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. Author manuscript; available in PMC: 2017 Mar 1.
Published in final edited form as: Dent Mater. 2016 Jan 12;32(3):363–372. doi: 10.1016/j.dental.2015.11.021

Tuning photo-catalytic activities of TiO2 nanoparticles using dimethacrylate resins

Jirun Sun 1,*, Stephanie S Watson 2, David A Allsopp 1, Debbie Stanley 2, Drago Skrtic 1
PMCID: PMC4764397  NIHMSID: NIHMS745185  PMID: 26792622

Abstract

Objectives

The unique photo-catalytic activities (PCAs) of titanium dioxide nanoparticles (TiO2 NPs) made them attractive in many potential applications in medical devices. The objective of this study is to optimize the benefits of PCAs of TiO2 NPs through varying chemical structures of dimethacrylate resins.

Methods

TiO2 NPs were functionalized to improve the PCAs and bonding to the resins. The PCAs of TiO2 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 TiO2 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% TiO2 NPs. Viscosity of the resins had minimal or no role in DC improvement through TiO2 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 TiO2 PCAs as co-initiators for photo-polymerization, additives for making super-hydrophilic materials and/or antibacterial agents.

Keywords: Titanium dioxide nanoparticles, free radical, photo-catalytic activities, dental resins, nano-composites, dimethacrylates, wettability

Graphical Abstract

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1. Introduction

The unique photo-catalytic activities (PCAs) of TiO2 nanoparticles [1-4] make them attractive in many potential applications including self-cleaning surfaces [5, 6], antibacterial medical devices [7] and high performance dental composites [8, 9]. Besides their PCAs, TiO2 NPs are inexpensive and chemically stable; they also have excellent mechanical properties (elastic modulus of TiO2 is 230 GPa) [1]. Traditionally, TiO2 is used as a pigment additive in materials, including cosmetic and constructive materials, because of its high refractive index (n); TiO2 particles are also used in dental composites to match the color, translucence and opalescence of natural tooth for aesthetic purposes [10, 11]; and its PCAs are attracting more and more attention because they are the basis of TiO2 NPs utility in antimicrobial agents [3, 7, 12], initiators for photo-polymerization [9], solar cells [4], and super-hydrophilic surfaces for self-cleaning materials [13]. The PCAs indicate the capability of TiO2 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) [2]. The processes of PCAs from light irradiation to final applications are: TiO2 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 [1, 14, 15], 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 [16, 17]. The two most important oxygen-centered radicals are superoxide and hydroxyl radical [17]. 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 [18, 19]. 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 [20-22].

Successful implementation of the unique properties of TiO2 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) TiO2-containing resins yielded significantly stronger dental adhesives (the mean shear bond strength increased approximately 30 % when 0.1 wt% of functionalized TiO2 NPs were added) [9] than the pure resin counterparts; 2) The addition of functionalized TiO2 NPs dramatically improved the performance of resins, including significantly enhanced degree of vinyl conversion (DC), elastic modulus (E), hardness (H) [9]. 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 TiO2 NPs. The PCAs of TiO2 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 TiO2 nanoparticles.

2. Materials and methods*

2.1 Materials and sample preparation

Resin 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 Figure 1. The initiators, camphorquinone (CQ) and ethyl 4-N, N-dimethylaminobenzoate (4E) were purchased from SigmaAldrich (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 SigmaAldrich. Titanium dioxide nanoparticles (P25, AEROXIDE TiO2), 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.

Figure 1.

Figure 1

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Chemical structure of the monomers employed in the study.

The P25 TiO2 NPs were functionalized with acrylic acid following a modified method described previously [9, 23]; 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 13000 rpm for 60 min. The AP25 precipitate was collected and redistributed in 25 mL of ethanol, which was centrifuged again at 13000 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.

Table 1.

Composition (mass fraction, %) of the experimental resins.

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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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/cm2) 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, SigmaAldrich, 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, SigmaAldrich, 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/cm2) 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) mm3). 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-TiO2 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 degree angle, 25 °C, in ethanol. A 15 mW solid state laser with a wavelength of 532 nm was used. The hydrodynamic radius (Rh) of particles and its distribution was calculated employing the Brookhaven Instruments-provided software by the method of cumulants using one- or two-exponential fits [24, 25]. 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) [26-28]. 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 4,623 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/cm2) for 20 min. UV irradiation was applied here to maximize the photo-catalytic activities of the TiO2 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.

3. Results and discussion

3.1 Functionalization and characterization of TiO2 nanoparticles

The presence and reactivity of C=C bond on AP25 was confirmed through FTIR and polymerization of BMA in the presence of P25/AP25 NPs. FTIR spectrum of AP25 [9, 23] indicated that the acrylic acid was successfully attached onto P25. The –C=C- signal at 1636 cm−1 in the FTIR spectrum of AP25 verified the existence of the double bonds on the surface of AP25. In order to better understand the reactivity of the –C=C- bond on AP25 during photo-polymerization, an experiment was designed to evaluate these double bonds on nanoparticles directly and determine their ability for copolymerizing with other monomers. The monomer used in this evaluation was BMA which had only one polymerizable C=C bond. The corresponding polymer, PBMA, was soluble in toluene.

FTIR indicated that the PBMA was attached on AP25 but not on P25. Figure 2 shows the FTIR spectra of particles after co-polymerization with BMA and isolated by rinsing with toluene and centrifugation. The carbonyl stretch (C=O) of ester on PBMA at 1727 cm−1 was only found on particles collected in the PBMA+AP25 mixture. When BMA monomers were polymerized in the presence of P25, the product, PBMA, was dissolved in toluene and separated from P25 particles through centrifugation; no PBMA was attached on P25. On the other hand, PBMA was found on functionalized nanoparticles of AP25. BMA copolymerized with C=C on AP25 and formed nano-composites, which could be dispersed in toluene and collected as particles through centrifugation.

Figure 2.

Figure 2

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FTIR spectra of P25/AP25 after co-polymerized with BMA.

This experiment confirmed that the C=C on AP25 copolymerized with resins during photo-polymerization, thus likely creating rigid, more cross-linked polymer network. The covalent bonds between the NPs and the resin via short-chain unsaturated groups are a powerful tool for enhanced mechanical performance of dental resins [9, 29]. Previous studies showed that the mechanical performance of dimethacrylate resins were dramatically enhancement by adding a very small amount (<0.1 mass %) of NPs functionalized with C=C bonds [9].

The PCAs of TiO2 NPs are generally evaluated by: 1) the NPs’ capability to absorb low energy light, especially visible light; and 2) the NPs’ ability to generate and maintain radicals [2]. In this work, we used EPR, UV-vis and degradation of methylene blue to compare the PCAs of the unfunctionalized P25 and functionalized AP25 TiO2 NPs.

Figure 3A and 3B show the EPR spectra of P25 and AP25, respectively. In both figures, the red line indicates the spectra of nanoparticles without UV irradiation and the blue lines are the spectra of nanoparticles after 1h UV irradiation. Both P25 and AP25 contained more radicals after UV irradiation, but their EPR spectra and the radical evolution over time of UV irradiation are different. Without UV irradiation, the EPR signal of P25 is low, below an intensity of 2.5. After UV irradiation, the EPR signal increases, and more electrons than holes are detected. Comparatively, before UV irradiation, AP25 starts with a stronger EPR signal, at an approximate intensity of 10. After UV irradiation, the EPR signal also increased; but more holes instead of electrons are detected. The EPR results indicate that P25 and AP25 are chemically different. The kinetics of radical production and radical life time are different as well.

Figure 3.

Figure 3

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A) EPR spectra of P25 NPs before and after UV irradiation; B) EPR spectra of AP25 NPs before and after UV irradiation; C) Radicals (electrons and holes) forming on P25 NPs (green) and AP25 NPs (orange) as a function of time under UV irradiation. Open symbols indicate the change of radicals over time after cessation of UV irradiation.

Figure 3C shows radical evolution over irradiation time and the lifetime of these radicals. In this figure, square symbols and circle symbols stand for the amount of radicals (electrons and holes) on P25 and AP25 as a function of time, respectively; solid symbols show the amount of radicals as a function of time under UV irradiation, and open symbols indicate the amount of radicals over time after irradiation. Both of the nanoparticles show exponential increase of radicals under UV irradiation. The amount of radicals grows fast initially and then plateau. P25 nanoparticles reach plateau within 30 min, while AP25 nanoparticles reach the maximum levels after 1h of UV irradiation. In addition, the maximum amount of radicals on AP25 nanoparticles is approximately 2.5 times the amount generated on P25. Moreover, after UV irradiation has stopped, the radicals on AP25 nanoparticles are stable for more than 30 min, while the radicals on P25 disappear instantly.

The functionalization of TiO2 NPs enhanced the capability of nanoparticles to produce radicals, and the life time of radicals on nanoparticles after functionalization was greatly improved. In the case of the unfunctionalized TiO2 NPs, electrons and holes combined with each other resulting in the reduced number of free radicals. However, in the case of the functionalized TiO2 NPs (AP 25), the recombination of electrons and holes appears to be prevented by the attached functional groups thus yielding NPs with the enhanced radical capacity and life time.

DLS results indicated that both particles formed small agglomerates in ethanol and the hydrodynamic radius of their agglomerate were the same, 77 ± 2 nm, with narrow sized distribution. In addition, TEM results showed that the functionalization of P25 did not alter the average diameter of individual nanoparticle [9].

The UV-vis absorbance spectra (Figure 4A) indicates slight absorbance increase in the visible light region from 400 nm to 540 nm, which is the active wavelength range for light curing unit, Triad 2000. The PCAs of nanoparticles were further evaluated by the decomposition of methylene blue (MB) under light irradiation in Triad 2000 unit which is widely used in dental research for curing dental resins and dental resin composites. MB has at least three forms in solution. As illustrated in Figure 4B, monomer MB (absorbance peak at 664 nm) is the majority, then dimer (absorbance peak at 606 nm), and then trimer (absorbance peak at 570 nm). Other forms of MB may also exist but in very low concentrations, so they weren't evaluated. The inset table in Figure 4B lists the percentage of the MBs degraded after 200 s light irradiation. Clearly, AP25 decomposed more MBs than P25. Above all, AP25 had better PCAs comparing with P25, and the presence of functional groups on the surface enhances the bonding of nanoparticles to the resin network. This study is focusing on the functionalized nanoparticles, AP25, and its PCAs in different resins.

Figure 4.

Figure 4

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A) UV-vis absorbance spectrum of P25 (green) and AP25 (orange) in ethanol. Inset: the absorbance of the same NPs in the visible light region. B) UV-vis spectra of MB in ethanol. Inlet (Table): the percentage of the MBs degraded after 200 s light irradiation.

3.2 Grouping resins based on chemical structure and viscosity of monomers

Superoxide and hydroxyl free radicals are both strong oxidants. Oppositely, hydroxyl and carboxyl groups are effective functional groups in many antioxidants. Therefore, the content of hydroxyl and carboxylic groups of the monomers was one of the major criteria for resin selection. Viscosity and hydrophilicity of resin monomers were also considered.

The eight resin compositions (Table 1) are divided into two sets based on the chemical structure and viscosity of monomers. One set includes EBPDMA, B/T=1/1, B/H=1/1, and H/P=1/1. In this set, the variation of chemical structures (the number of hydroxyl and carboxyl groups) lead the trend in radical intensity under strong magnetic fields, LogP, and hydrophilicity of these resins (Table 2). The other set contains six resin compositions of B/T and P/H systems. By varying the mass ratio (3/1, 1/1 and 1/3) of base resin (Bis-GMA or PMGDM) and the corresponding diluent resin (TEGDMA or HEMA), three distinguishable viscosities are created in two chemical systems. P/H systems contain more hydroxyl and carboxyl groups than B/T systems.

Table 2.

Trends in the number of hydroxyl and carboxyl groups, radical intensity under strong magnetic fields, LogP, and hydrophilicity of the resins in set one.

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Table 2. shows the trends in chemical structure change (the number of hydroxyl groups and carboxyl groups), radical concentration, partition-coefficient (LogP), and hydrophilicity [30, 31] of the selected experimental resins in set one. The number of hydroxyl groups and carboxyl groups was calculated (monomer structures are indicated in Figure 1), and the LogP of resin monomers was predicted using ChemBioDraw (verision 14, ParkinElmer Informatic, USA). The radical level in these resins, measured by EPR, revealed that the trend was identical to the change in number of hydroxyl and carboxyl groups (Figure 5). In addition, these four resins also shared the same trend in hydrophilicity (experimentally evaluated through WCA measurements) as that in the number of hydroxyl and carboxyl groups.

Figure 5.

Figure 5

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EPR spectra of the selected experimental resins in set one under strong magnetic field.

An important property that could affect monomer conversion in photo-polymerization is the viscosity of the resin monomers. To emphasize the impact of viscosity on DC, two categories of resin systems with distinguished chemical differences and containing three viscosity variations were compared: B/T and P/H resin systems. The mixtures in the B/T systems were relatively hydrophobic with fewer hydroxyl or carboxyl groups. The P/H systems were generally more hydrophilic with more hydroxyl and carboxyl groups. In each catagory, there was a base monomer, Bis-GMA or PMGDM, which was more viscous than the other monomer, the diluent monomer, TEGDMA or HEMA. By changing the mass ratios of the base and diluent monomers, three distinguishable viscosities were attained within B/T or P/H systems. The viscosity of the resins increased with the content of Bis-GMA or PMGDM in the following order: B/T or P/H =1/3 < B/T or PH =1/1 < B/T or P/H = 3/1.

3.3. The effects of chemical structure and viscosity on DC improvement

The effect of chemical structure of constituent monomers on DC improvement was evaluated in resins indicated in Table 2 (EBPADMA, B/T, B/H and P/H resins). DC of the composites generally increased with the increasing levels of AP25 incorporated into resins (Figure 6A), furthermore, the extent of the increase varied from resin to resin. EBPDMA showed the highest increase (22.9 %) in DC with the addition of A25. Lesser effect on DC was seen in B/T resins (increase in DC of up to 14.9 %) and in P/H or B/H resins (less than 10 % increase in DC - both resins). Consequently, a following order of DC improvement was obtained due to the addition of TiO2 NPs to the resin compositions of comparable viscosities (200-500 cP): EBPDMA > B/T > (P/H • • B/H), which is in good agreement with the trend of chemical structure and properties listed in Table 2.

Figure 6.

Figure 6

Figure 6

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A) The DC of resins increased as a function of mass % of AP25; B) It shows DCs of two groups of B/T and P/H monomers with/without 0.5 mass % AP25 NPs.

Viscosity effects on DC improvement attained in B/T and P/H resin systems with and without NPs are shown in Figure 6B. Chosen 3/1, 1/1 and 1/3 mass ratios of the base (BisGMA or PMGDM) vs. diluent monomer (TEGMDA or HEMA) yielded three distinct levels of resin viscosity, categorized as high viscosity (more than 1000 cP), medium viscosity (200-500cP) and low viscosity (below 100 cP). Without the NPs, in both B/T and P/H series, DC increased from the high viscosity resins to low viscosity ones. Bis-GMA- and PMGDM-rich mixtures exhibited the lowest DC while diluent monomer (TEGDMA or HEMA)-rich resins exhibited the highest DC. The above results agree well with the findings in free radical polymerization of methacrylate monomers, where in the later stage of polymerization, the chain termination reaction and chain propagation reaction become diffusion controlled; as a result, less mobile monomers (higher viscosity monomers) have a lower double bond conversion [32-34].

One would expect that any observable improvement in DC resulting from the introduction of TiO2 NPs into the resins at levels employed in this study (up to 0.5 mass %), would be due to the chemical interactions with the resin monomers rather than viscosity alterations. In the P/H system, no statistically significant difference (p < 0.05) in DC was observed when comparing pure resins and composites with 0.5 mass % NPs (Figure 6B). While in the B/T system, adding NPs to the resins had multifaceted effects. For B/T=3/1 and B/T=1/1 monomers, addition of AP25 resulted in a significant improvement in DC (maximum DC values were comparable in these two systems), but the B/T=1/3 monomer showed no improvement in DC following the addition of 0.5 mass % AP25. This may be due to the intrinsically higher DC of B/T =1/3 (85.7 ± 1 %) leaving a small amount of available monomers for further DC improvement. The DC of B/T=3/1 and B/T=1/1 without AP25 addition was lower, 73.4 ± 3.3 % and 78.9 ± 3.3 %, respectively. In both compositions, the unreacted monomers react with added AP25 NPs and yield 11 -16 % DC increase in the ensuing composites.

The TiO2 NPs further enhanced the DC of dental resins in addition to the double bond conversion initiated by CQ/4E system. The effects were more pronounced in monomers that are more hydrophobic/containing less hydroxyl groups or/and carboxyl groups. Unlike the CQ/4E initiator system, the viscosity of resin monomers played minimum or no roles in the DC improvement by adding TiO2 NPs. Beneficial effects of the increased DC of TiO2-containing nano-composites include reduced amount of unreacted monomers, which may leach into bio-environment, and potentially more rigid and harder composites [9]. However, resins with improved DC are not necessary superior in mechanical performance. Depending on the application, the overall performance of the nanocomposites need to be balanced with other factors including but not limited to: wettability, dispersion of NPs, interaction of NPs with resin monomers, and polymerization shrinkage.

3.4 Water contact angle measurements and wettability of nanocomposites

Water contact angle (WCA) measurement is a simplified way to directly compare the hydrophilicity (surface wettability to water) of material surfaces. Lower WCA indicates relatively more hydrophilic surface; superhydrophilic surfaces have WCA below five degree [35]. Biologic and clinical studies indicated that surface hydrophilicity of materials is one of the key factors to improve the performance of tissue engineering materials [36]. Hydrophilic surfaces tend to enhance the early stages of cell adhesion, proliferation, differentiation and bone mineralization compared to hydrophobic surfaces [37].

The WCAs of the resins and their AP25-containing composites before and after UV irradiation are compared in Figure 7A and 7B. Before UV irradiation, the WCAs of both pure resins and their AP25 composites followed the order predicted by LogP (Table 2): EBPDMA > B/T=1/1 > P/H=1/1. Adding nanoparticles to the resins increased the hydrophilicity, and the composites (resin with 0.5 mass % nanoparticles) had smaller WCAs angles than pure resins. The WCAs of P/H and EBPDMA resins were significantly greater than the WCAs of their corresponding AP25 composites. However, the WCAs of B/T resins and their AP25 composites were statistically indistinguishable according to one-way ANOVA, even the mean value of the WCAs of the B/T resin was bigger.

Figure 7.

Figure 7

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A) The WCAs of the resins (without AP25) and their corresponding composites containing 0.5 mass % AP25 NPs - before and after UV irradiation. B) The WCAs of the P/H=1/1 resin and the composites with different AP25 NPs content - before and after UV irradiation.

After UV irradiation, the WCAs of all resins and their AP25 composites decreased to different degrees. The WCAs of EBPDMA, B/T and P/H resins were the same suggesting that the relatively more hydrophobic EBPDMA resin changed more than the relatively more hydrophilic B/T and/or P/H resins upon UV irradiation. On the other hand, UV irradiation had dramatically different impacts on AP25 composites (Figure 7A). The P/H =1/1 composite had the largest WCA drop, followed by B/T=1/1 composite and EBPDMA composite; the order coincides with a decrease in resin's hydrophilicity. The most hydrophilic composite showed the largest improvement in surface hydrophilicity by adding AP25 NPs. In addition, the WCAs also changed with concentration of added AP25 (Figure 7B). In P/H=1/1 composites, more AP25 NPs resulted in greater hydrophilicity upon UV irradiation. These findings suggest that the radicals generated through AP25 NPs could modify surface hydrophilicity more than UV irradiation alone. More significantly, the extent of this modification could be controlled by varying the amount of NPs and the composition of resins. The hydrophilicity change induced by the UV irradiation preserved in the resins and composites. After UV irradiation, the specimens were stored in dark for one month. No statistically significant difference (p > 0.05) in WCAs was observed when comparing the WCAs of the specimens measured immediately after and one month after UV irradiation.

UV irradiation may cause chemical changes in both resins and composites. FTIR was not sensitive enough to identify the chemical changes on pure resins. EPR spectra indicate no chemical change in EBPDMA resin nor chemical changes in EBPDMA composites after UV irradiation. However, FTIR shows that the absorbance at 1561 cm−1, corresponding to the O=C-O functional group, in P/H composites disappeared after UV irradiation. The absence of this particular peak in FTIR spectrum is most likely due to carboxylate/TiO2 interaction in P/H –AP25 composites. In addition, a red-shift of –OH groups above 3400 cm−1 was observed, possibly due to hydrogen bonding [38]. The WCA results and FTIR findings combined indicate that the radicals produced by NPs generate permanent chemical changes in the composites. These changes may be triggered by decomposition of carboxyl groups and forming of hydrogen bonds.

4. Conclusions

Functionalizing TiO2 NPs with short-chain unsaturated carboxylic acid enhanced radical loading on NPs by 2.5 times under light irradiation, and life time of radicals was increased over 10 times. The double bonds on the functionalized TiO2 NPs (AP25) were active in copolymerizing with other monomers through free radical photo-polymerization. The addition of AP25 enhanced DC in an order: EBPDMA > B/T > (P/H • •B/H), which was in a reverse trend of chemical structure variation, i.e, increasing number of hydroxyl and carboxyl groups in monomers, and hydrophilicity. As co-initiator to CQ/amine initiator systems, TiO2 NPs were more effective in promoting conversion of monomers into polymers in the resin systems with less hydroxyl and carboxyl groups. Viscosity plays minimal or no role in DC improvement through TiO2 NPs. This finding differs from the viscosity effects seen in the pristine CQ/amine resin systems, in which high viscosity resin monomers yielded low DCs. When using TiO2 NPs to modify wettability of resins under UV irradiation, the AP25 was more active in resin systems with more hydroxyl and carboxyl groups. The hydrophilicity of P/H=1/1 increases as a function of mass fraction of TiO2 NPs thus providing a potential to design materials with controllable wettability, desirable in many applications, including implants and scaffolds for tissue engineering.

Above all, TiO2 NPs are useful additives in achieving dramatic performance improvement at very small fraction in the resins. Furthermore, their applications can be tuned through the functional groups including hydroxyl and carboxyl groups. The findings of this study open new possibilities to balance material performance without significantly changing the composition of the materials.

Table 3.

Viscosity, LogP and number of OH/COOH groups of monomers.

Monomer Viscosity (cP)* LogP OH/COOH groups/molecular
PMGDM solid 3.73 2 COOH groups
Bis-GMA 700000 5.05 2 OH groups
EBPDMA 450 6.17 0
B/T=1/1 225 - 1 OH group
TEGDMA 12 1.27 0
HEMA 5.9 0.47 1 OH group
Open in a new tab
*

Note: The viscosity values were provided by the vender of the chemicals, Esstech Inc.

Highlights.

  • Application of photo-catalytic activities of TiO2 NPs could be tuned through dental resins

  • Adding TiO2 NPs in hydroxyl-rich resins may produce materials with controlled wettability

  • The degree of vinyl conversion could be increased up to 22 % by adding only 0.1 wt% TiO2 NPs

  • Viscosity of resins had minimal or no role in enhancing degree of vinyl conversion through TiO2 NPs

  • Attaching acrylic acid on TiO2 NPs doubled radical loading and extended radical life over 10 times.

Acknowledgement

This work is partially funded by the National Institute of Dental and Craniofacial Research (U01DE023752). Financial support was also provided through ADA Foundation. The authors would like to thank Mr. Anthony Giuseppetti and Dr. Joseph Antonucci for their technical recommendations. We also would like to thank Center for Nanoscale Science and Technology (CNST) at NIST for their technical support.

Footnotes

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Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States

*

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.

References

  • 1.Carp O, Huisman CL, Reller A. Photoinduced reactivity of titanium dioxide. Progress in Solid State Chemistry. 2004;32:33–177. [Google Scholar]
  • 2.Chen XB, Liu L, Yu PY, Mao SS. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science. 2011;331:746–50. doi: 10.1126/science.1200448. [DOI] [PubMed] [Google Scholar]
  • 3.Fujishima A, Honda K. Electrochemical photolysis of water ata semiconductor electrode. Nature. 1972;238:37–8. doi: 10.1038/238037a0. [DOI] [PubMed] [Google Scholar]
  • 4.Oregan B, Gratzel M. A low-cost, high-efficiency solar-cell based on dye-sensitized coloidal TiO2 films. Nature. 1991;353:737–40. [Google Scholar]
  • 5.Qiu XQ, Miyauchi M, Sunada K, Minoshima M, Liu M, Lu Y, et al. Hybrid CuxO/TiO2 nanocomposites as risk-reduction materials in indoor environments. Acs Nano. 2012;6:1609–18. doi: 10.1021/nn2045888. [DOI] [PubMed] [Google Scholar]
  • 6.Karimi L, Yazdanshenas ME, Khajavi R, Rashidi A, Mirjalili M. Using graphene/TiO2 nanocomposite as a new route for preparation of electroconductive, self-cleaning, antibacterial and antifungal cotton fabric without toxicity. Cellulose. 2014;21:3813–27. [Google Scholar]
  • 7.Campoccia D, Montanaro L, Arciola CR. A review of the biomaterials technologies for infection-resistant surfaces. Biomaterials. 2013;34:8533–54. doi: 10.1016/j.biomaterials.2013.07.089. [DOI] [PubMed] [Google Scholar]
  • 8.Xiao J, Chen WQ, Wang FYK, Du JZ. Polymer/TiO2 hybrid nanoparticles with highly effective UV-screening but eliminated photocatalytic activity. Macromolecules. 2013;46:375–83. [Google Scholar]
  • 9.Sun JR, Forster AM, Johnson PM, Eidelman N, Quinn G, Schumacher G, et al. Improving performance of dental resins by adding titanium dioxide nanoparticles. Dental Materials. 2011;27:972–82. doi: 10.1016/j.dental.2011.06.003. [DOI] [PubMed] [Google Scholar]
  • 10.Yoshida K, Taira Y, Atsuta M. Properties of opaque resin composite containing coated and silanized titanium dioxide. Journal of Dental Research. 2001;80:864–8. doi: 10.1177/00220345010800030401. [DOI] [PubMed] [Google Scholar]
  • 11.Yu B, Ahn JS, Lim JI, Lee YK. Influence of TiO2 nanoparticles on the optical properties of resin composites. Dental Materials. 2009;25:1142–7. doi: 10.1016/j.dental.2009.03.012. [DOI] [PubMed] [Google Scholar]
  • 12.Fu GF, Vary PS, Lin CT. Anatase TiO2 nanocomposites for antimicrobial coatings. Journal of Physical Chemistry B. 2005;109:8889–98. doi: 10.1021/jp0502196. [DOI] [PubMed] [Google Scholar]
  • 13.Feng XJ, Zhai J, Jiang L. The fabrication and switchable superhydrophobicity of TiO2 nanorod films. Angewandte Chemie-International Edition. 2005;44:5115–8. doi: 10.1002/anie.200501337. [DOI] [PubMed] [Google Scholar]
  • 14.Mills A, LeHunte S. An overview of semiconductor photocatalysis. Journal of Photochemistry and Photobiology a-Chemistry. 1997;108:1–35. [Google Scholar]
  • 15.Mills A, Lee SK, Lepre A. Photodecomposition of ozone sensitised by a film of titanium dioxide on glass. Journal of Photochemistry and Photobiology a-Chemistry. 2003;155:199–205. [Google Scholar]
  • 16.Darrah PA, Hondalus MK, Chen QP, Ischiropoulos H, Mosser DM. Cooperation between reactive oxygen and nitrogen intermediates in killing of Rhodococcus equi by activated macrophages. Infection and Immunity. 2000;68:3587–93. doi: 10.1128/iai.68.6.3587-3593.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Pacher P, Beckman JS, Liaudet L. Nitric oxide and peroxynitrite in health and disease. Physiological Reviews. 2007;87:315–424. doi: 10.1152/physrev.00029.2006. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Fang YZ, Yang S, Wu GY. Free radicals, antioxidants, and nutrition. Nutrition. 2002;18:872–9. doi: 10.1016/s0899-9007(02)00916-4. [DOI] [PubMed] [Google Scholar]
  • 19.Halliwell B. Oxidative stress and neurodegeneration: where are we now? Journal of Neurochemistry. 2006;97:1634–58. doi: 10.1111/j.1471-4159.2006.03907.x. [DOI] [PubMed] [Google Scholar]
  • 20.Britton G. Structure and properties of carotenoids in relation to function. Faseb Journal. 1995;9:1551–8. [PubMed] [Google Scholar]
  • 21.Heim KE, Tagliaferro AR, Bobilya DJ. Flavonoid antioxidants: chemistry, metabolism and structure-activity relationships. Journal of Nutritional Biochemistry. 2002;13:572–84. doi: 10.1016/s0955-2863(02)00208-5. [DOI] [PubMed] [Google Scholar]
  • 22.RiceEvans CA, Miller NJ, Paganga G. Structure-antioxidant activity relationships of flavonoids and phenolic acids. Free Radical Biology and Medicine. 1996;20:933–56. doi: 10.1016/0891-5849(95)02227-9. [DOI] [PubMed] [Google Scholar]
  • 23.Vo DQ, Kim EJ, Kim S. Surface modification of hydrophobic nanocrystals using short-chain carboxylic acids. Journal of Colloid and Interface Science. 2009;337:75–80. doi: 10.1016/j.jcis.2009.04.078. [DOI] [PubMed] [Google Scholar]
  • 24.Koppel DE. Analysis of macromolecular polydispersity in intensity correlation spectroscopy-method of cumulants. J Chem Phys. 1972;57:4814–20. [Google Scholar]
  • 25.Riseman J, Kirkwood JG. The intrinsic viscosity, translational and rotatory diffusion constants of rod-like macromoleculres in solution. J Chem Phys. 1950;18:512–6. [Google Scholar]
  • 26.Stansbury JW, Dickens SH. Determination of double bond conversion in dental resins by near infrared spectroscopy. Dental Materials. 2001;17:71–9. doi: 10.1016/s0109-5641(00)00062-2. [DOI] [PubMed] [Google Scholar]
  • 27.Eidelman N, Raghavan D, Forster AM, Amis EJ, Karim A. Combinatorial approach to characterizing epoxy curing. Macromol Rapid Commun. 2004;25:259–63. [Google Scholar]
  • 28.Antonucci JM, Fowler BO, Weir MD, Skrtic D, Stansbury JW. Effect of ethyl-alpha-hydroxymethylacrylate on selected properties of copolymers and ACP resin composites. Journal of Materials Science-Materials in Medicine. 2008;19:3263–71. doi: 10.1007/s10856-008-3463-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Wilson KS, Allen AJ, Washburn NR, Antonucci JM. Interphase effects in dental nanocomposites investigated by small-angle neutron scattering. Journal of Biomedical Materials Research Part A. 2007;81A:113–23. doi: 10.1002/jbm.a.30975. [DOI] [PubMed] [Google Scholar]
  • 30.Nishitani Y, Yoshiyama M, Donnelly AM, Agee KA, Sword J, Tay FR, et al. Effects of resin hydrophilicity on dentin bond strength. Journal of Dental Research. 2006;85:1016–21. doi: 10.1177/154405910608501108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Yiu CKY, King NM, Pashley DH, Suh BI, Carvalho RM, Carrilho MRO, et al. Effect of resin hydrophilicity and water storage on resin strength. Biomaterials. 2004;25:5789–96. doi: 10.1016/j.biomaterials.2004.01.026. [DOI] [PubMed] [Google Scholar]
  • 32.Khatri CA, Stansbury JW, Schultheisz CR, Antonucci JM. Synthesis, characterization and evaluation of urethane derivatives of Bis-GMA. Dental Materials. 2003;19:584–8. doi: 10.1016/s0109-5641(02)00108-2. [DOI] [PubMed] [Google Scholar]
  • 33.Anseth KS, Wang CM, Bowman CN. Reaction Behavior and kinetic constants for photopolymerizations of multi(meth)acrylate monomers. Polymer. 1994;35:3243–50. [Google Scholar]
  • 34.Brooks BW. Viscosity effects in the free-radical polymerization of methyl methacrylate. Proceedings of the Royal Society of London A Mathematical and Physical Sciences. 1977;357:183–92. [Google Scholar]
  • 35.Drelich J, Chibowski E, Meng DD, Terpilowski K. Hydrophilic and superhydrophilic surfaces and materials. Soft Matter. 2011;7:9804–28. [Google Scholar]
  • 36.Gittens RA, Scheideler L, Rupp F, Hyzy SL, Geis-Gerstorfer J, Schwartz Z, et al. A review on the wettability of dental implant surfaces II: Biological and clinical aspects. Acta Biomaterialia. 2014;10:2907–18. doi: 10.1016/j.actbio.2014.03.032. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Bornstein MM, Valderrama P, Jones AA, Wilson TG, Seibl R, Cochran DL. Bone apposition around two different sandblasted and acid-etched titanium implant surfaces: a histomorphometric study in canine mandibles. Clin Oral Implant Res. 2008;19:233–41. doi: 10.1111/j.1600-0501.2007.01473.x. [DOI] [PubMed] [Google Scholar]
  • 38.Kuo SW, Chang FC. Studies of miscibility behavior and hydrogen bonding in blends of poly(vinylphenol) and poly(vinylpyrrolidone). Macromolecules. 2001;34:5224–8. [Google Scholar]

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