Modification of Glassy Carbon Electrodes on Starch-Based for Detection of Chromium Hexavalent

This study aimed to investigate the effect of the percentage addition of conductivity material on phosphorylated starch to modify a Glassy Carbon Electrode (GCE) for detecting Cr(VI). The conductivity materials used are activated carbon from rice husk and Fe 3 O 4 nanoparticles. Phosphorylated starch was prepared by adding STPP as a cross-linker to starch. The method used for optimising the conductivity material in the phosphorylated starch. The sensor performance was determined by the limit of detection, sensitivity, and range of linear concentration. Data were collected by measuring Cr(VI) in the electrolyte H 2 SO 4 : K 2 SO 4 using Differential Pulse Voltammetry (DPV). As a result, adding conductivity material increases the Cr(VI) current and shifts the peak potential to the left. The amorphous form of activated carbon and trigonal Fe 3 O 4 nanoparticles affect the measured Cr(VI) current on the percentage of conductivity material in the phosphorylated starch matrix. The percentage of optimum conductivity of activated carbon and Fe 3 O 4 nanoparticles in the phosphorylated starch matrix is 5%. The phosphorylated starch composition with Fe 3 O 4 nanoparticles obtained the best sensor performance. The sensor performance obtained is the limit of detection 3.48 ppm; sensitivity 0.2120 ppm/µA in a linear concentration range of 2.6 – 18.2 ppm.


INTRODUCTION
The development of electrode modification refers to the use of renewable natural materials. Modification electrodes can be reached by renewable natural materials such as starch (Mulyasuryani et al., 2019). Starch has the limitations, such as forming unstable paste at low or high temperatures, low resistance to mechanical treatment, and low viscosity under acidic conditions (Herawati, 2016). The phosphorylation process can increase the ability of starch with the insertion of a phosphate group. Phosphorylation of starch increase material properties such as thermal resistance properties, acid stability, paste stability, and improves starch's texture properties (Sechi et al., 2017). However, applying phosphorylation starch in the electrochemical method still has limitations. The phosphorylation of starch leads to a decrease in the conductivity of starch (Dong et al., 2020). As a result, the modifier has good electrical conductivity is needed.
Activated carbon from rice husk and Fe 3 O 4 nanoparticles are modifiers that have good conductivity to increase the electrical conductivity of the polymer. Both materials have a specific surface area, a microporous structure, good conductance, and a high value of specific capacity (Yi et al., 2016). Some research to synthesize activated carbon from rice husks was carried out. Yi et al., (2016) got activated carbon with a specific capacitance of 147 F g -1 and a high specific surface area of 2696 m 2 g -1 . Xu et al., (2014) obtained activated carbon with a specific capacity of 250 Fg -1 and a high specific surface area of 2523.4 m 2 g -1 . In another research, Le Van et al., (2014) received activated carbon from rice husk with a specific surface area is 2681 m 2 g -1 and a specific capacitance of 198.4 F g -1 . In another way, Fe 3 O 4 nanoparticles have a specific surface area of 165.05 m 2 g -1 , and a specific capacitance is 207.7 F g -1 (Wang et al, 2013). Some research on adding Fe 3 O 4 nanoparticles for modifier electrodes has been done. Andawiyah et al., (2020) show that adding Fe 3 O 4 nanoparticles to PVA for paracetamol detection increases the sensors' sensitivity and decreases LOD. Mulyasuryani et al., (2019) show adding Fe 3 O 4 nanoparticles on cassava starch membrane to detect acetaminophen and caffeine. The obtained sensor has an accurate measurement of 96-99%. It shows that activated carbon and Fe 3 O 4 nanoparticles can be used as conductive materials to improve electrical conductivity.
Chromium (Cr) is one of the most earth elements with numerous 100 mg/kg crust of the earth. Chromium is present in a stable form as trivalent and hexavalent. Furthermore, Cr(VI) has a robust oxidizing ability. This character makes Cr(VI) have toxic, mutagenic, and carcinogenic effects (Xu et al, 2019) (Tu et al, 2018). The toxicity of Cr(VI) is 100 times higher than Cr(III) (Filik et al, 2019). WHO (World Health Organization) assign limited quality standard Cr(VI) in groundwater is 50 ppb (Cuéllar et al, 2016). Standard Nasional Indonesia (SNI) standardized analysis for Cr(VI) is spectrophotometry (with diphenylcarbazide agent) and Atomic Absorption Spectroscopy (AAS). They have limitations. There is an analysis for total Cr (Wiryawan et al, 2018). This is causing a high selectivity detection for analysis Cr(VI) was needed.
Voltammetry is an ion-specific method that measures the current at the reduction potential of the analyte. Measurement of Cr(VI) with the existence of Cr(III) will not be disturbed. It is because they have different reduction potentials. The reduction potential of Cr(VI) is at 1.33 V, while the reduction potential of Cr(III) is at -0.74 V. The specific Cr(VI) measurement can be conducted by arranging the potential used (Tu et al., 2018).
The development of voltammetric measurements is carried out by modifying the electrodes to increase sensitivity and selectivity. Several studies of electrode modification to detect Cr(VI) have been carried out using gold nanoparticles and bismuth. For example, modification of the SPCE (Screen-Printed Carbon Electrode) using gold nanoparticles showed a high increase in sensitivity, namely LOD (Limit Of Detection) 5.4 g/L and a concentration range of 20-200 g/L (Tu et al., 2018). While the modification of the GCE with bismuth also obtained high sensitivity with a LOD of 0.6 ppb, LOQ (Limit Of Quantitative) 2 ppb, and a concentration range of 2-12 ppb (Th Hue et al., 2020). However, modification using gold nanoparticles has a weakness, whereas the synthesis of gold nanoparticles takes much time, effort, and cost with not environmentally friendly (Yasser et al., 2019). In comparison, using bismuth is expensive and susceptible to temperature instability (Rossel et al, 2010). In this study, modification of GCE was carried out with starch as a renewable material.
This research aimed to investigate the influence of adding mass conductive material on a phosphorylated starch to modifier GCE and determine quantitative analysis from obtained sensors. In this research, the conductive material used was Fe 3 O 4 nanoparticles and activated carbon.

Prepared of Phosporylation Starch (PS)
Preparing phosphorylation starch (PS) was adapted from Mulyasuryani's experiment (Mulyasuryani et al., 2019). 2 g of cassava starch was added to 50 mL of boiling water and stirred until homogeneous. Next, 50 mL water and 8 drops of 2 M HCl were added and kept the solution at 90 °C for one hour, then add 2 M NaOH until pH 8. Next, a 70 mL solution of starch was put into another beaker glass and added 20 mL 2% STPP. Stir this mix solution at 90 °C overnight. Finally, the solution was placed in a petri dish and dried at 60 °C for 4 hours.

Modified of Glassy Carbon Electrode (GCE)
The modification of GCE was carried out by 0.1 g of PS, and 1 mL of hot water was mixed and stirred for 5 min to form a homogenous solution. Next, activated carbon and Fe 3 O 4 nanoparticles were added to the solution and stirred. Finally, the GCE surface was lubricated with a solution and dried for 4 min at 50 °C.

Differential Pulse Voltammetry (DPV) Measurement
Quantitatively measured Cr(VI) in mixed solution 0.05 M H 2 SO 4 and 0.05 M K 2 SO 4 using differential pulse voltammetry (DPV). The setting were potential range 0.6-1.4 V (vs Ag/AgCl), potential step 0.01 V, modulation amplitude 0.1 V; modulation time 0.05 V, interval time 0.5 V and scan rate 0.02 V/s.

Influence Mass Ratio Membran and Conductive Material
The percentage of activated carbon in the PS matrix produces the highest current on the PSC1 sensor, shown in Figure 1a-b. Furthermore, an increased percentage of activated carbon in the PS matrix indicates a decrease in Cr(VI) measurement current. According to Koncar (2019), decreasing peak current occurs due to the addition of conductive material in the polymer matrix being elongated and breaking some conductivity paths. That causes an increase in the electrical resistivity of the polymer matrix. Consequently, the current decreases as the mass of activated carbon increases.
The increase in the percentage of Fe 3 O 4 nanoparticles in the PS matrix shows the best current peak of Cr(VI) by the PSN1 sensor (Figure 2a-b). Therefore, adding Fe 3 O 4 nanoparticles reduces the current, then increasing current from PSN2 to PSN4. The result occurred because, on PSN1, numerous Fe 3 O 4 nanoparticles and PS matrix were proportional electrically. Then, the decrease of current at PSN2 occurs because adding Fe 3 O 4 nanoparticles came down the cross-section of PS (membrane matrix) while conductivity paths were still low. Then, intensifying of current occurs because adding material conductive like Fe 3 O 4 nanoparticles create more conductivity paths. Therefore, it caused an increase in the current (Koncar, 2019). Activated carbon and Fe 3 O 4 nanoparticles were analyzed with XRD. The XRD pattern of activated carbon used in this research shows two peaks at 23° dan 44°. This result shows that activated carbon was amorphous (Riyanto et al., 2020). In contrast, the XRD pattern of Fe 3 O 4 nanoparticles used in this research shows seven peaks at 18.3°; 30.1°; 35.5°; 37.1°; 43.1°; 53.5°; and 57°. According to Inorganic Crystal Structure Data (ICSD): 196988, this peak was Fe 3 O 4 with crystal system trigonal. Therefore, the results obtained differ based on the structure difference between activated carbon and Fe 3 O 4 nanoparticles. Figure 3 shows that modifying GCE with PS can increase the measurement current of Cr(VI). Moreover, this result proves that adding mass of the conductivity material like activated carbon and Fe 3 O 4 nanoparticle in PS can increase the analysis current of Cr(VI). Rising the current of Cr(VI) can be explained because activated carbon and Fe 3 O 4 nanoparticle, as conductivity properties, has high specific capacitance (Le et al., 2014;Pai et al., 2021).   Figure 4, which shows that adding conducting material in the PS. The addition of activated carbon to the PS revealed that the arrangement of activated carbon was dispersed irregularly due to differences in the size and shape of the activated carbon. Meanwhile, adding Fe 3 O 4 nanoparticles to the PS showed that Fe 3 O 4 nanoparticles were regularly dispersed on the PS. Table 3 shows that modification with PS can sift the peak of potential to the left. It occurs because the starch membrane has some carboxyl groups. This group can to reduced Cr(VI) at measurement. In addition, adding activated carbon and Fe 3 O 4 nanoparticles can swipe the peak potential to the left and plays a role in increasing the conductivity. Besides that, adding conductive material will facilitate electrons to move in the modifier film (Koncar, 2019).

Performance of Sensor
The performance of PSC1 and PSN1 was determined to understand the relationship between the concentration of Cr(VI) and the current peak using the DPV technique. The result of this experiment is a voltammogram and standard curve shown in Figure 5. The parameters of analysis Cr(VI) are shown in Table 4. Results show that adding Fe 3 O 4 nanoparticles in the PS has better parameter analysis of Cr(VI) than adding activated carbon.

CONCLUSIONS
The addition of activated carbon and Fe 3 O 4 nanoparticles in the PS matrix has increased the sensor's performance. The optimum percentage of both activated carbon and Fe 3 O 4 nanoparticles is 5%. Different kind of conductive material causes different impact increase percentage in the PS matrix for the current peak of Cr(VI). Enhancing activated carbon with amorphous shapes causes decreasing Cr(VI) current. While increasing Fe 3 O 4 nanoparticles with trigonal structure increase the current of Cr(VI). The obtained sensor shows that the best conductive material for sensor Cr(VI) is Fe 3 O 4 nanoparticles. To improve the performance of the sensor, further study needs to experiment with optimum electrolyte concentration.