Anal. Chem. 1905, 85, 255-258
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Luminescent Dicyanoplatinum(I I) Complexes as Sensors for the Optical Measurement of Oxygen Concentrations Weekey Wai-San Lee, Kwok-Yin Wong,' and Xiang-Ming Li Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic, Hunghom, Kowloon, Hong Kong
The emlsdon spectra of [Pt(L)(CN)*] (L = 4,7-dlphenyi-l,IO-phenanthrdlne or 4,4'-dCterl-bulyC2,2'-blpyrldlne) ImmoMlized in dllcone rubber are concentratlon dependent. At low concentratlons of platlnum complex a dngle emklon peak b observed. Oxygen quenchingof the e m h k n Isshown to be a sendtivemethodfor measurlngoxygen concentratlons In the gas phase. The polymer flkns contalnlng the platlnum complex are very stable, and no photochemlcal degradatlon was observed when they were subjected to step changes In the oxygen concentratlon. The fllms are not sendtlve to alkyl halldes such as chloroform and dlchloromethane. Sulfur dloxide, on the other hand, Is a fairly severe interferent. At hlgh concentratlons of the complex a second eml+rlon peak assignable to exclmer emledon Is o k r v e d . The exclmer emlsrlon lntenstty strongly depends on the structure of the polymer. Thb flndlng Indkatesthat [Pt(L)(CN)*] is a potentlal spectmscopk probefor the mlcroenvlronmentIn polymer fllms.
INTRODUCTION Considerable effort has been devoted to the development of luminescence-based optical sensors that respond to oxygen on an equilibrium basis.1-8 Unlike the Clark electrode? the optical sensor does not require a steady-state supply of oxygen to the sensor surface for constant response which makes it immune to the interference of flow conditions. Polyaromatic organic species have been widely used as oxygen-sensingdyes in the development of optical However, these systems usually have limited The use of luminescent transition-metal complexes as spectroscopic probes and sensor materials has received considerable attention in recent years.lO These materials have very desirable features including long lifetimes, intense visible absorptions, and resistance toward degradation. The spectroscopic and chemical properties can be systematically tuned through modification of the structure of the complexes which allows the design of systems for specific purposes. Wolfbeis and coworkers have used Ru(bpy)S2+(bpy = 2,2'-bipyridine) as a material for optical oxygen meas~uernent.~~~ The photophysics and photochemistry of oxygen sensors based on Ru(bpyh2+ and ita analogues have been studied in detail by Demas, DeGraff, and Bacon." Recently, an oxygen-sensitive paint (1)Luebbers, D. W.; Opitz, N. Sens. Actuators 1983,4 , 641. (2)Kroneis, H.W.; Marsoner, H. J. Sens. Actuators 1983,4 , 587. (3)Peterson, J. I.; Fitzgerald, R. V.; Buckhold, D. K. Anal. Chem. 1984,56,62. (4)Wolfbeis, 0.S.;Poach, H. E.; Kroneis, H. W. Anal. Chem. 1985, 57,2556. (5)Wolfbeis, 0.S.;Leiner. M. J. P.; Posch, H. E. Mikrochim. Acta 1986,3,359. (6)Bacon, J. R.; Demas, J. N.Anal. Chem. 1987,59,2780. (7)Lee, E.D.; Werner, T. C.; Seitz, W. R. Anal. Chem. 1987,59,279. (8) Wolfbeis. 0.S.:Weis,. L. J.:. Leiner. M.J. P.: Zieder, - -W. E. Anal. Chem. 1988,60,2028. (9)Clark, L. C. J. Tram. Am. Artif. Intern. Organs 1956,2,41. (10)Demas. J. N.: DeGraff. B. A. Anal. Chem. 1991.63.829A. (11)Carraway, E.'R.; Demak, J. N.;DeGraff, B. A.;Bacon, J. R. Anal. Chem. 1991,63,337. 0003-2700/93/0365-0255$04.00/0
based on platinum octaethylporphyrin has been developed by Gouterman and co-workers.12 In this article we will report some luminescent Pt(I1)complexes of the class [Pt(L)(CN)2] (where L is an a-diimine such as substituted 2,2'-bipyridine or 1,lO-phenanthroline) as sensors for optical determination of oxygen concentrations in the gas phase. These complexes are of particular interest because they have long excitedstate lifetimes (microseconds) and are resistant toward photochemicaldegradation.13-16These complexesare neutral which makes them more soluble in silicone rubber than the ionic Ru(bpy)S2+.Moreover, [Pt(L)(CN)21is known to form excimer upon irradiation.15Je By monitoring of the excimer emission, these complexesmay be used as spectroscopicprobes of the microenvironment inside polymers.
EXPERIMENTAL SECTION Materials. KzPtCL was purchased from Aldrich Chemical Co. [Pt(bathophen)(CN)2] (bathophen = 4,7-diphenyl- 1,lOphenanthr~line)'~ and [Pt(dtbpy)(CN)tl (dtbpy = 4,4'-di-tertb~tyl-2,2'-bipyridine)~~ were synthesized and purified as described in the literature. The silicone rubber RTV-732 (a one-part polymer containing 100% silicone rubber) was obtained from VereoChem Co. Oxygen and nitrogen gases (99.9%) were purchased from Hong Kong Oxygen Co. All other chemicals were analytical reagents and were used without further purification. Preparation of the Film. A piece of copper foil spacer (0.2 mm thick) with a square hole (15 mm X 15 mm) in the middle was placed on amicroscopicglass slide. RTV-732silicone rubber (-0.02 g) was placed inside the hole on the glass slide. The surface of the polymer was covered with a Teflon f i on top of which was placed another microscopic glass slide. The polymer was clamped between the two glass slides until cured. As the cured polymer does not stick to Teflon,this method can produce thin polymer filmson glass slides with good optical quality. The silicone rubber film will have a thickness equal to the thickness of the copper foil spacer. The silicone rubber film was impregnated with the platinum complexes by either one of the following methods: (i) A 10-pL aliquot of platinum complex solution in dichloromethane (concentrations 10*10-6 M)was mixed with 0.02 g of silicone rubber thoroughly before transfer to the glass slide. (ii)After the silicone rubber f i was cured, it was immersed in a platinum complex solution in dichloromethane with the concentration range mentioned in (i). The films rapidly swelled and took up the complex. After 30 min, the films were removed, rinsed rapidly with CHP Cln to remove any surface contamination, and evacuated for 24 h under vacuum. The concentrationof the platinum complex in the f i i prepared by both methods waa controlled by adjusting the concentration of the complex solution in CHZClz. The f i i prepared by both methods have similar optical quality. Method (12)Kavandi, J.; Callis, J.; Gouterman, M.;Khalil, G.; Wright, D.; Green, E.; Burns, D.; McLachlan, B. Rev. Sci. Imtrum. 1990,61,3340. (13)Che, C. M.;He, L. Y.; Poon, C. K.; Mak,T. C. W. Inorg. Chem. 1989,28,3081. (14)Che, C. M.;Wan, K. T.; He, L. Y.; Poon, C. K.; Yam,V. W. W., J. Chem. SOC.,Chem. Commun. 1989,943. (15) Kunkely, H.;Vogler, A. J. Am. Chem. SOC.1990,112,5625. (16)Wan, K. T.;Che, C. M.;Cho, K. C. J.Chem. SOC.,Dalton Trans. 1991,1077. 0 l9Q3 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
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Flgure 1. Absorption (- -) and corrected excitation (-) spectra of InsiUcone (A,top) [Pt(bathophen)(CNk]and(B,bottom)[Pt(~%bpy)(CN)~] rubber RTV-732. The emission wavelength was monitored at 520 nm for [Pt(bath~phen)(CN)~] and 486 nm for [Pt(dtbpyXCN)~].
ii is more convenient but has the disadvantage that the concentration of the complex inside the polymer film is difficult to estimate. Instrumentation. Luminescence intensity measurements were made with a Perkin-Elmer LS-5 microcomputerizedspectrofluorometer. Laser flash photolysis was made on a SpectraPhysics Quanta-RayDCR-3 pulsed Nd:YAG laser excited at 355 nm. The laser energywas set at 3 mJ/pulse. Two gas flow meters (Cole Parmer Co.), each individuallycalibratedby the volumetric method, were utilized to measure the flow rates of oxygen and nitrogen. The oxygen and nitrogen gases were mixed in l-mlong Tygon tubing and then fed into a flow cell in which the silicone rubber film with immobilized platinum complex was exposed to the mixed gas stream, and the glass slide was facing the excitation light source in the spectrofluorometer. The oxygen concentration (% v/v) was calculated by dividing the oxygen flow rate by the sum of the oxygen flow rate and the nitrogen flow rate. All the measurements were made at room temperature (25 2 "C)and atmospheric pressure.
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Flgure 2. Emission spectra of (A) [Pt(bathopher~XCN)~] and (B) [Pt(dtbpy)(CN)Z] in silicone rubber RTV-732. Concentration of complex in polymer: (A) (-) 0.13 mM, (- - -) 0.42 mM, (B) (-) 0.22 mM, (- - -) 0.43 mM. The excitation wavelength was 360 nm for [Pt(bathophenb
(CN)2] and 318 nm for [Pt(dtb~y)(CN)~].
of the complex inside the polymer film is increased, a second emission peak is observed at about 620 nm. Similar concentration dependence is observed in the emission spectra of [Pt(dtbpy)(CN)~]. The excitation spectra for [Pt(bathophen)(CN)zI are identical irrespective of whether the emission is monitored at 520 or 620 nm. These findings are consistent with the solution chemistry that the excited Pt" complex would dimerize with another Ptl*m01ecule.l~The emission a t 620 nm for [Pt(bathophen)(CN)z] and 565 nm for [Pt(dtbpy)(CN)zl can be attributed to emissionsof the excimers.
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RESULTS AND DISCUSSION While some [PtI1(L)(CN)2]complexes have been reported to emit in the solid state,13 the incorporation of these complexes inside polymer films is hampered by their very low solubility in all common solvents. Recent studies indicate that the presence of bulky substituents on the bipyridine or phenanthroline ligand can enhance the solubility of this kind of complex in nonaqueous ~ o l v e n t a . ~The ~ J ~complexes [Pt(bathophen)(CN)zl and [Pt(dtbpy)(CN)z] were therefore chosen in this study. The absorption, excitation, and emission spectra of [Pt(bathophen)(CN)nl and [Pt(dtbpy)(CN)z] inside silicone rubber film are shown in Figures 1and 2, respectively. For both complexes, the spectra inside silicone rubber film bear close resemblance to the spectra in solutions.15J6 For [Pt(bathophen)(CN)zl, an emission maximum is observed a t 520 nm a t concentrations below 0.2 mM. When the concentration
The time-dependent emission of the platinum complexes after a laser flash have been measured. For monomer emission, the emission maximum intensity is observed immediately followingthe laser excitation pulse. The excimer emission, on the other hand, does not reach ita maximum until a certain time delay (-60 ns) after the laser flash. Figure 3 shows a typical decay curve of 0.45 mM [Pt(bathophen)(CN)z] inside silicone rubber RTV-732 monitored at 620 nm. A similar time delay was observed for the platinum complex in solution,16and this has been attributed to the time required for migration of two platinum complex molecules. However, in polymer films the movement does not necessarily involve a fast diffusion of platinum molecules in the polymer matrix. It is generally accepted that certain polymer chain conformations are geometrically more suitable for excimer forma-
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, lQQ3 257 87.50 37.50
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Flgure 6. Emission spectra of the [Pt(bath~phenNCN)~] sensing film during differentstages of the curing of silicone rubber RTV-732. Before curing (- -): partially cured after 15 min at room temperature (- -): completely cured after 24 h (-). Excitation wavelength: 360 nm.
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tion.17 The time delay could be due to the reorientation of polymer chains upon irradiation to the desired conformations for platinum dimer formation. The monomer emission is very sensitive to quenching by oxygen. For a sensing film prepared by method ii with a 0.35 mM [Pt(bathophen)(CN)z]solution, the emission a t 520 nm was quenched to 7 % of its original intensity in pure oxygen. The polymer film containing [Pt(dtbpy)(CN)2] prepared under similar conditions is less sensitive to oxygen than [Pt(bathophen)(CN)z]. The monomer emission for [Pt(dtbpy)(CN)z] at 488 nm was quenched to -20% of its original intensity in pure oxygen. The Stern-Volmer plot for [Pt(bathophen)(CN)z] is shown in Figure 4. Similar to other luminophores in heterogeneous ~ y s t e m s , ' - ~ * the ~ J ~SternJ~ Volmer curve is not linear. The nonlinear Stern-Volmer response of luminophores inside the polymer has been studied by various workers. The phenomenon can be explained by a multiple emitting sites model1lJ8 or by considering a nonlinear solubility equation of the gas-phase oxygen dissolved in the polymer support.lg Nevertheless, the SternVolmer plots were highly reproducible and no hysteresis was observed. The excimer emission on the other hand, is less sensitive to quenching by oxygen. For a sensing film containing 0.65 mM [Pt(bathophen)(CN)z],the emission at 620 nm decreased to -86% of the original intensity in the presence of pure oxygen. The decrease in excimer emission
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(17) Guillet, J. Polymer Photophysics andPhotochemistry; Cambridge University Press: Cambridge, U.K., 1985; Chapter 7. (18)Lehrer, S. S, Biochemistry 1971, 10, 3254. (19) Li, X.M.; Wong, K. Y. Anal. Chirn. Acta 1992, 262, 27.
intensity is partly a result of the decrease in concentration of the excited monomer as a result of quenching with oxygen molecules. The film has a fast response toward changes in oxygen concentration. When oxygen is suddenly introduced into the flow cell, the luminescence intensity drops to 95% of the final value in less than 3 s. Figure 5 shows the response time of a 0.2-mm-thick sensing film prepared by method ii when subjected to step changes in the oxygen concentration. The time taken to reach the final intensity for the nitrogen-tooxygen transition is about 8 s. The response for the oxygento-nitrogen transition, on the other hand, takes about 50 s to reach the final luminescence intensity. Our results match the commonly observed fact that the response is much longer for the oxygen-to-nitrogen transition^.^^ We expect the response time can be further improved if the film thickness is decreased. Both platinum complexes immobilized in silicone rubber films appear to be very stable. There is no sign of photochemical degradation when irradiated with a 8.3-W xenon discharged lamp equipped with a F/3 MonkGillieson type monochromator and subjected to step changes in the oxygen concentration for over 100 cycles, as that in Figure 5. Thus this system is superior over the platinum porphyrins such as Pt(0EP) and Pt(TPP) (OEP = octaethylporphyrin, TPP = tetraphenylporphyrin) which would degrade rapidly after several cycles under similar conditions. No diminution of luminescence intensity was observed when the film had been stored in darkness under ambient conditions for over 6 months. Preliminary interference studies indicate that no interference was observed with halocarbons such as chloroform
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and dichloromethane. This is probably because the excited platinum complex is a stronger oxidant than reductant. The E0(Pt1I*-Pt1)and E0(Pt1ILPt1I*)for [Pt(dtbpy)(CN)z],for example,have been estimated to be 2.1and
