Ionic contrast across a lipid membrane for Debye length extension: towards an ultimate bioelectronic transducer

By resolving the chronic hurdles of molecular detection in physiologically relevant conditions, lipid membrane-assisted potentiometric devices and exceptional performance as well as first experimental observation of the SLB conformational changes upon molecular binding events was demonstrated.

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Detection of molecules via surface-sensitive bioelectronics have allowed for portable biomedical devices to implement point-of-care diagnostics to prescreen diseases without temporal and spatial limitations. Despite technological advances in biomolecule detections, evaluation of molecular interactions via potentiometric devices under ion-enriched solutions has remained a long-standing problem. As one of most well-known biomedical platform, field effect transistors (FETs) has attracted considerable attention because of their strength in miniaturization, cost effective mass production, and seamless integration with manufacturing processes. However, , molecular detection with FETs in ionic solutions at physiologic concentrations remains an ongoing challenge, mainly because of the formation of an “electrical double layer (EDL)” over the active sensing probe (Fig. 1a). That is, the EDL means effective and available area for molecular detection. For example, effective detection range, termed Debye length (λD), becomes <1 nm in physiological conditions. This means that molecular detection above 1 nm is impossible under physiological buffer condition using potentiometric devices.

Fig. 1. Electrical sensing in ionic environments and schematic of the SLB-assisted FET measurement set-up.

To overcome this issue, we propose an ion-impermeable SLB-assisted FET (SLB-FET) platform for molecular detection with high degree of consistency irrespective of environmental ionic conditions. The potential of supported lipid bilayers (SLBs), a mimicry of cellular membranes, as a bioelectronic transducer has been glimpsed. Such a prospective conjecture in their biosensing applications originates from their nature: i) high resistivity to nonspecific protein adsorption, ii) surface passivation with ion-impermeability, and iii) morphological preservation of receptors. By turning the fascinating glimpse into the realization, we demonstrate robust molecular detection under biologically relevant conditions, characterize its sensing capability in terms of sensitivity and selectivity, and finally investigate the signal transfer mechanism in our SLB-FET platform, which is triggered by real-time molecular bindings.

Key finding of our research can be obtained by engineering the buffer conditions across the SLB, which results in fascinating device performance as well as providing methodological tools for understanding the microscopic mechanism during electro receptor-ligand bindings. By placing deionized water (DIW) underneath the SLB, we successively confirmed the role of SLB for improved molecular detectability: i) defect-free coverage of lipid membrane, ii) directional alignment of receptors, iii) prohibition of non-specific binders on the SLB, and most importantly, iv) ion-impermeability of the SLB. Note that the combination of all the ingredients described above resulted in the realisation of reliable signal acquisition in solution at physiological ionic strength via the extension of the effective detection range (=Debye length extension). We found that placing the DIW underneath the SLB is the key for shifting the EDL above the outmost surface of the SLB, translating the lipid membrane into a faultless transducer.

Table 1. Comparison of FET sensors for molecule detection

In particular, striking FET responses with coterminous amplitudes upon protein bindings with distinct pIs make the authors being confused. Repeated results of identical amplitudes signal irrespective of different PIs of proteins forced the authors to investigate underlying mechanisms and origin of the electric signals above FET surface. X-ray reflectometry and the corresponding equivalent circuit analysis clearly explained that sub-nanoscale conformational change of lipids during binding events triggers major part of the electrical signals. And this explains why the identical amount of signals from Avidin/StreptAvidn/Neutravidin over the biotinylated- lipids. Note here that this sensing mechanism by the conformational change in the SLB allows for the detection of electroneutral targets.  Not only finding the roles of SLB as EDL extensor, we also found that our proposed SLB-FET platform showed “world’s lowest level of the limit of detection (LOD)” in physiologic conditions of 1× PBS (Table 1). 

Fig. 2. Schematic illustration of effective Debye length (λD) with ionic contrast across the SLB and conformational change upon avidin binding.

 To the best of our knowledge, our SLB-FET platform presents the first experimental observation of the SLB conformational changes upon molecular binding events with an in-vitro electrical detector, which has been long proposed as a theory but was never experimentally validated. This opens up an opportunity to analyse the physicochemical modulation of lipids at the molecular level, ruling out any possible contribution of the effective charge of target proteins in the signal transducing mechanism (Fig. 2). By resolving the chronic concern of molecular detection in physiologically relevant conditions, our SLB-FET platform and resultant exceptional performance can be extended to identify essential roles of membrane-related pathogenic proteins, and stages/factors of diseases and apoptosis of cells that experiences lipid membrane rupture processes. This has wide ranging implications for conditions such as neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases, virus-cell membrane interaction, and the impact of micro-particles on the human respiratory system.

Yong-Sang Ryu

Senior Researcher, Korea Institute of Science and Technology