Electro-plasmonic nanoantenna: A nonfluorescent optical probe for ultrasensitive label-free detection of electrophysiological signals

Abstract

Harnessing the unprecedented spatiotemporal resolution capability of light to detect electrophysiological signals has been the goal of scientists for nearly 50 years. Yet, progress toward that goal remains elusive due to lack of electro-optic translators that can efficiently convert electrical activity to high photon count optical signals. Here, we introduce an ultrasensitive and extremely bright nanoscale electric-field probe overcoming the low photon count limitations of existing optical field reporters. Our electro-plasmonic nanoantennas with drastically enhanced cross sections (~10⁴ nm² compared to typical values of ~10^-2 nm² for voltage-sensitive fluorescence dyes and ~1 nm² for quantum dots) offer reliable detection of local electric-field dynamics with remarkably high sensitivities and signal-to-shot noise ratios (~60 to 220) from diffraction-limited spots. In our electro-optics experiments, we demonstrate high-temporal resolution electric-field measurements at kilohertz frequencies and achieved label-free optical recording of network-level electrogenic activity of cardiomyocyte cells with low-intensity light (11 mW/mm²).

Fig. 1. Electro-plasmonic nanoantenna.

(A) SEM image of cardiomyocyte cells cultured on an array of electro-plasmonic nanoantennas. Considerable size difference between loaded nanoantennas (height, 45 nm; diameter, 90 nm) and electrogenic cells is shown. A total of 2.25 million electro-plasmonic nanoantennas are incorporated on a transparent substrate with nanometer spatial resolution, allowing measurement of electric-field dynamics from diffraction-limited spots over a large surface area. (B) Side view of near-field enhancement |E/E₀|² along the pristine nanoantenna at 678.8 nm. FDTD simulations show that plasmonic excitations lead to strong confinement of the light within the 20-nm-thick electrochromic layer. (C) Top view of the near-field enhancement |E/E₀|² profile along the center of the pristine nanoantenna at 678.8 nm. (D) Equivalent nanocircuit model of the electro-plasmonic nanoantenna. Electrochromic doping is incorporated through tunable resistor and capacitor elements. (E) Susceptances of the gold nanoantenna and the PEDOT:PSS load for doped (red) and dedoped states (blue) are shown. Intersections (indicated by the circles) correspond to the open-circuit condition, the plasmonic resonance. For the doped (dedoped) electrochromic load, the resonance condition occurs at the shorter (longer) wavelength intersection due to the diminished resistance (losses) of the electrochromic load. (F) Far-field response of the electro-plasmonic nanoantenna to the doping state of electrochromic load. Electrochromic switching of the load from the doped (red curve) to the dedoped (blue curve) state leads to red shifting of the plasmonic resonance. FDTD stimulations (solid curves) and lumped nanocircuit model (dashed curves) are compared. The inset depicts the chemical structure of PEDOT for the doped (left) and dedoped (right) state. A⁻ represents the counterions.

Fig. 2. Electrochromic loading.

(A) Equivalent circuit model of the (Au) electrode-PEDOT:PSS system used in EIS measurement (top). PEDOT:PSS layer electropolymerized on an Au surface is illustrated in an electrolyte solution (bottom). (B) Cyclic voltammograms of 10-nm-thick (blue curve) and 20-nm-thick (red curve) PEDOT:PSS-coated Au electrodes. (C) Bode impedance plot of the (Au) electrode-PEDOT:PSS system. Excellent agreement is observed between EIS measurements and the equivalent circuit model. (D) Potential step voltammetry measurements to analyze the temporal response of the PEDOT:PSS film. Linear scaling of the electrochromic switching speed with the active area for fixed thickness t = 20 nm (blue curve) and thickness for fixed area √Area = 7 mm (red curve) is shown. Our electrochemical analysis suggests that it is advantageous to use a thinner and smaller surface area PEDOT:PSS load to achieve fast response times. (E) Selective electropolymerization of EDOT monomer in NaPSS aqueous solution under potentiostatic conditions. First, CV is used to characterize electropolymerization of PEDOT:PSS on Au (red curve) and ITO (blue curve) substrates. The nucleation point difference between Au and ITO surfaces is exploited for selective deposition of PEDOT:PSS on Au nanoantenna (insets).

Fig. 3. Field sensitivity and signal-to-noise ratio.

(A) Differential scattering signal versus applied electric-field strength. Electro-optic measurements are performed at a modulation frequency of 500 Hz. Absolute values of the differential scattering signals are compared for the pristine (red curve) and electro-plasmonic (blue curve) nanoantennas. Approximately 3.25 × 10³ times enhanced field sensitivity is shown for the electro-plasmonic nanoantenna. For low field values (2 × 10² to 8 × 10² V/cm), we observed large intensity changes (1 to 7%) in scattering signal of the electro-plasmonic nanoantenna. (B) Detection limits of single electro-plasmonic and plasmonic nanoantennas. SSNR ratios are compared for single field probes at an illumination intensity of 300 W/cm². An integration time of 1 ms is considered. High SSNRs (~60 to 220) are shown for the electro-plasmonic nanoantenna even for low field values (2 × 10² to 8 × 10² V/cm). Reference field direction corresponding to positive electric-field is shown (inset).

Fig. 4. Optical recording of electrogenic activity.

(A) Schematics of the transmission dark-field measurement setup. Strong light scattering contrast in between the spatial regions with (green) and without (dark) electro-plasmonic (EP) nanoantenna is observed (inset). (B) Temporal response of the electro-plasmonic nanoantenna obtained using a square wave voltage for spectroelectrochemical recording. Optical response of the electro-plasmonic nanoantenna is shown (red curve) for potential steps (blue curve) in between −500 mV (versus Ag/AgCl) and 500 mV (versus Ag/AgCl) with a residence time of 5 ms. A switching time of 191 μs is obtained after fitting a decaying-exponential function to the scattered light intensity. (C) False-color scanning electron micrograph of hiPSC-derived iCMs (colored purple) cultured on electro-plasmonic nanoantenna array. (D) Differential scattering signal in response to electrogenic activity of a network of cardiomyocyte cells. Strong far-field signal allowing label-free and real-time optical detection of electrogenic activity of iCMs is obtained from substrates with electro-plasmonic nanoantennas (red curve). Control measurements are performed to verify the origin of the electro-optic signal. In the absence of electro-plasmonic nanoantennas, no far-field signal is detected (blue curve).