Promising post-consumer PET-derived activated carbon electrode material for non-enzymatic electrochemical determination of carbofuran hydrolysate

In this work, activated carbon (AC) materials, prepared from polyethylene terephthalate (PET) waste bottles were used as the sensing platform for the indirect detection of carbofuran. The morphology and surface properties of the PET-derived AC (PET-AC) were characterized by N2 adsorption/desorption isotherm, X-ray diffraction (XRD), field-emission scanning/transmission electron microscopy (FE-SEM/TEM) and Raman spectroscopy. The electrochemical activity of the PET-AC modified glassy carbon electrode (GCE) (PET-AC/GCE) was measured by cyclic voltammetry and amperometry. The enhanced surface area and desirable porosities of PET-AC are attributed for the superior electrocatalytic activity on the detection of carbofuran phenol, where, the proposed sensor shows low detection limit (0.03 µM) and remarkable sensitivity (0.11 µA µM−1 cm−2). The PET-AC/GCE holds high selectivity towards potentially interfering species. It also provides desirable stability, repeatability and reproducibility on detection of carbofuran phenol. Furthermore, the proposed sensor is utilized for the detection of carbofuran phenol in real sample applications. The above mentioned unique properties and desirable electrochemical performances suggest that the PET-derived AC is the most suitable carbonaceous materials for cost-effective and non-enzymatic electrochemical sensor.

However, serious drawbacks associated with enzymes such as poor stability, complicated immobilization procedures, critical operational conditions, and difficulties in handling and storing make these systems more difficult to work. Therefore, simple enzyme-free electrochemical sensor is highly desirable to alleviate the drawbacks of enzyme-based one. Several non-enzymatic sensors for carbofuran detection include cobalt oxide-reduced graphene oxide modified glassy carbon electrode 12 , hemin and nickel-graphene oxide modified carbon paste electrode 13 , screen-printed carbon electrodes modified with gold nanoparticles and graphene oxide 14 and disposable screen-printed carbon electrode 15 . Literature studies prove that the preparations of these above materials involve complicated procedures and need complex calibration. Moreover, it cannot be utilized for on-field applications. In recent times, activated carbon (AC) has become an interesting catalytic material as electrochemical sensor due to their exclusive properties, such as increased surface area, well-developed porosity, exceptional electrical conductivity, good mechanical property and chemical stability [16][17][18][19][20] . Fascinatingly, the method of preparation for AC is simple, more straightforward, low cost and environment friendly when compared to the other carbon based materials.
Plastics create considerable amount of solid waste in the world on account of their application in many areas such as building, packaging, automotive, electric and electronics. Since they possess high decomposition temperature, enough resistance to ultraviolet radiation and are mostly not biodegradable, they can remain on both land and sea for several years causing environmental pollution. Hence, post utilization these plastics become waste and recovery of this ecologically hazardous waste should be taken into account instead of being left freely in nature 21 . In particular, polyethylene terephthalate (PET) bottles, being lighter, more durable and less bulky than many alternative materials find significance in the plastic industry sector. Single-use PET bottles have a short service life and therefore turn into residential (post-consumer) plastic waste in a short period of time. As a result, it would be worthwhile to find out new application areas for PET bottle wastes to maximize their end-of service life management effectively 22 . The present work deals with conversion of this waste product in to activated carbon material for the fabrication of electrochemical sensor platform In this work, for the first time, an electrochemical sensor for indirect determination of carbofuran was fabricated based on PET derived activated carbon (PET-AC) modified glassy carbon electrode (GCE) (PET-AC/GCE). The AC had been prepared by the chemical activation of post-consumer PET bottles with potassium hydroxide (KOH). The morphology and surface properties of PET-AC were investigated by N 2 adsorption/desorption isotherm, X-ray diffraction (XRD), field-emission scanning/transmission electron microscopy (FE-SEM/TEM) and Raman spectroscopy. Cyclic voltammetry and amperometry were used for studying the electrochemical properties of the prepared PET-AC/GCE. The experimental results suggest that PET-AC/GCE not only exhibits good selectivity, repeatability and reproducibility but also shows excellent stability for the detection of carbofuran phenol. The PET-AC/GCE provides great sensitivity and low limit of detection as an amperometric sensor, which is comparable or even superior to the results reported in the literature. The sensor also offers a noteworthy performance in the analysis of real sample.

Results and Discussion
Structural and textural properties of ACs. The microstructures of the raw PET and KOH treated AC are shown in Fig. 1a and b. Unlike raw-PET which shows the homogeneous and, smooth surface, KOH treated PET-AC clearly exhibits several pores with different sizes and shapes. The major surface deterioration occurs with the PET-AC owing to the discharge of volatile compounds 23 . Additional FE-TEM images of the PET-AC in Fig. 1c and d display a multi-dimensional wormhole-like pore structure 24,25 . Figure 2a shows N 2 adsorption-desorption isotherms of ACs prepared at different carbonization temperatures. It is apparent that the AC prepared at low-temperature exhibits type I isotherm with substantial increase in adsorption of adsorbate below the relative pressure (P/P 0 < 0.1), and a long plateau at high relative pressures, indicating the presence of microporous structure. An increase in carbonization temperature provokes an increase in the amount of N 2 adsorbed at low relative pressure together with distinct hysteresis loop of H4 type for capillary condensation at high pressure revealing the simultaneous presence of micro and mesopores 26,27 . Thus, the isotherms belong to a mixed type in the IUPAC classification, combination of type I and type IV. Moreover, increase of the temperature from 900 to 1000 °C enhances the release of volatile matters from precursor, leading to the creation of new pores as well as widening of existing pores resulting in increment of both surface area and pore volume ( Table 1). This effect is associated with the enlargement of both micropores and mesopores. However, further increase of the activation temperature to 1100 °C results in slight reduction in both surface area and pore volume. This may be due to the degradation of both pore structure and structural integrity of the activated carbon at high temperature. In general, the known chemistry of KOH activation and the development of porosity involve a chain of reactions which include dehydration, water-gas reaction, water-gas shift reaction, reduction and carbonate formation. During carbonization, KOH dehydrates to form K 2 O at around 400 °C, which further reacts with CO 2 to form K 2 CO 3 . The as-formed K 2 CO 3 is decomposed into K 2 O and CO 2 at temperature above 700 °C. The K 2 O is reduced by carbon to produce metallic potassium at temperature over 700 °C.
Liberated potassium metal is intercalated to the carbon matrix. Elimination of this intercalated metallic potassium by washing leads to the formation of pore structures 28,29 . The pore size distribution (PSDs) derived from DFT method is depicted in Fig. 2b. It is clear that with the exception of PET-AC-900 shows a micropore distribution; the other PET-AC samples reveal distribution of micro and mesopore sizes. Figure 2c depicts the XRD spectrum of the as-prepared PET-AC-1000. It exhibits two broadened diffraction peaks centered at 24° and 43°, corresponding to the (0 0 2) and (1 0 0) planes, respectively. These peaks are not very intense, but well defined, indicating the negligible ordered crystalline phase. It is due to the collapse of the ordered frameworks with increase in the degree of heat treatment. These results further confirm the existence of an amorphous structure in the prepared carbon 29,30 . Figure 2d shows the Raman spectra of PET-AC taken at different temperatures. The board peak obtained at 1340 cm −1 corresponds to defect induced band (D) which could be concomitant to in-plane substitution as well as breaking of sp 2 symmetry. Similarly, the first order scattering of in-plane vibration of graphitic band is observed at 1582 cm −1 . The ID/IG ratio directly relates the index of turbostratic disorder, where in the range of 1.01 to 1.03 associates with 900 to 1100 cm −1 respectively. Further, as the  Fig. 3a and the inset shows the background voltamograms of the two electrodes. In the absence of carbofuran, PET-AC/GCE shows high background current due to the presence of non-faradaic process, when compared to bare GCE. On the other hand, PET-AC/GCE exhibits sharp and well defined anodic peak at low oxidation potential in the presence of carbofuran-phenol than that of bare GCE indicating excellent elctrocatalytic activity. Interestingly, the voltammograms recorded on the modified electrode exhibits 15 fold higher signal to background (S/B) ratio than that on GC electrode. The enhanced electrochemical activity is ascribed to the fast diffusion, good conductivity and excellent electron transfer rate of PET-AC/GCE, as a result of enriched porosity and high surface area of PET-AC 31,32 .
Effect of scan rate. Figure Fig. 3b). Eventually, the results indicate that the kinetics of the oxidation peak current is controlled by the surface-controlled process 33,34 .  Table 1. Physical Properties of the PET-AC. S BET -Brunauer-Emmet-Teller (BET) Surface area (m 2 g −1 ). S micro -Microporous surface area (m 2 g −1 ). V tot -Total pore volume (cm 3 g −1 ). V micro -Micropore volume (cm 3 g −1 ). V meso -Mesopore volume (cm 3 g −1 ). D p -Average pore diameter (nm).  Fig. 4a depicts the relationship curve for pH vs I pa and pH vs E pa . It can be seen that the oxidation peak current of carbofuran-phenol increases with increase of pH, reaches to maxima at 7.0 and then decreases. From the above investigations, a neutral pH of 7.0 is chosen optimal for the electrochemical determination of carbofuran-phenol.
Electrocatalytic activity of PET-AC/GCE towards carbofuran-phenol determination. Figure 4b shows    Table 2). The stable and quick amperometric response is attributed to the high surface area and enhanced porosities of PET-AC, which clearly plays a significant role in the electrocatalytic oxidation of carbofuran-phenol.

Reproducibility, repeatability and stability. To estimate the fabrication reproducibility, six independent
PET-AC/GC electrodes were tested in the presence of 50 µM carbofuran-phenol in 0.1 M PBS (pH 7.0). The measurements reveal an acceptable reproducibility with the relative standard deviation (RSD) of 3.51%. Additionally, 10 sequential measurements were carried out with RSD 3.18% to determine 50 µM carbofuran-phenol indicating the excellent repeatability of the proposed sensor. For storage stability analysis, an initially tested PET-AC/GCE was intentionally stored at room temperature and monitored for variation in the oxidation peak current over a period of about 15 days. The sensor preserves approximately 90% of its original oxidation peak current, suggesting good storage stability of the sensor (Fig. 5b and its inset).   Table 2. Comparison on the performance of different electrochemical methods for the determination of carbofuran by using various modified electrodes. a Glassy carbon electrode modified with acetylcholinesterase and iron oxide nanocomposite. b Glassy carbon electrode modified with cobalt (II) oxide and reduced graphene oxide. c Carbon paste electrode modified with hemin and nickel. d Gold electrode modified with gold nanoparticle (GNPs) and L-cysteine. e Glassy carbon electrode modified with molecularly imprinted polymer reduced graphene oxide and gold nanoparticles. Real sample tests. To further illustrate the practical applicability of the PET-AC/GCE, the electrode was tested with real sample, which was collected from the agricultural fields near Karaikudi, India. The amount of carbofuran in the real sample was pre-determined by UV-spectrophotometer and it was found to be 108 µM. Before the real analysis by the proposed sensor, all the carbofuran in the collected sample was converted to carbofuran phenol by hydrolyzing it in alkalescent solution at high temperature. Under the optimized conditions, 100 µl of the sample was added at regular intervals of time (50 s) in 10 ml PBS solution. For every addition of collected sample, a quick response of 0.034 µA current is observed (Fig. 6b). From the amperometry results, the concentration of collected real sample is found to be 120 µM. The results show that the PET-AC/GCE could be efficiently applied for real sample analysis with good accuracy.

Conclusions
In summary, activated carbon with high surface area and good porosities were successfully prepared from post-consumer PET bottles by simple and cost-effective chemical activation method with KOH. The materials were characterized by N 2 adsorption/desorption isotherm, XRD, FESEM, TEM and Raman spectroscopy. The fabricated PET-AC/GCE exhibits enhanced electro catalytic activity as non-enzymatic electrochemical sensor for the trace level determination of carbofuran phenol, as assessed by CV and amperometry. Notably, the PET-AC/ GCE shows excellent detection limit of 0.03 µM and ultrahigh sensitivity 0.11 µA µM −1 cm −2 for the detection of carbofuran phenol. The proposed sensor is a good alternative to the conventional GCE and most other previously reported modified electrodes, as it offers superior stability, sensitivity, detection limit, reproducibility and selectivity. Furthermore, the amperometric sensor provides remarkable results in real sample applications.

Experimental Section
Materials and Chemicals. Post-consumer PET bottles were collected from the premises of National Institute of Technology Rourkela, India. Analytical grade carbofuran and potassium hydroxide (KOH) were commercially obtained from Sigma-Aldrich and Merck respectively. The supporting electrolytes (phosphate buffer solution (PBS)) at pH 7.0 were prepared using 0.05 M Na 2 HPO 4 and NaH 2 PO 4 solutions and the pH was adjusted with 0.1 M H 3 PO 4 and 0.1 M NaOH. All other chemicals used were of analytical reagent grade and were used as received without further purification. All experiments were performed with ultrapure water (Millipore) at room temperature. According to earlier research, carbofuran with limited electrochemical activity could be converted into carbofuran-phenol by hydrolyzing it in alkaline solution to increase electrochemical activity, where its complete conversion was ensured 12,15 . Carbofuran solution of 1.0 × 10 −3 M was prepared by dissolving 22 mg of the compound in 100 ml of 0.1 M NaOH solution and heated for 1 h to hydrolyze all the carbofuran to carbofuran phenol (Fig. 7) 15 .
Preparation of PET-AC material. Raw material, PET, was properly cleaned to remove the impurities and then dried in oven to remove the moisture content. The dried sample was chopped to a particle size range of 5-10 mm.  Then, 10 g of PET granules were put in 100 ml of dilute KOH with an impregnation ratio 5. It was kept at 85 °C for 24 h, followed by drying at 120 °C for 4 h. The resultant samples were carbonized in a horizontal tubular furnace at final temperature in the range from 900 °C to 1100 °C with a definite heating rate (10 °C/min) in N 2 -atmosphere (100 ml/min). The samples were kept for 60 min at the final temperature. After cooling, the products were rinsed with 0.1 M HCl and hot water (80 °C) until the pH became neutral. Finally, The rinsed carbons were dried at 120 °C for 24 h in an oven and stored in air tight container.
Preparation of AC-modified electrode. As prepared PET-AC (5.0 mg) was dispersed in 1 ml ethanol, under sonication for 3 h. Meanwhile, the surface of the GCE was mirror polished with 0.3 and 0.05 µm alumina powder and ultra-sonicated for several minutes with ethanol and ultrapure water before modification. An aliquot of 5 µl ethanol/PET-AC suspension was introduced onto the surface of GCE using the drop casting method, followed by drying at 50 °C for 2 h. Subsequently, the PET-AC/GCE was gently rinsed with ultrapure water again to remove loosely bound ACs. Finally, the fabricated PET-AC/GCE was explored as the working electrode for further electrochemical measurements.
Characterization techniques. X-ray powder diffraction (XRD) experiment was carried out in Rigaku ultima IV diffractometer equipped with Cu Kα radiation. The surface morphologies of the ACs were studied using field emission scanning electron microscope (FE-SEM) (FEI, Nova NanoSEM 450) & transmission electron microscope (TEM) (FEI, Tecnai S-TWIN). Raman spectrum was obtained with BRUKER RFS 27 stand-alone laser Raman spectrometer using 1064 nm Nd:YAG laser source at a spectral range of 300-2500 cm −1 . The N 2 adsorption-desorption isotherms of the AC were analyzed at 77 K using Quantachrome (Autosorb-1) surface area analyzer. Prior to effecting adsorption measurements, samples were outgassed overnight at 200 °C under helium. The apparent surface area was derived according to the BET (Brunauer-Emmet-Teller) method. The pore size distribution of AC was calculated by Density functional theory (DFT) method. All electrochemical experiments were conducted using electrochemical work station (Autolab PGSTAT 30, Eco Chemie, Netherlands). A conventional three electrode setup was utilized using bare and modified GCE as the working electrode, Ag/AgCl (in saturated KCl) as the reference electrode and a large platinum (Pt) foil as the counter electrode. All electrochemical experiments were carried out under inert atmosphere at room temperature.