Adsorption of Alkanolamines onto Semiconductor Surfaces: Cadmium

The PL changes can be fit to a dead-layer model and correspond to an .... and Evidence for the Aversion of an Analyte to a Buried Semiconductor−Film...
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J. Phys. Chem. B 1999, 103, 995-1001

995

Adsorption of Alkanolamines onto Semiconductor Surfaces: Cadmium Selenide Photoluminescence as a Probe of Binding and Film Reactivity toward Carbon Dioxide Kathleen Meeker and Arthur B. Ellis* Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed: August 12, 1998; In Final Form: NoVember 2, 1998

Films of ethanolamine, 3-aminopropanol, and 4-aminobutanol, 1-3, serve as transducers for the detection of CO2 when deposited onto emissive CdSe substrates: the band-gap photoluminescence (PL) intensity of the semiconductor is reversibly quenched by exposure to CO2, while uncoated CdSe samples display no response relative to a nitrogen reference ambient. Infrared (IR) spectra obtained with films of these alkanolamines and related alcohols and amines, 4-13, are consistent with selective reaction of the amine functionality in these films to form carbamates. The direction of the PL response corresponds to an enhancement of Lewis acidity accompanying the reaction with CO2, and use of a dead-layer model permits an estimation of the adductinduced expansion of the depletion width as being roughly a few hundred Angstroms. Both IR and PL data reveal that the film response to CO2 begins at ∼0.1 atm and saturates by ∼0.3 atm, and their similarity indicates that the partition coefficient for binding CO2 to the bulk film compared to the CdSe-film interface is roughly unity. Adsorption of 1-3 onto CdSe substrates from N2-saturated THF solution causes reversible enhancement of the semiconductor’s PL intensity, consistent with a Lewis-basic interaction with the surface. The PL changes can be fit to a dead-layer model and correspond to an adduct-induced contraction of the depletion width of ∼200 Å for 1 and 3 but ∼600 Å for 2. The solution concentration dependence of PL changes for 1-3 could be well fit to either single-site or multisite Langmuir adsorption isotherm models, yielding modest binding constants of ∼102 M-1. Steric and electronic factors contributing to these effects are discussed based on the film and solution PL responses of the other amines and alcohols investigated.

Introduction Adsorption of bifunctional molecules such as alkanolamines can lead to rich ligation and reaction chemistry with applications to the design of chemical sensors. As shown in Figure 1 for such a molecule, surface binding can occur through monodentate coordination of either functional group to the surface or through bidentate ligation involving bridging and chelating structures. An appealing feature of these adsorbates is that for monodentate coordination, one functional group can anchor the molecule to the surface, leaving the other functional group free to engage in characteristic reaction chemistry. Analytes that serve as reaction partners for these accessible functional groups can be detected if a transduction mechanism has been incorporated into the binding platform. This strategy has been implemented in a variety of ways. For example, Crooks and co-workers have created acidterminated self-assembled monolayers for sensing bases in the vapor phase.1 They have also deposited dendrimers onto surfaces in order to detect various analytes.2 A strategy for selective protein adsorption onto gold has been developed by Whitesides and co-workers using self-assembled monolayers of oligo(ethylene glycol).3 Bifunctional molecules on surfaces can be linked with reaction partners, as demonstrated by Bitzer and co-workers: While one functional group on each molecule tethers a layer of diamines to a silicon surface, the other functional group reacts with a dianhydride to form a siliconbound polyimide chain.4 Recently, Mallouk and co-workers utilized bifunctionality by depositing copper phosphonate host solids onto quartz crystal microbalances (QCMs) and using intercalated alkanolamines to

Figure 1. Idealized ligations for an alkanolamine deposited onto a CdSe surface. From top to bottom, these ligations include monodentate binding through the amine group or through the hydroxyl group and bidentate binding modes, which include bridging two sites and chelating to a single site.

detect CO2.5 Both primary and secondary amines as well as alcohols can react with CO2 to form carbamates and carbonates, respectively (eqs 1 and 2).5-10 These reactions with CO2,

10.1021/jp9833552 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/26/1999

996 J. Phys. Chem. B, Vol. 103, No. 6, 1999 CHART 1

Meeker and Ellis CO2 by PL when deposited onto CdSe. Our results indicate that when CO2 binding occurs, it does so selectively with the amine functionality. Moreover, through comparisons of film PL and infrared data, we deduce that there is no significant preference for CO2 to bind at the semiconductor-film interface as compared to the bulk film. Adsorption studies of these molecules in THF solution help to identify the steric and electronic origins of these effects. Experimental Section

conducted in aqueous media, are industrially important, as represented by scrubbing technology, and synthetically valuable, as represented by their use in alkylations.11,12 We demonstrate in this paper that films of alkanolamines deposited onto emissive CdSe surfaces can function as transducers for the detection of CO2. In previous studies we have used the photoluminescence (PL) of single-crystal CdSe samples to characterize adsorption of various families of monofunctional compounds onto the surface of this semiconductor substrate. Amines have proven to be particularly good adsorbates, and in one study with bifunctional diamines, we found evidence for chelation from such species as ethylenediamine and o-phenylenediamine.13,14 The direction of the PL response has been used to define a “luminescence litmus test”: adsorbed Lewis bases enhance the PL intensity and adsorbed Lewis acids quench it relative to a reference ambient.14 Our working hypothesis has been that weak chargetransfer complexes are formed between the adsorbate and semiconductor surface atoms. These complexes enhance or diminish the thickness of the semiconductor’s electric field, the depletion region, by shifting electron density into surface electronic states (Lewis acids) or out of these states (Lewis bases), respectively. If a region on the order of the depletion width is regarded as nonemissive, a “dead layer”, because electron-hole pairs photogenerated in this region are prevented from recombining therein, this control of the depletion width by adduct formation accounts for the direction of PL responses. Many compounds, including CO2, elicit no appreciable PL response from CdSe upon adsorption. In related studies, we have demonstrated that films of coordination complexes such as Vaska’s complex, Jacobsen’s catalyst, and a Co(methoxysalen) complex can act as transducers on CdSe: the reaction of these films with small molecules, such as carbon monoxide, oxygen, and volatile epoxides, can perturb the electric field of the underlying semiconductor, affecting the PL of the CdSe substrate so as to permit detection of the analyte.15-17 In this paper, we extend these surface studies to the bifunctional molecules 1-7 and related compounds 8-13 shown in Chart 1. We demonstrate that there is substantial variation in the ability of films of the bifunctional molecules to sense

Materials. Single-crystal, vapor-grown c-plates of n-CdSe, having a resistivity of ∼2 Ω cm, were obtained from Cleveland Crystals, Inc. After mechanical polishing with 5 µm alumina and rinsing with refluxing methanol and heptane, the samples were etched in 1:15 (v/v) Br2/MeOH for 30 s, revealing the shiny Cd-rich (0001) face, which was illuminated in these PL experiments. Ethanolamine (1; 99.5+%, redistilled), 3-aminopropanol (2; 99+%), 4-aminobutanol (3; 98%), 4-aminophenol (4; 98+%), trans-1,4-diaminocyclohexane (6; 98+%), ethylene glycol (7; 99+%), diethanolamine (12; 99%), and triethanolamine (13; 98%) were all purchased from Aldrich and used as received. They were handled in a N2-filled glovebag. Solutions were prepared with tetrahydrofuran (EM Science; 99%), which was freshly distilled over Na/benzophenone. p-Phenylenediamine (5; 97%) was recrystallized from absolute ethanol prior to use. n-Propanol (9; EM Science; 99%) and n-butanol (11; Fisher; 99%) were distilled over Mg/I2 prior to use. nPropylamine (8; 99+%) and n-butylamine (10; 99+%) were purchased from Aldrich and distilled over KOH prior to use. Carbon dioxide (99+%) was obtained from Liquid Carbonic Specialty Gas Corp. and used without further purification. Film Preparation. Films of the alkanolamines and related compounds used for studies of CO2 detection were prepared by depositing the material onto the CdSe surface in one of two ways, depending on whether the compounds were liquids or solids. Liquid compounds 1-3, 7, and 10-13, with low volatility, were deposited by placing a drop of the neat liquid directly onto the surface of the semiconductor under a stream of flowing N2. The crystal was then mounted and the PL monitored under flowing N2 until the PL base line stabilized. Over the course of 1 h, these films, which are tacky, appeared unchanged. While some film material may have evaporated over the course of the PL experiment, the bulk of it remained (boiling points for 1-3, e.g., range from 170 to 206 °C), and the PL response was stable. The solid compounds 4-6 were deposited via solutions: 200 µM THF solutions were prepared in dry THF, and either 1-2 or 10 drops were placed directly on the etched semiconductor surface under a stream of flowing N2, yielding films of ∼0.1 or 1 µm in thickness, respectively, assuming uniform coverage. Films of the liquid compounds noted above were prepared from THF in this way as well, but no appreciable difference in PL behavior was observed relative to the preparation from neat liquids. Apparatus. The CdSe sample was mounted on a glass rod between two Teflon spacers in a glass cell that permitted both solution studies and gas-flow studies to be performed. The cell was fixed such that the introduction of various solutions did not alter the optical alignment nor did the various flow rates used in the film studies. In film experiments, the gas-flow rate was 100 mL/min and the total gas pressure was 1 atm. Optical Measurements. PL measurements were made using either a Coherent Innova model 90C 5.0W Ar+ laser (458, 514 nm) or a Melles-Griot HeNe laser (633 nm). Data were collected with an Oriel Instaspec IV CCD detector and processed by a

Adsorption of Alkanolamines PC. The red band-edge PL of n-CdSe was monitored at the band maximum of 720 nm with filtering of exciting wavelengths. Absorption spectra of these molecules, obtained using a HewlettPackard 8452 diode array spectrometer, reveal no absorption at any of the exciting or emitting wavelengths. Using the lowresolution conditions employed (∼1 nm), there was no change in the PL spectral distribution upon adsorption of any of the analytes studied, so that the PL could be monitored at a single wavelength, the band maximum. Nuclear Magnetic Resonance Spectroscopy. 1H and 13C NMR measurements were performed on a Bruker AC-300 spectrometer. Samples of 2 were dissolved in THF and reacted with gaseous CO2. The precipitate that formed was collected, and its 1H and 13C NMR spectra were obtained by dissolving it in CD3OD. Data for 2 prior to CO2 reaction: 1H NMR (CD3OD) δ 4.79 (s, 3H, NH2, OH), 3.57 (t, J ) 6 Hz, 2H, CH2OH), 2.69 (t, J ) 7 Hz, 2H, CH2NH2), 1.64 (tt, J ) 6 Hz, J ) 7 Hz, 2H, CH2-CH2-CH2); 13C NMR δ 60.96 (s, CH2-OH), 39.71 (s, CH2-CH2-CH2), 36.43 (s, CH2-NH2). Data for 2 after CO2 reaction: 1H NMR δ 4.23 (s, 3H, NH, OH, OH), 3.63 (t, J ) 5 Hz, 2H, CH2OH), 3.05 (t, J ) 7 Hz, 2H, CH2NH(CdO)OH), 1.82 (br, 2H, CH2-CH2-CH2); 13C NMR δ 161.43 (s, NH(Cd O)OH), 60.34 (s, CH2-OH), 38.84 (s, CH2-CH2-CH2), 31.30 (s, CH2-NH(CdO)OH). Infrared Spectroscopy. IR measurements were made using a Nicolet 740 spectrometer. Samples were tested for reactivity with CO2 by depositing films of both 0.1 and 1 µm thickness from neat liquid and from solution (see above) between two NaCl plates or by preparing fluorolube mulls of solid samples. Samples were then placed in a chamber that was purged for between 15 and 30 min with mixtures of CO2 and N2, whose compositions matched ambients used in PL experiments. The samples were then removed from the chamber and placed in the instrument. Data were processed with Nicolet Advantage software. Many peaks in the CO2-bound form of the alkanolamines are found in the same range as those for the alkanolamines themselves. Quantitative measures of CO2 binding were made by choosing a peak in the spectrum of 2 prior to CO2 exposure and monitoring its absorbance as CO2 was dosed into the cell. This peak, at 909 cm-1, decreased with increasing amounts of CO2 to a constant absorbance at CO2 pressures g0.3 atm. Another peak, at 821 cm-1, which grows in with increasing CO2 concentrations, was also monitored. These two peaks exhibited similar fractional changes in absorbance as a function of CO2 pressure. Another spectral position, 3800 cm-1, far from strong absorbances, was monitored to establish a base line intensity; the absorbance here was independent of CO2 partial pressure, indicating that the film thickness did not change appreciably throughout the series of measurements. A similar analysis was conducted with films of 1, 3, and 5. Results and Discussion Samples of single-crystal n-CdSe emit red band-edge PL at ∼720 nm when excited with ultraband-gap light at room temperature. By monitoring the intensity of this PL, we can develop qualitative and quantitative descriptions of the semiconductor’s surface interactions with a variety of bifunctional adsorbates. In sections below, we describe the reactivity of the compounds in Chart 1 toward CO2. The PL response induced by reaction of CO2 with films of these compounds deposited onto CdSe is then characterized. We describe the PL response of the CdSe surface to THF solutions of the bifunctional molecules 1-7 and related compounds 8-13 and discuss how

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Figure 2. IR spectra for a film of 2 (∼1 µm in thickness, assuming uniform coverage), deposited onto a NaCl plate in 1 atm N2 (bottom), and the CO2-bound form of 2 (top), prepared by exposing the film to 1 atm of CO2. The dashed lines indicate the absorptions at 909 and 821 cm-1, where the peaks used in quantitative binding analyses are found (see Experimental Section and Figure 5).

these interactions might contribute to film reactivity, using a dead-layer model and the Langmuir adsorption isotherm model to estimate the effect of adsorption on the substrate’s depletion width and the adsorbate binding constant, respectively. These results permit us to infer adsorbate structural features and interpret some of the film reactivity results. Film Experiments. Reactions with CO2. As noted in the Introduction, films of the alkanolamines can potentially react with CO2 to yield carbamates and carbonates. Either functional group could in principle bind to the surface, and either functional group could potentially bind CO2, although solution studies found in the literature describe a preference for carbamate formation.7 To investigate transducing behavior, we studied films of the compounds of Chart 1 by IR under N2 and CO2 ambients of 1 atm. All of the compounds reacted with CO2 with the exceptions of 4, 7, 9, 11, and 13, as evidenced by IR spectral changes in the 800-1800 cm-1 region; typical changes are shown for 2 in Figure 2. The compounds that contain only hydroxyl groups did not appear to react with CO2 by IR, which indicates that having a primary or secondary amine group is a necessary structural feature for reaction. That this is not sufficient for ensuring reactivity, however, is shown by the lack of reactivity by films of 4: While 4 does contain an amine group, we believe that the coupling of the amine group with the phenol alters the basicity of the amine enough to inhibit the reaction with CO2. Photoluminescent Response. When we prepared films of the bifunctional compounds 1-7 on CdSe, we found that only films of the aliphatic alkanolamines 1-3 and of p-phenylenediamine (5) serve as chemically sensitive transducers on CdSe: Although there is no appreciable change in PL when the bare CdSe surface is in the presence of CO2 relative to the nitrogen reference ambient, when the surfaces are coated, exposure to CO2 causes reversible quenching of the PL intensity by up to 50% when excited by 633 nm light, as illustrated for a film of 2 in Figure 3. The figure also shows that adsorption times are on the order of seconds and desorption times are considerably longer, on the order of tens of minutes. As expected from their lack of reactivity, neither films of 4 nor 7 yielded a PL response to CO2. More surprising was the lack of response by 6, which had displayed reactivity from IR data. Solution experiments (vide infra) suggest that the lack of a PL response from films of 6 may be due to poor coupling to the CdSe surface.

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Figure 3. PL intensity changes of an etched n-CdSe crystal coated with a film of 2 (∼1 µm in thickness, assuming uniform coverage), when exposed to various concentrations of carbon dioxide, relative to a nitrogen reference ambient. PL was excited with 633-nm light and monitored at 720 nm. The percentages correspond to the proportion of CO2 in CO2/N2 mixtures at a total pressure of 1 atm.

Films of 10 and 11 were also examined. These are monofunctional analogues of 4-aminobutanol, 3, but neither film gave an appreciable response. These data suggest that bifunctionality is a prerequisite for this kind of transduction: one end of the molecule, the alcohol, is bound to the surface, providing the ligation, and the other end of the molecule, the amine, reacts with CO2. Films of compounds 12 and 13, which are industrially significant and are typically used in aqueous solution, gave an erratic or no PL response to CO2.11,18 The lack of response from 13 is not surprising, as it contains a tertiary amine and did not exhibit a response by IR. The PL quenching observed with films of 12 was highly variable in magnitude and often irreversible, probably reflecting a multiplicity of binding modes. Dead-Layer Model. The quenching of PL intensity induced by the exposure of films of 1-3 and 5 to CO2 is consistent with the notion that the CO2 adduct makes the adsorbate more Lewis acidic: If the amine donates electron density to the semiconductor (vide infra), its conversion to the carbamate (eq 1) would diminish its donor strength. As noted in the Introduction, a dead-layer model can be used to estimate adduct-induced changes in depletion width by assuming that a region on the order of the depletion region is nonemissive.19,20 This assumption leads to a relationship between fractional PL changes and the change in the dead-layer thickness, ∆D:

PLref/PLx ) exp(-R′∆D)

(3)

PLref is the PL intensity in the N2 reference ambient, PLx is the PL intensity in the presence of the CO2 analyte, and R′ ) (R + β), where R and β are the absorptivities for the exciting and emitted wavelengths for CdSe. An assumption that is implicit in the dead-layer model is that surface recombination velocity S remains constant after adsorption or is very large in both ambients (S . L/τ and S . RL2/τ, where L and τ are the minority carrier diffusion length and lifetime, respectively). To test the model, the sample is excited by several different excitation wavelengths and the fractional PL changes measured. A good fit to the model is evidenced by rough agreement in ∆D values obtained from several excitation wavelengths. Figure 4 demonstrates that the films’ responses to CO2 are well fit by the dead-layer model. Film-to-film variations make it difficult to rank the compounds, except to note that 3 consistently has the smallest PL response, as typified by the data in Figure 4. Thus, while a four-carbon chain does provide some transducing ability, two or three methylene units in the

Figure 4. Maximum ∆D values, calculated from PL quenching data using eq 3, for films of 1-3 and 5 (∼0.1 µm in thickness, assuming uniform coverage) upon exposure to CO2 relative to a nitrogen ambient. The horizontal lines on the right-hand side of each bar represent the error associated with the ∆D value. These data were collected from different sample surfaces.

aliphatic connecting chain appear to be significantly better. The rigid aromatic phenyl ring of the diamine 5 appears to provide transduction effectiveness that is comparable to that of alkanolamines 1 and 2. Sensor Issues. In terms of practical sensing, it is noteworthy that the onset and saturation points for these PL responses to CO2 occur over a fairly small concentration range of 10-30% and the concentration profiles do not fit the simple Langmuir adsorption isotherm model, described below. However, because the profiles are similar for each transducing film (i.e., the onset and saturation points are essentially the same), we can conclude that the binding of CO2 to these films is approximately equally strong for each film, which is consistent with a common transformation from amine to carbamate. It is also significant that O2 does not appear to perturb the PL response of these systems on the time scale of the experiments but that water does have a deleterious effect: Films that are exposed to significant amounts of water vapor do not respond as well to CO2. Partition Equilibria. Quantitative comparisons of CO2induced IR and PL changes were made with films of 1-3 and 5 to examine partitioning of CO2 between the bulk film and the semiconductor-film interface. Previous work showed that a CdSe sample coated with a film of Vaska’s complex binds O2 preferentially at the interface, indicating a different binding mode at the interface as compared to the bulk film.17 The following equations provide a framework for this analysis: eq 4 represents binding of CO2 to the amine functionality in the bulk film, which is monitored in the IR experiment. Equation 5 represents the equilibrium associated with transferring a film-bound CO2 molecule to a surface-bound alkanolamine molecule, denoted by 2•σ. This is the distribution or partition equilibrium. Finally, eq 6 represents binding of the CO2 at the semiconductor-film interface, which we believe is monitored by PL. Note that eqs 4 and 5 add to eq 6. Figure 5 shows that the fractional changes in the IR spectrum for binding CO2 to 3-aminopropanol (2) occur over the same pressure range as the changes seen by PL within experimental error. This observation demonstrates that there is no preference for CO2 binding in the bulk film compared to binding at the semiconductor-film interface, i.e., the equilibrium constant for the distribution equilibrium of eq 5 is roughly unity. Although the Figure 5 data were obtained for films ∼1 µm in thickness, assuming uniform coverage, similar results were obtained for

Adsorption of Alkanolamines

Figure 5. Normalized infrared responses of a film of 2 on a NaCl plate and of PL responses of a film of 2 (both ∼1 µm in thickness, assuming uniform coverage) on CdSe to changes in CO2 partial pressure. Responses were normalized so that values of the absorption peak intensity (for the IR experiments) and dead-layer thickness (for the PL experiments) at saturation were set to be 100%; intermediate values were expressed as fractions of the saturation value. Squares represent IR data while circles represent PL data.

films of 2 ∼0.1 µm in thickness and for samples of 1, 3, and 5 with these two approximate film thicknesses. Solution Experiments. PL Response. All of our solution adsorption studies have been conducted in N2-saturated THF, in which the amine-containing bifunctional analytes of this study, 1-6, were found to be soluble. These compounds were all found to enhance CdSe PL reversibly, relative to a N2saturated THF base line, as illustrated for 3-aminopropanol, 2, in Figure 6. The magnitude of the PL enhancement was found in several experiments conducted with a common CdSe sample to follow the order 6 < 1, 3, 4, 5 < 2. 3-Aminopropanol, 2, exhibited the largest PL response, which reached enhancements of ∼50% when the CdSe surface was excited with 633 nm light. Attempts to conduct these experiments in CO2-saturated THF solutions with these compounds were unsuccessful owing to the formation of precipitates. The precipitate isolated from reaction of 2 with CO2 in THF solution appears to be the carbamate based on 1H and 13C NMR spectral data taken in CD3OD (see Experimental Section). For the alkanolamines 1, 2, and 3, we performed experiments using monosubstituted analogues of 2sn-propylamine, 8, and n-propanol, 9sto assess whether the PL response elicited by

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Figure 6. Changes in PL intensity of an etched n-CdSe crystal resulting from exposure to the indicated concentrations of 2, 3-aminopropanol, in a N2-saturated solution of THF. The downward spikes are transients caused by draining the sample cell when changing solutions. The PL was excited by 633-nm light and monitored at 720 nm. Concentration values are in units of millimolar.

the bifunctional compound could be regarded as simply the sum of monofunctional contributions. In striking contrast to the reversible enhancements we find for 2 in THF, the alcohol 9 yields modest, reversible PL quenching relative to a THF base line of about 10%, and the amine 8 gives irreproducible PL responses that include both quenching and enhancement relative to the reference solvent. The latter is particularly surprising given our prior observation that the amine causes reversible enhancements in cyclohexane solution.13 Conceivably, hydrogen-bonding interactions involving the amine and THF could contribute to the erratic PL responses observed with 8. The fact that the PL response of 2 cannot be derived by simply combining those of 8 and 9 suggests that the alkanolamines do not bind to CdSe exclusively by a mixture of monodentate alcohol- and aminebound molecules but may use bidentate ligation alone or in combination with monodentate binding. Analysis of these binding profiles using the Langmuir adsorption isotherm models is described below. For the aromatic alkanolamine 4, a similar PL response and binding constant (vide infra) observed with the aromatic diamine 5 suggests that binding to CdSe occurs through the amine group. PL data obtained for 12 and 13 were not reproducible in THF. It is possible that the multiple functional groups of 12 and 13 and their sterically demanding nature may lead to considerable surface-to-surface variation in binding. Most commonly, the PL signature for triethanolamine 13 displays quenching at low concentrations of this analyte, while at higher concentrations enhancements are seen. Dead-Layer Model. Adsorption of the bifunctional molecules 1-6 from solution onto CdSe causes a contraction of the depletion width consistent with their acting as Lewis bases toward the surface. The solution experiments performed here with the alkanolamines and diamines give reasonable fits to the dead-layer model (eq 3, with THF as the reference ambient), as shown in Figure 7. All of the compounds 1-6 gave ∆D values of ∼200 Å with the exception of 2, which gave by far the largest reduction in dead-layer thickness, ∼600 Å. This large value could reflect a particularly favorable binding interaction for 2, but it could also reflect greater surface coverage relative to the other compounds examined, since our measurements do not reveal absolute coverage. In this regard, it is important to note that the etched surfaces that we employ may contain impurity atoms and/or oxide phases that may also contribute to the effects observed.

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Figure 7. Maximum ∆D values for an etched n-CdSe crystal in the presence of nitrogen-saturated THF solutions of the indicated analytes. These values were calculated from PL responses using eq 3 at the three indicated excitation wavelengths. These data were collected on the same sample surface and remained consistently ordered from surface to surface.

Langmuir Adsorption Isotherm Model. The Langmuir adsorption isotherm model can be used to obtain quantitative estimates of analyte binding constants. We present two forms of the isotherm: one that models either monodentate or chelating coordination at single sites on the surface and a second that models bridging coordination. Fits to both of the Langmuir models are based on the concentration dependence of PL changes. Using θ to represent the fractional surface coverage, the quantitative Langmuir model for monodentate or chelating binding is represented by:

θ ) [(KC)/(1 + KC)] or 1/θ ) (1/KC) + 1

(7)

Figure 8. Comparison of single-site and multisite Langmuir adsorption isotherms for adsorption of 3-aminopropanol (2) from THF onto CdSe. The data in the top panel, obtained from the trace of Figure 6, correspond to a plot of the fractional surface coverage θ versus concentration of 2 in N2-saturated THF; values of θ are estimated from eq 8. The excitation wavelength employed was 633 nm. The left and right lower panels correspond to fits to a single-site binding model, eq 7, and a multisite binding model, eq 9, respectively. These fits give R2 values of 0.997 and 0.995 and y-intercept values of 0.96 and -0.36, respectively. The binding constant, K, for the single-site fit is 40 ( 10 M-1, while K ) 20 ( 5 M-1 for the multisite fit.

TABLE 1: Binding Constants for Adsorption onto CdSea

K is the equilibrium constant for binding the analyte to the surface, and C is the molar concentration of the analyte. The PL intensity in the reference ambient corresponds to θ ) 0, and the maximum change in PL intensity, PLsat, is assumed to correspond to θ ) 1. Intermediate values for θ are estimated as the fractional change in dead-layer thickness:

θ ) ln[PLref/PLx]/ln[PLref/PLsat]

(8)

For simple bidentate bridging coordination, the multisite adsorption isotherm applies:21,22

(1 - θ)2/θ ) 1/(2KC)

(9)

A comparison of typical fits of data using eqs (7) and (9) for the adsorption of 2 onto CdSe can be found in Figure 8. Both multisite and single-site models give comparably good fits to the data, suggesting there may be a mixture of binding modes on the surface upon adsorption. We were, in fact, unable to discriminate between the single-site adsorption model, which includes chelation, and the multisite adsorption model for any of the bifunctional molecules 1-6. In considering bidentate binding, compounds 1-3 are all capable of chelating to a single Cd atom, the likely site of surface binding. With respect to bridging, even compound 1, with the shortest distance between the two functional groups of compounds 1-3, can span the 4.3 Å distance between two Cd atoms that are the likely sites of surface binding, albeit with some distortion.13 Compounds 2 and 3 could easily span the distance.

a Values of K, calculated from the multisite Langmuir adsorption isotherm model, eq 9, with the exception of 9, which was calculated from the single-site model, eq 7; values of K calculated from the singlesite model for 1-6 are about a factor of 2 larger. All PL data from which these equilibrium constants were obtained were on the same sample surface.

The binding constants obtained from these experiments are summarized in Table 1. None of the aliphatic alkanolamines are bound especially strongly, but 3-aminopropanol, 2, has the largest value of K among the trio by about an order of magnitude. By far, the largest binding constants of ∼104 M-1 were observed for the two aromatic adsorbates, 4 and 5. The smallest value of K of ∼10 M-1was observed for trans-1,4diaminocyclohexane, 6.

Adsorption of Alkanolamines Implications for Film ReactiVity. The possible bidentate forms of surface ligation for 1-3 inferred from solution results may also be present in the films we have studied. If so, we believe that in the presence of CO2, there is a sufficiently strong driving force for the amine to detach from the surface to react with CO2 while the hydroxyl end of the molecule remains bound. The formation of the carbamate on exposure of the film to CO2 suggests that monodentate binding of the alkanolamines 1-3 to the surface occurs through the hydroxyl group while the amine is available for reaction. A simple experiment conducted by bubbling CO2 through neat samples of 1-3 revealed that the enthalpic driving force is quite strong for CO2 binding, as the test tubes containing these samples become very warm. Although 4-aminophenol (4) and p-phenylenediamine (5) are sterically inhibited from bridging, these compounds yielded indistinguishable fits to eqs 7 and 9 that corresponded to significantly higher binding constants in THF solution. This suggests that the aromatic derivatives have a different interaction with the surface than 1-3, based presumably on their aromaticity and/or planarity. Note that while these species might be expected to bind in a monodentate fashion, they could still potentially bridge surface sites if appropriate topographic features are present. The contrast in binding constants between the aromatic compound 5 and the cyclohexane derivative 6 is especially dramatic and may reflect an unfavorable binding geometry associated with the conformations of 6. We believe that this weak binding may explain the inability of 6 to function as a transducing film, despite its reactivity with CO2 seen in IR experiments. Acknowledgment. We gratefully acknowledge the National Science Foundation for financial support. We thank Prof. Thomas Mallouk and the reviewers for very helpful comments. References and Notes (1) Yang, H. C.; Dermody, D. L.; Xu, C.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726-735.

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