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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.31 No.3 pp.237-257
DOI : https://doi.org/10.14478/ace.2020.1034

Bio-Composite Materials Precursor to Chitosan in the Development of Electrochemical Sensors: A Critical Overview of Its use with Micro-Pollutants and Heavy Metals Detection

Sarikokba‡, Diwakar Tiwari‡, Shailesh Kumar Prasad*, Dong Jin Kim**, Suk Soon Choi***, Seung-Mok Lee****†
Department of Chemistry, School of Physical Sciences, Mizoram University, Aizawl-796004, India
*Department of Chemistry, National Institute of Technology, Jamshedpur-831043, India
**Department of Environmental Science & Biotechnology, Hallym University, Chuncheon 24252, Republic of Korea
***Department of Biological and Environmental Engineering, Semyung University, Jecheon 27136, Republic of Korea
****Department of Health and Environment, Catholic Kwandong University, Gangneung 25601, Republic of Korea
†Corresponding Author: Catholic Kwandong University, Department of Health and Environment, Gangneung 25601, Republic of Korea Tel: +82-33-649-7535 e-mail: leesm@cku.ac.kr
Corresponding Author: Mizoram University, Department of Chemistry, School of Physical Sciences, Aizawl-796004, India Tel: +91-9862323015 e-mail: diw_tiwari@yahoo.com
May 12, 2020 ; May 15, 2020 ; May 19, 2020

Abstract


The role of nano bio-composites precursor to chitosan are innumerable and are known for having different applications in various branches of physical sciences. The application to the sensor development is relatively new, where only few literature works are available to address the specific and critical analysis of nanocomposites in the subject area. The bio-composites are potential and having greater affinity towards the heavy metals and several micro-pollutants hence, perhaps are having wider implications in the low or even trace level detection of the pollutants. The nano-composites could show good selectivity and suitability for the detection of the pollutants as they are found in the complex matrix. However, the greater challenges are associated using the bio-composites, since the biomaterials are prone to be oxidized or reduced at an applied potential and found to be a hinderance for the detection of target pollutants. In addition, the materials could proceed with a series of electrochemical reactions, which could produce different by-products in analytical applications, resulting in several complex phenomena in electrochemical processes. Therefore, this review addresses critically various aspects of an evaluation of nano bio-composite materials in the electrochemical detection of heavy metals and micro-pollutants from aqueous solutions.


Electrochemical sensor , Bio-composite materials , Chitosan , Detection limit , Glassy carbon electrode , Heavy metals , Micro-pollutants

초록

    1. Introduction

    The presence of micro-pollutants in the aquatic environment can lead to serious environmental issues. These micro-pollutants are persistence and relatively toxic in nature and are found to be emerging as water contaminants. It includes a variety of anthropogenic and natural occurring substances. Anthropogenic substances are those which include day to day consumables viz., cosmetics, pharmaceutical products, pesticides used by cultivators and variety of industrial wastes emanating from chemical industries. These pollutants are detected in water bodies at very low level in concentrations (ng/L to μg/L) hence, it makes difficult to detect and complicates the process further in regards to treating these compounds in the wastewater treatment plants (WWTPs)[1]. Since these micro-pollutants are persistence hence, it is removed/degraded partly in the WWTPs and escaped through the WWTPs. Therefore, it resulted a continuous introduction of these pollutants into the aquatic environment[2]. Micro-pollutants which escape through WWTPs in an aquatic environment becomes a serious threat to human and other living beings because of its long- or short-term toxicity, endocrine disrupting effects and antibiotic resistance. In this respect, since there are no intended guidelines laid down by the regulatory bodies for discharge of these micro-pollutants into the water bodies, there will be problems going forward related to this issue[1]. However, many of these contaminants are toxic to living beings as well and need to be continually monitored and researched to preserve human health outcomes in the future.

    It is known that the pharmaceuticals/drugs which are administered to the human body could metabolize only up to 30% hence, and therefore the major pharmaceutical compounds are either unmetabolized or metabolized as by-products, and are excreted through urine or faeces which eventually enters into the receiving waste water treatment plants (WWTPs)[3-5]. Micro-pollutants are included with the endocrine disrupting chemicals (EDCs) which is known to be a serious health hazard. It is therefore, responsible of disrupting the endocrine system as well as nervous system of living organisms[1]. Apart from pharmaceuticals, the prolonged usage of pesticides in agricultural lands for controlling crops damaging insects, fungus etc. also contributes a sig-nificant burden to the aquatic environment. These compounds through the washes or leaches from the farm land enters into the aquatic environment, followed by to the drinking water system and is known for causing issues to disturb the flora-fauna of the natural environment[6]. There are a variety of synthetic pesticides that are widely employed for controlling different types of pests, insects, fungi, etc. Atrazine which is an herbicide that is used to control weeds, having half-life period of 20~100 days and found to be a potent type-C carcinogen, it is noted that they are also listed in EDC even at very low concentrations. Insecticides are commonly used chemicals which are often used to control various insects. For example, diuron and paraoxon are the most common insecticides, and it was reported that the oral intake of paraoxon even at very low concentration is highly toxic for mammals as well as humans[7-9]. Additionally, Clotrimazole and Tebuconazole are commonly used fungicides and are known to be a potential health hazard. Continuous intake of these chemicals by any other means (ingestion) by living organisms is known to have a result of hampering the entire nervous system[7-11]. There are many water pollutants viz., anti-inflammatory drugs, β-blockers, antiepileptic, antimicrobial, cystostatic agents and 17 β-estradiol, etc. which are posing a serious threat to the aquatic environment at low concentrations (i.e., ng/L to μg/L)[12]. Although, it was reported that the presence of these chemicals is currently at very low numbers in concentration, however, a continued intake leads to serious bio-chemical effect and could be responsible for causing varied biological disorders. Most of the micro- pollutants are not listed as hazardous chemicals by existing regulatory bodies viz., EPA or WHO however, their impact is significant towards the human beings. Although there are sophisticated analytical techniques used for the efficient and accurate separation/detection of these micro-pollutants viz., high performance liquid chromatography (HPLC), liquid chromatograph mass spectrometer (LC-MS), LC/MSMS etc. are the best tools to date. However, these techniques are highly expensive, time consuming, tedious in operations and devoid with on-site detection/determination. This result notes obvious practical implications of these analytical methods for use with its wider applications. Therefore, several attempts are made for the development of a sensitive, robust miniaturize devise for the low level detection of these micro- pollutants from aqueous solutions[8]. Moreover, the on-site detection is shown to be of use with greater applicability in a practical implication of deploying this device.

    On the other hand, several metal ions are essential for living organisms at trace or lower levels however; the higher concentrations shown potential for its toxicity. These heavy metal ions are increasingly increasing in aquatic environments, which is a serious concern for human as well as to the preservation of aquatic life[13,14]. An enhanced level of heavy metal toxic ions in an aquatic environment is mainly due to the anthropogenic activities and natural itself[15]. The heavy metal ions are inherently persistence in nature and hence, are non-biodegradable[ 16]. Among these, cadmium, lead, arsenic, mercury and chromium are highly toxic even at very low concentrations of exposure, and for that reason poses a serious environmental and human health hazard[15,17]. Based on the toxicity and penitential health hazard of these heavy metal toxic ions, the regulatory bodies have laid down the maximum contamination level that can be found to be present in the drinking water system (Cf. Table 1)[18]. Therefore, based on these strict regulatory limits it has worked to compel us to detect these ions at even a much lower level.

    Further, the presence of these contaminants in an aquatic environment makes it complex and difficult to detect, since it is shown that different complex species of these ions are present in aqueous media. Hence, the determination of several heavy metal toxic ions from the complex matrix required highly sophisticated and sensitive analytical instruments such as with the use of atomic absorption spectroscopy (AAS)[15,19], inductively coupled plasma optical emission spectroscopy (ICP-OES) or inductively coupled plasma mass spectroscopy (ICP-MS)[19], X-ray fluorescence spectrometry (XRF)[20], neutron activation analysis (NAA) etc.[21]. Although, these instruments are highly sensitive, accurate and provides the required low level detection limit i.e., mg/L to μg/L however, the high cost of instrumentation, off site detection and tedious operations makes limited use of this technology. Therefore, the efficient, robust, cost effective and more importantly the on-site low level detection methods are inevitable for wider practical applications going forward[22].

    The role of nano bio-composite materials are having very wide applications in various branches of science, including the biomedical, environmental, agricultural, materials, electronics, and biological sciences, etc. Therefore, the present review aims for the nano bio-composites precursors to the natural chitosan, which is intended for its applications in the use and research of electrochemical sensors. This review highlights intriguing properties of nanocomposite materials precursor to chitosan and their applications in the field of electrochemical sensors, as seen in the last few decades.

    Broadly speaking, chitosan is a known compound for centuries and widely known to be useful when applied in biomedical applications. It has received greater attention in the 19th century when Rouget discussed the deacetylation of chitin to chitosan[23]. Chitosan is an important and ubiquitous polysaccharide biopolymer which was obtained by the use of a partial alkaline N-deacetylation of chitin[23]. The chitin is known to be available and extracted from the shrimp and crab shells. Chitosan is also found in nature, such as in the cell walls of fungi of the class Zygomycetes and in insect cuticles[24]. Chitosan is a natural biopolymer and is considered the most abundant polymer in availability naturally found after cellulose. By the same token, it is widely obtained from the available natural products found in nature such as the exoskeleton of insects, arthropods or as a by-product of seafood processing industry[25,26]. Chitosan possesses wide medical applications because of its intriguing properties as biocompatibility, biodegradability, bio-renewable and non-toxic nature, low cost etc.[27]. Chitosan also has a close structure with cellulose because of their linear β-(1 → 4) glycosidic linkages [Figure 1(a)]. The two unit of chitosan i.e., 2-acetamido-d-glucose and 2-amino-d-glucose are combined with glycosidic linkage. Through removing an acetate moiety from chitin, chitosan is produced by hydration or through enzymatic hydrolysis in the presence of chitin deacetylase [Figure 1 (b)][27,28].

    Generally speaking, amino and hydroxyl groups present in the chitosan compound makes it a potential use for several applications, and it is known to characteristically exhibiting polycationic character as compared to other natural polymers[29]. Chitosan is found to be pH sensitive and it can easily be dissolved in acidic media[30], because of their glycosidic linkage which results in decreasing the molecular weight of the material and its viscosity. The physical and chemical properties mainly depend on the degree of deacetylation and molecular weight of chitosan. In general, chitosan being sensitive in lower pH and has low settling properties and lower mechanical strength which hampers its varied applications[27]. A simple mixing of two polymers may enable the researcher to modify the chitosan physical properties. Similarly, the chemical modifications utilizing the functional groups of chitosan viz., carboxylic, hydroxyl and lead to the formation of hydrogen bonding and facilitating the chemical bonding to obtain the desired compound with required properties[27]. Moreover, it is emphasized that chitosan possesses an excellent film-forming ability, high permeability, good adhesion and nontoxicity helps in various applications including the sensor development research. Additionally, it is noted that the swelled chitosan eases the electron transfer reaction because of its hydrophilic nature[ 31,32].

    This review emphasised specifically on the role of nano bio-composites in the development of electrochemical sensors. The updated literature critically evaluates the challenges and opportunities of utilizing nano bio-composites in the development of sensors for micro-pollutants and some of the heavy metals. The future aspects of research are included to pave the way for possible implications of nano composites in the sensor development industry.

    2. Electrochemical Detection of Micro-pollutants

    2.1. Pharmaceutical drugs detection

    Diclofenac (sodium[o-(2,6-dichloroanilino) phenyl] acetate) [Figure 2(a)] is a non-steroidal anti-inflammatory drug (NSAIDs). It is widely employed for the treatment of inflammatory and painful diseases viz., rheumatic, nonrheumatic and antiarthritic origin or also reduces the effects of menstrual pain, dysmenorrhea etc.[33]. It was reported previously that Ca 940 tons of diclofenac is consumed annually around the globe having a prescribed dose of 100 mg/day[34,35]. On the other hand, the excess intake of diclofenac by humans is shown to have caused several adverse biochemical effects e.g., cytotoxicity to liver, kidney and gill cells as well the renal lesions even at a concentration of 1.0 mg/L[36-38]. It may also influence the biochemical functions of fish which leads to tissue damage in those animals[39]. Similarly, the diclofenac sodium is known to cause life-threatening cardiac problems viz., heart attack and stroke with the patients who are already having a diagnosis of diabetes and also Shy-Drager syndrome[40]. Therefore, the detection of diclofenac at low levels is an marker important for safeguarding the aquatic environment. The nano composite contained with the multi-walled carbon nanotubes (MWCNTs) composed with chitosan-copper complex (MWCNTs/CTS-Cu/GCE) was utilized for the modification glassy carbon electrode (GCE) using the drop cast method. The modified electrode was then employed for electrooxidation of diclofenac using the cyclic voltammetric studies as shown in Figure 2(b).

    Further, the detection of diclofenac sodium is conducted using the square wave voltammograms (SWV) which showed that a good linearity was obtained with the equation Ip (mA) = 1.139 CDS (μmol/ L) + 0.128 (R2 = 0.997) with the limit of detection (LOD) = 0.021 mol/L. It is interesting to note that the real samples of human urine was intended for diclofenac detection at varied concentrations of diclofenac sodium using human urine, and indeed a 99.7 to 98.88% recovery was achieved upon review of these samples[41]. Similar to the diclofenac, the paracetamol (N-acetyl-paminophenol) or acetaminophen is having widespread use as analgesic and anti-pyretic drug[42,43]. It gives relief in the experience of pain of different types viz., arthralgia, neuralgia, migraine, joint pain, general pain, cancer pain, headache, toothache, backache, postoperative pain etc.[44]. An excessive dose of paracetamol causes life threatening health hazards such as liver damage, kidney failure and severe hypersensitivity reaction[45]. Further, it is noted that the nano bio-composite of f-MWCNTs-chitosan-Co was synthesized by a self-assembly process[46]. The glassy carbon electrode which was modified by the nano bio-composite and employed for the electrochemical oxidation/reduction of paracetamol in the well demonstrated cyclic voltammetric studies is shown in equation (1):

    Eq1.gif
    (1)

    Further, a low level detection of paracetamol was conducted using differential pulse voltammetry (DPV) for varied concentrations of paracetamol (Cf. Figure 3). The results showed that reasonably a good calibration was obtained having a calibration line Ipa (μA) = 0.636 CPR (μmol/L) + 0.273 (R2 = 0.99) with the limit of detection 0.01 μM/L, and it was further noted the sensitivity of 0.273 μA/μmol/L[46].

    Generally speaking, Bisphenol A (BPA) is one of the potential endocrine disrupters which mimics female estrogen 17-β-estradiol and it alters the normal hormonal system of animals. The biochemical effects of bisphenol A includes the morphological alterations and interferences in sex differentiation, and along with it is shown to be an agent that reduces fertility[47,48]. This result comes about as it causes the proliferation of MCF-7 human breast cancer cells[49], and exert a mutagenic action on human RSa cells[50]. In addition to this the in vitro and in vivo studies showed that bisphenol A causes an enhanced infertility, genital tract abnormalities and breast cancer[51,52]. In a line the US EPA (US Environmental Protection Agency) suggested a maximum acceptable or reference dose of BPA intake of 0.05 mg/kg of body weight/day[53,54]. Therefore, the low level detection of bisphenol A is conducted utilizing the composite materials Chitosan-Fe3O4 in the modified glassy carbon electrodes. Furthermore, amperometric sensors were used for the determination of bisphenol A (BPA) by surface modification of GCE using chitosan-Fe3O4 (CS-Fe3O4) nanocomposite material[ 56]. Likewise, the XPS data further demonstrated that Fe (2p) was changed from two characteristic bands (711.16, 723.27 eV) in Fe3O4 NPs to four characteristic bands (710.96, 714.09, 721.28, 724.82 eV) in the CS-Fe3O4 nanocomposite (Cf. Figure 4), and it was inferred that the lone pair electrons of N atom in chitosan was interacted with 3d orbital of Fe atom in Fe3O4 and hence forming a coordinate bond[55,56]. It was further observed that the surface modification of GCE by this composite material successfully enhanced its sensitivity toward the BPA detection conducted in plastic samples[56]. Moreover, the analytical method was employed for the detection of bisphenol A from the plastic samples and it was noted that reasonably a good recovery was achieved (Cf. Table 2)[56]. Further, it was stated that the doping of Au nanoparticles into the substrate materials were able to enhance the specific electro active surface area along with the conductivity of the materials hence, this eventually enhanced the peak current and was shown to be useful for identifying the trace detection of pollutants[57]. Next, the hydrothermally flower like nano molybdenum disulphide (MoS2) was obtained and the nanosheets of MoS2 was introduced with chitosan and gold nanoparticles to obtain the nanocomposite materials (MoS2-chitisan-Au). The nanocomposite material was utilized to modify the glassy carbon electrode which was introduced for the trace detection of bisphenol A using the cyclic voltammetric studies. The electrochemical studies showed that a two electron oxidation of bisphenol A takes place as shown in equation (2):

    Eq2.gif
    (2)

    Further, utilizing the oxidation peak current the detection of bisphenol A was conducted as shown in Figure 5. It is evident in this case that a good linearity of peak current against the concentration was obtained having the linear line equation ip (mA) = 88.87 + 29.79 log C (mM) with correlation coefficient 0.9896 and LOD 5 nmol/L[58].

    Theophylline is widely prescribed as bronchodilators and respiratory stimulator drug in order to treat the infant apnea, the asthmatic acute phase in children[59]. However, it was suggested that an accurate and careful dose needs to be adjusted for safe and effective use of this drug, which requires often determination of theophylline concentration in the blood serum[60]. Therefore, this reliable and miniaturized detection device is useful for an accurate detection and determination of theophylline from various biological samples. It was further reported previously that the oxidation of theophylline takes place at relatively high applied potential which restricted its detection using the electrochemical methods[59]. However, a useful study was carried out using the MnO2 (NPs)/ionic liquid/chitosan nanocomposite modified glassy carbon electrode using the electrodeposition method. The fabricated electrode showed the oxidation of theophylline at an applied potential of Ca 1.0 V in the DPV measurements (Cf. Figure 6). In other words, it was inferred that the presence of MnO2 catalyses the oxidation of drug as shown in equations (3 & 4):

    Eq3.gif
    (3)

    Eq4.gif
    (4)

    Further, a linear least square calibration line for the studied concentration of theophylline 0.2~0.6 mM was found to be I (mA) = 0.804 C (μmol/L) - 0.0049 (mA) (R2 = 0.996). Similarly, the detection limit and sensitivity were found to be 50 nM and 804 nA/nM, respectively [61]. In this relation, it is known that acetaminophen (ACT) is an extensively prescribed drug for relief of moderate pain, fever, lumbar pain, migraine or even non-specific indications[62] and often replaces the use of aspirin. Similarly, the Mefenamic acid (MEF) which is a nonsteroidal anti-inflammatory medicine is possessed with analgesic and antipyretic properties and is useful while employed for several diseases related to rheumatoid arthritis, osteoarthritis, nonarticular rheumatism, and sports injuries[63,64]. In what follows, the overdose of these medicines causes serious biochemical disorders in the human being which includes accumulation of toxic metabolites, that may cause severe and fatal hepatoxicity[65,66]. Therefore, the simultaneous determination of these two drugs i.e., acetaminophen and mefenamic acid are noted when conducted by the electroanalytical method[67]. A composite of multiwalled carbon nanotubes (MWCNTs) with chitosan was obtained and it was employed for the modification of a glassy carbon electrode. The modified electrode showed distinct oxidation peaks for the acetaminophen and mefenamic acid, and therefore makes it possible for their simultaneous detection (Cf. Figure 7). These two drugs were satisfactorily detected in the real matrix samples (human serum, human urine) with the recovery of 96 to 97%[67]. Similarly, the metronidazole, 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole is an antimicrobial drug and given for the treatment of trichomonas, Vincent's organisms, and anaerobic bacteria[68-71]. Although the drug has somewhat higher tolerable limits when used in humans (< 2 g per day) however, it has shown toxic effects at a higher intake. The detection/determination of metronidazole was conducted using the chitosan protected tetrasulfonated copper phthalocyanine coated glassy carbon electrode in the electrochemical studies. The modified electrode was subjected for the electrochemical impedance spectroscopic measurements and the results are shown in Figure 8. The Rct value was estimated for the modified electrode and found to be 100 Ω. This result infers that the electro chemical redox behaviour of the probe to the substrate electrodes is known to be accelerated[72]. Further, the metronidazole was estimated using the modified electrode in the differential pulse voltammetry and reasonably a good calibration line was obtained y = -0.2106 (nM) - 0.0386 (R2 = 0.9976). The method hereby was greatly applied for the determination of metronidazole in urine samples[72]. In this respect, the ruthenium complexes are found with a greater attention because of its reactivity and widespread applications. Therefore, chitosan supported ruthenium complex was impregnated with the glassy carbon electrode and successfully applied for the electrochemical detection/determination of sildenafil citrate (Viagra® 50 mg) and acetaminophen (Tylenol®) from aqueous solutions[73].

    To begin with, the decarboxylation of amino acids is naturally producing the biogenic amines which are involved in several metabolic processes in the living being. However, it was reported that food contained with excessive amounts of biogenic amines may lead to the diseases viz., hypotension, headaches and diarrhoea[74-77]. In a line the decarboxylation proceeded by microbial activity of tyrosine results the formation of tyramine, or p-hydroxyphenyl ethylamine which is important biogenic amine, it is shown that Tyramine is often occurred in fermented foods and beverages, meat, fish and other dairy products[ 78-80]. The measurement of tyramine is challenging using the robust analytical methods since the available techniques viz., HPLC, LC-MS etc. are sophisticated, expensive and more importantly time consuming. In this context, the multiwalled carbon nanotube doped with gold nanoparticle (MWCNT-AuNP) composite was employed for the molecularly imprinted polymer (MIP) using the chitosan as a bridge material to imprint the nanocomposite. The cyclic voltammetric results indicated that the imprinted film possessed an excellent selectivity for tyramine, and were shown to have displayed a good linear response with the current and concentration of tyramine in the amperometric measurements within the concentration range 1.08 × 10-7 to 1 × 10-5 mol/L showed the LOD 5.7 × 10-8 mol/L[81]. The study was further extended for use with the real matrix measurement using the spiked yogurt samples with varied concentrations of tyramine from 1.0 × 10-6 to 9 × 10-6 mol/L and showed the recovery of tyramine from 107.7.7% to 92.1%[81]. Similarly, generally speaking the warfarin[3-(α-acetonylbenzyl)-4-hydroxycoumarin, WAR] is one of potential oral anticoagulant employed for the cardiovascular and cerebrovascular problems viz., venous thromboembolism, pulmonary embolism, atrial fibrillation, valvular heart disease and coronary heart diseases[82-84]. The effectiveness of the drug is based on the regulated dose hence; the therapeutic window of warfarin is noted to be very narrow. Moreover, the excessive of this drug causes unwanted bleedings[85]. In this context, the determination of warfarin is conducted using the CdS-quantum dots (CdS-QDs), MWCNTs and chitosan modified GCE. Hence, a well-de-fined oxidation peak of warfarin was occurred around applied potential of 1.0 V. Additionally, it was noted that a linear relationship was obtained between the peak oxidation current of warfarin against the pH of the solution (Cf. Figure 9). The obtained equation was obtained as:

    Eq4a.gif
    (4)

    Therefore, the slope was found to be -49 mV/pH unit which was close to the theoretical value of -59 mV/pH unit. Further, the CdS-QDs/ CS/MWCNTs/GCE composite material shows a low detection limit (3 nmol/L) having high sensitivity and selectivity. Moreover, the real sample analysis was conducted using the urine, serum and milk samples and the recovery of warfarin was satisfactorily obtained[86]. Notably, cefpirome is a new C-3’quaternary ammonium cephalosporin and known to be newer cephalosporin drug, and is shown to be highly active against both gram negative organisms including Pseudomonas aeruginosa and gram positive organisms including staphylococci. The drug is prescribed for the treatment of upper and lower urinary tract; lower respiratory tract, skin and soft tissue infections[87-89]. The detection of the drug was conducted using the multiwalled carbon nanotube modified glassy carbon electrode. Therefore, the square wave voltammetric determination of the Cefpirome showed the regression equation ip = 148.2 C - 19.12 (R2 = 0.991) with the LOD of 0.647 mg/mL. These various studies enabled that the glassy carbon electrode modified with several nanocomposites are found to be useful materials and the chitosan, which provides a bridging polymer to enhance the impregnation of nanocomposites onto the electrode surface and provides the strength and stability of thin film onto the electrode surface.

    2.2. Pesticide pollutants

    Widespread use of variety of pesticides in the agricultural applications has caused a serious environmental concern and greater threat to the human being. The low level detection in the aquatic environment poses innumerable health hazards. Organophosphates are widely employed as pesticides but these compounds are highly toxic in nature [90], due to their ability to irreversibly modify the catalytic serine residue in acetyl-cholinesterases (AChE). Further, the inhibition of AChE causes the obstruction in nerve transmission by blocking breakdown of the transmitter choline[91]. Therefore, the trace measurement of organophosphates are important in view of several health and environmental concerns. The immobilization of AChE by using glutaraldehyde as cross-linker to multiwall carbon nanotubes (MWNTs)-chitosan (MC) composite was utilized in the modification of the glassy carbon electrode and utilized for the low level quantification of triazophos from aqueous solutions. MWNTs which show catalytic properties and improves the cathodic current in the triazpphos detection. The detection limit of triazophos was found to be 0.01 μmol/L[92]. In a line the biosensor made of immobilizing acetylcholinesterase (AChE) through covalent bond to oxidize exfoliated graphite nanoplatelets (xGnPs)-chitosan cross-linked composite is employed to modify the glassy carbon electrode. The sensor was then employed for the detection of one of potential pesticide chloropyrifos (CPF). The enhanced electrochemical signal was received which was mainly due to the increased surface area and improved conductivity of the composite material and interestingly noted that the electrode was stable and sensitive for the longer operations[93]. The methyl parathion (O,O-dimethyl O-(4-nitrophenyl) phosphonothioate) is extensively used in agricultural crops as insecticide, acaricide and herbicides[94]. It is again organophosphorus type of pesticide having extensive toxic effects to the biosystem once entered into the biosystem even at low level. A novel acetylene black-chitosan composite immobilized glassy carbon electrode was fabricated and the electrochemical studies indicated the redox behaviour of parathion on the electrode surface as demonstrated in equation (5):

    Eq5.gif
    (5)

    The differential pulse voltammetric studies indicated that a good linear relationship was obtained between the parathion concentration and the reduction current (Cf. Figure 10). The regression equation was found to be ipc (mA) = 0.048 + 0.2528 C (μmol/L) (R2 = 0.993) having the LOD of 2.0 × 10-9 mol/L[95].

    The chlorinated herbicides such as picloram (4-amino-3,5,6-trichloro- 2-pyridinecarboxilic acid) is although banned but many countries are still using it to control broadleaf weeds. Through the agricultural lands it enters into the water bodies through run off and poses serious environmental issues[96]. In order to detect the picloram, an efficient, cost-effective, sensitive and selective immunosensor is developed. Indigenously a conductive chitosan/gold nanoparticles composite membrane was utilized in the modification of glassy carbon electrode. The picloram showed good redox behaviour on the modified electrode hence, it was detected at low level[97]. 4-dichlorophenol (2,4-DCP) is a precursor of herbicides and widely used in the industries related to the pharmaceuticals, pesticides, fungicides and insecticides[98]. The compound is highly toxic and persistent in nature and showed carcinogenic[ 99]. Therefore, an efficient electrochemical method was proposed using the nanocomposite of carbon dots, hexadecyltrimethyl ammonium bromide and chitosan for the low level detection of 2,4-DCP. The electrochemical response of the different electrodes for 2,4-DCP is shown in figure (Figure 11) using the 0.1 mol/L PBS at pH 7.0. A significant oxidation peak of 2,4-DCP was occurred at the applied potential of Ca 0.78 V and the oxidation peak current was significantly increased for the CDs-CTAB/GCE electrode. Further, it was suggested that the CDs-CTAB nanocomposite enables synergistic effect for efficient electron transfer reactions[100]. Further. the efficiency of these electrodes in terms of their detection limit with the studied concentration range for various micro-pollutants and other potential are summarized in the Table 2.

    2.3. Heavy metal toxic ions determination

    Presence of heavy metal toxic ions in the aquatic environment is long known a serious environmental concern. The detection and determination of these pollutants in the complex matrix poses several complicated issues of its measurements. Although, the advanced and sophisticated instrumentation could enable to measure it at trace level. However, the instrumentation is highly expensive, time consuming and more importantly it requires off site determination. This eventually lacks the practical implications in such detection. Therefore, fast and cost effective electrochemical methods are developed and precisely reviewed here for wide understanding of subject and possible implications in practical applications. In a line Au(NPs)/Chitosan modified glassy carbon electrode was employed in the detection of Cu(II). It was noted that Cu(II) is oxidized at an applied potential of Ca 0.28 V. The differential pulse voltammograms indicated that reasonably a good linearity was obtained between the concentration of Cu(II) and oxidation peak current. The results showed that the regression line was followed the equation: I(mA) = 1.665 log[Cu2+] + 13.162 (R2 = 0.9927). The Ch/Au(NPs)-modified electrode possessed with enhanced active surface area compared to the bare chitosan electrode since Au(NPs) are bound with chitosan and facilitated the surface adsorption and redox reactions on the surface (Cf. Figure 12)[123]. Similarly, the graphene/chitosan composite was utilized to modify the glassy carbon electrode at the low level determination of Cu(II). The modified electrode enhanced significantly the electrochemical signal which enabled the low level detection of Cu(II)[124].

    Amino-functionalized graphene/chitosan composite modified glassy carbon electrode showed a greater application in the detection of Cu(II). The differential pulse voltammetric curves along with the calibration curves are shown in Figure 13. Interesting to note that two linear equa-tions are obtained at varied concentration range of Cu(II): ipc = 1.0553 C + 2.9608 (ipc in μA, C in μmol/L)[Cu2+]: 0.4 to 40 μmol/L and ipc = 0.3745 C + 30.6545 (ipc in μA, C in μmol/L) in the range of 40 μmol/L to 140 μmol/L. Further, the detection limit of Cu2+ was found to be 0.064 μmol/L[125].

    A newer composite material reduced graphene oxide-chitosan/ poly-L-lysine precursor to chitosan is synthesized and further utilized for the modification of glassy carbon electrode. It was reported that at the electrode surface heavy metals forming complexes with the immobilized chitosan (CS) and poly-L-lysine (PLL) followed by the redox reactions on the electrode surface. The studied heavy metals are oxidized at different applied potentials i.e., Cd(II), Pb(II) and Cu(II) are detected at potentials of -0.82, -0.54 and -0.11 V, respectively. This gives the advantage of their detection simultaneously in the electrochemical studies (Cf. Figure 14). Interesting to note that the regression line depends on the concentration range as shown in equations (7-12).

    Eq7.gif
    (7)

    Eq8.gif
    (8)

    for Cd(II),

    Eq9.gif
    (9)

    Eq10.gif
    (10)

    and for Pb(II),

    Eq11.gif
    (11)

    Eq12.gif
    (12)

    The method was applied to analyse the tap water and the recovery was within the range of 96.00~103.33% for Cd(II), 93.33~97.6% for Pb(II), and 96.66~101.33 for Cu(II)[126]. Similarly, the Bi doped mesoporous carbon xerogel (Bi-CX) mixed with chitosan to obtain the composite material BiONPs-CS. The material was coated onto the surface of glassy carbon electrode and attempted for the simultaneous de-termination Cd(II) and Pb(II). The Pb(II) was detected at the applied potential of -0.40 whereas Cd(II) was detected at the potential of -1.4 V against the Ag/Ag,KClsat[127]. On the other hand, the glassy carbon electrode was suitably modified with the composite material viz., fluorinated multiwalled carbon nanotubes magnetite (Fe3O4). The electrode was suitably for the detection of multielements viz., Zn(II), Cd(II), Pb(II), Cu(II) and Hg(II) since they showed the oxidation peaks at the applied potential of -1.11, -0.70, -0.53, +0.01 and + 0.37 V, respectively in the stripping wave voltammetry at pH 5.0 acetate buffer (Cf. Figure 15). Further, the stable electrode was employed for the real matrix sample analysis in monitoring water and rice samples which showed almost 96.0% to 101.5% recovery as compared to the standard analytical methods applied i.e., ICP-MS and AFS[128]. The one pot facile synthesis carried out to obtain the reduced graphene oxide with Fe3O4 nanocomposite (rGO-Fe3O4). Further, nanocomposite modified glassy carbon electrode was intended for the low level and simultaneous detection of Cd2+, Pb2+ and Hg2+ using the stripping voltammetry. The stripping peak potentials were recorded for Cd2+, Pb2+ and Hg2+ at the applied potential of -0.79, -0.59, and +0.25 V, respectively. The method was employed for the analysis of lead in soil sample and results were found within the permissible range[129]. Similar studies were conducted using the Fe3O4/multiwalled carbon nanotube/laser scribed graphene composites functionalized with chitosan modified glassy carbon electrode (Fe3O4/MWCNTs/LSG/CS/GCE). The modified electrode showed a sensitive sensor for the simultaneous detection of Cd(II) and Pb(II). The LOD of Cd(II) and Pb(II) was found to be 0.1 and 0.07 μg/L. Interesting to found that selectivity for several cations/ anions and the stability of electrode was fairly good. The 10 run of detection showed reasonable stability of electrode at least for the detection of Cd(II) and Pb(II) (Cf. Figure 16)[130]. Similarly, ZnO anchored with chitosan was useful for low level detection of Cd(II) using the modified glassy carbon electrode[131].

    A recent study utilized the zeolitic imidazolate framework ZIF-8 and mixed with chitosan (CS) solution which resulted a homogeneous dispersion (ZIF-8-CS) which was then utilized to fabricate the glassy carbon electrode. The stripping voltammetric analysis showed fairly a good response to detect the cations Hg2+, Cu2+, Pb2+ and Cd2+ simultaneously (Cf. Figure 17)[132]. The regression lines were obtained and shown in equations (13-16):

    Eq13.gif
    (13)

    Eq14.gif
    (14)

    Eq15.gif
    (15)

    Eq16.gif
    (16)

    As(III) detection was carried out using variety of materials modified glassy carbon electrode which includes silver nanoparticles built-in chitosan[ 133], Chitosan-Fe(OH)3[134], and gold nanoparticles[135]. It was reported that the redox behaviour of As takes place with 3 electron exchange as given below (Equations 17, 18)[133]:

    Eq17.gif
    (17)

    Eq18.gif
    (18)

    Differential pulse anodic stripping voltammograms for As(III) are shown in Figure 18. There was a shoulder peak at –0.3 V was due to the chitosan-Fe(OH)3 complex and the As(III) showed a distinguished peak at 0.55 V. The limit of detection for As(III) was found to be 0.072 ppb[134]. Total arsenic was estimated using variety of materials precursor to natural clay bentonite in the modification of carbon paste electrode (CPE). The presence of several cation and anions were studies in addition to real matrix analysis taking the river water. The LOD was obtained as 2.214, 1.502 and 1.408 μg/L respectively for the BCH, LCH and LCAH-modified CPEs, respectively (where BCH (HDTMA-loaded bentonite), LCH (HDTMA-loaded local clay) and LCAH (aluminium pillared-HDTMA-loaded local clay); HDTMA: hexadecyl trimethyl ammonium bromide)[136]. Similarly, the silane [trichloro(octadecyl)silane (TCODS)] grafted bentonite modified carbon paste electrode was utilized for the low level detection of As(III). It was shown that single step oxidation/reduction reaction occurred for As(III) on the surface of electrode with the ΔE value 0.36 V at pH 2.0. A possible mechanism of sorption of As(III) followed by the redox reactions occurred on the electrode surface as shown in Figure 19. Moreover, the anodic stripping voltammetric analyses showed the detection limit of As(III) 0.00360 ± 0.00002 μg/L[137].

    A nanocomposite materials contained with the manganese oxide nanoflakes/multi-walled carbon nanotubes/chitosan utilized to modify the glassy carbon electrode. The detection of Cr(III) was conducted using the modified electrode and the results are shown in Figure 20. On the other calibration line was found to be I (mA) = 0.0073 [Cr(III)] (μmol/L) + 1.5584 (mA) (R2 = 0.9906). It was further proposed that the a synergistic effect occurred for both MWCNTs and MnOx which enabled an efficient electrocatalytic activity towards Cr(III) oxidation at reduced over potential[138].

    Several studies are conducted for the low level detection of lead(II) using the modified glassy carbon electrodes. The nanocomposite materials viz., graphene, gold nanoparticles and chitosan[139], heparin modified chitosan/graphene nanocomposite[140] and Co3O4 nanoparticles, reduced graphene oxide and chitosan[141] were employed for modification of electrode. The lead was detected at the potential of -0.556 V and the DPASV response showed that a good linearity of the Pb(II) concentration and anodic current was obtained having the linear equation: y = 0.02459X + 0.1294 (R2 = 0.9962)[141]. On the other hand, the mechanism of accumulation and anodic stripping of Pb(II) during SWAS measurements is demonstrated as equations (19 & 20) [140].

    Eq19.gif
    (19)

    Eq20.gif
    (20)

    The detection of several heavy metals using the electrochemical methods are summarized in the Table 2. It includes the nanocomposite materials which were utilized in the modification of electrodes and studied for the varied concentrations of pollutants. Further, the LOD calculated was included in the Table 3. These several studies indicated that the variety of hybrid materials were useful in the development of sensors for low level detection of heavy metal toxic ions in the pure form or even in the complex matrix as well. The attempts of simultaneous detection further enhanced the applicability of electrochemical methods. The role of chitosan was numerous, providing the functional groups, binding the target materials or even possessed the conducting behaviour to ease the electron transfer reactions. However, there were several innumerable challenges ahead of utilizing the basic data for technology transfer.

    3. Conclusion

    Finally, the review as noted extensively emphasized the detection of several micro-pollutants including the pharmaceuticals, pesticides, herbicides along with the heavy metal toxic ions using the nanocomposites precursors to the chitosan modified electrodes. It has shown that the biopolymer chitosan possesses useful properties including the biocompatible, biodegradable, non-toxic, low cost contained with functional groups viz., -NH2 and -OH which enables it useful for wider applications in the sensor development. It is also revealed that the bare chitosan does possess the drawbacks such as settling properties, pH sensitivity and physical/mechanical strength which restricts its use in the practical applications. However, the composite materials contained with inorganic nanomaterials enables greater applicability in the specific sensor purposes. Furthermore, the presence of functional groups enabled the composite materials to functionalize it, and to make it more selective and suitable for specific target pollutants. Moreover, the nanocomposite materials enhance the sensitivity of electrode for pollutants, and the conducting behaviour of chitosan makes efficient electron transfer reactions on the surface of modified electrodes. This eventually allows to develop an efficient sensor for different organic of inorganic water pollutants.

    The nanocomposite materials are useful even for simultaneous detection of several heavy metals and the studies are enabled for developing a real matrix analysis. Various studies indicated that the laboratory scale results are encouraging however, the real implications needs several scientific and technological trials, which need to be conducted extensively for manifesting the real implications of the application in the miniaturized device development. The real technology development requires detailed trials in specific terms and the specific matrix also needs to be optimized. Additionally, the role of chitosan in the composite materials on the electrode surface need to be studied, since, it is possible that chitosan may take part in the electrochemical processes and may influence the detection of the target pollutant in the real matrix studies. The stability of bio-materials or bio nanocomposites employed for the working electrode preparation is having a greater concern which need to be studied for real implications.

    Acknowledgement

    This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1I1A3A01062424).

    Figures

    ACE-31-3-237_F1.gif
    (a) Structure of chitosan, chitin and cellulose, (b) deacetylation of chitin to chitosan[27].
    ACE-31-3-237_F2.gif
    (a) Diclofenac sodium, (b) CV of the bare GCE (curve a), MWCNTS/GCE electrode (curve b) and MWCNTs/CTS-Cu/GCE (curve c) in the solution containing 50 μmol/L diclofenac sodium in 0.2 mol/L PBS (pH 4.0) at 100 mV/s scan rate[41].
    ACE-31-3-237_F3.gif
    (a) DPVs response at f-MWCNTs/CTS-Co/GCE for different concentration of paracetamol between (0.01 μM~400 μM) in 0.2 M phosphate buffer solution (pH 7.0), and (b) calibration curve of peak current with PR concentration[46].
    ACE-31-3-237_F4.gif
    XPS spectra of the N element in (A) CS and (B) CS-Fe3O4 nanocomposite, Fe element in (C) Fe3O4 NPs and (D) CS-Fe3O4 nanocomposite[ 56].
    ACE-31-3-237_F5.gif
    (A) Cyclic voltammograms of BPA at AuNPs/MoS2/GCE in 0.1 M PBS (pH 7.0) containing different concentrations of BPA (from a to j: 0.05, 0.1, 0.2, 0.5, 0.8, 1.0, 3.0, 15.0, 25.0, 50 and 100 mmol/L); (B) the relationship of the peak current with the concentration of BPA[58].
    ACE-31-3-237_F6.gif
    DPVs recorded at bare GC (a and b) and GC/Chit/NH2-IL/ MnOx (c and d) electrodes in the absence (a and c) and presence (b and d) of 1 μmol/L TP in 0.1 M acetate buffer solution (pH 5)[61].
    ACE-31-3-237_F7.gif
    Cyclic voltammograms of 100 μmol/L ACT and 100 μmol/L MEF at MWCNTs-CHT/GCE in 0.1 mol/L phosphate buffer solution (pH 7) at scan rate 50 mV/s[67].
    ACE-31-3-237_F8.gif
    Electrochemical impedance studies (EIS) of a) bare GCE and b) Chit/CuTsPc/GCE in the presence of 0.1 mol/L KCl and 5 mM K3[Fe(CN)6]3-/4-[72].
    ACE-31-3-237_F9.gif
    The effect of pH on the peak current of 5 μmol/L warfarin at the surface of QDs/CS/MWCNTs/GCE; scan rate = 100 mV/s. Inset: dependence of the peak potential (EPA) with pH solution[86].
    ACE-31-3-237_F10.gif
    Differential pulse voltammograms of the sensor at the different MP concentrations. Inset: calibration plot of the dependence of the measured current on MP concentrations. Supporting electrolyte: MPS (pH 5.6), scan rate: 50 mV/s[95].
    ACE-31-3-237_F11.gif
    Cyclic voltammetric behaviours of 2,4-DCP (2.0 μmol/L) in 0.1 mol/ L PBS (pH 7.0) on the bare GCE, CTAB/GCE, CDs/GCE, CS/GCE, CDs-CTAB/GCE and CS/CDs-CTAB/GCE. Accumulation time: 120 s; scan rate: 100 mV/s[100].
    ACE-31-3-237_F12.gif
    Representation of Cu(II) trapping in the Ch/AuNPs matrix[ 123].
    ACE-31-3-237_F13.gif
    (a) DPV curves at NH2-G/Cs/GCE in 0.1 mol/L CH3COOH-CH3COONa (pH 4.6) containing Cu2+ with different concentrations, from a to o: 0.4, 1.6, 2.8, 5.2, 10, 20, 30, 40, 60, 80, 100, 120, and 140 μmol/L. Deposition time, 300 s; deposition potential, -0.7 V; amplitude, 50 mV; increment potential, 4 mV; pulse period, 0.5 s; sensitivity, 10-5 A/V; vs SCE. (b) Plot of the DPV peak current vs. the concentration of Cu2+. Insets: the linear fit of the dependence in two concentration ranges[125].
    ACE-31-3-237_F14.gif
    DPASV of heavy metal ions at RGO-CS/PLL/GCE (a) 8.0 mg/L Pb(II) and 8.0 mg/L Cu(II) with concentrations varying in the range of 0~10.0 mg/L Cd(II); (b) 8.0 mg/L Cd(II) and 8.0 mg/L Cu(II) with concentrations varying in the range of 0~10.0 mg/L Pb(II); (c) 8.0 mg/L Cd(II) and 8.0 mg/L Pb(II) with concentrations varying in the range of 0~10.0 mg/L Cu(II). Supporting electrolyte: 0.1 M acetate buffer (pH 4.5); deposition potential: -1.2 V; deposition time: 180 s; quiet time: 10 s. (d) Adsorption isotherm of RGO-CS/PLL for Cd(II), Pb(II) and Cu(II). Adsorbent dose = mg/L, contact time = 24 h, T = 298 K[126].
    ACE-31-3-237_F15.gif
    (a) SWVs of Fe3O4/FMWCNT/GCE for the simultaneous determination of Zn(II), Cd(II), Pb(II), Cu(II) and Hg(II) (from the bottom to top: 5, 10, 20, 50 nmol/L; 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, 20, 30, 31.5, 32.5, 33.5 μmol/L); corresponding calibration plots of (b) Zn(II), (c) Cd(II), (d) Pb(II), (e) Cu(II) and (f) Hg(II)[128].
    ACE-31-3-237_F16.gif
    (A) Selectivity of Fe3O4/MWCNTs/LSG/CS/GCE for simultaneous detection of Cd2+ and Pb2+; (B) Ten replicated SWASV peak currents recorder for 60 μg/L Cd2+ and Pb2+ in 0.1 mol/L HAc-NaAc buffer containing 500 μg/L Bi3+ (Insert: Corresponding SWASV curves of Fe3O4/MWCNTs/LSG/CS/GCE)[130].
    ACE-31-3-237_F17.gif
    (A) Current density response of the ZIF-8-CS/GCE for the simultaneous detection of Hg2+, Cu2+, Pb2+ and Cd2+ over a concentration range from1.0 μmol/L to 100 μmol/L for each metal ion. From top to bottom, 1.0, 3.0, 5.0, 7.0, 10, 30, 60, 80, 100 μmol/L. (B) The linear plots of the stripping peak current density versus concentrations of Hg2+, Cu2+, Pb2+ and Cd2+ obtained from the data in Figure (A)[132].
    ACE-31-3-237_F18.gif
    Anodic stripping voltammetric response of CFC modified electrode for various concentration (2~100 ppb) of As(III) solution in 0.1 mol/L acetate buffer (Inset: calibration plot of anodic peak current as a function of concentration)[134].
    ACE-31-3-237_F19.gif
    (a) CV response of As(III) using the nanocomposite electrode as a function of pH, and (b) Scheme of As(III) sorption onto the electrode surface[137].
    ACE-31-3-237_F20.gif
    CVs of Chit/MWCNTs/MnOx modified GC electrode in acetate buffer solution (pH 5) at scan rate 10 mV/s with increasing Cr(III) concentration from 40.0 to 360 μmol/dm3. Inset, plot of peak current vs. Cr(III) concentrations[138].

    Tables

    Permissible Limits of Several Heavy Metal Toxic Ions in Drinking Water Quality based on Several Regulatory Bodies[18]
    Determination of BPA in Plastic Samples[56]
    The use of Nano Composites in the Determination of Various Micro-pollutants from Aqueous Solutions
    The use of Nano Composites in the Determination of Various Heavy Metal Toxic Ions from Aqueous Solutions

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