The Fabrication Of Novel Electrochemical Sensors And Their Applications In Environmental Detection

An electrochemical sensor, which combines a sensing element and an energy transducer, is an analytic device that basing on the electrochemical property of the analyte. As an important technology of analytical chemistry, electrochemical sensor has been widely used in various areas, such as environmental monitoring, food testing industry, biomedical science, fermentation industrial production and so on, due to its simplicity, low-cost, high sensitivity, high selectivity, rapidity and potential ability for real-time and on-line analysis. Our work focused on the key issues of the fabrication of electrochemical sensor and its analysis detection in environment, that is, how to immobilize component onto the energy transducer surface efficiently and cleanly, how to detect the environmental pollutants including pentachlorophenol, methyl parathion and nitrite sensitively and rapidly. This paper is concentrated on the preparations of novel electrochemical sensors using various modified materials and modified methods, and their aplications in the field of environmental analysis detection. The electrochemical sensors have been investigated by cyclic voltammetry (CV), square wave anodic stripping voltammetry (SWV), current-time technique (i-t), differential pluse voltammetry (DPV), electrogenerated chemiluminescence (ECL), ultraviolet-visible spectrophotometry (UV-vis), fourier transform nfrared spectroscopy (FTIR), fluorescence spectrophotometry (FL), scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. The main points of this dissertation are summarized as follows:1. The oxidized carbon quantum dots/graphene oxide (OCQDs/GO) mixture has been reduced into carbon quantum dots/graphene (CQDs/GR) hybrid by one-step electrochemical reduction technology. The obtained CQDs/GR hybrid is attached onto the electrode surface directly, utilizing the difference of the solubility between OCQDs/GO and CQDs/GR. The proposed CQDs/GR hybrid has been investigated by SEM, TEM, FL, UV-vis, CV and ECL. The experimental results show that the structure of the CQDs/GR hybrid is very stable which ensures the stability of the ECL sensor in practical application. The sensitivity of the ECL sensor has been greatly enhanced, owing to the good conductivity of GR which can amplify the signal, meanwhile, a large amount of CQDs can be absorbed onto GR due to the large specific surface area of GR. The CQDs/GR sensor shows good ECL property and enables the real-time detection of pentachlorophenol (PCP) with unprecedented sensitivity reaching 1.0×10-12M concentration in a wide linear range from1.0x10-12～1.0x11-8M. The ECL sensor shows high selectivity to chlorophenols (CPs)，especially to PCP. The practicability of the sensing platform in real soil samples shows ideal recovery rates. Herein, for the first time, through one-step electrochemical reduction technology, GR serves as both the ECL amplification reagent and the immobilization platform for CQDs, which not only enhance the detection sensitivity but also achieve the recyclability of CQDs. This work may present an important strategy to design QDs ECL sensors and expand their applications in ultratrace environmental sensing.2. Firstly, the glassy carbon electrode is immersed into the mixture of graphene oxide (GO) and chitosan (CS), and then the stable graphene/chitosan (GR/CS) composite is obtained by one-step electrochemical reduction technology. The proton consumption during electroreduction of GO increases the local solution pH near the electrode surface, leading to the insolubility of CS. Meanwhile, the obtained GR is insoluble in aqueous solution, thus co-deposition of GR and CS is achieved. The GR/CS composite can capture methyl parathion (MP) efficiently and be used as solid phase extraction, thus the goal for MP sensing is achieved. Under the optimal conditions, the proposed sensor exhibits a wide linear range from4.0～400ng/mL, and a low detection limit of0.8ng/mL. Moreover, the proposed sensor shows good reproducibility, long-time stability and satisfactory anti-interference ability. The obtained GR/CS sensor opens a new opportunity for green, fast, simple and sensitive detection of organophosphate pesticides.3. Graphene oxide (GO) and carbon nanotube (CNTs) are dispersed in chitosan (CS) solution to form a GO/CNTs/CS mixture. The proton consumption during electroreduction of GO increases the local solution pH near the electrode surface, leading to the insolubility of CS. The obtained graphene (GR) is also insoluble in aqueous solution. Meanwhile, the surface of CNTs owns large amounts of oxygen-containing functional groups after acidification, which can be reduced during the electroreduction process, resulting in the insolubility of CNTs. Thus, co-electrodeposition of GR. CNTs and CS is achieved. The GR/CNTs/CS composite can capture methyl parathion (MP) efficiently and be used as solid phase extraction, thus the goal for MP sensing is also achieved. Under the optimal conditions, the proposed sensor exhibits a wide linear range from2.0～500ng/mL, and a low detection limit of0.5ng/mL. The proposed sensor shows good reproducibility, long-time stability and satisfactory anti-interference ability.4. Carbon nanotube (CNTs) are functionalized by DNA through the π-π interactions between the nanotube sidewalls and the nucleic acid bases, and then the DNA/CNTs/Cu2+hybrid is fixed onto glassy carbon electrode through electrodeposition under controlled dc potential. Electrochemical experiments reveal that the DNA/CNTs/Cu2+hybrid showed high electrocatalytic activity to the reduction of nitrite (NC2-), thus a sensitive sensor for the determination of NO2-is constructed. Effects of the electrodeposition conditions such as the concentration of Cu2+, DNA, CNTs and electrodeposition time and the determination conditions such as applied potential and pH value on the current response of the proposed DNA/CNTs/Cu2+sensor toward NO2-have been optimized in order to obtain the maximal sensitivity. Under the optimal conditions, the response is fast (less than3s), the linear range of the determination of NO2-is from3.0×10-8～2.6×103M, and the detection limit is3.0×10-8M. Moreover, the DNA/CNTs/Cu2+sensor shows good stability, high reproducibility and well antijamming capability, thus the proposed sensor may have great actual application prospect.5. Graphene oxide (GO), chitosan (CS) and glucose oxidase (GOx) are mixed together directly to form a graphene oxide/chitosan/glucose oxidase (GO/CS/GOx) solution. The stable graphene/chitosan/glucose oxidase (GR/CS/GOx) film can be obtained on the glassy carbon electrode by one-step electrochemical reduction technology. The procedure takes only several minutes, and the thickness of the resulting film is uniform and controllable. Since good bioactivity of GOx is achieved and the reversible2-proton and2-electron transfer between glucose and GOx is also exhibited, the GR/CS/GOx hybrid can be used for glucose sensing. The biosensor has a detection limit of4.0×10-7M (50-fold lower compared to the biosensor prepared by drop-casting method), and the response linear range is4.0×10-7～2.0×10-3M. The GR/CS/GOx biosensor shows good stability, high reproducibility and well anti-interference ability.