Structural, Optical, and pH-Stimulus Response Properties of Cresol

Article pubs.acs.org/IECR

Structural, Optical, and pH-Stimulus Response Properties of Cresol Red Immobilized Nanocomposite Silica Films Derived by a Sol−Gel Process Employing Different Synthetic Routes Shobhit Singh Chauhan,*,† R. V. Jasra,*,‡ and A. L. Sharma*,§ †

Analytical Discipline and Centralized Instrument Facility, CSIR-Central Salt and Marine Chemicals Research Institute, GB Marg, Bhavnagar 364002, Gujarat, India ‡ Reliance Technology Group, Reliance Industries Ltd., Vadodara 391 346, Gujarat, India § Department of Electronics and Instrumentation, Medi-Caps Instititute of Technology and Management, AB Road, Indore-453331, MP, India ABSTRACT: Formation of cresol red dye immobilized silica films derived by conventional as well as CTAB and P123 templated sol−gel is discussed. Comparative structural, textural, and optical properties of films derived by these three different methodologies were determined using analytical instrumental techniques like X-ray Diffraction (XRD), Atomic Force Microscope (AFM), Scanning Electron Microscope (SEM), surface area, and porosity measurements. Optical and pH sensing response properties of films were monitored by visible spectrophotometer. An attempt is made to demonstrate how incorporation of dyes affects the structure and interactions in films formed by different described methods leading to functional benefits in pH stimuli response behavior.

1. INTRODUCTION Since the discovery of the MCM41 class of mesoporous materials employing a long chain of alkylammonium salts,1 these materials have attracted the interest of the research community that has led to the development of mesoporous materials in hexagonal, cubic, and lamellar phases.2 These materials are synthesized, in general, by using surfactants that perform the function of architecting their structures in two- or three-dimensional formats. The cationic surfactant like cetyltrimethylammonium bromide (CTAB) has been used for the synthesis of hierarchically engineered structures like MCM41, MCM48, and MCM50 materials having varying pore dimensions from 2 to 10 nm, and underlying formation mechanistic pathways have been well understood.3,4 These materials can be constructed in various topologies like powders, monoliths, fibers, and films.5 The use of neutral diblock and triblock polymeric surfactants respectively of Brij and Pluronic classes, also, proved to be a new synthetic approach for the formation of mesoporous silica materials in 2D- and 3Dhexagonal, cubic, and lamellar format.6,7 The pore size could be tailored up to 30 nm by using higher EO:PO ratios in block copolymers that also adjusts mesoscopic-length scales of the formed structures. The development of mesostructured films in the supported format8 has emerged as the breakthrough in fabrication of devices where the materials in the powder form could not ever find the possibility of use. The coating of the film on substrates could be done either by dip- or spin-coating technique. The films are formed by an Evaporation Induced Self-Assembly (EISA) process that pronounces on consecutively increasing silica−surfactant concentration of the solution during coating, stimulated by graduated evaporating solvent and water as the film advances to its final cast state.9 The silica to surfactant © 2014 American Chemical Society

fraction, type, and chain length of hydrocarbon moiety in the surfactant and coating speed effect the formation of structures in 2D or 3D classified profile on the substrate.10,11 The methodology for incorporation of organic functions in the pores of mesoporous materials opened up a new area in materials synthesis and their applications. With the presence of organic species during the preparation of coating solution, the films can be imparted with functional features, and these find potential use in applications like pH sensing, optical filter, ion sensor, lasing, optical chemical, and biosensors etc.12−16 The silica materials and films can also be imparted functional features by a postgrafting method where the surface silanol groups are covalently attached to desired functional groups and its effect on host−guest interactions is also studied.17 The present article focuses on the comparative study in the differences brought about on the structural and optical properties of cresol red incorporated silica films formed by the traditional sol−gel method and also derived using CTAB and P123 templates leading to their merit for pH sensing response properties. Despite various works on the functionalization of silica films, reports on a similar study are scarce.

2. EXPERIMENTAL DETAILS 2.1. Reagents and Chemicals. Tetraethylorthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB), and Pluronic 123 (P123) were obtained from Sigma-Aldrich. Cresol Red (CR) was procured from BDH, India, and ethanol (EtOH) Special Issue: Ganapati D. Yadav Festschrift Received: Revised: Accepted: Published: 18863

February April 17, April 17, April 17,

27, 2014 2014 2014 2014

dx.doi.org/10.1021/ie500846z | Ind. Eng. Chem. Res. 2014, 53, 18863−18872

Industrial & Engineering Chemistry Research

Article

3. RESULTS AND DISCUSSIONS

was procured from SpectroChem, India. All the other chemicals were high purity commercially available products. Standard solutions and buffers were prepared with Millipore grade water. 2.2. Preparation of Sols. 2.2.1. By Sol−Gel Method. In a round-bottom flask of 100 mL capacity, 30 mL of ethanol, 30 mL of TEOS, and 10 mL of water were added with solution pH adjusted to 2 with 0.1 M HCl. After the solution was refluxed for 1 h, 190 mg of CR dye was added and stirred until the solution was homogeneous. The resulting solution was kept at room temperature for 4 days. The apparent viscous sol bearing an orangish-maroon color was termed as SG-CR sol. 2.2.2. By the CTAB Template Assisted Method. The hydrolysis of silica was carried out, first, by refluxing for an hour, 1 TEOS:3 EtOH:0.0005 HCl:1 H 2 O, in molar composition. Then, the solution of CTAB dissolved in ethanol separately was added to the prehydrolyzed solution. To attain the final composition of solution in molar ratio, 1 TEOS:20 EtOH:0.004 HCl:5 H2O:0.10 CTAB, additional water and HCl were subsequently added. This was followed by the addition of 40 mg of CR dye and stirring for several hours until the solution became homogeneous. The resulting sol was then stirred for 4 days at room temperature. The orange-yellow color of the resulting viscous sol presented the typical acidic nature of the CR dye which was termed as CTAB-CR sol. 2.2.3. By the P123 Template Assisted Method. The starting solution was prepared by mixing 2.08 g of TEOS, 5 mL of EtOH, 3 g of 0.2 M HCl, and 1.8 g of H2O which was refluxed at 80 °C for 1 h. Then, 20 mL of 5% solution of P123 prepared in ethanol was added to the prehydrolyzed silica solution along with 10 mL of ethanol. To this, 200 mg of CR dye was added with stirring for several hours until the solution became homogeneous. The resulting solution was then stirred for 4 days at room temperature. The color of the resulting apparent viscous solution was orangish-maroon typical of an acidic pH and was termed as P123-CR sol. 2.2.4. Properties of Sols. The molecular weight distribution and polydispersity index for SG-CR sol, CTAB-CR sol, and P123-CR sol as listed in Table 1 were determined by Gel

3.1. Structural Properties of Films. 3.1.1. XRD Study of Films. The structure of dye immobilized films was determined by X-ray diffraction (Philips X’Pert MPD) using Cu−Kα

Table 1. Viscosity, Molecular Weight Distribution, and Polydispersity Index Data of Sols SG-CR sol viscosity (cP) molecular weight distribution (Dalton) polydispersity index

1.4−1.5 800−2000 1.1−1.3

CTAB-CR sol 1.4−1.5 23000− 83500 1.1−1.2

P123-CR sol 2.8−3 2800− 9600 1.0−1.3

Permeation Chromatograph (Waters Alliance) from the calibration curve obtained from retention time of standard polystyrene dissolved in dimethylformamide. The difference in molecular weight distribution is accounted for the formation of different fractions of oligomers in respective sols. The polydispersity index from 1.1 to 1.3 for all three cases showed narrow molecular weight distribution of the siloxane oligomers in the sol. 2.3. Preparation of Films. The films were formed by spin coating at 2000 rpm on precleaned planar glass slides (75 × 25 × 10 mm3) using 100 μL of each sol. Film coated glass slides were left covered for 4 days at ambient conditions to achieve better condensation and polymerization reactions. The resulting films are correspondingly termed as the SG-CR film, the CTAB-CR film, and the P123-CR film.

Figure 1. XRD of the (a) SG-CR film, (b) CTAB-CR film, and (c) P123-CR film. 18864



Article

Figure 2. AFM image of the (a) SG-CR film, (b) CTAB-CR film, (c) P123-CR film, and (d) silica film.

3.1.2. AFM Study of Films. The influence of dye immobilization on the surface topography of films was examined by tapping-mode Atomic Force Microscope (NTMDT) at room temperature in air. The recorded images in 2D and 3D presentations were as shown in Figure 2. The dark regions can be assigned to the immobilized dye molecules resident in the porous voids of silica matrix. The CTAB-CR (Figure 2b) and P123-CR (Figure 2c) films were dense as compared to the SG-CR films (Figure 2a). Also, these films exhibited larger pores on surfaces as opposed to the SG-CR films which presented less porous structures. All three films were crack-free and bear discernible porous cavities. The open pore mouths were more visible in CTAB-CR and P123-CR films. These dye immobilized films were compared with silica films derived using CTAB (Figure 2d). It can be inferred that the silica film was the densest film where a less porous structure was visible and the surface roughness was under 65 nm. The section analysis of SG-CR films revealed the surface roughness was well below 10 nm indicating smoother coating surfaces. The surface roughness of CTAB-CR films was under 54−60 nm. The depth measurement of surface features revealed an average distance between elevations that varied from 35 to 50 nm for silica and the CTAB-CR film surface, indicating a marked difference in the surface morphology. P123-CR films had an average RMS surface roughness under 1.5 nm over the

radiation in the 2θ range 1−10°. The broad peak for the CRSG film was observed at 2θ = 2.52° (Figure 1a) corresponding to (100) reflection with an interplanar distance d100 = 35.05 Å. For the CTAB-CR film (Figure 1b), the peak at a 2θ value of 2.25° leads to an interplanar distance d100 = 39.22 Å and hexagonal unit cell length a = 2d100/√3 = 45.29 Å. There appeared only one prominent peak at 2θ = 1.51° for the P123CR film (Figure 1c) which can be ascribed to (100) reflections corresponding to a hexagonally arranged pore system of mesoporous materials.18 The interplanar distance d100 = 58.4 Å was found for the P123-CR film, which was basically the same as those of silica based mesoporous materials. The hexagonal unit cell length for the P123-CR film was a = 2d100/ √3 = 67.4 Å. These dye immobilized films exhibited only one prominent reflection in the low angle region corresponding to the mesoporous materials. The absence of reflections (110) and (210) implied that the channels run parallel to the surface of the film.18 The weak and broad diffraction patterns associated with all films were attributed to the reduced porosity owing to the pendent dye moieties that extended into the pores of the silica network, indicative of lower crystalline nature of films with short-range mesostructured order. Additionally, it was inferred from XRD results that the mesostructure was prevalent in dye immobilized silica films.19 18865



Article

Figure 3. SEM image of the (a) SG-CR film (Mag. 25K), (b) SG-CR film cross-section view (Mag. 20K), (c) CTAB-CR film (Mag. 25K), (d) CTAB-CR film cross-section view (Mag. 20K), (e) P123-CR film (Mag. 20K), and (f) P123-CR film cross-section view (Mag. 20K).

densest (Figure 3e). Hardly any grains like feature was evident from an image which, in fact, is observed in the AFM image of all the films. The presented high magnification images of P123CR films recorded at 20KX without template extraction displayed much better surface features and quality than the images shown at 10KX magnification after extraction of the P123 template from an amino-functionalized mesoporous silica film.21 This can be attributed to higher solubility and stabilization of dyes in the hydrophobic domains within the self-assembled mesostructured silica network. The thickness of films was estimated by observing their cross-section view. The thickness of the SG-CR film recorded

scanned area. This indicated that there were the least surface defects among the films under study, due to the film inorganic/ organic hybrid nature with Pluronic P123 functioning as a drying control agent.20 These results indicate the immobilization of dyes in films had brought about a significant impact on their surface properties. 3.1.3. SEM Study of Films. The Scanning Electron Microscope (Leo 1430VP) micrograms (Figure 3) of all films were recorded to assess the differences in surface morphological features. The surface of all films appeared to be smooth and crack free with uniform distribution of dye particles throughout the film surface. Of all films, the P123-CR films were the 18866



Article

Figure 4. Thermal gravimetric curves for SG-CR, CTAB-CR, and P123-CR film materials.

Table 2. Weight Loss and Associated Temperature for Dye Immobilized Film Materials SG-CR

CTAB-CR

P123-CR

temp (°C)

weight loss (%)

temp (°C)

weight loss (%)

temp (°C)

weight loss (%)

68 285 432 -

5.8 11.1 19.7 -

50 299 488 665

8.2 31.6 6.5 5

72 267 422 -

7 14.7 15.3 -

Table 3. Surface Area and Pore Size of Dye Immobilized Film Materials and Their Calcined Equivalents surface area (m2/g)/pore size (nm) uncalcined

calcined

SG-CR

CTAB-CR

P123-CR

SG-CR-c

CTAB-CR-c

P123-CR-c

0.8/1

1.4/3.88

2.1/1.84

913/3.54

933/3.45

856/5.28

was 515 nm (Figure 3b), while that of CTAB-CR and P123-CR films was 519 nm (Figure 3d) and 1.94 μm (figure not shown), respectively. The thickness in micrometer regime was attributed to the relatively high viscosity of sol ca. 3 cP and greater degree of polymerization owing to higher molecular weight P123. To assess the impact of sol dilution on the film thickness, the P123CR sol was diluted with ethanol arbitrarily in a 1:5 ratio, and the film was again formed by spin coating at 2000 rpm. The thickness of the film observed was 308 nm (Figure 3f). 3.1.4. Thermal Gravimetric Analysis Study of Film Materials. In order to understand the thermal stability of chemical species present in dye immobilized silica films, corresponding powder materials obtained after drying of SGCR, CTAB-CR, and P123-CR sol were analyzed by thermogravimetric analysis (Mettler Toledo TGA851) in 50− 800 °C temperature range as shown in Figure 4. In all three weight loss curves, the first regime corresponded to removal of solvent residues and adsorbed water, and the second regime corresponded to thermal decomposition of dye in consonance with the melting point of dye22 followed by water removal from framework hydroxyls. The final regime of weight loss was indicative of calcination of residual organics. The first order

Figure 5. Absorption spectra of the (a) SG-CR sol, (b) SG-CR film, (c) CTAB-CR sol, (d) CTAB-CR film, (e) P123-CR sol, and (f) P123CR film.

mathematical derivative operation was applied on all curves to pin−out the temperature corresponding to weight loss. The 18867



Article

Figure 6. Absorption vs pH response curves of the (a) aqueous CR, (b) SG-CR film, (c) CTAB-CR film, and (d) P123-CR film.

summary of these results is presented in Table 2. The lower weight loss in all cases was presumably because of good degree of polymerization for siloxane network formation. It can be inferred from the thermal decomposition data that thermal stability of cresol red dye improved upon immobilization in silica materials which could be extended to all three cases. 3.1.5. Surface Area and Pore Size Distribution Characteristics. Nitrogen sorption measurements at 77 K were performed with Micromeritics ASAP 2010 on all materials obtained after drying the sol under ambient conditions. These samples were subsequently calcined at 500 °C for 5 h to obtain materials termed as silica-c, CTAB-CR-c, and P123-CR-c. The surface area and pore size results of these materials were as listed in Table 3. The low surface area and reduced porosity in native materials was suggestive of incorporation of dye in the pores of these materials. However, dye and template got carbonized when these materials were calcined at 550 °C as evidenced by TGA results leading to porous silica framework of higher surface area and pore diameter. 3.2. Optical Properties of Films. Transmission properties of films are affected by optical inhomogeneities that results due to variations in structure of films and their thickness. This is

caused either by physical properties of the starting materials or by parameters governed during film deposition. The extinction in transmission is related to the absorption by the medium in question. The optical absorption property of all films was studied by spectrophotometer (Shimadzu UV−vis 2550) in the range 350−750 nm and was compared with respective sols to study the effect of immobilization of dye. Figure 5 shows the comparative absorption spectra of the SG-CR film and corresponding sol. The optical spectrum of the SG-CR film (Figure 5b) exhibited absorption bands corresponding to the acidic form at 441 nm as compared to 449 nm of another study,23 while the absorption peaks for the corresponding SGCR sol were found at 422 nm and hump centered at 533 nm (Figure 5a). The absorption peaks for the CTAB-CR film were at 434 nm (Figure 5d) which is close to 445 nm of another study,23 while that of corresponding CTAB-CR sol were 395 and 524 nm (Figure 5c). The absorption peaks for the P123CR film were at 438 nm (Figure 5f) while that of corresponding P123-CR sol were 404 and 523 nm (Figure 5e). There was shift in absorbance maxima for all dye immobilized films when compared to 432 and 570 nm dye absorption peaks for CR.22 The shifts in absorption bands 18868



Article

films formed by different synthetic methods had altered the spectral maxima wavelengths of these films, implying the dye has interacted differently with the host silica framework. 3.3. pH-Stimuli Response Properties of Films. The acid−base equilibrium of CR dye used in the study can be expressed by equation: HCR ↔ H+ + CR− where HCR represents the protonated form and CR− represents the deprotonated form of dye. In SG-CR films, the protonated form HCR corresponded to λmax = 411 nm, while the deprotonated form CR− corresponded to λmax = 565 nm. Similarly, for CTAB-CR and P123-CR films, λmax = 431 and 438 nm respectively represents the protonated form, while the deprotonated form for these two films was at λmax = 520 nm and λmax = 574 nm. As opposed to these, the absorption peaks for CR in aqueous solution lies at 432 and 570 nm.22 The optical pH response of all films was determined upon immersion in solutions having pH in the range of 1−13 by UV−vis spectrophotometer. All films were initially yelloworange in color which changed to red-violet upon immersion in 10 pH buffer and exhibited red-yellow color in 2 pH buffer, implying characteristic solution color properties of the dye were retained in doped films. For the SG-CR film, the absorption maxima at 565 nm (λmax) for different pH buffer solutions were recorded and plotted as shown in Figure 6b. These values were similarly recorded and plotted at 520 nm (λmax) for the CTAB-CR film shown in Figure 6c and for P123-CR films at 574 nm (λmax) shown in Figure 6d. These absorption maxima were close to 575 nm,26 525 nm,23 and 570 nm27 of other works. The pH response of all the films was found to extend upon immobilization in the silica film as opposed to solution state response of CR dye (Figure 6a). It was found that the sensor responds fairly well in the range 2−12 pH units in all cases. This dynamic range is large as compared to 9−13 of other studies.28 P123-CR films had better pH measurement range than 4.5−13 pH range where CR along with the other two dyes were used in films formed with neutral Triton X-100 surfactant.29 The pH response curve was fitted to third-order polynomial, and the point of inflection to the original curve was determined as the pKa value of CR in the film. It was found that the pKa value of CR dye upon immobilization in SG-CR was 8.7, while it was 9.7 and 9.2 respectively for CTAB-CR and P123-CR films. The pKa shifted by 2−2.5 pH units in dye doped films as opposed to known 7.9 in aqueous solution.22 In other studies, pKa for the immobilized CR film was 10.530 and 9.07 for CR the immobilized monolith,23 while for the CTABCR immobilized monolith, pKa was 10.5.23 These studies employed different precursors, reactant concentrations, and reaction conditions resulting in varying material topologies; hence there are slight differences in these values as compared to our work. It can be inferred from these results that material topologies also played an important role in sensor response characteristics and pKa of dye in the film.31 The pH response time and repeatability of the all dye immobilized films to pH step change from 10 to 5 and vice versa was as shown in Figure 7. The SG-CR film responded reversibly and instantaneously in ca. 1.5 s in both directions (Figure 7a) which is much better than 120 s28 and 60 s30 response time reported in other studies. Figure 7a also showed the repeatability in pH response which was recorded after ca. 20 pH measurements. Thus, with SG-CR films, 6 more pH measurements could be done beyond which the signal diminished less than 10% of its maximum indicated leaching

Figure 7. Time response curves to alternate pH 10 and pH 5 solutions for the (a) SG-CR film, (b) CTAB-CR film, and (c) P123-CR film.

associated with dye immobilized films imply that dye molecules have chemically interacted with silica network differently depending on their polarity, which altered the film matrix polarity.24,25 It can be observed that CR dye retained absorption response characteristics though at different wavelengths even when immobilized in silica matrix either in the form of film or liquid sol. The spectral transmittance depends on the thickness of the film which is dependent on the ratio of participating EtOH/ TEOS contents of film forming sol.14 Of all the formed films, P123-CR films were thickest (1.94 μm), while the thickness of SG-CR and CTAB-CR was of the order ca. 400 nm. In spite of the P123-CR film being thicker than SG-CR and CTAB-CR films, P123-CR films exhibited 98% transmittance in CR insensitive spectral band. Similar results were also obtained for SG-CR and CTAB-CR films. This indicated homogeneous formation of CR immobilized films even though derived by three different methods. The immobilization of CR dye in these 18869



Article

Figure 8. SEM images of films after pH measurements: the (a) SG-CR film after 45 cycles of pH measurement-Mag. 29K, (b) leached SG-CR filmMag. 25K, (c) CTAB-CR film after 55 cycles of pH measurement-Mag. 25.9K, (d) leached CTAB-CR film-Mag. 25K, (e) P123-CR film after 30 cycles of pH measurement-Mag. 20K, and (f) leached P123-CR film-Mag. 25K.

of dye from the film. Similarly, the relative signal levels, response time in both directions, and reversibility of the CTABCR film to pH 5 and 10 solutions were as shown in Figure 7b. The transitions in both directions were 1.2 s and nearly the same, which is better than the 12 s response time of fluorescent HPTS in 1 μm film thickness derived with CTAB.32 These transitions were recorded after ca. 30 pH measurements. It was found that the repeatable response of the CTAB-CR film to pH estimations lasted to only 35 cycles of pH measurements beyond which dye leaching caused unpredictable pH sensory response. P123-CR films (Figure 7c) displayed repeatable

transitions in both directions in 1.1 s as opposed to 3 min with a polymeric agarose film doped with congo red33 and 1.5 min to 2 min with neutral red.34 Figure 7c shows pH transitions after pH measurements to a count of ca. 10 with inference that P123-CR could be used for 12−15 cycles of pH measurements. The shifts in absorption maxima and pKa of dye in all three films were on account of varied amounts of protonation− deprotonation of dye in microheterogeneous structured sites in the film, leading to alterations in acid−base equilibrium and chemical structure of dye.35 It is known that these are brought about by the number of participating silanol groups and their 18870



Article

methods. The larger pore dimensions of P123 films deduced by AFM defied their utility to lesser pH measurement counts as opposed to other two films. The pH sensing dynamic range was quite similar in all three types of films. The optical absorbance maxima of cresol red got shifted in the film state as compared to its aqueous counterpart. There was over 98% transmission with no significant optical transmission loss through all three films in spectral insensitive regions of cresol red. The synthesis methodology also affected the absorption maxima in all three cases differently. Based on the outcomes of this report, a judicious choice of synthetic strategy can be employed to strike a trade-off between optical, physical stability, and pH response properties of films correlated to their structural characteristics.

surface chemistry, ion-pair formation, and interactions with surfactants.36 These interactive progressions render modulations in its polarity and hydrophobicity that drives dye molecules to spatially reorient in charge favorable locations within the heterogeneous silica compartments spread throughout the film,37 affecting the sensor response range and time. The wide-ranging textural and structural properties of these films had also affected in favor of this extended pH measurement dynamic range along with sensor response time as evident from the present study. 3.4. Physical Stability of Films. The physical adherence of all films to glass substrate was checked by the scratch test method. The 3 M Scotch tape was stuck uniformly on the films by rubbing it by hand ensuring no trapped air bubbles. The tape was then peeled-off suddenly in a single stretch at ca. 180° to the film. There were no visible film flakes either on the tape nor did the film appear scratched from any place on the glass substrate. This test indicated that all three films were intact on the glass surface and scratching by tape did not destroy the coated films displaying good degree physicochemical interactions between the glass substrate and the film material. During experiments it was found that the thinner films were less susceptible to peel-off during the tape test or their utilization as pH sensor. The solvent rich synthesis condition in the present case led to less viscous sol, resulting in thinner SGCR and CTAB-CR films. P123-CR films of 1.94 μm thickness did not exhibit good physical adherence to glass substrate during the tape test. However, when P123-CR sol was diluted to form ca. 500 nm thickness P123-CR films, they were resistant to peel test. The films reported in other studies were derived in low solvent condition leading to thicker films,30,38 and in another study the sol was diluted to achieve thinner films39 with no reference on film stability. The cleaning of glass substrate also played an important role for the formation of uniform and crack-free films imparting better stability to films in this study. To assess how the physical features of films got mutilated, all films were recorded with SEM after they had been subjected to pH measurements. The images shown in Figure 8(a, c, e) were recorded for SG-CR, CTAB-CR, and P123-CR films after they were used for 45, 55, and 30 cycles of pH measurements. It could be observed that the dye no longer exist in a homogeneous distribution pattern in the film. Rather, dye had leached out from the film, and, also at many places, the film fragments appeared to have peeled-off. As can be seen from SEM image Figure 8 (b, d, f), the continuous use of these films made them scantly populated with sensory dye molecules. The difference in usage cycles of pH measurements are attributed to the differences in film structure, size of pores, and varied interaction of cresol red dye with the local heterogeneous environment of the silica films derived by different methods.

■

AUTHOR INFORMATION

Corresponding Authors

*(S.S.C.) Phone: +91 278 2567760. Fax: +91 278 2566970. E‑mail: [email protected]. *(R.V.J.) Phone: +91 265 6693935. Fax: +91 265 6693934. E‑mail: [email protected], [email protected]. *(A.L.S.) Phone: +91 7292 424605. Fax: +91 7292 424501. E‑mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS CSIR-CSMCRI Communication Number: 038/2014. S.S.C. acknowledges the encouragement and support of Dr. P. K. Ghosh, Director, CSIR-CSMCRI and Dr. P Paul, Discipline Coordinator, ADCIF. S.S.C. expresses his thankfulness to Dr. Babulal Rebary, Dr. Pragnya Bhatt, Mr. Harshad Bhrambhatt, Mr. Jayesh Chaudhary, Ms. Sheetal, and Mr. Mahesh Sanghani for analytical support and Dr. Kamesh Prasad for viscosity measurements. S.S.C. also acknowledges useful discussions with Dr. H. C. Bajaj and Dr. Rajesh Tayade.

■

REFERENCES

(1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCulen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1991, 114, 10834−10843. (2) Huo, Q.; Margolese, D. I.; Stucky, G. D. Surfactant Control of Phases in the Synthesis of Mesoporous Silica-Based Materials. Chem. Mater. 1996, 8, 1147−1160. (3) Lin, H. P.; Mou, C. Y. Structural and morphological control of cationic surfactant-templated mesoporous silica. Acc. Chem. Res. 2002, 35, 927−935. (4) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chemical strategies to design textured materials: from microporous and mesoporous oxides to nanonetworks and hierarchical structures. Chem. Rev. 2002, 102, 4093−4138. (5) Zhao, D.; Yang, P.; Huo, Q.; Chmelka, B. F.; Stucky, G. D. Topological construction of mesoporous materials. Curr. Opin. Solid State Mater. Sci. 1998, 3, 111−121. (6) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chemlka, B. F.; Stucky, G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548−552. (7) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures. J. Am. Chem. Soc. 1998, 120, 6024−6036.

4. CONCLUSIONS In summary, the undertaken study presents the formation of dye immobilized films by the sol−gel process employing three different routes and offers a unique comparative presentation on the optical, structural, and pH response properties of these derived films. The films formed by simple sol−gel and CTAB displayed better physical stability than those derived by P123. The synthetic strategy played a role in defining structural features of films which affected their pH response performance since it was found that P123 films could be used for fewer tallies to pH measurements than the films derived by other two 18871



Article

(8) Ogawa, M. Formation of Novel Oriented Transparent Films of Layered Silica-Surfactant Nanocomposites. J. Am. Chem. Soc. 1994, 116, 7941−7942. (9) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Evaporation-induced self-assembly: nanostructures made easy. Adv. Mater. 1999, 11, 579− 585. (10) Besson, S.; Gacoin, T.; Ricolleau, C.; Jacquiod, C.; Boilot, J. P. Phase diagram for mesoporous CTAB−silica films prepared under dynamic conditions. J. Mater. Chem. 2003, 13, 404−409. (11) Alberius, P. C.; Fridell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. General predictive syntheses of cubic, hexagonal, and lamellar silica and titania mesostructured thin films. Chem. Mater. 2002, 14, 3284−3294. (12) Carrington, N. A.; Xue, Z. L. Inorganic sensing using organofunctional sol-gel materials. Acc. Chem. Res. 2007, 40, 343−350. (13) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P. Optical properties of functional hybrid organic-inorganic nanocomposites. Adv. Mater. 2003, 15, 1969−1994. (14) Chauhan, S. S.; Jasra, R. V.; Sharma, A. L. Phenol red dye functionalized nanostructured silica films as optical filters and pH sensors. Ind. Eng. Chem. Res. 2012, 51, 10381−10389. (15) Guan, X.; Li, D.; Song, J.; Ji, Y.; Xiao, F. S. One-pot synthesis of fluorescent mesoporous materials for detection of the presence of Be2+ ion. J. Porous Mater. 2008, 15, 527−533. (16) Appert, E. B.; Monnier, V.; Duong, T. H.; Pansu, R.; Ibanez, A. Polyaromatic Luminescent Nanocrystals for Chemical and Biological Sensors. Chem. Mater. 2004, 16, 1609−1611. (17) Park, J. W.; Park, Y. J.; Jun, C. H. Post-grafting of silica surfaces with pre-functionalized organosilanes: new synthetic equivalents of conventional trialkoxysilanes. Chem. Commun. 2011, 47, 4860−4871. (18) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Continuous formation of supported cubic and hexagonal mesoporous films by sol−gel dip-coating. Nature 1997, 389, 364−368. (19) Zhou, H. S.; Sasabe, H.; Honma, I. Synthesis of phthalocyaninedoped silica mesostructured materials by ferrocenyl surfactant. J. Mater. Chem. 1998, 8, 515−516. (20) Rossi, U. D.; Daehne, S.; Reisfeld, R. Photophysical properties of cyanine dyes in sol-gel matrices. Chem. Phys. Lett. 1996, 251, 259− 267. (21) Zhang, X.; Wang, J.; Wu, W.; Liu, C.; Qian, S. Preparation of amino-functionalized mesoporous silica thin films with highly ordered large pore structures. J. Sol-Gel Sci. Technol. 2007, 43, 305−311. (22) Sabnis, R. W. Handbook of Acid-Base Indicators; CRC Press: Boca Raton, FL, 2008. (23) El Nahhal, I. M.; Zourab, S. M.; Kodeh, F. S.; Babonneau, F.; Hegazy, W. Sol−gel encapsulation of cresol red in presence of surfactants. J. Sol-Gel Sci. Technol. 2012, 62, 117−125. (24) Lobnick, A.; Wolfbeis, O. S. Probing the Polarity of Sol-Gels and Ormosils via the Absorption of Nile Red. J. Sol-Gel Sci. Technol. 2001, 20, 303−311. (25) Moreno, E. M.; Levy, D. Role of the comonomer GLYMO in ORMOSILs as reflected by nile red spectroscopy. Chem. Mater. 2000, 12, 2334−2340. (26) Seki, A.; Katakura, H.; Kai, T.; Iga, M.; Watanabe, K. A heterocore structured fiber optic pH sensor. Anal. Chim. Acta 2007, 582, 154−157. (27) Jerónimo, P. C. A.; Araújo, A. N.; Montenegro, M. C. B. S. M.; Satinský, D.; Solich, P. Flow-through sol−gel optical biosensor for the colorimetric determination of acetazolamide. Analyst 2005, 130, 1190−1197. (28) Wu, S.; Cheng, W.; Qiu, Y.; Li, Z.; Shuang, S.; Dong, C. Fiber optic pH sensor based on mode-filtered light detection. Sens. Actuators, B 2010, 144, 255−259. (29) Dong, S.; Luo, M.; Peng, G.; Cheng, W. Broad range pH sensor based on sol−gel entrapped indicators on fibre optic. Sens. Actuators B 2008, 129, 94−98. (30) Makote, R.; Collinson, M. M. Organically Modified Silicate Films for Stable pH Sensors. Anal. Chim. Acta 1999, 394, 195−200.

(31) Jurmanovic, S.; Kordic, S.; Steinberg, M. D.; Steinberg, I. M. Organically modified silicate thin films doped with colourimetric pH indicators methyl red and bromocresol green as pH responsive sol−gel hybrid materials. Thin Solid Films 2010, 518, 2234−2240. (32) Wencel, D.; MacCraith, B. D.; McDonagh, C. High performance optical ratiometric sol−gel-based pH sensor. Sens. Actuators, B 2009, 139, 208−213. (33) Hashemi, P.; Abolghasemi, M. M. Preparation of a novel optical sensor for low pH values using agarose membranes as support. Sens. Actuators, B 2006, 115, 49−53. (34) Hashemi, P.; Zarjani, R. A.; Abolghasemi, M. M.; Olin, Å. Agarose film coated glass slides for preparation of pH optical sensors. Sens. Actuators, B 2007, 121, 396−400. (35) Rottman, C.; Avnir, D. Getting a library of activities from a single compound: tunability and very large shifts in acidity constants induced by sol-gel entrapped micelles. J. Am. Chem. Soc. 2001, 123, 5730−5734. (36) 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. (37) Rottman, C.; Grader, G.; Hazan, Y. D.; Melchior, S.; Avnir, D. Surfactant-Induced Modification of Dopants Reactivity in Sol−Gel Matrixes. J. Am. Chem. Soc. 1999, 121, 8533−8543. (38) Wang, E.; Kwok-Fan, C.; Kwan, V.; Chin, T.; Wong, C.; Bocarsly, A. Fast and Long Term Optical Sensors for pH Based on Sol-Gels. Anal. Chim. Acta 2003, 495, 45−50. (39) Gupta, G.; Atanassov, P.; López, G. P. Robust hybrid thin films that incorporate lamellar phospholipid bilayer assemblies and transmembrane proteins. Biointerphases 2006, 1, 6−10.

18872


Structural, Optical, and pH-Stimulus Response Properties of Cresol

Recommend Documents