| Background: In the clinical field of dentistry,brackets and other devices are common auxiliary devices in oral cavity.However,the presence of these attachments tends to cause the retention of food debris,leading to the attachment of plaque on the tooth surface and the rapid proliferation of pathogenic bacteria in the local microenvironment further causing oral problems such as tooth demineralization,caries,and gingival redness and bleeding.Currently,the use of antimicrobial coatings modified on the surface of oral attachments such as brackets is one of the effective strategies to address the adhesion of pathogenic bacteria.It is worthwhile to consider which antimicrobial agent to use to develop antimicrobial coatings.Carbon dots（CDs）,as new fluorescent carbon nanomaterials,have similar antimicrobial mechanisms to other antimicrobial nanomaterials,are less likely to induce bacterial resistance development,and also have advantages such as fluorescent labeling properties and good biocompatibility.Therefore,the development of antimicrobial coatings to modify brackets and other attachments based on antimicrobial carbon dots is an ideal solution for the adhesion and growth of pathogenic bacteria on the surface of oral orthodontic brackets.Objective: In this study,we synthesized honokiol carbon dots（HCD）using honokiol,the main active ingredient of Magnolia officinalis,as the carbon source.The antibacterial properties and antibacterial mechanism of HCD were investigated.Based on the successful synthesis of antimicrobial carbon dots,we formed an antimicrobial coating of PDA-HCD on the surface of orthodontic attachments-brackets by using the adhesive property of polydopamine（PDA）particles,which provides a new idea and method to solve the problem of bacterial adhesion and growth on the surface of oral orthodontic brackets.Methods: We refer to the one-step hydrothermal method to synthesize and thicken the phenol carbon dots and physically characterize them to investigate their microstructure and optical properties.The antibacterial properties of HCD against Escherichia coli and Streptococcus pyogenes were examined by WST-8 kit.The antibacterial mechanism was investigated by zeta potential,scanning electron microscopy,and reactive oxygen species production assay.On the basis of the above,the PDA-HCD coating was formed on the surface of the brackets by means of the superior adhesive properties of polydopamine（PDA）particles and characterized by scanning electron microscopy and X-ray diffraction mapping.Using artificial saliva to simulate the oral environment,the brackets modified with PDA-HCD coating were immersed in artificial saliva and their antibacterial properties were investigated by WST-8 kit assay and bacterial smear plate count after 0,7 and 14 days of immersion,respectively.In addition,the relationship between coating thickness and coating stability and its effect on antimicrobial properties were further investigated by preparing PDA-HCD coatings of different thicknesses.To assess the biocompatibility of the coating,the immersion solution of the coating was co-cultured with L929 cells and the cellular activity was measured with CCK8.Results: The transmission electron microscopy results showed that the HCD has nanoscale size,spherical morphology,no obvious agglomeration and uniform particle size.The zeta potential results showed that the HCD was positively charged（+27.63 m V）,and the fluorescence pattern reflected the fluorescence property of the synthesized HCD.The wavelength of excitation light changed,and the wavelength of emission light changed accordingly,reflecting the photoluminescence property of HCD.Under UV light,HCD has fluorescence visible to the naked eye.The results of antibacterial experiments showed that HCD has broad-spectrum and excellent antibacterial performance,and the antibacterial rate of both E.coli and Streptococcus mutans at a concentration of 100 μg/m L was over 70%.The antibacterial mechanism of HCD is the positive charge on the surface and the induction of reactive oxygen species production under natural light to kill bacteria.The brackets modified with PDA-HCD coating maintained good antibacterial properties after 7 days of immersion in artificial saliva,with antibacterial rates above 80% for both E.coli and Streptococcus pyogenes,and the antibacterial rate of the coating decreased slightly after 14 days of immersion.Moreover,when the coating thickness of the bracket surface is too thick,the antibacterial properties of the coating will be significantly weakened after artificial saliva soaking,which indicates that the stability of the coating in the oral environment will be reduced when it is too thick.Cytotoxicity experiments showed that the PDA-HCD coating had no significant cytotoxicity.Therefore,PDA-HCD antimicrobial coating is promising for controlling the growth of pathogenic bacteria on the surface of oral orthodontic brackets.Conclusion: In this study,we have synthesized honokiol carbon dots with good antibacterial activity against both Gram-negative bacteria-Escherichia coli and Grampositive bacteria-Streptococcus mutans,The antibacterial mechanism is mainly the disruption of bacterial cell membranes by the positive charge carried by the surface,and the induction of reactive oxygen species production in natural light,which produces irreversible killing effects on bacteria.With the antibacterial property of HCD and the adhesion property of PDA,PDA-HCD coating with antibacterial activity is formed on the surface of the bracket.With this new coating,which has good and stable antibacterial effect in the oral environment,we can provide new ideas and solutions to solve the problem of adhesion and growth of pathogenic bacteria on the surface of oral orthodontic brackets. |
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