Novel Applications Of Graphene And Nanomaterials In Electrochemical Biosensors And Detection Of Organic Pollutants

In this dissertation, we construct five new electrochemical sensors based on graphene, making full use of the significantly different adsorption affinity of GO for ss-DNA, ds-DNA and G-quadruplex, the highly aptamer-target binding affinity with excellent specificity and selectivity, and outstanding electro-catalytic activity of PEI@AgNCs and AuNPs, respectively. Additionally, some new valuable explorations have been carried out on the preparation of graphene oxide (GO) or electrochemically reduced graphene oxide (ERGO) modified electrode.1. Probe-label-free electrochemical aptasensor based on graphene oxide amplificationTwo novel signal-off probe-label-free electrochemical aptasensors based on GO amplification were developed for thrombin (TB) and ATP detection, providing a new simple, feasible way of fixing GO on the electrode by self-assembling.(1) Probe-label-free thrombin electrochemical aptasensor based on GO amplification:Firstly, a thiolated modifed thrombin-binding aptamer (TBA) was self-assembled on the gold electrode by gold-sulfur affinity, then GO could be readily fixed on the electrode by the strong π-π interaction of ss-DNA and GO. Next, abundant MB molecules were adsorbed and anchored by GO due to the strong electrostatic adsorption affinity, leading to a strong differential pulse voltammetry (DPV) signal in the absence of TB. When TB is present, the formation of TBA-TB G-quadruplex structure prohibits π-π stacking interactions between TBA and GO, and consequently draws GO together with plenty of MB far away from the electrode, resulting in a sharply decreased DPV signal of MB. By monitoring the change of the reduction peak current of MB (ip) before and after target binding, TB detection could be realized. The linear range for TB detection is from2.8x10-11to3.7x10-9mol.L-1, and the detection limit is3.05x10-12mol.L-1(3a). It may be ascribed to the bulky G-quadruplex structure tethered on electrode after TB binding, which blocked the further binding of TB with TBA, so the linear range is relatively narrow although with higher sensitivity.(2) Probe-label-free ATP electrochemical aptasensor based on GO amplification:Firstly, a thiolated modifed12-mer anchored DNA (ANDA) was self-assembled on the gold electrode by Au-S bond, then a32-mer ATP-binding aptamer (ABA) was immobilized via the partial hybridization reaction between ABA and ADNA, remaining20free bases on ABA. Next, GO could be readily fixed on the electrode by the strong π-π stacking interaction between the twenty free bases and GO. Finally, abundant MB molecules were adsorbed and anchored by GO via the strong electrostatic interactions, leading to a strong DPV signal in the absence of ATP. However, in the presence of ATP, the ABA switches its structure to bind ATP and prefers to form a stable ATP-aptamer complex rather than an aptamer-DNA duplex, resulting in the dissociation of ABA from ADNA strand. The release of ABA is accompanied by the extrication of abundant GO and MB anchored by GO, which offers a significant decreased DPV signal of MB in the capture event. For the ATP sensor, the linear range is from1.0x10-10to5.0x10-7mol.L-1, and the detection limit is2.91x10-11mol.L-1(3o).Because ABA, GO and MB could be released from the electrode surface, the steric hindrance on the electrode would be decreased gradually with ATP-binding, benefiting the further binding of ATP to aptamer. So, the linear range of ATP detection is relatively wider.Because there are no special requirements for the aptamer part, these two strategies are also versatile and may therefore be suitable for many other proteins, metal ions or small biomolecules detection.2. Label-free electrochemical aptasensor based on ERGO and PEI@AgNCs probe Polyethyleneimine (PEI) is a hyperbranched polymer containing a large number of amino groups, having three-dimensional network structure and good film-forming properties. It can be easily protonated and then positively charged in the acidic solution. And the GO aqueous dispersion is just acidic (pH4.0). So, PEI was firstly self-assembled on glassy carbon electrode (GCE), after that abundant GO can be easily self-assembled on GCE via the strong electrostatic adsorption affinity of protonated PEI and negatively charged GO. Next, ERGO modified electrode can be prepared through the electrochemically reduction of cyclic voltammetry, which having good stable and better electrochemical activity. Using the electrochemical method and resonance light scattering spectroscopy, it can be found that the prepared ERGO modified electrode has a strong interaction with ss-DNA aptamer through π-π stacking and with PEI@AgNCs via electrostatic adsorption. But the aptamer modified ERGO electrode has a very weak interaction with PEI@AgNCs. Thus, if there are no aptamers and analytes, PEI@AgNCs can self-assembled on the electrode easily, resulting in a very strong electrochemical signal of Ag in PEI@AgNCs. Whereas aptamer bound on the electrode can prohibit the self-assembly of PEI@AgNCs, resulting in a very weak signal. When the analyte is present, aptamer will specially recognize it and be dissociated from the electrode, then PEI@AgNCs can be self-assembled on the electrode further, resulting in an enhanced electrochemical signal of Ag. Utilizing the different interactions of ERGO, aptamer, PEI@AgNCs and the highly aptamer-target binding affinity with excellent specificity and selectivity, we construct a general, label-free and signal-turn-on electrochemical aptasensor in this chapter, using for a rapid and sensitive detection of ATP or thrombin respectively. The linear range is from5.0x10-10to5.0x10-6mol.L-1for ATP, and from5.0x10-11to2.0x10-8mol.L-1for thrombin. The detection limit for ATP and thrombin is9.9x10-11mol.L-1and4.0x10-12mol.L-1respectively (3o).3. Enzyme-free H2O2sensor based on electrocatalytic ERGO and PEI@AgNCsMany reports about chemically modified electrodes proves that graphene and metal nanomaterials can improve the electrocatalysis of the reduction of H2O2, and the combination of both of them can efficiently enhance the ratio of surface area of the catalysts and can further improve their electrocatalytic performance. Thus, in this chapter, ERGO modified electrode was prepared firstly, and then PEI@AgNCs was self-assembled on the ERGO modified electrode, thus an enzyme-free H2O2sensor based on ERGO and PEI@AgNCs electrocatalyst was constructed finally. The experiment results shows that ERGO and PEI@AgNCs have excellent synergistic effect for the electrocatalytic reduction of H2O2, which improved the sensitive detection of H2O2greatly and rapidly.4. Study on preparation of ERGO-AuNPs composite modified electrode and detection for4-nitrophenolAuNPs assembled with graphene can improve the electrocatalytic activity and sensitivity of sensor greatly. In this chapter, by combining the self-assembly-electrochemical reduction method of GO with electrodeposition technology of AuNPs, we developed a two-step electrochemical reduction method for preparation of ERGO-AuNPs composite modified electrode, which can be used for detection of organic pollutant4-nitrophenol (4NP). We compared our prepared target electrode with other modified electrodes for the electrocatalytic performance of the reduction of4NP, and further optimized the preparation process and conditions of our target modified electrode in details. PEI is a dendritic polymer containing a large number of amino groups and can be used as an efficient self-assembly linker of GO. PEI was firstly self-assembled on GCE, and then abundant GO can be easily self-assembled on GCE with a three-dimensional shape distribution via the strong electrostatic adsorption or hydrogen bonding affinity of PEI and GO. After the electrochemically reduction, the prepared ERGO can increase the amount of AuNPs electrodeposited subsequently on the electrode greatly. It can be found that ERGO and AuNPs on the electrode surface have even a regular uniform leaves-pattern distribution from SEM images. The prepared ERGO-AuNPs composite modified electrode has a better electrocatalytic activity, a rapid, highly sensitive response to the reduction of4NP and a wider linear range for4NP detection.