Synthesis, characterization and evaluation of azobenzene nanogels for their antibacterial properties in adhesive dentistry

Abstract

The presence of cariogenic bacteria within the prepared tooth cavity at the adhesive resin-dentin interface is detrimental to the long-term stability and function of composite restorations. Here, we report the synthesis and incorporation of methacrylated azobenzene nanogels within bisphenol A-glycidyl methacrylate/hydroxyethyl methacrylate/ethanol (B/H/E) adhesive resins and evaluate their ability to reduce the bacterial invasion of cariogenic Streptococcus mutans biofilms while preserving the mechanical strength and structural integrity of the critical interfacial connection between the restoration and the tooth. The azobenzene nanogel, with a hydrodynamic radius of < 2 nm and a molecular weight of 12,000 Da, was polymerized within B/H/E adhesive formulations at concentrations of 0.5 wt.%, 1.5 wt.%, and 2.5 wt.%. While the double-bond conversion, cytocompatibility, water solubility, and sorption of the adhesive networks were comparable, azobenzene nanogel networks showed improved hydrophobicity with a ≥ 25° increase in water contact angle. The polymerized adhesive surfaces formulated with azobenzene nanogels showed a 66% reduction in bacterial biofilms relative to the control while maintaining the mechanical properties and micro-tensile bond strength of the adhesive networks. The increased hydrophobicity and antibacterial activity are promising indicators that azobenzene nanogel additives have the potential to increase the durability and longevity of adhesive resins.

Keywords: Streptococcus mutans; caries resistance; composite; mechanical testing; nanoparticles.

FIGURE 1

The study design and experiments that were carried out in this work. The azobenzene nanogels (AB-NG) were synthesized and incorporated at 0.5 wt.%, 1.5 wt.%, and 2.5 wt.% within conventional BisGMA/HEMA/ethanol adhesive networks

FIGURE 2

The monomers and initiators used in the study (A). The azobenzene nanogel was synthesized using an isocyanate-alcohol solution polymerization protocol at 20°C (B)

FIGURE 3

The incorporation of azobenzene nanogel up to 2.5 wt.% did not impact the viscosity of the B/H/E in the absence or presence of light (A). UV-Vis spectroscopy quantifies the increasing concentration of azobenzene nanogel in the B/H/E formulations via the increased absorption observed between 300 and 450 nm (B). (Error bars denote the SD)

FIGURE 4

Flexural modulus and flexural strenght tests in both conditions indicate that the mechanical strength of the material was retained in comparison to the control. Condition 1: immediately after applying the adhesive (A), and condition 2: at the end of 7 days in distilled water at 37°C (B). Mean values (Error bars denote the SD)

FIGURE 5

The microtensile bond strength studies indicate that the strength of the azobenzene nanogel substrates is maintained in comparison to the control shown in both conditions. Condition 1: immediately after applying the adhesive (A), and condition 2: at the end of 7 days of storage in distilled water at 37°C (B). Mean values (Error bars denote the SD)

FIGURE 6

The MTT assay demonstrates that cell viability upon exposure to different concentrations of azobenzene nanogel samples is comparable to the control adhesive formulation. Mean values (Error bars denote the SD)

FIGURE 7

Reduced S. mutans CFU on azobenzene nanogel substrates in comparison to the control. Mean values (Error bars denote the SD). ****p < 0.0001.