Photoluminescence-based oxygen sensors

   There is a growing need for low-cost compact chemical and biological sensor platforms for commercial, including biomedical, applications. This need has resulted in efforts to develop structurally integrated oxygen sensors as well as platforms suitable for multianalyte detection that are efficient and easily fabricated [14].

   Extensive studies of optical O2 sensors are still continuing in an effort to enhance sensor performance, reduce sensor cost and size, simplify fabrication, and develop an O2 sensor that is compatible with in vivo biomedical monitoring [14,15].

   Fast and reliable measurernent of dissolved oxygen (DO) in water is important for biological, medical, environmental, and industrial monitoring. Most commercial DO sensors are based on electrochemical techniques that suffer from shortcomings related to oxygen consumption, solution stirring, and electrode poisoning. Photoluminescence(PL)-based DO sensors, which do not suffer from such shortcomings, have also been studied [15].

   Indeed a welI-known approach for gas-phase and solution O2 sensing is based on the dynamic quenching of the photoluminescence (PL) of oxygen-sensitive dyes such as Ru-complexes (the most used is Tris(2,2'-bipyridine)ruthenium(II)) and Pt or Pd porphyrins. Collisions with increasing levels of O2 result in a decrease in the PL intensity I and PL lifetime τ. In a homogeneous matrix, the O2 concentration can be determined ideally by monitoring changes in I under steady-state conditions or in τ using the Stern-Volmer (SV) equation:

I0/I = τ0/τ = 1 + KSV [O2](1)

where I0 and τ0 are the values in the absence of oxygen, KSV is the SV constant and [O2] the concentration of oxygen [14,15].

Fig. 1. The luminescence is efficiently quenched by oxygen which results in a decrease in luminescence signal directly related to the oxygen partial pressure.

Fig. 2. System response to step variations of O2 concentration level: (a) phase response; (b) fluorescence intensity response.

   Direct measurement of either the intensity or the lifetime has problems associated. Intensity measurements demand the implementation of reference schemes in order to compensate for optical power drift due to optical source instability, coupling efficiency fluctuations and possible leaching and photo bleaching of the fluorophore. Direct lifetime measurements demand for high-speed electronics and fast pulsed optical sources. All these problems can be avoided using a phase-modulation technique Applying a sinusoidal modulation to the optical source results in a phase delay (φ) in the fluorescent emission that can be related to the lifetime by the equation:

tan (φ) = 2 π f τ(2)

where f is the modulation frequency, which can be tuned, according to the luminescence lifetime, for optimum sensor sensitivity. Using Eqs. (1) and (2) the phase delay information can easily be related with oxygen concentration [15].

   With this technique, low cost high brightness optical sources, like blue LEDs, which are suitable for excitation of many important fluorophores, can be used in association with standard photodetection [14].

   Also organic light emittitting devices (OLEDs) are used as light source for fluorophore excitation. Structurally integrated OLED-based sensors, using the PL lifetime detection mode, exhibite responses comparable to those obtained using other excitation sources and more elaborated experimental designs [15].

   Silicone is the preferred matrix for oxygen sensors because of its distinctly higher solubility and permeability for oxygen compared to other organic polymers. However silicone rubber is a poor solvent for ionic and polar substances. Hence an adsortion of [Ru(bpy)3]2+ on zeolite Y, which then may be dispersed into silicone, is considered to be a useful method for physical immobilization. Oxygen-sensitive dyes are also embedded in other porous structures such as poystyrene films or sol-gel silica films [14,15,16].