Thermo stable ZnO NPs/Asiatic acid nanocomposites for acidogenic neutralization, anti-biofilm, and enamel protection in dental enamel reinforcement

Abstract

Objectives: Dental caries, a pervasive oral health issue, is driven by Streptococcus mutans-mediated biofilm formation and acidogenesis, culminating in enamel demineralization and structural degradation. This study evaluates the efficacy of thermostable ZnO NPs/AA nanocomposites in suppressing S.mutans acid production, disrupting its biofilm matrix, and strengthening enamel integrity, with an emphasis on its potential as a novel dental healthcare material.

Methods: This study encompassed the synthesis of zinc oxide nanoparticles functionalized with Asiatic acid (ZnO NPs/AA) using a co-precipitation method. The physicochemical properties of ZnO NPs/AA were characterized using FE-SEM with EDS, XRD, FT-IR, and UV-DRS, confirming structural integrity and functional modifications. Thermal stability was assessed via TGA and DSC, demonstrating robust performance suitable for biomedical applications. The antibacterial activity, anti-biofilm efficacy of ZnO NPs/AA including, extracellular polymeric substance inhibition, and acidogenic activity modulation were evaluated through microdilution methods, biofilm biomass quantification assays, Congo red binding studies, and pH analysis. In ex-vivo studies, ZnO NPs/AA treated sectioned tooth enamel was exposed to S. mutans to evaluate its effects. The mechanical properties, including microhardness and surface morphology, were analyzed using Vickers microhardness testing and Atomic Force Microscopy (AFM). Additionally, the controlled release kinetics of Asiatic acid were analyzed under physiological (pH 7.4) and acidic (pH 5.0) conditions to elucidate its pH-responsive drug delivery potential.

Results: A precisely synthesized ZnO NPs/AA with a sheet-assembled flower-like structure was observed through SEM analysis, while its composition and functionalization were further confirmed by FTIR and UV-DRS. Thermal stability was validated through TGA and DSC analyses, establishing ZnO NPs/AA as a highly thermally stable material for biomedical applications. ZnO NPs/AA exhibited remarkable multi-functional properties, including potent antibacterial activity, leading to an 85.25 % reduction in S. mutans biofilm biomass and an 81 % inhibition of EPS production. pH modulation studies demonstrated effective neutralization of acidogenic activity, maintaining a near-neutral pH (7.01 at 48 h), significantly outperforming ZnO NPs and the untreated control. Enamel treated with ZnO NPs/AA following exposure to S.mutans showed a 72.6 % increase in microhardness and a 80.93 % reduction in surface roughness, highlighting its ability to combat S.mutans induced demineralization and acid formation, thereby preserving the enamel integrity.

Conclusion: This study establishes ZnO NPs/AA as a promising biomaterial with potent antibacterial, anti-biofilm, and enamel-protective properties. These findings highlight ZnO NPs/AA as a promising and innovative approach for mitigating enamel demineralization and combating biofilm-associated dental challenges.

Clinical relevance: ZnO NPs/AA is a promising therapeutic option for protecting enamel, combating S. mutans biofilm damage, and improving dental health due to its stability, durability, and pH-responsive drug release.

Keywords: Acidogenic activity modulation; Asiatic acid; Nanomaterials; S.mutans; Tooth enamel.