1 Nanobioelectronics & Biosensors Group, Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, BIST, Campus UAB, 08193, Bellaterra, Barcelona, Spain; ICREA, Institució Catalana de Recerca I Estudis Avançats, Barcelona, Spain. Electronic address: arben.merkoci@icn2.cat.
2 Center of Cellular and Molecular Diagnosis, Tulane University School of Medicine, New Orleans, LA, 70112, USA. Electronic address: chenzhongbiosensor@gmail.com.
3 Nanobiosensors and Bioanalytical Applications (NanoB2A), Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, BIST and CIBER-BBN, 08193, Bellaterra, Barcelona, Spain. Electronic address: laura.lechuga@icn2.cat.
4 Electrical & Computer Engineering and Bioengineering Departments, University of California, Los Angeles (UCLA), CA, 90095, USA. Electronic address: ozcan@ucla.edu.