Two-photon excitation of ethidium bromide labeled DNA

A small-molecule with a large two-photon absorption cross-section serves as a membrane-permeable ribonucleic acid (RNA) probe for live cell imaging

DMI could light up the RNA of the nucleus and the cytoplasm in living systems, which not only exhibits larger two-photon absorption cross-sections (981 GM), but also displays high-permeability to plasma membranes of vigorous cells.

Turn-on fluorescent probes that can light up endogenous RNA in nucleoli and cytoplasm of living cells under a two-photon microscope

We report two turn-on RNA probes that can light up endogenous RNA in the nucleoli and cytoplasm of living cells under a two-photon microscope.

Novel mono-cationic fluorescent probes based on different central π-conjugated bridges for two-photon bioimaging of cellular nuclei

A series of novel pyridine mono-cationic two-photon fluorescent probes based on different central π-conjugated bridges, fluorenone (W-pyI), dibenzothiophene (S-pyI), and dibenzofuran (F-pyI), were prepared and studied for improving photostability in bioimaging applications.

Toward the measurement of multiple fluorescence lifetimes in flow cytometry: maximizing multi-harmonic content from cells and microspheres

Flow cytometry is a powerful means for in vitro cellular analyses where multi-fluorescence and multi-angle light scattering can indicate unique biochemical or morphological features of single cells. Yet, to date, flow cytometry systems have lacked the ability to capture complex fluorescence dynamics due to the transient nature of flowing cells. In this contribution we introduce a simple approach for measuring multiple fluorescence lifetimes from a single cytometric event. We leverage square wave modulation, Fourier analysis, and high frequency digitization and show the ability to resolve more than one fluorescence lifetime from fluorescently-labelled cells and microspheres. Illustration of a flow cytometer capable of capturing multiple fluorescence lifetime measurements; creating potential for multi-parametric, time-resolved signals to be captured for every color channel.

Conjugated oligoelectrolyte harnessed polyhedral oligomeric silsesquioxane as light-up hybrid nanodot for two-photon fluorescence imaging of cellular nucleus

A water-soluble organic/inorganic hybrid nanodot based on polyhedral oligomeric silsesquioxane (POSS) and conjugated oligoelectrolyte is designed and synthesized for two-photon fluorescence imaging of cellular nucleus, which takes advantage of its small size (∼3.3 nm) that imparts nucleus permeability and substantial DNA-enhanced two-photon excited fluorescence that allows illuminating the nucleus with a high contrast.

Two-photon microscopy of deep intravital tissues and its merits in clinical research

Multiphoton excitation laser scanning microscopy, relying on the simultaneous absorption of two or more photons by a molecule, is one of the most exciting recent developments in biomedical imaging. Thanks to its superior imaging capability of deeper tissue penetration and efficient light detection, this system becomes more and more an inspiring tool for intravital bulk tissue imaging. Two-photon excitation microscopy including 2-photon fluorescence and second harmonic generated signal microscopy is the most common multiphoton microscopic application. In the present review we take diverse ocular tissues as intravital samples to demonstrate the advantages of this approach. Experiments with registration of intracellular 2-photon fluorescence and extracellular collagen second harmonic generated signal microscopy in native ocular tissues are focused. Data show that the in-tandem combination of 2-photon fluorescence and second harmonic generated signal microscopy as two-modality microscopy allows for in situ co-localization imaging of various microstructural components in the whole-mount deep intravital tissues. New applications and recent developments of this high technology in clinical studies such as 2-photon-controlled drug release, in vivo drug screening and administration in skin and kidney, as well as its uses in tumourous tissues such as melanoma and glioma, in diseased lung, brain and heart are additionally reviewed. Intrinsic emission two-modal 2-photon microscopy/tomography, acting as an efficient and sensitive non-injurious imaging approach featured by high contrast and subcellular spatial resolution, has been proved to be a promising tool for intravital deep tissue imaging and clinical studies. Given the level of its performance, we believe that the non-linear optical imaging technique has tremendous potentials to find more applications in biomedical fundamental and clinical research in the near future.

Biomolecule labeling and imaging with a new fluorenyl two-photon fluorescent probe

Closely involved in the progression of nonlinear bioimaging is the development of optical probes for investigating biological function and activity. Introduction of new fluorescent compounds possessing enhanced nonlinearities is essential for advancing the utility of two-photon absorption (2PA) processes in the biological sciences. Herein, we report the synthesis of fluorene-based fluorophores tailored for multiphoton imaging, incorporating the succinimidyl ester and thioester functionality as reactive linkers for further coupling with a wide variety of biologically relevant molecules. The succinimidyl ester amine reactive probe was conjugated with the cyclic peptide RGDfK and polyclonal antirat IgG protein. Upon conjugation, the basic molecular architecture and photophysical properties of the active 2PA chromophore remain unchanged. Conventional and two-photon fluorescence microscopy (2PFM) imaging of COS-7 and HeLa cells, incubated with either the fluorene-RGD peptide conjugate or the fluorene-IgG conjugate, was demonstrated. The fluorene-IgG conjugate was used to image cell spindles at early mitotic developmental stages.

Vinyl-Pyridinium Triphenylamines: Novel Far-Red Emitters with High Photostability and Two-Photon Absorption Properties for Staining DNA

A series of mono-, bis- and trisvinyl-pyridinium triphenylamines (TP-py) has been synthesised and evaluated for its one- and two-photon absorption (2PA) induced-fluorescence properties under biological conditions. Interestingly, these compounds are only weakly fluorescent in water, whereas their fluorescence emissions are strongly restored (exaltation factors of 20-100) upon binding to double-stranded DNA. Additional measurements in glycerol indicate that the fluorescence increases are the result of immobilisation of the dyes in the DNA matrix, which inhibits rotational de-excitation modes. This particular feature is especially remarkable in the case of the bis and tris derivatives (TP-2 py, TP-3 py), which each display a high affinity (K(d) ~ microM) for dsDNA. TPIF measurements have shown that TP-2 py and TP-3 py each have a large 2PA cross section (delta up to 700 GM) both in glycerol and in the presence of DNA, which ranks them amongst the best 2PA biological fluorophores. Finally, one- and two-photon confocal imaging in cells revealed that these compounds perform red staining (lambda(em)=660-680 nm) of nuclear DNA with excellent contrast. The remarkable optical properties of the TP-py series, combined with their high photostability and their easy synthetic access, make these compounds extremely attractive for use in confocal and 2PA microscopy.

One- and two-photon induced fluorescence of Pacific Blue-labeled human serum albumin deposited on different core size silver colloids

We studied one- and two-photon induced fluorescence of Pacific Blue (PB)-labeled human serum albumin (HSA) in the presence of different size silver colloids. The PB fluorescence emission intensity was observed with small (30-40 nm) and large (about 120 nm) colloids and compared with PB emission in absence of colloids. For the system with a small core size colloids we did not detect any fluorescence enhancement with one-photon excitation and the enhancement observed with two-photon excitation was about 2.5-fold. In contrast, for large silver colloids we observed about a 2-fold increase in PB fluorescence brightness for one-photon excitation, and the enhancement with two-photon excitation excided 13-folds. Much stronger increases in brightness observed with two-photon excitation, compared to one-photon excitation, indicate a dominant role of enhanced local field in fluorescence enhancement on silver colloids in solutions.

Directional two-photon induced surface plasmon-coupled emission

We measured a directional surface plasmon-coupled emission (SPCE) induced by a two-photon absorption. A 60 nm thick layer of poly(vinyl alcohol) film doped with rhodamine 123 was deposited on a silvered (50 nm Ag) glass slide, which was attached to a hemicylindrical glass prism. The 820 nm excitation from a femtosecond Ti:Sapphire laser was used either in reverse Kretschmann or Kretschmann configuration. The angular distribution of two-photon induced SPCE does not depend on the used configuration. The two-photon induced SPCE can be applied to improve immunoassays and deoxyribonucleic acid detection.

Two-photon induced fluorescence of Cy5-DNA in buffer solution and on silver island films

We report the observation of a strong two-photon induced fluorescence emission of Cy5-DNA within the tunable range of a Ti:Sapphire laser. The estimated two-photon cross-section for Cy5-DNA of 400GM is about 3.5-fold higher than it was reported for rhodamine B. The fundamental anisotropies of Cy5-DNA are close to the theoretical limits of 2/5 and 4/7 for one- and two-photon excitation, respectively. We also observed an enhanced two-photon induced fluorescence (TPIF) of Cy5-DNA deposited on silver island films (SIFs). In the presence of SIFs, the TPIF is about 100-fold brighter. The brightness increase of Cy5-DNA TPIF near SIFs is mostly due to enhanced local field.

Rotation of the lever arm of Myosin in contracting skeletal muscle fiber measured by two-photon anisotropy

The rotation of the lever arm of myosin cross-bridges is believed to be responsible for muscle contraction. To resolve details of this rotation, it is necessary to observe a single cross-bridge. It is still impossible to do so in muscle fiber, but it is possible to investigate a small population of cross-bridges by simultaneously activating myosin in a femtoliter volume by rapid release of caged ATP. In earlier work, in which the number of observed cross-bridges was limited to approximately 600 by confocal microscopy, we were able to measure the rates of cross-bridge detachment and rebinding. However, we were unable to resolve the power stroke. We speculated that the reason for this was that the number of observed cross-bridges was too large. In an attempt to decrease this number, we used two-photon microscopy which permitted observation of approximately 1/2 as many cross-bridges as before with the same signal/noise ratio. With the two-photon excitation, the number of cross-bridges was small enough to resolve the beginning of the power stroke. The results indicated that the power stroke begins approximately 170 ms after the rigor cross-bridge first binds ATP.

A monomethine cyanine dye Cyan 40 for two-photon-excited fluorescence detection of nucleic acids and their visualization in live cells

Monomethine cyanine dye 4-((1-methylbenzothiazolyliliden-2)methyl)-1,2,6-trimethylpyridinium perchlorate (Cyan 40) was investigated as a two-photon-excited fluorescence probe for nucleic acids (NA). Cyan 40 has been shown to demonstrate efficient two-photon-excited fluorescence in the presence of NA in vitro in contrast to solutions without NA. Two-photon confocal laser scanning microscopy (TPCLSM) and two-photon laser scanning microspectrofluorometry were used to check the possibility of using Cyan 40 as two-photon-excited fluorescence label for NA in living cells. Study of dye effect on viability of cells was also carried out. We ascertained that Cyan 40 is a cell-permeant dye, manifesting efficient two-photon-excited fluorescence when bound to NA in living cells, without any significant influence on viability of cells. TPCLSM images obtained from stained cells indicate preferential RNA staining by Cyan 40 compared with DNA.

One- and two-photon excited fluorescence lifetimes and anisotropy decays of green fluorescent proteins

We have used one- (OPE) and two-photon (TPE) excitation with time-correlated single-photon counting techniques to determine time-resolved fluorescence intensity and anisotropy decays of the wild-type Green Fluorescent Protein (GFP) and two red-shifted mutants, S65T-GFP and RSGFP. WT-GFP and S65T-GFP exhibited a predominant approximately 3 ns monoexponential fluorescence decay, whereas for RSGFP the main lifetimes were approximately 1.1 ns (main component) and approximately 3.3 ns. The anisotropy decay of WT-GFP and S65T-GFP was also monoexponential (global rotational correlation time of 16 +/- 1 ns). The approximately 1.1 ns lifetime of RSGFP was associated with a faster rotational depolarization, evaluated as an additional approximately 13 ns component. This feature we attribute tentatively to a greater rotational freedom of the anionic chromophore. With OPE, the initial anisotropy was close to the theoretical limit of 0.4; with TPE it was higher, approaching the TPE theoretical limit of 0.57 for the colinear case. The measured power dependence of the fluorescence signals provided direct evidence for TPE. The general independence of fluorescence decay times, rotation correlation times, and steady-state emission spectra on the excitation mode indicates that the fluorescence originated from the same distinct excited singlet states (A*, I*, B*). However, we observed a relative enhancement of blue fluorescence peaked at approximately 440 nm for TPE compared to OPE, indicating different relative excitation efficiencies. We infer that the two lifetimes of RSGFP represent the deactivation of two substates of the deprotonated intermediate (I*), distinguished by their origin (i.e., from A* or B*) and by nonradiative decay rates reflecting different internal environments of the excited-state chromophore.