Analytical grade pyrrole (98%) was purchased from Sigma-Aldrich (United Kingdom), stored at 0°C and was freshly distilled before use. Sodium dodecyl sulfate (SDS), potassium ferricyanide (K₃ [Fe(CN)₆]), potassium chloride (KCl), paracetamol, dopamine hydrochloride and l-ascorbic acid were also purchased from Sigma-Aldrich (United Kingdom). Phosphate buffered saline (PBS) tablets were purchased from Fisher Scientific (United Kingdom). Carbon fibre (CAS nuber 7,440–44–0) was purchased from Alfa Aesar (United Kingdom).

Cyclic voltammetry (CV) was performed using an eDAQ EA161 potentiostat combined with e-corder 401 and EChem V2.1.16 software. Differential pulse voltammetry (DPV) was performed using a Palmsens4 potentiostat with PS Trace software. A Basi^® Ag/AgCl (3.0 M NaCl internal solution) was used as a reference electrode and a platinum mesh was used as a counter electrode. Raman spectroscopy was performed using a Renishaw inVia confocal Raman microscope with a 785 nm laser. Scanning electron microscopy (SEM) images were obtained using a JEOL United States JSM-7100 F field emission electron microscope combined with Pathfinder 2.5 X-ray microanalysis software for SEM/EDS. X-ray photoelectron spectroscopy (XPS) data was obtained using a ThermoFisher Scientific Theta Probe spectrometer with a monochromated Al K X-ray source. An X-ray spot of ∼400 μm radius was employed in the acquisition of all spectra. Survey spectra were acquired employing a pass energy of 300 eV. High resolution, core level spectra for S2p were acquired with a pass energy of 50 eV.

Graphene Oxide (GO) dispersion was prepared using the modified Hummers method (Marcano et al., 2010; Zhu et al., 2012; Liu et al., 2015). Firstly, a mixture of concentrated H₂SO₄/H₃PO₄ (360:40 ml) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv.) and KMnO₄ (18.0 g, 6 wt. equiv.). The reaction was heated to 55°C and stirred for 14.5 h, then cooled to room temperature, and poured onto ice (∼400 ml) with 30% H₂O₂ (20 ml). The mixture was then stirred for 30 min and centrifuged at 4,400 rpm for 20 min. The precipitate was washed and centrifuged with HCl solution (9:1 water/HCl by volume) twice and then dispersed in DI water and dialyzed for 7 days. The graphene oxide dispersion was finally obtained by probe sonicating (Branson Digital sonifier, 400 W, 38% amplitude) the purified graphite oxide dispersion for 1 h, with a pulse of 2 seconds on and one second off, totally 1.5 h. To prepare the dried GO powder, the prepared GO dispersion (12.3 mg/ml) was then put into Freeze-dryer Alpha 1-4 LDplus (48 h, −55°C) to make dried GO powder.

Dried GO powder was dissolved in an aqueous solution containing 0.1 M PBS solution used as an electrolyte, resulting in an electrodeposition solution at a concentration of 0.5 mg ml⁻¹. Deposition techniques used in this study were potentiostatic deposition (a constant potential applied at −1.2 V vs. Ag/AgCl for 5 min) and CV (a deposition potential range of −1.4 to +1.4 V vs. Ag/AgCl at a scan rate of 50 mV s⁻¹; the number of scans varied from 3 to 20). The electropolymerisation solution containing pyrrole (0.2 M) and SDS (0.2 M) were used to optimise the PPy-rGO sensing layer, performed by five scans CV (an electropolymerised potential range of −0.2 to +1.0 V vs. Ag/AgCl and a scan rate of 50 mV s⁻¹). All of the as-prepared fibre electrodes were washed with deionised water and then dried under ambient conditions before further sensing.

The ECSA (A) of the sensing fibre was measured using CV at different scan rate, and 5 min N₂ purged aqueous solution of 10 mM K₃ [Fe(CN)₆] in 0.1 M KCl supporting electrolyte, and calculated using the Randles - Sevcik equation as follows (Nayak and Shetti, 2016; Shetti et al., 2017): I p = ( 2 . 69 × 10 5 ) n 3 / 2 AD 0 1 / 2 v 1 / 2 C 0 [1] where I_p is the anodic peak current, n refers to the number of electrons transferred during the electrode reaction (n = 1 in this study), D₀ is the diffusion coefficient (7.6 × 10^–6 cm² s⁻¹), ν is the potential scan rate, and C₀ is the concentration of K₃ [Fe(CN)₆].

The rGO-coated and PPy-rGO-coated carbon fibre electrodes were examined in the presence of paracetamol, dopamine and ascorbic acid (an interferents commonly found in the blood). The current response of the fibre electrodes to paracetamol and interferents was tested (Ghanbari and Bonyadi, 2018). Stock solutions 1 mM analytes (drug or interferent) was prepared in 0.1 M PBS solution. The analyte solutions (0–500 µM) used in the electrochemical experiments were prepared by dilution of these stock solutions. CV was conducted with a potential range of −0.6 to +0.9 V vs. Ag/AgCl and a scan rate of 50 mV s⁻¹. For DPV, the following parameters were used: pulse amplitude = 50 mV; pulse width = 40 ms; voltage step = 50 mV, and a scan rate = 10 mV s⁻¹ (potential range uses was −1.0 to +1.0 V vs. Ag/AgCl).

The electrochemical reduction of GO was carried out using a neutral (pH = 7.4) aqueous GO dispersion (0.5 mg ml⁻¹) by either potentiostatic deposition or cyclic voltammetry (CV). For potentiostatic deposition, an electrode potential of −1.2 V (vs Ag/AgCl) was applied for 5 min. During CV, a potential range of −1.4 to +1.4 V (vs Ag/AgCl) was scanned for 5, 10, 15 and 20 cycles to deposit rGO onto carbon fibres (Lin et al., 2019). The electrochemical reduction of GO is described by the equation below (Selvam et al., 2016): GO  +  aH +   +  be -   =  rGO  +  H 2 O [1]

Supplementary Figure S1A shows voltammograms during 20 cycles of GO reduction onto carbon fibres. Two cathodic peaks appeared at approximately −0.45 and 0.2 V (vs Ag/AgCl) and the increase in the peak currents were attributed to the deposition of rGO on the bare carbon fibre surfaces (Selvam et al., 2016). The reduction peak was at a similar potential to that previously reported for the electrochemical reduction of GO in PBS on a glassy carbon electrode (Chen et al., 2011) (approximately 0.30 V vs SCE which is equivalent to ca. 0.35 V vs Ag/AgCl). The reduction peak of the GO layer deposited onto a roughened Au electrode was observed at approximately −0.7 V (vs Ag/AgCl) (Olejnik et al., 2014). PPy was electrochemically deposited using CV onto a subset of rGO-coated carbon fibres (made using 20 CV scans) to give a PPy.SDS coating (main graph in Supplementary Figure S1B). CVs following polymerisation gave an oxidation peak potential at 0.2 V (vs. Ag/AgCl) for the PPy.SDS-coated rGO on carbon fibres, with no clear reduction peaks observed (Supplementary Figure S1B inset). These results compare to an oxidation peak potential of 0.09 V (vs. Ag/AgCl) found for a PPy.KNO₃. SDS film reported by Nikoofard et al. (2014).

The Raman spectra of rGO-coated carbon fibres and PPy‒rGO-coated carbon fibres are shown in Figure 1. Characteristic D and G bands of the carbonaceous materials of bare carbon fibre appear at 1,350 and 1,587 cm⁻¹ respectively. In comparison, the rGO-coated carbon fibres exhibit dominant peaks corresponding to D and G bands in the range of 1,348–1,363 cm⁻¹ and 1,589–1,599 cm⁻¹, respectively (Farajikhah et al., 2019; Shukla et al., 2020). The Raman spectra of potentiostatic deposition of rGO on carbon fibre and three cycles CV rGO on carbon fibre are displayed in Supplementary Figure S2. Potentiostatic deposition of rGO and three cycles CV rGO on carbon fibre also exhibit two dominant peaks corresponding to D and G bands at 1,337 and 1,592 cm⁻¹, and 1,325 and 1,598 cm⁻¹, respectively. The D band is attributed to disordered structural defects that relate to the conversion of sp² hybridised carbon to sp³ hybridised carbon, whereas the G band is assigned to the vibrational mode of E2g phonon of sp² carbon atoms (the stretching of the C=C bond) (Atikah Md Jani et al., 2017; Ramesh et al., 2018; Yang et al., 2018; Zhang et al., 2018). The Raman spectra of all samples show a shift for the D and G bands corresponding to the increase in the electrodeposition scans.

The ratio of D/G bands (I_D/I_G) was measured to be 0.97 for bare carbon fibre. The variation in I_D/I_G intensity with respect to the different electrodeposition conditions was investigated and an increase in the I_D/I_G ratio was found following 5, 10 and 15 cycles of rGO electrodeposition. This corresponds to the formation of new smaller graphitic domains, indicating a reduction of rGO (Ramesh et al., 2018; Farajikhah et al., 2019). In contrast, the I_D/I_G ratio of an rGO-coated carbon fibre made from 20 cycles decreased to 0.89 indicating that oxygen functional groups of GO were removed, which led to a decrease in the I_D/I_G ratio compared to the bare carbon fibre. This suggests that the initial layers of rGO deposited on carbon fibre have more structural defects (rGO character) than the subsequent electrodeposited layers. Johra et al. (2014) suggested that the I_D/I_G ratio decreased during the reduction of graphene oxide compared to GO, due to the repair of defects by recovery of aromatic structures.

Two additional peaks were observed following rGO depostion at approximately 2,700 and 2,900 cm⁻¹, corresponding to the 2D band and (D + G) combination mode, respectively (Antony et al., 2015). The 2D band relates to the number of graphene layers and a broadened band is attributed to rGO consisting of few layers with some defects. The (D + G) combination mode is a second‒order peak derived from the combination of D‒G peak. The electrochemical reduction is accompanied by a lower oxygen content in graphene. The intensities of both peaks increased in the case of graphene compared to those of GO, indicating better graphitisation (Johra et al., 2014).

The Raman spectra of the PPy‒rGO coating at the surface of the fibre electrode (electropolymerised from 5 CV scans in 0.2 M pyrrole and 0.2 M SDS solution onto carbon fibre coated with rGO using 20 CV cycles) are depicted in Figure 1B. The growth of PPy at the surface of the modified rGO electrode is confirmed from the Raman spectra of the PPy‒rGO coated carbon fibre, showing peaks characteristic of PPy (grey line). The C=C stretching mode associated with the polymer backbone overlaps with the G-band of rGO at 1,578 cm⁻¹. The skeletal and antisymmetrical C-N stretching modes are observed at 1,488 cm⁻¹ and 1,382 cm⁻¹, respectively. The peaks at 932 cm⁻¹ and 982 cm⁻¹ are associated with ring distortion, while the peaks at 1,052 cm⁻¹ and 1,079 cm⁻¹ are due to symmetric C-H in-plane bending modes. The ring distortion (932 cm⁻¹) and C-H in-plane bending (1,079 cm⁻¹) modes are related to the PPy bipolaron structure, while the peaks at 982 cm⁻¹ and 1,052 cm⁻¹ are related to the PPy polaron structure (Lynam et al., 2007; Garcia-Torres and Crean, 2018). The Raman spectra show that both polaron and bipolaron charge carriers are present in the PPy‒rGO coated carbon with relative bipolaron to polaron peak intensity ratios I₉₃₂/I₉₈₂ of 0.82 (peak intensities of 9,715 and 11,813 arb.) and I₁₀₇₉/I₁₀₅₂ of 0.8 (peak intensities of 9,131 and 11,409 arb.). This indicates that the polaron charge carriers dominate in these electropolymerised coatings.

The surface morphology is an important parameter of coatings used as electrode materials in sensors. SEM images (Figure 2A) illustrate how the electrodeposited rGO interphases vary with the deposition method and materials used (e.g., PPy-rGO fibre electrode). Bare carbon fibre has a clear and smooth surface. After electrodeposition, rGO has a loose structure on the outer surface of the carbon fibre. The surface morphology of the rGO material deposited potentiostatically and the rGO deposited after 10 CV scans (Supplementary Figure S3) show an area of the carbon fibre that is covered with rGO, while the deposition of rGO (from both 15 and 20 CV scans) shows an increase in the area coated with rGO due to the higher number of CV cycles. PPy-rGO coated carbon fibre has a very different surface structure showing microspheroid particles of PPy grown on the rGO surface. This suggests that the polymerisation of pyrrole occurred on the surface of rGO from the π-π interaction and hydrogen bond between rGO and PPy (Pruna et al., 2020).

Supplementary Figure S4 gives EDS elemental mapping of an SEM image of rGO coated onto carbon fibre from 20 CV scans; composed of C, O, Na, Cl, P and K elements. The C and O mapping suggest the successful formation of rGO onto the carbon fibre. The EDS results presented in Supplementary Table S1 allow comparison of deposition methods and show that carbon and oxygen are the major elements on the rGO electrode surfaces with sodium, chlorine, phosphorous and potassium present due to the PBS solution used. The existence of oxygen indicates the electrodeposition of rGO on the surface of the carbon fibre yarn. EDS results for PPy‒rGO coated on the carbon fibre electrode, show carbon, oxygen, nitrogen, sulphur, sodium and chlorine. An increase in N atomic composition was observed after PPy electropolymerisation with the presence of sulfur and increased O due to the sulfate counterion of PPy.SDS on the surface of the rGO fibre electrode. The EDS results confirm that the surface of the carbon fibre was modified with PPy and rGO.

Figure 2B shows EDS elemental mapping of an SEM image of PPy‒rGO-carbon fibre, composing of C, O, N, S, Na, Cl and P. The C and N mapping suggest the successful formation of PPy onto rGO coated carbon fibre after PPy electropolymerisation. The presence of sulfur shows the incorporation of the sulfate anion dopant in PPy.SDS on the surface of the fibre electrode. EDS elemental mapping results confirm the presence of carbon and oxygen groups of rGO and carbon, nitrogen and sulfur groups of PPy‒rGO which comprise the surface of the fibre electrode.

XPS spectra of rGO coated on carbon fibre (using 20 CV scans; Figure 3A) exhibit two major peaks in the wide survey scan of C1s at 285 eV and O1s at 533 eV. The characteristic C1s peak corresponds to sp² carbon‒carbon bonding, while the O1s is the result of the different sp³ carbon‒oxygen bonding (Shin et al., 2014; Aunkor et al., 2016). The Na1s peak at 1,072 eV and Cl2p peak at 200 eV are the components of the electrolyte PBS solution used during rGO coating. High resolution spectra show characteristic C1s, O1s, Na1s and Cl2p core level peaks for rGO coated on carbon fibre (using 20 CV scans). The C1s peak shows an intense peak at 285 eV, corresponding to carbon atoms in different functional group which are proposed as C=C (sp²), C–C or C–H (sp³), C–O (epoxyl group), C–OH (hydroxyl group) and C=O or O–C=O (carboxyl group) (Shin et al., 2014; Kar et al., 2016). Such functionality, including epoxy, hydroxyl and carboxyl groups are present in graphene oxide and partially remain after incomplete reduction of graphene oxide (Hilder et al., 2011; Krishnamoorthy et al., 2013). The O1s peak represents oxygen in the form of C=O, C–O and O–C=O, due to the addition of oxygenated functional groups present in rGO (Aunkor et al., 2016).

The XPS spectrum of PPy‒rGO coated carbon fibre is shown in Figure 3B and consists of four main peaks: C1s at 285 eV, O1s at 532 eV, N1s at 400 eV and S2p at 169 eV. Carbon and nitrogen elements result from the PPy backbone, whereas the S2p region is due to the SDS dopant anions that are incorporated into PPy during the polymerisation process (Abdul Bashid et al., 2017). The C1s peak from PPy and rGO correspond to C–C and C=C (the chemical reduction of GO to rGO), C–N (the existence of PPy), C–O, C=O, –COO–, C–S (possible bonding structures) (Zheng et al., 2014; Chen et al., 2017). The O1s peak represents oxygen in the form of C–O and COOH (rGO functional groups), C–O–C, C=O and C–OH (the possible occurrence bonding structures) and S–O (sulphur dopant from SDS) (Abdul Bashid et al., 2017). Whereas, the N1s peak is assigned to–NH structures which are from nitrogen in the PPy ring, including –NH⁺– (the polaron state) and =NH⁺– (the bipolaron state) (Chen et al., 2017; El Jaouhari et al., 2017). The S2p peak for PPy-rGO is attributed to the S–O–, SO₂ and C₄H₄S species after the PPy polymerisation (Abdul Bashid et al., 2017).

After confirming using cyclic voltammetry, Raman spectroscopy, SEM-EDX and XPS results that the carbon fibre electrodes were functionalised with rGO and rGO-PPy, experiments to characterise the electrochemical performance of these fibre electrodes were carried out. Figure 4A shows cyclic voltammograms of rGO-coated fibre electrodes in aqueous solutions containing potassium ferricyanide (10 mM) and KCl (0.1 M) at a scan rate of 100 mV s⁻¹. The rGO fibre electrodes (20 CV scans) give larger oxidation peak currents compared to bare carbon fibre electrodes that are shifted to a lower peak potential which indicate faster electron transfer kinetics and improved electrochemical properties (Kumar et al., 2019). Electrochemical surface area (ECSA) measurements were performed the on PPy-rGO coated carbon fibre and Figure 4B shows cyclic voltammograms of PPy-rGO fibre electrodes in aqueous solutions containing potassium ferricyanide. The ECSA of the fibre electrodes was calculated using the Randles - Sevcik equation. Figure 4C shows how the ECSA of the bare carbon fibre increased with rGO coating, likely due to the nanostructure imparted by the reduction of rGO, while retaining the original carbon fibril surface as shown in SEM results. The potentiostatic deposition rGO on carbon fibres were found to have the lowest ECSA value compared to rGO deposition on carbon fibres using CV. The ECSA value increased with increasing numbers of CV scans during rGO deposition. rGO deposited using 10, 15 and 20 scans gave a 2-fold, 3-fold and 3.5-fold increase in ECSA value compared to the bare carbon fibre, respectively. The PPy-rGO coating gave a 7-fold increase in ECSA value compared to the bare carbon fibre. The polymerisation of pyrrole occurred on the rGO surface owing to the π-π interaction and hydrogen bond between rGO and pyrrole. These interactions enhance the ECSA of the fibre electrode which gave the PPy-rGO fibre electrode a 2-fold increase in ECSA value compared to the 20 CV scans rGO carbon fibre electrode (Atikah Md Jani et al., 2017).

The electrochemical response of the rGO fibre electrode in the presence of paracetamol (as model drug) was investigated initially by cyclic voltammetry. The number of cycles for deposition by CV was varied from 3 to 20 to determine the effect this had on the ability of rGO fibre electrodes to sense paracetamol and interferents. As expected, increasing the number of cycles during deposition resulted in more rGO being reduced on the fibre surface, increasing the rGO domain on fibre electrodes. Cyclic voltammograms for rGO coated carbon fibres in the presence of 0–500 µM paracetamol in 0.1 M PBS were recorded (Figure 5). At neutral pH, paracetamol is oxidised to N-acetyl-p-quinoneimine (Özcan and Şahin, 2007). Increasing oxidation currents of paracetamol and a cathodic shift in oxidation peak potential were observed with increasing cycles for rGO deposition (3–20 deposition scans). CVs of rGO carbon fibre (15 cycles) give an oxidation peak of paracetamol at 0.54 V (vs Ag/AgCl), and the reduction peak was found at 0.03 V (vs Ag/AgCl) as shown in Supplementary Figure S5A. The oxidation peak potential of paracetamol is shifted compared to that previously reported using rGO on a glassy carbon electrode at 0.63 V (vs Ag/AgCl) (Ghanbari and Bonyadi, 2018). The oxidation peak current measured at the rGO-carbon fibre electrode (15 cycles) versus the concentration of paracetamol is presented in Supplementary Figure S5B, with a linear range between 50–500 µM (R ² = 0.9901) and a sensitivity of 0.32 μA cm⁻² μmol⁻¹ L.

Well-defined redox peaks of paracetamol with increased peak currents were observed at rGO on carbon fibre deposited after 20 cycles as displayed in Figure 5A, suggesting a better electron transfer rate at the electrode surface compared to the 15 cycle equivalent (Palakollu et al., 2020). The oxidation and reduction peaks of paracetamol were observed at 0.49 and 0.11 V (vs Ag/AgCl), respectively. Figure 5B shows a calibration curve of the oxidation peak current measured at the rGO-carbon fibre electrode (20 cycles) versus the concentration of paracetamol with a linear region in the concentration range of 50–500 µM (R ² = 0.9961) and an improved sensitivity of 0.44 μA cm⁻² μmol⁻¹ L. The limits of detection for paracetamol of 15, and 20 CV scans of rGO-coated carbon fibre were 19.0 µM, and 17.5 µM (S/N = 3), respectively.

Differential pulse voltammetry (DPV) was examined in an effort to increase the fibre electrode sensitivity by reducing background capacitance observed during CV. Again an increase in peak current and a negative shift of the peak potential was found with increasing rGO deposition (Figure 5F). Figure S5c shows DPV detection of 10–500 µM paracetamol concentrations at the rGO-coated carbon fibre (15 cycles). Anodic current peaks were centred at 0.5 V (vs Ag/AgCl) and a linear correlation to paracetamol concentration was observed between 50–500 µM (R ² = 0.9946), as shown in Supplementary Figure S5D. In comparison, the 20-cycle rGO-coated carbon fibre (Figure 5C), shows a negative shift in the anodic peak of paracetamol at 0.45 V (vs Ag/AgCl). The corresponding plot of peak current vs paracetamol concentration shows two linear ranges between 20–100 µM (R ² = 0.9965) and 100–500 µM (R ² = 0.9933) as shown in Figure 5D. The limits of detection for paracetamol of 15 and 20 CV scans of rGO-coated carbon fibre were 38.6 and 21.1 µM (S/N = 3), respectively. The sensitivity of the calibration (20 cycles) is 0.3 μA cm⁻² μmol⁻¹ L, which is a slight improvement over the 15-cycle equivalent at 0.2 μA cm⁻² μmol⁻¹ L. These results demonstrate that the rGO coating on a carbon yarn electrode, after 20 deposition cycles, enhances the electro-oxidation of paracetamol.

Dopamine is a common interferent in biological fluids when measuring paracetamol (Farajikhah et al., 2019). As shown in Figure 6A, cyclic voltammograms of rGO carbon fibre (20 cycles) show an oxidation peak for dopamine at 0.25 V (vs Ag/AgCl), with two reduction peaks observed at −0.25 and 0.10 V (vs Ag/AgCl), respectively. The oxidised o-dopaminoquinone undergoes intramolecular cyclisation to form leucodopaminochrome, which exhibits two cathodic peaks related to the reduction of o-dopaminoquinone to dopamine and leucodopaminochrome to dopaminochrome (Daniel Arulraj et al., 2016). The oxidation peak current increased linearly with dopamine concentration over the concentration ranges of 0–50 µM with a correlation coefficient of 0.9927 and 50–500 µM with a correlation coefficient of 0.9955 (Figure 6B) and the detection limit (3σ) is 5.1 µM (S/N = 3) which is lower than that of paracetamol sensing. DPV responses of dopamine concentrations from 0–500 μM at the rGO carbon fibre (20 cycles) are displayed in Supplementary Figure S6A. Anodic peaks were centred at 0.2 V (vs. Ag/AgCl) and the peak current increased with increasing dopamine concentration. The corresponding calibration plot between the oxidation peak current and concentration of dopamine again shows two linear correlations with dopamine concentration observed between 0–50 µM (R ² = 0.9866) and 100–500 µM (R ² = 0.9930), as illustrated in Figure 6C. The detection limit (3σ S/N) of paracetamol with DPV was 9.0 µM, which is higher compared to CV detection. In addition, CV analysis gave a broader linear range for dopamine analysis with more sensitive detection (0.22 μA cm⁻² μmol⁻¹ L) compared to the detection using DPV (0.09 μA cm⁻² μmol⁻¹ L).

Generally, dopamine and ascorbic acid coexist as electro-active interferents in the electrochemical detection of paracetamol. Interestingly, when the rGO carbon fibre electrodes were studied in a solution containing both dopamine and ascorbic acid, only a response to dopamine was observed (results not shown). Good selectivity of paracetamol over interferents is demonstrated using DPV in Figure 6D, where simultaneous detection of 100 µM paracetamol in the presence of 100 µM dopamine (blue line) is shown. Two clearly distinguished anodic peaks are observed, with the oxidation peak of paracetamol at 0.49 V (vs. Ag/AgCl) separated from the oxidation peak of dopamine at 0.25 V (vs. Ag/AgCl), while ascorbic acid is not detected (yellow line). The sensitivity for paracetamol (calculated using DPV response) is higher at 0.3 μA cm⁻² μmol⁻¹ L compared to 0.09 μA cm⁻² μmol⁻¹ L for dopamine. These results indicate that the simultaneous determination of paracetamol and dopamine is possible at the rGO-coated carbon fibre electrode.

Polypyrrole has been shown to enhance the performance of carbon yarn electrodes towards sensing of paracetamol. PPy‒rGO coated carbon yarns were developed by electropolymerisation of PPy onto rGO-coated carbon fibre to further enhance the electrochemical surface area and therefore the performance of the fibre electrode. CV analysis using a PPy-rGO carbon fibre electrode (PPy deposited using 5 CV cycles from a 0.2 M pyrrole and 0.2 M dopant solution onto 20-scan-rGO fibre), gave an oxidation peak for paracetamol at 0.55 V vs. Ag/AgCl with the reduction peak obscured due to the background capacitance of PPy (Figure 7A). The oxidation peak was at a similar potential to that previously observed with PPy coated onto reduced graphene oxide/glassy carbon electrodes (0.57 V vs. Ag/AgCl) (Ghanbari and Bonyadi, 2018). A calibration curve of the oxidation peak current versus paracetamol concentration (0–500 µM) gives two linear regions in the range of 10–50 µM with a correlation coefficient of 0.9999 and 50–500 µM with a correlation coefficient of 0.9924 (Figure 7B). The detection limit (3σ S/N) is 9.1 µM. Figure 7C shows DPV detection of paracetamol at the PPy-rGO fibre electrode and the anodic peak current appeared at 0.45 V vs. Ag/AgCl. Two different linear correlations with paracetamol concentration were observed: the first between 0–50 µM (R ² = 0.9960) and the second between 100–500 µM (R ² = 0.9862) as shown in Figure 7D. Using DPV and the PPy-coated rGO carbon yarns, the detection limit of paracetamol sensing was notably improved (3σ S/N) to 3.7 µM. Table 1 lists a series of published electrochemical sensors based on rGO and PPy for the detection of various important drugs and compares them to the performance of the PPy-coated rGO carbon yarn reported here. Electrode material, synthesis approach, analyte, determination technique and LOD are given as well as the reference where the information is available.

Figure 8A shows cyclic voltammetric responses of dopamine (0–500 µM) at the PPy-rGO carbon fibre electrode, the oxidation peak of dopamine was found at 0.2 V (vs Ag/AgCl). The oxidation peak current increased linearly with dopamine concentration over the concentration ranges of 10–50 µM with a correlation coefficient of 0.9891 and 100–500 µM with a correlation coefficient of 0.9915 (Figure 8B). The detection limit is 6.7 µM (S/N = 3) which was lower than that of paracetamol sensing. DPV analysis of dopamine was also carried out at the PPy-rGO coated carbon fibre electrode (results not shown). The corresponding calibration plot between the oxidation peak current and concentration of dopamine shows two linear correlations with dopamine concentration observed between 2.5–50 µM (R ² = 0.9901) and 100–500 µM (R ² = 0.9974), as illustrated in Figure 8C. The detection limit (3σ S/N) of dopamine was 6.0 µM which was similar to the detection limit for dopamine sensing using CV. Table 2 shows a comparison of the limit of detection of various electrodes described in this study obtained using CV, DPV and amperometry for paracetamol and dopamine sensing.

Figure 8D shows good selectivity of paracetamol over dopamine and ascorbic acid using DPV, where two clearly distinguished oxidation peaks are observed during simultaneous detection of paracetamol (100 µM) and dopamine (100 μM; blue line). In addition, two anodic peaks were observed towards paracetamol (100 µM) and dopamine (100 µM) in the presence of ascorbic acid (100 μM; yellow line). These results indicate that the proposed PPy-rGO coated carbon fibre electrode enabled the simultaneous determination of paracetamol and dopamine.

The amperometric response of the rGO carbon yarn (20 cycles) to paracetamol (0–500 μM) was investigated at an applied potential of 0.6 V (vs. Ag/AgCl), as depicted in Figure 9A. The amperometric current shows a step increase after the addition of increasing concentrations of paracetamol. A linear relationship between the oxidation current and paracetamol concentration is obtained for concentrations ranging from 50 to 500 μM of paracetamol with a detection limit (3σ S/N) of 63.0 µM (Figure 9B). The PPy‒rGO coated carbon yarn shows a step increase with successive addition of 0–500 μM paracetamol (Figure 9C). From the calibration curve shown in Figure 9D, there is a linear region over a concentration range of 35–500 μM (R ² = 0.9914). The detection limit for the PPy‒rGO coated carbon fibre electrode (3σ S/N) is 48.0 μM.