Preparation And Surface/Interfacial Properties Of Amphiphilic Fluorescent Carbon Dots

Carbon dots（CDs）are new fluorescent carbon nanomaterials in the size range of1–10 nm.Given their low toxicity,biocompatibility,and stable fluorescence property,CDs have shown great potential applications in biological imaging,fluorescence sensing,photoelectric energy,and other fields.However,currently reported CDs usually only show a single hydrophilicity or lipophilicity,and there is little discussion on amphiphilic CDs.Moreover,the relative researches mainly focus on the optical properties and biological drug loading of CDs,and there are comparatively few reports on their surface and interface properties.Therefore,developing a preparation method for amphiphilic CDs and studying their surface and interface properties as amphiphilic nanomaterials is the key to broadening the CDs application.In this paper,a series of amphiphilic CDs with surface activity were prepared by the one-step pyrolysis method and two-step modification.The effects of sulfur on their acid and alkali resistance,salt resistance,and micellization behavior were studied,and the possibility of applying amphiphilic CDs as a nano surfactant in many fields was proposed.The relationship between the hydrophobicity and the inhibition performance was studied,and the corresponding inhibition mechanism was established.The details are as follows:1)First,nitrogen-doped amphiphilic CDs(C₁₂-CDs)with long carbon chains were prepared by one-step pyrolysis using citric acid and dodecylamine.Then,C₁₂-CDs were sulfonated with sulfamic acid to obtain nitrogen and sulfur co-doped amphiphilic CDs(C₁₂-SCDs).In order to explore the effect of carbon chain length on the amphiphilic properties of CDs,n-hexane and n-octylamine were selected to replace dodecylamine to prepare C₆-CDs and C₈-CDs.Then the morphology,structure,and optical properties of the amphiphilic CDs were characterized by transmission electron microscopy,infrared spectroscopy,X-ray photoelectron spectroscopy,and steady-state,transient fluorescence spectroscopy.It was found that the content of the element sulfur in C₁₂-SCDs reached13.219%,which proved that the element sulfur was successfully doped into CDs without changing the particle size,graphite-like structure,and fluorescence emission peak position.The ratio of hydrophilic and hydrophobic functional groups of CDs and the graphitization degree of CDs can be changed through the carbon chain length of the raw material.Then the hydrophilic and hydrophobic properties of CDs can be effectively controlled.The amphiphilic CDs have high absolute fluorescence quantum yield（QY）in water and various organic solvents.Due to the highest degree of graphitization,C₁₂-CDs have the highest QY in water and cyclohexane,18.40%and10.67%,respectively.2)The surface and interfacial activities of C₁₂-CDs and C₁₂-SCDs were studied by surface tension meter and rotary-drop interface tension meter,and the effects of temperature,p H,and inorganic salts on their surface activities were investigated.The results showed that the critical micelle concentrations（CMC）of C₁₂-CDs and C₁₂-SCDs are 111.64 mg/L and 150.26 mg/L,respectively,and the surface tension(γ_CMC)are 27.45m N/m and 28.41 m N/m;The surface tension of C₁₂-CDs and C₁₂-SCDs reaches the minimum at p H=3,which is close to the isoelectric point of CDs.As p H increases,the water solubility of CDs increases,and thus the surface tension increases;C₁₂-CDs can still maintain high activity at c _{Na Cl}=250 g/L,but their resistance to Ca Cl₂is poor.The addition of the sulfonate groups enhances the tolerance of CDs to both Na Cl and Ca Cl₂.At c _{Na Cl}=360 g/L and c _{Ca Cl2}=750 g/L,C₁₂-SCDs still have high surface activity.The micellization behavior of C₁₂-CDs and C₁₂-SCDs was determined by dynamic light scattering,scanning electron microscopy,and solubilization experiments:above the CMC,C₁₂-CDs aggregate into an irregular spherical structure like micelle with an average diameter of about 90 nm,whereas C₁₂-SCDs do not form aggregates.The removal efficiency of oil film and contact angle experiments show that compared to the traditional surfactant,C₁₂-CDs,and C₁₂-SCDs have an excellent wettability alternation due to their abundant amine,hydroxyl,carboxyl,and sulfonate groups.Among them,the wettability of C₁₂-SCDs is better than that of C₁₂-CDs due to their wealthy sulfonate groups,which are hydrophilic.In addition,the Pickering emulsions stabilized by C₁₂-SCDs have rapid p H&CO₂-response performance resulting from the hydrophile-lipophile balance of C₁₂-SCDs repeatedly adjusted by changing p H values.Compared with traditional anionic and non-ionic surfactants,CDs have an excellent dispersion performance to multi-walled carbon nanotubes（MWCNTs）.Unlike the dispersion mechanism of cationic surfactant（solid electrostatic attraction）,CDs are adsorbed on the surface of MWCNTs through their hydrophobic effect andπ-πstacking effect to achieve effective dispersion.Among them,because the sulfonate groups have excellent steric resistance and electrostatic repulsion,the dispersion performance of C₁₂-SCDs is better than C₁₂-CDs.3)The corrosion inhibition efficiency of C₆-CDs,C₈-CDs,and C₁₂-CDs for N80steel in hydrochloric acid solution was evaluated using the weight-loss and electrochemical methods.The results showed that the inhibition performance of these three amphiphilic CDs was excellent,and the inhibition efficiency was 85.7%,88.0%,and 92.9%,respectively.The longer the carbon chain was,the better the inhibition effect was.Moreover,due to the fluorescence characteristics of CDs,the changes in adsorption film during corrosion could be directly observed under UV light.Then the corrosion morphology of the metal surface and the composition of the adsorption film were analyzed through a scanning electron microscope and X-ray photoelectron spectroscopy.Finally,a corrosion inhibition mechanism model of amphiphilic CDs was established:a dense adsorption film was formed through the electrostatic interaction with CDs and the metal surface.Long carbon chains formed a hydrophobic layer on the surface of N80 to isolate the corrosive medium.In conclusion,a series of amphiphilic CDs were prepared by one-step pyrolysis and two-step modification.Their surface and interface properties were evaluated from the macro and micro perspectives through experimental means.The feasibility of amphiphilic CDs as nano-surfactants and corrosion inhibitors was proposed,which provided a reference for the future development of multi-field applications of CDs.