Silver nanoparticles added to a commercial adhesive primer: Colour change and resin colour stability with ageing
Introduction
The antimicrobial property of silver is well known since ancient Greece [1], and the use of silver nanoparticles (NAg) applied to Dentistry started its spreading in 19th century [2]. These nanoparticles’ biological, chemical, and mechanical properties have been widely investigated with concern to their use to inhibit biofilm formation [3], to achieve an antimicrobial effect [[4], [5], [6], [7], [8]], and to avoid hard dental tissue demineralisation whilst preserving their properties [9]. Its small size and associated large superficial area make NAg effective against many microorganisms [2] and suitable for use with different dental materials [3,8,9].
Evidence that the addition of 0.05–0.1 wt% of NAg to adhesives did not interfere in the adhesive bond strength have been shown [6]. Besides presenting an antimicrobial effect against S. mutans [2,3,8], such nanoparticles did not exhibit significant cytotoxicity and have been safely used over fibroblasts [6] and human dental pulp stem cells [3]. However, consensus still lacks regarding the maximum concentration of NAg that can be safely applied, as it is known that higher concentrations may interfere on material's mechanical and biological properties whilst inducing undesirable colour changes [6,10].
The incorporation of metallic nanoparticles might interfere on light absorption and refraction [11], jeopardizing the visual properties of restorative materials. The colour of nanoparticles depends on which material the core is made of. It also depends on the production and incorporation process, since their size and shape, as well as aggregation state, are strongly influenced by those parameters regardless of origin [11]. Depending on the heating time and temperature used in the silver nanoparticles obtention, their shape might change and so does their colour. Rounded corners or spherical shaped nanoparticles have a light yellow-orangish colour [11], whilst cubic- and tetrahedron-shaped nanoparticles present a brighter yellow colour and darkish red hues [12]. Some authors report colour alterations during the NAg synthesis [9], or when applied to thermally activated acrylic resins [13]. However, literature lacks information concerning colour change related to NAg-modified adhesive systems.
In vitro ageing methods are used to evidence colour change on resin-based materials. Seven-to thirty-day water ageing promotes water saturation, but it does not always induce colour change [10]. Thermocycling (T) may cause superficial microcracks and silane deterioration [[14], [15], [16]], as the matrix and inorganic fillers of resin-composites have different linear thermal expansion coefficients. Furthermore, upon exposure to 300 h of accelerated artificial ageing (AAA), the ultraviolet light can be absorbed by non-polymerized amines [10,17,18], and that process can be used to simulate one year of clinical service [19].
After a cavity preparation, the primer is usually the first agent to get in contact with the dental tissue. The eventually remaining bacteria can compromise the restoration durability, and for that reason, a primer with antimicrobial properties is highly desirable. To assure that the addition of NAg into the primer of a three-step adhesive system will not interfere in the colour of the restoration, the interaction between these nanoparticles and this type of materials needs to be investigated.
Thus, this study aims to evaluate the influence of different concentrations of NAg (0.005 wt% - 0.025 wt%) incorporated into a primer of a three-step adhesive system on the colour of the resin composite (simulating a bonded restoration), before and after ageing.
Section snippets
NAg synthesis and experimental primers preparation
Monodisperse and fully dispersible 20 nm NAg were prepared according to the previously described method [3]. Briefly, 11.7 mL of an ethanolic solution containing 5 wt% of polyvinyl alcohol (MM 100,000) were heated at reflux, for 5 min. Then, 1.5 mL of an aqueous AgNO3 (0.171 g) solution was added and the mixture was kept under reflux for 10 min, leading to a yellowish-brown hue solution. After that, the reaction medium was immediately cooled to room temperature, filtered through a 0.22 μm PVDF
Results
The colour analysis of the primers incorporated with different concentrations of NAg is presented in Fig. 2. The UV–vis electronic spectra were obtained directly from experimental primers, without dilution, in a 1-mm path quartz cuvette. The experimental primers’ UV–vis spectra showed a characteristic band, centred at 421 nm that was proportional to the concentration of NAg. This band is missing in the SBMP primer alone and can be attributed to the confined Surface Plasmon Resonance Band (SPB)
Discussion
The present study investigated the effect of NAg-primers and different ageing in the colour of resin composite simulated restorations. The ΔE variation was clinically acceptable (CIELab ΔE≥3.3; CIEDE ΔE≥1.8) when adding up to 0.025 wt% of NAg. The colour after ageing was different according to each ageing method. Although three different ageing methods were chosen in this study, there is still no method that could faithfully represent the clinical ageing condition. Also, the methodology of NAg
Conclusions
The addition of NAg into the primer of SBMP increased the ΔE; however, it remained under the acceptable limit reported in the literature of 2.7 - CIELab and 1.8 - CIEDE 2000, as far as colour change is concerned. Therefore, within the limitations of an in vitro set-up, the present study demonstrated that the use of primers with NAg up to 0.025 wt% in cavities more than 1 -mm deep does not change the colour perception significantly and is colour safe. CIEDE 2000 and CIELab methods presented
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported in part by the Coordination for the Improvement of Higher Education Personnel (CAPES 1741054) and São Paulo Research Foundation (FAPESP 201818/21489-1) and National Council for Scientific and Technological Development (CNPq 401581/2016-0 and 303137/2016-9).
S. H. Toma would like to thank the Brazilian Nanotechnology National Laboratory (LNNano) for the use of TEM facilities [TEM-21785] and the National Council for Scientific and Technological Development (CNPq)
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