and LD staining persisting for nearly two weeks.

New Fluorophore and Its Applications in Visualizing Polystyrene Nanoplastics in Bean Sprouts and HeLa Cells

In the domain of environmental science, pollutants of nanoscale plastic dimensions are acknowledged as subjects of intricate significance. Such entities, though minuscule, present formidable challenges to ecological systems and human health. The diminutive dimensions of these contaminants render their detection arduous, thus demanding the inception of avant-garde methodologies. The present manuscript postulates the employment of the tetraphenylethylene functional group with a fused xanthene (TPEF), a distinguished fluorophore, as an exemplary system for the discernment of nanoplastic particulates. The synthesis and characterization of TPEF have been exhaustively elucidated, revealing its paramount fluorescence attributes and inherent affinity for interaction with nanoplastics. When subjected to comparison with TPEF, nanoplastics are observed to manifest a more pronounced fluorescent luminescence than when associated with the conventional Nile Red (NR). Particularly, the TPEF has shown exceptional affinity for polystyrene (PS) nanoplastics. Further, the resilience of nanoplastics within the hypocotyl epidermis of soybeans, as well as their persistence in mung bean sprouts subsequent to rigorous rinsing protocols, has been meticulously examined. Additionally, this investigation furnishes empirical data signifying the existence of nano-dimensional plastic contaminants within HeLa cellular structures. The urgency of addressing the environmental ramifications engendered by these diminutive yet potent plastic constituents is emphatically highlighted in this manuscript. TPEF paves the way for prospective explorations, with the aspiration of devising efficacious mitigation strategies. Such strategies might encompass delineating the trajectories undertaken by nanoplastics within trophic networks or their ingress into human cellular architectures.

Julolidine-based small molecular probes for fluorescence imaging of RNA in live cells

Intracellular RNA imaging with organic small molecular probes has been an intense topic, although the number of such reported dyes, particularly dyes with high quantum yields and long wavelength excitation/emission, is quite limited. The present work reports the design and synthesis of three cationic julolidine-azolium conjugates (OX-JLD, BTZ-JLD and SEZ-JLD) as turn-on fluorescent probes with appreciably high quantum yields and brightness upon interaction with RNA. A structure-efficiency relationship has been established for their potential for the interaction and imaging of intracellular RNA. Given their chemical structure, the free rotation between the donor and the acceptor gets restricted when the probes bind with RNA resulting in strong fluorescence emission towards a higher wavelength upon photoexcitation. A detailed investigation revealed that the photophysical properties and the optical responses of two probes, viz. BTZ-JLD and SEZ-JLD, towards RNA are very promising and qualify them to be suitable candidates for biological studies, particularly for cellular imaging applications. The probes allow imaging of intracellular RNA with prominent staining of nucleoli in live cells under a range of physiological conditions. The results of the cellular digest test established the appreciable RNA selectivity of BTZ-JLD and SEZ-JLD inside the cellular environment. Moreover, a comparison between the relative intensity profile of SEZ-JLD before and after the RNA-digestion test inside the cellular environment indicated that the interference of cellular viscosity in fluorescence enhancement is insignificant, and hence, SEZ-JLD can be used as a cell membrane permeable cationic molecular probe for deep-red imaging of intracellular RNA with a good degree of selectivity.

Quantifying cell viability through organelle ratiometric probing

Detecting cell viability is crucial in research involving the precancerous discovery of abnormal cells, the evaluation of treatments, and drug toxicity testing. Although conventional methods afford cumulative results regarding cell viability based on a great number of cells, they do not permit investigating cell viability at the single-cell level. In response, we rationally designed and synthesized a fluorescent probe, PCV-1, to visualize cell viability under the super-resolution technology of structured illumination microscopy. Given its sensitivity to mitochondrial membrane potential and affinity to DNA, PCV-1's ability to stain mitochondria and nucleoli was observed in live and dead cells, respectively. During cell injury induced by drug treatment, PCV-1's migration from mitochondria to the nucleolus was dynamically visualized at the single-cell level. By extension, harnessing PCV-1's excellent photostability and signal-to-noise ratio and by comparing the fluorescence intensity of the two organelles, mitochondria and nucleoli, we developed a powerful analytical assay named organelle ratiometric probing (ORP) that we applied to quantitatively analyze and efficiently assess the viability of individual cells, thereby enabling deeper insights into the potential mechanisms of cell death. In ORP analysis with PCV-1, we identified 0.3 as the cutoff point for assessing whether adding a given drug will cause apparent cytotoxicity, which greatly expands the probe's applicability. To the best of our knowledge, PCV-1 is the first probe to allow visualizing cell death and cell injury under super-resolution imaging, and our proposed analytical assay using it paves the way for quantifying cell viability at the single-cell level.

Recent Development of Lysosome-Targeted Organic Fluorescent Probes for Reactive Oxygen Species

Reactive oxygen species (ROS) are extremely important for various biological functions. Lysosome plays key roles in cellular metabolism and has been known as the stomach of cells. The abnormalities and malfunctioning of lysosomal function are associated with many diseases. Accordingly, the quantitative monitoring and real-time imaging of ROS in lysosomes are of great interest. In recent years, with the advancement of fluorescence imaging, fluorescent ROS probes have received considerable interest in the biomedical field. Thus far, considerable efforts have been undertaken to create synthetic fluorescent probes for sensing ROS in lysosomes; however, specific review articles on this topic are still lacking. This review provides a general introduction to fluorescence imaging technology, the sensing mechanisms of fluorescent probes, lysosomes, and design strategies for lysosome-targetable fluorescent ROS probes. In addition, the latest advancements in organic small-molecule fluorescent probes for ROS detection within lysosomes are discussed. Finally, the main challenges and future perspectives for developing effective lysosome-targetable fluorescent ROS probes for biomedical applications are presented.

Modulating the Luminescence, Photosensitizing Properties, and Mitochondria-Targeting Ability of D-π-A-Structured Dihydrodibenzo[a,c]phenazines

Herein, pyridinium and 4-vinylpyridinium groups are introduced into the VIE-active N,N'-disubstituted-dihydrodibenzo[a,c]phenazines (DPAC) framework to afford a series of D-π-A-structured dihydrodibenzo[a,c]phenazines in consideration of the aggregation-benefited performance of the DPAC module and the potential mitochondria-targeting capability of the resultant pyridinium-decorated DPACs (DPAC-PyPF6 and DPAC-D-PyPF6). To modulate the properties and elucidate the structure-property relationship, the corresponding pyridinyl/4-vinylpyridinyl-substituted DPACs, i.e., DPAC-Py and DPAC-D-Py, are designed and studied as controls. It is found that the strong intramolecular charge transfer (ICT) effect enables the effective separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of DPAC-PyPF6 and DPAC-D-PyPF6, which is conducive to the generation of ROS. By adjusting the electron-accepting group and the π-bridge, the excitation, absorption, luminescence, photosensitizing properties as well as the mitochondria-targeting ability can be finely tuned. Both DPAC-PyPF6 and DPAC-D-PyPF6 display large Stokes shifts (70-222 nm), solvent-dependent absorptions and emissions, aggregation-induced emission (AIE), red fluorescence in the aggregated state (λem = 600-650 nm), aggregation-promoted photosensitizing ability with the relative singlet-oxygen quantum yields higher than 1.10, and a mitochondria-targeting ability with the Pearson coefficients larger than 0.85. DPAC-D-PyPF6 shows absorption maximum at a longer wavelength, slightly redder fluorescence and better photosensitivity as compared to DPAC-PyPF6, which consequently leads to the higher photocytotoxicity under the irradiation of white light as a result of the larger π-conjugation.

Naphthyridine-Based Electron Push-Pull-Type Amine-Reactive Fluorescent Probe for Sensing Amines and Proteins in Aqueous Media

In bioengineering, fluorescent amine-reactive probes are invaluable for the detection of amine species. In particular, targeting probes for lysine, which has a free amino group in amino acids, are a valid method for protein detection. For this purpose, many fluorescent "turn-on type" probes with amine reactivity have been developed; however, they require improvements. In the typical florescence probes, BODIPY and NBD analogs have small Stokes shifts based on absorption and emission and lability in an aqueous environment, respectively. In this study, a new class of fluorescent probes, 1,8-Nap-F, based on the electron push-pull-type 1,8-naphthyridine framework, was designed and investigated as an amine-reactive probe. Generally, electron push-pull-type fluorophores exhibit a large Stokes shift at the expense of fluorescent enhancement in aqueous media; thus, there is a trade-off between possessing a large Stokes shift and intense emission. However, 1,8-Nap-F reacts with primary amines, yielding emissive amine products with a large Stokes shift (>70 nm) without fluorescence quenching and side products, even in an aqueous environment, thereby overcoming the disadvantages of electron push-pull-type fluorophores and lability in aqueous conditions. By applying the specific features of 1,8-Nap-F, we achieved selective lysine detection and fluorescence bioimaging, such as endoplasmic reticulum-selective protein labeling and organelle staining, in living cells by utilizing amine-substituted derivatives.

Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes (1a-1b) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b, exhibited interesting photophysical properties including, longer wavelength excitation (λex ≈ 530-560 nm), bright near-infrared emission (λem ≈ 690-720 nm), high-fluorescence quantum yields (ϕfl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes (1a-1b) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a, due to the regio-effect. Probes (1a-1b) exhibited an excellent ability to visualize bacteria (Bacillus megaterium, Escherichia coli), and yeast (Saccharomyces cerevisiae) via fluorescence microscopy.

A novel benzo hemicyanine-based fluorescent probe for susceptible visualizing detection of phosgene

Leakage and misuse of phosgene, a common and highly hazardous industrial chemical, have always constituted a safety risk. Therefore, it is crucial to develop sensitive detection methods for gaseous phosgene. This work describes the design and development of a new fluorescent dye based on benzohemicyanine, as well as the synthesis of fluorescent probes for the sensitive detection of gaseous phosgene. Due to the excellent intramolecular charge transfer (ICT) effect from the strong electron-donating impact of the o-aminophenol group on benzo hemicyanine, the probe does not emit fluorescence. When the probe reacts with phosgene, the ICT effect is inhibited, and the result exhibits observable green fluorescence, thereby visualizing the response to phosgene. The probe offers exceptional sensitivity, a rapid response, and a low phosgene detection limit. In addition, we developed probe-loaded, portable test strips for the quick and sensitive detection of phosgene in the gas phase. Finally, the constructed probe-loaded test strips were utilized effectively to monitor the simulated phosgene leakage.

Excitation dependent emissive multi stimuli responsive ESIPT organic luminogen for monitoring sea food freshness

Excited state intramolecular proton transfer (ESIPT) organic luminophores with excitation wavelength-dependent color tunability have drawn significant attention due to their exceptional photoluminescent properties in solution and solid state. A novel salicylaldehyde-based Schiff's base molecule, (E)-N'-(3,5-dibromo-2-hydroxybenzylidene)benzohydrazide (BHN) exhibited stimuli (excitation wavelength and pH) induced changes in fluorescence properties which was utilised for applications like trace level water sensing in organic solvents (THF, acetone and DMF), detection and quantification of biogenic amines and anticounterfeiting. In the solution state, BHN rendered a ratiometric detection and quantification of ammonia, diethylamine and trimethylamine, which is further supported by DFT studies. The photoluminescent response of BHN towards various biogenic amines was later utilised to monitor shrimp freshness. The investigation carried out highlights the potential versatility of ESIPT hydrazones, which renders multi stimuli responsive behaviour that can be utilised for water sensing, anticounterfeiting and the detection and quantification of biogenic amines.

Quantifying cell viability through organelle ratiometric probing

Detecting cell viability is crucial in research involving the precancerous discovery of abnormal cells, the evaluation of treatments, and drug toxicity testing. Although conventional methods afford cumulative results regarding cell viability based on a great number of cells, they do not permit investigating cell viability at the single-cell level. In response, we rationally designed and synthesized a fluorescent probe, PCV-1, to visualize cell viability under the super-resolution technology of structured illumination microscopy. Given its sensitivity to mitochondrial membrane potential and affinity to DNA, PCV-1's ability to stain mitochondria and nucleoli was observed in live and dead cells, respectively. During cell injury induced by drug treatment, PCV-1's migration from mitochondria to the nucleolus was dynamically visualized at the single-cell level. By extension, harnessing PCV-1's excellent photostability and signal-to-noise ratio and by comparing the fluorescence intensity of the two organelles, mitochondria and nucleoli, we developed a powerful analytical assay named organelle ratiometric probing (ORP) that we applied to quantitatively analyze and efficiently assess the viability of individual cells, thereby enabling deeper insights into the potential mechanisms of cell death. In ORP analysis with PCV-1, we identified 0.3 as the cutoff point for assessing whether adding a given drug will cause apparent cytotoxicity, which greatly expands the probe's applicability. To the best of our knowledge, PCV-1 is the first probe to allow visualizing cell death and cell injury under super-resolution imaging, and our proposed analytical assay using it paves the way for quantifying cell viability at the single-cell level.

Bathochromic Shift of Fluorescence Peak in Dipyrrolo[1,2-a:2',1'-c]quinoxaline by Introducing Each of Electron-Donating and Electron-Withdrawing Substituent

Development of organic fluorophore is an important theme. Especially, the fluorophores with longer fluorescence peaks are useful to biological probes. One of the methods to change the fluorescence peak is the introduction of substituents. However, opposing characteristics of the substituents lead to different changes in the fluorescence peaks. Furthermore, the introduction of the substituent also affects their electric properties. Thus, if the materials were developed with the substituent effect on the optical and electric properties separately, it will be useful to design the functional materials related to both optical and electric properties. Herein, we investigated the substituent effect of dipyrrolo[1,2-a:2',1'-c]quinoxalines on fluorescence properties. We synthesized the compounds bearing electron-donating or electron-withdrawing substituents at the benzene ring on dipyrrolo[1,2-a:2',1'-c]quinoxaline, which would have more direct influence on the optical properties. By introducing each substituent at the 6 position of dipyrrolo[1,2-a:2',1'-c]quinoxaline, the bathochromic shift was observed in the fluorescence spectra. In the case of fluorine substituent, the change of the fluorescence peak reached was about 19 nm. Using a TDDFT calculation, we explained the reason for such a substituent effect that large on the increment of LUMO energy or decrement of HOMO energy occurred by introducing electron-withdrawing or electron-donating substituents at the 6 position, respectively. The substituent effect on the change of orbital energies is typical although the different characteristics of substituents resulted in the similar tendency about the change of fluorescence peak. Furthermore, with the introduction of phenyl substituents at the 3 and 10 positions, we achieved 40-50 nm longer fluorescence peaks compared with that of the original dipyrrolo[1,2-a:2',1'-c]quinoxaline.

Multifunctional AIE Nanosphere-Based "Nanobomb" for Trimodal Imaging-Guided Photothermal/Photodynamic/Pharmacological Therapy of Drug-Resistant Bacterial Infections

Injudicious or inappropriate use of antibiotics has led to the prevalence of drug-resistant bacteria, posing a huge menace to global health. Here, a self-assembled aggregation-induced emission (AIE) nanosphere (AIE-PEG₁₀₀₀ NPs) that simultaneously possesses near-infrared region II (NIR-II) fluorescence emissive, photothermal, and photodynamic properties is prepared using a multifunctional AIE luminogen (AIE-4COOH). The AIE-PEG₁₀₀₀ NPs were encapsulated with teicoplanin (Tei) and ammonium bicarbonate (AB) into lipid nanovesicles to form a laser-activated "nanobomb" (AIE-Tei@AB NVs) for the multimodal theranostics of drug-resistant bacterial infections. In vivo experiments validate that the "nanobomb" enables high-performance NIR-II fluorescence, infrared thermal, and ultrasound (AB decomposition during the photothermal process to produce numerous CO₂/NH₃ bubbles, which is an efficient ultrasound contrast agent) imaging of multidrug-resistant bacteria-infected foci after intravenous administration of AIE-Tei@AB NVs followed by 660 nm laser stimulation. The highly efficient photothermal and photodynamic features of AIE-Tei@AB NVs, combined with the excellent pharmacological property of rapidly released Tei during bubble generation and NV disintegration, collectively promote broad-spectrum eradication of three clinically isolated multidrug-resistant bacteria strains and rapid healing of infected wounds. This multimodal imaging-guided synergistic therapeutic strategy can be extended for the theranostics of superbugs.

Organelle-Targeted Fluorescent Probes for Sulfane Sulfur Species

Sulfane sulfurs, which include hydropersulfides (RSSH), hydrogen polysulfides (H2Sn, n > 1), and polysulfides (RSnR, n > 2), play important roles in cellular redox biology and are closely linked to hydrogen sulfide (H2S) signaling. While most studies on sulfane sulfur detection have focused on sulfane sulfurs in the whole cell, increasing the recognition of the effects of reactive sulfur species on the functions of various subcellular organelles has emerged. This has driven a need for organelle-targeted detection methods. However, the detection of sulfane sulfurs, particularly of RSSH and H2Sn, in biological systems is still a challenge due to their low endogenous concentrations and instabilities. In this review, we summarize the development and design of organelle-targeted fluorescent sulfane sulfur probes, examine their organelle-targeting strategies and choices of fluorophores (e.g., ratiometric, near-infrared, etc.), and discuss their mechanisms and ability to detect endogenous and exogenous sulfane sulfur species. We also present the advantages and limitations of the probes and propose directions for future work on this topic.

Fluorescence, ultrasonic and photoacoustic imaging for analysis and diagnosis of diseases

Biomedical imaging technology, which allows us to peer deeply within living subjects and visually explore the delivery and distribution of agents in living things, is producing tremendous opportunities for the early diagnosis and precise therapy of diseases. In this feature article, based on reviewing the latest representative examples of progress together with our recent efforts in the bioimaging field, we intend to introduce three typical kinds of non-invasive imaging technologies, i.e., fluorescence, ultrasonic and photoacoustic imaging, in which optical and/or acoustic signals are employed for analyzing various diseases. In particular, fluorescence imaging possesses a series of outstanding advantages, such as high temporal resolution, as well as rapid and sensitive feedback. Hence, in the first section, we will introduce the latest studies on developing novel fluorescence imaging methods for imaging bacterial infections, cancer and lymph node metastasis in a long-term and real-time manner. However, the issues of imaging penetration depth induced by photon scattering and light attenuation of biological tissue limit their widespread in vivo imaging applications. Taking advantage of the excellect penetration depth of acoustic signals, ultrasonic imaging has been widely applied for determining the location, size and shape of organs, identifying normal and abnormal tissues, as well as confirming the edges of lesions in hospitals. Thus, in the second section, we will briefly summarize recent advances in ultrasonic imaging techniques for diagnosing diseases in deep tissues. Nevertheless, the absence of lesion targeting and dependency on a professional technician may lead to the possibility of false-positive diagnosis. By combining the merits of both optical and acoustic signals, newly-developed photoacoustic imaging, simultaneously featuring higher temporal and spatial resolution with good sensitivity, as well as deeper penetration depth, is discussed in the third secretion. In the final part, we further discuss the major challenges and prospects for developing imaging technology for accurate disease diagnosis. We believe that these non-invasive imaging technologies will introduce a new perspective for the precise diagnosis of various diseases in the future.