Graphene Ink Film Based Electrochemical Detector for Paracetamol Analysis

Graphene ink is a commercialized product in the graphene industry with promising potential application in electronic device design. However, the limitation of the graphene ink is its low electronic performance due to the ink preparation protocol. In this work, we proposed a simple post-treatment of graphene ink coating via electrochemical oxidation. The electronic conductivity of the graphene ink coating was enhanced as expected after the treatment. The proposed electrochemical oxidation treatment also exposes the defects of graphene and triggered an electrocatalytic reaction during the sensing of paracetamol (PA). The overpotential of redox is much lower than conventional PA redox potential, which is favorable for avoiding the interference species. Under optimum conditions, the graphene ink-based electrochemical sensor could linearly detect PA from 10 to 500 micro molar (μM), with a limit of detection of 2.7 μM.


Introduction
Conductive graphene ink has been commercialized and widely applied for fabricating electronic devices such as lithium battery [1,2], supercapacitor [3,4], and distributed sensor network [5][6][7][8].Graphene ink possesses desirable properties combination, including chemical internees, environmental sustainability, and good electronic conductivity [9][10][11][12][13][14][15][16][17][18].Compared with other solution-processed conductive inks, such as transparent conductive oxides, the abundance of graphene precursor meets sustained global consumer demands.The continuous decline of production cost of graphene showed more competitive than metal nanoparticles and nanowires based inks.The graphene also shows better competitive chemical and thermal stability than conductive polymer based inks.In order to keep the shelf life of the graphene ink, many works have been devoted using graphene oxide and its derivatives as the ink raw materials due to their well dispersibility compared with that of the graphene sheets [19][20][21][22][23]. Later on, exfoliated graphene sheets with different stabilizer agents also have been mixed to make stable inks [24][25][26].However, these chemically modified graphene inks always suffer from the limitation of conductivity.For example, Shin et al. [27] demonstrated a graphene ink based film with thickness-dependent sheet resistance (R s ) spanning from 10 7 kΩ/sq before post treatment.Wang et al. [28] used Meyer rod coating to fabricate graphene ink film with R s of 20.1 and 1.68 kΩ/sq at 92.6% and 64.6%, respectively.
Carbon-based materials have been widely used for electrode fabrication due to their excellent electric property.Graphene-based electrodes have shown superior performance in terms of electrocatalytic activity and macroscopic scale conductivity than other carbon-based materials [29,30].In comparison with carbon nanotubes, graphene exhibits potential advantages of low cost, high surface area, ease of processing and safety [31].Moreover, the synthesis of graphene does not require the metal catalyst, which provides a good platform to study the electrocatalytic effects of carbon materials.For example, Shuai and co-worker [32] demonstrated a graphene ink-fabricated, screen-printed electrode for sensitive determination of Cd 2+ and Pd 2+ in water.Secor and co-workers proposed a graphene ink-nitrocellulose electrochemical sensor for H 2 O 2 determination [33].Several works have demonstrated that a post treatment of the graphene ink could improve its conductivity through thermal reduction process [12,34,35].More specifically, Arapov et al. [12], Secor et al. [34], and Del et al. [35] enhanced graphene ink films to sheet resistances of 1.4 Ω/cm, 21.4 Ω/cm, and 30 kΩ/cm, respectively.However, these post-treatment methods usually need expensive instruments such as a laser.
In this work, we proposed a simple graphene ink coating post-treatment after the coating process via an electrochemical oxidation.Graphene ink has been firstly coated on a glassy carbon electrode (GCE).Then, the graphene ink modified glassy carbon electrode (GI/GCE) underwent an electrochemical oxidation process at 1 V vs. Ag/AgCl (3 M) for additives oxidation in a phosphoric acid environment.The electric conductivity of the graphene ink was restored after the treatment due to the additives removal.Quite unexpectedly, the electrochemical oxidation process treatment exposes the surface defects of the graphene, which results in an outstanding electrocatalytic performance.Therefore, we used paracetamol (PA), a widely used antipyretic and analgesic drug, as the analyte for evaluating the electrocatalytic activity of the GI/GCE.The influence of the electrochemical oxidation parameters were studied in detail.Under optimization condition, the GI/GCE could linearly detect PA at concentration range from 10 µM to 500 µM, with a limit of detection of 2.7 µM.

Materials and Methods
Graphene ink was purchased from Suzhou Hengqiu Tech Co., Ltd.(Suzhou, China), (containing 5% graphene).Paracetamol (PA) was purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).The 0.1 M standard phosphate buffer solutions were made from K 2 HPO 4 and KH 2 PO 4 by adding 1.0 M KOH aqueous solution.All reagents were of analytical grade, and Millipore Milli-Q water (18 MΩ cm) was used throughout.All electrochemical measurements were performed using a CHI 832 electrochemical workstation with a conventional three-electrode system comprised of platinum wire as auxiliary electrode, a 3 M Ag/AgCl electrode as reference, and a graphene ink-coated glass as working electrode (digital photo of the electrochemical cell was supplied in Figure S1).A four point probe resistance meter (RTS-9, Four Probes Tech.) has been employed for measuring the sheet resistance.
A GCE (3 mm diameter) was polished with alumina-water slurry followed by thorough rinsing with ethanol and water.For the electrode surface modification, 5 µL of graphene ink dispersion (1%) was dropped onto the GCE and dried at room temperature (denoted as GI/GCE).Prior to electrochemical oxidation treatment, the modified electrode was carefully rinsed with water to remove the soluble additives, and dried in an air stream.Then, the graphene ink-modified glassy carbon electrode (GI/GCE) underwent an electrochemical oxidation process at 1 V vs. Ag/AgCl (3 M) for additives oxidation in a phosphoric acid environment.For electrochemical oxidation process, the GI/GCE underwent a potentiostatic treatment in a 0.1 M HNO 3 .The treated GI/GCE was rinsed with water and dried at room temperature for PA determination.
For real sample analysis, paracetamol tablet was first ground to powder.Then, an adequate amount of the PA powder was weighed and dissolved into 50 mL water with the assistance of the sonication.Standard addition protocol has been used for calculating the recovery of the sensor.

Results
The graphene ink was firstly characterized by Raman spectroscopy.As shown in the Figure S2, the large D band indicates the graphene containing surface functional groups and defects; probably, some GO was added.Since the layer number estimated from Raman spectrum is also suitable for CVD prepared graphene, the I 2D /I G ratio deduced from the spectrum only confirmed that the layer number of graphene used in this work is around 10.The electrochemical property of the GI/GCE, after different electrochemical oxidation times, was studied using 5 mM [Fe(CN) 6 ] 3−/4− in 0.1 M KCl.As shown in the Figure 1A, significant difference can be observed in CV profiles of GI/GCE before and after 5 min electrochemical oxidation treatment.The current response toward [Fe(CN) 6 ] 3−/4− redox largely enhanced with a narrower peak to peak splitting, suggesting the graphene ink exhibited a superior electric conductivity after the electrochemical oxidation.We believe the electrochemical oxidation process could partially remove or destroy the stabilizer and restore the intrinsic property of the graphene ink coating.The removal of the stabilizers also increases the surface roughness of the graphene, resulting in a higher electro-active surface area for an electrochemical reaction to take place.We also observed the electronic conductivity restoration was time dependent.As shown in Figure 1A, the peak current continuously raises, along with the electrochemical oxidation time, from 5 min to 20 min.A further increasing of treatment time shows negligible changes.Therefore, a 20 min electrochemical oxidation has been used in this work.We also measured the sheet resistance of the graphene ink coating before and after the electrochemical oxidation process.The pristine graphene ink coating exhibited a sheet resistance of 124.6 Ω/cm.The same coating exhibited a sheet resistance of 88.6 Ω/cm after 20 min electrochemical oxidation, suggesting the partial restoration of electro-conductivity.The morphology of the graphene ink before and after 20 min electrochemical oxidation was observed using SEM.As shown in Figure S3, the graphene ink shows no significant morphology change except a slight increase of roughness.The stability of the graphene ink coating after treatment has been evaluated by 15 excessive scans toward [Fe(CN) 6 ] 3−/4− redox.As shown in Figure S4, the proposed GI/GCE showed good stability.The potential window of the GI/GCE has been studied as well.As shown in Figure S5, the graphene showed a stable performance from −1.5 V to 1.5 V.It is pertinent to note that the working potential plays an important role for treatment performance.As shown in Figure 1B, positive potential shift could result enhanced electronic conductivity of the graphene ink coatings.However, a decreasing of electronic conductivity was observed when the working potential larger than 1 V.It is probably due to the graphene damage or detachment driven by a higher working potential.Therefore, working potential of 1 V has been chosen in this work for graphene ink treatment.Due to the enhancement of the electronic conductivity, the electrochemical-oxidized graphene ink is expected to have a higher current response towards electro-active molecules.In this work, we used PA as model molecule for evaluating the sensing performance of the graphene ink after the proposed treatment.Figure 2A shows the electrochemical behavior of PA on the GI/GCE with different treatments condition.As shown in the figure, bare GCE shows an irreversible oxidation of PA during the CV scan.The CV profile of GI/GCE in PBS without PA showed no noticeable peaks.In contrast, it is pertinent to note that the PA redox could be observed in GI/GCE-20 min, suggesting the graphene could trigger an electrocatalytic reaction of PA molecules.More interestingly, an additional redox peak has been noticed when the graphene ink underwent a post electrochemical oxidation treatment.As shown in the figure, a pair of small redox peaks was located at 0.27 V and 0.21 V, implying additional electro-redox was occurred for analyte.This redox mechanism of PA can be attributed to 1 protons and 1 electrons process (Figure 2B) due to the increases of the heterogeneous rate constant for the electron transfer.Similar phenomena were reported by other studies as well [36][37][38].It is well known that the graphene used for ink preparation was usually produced using top-down exfoliation methods in order to control the cost [39,40].However, the exfoliation technique inevitably introduced many defects into graphene.Although these defects affect the electronic performance of graphene, studies also noted that the outstanding activity of electrocatalysis could be produced [41][42][43].Therefore, the new redox peak observed in PA sensing could be ascribed to the surface electrocatalytic sites exposure during the additives removal.A lower oxidation potential is very important for practical sensing application, because it could avoid many interference species responses at a similar working potential.We further tested the analytic performance of the GI/GCE towards PA sensing.Based on the above investigation, the GI/GCE with 20 min electrochemical oxidation was used for differential pulse voltammetry (DPV).Figure 3 shows the DPV profiles of the GI/GCE towards PA concentration from 10 to 500 µM.It can be seen that a linear relationship between the current response and the PA concentrations was observed in the range from 10 to 500 µM with a correlation coefficient of 0.997.The linear regression equation can be expressed as: I = 29.9390+ 0.05442C (PA) .The limit of detection can be calculcated to be 2.7 µM base on S/N = 3.Table 1 summarized the sensing performance comparision using proposed GI/GCE with other previously reported, carbon-based electrochemical sensors.It is pertinent to note that the proposed GI/GCE shows a comparable sensing performance.Considering that the graphene ink was already commercialized and cheaper than other noble metal-involved electrocatalyst, this electrochemical oxidation treatment shows brightening prospects of graphene ink coatings for sensing application.Moreover, this work demonstrated other 2D material-based inks also could be applied for sensing application [44,45].In order to verify the practical performance of the proposed method, the GI/GCE was used for determining PA in tablet.Woolworth home brand paracetamol tablet (500 mg per tablet) was ground to powder and dissolved into water and followed by filtration for removing solid.Standard addition method was adopted for testing the recovery rate.As shown in Table 2, the detection result showed a great agreement with the value provide by manufactory.The recovery value of the GI/GCE was in the range between 99.40% and 101.48%.The precision of the proposed GI/GCE electrochemical sensor was tested using a GI/GCE for continuous 8 replicate detection of 0.1 mM PA. 4.3% current decreasing was observed in 8th scan compared with that of the original response, suggesting good stability.For temporal stability test, we conducted 5 replicate detections of 0.1 mM PA using a GI/GCE with 2 h interval.A 7.9% current decrease was observed, probably due to the graphene detachment in the drying and wetting process between each detection.The reproducibility of the GI/GCE was studied using 6 individual GI/GCEs for 0.1 mM PA detection.An acceptable RSD of 4.5% was found within 6 determinations.For interference study, three common species including glucose, ascorbic acid, and uric acid were tested.As shown in Figure 4, the current response of PA with the presence of 5 fold interference species shows negligible changes.Based on our previous study [48], the selectivity of the GI/GCE can be further improved by surface functionalization of beta-cyclodextrin due to its high supramolecular recognition and enrichment capability.It was also found that 20 folds of K + , Na + , SO

Conclusions
In this work, we proposed a simple electrochemical oxidation treatment method for restoring the electrochemical performance of the graphene ink.Due to the removal of additives and exposure of the electrocatalytic sites, the proposed GI/GCE has been successfully used for PA determination.Under optimized conditions, the proposed GI/GCE exhibits a linear PA detection range from 10 to 500 µM with a detection limit of 0.27 µM.This simple treatment procedure extended the potential application of graphene ink.It also provides a low cost approach for sensor fabrication.

Figure 2 .
Figure 2. (A) Cyclic voltammograms of 0.2 mM PA at bare GCE and the GI/GCE with different treatment times.(B) Proposed redox reaction mechanism of PA.

Figure 3 .
Figure 3. Differential pulse voltammograms of different concentration of PA at the GI/GCE.
− did not interfere in the determination, suggesting the treatment-deduced electrocatalytic oxidation of PA could effectively avoid the interference.

Figure 4 .
Figure 4. Relative PA oxidation current changes using the GI/GCE in the presence of interference species.

Table 1 .
Comparison of analytical performance between GI/GCE and other carbon material modified electrodes for PA detection.

Table 2 .
Determination of PA in tablet using the proposed GI/GCE electrochemical sensor.