Construction Of Electrochemical Sensors For Heavy Metal Ions And Its Application

Heavy metal pollution has become an important environmental problem in human health. It is highly necessary to develop rapid, simple and sensitive methods to detect trace metal ions in the environmental, medical and food fields. Taking the advantages of simplicity, low cost, high sensitivity and easy miniaturization, electrochemical detection methods for heavy metal have gotten widely attention. Electrochemical sensors with the high sensitivity can utilize various small chemical molecules and biological materials as receptors to realize the detection of trace heavy metal ions. In recent years, with the development and application of nano-materials in the fields of electroanalysis and biosensor, great breakthroughs have been made in analytical methods. Especially, with high specific surface area and fast electronic transfer ability, inorganic nano-materials greatly increase the adsorption sites for metal ions, which provides the possibility for continuous on-line detection of heavy metals. With the special interaction between DNA and heavy ions, the signals can be generated from the complementary base pairing, hydrolysis nuclease fracture and three-dimensional conformation change to realize the indirect detection of heavy metal ions. Utilizing variuous nanoparticles and DNA probes, the thesis constructs six electrochemical sensors for heavy metal ions and develops rapid, simple, highly selectivity quantitative analysis methods. This thesis includes the following six parts:1. Nitrogen-Doped Porous Carbon Derived from Metal-Organic Gel for Electrochemical Analysis of Heavy-Metal IonA nitrogen-doped porous carbon material (N@MOG-C) was prepared by simple pyrolysis of polypyrrole doped Al-based metal-organic gel (PPy@MOG) at 800℃. The N@MOG-C was characterized with morphological, spectroscopic and electrochemical techniques, and possessed a uniform 3-D interconnected mesoporous structure with high surface area of 1542.6 m2 g-1 and large pore volume of 0.76 cm3 g-1. By using ionic liquid (IL) to immobilize N@MOG-C on electrode surface, the N@MOG-C was further used for sensitive detection of heavy metal ion. The doping of nitrogen endowed N@MOG-C with faster electron transfer kinetics than other carbon materials such as MOG-C, multi-walled carbon nanotube and graphene. The N@MOG-C modified electrode showed a high effective area due to the porous structure. Under optimized conditions, the N@MOG-C based sensor could detect cadmium ion from 0.025 to 5 μM with a detection limit of 2.2 nM. The mesoporous structure, fast electron transfer ability, and simple and green synthesis of N@MOG-C made it a promising electrode material for practical applications in heavy metal ion sensing.2. Synthesis of Bismuth-Nanoparticles-Enriched Nanoporous Carbon on Graphene for Efficient Electrochemical Analysis of Heavy Metal IonsA novel nanocomposite BiNPs@NPCGS was designed for highly efficient multiple detection of heavy metal ions by in situ synthesis of bismuth-nanoparticles-enriched nanoporous carbon on graphene sheet. The NPCGS was prepared by pyrolysis of zeolitic imidazolate framework-8, ZIF-8 nanocrystals, deposited on graphene oxide, and displayed a high surface area of 1251 m2 g-1 and a pore size of 2.6 nm. BiNPs were in situ deposited on NPCGS by chemical reduction of Bi3+ with NaBH4. Due to the restriction effect of the pore/surface structure of NPCGS, BiNPs were uniform and well dispersed on NPCGS. The BiNPs@NPCGS showed good conductivity and high effective area, and the presence of BiNPs make it act as an efficient material for anodic stripping voltammetric detection of heavy metal ions. Under optimized conditions, the BiNPs@NPCGS based sensor could simultaneously determine Pb2+ and Cd2+ with the detection limits of 3.2 and 4.1 nM, respectively. Moreover, the proposed sensor could also differentiate Tl+ from Pb2+ and Cd2+. The advantages of simple preparation, environmental friend, high surface area, and fast electron transfer ability of BiNPs@NPCGS showed promising practical application in heavy-metal-ions sensing.3. Highly Sensitive Electrochemical Detection of Mercury (Ⅱ) via Single Ion-Induced Three-way Junction of DNAA "signal-on" electrochemical sensing strategy was designed for highly sensitive and selective detection of mercury (Ⅱ) via its induction to three-way junction of DNA (DNA-TWJ). The three-way junction consisted of 3 DNA probes, among which the capture probe was self-assembled on a gold electrode surface through S-Au bond, and the signal probe labeled with ferrocene (Fc) contained single T-T mismatch to capture probe. In the presence of mercury (Ⅱ), single Hg2+ -mediated base pair triggered the formation of DNA-TWJ on the electrode surface. The three-way junction was theoretically discussed and confirmed with polyacrylamide gel electrophoresis. This process caused the Fc tag approaching the electrode for fast electron transfer and thus increased the oxidation current. The "signal-on" sensing method showed high sensitivity for the detection of Hg2+ ranging from 0.005 to 100 nM without any additional amplification step. The assay was simple and fast. It showed high selectivity in the presence of other metal ions at even 100-fold higher concentration. The proposed method could detect Hg2+ in natural samples with good accuracy, showing potential application in on-site and real-time Hg2+ detection.4. Electrochemical Sensor for Lead Cation Sensitized with DNAzyme Functionalized Porphyrinic Metal-Organic FrameworkAn efficient electrochemical sensor was presented for lead cation detection using a DNA functionalized iron-porphyrinic metal-organic framework (GR-5/(Fe-P)n-MOF) as a probe. The newly designed probe showed both the recognition behavior of GR-5 to Pb2+ with high selectivity and the excellent mimic peroxidase performance of (Fe-P)n-MOF. In the presence of Pb2+, GR-5 could be specifically cleaved at the ribonucleotide (rA) site, which produced the short (Fe-P)n-MOF-linked oligonucleotide fragment to hybridize with hairpin DNA immobilized on the surface of screen-printed carbon electrode (SPCE). Due to the mimic peroxidase property of (Fe-P)n-MOF, enzymatically amplified electrochemical signal was obtained to offer the sensitive detection of Pb2+ ranging from 0.05 to 200 nM with a detection limit of 0.034 nM. In addition, benefiting from the Pb2+ -dependent GR-5, the proposed assay could selectively detect Pb2+ in the presence of other metal ions. The SPCE based electrochemical sensor along with the GR-5/(Fe-P)n-MOF probe exhibited the advantages of low-cost, simple fabrication, high sensitivity and selectivity, providing potential application of on-site and real-time Pb2+ detection in complex media.5. Label-free Aptamer Biosensor for Sensitive "Signal-on" Electrochemical Detection of Arsenic (Ⅲ)A simple and efficient electrochemical aptamer biosensor was fabricated for the highly sensitive and selective determination of arsenite in water samples. The aptamer with high affinity and specificity to arsenite was first immobilized on the gold electrode by a self-assembly approach via an Au-S bond. The binding of arsenite to the aptamer immobilized on the electrode surface induced the alteration of aptamer conformation,which caused the less hybridization with the cationic polymer (PDDA) through the electrostatic interaction. Using [Ru(NH3)6]3+ as an electrochemically active indicator to interact with hybridization reaction product, the amperometric response demonstrated the formation of an aptamer/arsenite complex. The constructed sensor showed a high sensitivity for the detection of arsenite in a linear range from 0.2 nM to 100 nM, and a detection limit down to 0.15 nM. The reported system also exhibited high reproducibility, excellent specificity against other heavy metal ions and could be widely applied in environmental detection.6. Electrochemical Detection of Cu2+ through Ag Nanoparticle Assembly Regulated by Copper-Catalyzed Oxidation of CysteamineA highly sensitive and selective electrochemical sensor was developed for the detection of Cu2+ by the assembly of Ag nanoparticles (AgNPs) at dithiobis[succinimidylpropionate] encapsulated Au nanoparticles (DSP-AuNPs), which was regulated by copper-catalyzed oxidation of cysteamine (Cys). The electrochemical sensor was constructed by layer-by-layer modification of glassy carbon electrode with carbon nanotubes, poly (amidoamine) dendrimers and DSP-AuNPs. In the absence of Cu2+, Cys could bind to the surface of citrate-stabilized AgNPs via Ag-S bond, thus AgNPs could be assembled on the sensor surface through the reaction between DSP and Cys. In contrast, the copper-catalyzed oxidation of Cys by dissolved oxygen in the presence of Cu2+ inhibited the Cys-induced aggregation of AgNPs, leading to the decrease of the electrochemical stripping signal of AgNPs. Under the optimized conditions, this method could detect Cu2+ in the range of 1.0-1000 nM with a detection limit of 0.48 nM. The propqsed Cu2+ sensor showed good reproducibility, stability and selectivity. It has been satisfactorily applied to determine Cu2+ in water samples.