Graphite pencil electrodes as electrochemical sensors for environmental analysis: a review of features, developments, and applications

Abstract

Graphite pencil electrodes (GPEs) are carbon-based electrodes that are recognized by their low cost, simplicity, commercial availability, ease of modification and disposability. GPEs are attractive substrates for electrochemical sensing because of their unique feature of “disposability” compared to other commonly used carbon-based electrodes. Mechanically rigid GPEs are easy to modify and miniaturize. The sensitivity and selectivity of GPE toward certain analytes can be enhanced by applying different modification materials. The primary focus of this review article is to highlight the applications of GPEs in the analysis of inorganic and organic pollutants in different environmental matrices. This review gives a brief overview of the various types of inorganic and organic pollutants and their impact on the environment. The key features of modified GPEs that enhance their electrocatalytic activity toward detection of certain target analytes are critically appraised. In the end, we summarize the current status, weaknesses and future prospects of GPE based sensors for environmental analysis.

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Abdel-Nasser Kawde

Abdel-Nasser Kawde received his B.S. (1991) and MS (1996) in chemistry from Assiut University, Egypt. He received his Ph.D. (2003) in analytical chemistry from New Mexico State University, New Mexico, USA under the supervision of Professor Joseph Wang. He has co-authored more than 100 journal and conference publications, and 15 US patents. His current research focuses on the development and characterization of electrochemical-based sensors and biosensors utilizing macro-, micro- and nanomaterials for various applications.

Nadeem Baig

Nadeem Baig received his B.S (Hons.) 2008 from University of the Punjab and M. Phil. from University of Engineering and Technology, Lahore in 2012. After that, he joined the doctoral program at King Fahd University of Petroleum and Minerals, Saudi Arabia on Jan. 2013. Mr Baig is working on the development of electrochemical sensors for various applications in the biological and environmental issues under the supervision of Professor A. Kawde. He is also interested in drug study. He has published five articles in ISI-indexed journals and has one patent.

Muhammad Sajid

Muhammad Sajid received his Ph.D. degree in Chemistry in May 2016 from King Fahd University of Petroleum and Minerals, Saudi Arabia. During his Ph.D., he worked on analytical methods development for trace level determination of endocrine disrupting compounds in biological samples. His research interests are focused on the development of miniaturized techniques for extraction of emerging pollutants in environmental, food and biological samples prior to their instrumental analysis. He is also interested in studying the impact of emerging pollutants on humans and the environment. He has co-authored more than 10 articles in ISI-indexed journals.

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1. Introduction

The Scopus database reveals that it was probably 1960 when for the first time a graphite electrode obtained from an HB pencil was used as the working anode in polarography without any modification.¹ Though this publication cites another work published in 1954, where a graphite pencil electrode was used as a reference electrode, this cited work is difficult to trace. Until the middle of the 1990s, there were only a few reports on modified GPEs for sensing of some analytes^2–4 e.g. an antibody immobilized GPE was utilized as a direct potentiometric immunoelectrode for detection of atrazine.³ However, these reports have yet not described any disposable and renewable GPEs. During the same period, some disposable electrodes such as metal electrodes or carbon materials deposited metal substrates were being used but their disposability was only good from an economic perspective and the issue of occupational and environmental safety was another aspect to be considered because of their applications in trace level detection of toxic substances.

To the best of our knowledge, renewable GPE was for the first time reported in 1997 for stripping voltammetry of cadmium and lead.⁵ Although for writing purposes, the renewable graphite pencils were introduced many years ago, but their potential as low-cost, readily available, and the renewable electrode was not realized up till 1997. Some notable works that were conducted in following years include detection of RNA and DNA,⁶ labels free detection of DNA hybridization,^7,8 and analysis of trace metals.⁹

The feature of renewable writing pencils where lead can be extruded to any desired length from the holder is very well utilized in GPEs. After each electrochemical measurement, the used surface of GPE can be easily renewed by removing the extruded part. Moreover, the sensing area of the pencil exposed in the solution can be easily controlled according to the requirement of analysis. From the green chemistry perspective, the only small material is used as an electrode and can be readily disposed of after use.

GPE is a subtype of the graphitic electrodes that has specialized characteristics of the high surface area, good conductivity and ease to use. However, the uniqueness of GPE is credited to some of its specific properties like it is cheap,¹⁰ commercially available and easily disposable. Moreover, GPEs are mechanically rigid, easy to modify and miniaturize.¹¹ GPEs are most attractive electrodes because they offer a simple and faster surface renewal compared to other commonly used electrodes which involve tedious surface polishing procedures. Because of renewable surfaces, GPEs are expected to give reasonably reproducible results. Additionally, GPEs show strong adsorption properties, low background current, and wide potential window.¹²

As a result of industrial revolution and urbanization, human environment is badly affected by the variety of inorganic and organic pollutants. These pollutants have been identified as a serious challenge for the survival of human and wildlife. Hence, some of these pollutants have been regulated by national and international environment protection agencies and their allowable limits have been defined. No doubt, last few decades have witnessed great progress in the instrumentation for analysis of a variety of inorganic and organic pollutants but the high cost of these sophisticated instruments itself a big challenge in low resource setups. In addition to cost, these instruments are not suitable for in field applications. Simple, low-cost, efficient, portable and commercially available instruments or electrochemical sensors are desired to fulfill the demand of worldwide monitoring of pollutants in different matrices.¹³ Because of their simplicity, low-cost, large-scale availability, GPE based electrochemical sensors can be a good choice for environmental analysis in limited resource setups.

In the recent years, there has been observed an increasing trend for the applications of GPE based electrodes in the environmental analysis. To the best of our knowledge, this is the first review article that covers environmental analytical applications of GPE. Hence, this review gives brief description of

(i) Highly toxic pollutants from inorganic and organic origin.

(ii) Applications of GPE based electrochemical sensors for detection of these pollutants in environmental matrices.

(ii) Properties of the GPE and modified GPEs which enhance the sensitivity and selectivity of the electrode toward particular target species.

Fig. 1 gives a pictorial description of the features of GPE that have been covered in this review article.

Fig. 1 Features, modification materials and applications of graphite pencil electrode.

2. Electrochemical analysis of inorganic pollutants at GPE

The presence of metals, metalloids and other inorganic pollutants in environmental matrices is the most important environmental problem of the modern age. Both natural and anthropogenic sources contribute significant amounts of inorganic pollutants in the environment. In the coming sections, we enlist and highlight the importance of studying inorganic pollutants in the environment. The applications and potential of GPE based sensors for detection of inorganic pollutants are critically discussed in each section.

2.1. Metallic pollutants

Heavy metal pollution is a global environmental challenge. Heavy metals are distributed in the environment as a result of their increased applications in domestic, industrial and agricultural processes. The heavy metals such as mercury, lead, cadmium, chromium and arsenic are considered priority pollutants and have been regulated by a number of national and international environmental agencies. Metallic pollutants are recognized as “systematic pollutants” and they cause various ailments in the human body including damage to different organs. Some metals have been classified as carcinogens (well known or suspected) by USEPA.¹⁴ Metal toxicity to an individual depends on upon many factors such as kind of exposure (environmental or occupational), dose and period of exposure and nature of the metal itself. However, it is obvious that continuous exposure to low-level concentrations of these pollutants may lead to serious health implications. From the polluted environment, these metals may enter to food chain and thus to the bodies of humans, other animals, and aquatic organisms. Therefore, sensitive analytical methods are needed to measure the levels of heavy metals in the environment. In order to deal with a global environmental challenge posed by heavy metal pollution, simple, efficient, portable and cost-effective analytical methods are always desired. The last few decades have witnessed some major advancements in analytical instrumentation for analysis of heavy metals. The instruments like atomic absorption spectroscopy (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS) have been widely used for determination of metals in different matrices. Although, these instruments are highly sensitive and selective, but some major issues are associated with them. They are very expensive, require a number of consumables, and their portability for in field applications is a big challenge. Before the analysis, extended procedures of sample preparation are needed. Moreover, analysis is destructive, time-consuming and expert operators are required. Electrochemical methods for detection of heavy metals are simple, efficient, cost-effective, non-destructive and do not require extended sample preparation procedures for common environmental matrices such as water and soil. Electrochemical methods can provide adequate sensitivity and selectivity for detection of metals. In addition, these methods can be used for speciation and multi-elemental analysis. GPE based electrodes are very cheap and easily available and can be used to perform cost-effective analysis in limited resources setups. Hence, in the following paragraphs, we described some applications of GPE and modified-GPE for sensing of metals in environmental samples. Table 1 lists applications of GPE based electrodes for detection of inorganic analytes in different environmental matrices.

Table 1 Applications of GPE based sensors for detection of various inorganic pollutants in environmental samples

Electrode	Analyte	Matrix	Medium/pH	Technique	Linear range	LOD	Detection in real samples	Ref.
PSS/CnP/GPE	Cu(II)	Mineral, sea and river water	0.1 M KCl/3.0	DPASV	—	0.11 μg L^−1	Yes	21
Cu-P4R/PPy/GPE	Cu(II)	Tap water	pH range 4.0–6.0	Potentiometry	1 × 10^−5 M to 5 × 10^−2 M	6.7 × 10^−6 M	Yes	22
Cu–Car/PA/GPE	Cu(II)	Green tea leaves, black currants, sour cherry juice	6.0	Potentiometry	5 × 10^−6 to 1 × 10^−1 M	2 × 10^−6 M	Yes	23
P4VP/GPE	Cd(II)	Soil, tea, cabbage, liver, kidney, heart	6.4	Potentiometry	1 × 10^−7 to 1 × 10^−1 M	2.51 × 10^−8 M	Yes	15
Bi-NPs/puMWCNT/GPE	Pb(II)	—	0.1 M NaOAc/4.5	DPV	0.4–10.8 μmol L^−1	1.7 nmol L^−1	No	17
DPC/PANI/GPE	Cr(VI)	Tap water	2.0	Potentiometry	1 × 10^−6 to 1 × 10^−1 M	8 × 10^−7 M	Yes	18
Tar/PPy/GPE	Zn(II)	Barley flakes, rice, dry milk	5.0	Potentiometry	1.0 × 10^−5 to 1.0 × 10^−1 M	8.0 × 10^−6 M	Yes	19
NG/PG/BiE	Pb(II), Cd(II), Zn(II)	Tap water	0.1 M acetate buffer/4.6	SWASV	2–20 μg L^−1, 10–100 μg L^−1	0.17 & 0.22 μg L^−1, 0.09 & 0.09 μg L^−1, 0.13 & 0.20 μg L^−1	Yes	20
Ag/FeOOH/GPE	H2O2	Disinfectant	0.1 M PBS/7.2	Amperometry	0.03–15.00 mM	22.8 μM	Yes	32
PtNPs/GPE	H2O2	—	0.1 M PB/7.0	Amperometry	10–110 μM	3.6 μM	No	30
PdNPs/GPE	H2O2	—	0.1 M PB/7.0	Amperometry	10–140 μM	0.045 μM	No	31
PVF^+/MWCNTs/GPE	NO2^−	Mineral water	0.05 M PB/7.4	DPV	1–400 μM	0.1 μM	Yes	33
Poly(PyY)/GPE	NO2^−	Salami	0.1 M PBS/4.0	Amperometry	1–100 μM	0.5 μM	Yes	34
Au NPs/GPE	N2H4	Drinking water	0.1 M PB/5.0	SWV/amperometry	0.05–1000 μmol L^−1, 25–1000 μmol L^−1	42 nmol L^−1, 3.07 μmol L^−1	Yes	12

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2.1.1. Cadmium. Different anthropogenic activities result in accumulation of cadmium in soil. This cadmium is then transferred to the human body through consumption of contaminated plants and food. The other sources may include smoking and occupational exposure. The fever and muscle ache that leads to respiratory problems and damage to kidneys. Exposure to cadmium has also been linked with lung and prostate cancer. So its environmental monitoring with simple, cheap and efficient methods is desired.

An ISE was fabricated by electropolymerization of 4-vinyl pyridine on 2B pencil graphite as ionosphere for Cd²⁺. The electrode gave good limit of detection (LOD) down to 2.5 × 10⁻⁸ M and relatively a broad pH range 4.0–7.5 for cadmium measurement. The major advantages of this modification are cheap material, easy modification and potential to apply in in situ measurements of cadmium in real samples. The developed electrode was used to determine Cd²⁺ by using the potentiometric method in different environmental (soil), food (tea, vegetables) and biological (cow liver, kidney, heart and chicken liver and heart) samples.¹⁵

2.1.2. Lead. Lead is soft, malleable, resistant to corrosion, ductile and has low melting points. Due to these features, it has been widely used in automobile, ceramics, plastic and paint industries. Lead poisoning is a serious issue, and accumulation of lead in the body may prove fatal in certain conditions. As a result of oxidative stress, lead poisoning can lead to disorders in central nervous system, renal and reproductive system.¹⁶ Recently, GPE was modified by the subsequent application of purified MWCNTS and bismuth NPs. The modified electrode was termed as Bi-NPs/puMWCNTS/GPE and was used for analytical measurement of Pb²⁺ ions. The electrode showed excellent electrocatalytic activity due to enhanced surface area of the modified surface and proper LODs down to 1.7 nM were obtained by using differential pulse voltammetry. As the major focus of this work was a synthesis of bismuth nanostructures, modified electrode was not used to demonstrate its feasibility for real environmental samples.¹⁷

2.1.3. Chromium. Chromium is also widely used in a range of industrial applications including leather tanning, manufacturing of alloys, electroplating, wood treatment and metal smelting. In the environment, chromium exists in two oxidation states, Cr(iii) and Cr(VI). Cr(VI) is considered toxic anion because of its high mobility and water solubility compared to Cr(iii). Cr(VI) is also reported to have carcinogenic effects. Hence, instead of measuring total chromium content in the environment, it would be more suitable to measure only hexavalent chromium. Electrochemical methods are thus capable of speciation analysis because of different redox potential values for various oxidation states of the same element. In order to detect Cr(VI) by potentiometric method, GPE was electrochemically modified with polyaniline in presence diphenyl carbazide and resulting modified GPE was termed as GPE/PANI/DPC. The resulting electrode gave a good linear range from 10⁻⁶ to 10⁻¹ M with a limit of detection 8.0 × 10⁻⁷ M at pH = 2. The electrode showed good tolerance against interfering ions and no significant decrease was observed in the response for the period of one month, showing a good shelf life.¹⁸

2.1.4. Zinc. Zinc and different compounds derived from it are extensively used in industrial processes such as alloy formation, wood treatment, electroplating, rubber processing, dye, pharmaceutical products, and paints. In medical, zinc is used in mouthwashes, disinfectants, antiseptics and mineral–vitamin formations because of its biocidal activity. It is a micronutrient whose small quantiles are essential to the human body, but its high concentrations can cause different ailments including vomiting, fever, pulmonary problems, nausea, and renal disorders. Therefore, highly efficient, cheap and simple methods are needed to monitor the zinc contents in the environmental and biological samples. A potentiometric ion-selective electrode for zinc was fabricated by modification of GPE with polypyrrole nano-film. Polypyrrole was electrochemically deposited on GPE in the presence of tartrazine as a dopant. This electrode showed analytical features such as wide linear range, high selectivity and good stability and reasonably low limit of detection of 8.0 μM at optimum pH. Applications of the modified electrodes were extended for testing of Zn²⁺ in barley flakes, rice and dry milk.¹⁹

There are only few reports which describe simultaneous detection of more than one metal ions on modified GPEs. The Nafion graphene nanocomposite pencil graphite bismuth film electrode (NG-PG-BiE) was used for simultaneous detection of Pb²⁺, Cd²⁺ and Zn²⁺ ions in tap water using square wave anodic stripping voltammetry. The NG-PG-BiE exhibited detection limits of 0.17, 0.09 and 0.13 μg L⁻¹ for Zn²⁺, Cd²⁺ and Pb²⁺ much lower than below the USEPA allowable limits of 5 mg L⁻¹, 5 μg L⁻¹ and 15 μg L⁻¹ for Zn²⁺, Cd²⁺ and Pb²⁺ respectively.²⁰

2.1.5. Copper. Copper is essential metal that is present in all organisms, and its excess or deficiency is associated with different diseases. Copper is released into the environment as a result of mining and industrial process. Human is exposed to copper through the environment and contaminated food. Hence, accurate monitoring of copper in the environment is desired. Polystyrene sulfonate–carbon nanopowders composite was used to modify GPE for determination of Cu²⁺ in mineral, river and seawater using differential pulse anodic stripping voltammetry. This simple to fabricate sensor showed good sensitivity toward Cu²⁺ and tolerance against interfering cations and anions. This method showed LOQ of 0.37 μg L⁻¹ and it was much lower than the allowable limit of copper in waters regulated by European (2 mg L⁻¹) and Spanish (1 mg L⁻¹) Drinking Water Directives.²¹

Ion-selective electrodes have shown some exciting applications in the current years because of their ability to analyze the target ions rapidly in small sample specimen with high accuracy and reproducibility. Potentiometric based ion selective electrodes are convenient because of their low cost, non-destructive analysis, and not affected by turbid or colored samples. Polypyrrole conducting polymer doped with Ponceau 4R azo dye based modified GPE was used for selective potentiometric determination of Cu²⁺ in water. The pyrrole monomer was electrochemically polymerized on GPE in the presence of Ponceau 4R azo dye as a dopant. The presence of the dopant in electropolymerization generates selective recognition sites in the formed polymer, and these sites can interact selectively with Cu²⁺ ions. This modified electrode showed good LOD values down to 6.7 μM. LODs remained unchanged for the shelf life of 21 days, and a slight change was observed after 60 days. The one limitation for this electrode is that it is only suitable in acidic medium (pH, 5).²² Another similar work is reported where GPE modified with conducting polyaniline doped with copper carmoisine dye complex was used for potentiometric monitoring of Cu²⁺ in green tea leaves, black currants, and sour cherry juice. This modified electrode showed a very fast response (20 s) and good limit of detection 2.0 μM.²³

2.2. Comparison of GPE based sensors and other instrumental methods for analysis of metals

Here it will be suitable to compare the GPE based electrochemical methods and other instrumental methods which are used for detection of metallic impurities in environmental samples. Flame atomic absorption spectrometry (FAAS) is one of the major technique used for determination of metals such as cadmium, lead, zinc and copper in different matrices. The disadvantages that are associated with FAAS are mainly related to matrix interference on the analyte signal and higher limit of detections than the normal concentration of metals present in real samples. Thus, removal of interferences and improvement in LOD is achieved through separation and preconcentration of the metals prior to analysis.²⁴ This kind of sample preparation itself is a tedious job because multistep processes are involved that may comprise reaction (derivatization), extraction, separation, preconcentration and determination. Moreover, the accuracy and reliability of sample preparation assisted FAAS depends upon the number of steps involved. It is matter of fact that these sample pretreatment methods have made trace element assays more convenient, but more often they require large sample or reagent volumes (several tens to hundreds of milliliters) to get high enrichment factors or preconcentration factors. However, when it is difficult to obtain such large sample volumes, these techniques led to low enrichment factors and high detection limits.²⁵ When dealing with trace elements, total content analysis as well as speciation analysis both are very important. Trace metals exist in different oxidation states and some metal species have been reported to show varying toxicities in their different oxidation states, for example, hexavalent chromium is much more toxic than trivalent chromium.²⁶ In order to find out the role of trace element species in different diseases, recent research in area of trace element analysis is shifting from total content to speciation analysis. Hence, very efficient extraction methods that would be capable of selective speciation and then highly selective detectors are desired in analytical instruments.²⁷

In general, no specialized sample preparation is necessary for electrochemical methods. In addition to that electrochemical methods are cost and time effective. For the trace elemental analysis, the speciation can be easily performed by simply altering the detection potential. Moreover, very low LODs can be achieved by electrochemical methods without performing any sample preconcentration. When particularly talking about graphite pencil electrodes, they are easily available in the market and cost much lesser than other carbon or metal-based electrodes. However, the automation of GPE based electrochemical methods presents a challenge that need to be addressed for successful commercialization of such analytical tools. Table 2 compares GPE based sensors with other instrumental methods.

Table 2 Comparison of GPE based electrochemical methods with other instrumental methods

Metal	Matrix	Sample preparation	Detection method	Detection technique	LOD	Ref.
Cadmium	Water	Pyrolysis vapor generation	Spectroscopic	AFS	2.2 ng mL^−1	77
	Water and food	Modified MNPs based SPE	Spectroscopic	FAAS	3.71 ng mL^−1	78
	Water and food	Nanomagnetic task specific ionic liquid as a selective sorbent in SPE	Spectroscopic	FAAS	0.5 ng mL^−1	24
	Water and food	Pyridine-functionalized magnetic nanoporous silica material	Spectroscopic	FAAS	0.04 ng mL^−1	79
	Soil, tea, cabbage, liver, kidney, heart	—	P4VP/GPE based electrochemical	Potentiometry	2.51 × 10^−8 M	15
Lead	Water and human air	Magnetic ion-imprinted polymer (MIIP) nanoparticles	Spectroscopic	GFAAS	2.4 ng L^−1	80
	Water samples	Air-assisted liquid–liquid extraction	Spectroscopic	FAAS	1.36 ng mL^−1	81
	—	—	Bi-NPs/puMWCNT/GPE based electrochemical	DPV	1.7 nmol L^−1	17
Copper	Water, food and hair samples	Switchable solvent-based liquid phase microextraction	Spectroscopic	FAAS	1.8 ng mL^−1	82
	Water samples	Temperature-assisted dispersive liquid–liquid microextraction	Spectroscopic	GFAAS	1.82 ng L^−1	83
	Mineral, sea and river water	—	PSS/CnP/GPE based electrochemical	DPASV	0.11 μg L^−1	21
	Tap water	—	Cu-P4R/PPy/GPE based electrochemical	Potentiometry	6.7 × 10^−6 M	22
	Green tea leaves, black currants, sour cherry juice	—	Cu–Car/PA/GPE based electrochemical	Potentiometry	2 × 10^−6 M	23
Zinc	Water samples	Temperature-assisted dispersive liquid–liquid microextraction	Spectroscopic	GFAAS	0.89 ng L^−1	83
	Water samples	Solid phase extraction	Spectroscopic	FAAS	0.24 ng mL^−1	84
	Water samples	Surfactant-based dispersive liquid–liquid microextraction	Spectroscopic	FAAS		85
	Barley flakes, rice, dry milk	—	Tar/PPy/GPE based electrochemical	Potentiometry	8.0 × 10^−6 M	19

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2.3. Other inorganic pollutants

In addition to the metals, other substances such as hydrazine, hydrogen peroxide, sulfides, cyanide, perchlorate, oxides of nitrogen and sulfur have a great environmental impact as inorganic pollutants. Few of these have been electrochemically detected in different environmental matrices using GP based electrodes. A brief account of these pollutants is provided in coming sections.

2.3.1. Hydrazine (N₂H₄). Hydrazine is an inorganic molecule with molecular weight 38 g mol⁻¹. Hydrazine is used in many industrial applications such as pesticide in agriculture, starting material for many chemicals and plastics, intermediate in pharmaceutical and corrosion control agent in the treatment of water boilers. As it is lightweight, volatile and water-soluble colorless liquid, it has the ability to be absorbed by oral, dermal and inhalation pathways in the living organisms. Hydrazine has been identified as a highly toxic substance to the living organisms because its exposure may lead to serious damage to kidney, liver, lungs and central nervous systems. It also affects negatively the reproduction system and its carcinogenic effects have also been reported. Hence, simple, low cost and reliable analytical methods are desired to monitor the environmental levels of hydrazine. Recently, AuNPs modified GPE was prepared by immersing the GPE in a test tube containing a solution of ascorbic acid and gold(III) chloride and heating it up to 75 °C for 15 minutes in water bath. This modification procedure does not require any cross-linking molecules and describes direct attachment of AuNPs on GPE. This modified electrode was used for detection of N₂H₄ in drinking water, and excellent limit of quantification down to 100 nM and limit of detection 42 nM was achieved using square-wave voltammetry as a mode of detection.¹²

Later on, Chelladurai Karuppiaha et al. have reported highly sensitive sensor Au NPs/AG/SPCE for detection of hydrazine. This sensor was more sensitive compared to simple Au NPs/GPE. The sensitivity of the SPCE was enhanced by coating the dispersed graphite and dried cast graphite was later on electrochemically activated. On AG/SPCE the Au NPs were electrochemically deposited and this activation process of graphite and the presence of Au NPs on the surface of SPCE made the sensor highly sensitive towards hydrazine and very low LOD down to 0.57 nM achieved.²⁸ However, in case of Au NPs/GPE no activation of graphite was involved prior to modification with Au NPs.

2.3.2. Hydrogen peroxide (H₂O₂). H₂O₂ has widespread applications in enzymatic reactions, food industry, organic products, disinfectants, bleaches, hair dyes and various other industrial, environmental and biomedical processes. It is also naturally present in some foods. It has been reported to have reasonable toxicity against living organisms.²⁹ Because of its increasing use in different applications; it is critical to monitor H₂O₂. For sensing of H₂O₂, compared to spectroscopic methods, electroanalytical methods are more attractive because of their fast response and high sensitivity.³⁰ Although conventional methods of H₂O₂ are based on enzyme based sensors but due to high cost of enzymes and complex synthetic procedures for modification, non-enzymatic electrochemical sensors are getting more popular.

Some sensitive analytical methods based on a modification of GPE have been developed for detection of H₂O₂. NPs and nanomaterials modified GPE showed good electrocatalytic activity toward H₂O₂ detection. Platinum NPs modified GPE was fabricated for highly sensitive detection of H₂O₂. This modification was achieved by a very simple process which involves dipping of GPE in an aqueous solution of 0.5 mM (NH₄)₂PtCl₄ and 0.55 mM ascorbic acid followed by heating at 75 °C for 15 min. The electrode gave excellent tolerance against interferes and LOD of 3.6 μM was achieved.³⁰ In another work, palladium NPs modified GPE was used for sensing of H₂O₂. This method also involves the facile synthesis of Pd NPs on the surface of GPE. In this Pd NPs were prepared by the very simple approach by adding aqueous solution of ascorbic acid (AA) to aqueous solution of (NH₄)₂PdCl₄ and stirring it for 15 min at room temperature. The color change of (NH₄)₂PdCl₄ from pale yellow to dark brown was indicative of NPs formation. GPE was then incubated in a solution of NPs for 15 min at room temperature. The H₂O₂ was determined in the aqueous medium using the amperometric technique. The modified electrode was capable of detecting H₂O₂ at nanomolar levels by amperometric method, and LOD of 45 nM was obtained which was much lower than the LOD obtained at bare GPE under the same set of experimental conditions (0.58 mM).³¹

In another work, Ag/FeOOH nanocomposites were immobilized on GPE for amperometric detection of H₂O₂. The rationale behind employing FeOOH nanomaterials was their high surface area which can provide better support for AgNPs immobilization. The developed electrode showed good long-term stability and was able to retain 90% of the signal after 3 weeks of modifications. LOD of 22.8 μM was achieved and method was used to measure H₂O₂ in disinfectants.³²

2.3.3. Nitrite ion (NO₂⁻). Nitrite ion is an intermediate in the nitrogen cycle. It results from oxidation of ammonia or reduction of nitrate. It is used as a food preservative in the food industry. It is hazardous because it combines with blood pigments to produce meta-hemoglobin, which in turn, depletes oxygen in tissues. It forms carcinogenic N-nitrosamines upon reaction with secondary amines. A polymer nanocomposite modified GPE was formed by single step electropolymerization of poly(vinyl ferrocene) (PVF) in the presence of MWCNTs. The resulting modified electrode was termed as poly(vinyl ferrocenium) (PVF⁺)/MWCNTS/GPE. The electrochemical detection of nitrite was carried out in commercial mineral water using DPV, and a LOD of 0.1 μM was achieved.³³ Similarly, in another work, pyronin y modified GPE was utilized for amperometric detection of nitrite and LOD of 0.5 μM was achieved.³⁴

3. Electrochemical analysis of organic pollutants at GPE

The rapid industrialization in the modern era resulted in the release of the toxic organic pollutants into the environment. Although most of the industries try to detoxify or degrade these organic pollutants by physical, chemical or biological means but still significant amounts of these pollutants are released into the environment. Organic pollutants are famous by different names including “emerging organic pollutants”, “priority pollutants”, “endocrine disrupting compounds” etc.³⁵ A continuous monitoring of these hazardous chemicals is necessary to take precautionary measures for sustainability of the healthy environment. In the coming section, we discuss the applications of GPE based sensors for detection of different classes of organic pollutants in environmental matrices.

3.1. Phenolic compounds

Phenolic compounds are very common and abundant environmental pollutants. Most of the phenolic compounds induce very severe health problems, and they have been classified as hazardous wastes and virulent pollutants by the United State Environmental Protection Agency.³⁶ They adversely affect human health by accelerating weight loss and weariness. Phenols are reported to cause respiratory cancer, cardiac and immune system disorders in case of severe exposures. The applications of GP based electrodes for high-sensitivity detections of different phenols are given in following sections.

3.1.1. Phenol. Recently, a very simple method for the determination of phenol was developed. In this method, GPE surface was charged by pretreating in NaOH solution. The charged GPE showed the capability of self-electropolymerization of phenol on its surface, and this led to the sensitive detection of phenol. The self-electropolymerized phenol was detected using square wave voltammetry. Very low limit of detection (4.17 nM) was achieved using this pre-charged sensor.³⁷

3.1.2. Nitrophenols. The simplest phenol derivative, 4-nitophenol, is a major threat to the environment. 4-Nitrophenol is widely used in the fungicides, pesticides, and organic dyes and also in some pharmaceutical products. Eventually, as a result of excessive use of these products, the 4-nitrophenol is released into the environment. It has good solubility in water,³⁸ and it does not degrade easily into other components, which in turn make it more persistent in the ground water. In addition, it is considered as carcinogenic and poses a serious threat to the human health.³⁹ A-N Kawde, and M. Aziz introduced a single step modification of GPE by electrochemical reduction of the Cu(II) on its surface. The Cu is more cost effective compared to Au, Ag, and Pt. The amperometric based electrochemical method was developed for the determination of 4-nitophenol and the wide linear range was obtained from 50 to 850 μM with LOD of 1.91 μM. The presence of potential interferences like phenol, 4-amino phenol and 3,4-dichlorophenol showed no effect on the detection of 4-nitopheneol.⁴⁰ In another method, the pencil graphite electrode was modified by the bismuth film and was used for the simultaneous analysis of 2-nitrophenol and 4-nitrophenol. Differential pulse voltammetry was used for the electrochemical analysis of 2-nitrophenol and 4-nitrophenol. Bismuth film electrodes are considered as alternate of the mercury film electrode due to environmental friendliness.⁴¹

3.1.3. Alkylphenols. Alkylphenols added into the environment due to the degradation of alkyl phenol polyethoxylates which is used in the detergent formation, fuel additive, fire retardant, antioxidant, thermoplastic elastomers, and phenolic resins. I. David et al. compared the response of the CNT/GPE and PGPE electrode for 4-tert-octylphenol. The CNT on GPE surface instead of increasing the current, the current was decreased while pretreatment significantly enhanced the current. The PGPE was used for the determination of 4-nonylphenol, 4-octylphenol and 4-tert-octylphenol by applying differential pulse voltammetry and developed method gave low detection limits 0.42, 0.25, and 0.77 μM.¹¹

3.1.4. Bisphenol A. Bisphenol A (BPA) belongs to the family of endocrine disruptors. Endocrine disrupters are the chemicals which interfere with normal hormonal activity in the human and wildlife. BPA is used as intermediate in the manufacturing of automotive lenses, building materials, thermal paper, compact disks, protective window glazing, adhesive, paper and protective coatings. It is also used in the processing of PVC plastics. The water solubility of the bisphenol A is 120–300 mg L⁻¹. The high solubility of the BPA in water is something which raises concerns about its toxicity to aquatic and human life.⁴² The striking effect of the BPA is on fetal and early childhood development. It also has an adverse effect on sexual differentiation and the brain development. It also reduces the sperm quality, enhances the risk of cancers and affects the thyroid hormone functions. The high level of BPA is observed in the condition of obesity, diabetes, and liver dysfunction. It is really dangerous for the fetus, pregnant women, and children.⁴³ It is highly important to find out the BPA concentrations in the water using efficient and cost-effective analytical strategies. The GPE was modified with polyaniline (PANI) and multiwalled carbon nanotubes (MWCNTs) for detection of BPA. PANI is extensively used conductive polymer for the electrochemical sensor fabrication due to its high electrocatalytic activity and the biocompatibility. On the other hand, the MWCNTs have great attraction for the sensing applications of the organic compounds due to high conductance, good adsorptive capability and high specific surface area. By using amperometric technique this modified GPE has shown wide linear range of 1–400 μM with LOD of 10 nM for BPA. Moreover, PANI/MWCNTs/GPE was applied to detect BPA in water extracted from baby bottles.⁴⁴ In another work, A. Ozcan used the synergistic effects of sodium hydroxide and the lithium perchlorate for the pretreatment of the GPE. The electrochemical pretreatment could enhance the selectivity and the sensitivity of the relevant analyte. In this case, pretreatment electrolyte improved the GPE surface significantly for the electrooxidation of BPA. The adsorptive stripping differential pulse voltammetry was used for the determination of BPA. The limit of detection was improved to 3.1 nM compared to above mentioned PANI/MWCNTs/GPE. This PTGPE was applied to determine BPA in tap and river water.⁴⁵

3.2. Organic dyes

A lot of dyes are released into the water as a residue during the coloring process in the textile industry and severely pollutes the environment. In dying process, almost 10 to 15% and in some conditions, up to 50% dyes are wasted and released to the aquatic bodies. Azo dyes are the commonly used dyes and contain azo group (–N=N–). The presence of azo dyes in the water has adverse effect on the aquatic life and the human health.⁴⁶

3.2.1. Chrysoidine. One of the azo dye is chrysoidine that has brownish red color. Chrysoidine is used in the dying of leather, plastic, natural and synthetic fiber, cosmetics, foodstuff, waxes, and papers. In the case of inhalation, dermal exposure, and oral ingestion, it induces acute and chronic toxicity. Hence its use in foodstuff has been banned. Moreover, it is also considered as carcinogenic. In some countries, it is being used to disinfect the fish skin. Hence, monitoring of its environmental concentrations is highly important from health and safety perspective. A. Ensafia et al., studied the interaction of ds DNA with the chrysoidine to develop a sensitive method for its detection in the aquatic system. It has been observed that DNA/MWCNTs-PDDA/GPE surface can adsorb chrysoidine more efficiently and strongly compared to the bare GPE surface. As the guanine and adenine are electrochemically active nucleotides their electrooxidation current reduced due to the adsorption of the chrysoidine, and this is another tool for the measurement of the analyte. A good linear range was obtained for DNA/MWCNTs-PDDA/GPE sensor ranging from 0.05 to 15.0 μg mL⁻¹ with LOD of 0.03 μg mL⁻¹ by using DPV. The developed method was applied on textile effluents, sauce, and the fish samples.⁴⁷

3.2.2. Sulfanilamide. Sulfanilamide is used in the synthesis of azo dye. Sulfanilamide is the first chemically synthesized antimicrobial agent, and it is also utilized in some drugs for the treatment of throat, urinary tract, and vaginal infections. Sulfanilamide is also found in the animal products and at high concentrations, it has been reported to induce allergic effects. Moreover, it is also toxic to the aquatic animals. Molecularly imprinted polymer (MIP) based electrochemical methods are highly selective and sensitive for the determination of the relevant analyte. In molecularly imprinted polymer based electrochemical methods, the complementary cavities are formed for the analyte, which makes it highly selective towards the target analyte. These cavities are formed by initiating the polymerization in the presence of the analyte and afterward the analyte is extracted out. Molecular imprinted polymer based graphite pencil electrode was prepared from the electropolymerization of the pyrrole for determination of sulfanilamide (Fig. 2 describes schematic of MIP modified GPE). DPV was used for the sensing of sulfanilamide. It resulted in two linear ranges 5.0 × 10⁻⁸ to 1.1 × 10⁻⁶ M and 1.1 × 10⁻⁶ to 48 × 10⁻⁶ M. The MIP/PPy/GPE applied to the ground water for detecting sulfanilamide and the fabricated sensor was highly sensitive with LOD of 2.0 × 10⁻⁸ M.⁴⁸

Fig. 2 Schematic representation of electrochemical detection of SN using molecularly imprinted PPy films on GPE. Reproduced from ⁴⁸ with permission. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.2.3. Sudan dyes. In chemical industries, the Sudan dyes are used as the coloring agent for waxes, fats, oils, shoes, petrol, spirit varnishing, petrol and printing inks. Sudan dyes are considered as carcinogenic by the International Agency for Research on Cancer (WHO). Due to carcinogenic nature of the Sudan dye, its use as a coloring agent in the food products is not allowed by national and international food regulations. The method development for the detection of Sudan dyes is very important due to its toxicity. A sensitive method has been developed for Sudan II dye. Interaction of dsDNA and the Sudan II was investigated on the pencil graphite electrode surface. The electrical response of the guanine and the adenine decreased in the presence of Sudan II, and this interaction was further confirmed by absorption spectrophotometry. The decrease in current may be due to the shielding of the oxidizable group of the guanine and the adenine by Sudan II. The linear range obtained by the interaction of dsDNA and Sudan II was 0.5–6.0 μg mL⁻¹ with LOD of 0.4 μg mL⁻¹. The direct determination of Sudan II was done by adsorptive stripping differential pulse voltammetry on the pretreated pencil graphite electrode. The pretreatment efficiently activated the surface for the analyte, and linear range obtained 0.0015–0.30 μg mL⁻¹ with a very low limit of detection 0.00007 μg mL⁻¹. The interferences have very small effect on the determination of Sudan II. Moreover, the satisfactory recoveries of the dye were obtained from the chili and ketchup sauce.⁴⁹

3.3. Pesticides

The pesticides represent a broad class of chemicals which is further divided into insecticides, herbicides, fungicides, nematicides, molluscicides, and rodenticides. No doubt, pesticides have played a major role in increasing food production worldwide but at the same time, their excessive use has led to serious health threats to the human and wildlife. The excessive use of pesticides results in their accumulation in food, soil, and water. It is reported that almost one million people die or suffer per year from chronic diseases resulted from exposure to pesticides.⁵⁰ Chlorpyrifos is the commonly used organophosphate pesticide, and its exposure can initiate disorders in neurological and the autoimmune system. A method has been developed for the trace level determination of the chlorpyrifos by fabrication the molecular imprinted polymer modified pencil graphite electrode. The impedance technique has been used for the measurement of the chlorpyrifos in the soil, tape water and corn leaves. The impedance increases as the concentration of the chlorpyrifos increases in the sample and wide linear range was observed from 20 to 300 μg L⁻¹ with a detection limit of 4.5 μg L⁻¹.⁵¹

3.4. Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants. PAHs have been added into the environment by human activities and also by natural incidents like volcanic eruption, forest fires, burning of fossils and the petroleum fuel. These compounds are considered highly toxic due to their mutagenic and carcinogenic behavior. The PAHs carcinogenicity has been related to the number of benzene rings in their structures and their transformation into the reactive electrophilic intermediates metabolites. The mammalian converts these compounds into the detoxification products in the liver and these metabolized products when excreted are more genotoxic and reactive. The PAH metabolites are responsible for carcinogenic process by covalently binding to the cellular DNA. These aromatic compounds become activated in the presence of sunlight and cellular destruction may start if skin comes into contact with these compounds in the presence of light.

3.4.1. 7,12-Dimethylbenz[a]anthracene. One of the PAHs is 7,12-dimethylbenz[a]anthracene (DMBA) that has been widely investigated for its biological activity. Because of its potential toxic and carcinogenic effects, many methods have been developed for its trace level quantitation. Y. Yardım et al., developed a highly sensitive method based on the disposable pencil graphite electrode and also studied the interaction of DNA and DMBA. The adsorptive stripping voltammetry was used for the low level determination of DMBA and an LOD of 0.194 nM was achieved.⁵²

3.4.2. Benzo[a]pyrene. One of the potent carcinogenic polyaromatic compounds is benzo[a]pyrene (BaP). The BaP is found in coke plants, steel foundries, and aluminum plants. The places where the coal fires is used for heating or cooking normally contains a higher concentration of BaP. The human exposure to the BaP occurs through water, foodstuff, and the air. Like other PAHs, BaP is hydrophobic compound and accumulate in the liver, kidney and the fat tissue. The mammalian metabolizes BaP with the help of epoxide hydrolase and cytochrome P450-dependent monooxygenase to primary and secondary metabolites like phenols, diols, quinones, water-soluble conjugates and arene oxide. BaP concentration in the environment is correlated to the total content of the PAHs. The adsorptive stripping square wave voltammetric based method was established by E. Keskin et al. using pretreated pencil graphite electrode for the determination of BaP. A small linear range has been observed from 0.25 to 1.25 μM with low detection limit 0.027 μM. The developed method was applied to find out the spiked BaP from the urine and satisfactory values for recoveries were obtained.⁵³ Table 3 lists applications of GPE based electrodes for detection of organic analytes in different environmental matrices.

Table 3 Applications of GPE based sensors for detection of various organic pollutants in environmental samples

Electrode	Analyte	Matrix	Medium/pH	Technique	Linear range	LOD	Detection in real samples	Ref.
pCu/GPE	4-Nitrophenol	—	0.1 M acetate buffer/4.8	Amperometry	50–850 μM	1.91 μM	No	40
ETGPE	Bisphenol A	Tap and river water	0.1 M PBS/2.0	ASDPV	0.05–5.0, 5–10 μM	0.0031 μM	Yes	45
DNA/MWCNTs-PDDA/GPE	Chrysoidine	Food products	0.5 M acetate buffer/4.8	DPV	0.05–15.00 μg mL^−1	0.03 μg mL^−1	Yes	47
MIP/PPy/GPE	Sulfanilamide	Blood serum, ground water	0.04 M BR buffer/2.0	DPV	5.0 × 10^−8 to 1.1 × 10^−6 M, 1.1 × 10^−6 to 48 × 10^−6 M	2.0 × 10^−8 M	Yes	48
PGPE	4-Nonylphenol, 4-octylphenol, 4-tert-octylphenol	Industrial water	PBS/7.4	DPV	1.2–94.0 μM, 0.6–78 μM, 2.38–243 μM	0.42 μM, 0.25 μM, 0.77 μM	Yes	11
MIP/PPy/GPE	Chlorpyrifos	Tap water, soil, corn leaves	0.1 M KCl	Impedance	20–300 μg L^−1	4.5 μg L^−1	Yes	51
PANI/MWCNTs/GPE	Bisphenol A	Extracted water from baby bottles	0.1 M glycine–NaOH/10.6	Amperometry	1–400 μM	10 nM	Yes	44
Ds-DNA/GPE	Sudan II	Chili sauce, ketchup sauce	0.5 M acetate buffer/4.8	DPV	0.5–6.0 μg mL^−1	0.4 μg mL^−1	Yes	49
PGPE	Sudan II	Chili sauce, ketchup sauce	0.1 M PBS/4.8	ASDPV	0.0015–0.30 μg mL^−1	0.00007 μg mL^−1	Yes	49
GPE	Thiourea	Waste water	PB/12.0	SWV	6.3–30 μM	1.29 μM	Yes	86
Ds-DNA/GPE	7,12-Dimethylbenz[a]anthracene	Urine	0.1 M acetate buffer/4.8	SWSV	2–10 nM	0.194 nM	Yes	52
PGPE	Benzo[a]pyrene	Urine	0.1 M acetate buffer/4.8	ASWV	0.25–1.25 μM	0.027 μM	Yes	53

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4. A brief overview of materials used for modification of GPE

In order to enhance their electrocatalytic activity toward certain inorganic or organic analytes, GP based electrodes were modified with different materials. These modification materials include metal nanoparticles, conducting polymers and carbon-based materials. In the following sections, we have discussed modification materials and their potential to increase electrocatalytic activity.

4.1. Nanoparticles

Nanoparticles (NPs) are characterized by their better physical, chemical, and electronic features than their bulk counterparts. It is the nature of the material used for synthesis that determines properties of NPs. Commonly, NPs are synthesized by chemical reduction of metal salts in the presence of a stabilizer. Stabilizers are attached to the surface of NPs and play a major role in defining their charge, solubility, and stability. NPs are extremely sensitive to the changes occurring on their surface. NPs modified electrodes may result in enhanced electron kinetics due to following

(i) High surface area.

(ii) Enhanced mass transport rates.

(iii) Better control of NP surface.

(iv) Functionalization of NPs with selective groups.

Recently, metal NPs (MNPs) have drawn significant attention in electrocatalysis because of their unique chemical and electronic conduct. The analytes that show sluggish electron kinetics at bare electrodes were reported to have reasonably enhanced electrocatalytic activity at MNP's modified electrodes. In addition, MNPs modified electrodes showed good peak separations for compounds having very close oxidation potentials.

Recently, the very simple procedure was adopted for immobilization of Au NPs on GPE for detection of hydrazine. The beauty of this modification is in the process that involves direct attachment of Au NPs on the surface of GPE without the use of any cross-linkers. Fig. 3 shows SEM images of Au NPs modified GPE at different magnification levels, and Fig. 4 clearly shows enhanced response at Au NPs modified GPE compared to bare GPE for detection of hydrazine.¹² Similarly, in another report, platinum modified GPE was synthesized by one-step reaction that involves reduction of metal salt while dipping the GPE in reaction media.³⁰ There are some examples where MNPs are used in combination with other materials such as CNTs. Such combinations are thought to contribute to sensing properties of electrodes through synergistic effect.

Fig. 3 FE-SEM images at two different magnifications, 2 μm and 200 nm of gold nanoparticle-modified GPE. Modified and reprinted from ¹² with permission. Copyright 2013 Elsevier.

Fig. 4 CVs in 0.1 mol L⁻¹ PBS (pH 7) in the absence (A) or presence (B) of 0.5 mmol L⁻¹ hydrazine at a bare GPE (a) and at a AuNP-GPE (b). Scan rate: 100 mV s⁻¹. Modified and reprinted from¹² with permission. Copyright 2013 Elsevier.

4.2. Conducting polymers

Different types of polymers are used in electrochemical sensing applications. Overall, these polymers can be categorized into four classes i.e. electroactive, polyelectrolyte, coordinating and biological polymers. The selection of a certain polymer for electrode modification is mainly dictated by the nature of the target analytes. The electrodes can be modified with polymers using different strategies. The frequently used methods include spin coating, dip coating, and electropolymerization. Conducting polymers allow the incorporation of counter ions or other functional moieties into the structure of polymers. In this way, modified electrodes can be made selective against target analytes. Their electronic properties have similarity with metals. The polymer coated electrodes can help in avoiding interferences because selective coating materials will distinguish between the analyte and other species via hydrophilic and hydrophobic interactions, electrostatic affinities and ion exchange capabilities.

The use of Ponceau 4R azo dye doped polypyrrole modified GPE for selective potentiometric determination of Cu²⁺ in water is an example of the polymer based ion selective electrode. In this work, the pyrrole monomers were electrochemically polymerized on GPE in the presence of Ponceau 4R azo dye as a dopant. The application of dopant in electropolymerization generates selective recognition sites in the coating which interact selectively with Cu²⁺ ions.²² Similarly, there are numerous other examples where polymer modified GPEs were employed as ISE for analysis of different metals.^15,19,23 MIPs⁴⁸ and polymer nanocomposites^33,44 are also used for modification of GPE for different sensing applications.

4.3. Carbon nanotubes

Carbon nanotubes (CNTs) have gained a great attraction since its discovery in 1991 by Iijima. CNTs being a sp² allotrope of carbon could be divided into two types (a) single-wall carbon nanotubes (SWCNTs) and (b) multiwall carbon nanotubes (MWCNTs). SWCNTs could be considered as the rolling of the single layer graphene while MWCNTs is the rolling of the multilayer graphene.⁵⁴ SWCNTs and MWCNTs could be synthesized by vapors deposition methods, electrical arc discharge, and laser vaporization.⁵⁵ The rapid development has been witnessed in the fabrication of the CNTs based sensor from past few years. Many studies have been reported in which CNTs based sensors has shown great electrocatalytic properties towards analytes. CNTs are considered as good electrode material due to fast charge transfer and also shown the compatibility and synergistic effect with the other electrode materials.⁵⁴

H. Safardoust-Hojaghan et al. has introduced a new device consist of purified MWCNTs and Bi NPs nanocomposite which was immobilized on the surface of GPE. The response of the Bi NPs modified GPE was increased by adding the purified MWCNTs. The MWCNTs increased the surface area with Bi NPs and facilitated the fast charge transfer. The electrochemical impedance study has shown the MWCNTs with Bi NPs have reduced the charge transfer resistance significantly.¹⁷ In another work, the MWCNTs was used in combination with electroactive polymer poly(vinyl ferrocenium). PVF⁺/MWCNTs/GPE was fabricated by single step electrooxidation of PVF/MWCNTs at +0.7 V for determination of nitrite. MWNCNTs and poly(vinyl ferrocenium) have increased the sensitivity of the GPE electrode for nitrite determination compared to bare GPE and PVF⁺/GPE.³³ MWCNTs were also used for the GPE based biosensors. A. Ensafi et al. reported work in which dsDNA was immobilized on the surface of MWCNTs-PDDA/GPE to develop a biosensor for the sensitive sensing of banned dye, chrysoidine. PDDA is a positive charge electrolyte which adsorbs or wraps the MWCNTs when sonicated together. This process leads to positive charge nanocomposite which provided a good effective surface area for the loading of dsDNA.⁴⁷ In another work, the MWCNTs with polyaniline nanorods was used for the surface modification of the GPE. PANI rods alone on the GPE surface has given broad and low oxidation peak current compared to MWCNTs which describes MWCNTs has more fast charge transfer capability. The combination of both PANI and MWCNTs has shown excellent sensitivity towards bisphenol A.⁴⁴

4.4. Pretreatment

This is a very common method to enhance the sensitivity of the carbon-based electrodes. This method is widely being used for increasing the sensitivity of the graphite pencil electrode. During electrochemical pretreatment, the electrode surface was cleaned, and some oxygen-containing groups also formed on the surface of GPE. The enhancement of the current could be attributed to the formation of oxygen-containing functional groups like carboxylic acid, phenolic, and carbonyl. In this process might be some graphite oxide films are also formed on the GPR surface which is also responsible for the current enhancement for the target analytes.⁵⁶ Electrochemical pretreatment is attractive choice due to the following reasons.⁵⁷

• Very simple.

• Less time-consuming.

• No complex material required.

• More applicable compared other modification materials.

For pretreatment, the number of electrolytes is being used like NaOH, the pretreated electrode was used for the determination of Sudan.⁴⁹ E. Keskin et al., pretreated the pencil electrode by dipping the electrode in DMSO/0.1 M LiClO₄ mixture at +1.6 V for sensing of benzo[a]pyrene.⁵³ Recently, the surface of the graphite pencil electrode was charged by pretreating the GPE in NaOH solution. The charged surface was used for the self-electropolymerization of the phenol on the electrode surface for the sensitive sensing of phenol. It is a very simple method for the determination of phenol.³⁷ The effect of different pretreatment solutions on the response of GPE surface charging for detection of phenol is shown in Fig. 5.

Fig. 5 (A) SWVs obtained from a 2 μM phenol solution in 0.1 M phosphate buffer, pH 7.2, in the presence of the uncharged (a) and charged GPEs in various 0.1 M media: (b) NaCl, (c) HCl, (d) Na₂HPO₄, and (e) NaOH solutions. (B) The corresponding histogram. Reproduced from³⁷ with permission Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

5. Comparison of GPE with other electrodes

Graphite pencil electrode is still an emerging electrode. There are few characteristics which make it unique over other carbon and metallic based electrodes and a good candidate for electrochemical sensing.

This single use electrode has the advantage to make the surface foul free.^37,58 Any electrode could be a single use electrode, but the cost is an issue. Glassy carbon electrode (GCE), Au and Pt disc electrode could not be employed as disposable electrodes due to high cost and their manufacturing style. Due to this reason, GCE, Au, and Pt disc electrodes are being used for multiple times after polishing the surface by number of different ways to get rid of adsorbed materials and to improve the electrochemical behavior.^59–61 GPE could be a single used electrode as the new surface is readily available after the measurement. However, screen printed electrode (SPE) is well established single use electrode, but it is costly compared to GPE. SPE surface cleaning is usually not required, but polishing could have some positive effect.⁶² For carbon paste electrode (CPE) fabrication, a typical preparation is needed which is time-consuming.⁶³

Control of sensing area is another attractive characteristic which is associated with GPE⁵⁸ and it is not much feasible for GCE, CPE and the SPEs where the active surface is provided by the supplier. In case of GPE the length of the extruded graphite pencil could be adjusted according to the requirement. Recently, the GPE was used for the sensing of α-naphthol. The response of the bare GPE towards α naphthol was compared with GCE, CPE, Au and Pt disc electrode and it was more sensitive.⁶⁴ Casting modification of GPE is difficult and challenging due to its elongated shape and small diameter compared to GCE, CPE, SPE, Au and Pt disc electrode.

The comparison of LODs for the detection of same analyte at different bare and modified electrode is nearly impossible task. The similar modifications of various electrodes can give real insight of the sensitivity of the electrodes toward certain analytes. However, it is rare to find similar modification on all electrodes for detection of same analyte. Moreover, the LOD of the H₂O₂ sensing on the different modified electrodes is compared (Table 4). While making this comparison, it was tried to choose the modification as similar as possible. Pd NPs/GPE has shown good response towards H₂O₂ (ref. 31, 60 and 65–67) compared to closely modified electrodes. All these characteristics made GPE a valuable electrode for electrochemical sensing.

Table 4 Comparison of different characteristics of GPE with other electrodes

Sr#	Characteristic	GPE	GCE	CPE	SPE	Au disc E	Pt disc E
1	Availability	Easily available	Easily available	Need preparation	Easily available	Easily available	Easily available
2	Cost	Very cheap	Costly	Average cost	Costly (due to single use)	Costly	Costly
3	Disposable (single use)	Yes	No	No	Yes	No	No
4	Surface polishing	Not required^58	Required^59	Required (removal and smoothing of surface)^63	Not required	Required^60	Required^61
5	Control of sensing area	Yes	No	No	No	No	No
6	Sensitivity (α-naphthol)	Very good^64	Good^64	Good^64	—	Low^64	Low^64
7	Casting modification	Difficult	Easy^59	Easy^87	Easy^88	Easy^89	Easy^90
8	(Modifications)/LOD analyte (H2O2)	(Pd NPs-GPE)/0.045 μM ^31	(Pd NPs-GCE)/0.34 μM ^65	(HRP-Au NPs/CCPE)/6.3 μM ^66	(Pt–PdBNC/SPGFE)/0.87 μM ^67	((MWCNT/Ag nanohybrids) modified gold electrode)/0.5 μM ^60	—

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5.1. Attractive features and challenges of graphite pencil electrodes

GPE possesses many attractive and unique features which make it an excellent choice for electrochemical sensing. These features are summarized below.

• A unique feature is the provision of the renewable surface after each measurement⁶⁸ just like the dropping mercury electrode. This feature makes the electrode surface foul free. But in the case of most of other electrodes like GCE, after each electrochemical measurement, a number of by-products are formed which can adhere to the surface of the electrode and could compromise the sensitivity of the electrode. However, here in case of GPE, the part of the used electrode can be easily removed, and the new surface is readily available.

• The surface polishing is generally not required prior to use.⁶⁹

• The GPE is easily available⁷⁰ as it is widely used for writing purposes as a graphite pencil.

• It is also disposable⁷⁰ like screen-printed electrode. However, it is extremely low cost⁷¹ compared to disposable SPE.

• Handling of GPE is relatively easy, and due to its elongated shape, the direct electro-modification methods are more facile.⁷²

• GPE is found more sensitive compared to GCE^64,71 (greater sensitivity is due to the presence of porosity on the surface of GPE).

• The control of the sensing area is also possible by changing the extruded pencil length.

Despite its excellent behavior and the advantages, the GPE is facing few problems. Still, it is not a well-established electrode, and it is not prepared for electrode applications, due to this reason, sometimes the electrode composition may vary. This could affect the sensor behavior. Such issues can be fixed by preparing the graphite pencil typically for electrode purposes. The modification by casting method is also challenging due to the smaller diameter of a pencil.

Although there are some challenges associated with GPE, the GPE can be an excellent commercial electrode due to the above attractive features for the study of environmental pollutants. It has the capability to overcome the regeneration problem of the other solid state electrodes.⁶⁹

6. Future scope and outlook

Point of care (POC) testing has become the most well-known way of diagnosis in clinical analysis, food safety, and the environment. The main advantage of POC lies in the fact that it provides results in very short time compared to centralized laboratories. These devices are helpful in making quick decisions.

The water that is considered to be a matrix of life has been highly polluted with inorganic and organic impurities in different parts of the globe due to the industrial revolution. This fact has already been indicated by a number of published reports.⁷³. Similarly, thousands of publications talk about analytical method development for analysis of inorganic and organic pollutants in water, food⁷⁴ and biological samples.⁷⁵ However, POC like devices that can readily analyze water or other environmental matrices for the presence of inorganics or organics is still not available to the public. Hence, there is very high stress on existing analytical technologies to meet the demands of low cost, availability, sensitivity and applicability in on field analysis.

GPE based electrochemical devices can have the potential to serve in POC applications. Their low cost, easy availability, and trait of disposability make them quite fit for POC based devices. Despite low cost and commercial availability of GP based electrodes, the commercialization of readily available GPE sensors for real life samples is still at some distance from the reality. Every lab prepared prototype cannot be brought to the market without considering its suitability for the desired assay, study of consumables and acceptability by the consumers. Although electrochemical methods involving modified GPEs demonstrate a grade of maturity good enough to implement them in real analytical applications but up till now, their applications are limited to proof of concepts. For a proof of concept, different materials such as nanoparticles, conductive polymers, and carbon-based materials have been used to modify the surface of GPEs. However, which of them can be employed for mass production of modified GPEs for certain analysis will be dictated by cost, ease of synthesis and stability of the material over the electrode surface. Moreover, the integration of such modified electrodes into miniaturized systems is nothing less than a challenge as commercialization needs careful designing of the product that should be compatible with market demands. Thus, these challenges need to be addressed in future.

7. Conclusive remarks

In the area of environmental analysis, a growing variety of low-cost analytical sensors is always welcomed. Useful sensors are recognized not only by their cost but the simplicity of fabrication and operation. The sensors which can serve in point of care or in field applications even for qualitative or semi-quantitative analysis are paramount for elevating the quality of life through quick analysis.⁷⁶ GPE based electrodes have been widely used for quantitative determination of inorganic and organic, environmental pollutants. GPEs are inexpensive and commercially available electrodes that can compete with other expensive metallic and non-metallic electrodes. GPE offers renewable surface, which can be easily modified with a number of materials including nanoparticles and conducting polymers, CNTs, and pretreatment. Modification of GPE with nanomaterials or direct formation of NPs on GPE surface enhances its surface area, which in turn, results in high sensitivity and improved electrochemical activity toward detection of target species. GPE based electrodes are easy to miniaturize. Due to high selectivity and sensitivity, portability, rapid analysis, and ease of availability, GP based electrodes offer excellent opportunities for decentralized environmental analytical labs. These electrodes can also provide an opportunity for the development of low-cost commercially available sensors for analysis of a number of analytes in the environment, e.g., customers by themselves can monitor toxic metals in their drinking water. Although, GPE based electrodes got a high degree of maturity from fabrication and modification point of view, but still they are at a significant distance from commercialization. The leading challenge, however, is to bring these GPE based electrochemical techniques in the hands of a common person without compromising accuracy and reliability. Indeed, technologies are needed that can help in the mass production of modified GPEs for reliable detection of certain analytes in environmental matrices. The miniaturized devices at mass level can only be generated by the integration between electronics, electrochemistry, electrical engineering and analytical chemistry.

Abbreviations

PSS/CnP/GPE	Polystyrenesulfonate/carbon nanopowders/pencil graphite electrode;
Ag/FeOOH/GPE	Silver/FeOOH/GPE
PtNPs/GPE	Platinum nanoparticle-modified GPE
Hb/PLE	Hemoglobin/modified pencil lead electrode
PVF^+/MWCNTs/GPE	Poly(vinylferrocenium)/multi-walled carbon nanotubes/GPE
Bi-NPs/puMWCNT/GPE	Bismuth NPs/purified multiwalled carbon nanotubes/GPE
NG/PG/BiE	Nafion graphene/pencil graphite/bismuth film electrode
Poly(PyY)/GPE	Poly pyronin Y/pencil graphite electrode
Cu-P4R/PPy/GPE	Copper-Ponceau 4R azo dye/polypyrol/GPE
Au NPs/GPE	Gold nanoparticles/GPE
P4VP/GPE	Poly(4-vinyl pyridine)/GPE
DPC/PANI/GPE	Diphenylcarbazide/polyaniline/GPE
Cu–Car/PA/GPE	Carmoisine–Cu(II) complex/polyaniline/GPE
(HRP/AuNPs)2/CS/GPE	Horseradish peroxidase/gold nanoparticles/chitosan/GPE
Tar/PPy/GPE	Tartrazine/polypyrrole/GPE
pCu/GPE	Porous copper/graphite pencil electrode
ETGPEE	Electrochemically treated pencil graphite electrode
MIP/PPy/GPE	Molecular imprinted polymer/polypyrol/graphite pencil electrode
PGPE	Pretreated pencil graphite electrode
PANI/MWCNTs/GPE	Polyaniline nanorods/multiwalled carbon nanotubes/GPE
LOD	Limit of detection
AFS	Atomic fluorescence spectroscopy
FAAS	Flame atomic absorption spectroscopy
GFFAAS	Graphite furnace atomic absorption spectroscopy
ISE	Ion selective electrode
HRP-Au NPs/CCPE	Horseradish peroxidase-gold nanoparticles/chitosan carbon paste electrode
Pt–PdBNC/SPGFE	Pt–Pd bimetallic nanoclusters/screen-printed gold nanofilm electrode
AuNPs/AG/SPCE	Gold nanoparticles/activated graphite/screen printed carbon electrode

Acknowledgements

The authors acknowledge the support provided by King Fahd University of Petroleum and Minerals for funding this work through project no. SB141010.

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