Coomassie brilliant blue G 250 modified carbon paste electrode sensor for the voltammetric detection of dihydroxybenzene isomers

In this present study, coomassie brilliant blue G-250 (CBBG) modified electrode was fabricated for the specific and simultaneous detection of three dihydroxybenzene isomers such as resorcinol (RS), catechol (CC) and hydroquinone (HQ). The fabrication of the modified electrode was carried out by electrochemical polymerization of CBBG on the surface of unmodified electrode. The surface structures of bare and fabricated electrode were studied by scanning electron microscope (SEM). The established electrode portrays the very fine interface with these isomers and displayed the sufficient sensitivity and selectivity. The specific parameters of pH solution, scan rate and varying the concentration of analytes were optimized at the modified electrode. The sensor process was originated to be adsorption-controlled activity and the low limit of detection (LOD) for RS and CC was attained at 0.24 and 0.21 µM respectively. In the simultaneous study, designed sensor clearly implies the three well separated anodic peaks for RS, HQ and CC nevertheless in unmodified electrode it failed. Also, the constructed electrode was applied for the real sample analysis in tap water and obtained results are agreeable and it consistent in-between 92.80–99.48%.

a traditional three-electrode cell of CH Instrument-660 electrochemical workstation (CHI-660c). Bare Carbon Paste Electrode (BCPE) and poly(CBBG)/MCPE were utilized as working electrodes, saturated calomel electrode (SCE) as a reference electrode and a platinum wire as a counter electrode. CC, RS and HQ gotten from Sigma Aldrich and standard solutions (25 × 10 -4 M) was make ready in distilled water. CBBG, sodium dihydrogen phosphate, disodium hydrogen phosphate was procured from Merck chemicals and all aqueous solution (0.2 M) were prepared in distilled water. Pure graphite powder of 50 μM particle size from Merck and high viscous paraffin oil from fluka were used for the preparation of carbon paste. All the chemicals are used in this experimentation is with analytical grade and employed as received.

Results and discussion
Construction of bare and poly(CBBG)/MCPE. The BCPE was made-up by hand mixing of 30:70 (w/w) silicone oil and graphite powder for about 35 min and get homogeneous mixture. The gotten mixture was then filled with homemade Teflon cavity having 3 mm interior diameter and copper wire was used for the electrical contact.
Electrochemical polymerisation method was applied for the construction of modified electrode. CBBG (2.0 mM) was carried out on the surface of CPE using cyclic voltammetry in the presence of NaOH (0.1 M) and cyclic the potential in-between − 0.4 to 1.5 V with scan rate 0.05 Vs −1 for 15 polymerization cycles as shown in Fig. 1. As observed in figure, the peak current gradually enhanced with increasing the polymer cycles this confirms the growth of polymeric films on CPE 25,26 . The deposition of CBBG (Scheme 1) on CPE was made by changing the polymer cycles (5 to 25 cycles) and applied to identify the electrochemical performance towards CC (10 µM) in PBS (0.2 M) of pH 7.4. The fifteen polymerization cycles depict maximum enhancement in peak   Figure 2 represents the acquired cyclic voltammograms (CVs) for BCPE (scattered streak) and solid streak for the poly(CBBG)/ MCPE with scan rate 50 mV/s. At BCPE, it indicates slight peak current and in the similar situation poly(CBBG)/ MCPE exhibits superior growth in peak current than BCPE. Therefore, this enhancement in current gave great electroactive superficial area and this was determined using Randles-Sevick's Eq. (1) 27 . Compared to unmodified CPE (0.027 cm 2 ) the fashioned poly(CBBG)/MCPE (0.053 cm 2 ) are accomplish more superficial surface area. The approximate adhered modifier thickness or surface average concentration on CPE was calculated using Eq.
(2) 28,29 and got at 0.183 × 10 −10 M/cm 2 . The surface changes of before and after modifying of CPE was characterized by SEM analysis using ZEISS Ultra-55. Figure 3a,b exposes the attained SEM pictures for BCPE and poly(CBBG)/MCPE. At BCPE, the layers shown as rough and wrinkle surface. After modification of CBBG on CPE it obvious changes was occurred and it looks like smooth and flat surface 30,31 .
(1) Ip = 2.69 × 10 5 n 3/2 AD 1/2 Coν 1/2   Fig. 4a,b, it clearly exposed that as the pH solution was varied then the peak potential (CC and RS) was moved towards the negative potential. This attained result was evidence for the directly involvement of proton in the electrochemical reaction 32,33 . The linear relationship between peak potential and varied pH solution for CC and RS was illustrated in inset Fig. 4a The transfer of electron was easier in constructed electrode than BCPE, because where ΔEp value is lower and then electron transfer rate will be greater. Therefore, the poly(CBBG)/MCPE act as moral and prominent sensor for the investigation of CC. The oxidation and reduction mechanism of CC was portrayed in Scheme 2.
Effect of scan rate and concentration variation on peak current of CC at poly(CBBG)/MCPE. The kinetics of poly(CBBG)/MCPE was examined by changing the scan rate. Figure 6a illustrated the obtained CVs for 10 μM CC in the existence of 0.2 M PBS (supporting electrolyte) with changed scan rates. As perceived in figure, the peak current of CC was subsequently enhanced with as raised in the scan rate (0.06 to 0.24 V/s) and tiny moved of their peak potential to positive and negative potential. The linear correlation between anodic peak current (Ipa) versus scan rate (ν) and Ipa versus square root of the scan rate was drawn in Fig. 6b,c. The gotten graph gave very fine linearity with correlation coefficient value (R 2 ) was originated to 0.999 and 0.998 respectively. Therefore, by noticing the above achieved outcome the kinetic characteristic of poly(CBBG)/MCPE was originated at surface controlled response 34,36 .
The detection limit of amended electrode was assessed by operating CV and DPV performance. Figures 7a  and 8b denotes the gained CVs and DPVs for different concentration of CC (10-90 μM) in the occurrence of 0.2 M PBS of pH 7.4 with scan rate 0.05 V/s. These figures clearly depict that the peak current was enhanced significantly when the concentration of analyte was increases. Inset Fig. 7a,b implies the correlation of Ipa and analyte concentration and it gave sufficient linearity with R 2 value of 0.999. By applying the standard deviation (S) and slope value (M) of the peak current (acquired from inset Fig. 7a,b) Fig. 9a, as the scan rates elevated the oxidation peak current was enhanced subsequently by tiny slide in their oxidation peak potential to positive potential. The interrelation between scan rate and oxidation peak current and square root of scan rate versus oxidation peak current is plotted in Fig. 9b,c respectively. The gotten graph gave fine linearity with R 2 was gotten at 0.998 and 0.995 respectively and the process of electrode was originated by surface-controlled phenomena.
LOD and LOQ was assessed by the Eqs.  Fig. 10a,b. As noticed in Fig. 10a,b, the oxidation peak current of RS was increases subsequently as the concentration raises and peak potential tiny moving to positive direction. The inset Fig. 10a,b depicts the correlation between concentration of RS and oxidation peak current and it offered very satisfactory linearity with R 2 value is 0.997. The gotten LOD and LOQ for RS is 0.24 and 0.79 μM separately. The poly(CBBG) MCPE gave less detection limit for CC and RS compared to other fabricated electrode shown in Table 1.

Simultaneous voltammetric detection of CC and RS in the presence of HQ at poly(CBBG)/ MCPE. The simultaneous detection of CC, RS and HQ was very problematic in mixed solution at BCPE.
As in the blending solution they are amalgamate each other owing to their closely similar oxidation potential 38 . Therefore, to validate the potentiality of modified electrode for the simultaneous investigations of CC and RS in the presence of HQ. Figure 11 reveals the gotten CVs for CC, RS and HQ (10 μM) in the existence of supportive electrolyte (0.2 M PBS of pH 7.4) with scan rate 0.05 Vs −1 . The BCPE (scattered streak) was unsuccessful to depicts distinguished the three peaks but it gave two oxidation peaks situated at 0.15 and 0.56 V correspondingly. Moreover, the established poly(CBBG)/MCPE (dashed line) clearly demonstrated the three separated oxidation peaks for CC, RS and HQ (0.13, 0.49 and 0.02 V) individually with exceptional improvement in peak current than BCPE. Therefore, the fabricated poly(CBBG)/MCPE was remarkable potentiality towards the simultaneous detection for CC and RS in the existence of HQ.

Selectivity, stability and real sample exploration of CC, RS and HQ at poly(CBBG)/MCPE. The
efficiency and selectivity detection of CC, RS and HQ was examined by utilizing the DPV system. Figure 12a represents the tracing of CC by reserved the concentration of RS and HQ (50 μM) was unchanged. As perceived in the figure, the peak current was enhanced by increasing the concentration of CC in the series 100-400 μM. Similarly, for RS the amount of analyte was varied in the series 100 to 400 μM by kept the unchanged solution of HQ and CC (50 μM) signified in Fig. 12b. Likewise, the amount of concentration for HQ (50-400 μM) was www.nature.com/scientificreports/ varied and the amount of RS and CC (50 μM) was unchanged. By seeing the overhead outcome, as the amount of concentration was enlarged, the peak current also enhanced gradually, nevertheless there was nothing variation in peak potential and current of reserved mixtures. This consequence specifies that the reserved analytes are no restrict in detection of mixtures. Thus, poly(CBBG)/MCPE consume admirable selectivity and they are traced individually in blended solution.
The steadiness of the poly(CBBG)/MCPE was estimated in RS, CC and HQ (10 µM) in 0.2 M PBS (pH 7.4) with the sweep rate 0.05 Vs −1 in ten consecutive cycles applying CV technique. As in the Fig. 13, the redox peak current remains steady and after accomplishment of 10 cycle the slight reductions in their oxidation current were 4.9%. The degradation percentage was estimated by equation % degradation = Ip n /Ip 1 39 , where Ip 1 and Ip n are the 1 st and n th cycle for Ipa separately. The regained steadiness of the poly(CBBG)/MCPE was got at 95.10% and this approves the fashioned electrode consumed adequate stability.
Finally, the recommended poly(CBBG)/MCPE was examined for the real sample applicability of CC, RS and HQ in a tap water utilizing standard addition method 40,41 . This advised the fabricated electrode portrays satisfactory retrieval for each sample adding and acquired outcomes are registered in Table 2. Therefore, this outcome authorized the accurateness of established electrode for the revealing CC, RS and HQ in tap water.

Conclusion
Present work outlined, poly(CBBG)/MCPE was operated as the sensor for the detection of CC and RS in presence of HQ. The surface property of BCPE and poly(CBBG)/MCPE was scanned by SEM exploration. This suggested probe exposed the strong electro-catalytic movement, great sensitivity, steadiness and gave improved electron transfer reaction than BCPE with respect to the oxidation of CC and RS in the presence of HQ. The influence of pH, analyte concentration variation and scan rate were surveyed at fashioned electrode. The poly(CBBG)/MCPE implies surface controlled manner and it exhibited less detection limits than other testified electrode. The sensor has a wide dynamic range that comprises both linear ranges and simultaneous tracking of CC and RS in the presence of HQ was tracked via CV technique. The adopted method of established electrode contributed noble selectivity, stability and gave agreeable retrieval in the real sample scrutiny of analytes. Hence, the mentioned poly(CBBG)/MCPE was remarkable potentiality for the specific and simultaneous analysis. This fabricated electrode can also be utilized for the supplementary examination of other electroactive molecules also.