Development and characterization of a diamond-insulated graphitic multi electrode array realized with ion beam lithography

Abstract

The detection of quantal exocytic events from neurons and neuroendocrine cells is a challenging task in neuroscience. One of the most promising platforms for the development of a new generation of biosensors is diamond, due to its biocompatibility, transparency and chemical inertness. Moreover, the electrical properties of diamond can be turned from a perfect insulator into a conductive material (resistivity ~mΩ·cm) by exploiting the metastable nature of this allotropic form of carbon. A 16‑channels MEA (Multi Electrode Array) suitable for cell culture growing has been fabricated by means of ion implantation. A focused 1.2 MeV He+ beam was scanned on a IIa single-crystal diamond sample (4.5 × 4.5 × 0.5 mm3) to cause highly damaged sub-superficial structures that were defined with micrometric spatial resolution. After implantation, the sample was annealed. This process provides the conversion of the sub-superficial highly damaged regions to a graphitic phase embedded in a highly insulating diamond matrix. Thanks to a three-dimensional masking technique, the endpoints of the sub-superficial channels emerge in contact with the sample surface, therefore being available as sensing electrodes. Cyclic voltammetry and amperometry measurements of solutions with increasing concentrations of adrenaline were performed to characterize the biosensor sensitivity. The reported results demonstrate that this new type of biosensor is suitable for in vitro detection of catecholamine release.

Figure 1.

Left: SRIM evaluation of vacancy density profiles generated in diamond by 1.2 MeV He⁺ irradiation after crossing a copper mask with increasing thicknesses. The graphitization threshold (9 × 10²² cm⁻³) is indicated with a dashed line. Right: schematics of the variable-thickness mask configuration, which allows the definition of the sub-superficial highly damaged layer at variable depth.

Figure 2.

(a) Top-view optical micrograph of the diamond-based biosensor: the 16 graphitic channels are ∼20 μm wide, ∼1 mm long, ∼250 nm thick, with sensing areas of ∼20 × 5 μm². The diamond substrate is mounted and wire-bonded on the chip carrier; (b) zoom optical micrograph of the central part of the sensor.

Figure 3.

(a) 3D schematics of the diamond-based biosensor configuration during I-V measurements; (b) I-V characteristics of the 16 graphitic channels.

Figure 4.

Cyclic voltammetric scans at 20 mV·s⁻¹ rate of a 0−1.2 V voltage applied to four representative sensing microelectrodes (channels #1, #4, #7, #8) with respect to the quasi-reference Ag/AgCl electrode in the presence of Tyrode buffer (a) and Tyrode spiked with: 10 μM adrenaline (b); 50 μM adrenaline (c); 100 μM adrenaline (d); 1 mM adrenaline (e) and 100 μM dopamine (f).

Figure 5.

(a) Amperometric chronograms in correspondence of the reported adrenaline concentrations, polarized to +800 mV with respect to the quasi-reference electrode. For each adrenaline concentration, the acquisition lasted 60 s, while in the graph only 1 s of the recording is reported. Consecutive recordings using increasing concentrations of adrenaline were separated by time breaks, in order to properly remove the previous solution and wash the device; (b) average amperometric signals resulting from 60 s time averages of the data reported in Figure 5a as a function of adrenaline concentration; experimental data and relevant standard error values are shown respectively in circular dots and error bars, while the linear fit is reported in continuous line.