Fabrication Of Functional Micro-Needle Electrodes And Their Application In Determination Of Typical Metals In Coastal Waters

Coastal zone,the field of interaction between ocean and land,is a complex and dynamic natural ecosystem with dual attributes of land and sea.It is also the core of land and oceanic economy and the base of marine exploitation activities.Coastal waters mainly include coastal rivers,bays,coastal seawater,lagoons,and so on,which are important part of coastal ecosystem.Metals play an important role in coastal waters and ecosystem,which are closely related to the ecological safety of coastal zone and human health.It is very important to detect the metals rapidly and study their species and concentration distribution in coastal waters for the protection of coastal ecosystem and human health.Among the methods for determination of metals,in addition to the advantages of simple operation,high sensitivity and fast analysis speed,electrochemical methods can also be used for the speciation analysis of metal elements.Mercury electrode shows high sensitivity and stability for the detection of metals.As a new working electrode,micro-needle electrode has the properties of easy fabrication,easy to use,non-toxicity,and so on.It can overcome the inconvenience of mercury electrode in preparation,storage,transportation,and on site application,which shows a broad application prospect.However,there are few reports about the determination of metal ions by micro-needle electrode,especially in the actual water environment.Furthermore,the unfunctionalized micro-needle electrode cannot be used for determination of metals directly in coastal waters due to its relatively low sensitivity.Therefore,the fabrication of new functional micro-needle electrodes for determination of metal ions in coastal waters will greatly promote the application of electrochemical sensors in the coastal environment analysis.In this thesis,a series of researches have been carried out,including the fabrication of functional micro-needle electrodes,the development of detection methods for different metals,and the speciation analysis of metals in coastal waters including coastal river water and coastal seawater.The main contents are as follows:1. Dendritic gold nanostructures/cation-exchange polymer functionalized micro-needle electrode for the speciation analysis of Cu in seawaterIn order to realize the speciation analysis of Cu in coastal waters,a new micro-needle electrode（MNE）was prepared through insulation,sealing,and other processes by using commercial stainless steel acupuncture needle as the matrix.The MNE was then functionalized with dendritic gold nanostructures（Au NDs）and cation-exchange polymer（Nafion）to prepare the functional MNE.Au NDs improved the conductivity and electrocatalytic activity of the electrode.Nafion could not only improve the stability of the electrode as a protective layer,but also improve the detection performance of the electrode through its enrichment ability.The sensing tip of the prepared MNE was about 1～2 mm,and the top width and length of the Au NDs were about 40～60 nm and 250～400,respectively.The detection mechanism of Cu²⁺with the functional MNE accorded with the“adsorption-catalysis”model.Under the optimal conditions,the functional MNE showed a linear response range of 0.05～1 n M dissolved acid extractable state and total acid extractable state)in coastal seawater samples were determined by combining different sample pretreatments and electrochemical anodic stripping voltammetry.Eventually,the speciation analysis of Cu in seawaters was realized with the so-prepared functional MNE.2. Porous gold/polydopamine functionalized micro-needle electrode for determination of Cu in seawaterThe surface of MNE prepared in the previous chapter was very smooth,which was not beneficial to the modification and attachment of gold nanomaterials.Therefore,in this chapter,a method of self-assembly was proposed to prepare the functional MNE.A layer of polydopamine（PDA）was first self-assembled on the surface of MNE for the surface modification by using the self-polymerization property of dopamine.Then a layer of porous gold（P-Au）was self-assembled on the surface of electrode by using the reducibility of PDA to fabricate the functional MNE.The self-assembled spherical PDA was partially oxidized and consumed with the reduction of chloroauric acid to gold nanomaterials,which resulted in the porous structure of gold nanomaterials covered on the PDA.As the surface modification material,PDA could not only provide more active binding sites for gold nanomaterials,but also act as the adhesion agent to prevent their from falling off from the electrode due to its adhesion property.In the absence of Nafion as the protective layer,the linear range and the detection limit of the functional MNE for Cu²⁺was 0.7～100 n M and0.24 n M,respectively.Furthermore,the functional MNE showed a good sensitivity and stability for determination of Cu in coastal seawater.3. Gold nanoclusters/conducting polymer film functionalized micro-needle electrode for determination of Fe in coastal watersAs a metal nutrient element,Fe plays an important role in coastal ecosystem and marine biogeochemical cyle.In this work,a functional MNE was fabricated for determination of Fe in coastal waters.The MNE was firstly modified with the conducting polymer film（PEDOT-PSS）to achieve the surface modification,and then functionalized with gold nanoclusters（Au NCs）.As a kind of conductive polymer film with excellent performance,PEDOT-PSS improved the conductivity of the electrode and increased the active sites for the combination of the Au NCs which had excellent catalytic performance for the voltammetric determination of Fe.Under the optimal conditions,the functional MNE showed a linear response range of 0.01～5μM and detection limit of 3.1 n M for Fe³⁺.The functional MNE was successfully applied to the determination of Fe in 19 samples from the coastal river of Yantai（Guangdang River）.4.3D reduced graphene oxide/flower-like bismuth nanosheets functionalized micro-needle electrode for determination of Pb in coastal watersIn order to realize the determination of Pb which is one of the heavy metal elements with relatively high toxicity and harm in coastal waters,a functional MNE based on 3D reduced graphene oxide（3DrGO）/flower-like bismuth nanosheets（F-Bi NSs）was fabricated through stepwise electrodeposition method.A layer of3 DrGO was first modified on the surface of MNE to improve the conductivity and specific surface area of the electrode and provide more binding sites for bismuth nanomaterials.Considering the properties of bismuth in easy formation of alloy like electrodeposited on the surface of 3DrGO to enhance the voltammetric response to Pb²⁺and improve the sensitivity of the electrode.The functional MNE showed a good electrochemical response to Pb²⁺with the linear range of 40～600 n M and detection limit of 12.5 n M.The determination of Pb in coastal waters was successfully realized by this functional MNE.5. Gold-mercury functionalized micro-needle electrode for the simultaneous determination of different metals in coastal watersThe main aim of this chapter is to investigate the practicability of the functional MNE for simultaneous determination of different metals in coastal waters.So,after the preparation of MNE,a gold-mercury（Au-Hg）functionalized MNE was constructed through stepwise electrodeposition method,and compared with the traditional mercury electrode in the simultaneous detection of metals.The results showed that the simultaneous detection of four metals of Cu,Pb,Cd and Zn in seawater can be realized by the mercury electrode.The so-fabricated functional MNE could be used for the detection of Cu,Pb,and Cd at the same time and simultaneous determination of total dissolved Cu and Pb,as well as the direct detection of active Cu in coastal waters.In the later stage,the micro-needle materials,functional materials and detection conditions will be further optimized,which is expected to achieve the simultaneous detection and analysis of active and total dissolved Cu,Pb,and Cd in coastal waters by the functional MNE.