Article pubs.acs.org/Langmuir
Single-Molecule Studies of Acidity Distributions in Mesoporous Aluminosilicate Thin Films Xiaojiao Sun,† Jingyi Xie,† Jiayi Xu,† Daniel A. Higgins,*,‡ and Keith L. Hohn*,† †
Department of Chemical Engineering and ‡Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States ABSTRACT: Solid acid catalysts are important for many petrochemical processes. The ensemble methods most often employed to characterize acid site properties in catalyst materials provide limited insights into their heterogeneity. Single-molecule (SM) fluorescence spectroscopic methods provide a valuable route to probing the properties of individual microenvironments. In this work, dual-color SM methods are adopted to study acidity distributions in mesoporous aluminosilicate (Al−Si) films prepared by the sol−gel method. The highly fluorescent pH-sensitive dye C-SNARF-1 was employed as a probe. The ratio of C-SNARF-1 emission in two bands centered at 580 and 640 nm provides an effective means to sense the pH of bulk solutions. In mesoporous thin films, SM emission data provide a measure of the effective pH of the microenvironment in which each molecule resides. SM emission data were obtained from mesoporous Al−Si films as a function of Al2O3 content for films ranging from 0% to 30% alumina. Histograms of the emission ratio reveal a broad distribution of acidity properties, with the film microenvironments becoming more acidic, on average, as the alumina content of the films increases. This work provides new insights into the distribution of Brønsted acidity in solid acids that cannot be obtained by conventional means.
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INTRODUCTION It is widely acknowledged that catalysts play a vital role in modern chemical industry,1,2 with more than 80% of all chemicals requiring at least one catalytic step during their synthesis.3,4 Compared to homogeneous systems, solid acid catalysts could help to reduce the environmental impact of chemical synthesis because they can serve as alternatives to hazardous mineral acids and eliminate a separation step to recover the catalyst.5 The Brønsted acidity of aluminosilicate (Al−Si) materials, such as zeolites and amorphous aluminosilicates, makes them suitable for use as solid acids in several petrochemical processes, including hydrocracking, isomerization and oligomerization of alkenes, and alkylation of aromatics, among other important reactions.6−9 The insertion of tetrahedral Al atoms into the silica matrix gives rise to catalytically active sites by compensation of the resulting excess negative charge by a proton.10,11 This Al-for-Si substitution model was proposed by Thomas12 and Tamele13 to explain the acidity of Al-containing materials. Many approaches have been used to study the acidity properties of Al−Si materials. Temperature-programmed desorption and FTIR are frequently used to measure materials acidity by probing interactions between their intrinsic acid sites and weak bases (e.g., ammonia and pyridine).14−19 NMR spectroscopy (e.g., 1H magic angle spinning) has also been employed to study the number and strength of Brønsted acid sites in solid acid catalysts.14,15 Classical titration measurements have also been reported.20 These techniques all provide valuable qualitative and sometimes quantitative data on solid acidity. However, it can be difficult to interpret the distribution of acid strengths using these techniques. Moreover, these techniques are incapable of probing individual acid sites or © XXXX American Chemical Society
providing location-specific information on acidity. Yet, it is the exact local properties of these individual microenvironments that govern the physical and chemical interactions between guest molecules and the solid matrix. Therefore, new methodologies are needed to probe the distribution of microenvironment acidity in heterogeneous catalysts. Recently, single-molecule (SM) fluorescence microscopy has been implemented in a number of catalyst studies because of its high temporal and spatial resolution and its ability to probe single-site properties.21−25 Valuable information can be accessed by analyzing the fluorescence intensity, emission spectrum, and polarization of individual guest molecules interacting with a catalyst medium.22−27 SM methods can also provide information on structural heterogeneities,23,28,29 reaction pathways30 and mechanisms,31 catalyst kinetics,23,28,30 and catalytic dynamics32 in and on solid materials. Several pHsensitive fluorescent dyes have been used to study acidity at the SM level in a variety of media, including biological systems,33 photoresists,34 and agarose gels.35 In previous work from the Higgins group,26,36 the pH-sensitive dye ((5′ and 6′)-carboxy10-(dimethylamino)-3-hydroxyspiro[7H-benzo[c]xanthene7,1′(3H)-isobenzofuran]-3′-one, C.SNARF-1)37 was used to explore the local acidity properties of pure silica films that had been exposed to solutions of different pH. The results showed that the C-SNARF-1 SMs were sensitive to the local pH, as determined by the treatment solutions and by surface silanol groups incorporated in the materials. To our knowledge, few SM studies have been devoted to probing microenvironmental acidity in common solid catalyst materials to date.22,38 Received: May 4, 2015
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DOI: 10.1021/acs.langmuir.5b01628 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Chemical structures of C-SNARF-1 in its protonated and deprotonated forms around the pH of its first protonation equilibrium (left, pKa ∼ 7.6) and its second and third protonation equilibria (right, pKa ∼ 3−4).
widths of the distributions are also assessed; these reveal significant variations in the effective local pH.
Ideally, the proper dye molecule for SM studies of materials acidity should exhibit clear and well-defined changes in its emission spectrum with pH. C-SNARF-1 is a suitable probe that emits into two bands centered at 580 and 640 nm. The ratio of fluorescence emission in these two bands changes with pH in bulk solution and thus provides a means to study the effective local pH in close proximity to a dye molecule in solid materials.35−37 C-SNARF-1 is most commonly employed to probe pH near (i.e., within 1−2 pH units) the pKa of its first protonation equilibrium (pKa ∼ 7.6),37 as shown in Figure 1 (left). In this range, the emission ratio I580/I640 shows a dramatic increase with decreasing pH. However, C-SNARF-1 also shows a more subtle response at lower pH, likely due to further protonation of the molecule, as shown in Figure 1 (right). The latter involves the sequential addition of two protons in two equilibria with pKa ∼ 3−4. These can be used to probe the properties of much more acidic environments. It should be noted that the traditional definition of pH is not directly applicable at either the single-molecule level or in nanoscale environments incorporating small numbers of water molecules.39 Nevertheless, spectroscopic probes can be used to directly observe protonation and deprotonation events for single molecules in highly confined environments. The response of the dye in this case is interpreted as if it were derived from an ensemble of molecules in bulk solution. Thus, when a microenvironment is concluded to have an “effective pH of 5”, for example, this means the dye is protonated to the same degree as would be expected in a bulk solution of pH = 5. In this paper, we study the acidity properties of individual microenvironments found within mesoporous Al−Si thin films by SM methods. These materials serve as models for Al−Si solid acid catalysts. The pH-dependent emission properties of C-SNARF-1 are first explored at low pH in solution, prior to its implementation in SM studies of mesoporous Al−Si films. The results of these studies show that C-SNARF-1 can serve as a viable probe of pH below pH ∼ 5. In all SM studies, the dye is immobilized in the matrix at nanomolar concentrations so that spatially well-separated SMs can be located and their spectral emission characteristics determined. A wide-field fluorescence microscope is employed to collect pairs of fluorescence images (one showing emission near 580 nm, the other near 640 nm) of ∼20 randomly selected 400 μm2 regions in each sample. The pairs of fluorescent spots (one in each of the two images) produced by each molecule are fit to Gaussian functions to determine their locations and emission ratios (I580/I640). Histograms of the emission ratios are then constructed to obtain acidity distribution information. The width and peak position of each histogram are then assessed. These data show clear changes as a function of Al2O3 content in each film. The results are interpreted to reflect a trend toward increased microenvironmental acidity with increasing Al2O3 content. The
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EXPERIMENTAL SECTION
Sample Preparation. C-SNARF-1 was purchased from Invitrogen and was used as received. For doping of the Al−Si films, a 22 μM stock solution of C-SNARF-1 was first prepared in absolute ethanol. This solution was then diluted with HPLC grade water to give a 0.5 nM CSNARF-1 solution. The latter yielded well-dispersed SMs when spincast onto the Al−Si films. Aluminosilicate samples were prepared by co-condensation of separately prehydrolyzed precursor sols.40−42 The silica sol was prepared by mixing 0.25 g of tetramethyl orthosilicate (TMOS), 9.4 mL of absolute ethanol, 0.37 mL of water, 0.30 mL of 0.1 M HCl, and 0.10 mL of ethyl acetoacetate (EAA). The mixture was stirred at room temperature for 1 h. For the alumina sol, 0.10 mL of EAA was first added to 0.32 g of aluminum isopropoxide in ∼7 mL of isopropanol and stirred at room temperature for 2 h. The EAA served as a chelating agent;41 the aluminum-to-ligand ratio was 2:1. A stoichiometric quantity of water was then added to yield an Al:H2O mole ratio of 2.5:3. In order to obtain a clear sol, 150 μL of concentrated nitric acid was added dropwise. More isopropanol was then added to bring the final sol volume to 10 mL. The mixture was then stirred at room temperature for 1 h. A series of Al−Si sols were subsequently obtained by mixing appropriate amounts of the above prehydrolyzed sols. These sols were stirred for 1 h and allowed to stand overnight. A 0.024 g quantity of cetyltrimethylammonium bromide (CTAB, Aldrich) was then dissolved in each of the 2 mL sols by stirring for 1 h. The cocondensed sols were spin-cast onto glass microscope coverslips (FisherFinest Premium) to obtain thin Al−Si films. These samples were stored in a desiccator overnight. Surfactant, solvent, and acid were next removed by calcination to form the mesoporous films.15 Calcination also rules out the influence of these substances on the acidity of the films. In this process, the films were first heated at 110 °C for 90 min. The temperature was then increased to 500 °C at a ramp rate of 1 °C/min, where it was held for 5 h. After cooling, the films were cleaned in an air plasma (Harrick Plasma) for 5 min to remove luminescent residues. The calcined films were then loaded with probe molecules by spin-casting a small aliquot of 0.5 nM CSNARF solution. The final thickness of the films was measured to be 30 ± 3 nm by spectroscopic ellipsometry. Instrumentation and Methods. All SM studies were conducted on a wide-field fluorescence microscope that has been described previously.43 This system is built on an inverted epi-illumination microscope (Nikon TiE). Light from a Nd:YVO4 laser (Coherent, Verdi, 532 nm) was used to excite the dye molecules. It was first focused into a spinning optical diffuser and then collected and passed through a polarization scrambler before being directed into the epiillumination port of the microscope. The light was then reflected from a dichroic beam splitter (Chroma Q555LP) and focused into the back aperture of an oil immersion objective (Nikon Apo TIRF 100×, 1.49 NA). The incident laser power was maintained between 0.75 and 1.5 mW. Fluorescence from the sample was collected with the same objective and separated from the incident laser light by passing back through the beam splitter and a 550 nm long pass filter. The fluorescence was subsequently directed into an image splitter (Cairn B
DOI: 10.1021/acs.langmuir.5b01628 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Research, OptoSplit II), where it was further split by a second dichroic beam splitter (Chroma, 610dcxr). The two signal beams were individually directed through two band-pass filters: one centered at 580 nm and the other at 640 nm, both having 40 nm passbands. Images at these two wavelengths were then simultaneously recorded on a back-illuminated EM-CCD camera (Andor iXon DU-897). The individual, well-separated dye molecules appeared as well-defined individual spots in each of the two images. Software written in house (on the National Instruments Labview platform) was used to extract the side-by-side images, correct for any distortion of these images, locate the individual spots, and, finally, to fit their profiles to Gaussian functions. Image distortion consisted only of translational offsets in X and Y and a slight rotation of the images. The rotation was always 5) will exhibit a relatively large increase in their emission ratios while others near to (or on the low pH side of) the peak exhibit very small changes in their emission ratios. Taken together, these trends suggest an initial decrease in the distribution width as the alumina content increases, as is observed. Additional studies of this same series
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CONCLUSIONS Single-molecule fluorescence spectroscopic methods have been employed to investigate the microenvironmental acidity distributions in mesoporous aluminosilicate thin films that serve as models for solid acid catalysts. C-SNARF-1 was shown to be an effective probe of the local pH in media of low pH (i.e., 1 < pH < 5). Histograms of the emission intensity ratios of C-SNARF-1 SMs were used to elucidate the acidity distributions in mesoporous aluminosilicate films of varying Al2O3 content. The SM data revealed an increase in film acidity with increasing Al2O3 content. The results were interpreted to depict a shift in microenvironmental pH downward from pH ∼ 5 (pure silica) to pH ≤ 3 as the Al2O3 content of the film increased from 0% to 30%. The distributions obtained were broader than expected for homogeneous samples, as estimated from the measurement noise, depicting a high degree of heterogeneity in the acidity properties of these films. In fact, the range of microenvironmental pH values found within individual films was concluded to be similar to the full range of ensemble pH values detected across the entire series of samples explored. This work provides important new insights into the distribution of Brønsted acidity in solid acids that cannot be obtained by conventional techniques. It demonstrates that SM methods can be used to characterize the heterogeneity of solid acid catalysts.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (D.A.H.). *E-mail [email protected] (K.L.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support from the America Chemistry Society-Petroleum Research Fund (ACS PRF# 50829-ND5). The U.S. Department of Energy (DE-FG0212ER16095) is acknowledged for providing the optical microscope employed in these studies. Prof. Takashi Ito is thanked for his help with the ellipsometry experiments.
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REFERENCES
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(26) Ye, F.; Collinson, M. M.; Higgins, D. A. What Can be Learned from Single Molecule Spectroscopy? Applications to Sol-Gel-Derived Silica Materials. Phys. Chem. Chem. Phys. 2009, 11, 66−82. (27) Higgins, D. A.; Tran-Ba, K.-H.; Ito, T. Following Single Molecules to a Better Understanding of Self-Assembled OneDimensional Nanostructures. J. Phys. Chem. Lett. 2013, 4, 3095−3103. (28) Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J. Spatially Resolved Observation of Crystal-Face-Dependent Catalysis by Single Turnover Counting. Nature 2006, 439, 572−575. (29) Esfandiari, N. M.; Wang, Y.; Bass, J. Y.; Cornell, T. P.; Otte, D. A. L.; Cheng, M. H.; Hemminger, J. C.; McIntire, T. M.; Mandelshtam, V. A.; Blum, S. A. Single-Molecule Imaging of Platinum Ligand Exchange Reaction Reveals Reactivity Distribution. J. Am. Chem. Soc. 2010, 132, 15167−15169. (30) Xu, W.; Kong, J. S.; Yeh, Y.-T. E.; Chen, P. Single-Molecule Nanocatalysis Reveals Heterogeneous Reaction Pathways and Catalytic Dynamics. Nat. Mater. 2008, 7, 992−996. (31) Esfandiari, N. M.; Blum, S. A. Homogeneous vs Heterogeneous Polymerization Catalysis Revealed by Single-Particle Fluorescence Microscopy. J. Am. Chem. Soc. 2011, 133, 18145−18147. (32) Buurmans, I. L. C.; Ruiz-Martinez, J.; Knowles, W. V.; van der Beek, D.; Bergwerff, J. A.; Vogt, E. T. C.; Weckhuysen, B. M. Catalytic Activity in Individual Cracking Catalyst Particles Imaged Throughout Different Life Stages by Selective Staining. Nat. Chem. 2011, 3, 862− 867. (33) Shi, X.; Lim, J.; Ha, T. Acidification of the Oxygen Scavenging System in Single-Molecule Fluroescence Studies: In Situ Sensing with a Ratiometric Dual-Emission Probe. Anal. Chem. 2010, 82, 6132− 6138. (34) Mason, M. D.; Ray, K.; Pohlers, G.; Cameron, J. F.; Grober, R. D. Probing the Local pH of Polymer Photoresist Films Using a TwoColor Single Molecule Nanoprobe. J. Phys. Chem. B 2003, 107, 14219−14224. (35) Brasselet, S.; Moerner, W. E. Fluorescence Behavior of SingleMolecule pH-Sensors. Single Mol. 2000, 1, 17−23. (36) Fu, Y.; Collinson, M. M.; Higgins, D. A. Single-Molecule Spectroscopy Studies of Microenvironmental Acidity in Silicate Thin Films. J. Am. Chem. Soc. 2004, 126, 13838−13844. (37) Whitaker, J. E.; Haugland, R. P.; Prendergast, F. G. Spectral and Photophysical Studies of Bezo[c]xanthene Dyes: Dual Emission pH Sensors. Anal. Biochem. 1991, 194, 330−344. (38) De Cremer, G.; Sels, B. F.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Fluorescence Micro(spectro)scopy as a Tool to Study Catalytic Materials in Action. Chem. Soc. Rev. 2010, 39, 4703−4717. (39) Crans, D. C.; Levinger, N. E. The Conundrum of pH in Water Nanodroplets: Sensing pH in Reverse Micelle Water Pools. Acc. Chem. Res. 2012, 45, 1637−1645. (40) Yoldas, B. E. Hydrolysis of Aluminum Alkoxides and Bayerite Conversion. J. Chem. Technol. Biot. 1973, 23, 803−809. (41) Nass, R.; Schmidt, H. Synthesis of an Alumina Coating from Chelated Aluminum Alkoxides. J. Non-Cryst. Solids 1990, 121, 329− 333. (42) Nampi, P. P.; Moothetty, P.; Wunderlich, W.; Berry, F. J.; Mortimer, M.; Creamer, N. J.; Warrier, K. G. High-Surface-Area Alumina-Silica Nanocatalysts Prepared by a Hybrid Sol-Gel Route Using Boehmite Precursor. J. Am. Ceram. Soc. 2010, 93, 4047−4052. (43) Tran Ba, K. H.; Everett, T. A.; Ito, T.; Higgins, D. A. Trajectory Angle Determination in One Dimensional Single Molecule Tracking Data by Orthogonal Regression Analysis. Phys. Chem. Chem. Phys. 2011, 13, 1827−1835. (44) Stoeckel, D.; Wallacher, D.; Zickler, G. A.; Perlich, J.; Tallerek, U.; Smarsly, B. M. Coherent Analysis of Disordered Mesoporous Adsorbents Using Small Angle X-ray Scattering and Physisorption Experiments. Phys. Chem. Chem. Phys. 2014, 16, 6583−6592. (45) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. H
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DOI: 10.1021/acs.langmuir.5b01628 Langmuir XXXX, XXX, XXX−XXX
Single-Molecule Studies of Acidity Distributions in Mesoporous Aluminosilicate Thin Films Xiaojiao Sun,† Jingyi Xie,† Jiayi Xu,† Daniel A. Higgins,*,‡ and Keith L. Hohn*,† †
Department of Chemical Engineering and ‡Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States ABSTRACT: Solid acid catalysts are important for many petrochemical processes. The ensemble methods most often employed to characterize acid site properties in catalyst materials provide limited insights into their heterogeneity. Single-molecule (SM) fluorescence spectroscopic methods provide a valuable route to probing the properties of individual microenvironments. In this work, dual-color SM methods are adopted to study acidity distributions in mesoporous aluminosilicate (Al−Si) films prepared by the sol−gel method. The highly fluorescent pH-sensitive dye C-SNARF-1 was employed as a probe. The ratio of C-SNARF-1 emission in two bands centered at 580 and 640 nm provides an effective means to sense the pH of bulk solutions. In mesoporous thin films, SM emission data provide a measure of the effective pH of the microenvironment in which each molecule resides. SM emission data were obtained from mesoporous Al−Si films as a function of Al2O3 content for films ranging from 0% to 30% alumina. Histograms of the emission ratio reveal a broad distribution of acidity properties, with the film microenvironments becoming more acidic, on average, as the alumina content of the films increases. This work provides new insights into the distribution of Brønsted acidity in solid acids that cannot be obtained by conventional means.
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INTRODUCTION It is widely acknowledged that catalysts play a vital role in modern chemical industry,1,2 with more than 80% of all chemicals requiring at least one catalytic step during their synthesis.3,4 Compared to homogeneous systems, solid acid catalysts could help to reduce the environmental impact of chemical synthesis because they can serve as alternatives to hazardous mineral acids and eliminate a separation step to recover the catalyst.5 The Brønsted acidity of aluminosilicate (Al−Si) materials, such as zeolites and amorphous aluminosilicates, makes them suitable for use as solid acids in several petrochemical processes, including hydrocracking, isomerization and oligomerization of alkenes, and alkylation of aromatics, among other important reactions.6−9 The insertion of tetrahedral Al atoms into the silica matrix gives rise to catalytically active sites by compensation of the resulting excess negative charge by a proton.10,11 This Al-for-Si substitution model was proposed by Thomas12 and Tamele13 to explain the acidity of Al-containing materials. Many approaches have been used to study the acidity properties of Al−Si materials. Temperature-programmed desorption and FTIR are frequently used to measure materials acidity by probing interactions between their intrinsic acid sites and weak bases (e.g., ammonia and pyridine).14−19 NMR spectroscopy (e.g., 1H magic angle spinning) has also been employed to study the number and strength of Brønsted acid sites in solid acid catalysts.14,15 Classical titration measurements have also been reported.20 These techniques all provide valuable qualitative and sometimes quantitative data on solid acidity. However, it can be difficult to interpret the distribution of acid strengths using these techniques. Moreover, these techniques are incapable of probing individual acid sites or © XXXX American Chemical Society
providing location-specific information on acidity. Yet, it is the exact local properties of these individual microenvironments that govern the physical and chemical interactions between guest molecules and the solid matrix. Therefore, new methodologies are needed to probe the distribution of microenvironment acidity in heterogeneous catalysts. Recently, single-molecule (SM) fluorescence microscopy has been implemented in a number of catalyst studies because of its high temporal and spatial resolution and its ability to probe single-site properties.21−25 Valuable information can be accessed by analyzing the fluorescence intensity, emission spectrum, and polarization of individual guest molecules interacting with a catalyst medium.22−27 SM methods can also provide information on structural heterogeneities,23,28,29 reaction pathways30 and mechanisms,31 catalyst kinetics,23,28,30 and catalytic dynamics32 in and on solid materials. Several pHsensitive fluorescent dyes have been used to study acidity at the SM level in a variety of media, including biological systems,33 photoresists,34 and agarose gels.35 In previous work from the Higgins group,26,36 the pH-sensitive dye ((5′ and 6′)-carboxy10-(dimethylamino)-3-hydroxyspiro[7H-benzo[c]xanthene7,1′(3H)-isobenzofuran]-3′-one, C.SNARF-1)37 was used to explore the local acidity properties of pure silica films that had been exposed to solutions of different pH. The results showed that the C-SNARF-1 SMs were sensitive to the local pH, as determined by the treatment solutions and by surface silanol groups incorporated in the materials. To our knowledge, few SM studies have been devoted to probing microenvironmental acidity in common solid catalyst materials to date.22,38 Received: May 4, 2015
A
DOI: 10.1021/acs.langmuir.5b01628 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Chemical structures of C-SNARF-1 in its protonated and deprotonated forms around the pH of its first protonation equilibrium (left, pKa ∼ 7.6) and its second and third protonation equilibria (right, pKa ∼ 3−4).
widths of the distributions are also assessed; these reveal significant variations in the effective local pH.
Ideally, the proper dye molecule for SM studies of materials acidity should exhibit clear and well-defined changes in its emission spectrum with pH. C-SNARF-1 is a suitable probe that emits into two bands centered at 580 and 640 nm. The ratio of fluorescence emission in these two bands changes with pH in bulk solution and thus provides a means to study the effective local pH in close proximity to a dye molecule in solid materials.35−37 C-SNARF-1 is most commonly employed to probe pH near (i.e., within 1−2 pH units) the pKa of its first protonation equilibrium (pKa ∼ 7.6),37 as shown in Figure 1 (left). In this range, the emission ratio I580/I640 shows a dramatic increase with decreasing pH. However, C-SNARF-1 also shows a more subtle response at lower pH, likely due to further protonation of the molecule, as shown in Figure 1 (right). The latter involves the sequential addition of two protons in two equilibria with pKa ∼ 3−4. These can be used to probe the properties of much more acidic environments. It should be noted that the traditional definition of pH is not directly applicable at either the single-molecule level or in nanoscale environments incorporating small numbers of water molecules.39 Nevertheless, spectroscopic probes can be used to directly observe protonation and deprotonation events for single molecules in highly confined environments. The response of the dye in this case is interpreted as if it were derived from an ensemble of molecules in bulk solution. Thus, when a microenvironment is concluded to have an “effective pH of 5”, for example, this means the dye is protonated to the same degree as would be expected in a bulk solution of pH = 5. In this paper, we study the acidity properties of individual microenvironments found within mesoporous Al−Si thin films by SM methods. These materials serve as models for Al−Si solid acid catalysts. The pH-dependent emission properties of C-SNARF-1 are first explored at low pH in solution, prior to its implementation in SM studies of mesoporous Al−Si films. The results of these studies show that C-SNARF-1 can serve as a viable probe of pH below pH ∼ 5. In all SM studies, the dye is immobilized in the matrix at nanomolar concentrations so that spatially well-separated SMs can be located and their spectral emission characteristics determined. A wide-field fluorescence microscope is employed to collect pairs of fluorescence images (one showing emission near 580 nm, the other near 640 nm) of ∼20 randomly selected 400 μm2 regions in each sample. The pairs of fluorescent spots (one in each of the two images) produced by each molecule are fit to Gaussian functions to determine their locations and emission ratios (I580/I640). Histograms of the emission ratios are then constructed to obtain acidity distribution information. The width and peak position of each histogram are then assessed. These data show clear changes as a function of Al2O3 content in each film. The results are interpreted to reflect a trend toward increased microenvironmental acidity with increasing Al2O3 content. The
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EXPERIMENTAL SECTION
Sample Preparation. C-SNARF-1 was purchased from Invitrogen and was used as received. For doping of the Al−Si films, a 22 μM stock solution of C-SNARF-1 was first prepared in absolute ethanol. This solution was then diluted with HPLC grade water to give a 0.5 nM CSNARF-1 solution. The latter yielded well-dispersed SMs when spincast onto the Al−Si films. Aluminosilicate samples were prepared by co-condensation of separately prehydrolyzed precursor sols.40−42 The silica sol was prepared by mixing 0.25 g of tetramethyl orthosilicate (TMOS), 9.4 mL of absolute ethanol, 0.37 mL of water, 0.30 mL of 0.1 M HCl, and 0.10 mL of ethyl acetoacetate (EAA). The mixture was stirred at room temperature for 1 h. For the alumina sol, 0.10 mL of EAA was first added to 0.32 g of aluminum isopropoxide in ∼7 mL of isopropanol and stirred at room temperature for 2 h. The EAA served as a chelating agent;41 the aluminum-to-ligand ratio was 2:1. A stoichiometric quantity of water was then added to yield an Al:H2O mole ratio of 2.5:3. In order to obtain a clear sol, 150 μL of concentrated nitric acid was added dropwise. More isopropanol was then added to bring the final sol volume to 10 mL. The mixture was then stirred at room temperature for 1 h. A series of Al−Si sols were subsequently obtained by mixing appropriate amounts of the above prehydrolyzed sols. These sols were stirred for 1 h and allowed to stand overnight. A 0.024 g quantity of cetyltrimethylammonium bromide (CTAB, Aldrich) was then dissolved in each of the 2 mL sols by stirring for 1 h. The cocondensed sols were spin-cast onto glass microscope coverslips (FisherFinest Premium) to obtain thin Al−Si films. These samples were stored in a desiccator overnight. Surfactant, solvent, and acid were next removed by calcination to form the mesoporous films.15 Calcination also rules out the influence of these substances on the acidity of the films. In this process, the films were first heated at 110 °C for 90 min. The temperature was then increased to 500 °C at a ramp rate of 1 °C/min, where it was held for 5 h. After cooling, the films were cleaned in an air plasma (Harrick Plasma) for 5 min to remove luminescent residues. The calcined films were then loaded with probe molecules by spin-casting a small aliquot of 0.5 nM CSNARF solution. The final thickness of the films was measured to be 30 ± 3 nm by spectroscopic ellipsometry. Instrumentation and Methods. All SM studies were conducted on a wide-field fluorescence microscope that has been described previously.43 This system is built on an inverted epi-illumination microscope (Nikon TiE). Light from a Nd:YVO4 laser (Coherent, Verdi, 532 nm) was used to excite the dye molecules. It was first focused into a spinning optical diffuser and then collected and passed through a polarization scrambler before being directed into the epiillumination port of the microscope. The light was then reflected from a dichroic beam splitter (Chroma Q555LP) and focused into the back aperture of an oil immersion objective (Nikon Apo TIRF 100×, 1.49 NA). The incident laser power was maintained between 0.75 and 1.5 mW. Fluorescence from the sample was collected with the same objective and separated from the incident laser light by passing back through the beam splitter and a 550 nm long pass filter. The fluorescence was subsequently directed into an image splitter (Cairn B
DOI: 10.1021/acs.langmuir.5b01628 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Research, OptoSplit II), where it was further split by a second dichroic beam splitter (Chroma, 610dcxr). The two signal beams were individually directed through two band-pass filters: one centered at 580 nm and the other at 640 nm, both having 40 nm passbands. Images at these two wavelengths were then simultaneously recorded on a back-illuminated EM-CCD camera (Andor iXon DU-897). The individual, well-separated dye molecules appeared as well-defined individual spots in each of the two images. Software written in house (on the National Instruments Labview platform) was used to extract the side-by-side images, correct for any distortion of these images, locate the individual spots, and, finally, to fit their profiles to Gaussian functions. Image distortion consisted only of translational offsets in X and Y and a slight rotation of the images. The rotation was always 5) will exhibit a relatively large increase in their emission ratios while others near to (or on the low pH side of) the peak exhibit very small changes in their emission ratios. Taken together, these trends suggest an initial decrease in the distribution width as the alumina content increases, as is observed. Additional studies of this same series
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CONCLUSIONS Single-molecule fluorescence spectroscopic methods have been employed to investigate the microenvironmental acidity distributions in mesoporous aluminosilicate thin films that serve as models for solid acid catalysts. C-SNARF-1 was shown to be an effective probe of the local pH in media of low pH (i.e., 1 < pH < 5). Histograms of the emission intensity ratios of C-SNARF-1 SMs were used to elucidate the acidity distributions in mesoporous aluminosilicate films of varying Al2O3 content. The SM data revealed an increase in film acidity with increasing Al2O3 content. The results were interpreted to depict a shift in microenvironmental pH downward from pH ∼ 5 (pure silica) to pH ≤ 3 as the Al2O3 content of the film increased from 0% to 30%. The distributions obtained were broader than expected for homogeneous samples, as estimated from the measurement noise, depicting a high degree of heterogeneity in the acidity properties of these films. In fact, the range of microenvironmental pH values found within individual films was concluded to be similar to the full range of ensemble pH values detected across the entire series of samples explored. This work provides important new insights into the distribution of Brønsted acidity in solid acids that cannot be obtained by conventional techniques. It demonstrates that SM methods can be used to characterize the heterogeneity of solid acid catalysts.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (D.A.H.). *E-mail [email protected] (K.L.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support from the America Chemistry Society-Petroleum Research Fund (ACS PRF# 50829-ND5). The U.S. Department of Energy (DE-FG0212ER16095) is acknowledged for providing the optical microscope employed in these studies. Prof. Takashi Ito is thanked for his help with the ellipsometry experiments.
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REFERENCES
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