PAPER
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meso-Tetraarylporpholactones as high pH sensors†‡ Gamal E. Khalil,x*a Pedro Daddario,{b Kimberly S. F. Lau,{ab Sayed Imtiaz,a Michelle King,b Martin Gouterman,a Alexey Sidelev,c Narissa Puran,c Masoud Ghandeharic and Christian Br€uckner*b Received 20th January 2010, Accepted 16th May 2010 DOI: 10.1039/c0an00018c The ability of meso-tetra(pentafluorophenyl)porpholactone (TFPL) and its Pt(II) complex [meso-tetra(pentafluorophenyl)porpholactonato]Pt(II) (TFPLPt) to function as optical high pH sensors is described. Under strongly alkaline or high methoxide conditions, their UV-vis spectra undergo dramatic and reversible red-shifts. The dynamic range for the sensor TFPLPt in solution is from pH 11.5 to 13.2. Using 1H, 19F, and 13C NMR, UV-vis and IR spectroscopy, mass spectrometry, and the use of model compounds, the molecular origin of this optical shift is deduced to be a nucleophilic attack of OH/MeO on the lactone carbonyl of the chromophore, representing a novel mechanism for porphyrin-based sensors. The sensing compound was solubilized with Cremophor EL for use in aqueous solutions and embedded in polymer matrixes for testing as optical fiber-based optodes and planar sheet optode materials.
Introduction The use of pH monitors is crucial for many applications in chemistry, engineering, biomedicine, and environmental control. The use of glass pH electrodes is widely favored for its rapid response and wide dynamic range. However, for applications for which the pH range of interest lies deep in the alkaline region, the performance of glass electrodes deteriorates—the so-called ‘alkaline error’. Because of this, optical fiber-based sensors measuring the response in absorbance, reflectance, fluorescence, refractive index, etc. upon changes in pH have been developed as an alternative. Optical fiber sensors offer the advantage of not requiring a reference element, having a potential for miniaturization, low costs, and ease of long distance signal transmission.1 However, only few optical sensors for high pH values (>pH 9) have been reported.2–6 Also, many of the sensors developed hitherto show considerable spectral overlap between their spectra at lower and high pH values. Some also face the problem of chemical instability of the chromophore at extreme pH values.7,8 Porphyrins or, more generally, porphyrinoids, such as corrole or expanded porphyrins, have figured prominently as
a Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA. E-mail:
[email protected] b Department of Chemistry, University of Connecticut, Unit 3060, Storrs, CT, 06269-3060, USA. E-mail:
[email protected]; Fax: +1-860486-2981; Tel: +1-860-486-2743 c Department of Civil and Environmental Engineering, Polytechnic Institute of New York University, Six MetroTech Center, Brooklyn, NY, 11201, USA † Electronic supplementary information (ESI) available: Details of the alternative preparations of TFPL and TFPLPt, 13C, 1H, and 19F NMR spectra, IR spectra, etc. of these and comparison compounds in the absence and presence of base, UV-vis spectra of TFPL in different solvents and the presence of different bases, UV-vis TFPL acid titrations, etc. See DOI: 10.1039/c0an00018c ‡ Part 3 of the series Oxazolochlorins. Part 2: J. Akhigbe, C. Ryppa, M. Zeller and C. Br€ uckner, J. Org. Chem., 2009, 74, 4927–4933. x Current address: Department of Aeronautics & Astronautics, University of Washington, Box 352250, Seattle, WA 98195-2250, USA. { Contributed equally to this work.
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chromophores in sensing applications.9 The availability of a wide range of derivatives combined with their high extinction coefficients, generally large fluorescence yields, and high chemical stability makes their use particularly attractive.10 Porphyrinoids and their metal complexes have been used as optical chemosensors for a range of analytes.11,12 Porphyrinoids have also been employed as pH sensors.13–20 Most sensors track the change of the optical properties (UV-vis absorption and fluorescence emission) of these chromophores upon protonation or deprotonation of attached functional groups. Most are developed for the sensing of neutral to acidic pH values, with a dynamic range commonly spanning 1–4 pH units, as dictated by the pKa/pKb of the indicator, with notable exceptions being examples that use multiple coupled acid/base functionalities.13,20 It was recognized 25 years ago that the oxidation of certain b-substituted porphyrins can lead to the loss of one b-carbon and formation of porpholactone TPL (Scheme 1).21 In this little explored class of porphyrinoids, one b,b0 -bond is formally replaced by a lactone moiety. Irrespective of this replacement, the
Scheme 1 Syntheses and structures of porpholactones. Reaction conditions: (i) for Ar ¼ Ph: 3–4 step synthesis;22–25 (ii) for Ar ¼ C6F5: AgNO3 in refluxing acetic acid containing oxalate;21,32 (iii) for Ar ¼ C6F5 or Ph: 1. OsO4/pyridine, followed by chromatography33 2.6 equiv. cetyl(CH3)3NMnO4, CH2Cl2, rt, 0.5–6 h;34 (iv) PtCl2, PhCN, reflux, 2d.35
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optical properties of free base porpholactones remain very porphyrin-like.21,22 Several oxidative reaction pathways toward porpholactones were discovered or rationally developed over the years, and some porpholactones have found applications.22–29 For instance, the Pt(II) complex of the fluorinated porpholactone, TFPLPt, has been prominent as the O2-sensitive phosphorescent component of pressure sensitive paints,30–32 allowing the imaging of air flow around objects. In this contribution, we present the hitherto unrecognized halochromic properties of porpholactones. Specifically, we will introduce TFPL and its Pt(II) complex, TFPLPt, as the first optical porphyrin-based high pH sensors (pH range from 11.5 to, depending on their formulation, nearly 14). We will detail the molecular mechanism for the hydroxide- and alkoxide-induced spectral changes, and we will present preliminary studies toward the incorporation of these sensors into polymer matrixes for their use as optodes.
Experimental section Materials All solvents and reagents were used as received. Chremophor EL was purchased from Acros. meso-Tetraphenylporphyrin (TPP),33 [meso-tetraphenylporphyrinato]Pt(II) (TPPPt),34 mesotetra(pentafluorophenyl)porphyrin (TFPP),35 [meso-tetra(pentafluorophenyl)porphyrinato]Pt(II) (TFPPPt),35 meso-tetraphenylporpholactone (TPL),36 and [meso-tetraphenylporpholactonato]Pt(II) (TPLPt),37 [meso-tetraphenyl-2-hydroxyoxazolochlorin]Ag(II) (HO-TPOAg),29 and [meso-tetraphenyl-2-ethoxyoxazolochlorin]Ag(II) (EtO-TPOAg)29,38 were prepared as described in the literature. Free base meso-tetra(pentafluorophenyl)porpholactone (TFPL) and its Pt(II) complex TFPLPt were purchased from Frontier Science, Inc., Logan, UT, USA, and used after plate chromatographic purification, or were prepared by stepwise oxidation of TFPP/TFPPPt,36 followed by insertion of Pt(II) using standard conditions,34 respectively (see ESI†). Poly(methylmethacrylate)/methylenebis-acrylamidemethacrylamidopropyl trimethylammonium chloride copolymer (PMMA/ MAPTAC) (95 : 5 polymer ratio) was provided by the Polymer Research Institute at Polytechnic Institute of NYU. Analytical TLC plates (aluminium backed, silica gel, 250 mm thickness), preparative TLC plates (20 20 cm, glass backed, silica gel 60, 500 or 1000 mm thickness), and the flash column ˚ , 40–75 mm) used were provided silica gel (premium grade, 60 A by Sorbent Technologies, Atlanta, GA. VWR clear pH buffers from pH 7.0, pH 9.0, pH 11.0 and pH 13.0 were used and adjusted with NaOH, as needed. Instruments 1 H, 19F, and 13C NMR spectra were recorded on a Bruker DRX400 spectrometer in the solvents indicated. IR spectra were recorded on Perkins Elmer 1600 Series or Thermo Nicolet Nexus 670 FTIR spectrometers. UV-vis spectra were recorded either on a Hewlett-Packard 8452A Diode array or on a Cary 50 (Varian) spectrometer. For the optical fiber instrumental set-up, see below.
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Preparation of a PMMA/MAPTAC–TFPL-based planar optode A solution of PMMA/MAPTAC (100 mg) and TFPL (2 mg) in CH2Cl2 (4 mL) was prepared. A 100 mL aliquot of this solution was spin-coated onto a 1 2 cm transparent polycarbonate coupon, forming an optical clear red film. The dried film was fitted into an optical cell filled with aqueous buffer solutions. Fiber optical sensor TFPL (20 mg, 2.0 105 mol) was dissolved in 25% PMMA/ MAPTAC copolymer in chloroform (CHCl3, 4 mL). Titanium dioxide (1.0 g, TRONOX, grade CR-828, Lot. number: G8281334) was added to the mixture that was stirred for 1.5 h. Optical glass fiber (Thorlabs, Inc. BFL600, 600 mm silica core and coated by a fluoropolymer cladding) was cut into 30 cm sections. 0.25 cm of the fluoropolymer cladding was removed from the fiber tips, and the tips were polished (diamond paste) and subsequently rinsed with acetone. The fibers were dip-coated with the PMMA/MAPTAC/TFPL/TiO2/CHCl3 solution twice, with a 30 min drying time between coatings. This process was monitored for an even coating using a Bodelin Proscope Microscope (at 50 magnification). After fibers were coated, they were cured for 24 hours at 70 C and then hydrated for 1.5 h in distilled water. The optical fibers were coupled into an optical fiber beam splitter. Ocean Optics USB2000 UV-vis Spectrometer and an Ocean Optics LS1 White light source were used to collect spectral data (450–800 nm). Media changes were accomplished by pipetting media in and out of a test tube without disturbing the position of the probe.
Results and discussion Syntheses of TFPL and TFPLPt meso-Tetra(pentafluorophenyl)porpholactone (TFPL) and its Pt(II) complex TFPLPt are known and commercially available.22,39 Alternatively, a literature-known method for the synthesis of the non-fluorinated porpholactone TPL (b,b0 osmylation, followed by MnO4-induced oxidative diol cleavage of the diolchlorin osmate ester)36,40 can be applied toward the syntheses of the fluorinated analogues TFPL and TFPLPt (Scheme 1).41,42 This step-wise oxidation is high-yielding and the products are readily purified. Colorimetric base sensing in organic solvents Fig. 1 shows the UV-vis spectra of the red solutions of TFPL and TFPLPt in THF, exhibiting the spectral shape and high extinction coefficients typical of porphyrins and metalloporphyrins, respectively.43 The addition of strong bases (such as NaOMe, i Pr4NOH, or DBU) turns the solutions green, with the UV-vis spectra of both species undergoing dramatic changes. These changes are only to a minor degree dependent on the nature of the base (Fig. S21†). Addition of a slight stoichiometric excess of the base NaOMe (in MeOH) to a solution of free-base TFPL (in THF) causes a bathochromic shift of the entire spectrum. The Soret band broadens, shifts 28 nm (to 436 nm), and its extinction coefficient drops by 40%. The four side bands collapse and a new This journal is ª The Royal Society of Chemistry 2010
Fig. 1 UV-vis spectra (3 mL THF + 5% DMF, 25 C) of (A) TFPL and (B) TFPLPt before (solid trace) and after (dotted trace) addition of NaOMe/MeOH (50 mL 1.0 M NaOMe in MeOH).
(lmax centered at 677 nm) and unusually broad (half-height width of 50 nm) band of approximately twice the extinction coefficient dominates the Q-band region of the spectrum. Under the same base conditions, the Soret band of the Pt(II) complex TFPLPt is split (with a new peak arising at 429 nm) and reduced to approximately a third of its original intensity. Its two side bands merge and red-shift 124 nm to a lmax of 697 nm. The spectral changes are fully reversible upon neutralization. Most importantly, there is little spectral overlap between the spectra of TFPL or TFPLPt and the two species that form in the presence of strong base, respectively, making these halochromic reactions particularly useful for sensing purposes. Weaker bases such as pyridine, Et3N, ammonia, or imidazole in a variety of solvents elicit no spectral changes. The baseinduced shifts are also observed in other metal complexes of TFPL (e.g., the Ni(II), Zn(II) and Pd(II) complexes) than the Pt(II) complex, but since TFPLPt is well studied, stable, and emissive, we will focus the following discussion on the UV-vis spectra of this metalloporpholactone and its corresponding free base (for emission data of TFPL and TFPLPt under base conditions, see Table S2 and Fig. S23†).
upon addition of base (Fig. 2C) but its Pt(II) complex TPLPt shows an optical response to base (Fig. 2D) that is very similar to that observed for the fluorinated porpholactones (cf. to Fig. 1). This finding is suggestive of two different mechanisms that are at the origin of the observed spectral shifts for TFPP/TPL and TFPL/TPLPt/TFPLPt, and points at the role the lactone moiety has in the process. What are the chemical transformations of the lactone moiety that are at the origin of the observed base-induced shifts? Both the lactone moiety and the pentafluorophenyl group affect the acid–base properties of the chromophore. The concentrations of TFA required for half-protonation of TPP, TPL, TFPP, and TFPL in CH2Cl2 (determined by UV-vis spectroscopy, Fig. S22 and Table S1†) reveal that the introduction of the lactone as well as the pentafluorophenyl moieties each decrease the virtual pKb values of the inner imine groups of the chromophore by 2. The effects are additive, rendering TFPL approximately 105-times less basic/more acidic than TPP. The same electron-withdrawing effects that make the inner NH protons more acidic might be expected to also increase the susceptibility of the lactone carbonyl toward nucleophilic attack. A number of additional findings described below support these mechanistic interpretations. A 1H NMR titration of TFPLPt with DBU shows the clean conversion of one species to another, with no change in the number of b-protons (Fig. S12–S14, ESI†). The spectra show conclusively that, first, the loss of a b-proton is not the underlying mechanism for the base sensing ability of the pentafluorophenylporpholactones and, second, that TFPLPt converts to the new species in a single step. The latter is also supported by the sharp isosbestic points in the UV-vis titrations (Fig. 4, to be discussed in detail below). This base-induced broadening of the chemical shift range is also prominent in the 13C and 19F NMR spectra of TFPLPt (Fig. S10 and S9, ESI†). Most significantly, the diagnostic signal for the lactone carbonyl carbon at 163.2 ppm vanishes upon addition of base. In fact, the up-field shifted and spread 13C NMR spectrum of the base-induced species is similar to the spectrum observed for the known compound HO-TPO or its Zn(II) complex.29,38,46,47 In HO-TPO, the sp2-hybridized C]O carbon of TPL is converted to a OH-substituted sp3-hybridized carbon. Note that a nucleophilic attack of OH/MeO onto the carbonyl group of TFPLPt induces a similar sp2-to-sp3 conversion.
Mechanistic considerations Particularly electron-poor free base porphyrins are readily deprotonated with DBU, whereupon they show altered optical spectra.44,45 The NH groups of TPP (Fig. S15†) and TFPP are deprotonated in THF by MeO (Fig. 2A), displaying distinct changes of their UV-vis (20 nm red-shift of the Soret band, reduction of the number of the Q-bands and blue-shift of lmax). Their Pt(II) complexes TPPPt (Fig. S15†) and TFPPPt (Fig. 2B) contain no NH protons and, therefore, exhibit no optical changes under the same conditions. Interestingly, the non-fluorinated porpholactone TPL also shows the spectral changes characteristic of NH-deprotonation This journal is ª The Royal Society of Chemistry 2010
The 19F NMR spectrum of TFPLPt shows three groups of signals, corresponding to the ortho-, meta-, and para-fluorine atoms of all four, non-equivalent pentafluorophenyl groups (Fig. S9†). Addition of base broadens the chemical shift range slightly but, most importantly, it also differentiates each signal group in a fashion consistent with a face-differentiation of the porphyrin. This face differentiation into an upper (same side as the hydroxy group) and lower (opposite) face is also Analyst, 2010, 135, 2125–2131 | 2127
Fig. 2 UV-vis spectra (3 mL THF + 5% DMF, 25 C) of (A) TFPP, (B) TFPPPt, (C) TPL, and (D) TPLPt before (solid trace) and after (dotted trace) addition of stoichiometric excess of NaOMe/MeOH (50 mL 1.0 M NaOMe in MeOH). [Porphyrin] in the range of 106 M.
characteristic for oxazolochlorin HO-TPO and other chlorins containing a sp3-hybridized b-carbon, such as tetraaryl-cis-2,3diolchlorins.29,40 Therefore, all NMR spectroscopic evidence is consistent with a nucleophilic addition of OH or RO to the carbonyl carbon (Scheme 2; a comparison of the 13C NMR of TPLZn and HO-TPOZn are shown in Fig. S11†). The conversion of the lactone carbonyl upon reaction with base is also detectable by IR spectroscopy. The distinct carbonyl stretch (nC]O) at 1778 cm1 in the IR of TFPLPt is lost upon addition of NaOMe (Fig. S18 and 19†). A pH-dependent ringopening is commonly observed for lactone-containing chromophores, such as for eosin and fluorescein. However, here we do not expect the ring-opened form to contribute to any significant degree (Scheme 2). This is because of the rigid porphyrinic ring structure and the absence of any extended resonance structure that could be the driving force for such an opening. Correspondingly, there is no evidence of the presence of any carboxylate/carbonyl functionality in the IR of the base reaction product of TFPLPt and NaOMe. Molecular modeling of the anionic species resulting of any ˚ of porpholactone with OH/RO suggests a distance of 3.38 A the anionic oxygen to the center of the adjacent pentafluorophenyl group. This proximity makes a stabilizing anion–p interaction possible,48 though we cannot prove its existence (Scheme 2). Mass spectrometric investigations of the reaction product of TFPLPt with NaOMe by ESI() show a prominent molecular peak that corresponds to the methoxide addition product [TFPLPt$OMe] (Fig. S20†). As our investigations refined our understanding of the nucleophilic addition of OH/MeO to porpholactones, we also began to comprehend our results using DBU as a base. DBU is, with rare exceptions,49 a non-nucleophilic base and, therefore, unlikely to be directly attacking the carbonyl group. However, it can act as a base by abstraction of H+ from water (from the wet solvents), generating OH. The latter interpretation is also supported by the ESI() mass spectra 2128 | Analyst, 2010, 135, 2125–2131
of a mixture of DBU and TFPL, i.e. the hydroxide and DBU adduct to TFPLPt. Other fragmentation peaks in the spectrum are illustrative of the stability of the hydroxide adduct [TFPLPt$OH]. The sum of the evidence conclusively shows that the nucleophilic addition of OH/MeO to the lactone carbonyl is the
Scheme 2 Nucleophilic attack of OH onto TFPLPt, highlighting the face differentiation brought about by this reaction, and the proximity of the hemiacetal oxide to the center of the flanking pentafluorophenyl group.
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underlying mechanism for the observed optical changes. Why, then, is there an apparent shift of mechanism when considering the spectral changes of TPL and TFPL? Evidently, the presence of the pentafluorophenyl group makes the lactone carbonyl more susceptible to nucleophilic attack than it increases the acidity of the NH group (cf. Fig. 2A and C). Are, in turn, the observed shifts of the UV-vis spectra of the porpholactones in the presence of base commensurate with expectations? This question can be answered affirmatively. Iterative extended H€ uckel calculations have shown that the oxazolone ring is fully conjugated into the porphyrinoid HOMO and LUMO p system, mimicking a regular porphyrin chromophore.22 Direct or formal reduction of the lactone to an hemiacetal functionality (in HO-TPO) establishes a chlorin-type chromophore.29,38,47 It is also well known that charge transfer perturbations caused by, for instance, protonation of a substituent attached to a porphyrin can lead to large spectral changes of the porphyrinic chromophore.50 Thus, the change of hybridization of the lactone carbonyl group by nucleophilic addition of OH/MeO with the concomitant build-up of an anionic charge at the benzylic porphyrinoid position readily rationalizes the observed bathochromic shifts. In fact, the Ag(II) complex of HO-TPO, HOTPOAg, in THF treated with NaOMe also shows dramatic shifts in its UV-vis spectrum that are reminiscent of those observed upon reaction of the porpholactones with this base (cf. Fig. 3A to Fig. 1A), though the presence of the pentafluorophenyl groups (or the Pt ion) in TFPLPt seem to amplify the effect. Considering the increase of acidity and the inverse polarization of the aryl group of the pentafluorophenyl-substituted systems and the higher electronegativity of Pt, this is not surprising. Inversely, when the hemiacetal hydroxy group is alkylated, as in EtOTPOAg, no change in the UV-vis spectrum is observed upon addition of base (Fig. 3B; see also Fig. S16† for results using the corresponding free bases and Zn(II) complexes of HO-TPO). Thus, the red-shifting effect of the anionic charge of the hydroxy
Fig. 3 UV-vis spectra (THF) of HO-TPOAg (A) and EtO-TPOAg (B) before (solid trace) and after addition of NaOMe/MeOH (dotted trace).
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group attached to a sp3-b-carbon of the chromophore is demonstrated. Base sensing in aqueous systems Neither TFPL nor TFPLPt is water-soluble but a solution of these compounds in DMF containing Cremophor EL (a synthetic, nonionic polyethoxylated castor oil-based surfactant) can be diluted with water to form stable emulsions that allow their pH titrations. Fig. 4 shows the result of a pH titration of TFPLPt. Most significantly, it shows an approximately 50-fold increase of absorbance at 708 nm in the pH range from 11.45 to 13.18, with sharp isosbestic points. A distinct hysteresis of the UV-vis/fluorescence response to the down- and upwards changing pH is notable (Fig. S17†). This is an indication for a slower rate of the reverse reaction of the nucleophile addition to the lactone. An effective pKa value of 12.6 can be computed for this indicator. However, since the physical basis for the operation of the indicator is not based on its Brønsted acidity but on the susceptibility of its lactone carbonyl group toward nucleophilic attack, this value cannot be directly compared to that of classic pH sensors. Toward optodes—aqueous base sensing with the sensors embedded in polymer matrixes Embedding the sensor dye into a polymer allows the construction of optodes and optical fiber-based sensors.51 In an attempt to evaluate the potential of TFPL as an optical sensor for high pH measurements, it was embedded into an ion permeable poly(methyl methacrylate)/methylenebis-acrylamide-methacrylamidopropyl trimethylammonium chloride copolymer (PMMA/ MAPTAC) film and spin-coated onto polycarbonate sheets.52 Fig. 5 shows the absorption spectra of the sensor film as a function of pH. The emergence of the base form of the sensor is visible from pH values of 10 upwards, with significant changes between pH 12 and 14. Not all chromophores present were, however, converted to the base form, likely as a function of the film thickness and slow OH permeability. We note a slight decrease in the apparent pKb value of TFPL when compared to the solution state data, an effect we can attribute to the polymer matrix.53 A sensor bead of TFPLPt, embedded in a PMMA/MAPTAC copolymer and mixed with alumina particles for better light
Fig. 4 UV-vis spectra (3 mL water + 5% Cremophor EL + 5% DMF, 25 C) of TFPLPt (2.4 106 M) calibrated with 0.1–5 M NaOH solution to the pH recorded.
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under aerobic conditions reduced the absorbance intensity by less than 3%, whereby a regular porphyrin or chlorin might be degraded under the same conditions by as much as 30%. We have also not found any base-induced degradation and TFPLPt/base solutions were stable over weeks. TFPLPt-based optodes were stable over months and could be reused repeatedly without any degradation in performance.
Summary and conclusions Fig. 5 UV-vis of TFPL embedded in a spin-coated layer of PMMA/ MAPTAC on glass and immersed in pH buffers indicated. Measurements after 5 min equilibration time at 25 C.
We have introduced porpholactone-based chemosensors for OH and MeO (and presumably also other alkoxides) that are endowed with a number of favorable chemical and physical properties: availability, high chemical stability, an absorption in the red and NIR region, high extinction coefficients, likely tunability by modification of their meso-aryl groups, a clear separation of the spectra of the neutral and base-sensing species, and a dynamic range in the unusual high pH range from, depending on the particular sensor and its formulation, 10 to 14. We are currently testing the luminescence properties of these, and related, chromophores, and their utility in technical applications.
Acknowledgements
Fig. 6 History plot of TFPLPt embedded in a 25% PMMA/MAPTAC copolymer bead containing alumina particles attached to the tip of an optical fiber submersed in the pH buffers indicated.
reflectance, can also be attached to the end of an optical fiber. This optical sensor is suitable for pH sensing much as shown for the planar optode. The response of the optical fiber pH sensor to a drastic pH change from pH 7 to 13 and then back to pH 9 in two steps is shown in Fig. 6. The initial steep pH gradient elicits the expected response (intensity increase of the band at 710 nm) and reaches within 30 min the near-maximum absorbance. This relatively slow response is a function of the polymer matrix as the nucleophilic addition reaction in solution is completed upon mixing of the reagents. The interference of the pH measurement by a complex and high ionic strength aqueous medium was tested by immersing the fibre optic sensor into a pH 13 Portland cement slurry and allowing the paste to harden. The fluorescence response of an analogous optode in a pH 13 buffer was also recorded. A comparison of the resulting spectra shows only minor differences (Fig. S24 and S25†), thus indicating that the complex medium did not perturb the optical properties of the sensor. Chromophore stability In our previous work on TFPLPt-based oxygen sensors,30–32 the pentafluorophenyl-substituted chromophore TFPLPt distinguishes itself through a much larger photo-stability than the corresponding tetraphenyl- or even pentafluorophenyl-porphyrin. This is due to the electron-withdrawing effects of the meso-aryl group but also because the porpholactones are already formally in a higher oxidation state than porphyrins. Indeed, constant irradiation of a film containing TFPLPt over 60 min 2130 | Analyst, 2010, 135, 2125–2131
This work was also supported by the Department of Defense Multi-Disciplinary University Research Initiative (MURI) Center on Polymeric Smart Skin Materials through the Air Force Office of Scientific Research (F49620-01-1-0364 to GK). CB thanks the National Science Foundation (CHE-0517782 and CMMI-0730826) and the University of Connecticut Research Foundation (IFP-090128) for financial support. MK was supported through an NSF-REU summer research fellowship (NSF CHE-0754580).
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