Anal. Chem. 2006, 78, 5645-5652
Controlling the Color Space Response of Colorimetric Luminescent Oxygen Sensors R. C. Evans and P. Douglas*
Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK
The color space response of colorimetric luminescent oxygen sensors is described in terms of the Commission Internationale de l’EÄ clairage (CIE) x,y color coordinates. We show how the color change response to oxygen can be obtained, provided the quenching kinetics for all lumophores involved can be represented mathematically. The theory is illustrated by analysis of examples of theoretical sensors in which lumophores are quenched by Stern-Volmer kinetics to give red to green, green to red, and red to green to blue color responses as the partial pressure of oxygen is increased. The effects of lumophore emission lifetime and the permeability of the polymer matrix to oxygen are discussed in terms of the control of sensor response, which variations in these parameters offer. In recent years, thin film optical oxygen sensors have emerged as attractive candidates for both gaseous and dissolved oxygen detection. In these devices, the sensor response is characterized by a change in some optical property, such as luminescence or absorbance, as a function of oxygen concentration. In most luminescence-based devices, the sensor response is determined by the quenching of photoluminescence of a lumophore immobilized in a polymer or sol-gel matrix by molecular oxygen.1-19 * Corresponding author. Tel.: +44-1792-513081. Fax: +44-1792-295747. Email:
[email protected]. (1) Bergman, I. Nature 1968, 218, 396. (2) Xu, W.; Schmidt, R.; Whaley, M.; Demas, J. N.; DeGraff, B. A.; Karikari, E. K.; Farmer, B. L. Anal. Chem. 1995, 67, 3172-3180. (3) Basu, B. J.; Thirumurugan, A.; Dinesh, A. R.; Anandan, C.; Rajam, K. S. Sens. Actuators, B 2005, 104, 15-22. (4) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal. Chem. 1991, 63, 337-342. (5) McMurray, H. N.; Douglas, P.; Busa, C.; Garley, M. S. J. Photochem. Photobiol., A 1994, 80, 283-288. (6) Klimant, I.; Wolfbeis, O. S. Anal. Chem. 1995, 67, 3160-3166. (7) Hartmann, P.; Leiner, M. J. P.; Lippitsch, M. E. Sens. Actuators, B 1995, 29, 251-257. (8) Papkovsky, D. B.; Ponomarev, G. V.; Trettnak, W.; O’Leary, P. Anal. Chem. 1995, 67, 4112-4117. (9) DiMarco, G.; Lanza, M. Sens. Actuators, B 2000, 63, 42-48. (10) Douglas, P.; Eaton, K. Sens. Actuators, B 2002, 82, 1-9. (11) Amao, Y.; Tabuchi, Y.; Yamshita, Y.; Kimura, K. Eur. Polym. J. 2002, 38, 675-681. (12) Eaton, K.; Douglas, B.; Douglas, P. Sens. Actuators, B 2004, 97, 2-12. (13) DeRosa, M. C.; Hodgson, D. J.; Enright, G. D.; Dawson, B.; Evans, C. E. B.; Crutchley, R. J. J. Am. Chem. Soc. 2004, 126, 7620-7626. (14) Ko ¨se, M. E.; Carroll, B. F.; Schanze, K. S. Langmuir 2005, 9110-9120. (15) Ko ¨se, M. E.; Carroll, B. F.; Schanze, K. S. Langmuir 2005, 21, 9121-9129. (16) Amao, Y.; Ishikawa, Y.; Okura, I. Anal. Chim. Acta 2001, 445, 177-182. (17) Huynh, K. L.; Wang, Z.; Yang, J.; Stoeva, V.; Lough, A.; Manners, I.; Winnik, M. A. Chem. Mater. 2005, 17, 4765-4773. 10.1021/ac060288w CCC: $33.50 Published on Web 07/14/2006
© 2006 American Chemical Society
Absorption-based sensors can be divided into two main groups, those using biological20 or synthetic21,22 oxygen binders and those using redox23,24 chemistry. In both groups of absorbance sensors, the indicator has a different color or intensity of color in the presence of oxygen. The relative advantages of luminescence and absorbance-based oxygen sensors with regard to a variety of specific applications have already been discussed by others.8,13-15,22,23,25-27 Luminescencebased devices provide a nondestructive means of quantitative O2 measurement and, to some extent, are tuneable with regard to oxygen sensitivity. However, because the sensor response is not visibly perceptible, spectroscopic instrumentation is required for measurements, and this both increases cost and requires the operator to have some scientific expertise. Absorption-based sensors offer the advantage of qualitative, and in some cases semiquantitative, oxygen detection by means of a simple color change response; however, absorbance-based sensors are inherently less sensitive, and fully quantitative measurements still require an external detection unit. For many commercial applications, the ideal optical oxygen sensor would combine the advantages of both luminescence- and absorbance-based devices. Incorporation of color change technology into a luminescence-based device would yield a sensor that is highly sensitive, inexpensive, and suitable for use by an untrained person, while at the same time also offering the potential for precise quantitative measurement. We recently reported a novel luminescence-based oxygen sensor incorporating this technology that is capable of both rapid qualitative/semiquantitative oxygen detection and quantitative measurements with high sensitivity, depending on the application requirements.28 The sensor is prepared by incorporating two lumophores with different oxygen sensitivities and emission colors in the same device. In the presence of oxygen, emission from one lumophore is quenched (18) Ko ¨se, M. E.; Crutchley, R. J.; DeRosa, M. C.; Ananthakrishnan, N.; Reynolds, J. R.; Schanze, K. S. Langmuir 2005, 21, 8255-8262. (19) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377-1380. (20) Zhujan, Z.; Seitz, W. R. Anal. Chem. 1986, 58, 220-222. (21) Chung, K. E.; Lan, E. H.; Davidson, M. S.; Dunn, B. S.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1995, 67, 1505-1509. (22) Choi, M. F.; Hawkins, P. Sens. Actuators, B 1996, 30, 167-171. (23) Wolthuis, R. A.; McCrae, D.; Hartl, J. C.; Saaski, E.; Mitchell, G. L.; Garcin, K.; Willard, R. IEEE Trans. Biomed. Eng. 1992, 39, 185-193. (24) Eaton, K. Sens. Actuators, B 2002, 85, 42-51. (25) Papkovsky, D. B. Sens. Actuators, B 1995, 29, 213-218. (26) Smolander, M.; Hurne, E.; Ahvenainen, R. Trends Food Sci. 1997, 8, 101105. (27) Mills, A. Chem. Soc. Rev. 2005, 34, 1003-1011. (28) Evans, R. C.; Douglas, P.; Williams, J. A. G.; Rochester, D. L. J. Fluoresc., in press.
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preferentially due to its higher sensitivity to oxygen. This results in a gradual, but dramatic, shift in the sensor emission color across the red-yellow-green spectral region with increasing oxygen concentration. The incorporation of a second sensing element into optical oxygen sensors to enable self-referencing is a relatively recent concept that has been most successfully applied to pressuresensitive paint.14,15,29 Pressure-sensitive paint (PSP) is essentially a luminescent oxygen-sensitive film that is used for the measurement of surface air-pressure distributions on aerodynamic models. Oxygen-quenching of luminescence is temperature-dependent, which may result in measurement errors in the air-pressure distribution if the temperature fluctuates during the experiment. This problem may be overcome by using dual-lumophore pressure-sensitive paint (DL-PSP), which contains one lumophore that is sensitive to both pressure and temperature variations and a second lumophore whose emission intensity varies only with temperature.14,15,29 The multilumophore oxygen sensors that we discuss in this paper differ from DL-PSP in that the use of mixed lumophores gives a luminescence colorimetric sensor that undergoes distinct color changes as a function of oxygen pressure or concentration. The response of multilumophore colorimetric sensors may be described in several ways. Since color is a very subjective phenomenon, the description of color differences can be quite challenging. The CIE (Commission Internationale de l’EÄ clairage) system of colorimetry provides a numerical description of color, known as x,y color coordinates, that is based on the sensitivity of the human eye to light across the visible spectral region.30,31 CIE x,y coordinates provide a particularly convenient system to describe the response of a luminescent colorimetric sensor when used in a qualitative or semiquantitative way. Fully quantitative measurements are also possible by kinetic or steadystate luminescence quenching measurements of one or both of the emissive components. Furthermore, measurement of both lumophores offers the additional advantage of internal referencing to minimize errors caused by fluctuations in excitation intensity. Multilumophore colorimetric oxygen sensors are attractive contenders for a wide range of commercial applications for which easy, semiquantitative oxygen detection is required, such as “intelligent inks” for modified atmosphere food packaging; oxygen levels for aquaculture, water treatment, fermentation, and other biotechnologies; medicine; and safety monitoring. In this paper, we will discuss the design of multilumophore devices in terms of sensitivity and color space response using x,y color coordinates. We will show how the x,y coordinates and quenching kinetics of each lumophore component determine the overall sensor film x,y color response. The sensor color space response for some hypothetical multilumophore devices will be discussed, and we will show how the lumophore optical characteristics and polymer host matrix can successfully be manipulated to yield a sensor with almost any required colorimetric response. (29) Khalil, G.; Costin, C.; Crafton, J.; Jones, G.; Grenoble, S.; Gouterman, M.; Callis, J.; Dalton, L. R. Sens. Actuators, B 2004, 97, 13-21. (30) Judd, D. B.; Wyszecki, G. In Color in Business, Science and Industry; Wiley: New York, 1962. (31) Hunt, R. W. G. Measuring Colour, 2nd ed.; Ellis Horwood: Chichester, 1991.
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PERCEPTION OF COLOR AND COLOR REPRESENTATION The CIE 1931 XYZ color space is defined in terms of three color-matching functions, jxλ, jyλ, and jzλ, that are closely related to the spectral response curves of the three cone cells responsible for color vision in humans. 30,31 The CIE system represents the color-matching response of a hypothetical standard observer. The jxλ, jyλ, and jzλ terms can be used as weighting functions to determine the X, Y, and Z tristimulus values that characterize the emission color of luminescence data across the visible spectrum according to30, 31
{ } 700
∑ jx ‚E ∆λ
X)
λ
λ
λ)380 700
∑ jy ‚E ∆λ
Y)
λ
λ
(1)
λ)380 700
Z)
∑ jz ‚E ∆λ λ
λ
λ)380
where E is the emission intensity at wavelength λ. The Y parameter is the luminosity of a color. The chromaticity of a color is specified by the two parameters, x and y, known as chromaticity or color coordinates, which are functions of the XYZ tristimulus values:
x)
X X+Y+Z
y)
Y X+Y+Z
(2)
The x,y color coordinates are usually represented in a twodimensional chromaticity diagram (Figure 1). This diagram represents all colors visible to the average person, (the gamut of human vision), with the most saturated colors on the line surrounding the horseshoe-shaped area of the diagram (the spectral locus). OXYGEN QUENCHING IN MULTILUMOPHORE SENSORS The color-change response in these multilumophore sensors is a consequence of a decrease in emission intensity of one or more of the lumophores due to oxygen quenching. Let us consider a thin film luminescence oxygen sensor consisting of a series of n layers, each layer containing a lumophore immobilized in a polymer matrix. The sensor is excited through one side of the film at wavelength λex and intensity Iex, and the luminescence response is detected at the other side. Each lumophore exhibits an emission spectrum that may be described in terms of its XYZ tristimulus values. Assuming a homogeneous system, then each lumophore is quenched bimolecularly by oxygen, and the emission intensity is described by the Stern-Volmer equation,
I0 ) 1 + kSVpO2 I
(3)
where I0 is the emission intensity in the absence of oxygen, I is the emission intensity in the presence of oxygen at pressure pO2,
1
Iinc i )
i-1 (∑i)1 εicili)
(6)
Iex
10
(We note here the possibility of reabsorption of emission from other lumophore layers, but leave this as a minor correction when i-1 εicili is the external excitation source is dominant, i.e., when ∑i)1 small, which is the optically dilute situation. However, we will return to this as a possible method of controlling sensor response characteristics in the discussion.) The relative emission efficiency of the lumophore in the ith layer, Fem is given by i
Fem i )
Φi
(7)
1 + kSV i pO2
where Φi is the emission quantum yield of the ith lumophore in the absence of oxygen, and kSV is the corresponding Sterni Volmer constant at pO2. Consequently, eq 4 may be rewritten as i)n
Xfilm )
∑ i)1
Figure 1. CIE chromaticity diagram mapping the x,y color coordinates of some standard lumophores (lumophore map). x,y coordinates are determined from uncorrected emission spectra according to eq 1. Abbreviations: PtOEP ) platinum octaethylporphyrin; PdOEP ) palladium octaethylporphyrin; Ru(bpy)3 ) [ruthenium(II) tris(2,2′bipyridine)]3+; Ptpyr ) chloro{N∧C∧N-1,3,5-tri-(2-pyridyl)phenyl}platinum.
and kSV is the Stern-Volmer constant, given by kSV ) τ0kq, where τ0 is the lifetime in the absence of oxygen, and kq is the bimolecular rate constant for quenching by oxygen in the polymer matrix. In the presence of oxygen at pO2, the sensor film XYZ tristimulus values are obtained from the sum of the XYZ values for each lumophore layer, that is, for X, i)n
Xfilm )
∑F X
(4)
i i
i)1
where Xi is the X tristimulus value for the ith lumophore obtained from the emission spectrum normalized to an emission quantum yield, Φ ) 1, and Fi is a factor that gives the contribution of the ith lumophore layer to the Xfilm value. Fi consists of two components: (i) the relative absorption efficiency of the lumophore, Fabs i , and (ii) the relative emission abs efficiency of the lumophore, Fem is determined from the i . Fi fraction of absorption of the ith lumophore layer according to
(
Fabs ) Iinc 1i i
)
1 10iciii
(5)
where , c, and l are, respectively, the molar extinction coefficient, the lumophore concentration, and the film thickness of the ith lumophore layer, and Iinc i is the fraction of the incident excitation energy arriving at the ith layer, given by
((
1
Iex
10
i-1 (∑i)1 εicili)
)( )( 1-
))
1
Φi
10icili
1 + kSV i pO2
Xi
(8)
and similarly for Yfilm and Zfilm. For a sensor film in which m lumophores are mixed homogeneously in the same polymer layer, rather than deposited as separate layers, the absorption efficiency terms are slightly different, and the film tristimulus values are given by (e.g., for Xfilm) m
Xfilm )
( ( )( )( ) )
∑I
ex
i)1
εici
m
∑(ε c ) i i
1
m ∑i)1 εicili
10
Φi
1 + kSV i pO2
Xi
(9)
i
Although we have assumed that lumophores are quenched following Stern-Volmer kinetics, in practice, it is often found that the situation is not so simple for lumophores dispersed in a polymer matrix, and a “nonlinear” Stern-Volmer response is common.4,5,10,12 This behavior is usually ascribed to heterogeneity in any one or more of τ0, kq, or the localized oxygen concentration at any pO2.10,12 We will not consider either the causes or effects of heterogeneity in sensor response in the analysis given here, since it may critically depend on the specific lumophore/polymer combination. However, we note that it is straightforward to obtain the equations comparable to those for Stern-Volmer quenching for more complex heterogeneous kinetics. Provided the quenching process can be represented mathematically, the appropriate equation, for example, a Freundlich isotherm,10 can easily be substituted for the Stern-Volmer term. We have also specified a linear geometry for excitation and detection, but the corresponding equations for other optical arrangements can be obtained simply by consideration of the excitation and emission paths. SENSOR DESIGN Color Space Response. Most people have a better spatial resolution for discrimination in the red-yellow-green region of the spectrum due to the greater number of yellow-green-sensitive Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
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cone cells present in the retina,30,31 and the spectral overlap of the “red”- and “green”-response cones. A sensor that generates a color change response that coincides with this spectral region is, therefore, desirable. This may be achieved simply by incorporating two lumophores with x,y coordinates in the red and green regions of the gamut, respectively, in a dual-lumophore device.28 The sensor color space response may be extended into the blue region by incorporating a third lumophore. This has the advantage of increasing the upper detection limit of the sensor, which may be a requirement for some applications. Lumophore Combination. In addition to the emission color, optical properties, such as emission quantum yields, SternVolmer quenching constants (kSV), and molar extinction coefficients, must also be considered when selecting the lumophore combination. If possible, both lumophores should exhibit high emission quantum yields in the absence of oxygen to give easy detection. Each lumophore layer should also exhibit a different oxygen-quenching efficiency, with the layer with the lowest kSV determining the limiting sensor color at high oxygen pressures. The range of color response over a given oxygen pressure range can then be controlled by the relative concentrations or molar extinction coefficients of the two lumophore components and the relative oxygen permeabilities of the polymers used. There are also several practical considerations to be made when making the final lumophore selection. Lumophores must be compatible with the host polymer matrix, and this generally restricts the lumophore selection to neutral compounds. However, this restriction can be relaxed by ion-pairing charged complexes with a hydrophobic counterion, yielding a lipophilic species that may easily be added to a hydrophobic polymer host.5 In addition, all of the chosen lumophores should ideally have overlapping excitation wavelengths in the near-UV/blue region, which would enable an inexpensive UV or blue LED to be used as a “single wavelength” excitation source, enhancing the commercial marketability of these devices. Potential Lumophores. Polyaromatic hydrocarbons (PAHs), such as pyrene, were the first lumophores to be extensively used in the development of thin film oxygen sensors.1-3 More recent studies have focused on transition metal complexes and metalloporphyrins. A variety of transition metal complexes are susceptible to oxygen quenching, with ruthenium diimine complexes being the most widely studied.4-7 More recently, some novel iridium(III)16-18 and rhenium(I)17 complexes have also been investigated as potential lumophores in thin film sensors, with varying degrees of success. With lifetimes on the order of a few microseconds, transition metal complexes have considerably longer natural lifetimes than PAHs and are, therefore, much more sensitive lumophores for oxygen detection. Of the metalloporphyrins, platinum and palladium derivatives remain the most commonly used porphyrin lumophores.8-15 They exhibit high quantum yields, are compatible with hydrophobic polymers, and have long lifetimes (80-1400 µs),10 making them more susceptible to oxygen quenching than many other transition metal complexes. In multilumophore colorimetric oxygen sensors, a combination of lumophores exhibiting both different emission colors and oxygen sensitivities are incorporated, suggesting that PAHs, transition metal complexes, and metalloporphyrins are all suitable lumophores, depending on the color space and sensitivity require5648
Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
ments of the application. Figure 1 maps the corresponding emission x,y coordinates for a variety of standard lumophores. We can consider the emission coordinates of the pure lumophores as anchor points between which the color changes will occur. Let us consider the design of a sensor film that exhibits a red-yellow-green color response with increasing oxygen concentration. The red lumophore anchor should be sensitive to oxygen quenching. Although both [ruthenium tris-(2,2′-bipyridyl)]3+ (Ru(bpy)3) and platinum octaethylporphyrin (PtOEP) are oxygen-quenched, using PtOEP as a red lumophore anchor extends the sensor color space response further than [Ru(bpy)3], because its x,y coordinates lie deep in the red region of the gamut. The limiting sensor color at high oxygen pressures is determined by the x,y coordinates of the lumophore that is least sensitive to oxygen quenching. Ideally, this component should be essentially nonquenchable, such as a short-lived fluorophore in an oxygen-permeable matrix. Examination of the lumophore map suggests that fluorescein is a good example of a green anchor for this purpose. Polymer Host Matrix. The choice of polymer matrix is a key issue to be considered, particularly in multilumophore systems. When two or more lumophores are dispersed together in a polymer host, they may interact via emission quenching, energy transfer, electron-transfer, etc. One approach used to eliminate these problems in DL-PSPs is to encapsulate one of the individual lumophore components in polymer nanospheres of polyacrylonitrate.15 The lumophore-loaded nanospheres and the second lumophore are then dispersed in the same polymer host matrix to form the sensor film. An alternative approach is to construct a multilayer device, in which each lumophore is immobilized individually in a polymer matrix and the sensor consists of a series of individual lumophore-polymer layers.28 It is the latter method that we consider here. The properties of the sensing film will depend strongly on the characteristics of the polymer matrix, in particular, the permeability of the polymer to oxygen, PO2. Permeation of oxygen through a polymer film is a two-step sequence consisting of solution of the O2 molecules in the polymer, followed by diffusion of the dissolved species, and consequently, PO2 is given by
PO2 ) DO2SO2
(10)
where DO2 and SO2 are the diffusion constant and oxygen solubility constant, respectively. The oxygen permeability of the polymer used to encapsulate each lumophore will, therefore, influence the quenching efficiency of each layer. Cellulose derivatives, such as ethyl cellulose and cellulose acetate butyrate, in conjunction with a long-lived lumophore typically yield highly sensitive films due to the high oxygen permeability of these matrixes.10,12 Incorporating the same lumophore into a glassy organic polymer, such as polystyrene or poly(vinyl chloride), generates a less sensitive device.10,25 The range of oxygen permeabilities of common filmforming polymers indicates that kq can be shifted by ∼4 orders of magnitude by variation in the polymer matrix. Using a multilayer system to isolate the lumophores introduces the possibility of fine-tuning the oxygen sensitivity of individual lumophore layers by varying the oxygen permeability of the polymer host used for that layer.
Figure 2. Stern-Volmer-type plots for (a) a dual-lumophore sensor containing a green fluorophore and a red lumophore as a function of the relative L0R/L0G ratio, and (b) a green lumophore and a red lumophore as a function of the relative L0R/L0G ratio. The color bars provide an indication of the sensor color response at any pO2 and were determined by selecting the corresponding pixels from the standard x,y chromaticity diagram for the L0R/L0G ratios considered.
(
Dual-Lumophore Sensors with a Red-Green Color Space Response. Consider a dual-lumophore sensor film, which consists of two lumophores, R and G, which emit predominantly in the red and green regions of the spectrum, respectively. The change in the sensor emission color with increasing pO2 is determined by two factors: (i) the ratio of R to G luminescence in the absence (L0R/L0G) of oxygen and (ii) the relative R, G oxygen quenching SV efficiency, kSV G /kR . Assuming that oxygen quenching of the individual R and G layers may be described in terms of SternVolmer kinetics, where L0 and L indicate the absence and presence of oxygen at pO2, respectively,
{
L0R ) 1 + kSV R pO2 LR L0G ) 1 + kSV G pO2 LG
}
(11)
then, oxygen quenching of the sensor film and, consequently, the relative contribution of R and G to the sensor response will be given by
LR L0R (1 + kSV G pO2) ) LG (1 + kSVpO ) L0G R 2
(12)
If the ratio of the Stern-Volmer constants is given by f ) SV kSV G /kR , then substitution into eq 12 gives
)
LR L0R 1 + fkSV R pO2 ) 0 SV LG L 1 + k pO G R 2
MULTILUMOPHORE SYSTEMS
(13)
which rearranges to
(
)
LR L0R f(1 + kSV R pO2) + (1 - f) ) 0 LG L 1 + kSVpO G
R
2
(14)
From this relationship, we can conclude the following: (i) if SV kSV R ) kG , no color change is observed; (ii) at zero pO2, the ratio 0 0 LR/LGdetermines the sensor film emission color; and (iii) at high pO2, f(L0R/L0G) determines the sensor color. The most rapid color change response for a dual-lumophore sensor will be observed when one layer contains a lumophore with kSV ∼ 0, for example, a fluorophore in an oxygen-impermeable polymer, while the second layer contains a lumophore that is highly sensitive to oxygen quenching. Figure 1 suggests that for a red-green sensor, a device incorporating PtOEP as the sensing lumophore (Llum) and fluorescein as the fluorophore (Lflu) should provide the desired color space response. Since the fluorophore Stern-Volmer constant, KSV flu , is essentially zero, the SternVolmer response of this type of sensor is given by
(
Lflu L0flu L0flu ) 0 + 0 kSV lumpO2 Llum L Llum lum
)
(15)
where L0flu and L0lum are the luminescence contribution from the Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
5649
fluorophore and the lumophore to the sensor film emission in the absence of oxygen, and kSV lum is the lumophore Stern-Volmer constant. Figure 2a shows the Stern-Volmer type plots obtained for this system with varying L0flu/L0lum ratios. The Lflu/Llum ratio varies linearly with increasing pO2. By increasing or decreasing the L0flu contribution, it is possible to control both the sensitivity and the color response range of these devices. Experimentally, this is achieved simply by adjusting the relative lumophore concentration of each layer or by using lumophores with different molar extinction coefficients at the excitation wavelength. The Stern-Volmer-type plots for a dual-lumophore R,G sensor in which both lumophores are quenched by O2, but with different sensitivities, is shown in Figure 2b. Adjusting the L0 contribution of either lumophore may again be used to tune the sensor color space response. However, in this situation, the LG/LR ratio does not vary linearly with pO2. The plots are characterized by a sharp increase at low pO2, which deviates with a downward curvature at higher pO2. The change in the sensor film x,y coordinates and, consequently, the sensor color change response for this two-lumophore sensor may be predicted by considering the influence of oxygen-quenching on the XYZ tristimulus values. In the presence of oxygen at pO2, the sensor film tristimulus values for a device containing lumophores R and G, are given by (e.g., for Xfilm)
Xfilm ) X0R
1 1 + X0G SV (1 + kR pO2) (1 + kSV G pO2)
(16)
X0R
and X0G are the SV and kSV R and kG
where X tristimulus values in the absence of oxygen are the corresponding Stern-Volmer constants of R and G respectively. If, at any pO2, the luminescence spectrum of the ith layer, Li, is described by Li ) Xi + Yi + Zi, where Xi, Yi, and Zi are the corresponding XYZ tristimulus values for that lumophore, the x,y coordinates of the sensor film in the presence of oxygen quenching will be given by i)n
∑X xfilm )
i)1
∑Y
∑L
i)1
(17)
i)n
∑L
i
i
i)1
i)1
If the response of the ith lumophore is referenced to a second lumophore whose luminescence spectrum at pO2 is described by L1, the film x,y color coordinates are given by i)n
x1 +
∑
i)n
xfilm )
∑S
i
i)2
∑y S
i i
i)2
yfilm )
i)n
1+
y1 +
xiSi
i)2
(18)
i)n
1+
∑S
i
i)2
where x1 and y1 are the x,y coordinates of the reference lumophore layer, xi and yi are the x,y coordinates of the ith lumophore layer, and Si ) Li/L1. 5650
∆x ) x∞film - x0film )
(
)(
SV xR + xGS0G(kSV R /kG ) SV 1 + S0G(kSV R /kG )
S0G SV S0G(kSV R /kG ))(1
+
-
)
xR + xGS0G 1 + S0G
)
SV (xR - xG)(1 - (kSV R /kG )) (19)
S0G)
i
yfilm )
i)n
For a dual-lumophore sensor consisting of two lumophores, R and G, the change in x,y coordinates, ∆xy, which describes the sensor color response from pO2 ) 0 to pO2 ) ∞ will be given by (e.g., for x)
(1 +
i)n
i
Figure 3. CIE chromaticity diagram tracking the x,y color coordinates of two dual-lumophore sensors as a function of pO2; (i) PtOEPPtpyr sensor with a red-green response; (ii) a Ptpyr-OEP sensor with a green-red response. pO2 is on a log2 scale; that is, 0, 2, 4, 8, 16, 32, 64, 128, 256, and 512 Torr. The x coordinates for the PtpyrOEP sensor response are shifted by -0.1 to allow comparison on the same diagram.
Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
where S0G ) L0G/L0R. From this relationship, we may conclude the following: If SV kSV /k R G ) 1, there is no color change, just a diminution in SV emission intensity. If kSV R /kG ) 0, that is, R is not quenched 0 (e.g., a fluorophore), and SG is small, that is, we have initially a lot of emission from this red fluorophore, then ∆x) S0G(xR - xG), and since S0G is small, then ∆x is small. Conversely, if S0G is large, SV that is, initially, there is a lot of green emission, and kSV R /kG ) 0, 0 then ∆x) (xR - xG). For SG ) 1, that is, equal emission from red SV and green emitters, and kSV R /kG ) 0, then ∆x ) (xR - xG)/2. Figure 3 shows the x,y chromaticity diagram for two hypothetical dual-lumophore sensors exhibiting a color space response in the red-green spectral region. Let us first consider the sensor that displays a red-green response with increasing pO2. We have previously illustrated this using a combination of PtOEP as the red lumophore and Ptpyr, a cyclometalated platinum(II) complex of the N∧C∧N-coordinating ligand 1,3,5-tri-(2-pyridyl)benzene, as the green lumophore, with both lumophores in ethyl cellulose.28 Using eq 16 to predict the
change in the x,y coordinates with oxygen-quenching, we expect a gradual shift in the color response through the red-orangeyellow range at low pO2. Above ∼140 Torr, the sensor response is green, and there is little change with increasing pO2. This predicted response for this sensor is analogous to the experimental response observed for the dual-lumophore sensor incorporating the same lumophore combination that we previously reported.28 Although this lumophore combination produces the required sensor spectral response, it is not ideal. With a natural lifetime of ∼9 µs, Ptpyr is moderately susceptible to oxygen quenching which leads to a reduction in the sensor luminance at high pO2. The second sensor shows a green-red color change with increasing pO2, for which a combination of the green lumophore Ptpyr and the red fluorophore, octaethylporphyrin (OEP) is considered. In comparison to PtOEP, Ptpyr is only moderately quenched by oxygen (kSV ) 0.06 and 0.007 Torr-1 in ethyl cellulose, respectively).28 Consequently, a high oxygen pressure is required to dramatically shift the sensor x,y coordinates. Up to pO2 ∼ 250 Torr, the sensor response is green; at oxygen pressures above this, the yellow-orange-red spectral response is observed. Although the CIE does not associate a specific color with any point on the chromaticity diagram, because the diagram does not take into account the dimension of brightness, the diagram is a useful tool when designing colorimetric luminescence sensors. By varying the lumophore combination and other factors, such as polymer host, it is possible to design a series of sensors with a red-green response that are suitable to detect oxygen up to atmospheric pressure. Multilumophore Sensors with a Red-Green-Blue Response. The introduction of a third lumophore component may extend the sensor color space response and also introduces the possibility of preparing sensors for a wide range of pressures. Let us consider the design of a multilumophore sensor exhibiting a red-green-blue (RGB) color space response with increasing pO2. The sensor should consist of three lumophore layers that emit individually in the red, green, and blue regions of the spectrum. The red lumophore should ideally exhibit a long natural lifetime (∼1-0.1 ms) and be significantly quenched by oxygen. To drive the sensor color space response, the green lumophore should also be moderately oxygen-quenched, although significantly less than the red component. The blue lumophore should have a very short natural lifetime (∼1 ns) and be insensitive to oxygen quenching. Examination of Figure 1 reveals PtOEP, Ptpyr, and Coumarin 110 satisfy the red, green, and blue x,y coordinate requirements, respectively. As discussed in the previous section, both PtOEP and, to a lesser extent, Ptpyr are susceptible to oxygen quenching. Coumarin 110 is a fluorophore with a lifetime in the nanosecond regime and relatively insensitive to quenching by oxygen. Therefore, a device comprising these three lumophore components at appropriate concentrations in appropriate polymer matrixes should yield a red-green-blue sensor response. At any pO2 the x or y response of a multilumophore sensor will be given by eq 17. The relative concentrations and quenching efficiencies of each layer will determine the range of the sensor color response. Figure 4 tracks the x,y coordinates of a hypothetical RGB sensor. In this example, both the green and blue lumophores are referenced to the red layer. The relative blue and red lumophore contributions are kept constant, but the green
Figure 4. CIE chromaticity diagram for a hypothetical RGB multilumophore sensor showing the sensor color change with pO2 as a function of varying R0(G) contribution. The responses of G and B are referenced to the R lumophore. R0(B) and R0(R) are kept constant. pO2 is on a log2 scale, that is, 0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, and 2048 Torr.
lumophore contribution is varied. When R0G ) 0.2, there is insufficient contribution from the green layer to produce a discernible green response. Increasing R0G has the effect of driving the color change into the green region of the gamut before curving downward into the blue. However, at high concentrations of the green lumophore (R0G > 0.9) the intense green transition dominates, preventing the extension of the sensor color response into the blue region. Figure 5 illustrates the influence of the relative oxygen quenching rate constants, kq, on the sensor response for the RGB sensor where R0G ) 0.45. The quenching rates of each lumophore layer may be easily manipulated by varying the permeability of the polymer host matrix. Since kq for the blue fluorophore is essentially negligible, the variation of the kq(R)/kq(G) ratio is considered. If the oxygen permeability of the green layer is significantly greater than the red layer, significant oxygen quenching of the green lumophore will prevent the sensor from exerting a purely green response. Conversely, if the green lumophore layer is too oxygen-impermeable, inhibited oxygen quenching will prevent the sensor response from reaching the blue spectral region. DISCUSSION The relatively simple configuration of multilumophore colorimetric sensors is an attractive feature. We have demonstrated how the sensor color space response is easily manipulated, with simple changes to the lumophore identity, concentration, or host matrix having dramatic effects. The facility to calculate the color response of a multilumophore sensor using x,y coordinates prior to experimentation is a useful design tool. If a lumophore with the exact color and quenching characteristics required to yield a Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
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component is excited by the source. An alternative approach is to design a sensor in which one of the lumophores is present at high optical density and, therefore, excited efficiently by the incident radiation (λex), and the other layers are excited by the emission from this lumophore. For example, a sensor containing a green (G) and a red lumophore (R) in which both layers are sensitive to oxygen quenching but only G is excited by λex, and R is excited by the emission from G. In the absence of oxygen, emission from both lumophores is observed, because R is excited by the emission from G, and the sensor color response is determined by the relative lumophore concentration. With increasing pO2, quenching of both layers occurs, but since the red layer is excited by the green emission, the quenching of the red emission follows a higher-order dependence on oxygen concentration, giving an upward-curving Stern-Volmer plot that may give rapid switching from red to green over a relatively small pO2 range, that is, a color “switch”. Such a device may well require careful manipulation of lumophore concentrations but does seem feasible.
Figure 5. CIE x,y chromaticity diagram for a hypothetical RGB multilumophore sensor, the sensor color change with pO2 as a function of varying R and G quenching rate constants (kq(R)/kq(G)). The responses of G and B are referenced to the R lumophore. R0(B), R0(G), and R0(R) are kept constant. pO2 is on a log2 scale, that is, 0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, and 2048 Torr.
specific color space response is not currently available, this will hopefully accelerate the preparation of new lumophores that exhibit the desired characteristics. The range of types of sensor response possible is quite wide. In addition to the simple red-green and red-green-blue 2- and 3-layer systems described above, the following sensor structures can be envisaged, all of which offer fine-tuning of the colorimetric response. (1) The introduction of an additional lumophore(s) to the red-green sensor to anchor the sensor emission color at some intermediate wavelength. For example, in the red-green sensor discussed previously, the introduction of a third layer containing the orange lumophore [Ru(bpy)3] would delay the red-yellowgreen transition until a higher pO2, yielding a less sensitive device, but one with a response profile that may be preferable for some applications. (2) Alternatively, introducing a second red lumophore that is insensitive to oxygen quenching, or a red lumophore in an oxygenimpermeable layer, to the red-green sensor will give, provided the green lumophore is moderately oxygen-quenched, a sensor with a red-green response that reverts back to red at very high pressures, thus offering a sensor with a specific color over a fixed intermediate range of oxygen pressures. (3) Devices that use the emission from one lumophore as the excitation source for adjacent lumophores. In the devices that we have considered so far, it is assumed that each lumophore
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CONCLUSIONS We have shown how the color response of luminescent colorimetric sensors can be represented using CIE x,y coordinates, how the change in color can be modeled, provided the oxygen quenching kinetics for all lumophores can be mathematically represented. We have illustrated this using lumophores that undergo Stern-Volmer quenching and show how choice of lumophore x,y coordinates and lifetime and the permeability of the polymer matrix to oxygen can be used to control the response profile. We have illustrated this in detail for two- and threelumophore sensors that give either red-to-green, green-to-red, or red-to-green-to-blue response as pO2 is increased. The response profiles of these sensors can be very finely tuned, and we suggest a number of other sensor arrangements to give a slower color response, a color response that stays at one color for an intermediate range of pO2, and a rapid color switch. The possibility to fine-tune sensor response using multiple lumophore/polymer layers to give a specific sensitivity and color space profile means that such sensors can, if required, be readily optimized for very specific applications. Such devices are potentially useful for a very wide range of technologies for which easy qualitative, easy semiquantitative, or precise quantitative measurement of O2 is required. Although here we have considered sensors made using multilayer arrangements of lumophore in thin polymer film, we are also working on the incorporation of these multilumophore sensors into polymer nanoparticles or onto polymer chains for color mapping of oxygen concentrations in in situ biomedical studies.
Received for review February 15, 2006. Accepted May 18, 2006. AC060288W
