Report
Oxygen Sensors Based on
Luminescence
Quenching Optical oxygen sensors are beingusedin applications ranging from blood gas analysis to pressure measurements in wind tunnels.
T
he design and development of molecular reporters or sensors present many challenges for chemists. The active element of these devices can range from a robust inorganic thin film to a delicately anchored enzyme (1). Often, the transduction-step response is based on a change in voltage or optical properties. The advent of low-cost, high-quality fiber-optic materials and sources has given considerable impetus to optical methods. Of the many strategies for optical detection, some of the most powerful are based on luminescence, which can provide exquisite sensitivity and selectivity (2). Luminescent
James N. Demas University of Virginia
B. A. DeGraff
sensing can be based on color, brightness, or lifetime changes, allowing a variety of sensing schemes. When coupled with fiberoptic technology, it is possible to build small, highly sensitive, low-cost probes. A useful type of luminescent probe is based on the quenching of a reporter molecule, M, by the analyte, Q. These luminescence-quenching systems are based on the following processes M + hv -> *M Photon absorption (7a)
(1)
*M -> M + hv Luminescence (kr)
(2)
*M -> M + A Nonradiative decay (knr)
(3)
James Madison University
Patricia B. Coleman Ford Motor Company
*M + Q -> M + Q Dynamic quenching (k )
(4)
The presence of a quencher in the system results in more rapid depletion of the excited-state population, which is detected as either a concomitant decrease in steadystate luminescence intensity or as a shorter emission decay time. Changes in either intensity or decay time can be used to quantitate the amount of analyte present. We will use the term "quenchometric" for such luminescence quenching-based systems The term is analogous to the wellaccepted term "fluorometric" The previous scheme gives rise to the Stern-Volmer (SV) expressions, which quantitatively relate lifetime and luminescence intensity to quencher concentration. I0/= i 1 + ^SV[Q]
(5)
T 0 /X = i + / y Q ]
(6)
K^ = k
XQ
= k (kr + kmY
(7)
Analytical Chemistry News & Features, December 1, 1999 793 A
Report Sensor molecules
The design of luminescent metal complexes is reasonably well-understood, and useful guidelines have been published (14). For oxygen sensors, Ru(II) cc-diimine complexes (15) and Pt(II)/Pd(II) porphyrin systems (8) have received the most study. Figure 1 shows the structures of some of the inorganic and organic molecules used as oxygen reporters. For solidphase, ambient temperature measurements, the platinum and palladium porphyrins (16) (with llfetimes of hundreds of microseconds) or Ru(II) and OsdD cx-diimine systems (with lifetimes of hundreds of nanoseconds to tens of microseconds) are the most attractive (Other systems are also under studv \ 17-19]) These complexes have lonp1 T 'S can be excited
in the i 'Lle
region and have which excitation source interference Manv are mienched at near divisional rates hv oxvp-pn and some of th c mple es have nnantum a-
Figure 1 . Representative molecules used as oxygen sensors.
in which Kav is the SV quenching constant; x and x0 are the quenched and unquenched excited state lifetimes, respectively; and / and I0 are the measured quenched and unquenched emission intensities, respectively. An ideal sensor based on SV kinettcs requires minimal calibration because the response to the quencher concentration is linear. Although static quenching, inhomogeneous systems, and short lifetimes can pose significant barriers, numerous senbased on this scheme (3) Among the quenchometric sensors, those used for oxygen analysis have attracted the most attention. In this case, quenching occurs by energy transfer to form singlet oxygen (4, 5). Oxygen analysis is important in biological oxygen demand (6), blood gas analysis (7), pressure-sensitive paints (8), in vivo analysis (9), and combustion (10). Optical oxygen sensors are more attractive than typical amperometric devices because they offer 794 A
faster responses, do not consume analyte, and lack electrical connections (11). Luminescence sensors respond to the partial pressure of oxygen in analyte media rather than to absolute concentrations. Suitable calibration curves are generated from measurements on the sensorfilmat different partial pressures of oxygen. Because Kav is proportional to x0, materials with long x0's will, in general, be more easily quenched by oxygen. Thus, organic fluorophores with short x0 values of —1-10 ns are seldom used as quenchometric sensors because of their poor response. Although longer-lived phosphorescent organics have been used (12,13), the most successful oxygen probes are based on luminescent transition metal complexes. This Report discusses the status of oxygen sensor molecules and their supports some key monitoring methods and their pitfalls data analysis and representative applications
Analytical Chemistry News & Features, December 1, 1999
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it. i
•
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efficiencies that rival the better organic lummophores (20). The metal complexes have different types of excited states. Visible excitation with the Ru(II) complexes promotes a metal-centered d electron to a ligand 7C* orbital. These transitions result in charge separation and are termed metal-to-ligand charge-transfer bands (14). Figure 2 demonstrates luminescence quenching of [Ru(bpy)3]2+ in methanol (21). The visible absorptions of metal porphyrins are primarily n -> s* in character and largely porphyrin-centered (19). The heavy metal ion facilitates intersystem crossing to the triplet state, which has a significantly longer x0 and thus enhances oxygen quenching. In addition, the atomic number for platinum and palladium enhances spinorbit coupling so that the triplet state x0 is not so long as to be totally quenched by local environmental interactions or trace quenchers. Sensor supports
Figure 3 shows typical fiber-sensor configurations. In Figure 3a thefiberis bifurcated with excitation along onefiberand emission monitoring through a second fiber. With a dichroic mirror, the same fiber can be used for excitation and emission
(Figure 3b). In the evanescent wave (Figure 3c), some of the excitation light in the fiber leaks into the reporter in the sample cladding and excites it. In this case, the sensor can be embedded, adsorbed, or covalently attached to the surface. Unfortunately, a problem remains. Sensor molecules must generally be held in a support material and the change from homogenous solution to solid matrix can cause various subtle and intriguing problems. In many regards, this is one of the most difficult problems. The support matrix must securely anchor the sensor material, provide reasonable and rapid analyte access to the sensor, be benign to the environment and sensor, be environmentally stable unwanted interferences and function as a barrier to the solvent during solution measurements Additionally heterogeneity of the sensor-sunnort system; physical parameters such as tackiness clarity and porositv and even photochem-
istry can thwart practical implementation. However, there are various strategies for dealing with most of these issues. Figures 4a and 4b show typical SV-quenching plots for three different Ru(II) complexes on a common solid support. The nonlinearity, which depends strongly on the complex, is typical of polymer-supported sensors. The emission intensity for [Ru(Ph2phen)3]2+ decreases 8-fold at 1 atm of pure air and 25-fold at 1 atm of pure oxygen. These large changes allow easy measurement of oxygen concentrations over the important
Figure 2. Luminescence oxygen quenching. Three identical [Ru(bpy)3]2+ samples are excited by a black light in front: (a) purged of oxygen, (b) air-saturated, and (c) saturated with 1 atm oxygen. The emission color of the left solution is reasonably accurate, but the limitations of color film distort the colors on the right.
ratige 0-1 atm
This sensitivity is qualitatively demonstrated in Figure 4c, in which the sensor film responds to a person's breath. The first exhalation after holding a breath gives the largest change because the air in the lungs has had the greatest time to exchange with the blood and is most oxygen-depleted. The re-equilibration of the lungs and the irregularity of the first few breaths caused by the hirfier C0 concentration are readily di
