br Materials and methods br Results and discussion

Materials and methods
Results and discussion
Conclusion
Conflict of interest
Acknowledgements We appreciate the help of staff from the University of South Florida Nanotechnology Research and Education Center, and also help of John Winskas and Surya Cheemalapati with device fabrication. E. Archibong was supported by the United States National Science Foundation Florida-Georgia Louis Stokes Alliance for Minority Participation (FGLSAMP) Bridge to the Doctorate grant (HRD #1139850) and the Alfred P. Sloan Minority PhD program.
Introduction Electrochemical biosensors are highly effective in detecting biomolecules due to the high sensitivity, real-time monitoring capability and low cost, compared to the relatively complex and expensive measurement techniques such as radioisotope tracing, NMR spectroscopy, and microfluorometry assay [12,25,18]. In recent years, electrochemical biosensors based on the enzymatic activity have received increasing interest, for its advantages of low cost, portability, fast response time, and ease-of-usage by non-specialist personnel [3]. Hydrogen peroxide (H2O2) is an electrochemical active species produced by various oxidase enzymes. Thus, the measurement of H2O2 in various enzymatic reactions can quantify the analyte for biomarker detections as shown in Eq. (1)[11]. The use of electrode or catalyst-modified electrode as a transducer was based on Eq. (2) of oxidation of H2O2: However, the total sensitivity of biosensors based on traditional materials is hindered due to restrictions in mass transport, enzyme loading, and electrochemical coupling, limiting the potential for miniaturization. This affects the limitation of detection of the analyte as well. Unconventional nanomaterials with good biocompatibility and electrocatalytic activities have been widely incorporated in biosensors to overcome these shortcomings [13,6,16,21]. Metal nanoparticles (NP), particularly Au and Pt, have been used in the development of electrochemical sensors and biosensors based on their catalytic activities [31,21]. They have the advantages of large surface-to-volume ratio and special binding site on the surface of nanoparticles, which lead to a fast communication between an enzymatic process and a nanoparticle response for signal transduction in biosensing or for catalytic reactions [2]. PtNPs has superior sensitivity because of its enhanced kainic acid transfer and reduction of overpotential for H2O2 oxidation [9,30]. However, electrodes modified with pure PtNPs requires a relatively high electrochemical potential (ca. +0.7V versus Ag/AgCl) to oxidize H2O2 generating the oxidation current. At relatively high potential, it will oxidize ascorbic acid (AA) and uric acid (UA) in human blood resulting in an interference of the detection of the analyte (e.g. glucose) [4]. Hence, gold nanoparticles (GNPs) are considered to be another potential candidate based on its good performance in H2O2 related sensors with lower potential [5]. Different from spherical GNP, gold nanorods (GNR) introduces more interesting functions based on its anisotropic shape having unique localized surface plasmon resonance (LSPR), which brings applications in cancer diagnosis and monitoring local environment changes [8,1]. This can potentially expand the diversity of biosensors. However, owing to its anisotropic shape, GNR tends to self-aggregate during surface processing forming side-by-side assemblies [19,27] Therefore, it is desirable to introduce selected conducting materials to support the GNR in order to induce decent dispersity. Carbon nanomaterials are excellent candidates as the supporting materials for metal NPs due to their unique structural, electrical, and mechanical properties [20,22]. The carbon nanomaterials can enhance the available electrochemical active surface area of electrocatalyst and provide high mass transport of reactants to the electrocatalyst. Among these carbon materials, graphene is a two dimensional monolayer of carbon atoms with high surface area, chemical stability, and thermal stability, making it as a useful substrate for electronics [24]. However, graphene sheets are difficult to exfoliate because their planar polycyclic aromatic structures favor tight packing as a result of strong π–π interactions. Also, even the graphene sheets are destacked, they can only support metal NPs on its two sides and these individual graphene-metal NP hybrid structures may lack connection to each other. Therefore, it requires a 3D structure to enhance the loading of metal NPs and interconnection between the metal NPs. In our previous work, a 3D graphene-GNR layer-by-layer nanostructure was constructed [28]. The GNRs dispersed well and linked the conductive reduced graphene layers through stable covalent Au-S bond in this 3D structure. Therefore, this architecture possesses ideal large surface area and conductive pathways for biosensing.