Design of organic molecules with large two-photon absorption cross sections

Probing single molecules in single living cells

Single-molecule detection in single living cells has been achieved by using confocal fluorescence microscopy and externally tagged probe molecules. The intracellular background fluorescence is substantially higher than that in aqueous buffer, but this background is continuous and stable and does not significantly interfere with the measurement of single-molecule photon bursts. As a result, single-molecule data have been obtained on three types of fluorescent probes at spatially resolved locations (e.g., cytoplasm and nucleus) inside human HeLa cells. First, the iron transport protein transferrin labeled with tetramethylrhodamine undergoes rapid receptor-mediated endocytosis, and single transferrin molecules are detected inside living cells. Second, the cationic dye rhodamine 6G (R6G) enters cultured cells by a potential-driven process, and single R6G molecules are observed as intense photon bursts when they move in and out of the intracellular laser beam. Third, we report results on synthetic oligonucleotides that are tagged with a fluorescent dye and are taken up by living cells via a passive, nonendocytic pathway.

High-throughput single-molecule spectroscopy in free solution

A high-speed high-throughput single-molecule imaging technique for identifying molecules in free solution based on differences in their fluorescence emission spectra is presented. Unlike previous reports, the entire spectrum, rather than selected wavelengths through optical filters, is recorded. Furthermore, the millisecond data acquisition time means that the molecules do not need to be immobilized or spatially confined. In one example, individual lambdaDNA molecules labeled with YOYO-I, POPO-III, or a combination of the two dyes can be distinguished from one another. In another example, biotinylated 2.1-kb DNA labeled with YOYO-I was reacted with avidin-conjugated R-phycoerythrin. The two different reactant molecules and the product molecule can be simultaneously imaged and identified by their spectroscopic characteristics. This technique can therefore be used for screening single molecules for disease markers and for monitoring individual molecular interactions at a rate of thousands of molecules per second.

Real three-dimensional microstructures fabricated by photopolymerization of resins through two-photon absorption

Effective energy windows for two-photon absorption (TPA) photopolymerization of resins were investigated and, with a properly selected laser pulse energy, exquisite three-dimensional (3D) microstructures with submicrometer spatial resolution were achieved. The results show the inherent utility of TPA in the fabrication of real 3D patterns. In particular, we propose and utilize a resin pre-exposure technique by which freely movable components affixed to an axle are built, demonstrating a new application of TPA in laser microfabrication.

Synthesis of new two-photon absorbing fluorene derivatives via Cu-mediated Ullmann condensations

The Ullmann amination reaction was utilized to provide access to a number of fluorene analogues from common intermediates, via facile functionalization at positions 2, 7, and 9 of the fluorene ring. Through variation of amine or iodofluorene derivative, analogues bearing substitutents with varying electron-donating and electron-withdrawing ability, e.g., diphenylamino, bis-(4-methoxyphenyl)amine, nitro, and benzothiazole, were synthesized in good yield. The novel fluorene derivatives were fully characterized, including absorption and emission spectra. Didecylation at the 9-position afforded remarkably soluble derivatives. Target compounds 4, 5, and 9 are potentially useful as fluorophores in two-photon fluorescence microscopy. Their UV-vis spectra display desirable absorption in the range of interest suitable for two-photon excitation by near-IR femtosecond lasers. Preliminary measurements of two-photon absorption indicate the derivatives exhibit high two-photon absorptivity, affirming their potential as two-photon fluorophores. For example, using a 1,210 nm femtosecond pump beam, diphenylaminobenzothiazolylfluorene 4 exhibited nondegenerate two-photon absorption, with two-photon absorptivity (delta) of ca. 820 x 10(-50) cm(4) s photon(-1) molecule(-1) at the femtosecond white light continuum probe wavelength of 615 nm.

High-throughput single-molecule DNA screening based on electrophoresis

In electrophoresis, the migration velocity is used for sizing DNA and proteins or for distinguishing molecules based on charge and hydrodynamic radius. Many protein and DNA assays relevant to disease diagnosis are based on such separations. However, standard protocols are not only slow (minutes to hours) but also insensitive (many molecules in a detectable band). We successfully demonstrated a high-throughput imaging approach that allows determination of the individual electrophoretic mobilities of many molecules at a time. Each measurement only requires a few milliseconds to complete. This opens up the possibility of screening single copies of DNA or proteins within single biological cells for disease markers without performing polymerase chain reaction or other biological amplification. The purpose is not to separate the DNA molecules but to identify each one on the basis of the measured electrophoretic mobility. We developed three different procedures to measure the individual molecular mobilities. The results correlate well with capillary electrophoresis (CE) experiments for the same samples (2-49 kb dsDNA) under identical separation conditions. The implication is that any electrophoresis protocols from slab gels to CE should be adaptable to single-molecule screening for disease diagnosis.

Two-photon excitation of fluorescence for three-dimensional optical imaging of biological structures

Techniques based on two-photon excitation (TPE) allow three-dimensional (3D) imaging in highly localized volumes, of the order of magnitude of a fraction of a femtolitre up to single-molecule detection. In TPE microscopy a fundamental advantage over conventional widefield or confocal 3D fluorescence microscopy is given by the use of infrared (IR) instead of ultraviolet (UV) radiation to excite those fluorophores requiring UV excitation, hence causing little damage to the specimen or to fluorescent molecules outside the volume of the TPE event and allowing a deeper penetration within the sample compared with conventional one-photon excitation of fluorescence. In our laboratory, within the framework of a national INFM project, we have realized a TPE fluorescence microscope, part of a multipurpose architecture also including lifetime imaging and fluorescence correlation spectroscopy modules. The core of the architecture is a mode-locked Ti:sapphire infrared pulsed laser pumped by a high-power (5 W, 532 nm) solid-state laser and coupled to an ultracompact scanning head. For the source we have measured a pulse width from 65 to 95 fs as a function of wavelength (690-830 nm). The scanning head allows conventional and two-photon confocal imaging. Point spread function measurements are reported with examples of applications to the study of biological systems.

Adapting a compact confocal microscope system to a two-photon excitation fluorescence imaging architecture

Within the framework of a national National Institute of Physics of Matter (INFM) project, we have realised a two-photon excitation (TPE) fluorescence microscope based on a new generation commercial confocal scanning head. The core of the architecture is a mode-locked Ti:Sapphire laser (Tsunami 3960, Spectra Physics Inc., Mountain View, CA) pumped by a high-power (5 W, 532 nm) laser (Millennia V, Spectra Physics Inc.) and an ultracompact confocal scanning head, Nikon PCM2000 (Nikon Instruments, Florence, Italy) using a single-pinhole design. Three-dimensional point-spread function has been measured to define spatial resolution performances. The TPE microscope has been used with a wide range of excitable fluorescent molecules (DAPI, Fura-2, Indo-1, DiOC(6)(3), fluoresceine, Texas red) covering a single photon spectral range from UV to green. An example is reported on 3D imaging of the helical structure of the sperm head of the Octopus Eledone cirrhosa labelled with an UV excitable dye, i.e., DAPI. The system can be easily switched for operating both in conventional and two-photon mode.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Highly Conjugated Molecules from Dibromonaphthyl Derivatives and 4-Vinylpyridine or 4-Acetoxystyrene by the Heck Reaction

Palladium-catalyzed coupling of 1,6- and 2,6-dibromonaphthyl derivatives with 4-vinylpyridine or 4-acetoxystyrene resulted in mono- and bis-arylvinylation depending on the choice of reaction conditions. For the 1,6-dibromoarenes, the mono-arylvinylated derivative at position-6 was the sole product in the presence of (o-tol)(3)P/Et(3)N. The bis-arylvinylated derivative was the major product, accompanied by a minor product, arylvinylated at position-6 and reduced at position-1, in the presence of (o-tol)(3)P/Et(3)N/MeCN. For the 2,6-dibromoarenes, the bis-arylvinylated derivative was the sole product in the presence of either Ph(3)P or (o-tol)(3)P, if provided with a small volume of Et(3)N/MeCN solvent, and the mono-arylvinylated derivative was the major product with larger solvent volume and higher haloarene ratio. Fluorescence intensities of 2,6-bis-arylvinylated products were 2 to 3 orders of magnitude higher than that of stilbene. Nematic phases, at relatively high temperatures, were observed for some of the rodlike molecules. Shown by (1)H NMR study, at the photostationary state of isomerization, smaller fractions of cis-form were obtained for molecules that exhibited larger fluorescence quantum yields. The results presented here are beneficial to the pursuit of nonlinear optical materials, fluorescent mesogens, photo- and electroactive materials.

Brominated 7-hydroxycoumarin-4-ylmethyls: photolabile protecting groups with biologically useful cross-sections for two photon photolysis

Photochemical release (uncaging) of bioactive messengers with three-dimensional spatial resolution in light-scattering media would be greatly facilitated if the photolysis could be powered by pairs of IR photons rather than the customary single UV photons. The quadratic dependence on light intensity would confine the photolysis to the focus point of the laser, and the longer wavelengths would be much less affected by scattering. However, previous caged messengers have had very small cross sections for two-photon excitation in the IR region. We now show that brominated 7-hydroxycoumarin-4-ylmethyl esters and carbamates efficiently release carboxylates and amines on photolysis, with one- and two-photon cross sections up to one or two orders of magnitude better than previously available. These advantages are demonstrated on neurons in brain slices from rat cortex and hippocampus excited by glutamate uncaged from N-(6-bromo-7-hydroxycoumarin-4-ylmethoxycarbonyl)-L-glutamate (Bhc-glu). Conventional UV photolysis of Bhc-glu requires less than one-fifth the intensities needed by one of the best previous caged glutamates, gamma-(alpha-carboxy-2-nitrobenzyl)-L-glutamate (CNB-glu). Two-photon photolysis with raster-scanned femtosecond IR pulses gives the first three-dimensionally resolved maps of the glutamate sensitivity of neurons in intact slices. Bhc-glu and analogs should allow more efficient and three-dimensionally localized uncaging and photocleavage, not only in cell biology and neurobiology but also in many technological applications.