Printed and flexible electronics is a rapidly advancing technology with the potential to develop into a multi-billion-dollar wearable electronics industry.[1] One of the main challenges limiting high volume manufacturing of wearable electronics is the lack of high performance and multifunctional inks compatible with existing direct write technologies such as inkjet printing (IJP). Graphene, a promising nano-material, is hypothesized to be great electrode material for flexible and wearable electrochemical sensors due to its flexible nature and high electrochemical activity at defect sites.[2] Furthermore, due to its high specific surface area, high carrier mobility, and compatibility with large scale manufacturing techniques, graphene has shown extraordinary properties and created tremendous breakthroughs in electronic related applications, particularly when it comes to trace gas and vapor sensing.[3]

To date, inkjet printing of graphene has been well established.[4] However, fully printed electrochemical sensors that offer high sensitivity and are operational under realistic environmental conditions utilizing graphene electrodes has not yet been accomplished. The complexity lies in decoupling the sensing mechanisms of printed graphene structures from environmental factors such as temperature, relative humidity, and radiation.[2] This is also complicated by the interaction of the printed graphene with the underlying substrate. Surface roughness, surface energies, adhesiveness, and chemical reactivity, can greatly affect the quality and properties of the printed structure.[5]

In this study, we investigated electrochemical properties of fully inkjet printed graphene electrodes on flexible substrate for pH and electrolyte sensing wearable devices. Graphene is obtained via solvent assisted exfoliation[6] and characterized via AFM, SEM, TEM, and Raman spectroscopy (Fig. 1a-f). We find the graphene flakes to range from an average thickness of 16nm +/- 15nm and crystal domains with lateral dimensions ranging from ~50 nm to 200 nm. To synthesize our graphene ink, we dispersed the flakes via sonication in a mixture of 85% cyclohexanone and 15% terpineol, which has been shown to be compatible with IJP.[7] This resulted in an ink concentration of 3.5mg/ml as confirmed by UV-Vis-IR absorption spectroscopy. The graphene dispersion was printed into (17 printed layers) of 1cm x 1cm squares with printed silver contact pads and printed SU-8 as a passivation later. These test structures were printed on polyimide substrates using a Dimatix 2850 materials inkjet printer. The tool platen temperature, nozzle diameter, and cartridge temperature were optimized to ensure the dimensions and material deposition were adequate to obtain uniform structures. The graphene printed structures were then annealed at about 390°C for one hour to remove solvents and binders in order to increase electrical conductivity (Fig. 2a, b).

To investigate the electrochemical activity of inkjet printed graphene electrodes, both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were carried out in a 10 mM ferricyanide aqueous solution (1 M KCl as the supporting electrolyte) at room temperature. A Ag/AgCl electrode and a platinum electrode were used as the reference and counter electrode, respectively. Inkjet printing of the electrodes assisted in developing a homogenous graphene layer on the surface, which is a key requirement to obtain an electrode with a good electrochemical reversibility.[8] To evaluate the performance of the electrode we measured the ratio of the anodic and cathodic peaks (I_pa/I_pc) and the peak-to-peak separation potential (ΔE_p). In the CV curve see in (Fig. 2c), clear maxima in the cathodic and anodic currents can be observed, as well the peak separation, suggesting IJP can enable a uniform printed graphene electrode. We then performed pH sensing experiments by coating the printed graphene electrodes with an inkjet printed polyaniline thin film. Potentiometric response of the electrodes was studied over three different pH values (4, 7 and 10). The pH response of our printed and flexible graphene electrodes was comparable to that of a conventional glass electrode.

In conclusion, our results provide new fundamental insights on role of structure-property-performance correlations in printed graphene electrodes for electrochemical sensing. We find that the key parameters important for sensing performance including uniformity of the printed electrodes, response time, stability and reproducibility, can be optimized via inkjet printing of graphene. Such findings have significant implications for miniaturization, reliability, and large-scale manufacturing of flexible and wearable electronics systems for electrochemical based human performance monitoring.

[1] Kenry et al., Microsystems Nanoeng., Sep. 2016.

[2] S. Cinti et al., Biosens. Bioelectron., 2016.

[3] F. Schedin et al., Nat. Photonics, 2016.

[4] J.Heinzl et al., Adv. Electron Phys., 1985

[5] A. Sridhar et al., Mater. Matters, 2009.

[6] J. N. Coleman et al.,Acc. Chem. Res., 2013.

[7] F. Torrisi et al., ACS Nano, 2012.

[8] B. Li, G et al., Biosens. Bioelectron., 2015.