Gaseous Synergistic Self-Assembly and Arraying to Develop Bio-Organic Photocapacitors for Neural Photostimulation

Abstract

Bioinspired supramolecular architectonics is attracting increasing interest due to their flexible organization and multifunctionality. However, state-of-the-art bioinspired architectonics generally take place in solvent-based circumstance, thus leading to achieving precise control over the self-assembly remains challenging. Moreover, the intrinsic difficulty of ordering the bio-organic self-assemblies into stable large-scale arrays in the liquid environment for engineering devices severely restricts their extensive applications. Herein, a gaseous organization strategy is proposed with the physical vapor deposition (PVD) technology, allowing the bio-organic monomers not only self-assemble into architectures well-established from the solvent-based approaches but morphologies distinct from those delivered from the liquid cases. Specifically, 9-fluorenylmethyloxycarbonyl-phenylalanine-phenylalanine (Fmoc-FF) self-assembles into spheres with tailored dimensions in the gaseous environment rather than conventional nanofibers, due to the distinct organization mechanisms. Arraying of the spherical architectures can integrate their behaviors, thus endorsing the bio-organic film the ability of programmable optoelectronic properties, which can be employed to design P-N heterojunction-based bio-photocapacitors for non-invasive and nongenetic neurostimulations. The findings demonstrate that the gaseous strategy may offer an alternative approach to achieve unprecedented bio-organic superstructures, and allow ordering into large-scale arrays for behavior integration, potentially paving the avenue of developing supramolecular devices and promoting the practical applications of bio-organic architectonics.

Keywords: bio‐photocapacitors; molecular manufacturing; neural photostimulation; physical vapor deposition; self‐assembly & arraying.

Figure 1

Gaseous organization strategy offers an alternative for bio‐organic self‐assembly as well as arraying. Middle panel: schematic cartoon depicting the bio‐organic gaseous organization with PVD technology. Left and right panels: diverse self‐assembling architectures, including i) plates, ii) fibers iii) membranes, and iv) spheres, either similar to or distinct from those gained in conventionally solvent situations, can be achieved. In addition, the supramolecular architectures can spontaneously arrange into large‐scale arraying films, allowing integration of the properties and endowing the feasibility to develop bio‐organic supramolecular devices. The insets display high‐magnification SEM images of the supramolecular architectures.

Figure 2

Architectural characterization of Fmoc‐FF spherical organization in the gaseous phase. a) Optical microscopy images of the bio‐organic supramolecular microspheres self‐assembled on diverse substrates. b) Left panel: SEM image showing the solid feature inside the microspheres. Right panel: high‐magnification SEM characterization of the captured rectangular framework in the left panel. c,d) SEM and AFM images depicting the co‐existence of large microspheres and a uniform film beneath. e) 2D (left) and 3D (right) viewpoint of high‐magnification AFM characterizations of the captured square area in (c) showing extensively‐collected nanoclusters inside the film. f) ω‐dependent diameter and amount evolution of the microspheres. g) Schematic cartoon depicting the hierarchical dynamics of Fmoc‐FF gaseous organization, experiencing several stages including adsorption, nucleation, ripening, and arraying.

Figure 3

Characterization and process analysis of Fmoc‐FF gaseous self‐assembly. a) MS profile of the supramolecular microspheres, showing the native MW of Fmoc‐FF. b) FTIR spectra of the fibrillar and spherical architectures self‐assembled by Fmoc‐FF in the aqueous solution and gaseous phase situations, respectively. The blue dotted line represents the wavenumber shift of the O–H/N–H stretching vibrations between the two supramolecular morphologies. c) Secondary structures statistics of the fibrillar and spherical architectures, through deconvolution of the amide I region marked in grey color in (b). d,e) Snapshots at six‐time points showing Fmoc‐FF forming (d) fibril‐like aggregates in solution at 300 K and (e) spherical assemblies in the gas phase at 673 K. Note that the functional moieties in the Fmoc‐FF molecule are simplified and marked in different colors for clarity, as shown in the inset at the right panel. f) Time evolution of the SASA fraction for Fmoc, side‐chain, and main‐chain groups relative to their initial randomly dispersed states and g) Snapshot and cross‐sectional view of the fibril‐like aggregate in solution at 300 K. h) Time evolution of the SASA fraction for Fmoc, side‐chain, and main‐chain groups relative to their initial randomly dispersed states and i) Snapshot and cross‐sectional view of the spherical aggregates in gaseous phase.

Figure 4

FES analysis of Fmoc‐FF self‐assembly. a–c) Fibril‐like aggregation in the solvents at 300 K. d–f) Spherical aggregation in the gaseous phase at 673 K. (a,d) FES of Fmoc‐Fmoc, Fmoc‐Phe, and Phe‐Phe aromatic stackings and (b,e) MC‐MC interactions as a function of centroid distance and angle between two aromatic rings/MCs for (top panel) fibril‐like and (lower panel) spherical aggregates, respectively. (c,f) Cross‐sectional views of the Fmoc‐FF fibril‐like and spherical aggregates, respectively, with Fmoc and Phe groups shown in stick representations.

Figure 5

Fmoc‐FF spheres arraying film shows intrinsic N‐type semiconductivity to develop bio‐organic photocapacitors. a) Contact angle evolution before and after Fmoc‐FF deposition by PVD, showing a significant hydrophobicity enhancement of the substrate upon supramolecular spheres arraying film modification. b) Box diagram representing Young's moduli evolution versus ω of Fmoc‐FF, showing a gradual attenuation of the mechanical rigidity with self‐assemblies ripening. c) (αhν) ^{[ 2 ]} profile versus hv of Fmoc‐FF spheres arraying film derived from the UV–vis spectrum, showing a calculated E_g of 2.65 eV by a linear fitting (R² = 0.99). d) UPS (left panel) and XPS (right panel) spectra of the Fmoc‐FF spheres arraying film, showing E₀ of 19.80 eV and E_vbm of 1.91 eV, respectively, by the intersection between the tangent and base lines. Note that the green and blue lines mark the baselines and tangent lines of the profiles. e) Schematic cartoon depicting the energy band gradient of the Fmoc‐FF/CuPc P‐N heterojunctions. f) Schematic diagram of the Fmoc‐FF/CuPc P‐N heterojunction‐based photocapacitors. The left and right panels show the SEM and photographic images of the device. g,h) Bode plots of (g) impedance and (h) phase frequency response under various irradiation conditions. The inset in (g) shows the equivalent circuit of the photocapacitor. i) Photocurrent curve of the photocapacitor under a pulsed irradiation (5ms, λ = 635 nm at 2.5 mW mm^{− 2}).

Figure 6

Light‐evoked neural stimulation using the bio‐organic arraying‐based photocapacitor. a) Photographic picture showing the cultured HT22 cells on the Fmoc‐FF/CuPc P‐N heterojunction‐based device during recording by patch clamping. b) Current traces recorded under “with cells” and “without cells” conditions. The red‐highlighted region indicates light stimulation (5 ms, λ = 635 nm, 2.5 mW mm^{− 2}). c) Peak inward current versus light intensity in the “with cells” condition.