At a Glance
Researchers at Colorado State University have developed a capacitive immunosensor with the ability to detect ultra-low concentrations of virus specific antibodies having direct application as a point-of-care diagnostic.
For illustration, robust capacitive biosensors have been designed to specifically detect ZIKV and Chikungunya (CHIKV) antibodies using a sensor modified with their respective envelope (E) protein. These devices directly measure monoclonal antibody with a lower boundary of approximately 10 antibody molecules in a 30 μL sample, demonstrating exquisite sensitivity. The antibody detection system discriminates between antibodies with minimal cross-reactivity and can even differentiate antibody isotypes, indicating marked selectivity.
The capacitive sensor platform can be designed for numerous different pathogen specific antibodies, having superior sensitivity and specificity than other diagnostic devices.
Analyzing the humoral antibody response in clinical samples is critical to diagnose infectious disease, understand pathogenesis and immune response kinetics, and develop vaccines. Currently, the enzyme-linked immunosorbent assay (ELISA) is used as the gold standard clinical diagnostic tool for antibody detection.
However, ELISAs require large instrumentation in centralized laboratories and specialized training to execute and interpret the results which limits the utility of ELISAs in low-resource settings. Many cases, therefore, go undiagnosed which indicates an urgent need for sensitive, robust assays that quickly diagnose infection at point-of-care and provide health-care providers with actionable information.
Capacitive biosensors employ direct sample application for label-free detection. Other electrochemical antibody sensors have been developed for serological analysis, but these designs incorporate enzymatic labels or redox couples that increase complexity and cost. Compared to other immunosensors, capacitive biosensors are ideal candidates for sensitive and label-free bioanalysis platforms. Capacitive sensing is based on the theory of the electrical double layer (DL), where the working electrode is conjugated with probe that binds a target to increase the length of the DL. Because capacitance is inversely proportional to the DL length, this increase produces a corresponding decrease in capacitance. Such capacitive signals provide a direct, rapid measure of target binding. Based on our previous work using capacitance to detect DNA (Biosens. Bioelectron. 2016, 87, 646), the sensitivity of capacitive biosensors is far superior to traditional diagnostic assays and is ideal to detect low antibody titers during early stages of infection. Capacitive biosensors are thus an attractive sensing modality that has not yet been fully explored for specific antibody detection.
The label-free capacitive immunosensor introduced here uses microwire electrodes to rapidly and sensitively detect antibodies produced during an immune response, in this case mouse antibodies against ZIKV. The device is comprised of low-cost, easily-acquired materials. A glass slide is used as the base substrate with a polydimethylsiloxane (PDMS) well for sample application. Au and Ag/AgCl microwires (working and reference electrodes, respectively) are immobilized across the PDMS well (Fig. 1a) and 30 μL of liquid sample is added to the well and incubated for 5 min. Measurements can then be taken in as quickly as one minute. Microelectrode wires, compared to other electrode fabrication methods like ink printing, paste, and sputter-coated electrodes, demonstrate increased mass transport rates due to radial diffusion (Aoki,1993; Liu et al., 2004). This increases the current density and consequently improves sensitivity and enhances detection limits (Salaün andVan Den Berg, 2006). Microelectrodes offer the additional benefits of simple fabrication without expensive equipment, ease of surface chemical modification, and availability in different pure and alloyed compositions (Adkins and Henry, 2015)
Fig. 1. Schematic of capacitive immunosensor design and working principles. (a) Device layers and resulting immunosensor shown from the top. RE: reference electrode, WE: working electrode; (b) Working electrode (Au microwire) surface chemistry and functionalized layers, with the corresponding equivalent circuit and total capacitance equation. DL capacitance, CDL, is placed in parallel with a leakage resistance, Rleak. CDL represents the total capacitance, Ctot, of the individual capacitance contribution from each surface layer.
- System can detect as few as 10 antibody molecules – orders of magnitude less than ELISA
- Ultra-low detection limit can allow for detection of antibody responses much earlier than traditional methods
- Earlier diagnosis of disease and improved patient outcomes
- Can be developed against any antigen/antibody pair desired
- Detection of pathogen-specific antibodies (diagnostic)
- Infectious disease monitoring
- Cancer screening
- Various other diagnostic devices/applications
- Point-of-care diagnostics – for physicians, clinics, or in the field