Review—Pencil Graphite Electrode: An Emerging Sensing Material

1945-7111/167/3/037501

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In the last few decades variety of electrochemical sensors have been developed and successfully applied to various fields over extensive surfaces of transducers and fabricating elements. Substantial efforts have been made to develop portable electrochemical sensors to analyze pollutant in the environment.1–4 In the development of these electrochemical sensors for the analyses of organic and inorganic pollutants, various sensing electrodes were investigated and often fabricated with appropriate modifier material to achieve desired enhancements in sensitivity and selectivity.5–8 Earlier techniques used for analysis of analytes were cyclic voltammetry (CV) and chronoamperometry (CA). However, these techniques are less sensitive and took longer time in analysis. Alternatively techniques like differential pulse voltammetry (DPV) and square wave voltammetry (SWV) were utilized which are sensitive and fast technique and the extent of over oxidation can be controlled in a facile manner.9–11 Voltammetric techniques has the benefits of easy, rapid and sensitive measurement upto nano level showing a promising future toward the analysis of various analytes. Development of newer materials as electrode material is successfully fulfilling the need of modern electrochemical technology. In recent years, application of carbon working electrode and its substrates modifications with various nanomaterials have found as promising technique in electrochemical sensing of environmental pollutants. Its wide application as electrochemical sensors is due to its higher sensitivity, better conductive property, durable strength and small residual current carbon paste electrodes have been extensively applied for electrochemical applications since their introduction by Adams in 1958.12–15

Glassy carbon, Carbon paste, carbon fiber, screen-printed carbon strips, etc. are the most frequently used carbonaceous electrodes for the electrochemical sensing of pesticides and other environmental pollutants. Despite these carbons based electrode a few important common materials are utilized to prepare electrodes and widely applied in various electro chemical sensing.16,17 In recent years, synthesis of huge variety of nanomaterials has been achieved and showing immense progress in the field of nanomaterials for their application as electrochemical sensors. Distinct electrocatalytic characteristics of nanomaterials in combination with recent electroanalytical methods have turn out to be a growing field for quantifying and characterizing electrochemical performance of an electroactive substance. The major problem associated with these commercial carbon based electrode is fouling of their surface during the voltammetric measurements which occurs due to the adherence of analyte or intermediate products formed during electrochemical reaction in the cell. This fouling of electrode surface suggests cleaning the electrode surface prior to each new electrochemical measurement for good results. Various modifications and new electrode materials have been developed to resolve the drawbacks of conventional glassy carbon-based sensors, together with metallization, derivatization and doping.18–20

To overcome this type of difficulty some disposable electrodes like the screen printed electrodes and the pencil graphite electrode (PGE) have been applied for voltammetric analysis PGE has the advantages of best mechanical resistance, inexpensive and readily available. The application of disposable sensing materials viz. PGE is most significant development to way out to the drawbacks of conventional gassy carbon sensor. Now a day's for on-site measurement demand of economical, field –utilizable easy-to fabricate type voltammetric sensors has been steadily increasing.21,22 In this context best known material is graphite based electrodes especially pencil graphite electrode due to their good stability, lower toxicity, easy disposability, reproducibility and with uniform quality. Therefore, Pencil Graphite electrodes (PGE) are often used as voltammetric sensor in analytical chemistry for various electrochemical measurements. In some cases Ag- and AgCl-doped pencil apparatus leads were applied as a reference electrode in a paper-based electrode system. PGEs have wide applications in the investigation of different environmental pollutants viz. pesticides, pharmaceuticals, food adulterants, metal ions, antioxidants etc. Fig. 3. The emergent interest in PGE is confirmed by ever-increasing number of research papers dealing with voltammetric quantification of different analyte on pencil graphite leads.23–26 In this review application of PGE to pharamaceuticals analysis have been complied in Table I and pesticide analysis in Table II.

Figure 3. Applications of PGEs in various Field.

Table I. Application of Pencil Graphite Electrode to Pharmaceutical analysis.

Target Analyte Modifier Experimental condition Peak Potential LOD Technique Real Sample [Ref] Linear Range Hydroxyurea PGE PBS, pH-8.0 630 mV,930 mV 7.89 μM. DPV Pharmaceutical, urine samples 15 0.01 -1.0mM Dacarbazine SWCNT/PGE PBS, pH 7.20 +1.019 1.10 mM DPV DNA 16 2 -10 mg/mL 6-Thioguanine SWCNT-PGE - +0.88 V 0.25 mM DPV, EIS - 19 - Amlodipine PGE BRB, pH -8.5 700 mV 0.21 pM. DPV,SWV human serum pharmaceutical samples 27 0.8 nM–51.2 nM Ifosfamide, Etoposide Au/Pd@rGO@p(L-Cys) BRB, pH- 6.0 211mV,467mV 9.210 nM 0.718 nM, DPV,CV Urine sample tablets 28 0.10 to 90.0 μM 0.01 to 40.0 μM Aceclofenac MCPGE PBS, pH 7.0 +120, +320 +510,260 mV 2.6nM CV Pharmaceutical, urine 42 1 μM- 60 μM Ledipasvir ɛ-MnO2/PGE 0.10 mol L−1 H2SO4 988 mV 5.10 nmol L−1 SWV Pharmaceuticals, rat plasma 65 0.025 -3.60 μmol L−1 Daunorubicin CHIT-PGEs. ABS,pH -4.80 +1040mV 0.60 μM DPV - 68 20-100μM Mitomycin CHIT-CNT/PGE ABS, pH- 4.8 +1.0 V - CV - 70 - Topotecan SWCNT-PGEs PBS, pH-4.8 +756 mV 0.51 μg/mL   Pharmaceuticals 73 0.05 to 10 μg/mL. 1,4dihydroxyanthraquinone MIPs/MWCNTs/PGE BRB, pH-2 -839mV 4.15 nmoL_1 DPV Serum & plasma 74 10 to 100 mmol L−1 2-thiouracil PA/GPE,PP/GPE PBS, pH -4.2. 200 mV 1.6 n M 1.8 nM. CV Pharmaceutical, Serum 75 1.0 × 10−8 - 1.5 × 10−7 M,1.0 × 10−8 - 1.3 × 10−7 M Acebutolol GPE PBS, pH -7.0 542 mV -321 mV 12.6 nM- 12.8 nM DPV, SWV Urine 76 1.00 - 15.0 μM Acebutolol PGE BRB, pH- 10.0 +78 0V 0.09 nM. SWV Urine samples 77 0.4–7 nM Chlorpromazine PGE PBS, pH -7.0 759 mV 0.003 mM. CV, DPV Pharmaceutical s 78 0.01 mM -0.08 mM Acyclovir PGE BRB, pH - 4.0 1070 mV 0.3 μM CV, DPV Pharmaceutical samples 79 1.0 Μm-100.0 μM Flutamide PGE BRB, pH 2.5 250m V. 34 pmol L−1 DPV human urine and plasma samples 81 0.10–100.0 nmol L−1 Flutamide &irinotecan PGE BRB, pH -9.0 540 mV, 950 mV. 1.68nM SWASV bulkform, human urine and serum samples 82 1.99 μM-53 μM Ceftizoxime HGNPs/rGO/PGE 0.35 M HClO4 900 mV 0.35μM AdDPV Pharmaceutical 86 1.0 × 10−12 M to 1.0 × 10–11 ML−1 Hydrochlorothiazide EPPGE PBS 842 mV, 1091 mV 3.25 μM L−1 0.421 μM L−1 CV, DPV SWV Pharmaceutical, urine samples 87 4 μM -140 μM 1 Μm-20 μM, Insulin CoOxNP/GO/PGE NaOH 0.1 M 580 mV 0.687μA/nmol dm−3 Amperometry Pharmaceutical 88 0.46 to 100 nmol dm−3 Ciprofloxacin hydrochloride MIP/PGE ABS, pH -3.5 268 mV 758 × 10pM SWV Real tablet samples 89 1 × 10−9 to 1 × 10−3 molL−1 Clozapine PGE 0.1 mM HCl +530 mV 0.9 ng mL−1 DPV,CV human plasma samples 90 3–1500 ng mL−1 Dantrolene sodium PGE BRB,pH- 9.0 -302, -249m V 0.09 μgmL−1 DPV, SWV Pharmaceutical, mother milk, urine 91 0.395-2.955 - 0.395-1.9 μgmL−1 0.09 Daunorubicin rGO–PGEs ABS,pH-4.80 +245 mV,+130 mV 0.55 μM CV - 92 1-6 μM Diazepam BiPPGE ABS, pH-4.8 -900 mV 1.1μM DPV, CV Human urine 93 1.4 to 16.7 μM Diclofenac sodium MWCNTs/PGE ABS, pH-4.0 +670m V 0.017 mM DPV Urine sample tablets 94 0.047–12.95 mM Dextromethorphan DDA/MWCNT/CD/PGE BRB, pH-8.0 750mV 0.2 μM. DPV,CV cough syrups, human urine 96 2.0-600 μM Famotidine PGE PBS, pH-6.81 856mV 0.151μM DPV pharmaceuticals 98 0.472 μM -49.5μM Fluoxetine,citalopram,sertraline PVC/PEDOT-C14/PGE pH2.0,10mM HCl 380mV flu,430V, ser 470 mV) 35, 45,25 nM ITSV Drinking and river water. 99 100 to 1000 nM Itraconazole UTGE,PGE BRB, pH -2 668mV 10.7, 9.1 ng mL−1 ASDPV ASWV Serum,urine 101 16-176.5, 32–169 ng mL−1 Ketamine pty)/fMWCNTs@AuNPs MIP/PGE PBS, pH -5.0 358mV 0.7 nM CV Human plasma sample, urine 102 1.0-1000.0 nM L-ascorbic acid or Dascorbic acid PANI-FSA-C-dots/ PGE PBS, pH -7.0 365mV 0.00073 nM-0.00016 nM DPV,EIS,CV Vitamin C tablet Multivitamin tablet Orange Juice 103 0.020-0.187 nM, 0.003-0.232 nM Lomustine MF/GPE pH 5.0,BRBS -949 mV, 81.3 nM SWASV Human blood and urine samples 104 1.92 × 10-7- 1.36 × 10–5 ML−1 Methadone MWCNT-PGE BRB, pH- 7 700 V 87 nM. DPV Human serum, urine 105 0.1 – 15 μM Metoprolol MIP/MWCNTs/PGE BRB, pH -6.0 1290 V 2.88 nmol L−1, DPV CV Serum sample Tablet 106 0.06-490nmol L−1 Nalbuphine hydrochloride PGE BRBS pH 6.00 472 mV 6.38 × μML−1 CV, DPV, SWV Pharmaceutical, human biological fluids. 107 16 -150 μML−1 Phenothiazine MIP/PGE BRB, pH -2.0 500m V 0.3μmol L−1 CV, DPV Beef, Sheep, Chicken 108 1–300, 0.5–10mmolL−1 Strontium Ranelate PGE BRB, pH -2.0 850 and 1000 mV 0.17, mg/mL 0.19 CV, DPV Pure and pharmaceutical dosage form 110 1.0–10.0, mg/mL Temozolomide PGE PBS, pH -7.4 408 mV 34.5 nM DPV - 111 40-100 mg/mL Thrombin TBA/GO/PGE PBS, pH -7.00 950m V, 0.07 nM. DPV Serum sample 112 0.1 to 10 nM Uric acid, Creatinine HF-QD/PGE PBS, pH -7.0 276, 1420m V 0.0833 μM DPV, CV Serum samples 113 0.297-2970 μM Valacyclovir Cu-PGE BRB, pH -2.7 1000 mV 0.178 nM ASWV Tablet amount 114 2.0n M -10 nM Valproic acid APTES-MNPs/PGE 0.1molL−1 KNO3 800m V 0.4 ppm CV human plasma 115 1.0 -100.0 ppm Methylergometrine Maleate PGE BRB, pH -5.0 740 – 780 mV 0.02 μg/mL, CV Ampoule,tablets 117 0.10 – 1.00 μg/mL,

Table II. Application of Pencil Graphite Electrode to Pesticide analysis.

Hydrazine CuNS–MWCNT/PGE PBS pH-5.0 280m V. 70 nM 4.3 μM. SWV water samples 118 0.1 to 800 μM Hydrazine. Pcv/PGE PBS, pH- 9.0 250 mV,600 mV 0.08 mM SWASV Sea water Tap water 119 0.25-500 mM Glyphosate HF-PGE/CuO/ MWCNTs–IL PBS, pH-7 834mV 1.3 nM DPV Soil and water sample 120 5 nM to 1.1 μM Dacarbazine CYN/GC/PGE 0.1 M H2SO4 997 mV 1.28 × 10−8 M SWV,CV - 121 7 × 10−8 to 5 × 10−6 M Diuron p-Phe-PGE BRB, pH- 8 0.820 mV 43.43 μM DPV,CV water samples 122 10–500 μM Malathion AuNP-CS-IL/PGE BRB, pH -7.0 -1000 mV 0.68 nM. SWV Tomato and apple samples 123 0.89–5.94 nM Mancozeb MISP–PGE 0.2 M NaOH, pH -7.0 +600 mV 0.96 μg L_1 SWV,CV Soil, Lady Finger Brinjal 124 5.96 to 257.0 μg L_1 metobromuron UTGE–MWCNTs UTGE–GNPs BRB, pH -2.0 +1250 mV 0.11, 0.048 μmol L−1 SWV Soil samples 125 0.5 – 34.0 μmol L– Pentachlorophenol GPE 0.1 mol L-1 KCl +990 mV 5.7 × 10–5 μM DPV,CV Water Sample 126 - Propham PG/p[(Thp)-Ox] PBS, pH-7.4). +1150 mV 1.0 nM ASWV potato, human urine,river 127 0.005 – 1.0μ M Fenitrothion PNT/PGE BRB, pH -2.0 −0.546 V 0.0196 μM CV Spiked water samples 128 0.114 μM to 1.712 μM

Abbreviations: Phosphate Buffer Solution (PBS), Britionson Robinson Buffer (BRB), Acetate Buffer Solution (ABS), Differential Pulse Voltammetry (DPV),Square Wave Voltammetry (SWV), Cyclic Voltammetry (CV), Square Wave Adsorptive Stripping Voltammetry (SWASV), Adsorptive Stripping Voltammetry (ASWV), Adsorptive Stripping Differential Pulse Voltammetry (ASDPV),Electrochemical Impedance Spectroscopy (EIS), Pencil Graphite Electrode/ Peptide Nanotubes (PGE/PNTs), Reduced graphene oxide /poly(E)-1-(4-((4 (phenylamino)phenyl)diazenyl)phenyl)ethanone/ pencil graphite electrode (r-RGO/DPA/PGE), Hollow fiber - pencil graphite electrode/ copper oxide nano/ multi-walled carbon nanotube-ionic liquid (HF-PGE/CuO/ MWCNTs–IL),Copper nanostructure-multiwalled carbon nanotubes /pencil graphite electrode (CuNS–MWCNT/PGE),Pyrocatechol violet /Pencil graphite electrode (Pcv/PGE),Gold nanoparticles – chitosan- ionic liquid /pencil graphite electrodes (AuNP-CS-IL/PGE), Molecularly imprinted star polymers (MISP–PGE), Graphite pencil electrode (GPE),Pencil graphite electrode/ over-oxidized poly(thiophene)- (PG/p(Thp)-Ox]), Multi-walledcarbon-nanotubes- Pencil Graphite Electrode (MWCNs–PGE), Polyaniline/graphite pencil electrodes (PA/GPE,) Polypyrrole/ graphite pencil electrodes (PP/GPE), Graphite Pencil Electrode (GPE), Multi-walled carbon nanotubes pencil graphite electrode (MCPGE), Reduced graphene oxide hollow gold nanoparticles (rGO/(HGNPs) /PGE), Molecularly imprinted polymer/ Pencil graphite electrode (MIP/PGE), Reduced graphene oxide / pencil graphite electrodes (rGO/PGEs), bismuth pretreated pencil graphite electrode (BiPPGE), poly (diallyl-dimethyl ammonium chloride) Multiwall carbon nanotubes/CD/ pencil graphite electrode (PDDA)/MWCNT/CD/PGE), 3, 4-ethylenedioxythiophene (PEDOT-C14), poly(vinyl chloride) (PVC), Ion transfer stripping voltammetry (ITSV), Electrochemically pretreated pencil graphite electrode (EPPGE), ultra-trace graphite (UTGE), pencil graphite (PGE), Magneic oxide nanoparticles/ pencil graphite electrode (MnO2 NPs/PGE), poly-aniline-ferrocene-sulfonic acid (PAFSA), Mercury film/ graphite pencil electrode (MF/GPE), multi-walled carbon nanotubes/ pencil graphite electrode (MWCNT-PGE), Molecularly imprinted polymer(MIP), graphene oxide/ thrombin binding aptamer (GO/TBA), cadmium selenide quantum dots /ionic liquid mediated hollow fiber-pencil graphite electrode (CD/QDs/IL/HF-PGE),3-aminopropyletriethoxy silane coated magnetic nanoparticles/pencil graphite electrode(ASMN/PGE), cobalt oxide nanoparticles/Graphene oxide/Pencil graphite electrode (CoOxNP/GO/PGE), Cyaniding/glassy carbon microparticles/Pencil Graphite Electrode (CYN/GC/PGE), p-phenylenediamine/Pencil Graphite/ Electrode ((p-Phe)-PGE)

Pencil Graphite Electrode

Pencil graphite leads applied as working electrodes are referred as pencil graphite electrodes (PGEs), first reports date back to the 50's/60's. A pencil, which is used by everyone on daily basis is a portable, low cost and simple graphite material found in various diameters and lengths in every part of the world27 (Fig. 1). The renewable graphite pencils are used for writing purposes since a being long time but identified as inexpensive, easily available and renewable electrode in late 1997. It is basically a nanocomposite of graphite which is produced by intercalation of clay particles on numerous substrates by the process of exfoliation applying mild force. These leads are identified as "graphite reinforcement carbon" and prepared by mixing of natural graphite (75-80%) with organic binder (13%) and spindle oil (8%). Above prepared mixture of graphite is squeezed into a rod shaped and dehydrated at the temperature of 100–300°C followed by sintering it at a temperature of 1000°C. This overall process is carried out will be proceed under an inert gas atmosphere to avoid the deformation.28 The small size highly conductive pencil leads are made up of numerous graphene layers which are interwoven of graphite chunk particles, in spite of the existence of non- conducting clay particles. In the intervening time, the graphitic particles are of low weight and found to be stable against heat, emission and chemical corrosively. As per the European tradition Letter Scale H (hardness) and B (blackness) letters are blotched on pencils are indicative of the degree of hardness or blackness from 9H (the hardest) to 8B (the softest). B type leads enclosed large amount of graphite and are softer, and the harder H type leads have large amount of lead, however HB type pencil leads hold same portions of graphite. Clay has significant effect on the ion exchange and structural properties viz. degree of disorder and surface properties of PGEs. On the other hand H 111 grade pencils due to lower background currents, lowest ΔEp and 112 peak-to-peak current ratios close to one are the finest alternative to obtain enhanced electrochemical properties. Renewable writing pencil is of great importance as it may be extruded to desired length from Pencil holder.29,30 Basically, electrochemistry is based on the interfacial phenomenon which signifies the importance of the nature of the electrode surface. Granularity of the conductive substances directly affects the roughness of the electrodes. So the characterizations of morphologies of electrode surfaces are of great importance. The pencil graphite is characterized through a surface formed by irregularly structured nanometer sized flakes of graphite which is confirmed by investigating the surfaces applying scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) (Fig. 2C) which indicate that the roughness of the electrodes is correlated to the granularity of the conductive materials. Other spectroscopic techniques such as electrochemical impedance spectroscopy (EIS) (Fig. 2B), XPS, Raman spectroscopy, X-ray diffraction (XRD) have also been applied to characterize the pencil graphite electrodes and confirmed the irregularly structured nanometer sized flakes of graphite. The pencil graphite for hardness 2B, 5B and 8B were characterized earlier by X-ray diffraction and results showed only the (002) and the (004) peaks of graphite that indicates that pencil lead is composed of natural graphite (Fig. 2D). These Pencil graphite electrodes (Leads diameters 0.3, 0.5 and 2 mm) were characterized electrochemically using potassium ferricyanide and hexaamineruthenium chloride (Fig. 2A).31–36 For redox behavior, the HB pencil revealed equal electron-transfer as that on other carbon based electrodes. Overall it may be concluded that PGE represents a cheap and disposable sensor for electrochemical measurements in comparison to other more expensive commercial carbon electrodes. Further, several studies have been carried out on rates of electron-transfer for the Ru(NH3)6 3+/2+ redox system and result indicate PGE exhibit similar rate of electron transfer (kinetics) on clean surface without the presence of surface oxide as in glassy carbon electrodes.37–40 Such kind of easily polarizable electrodes has become a promising tool for electrochemists, due to their electrical conductivity, inexpensive, small size and ubiquity. In order to determine the amount of different analytes in various samples, in various voltammetric techniques PGEs are used as working electrode and produce reproducible signals, producing well-defined voltammetric peaks. Other one time used disposable solid electrodes has limitations of regeneration which could be overcome by disposable pencil graphite electrode which offers a renewable surface for each run that is simpler and easier than tedious polishing process.41–43 PGE has several benefits over GCE in terms of higher sensitivity, narrower and sharper peaks, lower deposition time, direct application in real samples as compared to GCE which didn't provide contented results, due to is disposability, availability and cheapness more than GCE it has becoming a promising tool for drug quantification.44–48

Figure 1. Schematic figure of the preparation of Nanocomposite modified Pencil Graphite Electrode (NCs /PGE Sensor).

Figure 2. [A] Surface characterization of bare PGE (curve a), PGE/NCs (curve b), in 1.0 mM K3[Fe(CN)6] in 0.1 M KCl by cyclic voltammetry [B] Nyquest plot for electrodes in 5 mM K3[Fe(CN)6] (a) Bare PGE (b) NCs/PGE by Electrochemical Impedance Spectroscopy [C] TEM images of Pencil graphite electrode of various magnification [D]XRD spectra of Pencil Graphite Electrode.

Based on the different studies it is observed that fabricated pencil graphite surface with various polymers, metal oxide and other fabrication material could be productively applied for the sensitive monitoring of biomolecules in various real sample solutions such as vegetables, human serum, food, beverage, industrial, sewage and various environmental samples.49–53 Other advantage of these electrodes is their selectivity toward the electrochemical recognition of target molecules from the solution. To assess the selectivity of the fabricated PGE, the sound effects of the general excipients, interfering metals, ions and biological compounds were examined in earlier reported papers which signifies that a good selectivity could be achieved for the quantification of target molecules in the presence of potential interfering substances with a good % recovery.54–56 Electrochemical reactivity, well-built adsorption characteristics, extensive potential window, high-quality mechanical rigidity, inexpensive, short technology, background current and ease of modification make it high quality working electrodes. A thin film of polyvinyl chloride dioctylphthalate plasticizer was used to cover the pencil surface in late 80s. Because of renewable surfaces, GPEs are expected to give reasonably reproducible results.57–60 Pencil graphite electrode used surface can be renewed by eliminating the extruded part and sensing surface area of the electrode. Additionally, the sensing area of the pencil rendering in the solution can be easily controlled as the need of analysis. From the point of view of green chemistry this tiny material electrode has unique properties like commercially available and easily disposable which is proving it as milestones in the field of electrochemistry.61

Pretreatment of PGE

The pretreatment conditions of PGE is much more rapid, simpler inexpensive and eco friendly than other surface modification processes. Pretreated PGE posses better electrocatalytic properties (redox) as compared to non treated electrode. In recent few decades, continuous deteriorating quality of environment is found due to human interferences and activities. Different type of pollutants viz. pesticides, fertilizers, heavy metals, dyes, and pharmaceuticals have been identified as major environmental pollutants, some of them may be carcinogenic if present beyond the permissible limits.62–65 Other sources of major pollutants comprise of effluents from textile, paints, cosmetics, and photography industries, as they emit organic and inorganic pollutants. Application of insecticides and pesticides in agricultural field has led to severe environmental problem. These molecules are highly toxic to animal, human health and aquatic system.66–70 This pencil lead electrode could be tailored to inexpensive, portable, and environmental-friendly paper-based analytical devices also.71 In few studies electroanalytical properties was improved by fabricated its surface using protein nanaoparticles which exhibited high surface to volume ratio and provide a higher electroactive surface of PGE. Hemoglobin nanoparticles modified pencil graphite electrode (HbNPs/PG) electrode was applied to detect the H2O2 in diabetic patients with a detection limit of 1 nM. Preparation of protein nanoparticles fabricated PGE sensor without incorporating nanomaterials facilitates a innovative easy approach toward the biosensor design in clinical applications.72 Different method has applied successfully for the quantification of drug in pharmaceutical dosage and in spiked human urine and blood samples at various fabricated PGE sensor and found to be a promising tool for quality control laboratories as well as pharmacokinetic studies where economy and time are essential.

Application of PGE in Pharmaceuticals Analysis

Over past few years the analysis and treatment of organic environmental pollutant is of major worldwide concern due to their huge impact on natural resources and human health. Amid varieties of pollutants pharmaceuticals are progressively more of rising concerns. These pollutants are resistant to the other conventional treatment techniques and harming the environment. For monitoring pharmaceuticals, application of electrochemical sensors such as pencil graphite electrode has become a promising tool for routine analysis. These disposable working electrodes are simple, easy to handle and inexpensive.73 A. Nezhadali et al. designed a disposable and highly sensitive molecularly imprinted sensor for the quantification of 1,4-dihydroxyanthraquinone(1,4- DAQ). To amplify the sensor signal functionalized multi-walled-carbon-nanotubes-modifier (MWCNTs) was electro- deposited on the surface of PGE which enhanced the electrode surface area as well. To establish its electrochemical characterstics such as linearity, selectivity, stability, repeatability, and reproducibility, it was applied to analyze 1, 4-DAQ in real samples.74 Due to high adsorption property of Graphite Pencil Electrode (GPEs) its surface modification with conductive polymers like polyaniline and polypyrrole is feasible and applied for the electrocatalytic quantification of pharmaceuticals. Pencil electrodes (GPE) have been applied to quantify the 2-thiouracil (2TU) in biological samples by applying cyclic and stair case voltammetric techniques. Modified GPE showed better electrocatalytic activity as compared to bare for electrochemical oxidation.75 A. M. Bagojia developed simple, fast, cheap, and precise with higher sensitivity method for the quantification of Acebutolol (ACBT) in urine samples on low cost GPE. Further, a fabricated pencil graphite electrode with MWCNTs was applied by A. Levent (2018) for the voltammetric quantification of ACBT in pharmaceutical formulations and in physiological samples which showed substantially improved electrocatalytic properties as compared to bare GPE in biological samples.76,77 Manjunatha et al. described the preparation of easy, inexpensive, less time-consuming, highly sensitive and selective voltammetric technique for the quantification of aceclofenac (ACF) applying modified multi-walled carbon nanotubes modified pencil graphite electrode (MCPGE). At fabricated PGE better electrocatalytic response for the ACF was observed. This may be due to its porous surface that a good linear response with a low detection limit of 0.3 μM could be achieved.78 A novel sensitive voltammetric sensor based on the cadmium selenide quantum dots/ionic liquid interceded hollow fiber-pencil graphite electrode (HF-QD/PGE) was developed and applied for the sensitive simultaneous quantification of uric acid and creatinine in urine and serum samples with the detection limits of 0.083 and 0.229μM.79 Further, Dilgin et al. reported electrochemical determination of acyclovir (ACV) at PGE as working electrode at differential pulse voltammetric technique. The effect of the pH was studied in the Britton–Robinson buffer solution (BRBS) on differential pulse voltammograms of 0.14 mM ACV.80 Flutamide (FLT), an anticancer drug, was investigated at hyper branched polyglycerol functionalized graphene oxide (ionic liquid mediated hollow fiber- pencil graphite electrode (HF/ HBP-GO/PGE)) under optimized experimental conditions with high sensitivity and reproducibility. In this case, HBP-GO fabricated PGE was supported by a macro-porous polypropylene membrane wall that impregnated with the ionic liquid (1-pentyl- 3-methylimidazoliumbromide). The developed procedure is simple, easy of low cost and disposable nature of the hollow fiber entirely eradicates the possibility of sample carry over and makes sure high reproducibility.81 Temerk et al. reported quantification of flutamide (Flu) and irinotecan (Irino) in biological fluids on bare PGE.A new approach, was also reported to quantify the Flu at the surface of PGE coated with sodium dodecyl (SDS)-modified silica thin film.82 Prasad et al. fabricated a molecularly imprinted polymer-based core-shells (solid and hollow) @ pencil graphite electrode (SCs-MIP/HCs-MIP/PGE)for the electrochemical quantification of anti-HIV drugs, lamivudine and zidovudine, in real samples with sensitivity of 2.23 and 1.26 (aqueous sample), 2.45 and 1.88(blood serum), and 2.52 and 1.77 ng mL-1 respectively. They compared both modified solid and hollow core-shells electrodes in term of analyte diffusion for binding and electrode kinetics using a ferricyanide probe.83,84 A non-toxic voltammetric disposable devices was built by acorrugated fiberboard substrate (PD-CFB), by direct transport of graphite through the painting with the pencil in the spaced elimited by an adhesive mask, was prepared for quantification of Catechol (CA) with a LOD of 0.01 mmol L−1by LSV.85 F. Azadmehr and K. Zarei reported quantification of ceftizoxime (CFX) by applying adsorption differential pulse stripping voltammetry (AdDPV) for measuring oxidation current on hollow gold nanoparticles/reduced graphene oxide/pencil graphite electrode (HGNPs/rGO/PGE).86 Purushothama et al. investigated anodic behavior of chlorpromazine (CPZ) using PGE and also compared the sensitivity of PGE with CPE (carbon paste electrode), BDDE (Boron doped diamond electrode), GCE (glassy carbon electrode) and other modified electrodes. They concluded that the PGE showed the good electrocatalytic activity toward the oxidation of CPZ.87 Clinical diagnostics of Insulin is of great significance as it is an important hormone and serves as interpreter of diabetes and controls glucose levels in blood. Razmi et al. has developed inexpensive and stable graphene oxide/cobalt oxide fabricated pencil graphite sensor CoOxNP/GO/PGE using potentiostatic method in sulfuric acid solution to sensitive electrochemical quantification of Insulin with a good sensitivity 0.687 μA/nmol dm−3.88 A highly selective molecularly imprinted sensor was fabricated by C. Yan et al. through electro polymerization of pyrrole (Py) and o-phenylenediamine (o-PD) on PGE using ciprofloxacin hydrochloride (CPX) as template molecular for the electrochemical quantification of CPX to overcome the limitation of the conventional techniques. The electrochemical properties were confirmed by SWV technique with low detection limit (7.58 × 10−11 mol/L).89 Rouhollahi et al. reported a new, portable, simple and efficient design, which can be developed by combination of electromembrane extraction (EME) with differential pulse voltammetry (DPV) and applied it for the in situ determination of clozapine (CLZ) from human plasma samples.90 Dantrolene sodium (Dan) was quantified by H A. Hendawy and coworkers in pharmaceutical formulation, human mother milk and urine samples at both PG and GC electrodes and the result was compared.91 DNA modified rGO-PGEs (Reduced grapheme oxide pencil graphite electrode) was fabricated for electrochemical quantification of Daunorubicin (DNR).92 They concluded that PGE as working electrode pioneers a suitable, fast and affordable surface which could be easily fabricated or pretreated in various ways and applied for analytical applications. Dehghanzade et al. studied the electrochemical reduction of diazepam (DZ) on bismuth modified pretreated pencil graphite electrode (BiPPGE) as sensor and also applied in real samples such as diazepam tablets and human urine.93 Voltammetric quantification of diclofenac sodium (DIC) was studied at MWCNTs fabricated pencil graphite electrode and quantification of DIC by spiking in real samples with good recovery.94 Rezaei et al. studied electrochemical oxidation of dextromethorphan (DXM) in aqueous universal buffer (pH 10.0) at (diallyl(dimethyl) ammonium chloride/ multiwall carbon nanotubes/ carbon quantum dots/PGE (PDDA/MWCNT/CD/PGE) sensor and developed sensor showed an excellent synergic. The modified electrode exhibited good performance in quantification of DXM and demonstrated excellent stability, repeatability and reproducibility. Abdel-aal and coworkers elucidated the oxidation behavior of ledipasvir (LED) on MnO2 nanoparticles (ɛ-MnO2 NPs) fabricated PGE electrode and results demonstrated sensitive and selective quantification LED in tablets dosage form and rat plasma samples with a high detection sensitivity of 0.002 ng mL−1.95–97 A disposable pencil graphite electrode (PGE) applied for the voltammetric determination of famotidine (FM) using cyclic voltammetric technique in different supporting electrolytes showed irreversible diffusion-controlled and pH-dependent oxidation process.98 A double-polymer-modified pencil lead electrode using ion transfer stripping voltammetry (ITSV) applying poly(vinyl chloride 3, 4-ethylenedioxythiophene (PVC/PEDOT-C14) modified electrode was applied for the quantification of fluoxetine, citalopram and sertraline with a limit of detection of 35, 45, and 25 nM for fluoxetine, sertraline, and citalopram, respectively.99 The voltammetric oxidation of hydroxyurea (HU) was investigated at pencil graphite electrode at optimized pH = 8.0 in phosphate buffer by applying cyclic, linear sweep and differential pulse voltammetric method in pharmaceuticals dosages and urine. Electrochemically pretreated pencil graphite (EPPG) sensor was reported by Purushothama and Nayaka for studying the anodic behavior of hydrochlorothiazide (HCTZ) applying different voltammetric techniques and two anodic peaks were observed at peak potential 842 mV and 1091 mV with a detection limit of 3.25 μM L−1 and 0.421 μM L−1, respectively.100 Shalaby et al. reported the electrooxidation of itraconazole (ITZ) at ultra-trace graphite (UTGE), PGE and CPE under optimized condition using anodic stripping differential pulse voltammetry (AS-DPV) and anodic stripping square wave voltammetry (AS-SWV) and the applicability of method was confirmed by quantifying the drug in human urine and pharmaceutical tablets.101 A new strategy was developed for the electrochemical quantification of ketamine (KT) through MIP composed of molecularly imprinted polymer (MIP) functionalized multiwall carbon nanotubes@gold nanoparticles (f-MWCNTs@AuNPs PGE) using cyclic voltammetric (CV) technique.102 Another MIP based polyaniline-ferrocene-sulfonic-acid)-C-dots/PGE (PANI-FSA-C-dots/ PGE) was developed by the Pandey et al. for chiral detection of D-ascorbic acid (D-AA) or L-ascorbic (L-AA) acid in aqueous solution in real and commercial samples with detection limit of 0.00073 nM and 0.00016 nM, respectively.103 In situ MF/GPE was developed by Temerk and applied for selective and sensitive quantification of anticancer drug Lomustine (LMT) with limit of detection and quantification of 8.13 × 10−8 M and 2.71 × 10−7. M. E. Alipour et al. demonstrated the development of MWCNT-PGE for monitoring the methadone (MTP) in the presence of ascorbic acid (AA) and uric acid (UA) in biological samples such as human urine and serum samples with excellent sensitivity and selective.104,105 Fast and economic molecularly imprinted polymer multi-walled carbon nanotubes pencil graphite electrode (MIP/MWCNTs/PGE) sensor was fabricated for the selective and sensitive quantification of metoprolol (MTP) in pharmaceutical and biological samples and sensor showed agreeable sensing activity with better imprinting effect and anti-interference properties.106 Elquda developed an accurate, simple, and sensitive method for the determination of Nalbuphine hydrochloride (NP⋅HCl) drug in real sample and recommended routine analysis of the drug using disposable PGE.107 A simple MIP film based PGE electrochemical sensor was for reported for the determination of Phenothiazines (PTZ) and sensor activity was investigated by CV and DPV and applicability was confirmed by quantification of PTZ in samples of beef, mutton, chicken and fish as well as human serum and plasma.108 A sensitive, selective, reproducible, disposable PPy doped with citrate anion at graphite pencil electrode surface potentiometric sensor fabricated was investigated for electrochemical quantification of sildenafilcitrate (SC) with a limit of detection (LOD) of 30 mmol−L.109 Strontium Ranelate (SR) was quantified at bare PGE by applying CV,DPV,SWV technique and two well-defined, irreversible, diffusion-controlled anodic peaks were observed using Britton-Robinson buffer, pH 2.0.110 Altay reported various nucleic acids viz. fish sperm single stranded DNA (fsDNA),calf thymus double stranded DNA(dsDNA) and calf thymus single stranded DNA (ssDNA) fabricated PGE for the voltammetric quantification of Temozolomide (TMZ). The impedimetric outcomes were also estimated to identify the interaction mechanisms occurred between TMZ and ssDNA or ds DNA.111 Ahour and Ahsani proposed a novel label-free electrochemical aptasensing biosensor based on graphene oxide nanosheets adsorbed aptamer for electrotrochemical sensitive quantification of thrombin (TBA) in biological samples. The prepared biosensor showed the detection limit of 0.07nM.112 Cadmium selenide quantum dots (QDs)/ionic liquid mediated hollow fiber-pencil graphite electrode (HF-PGE) sensor has been reported for the simultaneous determination of uric acid (UA) and creatinine (Crn) in urine and serum samples.113 Saleh et al. investigated disposable sensor based on electrodeposition of porous Cu-microparticles on pencil graphite electrode (Cu-PGE) for sensing the antiviral compound valacyclovir (VAL). Developed sensor showed distinct electrocatalytic activities in redox reaction of Cu2+ ion in the Cu- VAL complex.114 Zabardasti et al. prepared a nano-material fabricated pencil graphite electrode for the electrochemical quantification of Valproic Acid (VA) through immobilization of 3-aminopropyletriethoxy silane coated magnetic nanoparticles (APTES-MNPs) on the pencil graphite surface (PGE). Their studies showed that the APTES-MNPs proficiently enhanced the electron transfer kinetics involving VA and the electrode.115 Anticancer pharmaceutical 6-thioguanine was investigated by electrochemical technique applying single stranded and double stranded oligonucleotides. The electrochemical quantification of interaction of 6-thioguanine by way of Oligo (A)25, or Oligo (A)25–Oligo (T)25 was carried out based on altered oxidation activity of 6-thioguanine and adenine by differential pulse and cyclic voltammetry.116 Rizk et al. employed accurate, simple and sensitive, cyclic and DPV quantification of Methylergometrine Maleate (MM) in pure and dosage forms using the most popular and simpler working electrodes; PGE, CPE and GCE.117

Application in Pesticide Analysis

Now a day's pesticide industry is one of the largest known industries over the world. Pesticides are applied to increase the quality of food by eradicating pest infestation in plant but they are very harmful to human and animal health and also degrading the water and soil quality. Excess use of these neurotoxic compounds and their intermediate products causes significant effect on non-target organisms rather than target species which causes serious health problems both in humans and other living organisms. Bioaccumulation of these pesticides in soil and water has direct effect to considerable environmental pollution and ecological problems.118,119

Hydrazine is broadly utilized as a pesticide, an intermediate in pharmaceuticals, a corrosion inhibitor, a reducing agent, a reagent etc in various fields though it is carcinogenic and causes toxic effects in the gastrointestinal tract, respiratory, nervous, lymph reticular, hematopoietic, cardiovascular, genitourinary, integumentary, musculoskeletal systems, etc. therefore quantification of this pesticide is matter of concern. Different researchers had reported various types of electrochemical sensors such as Copper (Cu) nanostructures (CuNS) were electrochemically deposited on a film of multiwall carbon nanotubes (MWCNTs) modified pencil graphite electrode (CuNS/MWCNT/ PGE) and pyrocatechol violet (Pcv/PGE).Heydari et al. developed the Cu NCs-MWCNT/PGE that showed major rise in the anodic current with a reduction in over-potential in comparison to a bare PGE because of its higher sensitivity for the quantification of hydrazine. Ayaz and Dilg investigated amperometric method for electrochemical determination of hydrazine in flow injection ampherometric (FIA) system. For the preparation of fabricated sensor, pencil leads was immersed into 0.01 M Pcv solution for 10 min at room temperature and electrochemical oxidation of HZ was observed at 100 mV in cyclic voltammetry.118–120 Diuron is phenyl group herbicides which are extensively applied in the agriculture and non agriculture field to control weeds and algae sector herbicide which is an extensive field in the phenyl urea group but potential negative potential impacts of diuron can be seen in environment. So, there is an urgent need of development of cheap methodology for its detection. B. Önde and M. Soysal has proposed p-phenylenediamine polymer imprinted pencil graphite electrode for the ultrasensitive detection diuron in water samples.121 F. A. M. Abdel-aalz and M. F. B. Ali has fabricated Pencil graphite electrode with glassy carbon Microparticles using red cabbage extract and applied this novel and eco-friendly electrochemical sensor for the electrochemical quantification of dacarbazine (DAC) with lower detection limit of 1.28 × 10−8 M. The developed method was successfully applied to examine the photolytic degradation of DAC in 5% dextrose and 0.9% sodium chloride infusions after exposure to daylight and UV light.122 Bolat et al. developed a novel green, cost-effective, simple, fast and sensitive peptide nanotubes/PGE sensor (PNT/PGE) for the electrochemical quantification of fenitrothion (FT). Preparation of diphenylalanine peptide nanotubes are known to be simpler and inexpensive viable as compared to other nanostructures such as carbon nanotube. Malathion (MLT) was investigated by Bolat et al. on ionic liquid (IL), chitosan and electrochemically synthesized gold nanoparticles on single use pencil graphite electrodes (AuNP-CS-IL-PGE). The suggested non-enzymatic, simple, proficient and cost-effective was established for efficient analysis of Malathion (MLT) in real samples such as tomato and apple samples.123 Kumar et al. proposed a new and inexpensive voltammetric sensor superparamagnetic iron oxide nanoparticles (SPIONs) and molecularly imprinted star polymers (MISP/PGE) to quantify and eliminate mancozeb (MCZ) from soil and vegetable samples. The imprinted star polymer was covered on the exterior of the magnetic core applying a surface imprinting loom. The MISPs bonded with MCZ could easily and rapidly eliminate from a sample solution with a simple laboratory magnet.124 Sipa et al. fabricated a simple, eco-friendly, and selective voltammetric sensors ultra trace graphite electrode (UTGE)modified with multi– walled carbon nanotubes (UTGE–MWCNTs), graphene nanoplatelets (UTGE-GNPs) to investigate the pesticide metobromuron (Mbn) with low limits of detection of 0.13, 0.11, 0.048 μmol L–1, respectively. Applicability of method was confirmed by the quantitative analysis of Mbn in soil samples with recovery of 99.1%.125 A three electrode system comprising pencil graphite with various hardness (2B, 5B and 8B) were compared for the quantification of pentachlorophenol (PCP) and the obtained results revealed that the PGE exhibited higher sensitivity than the powder electrodes. The electrochemical usability was raised with reducing hardness (2B> 5B> 8B).126 Özcan investigated poly(thiophene) modified pencil graphite sensor (PG/p(Thp)-Ox) for the electrochemical quantification of propham (PRO) on modification with p(Thp) film at PG surface. An increase in the voltammetric response of PG by 14 times was observed and the developed sensor was applicable for recovery studies in PRO-spiked potato, urine and river water samples.127 The developed PNT/PGE sensor was applied for the analysis of low levels of fenitrothion (FT) for water samples, which offered reliable outcomes in terms of reliability, sensitivity, low detection limit and simple/minimal sample preparation. Method can quantify other pesticide residues and other environmentally pollutants also.128

Conclusions

The PGE is a promising tool for electrochemical detection of various environmental pollutants. PGEs are reliable, feasible, disposable, and inexpensive alternative to conventional carbon based sensors for quantification of analyte in complex samples such as biological fluids and waste water. Fabricated PGE could thus potentially substitute for the GCEs and other expensive electrode in electrochemical analyses. As these electrodes are easy to fabricate, they have found versatile application in sensing of environmental pollutants. The electron transfer efficiency differs according to pencil lead hardness and also, the pre-treatment offered active surface of the electrode which plays crucial role in the outcome of the analysis. The present review describes the application of PGE in biosensing analysis of environmental pollutants specifically pesticides and pharmaceuticals. PGE shows higher sensitivity, selectivity and cost effectiveness sensing methods. Easy polishing of fabricated PGE eliminated the effect of fouling in the determination of analyte with good reproducibility.

Acknowledgments

Present work was supported by the Department of Science and Technology [Annu- IF150764, 2016].

ORCID

Annu 0000-0002-8987-8914

Swati Sharma 0000-0003-1920-2203

Rajeev Jain 0000-0001-5418-6400