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    • Carbon nanotubes CNTs have been widely utilized as component

    2018-11-01

    Carbon nanotubes (CNTs) have been widely utilized as components for nanoscale electronic devices and biosensors due to their high electrocatalytic property, their ability to promote faah inhibitors transfer, and their high thermal capacity. The CNTs-modified electrodes are reported to allow direct electron transfer (DET) to glucose oxidase [15,16]. The intrinsically hydrophobic carbon nanomaterials can decrease the bioactivity of GOx [17], however, functionalized carbon nanotubes have been proven to improve the glucose sensing performance and to facile direct electron transfer between GOx and the electrode surface [16,18]. Chitosan (CS), a linear polysaccharide, has a repeating hexosamide residue unit of one amino group and two hydroxyl groups permitting chemical modifications. It is nontoxic, biocompatible and economic. Chitosan is widely applied to immobilize biomolecules, especially in the assembly of enzymes and fabrication of amperometric biosensors, due to its excellent film forming and adhesion abilities, and its easiness for chemical modifications [19,20]. Gold nanoparticles (GNp) are very attractive in constructing electrochemical biosensors due to their unique physical and chemical properties. In particular, GNp are applied in electrode functionalization to catalyze electrode chemical reactions and conduct DET, and thus to increase the sensitivity of biochemical detection [21]. They are used as one negatively charged nanomaterial for electrostatic adsorption in LBL process. GNp not only increase the surface area to allow more enzyme to be immobilized, but also provide a mild microenvironment and give the biomolecules more freedom in orientation [22]. Tremendous work has been done to develop highly selective and sensitive glucose electrochemical biosensors, as reviewed by Professor Yao’s group [23]. Glucose biosensors functionalized with CNTs or GNp have the ability to linearly detect glucose down to very low levels [24–29]. Chitosan, or polymers like poly(diallydimethylammonium chloride) (PDDA), poly(ethylenedioxithiophene) (PEDOT), has been commonly used in enzyme immobilization or as a linker molecule in the fabrication of amperometric biosensors [6,24,25,27–30]. However, we have developed an accurate and reliable saliva glucose sensor using direct electron transfer enabled by using SWNT. Our on-chip electrochemical sensing device contains at least one working electrode, a counter electrode and a reference electrode. It is manufactured through several micro-fabrication procedures. The metal for all electrodes is Pt, which is widely applied for glucose sensing [20]. Pt provides significant advantages, such as much better conductivity, signal stability, and analytical response, over the other electrode materials, like Au, Ti, Ag [31,32]. Jin et al. have also proved that Pt’s deposition onto the gold electrode can increase the electrocatalytic properties of the electrodes for glucose oxidation [33]. Pt can also be used for reference electrode [34]. Thus, it is highly feasible to integrate three Pt electrodes onto one single chip to realize on-chip electrochemical sensing. Further, this single chip can not only be used for glucose detection, but also provides an innovative platform for on-chip electrochemical sensing of other chemicals and biomolecules.
    Experimental design and procedure
    Results and discussion
    Conclusion In summary, we have developed a simple and economic on-chip electrochemical sensing system containing at least one working electrode, a counter electrode and a reference electrode. It is currently used as a disposable nano-biosensor for glucose detection with the sensor electrode functionalized with SWNT and multilayered film composed of CS–GNp–GOx. The most important features are: (1) direct electron transfer between GOx and the electrode surface; (2) on-a-chip; (3) glucose detection down to 0.1mg/dL (5.6μM); (4) good sensing linearity over 0.017–0.81mM; (5) high sensitivity (61.4μA/mM-cm2) with a small reactive area (8mm2); (6) fast response; (7) high reproducibility and repeatability; (8) reliable and accurate saliva glucose detection. It can also be further miniaturized and maintain high reproducibility and repeatability. It is an alternative for real time tracking of glucose levels from body fluids, e.g. saliva, in a noninvasive, pain-free, accurate and continuous way. Furthermore, this on-chip electrochemical system is a platform suitable for on-chip electrochemical sensing of other chemical agents and biomolecules.