Nanotechnology: Delivering on the Promise Volume 2 : Applications of

Chapter 5

Applications of Nanoparticles through Surface Functionalization Downloaded by CORNELL UNIV on September 6, 2016 | http://pubs.acs.org Publication Date (Web): August 29, 2016 | doi: 10.1021/bk-2016-1224.ch005

Khaled Rashwan and Grigoriy Sereda* Department of Chemistry, University of South Dakota, 414 E Clark Street, Vermillion, South Dakota 57069, United States *E-mail: [email protected].

Modification of the titania surface with organic and inorganic groups can significantly improve the practical potential of this material as a component of sunscreen and toothpastes. Functionalization of titania and silica nanoparticles improves their adhesion to human dentin, which opens their future use for treating tooth hypersensitivity by occlusion of dentin tubules and carriers of remineralizing and other active components. The affinity of organic and inorganic molecules to titania can suppress photodegradation of caffeic acid and work toward new formulations of sunscreens.

Introduction The booming field of applied nanotechnology spurs the growing interest to the surface functionalization as a versatile tool for fine-tuning of nanoscale devices and compositions, and for tailoring their properties to a specific practical application (1–4). Among those properties is conjugation with different materials by electrostatic (5–7) or covalent (8, 9) interaction. One of the most common nanomaterials is titania (TiO2), also known as titanium dioxide, an inorganic compound occurring in three different polymorphs: anatase, brookite and rutile (10–12) which have found applications in catalysis (13, 14), water purification (15, 16) medicine (17–19) optoelectronics (20) and in charge-transfer process (21). The least expensive commercial rutile-anatase nanopowder is widely used as a constituent of sunscreens (22) due to its ability to absorb (23) and scatter UV-light (24), and as a component of toothpastes due to its physiochemical properties (25). © 2016 American Chemical Society Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on September 6, 2016 | http://pubs.acs.org Publication Date (Web): August 29, 2016 | doi: 10.1021/bk-2016-1224.ch005

Our recent results demonstrate that modification of the titania surface with organic and inorganic groups can significantly improve the practical potential of this material as a component of sunscreen and toothpastes. This study is based on the known affinity of the caffeic acid’s catechol group to titania (26) (Figure 1), which can be fine-tuned by the competitive binding of other ligands to the titania surface.

Figure 1. Binding of caffeic acid to titania particles.

Results and Discussion Application of the Affinity of Caffeic Acid toward Titania Occlusion of the dentin tubules is a known approach to the treatment of tooth hypersensitivity (27, 28). Hydroxyapatite crystallizing in the crystal unit Ca10(PO4)6(OH)2, is the major component of human dentin (29), which can bind with the carboxy-groups of aminoacids (30, 31). Functionalization of nanoparticles with carboxy-groups as a method of increasing their affinity to human dentin and, therefore, ability to occlude the dentin tubules, have been explored. First, adsorption of caffeic acid on titania increases its affinity to the hydroxyapatite component of dentin and capability to occlude the tubules, due to the binding of the imparted carboxy-groups with calcium ions. For comparison, affinity of non-functionalized and covalently functionalized by carboxy-groups silica nanoparticles to dentin has been also tested. The SEM images of cross-sections of human dentin before and after treatment with titania nanoparticles are presented in Figure 2. Sonication was selected as a reproducible method to access how well the particles are retained by the dentin surface and tubules. 92 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. (A) Untreated dentin; (B) Dentin exposed to untreated titania after rinsing with water; (C) Dentin exposed to caffeic acid-functionalized titania after rinsing with water; (D) Dentin exposed to caffeic acid-functionalized titania after rinsing with water and sonication for 10 s. As expected, surface functionalization of titania with caffeic acid increased adhesion of titania to hydroxyapatite. As opposed to untreated titania, after sonication, a significant fraction of the applied caffeic acid – functionalized titania particles still remained on the dentin surface (Figure 2). Next, the possibility of harnessing another common component of toothpastes – silica nanoparticles – for the treatment of tooth hypersensitivity was explored. However, unfunctionalized silica nanoparticles did not exhibit a sustainable affinity to dentin, and have been mostly removed from the surface after 10 seconds sonication (Figure 3A). This explains why ordinary silica-containing toothpastes do not address the hypersensitivity problem. The modification of silica nanoparticles with carboxy-groups (by APTES amination followed by carboxylation using succinic anhydride) noticeably improve their adhesion to dentin and occlusion of the tubules (Figure 3B). 93 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 3. (A) Dentin exposed to unfunctionalized silica nanoparticles after rinsing with water and sonication for 10 s; (B) Dentin exposed to carboxylated silica nanoparticles after rinsing with water and sonication for 10 s.

While carboxylated silica nanoparticles are not currently used in dentifrices, it was found that modification of titania with caffeic acid can be immediately employed for improving existing toothpastes. In order to avoid using any chemicals outside of the realm of approved dentifrice components and food supplements, a composition containing titania nanoparticles from an existing toothpaste Crest® - Cavity Protection), and caffeic acid from a green coffee bean extract (a food supplement) was tested. The SEM images in Figure 4 illustrate that the Crest® toothpaste forms a layer, covering most of the dentin tubules after rinsing with water (Figure 4A), but was completely cleared out after sonication (Figure 4B). In contrast, the toothpaste – green coffee bean extract composition was retained by the dentin surface even after sonication (Figure 4C and 4D).

Suppressing the Affinity of Caffeic Acid toward Titania While adsorption of caffeic acid on titania may lead to novel therapeutical toothpastes, it also makes caffeic acid unusable in titania-containing sunscreen compositions. Caffeic acid has been patented as a component of sunscreen composition due to its ability to adsorb the most damaging to skin UV-bands, and provide additional health benefits due to its anti-oxidant properties (32). However, to the best of our knowledge, caffeic acid has not been used in any commercial sunscreen compositions, perhaps, because of its low photostability, especially in the presence of a titania photocatalyst. In addition, the reactive oxygen species generated upon photoexcitation of titania, are damaging to skin (33). We have demonstrated that competitive adsorption of phosphate-ions and caffeic acid on titania significantly slows down photodecomposition of caffeic acid, perhaps, due 94 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


to dissociation of photoactive titania-caffeic acid surface complexes that separates caffeic acid from the photocatalyst. Sunlight-induced photodegradation of caffeic acid has been mimicked for 5h in a solar simulator. Concentration of caffeic acid in a solution of centrifuged samples was monitored by measuring its optical density at 328 nm.

Figure 4. (A) Dentin exposed to Crest® - Cavity Protection toothpaste after rinsing with water; (B) Dentin exposed to Crest® - Cavity Protection toothpaste after rinsing with water and sonication for 10 s; (C) Dentin exposed to the Crest® - Cavity Protection toothpaste and green coffee bean extract after rinsing with water; (D) Dentin exposed to the Crest® - Cavity Protection toothpaste and green coffee bean extract after rinsing with water and sonication for 10 s.

In order to mimic stability of sunscreen compositions containing caffeic acid and titania, photodegradation of caffeic acid in the presence of titania for about 3h was followed. As shown in Figure 5, the process follows the zero-order kinetics, which is consistent with literature (34). 95 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 5. Photodegradation of caffeic acid in the presence of titania (zero order plot.)

Interestingly, the kinetic of photodegradation also fits the first-order kinetic law (Figure 6). Further discussion regarding these reaction kinetics is provided in a subsection entitled “Further discussion on the kinetics of decomposition of caffeic acid over titania”

Figure 6. Photodegradation of caffeic acid in the presence of titania (first order plot.)

Regardless of the reaction order, the effect of addition of phosphate-ions, chondroitin sulfate, and β-cyclodextrin on the photodegradation rate of caffeic acid in the presence of titania was explored (Figures 7,8.) 96 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. Titania-catalyzed photodegradation of caffeic acid in the absence (●) and presence of phosphate-ions (▴), chondroitin sulfate (♦), and β-cyclodextrin (▪) additives (zero order plot.)

Figure 8. Titania-catalyzed photodegradation of caffeic acid in the absence (●) and presence of phosphate-ions (▴), chondroitin sulfate (♦), and β-cyclodextrin (▪) additives (first order plot). 97 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Not surprisingly, neither photodegradation process exhibits a distinctive zero or first order kinetics. However, addition of 4.9*10-5 M of phosphate significantly suppresses photodegradation of caffeic acid, increasing its concentration in solution roughly by the factor of 2. A similar effect of phosphate is known for photooxidation of ferulic acid by KBrO3 under anaerobic conditions (35). Perhaps, this suppressing effect is due to adsorption of phosphate on the surface of titania, which decreases the surface concentration of caffeic acid on the titania and, therefore, reduces effectiveness of photocatalysis. This is consistent with our observation that adsorption of caffeic acid on titania is significantly suppressed by the presence of phosphate ions. When 11 g of commercial titania was mixed with 2 mL of 6.9 mmol/L solution of caffeic acid in an ~20 % aqueous methanol for ~20 minutes, concentration of caffeic acid dropped to 6.4 mmol/L (by 6%) because of adsorption on titania. When 2 mL of 0.1 M KH2PO4 was added into the mixture, the concentration of caffeic acid increased nearly to the original value. Although we used about 2.8 times higher concentration of caffeic acid for the photodegradation experiments for the practical purposes, the effect of phosphate ions on the photostability of caffeic acid was quite evident. The observed competitive adsorption of caffeic acid and another solution component demonstrate that any reliable study of adsorption or photodegradation in the presence of titania must specify type of the employed buffer. Interestingly, neither chondroitin sulfate nor b-cyclodextrin significantly affected the photodegradation rate, despite their known affinity to titania (36, 37). It is consistent with our observation that either of those additives released a noticeable amount of the adsorbed caffeic acid on titania into the solution (Figure 7). The effect of desorption of caffeic acid from titania was likely counteracted by the formation of supramolecular complexes interfacing titania and caffeic acid and stimulating photodegradation. Such an equivocal effect on photodegradation of bisphenol has been reported for several cyclodextrins (38). In addition, the exact orientation of the molecule of caffeic acid at the surface of titania may be affected by the surface supramolecular complexes. According to our recent computational study (39), the mode of adsorption of a molecule of caffeic acid at the surface of titania significantly affect the energy of adsorption essential for the photoexcited state dynamics. Further Discussion on the Kinetics of Decomposition of Caffeic Acid over Titania Apparently, at the studied range of concentrations for caffeic acid and titania mixture, the difference between the zeroth and first kinetics is within the experimental error, which is illustrated in Figure 7, which shows lnC plotted against C, and one calculated point (lnC = 0 at C = 1). Figure 9 shows that the zeroth and first kinetic orders of photodecomposition cannot be confidently distinguished in the studied concentration range. According to another study (40), even in lower concentrations, the first reaction order becomes evident only after extended reaction times (several hours). The zeroand first order photodegradation rate constants calculated (36, 37) from the linear regression parameters, are summarized in Table 1. For zero-order; [C] versus t 98 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


derived from the equation [C]= [C]0 – kt; where [C] is the concentration of caffeic acid remaining in the solution (mol/L) , [C]0 is the initial concentration of caffeic acid (mol/L), k is the rate constant (mol/L.min) and t is time (min.) For first-order kinetics; Ln [C] versus t derived from the equation [C] = [C0] e- k t; where k is the rate constant (min-1.)

Figure 9. Plot of the logarithm of caffeic acid concentration against the concentration. One calculated point has been added (marked by a triangle).

Table 1. Titania-Catalyzed Photodegradation of Caffeic Acid Rate Constants (K), Their Margin Errors (±) and Line Regression Fitting (R2) in the ▴), Chondroitin Sulfate (♦), Absence (●) and Presence of Phosphate-Ions (▴ and β-Cyclodextrin (▪) Additives (First Order Plot) Zero Order

First Order

k (moL/L.min) *10-3

±

R2

k (min-1) *10-4

± *10-5

R2

●

12.1

1.7

0.90

14.2

17.9

0.91

▴

7.5

0.7

0.95

8.1

6.6

0.96

♦

10.3

2.5

0.81

12.3

23.3

0.85

▪

11.8

1.6

0.92

14.0

15.6

0.94

Conclusion Controlled adsorption of caffeic acid on titania can markedly increase its practical applicability for biomedical applications. The bifunctional molecules of caffeic acid can serve as a linker between titania and human dentin, enabling application of this material for the treatment of tooth hypersensitivity by dental 99 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

tubule occlusion. On the other hand, modification of titania surface significantly affects the photodegradation rate of caffeic acid, which will aid in developing new sunscreen formulations that combine advantages of both organic and inorganic UV-protectors.


Experimental Section Caffeic acid, (3-aminopropyl) triethoxysilane, 98% (APTES) and succinic anhydride were purchased from Alfa Aesar, tetraethoxysilane (TEOS) from Sigma-Aldrich, chondroitin sulfate from Bioalternatives, β-cyclodextrin from Lancaster, potassium phosphate monobasic from J. T. Baker Chemical Co., titania from Fisher Scientific (Cat. No. 79586). The experiments were conducted with the solar stimulator (Arc Lamp Housing, Single Element FS, F/1.5, Collimated, 50-500W,1.5 In, Newport co.), Cary® 50 UV-Vis Spectrophotometer, and FEI Quanta 450 FEG Environmental Scanning Electron Microscope (SEM).

Preparation of Tooth Samples A human tooth was rinsed with tap water. The outer enamel layer was removed using a dental bur. Next, the tooth was wrapped in a paper towel and split with a hammer into pieces about 2X2 mm, which were shaken 5 mL water and sonicated for 20 s. The turbid liquid was drained out, and sonication was repeated with a fresh 5 mL of water. The sonication step was repeated until no more turbid liquid was produced (usually 2-3 steps). The purified samples were dried on air overnight at 35°C.

Application of Materials to a Tooth Sample (The General Procedure) Approximately A 2 mL portion of a ~10 mg/mL of an aqueous nanoparticle suspension or 50 mg of the paste of a solid material mixed well with 1 mL of water was applied to the tooth sample followed by brushing with a toothbrush for 1 min. The treated tooth sample was washed by a medium stream of tap water for 20 s. Persistence of adhesion and occlusion was tested by sonicating the sample in 2 mL of water for 20 s.

Functionalization of Titania with Caffeic Acid A 30 mg portion of titania was added to a solution of 40 mg of caffeic acid in 20 mL of 50% v/v aqueous ethanol. The mixture was left for 48 h. Then 4 mL of the mixture was centrifuged at 8K RPM for 10 minutes, and the precipitate was washed with DI water (4 mL X 3), using centrifugation on each step. The titania-caffeic acid conjugate was stored in 2.5 mL of water. 100 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Synthesis of Silica Nanoparticles


Silica nanoparticles (100 nm average size) were prepared according to a known procedure (41, 42). Briefly, in a plastic flask, 2 mL of TEOS were added to 20 mL of absolute ethanol at stirring. After another 15 min, 2 mL of 30% ammonium hydroxide were added, and the mixture was stirred for 2 h. The white turbid solution of silica nanoparticles was centrifuged at 8K RPM for 10 minutes, and the precipitate was washed with absolute ethanol (10 mL X 3), using centrifugation on each step. The silica nanoparticles were suspended in 15 mL of absolute ethanol and stored in a plastic vial.

Carboxylation of Silica Nanoparticles Carboxylated silica nanoparticles were prepared by amination of silica nanoparticles followed by the reaction with succinic anhydride, according to the known method (43). Briefly, 15 mL of the ethanol suspension of silica nanoparticles, 150 uL of APTES, and 50 ul of 30% aqueous ammonium hydroxide was stirred overnight. The white turbid solution of silica nanoparticles was centrifuged at 8K RPM for 10 minutes, and the precipitate was washed with absolute ethanol (10 mL X 3), then DMF (dimethylformamide) (10 mL X2) using centrifugation on each step. This material (aminated silica nanoparticles) was carboxylated by stirring overnight with a solution of 0.266 mg of succinic anhydride in 10 mL DMF. The mixture was centrifuged at 8K RPM for 10 minutes, and the precipitate was washed with DMF (10 mL X 3) using centrifugation on each step.

Modification of a Toothpaste A solution of 0.225 g potassium citrate (a desensitizing agent (44–46) that can inhibit tooth pain via its capability of passing through the exposed dentin tubules to reach the tooth nerve) and 0.155 g citric acid in 0.5 mL of DI water was added to 4.5 g of a titania-containing toothpaste (Crest®) at stirring. Next, the mixture was thoroughly mixed with 0.022 g of caffeic acid.

Photodegradation of Caffeic Acid A solution of ~0.035 g of caffeic acid and an additive (~0.57 g of KH2PO4, or 1.1 g of β-cyclodextrin or 0.2 g of chondroitin, or no additive) in 10 mL of 55% v/v aqueous ethanol was stirred overnight with 30 mg of titania in a light protected quartz tube. The quartz tube was subjected to UV-light at 15 cm from a 300 watt solar simulator under magnetic stirring. After the specified time, a 0.5 mL sample was withdrawn from the reaction tube and centrifuged at 8000 RPM for 10 min. A 0.03 mL portion of the supernatant solution was diluted with DI water to 4 mL, and concentration of caffeic acid was determined photometrically. 101 Cheng et al.; Nanotechnology: Delivering on the Promise Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Acknowledgments The purchase of the solar simulator is supported by the Director, Office of Science, Office of Biological & Environmental Research, Biological Systems Science Division, of the U.S. Department of Energy under Contract No. DE-FG02-08ER64624, DE-EE0000270. The SEM instrument was funded by the Health Resources and Services Administration (HRSA) grant #C76HF09149 entitled “Imaging Facility at GEAR”. We also appreciate the NSF-EPSCoR PANS grant 0903804 for the Cary UV-vis spectrometer, the Materials Science Ph.D. program of the State of South Dakota, and the Joseph F. Nelson and Martha P. Nelson Scholarship Endowment for the support of Khaled Rashwan. We are grateful to Dr. Brower and the “SmilesForSiouxland” for financial support, providing human teeth and help in preparation of tooth samples. Finally, we thank David Christianson, Brandon Burum, Wendi Sapp, and Sunny Patel for technical contribution to this work.

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Nanotechnology: Delivering on the Promise Volume 2 : Applications of

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