Near-infrared excitation of nitrogen-doped ultrananocrystalline diamond photoelectrodes in saline solution
Graphical abstract
Introduction
The use of light-based techniques for neural stimulation is an area of growing interest, with potential applications in the treatment of neurological disorders and in implantable bionic devices [[1], [2], [3], [4]]. In particular, optically-driven electrodes have the potential to offer wireless stimulation with much greater spatial resolution than conventional electrically-driven electrodes [2]. This approach relies on the transduction of light into electrical signals in order to stimulate neural tissue, caused by the separation of photo-excited charge carriers in a semiconducting electrode [2]. Materials such as photoconductive silicon [[5], [6], [7], [8]], conductive polymers [[9], [10], [11]], and quantum dots [[12], [13], [14]] have been extensively studied for this purpose. However, these photoactive surfaces have often been found to exhibit limited biostability or produce cytotoxic reactions [[15], [16], [17], [18], [19], [20]].
Diamond is a material with the potential to address these issues, due to its well-known durability and biocompatibility [[21], [22], [23], [24], [25]]. Single crystal diamond is a wide-gap semiconductor with an intrinsic photoresponse band at unsafe ultraviolet frequencies and hence is not useful for neural stimulation applications. In contrast, nitrogen-doped ultrananocrystalline diamond (N-UNCD) is highly conductive due to the presence of sp2 bonded carbon at the diamond grain boundaries, and has been shown to exhibit a photoresponse at much longer wavelengths [26,27], making it a material well-suited for photostimulation [28]. In addition, the surface chemistry of N-UNCD may be altered to exhibit high electrochemical capacitance, a desirable attribute for neuromodulation electrodes [23,29].
The potential of N-UNCD for use as a photoelectrode material has been previously investigated, with the finding that it exhibits a photoresponse to wavelengths of 450 nm or shorter, meeting the requirements for extracellular and intercellular stimulation within the safe optical exposure limit [28]. In the present study, the feasibility of extending the wavelength range of the photoresponse is investigated. Longer wavelengths have greater optical penetration depth in biological tissue, and reduce the potential for phototoxic effects resulting in higher safe optical exposure limits [19,30]. To test this, the spectral response of N-UNCD is measured and analysed with reference to the known band structure. The effect of the surface chemistry on the electrochemical capacitance and charge transfer mechanisms was also examined. Finally, the capability of this technique to achieve threshold charge injection for the stimulation of neurons is evaluated, taking into account various parameters such as cell type, laser pulse parameters, and the size of the stimulating electrodes.
Section snippets
Experimental
The N-UNCD thin-film samples used in this study were grown in an Iplas microwave plasma-assisted CVD system on polycrystalline diamond (PCD) and nanodiamond-seeded silicon substrates. Films were grown to a thickness of approximately 30 μm. Details of the N-UNCD seeding and deposition processes have been reported elsewhere [23]. Samples underwent further plasma treatment to terminate the surface with either hydrogen or oxygen. The treatment conditions have also been reported in earlier works [23,
Electrochemical properties of N-UNCD
To investigate the suitability of N-UNCD as a photoelectrode material, the electrochemical properties were investigated using cyclic voltammetry and photocurrent measurements for different chemical surface terminations. As shown in Fig. 2(a), the electrochemical capacitance of N-UNCD measured by cyclic voltammetry is highly dependent on the chemical termination of the diamond surface. As-grown and hydrogen terminated N-UNCD samples exhibit similar electrochemical behaviour, with both
Summary and conclusions
In summary, the performance of N-UNCD as an optically-driven electrode for neural stimulation applications was investigated over a range of wavelengths. It was determined that N-UNCD exhibits a sub-bandgap photoresponse which diminishes with increasing wavelength, an effect attributed to transitions between mid-gap defect states associated with the graphitic grain boundary regions. It was also found that oxygen surface termination enhances the photoresponse through a capacitive charge transfer
CRediT authorship contribution statement
Andre Chambers: Formal analysis, Investigation, Methodology, Writing - original draft, Writing - review & editing. Arman Ahnood: Conceptualization, Funding acquisition, Methodology, Writing - original draft, Writing - review & editing, Supervision, Validation, Project administration. Samira Falahatdoost: Methodology. Steve Yianni: Formal analysis, Investigation, Methodology. David Hoxley: Resources. Brett C. Johnson: Resources, Formal analysis. David J. Garrett: Resources. Snjezana
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: S.P. is a shareholder, director and chief technology officer of iBIONICS. S.P. and D.J.G. are directors and shareholders in Carbon Cybernetics. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgements
This work was performed in part at the Australian National Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy, through the La Trobe University Centre for Materials and Surface Science. The authors also wish to acknowledge the technical assistance of Dr. Matias Maturana at Clinical Sciences, Department of Medicine, University of Melbourne, as well as helpful discussions with Alastair Stacey and Nikolai Dontschuk from the School of
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