Two‐photon excitation fluorescence microscopy using a semiconductor laser

Linear and Nonlinear Thermal Lens Signal of the (Δν = 6) C-H Vibrational Overtone of Naphthalene in Liquid Solutions of n-Hexane

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of naphthalene (C₁₀H₈) in liquid hexane. The C-H fifth vibrational (Δν = 6) overtone spectrum of C₁₀H₈ is detected at room temperature for mole fractions from 0.08 to 19 × 10^-6 using n-C₆H₁₄ as solvent. By detecting the absorption band in a 19 ppm (parts per million) solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10^-7cm^-1. A plot of normalized integrated intensity as a function of the mole fraction of naphthalene in solution reveals a dependence of the magnitude of the signal with the probe laser wavelength. If the wavelength of the probe laser is 568 nm, the thermal lens signal (TLS) is linear as a function of the mole fraction of the solution. When the wavelength of the probe laser is 488 nm, the TLS is nonlinear as a function of the concentration. Three different models of nonlinear absorption are discussed. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers could explain the enhanced magnitude and the nonlinear behavior of the TLS for solutions of mole fraction < 0.1%.

Advances in laser sources for confocal and multiphoton microscopy

The illumination source for all high-resolution, optical sectioning, scanning microscopes is crucially important to the overall performance of the system. We examine advances that have been made in laser sources for both confocal and multiphoton microscopy where the emphasis has been on the development of potentially low-cost, easy to use sources. Growing interest in temporally and spatially resolved techniques has directed laser research towards addressing these challenges. We present the most recent developments in sources for confocal and multiphoton microscopy along with the considerations that should be made when a new source is being considered.

Photostability of a fluorescent marker under pulsed excited-state depletion through stimulated emission

Saturated stimulated-emission depletion (STED) of a fluorescent marker has been shown to break the diffraction barrier in far-field fluorescence microscopy and to facilitate spatial resolution down to a few tens of nanometers. Here we investigate the photostability of a fluorophore that, in this concept, is repeatedly excited and depleted by synchronized laser pulses. Our study of bacteria labeled with RH-414, a membrane marker, reveals that increasing the duration of the STED pulse from approximately 10 to 160 ps fundamentally improves the photostability of the dye. At the same time the STED efficiency is maintained. The observed photobleaching of RH-414 is due primarily to multiphoton absorption from its ground state. One can counteract photobleaching by employing STED pulses that range from 150 ps to approximately half of the lifetime of the excited state. The results also have implications for multiphoton excitation microscopy.

A new microvolume technique for bioaffinity assays using two-photon excitation

Bioaffinity binding assays such as the immunoassay are widely used in life science research. In an immunoassay, specific antibodies are used to bind target molecules in the sample, and quantification of the binding reaction reveals the amount of the target molecules. Here we present a method to measure bioaffinity assays using the two-photon excitation of fluorescence. In this method, microparticles are used as solid phase in binding the target molecules. The degree of binding is then quantified from individual microparticles by use of two photon excitation of fluorescence. We demonstrated the effectiveness of the method using the human alpha-fetoprotein (AFP) immunoassay, which is used to detect fetal disorders. The sensitivity and dynamic range we obtained with this assay indicate that this method can provide a cost-effective and simple way to measure various biomolecules in solution for research and clinical applications.

Cell Viability and DNA Denaturation Measurements by Two-Photon Fluorescence Excitation in CW Al:GaAs Diode Laser Optical Traps

Cell viability and DNA denaturation are measured through two-photon fluorescence excitation using a single diode laser beam as the trapping and exciting source simultaneously. Two-photon fluorescence emission spectra are presented for CHO cells and T lymphocytes loaded with various fluorescent probes. This single beam method is demonstrated to be a safe tool to monitor the viability of optically trapped cells, even under intense 809 nm diode laser illumination (∼106 W/cm2). The dynamics of cellular necrosis is monitored by adding ethanol to the cell suspension during trapping. Thermal damage to heat-treated samples is assessed by recording shifts in the emission spectra from trapped cells loaded with the nucleic acid probe, acridine orange. © 1999 Society of Photo-Optical Instrumentation Engineers.

Spatially localized ballistic two-photon excitation in scattering media

We describe spatially localized two-photon excitation in scattering media. Using femtosecond pulses at 770 nm from a Ti: Sapphire laser, we were able to excite fluorophores in capillary tubes under up to 1.5 mm of 0.5% intralipid. Displacement of the laser beam relative to the embedded samples indicates that highly localized excitation was possible with two-photon excitation, whereas one-photon excitation resulted in loss of spatial resolution due to excitation by the diffusely scattered photons. These results indicate that two-photon excitation in the scattering solution is due only to the ballistic photons, a result confirmed by frequency-domain time-resolved measurements. Selective excitation of adjacent embedded samples was found possible for two but not one-photon excitation.

Two-photon near- and far-field fluorescence microscopy with continuous-wave excitation

We report on scanning far- and near-field two-photon microscopy of cell nuclei stained with DAPI and bisbenzimidazole Hoechst 33342 (BBI-342) with the 647-nm laser line of a cw ArKr mixed-gas laser. Two-photon-excited fluorescence images are obtained for 50-200 mW of average power at the sample. A nearly quadratic dependence of fluorescence intensity on laser power confirmed the two-photon effect. The nonlinearity was further supported by evidence of three-dimensional sectioning in a scanning far-field microscope. We find that the cw two-photon irradiation sufficient for imaging within typically 5 s does not significantly impair cell cycling of BBI-342-labeled live cells. Finally, high-resolution imaging in scanning near-field microscopy with good contrast is demonstrated.

Light quenching of pyridine2 fluorescence with time-delayed pulses

We describe the effects of time-delayed long-wavelength pulses on the intensity and anisotropy decays of pyridine2. The sample was exposed to a continuous train of 360 nm excitation pulses and time-delayed 720 nm pulses. The long-wavelength pulses, which overlapped the emission spectrum of pyridine2, resulted in a spatially localized decrease in intensity at the point of beam overlap. The time-delayed quenching pulses caused a stepwise decrease in the intensity and anisotropy decays, as seen by oscillations in the frequency-domain data. The time-resolved anisotropy was shown to decrease below zero (-0.2) following the vertically polarized quenching pulse. The extent of light quenching depended on the time delay between the excitation and quenching pulses, and can be used to measure the decay time. Light quenching and/or multipulse methods may provide a new class of experiments for fluorescence spectroscopy and imaging.