Fundamental design considerations and scaling principles for nanobiosensors: A geometro-physical perspective

Biotechnology and bio-sensing are often mentioned as the next frontier of electronics that could rival semiconductor industry’s broad and revolutionary impact on society. Any disease is a signature of either genetic defects or broken protein signaling pathways that occur far in advance of overt symptoms of the disease. Hence, one of the grand challenges of modern bio-sensing is to find cost-effective, reliable, and fast methods for gene sequencing (as an ultimate ‘Finger-print’ of one’s biological make-up that could allow an early intervention for genetic anomalies), and the detection/identification of the irreducible and emergent protein networks for applications in proteomics and systems biology. Modern bio-sensors based on nanoscale electrical devices promise highly sensitive detection of bio-molecules, unmatched by existing classical techniques. The emergence of nanometer-scale fabrication techniques have enabled a dramatic reduction in device dimensions, so much so that the bulk conduction mechanisms can now be controlled by surface properties. Reduced dimensionality has enabled Silicon nanowire and Carbon nanotube sensors to emerge as highly sensitive, label-free, and dynamic detectors for chemical and biological molecules that are ideally suited for integration in an array format. Despite tangible research accomplishments by various groups, the elements that dictate the response of a nanoscale biosensor has remained, until recently, poorly understood. In this thesis, we develop a predictive theoretical framework for the design and optimization of nano-bio sensors, identify key functional variables, and provide unifying principles for rapid and simple interpretation of experimental data. Specifically, we show how the elementary use of fractal geometry of diffusion, percolative transport in random networks, electrolyte screening-limited response, excluded interaction of bio-molecules, etc. are finally allowing us to establish the limits of performance and scalability for nanobiosensors. Regardless of the specific physical aspects of the system, our conclusions are broadly applicable and this thesis provides a unified geometro-physical perspective of nanobiosensors based on the geometrical considerations of diffusion, screening, and surface exclusion