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Multiphoton laser scanning microscope

Description

Multiphoton or 2-photon microscopy (MPM / 2PM) combines confocal resolution with in depth imaging in turbid media. Typically, it is used to image multicellular systems like the roots or the leaves of plants, or strongly scattering media. As a special feature, it is equipped with fluorescence lifetime imaging (FLIM) detection.

Technical Details

When femtosecond pulsed near infrared (NIR) light is focused to a diffraction limited spot of ~200 nm diameter, there is a probability that the energy of the photons is combined to excite a molecule. For example, two NIR photons of 800 nm can be simultaneously absorbed for a transitions that would normally require a single 400 nm photon.
The MPM has a Ti:sapphire laser (tunable from 700-1000 nm) for excitation. A confocal scanhead (Bio-Rad Radiance) scans the beam through the sample, like a confocal laser scanning microscope (CLSM). The imaging depth can be changed by adjusting the height of the objective with respect to the sample.
For detection, the layout is different from CLSM. As multiphoton excitation is only possible in the focal spot of the objective, there is no need to place a pinhole before the detector (and descan the optical beam). Therefore, a dichroic mirror is placed just below the objective to reflect the emission directly to one of the detectors. The optical configuration and the size of the detectors is such that the loss of photons due to scattering (i.e. by changes in refractive index) is minimized.
Multiphoton microscopy is combined with fluorescence lifetime imaging (FLIM). FLIM monitors the distribution of the fluorescence lifetimes of a fluorophore at the different locations within the sample. The fluorescence of a sample is monitored as a function of time after excitation by a flash of light. We use the Time Correlated Single Photon Counting (TCSPC) method to record the fluorescence time trace for every pixel in the image. The lifetime may be sensitive to environmental factors like Ca2+ concentration, pH and polarity but is independent of dye concentration or light path length. The lifetime can be significantly reduced if excited state processes like FRET occur. Moreover, it can be used to measure the ultrafast dynamics of photosynthesis in leaves, algae or bacteria.

Applications

MPM is ideally suited for imaging turbid / opaque samples, including tissues (plant roots and leaves) or food samples. The absence of out-of-focus excitation improves imaging depth and minimizes photobleaching, which makes it well suited for 3D imaging. The nondescanned layout (i.e. the absence of a pinhole) reduces the loss of fluorescence due to scattering in the sample.
The combination with FLIM allows for Förster Resonance Energy Transfer (FRET) imaging, as well as pH / viscosity / polarity imaging.
The main disadvantage is that MPM is less user-friendly than regular CLSM. Note that high energy pulsed laser light (CLASS 4) is used that is almost invisible but can cause severe damage to the human eye. Support of an operator is therefore required; contact Arjen Bader or Jan Willem Borst for more information.

Contact Arjen Bader

Brand

Nikon / Bio-Rad

Type

Radiance 2100MP

Organisation

Agrotechnology and Food Sciences

Department

MICROSPECTR. CENTR. (MSC)

Last edited by Oscar de Vos on 2017-11-24