Titania Nanocrystal Surface Functionalization ... - ACS Publications

Research Article www.acsami.org

Titania Nanocrystal Surface Functionalization through Silane Chemistry for Low Temperature Deposition on Polymers Jonathan Watté, Wouter Van Gompel, Petra Lommens, Klaartje De Buysser, and Isabel Van Driessche* SCRiPTS, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium

ABSTRACT: A method to obtain photocatalytically active thin films of anatase nanocrystals on polymer substrates was explored. Anatase nanocrystals were synthesized by a fast hydrolysis synthesis in an apolar solvent and characterized with regard to their crystallinity, size, and dispersibility and the stability of the resulting suspensions. The stable titania nanocrystal suspensions were further processed for their use in polar solvents using ligand exchange. Oleic acid was exchanged for 3aminopropyltriethoxysilane (APTES), resulting in aqueous suspensions of charge-stabilized nanocrystals. These were adapted for use as coating suspensions for surface-treated PMMA substrates in order to obtain thin films containing anatase nanocrystals covalently coupled to the surface of the PMMA substrates. Thereby, the ligand exchange was beneficial for increasing the compatibility and durability of the inorganic/organic composite, by the formation of a covalent amide bond between the silane ligands on the nanocrystals and the carboxylic acid groups on the polymer substrate. The surface morphology, transparency, and photocatalytic activity toward the degradation of organic pollutants of the coatings, obtained through dip-coating, were evaluated. KEYWORDS: TiO2, nanoparticles, ligand exchange, surface functionalization, polymer substrates, photocatalysis

■

tion, and spray-coating7−9 are described. However, these techniques are often time-consuming and expensive, resulting in an undesired synthesis route for industrial upscaling. A second challenge is the limited wettability of the polymer substrates by the used coating suspensions. A third one is the improvement of the durability of titania coatings on polymer substrates.1,10,11 Silanes were chosen as chemical linkers because of their established use as linkers at organic−inorganic interfaces and their proven compatibility with titania12−15 as well as polymer surfaces.16 To our knowledge, the use of ligands at the surface of anatase nanocrystals to act as chemical linkers between these nanocrystals and the surface of a polymer substrate has not been described in the literature. For our research, PMMA was chosen as a substrate because of its transparency and the property that its surface functional groups (methyl ester units) can be converted into carboxylic acid groups.14 Thereby, nanoparticles functionalized with ligands that possess an amine functionality can be coupled to the surface via the formation of an amide bond (Figure 1). Next to

INTRODUCTION Titanium dioxide can be used to create transparent, photocatalytically active thin films toward the degradation of organic matter, and antibacterial coatings.1 Under the influence of UV light, electron−hole pairs are created. The electrons can reduce electron acceptors while the holes can oxidize electron donors. The resulting radicals can subsequently degrade organic pollutants.2 Self-cleaning coatings can mitigate staining, fogging, and the odor and deterioration caused by dirt. Furthermore, some of these coatings have antibacterial properties. Selfcleaning coatings are commercially highly relevant on glass;3 however, the transition to polymer substrates would open up a large and growing market for coatings on (touch)screens, signs, visors, sunglasses, and noise barriers on highways.3,4 This transition to polymer substrates poses a couple of challenges, a first one being the thermal sensitivity of polymers, which excludes the use of high temperature processes to obtain crystalline titanium dioxide coatings.5,6 This problem was circumvented by synthesizing suspensions that already contain crystalline titania nanocrystals. In literature, research on titania coatings deposited on unmodified and surface-treated polymer substrates by using a variety of deposition techniques, such as radio frequency magnetron sputtering, chemical vapor deposi© 2016 American Chemical Society

Received: July 20, 2016 Accepted: October 13, 2016 Published: October 13, 2016 29759

DOI: 10.1021/acsami.6b08931 ACS Appl. Mater. Interfaces 2016, 8, 29759−29769

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the route followed to obtain thin films containing titania nanocrystals covalently coupled to the surface of PMMA substrates: (1) precursor solution containing stabilizing ligands and a titanium alkoxide; (2) suspension containing crystalline nanocrystals, in situ stabilized by ligands; (3) ligand exchange for APTES (Y = NH2); (4, 5) nanocrystals coupled to PMMA substrates via the formation of an amide bond (X = COO−, Z = NHCO); (6) photocatalytically active anatase thin films on polymer substrates. according to an adaption of a ligand exchange performed on iron oxide nanoparticles.18 The starting point for ligand exchange was an optically clear suspension of nanocrystals in toluene containing 100 mg of nanopowder (Figure 2a). Subsequently, the volume of toluene was

these practical reasons, PMMA was chosen because it is relatively cheap and a (durable) titania coating on this polymer would be highly relevant from a commercial point of view. PMMA is frequently used as a glass substitute (i.e., Plexiglas), and its major applications include (automotive) glazing, lighting fixtures, signs, and displays. Despite the importance, few researchers have investigated the (long-term) durability of their titania coating on a polymer substrate. The covalent linking of the nanocrystals to the surface of the polymer substrate that we here describe is envisioned to improve the durability of the final titania coating. 3-Aminopropyltriethoxysilane (APTES) ligands, obtained through exchange with the original ligands, were chosen as chemical linkers. In this work, a contribution was made to the development of transparent photocatalytically active coatings on polymer substrates. Aqueous suspensions of APTES functionalized nanocrystals were used to deposit coatings on PMMA substrates via dip-coating. These coatings were analyzed with regard to transparency, surface morphology, and photocatalytic activity. Raman and FTIR spectroscopy were used to chemically characterize the coatings.

■

Figure 2. Suspensions obtained in different steps of the ligand exchange procedure: (a) suspension of nanorods with OLAC in toluene; (b) suspension of nanorods functionalized with APTES in toluene (ligand exchange mixture); (c) suspension of nanorods functionalized with APTES transferred to water after workup, at initial pH 10.3; (d) clear suspension of nanorods functionalized with APTES, in water with the pH altered to ∼2.

EXPERIMENTAL SECTION

increased to a total of 25 mL, and 3.56 mol of Et3N/(mol of TiO2) was added. Next, a small amount of Milli-Q water (0.55 mol/(mol of TiO2)) was added in order to hydrolyze the silane alkoxy groups. The flask was then flushed with argon, and 0.43 mol of APTES/(mol of TiO2) was injected. The mixture was left stirring for 5 h at 50 °C under an argon atmosphere. Afterward, the mixture became very turbid (Figure 2b), and precipitation occurred after standing for ∼10 min without stirring. The suspension was centrifuged at 5000 rpm for 5 min, and the resulting powder was washed three times with acetone to remove unreacted silane. Subsequently, the powder was redispersed in Milli-Q water. After sonication, a turbid suspension with pH 10.5 was obtained as can be seen in Figure 2c. The pH of this suspension was lowered with diluted hydrochloric acid to pH ∼ 2, to obtain an optically clear dispersion (Figure 2d). The procedure to obtain the aqueous suspensions, from the synthesis of the nanorods to dispersing the nanorods in (acidic) water to obtain clear suspensions, was found to be very reproducible. Surface Treatment of the PMMA Substrate. The substrates used were PMMA substrates of 2 × 4 or 2 × 2 cm2 and 2 mm thick. First the PMMA substrates were rinsed with Milli-Q water, next with isopropanol, and then again with Milli-Q water. Subsequently they were sonicated in a 50% (v/v) 2-propanol/Milli-Q solution for 20 min. Next, the substrates were dried and placed 1.5 cm below an intense UV lamp (type Pen-Ray mercury lamp, 0.75 μW/cm2, Mid-IR, most

Anatase Nanorod Synthesis. This synthesis was carried out as described by Cozzoli et al.17 In a three-neck flask, 124 mmol of oleic acid (OLAC; 90%, Sigma-Aldrich) was degassed at 120 °C under vacuum for 60 min. The flask was then allowed to cool to 100 °C, and 3.33 mmol of titanium tetraisopropoxide (TTIP; ≥97%, SigmaAldrich) was injected under an argon atmosphere. After stirring for 10 min, either 5 mL of a 2 mol/L trimethylamine-N-oxide (TMAO; ≥98%, Alfa Aesar) aqueous solution or 10 mmol of triethylamine (Et3N; ≥99%, Sigma-Aldrich) followed by 5 mL of H2O (Milli-Q quality) was injected. Afterward, the mixture was stirred under an argon atmosphere for 6 h at 100 °C. Contrary to the work presented by Cozzoli, it was not possible to obtain a clear reaction mixture upon removal of water under vacuum. In the subsequent workup procedure, the nanocrystals were precipitated by adding methanol (in a ratio of 2:1) to the reaction mixture followed by centrifugation at 4000 rpm for 3 min. The resulting white precipitate was washed 5 times with methanol and was subsequently dispersed in toluene to produce optically clear and colorless suspensions after sonication for 15 min. Dispersions in apolar solvents (i.e., toluene and chloroform) remained stable for more than 3 months. Ligand Exchange. A ligand exchange was performed on the original oleic acid capped titania nanorods. Oleic acid was exchanged with (3-aminopropyl)triethoxysilane (APTES; 98%, TCI Chemicals) 29760


Research Article

ACS Applied Materials & Interfaces intense at 245 nm) for 20 min up to 6 h in order to induce reactive carboxylate function onto the PMMA surface. Chemical Coupling of TiO2 Nanoparticles to the Polymer Substrates: Low Temperature Deposition. The procedure used to couple the APTES functionalized nanoparticles to the surface of the PMMA substrate was adapted from literature methods to couple biomolecules to PMMA substrates or to couple biomolecules to nanoparticles functionalized with APTES.19,20 In this experiment, 20 mL of an aqueous suspension of APTES functionalized nanorods in 0.1 mol/L 2-(N-morpholino)ethanesulfonic acid (MES) buffer (Sigma-Aldrich, ≥99.5%; a nonamine, noncarboxylate buffer) at pH 4.75 was prepared. Subsequently, a 10 times molar excess of 1-ethyl-3(3-(dimethylamino)propyl)carbodiimide (EDC; TCI, 98%) was added and the mixture was allowed to stir for 5 min. Next, a 25 times molar excess of N-hydroxysuccinimide (NHS; Sigma-Aldrich, 98%) was added, followed by another 5 min of stirring. Afterward, the surfacetreated substrates were immersed in this coating suspension using a dip-coater apparatus and were left immersed in this suspension for 2 min, after which the substrates were removed from the suspension at a controlled speed (e.g., 50 mm/min). Next, the coated substrates were dried, either at 60 °C for 1 h in a drying furnace or alternatively with use of an IR lamp or a hot plate immediately after dip-coating. This was followed by thermal treatment in a muffle furnace, consisting of heating from room temperature to 180 °C in 30 min, followed by 1 h at 180 °C, and air-cooling by removal of the coated substrate immediately from the furnace. Spectroscopic Characterization. Characterization of the suspensions and coated polymer substrates was performed by dispersive Raman spectroscopy (RamanRxn, Kaiser Optical Systems Inc., 532 nm) and ATR-FTIR (PerkinElmer Spectrum 1000). The contact angle is a measure of the wettability of the surface by a solution (i.e., how effectively a liquid will spread over the surface). The contact angles were determined using an optical tensiometer (Kruss DSA30). Droplets with a volume of 5 μL were deposited on the surface using a syringe with a hube diameter of 4.717 mm and a needle with a diameter of 0.506 mm. The contact angle was determined using the drop shape analysis software delivered with the apparatus, using the Laplace−Young fitting method. The transparency of the deposited films on glass substrates was determined using a UV−vis Spectrophotometer (PerkinElmer Lambda 950). Structural Characterization. XRD analysis on precipitated nanopowders was collected on a Thermo X’tra diffractometer (Cu Kα, 1.5405 Å) with a solid state Si−Li detector. Samples were measured in a θ−2θ geometry over an angular range of 5−70° using a 0.02° step size and a 1 s step counting time. The internal standard approach was selected for the determination of the amorphous content by XRD analysis and Rietveld refinement. The method is based on the use of model structures that are refined using a least-squares procedure to improve the agreement between the experimental diffraction pattern and the pattern calculated from the model structures. Background function, scale factor, size broadening, and cell parameters were refined. Rietveld refinement was also used for the determination of the percentage crystallinity of the sample. For this purpose, a known amount of the powder sample is mixed with a known amount of pure crystalline powder of another crystalline phase that serves as an internal standard. A 10 wt % amount of zincite (ZnO) was mixed with the powder samples. The presence of organic matter left in the sample after workup of the nanoparticles was corrected through the use of thermogravimetric analysis (TGA; NETZSCH STA 449F3 Jupiter, with a heating rate of 2 °C/min, under O2 flow). Topas Academic V4.1 software was used for Rietveld refinement.21 high resolution transmission electron microscopy (HR-TEM, JEM-2200FS with Cs corrector) was used to investigate particle sizes, morphology, and crystallinity.

Figure 3. HR-TEM image of nanorods obtained by the fast hydrolysis procedure. The yellow rectangle encompasses a single anatase nanorod.

collected from a synthesis for 6 h at 100 °C using 3.3 mmol of TTIP, 10 mmol of TMAO in 5 mL of Milli-Q (2 mol/L) and 124 mmol of OLAC). Raman (Figure 4, left) and XRD (Figure 4, right) measurements indicate that the nanorods belong to the anatase crystal phase.22 By applying Rietveld refinement on collected XRD spectra, a crystallinity percentage of 45.5%, corrected using TGA, was obtained. The yield of the synthesis was calculated to be 74%. The fact that nanorods were obtained can be derived from the diffractogram as well (Figure 4, right). It is clear that the (004) reflection is more intense relative to the (101) reflection than in the case of the reference pattern, pointing to the presence of nanorods with a preferred growth orientation along the c-axis of the anatase crystal lattice. The infrared spectrum of powders collected from the fast hydrolysis after washing is represented in Figure 5. All the peaks in this spectrum can be assigned to vibrations of oleic acid17,23−25 (Table 1). The suspensions of nanorods in toluene remain present on the surface of the nanorods for more than 3 months. Ligand Exchange. After the ligand exchange of OLAC by APTES on nanorods obtained from the fast hydrolysis synthesis, suspensions were obtained from a ligand exchange procedure using 100 mg of OLAC functionalized nanopowder, 1.07 mmol of APTES, 8.9 mmol of Et3N, and 1.39 mmol of H2O in 25 mL of toluene. After that procedure, the suspension of nanorods in toluene becomes turbid and a precipitate forms upon standing (Figure 2b). This is a first indication that ligand exchange was successful, due to the fact that the original OLAC ligands, which provided steric stabilization to the nanorods, were exchanged for the significantly shorter APTES ligands that do not provide sufficient steric stabilization for the nanorods to be stable in toluene. The resulting precipitate was washed and transferred to water, and a turbid suspension was obtained at pH 10.3. Upon lowering the pH using diluted hydrochloric acid, the suspension changes from turbid to clear (Figure 2d). The optimal value (on the basis of ζ potential measurements and visual observations; see further) was found to be pH ∼ 2. This pH dependent dispersibility indicates that the nanorods are charge stabilized by the protonated amino groups of APTES. Measurement of the ζ potential of the aqueous suspension as a function of the pH (Figure 6) shows that the nanorods possess an isoelectric point (pI) between pH 10.5 and 11. This is close to the pI of 10.4 reported for CoFe2O4 nanoparticles functionalized with APTES.26 The isoelectric

■

RESULTS AND DISCUSSION Titania Nanorods Synthesis. Following the fast hydrolysis synthesis by Cozzoli et al.,17 we produced nanorods of ∼20 nm in length and ∼2.5 nm in diameter (Figure 3). Data were 29761


Research Article


Figure 4. Raman spectrum with the Raman-active modes of anatase indicated (left) and XRD pattern (right) of powder from the fast hydrolysis.

Figure 6. ζ potential of suspensions of APTES functionalized nanorods as a function of pH. Figure 5. FTIR spectra of powder obtained from the fast hydrolysis.

Table 1. Assignment of Peaks in the FTIR Spectrum of Powder Obtained from the Fast Hydrolysis Synthesis peak no.

peak position (cm‑1)

assignment

1 2 3 4 5 6 7 8 9 10 11 12 13 14

3200 (br) 3004 2918 2850 2360 1712 1628 1520 1430 1310 1088 908 726 628

ν(O−H) ν(CC−H) νas(CH2) νs(CH2) CO2 residual oleic acid CO νas(COO−) free oleate νas(COO−) bound oleate νs(COO−) bound oleate νs(COO−) free oleate ν(C−O) of COO δγ(CH) alkene δβr(CH2) OH rock

protonated amino functions as surface groups. The amino functions of APTES impart a continuous positive charge over the entire lower pH range (Figure 6) due to the presence of NH3+ groups. This pI is in good agreement with the basic character of amino functions.26 The stability of the suspensions as a function of the pH was also determined visually; below a pH ∼ 5 the suspension was completely transparent, between pH 5 and 8 the suspension immediately became turbid, between pH 8 and 10 the suspension became very turbid, and between pH 10 and 12 agglomerates were visible throughout the suspension. Only at pH 2 did the average ζ potential reach a value >30 mV, indicating that long-term stability might only be obtained at this pH. The aggregation close to the pI can be understood by the low amount of repulsive charges, and consequently the attractive van der Waals interactions will lead to aggregation. These findings implicate that amino, or more specific APTES, modified titania nanoparticles are mainly stabilized by electrostatic repulsions.26 Aqueous suspensions at pH 2 remained visibly stable for more than 5 weeks (i.e., no turbidity or precipitation was observed). After 5 weeks the ζ potential was measured to be +28.6 mV, which can indicate that a slight aggregation might have occurred. At pH ∼ 5, the suspensions remain visibly stable for 1 day at room temperature; when put in a refrigerator at 7 °C the stability was prolonged for more than 1 week. Turbid suspensions obtained at higher pH values could be changed into clear suspensions by

point of bare anatase nanoparticles (due to the presence of an equal amount of Ti−OH2+ and Ti−O− groups on the surface) is reported in literature to be in the range of 4.7−6.7.27 For commercial titania nanoparticles, Degussa P25, it is reported as 6.48,28 which is in the same range as that of bare cobalt ferrite nanoparticles.29 This is why we observe a discrepancy in measured pI. Due to the capping of APTES to the titania nanoparticles, the pI is shifted to higher values because of the 29762


Research Article

ACS Applied Materials & Interfaces lowering the value again to pH ∼ 2. Therefore, the nanorods can be precipitated and re-dispersed reversibly in water. FTIR spectra, obtained from a suspension of a ligand exchange procedure using 100 mg of OLAC functionalized nanopowder, 0.53 mmol of APTES, 4.48 mmol of Et3N, and 0.69 mmol of H2O in 25 mL of toluene, indicate that the exchange of OLAC by APTES was successful. The peaks that have been assigned (Table 2) to OLAC (Figure 5) have

exchange procedure, but in the absence of the titania nanorods. The powder obtained from this reference sample is assumed to correspond to clusters of silane molecules formed through condensation of the APTES molecules with each other (Si−O− Si bond formation). As can be observed from the FTIR spectrum, the silane clusters differ from that of APTES bound to the titania nanorods. The most important peak in the spectra of the APTES functionalized nanorods, is the reflection at 910 cm−1, which has been assigned to the characteristic stretching vibration of Ti−O−Si bonds.12,30−32 In the reference spectrum (Figure 7, right), a peak at 920 cm−1 can be observed, which can be assigned to silanol (Si−OH) asymmetric stretching.33 If silanol species are present in the mixture after ligand exchange, the peak corresponding to the silanol asymmetric stretch would overlap with the peak belonging to Ti−O−Si. The peak assigned to Ti−O−Si stretching is indeed quite broad (860− 945 cm−1), but no shoulder belonging to Si−OH can be distinguished. Combined data from FTIR, ζ potential measurements, and the pH dependency of the dispersibility of the nanorods in water after ligand exchange clearly indicate the successful exchange of OLAC ligands for APTES on TiO2 nanorods. PMMA Substrate Activation. Surface activation of the PMMA substrate serves two purposes: it should enhance the wettability of the substrate, and it should induce the presence of carboxylic acid groups on the surface of the polymer. Given the polar nature of carboxylic acid groups, an increase in surface carboxylic acid groups should also result in an enhanced wettability for aqueous suspensions. The contact angle of the aqueous coating suspension on untreated PMMA (cleaned with Milli-Q water and isopropanol) was measured to be 79.0 ± 3.4°. Because of the importance of the wettability of the substrate to obtain more homogeneous coatings, UV treatment has been selected as the method of choice to improve the wettability of PMMA substrates.34 Under UV radiation, radicals are formed due to photodegradation. Polar groups, such as carboxylic acids, are formed on the surface of the substrate due to reaction with oxygen and/or water molecules from the atmosphere. The increase in the amount of polar surface groups decreases the contact angle of the aqueous coating suspension. UV irradiation had a significant effect on the wettability of the surface of the substrate, the contact angle decreased from 79.0 ± 3.4° for untreated PMMA to 25.2 ± 0.7° after 20 min, 20.8 ±

Table 2. Assignment of Peaks in the FTIR Spectrum of Powder Obtained from the Ligand Exchange of OLAC by APTES peak no.


assignment

1 2 3 4 5 6 7

1562 + 1484 1432 1300 1120 1040 910 764

NH2 deformation modes δβS(CH2) CH2 wagging symmetric Si−O + C−N antisymmetric Si−O−Si Ti−O−Si + (Si−OH) CH2 rock in Si−CH2

Table 3. Assignment of Peaks in the FTIR Spectrum of Powder Obtained from an Identical Exchange Procedure Performed in the Absence of TiO2 Nanorods peak no.


assignment

1 2 3 4 5 6 7 8 9

1562 + 1484 1406 1308 1120 1020 920 860 762 692

NH2 deformation modes CH2 bending in Si−CH2 CH2 wagging symmetric Si−O + C−N antisymmetric Si−O−Si Si−OH Si−O stretch of Si−O−Si CH2 rock in Si−CH2 Si−C

disappeared and peaks that can be assigned (Table 3) to vibrations of (bound) APTES appear (Figure 7, left). For comparison, a reference sample was prepared (Figure 7, right). This reference sample was obtained using an identical ligand

Figure 7. FTIR spectrum of powder obtained from the ligand exchange of OLAC with APTES (left). FTIR spectrum of powder obtained from a reference procedure using the same amounts of reagents but in the absence of nanopowder (right). 29763


Research Article

ACS Applied Materials & Interfaces 1.2° after 2 h, 12.8 ± 0.99° after 3 h, and 6.3 ± 0.9° after 6 h of irradiation, as shown in Figure 8. A downside to this surface

Forming an Amide Bond between the PMMA Substrate and Silane Functionalized Titania Nanoparticles. When an amine and a carboxylic acid are mixed, an acid−base reaction occurs and a salt is formed, which can react to an amide. However, the equilibrium lies strongly to the side of hydrolysis rather than amide bond formation. The direct condensation of the salt to an amide can only be achieved at high temperatures (160−180 °C). Therefore, carboxylic acids are usually activated prior to reaction with an amine using coupling reagents that react with the carboxylic acid in such a way that a good leaving group is formed on the acyl carbon of the acid.36 It was opted to use the combination of EDC and NHS. The reactions involved in this coupling are shown in Figure 9. The formed O-acylisourea ester through the reaction of EDC with the carboxylic acid is rather susceptible to hydrolysis (2−3 s−1 at pH 4.75),37 such that the activated carboxylic acid is deactivated fast and the amine has less chance to attack and can moreover undergo cyclic electronic displacement (N → O displacement), giving the energetically more favored Nacylurea38 (Figure 10). This N-acylurea is not reactive toward primary amine groups.39

Figure 8. Contact angle of the coating suspension with PMMA subjected to UV irradiation.

modification procedure is the slight yellowing of the PMMA substrates by prolonged intense UV irradiation. This has been linked to the formation of double bonds due to side-chain scission.35 For the majority of the substrates to be coated, a compromise was taken at 2 h of irradiation. To test if the wettability of the UV-treated substrates changes over time, the contact angle of a sample treated for 20 min under UV was measured again after being stored for 5 h in a sealed Petri dish. The contact angle did not increase drastically (from 25.2 ± 0.7° to 28.4 ± 2.99°).

Figure 10. Formation of an N-acylurea from the O-acylisourea formed upon reaction of EDC with a carboxylic acid.

To avoid this, NHS is used. NHS attacks the EDC-activated carboxylic acid nucleophilicly to form an NHS ester that is more stable toward hydrolysis, thereby hindering the displacement reaction. The coupling reaction between EDC and

Figure 9. Reaction scheme of the linking of the nanoparticles to the polymer substrates via an amide bond. EDC is added to a PMMA substrate with carboxylic acid groups on the surface (1), forming an O-acylisourea ester (2). This ester reacts with NHS to form a NHS ester (3), which reacts with the primary amine function of APTES forming an amide bond (4). 29764


Research Article

ACS Applied Materials & Interfaces carboxylic acids requires slightly acidic conditions, and pH 4.75 can be used (e.g., using a MES buffer). For the formation of an amide bond, however, the pH should ideally be increased to 7.2−7.5 (e.g., through the addition of a phosphate buffer) to suppress ionization of the amine (since protonated amines are much less nucleophilic than deprotonated amines). A difficulty in this procedure is the fact that the nanoparticles are stabilized in water by the presence of the positively charged protonated ammonium groups of the ligands (NH3+). Therefore, the pH could not be increased above pH 5; otherwise turbid and unstable suspensions were obtained. At pH 5, only a fraction of the amine groups of the ligands around the nanorods will be deprotonated and able to react with the activated carboxylic acid groups. If the pH of an aqueous suspension would be chosen such that part of the amine groups of the APTES ligands on a particle are protonated while the rest are deprotonated, there might be sufficient stabilization as well as enough deprotonated amines that can react with the carboxylic acid groups on the polymer. Therefore, a trade-off between the stability of the suspension on the one hand and the amount of deprotonated amine groups available for coupling on the other hand needs to be determined. pH 4.75 was selected, since this is the lowest pH used for coupling between an amine and an EDC- and NHS-activated carboxylic acid, and the coating suspensions are clear at this pH value.19,40 Low Temperature Deposition on PMMA. The synthesis route for chemically coupling titania nanoparticles through silanes to a polymer surface provides the possibility to produce optically clear and photocatalytically active thin films on PMMA substrates. A temperature of 180 °C was determined to be the highest temperature that could be used in order to avoid deformation of the PMMA substrate upon removal from the muffle furnace. At this temperature the maximum treatment time was determined to be 1 h (after heating to 180 °C in 30 min). It must be noted that this temperature treatment is not intended to induce crystallinity in the deposited thin film, since temperatures of ∼400 °C would be necessary to induce the formation of anatase. The temperature treatment was applied to remove volatiles and organic material. The homogeneity of the coatings of UV-treated substrates was linked to the enhanced wettability of the substrates by the coating suspension. However, the coatings still showed an opaque white color (Figure 11), likely due to scattering linked to the roughness of the coating (this can be observed clearly for a substrate coated at 50 mm/min). This was thought to be the result of the precipitation of the nanorods out of the liquid film after deposition triggered by the fast evaporation of water out of the films upon drying. In order to reduce this effect, an additive with a higher boiling point than water was added such that the film evaporates more slowly. Ethylene glycol (EG) was chosen for this purpose.3 Adding EG to the suspension also increased the viscosity of the coating suspension (from 2.32 cP without EG to 2.82 cP with 5% EG). As a proof of concept, 2 mL of suspension containing different volume percentages of EG (0%, 1%, 5%, and 10%) was pipetted into a Petri dish and dried in a laboratory furnace at 60 °C for 3 h. The suspensions with 0% and 1% EG formed a hard translucent structure on the bottom of the Petri dish while the suspension containing 5 and 10 vol % EG formed a transparent gel. It is thought that the formation of the gel-like substance is the result of the evaporation of water such that a residual highly viscous substance remains. Coatings formed using suspensions

Figure 11. TiO2−APTES-coated PMMA substrate, with a dip-coating speed of 50 mm/min, dried at 60 °C in a laboratory furnace, and subjected to thermal treatment at 180 °C.

containing 5 vol % ethylene glycol were nearly macroscopically homogeneous and more transparent (Figure 12).

Figure 12. Coated PMMA substrates: (a) 50 mm/min, 5 vol % EG, 150 °C IR drying, 180 °C furnace; (b) 50 mm/min, 5 vol % EG, 150 °C IR drying, 180 °C furnace; (c) 100 mm/min, 5 vol % EG, 150 °C IR drying, 180 °C furnace; (d) 50 mm/min, 5 vol % EG, 100 °C IR drying, 180 °C furnace.

For most of the self-cleaning applications, it is important that the coatings are transparent in the visible range. To evaluate this quantitatively, the transmittance of coated substrates in the visible range was measured using a UV−vis spectrometer (Figure 13). The clean PMMA substrates had a transmittance of ∼93% in the visible region. After UV irradiation for 2 h, the transmittance of the substrates decreased to ∼91% between 500 and 800 nm and more drastically in the range between 400 and 500 nm with a transmittance of 82% at 400 nm. This is explained by the yellowing of the substrate. Substrates coated with a suspension containing 5 vol % EG that were immediately dried under an IR lamp at 150 °C for 30 s were the most transparent, with a transmittance of ∼87−84% between 500 and 800 nm and 79% at 400 nm. This coating reduced the transparency of the substrates by only ∼5% when compared to the UV-treated substrate. Substrates coated without the addition of EG to the coating suspensions were clearly less transparent (∼80−74% between 500 and 800 nm and ∼69% at 29765


Research Article


phase), one can observe that it coincides with the most intense characteristic vibration of anatase. An ATR-FTIR spectrum of a coated PMMA substrate was compared to a spectrum of a clean PMMA substrate. Peaks corresponding to vibrations of the PMMA substrate underneath the coating can be detected in the spectrum of the coated substrate but also some peaks unique to the spectrum of the coated substrate can be distinguished (Figure 15). These peaks

Figure 13. UV−vis spectrum of a clean PMMA substrate; PMMA substrate subjected to UV irradiation for 2 h; substrate (PMMA coated [1]) coated with a suspension containing 5 vol % of EG using a dipcoating speed of 50 mm/min, dried at 150 °C under an IR lamp for 30 s, and subjected to thermal treatment at 180 °C (yellow); substrate (PMMA coated [2]) coated using the same treatment as PMMA coated [1]; substrate (PMMA coated [3]) coated using the same treatment as PMMA coated [1] using a suspension without EG. All of the coated substrates were subjected to 2 h of UV treatment before coating.

400 nm). This can be explained by the white tinge of the coating due to scattering of visible light. A Raman spectrum of a TiO2-coated PMMA substrate was measured and compared to a spectrum of a clean PMMA substrate and a powder obtained from the fast hydrolysis (corresponding to the anatase crystal phase) (Figure 14). Most of the bands correspond to vibrations of the PMMA substrate; the only marked difference is a peak at 150 cm−1 in the spectrum of the coated substrate that is absent in the spectrum of the clean substrate. Comparing this to a spectrum of powder from the fast hydrolysis (corresponding to the anatase crystal

Figure 15. Comparison of ATR-IR spectra of coated (blue) and uncoated (black) PMMA substrates, coated with the standard coating suspension, dried at 60 °C in a laboratory furnace, and treated at 180 °C in a muffle furnace.

can be assigned to vibrations related to APTES and silane networks (Table 4).41 In summary, the combination of the Table 4. Assignment of Peaks in the FTIR Spectrum of a Coated PMMA Substrate peak no.


assignment

1 2 3 4 5 6

630 796 870 1034 1120 1168

OH rock Si−O−Si bending Si−O stretch of Si−O−Si antisymmetric stretch Si−O−Si symmetric Si−O + C−N Si−O

Raman and the FTIR spectra of the coated substrate indicate the presence of anatase (from the Raman spectrum) as well as silane components (from the ATR-FTIR spectrum). Although the presence of amide bonds could not be proven, these measurements indicate the successful deposition of the silane capped anatase nanorods on the PMMA substrate. The absence of the amide bond vibrations in the IR spectra does not necessarily mean that the coupling reaction was unsuccessful, however. The amide bonds, if present, form a molecular layer at the interface between two considerably thicker layers (the substrate and the coating) both of which possess infrared-active components. It is plausible that the peaks belonging to vibrations of the amide bonds are not intense enough to be distinguished. Photocatalytic Activity. The photocatalytic activity of a model coating was evaluated according to an ISO certified test (ISO 10678:2010(E)) based on the photocatalytic degradation

Figure 14. Raman spectra of coated PMMA (dried and heat treated) in blue, cleaned uncoated PMMA (in black) and powder from the fast hydrolysis (corresponding to anatase; in green). The substrate was coated with the standard coating suspension, dried at 60 °C in a laboratory furnace, and treated at 180 °C in a muffle furnace. 29766


Research Article


compared to bare nanoparticles has been obtained.42 The possible presence of silane clusters in the coating next to the nanorods might further increase this effect. However, in order to enhance the activity of the coatings, it can be suggested that a second layer of titania nanocrystals is deposited on the obtained layers. The relatively low activity of the first layers might reduce potential effects of photocatalytic degradation of the polymer substrate, such that this layer acts as a buffer layer between the more active second layer and the polymer substrate. The first layer then also acts as a coupling layer, to enhance the durability of the final coating. As a proof of concept, a second layer of titanium oxide has been deposited onto the coated PMMA. Highly crystalline titania nanoparticle suspensions, synthesized in a previous publication,5 were dipcoated at room temperature and at a coating speed of 60 mm/s, by means of a computer controlled dip-coating unit (KSV Instruments) in a clean room facility (class 100,000/1000). The subsequent thermal processing was performed in a Carbolite tube furnace at 200 °C for 1 h, with a heating rate of 2 °C/min and under a 0.5 L/min air flow. Analysis of this titania top layer has already been described in detail in our recent publication.5 Results of photocatalytic-activity testing have been added in Figure 16. This clearly indicates a higher photocatalytic activity when a top titania layer is added in comparison to the APTES− TiO2 coating. UV−vis spectroscopy was applied to determine the transparency of both the APTES−TiO2 coating on PMMA and of an added TiO2 thin film on top (Figure 17). For both

of methylene blue. It is generally accepted that bleaching of MB aqueous solutions exhibits a pseudo-first-order kinetic mechanism as described by the equation ln(C/C0) = −kt. In which C is the concentration of methylene blue after a specific UV irradiation time t, C0 is the initial MB concentration (10−5 mol/ L), and k is the rate constant of the reaction. According to another ISO standard (ISO 10677:2011), the ultraviolet light source for testing the performance of semiconducting photocatalytic materials was assessed for radiation intensity. The MB decolorization measurement setup was equipped with a Vilber Lourmat VL-315BLB blacklight blue fluorescent light tube. The photon source has a maximum emission at 365 nm and emits 10 W/m2. The coated PMMA samples were accurately cut at 4.0 cm2 and inserted into the holder cell. A least-squares linear fit of the data points after 1 h is shown. The sample is a substrate coated with a suspension containing 5 vol % EG using a dip-coating speed of 50 mm/min, dried at 150 °C under an IR lamp for 30 s and subjected to thermal treatment at 180 °C. The coated PMMA substrate clearly shows some photocatalytic activity (Figure 16). After 1 h, the curve for the blank

Figure 16. Logarithmic plot of the decomposition of methylene blue as a function of UV exposure time for a blank sample, a coated APTES−TiO2−PMMA sample, and the same sample coated with a titania top layer. Figure 17. UV−vis spectrum of a APTES−TiO2-coated PMMA sample and the same sample coated with a titania top layer.

stabilizes (fluctuates) around −0.055 but the curve for the coated sample continues to drop further. This indicates that the decrease observed after 1 h for the coated substrate is linked to actual photocatalytic degradation of methylene blue by the anatase nanocrystals in the coating. The initial steep drop in the curve for the coated substrate and the drop for the blank sample might be attributed to adsorption of methylene blue, although the samples were conditioned for 12 h before the measurements, as described in the ISO test, in order to limit this effect. Such a steep initial decrease has also been observed in literature.3 A specific degradation rate was calculated for this coated PMMA sample (as an average over the data points after 1 h of irradiation) as 1.45 × 10−5 mol/(m2 h). This is a relatively low value as values up to 3.8 × 10−5 mol/(m2 h) have been obtained using aqueous suspensions on glass substrates.3 A possible explanation for this relatively low activity is the presence of APTES around the nanorods. It has been shown in literature that the presence of APTES ligands around anatase nanoparticles can significantly suppress their photocatalytic activity. For high ligand densities of APTES (6.2 nm−2), a decrease in activity of up to 75%

samples, the steep decrease in transparency below 380 nm is due to absorption of UV light as a result of electron excitation from the valence band to the conduction band of TiO2, corresponding to the bandgap of the anatase polymorph.43 For the APTES−TiO2 coating on PMMA, the average transmittance at wavelengths above 400 nm is around 80%. As can be seen from Figure 17, the added titania top coating results in a transmittance, reaching a value of 80% at 800 nm. The wavy fluctuation of the transmittance curve over the 350−800 nm spectral region is due to interference effects which are mainly determined by the thickness of the thin film and the refractive index of the material and substrate.3 Thus, a transparent and photocatalytically active TiO2 has been successfully deposited on a PMMA−APTES−TiO2 buffer layer system. This synthesis route provides a means of a low temperature chemical solution deposition method of titania on polymer substrates. 29767


Research Article


■

(6) Avci, N.; Smet, P.; Poelman, H.; Van de Velde, N.; De Buysser, K.; Van Driessche, I.; Poelman, D. Characterization of Tio2 Powders and Thin Films Prepared by Non-Aqueous Sol−Gel Techniques. J. Sol-Gel Sci. Technol. 2009, 52 (3), 424−431. (7) Carneiro, J. O.; Teixeira, V.; Portinha, A.; Magalhães, A.; Coutinho, P.; Tavares, C. J.; Newton, R. Iron-Doped Photocatalytic Tio2 Sputtered Coatings on Plastics for Self-Cleaning Applications. Mater. Sci. Eng., B 2007, 138 (2), 144−150. (8) Yang, J.-H.; Han, Y.-S.; Choy, J.-H. Tio2 Thin-Films on Polymer Substrates and Their Photocatalytic Activity. Thin Solid Films 2006, 495 (1−2), 266−271. (9) Loddo, V.; Marcì, G.; Palmisano, G.; Yurdakal, S.; Brazzoli, M.; Garavaglia, L.; Palmisano, L. Extruded Expanded Polystyrene Sheets Coated by Tio2 as New Photocatalytic Materials for Foodstuffs Packaging. Appl. Surf. Sci. 2012, 261, 783−788. (10) Blossey, R. Self-Cleaning Surfaces [Mdash] Virtual Realities. Nat. Mater. 2003, 2 (5), 301−306. (11) Allen, N. S.; Edge, M.; Verran, J.; Stratton, J.; Maltby, J.; Bygott, C. Photocatalytic Titania Based Surfaces: Environmental Benefits. Polym. Degrad. Stab. 2008, 93 (9), 1632−1646. (12) Zhao, J.; Milanova, M.; Warmoeskerken, M. M. C. G.; Dutschk, V. Surface Modification of Tio2 Nanoparticles with Silane Coupling Agents. Colloids Surf., A 2012, 413, 273−279. (13) Sabzi, M.; Mirabedini, S. M.; Zohuriaan-Mehr, J.; Atai, M. Surface Modification of Tio2 Nano-Particles with Silane Coupling Agent and Investigation of Its Effect on the Properties of Polyurethane Composite Coating. Prog. Org. Coat. 2009, 65 (2), 222−228. (14) Siwiń s ka-Stefań s ka, K.; Ciesielczyk, F.; Nowacka, M.; Jesionowski, T. Influence of Selected Alkoxysilanes on Dispersive Properties and Surface Chemistry of Titanium Dioxide and Tio2–Sio2 Composite Material. J. Nanomater. 2012, 2012, 316173. (15) Pujari, S. P.; Scheres, L.; Marcelis, A. T. M.; Zuilhof, H. Covalent Surface Modification of Oxide Surfaces. Angew. Chem., Int. Ed. 2014, 53 (25), 6322−6356. (16) Xie, Y.; Hill, C. A. S.; Xiao, Z.; Militz, H.; Mai, C. Silane Coupling Agents Used for Natural Fiber/Polymer Composites: A Review. Composites, Part A 2010, 41 (7), 806−819. (17) Cozzoli, P. D.; Kornowski, A.; Weller, H. Low-Temperature Synthesis of Soluble and Processable Organic-Capped Anatase Tio2 Nanorods. J. Am. Chem. Soc. 2003, 125 (47), 14539−14548. (18) Bloemen, M.; Brullot, W.; Luong, T. T.; Geukens, N.; Gils, A.; Verbiest, T. Improved Functionalization of Oleic Acid-Coated Iron Oxide Nanoparticles for Biomedical Applications. J. Nanopart. Res. 2012, 14 (9), 1100. (19) Barbucci, R.; Pasqui, D.; Giani, G.; De Cagna, M.; Fini, M.; Giardino, R.; Atrei, A. A Novel Strategy for Engineering Hydrogels with Ferromagnetic Nanoparticles as Crosslinkers of the Polymer Chains. Potential Applications as a Targeted Drug Delivery System. Soft Matter 2011, 7 (12), 5558−5565. (20) Chen, Y.-W.; Wang, H.; Hupert, M.; Soper, S. A. Identification of Methicillin-Resistant Staphylococcus Aureus Using an Integrated and Modular Microfluidic System. Analyst 2013, 138 (4), 1075−1083. (21) Coelho, A. A. Topas-Academic, Version 4.1; Coelho Software: Brisbane, Australia, 2007. (22) Wang, D.; Zhao, J.; Chen, B.; Zhu, C. Lattice Vibration Fundamentals in Nanocrystalline Anatase Investigated with Raman Scattering. J. Phys.: Condens. Matter 2008, 20 (8), 085212. (23) Joo, J.; Kwon, S. G.; Yu, T.; Cho, M.; Lee, J.; Yoon, J.; Hyeon, T. Large-Scale Synthesis of Tio2 Nanorods Via Nonhydrolytic Sol−Gel Ester Elimination Reaction and Their Application to Photocatalytic Inactivation of E. Coli. J. Phys. Chem. B 2005, 109 (32), 15297−15302. (24) Carlucci, C.; Xu, H.; Scremin, B. F.; Giannini, C.; Sibillano, T.; Carlino, E.; Videtta, V.; Gigli, G.; Ciccarella, G. Controllable One-Pot Synthesis of Anatase TiO 2 Nanorods with the MicrowaveSolvothermal Method. Sci. Adv. Mater. 2014, 6 (8), 1668−1675. (25) Nakayama, N.; Hayashi, T. Preparation of Tio2 Nanoparticles Surface-Modified by Both Carboxylic Acid and Amine: Dispersibility

CONCLUSIONS Anatase nanocrystals with good crystallinity that can be used to obtain clear and stable suspensions were reproducibly synthesized. The nanorods synthesized through the fast hydrolysis procedure were used in a ligand exchange procedure to obtain APTES capped nanorods dispersible in aqueous solutions. The stability of the resulting suspensions is pH dependent, and at pH ∼ 2, clear suspensions can be obtained that remain stable for more than a month. Indications that the ligand exchange was successful are the pH dependent dispersibility of the nanorods in water after the ligand exchange procedure and the ζ potential as a function of pH and FTIR measurements. These aqueous suspensions were subsequently used to coat PMMA substrates. In order to improve the wettability of the substrates, UV irradiation proved to be an effective surface treatment, resulting in homogeneous coatings. The addition of ethylene glycol to the suspensions further improved the transparency of the coatings. Coatings with only a slightly reduced transparency compared to uncoated PMMA substrates were obtained. Raman and infrared spectroscopy measurements provide indications that the coupling of the nanocrystals to the PMMA substrate was successful. A transparent photocatalytically active coating was obtained through the use of a low temperature procedure compatible with the thermal sensitivity of the PMMA substrates.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +32(0) 92644433. Fax: +32(0)92644983. Funding

J.W. thanks the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT) for funding. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Research group COMOC (Prof. Dr. P. Van der Voort) for use of the Raman setup and Prof. Dr. Van der Eycken for use of the infrared equipment.

■

REFERENCES

(1) Roach, P.; Shirtcliffe, N. Superhydrophobicity and Self-Cleaning. In Self-Cleaning Materials and Surfaces: A Nanotechnology Approach; Daoud, W. A., Ed.; John Wiley & Sons: Chichester, U.K., 2013; pp 1− 32, DOI: 10.1002/9781118652336.ch1. (2) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107 (7), 2891−2959. (3) Arin, M.; Watté, J.; Pollefeyt, G.; De Buysser, K.; Van Driessche, I.; Lommens, P. Low Temperature Deposition of TiO2 Layers from Nanoparticle Containing Suspensions Synthesized by Microwave Hydrothermal Treatment. J. Sol-Gel Sci. Technol. 2013, 66 (1), 100− 111. (4) Arin, M.; Lommens, P.; Hopkins, S. C.; Pollefeyt, G.; Van der Eycken, J.; Ricart, S.; Granados, X.; Glowacki, B. A.; Van Driessche, I. Deposition of Photocatalytically Active TiO2 Films by Inkjet Printing of TiO2 Nanoparticle Suspensions Obtained from Microwave-Assisted Hydrothermal Synthesis. Nanotechnology 2012, 23, 165603. (5) Watté, J.; Lommens, P.; Pollefeyt, G.; Meire, M.; De Buysser, K.; Van Driessche, I. Highly Crystalline Nanoparticle Suspensions for Low-Temperature Processing of Tio2 Thin Films. ACS Appl. Mater. Interfaces 2016, 8 (20), 13027−13036. 29768


Research Article

ACS Applied Materials & Interfaces and Stabilization in Organic Solvents. Colloids Surf., A 2008, 317 (1− 3), 543−550. (26) De Palma, R.; Peeters, S.; Van Bael, M. J.; Van den Rul, H.; Bonroy, K.; Laureyn, W.; Mullens, J.; Borghs, G.; Maes, G. Silane Ligand Exchange to Make Hydrophobic Superparamagnetic Nanoparticles Water-Dispersible. Chem. Mater. 2007, 19 (7), 1821−1831. (27) Kobayashi, M.; Osada, M.; Kato, H.; Kakihana, M. Design of Crystal Structures, Morphologies and Functionalities of Titanium Oxide Using Water-Soluble Complexes and Molecular Control Agents. Polym. J. 2015, 47 (2), 78−83. (28) Long, T. C.; Saleh, N.; Tilton, R. D.; Lowry, G. V.; Veronesi, B. Titanium Dioxide (P25) Produces Reactive Oxygen Species in Immortalized Brain Microglia (Bv2): Implications for Nanoparticle Neurotoxicity. Environ. Sci. Technol. 2006, 40 (14), 4346−4352. (29) de Vicente, J.; Delgado, A. V.; Plaza, R. C.; Durán, J. D. G.; González-Caballero, F. Stability of Cobalt Ferrite Colloidal Particles. Effect of Ph and Applied Magnetic Fields. Langmuir 2000, 16 (21), 7954−7961. (30) Gao, X.; Wachs, I. E. Titania−Silica as Catalysts: Molecular Structural Characteristics and Physico-Chemical Properties. Catal. Today 1999, 51 (2), 233−254. (31) Aizawa, M.; Nosaka, Y.; Fujii, N. Ft-Ir Liquid Attenuated Total Reflection Study of Tio2-Sio2 Sol-Gel Reaction. J. Non-Cryst. Solids 1991, 128 (1), 77−85. (32) Li, Z.; Hou, B.; Xu, Y.; Wu, D.; Sun, Y.; Hu, W.; Deng, F. Comparative Study of Sol−Gel-Hydrothermal and Sol−Gel Synthesis of Titania−Silica Composite Nanoparticles. J. Solid State Chem. 2005, 178 (5), 1395−1405. (33) Klein, S.; Thorimbert, S.; Maier, W. F. Amorphous Microporous Titania−Silica Mixed Oxides: Preparation, Characterization, and Catalytic Redox Properties. J. Catal. 1996, 163 (2), 476−488. (34) Hollins, B. C.; Soper, S. A.; Feng, J. Microfluidic Carbonylated Protein Capture. In World Congress on Medical Physics and Biomedical Engineering, May 26−31, 2012, Beijing, China; Long, M., Ed.; Springer: Berlin, Heidelberg, 2013; pp 270−273. (35) Pethrick, R. A. Compositional and Failure Analysis of Polymers: A Practical Approach. Polym. Int. 2001, 50 (9), 1054−1054. (36) Montalbetti, C. A. G. N.; Falque, V. Amide Bond Formation and Peptide Coupling. Tetrahedron 2005, 61 (46), 10827−10852. (37) Hoare, D. G.; Koshland, D. E., Jr. A Method for the Quantitative Modification and Estimation of Carboxylic Acid Groups in Proteins. J. Biol. Chem. 1967, 242 (10), 2447−2453. (38) Kurzer, F.; Douraghi-Zadeh, K. Advances in the Chemistry of Carbodiimides. Chem. Rev. 1967, 67 (2), 107−152. (39) Bulpitt, P.; Aeschlimann, D. New Strategy for Chemical Modification of Hyaluronic Acid: Preparation of Functionalized Derivatives and Their Use in the Formation of Novel Biocompatible Hydrogels. J. Biomed. Mater. Res. 1999, 47 (2), 152−169. (40) Pasqui, D.; Atrei, A.; Giani, G.; De Cagna, M.; Barbucci, R. Metal Oxide Nanoparticles as Cross-Linkers in Polymeric Hybrid Hydrogels. Mater. Lett. 2011, 65 (2), 392−395. (41) Einarsrud, M.-A.; Haereid, S. Preparation of Transparent, Monolithic Silica Xerogels with Low Density. J. Sol-Gel Sci. Technol. 1994, 2 (1-3), 903−906. (42) Ukaji, E.; Furusawa, T.; Sato, M.; Suzuki, N. The Effect of Surface Modification with Silane Coupling Agent on Suppressing the Photo-Catalytic Activity of Fine Tio2 Particles as Inorganic Uv Filter. Appl. Surf. Sci. 2007, 254 (2), 563−569. (43) Fujishima, A.; Zhang, X.; Tryk, D. A. Tio2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582.

29769
