Novel Dentin Adhesive Containing Silver Exchanged EMT Zeolites Against Cariogenic Biofilms

Background:Secondary caries is the most common and major cause of clinical restoration failure.It is mostly caused by microleakage at the restoration-dentin bonding interface and further bacterial invasion.The primary goal of secondary caries prevention is to inhibit the accumulation of caries-related bacteria at the interface between restoration and dental tissue.In recent years,a large number of studies have been committed to developing antibacterial dentin adhesives by incorporating of antibacterial substances such as quaternary ammonium salts,zinc oxide,titanium dioxide nanoparticles,which could achieve rapid and effective inhibition of caries-related pathogens.Silver nanoparticles（NAg）may also be added to impart antibacterial properties into dentin adhesive.However,there are several challenges for incorporating NAg into dentin adhesives.Therefore,a reasonable Ag delivery system should be developed to solve aforementioned problems.Zeolite is a kind of hydrated sodium aluminosilicate crystalline material with highly regular microporous structure,composed of MO₄ tetrahedra（M=Si or Al）linked to each other by oxygen ions.Among them,nanosized EMT zeolite with uniform pore structure,high porosity,large pore volume and high specific surface areas,has the strong ability of ion exchange,thus it can be as an ideal carrier of silver ions.It can be added into dentin adhesive,and thus play a great role in antibacterial performance and achieve a long-term antibacterial effect,which provides a new way for the clinical prevention of secondary caries.Objective:The nanosized EMT zeolite was synthesized.EMT zeolite with different silver ion exchange time was incorporated into dentin adhesive to develop an antibacterial adhesive,which investigates the inhibition of biofilm formation at the dentin-restoration margin and to prevent the formation of secondary caries.Methods:1.Nanosized EMT zeolite was prepared with template-free at low temperature.Silver ions were loaded into the channels of EMT zeolite through ion exchange reaction.2.The structures of EMT zeolite loaded with silver before and after were characterized by transmission electron microscopy（TEM）,X-ray diffraction（XRD）,UV-visible absorption spectroscopy（UV-vis）,nitrogen adsorption method,inductively coupled plasma-atomic emission spectrometer（ICP-AES）.3.The EMT zeolite and the EMT zeolite exchanged with silver ion for 10,20,and 40min（referred to as:Ag⁺-EMT-1,Ag⁺-EMT-2,Ag⁺-EMT-3）were incorporated into the dentin adhesive respectively,and the adhesive-resin layered disks were prepared.And the biosafety of all samples was further evaluated by cytotoxicity test.4.The inhibitive effect of pure adhesives and adhesives containing EMT,Ag⁺-EMT-1,Ag⁺-EMT-2 and Ag⁺-EMT-3 on bacterial biofilms related to caries were investigated.Streptococcus mutans（S.mutans）,Streptococcus gordonii（S.gordonii）,and Streptococcus sanguinis（S.sanguinis）were selected as representative strains of cariogenic pathogens and to form single-species biofilms.Bacterial early attachment,colony-forming units（CFU）,live/dead staining,and metabolic activity were measured.Results:1.The hexagonal EMT zeolites have been successfully prepared.The structure of nanosized EMT zeolite is metastable and easy to occur ion exchange.2.The ion-exchange reaction between Ag⁺and nanosized EMT zeolite was time-dependent.The content of Ag⁺in the EMT zeolites with 40-min silver ion exchange was the highest,up to 6.40 wt%.While the content of Ag⁺in the EMT zeolites with 10-min silver ion exchange was the lowest,up to 1.60 wt%.3.The five groups of adhesive samples all presented good biosafety.4.The adhesive containing EMT zeolites exchanged with silver ions showed good anti-bacterial capability.The adhesive containing the EMT zeolite with the longest silver ion exchange time（40 min）showed the greatest reduction in early bacterial attachment,bacterial biomass,biofilm growth and metabolic activity.The CFU counts of bacterial biofilms decreased by about 2 orders of magnitude.Conclusions:This study synthesized the nanosized silver loading EMT zeolite which was further incorporated into the dentin adhesive to combat cariogenic biofilms.The dentin adhesives containing Ag⁺exchanged zeolites had remarkable antibacterial properties without affecting the color and aesthetic properties of dental restorations.It can be served as“bioactive”adhesive materials and revealed its potential value for anti-biofilms and anti-caries clinical applications.