Sol-Gel Derived Titanium Carboxylate Thin Films for Optical Detection

Anal. Chem. 1994,66, 2739-2744

Sol-Gel Derived Titanium Carboxylate Thin Films for Optical Detection of Analytes Dilum D. Dunuwila,t B. A. Torgerson,t C. K. Chang,* and Kris A. Berglund'ltl* Department of Chemical Engineering and Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 The feasibility of using a sol-gel derived titanium carboxylate thin film, which can be supported on glass, for the fabrication of optical test strips was investigated using a model probe/ analyte combination. The porous films, being optically transparent and having the capability to host probe molecules, provided an excellent system to investigate the possibility of making optical test strips. A colorimetric reagent, an iron(111) porpohyrin, was chosen as the probe molecule to detect free cyanide ion concentrations in aqueous solutions. Entrap ment of the porphyrin in the film was accomplished by direct dissolutionof the porphyrinin the sol-gel solutions. Chemically induced structural modifications of the polymer were carried out to stabilize the encapsulated metalloporphyrinwithin the sol-gel derived matrix. The syntheses of this model analyte detection system and its response are presented. An optical parameter reflective of the chemical changes that occur in the system was selected as the measurement tool; its response asymptotically increased over the cyanide ion concentration range of 40-25 000 ppm. Development of the sol-gel process' has paved the way to processing novel materials having a variety of chemical and physical properties suitable to many applications including optical sensors and detection systems. The sol-gel technique, in addition to providing stable, transparent, and porous materials, offers a multitude of modifying techniques' that may be necessary to sustain foreign molecular probes in good optical condition. The porous matrix should contain probe molecules with minimal leakage, but at the same time allow maximum access for analytesto the immobilized probe through favorable diffusion properties that minimize barriers to mass transport. The sol-gel process allows adjustment of diffusion properties through structural modifications.If Previous work has demonstrated the general usefulness of sol-gel derived support structures for various guest molecules.2J An ideal optical sensor should have the capability to detect fluctuations in analyte concentration continuously; i.e., the t Department of Chemical Engineering. 8 Department of Chemistry. (1) (a) Reuter, H. Adv. Mater. 1991, 3 (5). 258. (b) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988,18,259. (c) Zheng, H.; Colby, M. W.; Mackenzie, J. D. Mater. Res. Soc. Symp. Proc. 1988,121,537. (d) Schmidt, H.; Rinn, G.; Nass, R.; Spom, D. Mater. Res. SOC. Symp. Proc. 1988, 121,743. (e) Babonneau, F.; Leaustic, A.; Livage, J. Mater. Res. Soc. Symp. Proc. 1988,121,317. (f) Melpolder,S. M.;Coltrain,B. K.Mater. Res. SOC.Symp. Proc. 1988, 121, 8 1 1. (2) (a) Dulebohn, J. I.; Van Vlierberge, B.; Berglund, K. A.; Lessard, R. B.; Yu, J.; Nocera, D. G. Mater. Res. Soc. Symp. Proc. 1990,180,733. (b) Lessard, R. B.; Berglund, K. A.; Noccera, D. G. Mater. Res. Soc. Symp. Proc. 1989, 155,119. (c) Newsham, M. D.; Cerreta, M. K.; Berglund, K. A.; Nocera, D. G. Mater. Res. Soc.Symp. Proc. 1988,121,627. (d) Dulebohn, J. I.; Haefner, S. C.; Berglund, K. A.; Dunbar, K. R. Chem. Muter. 1992, 4(3), 506.

0003-2700/94/03682739$04.50/0 0 1994 American Chemical Society

. . .. .. .UV/VIS .. Ligt7 t .. .. .. ...

To Detect f--

. .. . .. ... . .. .. . .

Diffusing Exogenous Species

.'

Substrate

Film

Bulk Solution

@ Probe Molecules Figure 1. Configuration of the sensor system.

optical probe should interact with the analyte in a reversible manner for long periods of time. However, most colorimetric and fluorimetric reactions are irreversible, because resulting products are tightly bound complexes. A pragmatic solution that avoids such problems is the construction of cheap, singleuse, and recyclable test strips. Sol-gel materials and processing are well suited for the manufacture of economically viable and environmentally friendly polymer matrix supports. Therefore, the focus of this investigationwas to study the feasibility of sol-gel derived thin films for sensory applications. The structure of the test strip is outlined in Figure 1. The molecular structure of the metalloporphyrin used as the molecular optical probe, tetrakis(pentafluoropheny1)porphine iron(II1) chloride (PFPP), is given in Figure 2. The titanium carboxylate thin film utilized to entrap the metalloporphyrin was developed prior to this study.4

MATERIALS AND INSTRUMENTATION Titanium(IV) isopropoxide and valeric acid were purchased from Aldrich Chemical Co. and used with no further purification. Absolute ethanol was purchased from the Quantum Chemical Corp. Deionized water with a resistance of 18 Mil was used. VWR precleaned microscope slides used as substratesfor films were cleaned prior to usage. A Scientific Industries Vortex Genie 2 stirrer was used to agitate the (3) (a) Ellerby, L. M.; Nishida, C. R.; Nishida, F.; Yamanaka, S. A.; Dunn, B.; Selverstonc-Valentine, J.; Zink, J. I. Science 1992, 255, 1113. (b) SlamaSchwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992,355,240. (c) Kuselman, I.; Kuyavskaya, B. I.; Lev, 0.Anal. Chim. Acto 1992,256.65. (d) Schowk, A.; Avnir, D.; Ottolenghi, M.J. Am. Chem.Soc. 1991,113,3984. ( e )Haruvy, T.; Webber, S. E. Chem. Mater. 1991.3.501. ( f ) Zusman, R.; Rottman, C.; Ottolenghi, M.; Avnir, D. J. Non-Crysr.Solids 1990,122,107. (8) Dunn, B.; Knobbe, E.; McKieman, J. M.; Pouxvicl, J. C.; Zink, J. I. Mater. Res. SOC. Symp. Proc. 1988, 121,331 (h) Avnir,D.;Lcvy,D.; Reisfe1d.R.J.Phys. Chem. 1984,88,5956.

Ana&ticalChemistty, Vol. 66, No. 17, September 1, 1994 2739

1: Figure 2. Molecularconf@ratlon of tetrakls(pentaflucn'0phenyi)porphine iron(II1) chloride (PFPP).

Table 1. Compolnlon of PFPP Incorporated Fllm Wutlon component molar ratio (R)O

PFPP valeric acid water

R, = 0.008 R, = 9.0 R, = 1.5

Molar ratio with respect to titanium(1V) isopropoxides PFPP = tetrakis(DcntafluoroDheny1borphine iron(II1) chloride.

reaction mixtures. The reaction mixtures were contained in Research Products International Corp. screw-top liquid scintillation glass vials. An International Clinical Centrifuge (Model CL 26802 M) was used for spin-casting films. The films were supported on microscope slides cut to fit 1 X 1 cm cuvettes. The films were dried using a Master heat gun. The metalloporphyrin was tetrakis(pentafluoropheny1)porphine iron(II1) chloride (PFPP) and was synthesized and provided by Chang.5 Absorbance measurements were taken in a Perkin-Elmer Lambda 3A UV/VIS spectrophotometer. Fisher Scientific 1 X 1 cm disposable polystyrene cuvettes were used to hold the films (supported on slides) vertically across the UV/VIS beam path. Spectra were recorded on a Perkin-Elmer R lOOA Recorder. Method: Encapsulation of Metalloporphyrin. A suitable film solution composition was determined prior to the commencement of this work.4 The film solution composition that includes PFPP is given in Table 1. The preparation of the solutions was as follows;first, PFPP was dissolved invaleric acid, and titanium( IV) isoproposidewas added to the porphyrin solution. After a few seconds of vigorous stirring, water was added, and the mixture was stirred again. Finally, ethanol was added to the mixture. The effect of added ethanol on encapsulated PFPP stability was studied. The range studied was between Re values (the molar ratio of ethanol to alkoxide) of 10 and 40. (4) Dunuwila, D. D.; Gagliardi, C. D.; Bcrglund, K. A. Chem. Mater., in press. ( 5 ) Chang, C.K. Department of Chemistry, Michigan State University personal communication.

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The film solutions were aged for at least 24 h prior to casting but were used within 1 week. The film solutions were cast on cut microscope sides for 3 min, and the cast films were dried for 1 min with a heater gun at 200 OC, primarily to drive off excess valeric acid which possesses an unpleasant odor. Both casting and drying were carried out in a ventilated environment. The stability of PFPP encapsulated within the polymer matrix was studied using UV/VIS absorbance spectroscopy. Cast films supported on cut slides were placed vertically inside polystyrene cuvettes in pairs. The pair was placed against the opposite walls of the cuvette such that both films were oriented toward the cavity of the cuvette and not the wall. The cuvette was placed in the spectrophotometer cells compartment such that the film surfaces were perpendicular to the beam path. In order to obtain porphyrin absorbance spectra that were as precise as possible, a reference cell of the same configuration, but with films free of porphyrin, was used in the reference compartment of the double-beam spectrophotometer. Method: Construction and Calibration of Test Strip. In addition to the components given in Table 1, the final film solution composition consisted of absolute ethanol at Re = 40. Film slideswere placed firmly against both cuvette walls across the beam path by wetting the contact surfaces such that the film slides were exposed to the cuvette cavity. Two films were used in order to increase the signal to noise ratio. Also, background spectral interferences were minimized by using a reference cell of the same configuration with films free of PFPP in the reference compartment of the double-beam spectrophotometer. All films used in this experiment were stored in a dark environment prior to usage; however, they were exposed to room conditions. The films were soaked in water for about 15 min, immediately preceding the contact with cyanide solutions, to avoid any contributions from spectral changes due to water. The experimental procedures was as follows. Both the active cell and the reference cell were placed in their respective compartments. Three milliliters of an aqueous cyanide solution of known concentration was introduced to the empty active cell, and timing was commenced immediately. After 15 min, the absorbance intensities at 410 and 430 nm were recorded. The ratio of absorbance at 430 nm to absorbance at 410 nm was recorded for cyanide concentrations ranging from 400 ppb to 25 000 ppm. From the time the active cell was placed in the cell compartment to the time the measurement was taken, the incident light beam was blocked as a precautionary measure against photodecomposition of PFPP. RESULTS AND DISCUSSION Encapsulation of Metalloporphyrin within Porous Film. Metalloporphyrins can be entrapped within the porous matrix of sol-gel derived thin films6,' by dissolvingin the film solution prior to spin-casting or any other thin-film processingtechnique that is preferred. This method requires the porphyrin to be sufficiently soluble in the solution such that a functional (6) Lesasard, R.B.; Wallace, M. M.; Oertling, W. A.; Chang, C. K.;Bcrglund, K. A,; Nocera, D. G. Mater. Res. Soc. Symp.Ser. 1988, 155, 109. (7) Gagliardi, C. D.; Dunuwila. D. D.; Chang, C. K.; Berglund, K. A. Mater. Res. SOC.Symp. Proc. 1992, 271, 645.

0.10

im?

e 51 c

0.05

1

I

350

450

550

Wavelength ( n m ) Flguro 8. Spectral dynamicsof PFPP entrapped In a valeric acid film. The film solution parameterswere R. = 9, R, = 1.5, and Rp = 0.008. The spectra were normalizedto Isolate the effect of aggregationfrom that of photodecomposltlon. The fihns were exposed to room light and ambient conditions during the period these changes were observed. The Inset Is a plot of A vs time: R. is the molar ratioof acid to alkoxide, RwIs the molar ratio of water to alkoxide, RpIs the molar ratio of PFPP to alkoxide, PFPP Is tetrakIs(pentaflwrophenyl)porphlne iron(II1) chloride, and A = absorbance at 350 nmlabsorbanceat 410 nm.

amount of porphyrin would be available in the thin film to react with the target analyte. Metalloporphyrins demonstrate a high degree of thermodynamic stability. This is due to the quadruple binding sites on the porphyrin ligand through which the metal complex is formed and the extensive resonance stabilization of the highly conjugated structure. However, there are instability phenomenon associated with metalloporphyrins. Among them are photodecomposition8and aggregation.” Decomposition upon exposure to light is common among metalloporphyrins. Dimerization and aggregationof free base porphyrins and their metal complexes are frequently encountered in solution, especially in aqueous Higher porphyrin concentrations promote aggregation. Generally, the blue-shifted dimer Soret band shifts to longer wave lengths upon d i l ~ t i o n . l ~ - ~ ~ When films were made with solutions of the composition given in Table 1, the Soret band of the encapsulated PFPP shifted to the blue slowly but continuously. The shift was from 410 to 350 nm. Figure 3 illustrates the observed spectral dynamics, and the inset of Figure 3 illustrates the steady accumulation of the blue-shifted peak. Demetalation of PFPP is unlikely since strong acidic conditionsare necessary to dislodge a metal from the porphyrin center. Any photoeffect was discounted since PFPP in films stored in the dark followed the same trend. It was deemed (8) Buchler, J. W. In The Porphyrins: SrrucrureundSynthesis, Purr A; Dolphin, D., Ed.;Academic Press: New York, 1978; Vol 1. Chapter 10. (9) Alexander, A. E. J. Chem. Soc. 1937, 1813. (IO) Bergeron, J. A.; Gain-, J. G. I.; Bellamy, W. D. J. Colloid Interface Sei. 1967. 25, 97. (1 1) White, W. I. In The Porphyrins: Physical Chemisrry, Par? C; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. V, Chapter 7. (12) Shelnutt, J. A.; Dobry, M. M.;Satterlee, J. D.J.Phys. Chem. 1984,88,4980. (1 3) Parternack, R. F.; Francarconi,L.; Raff, D.; Spiro, E. Inorg. Chem. 1973.12 (11). 2606. (14) Kassner, R. J.; Wang, J. H.J . Am. Chem. Soc. 1966,88,5170. (15) White, W. I.; Plane, R. A. Eioinorg. Chem. 1974, I , 21.

unlikely that an alteration of the PFPP structure due to a reaction was taking place since controlled conditions are necessary to cause changes to the thermodynamically stable porphyrin molecule (unless it reacts with strong donor ligands such as cyanide or carbon monoxide). Therefore, by the process of elimination, we attributed the instability demonstrated by spectral changes given in Figure 3 to aggregation of PFPP within the polymer matrix. Spectral changes are consistent with that observed in the literature with aggregating systems.l2-l5 Sol-gel materials by classification are fluids. Therefore, we speculated that the fluidity of the film structure may allow the porphyrin molecules to be mobile, thus promoting aggregation. Further, it is possible that the polymer matrix may not have reached thermodynamic equilibrium in terms of matrix formation and is thus a dynamic structure. Long equilibration times for matrix formation in sol-gel systems are not surprising due to the complexity of the reaction dynamics. Stabilization of the matrix was of primary importance to the implementation of a successful detection system since preliminary investigations proved that excessivelyaggregated systems are not responsive to cyanide ions. Experimentation with modification techniques available in the sol-gel process was the logical next step toward matrix stabilization. One such technique was Often in the sol-gel process, alcohols are used as the solvent. However, unlike most other solventsthat are chemically inert, alcohols partake in the schemeof reactionsthat takes place in thesol-gel process. Therefore, alcohols are very effective in controlling the final structural properties of sol-gel derived materials. The sensible selection of alcohols can lead to tailor-made sol-gel material. A review of the porphyrin literature revealed that ethanol was capable of dispersing porphyrin aggregates in s o l ~ t i o n . l 5 ~ ~ Lowering of the dielectric constant of the solution upon the addition of ethanol is responsible for this effect. As discussed above, ethanol, in addition to being a dispersive agent, can participate in alcoholysis in sol-gel systems. Consequently, the 2-fold effectiveness of ethanol rendered it a candidate for initial stabilization experiments. Even though aggregation of PFPP in the film solution was not observed experimentally due to instrumental limitations, it is reasonableto supposethat it originates in the film solution. The high porphyrin concentration, approximately 2 X M, is likely to initiate such an event. It is also likely that the control of aggregation implemented in the film solution can be extended to the film phase. Ethanol was added to the film solution containing PFPP soon after the addition of water. The film composition was given in Table 1. The effect of varying ethanol concentrations on the system is presented in Table 2. Along with aggregation, photodecomposition of encapsulated PFPP was observed in preliminary experiments. However, theresultspresented inTable 2 indicate that modifications (16) Woignier, T.; Phalippou, J.; Zarzycki, J. J . Non-Crysr. Solfds 1984,63, 117. (17) (a) Gugliclmi, M.; Carturan, G. J. Non-Crysr. Solids 190. 100, 16. (b) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, F. J. Non-Crysr.Solids 1988, 100, 65. (18) Vcrma, I. D.; Mehrotra, R. C. 1. Chem. Soc. 1960, 2966. (19) (a) Smith, M. H. Eiochem. J . 1959,73,90. (b) Gallagher,W. A,; Elliott, W. 8.Eiochem.J.1%5,97,187. (c) Shack, J.; Clark, W. M. J.Eiol. Chcm. 1947, 171. 143.

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Tablr 2. E f l M of Ethanol on Photodacomporltlon of Entrapped PFPP absorbance at 410 nmb ethanol‘ (mL) 1st day 7th day 2 l s t day 42nd day total loss (%)c

1 2 3 4

0.057 0.097 0.214 0.332

0.026 0.065 0.220 0.332

0.026 0.055 0.300

0.017 0.042 0.160 0.280

70.2 56.7 25.2 15.7

a Ethanolwasadded tofilmsolutionscomprisedof1 mLoftitanium(1V) isopropoxide, 3.29 mL of valeric acid, 92 fiL of water, and 0.01 g of PFPP. This composition corresponds to that given in Table 1 . The day on which theabsorbanceof theencapculated PFPP wasobservedcounting from the day the films were cast. Loss percent in absorbance intensity was calculated after 42 days. The entrapped PFPP was exposed to room light and ambient conditions.

to the sol-gel system initiated by ethanol contribute to the partial control of the rate of photodecomposition of encapsulated PFPP. One milliliter of ethanol is accompanied by a 70% loss in absorbance intensity over a period of 42 days. In contrast, only a 15% reduction was observed when 4 mL of ethanol was used. It is possible that an alteration to the extent or the nature of electronic interaction between the porphyrin and the titanium based polymer matrix may have contributed to this stabilization effect. It may also be possible that the new material is capable of acting as a filter at critical wave lengths that decompose the porphyrin. The primary objective of this segment of the project was to study the possibility of controlling aggregation of encapsulated PFPP. The absorbance PFPP encapsulated in films made from solutions containing ethanol at R, = 40 (Reis the molar ratio of absolute ethanol to alkoxide; the overall composition corresponds to that given in Table 2 where 4 mL of ethanol was used) was monitored for a period exceeding 1 month. Over the span of this experiment, some films were stored in a dark environment and some were exposed to room light in order to avoid ambiguities that may arise from untested photo effects. None showed any sign of aggregation; that is, the growth of the band of PFPP at 350 nm was completely curtailed in films modified by ethanol. Clearly, the modified chemical composition of the film solution has been effective in converting the polymer matrix to a less fluid host. Consequently, PFPP is held in place as monomers. Presumably, PFPP exists as dispersed monomers in film solutions containing ethanol, since ethanol is effective in dispersing aggregates. In turn, this would help give a more dispersed porphyrin distribution in the film, further deterring the aggregation process. The stability of the guest-host system proved that the modification technique adopted was apprppriate. Although, the exact course through which ethanol executed the stabilization process is yet unknown, it is reasonable to suppose that both dispersion of aggregates and structural modification in combination were able to impart stability to the system. Alcoholysis of the propoxy groups of the titanium alkoxide by ethoxide groups potentially changes the reactivity of the alkoxide; consequently, hydrolysis and polycondensation are a f f e ~ t e d . ~ J ”The ’ ~ compounded outcome is an alteration to the polymer configuration. In addition, since ethanol is used in excess, the dilution of the reacting species in ethanol may change reaction kinetics and thermodynamics significantly, 2742

AnaIytIcaIChemIstty, Vol. 66,No. 17, September 1, 1994

giving an alternative equilibrium composition. A detailed characterization is beyond the scope of this project. The film solutions can be used for approximately 3 weeks. Films made with solutions aged over 4 weeks exhibit increasingly poorer adhesion properties. The cause of poor adhesion properties of films made with solutions aged over 4 weeks is not clear. However, it is possible that due to the presence of an excess amount of carboxylic acid groups, the polymer structure is broken down at the titanium metal by further carboxylate substitutions. This could lead to discontinuity in the polymer structure and more hydrophobicity, thus resulting in poorer adhesion. Further details on the stability and factors contributing to stability of these films are given by Dunuwila et al.4 Constructionof CyanideTest Strip. Iron porphyrins readily bind ligands at axial positions to form octahedral complexes. Among these ligands are molecules such as water, pyridine, and carbon monoxide or ions such as halides, hydroxide, and cyanide.20 These reactions are accompanied by spectral changes that reflect the character of the altered electronic environment imposed by the incoming ligand. For the purpose of fabricating functional detection systems, changes in the Soret region20 are the most practical due to the extent of relative changes that are normally observed in this region and due to the high Soret absorbance intensity. Cyanide ions, placed at the high end of the spectrochemical series, have a particularly high affinity toward metalloporphyrim2’ Therefore, the cyanide and metalloporphyrin system having favorable optical, physical, and chemical properties was chosen for this investigation. It was our intention to calibrate the changes in the Soret region in the absorption spectrum of a metalloporphyrin immobilized in a sol-gel film upon its reaction with cyanide to reflect free cyanide ion concentrations in aqueous media. In order to determine the appropriate parameters for the test strip calibration, films were treated with a 400 ppm cyanide solution. Spectral changes were recorded at steady time increments. Spectra recorded over a period of 15 min are given in Figure 4. The Soret absorbance maximum of encapsulated PFPP is at 410 nm. Upon the introduction of aqueous cyanide, the Soret absorbance maximum red shifts to 430 nm. The isosbestic point verifies the presence of two absorbing species; presumably, the second being the cyanide complexed PFPP. When films containing PFPP were soaked in water, no detectable amounts of PFPP were observed to leach to the solution. However, upon complexation with cyanide, trace amounts of complexed PFPP were detected in the aqueous cyanide solution. At 25 000 ppm cyanide concentration, the percentage leached to the solution was approximately 5% with respect to the total absorbance measured at 430 nm. Leaching was undetectable at low concentrations of cyanide. Even though the leached porphyrin was quantitatively significant at high concentrations of cyanide, the leaching was not considered to be restrictive toward a reasonable calibration. Figure 4 suggested that the obvious choice for an internally standardized calibration parameter for the test strip is (20) Falk, J. E. Porphyrins and Mefolloporphyrins;Elscvicr Publishing Co.: N e w York, 1964; Vol. 2, Chapter 6. (21) Huheey, J. E. Inorganic Chemistry, 3rd 4.; Harper & Row: New York, 1983.

0.50

1I

1

,

18 1

l/”’I

f

0.25

i

, +

0 00 350

450

0.50 550

Wavelength (nm) Flguro 4. Spectraldynamics of PFPP entrapped in a valeric acld film upon contacting the flim surface wlth an aqueous cyanide solution. Film solution parametersare as follows: Re = 9, & = 1.5,& = 40, and Rp = 0.008. The presented changes in the PFPP spectrum took place over a 15-min period. The initla1bulk cyanide ion concentratlon was 400 ppm. The scan tlme was approxlmatety 1.6 min. The inset is a plot of 0 vs time at cyanide bn concentratbnsof 40 and 25 000 ppm. R. is the molar ratio of acid to aikoxide, & is the molar ratio of water to alkoxide, Re is the molar ratio of ethanol to aikoxide, Rp is the molarratk of ff PPto akoxide,PFPP = tetrakispentafluoropbnfi porphineiron(III)chkride,and0= absorbanceat430nmlabsorbance at 410 nm.

the ratio of absorbance at 430 nm (PFPP complex) to absorbanceat 410nm (PFPP). Thechoiceofa ratioreflecting the relative amounts of both product and reactant as the calibration parameter is particularly advisable since it will not be affected by instrumental drift. In the following discussion this calibration parameter will be identified by the symbol 0. The plot of 0 vs time is given in the inset of Figure 4. A 15-min test strip response time was used. The inset of Figure 4 suggests that 0 levels off at values proportional to the cyanide ion concentration. This can be due to either of two partitioning effects exerted by the polymer matrix. The polymer-solutioninterface can partition cyanide ions in the polymer phase proportional to the bulk solution concentration; Le., the uptake of cyanide ions into the film is dependent on the bulk solution concentration. Also, the heterogeneous environment within the film may result in a partitioning between the bound and the free cyanide ions. Spectra of film-encapsulated PFPP and that of PFPP dissolvedin a solution composedof 10%(v/v) water in ethanol were similar in appearance. The response of PFPP to cyanide ions, in both situations, was also similar. The similarity of the PFPP spectra and spectral dynamics in solution and in the heterogeneouspolymeric environmentindicated that the major characteristics of PFPP have been retained. The time scale of the response in solution was less than 1 s as compared to 15 min required in the film. Mass transport considerations are mostly likely responsible for the sluggishness of the test strip response. Construction of the Calibration. A linear calibration for the detector was obtained by plotting 0, measured after 15 min of treatment, vs the logarithm of the CN- ion concentration. The calibration is given in Figure 5. The data are compiled from analysis of six different sets of films at each

10

io0

1000

io4

io5

Cyanide Ion Concentration (ppm) Figure 6. Sensor caiibratlon. The parameter 0 was m s u r d after 15 min of treatment. The data are complied from anatysls of six different sets of films at each glven concentration. 0 = absorbance at 430 nm/abswbence at 410 nm.

given concentration. In addition, the films used for analysis were aged between 3 and 30 days. The calibration indicates that the test strip has a large dynamic range for the detection of CN- ions. The test strip is stable and functional over a periodof 1 month (stability beyond 1 month has to be tested), suggesting that the general approach can be applied to manufacturing test strips with long shelf lives. The lower limit of detection is approximately 10 ppm, and the upper limit is approximately 25 000 ppm. Below 10 ppm, changes in CN- ion concentration do not inflict significant changes to 0. At 25 000 ppm, 0 reaches an asymptote, suggesting that the encapsulated PFPP available for complexation is approaching saturation. The work presented herein has clearly demonstrated the viability of integrating sol-gel polymer matrices and relevant chemistries to construct detection systems. The method is particularly advantageous for analytes for which there are no direct indicators. The system possesses some flexibility to adjust the range of analyte concentration within which the sensor is responsive. For instance, a higher concentration PFPP within the polymer matrix may have responded to lower concentrations of cyanide. However, using higher concentrations of porphyrins may be conterproductive since they tend to aggregate more readily at higher concentrations. The remarkable success in controlling aggregation accomplished in this work, in part through structural modifications initiated by alcoholysis,suggests that overcoming aggregation at higher concentrations would not be an insurmountable task. It is also possible to select a probe molecule that is inherently stable. The kinetics and thermodynamics of anation reactions are affected by, among other things, the nature of the porphyrin macrocycle and the peripheral g r o ~ p s . ~Therefore, ~ - ~ ~ an (22) (a) Clark, W.M.;Pcrkins, M.E.J. Biol. Chem. 1940,135,643. (b) Cowgill, R. W.;Clark, W . M.J. Biol. Chem. 1952, 198, 33. (23) Hambright, P.; Chock, P. B. J. Inorg. Nucl. Chem. 1975, 37, 2363. (24) (a) Strycr, L.; Kendrcw, J. C.; Watson, H. C. J. Mol. Biol. 1964.8, 96. (b) Nobbs, C. L.;Watson, H.C.; Kendrcw, J. C. Narure(bndon)1966,209,339. (c) Muirhcad, H.; Green, J. N a m e (London) 1970, 228, 516.

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alternative to using higher concentrations of metalloporphyrins is to investigatethe possibilityof using a porphyrin macrocycle that would favor a higher extent of complexation at the same conditions. For a example, a weak donor porphyrin macrocycle would promote axial binding of donor ligands. However, it would have limited capability to stabilize the axial bond through back-bonding and therefore would not lead to higher extents of reaction. In contrast, a porphyrin macrocycle that has the ability to donate electrons to the empty ?r-orbitalsof the donor ligand would strengthen the axial bond, resulting in a higher extent of complexation. Therefore, the utilization of an alternate metalloporphyrin possessing favorable thermodynamic properties may lead to the development of a cyanide system with an improved sensitivity.

CONCLUSIONS Probe molecules having good solubility properties can be encapsulated within titanium carboxylate thin films such that they are accessible to reactive exogenous species. The metalloporphyrin-cyanide ion combination used here serves as an example for other probe-analyte systems. The control of aggregation of PFPP by ethanol-induced alcoholysis on

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modified titanium alkoxides manifests the versatility of the modification techniques available in the sol-gel process toward making functional sensor systems. The success of this work indicates that spectroscopic changes incurred by probe molecules encapsulated in sol-gel derived films can be conveniently calibrated to quantify unknown amounts of analyte. It is expected that this approach can be used on a number of relevant chemical systems.

ACKNOWLEDGMENT The authors acknowledge financial support from the Cooperative State Research Service (Grant 90-24189-5014) of the United States Department of Agriculture and the Crop and Food Bioprocessing Center/Research Excellence Fund at Michigan State University. Salary support for K.A.B. by the Michigan Agricultural Experiment Station is also acknowledged. Received for review February 14, 1994. Accepted May 20,

1994.B *Abstract published in Aduance ACS Absrracrs. July 1, 1994.