Effect of combining photoinitiators on cure efficiency of dental resin-based composites

Abstract Camphorquinone is the most conventionally used photoinitiator in Dentistry. Although different alternative photoinitiators have been proposed, no photoinitiator was capable of completely substituting camphorquinone. The combination of photoinitiators has been considered the best alternative. Objectives: To evaluate the effect of combining Norrish type I and II photoinitiators on the cure efficiency of dental resin-based composites. Methodology: Experimental composites were produced containing different photoinitiator systems: Norrish type I-only, mono-alkyl phosphine oxide (TPO); Norrish type II-only, camphorquinone (CQ); or its combination, CQ and TPO, in a 1: 1 molar ratio. UV-vis absorption spectrophotometry was performed to assess the consumption of each photoinitiator after curing (n=3). A multi-wave LED (Bluephase® G2, Ivoclar Vivadent) was pre-characterized and used with a radiant exposure of 24 J/cm2. The degree of conversion was evaluated by Raman spectrometry, and the elution of the monomers by nuclear magnetic resonance analysis (n=3). Data were analyzed using ANOVA and Tukey's test (α=0.05; β=0.2). Results: The combination of CQ and TPO increased the consumption of the photoinitiator system compared to CQ-only (p=0.001), but presented similar consumption compared to TPO-only (p=0.52). There was no significant difference in the degree of conversion between the composites regardless of the photoinitiator system (p=0.81). However, the elution of the monomers was reduced when both photoinitiators were combined. TPO-based material presented the highest elution of monomers. Conclusions: The combination of the photoinitiator systems seems to be beneficial for the cure efficiency of dental resin-based composites.


Introduction
Light cured resin-based materials are composed of monomers that, after exposure to light, form a polymer. This process of building a polymer through the combination of monomers is called polymerization. When polymerization is triggered by a physical medium, such as light, this process is called photopolymerization. 1 Photoactivation promotes the excitation of the photoinitiators. After being excited, the photoinitiators react, generating free radicals. 2 The free radicals, in turn, are responsible for breaking the double bonds of the monomers. So that for the chemical stabilization of the molecule, the monomers bind together, forming larger units and the polymers. 1 Camphorquinone is the most used photoinitiator system in the manufacture of dental resin-based materials since 1970. 3 Camphorquinone is a Norrish type II photoinitiator. This classification is due to the need to be combined with a reducing agent to generate free radicals and initiate the polymerization reaction. [4][5][6] In the case of camphorquinone, the most common reducing agents are tertiary amines. 2 On the other hand, Norrish type I photoinitiators are capable of generating free radicals after photoactivation without the need for a reduction agent. Generation of free radicals occurs through the self-cleavage of the photoinitiator molecule itself, creating at least two free radicals from this self-cleavage. The mono-alkyl phosphine oxide (TPO) is a well-known tested Norrish type I photoinitiator in Dentistry. [5][6] Several studies have demonstrated the curing efficiency of mono-alkyl phosphine oxide for application in some dental resin materials. 3-7 However, it is also known that there are limitations for its use combined with other photoinitiators, such as camphorquinone.
Mono-alkyl phosphine oxide is a much more reactive molecule than camphorquinone. The mono-alkyl phosphine oxide can generate two active free radicals that can initiate the polymerization reaction. At the same time, the camphorquinone-based system, combined with a reducing agent, is only capable of producing one active free radical. 5-8 On the other hand, camphorquinone is activated by the blue wavelength spectrum, while the mono-alkyl phosphine oxide, by the violet wavelength spectrum. 9 The blue wavelength spectrum can penetrate deeper through the composite compared to the violet wavelength spectrum. Thus, for resin-based materials that need to be photoactivated to a certain depth or thickness, mono-alkyl phosphine oxide may present a certain disadvantage compared to the camphorquinone-based system. 4,7,9 Still, the quality of the polymer not only depends on the degree of conversion the material can achieve, but the kinetics of conversion from the photoinitiators or their combination. Thus, the monomer elution is an important parameter to evaluate the quality of the polymeric chain formed with the presence -or not -of branches or reticulations between the polymers. 10,11 Recent studies have shown the combination of Norrish type I and II photoinitiators can be even more efficient compared to Norrish type I photoinitiators. 7,9 This fact seems to be related to a possible synergy effect when the two photoinitiator systems are combined. 9 However, further research on the impact of combining

Curing light characterization
A multi-wave curing light (Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein) with a standardized tip (9 mm diameter) was used in this study. First, the light tip active area of emission was measured using a bean profile. 7,9 The output power (mW) was measured with a calibrated power meter (Ophir Optronics, Har-Hotzvim, Jerusalem, Israel). The light irradiance (mW/ cm²) was calculated by dividing the output power by the area of the light tip. The spectral distribution was obtained by using a pre-calibrated spectrometer (USB2000, Ocean Optics, Dunedin, FL, USA), and the spectral distribution data were integrated using Origin 6.0 software (OriginLab, Northampton, MA, USA).

Material
Chemical   The Bluephase ® G2 had an active area of emission of 0.646 cm 2 . The mean irradiance of the Bluephase ® G2 was 1195 mW/cm 2 ± 17 mW/cm 2 and had a total radiant exposure of 24 J/cm 2 ± 0.5 J/cm 2 after 20 seconds of exposure, with 19.4 J/cm 2 ± 0.6 J/cm 2 being generated over the blue wavelength range of 420-495 nm and 4.6 J/cm 2 ± 0.3 J/cm 2 over the violet wavelength range of 380-420 nm. The specimens had a surface area of 0.196 cm 2 . The mean irradiance received by the specimens was 888 mW/cm 2 ± 10 mW/ cm 2 and had a total radiant exposure of 18 J/cm 2 ± 0.2 J/cm 2 after 20 seconds of exposure, with 15 J/cm 2 ± 0.1 J/cm 2 being generated over the blue wavelength range of 420-495 nm and 3 J/cm 2 ± 0.0 J/cm 2 over the violet wavelength range of 380-420 nm. Figure 2 illustrates the spectral power (mW) distribution according to each wavelength (nm). As it can be observed, the Bluephase ® G2 is a dual peak multi-wave curing light, with one LED chip emitting "violet" light with peak at 410 nm, and three LED chips emitting "blue light" with peak at 460 nm. The reason for using a multiwave curing light in this study is because most of the absorption of CQ is within the 430-490 nm range, or the "blue light" range, with absorption peak approximately at 470 nm, whereas the absorption peak of TPO is mainly in the near UV-A region and extends to the violet spectrum range (380-420 nm).

Photoinitiators consumption by absorption spectrophotometric analysis
First, a calibration curve was created by first preparing a set of standard solutions with known concentrations of each photoinitiator and its combination. All solutions were prepared with 0.1 ml of the monomer blend presented in Table 1 as the diluent. For each solution, the absorbance at a similar wavelength was measured, and a graph of absorbance against concentration was plotted. All spectra were collected in the 200-600 nm wavelength range using a UV-Vis spectrophotometer (U-2450, Hitachi High-Technologies, Chiyoda, Tokyo, Japan). The spectra were collected using a disposable cell with a path length of 1 cm. Then, an initial spectrophotometric analysis of each photoinitiator diluted in 0.1 ml of the same monomer blend at the concentrations stated in Table 2

Degree of conversion analysis
The cure efficiency for each resin was measured using a µ-Raman spectrometer (Xplora, Horiba, Kyoto, Japan) (n=3). Each experimental resin-based composite was placed in a silicon rubber mold (Ø=5 mm, 1 mm thick) sandwiched between two polyester strips. First, the unpolymerized blends were scanned, then light cured with 24 J/cm 2 , and immediately rescanned. All light curing procedures were performed with the curing light tip positioned in the center of the specimen. All spectra were obtained by the coaddition of 32 scans at a resolution of 4 cm -1 . Data were exported to a software (SpectraGryph 1.2, Effemm, Oberstdorf, Germany), and the derivative of the 1,610 cm-1 and 1,640 cm-1 peaks corresponded to the phenyl CC peak and the vinyl CC peak, respectively.
The degree of conversion (DC) was calculated using the equation:

Statistical analyses
Power analysis was conducted to determine the sample size for each experiment to provide a power of at least 0.8 at a significance level of 0.05 (β=0.2).
Data were checked for normality by Shapiro-Wilk's test and homoscedasticity of variances by Levene's test. All data were analyzed using a one-way ANOVA test, followed by Tukey's post-hoc test for multiple comparisons. A 95% significance level was considered for all analyses. Degree of conversion Monomer elution Table 4 also shows the monomer elution (µg/ml) of the experimental composites containing the different photoinitiator systems. The elution of the monomers was reduced when both photoinitiators were combined.

Photoinitiators consumption
TPO-based material presented the highest elution of monomers.

Discussion
The objective of this study was to evaluate the effect of combining photoinitiators type I (mono-alkyl phosphine oxide -TPO) and II (camphorquinone -CQ) on polymerization efficiency of dental resins. The first tested hypothesis that the combination of Norrish type I and II photoinitiators would increase the consumption of the photoinitiator system was accepted. As observed in the results, the combination of camphorquinone and TPO increased the consumption of the photoinitiation system compared when the camphorquinone was used alone.
The reaction of camphorquinone with a tertiary amine result in the consumption of part of the total amount of the photoinitiator present in the material. 6,12 As it is known, camphorquinone is a yellow-colored substance, which limits the production of certain shades, especially less yellowish shades, and bleaching shades. Also, with its consumption, a phenomenon also known as photobleaching effect occurs during the reaction. Despite the decrease in the yellow appearance of the material due to the consumption of camphorquinone, this phenomenon makes the clinical selection of color more difficult. 4,13 TPO, on the other hand, is a whitish substance, and its combination with camphorquinone reduces the overall yellowness of the material as well as the color change throughout the curing reaction. 4,7 Besides, this lower yellowness of the material does not only contribute to the color of the material, but better the light transmittance of the light through the material during curing. 7,9 Thus, favoring the activation of more of the photoinitiator system, as observed in the results. Another fact that can contribute to that is the combination of camphorquinone and TPO allowed the photon absorption efficiency to increase, that is, more photons are absorbed due to the broad spectrum of the curing light used in the experiment. Thus, there is an increase in the yield of photoinitiators, especially camphorquinone. [5][6]9 However, the second tested hypothesis that the combination of Norrish type I and II photoinitiators increases the degree of conversion of dental resinbased composites was rejected. There was no significant difference in the degree of conversion between the composites regardless of the photoinitiator system. Therefore, although the higher consumption of the photoinitiator system, the number of monomers linked to form the polymer was the same. The primary reason for this is the similar viscosity of the    It is worthwhile to mention the BisGMA is even more viscous than the BisEMA due to the presence of the -OH terminals in the BisGMA structure ( Figure 1).
These terminals tend to form hydrogen interactions between these monomers leading to a very high intermolecular interaction energy, thus contributing to the high viscosity of the BisGMA. 20,23,25 This explains the higher levels of BisGMA and BisEMA in comparison to UDMA and TEGDMA. However, the higher levels of BisEMA found in the composite containing CQ and TPO combined may be due to differences in the kinetics reactive of the CQ when alone or in combination with TPO.
It is known that composites with a low level of crosslinking tend to be weaker than those with a high level of crosslinking. As a limitation of this study, Conclusion Within the limitations of this in vitro study it was possible to conclude that the combination of the photoinitiator systems seems to be beneficial for the cure efficiency of dental resin-based composites. The combination of Norrish type I and II photoinitiators increased the consumption of the photoinitiator system; and, however it did not increase the degree of conversion of dental resin-based composites; it did reduce monomer elution.