Fabrication of Electrochemical Sensors for the Sensing of Hazardous Compounds
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Environmental pollution is a big threat for the world. There are various inorganic/organic pollutants which derailed out from the different industries and have toxic nature. Many compounds such as toluene, hydrazine, nitrite, hydrogen peroxide, resorcinol, 4-chlorophenol, hydroquinone, catechol, phenol, nitro-phenol, and nitrobenzene are widely used in the industries and these compounds are showed hazardous effects on humans, animals, as well as environments. Some of these compounds even in trace amount may harm the human beings. Therefore, the determinations of such compounds are of great importance. Previously various approaches have been made to detect these organic/inorganic compounds. Various research groups have employed different detection techniques which showed good results. Recently, electrochemical approach attracted much attention of the researchers due to its excellent sensitivity, good detection limit, reproducibility, repeatability, simple fabrication procedure, low cost, and high selectivity. In this chapter, fabrication and advantages of electrochemical sensor for the sensing of different analytes have been discussed. Moreover, the role of newly designed and different electrode modifiers for the fabrication of electrochemical sensors has also been discussed.
In present time environmental pollution is a great threat for the whole world [1, 2, 3, 4, 5, 6, 7, 8, 9]. Environmental pollution rapidly increasing, and various factors are responsible for this enhanced environment pollution. There are various organic, inorganic, or other toxic compounds which possess hazardous impact on animals as well as humans and environment [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. Phenol family is toxic in nature and widely used in various applications. Catechol has been used in chemical, textiles, oil refinements, plastic industries, and agricultural fields . Similarly, hydroquinone which is also isomer of catechol has been applied in various applications such as paints, cosmetics, antioxidants, pesticides, oil industries, photography and pharmaceuticals etc. . There is also another form of phenol derivative known as resorcinol (1,2-benzenediol) has been used in food, dye, and pharmaceutical industries. These phenolic compounds such as catechol and resorcinol have hazardous impact on the environment as well as plants, animals, and human beings. Another form of phenol family is 2-phenylphenol and 4-chlorophenol [1, 5] which also has toxic nature. These phenolic compounds are also used as disinfectant in nursing homes, households, fungicides, food processing plants, hospitals, barbershops, industries, and laundries . These phenolic compounds are also hazardous to the skin, eyes, and responsible for other health issues. Nitrite is another source of nitrogen for green plants and known as intermediate byproduct in the nitrogen cycles. Nitrite is also present in soil, water, and environment. Nitrite is also employed in food industries as preservative. Although nitrite does not cause harmful effects in moderate concentrations but at higher concentrations it may interact with hemoglobin to produce methemoglobin which inhibited the hemoglobin to transport the oxygen throughout the human body and can cause tissue hypoxia . Nitrite may also interact with amide, secondary amines, and tertiary amines to produce nitrosamines which are a carcinogenic compound. Thus, detection of nitrite is important for human as well as environmental concerns. Another compound urea is an organic compound which is used as fertilizer and converted to ammonia and polluted the environment. Urea is also present in protein metabolism and its presence in high concentration in the blood or urine may cause urinary tract obstruction, dehydration, shock, burn, kidney damage, and gastrointestinal bleeding. However, its presence in low concentration may also cause nephritic syndrome, cachexia, and hepatic failure. Urea is also used in milk and its higher concentration in the milk can causes ulcers, indigestion, acidity, etc. Therefore, the detection of urea is important for its use in dairy products, clinical diagnostics, fertilizer plants, or environment monitoring. Hydrazine is also an unstable and highly toxic compound and widely used in rocket fuels, duel cells, chemical reactions, catalyst, and other applications . The long-term contact with hydrazine may have carcinogenic effects due to containing of neurotoxin. Hydrogen peroxide (H2O2) plays crucial role physiological processes. H2O2 is an analyte used in clinics, drugs, chemical, and food industry . H2O2 also has negative impacts on the environments and human beings. So, the detection of H2O2 is also a necessary task. The nitro group containing aromatic compounds such as nitrophenol and picric acid (2,4,6 trinitrophenol) are highly toxic and explosive in nature [15, 17]. Picric acid is used in pharmaceutical, dye, and leather industries . It has toxic nature and has hazardous effects on the environment, plants, and human beings due to its bio-toxicity. Thus, the above-discussed compounds are highly toxic and have hazardous effects. Therefore, the determinations of such compounds are important task. In previous reports, various approaches have been used to the determination of such toxic compounds. The conventional methods such as high-performance liquid chromatography, spectrophotometry, quartz crystal microbalance, spectrofluorometry, surface plasmon resonance, electrophoresis, and flow injection chemiluminescence have been employed for the determination of toxic compounds [9, 28]. In last few years, electrochemical methods have attracted the scientific community for the sensing of toxic and hazardous compounds due to its simple fabrication, cost effectiveness, high sensitivity, selectivity, and repeatability [29, 30, 31]. Hence, in this chapter, we have discussed the recent advances in the sensing of different toxic and hazardous compound employing electrochemical methods.
How to Prepare the Electrochemical Sensors?
The fabrication of the electrochemical sensors is a simple task. Generally a glassy carbon electrode (GCE) or screen printed electrode (SPE) has been widely used as electrode substrate. The working area of the screen printed or glassy carbon electrode cleaned with alumina slurry and the electrode modifier has to be deposited on to the cleaned active area of the working electrode substrate. Further, this modified electrode to be dry in air for several hours and further used as working electrode.
Electrochemical Sensing of Hazardous Compounds
There are numerous toxic and hazardous compounds which are harmful to the humans, plants, animals, and environment. Herein, we have summarized the recent advances in the field of electrochemical sensing of different toxic compound using electrochemical methods.
Sensing of Catechol
Catechol is a derivative of the phenol and is highly toxic in nature. Nazari et al. prepared an electrochemical sensor for the sensing of catechol using ZnO-Al2O3 ceramic nanofibers electrode modifier while glassy carbon electrode was used as working substrate . The prepared ZnO-Al2O3 ceramic nanofiber was checked by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Energy-dispersive X-ray spectroscopy (EDX).
The obtained results showed good electrochemical activity of the AuNP/ZnO/Al2O3/GO/chit/GCE towards determination of catechol. The detection limit of 3.1 μM was obtained for catechol sensing using AuNP/ZnO/Al2O3/GO/chit/GCE.
In another report, Liu et al. employed F, N-doped carbon dots/laccase composite for the sensing of catechol . The authors of this work have prepared F, N-doped carbon dots decorated laccase using benign approach. The formation of the F, N-doped carbon dots/laccase composite was confirmed by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectroscopy, and photoluminescence (PL) spectroscopy which clearly showing the formation of F, N-doped carbon dots/laccase composite.
Sensing of Hydroquinone
Hydroquinone is another isomer form of catechol and derivative of phenol compound which is also toxic in nature and widely used in cosmetics. The accurate detection is necessary to avoid the carcinogenic effect of the hydroquinone.
The CV pattern of the bare GCE showed least current whereas Cu-MWCNTs@Chi/GCE exhibited the highest current towards the sensing of hydroquinone using cyclic voltammetry. This higher current activity or electrochemical behavior of the Cu-MWCNTs@Chi/GCE may be due to the synergistic interactions. The lowest detection limit of 0.04 μM was obtained with excellent reproducibility. The prepared Cu-MWCNTs@Chi/GCE was also employed for real sample analysis and obtained results showed satisfactory performance for practical applications.
Sensing of Hydrazine
Hydrazine is a hazardous compound and the determination of hydrazine is an important tool. Therefore, Beduk et al. have designed and fabricated a novel sensor for the detection of hydrazine . In this work, authors developed inkjet-printed paper using poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) = PEDOT:PSS decorated ZnO with nafion matrix sensor for the sensing of hydrazine.
Sensing of 2-Phenylphenol and Chlorophenol
Karimi-Maleh et al. developed the electrochemical sensor for the sensing of 2-phenylphenol (water pollutant) in presence of 4-chlorophenol using voltammetric measurements .
Karimi-Maleh et al. synthesized Fe3O4 nanoparticles decorated n-hexyl-3-methylimidazolium hexafluorophosphate composite. The n-hexyl-3-methylimidazolium hexafluorophosphate is denoted with HMPF6 whereas carbon paste electrode denoted with CPE. The CPE was modified with Fe3O4-NPs/HMPF6 denoted as Fe3O4-NPs/HMPF6/CPE. This modified Fe3O4-NPs/HMPF6/CPE was further employed for the sensing of 2-phenylphenol using voltammetry investigations. The fabricated sensor showed descent electrochemical performance.
The electrode modified with CNTs-OH/PtNP/RhB showed the highest electrochemical activity compare to the other four electrodes (GCE, CNTs-OH/GCE, CNTs-OH/RhB/GCE, and CNTs-OH/PtNPs/GCE). This improved electrochemical activity attributed to the synergistic effects.
The CNTs-OH/PtNPs/RhB/GCE exhibited higher current response compare to the other four electrodes (GCE, CNTs-OH/GCE, CNTs-OH/RhB/GCE, and CNTs-OH/PtNPs/GCE). However, GCE showed the least current response which is due to the poor and bare surface area of the electrode. The developed sensor showed the detection limit of 3.69 μM for 4-CP whereas 1.55 μM for 2,4,6-TCP, respectively. This electrode CNTs-OH/PtNPs/RhB/GCE also showed potential for real sample analysis. Thus it can be said the proposed electrode CNTs-OH/PtNPs/RhB/GCE possess excellent electrochemical activity and can be further employed in the detection of other hazardous compounds.
Sensing of Hydrogen Peroxide
The modified electrode with AuNPs-NH2/Cu-MOF showed the wide linear range from 5 μM to 850 μM. The detection limit using AuNPs-NH2/Cu-MOF modified electrode was calculated to be 1.2 μM. The obtained results were really impressive and could be further employed for practical applications.
Sensing of Nitrite
The observations revealed that bare GCE has poor electrochemical activity while the AuNPs/MoS2/GN/GCE possesses excellent electrochemical activity. The highest current response was obtained for the AuNPs/MoS2/GN/GCE in 1 mM NaNO2 in 0.1 M PBS at pH = 4. Further the effect of different scan rates was also investigated in 1 mM NaNO2 in 0.1 M PBS. The recorded CV graphs have been inserted in Fig. 14b. The obtained results showed that the current enhanced in a linear way with increase in the scan rate. This suggested the diffusion controlled process for the sensing of nitrite. The detection limit of 1 μM was obtained with wide linear range using AuNPs/MoS2/GN/GCE. The obtained detection limit for the sensing of nitrite was quite interesting and showed the potential use of AuNPs/MoS2/GN/GCE as a suitable electrode material for electrochemical sensing applications.
Conclusions and Further Outlook
There are so many toxic and hazardous compounds which are frequently used in various industries, cosmetics, and also used as preservatives. These compounds like catechol, hydroquinone, hydrazine, hydrogen peroxide, nitrite, chlorophenol etc. has negative impacts on the human beings and environment including animals and plants. There were conventional approaches to detect these toxic compounds but electrochemical has shown excellent performance with good detection limit and sensitivity. However, there are few challenges for the electrochemical approaches for the sensing of such hazardous compounds. Since the performance of the electrochemical detecting devices depends on the working substrate and electrode modifier, a highly efficient sensor needs to be developed.
The electrochemical sensitivity and detection limit can also be improved by applying new electrode materials such as highly conducting metal oxides, two-dimensional materials such MXene decorated metal oxides or polymer decorated metal oxides. Moreover, some new working electrode substrate consists of highly conducting materials are desirable for the construction of the sensors for electrochemical sensing of toxic or hazardous compounds.
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