No abstract

We report here on a low-cost, optical oxygen sensor as an attractive alternative to the widely used amperometric Clark-type oxygen electrode for measuring dissolved oxygen tensions in cell cultures and bioreactor. Our sensor is based on the defferential quenching of the fluorescence lifetime of chromophore in response to the partial pressure of oxygen. This is measured as a phase shift in fluorescence emission from the chromophore due to oxygen quenching when excited by an intensity modulated beam of light. In this article we demonstrate the advantages of lifetime-based optical methods over both intensity based optical methods and amperometric electrodes. Our sensor is particularly suitable for measuring dissolved oxygen in bioreactors. It is autoclavable, is free of maintenance requirements, and solvents the problems of long-term stability, calibration drifts, and reliable measurement of low oxygen tensions in dense microbial cultures that limit the utility of Clark-type elcectordes. (c) 1994 John Wiley & Sons, Inc.

We have characterized the output of inexpensive, commercially available single quantum well (SQW) blue and green light emitting diodes (LEDs). The SQW LEDs were amplitude modulated with the output from a frequency generator while biased through a bias tee with 5 mA of current. The blue SQW LED produced 800 μW of light centered at 466 nm, with a −3 dB bandwidth of 58 MHz. The green SQW LED produced 543 μW of light centered at 522 nm, with a −3 dB bandwidth of 26 MHz. Modulated light was available to approximately 100 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetime of a standard fluorophore (fluorescein) was measured in the frequency domain using the phase-modulation technique, and gave results similar to those obtained with a 488 nm argon ion laser modulated with a Pockels cell. To demonstrate the usefulness of the SQW LED source, we also performed measurements with the fluorescent pH indicator SNAFL-2. Again, these results compared favorably with those obtained with the laser. When compared to a laser modulated with a Pockels cell, the SQW LEDs were smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the SQW LEDs at high frequencies, along with the blue or green outputs, allow their use as inexpensive light sources in fluorescence lifetime optical sensors and even fluorometers.

We present a side-by-side comparison of the nonlinear behavior of four passive AlGaAs ridge waveguides where the bandgap energy of the core layers ranges from 1.60 to 1.79 eV. By engineering the bandgap to suppress two-photon absorption, minimizing the linear loss, and minimizing the mode area, we achieve efficient wavelength conversion in the C-band via partially degenerate four-wave mixing with a continuous-wave pump. The observed conversion efficiency [Idler(OUT)/Signal(IN)=-6.8  dB] is among the highest reported in passive semiconductor or glass waveguides.

Selective excitation of tryptophan fluorescence decay in proteins using a subnanosecond 295 nm light-emitting diode and time-correlated single-photon counting Appl. Phys. Lett. 86, 261911 (2005); 10.1063/1.1984088 Ultraviolet light-emitting diodes operating in the 340 nm wavelength range and application to time-resolved fluorescence spectroscopyWe have produced amplitude-modulated near-ultraviolet light, centered at 390 nm, using an inexpensive, commercially available blue light-emitting diode ͑LED͒. The LED was amplitude modulated with the ϩ13 dBm ac output from a frequency generator while biased through a bias tee with 60 mA of dc current. The LED produced 45 to 54 W of UV light over the modulation bandwidth of 0.01 to 200 MHz, when measured after optical filters to remove the residual blue output. Since the filter attenuated the UV output about 3 dB, more than 100 W of UV light was initially produced. Modulated UV light was available to approximately 200 MHz, with a Ϫ3 dB point of 31 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetimes of standard fluorophores ͑9-cyanoanthracene and green fluorescent protein͒ were measured in the frequency domain using the phase-modulation technique, producing lifetimes that closely agree with those reported in the literature, confirming that the UV-emitting blue LED is practical for spectroscopic and sensor applications. When compared to a laser modulated with a Pockels cell, the LED was smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the LED at high frequencies, along with the UV output, allows its use as an inexpensive UV light source in fluorescence lifetime optical sensors and even frequency-domain fluorometers.