7852
Langmuir 2000, 16, 7852-7858
Linker-Enhanced Binding of Metalloporphyrins to Cadmium Selenide and Implications for Oxygen Detection Albena Ivanisevic and Arthur B. Ellis* Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin 53706
Gonen Ashkenasy and Abraham Shanzer* Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
Yossi Rosenwaks Department of Physical Electronics, Tel-Aviv University, Ramat-Aviv 69978, Israel Received April 19, 2000. In Final Form: June 20, 2000 Adsorption of trivalent metalloporphyrins from nitrogen-saturated chloroform solution onto etched n-CdSe crystals causes a profound reversible quenching of the semiconductor’s photoluminescence (PL). The PL responses due to the presence of MIIIDMPPCl and MIIITPPCl (DMPPCl ) protoporphyrin IX dimethyl ester chloride; TPP ) tetraphenylporphyrin; M ) Fe, Mn) exhibit a concentration dependence that can be fit to the Langmuir adsorption isotherm model to yield binding constants of 104-105 M-1. The CdSe surface may be modified by adsorption from solution of specifically designed linker ligands (1-4). These ligands are able to bind to the semiconductor surface through one end and to ligate a heme analogue axially on the other end. Surfaces derivatized by each of the linkers showed concentration-dependent metalloporphyrininduced PL changes, corresponding to roughly order-of-magnitude increases in binding constants to 105 to 106 M-1. Films of linker-metalloporphyrin complexes were coated onto the semiconductor substrates and characterized by X-ray photoelectron (XPS) spectroscopy. The linker-metalloporphyrin films can be used as transducers for dioxygen detection. Relative to a nitrogen ambient, the PL of CdSe samples coated with 1-3 is reversibly quenched by exposure to oxygen (binding constants of ∼1-10 atm-1; detection limit of ∼0.1 atm), while bare CdSe surfaces show no response to dioxygen. These coated CdSe samples were further characterized by contact potential difference (CPD) and time-resolved photoluminescence (TRPL), which suggest that oxygen-induced PL changes are due to variations in the electric field present in the semiconductor substrate.
Introduction The ability to control the local environment and adsorption properties of solid surfaces is a subject of intense investigation.1 The development of successful strategies for adsorption has led to the fabrication of various sensor devices that take advantage of the interaction between analytes and adsorbents.2 Methods utilized to tailor surfaces for sensing include direct adsorption of compounds onto substrates, electrochemical deposition, and the formation of a variety of well-ordered monolayers.3,4 Metalloporphyrins (MPs) and related complexes show promise as a class of compounds that can be used to alter the binding properties of solid supports.5,6 The rich redox and coordination properties that MPs provide as adsorbates permit substantial flexibility in customizing surface binding characteristics.7 (1) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (2) Janata, J.; Josowicz, M.; Vanysek, P.; Devaney, D. M. Anal. Chem. 1998, 70, R179. (3) Ricco, A. J.; Crooks, R. M.; Osbourn, G. C. Acc. Chem. Res. 1998, 31, 1, 289. (4) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (5) Offord, D. A.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478. (6) Katz, E.; Heleg-Shabtai, V.; Willner, I.; Rau, H. K.; Haehnel, W. Angew. Chem., Int. Ed. Engl. 1998, 37, 3253. (7) Bertini, I.; Gray, H. B.; Lippard, S. J.; Valentine, J. S. Bioinorganic Chemistry; University Science Books: Sausalito, CA, 1994.
Previously, we and others have used semiconductor substrates such as CdS and CdSe to demonstrate that MPs can bind strongly to the surfaces of such substrates and that their adsorption properties can profoundly be influenced by the presence of small gaseous molecules such as dioxygen and nitric oxide.8-13 The primary tool we have used to track adsorption is the band-edge photoluminescence (PL) intensity of single-crystal II-VI semiconductor substrates.14 We have reported that the PL of the emissive semiconductor can be reversibly altered by adsorption of MPs from solution or by binding gaseous analytes to MP film transducers used to coat the semiconductor. Recently, we used PL changes to show that adsorption of linker ligands 1 and 3 (Chart 1A) onto CdSe can enhance the binding of FeDMPPCl (Chart 1B; DMPPCl is protoporphyrin IX dimethyl ester chloride) from chloroform solution by roughly 1 order of magnitude.15 These linkers (8) Chrysochoos, J. J. Phys. Chem. 1992, 96, 2868. (9) Bhamro; Chrysochoos, J. J. Lumin. 1994, 60-61, 359. (10) Isarov, A. V.; Chrysochoos, J. Langmuir 1997, 13, 3142. (11) Isarov, A. V.; Chrysochoos, J. Proc. Indian Acad. Sci., Chem. Sci. 1998, 110, 277. (12) Ivanisevic, A.; Ellis, A. B. J. Phys. Chem. B 1999, 103, 1914. (13) Ivanisevic, A.; Reynolds, M. F.; Burstyn, J. N.; Ellis, A. B. J. Am. Chem. Soc. 2000, 122, 3731. (14) Ellis, A. B.; Brainard, R. J.; Kepler, K. D.; Moore, D. E.; Winder, E. J.; Kuech, T. F.; Lisensky, G. C. J. Chem. Educ. 1997, 74, 680. (15) Ashkenasy, G.; Ivanisevic, A.; Cohen, R.; Felder, C. E.; Cahen, D.; Ellis, A. B.; Shanzer, A. J. Am. Chem. Soc. 2000, 122, 1116.
10.1021/la000580d CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000
Binding of Metalloporphyrins to CdSe Chart 1
bind strongly to the CdSe surface through their disulfide group, and their arms possess imidazolyl groups capable of axially binding heme analogues and other MPs. Due to this ligation by pairs of imidazolyl groups, the MPs are oriented, on average, perpendicular to the surface and are spatially isolated from adjacent molecules. The binding enhancement was obtained either in two steps, by first adsorbing the linker molecule and then binding the MP or by preforming the 1:1 linker:MP complex in solution and then adsorbing it onto CdSe. Films of the linker:MP complex prepared by either method on CdSe served as transducers for gaseous oxygen detection: reversible oxygen-induced PL quenching of the CdSe substrate was observed with the coated surface relative to nitrogen, while no effect was seen with bare CdSe surfaces. In this paper we extend this methodology to two additional linker molecules, 2 and 4 (Chart 1A), into which variations in the imidazolyl or in the surface binding groups were introduced. In addition, we expand our use of MPs to MnDMPPCl and to the synthetic TPPs (tetraphenylporphyrins), FeTPPCl, and MnTPPCl (Chart 1B). By an extensive surface study, with PL again as the primary tool, we demonstrate that in all cases at least 1 order-of-magnitude binding enhancement of the MP to the CdSe surface is observed relative to the linker-less system. When deposited as films, most of these systems yield reversible PL quenching by oxygen relative to the PL intensity in a nitrogen ambient. Experimental Section Materials. Samples of FeTPPCl and MnTPPCl were purchased from Porphyrin Products, Logan, UT, and used without further purification. MnDMPPCl and FeDMPPCl were synthesized in our laboratories.15 Oxygen gas was obtained from Liquid Carbonic Specialty Gas Corp. and used as received. Chloroform for the PL experiments (Aldrich, 99+%) was analytical grade and used without further purification. Toluene (Fisher) was distilled over Na/benzophenone before being used in PL and XPS
Langmuir, Vol. 16, No. 20, 2000 7853 experiments. Chloroform, methylene chloride, and DMF for the UV/vis titrations were all of spectroscopic grade (Merck) and used as obtained. Single-crystal, vapor-grown c-plates of n-CdSe, having a resistivity of ∼2 Ω‚cm, were obtained from Cleveland Crystals, Inc. Prior to PL, CPD, XPS, or TRPL experiments (vide infra) the crystals were etched in Br2/MeOH (1:15 v/v), allowing the shiny Cd-rich (0001) face to be revealed and later monitored in each experiment. Each solution used was degassed with dry nitrogen prior to conducting the PL experiment. Linker Syntheses. The synthesis of ligands 1-3 was previously reported.15,16 Synthesis of ligand 4 proceeds as follows (see Scheme 1): Synthesis of I. An acetonitrile solution of N-Cbz-(L)-alanine (2.07 g, 9.3 mmol), 1-(3-aminopropyl)-imidazole (1.22 mL, 10.2 mmol), and 4-(dimethylamino)pyridine (0.34 g, 2.8 mmol) was stirred at 0 °C. N,N’-Diisopropylcarbodiimide (DICD, 1.82 mL, 11.6 mmol) and 1-hydroxybenzotriazole (ca. 1.0 mmol) were added, and the reaction was stirred overnight at room temperature. The mixture was concentrated and purified by flash chromatography (silica gel chloroform:methanol, 9:1) to yield 2.42 g (80%) of the Cbz-protected I. Deprotection of the amine was achieved by hydrogenolysis in ethanol to yield 1.42 g (99%) of I. I: 1H NMR (CDCl3) δ 7.50 (s, 1H, Im), 7.40 (br, 1H, NHCO), 7.07 (s, 1H, Im), 6.95 (s, 1H, Im), 4.00 (t, J ) 6.0 Hz, 2H, CH2-Im), 3.49 (q, J ) 7.0 Hz, 1H, CH-R), 3.31 (dt, 2H, CH2NHCO), 2.02 (qui, J ) 7.0 Hz, 2H, CH2CH2-Im), 1.33 (d, J ) 8.0 Hz, 3H, CH3-CR). Synthesis of II. A solution of di-tert-butyl malonate (3.2 g, 14.8 mmol) in THF (20 mL) was treated dropwise with sodium hydride (0.65 g, 16.2 mmol) and after 10 min with methyl 3-bromopropionate (1.77 g, 16.2 mmol). After 1 h at room temperature, the reaction mixture was treated again with NaH and the propionate in the same manner and stirred for 12 h. Hexane (250 mL) was added, and the solution was washed with water (3 × 20 mL). The mixture was concentrated and purified by flash chromatography (silica gel; hexane:chloroform:ethyl acetate, 7.5:1.5:1) to yield 2.8 g (50%) of the dimethyl ester derivative. Subsequently, 1.7 g (4.4 mmol) was hydrolyzed using a conventional procedure, by 1 N NaOH in methanol, to yield 1.55 g of the di-tert-butyl malonate elongated by diacid, II. II: IR (CHCl3) ν 1715 cm-1 (COOH and COO-t-Bu); 1H NMR (CDCl3) δ 2.36 (t, J ) 7.0 Hz, 4H, CH2COOMe), 2.13 (t, J ) 7.0 Hz, 4H, (CCH2)2), 1.45 (s, 18H, t-Bu). Synthesis of 4. A THF solution of I (0.52 g, 2.65 mmol) and II (0.43 g, 1.2 mmol) was stirred at 0 °C. N,N’-Diisopropylcarbodiimide (0.46 mL, 3.0 mmol) and 1-hydroxybenzotriazole (90 mg, 0.6 mmol) were added, and the reaction was stirred overnight at room temperature. The mixture was concentrated and purified by flash chromatography on silica gel (chloroform:methanol, 8:2) to yield 0.5 g (58%) of III. Removal of the di-tert-butyl protection of the malonate was done by stirring III for 2 h in 20% TFA in methylene chloride. Crystallization from a methanol:ether mixture produced the final product 4. III: 1H NMR (CDCl3) δ 7.90 (t, 2H, NH-CH2), 7.82 (d, J ) 6.0 Hz, 2H, NH-CR), 7.60 (s, 2H, Im), 7.03 (s, 2H, Im), 6.95 (s, 2H, Im), 4.42 (qui, J ) 7.0 Hz, 2H,CH-R), 4.01 (m, 4H, CH2-Im), 3.28 (m, 2H, CH2NHCO), 3.08 (m, 2H, CH2NHCO), 1.80-2.10 (m, 12H, CH2CH2-Im + CH2NHCO + C(CH2)2), 1.42 (s, 18H, t-Bu), 1.37 (d, J ) 8.0 Hz, 6H, CH3-CR). 4: 1H NMR (CD3OD) δ 8.20 (s, 2H, Im), 7.30 (s, 2H, Im), 7.10 (s, 2H, Im), 4.12 (br, 6H, CH2-Im + CH-R), 3.19 (t, J ) 7.0 Hz, 4H, CH2NHCO), 2.19 (t, 4H, CH2CONH), 2.12 (t, 4H, C(CH2)2), 1.99 (qui, J ) 7.0 Hz, 4H, CH2CH2-Im), 1.30 (d, J ) 8.0 Hz, 6H, CH3-CR); FAB MS m/e 604.8 (M + H+). Titrations. Complexes of 1:1 ligand-to-metalloporphyrin (L: MP, e.g., 1:MP) or, equivalently here, 2:1 imidazole-to-metalloporphyrin (Im:MP) stoichiometry were characterized by 1H NMR and by UV/vis measurements. 1H NMR spectra (Bruker AMX-400) of the complexes, of either FeTPPCl or FeDMPPCl with the linkers, were made at variable temperatures, in chloroform (complexes of 1) or in chloroform/methanol (complexes of 2-4) solutions. The MPs (10-5-10-4 M) were also titrated with an excess of the symmetric ligands 1, 2, or 4, at room temperature, in DMF, chloroform, or methylene chloride. After the samples reached equilibrium, the changes in the Q-bands of (16) Ashkenasy, G.; Kalyuzhny, G.; Libman, J.; Rubinstein, I.; Shanzer, A. Angew. Chem., Int. Ed. Engl. 1999, 38, 1257.
7854
Langmuir, Vol. 16, No. 20, 2000
Ivanisevic et al. Scheme 1
the UV/vis absorption spectra were followed to calculate binding constants of the bis(imidazolyl) binding sites to the central metal ions.17 There is substantial overlap between spectra of the linkerto-metalloporphyrin complexes and the free MPs, but the spectral changes are consistent with the 2:1 Im:MP (1:1 L:MP) stoichiometry. Film Adsorption. All films were prepared from chloroform MP solutions. Films of various MP and linker combinations in Chart 1 were prepared by two previously reported methods.15,16 In the two-step method, adsorption of linker from 3 mM chloroform (1 and 2) or methanol (3 and 4) is followed by removal of the derivatized CdSe sample and subsequent adsorption of MP from 3 mM chloroform solution. In the preformed method, adsorption occurs from 15 mM 1:1 linker:MP chloroform solutions (1 and 2) or 9:1 chloroform:methanol solutions (3 and 4).15,16 All films were prepared in a nitrogen-filled glovebox and stored under nitrogen prior to use in the PL experiment. Steady-State Photoluminescence (PL). A Melles Griot He-Ne laser (633 nm) provided excitation of the semiconductor. Incident intensities ranged from 5 to 20 mW/cm2. Emission spectra were monitored using an Oriel Instaspec II silicon diode array spectrophotometer or Oriel Instaspec IV CCD spectrophotometer. A cutoff filter was used in the spectrophotometer to permit collection of the band-edge PL at λmax ∼720 nm (Eg ∼1.7 eV). The entire PL spectrum was monitored, while tracking the PL intensity at a particular wavelength, typically the band maximum, as a function of time; the band maximum did not shift under the low-resolution (0.5 nm) spectral conditions employed, and stable PL signals were typically obtained within a few minutes. The signal collected was analyzed utilizing InstaSpec1.1 for Windows 95. A gas flow apparatus was assembled using Tygon tubing, thus allowing mixtures of nitrogen and oxygen to flow over the semiconductor surface while the solid was illuminated.18 Flow meters were used to introduce variable percentages of N2 and O2 into the sample cell. Partial pressures of the gases were controlled by adjusting the flow rates of the incoming gases. The total gas flow rates varied from 80 to 250 mL/min, and the total gas pressure of 1 atm was maintained throughout the duration of the experiment. The sample of CdSe was mounted on a glass rod between two Teflon rings within a glass cell. The cell was equipped with a sidearm, through which either solutions or gases could be introduced without disturbing the optical alignment.12 Time-Resolved Photoluminescence (TRPL). The TRPL measurements were performed on film-coated CdSe surfaces in air using the time-correlated single photon counting technique. The excitation source was a continuous wave mode-locked Nd: YAG pumped dye laser at a wavelength of 617 nm. The detection wavelength was 720 nm, and the overall instrument response (17) (a) Walker, F. A.; Lo, M.-W.; Ree, M. T. J. Am. Chem. Soc. 1976, 98, 5552. (b) Kelley, S. L.; Kadish, K. M. Inorg. Chem. 1982, 21, 3631. (c) Hasegawa, E.; Kaneda, M.; Nemoto, J.; Tsuchida, E. J. Inorg. Nucl. Chem. 1978, 40, 1241. (d) Ashkenasy, G. M.Sc. Thesis, Weizmann Institute of Science, 1997. (18) Brainard, R. J.; Ellis, A. B. J. Phys. Chem. B 1997, 101, 2533.
(full width at half-maximum) was about 40 ps. Additional details of the experimental setup have been described.19 Contact Potential Difference (CPD) Measurements. CPD data were acquired on film-coated CdSe surfaces both in air and in dry nitrogen ambients using a Kelvin probe apparatus (DeltaPhi Elektronik). The method has been previously described and its principles summarized elsewhere.20 The CPD is defined as the work function difference between the sample surface and an inert reference electrode. In this case the reference electrode was made of Au with known work function of 5.1 eV.21 X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS data were collected on film-coated CdSe samples prepared with Fe MPs using a Perkin-Elmer PHI 5400 ESCA system with a Mg KR X-ray source powered at 300 W. The aperture size was 1 mm × 3 mm and the pass energy was 89.45 eV. The data acquisition time for each run was 4.17 min. Samples were rinsed briefly with either chloroform or toluene to remove uncomplexed MP prior to conducting the XPS experiments. Additional rinsing with toluene was performed with FeDMPPClcontaining films to demonstrate, by XPS, complete removal of the MP. These samples were also subsequently reexposed to a chloroform solution of FeDMPPCl (followed by brief rinsing with chloroform) to demonstrate readsorption. Electronic Spectroscopy. The air-saturated solution electronic spectrum of each MP, in chloroform or chloroform/methanol mixtures and with and without linker, was obtained using a Hewlett-Packard HP89530A spectrophotometer at room temperature between 300 and 800 nm. The spectra of all compounds of this study had negligible absorbance at the 633 nm excitation and 720 nm emission wavelengths.
Results and Discussion Linker molecules 1-4 provide a range of MP binding platforms. Linkers 1 and 2 should hold the MP at distances of ∼14 and 12 Å from the disulfide anchoring groups, respectively.16 The asymmetry of linker 3 could in principle permit easier detachment of an imidazole ligand that could be advantageous for oxygen binding. In 4, the anchoring groups are changed to be dicarboxylic acids, which have previously been shown to bind strongly to semiconductor surfaces.22 The quartet of MPs selected for the study provide variation in the metal center and porphyrin ring substituents. In sections below we describe UV/vis and 1H NMR analysis of the complexation process of MPs and the linkers (19) Poles, E.; Huppert, D.; Rosenwaks, Y. Semicond. Sci. Technol. 1997, 12, 1252. (20) Bruening, M.; Moons, E.; Yaron-Marcovich, D.; Cahen, D.; Libman, J.; Shanzer, A. J. Am. Chem. Soc. 1994, 116, 2972. (21) Cohen, R.; Kronik, L.; Shanzer, A.; Cahen, D.; Liu, A.; Rosenwaks, Y.; Lorenz, J. K.; Ellis, A. B. J. Am. Chem. Soc. 1999, 121, 10545. (22) Vilan, A.; Ussyshkin, R.; Gartsman, K.; Cahen, D.; Naaman, R.; Shanzer, A. J. Phys. Chem. B 1998, 102, 3307.
Binding of Metalloporphyrins to CdSe
Langmuir, Vol. 16, No. 20, 2000 7855
Table 1. Formation Constants for Linker-MP Complexesa MP
linker
solvent
β2 × 10-4 (M-2)b
FeTPPCl FeTPPCl FeTPPCl MnTPPCl FeDMPPCl
1 2 4 1 1
DMF DMF DMF CH2Cl2 CHCl3
1.1c 2.2c 1.5 0.18 2.4
a Equilibrium binding constants for complexation of the indicated metalloporphyrin (MP) and linker in the indicated solvent, obtained at room temperature by monitoring the Q-band of the metalloporphyrin as the solution is titrated with the linker molecule. b Binding constant as defined by eq 1 in the text, with estimated errors of (10%. c Reference 16.
in solution. PL solution studies are then presented for both bare and linker-derivatized CdSe surfaces. In the final section we describe linker-MP films, including characterization by XPS and by their PL response to oxygen. We present complementary characterization of these sensor platforms using time-resolved PL (TRPL) and contact potential difference (CPD) measurements. I. Solution Studies. Formation of the Linker-Metalloporphyrin Complexes. Complexes of the desired 1:1, ligand-to-metalloporphyrin (L:MP) stoichiometry were characterized in DMF, chloroform, or methylene chloride solution by 1H NMR and UV/vis spectroscopy measurements. 1H NMR measurements yielded spectra that belong to only a single species for each low-spin complex of the FeIII-porphyrins, FeTPPCl or FeDMPPCl, with ligands 1-4. Disappearance of the high-spin chemical shifts, e.g., the TPP pyrrole protons at 80-110 ppm, was observed in all spectra upon mixing equimolar solutions at concentrations corresponding to complete complexation (>20 mM). The measurements were done at various temperatures from -100 to +50 °C. All the proton chemical shifts could be assigned by following the changes with temperature in the spectra of FeTPPCl complexes with 1, 2, and 4, because of simplification due to their C2 symmetry structures. Complexes of FeDMPPCl were compared to previously reported systems in order to resolve the chemical shifts in the high (10 ppm) field spectra.23 Interestingly, one can also assign the peaks of protons in the vicinity of the paramagnetic center for the spectrum of the nonsymmetric complex 3-FeTPPCl. Different chemical shifts were observed for the two C2-imidazole protons (-7.8 and -15.6 ppm, DMF-d7, 240 K) and for the CH2-imidazole protons (17.9 and 19.2 ppm), which are located on opposite arms of the ligand. This shows that the binding of the N-alkylimidazolyl is different from that of the NH-imidazolyl in the complex. Titrations of MPs with the symmetric ligands 1, 2, and 4 were monitored by UV-vis spectroscopy at room temperature, using changes in the Q-bands to calculate binding constants β2 of the bis(imidazolyl) binding sites to the central metal ions, corresponding to the following representative reaction:17
Figure 1. Changes in the PL intensity of a n-CdSe sample resulting from exposure to MnTPPCl in N2-saturated CHCl3 for (A) an etched surface and (B) a surface derivatized with 4.
This equation reflects the spectrophotometrically derived stoichiometry that both imidazoles (Im) of a single linker molecule L bind to the MP. The results are shown in Table 1, and all the values fall in the range of 103-104 M-2. Due to the low solubility of MnTPPCl and of 1-FeDMPPCl in DMF, the titrations of these systems were done in
chlorinated solvents. The larger binding constant of 2 for FeTPPCl relative to 1 suggests a modest preference for NH-imidazolyls as metal binding sites. PL Results with Bare Surfaces. All PL measurements recorded in this study were done at room temperature using the (0001) surface of etched n-type CdSe crystals. The intensity of the CdSe band-edge PL at 720 nm was monitored throughout each experiment and was excited by ultra-band-gap 633 nm light, at which wavelength the MP, linker, and MP-linker adducts have minimal absorption. A representative PL trace for the adsorption of MP onto the etched CdSe surface is shown in Figure 1A. The presence of MP in nitrogen-saturated chloroform solutions results in the quenching of the CdSe PL intensity. The changes recorded are readily reversible and concentration dependent. For the case of MnTPPCl, shown in Figure 1A, the onset of the response occurs at 45 µM and saturates at ∼400 µM. Similar data were obtained for all four MP complexes. We have previously correlated PL quenching with Lewis acidity of the adsorbates, which have the ability to cause a shift of electron density from the semiconductor bulk toward the surface.14,24 Such a shift in electron density will cause an expansion in the depletion width of the solid. If a region on the order of the depletion width is treated as nonemissive, a so-called “dead layer”, then this adsorbate-induced expansion of the electric field will result in a quenching of PL intensity.24 This model can be tested quantitatively by measuring the magnitude of PL quenching, with different exciting wavelengths providing a variation in optical penetration depth. Although the strong visible absorption of MPs precludes such a quantitative test of the dead layer model, CPD measurements (vide supra) support the notion that the observed PL quenching behavior in our studies arises from such an electric fielddriven mechanism.
(23) La-Mar, G. N.; Frye, J. S.; Satterlee, J. D. Biochim. Biophys. Acta 1976, 428, 78.
(24) Seker, F.; Meeker, K.; Kuech, T. F.; Ellis, A. B. Chem. Rev. 2000, 100, 2505.
FeTPPCl + 2Im S [FeTPP-Im2]+Cl-
(1)
7856
Langmuir, Vol. 16, No. 20, 2000
Ivanisevic et al.
Figure 2. Plot of θ, based on fractional PL changes (eq 3), as a function of concentration for an etched n-CdSe sample exposed to a N2-saturated CHCl3 solution of FeDMPPCl. The inset presents the same data as a double-reciprocal plot (eq 2), yielding an equilibrium constant of (9.3 ( 0.6) × 104 M-1. The PL was excited with 633 nm light and monitored at 720 nm.
Figure 3. Plot of θ, based on fractional PL changes (eq 3), as a function of concentration for a sample of n-CdSe derivatized with 1 and then exposed to a N2-saturated CHCl3 solution of FeDMPPCl. The inset presents the same data as a doublereciprocal plot (eq 2), yielding an equilibrium constant of (3.0 ( 0.3) × 106 M-1. The PL was excited with 633 nm light and monitored at 720 nm.
Table 2. Solution Binding Constants metalloporphyrin
K (M-1) to CdSea
K (M-1) to 1-CdSeb
MnTPPCl MnDMPPCl FeTPPCl FeDMPPCl
9× 8 × 104 5 × 103 9 × 104
3× 1 × 106 2 × 105 3 × 106
103
105
a Equilibrium binding constant for adsorption of the indicated analyte in nitrogen-saturated CHCl3 solution onto etched CdSe, obtained from fits to the Langmuir adsorption isotherm model, eq 2. The error was generally less than 10%. b Equilibrium binding constant, obtained as described in footnote a, except that the CdSe surface was first derivatized with linker 1 prior to adsorption of the indicated metalloporphyrin, as described in the Experimental Section and text.
The concentration dependence of the PL changes allows us to estimate rough binding constants for adsorption onto the CdSe surface using the Langmuir adsorption isotherm model. The quantitative form of the model can be expressed by eq 2,2525
θ ) KC/(1 + KC) 1/θ ) 1 + (1/KC)
(2)
where K is the equilibrium constant for adsorption, θ is the fractional surface coverage for active binding sites, and C is the molar concentration. The values of θ can be estimated from the fractional PL changes: the PL intensity in the reference ambient, PLref, corresponds to θ ) 0; when the PL changes have saturated, PLsat, θ ) 1. At intermediate coverages, the PL intensity is designated as PLx, and θ is calculated from the fractional PL changes:26
θ ) |(PLx - PLref)|/|(PLsat - PLref)|
(3)
Figure 2 shows a representative plot of 1/θ vs 1/C, which gives a reasonably good fit to the Langmuir model, corresponding to a value for K of ∼9 × 104 M-1. Table 2 demonstrates that the binding constants for the metalloporphyrin analytes are all on the order of 104-105 M-1. For a given porphyrin, binding constants show little dependence on the metal. However, the DMPP derivatives yielded larger binding constants by about 1 order of (25) Atkins, P. W. Physical Chemistry, 6th ed.; W. H. Freeman and Co.: New York, 1998. (26) Winder, E. J.; Kuech, T. F.; Ellis, A. B. J. Electrochem. Soc. 1998, 145, 2475.
magnitude compared to the TPP analytes. This may reflect a more favorable interaction with the CdSe surface through the ester groups and/or less steric hindrance associated with binding. PL Results with Derivatized Surfaces. The linkers shown in Chart 1A were used to derivatize the CdSe surfaces. Only for linker 4 could we observe surface modification directly by PL: The PL intensity did not change significantly when the linkers were introduced into contact with the semiconductor surfaces from chloroform (1) and methanol (2 and 3) solutions, but for linker 4 in methanol, a large PL quenching was found initially (∼50%), and a net quenching of about 15% was observed after this sample was rinsed repeatedly with methanol. This indicates that irreversible adsorption chemistry has taken place. Although we were unable to see corresponding irreversible PL changes for linkers 1 and 2, we established that an irreversible modification of the surface took place by observing the appearance of a sulfur peak in XPS (vide infra). To determine how the linker influences adsorption of the MPs, we initially prepared four separate CdSe samples, each modified with linker 1. Figure 3 presents a typical Langmuir plot for the interaction of FeDMPPCl with 1-modified CdSe, corresponding to a binding constant of ∼3 × 106 M-1. As Table 2 reveals, all four MPs exhibited enhancement in their binding to the surface by factors of on the order of 10-40. Because of this similarity, we then selected one of the MPs, FeDMPPCl, and used it to evaluate the effects of each of the four linker molecules. In each case, we monitored the semiconductor’s PL as we titrated with FeDMPPCl. Table 3 summarizes the binding constants obtained, which are all in the range of 105-106 M-1. Figure 1B presents typical data for a 4-modified surface that is subsequently exposed to MnTPPCl. The figure illustrates the reversible quenching of PL intensity. Reflecting the larger binding constant of the derivatized solid, note that the PL changes begin and saturate at lower concentrations compared to the bare surface, whose response to MnTPPCl is shown in Figure 1A. The larger binding constants of the metalloporphyrin with the derivatized surface demonstrate that all four ligands in Chart 1A can be used to modify the surface so that the metalloporphyrin analytes bind more tightly. We
Binding of Metalloporphyrins to CdSe
Langmuir, Vol. 16, No. 20, 2000 7857
Table 3. Solution Binding Constants for Adsorption of FeDMPPCl onto Derivatized CdSe surface modificationa
K (M-1)b
etched CdSe 1-CdSe 2-CdSe 3-CdSe 4-CdSe
9 × 104 3 × 106 3 × 105 2 × 106 3 × 106
a The etched CdSe surface was modified by the indicated linker, as described in the Experimental Section, prior to the adsorption of FeDMPPCl in nitrogen-saturated CHCl3 solution. b Equilibrium binding constants for adsorption of FeDMPPCl from nitrogensaturated CHCl3 solution onto the indicated derivatized CdSe surface, obtained from fits to the Langmuir adsorption isotherm, eq 2. The error was generally less than 10%.
attribute this difference in binding affinity to the formation of a chemical bond between the metal in the porphyrin ring and the linker ligand, which results in a stronger interaction of the metalloporphyrin with the surface. II. Film Studies. Films could be prepared by either of two methods, summarized as follows:15,16
MP + L f MP·L
(4)
MP·L + σ f σ·L·MP preformed method
(5)
σ + L f σ·L
(6)
σ·L + MP f σ·L·MP two-step method
(7)
Equations 4 and 5 refer to a preformed method of film preparation, wherein the MP and linker solutions are combined to yield the 1:1 MP·L adduct in solution, eq 4. Exposure of the CdSe surface to this mixture permits the adduct to bind at a surface site (σ), eq 5. The second method, represented by eqs 6 and 7, is a two-step procedure. First, the linker adsorbs onto the CdSe surface, eq 6. After being rinsed with the solvent, the derivatized CdSe is immersed in a solution of the MP, eq 7. Derivatized surfaces prepared by each method were subsequently rinsed with solvent prior to film characterization and used for oxygen-detection experiments. Washing the linker-MP-derivatized surfaces (σ·MP·L), prepared by either method, with toluene removes only the MP molecules from the surface. The resulting linker-derivatized surface (σ·L) can subsequently be used for readsorption of the MPs. XPS Results. We employed XPS to establish the presence of linker and/or MP in films prepared by the two-step method. The S2p peak and the Fe3+ 2p3/2 peaks were used to establish adsorption of linker and Fe-containing MPs, respectively.27 The S2p peak was present on a sample that was treated only with 1, washed with chloroform, and allowed to dry under a stream of nitrogen (i.e., after the first step). A CdSe surface treated only with FeDMPPCl, followed by a wash with chloroform, showed no Fe using XPS. However, a film prepared by the two-step method using 1 and FeDMPPCl exhibited the Fe3+ 2p3/2 peak. Our XPS data thus confirm the irreversible adsorption of the linker and linker-MP adduct to the surface and show that the linker helps bind the MP more tightly to the semiconductor. We demonstrated using XPS that we can remove the complexed MP from the film by repeated washing with toluene. Following that treatment, we observed approximately the same amount of S on the surface but no Fe. The MP can be reattached to this film again by treating the crystal with a solution of the MP in chloroform, resulting, after a brief rinse in chloroform to (27) Guo, L. H.; McLendon, G.; Razafitrimo, H.; Gao, Y. J. Mater. Chem. 1996, 6, 369.
Figure 4. PL intensity changes due to exposure to gaseous oxygen of an etched n-CdSe crystal coated with a film of 1-FeDMPPCl, prepared by the (A) preformed and (B) two-step method, as described in the text and eqs 4-7. The mixtures of N2/O2 were prepared as described in the Experimental Section; the partial pressures of O2 are indicated in the figure, and the total pressure for all experiments was 1.0 atm. After each exposure to O2, the ambient was restored to 1 atm N2. PL was excited with 633 nm light and monitored at 720 nm.
remove excess MP, in the reappearance of a XPS Fe3+ 2p3/2 signal. PL Studies. When gaseous O2 is allowed to come into contact with the surface of CdSe that is only etched, or is etched and derivatized with any of the four linkers, the PL intensity of the semiconductor remains constant within experimental error of its intensity in the reference N2 ambient.18 To test the ability of linker-MP coatings to act as transducers for molecular oxygen detection, films were made by either eq 4 or 5. By either method, reversible PL responses to oxygen were routinely obtained. Representative PL traces are shown in Figure 4A,B for coatings resulting from the preformed and two-step method, respectively. As is evident from the data, O2 causes a quenching in the PL intensity of the transducer-modified CdSe substrate. Quenching of the PL intensity by molecular oxygen can be attributed to the conversion of the film to a more electron-withdrawing adduct after the addition of O2, perhaps mediated by Fe redox chemistry.15 The changes in PL intensity varied with the method used to deposit the transducer. In general, the preformed method resulted in smaller quenching of the PL intensity. The data presented in Figure 4 show O2-induced PL changes that are readily reversible once the ambient is returned to pure N2, although we do observe some baseline drift over the 10-15 min duration of most experiments. Similar effects were found with all linker-MP combinations, save films using 4, which typically displayed irreversible PL changes with exposure to oxygen, perhaps reflecting linker-semiconductor chemistry associated with the carboxylic acid functionality. We have previously seen evidence by IR spectroscopy for strong binding of this functionality to semiconductor surfaces.20 Figure 5 presents a Langmuir plot of concentrationdependent PL quenching for a film of 2-FeDMPPCl, which yields a binding constant estimate of ∼1.9 atm-1.
7858
Langmuir, Vol. 16, No. 20, 2000
Ivanisevic et al. Table 4. Contact Potential Difference Measurementsa surfaceb
Figure 5. Plot of θ, based on fractional PL changes (eq 3), for CdSe coated with a film of 2-FeDMPPCl (preformed method) vs partial pressure of O2. The inset shows the double-reciprocal plot of the same data (eq 2), yielding a binding constant of 1.9 ( 0.2 atm-1.
In general, all of the films gave binding constants on the order of 1-10 atm-1. These values are similar to oxygen binding equilibrium constants we have previously reported for films of divalent MPs on CdSe substrates.12 CPD. The contact potential difference (CPD) was measured under photosaturation conditions (to produce band flattening), in ambient atmosphere, for CdSe derivatized with films of FeDMPPCl, linkers 1 and 4, and linker-FeDMPPCl complexes. The results are summarized in Table 4. For films of FeDMPPCl alone, an increase in band bending is seen, in agreement with the PL quenching data. Adsorption of linkers 1 and 4 causes only modest increases and decreases, respectively, in Vs, relative to the bare etched n-CdSe surface. Exposure of these surfaces to FeDMPPCl to form films of linkerFeDMPPCl causes an increase in Vs, with the change about twice as large for linker 1. Consistent with the XPS data, washing with toluene reduces Vs toward a value characteristic of the linker-derivatized surfaces. These trends in Vs values were observed in several measurements of each system, although Vs of the bare surface varied somewhat from one CdSe sample to another. TRPL. The surface recombination velocity (SRV) of CdSe is relatively low, below 103 cm/s.21 We found nonexponential PL decays occurring over a time scale of roughly tens of nanoseconds for bare CdSe, for an FeDMPPCl film on CdSe, for 1-CdSe and 4-CdSe, and for 1-FeDMPPCl, 4-FeDMPPCl, and 1-MnDMPPCl (all prepared by the two-step method) on CdSe. Within experimental error, adsorption of the linker monolayers or of linker-MP monolayers had a negligible effect on the PL decay traces
∆Vs (mV)c
etched CdSe FeDMPPCl-CdSe
0 (445) 115 (560)
etched CdSe 1 - CdSe 1 + FeDMPPCl-CdSe 1 + FeDMPPCl-CdSe after toluene wash
0 (445) 10 (455) 125 (570) 60 (505)
etched CdSe 4 - CdSe 4 + FeDMPPCl-CdSe 4 + FeDMPPCl-CdSe after toluene wash
0 (435) -30 (405) 20 (455) 0 (435)
a Contact potential difference (CPD) measurements for etched and coated CdSe samples. b Sample surface used for CPD measurement, where 1- and 4-CdSe refer to linker-modified surfaces. The descriptions 1 + FeDMPPCl-CdSe and 4 + FeDMPPCl-CdSe refer to linker-treated surfaces that were subsequently exposed to FeDMPPCl. These same surfaces were then remeasured after repeated toluene washings to remove the metalloporphyrin. c Change in surface voltage derived from the CPD measurement. For each set of experiments, the untreated etched CdSe sample is taken as a reference and absolute values ((5 mV) are shown in parentheses.
and SRV. This result supports the conclusion that the quenching observed in the steady-state PL measurements, following MP adsorption, is mainly due to the increase in band bending that was evidenced by the CPD measurements. Summary We have demonstrated using PL that a variety of linker molecules with a propensity to bind irreversibly to the CdSe surface can be used to anchor trivalent metalloporphyrins, enhancing their adsorption binding constant by 1 order of magnitude relative to adsorption onto the bare semiconductor surface. Moreover, films prepared from these linker-immobilized MPs can serve as transducers: exposure of the linker-MP-coated CdSe surfaces to oxygen leads for most linker-MP combinations to reversible quenching of the semiconductor PL intensity. Acknowledgment. We thank Prof. David Cahen for fruitful discussions and suggestions throughout the work and Rami Cohen for his help in the CPD measurements. We are grateful to the National Science Foundation and U.S.-Israel Binational Science Foundation for generous support of this work. The NSF-MRSEC at UW-Madison provides support for the Materials Science Center facilities used in this work. A.S. holds the Siegfried and Irma Ullman professorial chair. LA000580D
