New approaches to lifetime-resolved luminescence imaging

Total internal reflection fluorescence imaging using an upconverting cover slip for multicolour evanescent excitation

Total internal reflection fluorescence microscopy is well known as a means of studying surface-bound structures in cell biology. It is usually measured either by coupling a light source to the sample using a prism or with a special objective where light passing through the periphery of the lens illuminates the contact region beyond the critical angle. In this study we present a new and simple approach to total internal reflection fluorescence microscopy where the sample is mounted on a cover slip prepared from a high-index upconverting glass-ceramic. Excitation of the cover slip with a low-cost near-infrared laser diode generates intense narrow-band visible emission within the cover slip, some of which is totally internally reflected. This emission gives rise to an evanescent wave at the interface and hence can excite surface-bound fluorescent species. Depending on the excitation conditions the cover slip can generate violet, green and red emission and hence can excite a wide range of fluorescent labels. Fluorescence emission from the sample can be detected in spectral regions where the direct emission from the cover slip is very weak. The advantages and limitations of the technique are discussed in comparison with conventional total internal reflection fluorescence microscopy measurements and prospects for novel total internal reflection fluorescence microscopy geometries are considered.

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.

Direct modulation of the effective sensitivity of a CCD detector: a new approach to time-resolved fluorescence imaging

In this paper a novel approach to frequency-domain fluorescence lifetime imaging (FLIM) is described. In a CCD camera a single pixel is defined by a charge pattern on a group of electrodes. By modulation of the pattern of voltages defining the pixel structure it is possible to modulate the sensitivity of the CCD at radio frequency. The modulation enhances the noise performance of the CCD, in contrast to the deterioration in performance seen when an intensifier stage is similarly modulated. The new technology has potential applications to a wide range of assays as well as in conventional FLIM applications. Unlike intensifier-based systems, the directly modulated CCD is physically small, inexpensive, robust and offers superior resolution and noise performance.

Fluorescent imaging of pH with optical sensors using time domain dual lifetime referencing

We present a referenced scheme for fluorescence intensity measurements that is useful for imaging applications. It is based on the conversion of the fluorescence intensity information into a time-dependent parameter. A phosphorescent dye is added in the form of approximately 10-microm particles to the sample containing the pH-sensitive fluorescent indicator. Both the reference dye and the pH probe are excited simultaneously by a blue LED, and an overall luminescence is measured. In the time-resolved imaging method presented here, two images taken at different time gates were recorded using a CCD camera. The first image is recorded during excitation and reflects the luminescence signal of both the fluorophore (pH) and the phosphor (reference). The second image, which is measured after a certain delay (after switching off the light source), is solely caused by the long-lived phosphorescent dye. Because the intensity of the fluorophore contains the information on pH, whereas phosphorescence is pH-independent, the ratio of the images displays a referenced intensity distribution that reflects the pH at each picture element (pixel). The scheme is useful for LED light sources and CCD cameras that can be gated with square pulses in the microsecond range. The fundamentals and potential of this new method, to which we refer as time domain dual lifetime referencing (t-DLR), are demonstrated.