Mechanism of Visible-Light Photooxidative Demethylation of Toluidine Blue O

Effects and avoidance of photoconversion-induced artifacts in confocal and STED microscopy

Fluorescence microscopy is limited by photoconversion due to continuous illumination, which results in not only photobleaching but also conversion of fluorescent molecules into species of different spectral properties through photoblueing. Here, we determined different fluorescence parameters of photoconverted products for various fluorophores under standard confocal and stimulated emission depletion (STED) microscopy conditions. We observed changes in both fluorescence spectra and lifetimes that can cause artifacts in quantitative measurements, which can be avoided by using exchangeable dyes.

Ratiometric electrochemical immunoassay based on 2D Co/Fe MOF decorated with toluidine blue and Fc-labeled Schiff base for accurate assay of alpha-fetoprotein in clinical serum

The high level of alpha-fetoprotein (AFP) expression is closely related to hepatocellular carcinoma (HCC). Herein, a dual signal ratiometric electrochemical immunosensor based on chitosan-ferrocenecarboxaldehyde-spindle gold (Chit-Fc-SAu) and Co/Fe metal-organic framework-toluidine blue/polydopamine (Co/Fe MOF-TB/PDA) was proposed for quantitative analysis of AFP. Specifically, Chit-Fc-SAu worked as a substrate to trap more primary antibodies (Ab1) generating the first electrochemical signal from Fc. Thanks to the large specific surface area, the synergistic and electronic effects of Co/Fe MOF nanosheets, and the rich functional groups of PDA, Co/Fe MOF-TB/PDA could load more secondary antibodies (Ab2) and signal molecules (TB) providing another amplified electrochemical signal. In the presence of AFP, Ab1-AFP-Ab2 formed a sandwich structure, and as the AFP concentration increased, the peak current ratio of TB to Fc (ITB/IFc) also increased. The dual signal ratiometric strategy can avoid environmental signal interference and achieve signal self-calibration, thereby improving the accuracy and reproducibility of detection. After a series of exploration, this self-calibrated ratiometric immunosensor exhibited a wide linear range (0.001-200 ng mL-1), a low detection limit (0.34 pg mL-1), and good repeatability. When applied to the assay of clinical serum samples, the detection results of ratiometric sensor were consistent with that of commercial electrochemiluminescence (ECL) immunoassay, significantly superior to that of non-ratiometric sensor. The self-calibrated strategy based on ratiometric sensor helps to improve the accuracy of AFP in clinical diagnosis.

On the mechanism of visible-light sensitized photosulfoxidation of toluidine blue O

We report on the formation of toluidine blue O (TBO) sulfoxide by a self-sensitized photooxidation of TBO. Here, the photosulfoxidation process was studied by mass spectrometry (MS) and discussed in the context of photodemethylation processes which both contribute to TBO consumption over time. Analysis of solvent effects with D2O, H2O, and CH3CN along with product yields and MS fragmentation patterns provided mechanistic insight into TBO sulfoxide's formation. The formation of TBO sulfoxide is minor and detectable up to 12% after irradiation of 3 h. The photosulfoxidation process is dependent on oxygen wherein instead of a type II (singlet oxygen, 1O2) reaction, a type I reaction involving TBO to reach the TBO sulfoxide is consistent with the results. Density functional theory results point to the formation of the TBO sulfoxide by the oxidation of TBO via transiently formed peroxyl radical or thiadioxirane intermediates. We discover that the TBO photosulfoxidation arises competitively with TBO photodemethylation with the latter leading to formaldehyde formation.

A Self-Sustaining Near-Infrared Afterglow Chemiluminophore for High-Contrast Activatable Imaging

Afterglow imaging holds great promise for ultrasensitive bioimaging due to its elimination of autofluorescence. Self-sustaining afterglow molecules (SAMs), which enable all-in-one photon sensitization, chemical defect formation and afterglow generation, possess a simplified, reproducible, and efficient superiority over commonly used multi-component systems. However, there is a lack of SAMs, particularly those with much brighter near-infrared (NIR) emission and structural flexibility for building high-contrast activatable imaging probes. To address these issues, this study for the first time reports a methylene blue derivative-based self-sustaining afterglow agent (SAN-M) with brighter NIR afterglow chemiluminescence peaking at 710 nm. By leveraging the structural flexibility and tunability, an activatable nanoprobe (SAN-MO) is customized for simultaneously activatable fluoro-photoacoustic and afterglow imaging of peroxynitrite (ONOO- ), notably with a superior activation ratio of 4523 in the afterglow mode, which is at least an order of magnitude higher than other reported activatable afterglow systems. By virtue of the elimination of autofluorescence and ultrahigh activation contrast, SAN-MO enables early monitoring of the LPS-induced acute inflammatory response within 30 min upon LPS stimulation and precise image-guided resection of tiny metastatic tumors, which is unattainable for fluorescence imaging.

Bluer Phototruncation: Retro-Diels-Alder of Heptamethine Cyanine to Trimethine Cyanine through an Allene Hydroperoxide Intermediate

The photoconversion of heptamethine to pentamethine cyanines and of pentamethine to trimethine cyanines was recently reported. Here, we report mechanistic studies and initial experimental evidence for a previously unexplored 4-carbon truncation reaction that converts the simplest heptamethine cyanine to the corresponding trimethine cyanine. We propose a DFT-supported model describing a singlet oxygen (1O2)-mediated formation of an allene hydroperoxide intermediate and subsequent 4-carbon loss through a retro-Diels-Alder process. Fluorescence and mass spectrometry measurements provide evidence of this direct conversion process. This 4-carbon truncation reaction adds to a growing body of cyanine reactivity and may provide an optical tool leading to a substantial blue-shift (Δλem) of ∼200 nm.

Photoinduced Ring-Opening and Phototoxicity of an Indolin-3-one Derivative

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity.

Binding of toluidine blue-myristic acid derivative to cucurbit[7]uril and human serum albumin: computational and biophysical insights towards a biosupramolecular assembly

A new toluidine blue-myristic acid photosensitizer derivate (TBOMyr) was investigated as a design molecule to bind simultaneously to cucurbit[7]uril (CB[7]) and human serum albumin (HSA) with the aim of constructing a biosupramolecular assembly. Molecular docking and dynamics calculations revealed the main supramolecular and bio-molecular interactions of TBOMyr with the macrocycle or the protein, respectively. The addition of the negatively charged myristic acid-like tail resulted in a unique conformation of the CB[7] complex where the phenothiazine core was included in the cavity of CB[7], leaving the fatty acid portion free to interact with the protein. A favorable ternary interaction between TBOMyr, CB[7] and HSA was suggested by the calculations, and an experimental binding affinity in the order of 10⁵ M^-1 was determined for the TBOMyr@CB[7] complex with HSA. The new TBOMyr derivative could find applications in photodynamic therapy benefiting from the biosupramolecular interactions as a transport system.

Biosupramolecular complexes of amphiphilic photosensitizers with human serum albumin and cucurbit[7]uril as carriers for photodynamic therapy

In the present work, we evaluated the supramolecular interactions between three photosensitizers, namely toluidine blue O (TBO, positively charged) and two fatty acid conjugates of 6 and 14 carbon atoms chain lengths (TBOC6 and TBOC14), with human serum albumin (HSA) and the macrocycle cucurbit[7]uril (CB[7]), alone or in combination within a biosupramolecular system as potential carriers of photosensitizers for Photodynamic therapy (PDT). Binding studies were carried out using photophysical and calorimetric techniques and accompanied with molecular docking simulations. Amphiphilic photosensitizers, particularly TBOC14, showed stronger binding to HSA and (CB[7]). Comparing the different delivery systems, (CB[7]) had a marginal effect on cell uptake and phototoxicity in HeLa cells, while HSA showed enhanced cell uptake with phototoxicities that depended on the photosensitizer. Despite low cell uptake, the combination of both (CB[7]) and HSA was the most phototoxic, which illustrates the potential of combining these systems for PDT applications.

Defining the Basis of Cyanine Phototruncation Enables a New Approach to Single-Molecule Localization Microscopy

The light-promoted conversion of extensively used cyanine dyes to blue-shifted emissive products has been observed in various contexts. However, both the underlying mechanism and the species involved in this photoconversion reaction have remained elusive. Here we report that irradiation of heptamethine cyanines provides pentamethine cyanines, which, in turn, are photoconverted to trimethine cyanines. We detail an examination of the mechanism and substrate scope of this remarkable two-carbon phototruncation reaction. Supported by computational analysis, we propose that this reaction involves a singlet oxygen-initiated multistep sequence involving a key hydroperoxycyclobutanol intermediate. Building on this mechanistic framework, we identify conditions to improve the yield of photoconversion by over an order of magnitude. We then demonstrate that cyanine phototruncation can be applied to super-resolution single-molecule localization microscopy, leading to improved spatial resolution with shorter imaging times. We anticipate these insights will help transform a common, but previously mechanistically ill-defined, chemical transformation into a valuable optical tool.

Fatty Acid Conjugates of Toluidine Blue O as Amphiphilic Photosensitizers: Synthesis, Solubility, Photophysics and Photochemical Properties†

Toluidine blue O (TBO) is a water-soluble photosensitizer that has been used in photodynamic antimicrobial and anticancer treatments, but suffers from limited solubility in hydrophobic media. In an effort to incrementally increase TBO's hydrophobicity, we describe the synthesis of hexanoic (TBOC6) and myristic (TBOC14) fatty acid derivatives of TBO formed in low to moderate percent yields by condensation with the free amine site. Covalently linking 6 and 14 carbon chains led to modifications of not only TBO's solubility, but also its photophysical and photochemical properties. TBOC6 and TBOC14 derivatives were more soluble in organic solvents and showed hypsochromic shifts in their absorption and emission bands. The solubility in phosphate buffer solution was low for both TBOC6 and TBOC14, but unexpectedly slightly greater in the latter. Both TBOC6 and TBOC14 showed decreased triplet excited-state lifetimes and singlet oxygen quantum yields in acetonitrile, which was attributed to heightened aggregation of these conjugates particularly at high concentrations due to the hydrophobic "tails." While in diluted aqueous buffer solution, indirect measurements showed similar efficiency in singlet oxygen generation for TBOC14 compared to TBO. This work demonstrates a facile synthesis of fatty acid TBO derivatives leading to amphiphilic compounds with a delocalized cationic "head" group and hydrophobic "tails" for potential to accumulate into biological membranes or membrane/aqueous interfaces in PDT applications.