Syntheses and Applications of Coumarin-Derived Fluorescent Probes for Real-Time Monitoring of NAD(P)H Dynamics in Living Cells across Diverse Chemical Environments

Fluorescent probes play a crucial role in elucidating cellular processes, with NAD(P)H sensing being pivotal in understanding cellular metabolism and redox biology. Here, the development and characterization of three fluorescent probes, A, B, and C, based on the coumarin platform for monitoring of NAD(P)H levels in living cells are described. Probes A and B incorporate a coumarin-cyanine hybrid structure with vinyl and thiophene connection bridges to 3-quinolinium acceptors, respectively, while probe C introduces a dicyano moiety for replacement of the lactone carbonyl group of probe A which increases the reaction rate of the probe with NAD(P)H. Initially, all probes exhibit subdued fluorescence due to intramolecular charge transfer (ICT) quenching. However, upon hydride transfer by NAD(P)H, fluorescence activation is triggered through enhanced ICT. Theoretical calculations confirm that the electronic absorption changes upon the addition of hydride to originate from the quinoline moiety instead of the coumarin section and end up in the middle section, illustrating how the addition of hydride affects the nature of this absorption. Control and dose-response experiments provide conclusive evidence of probe C's specificity and reliability in identifying intracellular NAD(P)H levels within HeLa cells. Furthermore, colocalization studies indicate probe C's selective targeting of mitochondria. Investigation into metabolic substrates reveals the influence of glucose, maltose, pyruvate, lactate, acesulfame potassium, and aspartame on NAD(P)H levels, shedding light on cellular responses to nutrient availability and artificial sweeteners. Additionally, we explore the consequence of oxaliplatin on cellular NAD(P)H levels, revealing complex interplays between DNA damage repair, metabolic reprogramming, and enzyme activities. In vivo studies utilizing starved fruit fly larvae underscore probe C's efficacy in monitoring NAD(P)H dynamics in response to external compounds. These findings highlight probe C's utility as a versatile tool for investigating NAD(P)H signaling pathways in biomedical research contexts, offering insights into cellular metabolism, stress responses, and disease mechanisms.

Near-Infrared Spontaneously Blinking Fluorophores for Live Cell Super-Resolution Imaging with Minimized Phototoxicity

Single-molecule localization microscopy (SMLM) requires high-intensity laser irradiation, typically exceeding kW/cm2, to yield a sufficient photon count. However, this intense visible light exposure incurs substantial cellular toxicity, hindering its use in living cells. Here, we developed a class of near-infrared (NIR) spontaneously blinking fluorophores for SMLM. These NIR fluorophores are a combination of rhodamine spirolactams and merocyanine derivatives, where the rhodamine spirolactam component converts between a bright and dark state based on pH-dependent spirocyclization and merocyanine derivatives shift the excitation wavelength into the infrared. Single-molecule characterizations demonstrated their potential for SMLM. At a moderate power density of 3.93 kW/cm2, these probes exhibit duty cycle as low as 0.18% and an emission rate as high as 26,700 photons/s. Phototoxicity assessment under single-molecule imaging conditions reveals that NIR illumination (721 nm) minimizes harm to living cells. Employing these NIR fluorophores, we successfully captured time-lapse super-resolution tracking of mitochondria at a Fourier ring correlation (FRC) resolution of 69.4 nm and reconstructed the ultrastructures of endoplasmic reticulum (ER) in living cells.

Design of an Acidic pH-Activated NIR Fluorescent Convertible Rhodamine-Hemicyanine Probe-Peptide Conjugate for Living Cancer Cell Active Targeted Selective Tracking of Lysosomes

We have synthesized an acidic pH-activatable dual targeting ratiometric fluorescent probe-peptide conjugate using the SPPS protocol on Rink amide AM resin. Living carcinoma cell specific active targeting, successive cell penetration, and selective staining of lysosomes are accomplished. Real-time monitoring of lysosomes, 3D, and multicolor cancer cell imaging are also attained. The de novo design consists of the integration of multifunctionality into a single molecular scaffold, e. g., RGDS peptide residue to target cancer cell surface overexpressed receptor αVβ3 integrin, live-cell penetrating organic unsymmetrical rhodamine-hemicyanine chromophore comprising a lysosome targeting morpholine group, and an acidic pH openable spiro-lactam ring for a visible-to-NIR switchable ratiometric response. Water-soluble fluorescent probe-peptide conjugate exhibits intramolecular spirolactamization at basic pH through Arg amide N. The visible spirolactam state predominantly exists at physiological and basic pH and can be switched to the highly conjugated NIR open amide state (λem=735 nm) through spiro-lactam ring opening triggered by acidic pH with a huge bathochromic shift (Δλabs=336 nm, ΔλFL=265 nm). Moreover, pH-sensitive ratiometric optical switching is achieved. This in situ acidic cancer cell lysosome activatable multifunctional fluorophore-peptide conjugate shows augmented molar absorptivity, enhanced quantum yield, and improved fluorescence lifetime at acidic lysosomal pH; negligible cytotoxicity; and dual targeted ratiometric imaging capability of living cancer cell selective lysosomes with a pKa value of 5.1.

Targeted theranostics: Near-infrared triterpenoic acid-rhodamine conjugates as prerequisites for precise cancer diagnosis and therapy

Pentacyclic triterpenoic acids have shown excellent potential as starting materials for the synthesis of highly cytotoxic agents with significantly reduced toxicity for non-malignant cells. This study focuses on the development of triterpenoic acid-rhodamine conjugates with fluorescence shifted to the near-infrared (NIR) region for theranostic applications in cancer research. Spectral analysis revealed emission wavelengths around λ = 760 nm, enabling stronger signals and deeper tissue penetration. The conjugates were evaluated using SRB assays on tumor cell lines and non-malignant fibroblasts, demonstrating low nanomolar activity and high selectivity, similarly to their known rhodamine B counterparts. Additional staining experiments proved their mode of action as mitocans.

Rhodamine-Based Cyclic Hydroxamate as Fluorescent pH Probe for Imaging of Lysosomes

Monitoring the microenvironment within specific cellular regions is crucial for a comprehensive understanding of life events. Fluorescent probes working in different ranges of pH regions have been developed for the local imaging of different pH environments. Especially, rhodamine-based fluorescent pH probes have been of great interest due to their ON/OFF fluorescence depending on the spirolactam ring's opening/closure. By introducing the N-alkyl-hydroxamic acid instead of the alkyl amines in the spirolactam of rhodamine, we were able to tune the pH range where the ring opening and closing of the spirolactam occurs. This six-membered cyclic hydroxamate spirolactam ring of rhodamine B proved to be highly fluorescent in acidic pH environments. In addition, we could monitor pH changes of lysosomes in live cells and zebrafish.

A methionine biomolecule-modified chromenylium-cyanine fluorescent probe for the analysis of Hg2+ in the environment and living cells

The objective of the study is, for the first time, to construct a new near infrared (NIR) fluorophore, spectrophotometric, colorimetric, ratiometric, and turn-on probe (CSME) based on chromenylium cyanine platform decorated with methionine biomolecule to provide an efficient solution for critical shortcoming to be encountered for analysis of hazardous Hg²⁺ in environment and living cell. The CSME structure and its interaction with Hg²⁺ ion were evaluated by NMR, FTIR, MS, UV-Vis and fluorescence methods as well as Density Functional Theory (DFT) calculations. The none fluorescence CSME having spirolactam ring only interacted with Hg²⁺ in aqueous solution including competing ions. This interaction caused the fluorescence CSME with opened spirolactam form which exhibited spectral and colorimetric changes in the NIR region. The probe based on UV-Vis and fluorescence techniques respond in 90 s, has wide linear ranges (for UV-Vis: 6.29 × 10^-8 - 1.86 × 10^-4 M; for fluorescence: 9.49 × 10^-9 - 1.13 × 10^-5 M), and has a lower Limit of Detection (LOD) value (for fluorescence: 4.93 × 10^-9 M, 0.99 ng/mL) than the value predicted by the US Environmental Protection Agency (EPA) organization. Hg²⁺ analysis was performed in drinking and tap water with low Relative Standard Deviation (RSD) values and high recovery. Smartphone and living cell applications were successfully performed for colorimetric sensing Hg²⁺ in real samples and 3T3 cells, respectively.

Constant Conversion Rate of Endolysosomes Revealed by a pH-Sensitive Fluorescent Probe

Endolysosome dynamics plays an important role in autophagosome biogenesis. Hence, imaging the subcellular dynamics of endolysosomes using high-resolution fluorescent imaging techniques would deepen our understanding of autophagy and benefit the development of pharmaceuticals against endosome-related diseases. Taking advantage of the intramolecular charge-transfer mechanism, herein we report a cationic quinolinium-based fluorescent probe (PyQPMe) that exhibits excellent pH-sensitive fluorescence in endolysosomes at different stages of interest. A systematic photophysical and computational study on PyQPMe was carried out to rationalize its highly pH-dependent absorption and emission spectra. The large Stokes shift and strong fluorescence intensity of PyQPMe can effectively reduce the background noise caused by excitation light and microenvironments and provide a high signal-to-noise ratio for high-resolution imaging of endolysosomes. By applying PyQPMe as a small molecular probe in live cells, we were able to reveal a constant conversion rate from early endosomes to late endosomes/lysosomes during autophagy at the submicron level.

Preparation and pH Detection Performance of Rosin-Based Fluorescent Polyurethane Microspheres

Rosin-based fluorescent polyurethane emulsion (FPU) was prepared using isophorone diisocyanate, ester of acrylic rosin and glycidyl methacrylate, 1,5-dihydroxy naphthalene (1,5-DN), and 1,4-butanediol as the raw materials. Then, rosin-based fluorescent polyurethane microspheres (FPUMs) were successfully prepared by suspension polymerization method using FPU as the main material, azodiisobutyronitrile as the initiator, and gelatin as the dispersant. FPUMs were characterized by Fourier transform infrared spectra, thermogravimetric analysis, optical microscopy, scanning electron microscopy and fluorescence spectra, and the response performance of FPUMs to pH was studied. The results showed that FPUMs were successfully prepared. With the increase of the level of 1,5-DN, the particle size of FPUMs increased gradually, and the fluorescence intensity increased first and then decreased. When the level of 1,5-DN was 3 wt.%, the average particle size was 49.3 μm, the particle distribution index (PDI) was 1.05, and the fluorescence intensity was the largest (3662 a.u.). The fluorescence intensity of FPUMs increased linearly with the decrease of pH, which can be used for pH detection in solution. Furthermore, the FPUMs exhibited good thermal stability, anti-interference and recoverability.

pH-Switched Near-Infrared Fluorescent Strategy for Ratiometric Detection of ONOO- in Lysosomes and Precise Imaging of Oxidative Stress in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is well-known as a kind of autoimmune disease, which brings unbearable pain to the patients by multiple organ complications besides arthritis. To date, RA can be hardly cured, but early diagnosis and standard treatment can relieve symptoms and pain. Therefore, an effective tool to assist the early diagnosis of RA deserves considerable attention. On account of the overexpressed ONOO^- during the early stage of RA, a near-infrared (NIR) receptor, Lyso-Cy, is proposed in this work by linker chemistry to expand the conjugated rhodamine framework by cyanine groups. Contributed by the pH-sensitive spiral ring in rhodamine, receptor Lyso-Cy has been found to be workable in lysosomes specifically, which was confirmed by the pH-dependent spectra with a narrow responding region and a well-calculated pK_a value of 5.81. We presented an excellent ratiometric sensing protocol for ONOO^- in an acidic environment, which was also available for targeting ONOO^- in lysosomes selectively. This innovative dual-targeting responsive design is expected to be promising for assisting RA diagnosis at an early stage with respect to the joint inflammatory model established in this work at the organism level.

Rational design of pH-activated upconversion luminescent nanoprobes for bioimaging of tumor acidic microenvironment and the enhancement of photothermal therapy

The development of effective and safe tumor photothermal therapeutic strategies has attracted considerable attention. Herein, we synthesized tumor microenvironment (TME)-activatable self-assembling organic nanotheranostics (NRhD-PEG-X NPs (X = 1, 2, 3, and 4)) for precise tumor targeting and upconversion image-guided photothermal therapy (PTT). The amphiphilic polymer NRhD-PEG-X consisted of upconversion luminescent probes (NRhD) modified with polyethylene glycol (PEG) of various lengths. The continuous external irradiation-free photothermal NRhD-PEG-4 NPs with pKa 6.70 displayed high sensitivity and selectivity to protons, resulting in the turn-on upconversion luminescence and enhanced photothermal properties in the acidic TME without asynchronous therapy and side effects. This nanotheranostic offers acidic activatability, tumor targetability, and PTT enhancement, thus allowing autofluorescence-free upconversion luminescent imaging-guided precision PTT. Our strategy affords a paradigm to develop activatable theranostic nanoplatforms for precision medicine. STATEMENT OF SIGNIFICANCE: As a hyperthermia-based treatment, activatable photothermal therapy (PTT) is highly significant in tumor treatment. Herein, we develop acidic tumor microenvironment-activatable nanotheranostics for upconversion luminescent imaging-guided diagnosis and precision tumor-targeted PTT. PEGylation of upconversion dyes not only could self-assemble to yield organic nanoparticles in water, but it could also significantly improve biocompatibility, stability, and circulation time and tune significantly the pKa value of nanoparticles. In an acidic tumor microenvironment, NRhD-PEG-4 NPs with pKa 6.70 show high sensitivity to release NRhDH⁺-PEG-4 NPs, which exhibit good upconversion luminescence and enhanced photothermal effect. Therefore, upconversion luminescence imaging-guided precision PTT has high potential to enhance cancer diagnostic and therapeutic efficiency.

Upconversion fluorescent nanoprobe based on 4-NP reversible structure for a wide range of pH determination

Accurate detection of pH value has received more and more attention in various fields. However, most reported probes show pH values in the acidic or alkaline range and work within...

Development of a Simple Dansyl-Based PH Fluorescent Probe in Acidic Medium and Its Application in Cell Imaging

A simple pH fluorescent probe based on dansyl derivative (Bu-Dns) was synthesized via one-step reaction between dansyl chloride and 4-bromobutan-1-amine hydrobromide. The obtained probe showed good selectivity and sensitivity toward H⁺ in acidic medium over two pH units (~4-2). At pH > 4, Bu-Dns solution emitted yellow fluorescent light, which became gradually weaker with decreasing pH value. At pH below 2, complete fluorescence quenching occurred. The pH response of Bu-Dns was ascribed to the protonation of dimethylamine group. The lack of influence of metal ion on pH response increases the prevalence of Bu-Dns in the potential detection of pH variation in acidic aqueous media. More importantly, it can sense the intracellular pH change in acidic range.

A ratiometric near-infrared fluorescent probe based on a novel reactive cyanine platform for mitochondrial pH detection

A near-infrared reactive cyanine platform (probe A) was prepared by condensation of 9-chloro-1,2,3,4-tetrahydro-10-methyl-acridinium iodide with Fisher's aldehyde. A near-infrared fluorescent probe (probe B) was prepared by modifying a reactive chlorine atom of probe A with tert-butyl(2-aminoethyl)carbamate through a substitution reaction. The deprotection of the Boc group of probe B was achieved under an acidic condition, affording an amine-functionalized cyanine dye (probe C). A near-infrared ratiometric fluorescent probe (probe D) for mitochondrial pH detection was synthesized by conjugating a FRET coumarin donor to a FRET cyanine acceptor (probe C) through an amide bond connection. Probe A shows low fluorescence of 2% due to an electron-withdrawing chlorine atom, while probes B-D display high fluorescence quantum yields of 60%, 32%, and 35% in aqueous solutions containing 10% ethanol, respectively. Probes B-D show strong fluorescence with push-pull molecular structures in neutral and basic pH conditions. However, protonation of the probe's second amine at the 9-position under acidic condition disrupts the push-pull feature of the probes, resulting in fluorescence quenching of the new cyanine fluorophores. The probes can selectively stain mitochondria, while probe D was employed to detect pH changes in HeLa cells and Drosophila melanogaster first-instar larvae.

Real-time tracking lysosomal pH changes under heatstroke and redox stress with a novel near-infrared emissive probe

Lysosomes are important subcellular organelles with acidic pH. The change of lysosomal pH can affect the normal function and activity of cells. To conveniently detect and visualize lysosomal pH changes, we designed herein a novel fluorescent probe NIR-Rh-LysopH. The probe is based on a Rhodamine 101 derivative, which was modified to include a fused tetrahydroquinoxaline ring to obtain near-infrared fluorescence and a methylcarbitol moiety to locate the lysosome. Based on the proton-induced spirolactam ring-opening mechanism, NIR-Rh-LysopH showed rapid, selective, sensitive, and reversible near-infrared fluorescence responses around 686 nm (Stokes shift 88 nm) with a pKa value of 5.70. From pH 7.4 to 4.0, about 285 folds of fluorescence enhancement was observed. Cell experiments showed that NIR-Rh-LysopH has low cytotoxicity and excellent lysosome-targeting ability. Moreover, NIR-Rh-LysopH was applied successfully to track lysosomal pH changes induced by drugs (such as chloroquine and dexamethasone), heatstroke, and redox stress. Thus, NIR-Rh-LysopH is very promising for conveniently tracking lysosomal pH changes and studying the related life processes.

Time-Resolved Luminescent High-Throughput Screening Platform for Lysosomotropic Compounds in Living Cells

Lysosomes are membrane-bound organelles that regulate protein degradation and cellular organelle recycling. Homeostatic alteration by lysosomotropic compounds has been suggested as a potential approach for the treatment of cancer. However, because of the high false-negative rate resulting from strong fluorescent background noise, few luminescent high-throughput screening methods for lysosomotropic compounds have been developed for cancer therapy. Imidazole is a five-membered heterocycle that can act within the acidic interior of lysosomes. To develop an efficient lysosomotropic compound screening system, we introduced an imidazole group to iridium-based complexes and designed a long-lifetime lysosomal probe to monitor lysosomal activity in living cells. By integrating time-resolved emission spectroscopy (TRES) with the novel iridium-based lysosomal probe, a high-throughput screening platform capable of overcoming background fluorescent interference in living cells was developed for discovering lysosomotropic drugs. As a proof-of-concept, 400 FDA/EMA-approved drugs were screened using the TRES system, revealing five compounds as potential lysosomotropic agents. Significantly, the most promising potent lysosomotropic compound (mitoxantrone) identified in this work would have showed less activity if screened using a commercial lysosomal probe because of interference from the intrinsic fluorescence of mitoxantrone. We anticipate that this TRES-based high-throughput screening system could facilitate the development of more lysosomotropic drugs by avoiding false results arising from the intrinsic fluorescence of both bioactive compounds and/or the cell background.

Lysosome-targeting pH indicator based on peri-fused naphthalene monoimide with superior stability for long term live cell imaging

Lysosomal pH is altered in many pathophysiological conditions. We describe synthesis and spectral properties of a new lysosomal fluorescent marker dye suitable for microscopic evaluation of lysosomal distribution and pH changes.

pH-Sensitive Near-IR Emitting Dinuclear Ruthenium Complex for Recognition, Two-Photon Luminescent Imaging, and Subcellular Localization of Cancer Cells

A dinuclear Ru(II) complex of [(bpy)₂Ru(Hdip)Ru(H₂bip)](ClO₄)₄ {bpy is 2,2'-bipyridine, Hdip is 2-(2,6-di(pyridin-2-yl)-pyridin-4-yl)-1H-imidazo[4,5-f]-[1,10]phenanthroline, and H₂bip is 2,6-bis(imidazole-2-yl)-pyridine} was synthesized and characterized by elemental analysis, mass spectrometry, and ¹H NMR spectroscopy. Spectrophotometric pH titrations in aqueous buffer and in vitro cell experiments indicated the response ability of the complex to pH fluctuations in the physiological pH range (6.0-8.0). The complex was found to be capable of differentiating live HeLa cells from healthy HEK293 cells by selectively accumulating in lysosomes of the HeLa cells. The low cytotoxicity (IC₅₀ > 100 μM), a large Stokes shift (∼200 nm), strong near-IR emission at ∼700 nm, a relatively long excited state lifetime, high photostability, and solubility make this complex considerably promising in real-time tracking and visualization of lysosomes in live cells. More interestingly, the tumor cell-specific two-photon luminescent imaging properties also endow this Ru complex with potential for applications in high-resolution tumor imaging and luminescence-guided tumor resection.

Multi-Dimensionally Extended Functionalization Innovates to an Entropy-Driven Detection of Multi-miRNAs for One-Step Cancer Screening and Diagnosis in Living Cells

Compared with tedious multi-step detections, multi-functional nanoprobes are effective for one-step screening and diagnosis of cancers by multi-detection of microRNAs (miRNAs). However, limited probe density, spatial mutual interference, and low target-triggered hybridization efficiency of nanoprobes will hinder intracellular applications. Here, for obtaining high loading density but low spatial mutual interference between functional biomolecules on nanoprobes, an extended biofunctionalization in three dimensions (the two-dimensional surface and a special "height" direction) is designed. Therefore, a multi-functional probe is constructed for one-step detection of multi-miRNAs for cancer screening and diagnosis. With linker-bridged multiple single-stranded DNAs swung out rigidly, multi-dimensionally extended upconversion nanorods (ME-UCNRs) covered by chitosan are constructed to load and deliver multiple biomolecules into living cells. Escaping from endolysosomes, ME-UCNRs maintain good biological activities of functionalized DNAs for effective detection of multi-miRNAs in living cells. Thereby, with multiple targets of miRNAs, toehold-mediated entropy-driven strand displacements are employed to give respectively changed fluorescent signals via fluorescence resonance energy transfer. Thus, a universal cancer biomarker of miR-21 and two specific liver-cancer biomarkers (miR-199a and miR-224) are efficiently detected through a one-step detection. By discriminating cancer cells from normal ones and determining liver-cancer cells simultaneously, this work innovates an efficient and definite one-step strategy for fast screening and early cancer diagnosis.

Ratiometric fluorescent probes based on through-bond energy transfer of cyanine donors to near-infrared hemicyanine acceptors for mitochondrial pH detection and monitoring of mitophagy

Two ratiometric near-infrared fluorescent probes have been developed to selectively detect mitochondrial pH changes based on highly efficient through-bond energy transfer (TBET) from cyanine donors to near-infrared hemicyanine acceptors. The probes consist of identical cyanine donors connected to different hemicyanine acceptors with a spirolactam ring structure linked via a biphenyl linkage. At neutral or basic pH, the probes display only fluorescence of the cyanine donors when they are excited at 520 nm. However, acidic pH conditions trigger spirolactam ring opening, leading to increased π-conjugation of the hemicyanine acceptors, resulting in new near-infrared fluorescence peaks at 740 nm and 780 nm for probes A and B, respectively. This results in ratiometric fluorescence responses of the probes to pH changes indicated by decreases of the donor fluorescence and increases of the acceptor fluorescence under donor excitation at 520 nm due to a highly efficient TBET from the donors to the acceptors. The probes only show cyanine donor fluorescence in alkaline-pH mitochondria. However, the probes show moderate fluorescence decreases of the cyanine donor and considerable fluorescence increases of hemicyanine acceptors during the mitophagy process induced by nutrient starvation or under drug treatment. The probes display rapid, selective, and sensitive responses to pH changes over metal ions, good membrane penetration, good photostability, large pseudo-Stokes shifts, low cytotoxicity, mitochondria-targeting, and mitophagy-tracking capabilities.

Hemicyanine based naked-eye ratiometric fluorescent probe for monitoring lysosomal pH and its application

Lysosome as the typical acidic organelle involved in many biological processes, the dysfunction of its pH would alter many diseases. Small-molecule fluorescent probe has been considered as vital tool for monitoring pH fluctuation in living cells. Herein, a hemicyanine based ratiometric fluorescent probe was synthesized, namely, 2,3-trimethy-3-[2-(dimethyl-amino-4-yl) vinyl]-3H-benzo[e]indole (BiDL), for rapidly detection pH under acidic conditions. BiDL exhibited ratio fluorescence emission (F_534nm/F_622nm) characteristic with pK_a 4.25. BiDL showed good linearly response in pH 3.4-4.82, indicating that the probe can be used for quantitative detection pH. The probe also displayed large Stokes shift (112 nm/201 nm) under neutral and acidic conditions, respectively, which could effectively reduce the excitation interference. BiDL had excellent cell membrane permeability, good photo-stability and low toxicity in living cells. The dual-channel confocal fluorescent microscopic ratiometric imaging application of pH in HeLa cells and lysosome were achieved successfully, indicating that BiDL has good potential for investigating lysosome-relevant pathological and physiological processes.

An electron-deficiency-based framework for NIR-II fluorescence probes

Increasing the electron withdrawing ability of substituents in monochlorinated BODIPY could vary the emission from the NIR-I to NIR-II region together with enhanced response rate, indicative of a promising approach for activatable NIR-II probes.

De Novo-Designed Near-Infrared Nanoaggregates for Super-Resolution Monitoring of Lysosomes in Cells, in Whole Organoids, and in Vivo

As the cleaners of cells, lysosomes play an important role in circulating organic matter within cells, recovering damaged organelles, and removing waste via endocytosis. Because lysosome dysfunction is associated with various diseases-lysosomal storage diseases, inherited diseases, rheumatoid arthritis, and even shock-it is vital to monitor the movement of lysosomes in cells and in vivo. To that purpose, a method of optical imaging, super-resolution imaging technology (e.g., SIM and STORM), can overcome the limitations of traditional optical imaging and afford a range of possibilities for fluorescence imaging. However, the short wavelength excitation and easy photobleaching of super-resolution fluorescence probes somewhat problematize super-resolution imaging. As described herein, we designed a low-toxicity, photostable, near-infrared small molecule fluorescence probe HD-Br for use in the super-resolution imaging of lysosomes. The interaction of lysosomes and mitochondria was dynamically traced while using the probe's properties to label the lysosomes. Because the probe has the optimal near-infrared excitation and emission wavelengths, liver organoid 3D imaging and Caenorhabditis elegans imaging were also performed. Altogether, our findings indicate valuable approaches and techniques for super-resolution 3D and in vivo imaging.

A dansyl fluorescent pH probe with wide responsive range in aqueous solution

A fluorescent pH probe based on diphenylalanine-modified dansyl (FF-Dns) was designed and synthesized. The probe showed sensitive fluorescence emission to pH change over a wide range of 1-13 in aqueous solution. At pH 4-10, FF-Dns displayed the characteristic fluorescent emission of dansyl group (yellow); while protonation of the dimethylamino group caused a very weak fluorescence emission at pH 1-4, and deprotonation of the sulfonamide group caused an emission blue-shift to the green region at pH 11-13. These properties endow FF-Dns with the potential to detect pH variation in physiological environments.

Fluorescent probes with high pKa values based on traditional, near-infrared rhodamine, and hemicyanine fluorophores for sensitive detection of lysosomal pH variations

Sterically hindered fluorescent probes (A-C) have been developed by introducing 2-aminophenylboronic acid pinacol ester to a traditional, A, a near-infrared rhodamine dye, B, and a near-infrared hemicyanine dye, C, forming closed spirolactam ring structures. Probe A was non-fluorescent under basic pH conditions whereas probes B and C were moderately fluorescent with fluorescence quantum yields of 9% and 5% in pH 7.4 PBS buffer containing 1% ethanol, respectively. With all probes increasing acidity leads to significant increases in fluorescence at 580 nm, 644 and 744 nm for probes A, B and C with fluorescence quantum yields of 26%, 21% and 10% in pH 4.5 PBS buffer containing 1% ethanol, respectively. Probes A, B and C were calculated to have pKa values of 5.81, 5.45 and 6.97. The difference in fluorescence under basic conditions is ascribed to easier opening of the closed spirolactam ring configurations due to significant steric hindrance between the 2-aminophenylboronic acid pinacol ester residue and an adjacent H atom in the xanthene derivative moiety in probe B or C. The probes show fast, reversible, selective and sensitive fluorescence responses to pH changes, and are capable of sensing lysosomal pH variations in living cells.

Near-infrared fluorescent probes with BODIPY donors and rhodamine and merocyanine acceptors for ratiometric determination of lysosomal pH variance

Three fluorescent probes have been developed by conjugating three different BODIPY donors to rhodamine and merocyanine acceptors for ratiometric determination of lysosomal pH variations. Probe A consists of a 1,3,5,7-tetramethyl-BODIPY donor and a near-infrared rhodamine acceptor bearing a lysosome-targeting morpholine residue. Probe B is composed of a 3,5-dimethyl-BODIPY donor and a near-infrared rhodamine acceptor modified with an o-phenylenediamine residue. Probe C contains a 3-styrene-functionalized BODIPY donor with longer wavelength emission and a near-infrared merocyanine acceptor containing a morpholine residue. Under neutral or basic pH conditions, the probes only show fluorescence from the BODIPY donors under BODIPY excitation because the rhodamine and merocyanine acceptors maintain closed spirolactam configurations. However, excitation at BODIPY absorption wavelengths concomitant with gradual pH decrease results in fluorescence decreases with the BODIPY donors and fluorescence increases from the rhodamine and merocyanine acceptors due to through-bond energy transfer from the donors to the acceptors. This is because the spirolactam ring opens under more acidic conditions and fluorescence of the acceptors results from significantly improved π-conjugation. These experimental results are substantiated with theoretical calculations on models of the different probes. The probes have all been used to determine lysosome pH variations in HeLa cells. Probe B was further utilized to successfully detect pH fluctuations in HeLa cells under oxidative stress and with treatment of NH4Cl and chloroquine.

Small-molecule fluorescent probes for specific detection and imaging of chemical species inside lysosomes

In the past few years, the preparation of novel small-molecule fluorescent probes for specific detection and imaging of chemical species inside lysosomes has attracted considerable attention because of their wide applications in chemistry, biology, and medical science. This feature article summarizes the recent advances in the design and preparation of small-molecule fluorescent probes for specific detection of chemical species inside lysosomes. In addition, their properties and applications for the detection and imaging of pH, H2O2, HOCl, O2˙-, lipid peroxidation, H2S, HSO3-, thiols, NO, ONOO-, HNO, Zn2+, Cu2+, enzymes, etc. in lysosomes are discussed as well.

Near-infrared fluorescent probes based on TBET and FRET rhodamine acceptors with different pK a values for sensitive ratiometric visualization of pH changes in live cells

Three near-infrared ratiometric fluorescent probes (A-C) based on TBET and FRET near-infrared rhodamine acceptors with different pK a values were designed and synthesized to achieve sensitive ratiometric visualization of pH variations in lysosomes in visible and near-infrared channels. Tetraphenylethene (TPE) was bonded to near-infrared rhodamine dyes through short electrical π -conjugation linkers to prevent an aggregation-caused quenching (ACQ) effect and allow highly efficient energy transfer of up to 98.9% from TPE donors to rhodamine acceptors. Probes A-C respond to pH variation from 7.4 to 3.0 in both buffer solutions and live cells with significant decreases of donor fluorescence and concomitant extraordinary increases of rhodamine acceptor fluorescence because of highly efficient energy transfer. In addition, probe C is capable of determining pH fluctuations in live cells treated with chloroquine. The probes show good photostability, excellent cell membrane permeability, high selectivity to pH, and two well-resolved emission peaks to ensure accurately comparative and quantitative analyses of intracellular pH changes.

Lysosome-specific sensing and imaging of pH variations in vitro and in vivo utilizing a near-infrared boron complex

A NIR lysosome-targeting boron complex has been developed based on hemicyanine for monitoring pH variations in vitro and in vivo.

A Near-Infrared Fluorescent Probe Based on a FRET Rhodamine Donor Linked to a Cyanine Acceptor for Sensitive Detection of Intracellular pH Alternations

A fluorescence resonance energy transfer (FRET)-based near-infrared fluorescent probe (B⁺) for double-checked sensitive detection of intracellular pH changes has been synthesized by binding a near-infrared rhodamine donor to a near-infrared cyanine acceptor through robust C-N bonds via a nucleophilic substitution reaction. To demonstrate the double-checked advantages of probe B⁺, a near-infrared probe (A) was also prepared by modification of a near-infrared rhodamine dye with ethylenediamine to produce a closed spirolactam residue. Under basic conditions, probe B⁺ shows only weak fluorescence from the cyanine acceptor while probe A displays nonfluorescence due to retention of the closed spirolactam form of the rhodamine moiety. Upon decrease in solution pH level, probe B⁺ exhibits a gradual fluorescence increase from rhodamine and cyanine constituents at 623 nm and 743 nm respectively, whereas probe A displays fluorescence increase at 623 nm on the rhodamine moiety as acidic conditions leads to the rupture of the probe spirolactam rings. Probes A and B⁺ have successfully been used to monitor intracellular pH alternations and possess pKa values of 5.15 and 7.80, respectively.

New Near-infrared Fluorescent Probes with Single-photon Anti-Stokes-shift Fluorescence for Sensitive Determination of pH Variances in Lysosomes with a Double-Checked Capability

Two near-infrared luminescent probes with Stokes-shift and single-photon anti-Stokes-shift fluorescence properties for sensitive determination of pH variance in lysosomes have been synthesized. A morpholine residue in probe A which serves as a targeting group for lysosomes in viable cells was attached to the fluorophores via a spirolactam moiety while a mannose residue was ligated to probe B resulting in increased biocompatibility and solubility in water. Probes A and B contain closed spirolactam moieties, and show no Stokes-shift or anti-Stokes-shift fluorescence under neutral or alkali conditions. However, the probes incrementally react to pH variance from 7.22 to 2.76 with measurable increases in both Stokes-shift and anti-Stokes-shift fluorescence at 699 nm and 693 nm under 645 nm and 800 nm excitation, respectively. This acid-activated fluorescence is produced by the breaking of the probe spirolactam moiety, which greatly increased overall π-conjugation in the probes. These probes possess upconversion near-infrared fluorescence imaging advantages including minimum cellular photo-damage, tissue penetration, and minimum biological fluorescence background. They display excellent photostability with low dye photobleaching and show good biocompatibility. They are selective and capable of detecting pH variances in lysosomes at excitation with two different wavelengths, i.e., 645 and 800 nm.

Fluorescent Probes Based on π-Conjugation Modulation between Hemicyanine and Coumarin Moieties for Ratiometric Detection of pH Changes in Live Cells with Visible and Near-infrared Channels

We report two ratiometric fluorescent probes based on π-conjugation modulation between coumarin and hemicyanine moieties for sensitive ratiometric detection of pH alterations in live cells by monitoring visible and near-infrared fluorescence changes. In a π-conjugation modulation strategy, a coumarin dye was conjugated to a near-infrared hemicyanine dye via a vinyl connection while lysosome-targeting morpholine ligand and o-phenylenediamine residue were introduced to the hemicyanine dye to form closed spirolactam ring structures in probes A and B, respectively. The probes show only visible fluorescence of the coumarin moiety under physiological and basic conditions because the hemicyanine moieties retain their closed spirolactam ring structures. However, decrease of pH to acidic condition causes spirolactam ring opening, and significantly enhances π-conjugation within the probes, thus generating new near-infrared fluorescence peaks of the hemicyanine at 755 nm and 740 nm for probes A and B, respectively. Moreover, the probes display ratiometric fluorescence response to pH with decreases of the coumarin fluorescence and increases of the hemicyanine fluorescence when pH changes from 7.4 to 2.5. The probes are fully capable of imaging pH changes in live cells with good ratiometric responses in visible and near-infrared channels, and effectively avoid fluorescence blind spots under neutral and basic pH conditions - an issue that typical intensity-based pH fluorescent probes run into. The probe design platform reported herein can be easily applied to prepare a variety of ratiometric fluorescent probes for detection of biological thiols, metal ions, reactive oxygen and nitrogen species by introducing appropriate functional groups to hemicyanine moiety.

New near-infrared rhodamine dyes with large Stokes shifts for sensitive sensing of intracellular pH changes and fluctuations

New near-infrared rhodamine dyes with large Stokes shifts were developed and applied for sensitive detection of cellular pH changes and fluctuations by incorporating an additional amine group with fused rings into the rhodamine dyes to enhance the electron donating ability of amine groups and improve the spectroscopic properties of the dyes.

Ratiometric Near-Infrared Fluorescent Probes Based On Through-Bond Energy Transfer and π-Conjugation Modulation between Tetraphenylethene and Hemicyanine Moieties for Sensitive Detection of pH Changes in Live Cells

In this paper, we present three ratiometric near-infrared fluorescent probes (A-C) for accurate, ratiometric detection of intracellular pH changes in live cells. Probe A consists of a tetraphenylethene (TPE) donor and near-infrared hemicyanine acceptor in a through-bond energy transfer (TBET) strategy, while probes B and C are composed of TPE and hemicyanine moieties through single and double sp2 carbon-carbon bond connections in a π-conjugation modulation strategy. The specific targeting of the probes to lysosomes in live cells was achieved by introducing morpholine residues to the hemicyanine moieties to form closed spirolactam ring structures. Probe A shows aggregation-induced emission (AIE) property at neutral or basic pH, while probes B and C lack AIE properties. At basic or neutral pH, the probes only show fluorescence of TPE moieties with closed spirolactam forms of hemicyanine moieties, and effectively avoid blind fluorescence imaging spots, an issue which typical intensity-based pH fluorescent probes encounter. Three probes show ratiometric fluorescence responses to pH changes from 7.0 to 3.0 with TPE fluorescence decreases and hemicyanine fluorescence increases, because acidic pH makes the spirolactam rings open to enhance π-conjugation of hemicyanine moieties. However, probe A shows much more sensitive ratiometric fluorescence responses to pH changes from 7.0 to 3.0 with remarkable ratio increase of TPE fluorescence to hemicyanine fluorescence up to 238-fold than probes B and C because of its high efficiency of energy transfer from TPE donor to the hemicyanine acceptor in the TBET strategy. The probe offers dual Stokes shifts with a large pseudo-Stokes shift of 361 nm and well-defined dual emissions, and allows for colocalization of the imaging readouts of visible and near-infrared fluorescence channels to achieve more precisely double-checked ratiometric fluorescence imaging. These platforms could be employed to develop a variety of novel ratiometric fluorescent probes for accurate detection of different analytes in applications of chemical and biological sensing, imaging, and diagnostics by introducing appropriate sensing ligands to hemicyanine moieties to form on-off spirolactam switches.

A cyanine-based fluorescent cassette with aggregation-induced emission for sensitive detection of pH changes in live cells

An aggregation-induced emission (AIE) cyanine-based fluorescent cassette with a large pseudo-Stokes shift was designed and prepared to sensitively image pH changes in live cells via through-bond energy transfer (TBET) from a tetraphenylethene (TPE) donor to a cyanine acceptor.

A review: the trend of progress about pH probes in cell application in recent years

Intracellular pH values are some of the most important factors that govern biological processes and the acid-base homeostasis in cells, body fluids and organs sustains the normal operations of the body. Subcellular organelles including the acidic lysosomes and the alkalescent mitochondria undergo various processes such as intracellular digestion, ATP production and apoptosis. Due to their precise imaging capabilities, fluorescent probes have attracted great attention for the illustration of pH modulated processes. Furthermore, based on the unique acidic extracellular environment of acidic lysosomes, fluorescent probes can specifically be activated in cancer cells or tumors. In this review, recently reported lysosome and mitochondria specific pH imaging probes as well as pH-activatable cancer cell-targetable probes have been discussed.

A rhodamine-based fluorescent probe for colorimetric and fluorescence lighting-up determination of toxic thiophenols in environmental water and living cells

Thiophenols are a class of highly toxic environmental pollutant, hence it is very necessary to monitor thiophenols in environment and living cells with an efficient and reliable method. Herein, a novel fluorescent probe for thiophenols has been developed, which exhibited a colorimetric and fluorescence turn-on dual response towards thiophenols with good selectivity and fast response. The sensing mechanism for thiophenols was attributed to nucleophilic substitution reaction, which was confirmed by HPLC. The probe exhibited good recovery (from 90% to 107%) and low limit of detection for thiophenols (37nM) in industrial wastewater. Moreover, the probe has been successfully employed to visualize thiophenol in living cells. Therefore, the fluorescent probe has good capability for monitoring thiophenols in environmental samples and biological systems.