Graphene Oxide Bulk-Modified Screen-Printed Electrodes Provide Beneficial Electroanalytical Sensing Capabilities

We demonstrate a facile methodology for the mass production of graphene oxide (GO) bulk-modified screen-printed electrodes (GO-SPEs) that are economical, highly reproducible and provide analytically useful outputs. Through fabricating GO-SPEs with varying percentage mass incorporations (2.5%, 5%, 7.5% and 10%) of GO, an electrocatalytic effect towards the chosen electroanalytical probes is observed, which increases with greater GO incorporated compared to bare/graphite SPEs. The optimum mass ratio of 10% GO to 90% carbon ink produces an electroanalytical signal towards dopamine (DA) and uric acid (UA) which is ca. ×10 greater in magnitude than that achievable at a bare/unmodified graphite SPE. Furthermore, 10% GO-SPEs exhibit a competitively low limit of detection (3σ) towards DA at ca. 81 nM, which is superior to that of a bare/unmodified graphite SPE at ca. 780 nM. The improved analytical response is attributed to the large number of oxygenated species inhabiting the edge and defect sites of the GO nanosheets, which are able to exhibit electrocatalytic responses towards inner-sphere electrochemical analytes. Our reported methodology is simple, scalable, and cost effective for the fabrication of GO-SPEs that display highly competitive LODs and are of significant interest for use in commercial and medicinal applications.


Electrode Production
The GO incorporated ink formulations described within the main manuscript were printed using the appropriate stencils by a DEK 248 screen-printing machine (DEK, Weymouth, U.K.) [1]. These electrodes have been used extensively in previous studies [2][3][4][5][6]. In their fabrication; first a carbon-graphite ink formulation (product code C2000802P2; Gwent Electronic Materials Ltd., U.K.) was screen-printed onto a polyester (Autostat, 250 μm thickness) flexible film This layer was cured in a fan oven at 60 °C for 30 min. Next, a silver/silver chloride reference electrode was included by screen-printing Ag/AgCl paste (product code C2030812P3; Gwent Electronic Materials Ltd., U.K.) onto the polyester substrates and a second curing step was undertaken where the electrodes were cured at 60 °C for 30 min. Finally, a dielectric paste (product code D2070423D5; Gwent Electronic Materials Ltd., U.K.) was then printed onto the polyester substrate to cover the connections. After a final curing at 60 °C for 30 min the SPEs are ready to be used and were connected via an edge connector to ensure a secure electrical connection [2]. The unmodified SPEs have been reported previously and shown to exhibit a heterogeneous electron transfer rate constant, k o , of ca. 10 -3 cm s -1 , as measured using the [Ru(NH3)6] 3+/2+ outer-sphere redox probe [3]. The GO was incorporated into the bulk of the SPEs on the basis of the weight percent of MP to MI, where MP is the mass of particulate (in this case the GO) and MI is the mass of the ink formulation used in the printing process, i.e. % = (MP / MI ) × 100. The weight percent of MP to MI varied from 2.5, 5, 7.5 and 10%, which resulted four separate inks that could subsequently be individually screen-printed on top of the working SPE electrode (see above) and cured as described earlier (60 °C for 30 min). Note that for the purpose of this work, electrochemical experiments were performed using the working electrode of the SPEs only and external reference and counter electrodes were utilised as detailed earlier to allow a direct comparison between all the electrodes utilised and with prior literature.

Experimental: Physicochemical Characterisation
Transmission electron microscopy (TEM) images were obtained using a 200 kV primary beam under conventional bright-field conditions. The 2D-MoSe2 sample was dispersed onto a holey-carbon film supported on a 300 mesh Cu TEM grid. Raman Spectroscopy was performed using a 'Renishaw InVia' spectrometer equipped with a confocal microscope (×50 objective) and an argon laser (514.3 nm excitation). Measurements were performed at a very low laser power level (0.8 mW) to avoid any heating effects. X-ray diffraction (XRD) was performed using an "X`pert powder PANalytical" model with a copper source of Kα radiation (of 1.54 Å) and Kβ radiation (of 1.39 Å), using a thin sheet of nickel with an absorption edge of 1.49 Å to absorb Kβ radiation. A reflection transmission spinner stage (15 rpm) was implemented to hold the commercially sourced GO nano-powder. The range was set between 5 and 80 2θ in correspondence with literature ranges [9]. Additionally, to ensure well defined peaks an exposure of 50 s per 2θ step was implemented with a size of 0.013°. The x-ray photoelectron spectroscopy (XPS) data was acquired using a bespoke ultra-high vacuum system fitted with a Specs GmbH Focus 500 monochromated Al Kα X-ray source, Specs GmbH Phoibos 150 mm mean radius hemispherical analyser with 9-channeltron detection, and a Specs GmbH FG20 charge neutralising electron gun [4]. Survey spectra were acquired over the binding energy range 1100 -0 eV using a pass energy of 50 eV and high resolution scans were made over the C 1s and O 1s lines using a pass energy of 20 eV. Under these conditions the full width at half maximum of the Ag 3d5/2 reference line is ca. 0.7 eV. In each case, the analysis was an area-average over a region approximately 1.4 mm in diameter on the sample surface, using the 7 mm diameter aperture and lens magnification of ×5. The energy scale of the instrument is calibrated according to ISO 15472, and the intensity scale is calibrated using an in-house method traceable to the UK National Physical Laboratory [5]. Data were quantified using Scofield cross sections corrected for the energy dependencies of the electron attenuation lengths and the instrument transmission [6]. Data interpretation was carried out using CasaXPS software v2.3.16 [7].

Scan Rate Study
Examining a 10% GO-SPE voltammetric response to 1 mM [Ru(NH3)6] 3+ in 0.1 M KCl at varying scan rates. The voltammetric peak height (IP) at each scan rate was monitored (υ) with a plot of peak height versus square-root of the scan rate revealing a linear response (Ip (A) = 2.81 x 10 -4 A/(Vs -1 ) 0.5 ; R 2 = 0.87) indicating a diffusional process; furthermore, as is expected for the case of the semi-infinite diffusion model as governed by the Randles-Ševćik equation, analysis of log Ip versus log n revealed a gradient of 0.4, indicating the absence of thin-layer effects. Figure S4 depicts the typical CVs obtained when utilising an SPE as a working electrode whilst the concentration of DA (in pH 7.4 phosphate buffer solution (PBS)) is increased from 5 μM to 50 μM. It is clear that the anodic oxidation peak obtained at 5 μM has a peak current of 0.29 μA at ca. + 0.198 V. Successive increases in the concentration of DA corresponded to an increase in the peak current as well as a slight anodic shift in the peak potential until at 50 μM they were observed to be 2.74 μA and + 0.257 V respectively.

Uric Acid Electrochemistry
We will now explore the GO-SPEs towards UA. Firstly it was essential to bench mark our system by using a bare/unmodified SPE and examining its ability to detect UA. Figure S5 depicts the typical CVs obtained when utilising an SPE as a working electrode whilst the concentration of UA (in pH 7.4 phosphate buffer solution (PBS)) is increased from 20 μM to 200 μM. It is clear that the anodic oxidation peak obtained at 20 μM has a peak current of 0.53 μA at ca. + 0.35 V. Successive increases in the concentration of DA corresponded to an increase in the peak current as well as a slight anodic shift in the peak potential until at 200 μM they were observed to be 5.17 μA and + 0.388 V respectively. The explanation for this anodic shift can be found within main manuscript.
Taking the 10% GO-SPE as an a representative example of all the GO-SPEs, Figure S3 shows that the oxidation peak current at a 20 μM UA concentration was 8.22 μA, which incrementally increased to 13.70 μA by 200 μM. There was a corresponding anodic shift in the onset potential from + 0.353 to + 0.406 V. It is clearly observable from Figure S3 that as with the 10% GO-SPE all the GO-SPEs display a greater anodic peak current than the bare SPE. This can be associated with the oxygenated species present on GO benefitting the oxygenated electro-catalytic reactions. This is further supported by the observation that the greater percentage incorporation of GO into the GO-SPE the larger the observed anodic peak current. However, as the percentage of GO within the electrode increases from 0 to 10% the activation potential for UA oxidation increases.