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

Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

Recent advances in FRET probes for mitochondrial imaging and sensing

Mitochondria, as essential organelles in cells, play a crucial role in cellular growth and apoptosis. Monitoring mitochondria is of great importance, as mitochondrial dysfunction is often considered a hallmark event of cell apoptosis. Traditional fluorescence probes used for mitochondrial imaging and sensing are mostly intensity-based and are susceptible to factors such as concentration, the probe environment, and fluorescence intensity. Probes based on fluorescence resonance energy transfer (FRET) can effectively overcome external interference and achieve high-contrast imaging of mitochondria as well as quantitative monitoring of mitochondrial microenvironments. This review focuses on recent advances in the application of FRET-based probes for mitochondrial structure imaging and microenvironment sensing.

Nitrogen, sulfur-doped carbon quantum dots with large Stokes shift for real-time monitoring of pH in living cells

Construction of carbon quantum dots-based (CQDs) fluorescent probes for real-time monitoring pH in cells is still unsatisfied. Here, we propose the synthesis of nitrogen, sulfur-doped CQDs (N,S-CQDs) using one-pot hydrothermal treatment, and serve it as fluorescent probes to realize the real-time sensing of intracellular pH. These pH-responsive N,S-CQDs were proved exhibited a diversity of admirable properties, including great photostability, nontoxicity, favorable biocompatibility, and high selectivity. Particularly, due to the doping of nitrogen and sulfur, N,S-CQDs possessed long-wavelength emission and large Stokes Shift (190 nm), which could avoid self-absorption of tissue to realize high contrast and resolution bioimaging. The response of the probes to pH showed a good linear in range of 0.93-7.00 with coefficient of determination of 0.9956. Moreover, with advantages of high signal-to-noise ratio and stability against photobleaching, the as-prepared N,S-CQDs were successfully applied to monitor pH in living cells via bioimaging. All findings suggest that N,S-CQDs have significant potential for practical application for sensing and visualizing pH fluctuation in living systems.

Hemicyanine-based sensor for mitochondrial viscosity imaging in BV2 cells

Mitochondrial viscosity is a critical factor affecting numerous physiological processes, including phagocytosis. Abnormal viscosity in mitochondria is related to some pathological activities and diseases. Evaluating and detecting the changes in mitochondrial viscosity in vivo is crucial. Thus, a mitochondria-targeted red-emitting fluorescent probe (VP) was prepared, and can be used to detect viscosity with high selectivity and sensitivity. The synthesis of probe VP was as simple as two steps and the cost was low. In addition, the fluorescence intensity (log I615) exhibited an excellent relationship with viscosity (log η) in the range of 0.5 - 2.5 (R2 = 0.9985) in water/glycerol mixture. It is noteworthy that the probe VP displayed the highest signal-to-noise ratio (about 50-fold) for viscosity in water and glycerol system. The probe VP can visualize the mitochondrial viscosity change in living cells. More importantly, phagocytic test for BV2 cells further demonstrated that phagocytosis decreased with increased viscosity. Furthermore, VP was successfully used for monitoring the mitophagy process induced by starvation, and mitochondrial viscosity exhibited enhancement during mitophagy. The probe was a potential tool for studying viscosity and phagocytosis.

Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.

Highly Sensitive Cyanine Dyes for Rapid Sensing of NAD(P)H in Mitochondria and First-Instar Larvae of Drosophila melanogaster

We have developed two highly sensitive cyanine dyes, which we refer to as probes A and B. These dyes are capable of quick and sensitive sensing of NAD(P)H. The dyes were fabricated by connecting benzothiazolium and 2,3-dimethylnaphtho[1,2-d]thiazol-3-ium units to 3-quinolinium through a vinyl bond. In the absence of NAD(P)H, both probes have low fluorescence and absorption peaks at 370 and 400 nm, correspondingly. This is because of their two electron-withdrawing acceptor systems with high charge densities. However, when NAD(P)H reduces the probes' electron-withdrawing 3-quinolinium units to electron-donating 1,4-dihydroquinoline units, the probes absorb at 533 and 535 nm and fluoresce at 572 and 586 nm for A and B correspondingly. This creates well-defined donor-π-acceptor cyanine dyes. We successfully used probe A to monitor NAD(P)H levels in live cells during glycolysis, under hypoxic conditions induced by CoCl2 treatment and after treatment with cancer drugs, including cisplatin, camptothecin, and gemcitabine. Probe A was also employed to visualize NAD(P)H in Drosophila melanogaster first-instar larvae. We observed an increase in NAD(P)H levels in A549 cancer cells both under hypoxic conditions and after treatment with cancer drugs, including cisplatin, camptothecin, and gemcitabine.

Fluorescence-On Imaging of Reticulophagy Enabled by an Acidity-Reporting Solvatochromic Probe

Aberrant autophagy of the endoplasmic reticulum (reticulophagy) is engaged in diverse pathological disorders. Herein, we reported sensitive imaging of reticulophagy with ER-Green-proRed, a diad combining a solvatochromic entity of trifluoromethylated naphthalimide for long-term ER tracking by green fluorescence and an entity of rhodamine-lactam fluorogenic to lysosomal acidity. Stringently accumulated in the ER to give green fluorescence, ER-Green-proRed exhibits robust red fluorescence upon codelivery with the ER subdomain into lysosomes. The relevance of turn-on red fluorescence to reticulophagy was validated by reticulophagy modulated by starvation, reticulophagic receptors, and autophagy inhibition. This imaging method was successfully employed to discern reticulophagy induced by various pharmacological agents. These results show the potential of ER-targeted pH probes, as exemplified by ER-Green-proRed, to image reticulophagy and to identify reticulophagy inducers.

A sensitive ratiometric fluorescent chemosensor for visual and wearable mercury (II) recognition in river prawn and water samples

A novel ratiometric fluorescent probe p-RPT was prepared with triphenylamine and a rhodamine derivative. The probe displays high sensitivity and selectivity for Hg²⁺, which was applied in real water sample detection and biologic cell imaging. Hydrogel-coated paper sensors were fabricated with p-RPT, which displayed fluorescent colour change upon Hg²⁺ ion contact with a detection limit of 1.2 × 10^-8 M (∼10 ppb). In addition, flexible fluorescent p-RPT gloves were developed for visible and wearable Hg²⁺ detection and applied to detect mercury (II) in river prawn samples. In summary, the p-RPT probe not only shows great potential in mercury (II) detection for food and water, but also provides a new perspective for wearable sensing apparatus.

A novel near-infrared fluorescent probe with hemicyanine scaffold for sensitive mitochondrial pH detection and mitophagy study

Mitochondria, as an energy-producing powerhouse in live cells, is considered to be directly linked to cellular health. However, dysfunctional mitochondria and abnormal mitochondria pH would possibly activate mitophagy, cell apoptosis and intercellular acidification process. In this work, we synthesized a novel near infrared fluorescent probe (FNIR-pH) for measurement of mitochondrial pH based on the hemicyanine skeleton as a fluorophore. The FNIR-pH probe functioned as a mitochondrial pH substrate and exhibited quick and sensitive turn-on fluorescence responses to mitochondrial pH in basic solution due to the deprotonation of hydroxy group in the structure. From pH 3.0 to 10.0, the FNIR-pH exhibited almost 100-fold increase in fluorescence intensity at 766 nm wavelength. The FNIR-pH also displayed superior selectivity to various metal ions, excellent photostability, and low cytotoxicity, which facilitated further biological application. Owing to the proper pKa value of 7.2, the FNIR-pH paved the way for real-time monitoring of mitochondria pH changes in live cells and sensitive sensing of mitophagy. Moreover, the FNIR-pH probe was also implemented for fluorescent imaging of tumor-bearing mice to validate its potential application for in vivo imaging of bioanalytes and biomarkers.

Vision for Ratiometric Nanoprobes: In Vivo Noninvasive Visualization and Readout of Physiological Hallmarks

Lesion areas are distinguished from normal tissues surrounding them by distinct physiological characteristics. These features serve as biological hallmarks with which targeted biomedical imaging of the lesion sites can be achieved. Although tremendous efforts have been devoted to providing smart imaging probes with the capability of visualizing the physiological hallmarks at the molecular level, the majority of them are merely able to derive anatomical information from the tissues of interest, and thus are not suitable for taking part in in vivo quantification of the biomarkers. Recent advances in chemical construction of advanced ratiometric nanoprobes (RNPs) have enabled a horizon for quantitatively monitoring the biological abnormalities in vivo. In contrast to the conventional probes whose dependency of output on single-signal profiles restricts them from taking part in quantitative practices, RNPs are designed to provide information in two channels, affording a self-calibration opportunity to exclude the analyte-independent factors from the outputs and address the issue. Most of the conventional RNPs have encountered several challenges regarding the reliability and sufficiency of the obtained data for high-performance imaging. In this Review, we have summarized the recent progresses in developing highly advanced RNPs with the capabilities of deriving maximized information from the lesion areas of interest as well as adapting themselves to the complex biological systems in order to minimize microenvironmental-induced falsified signals. To provide a better outlook on the current advanced RNPs, nanoprobes based on optical, photoacoustic, and magnetic resonance imaging modalities for visualizing a wide range of analytes such as pH, reactive species, and different derivations of amino acids have been included. Furthermore, the physicochemical properties of the RNPs, the major constituents of the nanosystems and the analyte recognition mechanisms have been introduced. Moreover, the alterations in the values of the ratiometric signal in response to the analyte of interest as well as the time at which the highest value is achieved, have been included for most of RNPs discussed in this Review. Finally, the challenges as well as future perspectives in the field are discussed.

Near-infrared rhodol dyes bearing salicylaldehyde moieties for ratiometric pH sensing in live cells during mitophagy and under hypoxia conditions

We describe a simple but efficient approach to make fluorescent probes A and B based on rhodol dyes incorporated with salicyaldehyde moiety for monitoring pH changes in mitochondria under oxidative stresses and hypoxia conditions, and for tracking mitophagy processes. Probes A and B possess pKa values (pKa ≈ 6.41 and 6.83 respectively) near physiological pH and exhibit decent mitochondria-targeted capabilities, low cytotoxicity, and useful ratiometric and reversible pH responses, which make the probes appropriate for monitoring pH fluctuations of mitochondria in living cells with built-in calibration feature for quantitative analysis. The probes have been effectively useful for the ratiometric determination of pH variations of mitochondria under the stimuli of carbonyl cyanide-4(trifluoromethoxy)phenylhydrazone (FCCP), hydrogen peroxide (H2O2), and N-acetyl cysteine (NAC), and during mitophagy triggered by cell nutrient deprivation, and under hypoxia conditions with cobalt chloride (CoCl2) treatment in living cells. In addition, probe A was efficient in visualizing pH changes in the larvae of fruit flies.

Visual monitoring of the mitochondrial pH changes during mitophagy with a NIR fluorescent probe and its application in tumor imaging

Mitophagy, a mitochondria-selective autophagy process, plays critical roles in maintaining intracellular homeostasis by removing the damaged mitochondria and recycling the nutrients in a lysosome-dependent manner. Mitophagy process could result in the changes of mitochondrial pH. So fluorescent probes for detecting mitochondrial pH during mitophagy are highly needed for exploring the functions of mitochondria. Herein, a series of near-infrared pH probes were designed based on the rhodamine framework. The probes showed high sensitivity for pH with the tunable pKa from 4.74 to 6.54. Particularly, for probe 5 (with the pKa of 6.54), a linear relationship between fluorescence intensity and pH in the range of 5.6-7.2 was observed, which was suitable for mitochondrial pH detection. The probe displayed excellent mitochondria-targeting ability. It was applied to monitor pH changes during mitophagy caused by starvation. Besides, in vivo non-invasive visualization of tumor pH variations was achieved via the fluorescence imaging in the near-infrared region. We anticipate that the probe may be a useful tool for revealing essential information about mitophagy-related research and clinical tumor diagnosis.

Synthesis and application of the fluorescent furan and imidazole probes for selective in vivo and in vitro cancer cell imaging

Development of imaging probes for identification of tumors in the early stages of growth can significantly reduce the tumor-related health hazards and improve our capacity for treatment of cancer. In this work, three different furan and imidazole fluorescent derivatives abbreviated as Cyclo X, SAC and SNO are introduced for in vivo and in vitro imaging of cancer cells. The fluorescence quantum yield values were 0.226, 0.400 and 0.479 for Cyclo X, SAC and SNO, respectively. The excitation and emission wavelengths of maximum intensity were (360, 452), (350, 428) and (350, 432) nm for Cyclo X, SAC and SNO, respectively. The MTT reduction assay was used to estimate the cytotoxic activity of the proposed derivatives against HT-29 (cancer) and Vero (normal) cell lines. Cyclo X showed no cytotoxic effect, while SAC and SNO showed significantly higher cytotoxicity against the tested cell lines than cisplatin as a well-known anticancer drug. In vitro fluorescence microscopic images obtained using HT-29 cells showed that Cyclo X produced very bright images. The in vivo cancer cell imaging using 4T1 tumor-bearing mice revealed that Cyclo X is selectively accumulated in the tumor without distribution in the mice body organs. The spectral and structural stability, large Stokes shift, non-cytotoxicity and high level of selectivity for in vivo imaging are properties that make Cyclo X a suitable candidate to be used for long-term monitoring of cancer cells.

Near-infrared Fluorescent Probe Based on Rhodamine Derivative for Detection of NADH in Live Cells

A near-infrared fluorescent probe was prepared for selective detection of reduced nicotinamide adenine dinucleotide (NADH) in live cells. The probe turns off the fluorescence with a closed spironolactone switch. However, reduction of the probe by NADH turns on fluorescence at 740 nm. Theoretical calculations suggest a more planar arrangement between the rhodamine and quinoline moieties with increased π-delocalization resulting from reduction.

Activity-based NIR fluorescent probes based on the versatile hemicyanine scaffold: design strategy, biomedical applications, and outlook

The discovery of a near-infrared (NIR, 650-900 nm) fluorescent chromophore hemicyanine dye with high structural tailorability is of great significance in the field of detection, bioimaging, and medical therapeutic applications. It exhibits many outstanding advantages including absorption and emission in the NIR region, tunable spectral properties, high photostability as well as a large Stokes shift. These properties are superior to those of conventional fluorogens, such as coumarin, fluorescein, naphthalimides, rhodamine, and cyanine. Researchers have made remarkable progress in developing activity-based multifunctional fluorescent probes based on hemicyanine skeletons for monitoring vital biomolecules in living systems through the output of fluorescence/photoacoustic signals, and integration of diagnosis and treatment of diseases using chemotherapy or photothermal/photodynamic therapy or combination therapy. These achievements prompted researchers to develop more smart fluorescent probes using a hemicyanine fluorogen as a template. In this review, we begin by describing the brief history of the discovery of hemicyanine dyes, synthetic approaches, and design strategies for activity-based functional fluorescent probes. Then, many selected hemicyanine-based probes that can detect ions, small biomolecules, overexpressed enzymes and diagnostic reagents for diseases are systematically highlighted. Finally, potential drawbacks and the outlook for future investigation and clinical medicine transformation of hemicyanine-based activatable functional probes are also discussed.

IndiFluors: A New Full-Visible Color-Tunable Donor-Acceptor-Donor (D1-A-D2) Fluorophore Family for Ratiometric pH Imaging during Mitophagy

Full-visible color-tunable new fluorophores are essential in bioimaging research. However, it is significantly challenging to design fluorophores with the desired optical and biological properties owing to their structural complexity. We report a unified design of an interesting molecular framework, IndiFluors, based on the principle of a donor-acceptor-donor (D1-A-D2) system. The IndiFluors comprise pyrylium, pyridinium, and pyridine derivatives, which exhibit full-visible emission color (375-700 nm) by varying donor and acceptor strengths of the core scaffolds. With a minimal change of structure, the bright fluorophores (Φ: 0.96) can be tuned to become nonfluorescent (Φ: 0.01), which is well explained by time-dependent density functional theory (TD-DFT/PCM) by oscillator strengths in the S1 state. Within IndiFluors, pyridinium offers several advantages, including a large Stokes shift (∼154 nm) and excellent stability, compared to pentacyclic pyrylium fluorophores. Especially, the designed probe, PM-Mito-OH, demonstrated specific colocalization in mitochondria and a monitored ratiometric pH change during mitochondrial damage, autolysosomes, and the mitophagy process. Hence, IndiFluors and the derived probe show great potential for cellular pH imaging in live cells while exhibiting minimal cytotoxicity.

Recent Advancements in Colorimetric and Fluorescent pH Chemosensors: From Design Principles to Applications

Due to the immense biological significance of pH in diverse living systems, the design, synthesis, and development of pH chemosensors for pH monitoring has been a very active research field in recent times. In this review, we summarize the designing strategies, sensing mechanisms, biological and environmental applications of fluorogenic and chromogenic pH chemosensors of the last three years (2018-2020). We categorized these pH probes into seven types based on their applications, including 1) Cancer cell discriminating pH probes; 2) Lysosome targetable pH probes; 3) Mitochondria targetable pH probes; 4) Golgi body targetable pH probes; 5) Endoplasmic reticulum targetable pH probes; 6) pH probes used in nonspecific cell imaging; and 7) pH probes without cell imaging. All these different categories exhibit diverse applications of pH probes in biological and environmental fields.

Recent progress of organic small molecule-based fluorescent probes for intracellular pH sensing

Fluorescent probes along with fluorescence microscopy are essential tools for biomedical research. Various cellular ubiquitous chemical factors such as pH, H₂O₂, and Ca²⁺ are labeled and traced using specific fluorescent probes, therefore helping us to explore their physiological function and pathological change. Among them, intracellular pH value is an important factor that governs biological processes, generally ∼7.2. Furthermore, specific organelles within cells possess unique acid-base homeostasis, involving the acidic lysosomes, alkalescent mitochondria, and neutral endoplasmic reticulum and Golgi apparatus, which undergo various physiological processes such as intracellular digestion, ATP production, and protein folding and processing. In this review, recently reported fluorescent probes targeted toward the lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and cytoplasm for sensing pH change are discussed, which involves molecular structures, fluorescence behavior, and biological applications.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

One-step fabrication of functional carbon dots with long wavelength emission for gene delivery and bio-imaging

As a new-type of fluorescent material, carbon dots (CDs) are promising nanoscale reagents with the potential to integrate the functions of bio-imaging and gene/drug delivery. Most of the reported CDs for nucleic acid delivery only emitted short-wavelength (blue and green) fluorescence, making them unsuitable for in vivo application. Herein, a one-step solvothermal method was applied to prepare CDs with long wavelength emission from low molecular weight PEI and rhodamine dyes for both bio-imaging and gene delivery. The structure of the CDs was confirmed by several analytical methods including ¹H NMR, FT-IR, TEM, and XPS. The results showed that the CDs possess excellent fluorescence properties, which enable their application in both in vitro and in vivo bio-imaging. Meanwhile, the CDs could also be used for the intracellular tracking of the gene delivery process. In vitro transfection results revealed that the CDs possessed high transfection efficiency, which was up to 162 times higher than that of the "golden standard" transfection reagent PEI 25 kDa. Besides, these CDs also exhibited better serum tolerance and lower cytotoxicity than PEI. A mechanistic study suggested that the CD/DNA complexes may release the nucleic acid cargo more effectively. This work provides a new method to prepare multi-functional CDs with non-viral gene delivery and long wavelength bio-imaging abilities.

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.

β-Naphthothiazolium-based ratiometric fluorescent probe with ideal pKa for pH imaging in mitochondria of living cells

The weakly alkaline microenvironment (pH ~8.0) in mitochondria plays a vital role in maintaining its morphology and function. Thus monitoring mitochondrial pH (pH_mito) is of great significance. Herein, a ratiometric fluorescent probe (ENBT) for pHmito imaging in mitochondria of living cells is reported. pH variation closely correlates to intramolecular charge transfer (ICT) from naphthol to β-naphthothiazolium. ENBT exhibits a remarkable decrease on ratiometric fluorescence at λ_em1/λ_em2 = F₅₉₅/F₇₀₀ in response to pH variation within 6.30-9.29. In addition, ENBT has an ideal pKa value of 7.94 ± 0.08, which is advantageous in accurate sensing of pH_mito. Moreover, ENBT has a Stokes shift of >150 nm, which effectively eliminates the potential interference from the excitation irradiation. ENBT shows excellent capability for specific staining of mitochondria with low cytotoxicity, which is most suitable for pH_mito imaging in live cells. The probe was applied for monitoring pH_mito variation in mitochondria of live cells caused by H₂O₂, NAC (N-Acetyl-l-cysteine), NH₄Cl, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and lactate/pyruvate. The morphological alterations of mitochondria in living cells after treatment by CCCP were further evaluated.

Ratiometric Near-Infrared Fluorescent Probes Based on Hemicyanine Dyes Bearing Dithioacetal and Formal Residues for pH Detection in Mitochondria

Ratiometric near-infrared fluorescent probes (AH⁺ and BH⁺) have been prepared for pH determination in mitochondria by attaching dithioacetal and formal residues onto a hemicyanine dye. The reactive formyl group on probe BH⁺ allows for retention inside mitochondria as it can react with a protein primary amine residue to form an imine under slightly basic pH 8.0. Probes AH⁺ and BH⁺ display ratiometric fluorescent responses to pH changes through the protonation and deprotonaton of a hydroxy group in hemicyanine dyes with experimentally determined pKa values of 6.85 and 6.49, respectively. Calculated pK_a values from a variety of theoretical methods indicated that the SMD_BONDI method of accounting for solvent and van der Waals radii plus including a water molecule located near the site of protonation produced the closest overall agreement with the experimental values at 7.33 and 6.14 for AH⁺ and BH⁺ respectively.

A near-infrared fluorescent probe based on a hemicyanine dye with an oxazolidine switch for mitochondrial pH detection

A near-infrared fluorescent probe (AH+) has been prepared by incorporating an oxazolidine switch into a near-infrared hemicyanine dye. The probe shows fast and sensitive responses to pH from an oxazolidine switch to the hemicyanine dye upon pH decreases from 10.0 to 5.0. The probe shows good photostability, low cytotoxicity, and reversible fluorescence responses to pH changes with a pKa value of 7.6. It has been successfully used to determine pH changes in mitochondria.

Organic fluorescent probes for monitoring autophagy in living cells

As a ubiquitous degradation process in cells, autophagy plays important roles in various biological activities. However, the abnormality of autophagy is closely related to many diseases, such as aging, neurological disorder, and cancer. Thus, monitoring the process of autophagy in living cells has high significance in biological studies and diagnosis of related diseases. In order to real-time and in situ monitor the process of autophagy, various organic fluorescent probes have been explored in recent years owing to the advantages such as handy staining processes, flexible molecular design strategies, and near-nondestructive detection. However, this interesting and frontier topic has not been reviewed so far. In this tutorial review, we will focus on the latest breakthrough results of organic fluorescent probes in monitoring autophagy of living cells, especially the probe design strategies based on the several microenvironment changes of the autophagy process, and the responding mechanisms and bio-imaging applications in the autophagy process. In addition, we will discuss the shortcomings and limitations of the probes developed, such as susceptible to interference, unable to monitor the whole process, and lack of clinical applications. Finally, we will highlight some challenges and further opportunities in this field. This tutorial review may promote the development of more robust fluorescent probes to further reveal the mechanisms of autophagy, which is the basis of degradation and recycling of cell components.

A benzothiazolium-based fluorescent probe with ideal pKa for mitochondrial pH imaging and cancer cell differentiation

A mitochondrial pH sensing fluorescent probe namely 2-(2-(6-hydroxynaphthalen-2-yl)vinyl)-3-(6-(triphenyl-phosphonio)hexyl)benzothiazol-3-ium bromide (HTBT2) was designed and facilely synthesized via the Knoevenagel condensation reaction. HTBT2 displayed a linear fluorescence enhancement at 612 nm in response to pH changes between 8.70 and 7.20. The pK_a value was determined to be 8.04 ± 0.02, which might be ideal for mitochondrial pH (pH_mito∼8.0) detection. HTBT2 also exhibited a remarkable large Stokes shift of 176 nm, which could diminish the interference of excitation light. The results of live cell imaging studies suggested that HTBT2 showed excellent targeting ability for mitochondria. Importantly, it was successfully applied to visualize mitochondrial pH changes in live cells and differentiate the pH_mito difference between cancer cell lines and normal cell lines. Our results consistently supported that HTBT2 held practical promise for the investigation of physiological processes related to pH_mito changes and clinical potential for cancer cell differentiation.

Strongly coupled exciton-plasmon nanohybrids reveal extraordinary resistance to harsh environmental stressors: temperature, pH and irradiation

Hybridized plexcitonic states have unique properties which have been widely studied in recent decades in many research fields targeted at both fundamental science and innovative applications. However, to make these applications come true one needs to ensure the stabilization and preservation of electronic states and optical transitions in hybrid nanostructures, especially under the influence of external stressors, in regimes, that have not yet been comprehensively investigated. The present work shows that the nanohybrid system, composed of plasmonic nanoparticles and J-aggregates of organic molecules, displays outstanding resistance to harsh environmental stressors such as temperature, pH and strong light irradiation as well as demonstrates long-term stability and processability of the nanostructures both in weak and strong coupling regimes. These findings contribute to a deeper understanding of the physicochemical properties of plexcitonic nanoparticles and may find important implications for the development of potential applications in optoelectronics, optical imaging and chemo-bio-sensing and, in general, in the field of optical materials science.

A red-emissive D-A-D type fluorescent probe for lysosomal pH imaging

Visual detection of pH changes in lysosomes is critical because lysosomes not only play an important role in diverse cellular functions but also are closely related to various physiological and pathological processes. Herein, we disclose a donor-acceptor-donor (D-A-D) type fluorescent probe DBTD for detecting pH fluctuation in lysosomes. DBTD was rationally designed by using benzothiadiazole as the electron acceptor and N,N-diethylamino groups as the electron donor. Owing to its unique D-A-D structure, DBTD displayed a red-emission centered at 614 nm. It showed a sensitive and a linear response to pH from 4.5 to 5.2 with a pKa of 5.0, which is very suitable for lysosomal pH imaging. The response was based on the intramolecular charge transfer (ICT) effect owing to the protonation of the diethylamino group. Furthermore, DBTD could accurately monitor lysosomal pH variations in SGC-7901 cells. More importantly, it was able to image the pH change in lysosomes during the autophagy process successfully, suggesting the great potential of DBTD acting as a powerful tool for monitoring lysosomal pH-related biological processes.