Graphical abstract
Highlights
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Application of photo-catalytic activities of TiO 2 NPs could be tuned through dental resins.
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Adding TiO 2 NPs in hydroxyl-rich resins may produce materials with controlled wettability.
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The degree of vinyl conversion could be increased up to 22% by adding only 0.1 wt% TiO 2 NPs.
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Viscosity of resins had minimal or no role in enhancing degree of vinyl conversion through TiO 2 NPs.
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Attaching acrylic acid on TiO 2 NPs doubled radical loading and extended radical life over 10 times.
Abstract
Objective
The unique photo-catalytic activities (PCAs) of titanium dioxide nanoparticles (TiO 2 NPs) made them attractive in many potential applications in medical devices. The objective of this study is to optimize the benefits of PCAs of TiO 2 NPs through varying chemical structures of dimethacrylate resins.
Methods
TiO 2 NPs were functionalized to improve the PCAs and bonding to the resins. The PCAs of TiO 2 NPs were evaluated using electron paramagnetic resonance (EPR) and UV–vis spectroscopy to determine the amount of the radicals generated and the energy required for their production, respectively. The beneficial effects of the radicals were assessed through: (1) the improvement of degree of vinyl conversion (DC) and (2) modification of resin hydrophilicity. One-way ANOVA with a 95% confidence interval was used to indicate the significant differences between the experimental groups.
Results
EPR and UV–vis results clearly showed that the functionalization of TiO 2 NPs enhanced PCAs in terms of generating radicals under visible light irradiation. The presence of hydroxyl and carboxylic acid functionalities played an important role in DC enhancement and hydrophilicity modification. The DC could be increased up to 22% by adding only 0.1 wt% TiO 2 NPs. Viscosity of the resins had minimal or no role in DC improvement through TiO 2 NPs. In resins with abundant hydroxyl groups, radicals were more effective in making the resin more hydrophilic.
Significance
Knowledge learned from this study will help formulating nano-composites with optimized use of TiO 2 PCAs as co-initiators for photo-polymerization, additives for making super-hydrophilic materials and/or antibacterial agents.
1
Introduction
The unique photo-catalytic activities (PCAs) of TiO 2 nanoparticles make them attractive in many potential applications including self-cleaning surfaces , antibacterial medical devices and high performance dental composites . Besides their PCAs, TiO 2 NPs are inexpensive and chemically stable; they also have excellent mechanical properties (elastic modulus of TiO 2 is 230 GPa) . Traditionally, TiO 2 is used as a pigment additive in materials, including cosmetic and constructive materials, because of its high refractive index ( n ); TiO 2 particles are also used in dental composites to match the color, translucence and opalescence of natural tooth for aesthetic purposes ; and its PCAs are attracting more and more attention because they are the basis of TiO 2 NPs utility in antimicrobial agents , initiators for photo-polymerization , solar cells , and super-hydrophilic surfaces for self-cleaning materials . The PCAs indicate the capability of TiO 2 NPs to utilize energy from light irradiation and generate resources for the above applications without consuming themselves, which are typically evaluated by the NPs’ capability to: (1) absorb light, especially visible light; and (2) produce and preserve radicals (in the form of electrons and holes) . The processes of PCAs from light irradiation to final applications are: TiO 2 NPs (<50 nm) utilize energy from light irradiation to generate electrons and holes; through them, water and oxygen are converted into powerful free radicals and oxidation agents: superoxide and hydroxyl radical , which are valuable in the above applications and biological processes.
Free radicals are vital in biological processes. They are used for intracellular killing of bacteria in phagocytic cells, including granulocytes and macrophages; also they are implicated in redox signaling processes . The two most important oxygen-centered radicals are superoxide and hydroxyl radical . Both of them are strong oxidants, e.g., hydroxyl radicals are four times stronger than hydrogen peroxides. Due to their reactivity, excessive amount of free radicals may cause unwanted side effects resulting in damage of cells and biological bodies . Because they are necessary for life, there are many biological defense mechanisms to minimize free-radical-induced damage and to repair damage that occurs, in which antioxidants, including vitamin A, vitamin C and vitamin E, play a key role; and some functional groups including hydroxyl groups, carboxyl groups, and amine groups are essential for their antioxidant purposes .
Successful implementation of the unique properties of TiO 2 NPs in resin can lead to significant enhancement of resin performance and provide flexibility in controlling the resin network structure and functionality, thus creating new materials capable of meeting the complex requirements for functionality and durability. Our previous studies demonstrated that: (1) TiO 2 -containing resins yielded significantly stronger dental adhesives (the mean shear bond strength increased approximately 30% when 0.1 wt% of functionalized TiO 2 NPs were added) than the pure resin counterparts; (2) The addition of functionalized TiO 2 NPs dramatically improved the performance of resins, including significantly enhanced degree of vinyl conversion (DC), elastic modulus ( E ), hardness ( H ) . Learning from the key functional groups of antioxidants in biological defense systems, experiments were designed to understand how the chemical structure of resins influences the application of the free radicals generated by TiO 2 NPs. The PCAs of TiO 2 NPs were evaluated using electron paramagnetic resonance (EPR) and UV–vis spectroscopy to determine the amount of radicals generated and the energy required for their production, respectively. The beneficial effects of the free radicals were assessed through the improvement of degree of vinyl conversion (DC; quantified by Fourier Transform Infrared spectroscopy) and the modification of hydrophilicity of dimethacrylate resins [quantified by measuring the water contact angle (WCA)]. The viscosity and the number of hydroxyl and/or carboxyl groups of the experimental resins were then correlated with the PCAs of TiO 2 nanoparticles.
2
Materials and methods
2.1
Materials and sample preparation
Resin 1
1 Certain equipment, instruments or materials are identified in this paper to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply the materials are necessary the best available for the purpose.
monomers, ethoxylated-bisphenol-A-dimethacrylate (EBPDMA), 2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane) (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA), pyromellitic-glycerol-dimethacrylate (PMGDM) and/or hydroxyethyl-methacrylate (HEMA), were gifts from Esstech Inc (Essington, PA, USA). Their chemical structures are shown in Fig. 1 . The initiators, camphorquinone (CQ) and ethyl 4-N, N-dimethylaminobenzoate (4E) were purchased from Sigma–Aldrich (St. Louis, MO, USA). CQ (0.2 wt%) and 4E (0.8 wt%) were mixed with resin monomers (99 wt%) before adding nanoparticles. Methylene blue (MB) and acrylic acid was also obtained from Sigma–Aldrich. Titanium dioxide nanoparticles (P25, AEROXIDE TiO 2 ), a known photo-catalytically active material composed of both anatase and rutile phases, were provided by Evonik Industries (Essen, Germany). All reagents were used as received.
The P25 TiO 2 NPs were functionalized with acrylic acid following a modified method described previously ; the product was labeled as AP25. Briefly, acrylic acid (7.2 g), water (0.8 g), hexane (8 g) and P25 (0.2 g) were combined and sonicated at 0 °C for 30 min using a 130 W ultrasonic processor (model# GEX 130PB, Cole-Parmer, USA). The mixture was then agitated at 500 RPM at 37 °C for 48 h. A resulting milky mixture was transferred into a 50 mL centrifuge tube and centrifuged at 13,000 rpm for 60 min. The AP25 precipitate was collected and redistributed in 25 mL of ethanol, which was centrifuged again at 13,000 rpm for 60 min. The redistribution and centrifugation steps were repeated two more times to remove the excess acrylic acid loosely absorbed on the NPs. The same sonication setup was also used to prepare AP25 organosols by dispersing AP25 powder in ethanol at 0.1% by mass. These AP25 organosols did not form any observable precipitate for months at room temperature.
The compositions of the experimental resins employed in the study are given in Table 1 . The experimental formulations activated for light photo-polymerization were stored in the dark before being utilized. The AP25 ethanol organosols were mixed into the activated resins by 10 min agitation at 500 rpm. The ethanol was then removed via air blowing at room temperature for 24 h. Resins with different mass fractions (0 mass %, 0.02 mass %, 0.05 mass %, 0.1 mass %, 0.2 mass %and 0.5 mass %) of AP25 were also prepared.
| Resin/monomer | Bis-GMA | EBPDMA | PMGDM | TEGDMA | HEMA | CQ | 4E |
|---|---|---|---|---|---|---|---|
| B/T = 3/1 | 74.25 | – | – | 24.75 | – | 0.2 | 0.8 |
| B/T = 1/1 | 49.50 | – | – | 49.50 | – | 0.2 | 0.8 |
| B/T = 1/3 | 24.75 | – | – | 74.25 | – | 0.2 | 0.8 |
| B/H = 1/1 | 49.50 | – | – | – | 49.50 | 0.2 | 0.8 |
| EBPDMA | – | 99.00 | – | – | – | 0.2 | 0.8 |
| P/H = 3/1 | – | – | 74.25 | – | 24.75 | 0.2 | 0.8 |
| P/H = 1/1 | – | – | 49.50 | – | 49.50 | 0.2 | 0.8 |
| P/H = 1/3 | – | – | 24.75 | – | 74.25 | 0.2 | 0.8 |
Disk specimens for degree of vinyl conversion (DC) and water contact angle (WCA) measurements (five disks/experimental group) were prepared in a following manner: a mixture of monomers and AP25 was pipetted into a TEFLON cylinder mold (6 mm in diameter and 1.5 mm thick) placed on top of a piece of Mylar film and a glass slide. Once filled, the top of the mold was covered sequentially with Mylar film and a glass slide, and the assembly was clamped together. The specimens were cured using a Triad 2000 visible light curing unit (Dentsply, York, PA, USA) with a tungsten halogen light bulb (75 W and 120 V, 43 mW/cm 2 ) for 2 min each from both open sides of the assembly.
2.2
Polymerization of BMA in the presence of nanoparticles
The benzyl methylmethacrylate (BMA, Sigma–Aldrich, St. Louis, MO, USA) was photo-polymerized in the presence of 0.1% by mass of P25 or AP25 NPs using 1 wt% of bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BPPO, Sigma–Aldrich, St. Louis, MO, USA) as an initiator. The mixture was cured by illuminating with a Blak-Ray 100 W UV lamp (365 nm) at a 20 cm distance (intensity of 10 mW/cm 2 ) for 3 min, then the polymer was dissolved in toluene and centrifuged to collect precipitates. The dissolving and centrifugation processes were repeated three times, and the final powder were collected, dried and evaluated using Fourier transform infrared (FTIR).
2.3
Characterization methods
FTIR spectroscopic measurements were carried out with the Nexus 670 FTIR spectrophotometer (Thermo Scientific, Madison, Wisconsin). NPs were mixed with KBr powder (1.5 mg AP25 and 150 mg KBr) and pressed into pellets. These pellets were examined in transmission mode. Resins and composites were evaluated through an attenuated total reflectance setup. A total of 64 scans were collected from 4000 cm −1 to 650 cm −1 at 4 cm −1 resolution. All measurements were done in triplicate ( n = 3).
EPR measurements were carried out on a BRUKER BioSpin ElexSys500 spectrometer, using an x-band square cavity (TE011 mode) operating under identical parameter settings: microwave frequency ≈9.38 GHz, field modulation = 100 kHz, and microwave power = 10 mW. Each specimen was placed in a capped quartz EPR tube (0.4 mm thin wall tube, 20 cm long) in the EPR cavity. A 500 W Xe Arc lamp was used as the UV source for in-situ irradiation experiments. All spectra were obtained at 77 K by sweeping the static magnetic field and recording the first derivative of the absorption spectrum. Unexposed specimen and empty EPR tubes were tested, respectively, for reference spectra. A weak pitch standard sample was measured under identical instrumental conditions periodically to correlate the position of EPR signals and double integrals of EPR spectra were analyzed to determine the intensity of peaks. Powder samples were dried before loading into the EPR tubes. The resin and resin composites were in the form of bars (25 × 2 × 2 mm 3 ). Measurements were repeated twice ( n = 2).
The UV–vis spectra were collected using a multimode microplate reader (SpectraMax, Model M5, Molecular Devices LLC, CA, USA). The NPs were dispersed in water via ultrasonication (100% amplitude, 130 W Ultrasonic processor, Cole Parmer, USA) for 30 min at a concentration of 0.01 mass %, and transferred to a 96-well plate. Each well contained 0.2 mL samples. The spectra were collected from 200 nm to 1000 nm at 10 nm interval at room temperature. Water was used as a background. Number of repetitive runs/group, n = 5.
In 2 mL quartz cuvettes, MB dye-TiO 2 suspensions were prepared to contain 5.5 μM of MB with P25 or AP25 1 mM in ethanol. These solutions were exposed to light irradiation using a Triad 2000 Curing Unit. Irradiation time interval was 20 s for a total exposure of 200 s. Hitachi U2000 Spectrophotometer (Hitachi instrument Inc, San Jose, CA, USA) was used to collect absorbance spectra. The ODs of MB monomer (at 664 nm), dimer (at 606 nm) and trimer (at 570 nm) were measured before and after 200 s irradiation, and the percentage of the OD decrease after light illumination were calculated as the MB degradation in each form.
Dynamic light scattering (DLS) measurements were performed on a Brookhaven instrument (90PLUS/BI-MAS mode, Brookhaven Instruments Cooperation, New York, USA) at 90° angle, 25 °C, in ethanol. A 15 mW solid state laser with a wavelength of 532 nm was used. The hydrodynamic radius ( R h ) of particles and its distribution was calculated employing the Brookhaven Instruments-provided software by the method of cumulants using one- or two-exponential fits . Measurements were repeated three times ( n = 3).
2.4
Degree of vinyl conversion (DC)
The DC of resin and composite specimens was determined using near infrared spectroscopy (NIR) . NIR spectra were acquired before photo cure and at 24 h post cure. DC was calculated as the percentage change in the integrated peak area of the 6,165 cm −1 vinyl on methacrylate absorption band normalized to the 4623 cm −1 aromatic C H absorption band area between the polymer (value after cure) and monomer (values before cure). The standard uncertainty associated with the DC measurement was <1%.
2.5
Water contact angle (WCA) measurement
The WCA measurements were carried out using the sessile drop method with a Kruss G2 system (Hamburg, Germany) at room temperature. The volume of the deionized water droplet was 2 μL, and the images of the sessile droplet were taken immediately after deposition on the substrate. Water contact angle on resins/resin composites were acquired before UV irradiation, 24 h post irradiation and one month after UV irradiation. The films were illuminated with a Blak-Ray 100 W UV lamp (365 nm) at a 20 cm distance (intensity of 10 mW/cm 2 ) for 20 min. UV irradiation was applied here to maximize the photo-catalytic activities of the TiO 2 nanoparticles in a reasonably short period of time.
2.6
Statistical analysis
One-way analysis of variance (ANOVA) with a 95% confidence interval was used to indicate significant differences between the experimental groups.
2
Materials and methods
2.1
Materials and sample preparation
Resin 1
1 Certain equipment, instruments or materials are identified in this paper to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply the materials are necessary the best available for the purpose.
monomers, ethoxylated-bisphenol-A-dimethacrylate (EBPDMA), 2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane) (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA), pyromellitic-glycerol-dimethacrylate (PMGDM) and/or hydroxyethyl-methacrylate (HEMA), were gifts from Esstech Inc (Essington, PA, USA). Their chemical structures are shown in Fig. 1 . The initiators, camphorquinone (CQ) and ethyl 4-N, N-dimethylaminobenzoate (4E) were purchased from Sigma–Aldrich (St. Louis, MO, USA). CQ (0.2 wt%) and 4E (0.8 wt%) were mixed with resin monomers (99 wt%) before adding nanoparticles. Methylene blue (MB) and acrylic acid was also obtained from Sigma–Aldrich. Titanium dioxide nanoparticles (P25, AEROXIDE TiO 2 ), a known photo-catalytically active material composed of both anatase and rutile phases, were provided by Evonik Industries (Essen, Germany). All reagents were used as received.
The P25 TiO 2 NPs were functionalized with acrylic acid following a modified method described previously ; the product was labeled as AP25. Briefly, acrylic acid (7.2 g), water (0.8 g), hexane (8 g) and P25 (0.2 g) were combined and sonicated at 0 °C for 30 min using a 130 W ultrasonic processor (model# GEX 130PB, Cole-Parmer, USA). The mixture was then agitated at 500 RPM at 37 °C for 48 h. A resulting milky mixture was transferred into a 50 mL centrifuge tube and centrifuged at 13,000 rpm for 60 min. The AP25 precipitate was collected and redistributed in 25 mL of ethanol, which was centrifuged again at 13,000 rpm for 60 min. The redistribution and centrifugation steps were repeated two more times to remove the excess acrylic acid loosely absorbed on the NPs. The same sonication setup was also used to prepare AP25 organosols by dispersing AP25 powder in ethanol at 0.1% by mass. These AP25 organosols did not form any observable precipitate for months at room temperature.
The compositions of the experimental resins employed in the study are given in Table 1 . The experimental formulations activated for light photo-polymerization were stored in the dark before being utilized. The AP25 ethanol organosols were mixed into the activated resins by 10 min agitation at 500 rpm. The ethanol was then removed via air blowing at room temperature for 24 h. Resins with different mass fractions (0 mass %, 0.02 mass %, 0.05 mass %, 0.1 mass %, 0.2 mass %and 0.5 mass %) of AP25 were also prepared.
| Resin/monomer | Bis-GMA | EBPDMA | PMGDM | TEGDMA | HEMA | CQ | 4E |
|---|---|---|---|---|---|---|---|
| B/T = 3/1 | 74.25 | – | – | 24.75 | – | 0.2 | 0.8 |
| B/T = 1/1 | 49.50 | – | – | 49.50 | – | 0.2 | 0.8 |
| B/T = 1/3 | 24.75 | – | – | 74.25 | – | 0.2 | 0.8 |
| B/H = 1/1 | 49.50 | – | – | – | 49.50 | 0.2 | 0.8 |
| EBPDMA | – | 99.00 | – | – | – | 0.2 | 0.8 |
| P/H = 3/1 | – | – | 74.25 | – | 24.75 | 0.2 | 0.8 |
| P/H = 1/1 | – | – | 49.50 | – | 49.50 | 0.2 | 0.8 |
| P/H = 1/3 | – | – | 24.75 | – | 74.25 | 0.2 | 0.8 |
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