Electrostatic counter ion screening is a phenomenon that is detrimental to the sensitivity of charge detection in electrolytic environments, such as in field-effect transistor-based biosensors. Using simple analytical arguments, we show that electrostatic screening is weaker in the vicinity of concave curved surfaces, and stronger in the vicinity of convex surfaces. We use this insight to show, using numerical simulations, that the enhanced sensitivity observed in nanoscale biosensors is due to binding of biomolecules in concave corners where screening is reduced. We show that the traditional argument, that increased surface area-to-volume ratio for nanoscale sensors is responsible for their increased sensitivity, is incorrect.I n recent years, there has been a major drive to use field-effect transistor (FET)-based devices to detect biological molecules in electrolytic environments (1). These biosensors use the charge of biomolecules to gate the current through a transistor (2). Frequently, the transistor is based on a quasi-1D nanostructure, such as a nanowire (NW) or nanotube, and the biomolecules bind directly to the surface of the nanoscale structure (1, 3). The use of such nanostructures is justified by the belief that nanoscale biosensors are more sensitive, with sensitivity defined as the relative change in drain current or a shift in threshold voltage in response to a change in bound biomolecule density. A few experiments specifically studied the effect of shrinking nanowire radii on sensitivity, albeit with varying structures, analytes, and sensing circumstances, and found that shrinking a sensor's dimensions indeed improves its sensitivity (4-6). The enhanced sensitivity has been loosely attributed to the increase in the sensor's surface area-to-volume ratio, which is a direct result of shrinking its dimensions. This argument has been analytically justified in the context of gas sensors (7). However, there is a fundamental difference between gas and biomolecule sensing: biomolecule sensing is performed in an electrolyte, and the ions therein will screen the charge of bound biomolecules in a phenomenon known as Debye screening (8, 9). The direct application of the gas sensing result to the biosensing environment implicitly assumes that the screening effect does not change with shrinking dimensions, an assumption we believe to be false. There have been studies that included a rigorous treatment of screening in biosensors, but they studied neither the specific cause of increased sensitivity at the nanoscale, nor the effect of varying size on screening behavior (10). We believe the phenomenon responsible for the increased sensitivity of nanowires in particular, and nanostructured biosensors in general, have not yet been uncovered by the research community.We have previously dissected the operation of biosensors into two independent parts to better understand the underlying physics (11): first, biomolecule charges cause a change in the local electrostatic potential at the outer surface of the gate dielectri...

No abstract

No abstract

No abstract

We have studied the electrostatics of nanowire bioFETs as their channel widths vary. It is commonly believed that smaller bioFET channel widths and heights result in better sensitivity, which is attributed to increased surface-area-to-volume ratios of the bioFET channels. The simulations and analytical arguments presented here show that this reasoning is flawed. Instead, our work suggests that the local curvature of the bioFET surface, especially the corners, affects the electrostatic potential that results from the captured target analytes. Nanowires of any width have the same number of corners, but as the nanowire width shrinks, the beneficial concave corners contain a larger fraction of the total surface area, and their relative importance increases, leading to a slight increase in the bioFET signal.BioFETs are field-effect transistors that have been chemically modified in order to be able to quantify protein, DNA, and RNA concentrations in aqueous physiological solutions (samples). Their operation can be briefly summarized as follows: biochemical capture probes, such as antibodies, are attached to the gate insulator of the FET, in a manner similar to the attachment of probe molecules to wells for ELISA. A sample with target analytes is then introduced to the chip surface, and the targets bind to the capture probes. Most target molecules carry charge in aqueous environments, and this charge contributes to the gating of the FET channel nearby. Since an intrinsic property of the captured analytes is detected, secondary and detection antibodies are not necessary.A few experiments have suggested that bioFETs in the form of nanowires are more sensitive than their planar counterparts. Although super-sensitive planar bioFETs do exist (1), the most sensitive bioFETs have employed nanoscale structures (2, 3). The smaller the diameter of the nanowires, the more sensitive they become (4, 5). This effect is commonly attributed to an increased "surface-area-to-volume ratio" (S/V) that accompanies "scaling down" of the bioFET channel dimensions (6). This argument goes as follows: the amount of electric charge brought to the bioFET surface by captured target molecules is proportional to the sensor surface. If the S/V ratio of the bioFET is increased while its channel volume is kept constant, the charge density at the bioFET surface will increase, but the volume of channel that is to be gated does not change, so the gating effect becomes stronger, and the bioFET becomes more sensitive. We use TCAD simulations and analytical arguments to show that this S/V ratio argument is flawed, and that alternative explanations must be considered. ECS Transactions, 72 (6) 11-17 (2016) 11 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.77.150.148 Downloaded on 2016-06-30 to IP