Highly Sensitive and Practical Detection of Plant Viruses via Electrical Impedance of Droplets on Textured Silicon-Based Devices

Abstract

Early diagnosis of plant virus infections before the disease symptoms appearance may represent a significant benefit in limiting disease spread by a prompt application of appropriate containment steps. We propose a label-free procedure applied on a device structure where the electrical signal transduction is evaluated via impedance spectroscopy techniques. The device consists of a droplet suspension embedding two representative purified plant viruses i.e., Tomato mosaic virus and Turnip yellow mosaic virus, put in contact with a highly hydrophobic plasma textured silicon surface. Results show a high sensitivity of the system towards the virus particles with an interestingly low detection limit, from tens to hundreds of attomolar corresponding to pg/mL of sap, which refers, in the infection time-scale, to a concentration of virus particles in still-symptomless plants. Such a threshold limit, together with an envisaged engineering of an easily manageable device, compared to more sophisticated apparatuses, may contribute in simplifying the in-field plant virus diagnostics.

Keywords: EIS; TYMV; ToMV; droplet-based device; label-free detection; plant viruses; surface texturing.

Figure 1

Transmission electron microscopy images of ToMV (a) and TYMV (b). Bar sizes are 500 nm and 10 nm in the main pictures and in the insets, respectively; (c) Simplified sketch of the droplet/ textured p-type silicon (T-pSi)-based device and the connection to the measurement setup; (d) Photograph representing the typical virus particles-in-solution droplet on top of the highly hydrophobic T-pSi surface and one of the two needle probes for the connection to the experimental setup (see text).

Figure 2

(a,b) Averaged NPs at V_DC = 0.0 V extracted from Z vs f spectra between 0.1 Hz and 10 MHz at four representative virus concentrations for ToMV and TYMV-in TRIS-HCl droplet suspension. (c,d) Detail of the data range in (a,b) in the higher frequency region. The frequency corresponding to the onset of the anomalous diffusion was 9.0 kHz in both virus-based suspensions and at all molar concentrations.

Figure 3

NPs read out at V_DC = −200 mV and t = 0 min, i.e., soon after the bias application in blank TRIS-HCl buffer and in ToMV and TYMV—Based suspension droplet at two representative concentrations (see legend). The NPs response was different depending on the virus embedded in the suspension and with respect to suspending medium.

Figure 4

Calibration curves for virus quantification represented as ReZ at 0.1 Hz vs. the molar virus concentration in the droplet suspension. The inset represents the relative variation of the ReZ values respect to the blank TRIS-HCl.

Figure 5

(a) Comparison between ToMV and TYMV capacitances (from Equation (2)) and phase angles frequency dispersion for a suspension at similar virus molarity; (b) Γ parameters extracted from capacitance values in Figure 5 at 9.0 kHz at the lowest virus molar concentrations corresponding to the plateau region in (a). The different capacitance variation vs virus types evidences the feasibility of virus identification by using this method. The full dataset has been reported in Figure S5 (Supplementary Materials).

Figure 6

3D sketch representing the virus in a droplet system on the high hydrophobic T-pSi surface. In the figure the electrical circuit bests fitting the experimental NPs; each circuit element has been superimposed on the corresponding device structure region.

Figure 7

Comparison between translational diffusion coefficients (D) extracted from Warburg impedance (Equation (6)) at different virus molar concentrations of ToMV (red squares symbols and lines) and TYMV (blue stars symbols and lines) and the corresponding theoretically-expected ones for nanoparticles diffusion driven by Brownian forces D_t,B = D₀ (from Equations (7b) and (8b)) (horizontal lines red ToMV, blue TYMV).