Resonant Enhancement of Polymer-Cell Optostimulation by a Plasmonic Metasurface

Abstract

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders.

Figure 1

Spectral characteristics of the designed metasurface. Experimentally measured optical reflection (black trace), transmission (blue trace), and absorption (red trace) spectra of the nanoslit metamaterial arrays fabricated for normally incident light with polarizations perpendicular (TM, left panel) and parallel (TE, right panel) to the slits.

Figure 2

Spectral response of the P3HT thin film (∼30 nm) spin-coated on the metasurfaces. Experimentally measured optical reflection for normally incident light on the unpatterned aluminum (black trace) and on the nanoslit metamaterial with TE-polarized (orange trace) and TM-polarized (blue trace) light.

Figure 3

Time-resolved photoluminescence. PL dynamics for 10 nm (a) and 150 nm (b) thick P3HT film. PL emission spectra of 10 nm (c) and 150 nm (d) thick P3HT film. Excitation wavelength 520 nm, P = 15 μW.

Figure 4

Visualization of cells grown on the metasurface. (a) High-resolution scanning electron microscopy image recorded in secondary electron imaging mode with 5 kV acceleration voltage and 17000× magnification. (b) Bright-field image of the patch-clamp measurement. (c) Confocal microscopy image on the metamaterial/on the control pad. Effect of photostimulation on HEK293T cells seeded on different substrates. (d) Representative traces of the photocurrents generated by HEK293T cells seeded on coverslips containing metasurface (black) or metasurface + P3HT (red) during light stimulation. (e) Histograms showing the maximal current density after 4 min of light stimulation (right) and cell membrane capacitance (left). The mean current of the baseline was subtracted from the peak current (pA) during the photostimulation and normalized against the cell capacitance (pF); N = 8, 12, 8, and 11 for cells plated on aluminum, metasurface, aluminum + P3HT, and metasurface + P3HT, respectively; *p < 0.05; **p < 0.01, ****p < 0.0001; Kruskal–Wallis test followed by Dunn’s correction. Data are presented as box plots with whiskers representing the minimum and the maximum values.