MnO2 Nanoflower Integrated Optoelectronic Biointerfaces for Photostimulation of Neurons

Abstract Optoelectronic biointerfaces have gained significant interest for wireless and electrical control of neurons. Three–dimentional (3D) pseudocapacitive nanomaterials with large surface areas and interconnected porous structures have great potential for optoelectronic biointerfaces that can fulfill the requirement of high electrode‐electrolyte capacitance to effectively transduce light into stimulating ionic currents. In this study, the integration of 3D manganese dioxide (MnO2) nanoflowers into flexible optoelectronic biointerfaces for safe and efficient photostimulation of neurons is demonstrated. MnO2 nanoflowers are grown via chemical bath deposition on the return electrode, which has a MnO2 seed layer deposited via cyclic voltammetry. They facilitate a high interfacial capacitance (larger than 10 mF cm−2) and photogenerated charge density (over 20 µC cm−2) under low light intensity (1 mW mm−2). MnO2 nanoflowers induce safe capacitive currents with reversible Faradaic reactions and do not cause any toxicity on hippocampal neurons in vitro, making them a promising material for biointerfacing with electrogenic cells. Patch‐clamp electrophysiology is recorded in the whole‐cell configuration of hippocampal neurons, and the optoelectronic biointerfaces trigger repetitive and rapid firing of action potentials in response to light pulse trains. This study points out the potential of electrochemically‐deposited 3D pseudocapacitive nanomaterials as a robust building block for optoelectronic control of neurons.

For optimization we varied the concentration of the coating solution of the photoactive layer while keeping the spin speed fixed at 1500 rpm (Figure S1).The increase of concentration directly affects the conversion of light to ionic currents and after the concentration of 60 mg ml -1 the photocurrent starts to drop possibly due to the limited electron and hole diffusion in thicker layers.According to Gaussian distribution, the mean is 279 nm, and the standard deviation is 169 nm.
Using our deposition technique, we observed nanoflowers of different sizes, and the average size is found to be 279 nm with a standard deviation of 169 nm (Figure S3).According to the XPS survey spectrum (Figure S4a), there are 3 main elements, which are Mn, O, and C. The peaks at 495-486 eV, and 451.9-444.8eV are attributed to the ITO substrate, which has Sn 3d and In 3d peaks.Figure S4b demonstrates the Mn 2p spectra of the manganese oxide flowers.Two signals are detected at around 641.9 and 653.6 eV due to the spin-orbit coupling [1], which can be ascribed to the binding energies of Mn 2p3/2 and Mn 2p1/2, respectively [2].The energy difference between these signals is 11.7 eV, consistent with the previously reported data for Mn 2p3/2 and Mn 2p1/2 in MnO2 [3].Moreover, we performed XPS analysis on the Mn 3s region.In this analysis, we observed two distinct peaks at energy levels of 84.4 eV and 88.9 eV.The multiplet splitting, measured at 4.5 eV (Figure S4c), suggests the predominant presence of Mn (IV) oxidation states, in agreement with the previous literature [4] that indicates the existence of MnO2.The high-resolution O 1s spectrum deconvoluted into two peaks, 529.6 and 531.1 eV, using the XPS best peak fitting with Gaussian modes (Figure S4d).These peaks can be attributed to Mn-O-Mn and Mn-O-H bonding according to the literature, respectively [5].Also, the high-resolution C 1s spectrum of MnO2 is shown in Figure S4.

Figure S2 .
Figure S2.The effect of layers on the capacitance of various return electrodes.

Figure S3 .
Figure S3.MnO2 nanoflowers size distribution histograms obtained from SEM images (n>2000).According to Gaussian distribution, the mean is 279 nm, and the standard deviation is 169 nm.
e C-C, C-O, and C=C peaks are deconvoluted at 284.5, 286.1, and 287.9 eV with the best fitting of Gaussian modes.During this analysis, all the peaks were adjusted based on the C 1s standard peak of 284.5 eV.

Figure S5 .
Figure S5.The current density of only MnO2 return structure under the dark and light.The blue square represents the light pulse.

Figure S7 .
Figure S7.Current clamp recording trace showing the capacitive stimulation artifacts at the light onset and offset.Blue bars show the 20 ms 'light on' period.The artifacts are eliminated via downsampling and smoothing, which remains the characteristics of the action potential, such as threshold voltage, peak magnitude, and latency intact.

Figure S8 .
Figure S8.Impedance of MnO2 seed, MnO2 NFs and RuO2 return in the frequency range of 1Hz -10 kHz

Figure S9 .
Figure S9.The SEM images of MnO2 NFs coated on a) FTO, b) gold, and c) stainless steel substrates.

Table S1 .
Photocurrent, charge density, excitation wavelength, light intensity and optical pulse duration in previous reports.

Table S2 .
Deposition parameters of MnO2 nanoflowers on different substrates.