This study used two commercial feldspathic dental porcelains, IPS Inline and Ex-3 Noritake, to evaluate the viability of the mixture between ceramic materials and the β-AgVO3 nanomaterial in an unprecedented way. One of the biggest challenges in incorporating antimicrobial compounds into ceramic materials is that the material is exposed to high temperatures during processing, which may impair the action of the added compound [22]. Positive results were obtained in this study, once the chemical composition confirmed the viability of the proposed mixture, with the unaltered atomic proportion of the porcelain elements, added to the beneficial antimicrobial effect of the β-AgVO3 addition, without major damage to the mechanical properties. Thus, the null hypothesis tested in this study was rejected.
The β-AgVO3 antimicrobial effect observed is due to the action of silver (Ag+) and vanadium (V4+ and V5+) ions released by the nanomaterial in contact with bacterial surfaces [10, 11]. Ag+ adheres to the cytoplasmic membrane or cell wall through electrostatic interaction with sulfur proteins, which can permeate the membrane and cause its rupture [23]. Vanadium ions in the 5 + state of oxidation and Ag+ interact with the thiol groups present in the enzyme’s bacterial metabolism, forming stable complexes. Ag+ still comes into contact with bacterial DNA preventing their replication, and oxidation-reduction between V4+ and V5+ leads to oxidative stress in the bacterial cell [11].
The pure β-AgVO3 demonstrated antimicrobial activity confirming its potential to be used as an antimicrobial agent, as observed in other studies [12, 17]. When incorporated into dental porcelains, it presented more effect against S. mutans and S. sobrinus, demonstrating a spectrum of action against Gram-positive and Gram-negative bacteria, which can prevent the incidence of dental caries. Oda et al. [24] related that in the presence of S. mutans and S. sobrinus the incidence of the dental caries is significantly higher than when in the presence of S. mutans only. The IPS Inline with 5% of β-AgVO3 presented higher action against S. mutans than S. sobrinus, which can be due to the greater resistance of the S. sobrinus whose virulence and acid-forming capacity is higher and faster than S. mutans [25, 26]. A positive effect against S. sobrinus was observed to Ex-3 Noritake incorporated with 5% of β-AgVO3.
IPS Inline with 2.5% of β-AgVO3 showed antimicrobial activity against A. actinomycetemcomitans. This microorganism is commonly found in aggressive periodontal disease [27, 28], and may cause extraoral infections such as infectious endocarditis, bacterial arthritis, pregnancy-associated septicemia, brain abscesses, and osteomyelitis, but its primary source is the oral cavity [28]. Acinetobacter spp. associate with P. aeruginosa participate in the etiopathogenesis of periodontal diseases and, P. aeruginosa, A. actinomycetemcomitans, A. baumannii and species of the red complex (Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola), increase the likelihood of developing aggressive periodontitis [29]. Against P. aeruginosa, the modified porcelains showed no antimicrobial activity. The resistance of A. actinomycetemcomitans and P. aerugionosa, both of which are Gram-negative, to the action of β-AgVO3 can be due to structural differences, as they have an additional external membrane that Gram-positive bacteria do not possess, resulting in different susceptibility and permeability to antimicrobial agents [30].
The pure β-AgVO3 demonstrated higher antimicrobial activity than when was incorporated into dental porcelains, mainly to P. aeruginosa. This can be due to the total non-release of the nanomaterial when incorporated, once all groups released more vanadium ions than silver ions. However, in the synthesis of the nanomaterial, 2 mmol of silver nitrate is added while 1 mmol of ammonium metavanadate is added [11], thus expecting a greater release of silver. This greater release of vanadium than silver was also observed in other dental materials incorporated with β-AgVO3 [31, 32].
The results also demonstrated that the IPS Inline presented better antimicrobial effect than Ex-3 Noritake. This difference can be due to the greater release of silver and vanadium ions by IPS Inline, and due to differences in this porcelain’s sintering cycle led to a change in the nanomaterial’s properties. During processing, Ex-3 Noritake reaches temperatures of 600 °C, as indicated by the manufacturer, whereas IPS Inline is limited to 403 °C. The high temperatures produced during porcelain sintering negatively impact the incorporation of antimicrobial agents [22]. In addition, the porcelain’s composition (Ex-3 Noritake presented less K and more Zn wt.% elements) and particle type may not have interacted or integrated with the nanomaterial and may have therefore inhibited effective antimicrobial release.
The β-AgVO3 incorporation changed the surface characteristics and higher roughness was observed. This higher roughness may have provided biofilm retention in the groups that did not show a reduction in the bacterial contingent. The roughness has a considerable influence on biofilm formation because it favors greater quantity and early maturation of bacteria that are lodged in pores [33]. Despite the higher roughness, the nanomaterial released demonstrated antimicrobial effect in some groups.
Roughness is also related to mechanical microretention between the cementing agent and the porcelain and is positively correlated with bond strength [34]. The proposed new material can be used on the inner face of porcelain restorations; thus, it may enhance their mechanical retention [35].
The photomicrographs of surface characteristics suggest that the incorporation interfered more in the material conformation when compared to the control groups and indicate that this interference was different from one porcelain to another, which corroborates the mechanical and antimicrobial results. Morphology may also have interfered with antimicrobial potential due to nanomaterial dispersion. When used in conjunction with SEM, dispersive EDS is a chemical microanalysis technique that facilitates the investigation of various samples’ compositional details [36]. Based on this analysis, the porcelains presented few differences in oxygen, potassium and zinc levels. The presence of silver and vanadium in the groups incorporated with β-AgVO3 indicates the presence of the nanomaterial in the new material’s structure, and this addition without changing the components inherent to the porcelain itself.
Several parameters, such as microhardness and fracture toughness, influence the porcelain’s mechanical properties and clinical use. A lower fracture toughness value can lead to poorer clinical performance [37]. When this property was evaluated, the incorporation of the nanomaterial in IPS Inline promoted an increase of fracture toughness, which can lead to a considerable increase in the material’s ability to absorb elastic energy, indicating a greater chance of surviving more severe impacts [38]. The fracture toughness values for the modified groups of IPS Inline are in accordance with results showed in literature for feldspathic porcelains [38]. In microhardness evaluation, the group incorporated with 2.5% presented the lowest value. However, this property did not change in the group incorporated with 5% β-AgVO3, demonstrating that the incorporation of higher concentrations of vanadate does not harm the mechanical properties and still favors the antimicrobial effectiveness.
Significant results were obtained by incorporating β-AgVO3 into dental porcelains, including imbuing materials with antimicrobial properties against microorganisms frequently associated with dental problems. Thus, the antimicrobial action identified in this study has high significance, as the nanomaterial was able to maintain its effectiveness after porcelain processing. Improving the fracture toughness property of IPS Inline and maintaining this property for Ex-3 Noritake through the incorporation of the nanomaterial suggests the possible viability of this material for clinical application. The porcelains used are commercial formulations, so this study is an initial analysis of the viability of mixing porcelain materials and β-AgVO3. Therefore, the development of specific formulations to receive this additive should be evaluated in the future.
This study has its limitations because it was performed strictly in vitro. The interaction between microorganisms in vivo may lead to different results, and further analysis is important to consider the clinical use of these porcelains.
Although the results regarding the mechanical properties were altered, the literature is not clear about the maximum acceptable alteration levels. Because the suggested use of this material is limited to the internal regions of prostheses, achieving antimicrobial results after firing processing, as well as processing the modified material itself and giving it shape, contour and strength, shows the great advance and potential use that this methodological proposal offers.