Electrochemical Sensors For Persistent Organic Pollutants Based On β-cyclodextrins And Aptamers

Persistent organic pollutants（POPs） are major environmental concern due to their persistence, long-range transportability, bio-accumulation and potentially adverse effects on living organisms. Some cancers, birth defects, dysfunctional immune and reproductive systems and even diminished intelligence are suspected to be related to an exposure to these chemicals. Due to their stability and long-range transport properties, they are now ubiquitous around the world. Much effort should be devoted to the development of faster, safer, more reliable and more sensitive analytical techniques for these pollutants.β-Cyclodextrins（β-CDs） are renewable cyclic oligosaccharides consist of seven, glucopyranose units, which are produced by enzymatic degradation of starch. CDs can be functionalized to encapsulate various organic or inorganic substrates in their hydrophobic microenvironments through the cooperative contribution of noncovalent forces, including hydrophobic, π-π stacking, van der Waals, and hydrogen bonding interactions. Aptamers are ligand-binding nucleic acids whose affinities and selectivities can rival those of antibodies. As a result, the electrochemical biosensors based on aptamers show excellent selectivity and sensitivity.Based on the selective combination between β-Cyclodextrins or aptamer and tracking POPs in the environment, as well as the excellent physical and chemical properties of nanomaterial such as Graphene, Carbon Nanotubes and Au Nanoparticles, we have developed several electrochemical sensors for sensitive and selective determination of some POPs:（1） Using hexamethylene diisocyanate（HMDI） as a crosslinker to polymerize β-CDs, Poly（β-CD-HMDI） noncovalently functionalized multi-walled carbon nanotubes（MWCNTs） composite（MWCNTs/Poly（β-CD-HMDI）） was successly prepared. The as-prepared MWCNTs/Poly（β-CD-HMDI） were characterized by Fourier transform infrared spectroscopy, X-ray Diffraction, scanning electron microscope, thermogravimetric analysis and electrochemical methods. Based on the MWCNTs/Poly（β-CD-HMDI） composite, we developed an electrochemical sensor for detection of non-electroactive activity 4,4′-Dichlorobiphenyl（DPCB）. Due to the host-guest interaction, nutruel red（NR） molecules could entry into the hydrophobic inner cavity of b-CDs, and the MWCNTs/Poly（β-CD-HMDI） modified glass carbon electrode displays a remarkable oxidation peak of NR. While in the presence of the analyte substrate, DPCB, competitive association to the β-CDs occurs and the NR molecules are displaced by DPCB. This results that the oxidation peak current of the NR decreases, and the changes of signal for NR are linear with the concentration of DPCB. This proposed sensor shows good perfermense, such as a wide linear range of 2 nM～80 uM, a low detection limit of 0.4 nM（S/N = 3） and good stability and reproducibility。（2） A ratiometric electrochemical sensor has been developed for highly sensitive and selective detection of bisphenol A（BPA）. The assay strategy was based on the competitive host–guest interaction between poly-β-cyclodextrin/electroreduced graphene（Pβ-CD/EG） and Rhodamine B（Rh B） probe or bisphenol A（BPA） target molecules. Rh B can enter into the hydrophobic inner cavity of β-CD and shows an obvious oxidation peak on the Pβ-CD/EG modified glassy carbon electrode（GCE）. In the presence of BPA, the Rh B molecules are displaced by BPA because the host –guest interaction between β-CD and BPA is stronger than that between β-CD and Rh B. As a result, the oxidation peak current of Rh B（ IRh B） decreases and the oxidation peak current of BPA(I_BPA) increases correspondingly. The logarithmic value of I_BPA/IRh B is linear with the logarithm of BPA concentration in the range of 1 to 6000 nM and the detection limit is 52 pM（S/N = 3）. This strategy provides a new approach for sensitive detection of BPA, and has promising applications in the detection of organic pollutants in real environmental samples.（3） Based on the interaction between BPA and the aptamer, a new ratiometric electrochemical biosensor was developed for sensitive and electivity determination of BPA. Firstly, gold nanoparticles（Au NPs） prepared by electrodepotion on GC electrode, named Au NPs/GCE. Then a specific aptamer against BPA and its complementary DNA probe modified with methylene blue（MB） were immobilized on the surface of the Au NPs/GC electrode via self-assembly and hybridization, respectively, so MB display a remarkable peak current on the electrode. In the present of BPA, the interaction between BPA and the aptamer lead to the dissociation of the duplex DNA, thereby the c DNA releases from the eledtrode, as a result, the signal of MB decreases, and that of BPA increases. The electrochemical aptasensor enables BPA to be detected with good sentivity and selectivity. The logarithmic value of I_BPA/I_MB is linear with the logarithm of BPA concentration in the range of 1 pM to 100 nM and the detection limit is 0.09 pM（S/N = 3）.