Construction and characterization of a frequency-domain fluorescence lifetime imaging microscopy system

Temperature-modulated fluorescence tomography based on both concentration and lifetime contrast

It is challenging to image fluorescence objects with high spatial resolution in a highly scattering medium. Recently reported temperature-sensitive indocyanine green-loaded pluronic nanocapsules can potentially alleviate this problem. Here we demonstrate a frequency-domain temperature-modulated fluorescence tomography system that could acquire images at high intensity-focused ultrasound resolution with use of these nanocapsules. The system is experimentally verified with a phantom study, where a 3-mm fluorescence object embedded 2 cm deep in a turbid medium is successfully recovered based on both intensity and lifetime contrast.

Photon budget analysis for fluorescence lifetime imaging microscopy

We have constructed a mathematical model to analyze the photon efficiency of frequency-domain fluorescence lifetime imaging microscopy (FLIM). The power of the light source needed for illumination in a FLIM system and the signal-to-noise ratio of the detector have led us to a photon "budget." These measures are relevant to many fluorescence microscope users and the results are not restricted to FLIM but applicable to widefield fluorescence microscopy in general. Limitations in photon numbers, however, are more of an issue with FLIM compared to other less quantitative types of imaging. By modeling a typical experimental configuration, examples are given for fluorophores whose absorption peaks span the visible spectrum from Fura-2 to Cy5. We have performed experiments to validate the assumptions and parameters used in our mathematical model. The influence of fluorophore concentration on the intensity of the fluorescence emission light and the Poisson distribution assumption of the detected fluorescence emission light have been validated. The experimental results agree well with the mathematical model. This photon budget is important in order to characterize the constraints involved in current fluorescent microscope systems that are used for lifetime as well as intensity measurements and to design and fabricate new systems.

Time-resolved luminescence imaging of hydrogen peroxide using sensor membranes in a microwell format

We demonstrate an optical imaging scheme for hydrogen peroxide in a microwell-based format using the europium(III) tetracycline complex as the fluorescent probe, which is incorporated into a polyacrylonitrile-co-polyacrylamide polymer matrix. The resulting sensor membranes are integrated into a 96-microwell plate. Hydrogen peroxide can be visualized by means of time-resolved luminescence lifetime imaging. The imaging system consists of a fast, gated charge-coupled device (CCD) camera and a pulsed array of 96 light emitting diodes (LEDs). Fluorescence lifetime images are acquired in different modes (rapid lifetime determination, RLD, and phase delay rationing, PDR) and compared with conventional intensity-based methods with respect to sensitivity and the dynamic range of the sensor. The lowest limits of detection can be achieved by the RLD method. The response time of the sensor is comparatively high, typically in the range of 10 to 20 minutes, but the response is reversible. The largest signal changes are observed at pH values between 6.5 and 7.5.

Homomultimerization of the coxsackievirus 2B protein in living cells visualized by fluorescence resonance energy transfer microscopy

The 2B protein of enteroviruses is the viral membrane-active protein that is responsible for the modifications in host cell membrane permeability that take place in enterovirus-infected cells. The 2B protein shows structural similarities to the group of lytic polypeptides, polypeptides that permeate membranes either by forming multimeric membrane-integral pores or, alternatively, by lying parallel to the lipid bilayer and disturbing the curvature and symmetry of the membrane. Our aim is to gain more insight into the molecular architecture of the 2B protein in vivo. In this study, the possible existence of multimers of the coxsackie B3 virus 2B protein in single living cells was explored by fluorescence resonance energy transfer (FRET) microscopy. FRET between fusion proteins 2B-ECFP and 2B-EYFP (enhanced cyan and yellow fluorescent variants of green fluorescent protein) was monitored by using spectral imaging microscopy (SPIM) and fluorescence lifetime imaging microscopy (FLIM). Both techniques revealed the occurrence of intermolecular FRET between 2B-ECFP and 2B-EYFP, providing evidence for the formation of protein 2B homomultimers. Putative models for the mode of action of the membrane-active 2B protein and the formation of membrane-integral pores by 2B multimers are discussed.