[show abstract][hide abstract] ABSTRACT: This paper investigates a new immobilization procedure for biological molecules that is based on the formation of reactive ω-functionalized-self-assembled thiol monolayers onto a gold electrode. The homogeneous self-assembled monolayer was characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The SAM modified gold electrode showed a clear peak corresponding to S2p that characterized the Au-thiolate bond, while cyclic voltammetry and electrochemical impedance spectroscopy measurements, in 10 mM phosphate buffer pH 7, in the presence of Fe(CN)6− 3/− 4 as redox probe, showed that these monolayers were densely packed and prevented electron transfer towards the gold surface. These homogeneous SAMs were used to immobilize biotin hydrazide by covalent attachment, after the nucleophilic attack of the amino group of biotin hydrazide on the ω-activated ester function of thiols. The biotin–avidin interaction was then examined as a model for an affinity biosensor with electrochemical impedance spectroscopy. A Randles equivalent circuit was used for the interpretation of impedance data and the change in the interfacial properties at the modified-electrode/electrolyte interface were monitored through charge-transfer resistance variation. The proposed affinity biosensor showed a detection range that was linear between 200 and 800 ng/ml for avidin. In order to improve the sensitivity the technique of mixed self-assembled monolayers was adopted. Mixed SAMs were elaborated by co-adsorption of two differently substituted thiols, one was substituted by a reactive group that was used to react with the amino group of biotin hydrazide, whereas the other was substituted by an hydroxyl group that was chosen to mimic protein resistance. In this study, we started with a 1:3 activated ester:hydroxyl-terminated alkanethiol ratio. The results obtained with the mixed SAMs appeared to be better than those obtained with the homogeneous SAMs, and the corresponding affinity biosensor presented two detection ranges that were linear between 20 and 100 ng/ml and between 100 and 1200 ng/ml, respectively, with two different slopes.
Materials Science and Engineering: C. 01/2008; 28:861-868.
[show abstract][hide abstract] ABSTRACT: We report a new approach for detecting DNA hybridisation using nonFaradaic electrochemical impedance spectroscopy. The technique was applied to a system of DNA probes bearing amine groups that are immobilized by covalent grafting on a supporting polypyrrole matrix functionalised with activated ester groups. Results show a significant modification in the Nyquist plot upon addition of the complementary target whereas, in the presence of the noncomplementary target, the Nyquist plot is not modified. The spectra, in the form of Nyquist plot, were analysed with the Randles circuit. The transfer charge resistance R<sub>2</sub> shows linear variation with the complementary target concentration.
Solid-State Sensors, Actuators and Microsystems, 2005. Digest of Technical Papers. TRANSDUCERS '05. The 13th International Conference on; 07/2005
[show abstract][hide abstract] ABSTRACT: An electronic device, namely suspended gate thin film transistor (SGTFT), for rapid and direct detection of specific DNA sequences with high sensitivity is presented. It is an usual P-type polysilicon TFT where the gate contact is a Si<sub>3</sub>N<sub>4</sub>/polysilicon/Si <sub>3</sub>N<sub>4</sub> bridge suspended at 0.5 mum above the SiO <sub>2</sub>/Si<sub>3</sub>N<sub>4</sub> gate insulator. The high field effect due to the low height of the gap induces high sensitivity of the characteristics to any charge variation in the space between the gate and the channel. Amino-substituted ODN is grafted on silicon nitride surface after glutaraldehyde activation. The presence of this grafted ODN is confirmed by the positive shift, meaning the presence of negative charge, of the SGTFT transfer characteristics. The characteristics do not shift with non complementary DNA target. On the contrary, hybridization with complementary DNA is evidenced by the positive shift as large as 0.35 V with 5nM DNA concentration and an effective volume of 7 times 10<sup>-10</sup> milliliter