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Near-infrared (NIR) organic materials have been widely developed for tumor phototherapy due to their deep tumor penetration, good biodegradability, and high photothermal conversion (PCE). However, most of the NIR organic dyes are easily destroyed by photooxidation due to their big and long conjugated structures, such as cyanine dyes. Under light irradiation, the reactive oxygen species (ROS) produced by these NIR dyes can easily break their conjugated skeleton, resulting in a dramatic decrease in phototherapeutic efficiency. Herein, an NIR organic dye cyanine dye (CyS) and a photosensitizer methylene blue (MB) were chosen to prepare nanocarrier CMTNPs by facile self-assembling with a natural antioxidant, tannic acid (TA). TA can greatly enhance the stability of NIR cyanine dyes by scavenging ROS. Furthermore, CMTNPs have a character of pH/thermal dual response, allowing for controlled release of MB in the slightly acidic tumor environment during photothermal therapy. The released MB can turn on both fluorescence and photodynamic therapy effects. In vitro and in vivo experiments demonstrated the remarkable tumor ablation ability of CMTNPs. Thus, our study provided an antiphotobleaching and controlled release photosensitizer strategy through the introduction of antioxidant TA into the nanocarrier for efficient collaborative photothermal/photodynamic therapy.

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Near-infrared (NIR) heptamethine cyanine (HCy) dyes are promising photothermal transducers for image-guided cancer treatment owing to their prominent photophysical properties and high photothermal conversion ability. However, HCy photothermal transducers usually have poor photostability due to degradation induced by the self-generated reactive oxygen species. Herein, a novel mitochondria-targeting dimeric HCy dye, named dimeric oBHCy, is rationally designed, exhibiting strong near-infrared II (NIR-II) fluorescence emission, high photothermal conversion efficiency (PCE), and excellent photostability. The large π-conjugation and drastic intramolecular motion of the diphenol rotor in the dimeric oBHCy enhance the nonradiative energy dissipation and suppress the intersystem crossing process, thereby achieving a high PCE (49.2%) and improved photostability. Impressively, dimeric oBHCy can precisely target mitochondria and induce mitochondrial damage upon NIR light irradiation. Under the guidance of in vivo NIR-II fluorescence imaging, efficient NIR light-activated photothermal therapy of 4T1 breast tumors is accomplished with a tumor inhibitory rate of 96% following a single injection of the dimeric oBHCy. This work offers an innovative strategy for designing cyanine photothermal transducers with integrated NIR-II fluorescence and photothermal properties for efficient cancer theranostics.

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A spectrophotometric method for the quantitative determination of nitrite was developed, based on the radical nitration of indopolycarbocyanine dyes in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). The rate of the reaction of the studied dyes with nitrite increases with the lengthening of the polymethine chain and the presence of hydrophilic sulfo groups in the side chain of the dye. TEMPO acts as a co-reagent, significantly accelerating the reaction rate and increasing the sensitivity of nitrite determination. The proposed reaction mechanism is supported by spectrophotometric and HPLC/MS studies. For Ind2 (tetramethine indocarbocyanine cationic dye), the limit of detection for nitrite is 0.50 µM within a linearity range of 1-13 µM. The developed method is sensitive, with a LOD 130 times lower than the maximum contaminant level (MCL) of nitrite in drinking water (65 µM), as specified by the WHO. The method is of low-toxicity and good selectivity, as the determination of nitrite is not significantly affected by the main components of water. The method was successfully applied for the analysis of nitrite in natural and bottled water.

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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.

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Advanced photosensitizers for high-performance fluorescence imaging-guided photothermal therapy demand excellent near-infrared (NIR) brightness [molar absorption coefficient (ε) × quantum yield (QY)] and exceptional photothermal performance [ε × photothermal conversion efficiency (PCE)]. However, integrating high brightness and potent photothermal performance within a single molecule faces a formidable challenge. This article proposes a method to address this issue by preparing J-aggregate nanoparticles (NPs) using molecules with high ε. J-aggregates effectively improve QY and induce molecular emission redshift, while high ε molecules play a crucial role in improving the brightness and photothermal performance. By optimizing the molecular structure based on the pyrrolopyrrole cyanine (PPCy), precise control over the QY and PCE of PPCy J-aggregates is achieved. Ultimately, PDDO NPs exhibiting superior brightness (ε × QY = 3.32 × 104 M-1 cm-1) and photothermal performance (ε × PCE = 1.21 × 105 M-1 cm-1) are identified as high-performance photosensitizers. Notably, each parameter represents one of the highest levels among the reported fluorescence or photothermal probes to date. The in vivo studies demonstrate that PDDO NPs possess exceptional NIR imaging capabilities and remarkable photothermal tumor inhibition rates. This study provides innovative insights into the development of high-performance multifunctional photosensitizers.

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We introduce a two-step silica-encapsulation procedure to optimize both the optical efficiency and structural robustness of 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), a two-dimensional sheet-like J-aggregate. We report a fluorescence quantum yield of ∼98%, the highest quantum yield recorded for any J-aggregate structure at room temperature, and a fast, emissive lifetime of 234 ps. Silica, as an encapsulating matrix, provides optical transparency, chemical inertness, and robustness to dilution, while rigidifying the J-aggregate structure. Our in situ encapsulation process preserves the excitonic structure in TDBC J-aggregates, maintaining their light absorption and emission properties. The homogeneous silica coating has an average thickness of 0.5-1 nm around J-aggregate sheets. Silica encapsulation permits extensive dilutions of J-aggregates without significant disintegration into monomers. The narrow absorbance and emission line widths exhibit further narrowing upon cooling to 79 K, which is consistent with J-type coupling in the encapsulated aggregates. This silica TDBC J-aggregate construct signifies (1) a bright, fast, and robust fluorophore system, (2) a platform for further manipulation of J-aggregates as building blocks for integration with other optical materials and structures, and (3) a system for fundamental studies of exciton delocalization, transport, and emission dynamics within a rigid matrix.

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Photothermal therapy (PTT) has attracted attention as a highly effective non-invasive treatment method. However, the high localized temperatures (>50 °C) required for its treatment will inevitably cause damage to the surrounding normal tissues. Therefore, it is important to develop novel and effective strategies to achieve mild photothermal therapy (mPTT). The overexpression of heat shock proteins (HSPs), a widespread heat stress protein, leads to the generation of heat resistance in cancer cells, which seriously affects the therapeutic effect. Thus, inhibiting the expression of HSPs to reduce the heat resistance of tumor cells is expected to enhance the therapeutic effect of mPTT. Here, we successfully synthesized a fluorescent probe bonded with an amphiphilic polypeptide to a cyanine dye and achieved physical encapsulation of the blocker SB705498 through a self-assembly process. SB705498 promotes transient receptor potential vanilloid member 1 (TRPV1) channel blockade that can inhibit the translocation of the heat shock transcription factor 1 (HSF 1) by blocking the influx of calcium and thus affecting the expression of HSPs, which has the potential to enhance the thermotherapy of cancer under mild conditions. In addition, the nanoparticles enabled NIR-II fluorescence imaging with good stability and high photothermal conversion efficiency (48.10 %). Therefore, this study provides a new strategy for realizing precise mPTT(<45 °C) guided by NIR-II imaging. STATEMENT OF SIGNIFICANCE: Inhibition of overexpression of heat shock proteins (HSPs) in cancer photothermal therapy (PTT) is expected to enhance the therapeutic effect of mild photothermal therapy (mPTT). In this study, we synthesized a fluorescent probe bonded to cyanine dyes with amphiphilic polypeptides and physically wrapped the blocker SB705498 through a self-assembly process. As a transient receptor potential vanillin 1 (TRPV1) channel blocker, SB705498 inhibits heat shock transcription factor 1 (HSF1) translocation by blocking calcium ion influx, thereby improving mPTT efficacy by inhibiting the expression of HSPs. The nanoparticles also enable NIR-II fluorescence imaging with good stability and high photothermal conversion efficiency (48.10 %). Thus, this study provides a new strategy for NIR-II mPTT.

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DNA nanostructures offer a versatile platform for precise dye assembly, making them promising templates for creating photonic complexes with applications in photonics and bioimaging. However, despite these advancements, the effect of dye loading on the hybridization kinetics of single-stranded DNA protruding from DNA nanostructures remains unexplored. In this study, the DNA points accumulation for imaging in the nanoscale topography (DNA-PAINT) technique is employed to investigate the accessibility of functional binding sites on DNA-templated excitonic wires. The results indicate that positively charged dyes on DNA frameworks can accelerate the hybridization kinetics of protruded ssDNA through long-range electrostatic interactions. Furthermore, the impacts of various charged dyes and binding sites are explored on diverse DNA frameworks with varying cross-sizes. The research underscores the crucial role of electrostatic interactions in DNA hybridization kinetics within DNA-dye complexes, offering valuable insights for the functionalization and assembly of biomimetic photonic systems.

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IR-783, a commercially available near-infrared (NIR) heptamethine cyanine dye, has been used for selective tumor imaging in breast, prostate, cervical, and brain cancers in vitro and in vivo. Although the molecular mechanism behind the structure-inherent tumor targeting of IR-783 has not been well-demonstrated, IR-783 has unique properties such as a good water solubility and low cytotoxicity compared with other commercial heptamethine cyanine dyes. The goal of this study is to evaluate the phototherapeutic efficacy of IR-783 as a tumor-targeted photothermal agent in human colorectal cancer xenografts. The results demonstrate that IR-783 shows both the subcellular localization in HT-29 cancer cells and preferential accumulation in HT-29 xenografted tumors 24 h after its intravenous administration. Furthermore, the IR-783 dye reveals the superior capability to convert NIR light into heat energy under 808 nm NIR laser irradiation in vitro and in vivo, thereby inducing cancer cell death. Taken together, these findings suggest that water-soluble anionic IR-783 can be used as a bifunctional phototherapeutic agent for the targeted imaging and photothermal therapy (PTT) of colorectal cancer. Therefore, this work provides a simple and effective approach to develop biocompatible, hydrophilic, and tumor-targetable PTT agents for targeted cancer phototherapy.

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Cyanine-based cationic dyes with different substituents in the donor unit were easily synthesized using readily available starting materials. The prepared dye molecules were spectroscopically characterized by NMR, FT-IR, and HR-Mass, and their thermal stability was measured by TGA, DSC, and XRD. Based on the TGA and DSC measurements, it was concluded that all the dyes are thermally stable up to 200 °C. Also, powder XRD was studied for all dyes to identify the explicit crystallinity and morphological nature of the dyes. A dye dispersion solution was prepared for the proper dyeing of modacrylic fabric and the dyed fabric showed good color strength K/S for dyes R1, R2, and R6 and fragile color strength for R3, R4,and R5. These dyes are also used for printing on substrates like paper and fabric using ink-jet printing. These dyes were also used for transferability printing applications on various fabrics.

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Photothermal agent accompanying with thermally responsive materials, displays well controlled drug release property, which is well-received as an outstanding design strategy for simultaneous photothermal/chemotherapy in cancer. Cyanine dye, as the prestigious photothermal agent has shown great potential due to its preeminent near-infrared absorbance and excellent thermal conversion efficiency. However, their inherent defect such as inferior photothermal stability, high leakage risk and poor therapy efficacy limit their further application in cancer therapy. Hence, a facile and universal strategy to make up these deficiencies is developed. Chemotherapeutic drug DOX and cyanine dye were loaded into polydopamine (PDA) nanoparticles. The PDA encapsulation dramatically improved the photothermal stability of cyanine dye. Attributed by the PDA structure feature, the thermo-sensitive small molecule glyamine (Gla) is introduced into the PDA surface to lessen leakage. The Gla can form a dense encapsulation layer on the dopamine surface through hydrogen bond. This newly fabricated Cyanine/DOX@PDA-Gla nanopaltform is characterized with NIR light/pH dual-responsive property, high NIR photothermal conversion performance and fluorescence guided chemo-photothermal therapy.

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Second near-infrared (NIR-II,1000 âˆ¼ 1700 nm) therapeutic window presents an increased tissue penetration and elevated maximal permissible exposure in the application of photothermal therapy (PTT). However, the lack of NIR-II photothermal conversion agents (PCAs) limit their further development. In this work, we rationally designed and successfully developed three novel indolium-like heptamethine cyanine dyes (NFs) by installing N,N-diethylamino on the terminal ends of a conjugated polyene backbone and replacing the middle chlorine atom with o-mercapto benzoic acid and p-mercapto benzoic acid. Notably, NF2 with stronger rotating group encapsulated in organic nanoparticles (NF2 NPs) exhibited high photothermal conversion efficiency (PCE), which could come up to (61.3 %). Then we conducted serial experiments to further investigate PTT capability of NF2 NPs 4 T1 cell line and nude mice bearing 4 T1 tumor. As expected, the resulting NF2 NPs presented the excellent photothermal conversion ability and superb PTT effect both in vivo and in vitro. This study will inspire more work for future design and clinical applications of NIR-II therapeutic agents.

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In non-viscous aqueous solutions, the cyanine fluorescent dyes Cy3 and Cy5 have rather low fluorescence efficiency (the fluorescence quantum yields of Cy3 and Cy5 are 0.04 and 0.3, respectively [1, 2]) and short excited state lifetimes due to their structural features. In this work, we investigated the effect of solubility and rotational degrees of freedom on the fluorescence efficiency of Cy3 and Cy5 in several ways. We compared the fluorescence efficiencies of two cyanine dyes sCy3 and sCy5 with the introduction of a sulfonyl substituent in the aromatic ring as well as covalently bound to T10 oligonucleotides. The results show that because of the different lengths of the polymethine chains between the aromatic rings of the dyes, cis-trans-isomerization has a much greater effect on the Cy3 molecule than on the Cy5 molecule, while the effect of aggregation is also significant.

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Real-time vascular positioning, postoperative flap monitoring, and vascular reconstruction assessment are of great importance in flap transplantation. Cyanine dyes offer the advantage of high resolution in the Near-infrared-II (NIR-II) imaging window. However, the nonspecific binding of many cyanine dyes to endogenous albumin leads to high organ accumulation and skin absorption, resulting in low-quality imaging and poor reproducibility of contrast during long-term (e.g., 7 days) postoperative monitoring. Here, a novel strategy is proposed that can be widely applied to prevent protein binding for NIR-I/II Cl-containing cyanine dyes. This strategy produces protein-escaping dyes, ensuring high fluorescence enhancement in the blood with rapid clearance and no residual fluorescence, allowing for short-term repeatable injections for vascular imaging. This strategy in the perioperative monitoring of pedicle perforator flap models in mice and rats is successfully applied. Furthermore, leveraging the universality of this strategy, multiple nonoverlapping protein-escaping probes that achieve dual-excitation (808 and 1064 nm) interference-free imaging of nerve-vessel and tumor-vessel simultaneously are designed and synthesized. These protein-escaping dyes enable long-term repeatable dual-color imaging of tumor localization, resection, and tumor-vessel reconstruction at the wound site.

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Prostate cancer (PCa) is considered to be the most prevalent malignancy in males worldwide. Abiraterone is a 17α-hydroxylase/C17, 20-lyase (CYP17) inhibitor that has been approved for use in patients with prostate cancer. However, several negative aspects, such as drug resistance, toxicity, and lack of real-time monitoring of treatment responses, could appear with long-term use. Therefore, the development of anticancer agents with specific targeting to avoid side effects is imperative. Here, we used MHI-148, a type of heptamethine cyanine (HC) near-infrared fluorescence dye (NIRF), as a prototype structure to synthesize two theranostic agents, Abi-DZ-1 and Abi-783. The new compound Abi-DZ-1 retained the excellent photophysical characteristics and NIRF imaging property of MHI-148, and it could preferentially accumulate in prostate cancer cells but not in normal prostate epithelial cells via the HIF1α/organic anion-transporting polypeptides axis. NIRF imaging using Abi-DZ-1 selectively identified tumors in mice bearing PCa xenografts. Moreover, Abi-DZ-1 treatment significantly retarded the tumor growth in both a cell-derived xenograft model and a patient-derived tumor xenograft model. This finding demonstrated that Abi-DZ-1 may hold promise as a potential multifunctional theranostic agent for future tumor-targeted imaging and precision therapy. Constructing theranostic agents using the NIRF dye platform holds great promise in accurate therapy and intraoperative navigation.

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The scientific literature contains many accounts of application of polymethine dyes, including cyanine dyes, as imaging agents, i.e., "biological stains," for microscopic investigation of biological materials. Currently, many such dyes are used as probes for living cells, i.e., "fluorescent probes." Polymethine dyes are defined here by two criteria. First, they possess a conjugated chain of (2n + 1) sp2-hybridized carbon atoms that connect a terminal π-electron-accepting (π-electron withdrawing) group with a terminal π-electron-donating group. Second, they have an odd number (2n + 3) of π-centers and an even number (2n + 4) of π-electrons in this chain, where n equals the number of -CR2=CR3- groups, usually vinylene groups -CH=CH-. Commercialization of diverse chemical types of many polymethine dyes has been attempted. The dyes that have achieved wide application, however, are limited in number and it is these dyes that are emphasized here. Because these polymethine dyes sometimes have been described by confusing, and sometimes confused, names, we clarify here the chemical categories and names of such dyes for the nonchemist, biomedical end user of such imaging agents. Nevertheless, the nomenclature presented here is not intended to replace the traditional "chromophore" categories of dyestuff chemistry, because the latter are held in place both by wide usage and by venerable authorities, such as the Colour Index.

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In search of new probes for biomolecules, the spectral fluorescent study of four monomethine cyanine dyes (MCD), both unsymmetrical and symmetrical, has been carried out in different organic solvents, in aqueous buffer solutions, and in the presence of DNA and HSA. The complexation of MCD with biomacromolecules leads to a steep growth of the fluorescence intensity. Complexes of MCD with dsDNA and HSA of various types were modeled in silico by molecular docking. Experiments on thermal dissociation of dsDNA in the presence of MCD showed the formation of intercalative complexes of MCD with DNA. Quenching of intrinsic fluorescence of HSA by MCD occurred with rate constants much higher than the diffusion limit, that is, in dye-HSA complexes. Effective constants of MCD complexation with the biomacromolecules were estimated. MCD 1 has the best characteristics as a possible fluorescent probe for dsDNA and can serve as a sensitive and selective probe for dsDNA in the presence of HSA. Photochemical properties of MCD complexed with DNA have been also studied. An increase in the quantum yield of the triplet states of MCD in complexes with DNA has been found, which may be important for using these dyes as potential candidates in photodynamic therapy.

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Despite the wide range of applications of bright NIR-II polymethine scaffolds in biomedical imaging, their solvatochromism and aggregation-caused quenching (ACQ) effects in aqueous solutions limit their inherent brightness using traditional encapsulation methods, and effective hydrophilization strategies are still scarce. Here, a new set of Flav dyes is synthesized and PEGylated, followed by manufacturing DSPE@FlavP2000 nanoparticles using a self-adaptive co-assembly strategy to overcome these limitations. FlavP2000 can autonomously adjust its conformation when co-assembled with DSPE-PEG2000 , resulting in high-efficiency luminescence (≈44.9% fluorescence of Flav in DMSO). DSPE@FlavP2000 enables NIR-IIb (>1500 nm) angiography with high signal-to-noise ratios. Notably, this co-assembly can occur in situ between FlavP2000 with proteins in the living body based on a novel mechanism of brightness activation induced by disassembly (BAD), achieving consistent brightness as DSPE@FlavP2000 in blood or serum. The self-adaptive co-assembly strategy can be enhanced by incorporating an IPA moiety, which dynamically binds to albumin to prolong the dye's blood circulation time. Thus, the "enhanced" BAD is successfully applied to long-term vascular imaging and sciatic nerve imaging. Both the self-adaptive co-assembly strategy and BAD phenomenon improve the selectivity and availability of the hydrophilization methods, paving the way for efficient biological applications of polymethine dyes.

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Background: Near-infrared cyanine dyes have high sensitivity and spatial resolution imaging capabilities, but they also have unavoidable drawbacks such as photobleaching, low water solubility, fluorescence quenching, and toxic side effects. As an effective biologic drug carrier, albumin combines with cyanine dyes to form albumin@dye nanoparticles. These nanoparticles can alleviate the aforementioned issues and are widely used in tumor imaging and photothermal therapy. Methods: Herein, a newly synthesized near-infrared dye IR-817 was combined with bovine serum albumin (BSA) to create BSA@IR-817 nanoparticles. Through the detection of fluorescence emission and absorption, the optimal concentration and ratio of BSA and IR-817 were determined. Subsequently, dynamic light scattering (DLS) measurements and scanning electron microscopy (SEM) were used for the physical characterization of the BSA@IR-817 nanoparticles. Finally, in vitro and in vivo experiments were conducted to assess the fluorescence imaging and photothermal therapeutic potential of BSA@IR-817 nanoparticles. Results: IR-817 was adsorbed onto the BSA carrier by covalent conjugation and supramolecular encapsulation, resulting in the formation of dispersed, homogeneous, and stable nanoparticles with a particle size range of 120-220 nm. BSA@IR-817 not only improved the poor water solubility, fluorescence quenching, and toxic side effects of IR-817 but also enhanced the absorption and fluorescence emission peaks in the near-infrared region, as well as the fluorescence in the visible spectrum. In addition, BSA@IR-817 combined with laser 808 irradiation was able to convert light energy into heat energy with temperatures exceeding 50 °C. By creating a mouse model of subcutaneous melanoma, it was discovered that the tumor inhibition rate of BSA@IR-817 was greater than 99% after laser irradiation and that it achieved nearly complete tumor ablation without causing significant toxicity. Conclusion: Our research, therefore, proposes the use of safe and effective photothermal nanoparticles for the imaging, diagnosis, and treatment of melanoma, and offers a promising strategy for future biomedical applications.

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In the light of recent retrovirus pandemics, the issue of discovering new and diverse RNA-specific fluorochromes for research and diagnostics became of acute importance. The great majority of nucleic acid-specific probes either do not stain RNA or cannot distinguish between DNA and RNA. The versatility of polymethine dyes makes them suitable as stains for visualization, analysis, and detection of nucleic acids, proteins, and other biomolecules. We synthesized the asymmetric dicationic homodimeric monomethine cyanine dyes 1,1'-(1,3-phenylenebis(methylene))bis(4-((3-methylbenzo[d]thiazol-2(3H)-ylidene)methyl)pyridin-1-ium) bromide (Т1) and 1,1'-(1,3-phenylenebis(methylene))bis(4-((3-methylbenzo[d]thiazol-2(3H)-ylidene)methyl)quinolin-1-ium) bromide (M1) and tested their binding specificity, spectral characteristics, membrane penetration in living and fixed cells, cellular toxicity, and stability of fluorescent emission. Mesenchymal cells have diverse phenotypes and extensive proliferation and differentiation properties. We found dyes T1 and M1 to show high photochemical stability in living mesenchymal stem cells from apical papilla (SCAP) with a strong fluorescent signal when bound to nucleic acids. We found M1 to perform better than control fluorochrome (Hoechst 33342) for in vivo DNA visualization. T1, on the other hand, stains granular cellular structures resembling ribosomes in living cells and after permeabilization of the nuclear membrane stains the nucleoli and not the chromatin in the nucleus. This makes T1 suitable for the visualization of structures rich in RNA in living and fixed cells.