This is to prove that the thesis work titled “A Novel Sensor for Simultaneous Detection of Dihydroxybenzene Isomers” was carried out by Nusrat Tazeen Tonu at the Department of Chemistry, Khulna University of Engineering and Technology, Khulna, Bangladesh. HIL-PGE showed excellent selectivity and strong anti-interference for the detection of HQ, CC and RS simultaneously in aquatic environments with excellent results.
List of Tables
114 4.66 Calibration curve for estimation of HQ in presence of RS (current . response with variation of concentration). 115 4.68 Calibration curve for estimation of HQ in presence of CC (current .response with variation of concentration).
Nomenclature
Introduction
General
In particular, electrochemical sensors and detectors are very attractive for on-site monitoring of priority pollutants as well as for addressing other environmental needs. Some electrochemical devices, such as pH or oxygen electrodes, have been routinely used for years in environmental analysis.
Principle of Electrochemical Sensors
In potentiometric sensors, analytical information is obtained by converting the recognition process into a potential signal that is proportional (in a logarithmic manner) to the concentration (activity) of the species generated or consumed in the recognition event. In the past, potentiometric devices were more commonly used, but the increasing number of researches on amperometric probes should gradually shift this balance [6-10].
Chemically Modified Electrodes for Environmental Monitoring
Thus, it reflects the rate of the recognition event and is proportional to the concentration of the target analyte. Modification of miniature screen-printed sensor strips can also be achieved by incorporating the desired reagent (eg, ligand, catalyst) into the ink used for the microfabrication process.
General methods of modification of electrodes
Electrodeposition: In this technique, the electrode is immersed in a concentrated solution (~10-3 molL-1) of the polymer, modifier or catalyst followed by repeated voltammetric scans. The first and second scans are similar, subsequent scans decrease with the peak current For example electrochemical deposition of poly(o-toluidine) on activated carbon fiber [44]. In this technique, the electrode is immersed in a polymer, modifier or catalyst solution, and layers of the electropolymerized material build up on the electrode surface.
Simultaneous Detection
In general, the peak current increases with each voltammetric scan such that there is a noticeable difference between the first and final scans indicating the presence of polymerized material. For example, the electropolymerization of aniline on a platinum electrode. 8 quantitatively determine CC, HQ or RS in the presence of other isomers without pretreatment or separation [55-56].
Prospect of Modified Electrodes in Simultaneous Detection
- Ionic liquid
Solubility and other physicochemical properties suitable for a particular application can be engineered by appropriate combinations of cations and anions. Recently, ILs have attracted the attention of analysts to also use them in various analytical applications.
Wooden Pencil graphite electrode
- Natural Occurrence of Catechol
- Uses of Catechol
Small amounts of CC occur naturally in fruits and vegetables along with the enzyme polyphenol oxidase (also known as catecholase or catechol oxidase). 3-trans-isocamphylcyclohexanol, widely used as a substitute for sandalwood oil, is produced from CC via guaiacol and camphor.
Hydroquinone
- Natural Occurrence of Hydroquinone
- Uses of Hydroquinone
14 CC is used as a black-and-white photographic developer, but except for a few special purposes, its use has been largely historical until recently. In human medicine, HQ is used as a topical application in skin bleaching to reduce the color of the skin, as it does not have the same propensity to cause dermatitis as methol does.
Resorcinol
- Natural Occurrence of Resorcinol
- Uses of Resorcinol
Chemical: RS is also used as a chemical intermediate for the synthesis of pharmaceuticals and other organic compounds. RS is an analytical reagent for the qualitative determination of ketosis (Seliwanoff test). It is the starting material for resorcinarene molecules and the initiator explosive lead styphnate[83].
Mass transfer process in voltammetry
- Migration
- Diffusion
- Convection
Under these conditions, the reducible ions are diffused from most of the solution and diffusion current is produced. Under such conditions, the current is unevenly controlled by diffusion of the reacting species through the concentration gradient adjacent to the electrode.
Differential Pulse Voltammetry
- Uses of DPV
By sampling the current just before the potential changes, the effect of the charging current can be reduced. This technique can be used to study the redox properties of extremely small amounts of chemicals due to the following two properties[86]:.
UV-Vis Spectrophotometry
The current is measured immediately before each potential change, and the current difference is plotted as a function of potential. In these measurements, the effect of the charging current can be minimized, so high sensitivity is achieved. ii) Faradic current is withdrawn, so electrode responses can be analyzed more precisely.
Scanning Electron Microscopy
Energy Dispersive X-ray Microanalysis
Literature Review
Aim of the present work
From the literature, it is seen that most of the work for the simultaneous detection has been done by modified GCE, where some can successfully detect the three isomers and some can only detect two. The sources of various chemicals, the instruments and a brief description of the methods are given below.
Chemicals
The selectivity of electrode reactions has been improved by modifying the electrodes with 1-hexylpyridinium hexafluorophosphate (HIL) and 1-butyl-3-methylimidazolium hexafluorophosphate (BIL). The modification of the electrodes has also been evaluated with the response of the isomers in UV-Vis spectroscopy.
Cyclic Voltammetry
The current at the working electrode is monitored as a triangular excitation potential is applied to the electrode. The potential at the working electrode is controlled against a surface electrode, Ag/AgCl(s)/Cl- electrode.
Important Features of Cyclic Voltammetry
Regardless of the k° value, such a maximum shift can be compensated for by an appropriate change in scan speed. Assuming α = 0.5, the ratio of reversible to irreversible current peaks is 1.27 (i.e. the peak current for the irreversible process is about 80% of the peak for a reversible process).
Differential Pulse Voltammetry
For quasi-reversible systems (with 10-1 > k° > 10-5 cm/s) the current is controlled by both charge transfer and mass transport. Another type of pulse voltammetry is square-wave voltammetry, which can be considered a special type of DPV in which equal time is spent at the raised baseline potential and the superimposed pulse potential.
Important features of differential pulse voltammetry DPV has these characteristics
By contrast, in normal pulse voltammetry the current resulting from a series of increasingly large potential pulses is compared to the current at a constant 'baseline' voltage. Unlike NPV, the current is sampled twice in each Pulse period (once before the pulse, and at the end of the pulse), and the difference between these two current values is recorded and displayed.
Electrochemical cell
- Counter electrode
- Reference electrode
It is also called auxiliary electrode and is used to balance the electrical current expected to flow through the working electrode. For accurate electrochemical measurements, we generally used a counter electrode made of platinum wire (spiral wire) with a surface area approximately 100 times larger than the working electrode.
Electrodes used in experiment
Preparation of Wooden Pencil Graphite Electrode
Modification of PGE with 1-hexylpyridinium hexafluorophosphate
Then electrochemical cell was prepared by connecting electrodes (working: PGE, reference: Ag|AgCl|Cl- and counter: Pt coil) to the computer controlled potentiostat as shown in Figure 3.12(i). Then electrochemical cell was prepared by connecting electrodes (working: PGE, reference: Ag|AgCl|Cl-(aq) and counter: . Pt coil) to the computer controlled potentiostat as shown in Figure 3.12(i).
Removing Dissolved Oxygen from Solution
The beaker was covered with parafilm and placed in an ultrasonic bath for 30 minutes. Before electrochemical modification, the bare PGE with a diameter of 2 mm was polished on a paper.
Electrode polishing
55 3.15 Preparation of different stock solutions .. i) Catechol solution: Catechol solutions of different concentrations (1 to 10 mM) were prepared in phosphate buffer. ii) Hydroquinone solution: Hydroquinone solutions of different concentrations (1 to 10 mM) were prepared in phosphate buffer solution. iii) Resorcinol solution: Solutions with different concentrations of Resorcinol (1 to 10 mM) were prepared in a phosphate buffer. iv) 1-hexylpyridinium hexafluorophosphate solution was prepared in PBS. v) 1-butyl-3-methylimidazolium hexafluorophosphate solution was prepared in PBS.
Preparation of Buffer Solutions
Standardization of the System
To determine the potential window the scan is initially performed with the supporting electrolyte solution to obtain the background voltammogram. The voltammogram containing the analyte in supporting electrolyte is taken in two different ways, at i) different scan rates and ii) different concentrations.
UV-Vis Spectrophotometry
Beer studied the influence of the concentration of the colored component in the solution on the transmission of light. He found the same relationship between transmission and concentration as Lambert found between transmission and layer thickness.
Scanning Electron Microscopy
Energy Dispersive X-ray Microanalysis
60 present) involves measuring line intensities for each element in the sample and for the same elements in calibration standards of known composition. New electrochemical sensors have been developed to simultaneously detect DHBIs in the environment.
SEM images of working electrodes
The electrode was made from locally available and inexpensive pencil graphite instead of conventional expensive platinum, gold or GCE. CV was delivered to analyte HQ, CC, and RS at different scan rates and concentrations.
EDX of bare PGE
The experimental part described the whole research work, but a graphical diagram is shown here. Due to an instrumental error in the UV-Vis spectrophotometer (Heios , Thermo Scientific, USA), the absorption maxima were obtained at wavelengths for DHBIs with some irregularities in the peak lines and also close to the previous ones.
Individual and simultaneous UV-Vis spectrum of CC and HQ
Individual and simultaneous UV-Vis spectrum of CC and RS
68 it can be decided that simultaneous detection of CC and RS in a binary mixture is impossible using UV-Vis spectrophotometry.
Individual and simultaneous UV-Vis spectrum of HQ and RS
When we took the UV spectrum for a mixture of CC, HQ and RS in PBS, we found one peak at 277.4 nm. Thus, we can decide that the simultaneous detection of CC, HQ and RS in the mixture is not possible by UV-Vis spectrophotometry.
Cyclic voltammetric behavior of catechol at bare PGE
- Effect of concentration
Because there is only CC in the buffer solution, an anodic peak and a cathodic peak must be observed for CC. It is seen that both anodic and cathodic currents increase with increasing CC concentration.
Cyclic voltammetric behavior of hydroquinone at bare PGE
- Effect of concentration
Since there is only HQ in the buffer solution, one anodic and one cathodic peak should be observed for CC. It can be seen that both anodic and cathodic currents increase with increasing HQ concentration.
Cyclic voltammetric behavior of resorcinol at bare PGE
The CV of RS solutions with different concentrations at a scan rate of 0.05 V/s and at pH 7.0 were shown in Figure 4.17. It is seen that the anodic current increases with increasing RS concentration.
Simultaneous detection of CC and HQ in PBH at bare PGE by CV
76 was also found at -0.262 V for the binary mixture which has a low potential than the peaks for individual CC and HQ. The anodic and cathodic peaks in the binary mixture are the combined peak of both species, which is due to the fouling effect.
Simultaneous detection of CC and RS in PBH at bare PGE by CV
Since they do not give individual responses when both are present in the mixture, their simultaneous detection in bare PGE is impossible. So they did not give individual responses when both are present in the mixture, their simultaneous detection in bare PGE is impossible.
Simultaneous detection of HQ and RS at bare PGE in PBS by CV
But there should be at least two cathodic peaks for CC compared to the response for single CC in bare PGE. Since they did not respond individually when both were present in the mixture, their simultaneous detection by bare PGE is impossible.
Simultaneous detection of HQ, CC and RS at bare PGE in PBS by CV
If they were all present in the mixture, simultaneous detection with bare PGE would be impossible.
Modification of PGE with Ionic Liquid
- Modification of PGE with 1-hexylpyridinium hexafluorophosphate solution Modification process: Prior to electrochemical modification, the bare PGE (grade 2B)
- Modification of PGE with 1-butyl-3-methylimidazolium hexafluorophosphate solution
It is assumed that a uniform and thin film was present on the PGE surface. It is assumed that a uniform and thin film was present on the PGE surface.
Cyclic voltammetric behavior of catechol at HIL-PGE
- Comparison of CV of CC at Bare PGE and HIL-PGE
- Effect of scan rate
- Effect of concentration
84 Table 4.1: Current potential data, peak potential separation, peak current ratio for the voltammograms of 5 mM CC in PBS at different scan rates. From Figure 4.27, it can be seen that the peak potential separation increases with the increase in scan speed.
Cyclic voltammetric behavior of hydroquinone at HIL-PGE
- Comparison of CV of HQ at Bare PGE and HIL-PGE
- Effect of scan rate
- Effect of concentration
The current potential data, peak potential separation, peak current ratio of the voltammograms at different scan rates are organized in Table 4.2. The anodic and cathodic currents gradually increase with increasing concentration of the analyte solution.
Cyclic voltammetric behavior of resorcinol at HIL-PGE
- Comparison of CV of RS at Bare PGE and HIL-PGE
- Effect of scan rate
- Effect of concentration
The current potential data, peak potential separation, peak current ratio of the voltammograms at different scan rates are tabulated in table 4.3. 96 Table 4.3: Current potential data, peak potential separation, peak current ratio of the voltammograms of 5 mM RS in PBS at different scan rates.
Simultaneous detection of CC and HQ at HIL-PGE in PBS by CV
Simultaneous detection of CC and RS at HIL-PGE in PBS by CV
From the positions of the anodic peaks, it can be said that the peaks of both CC and RS were found. The HIL-PGE electrode can separate the anodic peaks of CC and RS even though they are present in a binary mixture.
Simultaneous detection of HQ and RS at HIL-PGE in PBS by CV
The HIL-PGE could not separate the anodic and cathodic peaks of CC, HQ and RS when they are present in a mixture. This resolving power of the HIL-PGE cannot be used to qualitatively detect CC, HQ and RS in the presence of others.
Simultaneous detection of CC and HQ at HIL-PGE in PBS by DPV
Simultaneous detection of CC and RS at HIL-PGE in PBS by DPV
HILL-PAGE could separate the peaks of CC and RS when present in a mixture. HILL-PAGE could separate the peaks of HQ and RS when present in a mixture.
Simultaneous detection of CC, HQ and RS at HIL-PGE in PBS by DPV
Quantitative estimation of CC in presence of HQ in PBS at HIL-PGE
The concentration of RS was kept constant while the concentration of CC was changed sequentially by adding a certain amount of the solution using a micropipette. 113 Figure 4.64: Calibration curve for CC estimation in the presence of RS (current response . with concentration variation).
Quantitative estimation of HQ in presence of RS in PBS at HIL-PGE
114 Figure 4.65: DPV for quantitative estimation of HQ in the presence of RS by HIL-PGE in.
Quantitative estimation of HQ in presence of CC in PBS at HIL-PGE
115 Figure 4.67: DPV for quantitative estimation of HQ in the presence of CC by HIL-PGE in.
Quantitative estimation of RS in presence of HQ in PBS at HIL-PGE
116 Figure 4.69: DPV for quantitative estimation of RS in the presence of HQ by HIL-PGE in.
Quantitative estimation of RS in presence of CC in PBS at HIL-PGE
117 Figure 4.71: DPV for quantitative assessment of RS in the presence of CC at HIL-PGE v.
This can be explained by SEM image data of the working electrode surface. There are so many pores and pits that the surface of the working electrode increases.
Simultaneous detection of HQ and CC at BIL-PGE in PBS by DPV
Simultaneous detection of CC and RS at BIL-PGE in PBS by DPV
Simultaneous detection of HQ and RS at BIL-PGE in PBS by DPV
BIL-PGE could separate CC and HQ peaks when present in a mixture.
Comparison of the response of HQ, CC and RS at HIL-PGE and BIL-PGE in PBS by DPV
The concentration of HQ, RS and CC was varied successively by adding a certain amount of solutions using a micropipette. This calibration curve can be used for quantitative estimation of RS, HQ and CC simultaneously from a ternary mixture.
Price of Conventional electrodes vs PGE
Comparison of HIL-PGE and BIL-PGE
Conclusions
101] Liu, W., Wu, L., Zhang, X., and Chen, J., 2014, “Simultaneous Electrochemical Determination of Hydroquinone, Catechol, and Resorcinol by Nitrogen-Doped Porous Carbon-Nanopolyhedrons-Multiwall Carbon-Nanotube Hybrid Materials Modified glassy carbon electrode”, Bull. 102] Liu, Y., Wang, W., Wei, H., Li, J., Lu, X., and Liu, X., 2014, “Simultaneous determination of dihydroxybenzene isomers based on thionine functionalized multiwalled carbon nanotube modified electrode”, Appl.
