Anal. Chem. 1999, 05, 3480-3483
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Design of Oxygen Sensors Based on Quenching of Luminescent Metal Complexes: Effect of Ligand Size on Heterogeneity LouAnn Sacksteder,?J. N. Demas,’*+and B. A. DeGrafflt Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, and Department of Chemistry, James Madison University, Harrisonburg, Virginia 22807
A detailed study of the oxygen quenching for luminescent ReL(CO)&NR+(L = a-diimineand R = alkyl) in silicone polymers shows a high degree of heterogeneity, which depends on the size of L but not of R. All quenchingis dynamic with little or no static quenching. Microheterogeneity is important in the nonlinear quenching responses. Quenching data are well described by a two-site model, although detailed lifetime measurements suggest a more complex underlying system. The appearance of two dominant sites indicates that the complex can bind in sites that are either readily or poorly quenched by oxygen. The variations of this ratio with the size of L suggests that there is a distributionof bindingsite sizes and that binding of L in these sites is necessary to protect the complex from quenching. Good quenchingoccurs when the L’s do not bind to protective sites and thus are left exposed to oxygen quenching. The use of ligand size to control response characteristics is discussed. Photochemistry is most pronounced in vacuum while oxygen or nitrogen enhances stability.
INTRODUCTION Luminescent transition metal complexes currently show great promise as sensor materials in fiber optic sensors.192 In particular, oxygen sensors based on luminescence quenching are attractive. As with other luminescent sensors, these systemsfrequently display microheterogeneity with nonlinear Stern-Volmer quenching curves and complex luminescence decay kinetics that can be characterized as sums of several exponentials or as distribution functions of exponentials.3-7 This complexity can frequently result in poorly understood sensors. An important class of luminescence sensors is based on the decrease of luminescence intensity and lifetime of the sensor
* Authors to whom correspondence should be addressed. t University of Virginia.
James Madison University. (1)Chemical, Biochemical, and Environmental Fiber Sensors; Lieberman, R. A., Wlodarczyk, M. T., Eds. R o c . SPZE Znt. Soc. Opt. Eng. 1989,1172. (2)Demas, J. N.; DeGraff, B. A. Anal. Chern. l991,63,829A. (3)Kalyansundaram, K. Photochemistry in Microheterogeneoua Systems; Academic Press: New York, 1987. (4)Simiarczuk, A.; Ware, W. R. J. Phys. Chem. 1989,93,7609. (5)Wong, A. L.; Hunnicutt, M.. L.; Harris, J. M. Anal. Chem. 1991, 63,1076. (6)Krasnansky, R.;Koike, K.; Thomas, J. K.J. Phys. Chem. 1990,94, 4521. (7)Carraway, E.R.;Demas, J. N. Langmuir 1991,7, 2991. t
0003-2700/93/0365-3480$04.00/0
material as a function of 0 2 tensionP16 In homogeneous media with only a single-component exponential decay, the intensity and lifetime forms of the Stern-Volmer equations with both static and dynamic quenching are
7d7 = 1+ &v[QI
(la)
Ksv = k27o
(lb)
where [QI is the quencher concentration, 7’s are lifetimes,I’s are intensities, KSvand k2 are the Stern-Volmer and bimolecular quenching constants, respectively, and Kq is the association constant for binding of the quencher to the luminescent species. The subscript 0 denotes the value in the absence of quencher. If plots of 7d7 or IdI versus quencher concentration are linear and match, quenching is purely dynamic (Le., Kw = 0). If Io/I is above 7o/7, static quenching is present. However, in many microheterogeneous systems, the multiexponentiality of the decay curves and the uncertainty of the fitting model preclude evaluating single-exponential 7’s for use in eq 1. Even the question of the relative contributions of static and dynamic quenching is difficult to address. Further, the Io/I plota are downward curved, which makes more accurate calibration difficult. We have reported on the preparation and properties of a homologous series of ReL(CO)aCNR+, where L = 2,2bipyridine or 1,lO-phenanthrolineor its substituted analogues and R is tert-butyl or CH3(CH2),,.l6 These complexes were exceptionally luminescent (quantum yields to 0.7) and were promising as sensors due to very high degrees of quenching. In preliminary experiments we examined their performance in GE RTV 118 silicone polymer. Up to this point, this support had yielded the best quenching responses of any support examined. The Stern-Volmer plots were all curved, and we wanted to understand the fundamental interactions responsible for the curvature. Since we had a wide range of homologous molecules that differed mainly in the size and hydrophobicity of the coordinating ligands, this seemed like an ideal model for testing changes in complex structure and response. (8)Peterson, J. I.; Fitzgerald, R. V.; Buckhold, D. K. Anal. Chem. 1984,56, 62. (9)Wolfbeis, 0.S.;Posch, H. E.; Kroneis, H. W. Anal. Chem. 1985, 57,2556. (10)Lee, E. D.; Werner, T. C.; Seitz, W. R.Anal. Chem. 1987,59,279. (11) Wolfbeis, 0. S.;Weis, L. J.; Leiner, M. J. P.; Ziegler, W. E. Anal. Chem. 1988,60,2028. (12)Wolfbeis, 0.S. Chem. Anal. 1988, 77, 129. (13)Li, X.M.; Ruan, F. C.; Wong, Y. Analyst 1993,118,289. (14)Bacon, J. R.;Demas, J. N. Anal. Chem. 1987,59,2780. (15)Carraway, E.R.;Demas, J. N.; B. A. DeGraff, Anal. Chern. 1991, 63.. 337. (16)Sacksteder,L. A.;Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1993,115,8230.
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@ 1993 I American
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
Earlier we demonstrated that the nonlinear responses of
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RULa2+ oxygen sensors could best be described using a two-
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site model with each site being quenched with a different quenching constant.15 As with these earlier systems, quenching in the Re(1) systems is purely dynamic with the downward curvature resulting from siteheterogeneity. Further, the very simple tweetate model givesexcellent fitsto our experimental intensity quenching results and provides insight into the nature of quenchable sites.
EXPERIMENTAL SECTION Chemicals. The complexes were prepared as described elsewhere.le The abbreviations for the ligands are bpy = 2,2'bipyridine,phen = 1,lO-phenanthroline,Phghen = 4,7-diphenyl1,lO-phenanthroline,5-Mephen = 5-methyl- l,l0-phenanthroline, 5-CUphen) = 5-chloro-l,lO-phenanthroline, 4,7-Mezphen = 4,7dimethyl-1,lO-phenanthroline, and Melphen = 3,4,7,&dimethyl1,lO-phenanthroline. Since all complexes have the same ReL(CO)&NR+structure and differ only in L and R, we will denote the complexes by indicating the L/R combination only. The one-part RTV 118 was from General Electric Corp. and the sealant was a general purpose sealant silicone, clear (Dow, 1677535-0481). Samples were prepared as described earlierl4.15 usFg =l mM solutions of the complex in CHzC12. Film thicknesses were typically 0.005-0.015 in. The recommended slow evaporation procedure was used to improve film quality.15 Absorption spectra for the Re(1) species in RTV-118 showed no new bands or significant spectral shifts. This, coupled with the similarity of the emission spectra in dilute solutions and in the films, indicates that aggregation is not significant. Experimentswere performed varying the concentrationof the impregnating solution from about 0.1 to 1 mM in CHZC12. No variation in the luminescence properties or quenching curves was observed. Photophysical Measurements. Static dc luminescence measurements were made on a SPEX Fluorolog system.1c16 Luminescence lifetimemeasurementswere made usinganitrogen laser based system.17 Solution spectra were measured at =lo0 M. All measurements were made at room temperature (22 f 2 O C ) . Air measurements used room air. Previous measurements have shown that the systems are not affected by variations in humidity, including putting the sample in water." For the pressure dependence studies, the gas was pure oxygen and the pressure was varied with a pump. Data Treatment. For ascertaining the relative contributions of static and dynamic quenching, we used the method described earlier.l8 The preexponential weighted mean lifetime, TM, 7M I
CaiTi/Cai
(2)
was computed. The necessary a's and 7's were extracted from the decays by nonlinear least squares using either a simplex or a Marquardt nonlinear least squares algorithm to a sum of exponentials.lQThe computed best fit a's and 7's were used to calculate TM from eq 2. Even if no physical significance can be ascribed to the a's and 7'8, the following is true if there is no static quenching: (3) Thus, if we use IM'S rather than single-exponential lifetimes in our Stern-Volmer equation,we can compare intensityand lifetime data in microheterogeneous systems to assess the presence of static quenching. We have demonstratedearlier that this method is insensitive to the details of the act.ual decay model.
RESULTS The emission spectra are all in the green-yellow region and show negligible solvent shifts. Typical solution spectra are shown elsewhere.l6 (17) Sacksteder, L.;Demas, J. N.; DeGraff, B. A. Znorg. Chem. 1989,
a,1787.
(18) Carraway, E.R.;D e w , J. N. A w l . Chem. 1991,63,332. (19) Demas, J. N.; Demas, S. E. Scientific Computing andznterfacing on Personul Computers: Allyn & Bacon: New York, 1990.
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Oxygen Pressure (Torr) Figure 1. Intensity Stern-Volmer quenching plot for bpy/t-Bu (A), phenlt-Bu (B), and Me,phen/t-Bu (C) In RW-118. The solid lines are the best flts for the two-slte model.
Figure 1shows the intensity quenching plots for the bpy/ t-Bu, phen/t-Bu, and Merphen/t-Bu complexes. The solid lines are the best fits for the two-component model described later. The estimated uncertainties on the IolI's are 1-2% based on the precision of the individual intensity measurements. These uncertainties also match the residuals in our modeling. Figure 2 shows 0 2 lifetime (TOM/TM) quenching data for the bpy/t- Bu complex. The solid line is the best fit for the
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993 2.20
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Table I. Oxygen QuenchingFitting Parameten for ReL(CO)&NR+Complexes in RTV-118 Polymer Films comples L/R f01 Ksvi foz Ksvi
1.90 -
2.00
1.70
-
1.60
-
1.80
bpy/t-Bu phen/tBu 4-Mephenlt-Bu 5-Mephenlt-Bu 5-Clphenlt-Bu Mezphenlt-Bu Mezphenln = 3 Mezphenln = 11 Merphenlt-Bu Merphenln = 3 Merphenln = 7 Merphenln = 11 PhzphenIt-Bu
1.40 1.30 1.50
1.20
@
1.10 1.00
4 0
I
I
150
'
I
360
460
600
750
Oxygen Pressure (Torr)
0.21 0.50
0.78 0.82 0.83 0.90 0.90 0.92 0.95 0.95 0.95 0.95 0.85
0.0083 0.0711 0.0750 0.0879 0.0064
0.1204 0.0909 0.0708 0.1568 0.1519 0.2043 0.1421 0.1823
0.79 0.50 0.22 0.18 0.17 0.10 0.10 0.08 0.05 0.05 0.05 0.05 0.15
0.0010 0.0030 0.0035 0.0039 0.0026 0.0034 0.0031 0.0018 0.0032 0.0033 O.OO60
0.0037 0.0108
RuLs2+systems, we find that the minimal model necessary to fit the experimental data assumes two independent sites with different sensitivities being quenched by oxygen. The equation is
Fburr 2. Intensity and llfetlme ( r M plots ) for the bpy/t-Bu complex In RTV 118.The symbols are the T M data while the solid line is the best
fR curve taken from the Intensity data of Figure 1A. The experimental points from Flgure 1A are omltted for clarity.
intensity data from Figure 1A. Similar results were obtained for the other complexes. Typically, for a completely satisfactory lifetime decay fit, it was necessary to fit the data to a triple-exponential decay. A two-component fit gave reasonable fits to the decay data, and this suggests that two components dominated the fit. However, given the complexity of decay time fitting to microheterogeneoussystems, any attempt to draw substantive conclusions from the decay parameters would be unwise.20 The complexes all show a significant rate of photodecomposition. It was necessary to use the minimum excitation slit widths and close the shutters between the measurements to minimize decomposition. Success was judged by a lack of hysteresis in the Stern-Volmer plots with both increasing and decreasing oxygen pressure. Surprisingly, decomposition was most rapid if the cell was evacuated. Introduction of either oxygen or nitrogen slowed the reaction significantly (a factor of -4) versus a vacuum.
DISCUSSION The broad structureless emissions as well as their energies show that all emissions arise from ligand-to-metal chargetransfer (MLCT) excited states where the emitting state is derived from a configuration involving promoting a metal d electron to a ligand P* antibonding orbital. The long microsecond lifetimesarise from the emitting states containing a high degree of triplet character; however, to denote them as triplets and the emissions as phosphorescences is probably a misnomer. Due to the high atomic number of Re, the emitting states are best described as spin-orbit states rather than as either singlets or triplets.21 The excellent agreement between Idland TMO/TMof Figure 2 shows that quenching in these systems is essentially purely dynamic. This is the same as we found earlier for RuLa2+ systems.18 The downward curvature of the Stern-Volmer plots necessitates a more complex model than a single species quenched bimolecularly (Eq 1). Microheterogeneity is required to explain these results. As with our earlier work with (20) Demae, J. N.; DeGraff, B. A. Sens. Actuators E 1993, 11, 35. (21) Mandel, K.; Peareon, T. D. L.; Krug, W. P.; Demae, J. N. J. Am. Chem. SOC.1983, 105,701.
where the fG's are the fraction of the total emission from each component under unquenched conditions and the K&s are the associated Stern-Volmer quenching constants for each component. Table I summarizes the best fit parameters. The fits are all excellent,and in all subsequent model discussion,we adopt this two-site model. In view of the ability of data described by complex decay kinetics to be fit by relatively simple decay schemes, it is entirely possible for this two-component model to fit the data but not be physically correct. However, as we have shown, such curved Stern-Volmer plota cannot be fit by a single Gaussian distribution of lifetimes. Two sites or two distributions are the minimal requirement.20 Photochemistry. The decrease of photochemical decomposition rate with gas pressure of nitrogen or the quenching oxygen was a novel result. We suggest that the photochemical decomposition is ejection of a CO, which is a common photochemical pathway for carbonylcomplexes. In a vacuum, the CO readily escapes. In the presence of any buffer gas, a cage effectoperates and someof the CO is trapped long enough to back react with the highly reactive coordinatively unsaturated site left on the metal. Comparison of Complexes and Supports. For the Merphen/t-Bu complex, the Dow polymer was clearly inferior to the RTV. For comparable thicknesses the Dow polymer has a response time that is at least a factor of 10 slower than the RTV, and the sensitivity is at least a factor of 3 lower. The lower sensitivity of the Dow polymer is, thus, probably due in part to its lower oxygen diffusion coefficient. The RTV 118 is not a pure polymer, but is filled with a hydrophobic silica. As shown by Klimant and Leiner,22 the early oxygen quenching with Ru(I1) complexes on RTV 118 was largely due to the complexes being adsorbed on the silica rather than homogeneously dispersed in the hydrophobic siloxane polymer. However, we note that there are very large differencesbetween Ru(I1)complexesin the RTV 118relative to their behavior when adsorbed on pure polar hydrophilic silica.' The polar silica shows far greater heterogeneity,much more curved Stern-Volmer plots, and more complex decay curvesthan does the RTV 118. For the current interpretation, we assume that the complexes are also predominantly adsorbed on the silica. (22) Klimant, I.; Leiner, M. J.P.Abstracts, let European Conference on Optical ChemicalSensors and Biosensors,Graz,Austria, April 12-15, 1992; p131.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
A
/
/
B
Figuro 5. Schematlc model for sire dependence of the complexes for quenching data. Both (A) and (B) show the same assortment of blndlngpockets. (A) shows representetivebindkrgof Re@henXCO~cNR+ to the pockets, and (B) shows binding of Re(Me2phenXCOkCNR+to the same pockets.
We turn now to the interpretation of the two-site model. Except for the Phzphen complex, there is a monotonic increase in the fraction of the easily quenched form (fox) versus the poorly quenched species as the size of the L increases (Table I). On the other hand, the size of the isonitrile seems to be unimportant. A plausible explanation can be rationalized using the following model: Quenching is through collision of oxygen with the a-diimine portion of the complex. There will be a distribution of sizes in the binding sites in the support with two possible binding modes. The complex can bind with the a-diimine inserted into the site or with the a-diimine projecting into the void space. Those metal complexes that are bound such that the a-diimineligand is physically shielded from oxygen will exhibit lower degrees of oxygen quenching
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than those where the a-diimine is directly exposed to oxygen. This model is shown schematicallyin Figure 3. We show the same collection of different-sized sites. In one case we show a small phen complex binding to this distribution of sites, and in the other we show a larger Mezphen complex. We make no claims about the intimate details of the sites. However, it is clear from Table I that the sites have different size requirements as shown by the pronounced size variation in the ratio of easily quenched to difficultly quenched components ( f d f o z ) . The larger the binding pocket, the larger the a-diimine ligand that can fit into it. For a larger L, there will be fewer binding sites that can accommodate it, and thus, a larger fraction of molecules will be easily quenched. In contrast, for smaller L's, a larger percentage can find a binding site that will accept the L and shield it from oxygen quenching. Thus, the smaller the L, the more complexes that can find a shielded site and the smaller the fraction of easily quenched molecules. This is shown in Figure 3 by comparing the orientations of the different-sized complexes. While the Phzphen/t-Bu complex does not follow the trend exactly, it has large substituents and a very high fol. We have no obvious interpretation of this small anomaly. The absence of any dependence of shielding with variation in R (isonitrile) is a consequence of two factors. Excitation is not on this portion of the complex, and shielding of R will not affect quenching. Also, long alkyl groups will have complete flexibility, can adapt to any conformation of the supporting environment and, thus, will not affect the orientation of the critical luminescent portion of the complex in the binding sites. The ability to systematically alter the fraction of easily versus difficultly quenched sites provides the toolsto engineer sensors with specific properties. The closer the fraction is to 0 or to 1 the more nearly linear the Stern-Volmer response, which simplifies calibration. Iff is near 0, the sensor is much less sensitive than if f is nearly unity. Thus, merely by controlling size one can alter the sensitivity. On the other hand, systems with nearly linear Stern-Volmer responses have the disadvantage of having a relatively limited dynamic oxygen concentration range; too high a degree of quenching results in small signals and unacceptable baseline corrections. However, a sensor with an f o l of 0.5 can respond well over a wider range. At low concentration the more sensitive sites provide good response; at higher pressures, where the sensitive site is essentially totally quenched, the less sensitive site will continue to be quenched appreciably and provide usable changes. Thus, by suitable ligand modifications, one can optimize for high pressure, low pressure, or wide range response.
ACKNOWLEDGMENT We gratefully acknowledge the support of the National Science Foundation (CHE 88-17809 and 91-18304).
RECEIVED for review July 22, 1993. Accepted August 24, 1993.' @
Abstract published in Advance ACS Abstracts, October 1,1993.
