Electrochemical sensors self-assembled monolayers

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

Preorganisation of redox anion sensors on electrode surfaces is a promising new technique for electrochemical anion sensing. Self-assembled monolayers or thin polymer films of metal-based receptors have the potential to generate an amplified response to anion binding akin to the dendritic effect. 

Electrochemical Sensors Based on Self-Assembled Monolayers (SAMs) of Metallomacrocycles... 

Composites of PANI-NFs, synthesized using a rapid mixing method, with amines have recently been presented as novel materials for phosgene detection [472]. Chemiresistor sensors with nanofibrous PANI films as a sensitive layer, prepared by chemical oxidative polymerization of aniline on Si substrates, which were surface-modified by amino-silane self-assembled monolayers, showed sensitivity to very low concentration (0.5 ppm) of ammonia gas [297]. Ultrafast sensor responses to ammonia gas of the dispersed PANI-CSA nanorods [303] and patterned PANI nanobowl monolayers containing Au nanoparticles [473] have recently been demonstrated. The gas response of the PANI-NTs to a series of chemical vapors such as ammonia, hydrazine, and triethylamine was studied [319,323]. The results indicated that the PANI-NTs show superior performance as chemical sensors. Electrospun isolated PANI-CSA nanofiber sensors of various aliphatic alcohol vapors have been proven to be comparable to or faster than those prepared from PANI-NF mats [474]. An electrochemical method for the detection of ultratrace amount of 2,4,6-trinitrotoluene with synthetic copolypeptide-doped PANI-NFs has recently been reported [475]. PANI-NFs, prepared through the in situ oxidative polymerization method, were used for the detection of aromatic organic compounds [476]. 

Ozoemena, K. and T. Nyokong (2002). Voltammetiic characterization of the self-assembled monolayer (SAM) of octabutylthiophthalocyaninatoiron(II) A potential electrochemical sensor. Electrochim. Acta 47(25), 4035-4043. 

Nakashima has prepared a phenylboronic acid terminated redox active self-assembled monolayer on a gold electrode as an electrochemical sensor for saccharides. Self-assembled monolayers of 89, a phenylboronic acid terminated viologen alkyl disulfide, function as a sensitive saccharide sensor in aqueous soluhon [168]. 

What one can consider as the first electrochemical sensor, i.e., a modified electrode surface dedicated to a specific target analyte, was a platinum electrode, covered by a protective membrane, the Clark electrode, for the determination of O2 in blood.The first biosensor was based on the determination with such Clark electrode of O2 depletion induced by glucose oxidase activity in the presence of glucose.These two examples show the importance of platinum as electrode material. It will be seen below that gold was also widely used for the development of chemically modified electrodes, especially due to the strong interaction with thiol-functionalized organic molecules allowing the formation of self-assembled monolayers (SAM). ... 

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