| Bacteria prefer attaching to solid substrates rather than dwelling in a planktonic state,eventually leading to the formation of biofilms and surface-bound communities of microbes may cause adverse consequences in the research and application fields,e.g.,biomedical devices,water storage and treatment systems,marine vessels and the food industry.The adhesion of bacteria onto solid surfaces is a complicated process that is influenced by various factors,including bacterial properties,material surface properties and environmental factors.Bacteria typically carry net negative charges in aqueous suspension;therefore,a deeper understanding how bacterial interact on different charged surfaces is very important for the further development of anti-biofouling surfaces.Due to its attractive physical properties,such as stability under physiological conditions,special transparent and conductive properties,the indium tin oxide(ITO)surface is suitable for studying the influence of surface modifications on bacterial adhesion.In this paper,we modify the ITO surfaces with sulfonic groups(negatively charged)and quaternary ammonium groups(positively charged).The ITO surfaces were fabricated via oxygen plasma pretreatment,followed by treatment with silane-coupling agents((3-mercaptopropyl)trimethoxysilane or(3-chloropropyl)trimethoxysilane).After the oxidation or quaternization treatment,a negatively charged surface and a positively charged surface were prepared.The modified surfaces were characterized by water contact angle,atomic force microscope,cyclic voltammetry,electrochemical impedance spectroscopy,X-ray photoelectron spectroscopy,TB colorimetric method and fluorescein sodium salt assay.In this study,Gram-negative Escherichia coli(E.coli K-12)cells were used to evaluate the antibacterial adhesion characteristics of the ITO surfaces.The spread plate method,confocal laser scanning microscopy(CLSM)and scanning electron microscope(SEM)were used to detect changes in the viability,morphology and structure of Gram-negative E.coli,which provided detailed information for understanding the interaction between the charged surfaces and bacteria.The main conclusions of this study are listed as follows:(1)The WCA and AFM measurements revealed that the wettability and roughness of ITO surfaces changed slightly after modification.However,the effect of wettability and surface roughness on the bacterial adhesion could be neglected in the present study ’because the bare and modified ITO surfaces were hydrophilic in general,and the size of E.coli was three order of magnitudes larger than the roughness values.(2)CVs and EISs showed that silane-coupling agents were successfully grafted onto bare ITO surfaces and acted as barriers for the electron transfer between the electrode and redox species in the solution.After the oxidation or quaternization reaction,ITO electrodes showed electrochemical response again,indicating that ITO surfaces had been successfully modified with sulfonic acid groups and quaternary ammonium groups.Through the detection of the specific elements,such as S,N and Cl,the XPS experiments further confirmed that the ITO surfaces had been modified by the target functional groups.(3)The TB colorimetric method allowed the quantification of the sulfonic acid groups on the ITO surface,indicating that the ITO surface had been successfully sulfonated.The cationic charge density of the modified glass slide was determined using fluorescein(sodium salt)staining.The results showed that ITO surface was successfully quaternized with a high degree of quaternization.(4)The viable bacterial count test showed that both negatively and positively charged surfaces can reduce the initial bacterial adhesion,and surfaces with negative charges showed more inhibitory effects on bacterial adhesion.CLSM suggested that the negatively charged surfaces did not ’kill’ the bacteria but that the positively charged surface did.The SEM images showed that the E.coli produced flagella on the bare ITO surface and no obvious flagella were observed on the charged surfaces,indicating that flagella are involved in bacterial attachment process and will help to control bioadhesion and biofilm at interfaces. |
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