Electrostatically Governed Debye Screening Length at the Solution-Solid Interface for Biosensing Applications

Abstract

Biosensors based on field-effect devices (bioFETs) offer numerous advantages over current technologies and therefore have attracted immense research over the decades. However, short Debye screening length in highly ionic physiological solutions remains the main obstacle for bioFET realization. This challenge becomes considerably more acute at the electrolyte−oxide interface of the sensing area due to high ion concentration induced by the charged amphoteric sites, which prohibits any attempt to employ the field-effect mechanism to "sense" any charged biomolecules. In this work, we present an electrostatic approach by which the double layer (DL) excess ion concentration is removed, thus forcing the DL ion concentration to match the bulk concentration, which subsequently forces bulk screening length at the DL, thereby "exposing" target biomolecules to the underlying bioFET. To this end, we employ local tunable surface electric fields, introduced to the DL using surface passivated-metal electrodes. We examine numerically and analytically the effect of these electric fields on the DL ion distribution. We also numerically demonstrate the feasibility of the proposed approach for a fully depleted silicon-on-insulator based bioFET and show how the threshold voltage shift induced by the presence of target molecules increases by almost two orders of magnitude upon the removal of the surface excess ion population.

A new approach towards the Debye length challenge for specific and label-free biological sensing based on field-effect transistors

“…The application of negative V GR removes the surface excess cations and forms hole accumulation at the Si/SiO 2 interface to maintain charge neutrality across the EOI. In this way the ion concentration at the double layer is reduced and λ s is increased (for stronger electrolytes, see reference 11 ).…”

“…A major recognized obstacle in the application of BioFETs is the screening length which is especially significant in highly-concentrated ionic solutions. 11–18 The screening length is identified with the Debye length ( λ D ) which at room temperature is represented as , where l B = 0.7 nm is the Bjerrum length, N Av is the Avogadro's number, z i and n B, i are the valence and solution concentration of ion type i , correspondingly, and the summation accounts for the various types of ions. 19 In a characteristic physiological solution, λ D = 1–2 nm which is significantly smaller than typical surface-bound antibody size (∼10 nm), for example, which seriously affects the possibility to sense target biomolecules.…”

A review of BioFET’s basic principles and materials for biomedical applications

“…This equation results in a value of 0.7 to 2.2 nm in a typical physiological sample, which is much smaller than the 5 to 10 nm length of an IgG antibody commonly used as a probe [39,40]. As a result, the charge of target molecules such as proteins or target DNAs is screened, making efficient sensing impossible [41]. This suggests that the size of the probe in BioFET can have a significant impact on sensitivity, so small probes like aptamers are often used recently rather than large probes, such as antibodies.…”

Surface Potential/Charge Sensing Techniques and Applications

“…Park et al [59] reported an interfacial charge regulation method by protein-blocking layers on ISFET for direct measurements in serum. Bhattacharyya et al [81] proposed a local electrostatic method to tune the Debye length, forcing the double layer ion concentration to match the bulk.…”