Label-Free, Electrical Biomarker Detection Based on Nanowire Biosensors Utilizing Antibody Mimics as Capture Probes
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Label-Free, Electrical Biomarker Detection Based on Nanowire Biosensors Utilizing Antibody Mimics as Capture Probes Hsiao-Kang Chang1, Fumiaki Ishikawa1 and Chongwu Zhou1 1
Department of Electrical Engineering, University of Southern California Los Angeles, CA 90089, U.S.A. ABSTRACT Antibody mimic proteins (AMPs) are poly-peptides that bind to their target analytes with high affinity and specificity, just like conventional antibodies, but are much smaller in size (2-5 nm, less than 10kDa). In this report, we describe the first application of AMP in the field of nanobiosensors. In2O3 nanowire based biosensors have been configured with an AMP (Fibronectin, Fn) to detect nucleocapsid (N) protein, a biomarker for severe acute respiratory syndrome (SARS). Using these devices, N protein was detected at sub-nanomolar concentration in the presence of 44 µM bovine serum albumin as a background. Furthermore, negative control experiment is carried out to confirm the role of AMPs in N protein detection. INTRODUCTION In less than a decade, biosensors based on nanowire/carbon nanotube transistors have successfully made the transition from proof of concept1 to highly selective, ultra sensitive devices capable of detecting specific proteins and DNA sequences.2-4 These devices utilize a capture agent on the sensor surface to selectively bind the target biomolecules. The captured biomolecules affect the electronic properties of the nanowires/nanotubes, resulting in an electronically readable signal. Capture agents commonly used in nanobiosensors include antibodies, oligonucleotides, and small ligands (e.g. biotin).2-4 Antibody mimic proteins (AMPs) are a class of affinity binding agents developed by in vitro selection techniques. These AMPs can be evolved/engineered to improve recognition properties such as selectivity and binding affinity, with the potential to surpass antibodies and nucleotide aptamers. In contrast to typical antibodies, AMPs are stable to a wide range of pH and electrolyte concentrations, and are relatively small (usually 2-5 nm, less than 10 kDa). Moreover, it is expected that these peptide based affinity agents can be produced in large quantity, at relatively low cost. The combination of low cost, high binding affinity, chemical stability, and small size makes AMPs particularly attractive for use with nanowire/nanotube biosensors. In this report, we introduce evolved AMPs as a new class of capture agents for nanowire/nanotube biosensors. These AMPs will allow us to build nanobiosensors for virtually any biomolecule with high sensitivity/selectivity, as demonstrated here for a protein related to severe acute respiratory syndrome (SARS), using devices based on In2O3 nanowires. Metal oxide nanowires, such as In2O3, ZnO, and SnO2, can be easily derivatized and their surface do not possess an insulating, native oxide layer (e.g. SiO2 on Si nanowires) that may decrease the nanowire sensitivity.4 Thus, it is worthwhile to investigate metal oxide nanowires as alternative nanomaterials to silicon nanowires for biosensin
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