Green Synthesis Of Novel Fluorescent Carbon Nanoparticles And Their Applications In Metal-ions Sensing

Objective:Fluorescent carbon nanoparticles have attracted great attention for their potential applications in the fields of sensing,biomarkers,bioimaging and drug delivery due to their excellent light stability,surface function tunability and photoluminescence?PL?characteristics.Among them,fluorescent organic nanoparticles and carbon dots?CDs?have low toxicity and good biocompatibility,which has become a focus of research in recent years.As a kind of naturally occurring melanin,polydopamine?PDA?nanoparticle is a hotspot in the research of fluorescent organic nanoparticles.PDA usually displays extremely weak fluorescent properties,which hinders its potential in more biomedical applications.Therefore,it is of great significance to develop effective methods to enhance the intrinsic fluorescence of PDANPs.However,compared with fluorescent organic nanoparticles,CDs are more valuable due to their smaller size and high quantum yield.However,a mass of external energy is needed for the synthesis of CDs and the synthesis steps are tedious,which limit its application prospects.In addition,there have been many studies on the fabrication of CDs derived from traditional Chinese medicine biomass.Most of these CDs are prepared by the method of pyrolysis,which has many shortcomings.For instance,the expensive equipment is needed and the steps of preparation and extraction are complicated.Therefore,a simple,green method for preparing CDs from traditional Chinese medicine is needed,which would pave the way for another important research in the later period.Therefore,it is of great practical significance to develop green,energy-saving synthetic methods to produce new fluorescent nanoparticles at low temperatures without requiring externally supplied energy.The purpose of this research mainly includes the following three aspects:?1?Polydopamine?PDA?,as a melanin analogue,usually exhibits its weak intrinsic fluorescence properties.To study the enhancement of the intrinsic fluorescence of PDA nanoparticles at room temperature,the fluorescent organic nanoparticles were prepared by redox modification.The resulting fluorescent nanoparticles are used for the detection of Fe³⁺ion.?2?The novel strategy of carbon source self-heating,which was simple,green,ultra-fast,and energy-efficient,was developed for the room temperature synthesis of highly fluorescent heteroatom-doped CDs without any external energy source,and further applied to the quantitative detection for Fe³⁺in biological samples.?3?The processed Codonopsis pilosula was utilized as the carbon source for the facile fabrication of green CDs via one-step solvent extraction method,which possessed great potential for the semi-quantitative visual detection and removal of Cr?VI?in industrial wastewater.Methods:?1?Dopamine?DA?was firstly oxidized and polymerized by using SP as oxidant under slightly acidic condition?pH 5.0?,resulting into the formation of intrinsic fluorescent periodate oxidized PDANPs?o-PDANPs?.The surface chemistry of the obtained fluorescent o-PDANPs was further regulated by the reduction of sodium borohydride?NaBH₄?,resulting into the formation of reduced PDANPs?r-PDANPs?.The morphology and optical properties of r-PDANPs were characterized.With Fe³⁺ion as the quencher for r-PDANPs,the quenched linear range and detection limit were determined.Furthermore,based on the specific quenching mechanism,this nanosensor was applied to the detection of Fe³⁺in actual water samples.?2?The ultrafast and spontaneous synthesis of highly fluorescent N,S-codoped CDs was based on the self-heating effect of tetraethylenepentamine?TEPA?triggered by the heat initiator sodium persulfate?Na₂S₂O₈?.After adding TEPA to Na₂S₂O₈ aqueous solution,lots of reaction heat derived from the Na₂S₂O₈-triggered oxidation of TEPA would be spontaneously produced at room temperature.Meanwhile,the oxidization,polymerization and carbonization of TEPA as well as the in-situ co-doping of sulphur atoms occurred simultaneously,resulting in the rapidly spontaneous formation of N,S-codoped CDs within 2 min.Subsequently,N,S-codoped CDs were morphologically and optically characterized,and the photostability of CDs was also investigated.In addition,the mechanism of fluorescence quenching induced by Fe³⁺was investigated,and the quenching linearity range and detection limit were determined.Furthermore,the practical application on the detection of Fe³⁺ion in environmental water and human serum samples were systematically studied.?3?After purchasing the rice fried Codonopsis pilosula from the Chinese herbal medicine manufacturer,the method of one-step ultrasonic extraction with methanol was used to obtain the CDs derived from rice fried Codonopsis pilosula?RFC-CDs?.The morphology and optical characterization of RFC-CDs were performed.Furthermore,the method for semiquantitative detection of Cr?VI?in industrial wastewater was established with the RFC-CDs as a nanoprobe.Furthermore,the RFC-CDs was doped into the agarose gel to study the effect of Cr?VI?adsorption in sewage.Results:?1?The quantum yield of the r-PDANPs synthesized under the optimal reduction conditions is 5.1%,which is the highest value recorded of all the reported intrinsic fluorescent PDANPs.Through a series of characterization methods,it was revealed that the chemical properties of the PDA surface are closely related to their fluorescent properties:the carboxyl and carbonyl groups on the PDA surface can induce more non-radiative transitions and intensify fluorescence quenching.The electron donors such as hydroxyl groups can help increase the fluorescence emission of PDAs.By adjusting the synthesis conditions of PDANPs,the oxidation/reduction states of the surface groups of PDANPs can be flexibly adjusted,and then the fluorescence properties of PDANPs can be significantly affected.In this study,the strong coordination interaction between phenolic hydroxyl groups on the surface of r-PDANPs and Fe³⁺was successfully used for the highly selective fluorescence sensing detection of Fe³⁺.The detection limit of this method is 0.15?M,and the linear range is 0.5-20?M.When used to detect Fe³⁺in environmental water samples such as tap water and lake water,the sample recovery rates were between91.3%and 101.8%,which had great practical application value.?2?The as-prepared N,S-codoped CDs exhibited excellent water solubility,high production yield?about 58.0%?,high photostability and a high QY up to26.4%under the optimized synthesis conditions.More intriguingly,the label-free N,S-codoped CDs showed a highly selective and sensitive fluorescence response to Fe³⁺ions under the masking effect of EDTA,in an ultra-wide range of 0.20-600?M,with a detection limit of 0.10?M.Furthermore,the N,S-codoped CDs based fluorescent nanoprobe was successfully utilized for the quantitative detection of Fe³⁺in environmental water and human serum samples,and the quantitative results were comparable to those obtained by ICP-MS.?3?In this study,N-doped fluorescent carbon dots with excellent fluorescence properties were successfully extracted from processed Chinese medicines.More importantly,the extracted carbon dots have high quantum yield,good detection sensitivity for Cr?VI?,and a linear range of 0.03-50?M.In addition,carbon dots were successfully applied in the visual detection of Cr?VI?,and the adsorption and removal effect on Cr?VI?in sewage reached 65%.Therefore,it is hoped to become a high-performance Cr?VI?adsorbent.Conclusion:?1?In short,this study modified the surface state of fluorescent nanoparticles through the redox effect of SP and sodium borohydride.Compared with other previous studies related to the intrinsic fluorescence performance of PDA,r-PDANPs prepared according to this strategy have the highest fluorescence quantum yield?5.1%?.Additionally,it was found that r-PDANPs has great potential in the detection of Fe³⁺.?2?The constructed carbon source self-heating synthesis strategy can achieve highly fluorescent N,S-codoped CDs within 2 minutes at room temperature.Compared with other methods for synthesizing fluorescent CDs,the carbon source self-heating synthesis strategy can completely avoid external energy supply,which is more efficient,energy-saving,and environmentally friendly;and the fluorescent properties of the synthesized carbon dots can be flexibly adjusted by changing the self-heating synthesis parameters.Moreover,the great application value of the N,S-codoped CDs in the fields of sensing and clinical diagnosis have been preliminarily confirmed.?3?A new method for preparing highly fluorescent CDs via one-step solvent extraction from processed Chinese medicines was established.Compared with other methods using traditional Chinese medicine biomass as the carbon sources to synthesize CDs,the presented strategy is easy to operate and does not require a long-time high-temperature heating process.Moreover,this study provides a new idea for the development and utilization of processed traditional Chinese medicine resources.The three new methods for synthesizing fluorescent organic nanoparticles and CDs have the advantages of green,environmentally friendly,high energy efficiency and strong practicability.They are in line with the concept of green chemistry and have broad application prospects in the fields of sensing,clinical diagnosis and environmental monitoring.