Electrochemical Sensor for Detection and Degradation Studies of Ethyl Violet Dye.

In this work, a simple and sensitive electrochemical method was developed to determine ethyl violet (EV) dye in aqueous systems by using square wave anodic stripping voltammetry (SWASV) employing a glassy carbon electrode modified with acidic-functionalized carbon nanotubes (COOH-fCNTs). In square wave anodic stripping voltammetry, EV exhibited a well-defined oxidation peak at 0.86 V at the modified GCE. Impedance spectroscopy and cyclic voltammetry were used to examine the charge transduction and sensing capabilities of the modified electrode. The influence of pH, deposition potential, and accumulation time on the electro-oxidation of EV was optimized. Under the optimum experimental conditions, the limit of detection with a value of 0.36 nM demonstrates high sensitivity of COOH-fCNTs/GCE for EV. After detection, it was envisioned to devise a method for the efficient removal of EV from an aqueous system. In this regard a photocatalytic degradation method of EV using Ho/TiO2 nanoparticles was developed. The Ho/TiO2 nanoparticles synthesized by the sol-gel method were characterized by UV-vis, XRD, FTIR, SEM, and EDX. The photocatalytic degradation studies revealed that basic medium is more suitable for a higher degradation rate of EV than acidic and neutral media. The photodegradation kinetic parameters were evaluated using UV-vis spectroscopic and electrochemical methods. The results revealed that the degradation process of EV follows first-order kinetics.

Introduction

In the present world, energy crises and pollution have become serious threats to humans and the environment. Industrialization and population expansion are major factors that contribute to environmental pollution and energy shortages.[1,2] With the advancement of industry, the overuse of chemicals and poisonous effluent discharge have become more significant contributors to degrading the natural environment.[3] The dumping of radioactive and nonbiodegradable industrial waste, the emission of hazardous gases, and contamination by toxic substances in the air and water pollute the entire ecosystem, leading to the extinction of marine life as well as serious respiratory and immune system health problems.[4,5] Contaminated drinking water is a serious threat to mankind and has become a political and social concern for nations around the world. Industrial wastewater includes dyes, pesticides, and nonbiodegradable organic and inorganic compounds.[6] The extensive use of various grades of chemical dyes to provide diverse colors in textiles, foodstuffs, and tanned products is the leading cause of drinking water pollution. Most of the dyes are carcinogenic and toxic to aquatic life, and their utilization produces waste that is released into the environment.[7−9]

Dyes released into the environment through wastewater are hazardous to aquatic life.[10] Owing to the hazardous effects of waste material dumped into the water by organic industries, it is essential to eliminate the waste and especially dyes. Due to the variability of their structural configuration, dyes such as anionic, cationic, naphtha-based, azo, acidic, basic, and metal complex dyes are chemically stable.[11] The toxicity and stability of these pollutants in the environment have become a complicated subject for regulatory bodies all over the world. Owing to their hazardous effects, it is essential to eliminate these dyes from drinking water. Dyes can be degraded photochemically, photocatalytically, or by using nanoparticles, enzymes, or bacteria.[12] For instance, ethyl violet dye, which is widely utilized in many industries, can be removed from the environment by biological treatment, adsorption, incineration, foam flotation, and/or photodegradation.[13,14] Modern electrochemical and spectroscopic techniques are more widely used for dyes monitering.[1]

In this work, we used electrochemical and spectroscopic techniques to detect and degrade ethyl violet dye. Square wave voltammetry (SWV), one of the electrochemical techniques, is a fast detection method that generates signals of the targeted analyte with high resolution. By incorporating acidic-functionalized carbon nanotubes (COOH-fCNTs) to the working electrode surface, we developed an electrochemical sensor for electrochemical studies. The CNTs are cylindrical tubes made up of layers of coaxially produced graphene sheets that have a large surface area and attractive electrical properties.[15−17] Further, acidification of CNTs enhances the surface area and active sites to accommodate the analyte molecules. In addition, a photocatalytic study of ethyl violet dye was carried out using titanium oxide (TiO2) nanostructure-based photocatalysts.[18] TiO2 exhibits greater stability, recyclability, and chemical and physical inertness.[19] The low toxicity allows it to be used in broader applications in the food and medical fields, such as in dentistry, bone tissue engineering, and nanocarriers for drug delivery.[20,21] During photodegradation, its photocatalytic ability becomes lower due to recombination on photogenerated charge carriers.[22] Consequently, to delay the recombination of charge carriers, doping with rare earth elements has been carried out.[23] Holmium (Ho3+) doped with TiO2, ZnO, and CdS attracted the interest of scientists due to its unique structural and optical properties. These semiconductors show promise for photodegradation applications, as the doped elements hinder the charge carrier recombination process.[24,25] Therefore, Ho/TiO2 nanoparticles can be used as a photocatalyst for the degradation of ethyl violet. Furthermore, TiO2 is the most efficient, cost-effective photocatalyst and has a bandgap of 3.2 eV. Photocatalysis of ethyl violet dye relies on the creation of electron–hole pairs in TiO2 NPs dispersed in aqueous medium by sufficient light energy, which breaks down the enlarged dye molecules.[26]

The work is novel, as the sensing platform has been employed for the first time to detect EV dye with a limit of detection down to 0.36 nM. The implementation of a smart approach to increase the GCE’s conducting ability resulted in a lower limit of detection for larger molecular compounds such as EV. Another step which imparted more sensitivity to the electrode surface for the targeted analyte is that we drop-casted the dye on the designed modified electrode and dipped into the solution of supporting electrolyte of known concentration and pH. In this way, closer accessibility of the dye molecules to the electrode surface was achieved as witnessed by the robust current signal. This approach has resolved the orientation issue of the electroactive moiety. Improper orientation of the oxidizable moiety hinders the closer accessibility of the molecule, and consequently a poor signal is obtained when dye solution is placed in a electrochemical cell. Optimized conditions also significantly improve the limit of detection. Hence, we investigated the dye in various conditions and selected the best sensing conditions. Another unique feature of this research is that the designed sensor was used to examine the kinetics of EV degradation. The targeted dye was degraded using a synthesized photocatalyst (3% Ho-doped TiO2 photocatalyst), and samples were taken at regular intervals and tested using the sensing platform. In this manner, the degradation was followed from the decrease in peak current height of EV. The degradation was also monitored spectrophotometrically by employing UV/vis spectroscopy. The dye was degraded up to 96%, and the degradation extent and kinetic results obtained from both techniques were found in close agreement. Hence, the developed method holds great promise for the treatment of dye-contaminated wastewater. This is the first report on an electroanalytical technique and designed sensor that demonstrates its uniqueness through considerable peak current magnification and plays an important role in dye degradation study. To the best of our knowledge, degradation monitoring of EV is not so far reported on an electrochemical sensor.

Experimental Details

Reagents and Materials

All chemicals used during the experiment were purchased from Sigma-Aldrich, Merck, or Alfa Aesar and were of analytical grade. The COOH-fCNTs were used as a modifier, and ethyl violet was used as the analyte. Holmium-doped titanium oxide nanoparticles (Ho/TiO2 NPs) were synthesized and characterized as a photocatalyst. The chemicals used for the synthesis of Ho/TiO2 NPs were holmium(III) nitrate pentahydrate, titanium(IV) isopropoxide, and 2-propanol. Different kinds of supporting electrolytes, including HCl, H2SO4, phosphate-buffered solution (PBS), NaCl, Britton–Robinson buffer (BRB), KCl, and NaOH, were prepared using analytical grade reagents.

Instrumentation

A Metrohm Autolab PGSTAT302N device running with NOVA 1.11 software was used to carry out the electrochemical studies. The cell assembly comprised a three-electrode system in which Ag|AgCl (3.0 M KCl) was used as reference, platinum wire as counter, and modified glassy carbon electron (GCE) as working electrode. The photodegradation study of ethyl violet was performed on a Shimadzu UV-1700 spectrophotometer in the wavelength range of 200 to 800 nm.

Synthesis of Holmium-Doped TiO2 NPs (Photocatalyst)

Ho-doped TiO2 NPs were synthesized by using the sol–gel technique. First, a specific amount (3%) of holmium(III) nitrate pentahydrate was dissolved in 200 g of 2-propanol and stirred until the holmium salt dissolved completely. To this solution was added 97 g of titanium(IV) isopropoxide (titanium precursor), and it was stirred for 10 min using a magnetic stirrer. As a result, we obtained an alkoxide solution. Then a mixture of 25.35 g of deionized water and 127 g of 2-propanol was prepared and added to the alkoxide solution dropwise. The mixture was stirred for 24 h at room temperature. Filtration was carried out, and then the precipitate was dried in an oven at 100 °C before being calcined in a muffle furnace at 500 °C.

Cleaning of the Working Electrode

Before the experiment, the surface of the glassy carbon electrode was cleaned physiochemically to activate it. A silver mirror-like surface of the electrode was accomplished by scrubbing the surface in a figure eight pattern on a pad with a 0.5 μm alumina slurry and rinsing it with distilled water.[27] Afterward, the cleaned electrode was chemically cleaned by dipping it in the supporting electrolyte while recording voltammograms at a scan rate of 100 mV/s in a potential window of 0 and 1.5 V. Voltammograms with no current variation indicated that the surface of the working electrode was clean.[28]

Preparation of the Modified Electrode

The GCE was modified through the layer-by-layer drop-casting method, by which modifier COOH-fCNTs were coated on the precleaned electrode surface. A 5 μL drop of COOH-fCNTs at a concentration of 1 mg/mL was drop-casted on the surface of the working electrode and air-dried. After that, a 5 μL drop of analyte (concn = 15 μM) was applied to the prepared sensor and air-dried again. The prepared electrode was used for electroanalytical studies, with 0.1 M PBS used as supporting electrolyte to record cyclic and square wave voltammograms during the sensor performance evaluation.

Results and Discussion

Characterization of Photocatalyst

UV–Vis Spectrum Analysis

The UV–vis spectrum of 3% Ho/TiO2 NPs is shown in Figure A. The aqueous suspension of the synthesized sample reveals an absorption peak at 311 nm, with a fundamental absorption edge at 520 nm. The energy gaps of holmium-doped TiO2 were calculated by using a Tauc plot (Figure B).where α denotes the absorption coefficient, h is the photon’s energy, A is the proportionality constant, and n is the electronic transition with 3% Ho/TiO2 NPs n = 1/2; therefore, the calculated band gap is 2.77 eV.[29,30]

Figure 1

(A) UV–vis spectrum and (B) Tauc plot of 3% Ho/TiO2 NPs.

XRD Spectrum Analysis

Figure A shows the XRD pattern of pristine and 3% Ho/TiO2 NPs, which corresponds to JCPDS card number 00-001-0562. In 3% Ho/TiO2 NPs XRD, peaks at 2θ = 25.10°, 37.91°, 48.10°, 54.79°, and 62.68° verify the same anatase structure as observed for pristine TiO2.[31] No major peak shift was observed in XRD, verifying the successful substitution of doped metal oxide into the crystal lattice of TiO2 to produce homogeneous Ho-doped TiO2 nanoparticles. In addition, no further holmium peaks were detected in the synthesized sample, indicating the absence of any impurities. The average crystallite size of the sample was determined by using the Debye–Scherrer equation:[32]where K indicates the Scherrer constant, β is full width at half-maximum, λ is the wavelength of light, and θ is Bragg’s angle. The average crystallite size of 3% Ho/TiO2 NPs was calculated to be 13.73 nm.[33]

Figure 2

(A) XRD pattern and (B) FT-IR spectrum of 3% Ho/TiO2 NPs.

FT-IR Spectrum Analysis

The FT-IR spectrum of pristine and 3% Ho/TiO2 NPs in the 4000–400 cm–1 region. For doped material (Figure B), a bending mode of Ti–O–Ti at 900–400 cm–1 indicates an anatase phase of titania NPs. Likewise, the O–H stretching mode at 3464 and 1367 cm–1, water adsorption peak at 1733 cm–1, metal oxide (Ho–O and Ti–O bands) bending mode peak at 507 and 445 cm–1, CO2 and CO adsorption peak at 2356 cm–1,[34] and Ti–O–CH vibration peak at 3026 cm–1 verify the successful synthesis of 3% Ho/TiO2 NPs.[25,35] Similar FT-IR signals were observed in pristine TiO2 with the absence of a peak around 507 cm–1 which particularly corresponds to the Ho–O bond present in the doped material.

SEM and EXD Analysis

Scanning electron microscopy was studied to evaluate the morphological characteristics of the synthesized nanoparticles. A SEM image of 3% Ho/TiO2 NP is shown in Figure A, where aggregated spherical 3% Ho/TiO2 NPs can be seen at a magnification of 500 nm. Following XRD and FT-IR investigations which showed that both TiO2 and Ho-TiO2 NPs have tetragonal (anatase) structures, morphological analysis illustrated that doping further enhanced the particle shape and reduced agglomeration between the NPs. The elemental composition of 3% Ho/TiO2 NPs was investigated by using energy-dispersive X-ray spectroscopy. The EDX result shown in Figure B reveals that Ti, Ho (holmium), and oxygen were present in the synthesized sample. An EDX table, revealing the quantification of holmium (Ho) dopant in TiO2, is shown in Table .

Figure 3

(A) SEM image and (B) EDX spectrum of 3% Ho/TiO2 NPs.

Table 1

EDX Data Showing the Quantification of Holmium (Ho) Dopant in TiO2

serial number	compound	wt % oxygen	wt % titanium	wt % dopant
1	Ho-TiO2	60.81	36.43	2.76

Electrochemical Characterization

Electrochemical characterization of the defined sensing platform was performed using EIS and SWV techniques. EIS gives useful information regarding charge transfer properties at the sensor surface. The EIS measurement of COOH-fCNTs/GCE was carried out in a 5 mM K3[Fe(CN)6] redox probe in 0.1 M KCl electrolyte at a frequency range of 14 kHz to 1 Hz. Figure A shows the Nyquist plots of bare and modified GCE. The largest semicircle diameter of bare GCE compared to COOH-fCNTs/GCE indicates a faster charge transfer at the COOH-fCNTs, while CNTs/GCE presents a charge transduction rate between COOH-fCNTs and bare GCE. The Rct values obtained for CNTs/GCE (10.5 kΩ) and COOH-fCNTs/GCE (4.6 kΩ) are lower than that for bare glassy carbon electrode (22.11 kΩ). The smallest Rct value of COOH-fCNTs/GCE indicate optimum charge transport of the developed sensor owing to an increased electroactive surface area.[36] The modifier imparts enhanced electrical properties to bare GCE by acting as a bridge between the analyte and the transducer.[37] The Randles equivalent circuit, shown in the inset of Figure a, was used to calculate Rct and other EIS parameters, as listed in Table S1.

Figure 4

(A) EIS Nyquist plots and (B) cyclic voltammograms recorded in solution of 5 mM K3[Fe(CN)6] and 0.1 M KCl on bare and modified GCE at a scan rate of 100 mV s–1.

The cyclic voltammograms of the bare and modified GCE were obtained in a 5 mM K3[Fe(CN)6] redox probe and 0.1 M KCl solution. The CV plots shown in Figure B were used to measure the active surface area of bare GCE, CNTs/GCE, and COOH-fCNTs/GCE by the Randles–Sevcik equation:For 5 mM K3[Fe(CN)6], n = 1, D = 7.6 × 10–6 cm2 s–1, and v = 100 mV/s; thereby, the Ip value from CV was used to find the surface area as tabulated in Table S2. The surface area values indicate a 5 times higher surface area of COOH-fCNTs/GCE than bare GCE. The CV results verify the EIS data, and COOH-fCNTs/GCE was selected as a sensing platform for further investigation.

Voltammetric Analysis of the Targeted Analyte

Square wave voltammetry (SWV) was successfully applied for the detection of ethyl violet dye on the surface of modified GCE under optimized conditions. The square wave anodic stripping voltammograms were recorded on bare/GCE, CNTs/GCE, and HOOC-fCNTs/GCE by using 15 μM ethyl violet dye solution at a potential range of 0.2 to 1.5 V, PBS pH 6.0, scan rate 100 mV/s, deposition potential 0.5 V, and deposition time 5 s. Figure shows the oxidation peak current of the targeted analyte on bare and modified GCEs. The peak current values were enhanced on CNTs/GCE and HOOC-fCNTs/GCE. The improved performance of CNTs and HOOC-fCNTs GCEs was attributed to the high surface area and superior catalytic activity of the designed sensor for the targeted analyte.[38] The results show that ethyl violet dye is oxidized at 0.87 V with an Ip of 12.12 μA on bare GCE, while the peak current was enhanced up to 25.06 and 31.40 μA on CNTs/GCE and HOOC-fCNTs/GCE, respectively. Therefore, acid-functionalized carbon nanotubes were chosen to develop an electrochemical sensor to detect ethyl violet dye.

Figure 5

Comparative SWVs of 15 μM ethyl violet on bare GCE, CNTs/GCE, and HOOC-fCNTs/GCE in a supporting electrolyte of 0.1 M PBS (pH = 6) at a scan rate of 100 mV/s.

Effect of Scan Rates

The effect of scan rates (25–150 mV/s) on the oxidation peak current of the analyte (Figure A) was studied by using cyclic voltammetry (CV). The cyclic voltammograms for ethyl violet dye were recorded in a potential window of −1.0 to 1.5 V. The anodic peak current was enhanced by increasing the scan rate. The relationship between scan rate and peak current provides information about the type of reaction (adsorption controlled or diffusion controlled) that occurred at the modified GCE surface. The Ip vs v plot with a correlation factor of 0.99 (Figure S1 A) is more linear than the Ip vs v1/2 plot with a correlation coefficient of 0.97 (Figure S1 B). Thus, the reaction on the modified electrode surface was adsorption controlled, which was further confirmed by the log Ip vs log v plot, where the slope was nearly equal to 1 (Figure B), verifying surface-assisted reaction at the sensor surface.[39]

Figure 6

(A) Cyclic voltammograms of ethyl violet (15 μM) at various scan rates in PBS (pH = 6.0). (B) Plot of log Ip vs log v.

Optimization of Experimental Parameters

Optimization of parameters including supporting electrolyte, pH of solution, deposition potential, and deposition time was performed at the designed sensor. The nature of the supporting electrolyte greatly affected the peak shape, position, and intensity of the desired analyte. To select the optimum supporting electrolyte, 0.1 M PBS, BRB, NaCl, KCl, and H2SO4 were checked. A well-defined peak shape and high peak current density were obtained with 0.1 M PBS electrolyte, as shown in Figure ; thus, 0.1 M PBS was chosen as the best supporting electrolyte for detection of ethyl violet on HOOC-fCNTs/GCE.

Figure 7

(A) Square wave voltammograms of ethyl violet (15 μM) on COOH-fCNTs/GCE in different supporting electrolytes at a scan rate of 100 mV/s, deposition potential of 0.5 V. (B) Bar graph of various supporting electrolytes vs peak current.

SWV was performed at pH 3–12 to check the involvement of protons in the electrochemical oxidation of ethyl violet dye on COOH-fCNTs/GCE. The SW voltammograms in Figure A shows maximum signals at pH 6.0. The anodic peak shifting linearly toward the lower potential with increased pH of the medium indicates the involvement of protons in the electrochemical process (Figure B). On the pH vs potential plot, the slope value 57 mV/pH (Figure B) is close to the Nernstian slope (59 mV/pH),[40] which indicates that equal numbers of electrons and protons were involved in the oxidation of ethyl violet. At the modified electrode, the fCNTs facilitate the electron transfer rate due to their conductive nature and support adsorption of EV during the preconcentration step of SWASV. Moreover, the oxygen functionalities of the fCNTs can develop hydrogen bonding with the EV nitrogen (H/N–H). In addition, the hydrogen bonds may also be developed between the surface-adsorbed analytes and their aqueous dissolved species. The rings of CNTs can offer additional π–π interactions to the benzene unit of EV via solute–sorbent interactions to boost their regional concentration and enhance electrocatalytic redox events.

Figure 8

(A) SWV peak currents of 15 μM ethyl violet as a function of pH of PBS (3–12) obtained at a scan rate of 100 mV/s. (B) Calibration plot of Ep vs pH.

The effect of deposition potential and accumulation time was investigated by using SWV. Figure S2A and S2B shows the highest peak current at −0.4 V deposition potential, indicating that the maximum number of molecules of the dye were oriented for oxidation at this deposition potential. The optimum accumulation time to obtain the maximum dye molecule deposition on the sensor surface in 5 s is shown in Figure S3A and S3B.

Analytical Performance of HOOC-fCNTs/GCE

The performance of HOOC-fCNTs/GCE was analyzed by SWV under optimized conditions. The recorded voltammograms with ethyl violet concentrations from 15 μM down to 0.01 μM are shown in Figure A. The limit of detection (LOD) of ethyl violet for the designed sensor was calculated according to IUPAC guidelines:[41] 3 σ/m, where m is the slope of a linear calibration plot (Figure B) and σ is the standard deviation of the blank solution. Based on peak current values of the blank solution (PBS, pH = 6.0), standard deviation was calculated for HOOC-fCNTs/GCE. The LOD of ethyl violet was found to be 0.36 nM, indicating that HOOC-fCNTs/GCE is a promising tool for trace level detection of targeted toxins. The calculated figures of merit especially the LOD value (0.36 nM) are far better than the reported LODs for the ethyl violet dye.[42−48]Table shows a comparison between the HOOC-fCNTs/GCE assay and other reported ethyl violet detection methods. Remarkably, the LOD for the target analyte attainable by using the stated platform is much better than reported methods. A literature survey reveals that our designed electrochemical nanosensor is a preferred platform in the perspective of sensitivity, stability, and selectivity, indicating it as a promising tool for the level detection of targeted toxin.

Figure 9

(A) SWV recorded for COOH-fCNTs/GCE by varying the concentration of ethyl violet in PBS at pH = 6.0, scan rate of 100 mV/s, deposition potential of −0.4 V, and deposition time of 5 s. (B) Calibration plots obtained by SWV data of a lower concentration range.

Table 2

Comparison between HOOC-fCNTs/GCE Assay and Other Reported Ethyl Violet Detection Methods

method	material	LOD (nM)	references
microcloud point extraction	Triton X-114	12	(42)
chemometric-assisted	HCl–KCl	22	(43)
surface-enhanced Raman scattering	Cu-nanoleaves	1 × 10–3	(44)
surface-enhanced Raman scattering	PSi/Ag NPs	0.1	(45)
surface-enhanced Raman scattering	Ag@Cu@CW	10–1	(46)
dispersive solid-phase extraction.	GO	9	(47)
electrochemiluminescence	CuS/GCE	3.2	(48)
electrochemical	COOH-fCNTs/GCE	0.36	this work

Validity of the Designed Sensor

The validity of the designed sensor was confirmed by its reproducibility and repeatability. For this purpose, four electrodes were modified in the same way, and voltammograms were recorded under optimized conditions, as shown in Figure A. Identical peak current values of ethyl violet indicate good reproducibility of the designed sensor, with RSD < 5%. A repeatability experiment was performed by taking several readings on the same electrode, and the HOOC-fCNTs/GCE was found to be stable, with RSD < 5%, as shown in Figure B.

Figure 10

SW voltammograms showing (A) reproducibility and (B) repeatability of the developed sensor in 0.1 M PBS electrolyte at pH = 6.0, scan rate of 100 mV/s, deposition potential of −0.4 V, and deposition time of 5 s.

Study of the Effects of Interferents for Validation of the Designed Sensor

To find out the effect of interferents on the anodic peak response of the sensing platform, different dyes and metal ions at 1 mM were added individually as interferents to the 15 μM analyte solution. The interferents include dyes such as methyl red (MR), metanil yellow (MY), orange II (Or-II), and erichrome black T (EBT) along with metal ions such as Cu2+, Ni2+, Ca2+, Zn2+, Mg2+, Na+, and K+. Under optimized conditions, the interfering species did not show any appreciable influence on the peak current response of EV as evident from Figure A. This behavior can be related to the strong affinity of the target analyte for the sensing platform. Signals for the interfering agents also appeared in the voltammogram along with the analyte’s oxidation peak, but they had no discernible impact on the analyte’s signal. These results confirm the EV selectivity of the sensing platform and its capability for detecting and identifying other dyes as well at potentials corresponding to their oxidation. The bar graph in Figure B suggests that the designed sensor has a % RSD value less than 3%, which points to its validity as a selective sensor. Thus, the designed sensing platform is capable of discriminating and sensing the target analyte from other species which may be present in real samples.

Figure 11

(A) Square wave voltammograms of 15 μM ethyl violet in the presence and absence of different interfering agents using 0.1 M PBS of pH 6.0. (B) Bar graph of the peak current of ethyl violet at the designed sensor.

Application of the Sensor in Real Samples

A real analysis was carried out to analyze the designed modifier’s precision and accuracy. For this purpose, the amounts of ethyl violet in industrial wastewater, fruit juice, and hospital wastewater samples were measured. Two milliliters of fruit juices was diluted to 10 mL by using 0.1 M PBS, and water samples were used as such. Initially, no contents of ethyl violet molecules were found in the real matrixes. Then a known amount of the targeted analyte was spiked by standard addition protocol. The recovered amount of ethyl violet was measured by using a calibration plot. All experiments were repeated four times, and the oxidation peak current corresponded to the Ip of Figure . Table reveals that the developed sensor is quite sensitive for ethyl violet dye with % RSD in the range of 2.2–3.4%. The percent recoveries of the targeted analyte in the range of 96.00–98.33% suggest the applicability of the designed sensor for real sample analysis.

Table 3

Real Sample Analysis of Ethyl Violet at the Designed Sensor under Optimized Conditions

dye	sample	initial amount (μM)	spiked amount (μM)	found (n = 4) (μM)	RSD (%)	recovery (%)
ethyl violet	industrial wastewater sample 1	0.0	15	14.5	2.2	98.33
	industrial wastewater sample 2	0.0	15	14.5	2.3	96.50
	fruit juice sample 1	0.0	15	14.80	2.8	96.00
	fruit juice sample 2	0.002	15	14.40 expected (15.002)	3.1	98.00
	hospital wastewater sample 1	0.005	15	14.9 expected (15.005)	3.4	96.36
	hospital wastewater sample 2	0.0	15	14.7	3.2	96.43

Photodegradation Studies on the Designed Sensor

Photodegradation of ethyl violet with Ho/TiO2 NP catalyst on the designed electrochemical sensor was studied under sunlight using an electrochemical system. For this purpose, 15 μM dye solution was prepared in aqueous (pH = 7.0), basic (pH = 13.0), and acidic (pH = 2.0) medium. The spectrophotometric study revealed that the rate of degradation was faster in basic medium. So, 0.1 mg of Ho/TiO2 nanoparticles was added to a 50 mL basic solution of dye and stirred for 10 min as the maximum number of molecules of dye were adsorbed on the nanomaterial. Then 5 μL of dye solution was drop-casted on the HOOC-fCNTs/GCE and dried under hot air. The SW voltammogram was recorded as shown in Figure A. Then the basic solution of dye and NPs was kept under direct sunlight, and voltammograms were recorded every 3 min by repeating the same procedure as described above. The oxidation current gradually decreased as the concentration of dye in the solution decreased due to the breakdown of ethyl violet molecules. After 30 min, 96% of dye molecules had disappeared from the solution. The percent degradation and extent of reaction are shown in Figure S4A and S4B. A kinetic study was also carried out by plotting a first-order kinetic graph. The rate constant K was calculated by the slope of log [Ip]t/[Ip]o vs time (min), as shown in Figure B.

Figure 12

(A) Square wave voltammograms for degradation of ethyl violet solution were recorded for COOH-fCNTs/GCE in 0.1 M PBS electrolyte at pH = 6.0, scan rate of 100 mV/s, deposition potential of −0.4 V, and deposition time of 5 s. (B) Plot showing first-order kinetics.

Spectrophotometric Investigations of Photodegradation of Ethyl Violet

Photodegradation of ethyl violet dye was performed in sunlight by varying the pH conditions, as pH greatly affects the degradation rate by the production of H+ and OH– ions in the solution. The amount of H+ and OH– ions in the solution controls the interaction between dye molecules and surface charge of the catalysts.[49] The absorption spectra of ethyl violet in acidic, neutral, and basic media are shown in Figure A–C. The rate of degradation of ethyl violet dye molecules increased as the medium was changed from acidic to basic, as shown in Figure D–F. Higher alkalinity increased the degradation rate, as positively charged dye molecules are adsorbed more strongly on the surface of the Ho/TiO2 photocatalyst through electrostatic forces.[50] The percent degradation was calculated by using the following formula:where Co is the initial concentration of the dye solution and C is the concentration after irradiation with sunlight at time t. The order of reaction was estimated by ln C/Co vs time plots. The rate constant (K) value was calculated from the slope: 0.009 min–1 in acidic, 0.033 min–1 in neutral, and 0.11 min–1 in basic medium. The percent degradation and extent of reduction in acidic, neutral, and basic media are shown in Figures S5–S7. The proposed photocatalytic degradation mechanism of ethyl violet can be seen in Schemes S1 and S2. This mechanism is consistent with literature reports.[51−53]

Figure 13

(A, D) UV–visible spectra and first-order kinetics plot of photocatalytic degradation of 15 μM ethyl violet dye at Ho/TiO2 NPs in acidic medium. (B, E) UV–visible spectra and first-order kinetics plot of photocatalytic degradation of 15 μM ethyl violet dye at Ho/TiO2 NPs in neutral medium. (C, F) UV–visible spectra and first-order kinetics plot of photocatalytic degradation of 15 μM ethyl violet dye at Ho/TiO2 NPs in basic medium.

Conclusion

An effective electroanalytical technique based on COOH-fCNTs was developed for detecting ethyl violet in aqueous environments. The modifier played a mediating role in the charge transfer process at the interface of modified GCE and significantly improved the target analyte’s current signals compared to bare GCE. The developed electrode presents excellent electrical features for analyte detection owing to its large electroactive surface area. To obtain exceptional sensitivity, experimental parameters including modifier concentrations, stripping electrolyte, medium pH, deposition potential, and accumulation period were optimized. Phosphate-buffered solution (PBS) was selected as the best electrolyte, because the highest peak current of the analyte on the designed electrode was obtained in PBS. The highest sensitivity of the sensor toward the analyte can be seen in its nanomolar level LOD value. Moreover, the photodegradation and decolorization of the analyte solution were also investigated using voltammetry and spectroscopic methods. The percent degradation of ethyl violet validates the claim that the proposed sensor has excellent performance and practical application in environmental water analysis.