Electrosynthesis and characterization of biotin-functionalized poly(terthiophene) copolymers, and their response to avidin

Abstract

Using a succinyl ester linker, a terthiophene bearing a pendant biotin hydrazide in the 3′ position (4-{N′-[5-(4,6-dioxo-hexahydrothieno[3,4-c]pyrrol-3-yl)-pentanoyl]hydrazino}-4-oxo-butyric acid 2-[2,2′:5′,2″]terthiophen-3′-yl-ethyl ester, 4) has been prepared in three steps from 2-[2,2′:5′,2″]-terthiophen-3′-yl-ethanol, 2, via succinic acid mono-(2-[2,2′,5′,2″]-terthiophene-3′-yl-ethyl) ester, 3. Using the ‘oligomer as monomer’ approach, polyterthiophene (co)polymer films have been generated, on Pt, Au and ITO-coated glass working electrodes, by repetitive scan cyclic voltammetry of solutions of 2,2′:5′,2″-terthiophene, or either 3 or 4 along with 2,2′:5′,2″-terthiophene itself. These films have been characterized by cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and in situ reflectance FTIR spectroscopy. FTIR spectroscopy suggests that the 2,2′:5′,2″-terthiophene : 4 copolymer contains intact biotin moieties. The incubation of poly-2,2′:5′,2″-terthiophene and 2,2′:5′,2″-terthiophene : 4 copolymer with aqueous buffer solutions, followed by transfer back to CH₃CN/0.1 M TEAT, results in only small changes in the film CV, whereas exposure of the biotin-functionalized 2,2′:5′,2″-terthiophene : 4 copolymer to avidin causes drastic changes to its CV and its EIS, consistent with the binding of avidin by biotin resulting in restricted ion transfer to and from the polymer. The minimum amount of avidin which causes a detectable change in the electrochemistry of a 0.03 cm² 2,2′:5′,2″-terthiophene : 4 copolymer-modified electrode is 5 × 10⁻¹⁴ moles. Exposure of the 2,2′:5′,2″-terthiophene : 4 copolymer to excess bovine serum albumin in buffer causes only the same small changes in its electrochemistry as exposure to buffer alone, ruling out nonspecific adsorption as a cause of the electrochemical changes on avidin exposure.