Synthesis and characterization of water-soluble two-photon excited blue fluorescent chromophores for bioimaging

Synthesis, Photophysics and PDT Evaluation of Mono-, Di-, Tri- and Hexa-PEG Chlorins for Pointsource Photodynamic Therapy

Pointsource photodynamic therapy (PSPDT) is a newly developed fiber optic method aimed at the delivery of photosensitizer, light and oxygen to a diseased site. Because of a need for developing photosensitizers with desirable properties for PSPDT, we have carried out a synthetic, photophysical and phototoxicity study on a series of PEGylated sensitizers. Chlorin and pheophorbide sensitizers were readily amenable to our synthetic PEGylation strategy to reach triPEG and hexaPEG galloyl pheophorbides and mono-, di-, triPEG chlorins. On screening these PEG sensitizers, we found that increasing the number of PEG groups, except for hexaPEGylation, increases phototoxicity. We found that three PEG groups but not less or more were optimal. Of the series tested, a triPEG gallyol pheophorbide and a triPEG chlorin were the most efficient at generating singlet oxygen, and produced the highest phototoxicity and lowest dark toxicity to Jurkat cells. A detailed kinetic analysis of the PEGylated sensitizers in solution and cell culture and media is also presented. The data provide us with steps in the development of PSPDT to add to the PDT tools we have in general.

Synthesis of a poly(ethylene glycol) galloyl sensitizer tip for an 'all-in-one' photodynamic device

This paper describes the synthesis of a specialized silica tip for an optical fiber device capable of delivering all components necessary for photodynamic therapy. Oxygen, light and a cleavable tripolyethylene glycol (PEG)-galloyl pheophorbide sensitizer are simultaneously delivered by the silica tip, where the tip was synthesized in six steps. A comparison of synthetic steps to reach PEGylated sensitizers bound to fluorinated silica and a previously reported Teflon/polyvinyl alcohol (PVA) nanocomposite ( Ghosh, G. et al. J. Phys. Chem. B 2015, 119, 4155- 4164) was made. The hydrolytic stability of the attached PEGs and the extent to which the PEG groups enhance solubility will also be discussed. The new triPEG-galloyl sensitizer has the potential for use in intraoperative pointsource photodynamic therapy which aims for precision treatment of residual disease. Schematic of the synthesis of a photoactive silica surface. It is composed of fluorinated silica connected to a photo-releasable sensitizer with short-chain PEGs.

Fluorescent Tobacco mosaic virus-Derived Bio-Nanoparticles for Intravital Two-Photon Imaging

Multi-photon intravital imaging has become a powerful tool to investigate the healthy and diseased brain vasculature in living animals. Although agents for multi-photon fluorescence microscopy of the microvasculature are available, issues related to stability, bioavailability, toxicity, cost or chemical adaptability remain to be solved. In particular, there is a need for highly fluorescent dyes linked to particles that do not cross the blood brain barrier (BBB) in brain diseases like tumor or stroke to estimate the functional blood supply. Plant virus particles possess a number of distinct advantages over other particles, the most important being the multi-valency of chemically addressable sites on the particle surface. This multi-valency, together with biological compatibility and inert nature, makes plant viruses ideal carriers for in vivo imaging agents. Here, we show that the well-known Tobacco mosaic virus is a suitable nanocarrier for two-photon dyes and for intravital imaging of the mouse brain vasculature.

A two-photon off-on fluorescence probe for imaging thiols in live cells and tissues

An effective off-on two-photon fluorescence imaging probe for thiols was developed and successfully applied to image thiols both in live cells and tissues.

Photodynamic therapy and two-photon bio-imaging applications of hydrophobic chromophores through amphiphilic polymer delivery

The synthesis and photophysical properties of two lipophilic quadrupolar chromophores featuring anthracenyl (1) or dibromobenzene (2) were described. These two chromophores combined significant two-photon absorption cross-sections with high fluorescence quantum yield for 1 and improved singlet oxygen generation efficiency for 2, in organic solvents. The use of Pluronic nanoparticles allowed a simple and straightforward introduction of these lipophilic chromophores into biological cell media. Their internal distribution in various cell lines was studied using fluorescence microscopy and flow-cytometry following a successful staining that was achieved upon 2 h of incubation. Finally, multiphoton excitation microscopy and photodynamic therapy capability of the chromophores were demonstrated by cell exposure to a 820 nm fs laser and cell death upon one photon resonant irradiation at 436 ± 10 nm, respectively.

Metal Ion-Responsive Fluorescent Probes for Two-Photon Excitation Microscopy

Metal ion-responsive fluorescent probes are powerful tools for visualizing labile metal ion pools in live cells. To take full advantage of the benefits offered by two-photon excitation microscopy, including increased depth penetration, reduced phototoxicity, and intrinsic 3D capabilities, the photophysical properties of the probes must be optimized for nonlinear excitation. This review summarizes the challenges associated with the design of two-photon excitable fluorescent probes and labels and offers an overview on recent efforts in developing selective and sensitive reagents for the detection of metal ions in biological systems.

Linear and nonlinear photophysics and bioimaging of an integrin-targeting water-soluble fluorenyl probe

Linear photophysical characterization and two-photon absorption (2PA) properties of a new water-soluble fluorene derivative, 3-(9-(2-(2-methoxyethoxy)ethyl)-2,7-bis{3-[2-(polyethyleneglycol-550-monomethylether-1-yl)]-4-(benzo[d]thiazol-2-yl)styryl}-9H-fluoren-9-yl)propanoic acid (1), were investigated in several organic solvents and water at room temperature. A comprehensive analysis of the steady-state absorption, emission and excitation anisotropy spectra revealed electronic structures of 1, including mutual orientation of the transition dipoles, relatively weak solvatochromic effects, high fluorescence quantum yield (approximately 0.5-1.0), and strong aggregation in water. The 2PA spectra of 1 were obtained in the 600-900 nm spectral range by two-photon induced fluorescence (2PF) and open aperture Z-scan methods using femtosecond laser sources. No discrete 2PA bands were apparent and values of the corresponding 2PA cross sections monotonically increased in the short wavelength range up to 3000 GM in organic solvents and approximately 6000 GM in aqueous solution, reflecting relatively high two-photon absorptivity. The 2PA efficiency of in water increased 2-3 times relative to aprotic solvents and can be explained by cooperative electronic effects of molecular aggregates of 1 produced in aqueous media. The carboxylic acid fluorenyl probe 1 was conjugated with the cyclic peptide RGDfK. Two-photon fluorescence microscopy (2PFM) imaging of U87MG cells (and MCF-7 as control), incubated with fluorene-RGD peptide conjugate 2, demonstrated high alpha(v)beta(3) integrin selectivity, making this probe particularly attractive for integrin imaging.

Synthesis and luminescence of CePO4:Tb/LaPO4 core/sheath nanowires

CePO(4):Tb/LaPO(4) nanowires with a core/sheath architecture have been successful synthesized by a facile aqueous chemical method mediated by original CePO(4):Tb aggregation seeds. The seed crystals serve as both a luminescence center and a nucleation site for epitaxial growth. The seed nanocrystals have an irregular sphere-like shape with an average size of around 6.8 nm and a narrow size distribution. When the seed crystals are coated with LaPO(4), the resulting core/sheath CePO(4):Tb/LaPO(4) nanowires have mean diameters of about 7.6 nm and lengths up to 331 nm. Both photo- and x-ray luminescence demonstrate that the LaPO(4) coating increases the luminescence efficiency. These core/sheath structured nanowires may find potential applications in solid state lighting, medical imaging and radiation detection.

Excited state intramolecular proton transfer and photophysics of a new fluorenyl two-photon fluorescent probe

The steady-state photophysical, NMR, and two-photon absorption (2PA) properties of a new fluorene derivative (1) containing the 2-(2'-hydroxyphenyl)benzothiazole (HBT) terminal construct is investigated for use as a fluorescence probe in bioimaging. A comprehensive analysis of the linear spectral properties reveals inter- and intramolecular hydrogen bonding and excited state intramolecular proton transfer (ESIPT) processes in the HBT substituent. A specific electronic model with a double minimum potential energy surface is consistent with the observed spectral properties. The 2PA spectra are obtained using a standard two-photon induced fluorescence method with a femtosecond kHz laser system, affording a maximum 2PA cross section of approximately 600 GM, a sufficiently high value for two-photon fluorescence imaging. No dependence of two-photon absorption efficiency on solvent properties and hydrogen bonding in the HBT substituent is observed. The potential use of this fluorenyl probe in bioimaging is demonstrated via one- and two-photon fluorescence imaging of COS-7 cells.

Photochemical tools to study dynamic biological processes

Light-responsive biologically active compounds offer the possibility to study the dynamics of biological processes. Phototriggers and photoswitches have been designed, providing the capability to rapidly cause the initiation of wide range of dynamic biological phenomena. We will discuss, in this article, recent developments in the field of light-triggered chemical tools, specially how two-photon excitation, "caged" fluorophores, and the photoregulation of protein activities in combination with time-resolved x-ray techniques should break new grounds in the understanding of dynamic biological processes.

Long-lived two-photon excited luminescence of water-soluble europium complex: applications in biological imaging using two-photon scanning microscopy

A new europium complex presenting good solubility and stability in water, intense emission in the red (616 nm), long luminescence lifetime and significant two-photon absorption cross-section in the biological window has been designed and successfully used for two-photon scanning microscopy bio-imaging experiments on fixed cancer cells.

A comprehensive investigation of the interrelationships between spectroscopy and photochemistry and substituents of some azulenic derivatives

Several azulenic dyes, including six azulene hydrocarbons, two azulene aldehydes, and two olefinic azulenes, have been synthesized to survey their photophysics and photochemistry. These azulenes display S(2)-->S(0) emission, but with several differences. This is the most remarkable characteristic of the effect of orbital control on color and excited state properties of the azulenic compounds. This paper emphasizes how emission spectra and photochemistry of azulenic compounds are influenced by their chemical structure and solvent. The emission spectra of the azulene hydrocarbons suggest that their excited state properties can be controlled by their molecular structure and size. It was confirmed by emission and (1)H NMR spectroscopy that azulene monoaldehyde is protonated in a strong acid, such as trifluoroacetic acid (TFA). Photochemistry of styrylazulenes was observed during irradiation. Azulenic compounds are thermally stable and color tunable, and hence they are good candidates as non-linear optical materials. Based on their unique photochemical and photophysical characteristics, novel azulenic dyes can be constructed for different uses.

Cell-permeant cytoplasmic blue fluorophores optimized for in vivo two-photon microscopy with low-power excitation

Because of the spreading of nonlinear microscopies in biology, there is a strong demand for specifically engineered probes in these applications. Herein, we report on the imaging properties in living cells and nude mice brains of recently developed water soluble blue fluorophores that show efficient diffusion through cell membranes and blood-brain barriers. They are characterized by two-photon absorption cross-sections of 100-150 Goeppert-Mayer range in the near IR and fluorescence efficiencies of up to 72% in water. They were found to stain homogeneously the cytoplasm of cultured living cells within minutes. Moreover, their diffusion times and fluorescence characteristics in the cytoplasm suggest a hydrophobic association with intracellular membranes. Their intracellular fluorescent decays were found to be almost mono-exponential, a very favorable feature for fluorescence lifetime imaging. Two photon images of living cells were obtained with a good signal to noise ratio using laser powers in the sub-milliwatt range. This allows continuous imaging without significant photobleaching for tens of minutes. In addition, these fluorophores allowed in vivo three-dimensional two-photon imaging of mice cortex vasculatures and extra vasculature structures, with no sign of toxicity.

Two-Photon Microscopy and Spectroscopy of Lanthanide Bioprobes

The linear and non-linear photophysical properties of tris-dipicolinate europium and terbium complexes (absorption, emission, lifetime, luminescence induced by two-photon absorption) are studied in the crystalline state as well as in protein derivative crystals and compared to those in solution. Upon laser irradiation at 532 nm, luminescence of terbium is induced by a two-photon antenna effect, whereas luminescence of europium results from one-photon absorption in forbidden f-f transitions. Finally, linear and two-photon microscopy imaging experiments on biological and bio-inspired crystals are performed. These first proof-of-concept experiments open the way for the development of time-resolved non-linear microscopy that should combine the advantages of lanthanide luminescence (long lifetime, sharp emission bands, insensitivity to oxygen) with those of confocal biphotonic excitation (near-IR excitation, 3D resolution and reduced photodamage).

Synthesis of chromophores combining second harmonic generation and two photon induced fluorescence properties

The design of new chromophores presenting simultaneous SHG and TPEF properties is reported.