Activatable Heavy-Atom-Free Photosensitizer with Large Stokes Shift and a NIR-II Emission Harnessing Rhodamine Ring-Opening Strategy

Activatable photosensitizers (PSs) generating 1O2 only under specific conditions can minimize concomitant injury to normal tissues. Heavy-atom-free PSs hold the merits of low dark toxicity, long triplet-state lifetimes, good photostability, and relatively low cost. PSs with emission in the second near-infrared (NIR-II) window are highly valuable for deep-tissue, high-contrast imaging. Herein, we have designed and synthesized a series of heavy-atom-free PSs by a one-step reaction between an easily accessible rhodamine derivative and commercially available thiophene aldehydes. One of the as-prepared PSs, 2b-3T, exhibits emission maxima at 810 nm and tails to the NIR-II region at 1140 nm, together with large Stokes shift (178 nm). Importantly, the newly developed PSs, featuring functional carboxylic acid groups, present promising opportunities as versatile platforms for creating activatable PSs. To validate our concept, we developed Cu2+/pH-activatable PSs using the spirocyclization mechanism of rhodamine. Ultimately, we showcased the effectiveness of these innovative PSs in photodynamic therapy through in vitro experiments.

A Nonlinear Photochromic Reaction Based on Sensitizer-Free Triplet-Triplet Annihilation in a Perylene-Substituted Rhodamine Spirolactam

Nonlinear photochromic reactions that work with weak incoherent light are important for molecular operations with high spatial resolution and multiple photofunctions based on single molecules. However, nonlinear photochromic compounds generally require complex molecular design, restricting accessibility in various fields. Herein, we report nonlinear photochromic properties in a perylene-substituted rhodamine spirolactam derivative (Rh-Pe), which is synthesized from rhodamine B in facile procedures. Direct excitation of Rh-Pe produces the triplet excited state via the charge-transfer (CT) state. The triplet excited state causes triplet-triplet annihilation to bring the generation of the intensely colored ring-open form with nonlinear behavior. Furthermore, green- and red-light-induced photochromism was achieved in Rh-Pe using triplet sensitizers, although Rh-Pe can be directly excited only by ultraviolet and blue light. Our findings are expected to contribute to the development of photofunctional materials showing nonlinear behavior and low-energy light responsivity.

Rational design of pH-responsive near-infrared spirocyclic cyanines: the effects of substituents and the external environment

pH-responsive spirocyclic cyanine dyes were designed and synthesized. The equilibrium constant for cyclization (pKcycl) could be rationally controlled by changing the nucleophilic moiety and the side chains. Encapsulation in polymeric micelles inhibited the H-aggregation of the dye, and the pKcycl could be shifted according to the amphiphilic polymer employed.

Fluorogenic and Cell-Permeable Rhodamine Dyes for High-Contrast Live-Cell Protein Labeling in Bioimaging and Biosensing

The advancement of fluorescence microscopy techniques has opened up new opportunities for visualizing proteins and unraveling their functions in living biological systems. Small-molecule organic dyes, which possess exceptional photophysical properties, small size, and high photostability, serve as powerful fluorescent reporters in protein imaging. However, achieving high-contrast live-cell labeling of target proteins with conventional organic dyes remains a considerable challenge in bioimaging and biosensing due to their inadequate cell permeability and high background signal. Over the past decade, a novel generation of fluorogenic and cell-permeable dyes has been developed, which have substantially improved live-cell protein labeling by fine-tuning the reversible equilibrium between a cell-permeable, nonfluorescent spirocyclic state (unbound) and a fluorescent zwitterion (protein-bound) of rhodamines. In this review, we present the mechanism and design strategies of these fluorogenic and cell-permeable rhodamines, as well as their applications in bioimaging and biosensing.

Spontaneously Blinking Rhodamine Dyes for Single-Molecule Localization Microscopy

Single-molecule localization microscopy (SMLM) has found extensive applications in various fields of biology and chemistry. As a vital component of SMLM, fluorophores play an essential role in obtaining super-resolution fluorescence images. Recent research on spontaneously blinking fluorophores has greatly simplified the experimental setups and extended the imaging duration of SMLM. To support this crucial development, this review provides a comprehensive overview of the development of spontaneously blinking rhodamines from 2014 to 2023, as well as the key mechanistic aspects of intramolecular spirocyclization reactions. We hope that by offering insightful design guidelines, this review will contribute to accelerating the advancement of super-resolution imaging technologies.

Accelerated MINFLUX Nanoscopy, through Spontaneously Fast-Blinking Fluorophores

The introduction of MINFLUX nanoscopy allows single molecules to be localized with one nanometer precision in as little as one millisecond. However, current applications have so far focused on increasing this precision by optimizing photon collection, rather than minimizing the localization time. Concurrently, commonly used fluorescent switches are specifically designed for stochastic methods (e.g., STORM), optimized for a high photon yield and rather long on-times (tens of milliseconds). Here, accelerated MINFLUX nanoscopy with up to a 30-fold gain in localization speed is presented. The improvement is attained by designing spontaneously blinking fluorescent markers with remarkably fast on-times, down to 1-3 ms, matching the iterative localization process used in a MINFLUX microscope. This design utilizes a silicon rhodamine amide core, shifting the spirocyclization equilibrium toward an uncharged closed form at physiological conditions and imparting intact live cell permeability, modified with a fused (benzo)thiophene spirolactam fragment. The best candidate for MINFLUX microscopy (also suitable for STORM imaging) is selected through detailed characterization of the blinking behavior of single fluorophores, bound to different protein tags. Finally, optimization of the localization routines, customized to the fast blinking times, renders a significant speed improvement on a commercial MINFLUX microscope.

A library of Rhodamine6G-based pH-sensitive fluorescent probes with versatile in vivo and in vitro applications

Acidic pH is critical to the function of the gastrointestinal system, bone-resorbing osteoclasts, and the endolysosomal compartment of nearly every cell in the body. Non-invasive, real-time fluorescence imaging of acidic microenvironments represents a powerful tool for understanding normal cellular biology, defining mechanisms of disease, and monitoring for therapeutic response. While commercially available pH-sensitive fluorescent probes exist, several limitations hinder their widespread use and potential for biologic application. To address this need, we developed a novel library of pH-sensitive probes based on the highly photostable and water-soluble fluorescent molecule, Rhodamine 6G. We demonstrate versatility in terms of both pH sensitivity (i.e., pK a) and chemical functionality, allowing conjugation to small molecules, proteins, nanoparticles, and regenerative biomaterial scaffold matrices. Furthermore, we show preserved pH-sensitive fluorescence following a variety of forms of covalent functionalization and demonstrate three potential applications, both in vitro and in vivo, for intracellular and extracellular pH sensing. Finally, we develop a computation approach for predicting the pH sensitivity of R6G derivatives, which could be used to expand our library and generate probes with novel properties.

Novel PET-pperated rosamine pH-sensor dyes with substitution pattern-tunable pKa values and temperature sensitivity

We present the synthesis and characterization of a family of regioisomerically pure pH-sensitive rosamine fluorophores consisting of xanthene fluorophore cores and differently substituted phenol moieties.

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

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

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Unusual red-orange emission from rhodamine-derived polynorbornene for selective binding to Fe3+ ions in an aqueous environment

Norbornene-based rhodamine derivative (NR) was observed as an "off-on" probe for Fe3+ ions both colorimetrically and fluorimetrically in an aqueous environment. NR and its homopolymer (PNR) were capable of detecting Fe3+ ions with high selectivity and sensitivity in an aqueous environment. For NR and PNR the limit of detection (LOD) towards Fe3+ ions was found to be 49 nM and 19 nM, respectively, making these materials highly efficient. Most interestingly, PNR has more efficacy towards the detection of Fe3+ ions than NR, which is expected due to the favorable side-chain interaction in the presence of multiple sensing motifs between the polymer chains. The sensing behavior was thoroughly studied via spectroscopic techniques. We hypothesized that the Fe3+ ion was expected to induce the spirolactam ring-opening of the rhodamine unit due to its Lewis acid nature and preferable interactions with the N and O atoms present in NR as well as PNR.

A sensitive and selective fluorescent probe for the detection of endogenous peroxynitrite (ONOO-) in living cells

Peroxynitrite (ONOO-) is one of reactive oxygen species, and plays a vital role in numeorus physiological and pathological processes. Given that the ONOO- level is closely related with various serious diseases, the in situ and real time detection of endogenous ONOO- is highly important for the in-depth study of its roles in living systems. Herein, we present a new fluorescent probe (RHPN) for the real-time detection of intracellular ONOO-. The probe RHPN consists of a rhodamine analogue and an arylhydrazide group as a response site for ONOO-. In response to ONOO-, the probe RHPN converts to an open-ring form and generates strong fluorescence. Moreover, the probe RHPN was successfully used for the imaging of the endogenous and exogenous ONOO- level changes in living cells.

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.

Tuning the pKa of Carboxyfluorescein with Arginine-Rich Cell-Penetrating Peptides for Intracellular pH Imaging

5-Carboxylfluorescein (FAM) is a conventional pH-responsive fluorophore widely used in fluorescence labeling and imaging. Because of its nonfluorescent structure under acidic conditions, FAM has long been limited to pH determination in a neutral-basic environment. Here, we modified the optical properties of FAM with cationic arginine-rich cell-penetrating peptides (CPPs), tuning the pK_a value of FAM to adapt well to pH measurement under diverse pH conditions. With increasing length of polyarginine, the pK_a value of FAM was tuned from 6.20 ± 0.06 to 5.17 ± 0.05. The key mechanism for pK_a variations was attributed to intramolecular electrostatic attraction and the positive charge of cationic CPPs tend to stabilize the fluorescent dianionic form of FAM. Apart from tunable pK_a, arginine-rich CPPs also improved the water solubility, membrane permeability, and organelle-specific localization of FAM. Two conjugated probes FAM-R₁₂ and FAM-(F_xr)₃ were selected to monitor intracellular pH fluctuations. Compared to FAM-(F_xr)₃, highly positively charged FAM-R₁₂ was more effective in lower pH condition and realized targeted visualization of lysosomal pH changes. The arginine-rich CPP-based strategy offers a promising approach to obtain optimized fluorescent pH probes with adjustable pK_a values for organelle-specific pH measurement.

In Vivo Multicolor Imaging with Fluorescent Probes Revealed the Dynamics and Function of Osteoclast Proton Pumps

In vivo two-photon fluorescence imaging is a powerful modality to monitor cell dynamics in biomedical studies. To detect protein functions in living animals in real-time, fluorescent probes must show a quick response to the target function in specific tissues. Here, we developed a rhodamine-based small-molecule fluorescent probe called Red-pHocas (red pH-activatable fluorescent probe for osteoclast activity sensing) to reversibly detect the acidic environments for the spatiotemporal analysis of the function of osteoclast proton pumps. The introduction of electron-withdrawing N-alkyl substituents in the rhodamine spirolactam fluorophore remarkably increased the kinetics of the fluorescence response to acidic pHs, which allowed the rapid and reversible monitoring of acidic compartments and the analysis of the dynamics of osteoclast proton pumps during osteoclastic bone resorption. In vivo multicolor two-photon imaging using Red-pHocas in fluorescent reporter mice revealed that bone acidification occurred synchronously with the accumulation of proton pumps onto the bone surfaces. To our knowledge, this is the first study to demonstrate the direct involvement of osteoclast proton pumps in bone acidification under intravital conditions by means of an imaging probe.

Novel polysiloxane-based rhodamine B fluorescent probe for selectively detection of Al3+ and its application in living-cell and zebrafish imaging

Polysiloxanes have excellent stability and biological relevance and are suitable for biological research. However, there were few polysiloxane-based fluorescent probes for bioimaging. This report successfully designed a new polysiloxane-based polymer fluorescent probe (RB-1) for the first time as a "turn-on" fluorescent probe response to Al³⁺ ion with highly sensitive and selectivity. Importantly, this probe could also apply both in cell and zebrafish imaging, indicating the huge application development prospects of polysiloxane-based fluorescent probes in future.

Novel two-photon fluorescent probe with high fluorescence quantum yields for tracking lipid droplets in biological systems

Lipid droplets (LDs) played an important role in storing neutral lipids process. In this work, we developed a series of fluorescent dyes PIE1-PIE4 with high fluorescence quantum yields based on a single phenanthrenequinone imidazole fluorophore core. Among these compounds, we introduced diethyl aniline group on the phenanthrenequinone imidazole core to provide the compound PIE1. It is firstly found that PIE1 could real-color image and track lipid droplets (LDs) in living cells. However, we modified nitrile, imidazole and methyl group on the same fluorophore core to afford three dyes PIE2, PIE3 and PIE4, respectively. Although PIE2, PIE3 and PIE4 possessed high fluorescence quantum yields in various organic solvents, they could not real-color image and track LDs in living cells.

A two-photon excited red-emissive probe for imaging mitochondria with high fidelity and its application in monitoring mitochondrial depolarization via FRET

Two-photon red-emissive fluorescent probes for imaging mitochondria with high-fidelity have been constructed, and mitochondrial depolarization has been visualized with the probe.

Development of a two-photon fluorescent probe for the selective detection of β-galactosidase in living cells and tissues

We have developed a two-photon fluorescent β-gal probe (G-GAL), which was demonstrated to be an efficient two-photon biosensor for β-gal in living cells and tissues.

A DNA-based fluorescent reporter maps HOCl production in the maturing phagosome

Phagocytes destroy pathogens by trapping them in a transient organelle called the phagosome, where they are bombarded with reactive oxygen species (ROS) and reactive nitrogen species (RNS). Imaging reactive species within the phagosome would directly reveal the chemical dynamics underlying pathogen destruction. Here we introduce a fluorescent, DNA-based combination reporter, cHOClate, which simultaneously images hypochlorous acid (HOCl) and pH quantitatively. Using cHOClate targeted to phagosomes in live cells, we successfully map phagosomal production of a specific ROS, HOCl, as a function of phagosome maturation. We found that phagosomal acidification was gradual in macrophages and upon completion, HOCl was released in a burst. This revealed that phagosome-lysosome fusion was essential not only for phagosome acidification, but also for providing the chloride necessary for myeloperoxidase activity. This method can be expanded to image several kinds of ROS and RNS and be readily applied to identify how resistant pathogens evade phagosomal killing.

Detection of Curvature-Radius-Dependent Interfacial pH/Polarity for Amphiphilic Self-Assemblies: Positive versus Negative Curvature

It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to ¹H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio ( w₀) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

A reaction-based luminescent switch-on sensor for the detection of OH- ions in simulated wastewater

A series of luminescent iridium(iii) complexes were synthesized and evaluated for their ability to interact with hydroxide ions in semi-aqueous media at ambient temperature. Upon the addition of OH^-, a nucleophilic aromatic substitution reaction takes place at the bromine groups of the N^N ligand of complex 1, resulting in the generation of a yellow-green luminescence. Complex 1 showed a 35-fold enhanced emission at pH 14 when compared to neutral pH, and the detection limit for OH^- ions was 4.96 μM. Complex 1 exhibited high sensitivity and selectivity, long-lived luminescence and impressive stability. Additionally, we have demonstrated the practical application of complex 1 to detect OH^- ions in simulated wastewater.

A two-photon fluorescent probe for detecting lipid droplet viscosity in living cells and zebra fish

A fluorescent probe CBA with high photostability to measure the viscosity changes of lipid drops.

A single rhodamine spirolactam probe for localization and pH monitoring of mitochondrion/lysosome in living cells

Rh-BMDZ with neutral pK_a6.9 succeeds in indicating and discriminating mitochondria and lysosomes simultaneously in MCF-7 cells.

Activatable fluorescence: From small molecule to nanoparticle

Molecular imaging has emerged as an indispensable technology in the development and application of drug delivery systems. Targeted imaging agents report the presence of biomolecules, including therapeutic targets and disease biomarkers, while the biological behaviour of labelled delivery systems can be non-invasively assessed in real time. As an imaging modality, fluorescence offers additional signal specificity and dynamic information due to the inherent responsivity of fluorescence agents to interactions with other optical species and with their environment. Harnessing this responsivity is the basis of activatable fluorescence imaging, where interactions between an engineered fluorescence agent and its biological target induce a fluorogenic response. Small molecule activatable agents are frequently derivatives of common fluorophores designed to chemically react with their target. Macromolecular scale agents are useful for imaging proteins and nucleic acids, although their biological delivery can be difficult. Nanoscale activatable agents combine the responsivity of fluorophores with the unique optical and physical properties of nanomaterials. The molecular imaging application and overall complexity of biological target dictate the most advantageous fluorescence agent size scale and activation strategy.

Microwave-assisted synthesis and photophysical studies of novel fluorescent N-acylhydrazone and semicarbazone-7-OH-coumarin dyes

Emissive 7-OH-coumarins were synthesized by a microwave-assisted protocol and spectral changes were induced after conformational changes in low polarity media.

An ultra-fast illuminating fluorescent probe for monitoring formaldehyde in living cells, shiitake mushrooms, and indoors

An ultra-fast illuminating fluorescent formaldehyde probe (R6-FA) was engineered for tracing formaldehyde in living cells, shiitake mushrooms and indoors.

Metal-free photoinduced electron transfer–atom transfer radical polymerization (PET–ATRP) via a visible light organic photocatalyst

This work developed the first example of PET-ATRP using a reductive pathway, which provides new opportunities for the synthesis of well-controlled polymer architectures through a photochemical approach.

Self-assembly of BODIPY based pH-sensitive near-infrared polymeric micelles for drug controlled delivery and fluorescence imaging applications

Responsive block copolymer micelles emerging as promising imaging and drug delivery systems show high stability and on-demand drug release activities. Herein, we developed self-assembled pH-responsive NIR emission micelles entrapped with doxorubicin (DOX) within the cores by the electrostatic interactions for fluorescence imaging and chemotherapy applications. The block copolymer, poly(methacrylic acid)-block-poly[(poly(ethylene glycol) methyl ether methacrylate)-co-boron dipyrromethene derivatives] (PMAA-b-P(PEGMA-co-BODIPY), was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, and the molecular weight distribution of this copolymer was narrow (Mw/Mn = 1.31). The NIR fluorescence enhancement induced by the phenol/phenolate interconversion equilibrium works as a switch in response to the intracellular pH fluctuations. DOX-loaded PMAA-b-P(PEGMA-co-BODIPY) micelles can detect the physiological pH fluctuations with a pKa near physiological conditions (∼7.52), and showed pH-responsive collapse and an obvious acid promoted anticancer drug release behavior (over 58.8-62.8% in 10 h). Real-time imaging of intracellular pH variations was performed and a significant chemotherapy effect was demonstrated against HeLa cells.