Advanced 3D-Printed Flexible Composite Electrodes\nof Diamond, Carbon Nanotubes, and Thermoplastic Polyurethane

In this work, we\n pioneered the preparation of diamond-containing\nflexible electrodes using 3D printing technology. The herein developed\nprocedure involves a unique integration of boron-doped diamond (BDD)\nmicroparticles and multi-walled carbon nanotubes (CNTs) within a flexible\npolymer, thermoplastic polyurethane (TPU). Initially, the process\nfor the preparation of homogeneous filaments with optimal printability\nwas addressed, leading to the development of two TPU/CNT/BDD composite\nelectrodes with different CNT:BDD weight ratios (1:1 and 1:2), which\nwere benchmarked against a TPU/CNT electrode. Scanning electron microscopy\nrevealed a uniform distribution of conductive fillers within the composite\nmaterials with no signs of clustering or aggregation. Notably, increasing\nthe proportion of BDD particles led to a 10-fold improvement in conductivity,\nfrom 0.12 S m<sup>–1</sup> for TPU/CNT to 1.2 S m<sup>–1</sup> for TPU/CNT/BDD (1:2). Cyclic voltammetry of the inorganic redox\nmarkers, [Ru(NH<sub>3</sub>)<sub>6</sub>]<sup>3+/2+</sup> and [Fe(CN)<sub>6</sub>]<sup>3–/4–</sup>, also revealed a reduction\nin peak-to-peak separation (Δ<i>E</i><sub>p</sub>)\nwith a higher BDD content, indicating enhanced electron transfer kinetics.\nThis was further confirmed by the highest apparent heterogeneous electron\ntransfer rate constants (<i>k</i><sup>0</sup><sub>app</sub>) of 1 × 10<sup>–3</sup> cm s<sup>–1</sup> obtained\nfor both markers for the TPU/CNT/BDD (1:2) electrode. Additionally,\nthe functionality of the flexible TPU/CNT/BDD electrodes was successfully\nvalidated by the electrochemical detection of dopamine, a complex\norganic molecule, at millimolar concentrations by using differential\npulse voltammetry. This proof-of-concept may accelerate development\nof highly desirable diamond-based flexible devices with customizable\ngeometries and dimensions and pave the way for various applications\nwhere flexibility is mandated, such as neuroscience, biomedical fields,\nhealth, and food monitoring.