Long‐Range Electrical Contacting of Redox Enzymes by SWCNT Connectors

Abstract

Dedicated to Prof. Helmut Schwarz on the occasion of his 60th birthday.The combination of biological molecules and novel nanomaterial components is of great importance in the process of developing new nanoscale devices for future biological, medical, and electronic applications.[1] The electrical contacting of redox enzymes with electrodes is a subject of extensive research over the last decade, with important implications for developing biosensing enzyme electrodes, biofuel cells and bioelectronic systems. [2][3][4] Tethering of redox-relay units to enzymes associated with electrodes, [5][6][7] the immobilization of enzymes in redox-polymers, [8] and the reconstitution of apo enzymes on relay-cofactor units associated with electrodes [9] were reported as means to establish electrical communication between redox proteins and electrodes. Recently, the reconstitution of the apo-flavoenzyme glucose oxidase, GOx, with a single Au nanoparticle functionalized with the flavin adenine dinucleotide (FAD) cofactor was reported.[10]The assembly of the Au-nanoparticle/GOx biocatalyst on an electrode led to an effective electrically contacted enzyme electrode. Single-walled carbon nanotubes (SWCNTs) exhibit unique structural, mechanical, and electronic properties, [11][12][13][14] and recent studies have demonstrated the use of SWCNTs in nanodevices and sensors. [15][16][17][18][19] Several research activities have addressed the generation of biomaterial-SWCNT hybrid systems, protein-linked CNT [20] and nucleic acid-functionalized SWCNT. [21,22] The oriented assembly of short SWCNT normal to electrode surfaces was accomplished by the covalent attachment of the CNT to the electrode surfaces. [23][24][25][26] This structural alignment of the SWCNTs allows the secondary association of redox-active components to the CNT, and the examination of charge transport through the SWCNT. Herein we wish to report on the structural alignment of the enzyme glucose oxidase, GOx, on electrodes by using SWCNTs as electrical connectors between the enzyme redox centers and the electrode. We demonstrate that the surfaceassembled GOx is electrically contacted to the electrode by means of the SWCNTs, which acts as conductive nanoneedles that electrically wire the enzyme redox-active site to the transducer surface. The effect of the length of the SWCNT on controlling the electrical-communication properties between the enzyme redox center and the electrode is discussed.SWCNTs (Carbolex, Sigma) were first purified by heating the as-received nanotubes in refluxing 3 m nitric acid for 24 h and then washing the resulting nanotubes with water by using a 0.6 mm polycarbonate membrane filter (Millipore). The purified long SWCNTs were chemically shortened by oxidation in a mixture of concentrated sulfuric and nitric acids (3:1, 98 % and 70 %, respectively) that was subjected to sonication for 8 h in an ice/water bath. This procedure yields shortened SWCNTs with a broad length distribution and that have terminal carboxyl functionalities. The shortened SWCNTs ...

Carbon Nanotubes for Electronic and Electrochemical Detection of Biomolecules

“…This is further augmented by the ballistic transport of electrons or holes through SWNTs, with current densities as high as 10 9 A cm −2 in the presence of oxygen, [18,25] along with prevention of current-induced migration of metal contacts. [26] The facile electron transfer between nanotubes and a variety of protein redox cofactors, [27] in conjunction with conductive polymers [28,29] and nanoparticles [30] of metallic, semiconducting, and magnetic nature provide the means to implement new biosensing schemes and enhance the sensitivity of current methodologies. [27,30,31] CNTs can be readily dispersed as individual [32] and lightly bundled nanotubes [33,34] and assembled, [35] screen-printed, [36] and potentially inkjet-printed to produce device configurations with controlled transparency.…”

“…[26] The facile electron transfer between nanotubes and a variety of protein redox cofactors, [27] in conjunction with conductive polymers [28,29] and nanoparticles [30] of metallic, semiconducting, and magnetic nature provide the means to implement new biosensing schemes and enhance the sensitivity of current methodologies. [27,30,31] CNTs can be readily dispersed as individual [32] and lightly bundled nanotubes [33,34] and assembled, [35] screen-printed, [36] and potentially inkjet-printed to produce device configurations with controlled transparency. This is further amplified by an assortment of covalent and noncovalent functionalization strategies to link a variety of biological entities onto CNTs.…”

Enhanced electrochemical activity of redox-labels in multi-layered protein films on indium tin oxide nanoparticle-based electrode

“…It has always been a great challenge for electrochemists to immobilize proteins on electrode surface and yet keep the electrode accessible for electron transfer reactions [24]. One possible solution is to keep the surface coverage of proteins below unity to leave out some unoccupied sites.…”

“…Because the redox center of an enzyme is usually buried deeply inside the protein matrix, efficient electric communication with an electrode is established either by surface modification or by enzyme engineering [21,22]. Due to the high electric conductivity of gold nanoparticles and carbon nanotubes, they were employed as electric connectors to electrochemically activate glucose oxidase in a glucose sensor [23,24].…”

Electrochemical Sensors

“…142 The new sensing scheme eliminated the need for electronic mediators and other membrane materials. The electrons can be effectively transported longer than 150 nm along the distance of a SWCNT as a nanoconnector, while the transfer rate is controlled by the length of the SWCNT.…”