*Harsha Devnani and Chetna Sharma*

### **Abstract**

The practical day to day life is largely affected by the products that we use, the air that we breathe, the soil that is used to grow crops, the water we drink and use for various household chores or industrial purposes. The purity analysis of these products or estimation of useful inorganic and organic analytes is of utmost importance for avoiding health and environment risk. Everyone wants to be aware that what they are eating or applying on their skin is safe for them. A diabetic patient needs to monitor their blood sugar levels constantly. The air, water and soil quality needs constant monitoring to avoid health hazards. Not just this, chemical analysis is crucial as a crime investigation technique to identify suspects. Fuel quality and storage needs to be tested for eliminating unwanted losses. The electrochemical techniques are inherently fast, selective and sensitive and some systems are portable as well which is a boon for on-site monitoring. Voltammetric techniques like cyclic voltammetry, stripping voltammetry, impedance spectroscopy, amperometry and other techniques provide specific information of the analytes to be tested. This chapter will highlight the voltammetric techniques used for different types of analyte sensing and the advances that have taken place recently related to voltammetric sensing.

**Keywords:** voltammetry, sensing, electrochemistry, cyclic voltammetry, stripping voltammetry

#### **1. Introduction**

The electrochemical sensor is a broad integrated area encompassing physical aspects, analytical science, material science, electronic fabrication, biochemistry along with statistical analysis. This chapter restricts to the electroanalytical methods involving analytical science and electrochemistry which lay the foundation for voltammetric sensing. Electroanalytical measurements are based on the measurement of potential (potentiometry), current (voltammetry) and the amount of electricity consumed or the matter transformed during electrolysis (coulometry). These techniques are inherently fast, sensitive, selective and offer low detection and quantification limits [1]. In contrast to other analytical techniques like chromatography, ICP-OES, MS and others, electrochemical analysis does not require heavy instrumentation and tedious sample preparation. In fact, miniaturized sensors are portable and are thus handy for on-site monitoring wherever required.

Each of the above mentioned electroanalytical measurement methods involve a specifically designed electrochemical cell. Although potentiometric sensors are more lucrative for on-site operations, voltammetric sensors are more sensitive and fast. Recently, there has been a surge in researching new underlying principles for electrochemical sensing. As a result, sensors are now being developed taking advantage of changes in ionic conductivity, resistivity and impedance [2]. An altogether new concept based on imaging of local electrochemical current utilizing the optical signal from the electrode surface (surface plasmon resonance) has been reported by Shan et al. in 2012 [3]. This derives foundation from the fact that the current density can be determined from the local surface plasmon resonance signal.

Electroanalytical techniques like linear sweep voltammetry (LSV), cyclic voltammetry (CV), pulse voltammetry (PV), stripping voltammetry (SV), chronoamperometry provide an in-depth qualitative and quantitative information for an electro-active species deeming them to be a potential alternative to more commonly used spectrometric or chromatographic techniques. The understanding of an electrode process helps to explore mechanisms for in vivo studies, which is beneficial in analyzing how a drug works when administered to a human body. It is being utilized to speciate and determine ultra trace analytes in complex clinical and industrial samples. Sensor development is a thrust area in the field of chemistry, biology and environmental sciences [4]. A chemical sensor is defined as, "a small device that as the result of a chemical interaction or process between the analyte and the sensor device, transforms chemical or biochemical information of a quantitative or qualitative type into an analytically useful signal" [5]. Chemical sensors find wide applications in industry, critical care, quality control, process controls, pollutant monitoring, safety alarms, clinical diagnostics, food adulteration testing, in agriculture and forensics.

With the advent of electrochemistry, initially the voltammetric methods posed a lot of difficulties as a result of which they could not be much benefited from at the time. Gradually, with significant advancements over a period of time, voltammetric sensing soon became popular as an attractive analytical method. All the voltammetric techniques fundamentally involve the application of an electrode potential (*E*) and measuring the corresponding current (*i*) in the respective electrochemical cell. In most voltammetric techniques, the applied potential (*E*) is varied and the resulting current (*i*) is measured over a period of time (*t*). The applied potential relates to the change in electroactive species which can undergo a redox reaction and thus there is change in its concentration, which can be related to the corresponding current of the cell [6].

The voltammetric sensors have lured the researchers as they offer high sensitivity for inorganic as well as organic species along with a wide linearity range using a variety of electrolytes over a wide range of temperatures. Moreover, it has a fast response time and offers simultaneous determination of analytes at times. Voltammetric studies extend a deep insight into the kinetics and mechanism of the electrochemical process under study, making them attractive for sensing applications. Stripping analysis by far is the most sensitive voltammetric technique which boasts the advantages that it does not require derivatization and is also less sensitive to matrix changes in comparison to other analytical techniques [7, 8]. It is being widely used for analysis of heavy metals present in trace amounts in environmental samples. Cathodic stripping voltammetry (CSV) and anodic stripping voltammetry (ASV) have also been used for achieving sensitive determination of various analytes like nucleotides, nucleosides and nucleobases [9].

#### **2. Voltammetric sensors**

The voltammetric sensors initially were developed from mercury, carbon materials and inert metals as working electrodes. The use of mercury has long been forbidden due to its toxic nature and the noble metals demand high cost. The mercury electrode is almost out of the picture due to its difficult handling and limited anodic potential range. The carbon electrodes are still in use in a number of forms viz. glassy carbon, diamond, fullerene, graphite, nanotubes, graphene and graphene oxide [2]. In the past few years, carbon paste electrodes have gained momentum as working electrodes as they can be screen printed for production at mass scale. Another set of electrodes is the screen printed electrodes which are used as a robust miniaturized version and are viable for commercial technology. Most of the electrochemical sensors in the market, like the glucose sensor, employ screen printed electrodes as the working electrode [10, 11]. There is immense potential for electrochemical sensors to be established in markets other than that of glucose. The recent COVID-19 pandemic has demanded an urgent need for fast diagnostic processes which can help prevent spread of infectious diseases and provide timely diagnosis of neurodegenerative disease which is a crucial factor [12–14]. The voltammetric sensors hold promise for such diagnostic methods as they are fast, simple in operation, offer real time analysis and can be mass-produced for portable use as they can be miniaturized compared to other existing diagnostic methods [15–17].

The research is now targeted in the direction of new electrode materials which could offer comparable sensitivity and cost economical. Carbon and gold based electrodes are more popular owing to their biocompatibility, stability and good electron transfer kinetics. The bare electrodes often lack sensitivity and selectivity needed for the analysis due to poor charge transfer at the electrode surface. This requires modification of electrode surfaces to enhance charge transfer kinetics and avoid interferences [18, 19]. In this light, the nanomaterials are paving the way for smart electrochemical sensors as they provide enhanced electrochemical surface area with large surface area to volume ratio allowing improved interfacial kinetics exhibiting electrocatalytic activity for sensitive and selective determination of analytes [18–20]. The synergistic effect of nanomaterials is also being actively pursued by combining two or more nanomaterials to form a composite which would derive advantages from all individual components based on the sensing need [7, 8, 21–24]. Although nanomaterials have proved to be beneficial, there are certain challenges yet to overcome like handling sophisticated instruments and the stability of nanoparticles utilizing stabilizers or capping agents in certain cases to avoid agglomeration which will disrupt electrode-electrolyte interfacial kinetics [25, 26].

There are mainly three major hurdles encountered in the development of electrochemical sensors: the attainment of a low limit of detection (LOD); limiting the interaction of unwanted interfering species; maintaining sensor stability and achieving reproducibility in complex real matrices [27]. The LOD indicates the lowest concentration/quantity that could be detected for an analyte and is a major criterion for a sensor performance as many times we are dealing with analytes that are present at trace levels in real samples. With the advancement of nanomaterial modified surfaces, picomolar level of detection has been achieved [28–30]. These modified surfaces still pose the challenge of stability and reproducibility. Moreover, the sensor needs to be validated for real samples otherwise it does not hold importance in the market. The test of an electrochemical sensor as a diagnostic tool is only validated if it is stable and functional in a real matrix [31–33]. Real matrices involve interferences that hinder electrode performance. In the area of medical applications, this is being

dealt with by exploring advanced materials to improve electrode surfaces. While the passive methods involve the use of polymers to create a hydrophilic and non-charged layer to limit protein absorption, active methods aim to develop stronger shear forces than the adhesion forces of the bound interferents on the surface [34, 35]. Recently, the use of sol–gel materials along with ceramics and nanomaterials for electrode surfaces have proven to enhance the stability of the sensor [36, 37]. The research has progressed over the years to overcome these challenges and establish effective diagnostic tools in the market for various applications. This chapter highlights the advancement that has taken place in recent years in the development of voltammetric sensors for a variety of applications.

### **2.1 Voltammetric food sensors**

Food is prone to contamination of heavy metals, pesticides and adulterants. Metals come from a variety of natural and anthropogenic sources. However, man-made activities such as urbanization and agriculture can increase their levels, pollute water and soil, and damage the environment. Moreover, pesticides and food additives when added beyond their permissible limits are making way through the food chain, affecting human health invariably [38]. Ingestion of dangerous metal-rich vegetables, crustaceans and other foods can damage stable cells, alter metabolism, lead to carcinogenic mutations and toxicological effects on human organs [39]. The consequences of these elements on human health have inspired extensive analysis of major contaminants in food samples. The well-established spectrometric techniques viz. atomic emission or absorption spectroscopy [40], mass spectroscopy (MS) and inductively coupled plasma-MS (ICP-MS) have been widely used for this analysis in the past decade. Locatelli and Melucci (2013) detected levels of copper, mercury, lead, zinc and cadmium in several vegetables using a rectangular ASV approach and a standard addition method. The study was performed using two electrodes, gold electrode for detection of mercury and the other made of mercury to determine lead, copper, zinc and cadmium. For these studies, lettuce, spinach, and tomatoes were differentiated as washed and unwashed categories. The two groups were mineralized after treatment with an acid mixture in different amounts [41, 42]. The investigation showed that the values for lead and cadmium exceed the regulatory limits of the European Council. **Table 1** lists the range of voltammetric techniques used recently to sense a variety of heavy metals with nanomolar detection limits being achieved at robust electrode systems.

Zabihpour et al. (2020) have reported a robust vanillin (flavoring agent) electrochemical sensor derived from carbon paste electrode (CPE) substrate enriched with NiFe2O4 nanoparticles and 1-hexyl-3-methylimidazolium chloride (1H3MCl). NiFe2O4 nanoparticles were formed using a co-precipitation approach and the characterization findings pointed to a spherical NiFe2O4 nanoparticle with a diameter of 22 nm [52]. The CV technique was used to measure the vanillin oxidation peaks at potentials of +690 and + 650 mV on the surface of CPE and NiFe2O4/1H3MCl/CPE. The DPV analysis at NiFe2O4/1H3MCl/CPE showed a strong electrocatalytic capacity towards the electrooxidation of vanillin and revealed two distinct oxidation signals at potentials of 640 and 1050 mV. NiFe2O4/1H3MCl/CPE was thus successfully used as an analytical sensor to determine vanillin and tryptophan levels in coffee, milk, chocolate and cookie samples [52, 53].

It is also required to estimate nutraceuticals or phytopotentials of certain plants for therapeutic purposes. Voltammetry provides a gateway to such analysis with high sensitivity and selectivity. Zheng et al. (2022) have reviewed the evaluation of


*LSASV: Linear Sweep Anodic Stripping Voltammetry, DPV: Differential Pulse Voltammetry, SWV: Square Wave Voltammetry, DPASV: Differential Pulse Anodic Stripping Voltammetry, GCE: Glassy Carbon Electrode, SPCE: Screen Printed Carbon Electrode, PGE: Pencil Graphite Electrode, MWCNT: Multiwalled Carbon Nanotubes, MOF: Metal Organic Framework.*

#### **Table 1.**

*Literature survey of metal analysis in food samples.*

antioxidant activity using electrochemical sensors. The development of research in this area has been outlined with its initial stages in 1999, thereafter gaining momentum in 2010 and has remained so ever since. A total of 758 articles were published during this period. Electrochemical methods were used for the first time mainly for quantitative analysis, as well as other analytical approaches. Subsequently, CV was used to directly measure the electrochemical properties of various antioxidants and evaluate their antioxidant capacity. There were several advantages in this scenario when compared to the conventional DPPH assay [54]. The most direct application scenario for the evaluation of antioxidant capacity is in the food industry as it can have a direct impact on their price and nutritional value. One of the most widespread and produced antioxidants present commonly in food is vitamin C, which is electrochemically active and thus can be detected by electrochemical sensors. The development of an electrochemical sensor for Vitamin C brought a major breakthrough in the field of analytical chemistry [55]. Bounegru and Apetrei (2020) reported a nanomaterial based voltammetric sensor for the qualitative and quantitative determination of caffeic acid using CV (**Figure 1**). Carbon nanofibre (CNF) and MWCNT modified carbon based SPEs (C-SPE) were utilized to study the electrochemical behavior of caffeic acid in aqueous solution (pH 3.6). The LOD and LOQ values were seen to be in the range of 10�<sup>7</sup> – 10�<sup>9</sup> M (**Figure 1**) which indicates good sensitivity and in fact the

**Figure 1.**

*(a) Zoomed-in view of the anodic peak zone of the CV registered with CNF/C-SPE immersed in caffeic acid solutions with the concentrations in the 0.1–40 μM range. (b) Linear dependence between the anodic peak current and the concentration of the caffeic acid solution [56].*

electrochemical results were also compared to the spectrophotometric data. Also, among the two naomaterials used CNF based sensor proved to be better in terms of sensitivity and performance. The optimized sensor (CNF/C-SPE) was also tested for real samples (Active Detox (Herbagetica), DVR-Stem Glycemo (DVR Pharm) and green tea (Alevia) as depicted in **Figure 2** yielding satisfactory results [56].

Qin et al. (2020) reported a TiO2/electro reduced graphene oxide (TiO2/ErGO) nanoparticles based electrochemical sensor for the simultaneous analysis of ponceau 4R and tartrazine. The nanocomposite was prepared by ultrasonically dispersing TiO2 nanoparticles in the grapheme oxide solution followed by an electro-reduction step. The TEM analysis (**Figure 3**) indicated the uniform distribution of the TiO2 nanoparticles in ErGO nanoflakes confirming the nanocomposite formation which resulted in enhanced adsorptive stripping DPV current response as is expected because of the synergistic effect of the nanomaterials. The two colorants could be detected in a nanomolar range using the respective sensor with selectivity, sensitivity and stability. The sensor was also applied for real sample which was orange juice in this case [57].

#### **2.2 Voltammetric sensing of drug & pharmaceuticals**

As pharmaceutical firms evolve to keep pace with expanding population of humans globally, the global pharmaceutical industry reached \$1.25 trillion in 2019 and is expected to reach \$1.5 trillion by 2023. The implications of this data suggest that the pharmaceutical sales grew by 207.9 percent between 2005 and 2019. As a result, two main problems emerged: many drugs are misused/abused thus pressing the need for researching economical, portable, and effective sensors for monitoring drug overdose or biomedical monitoring; and secondly, tracking or monitoring of pharmaceutical or other contaminants in water sources to maintain human and ecological health is also necessity of today's world [58]. Based on the unique electrochemical interfaces between the nanomaterials and the analyte at hand, there are several approaches that may be more suitable for the detection of pharmaceuticals [59]. Voltammetry, potentiometry, amperometry, and electrochemical impedance spectroscopy (EIS) are examples of common electrochemical methods. Due to their higher sensitivity than CV, pulse methods such as normal pulse voltammetry (NPV), DPV and SWV are more commonly utilized for electrochemical detection. These pulse approaches

*Recent Advances in Voltammetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.108595*

#### **Figure 2.**

*CVs of CNF/C-SPE immersed in solutions of (a) active detox, (b) DVR-stem Glycemo and (c) green tea, recorded at scan rates between 0.1 and 1.0 Vs <sup>1</sup> [56].*

#### **Figure 3.**

*TEM images of TiO2 nanoparticles (A) and TiO2/GO nanocomposites (B); XRD patterns of nanomaterials (C) [57].*

significantly increase the signal of interest corresponding to the analytes compared to the capacitive current response [60].

Voltammetry and polarography are the most commonly used electroanalytical methods in pharmaceutical and biological analysis. In the 1930s and 1940s, the first examples of pharmaceutical analysis using polarographic techniques were documented. Most pharmaceutically active chemicals have been found to be electrochemically active [61]. Electrochemistry is a well-established and rapidly growing field with numerous potential applications in the pharmaceutical industry as is evident from **Table 2** [56]. The use of solid electrodes in the voltammetric determination


#### **Table 2.**

*Literature survey of electrochemical analysis in drug & pharmaceuticals.*

of drugs is gaining popularity due to its simple modification providing a large electrochemical active surface area for fast charge transfer kinetics.

Various modifiers were used to improve the recorded current intensity and the sensitivity of the electrode for the detection of pharmaceutical traces. Additionally, since the solid electrode is non-toxic, it can be used with minimal safety precautions. The phrase disposable electrode makes electrochemical drug determination easier than it was in the 1940s. This word is associated with solid electrodes, the most common home-made conventional solid electrodes, including carbon paste electrodes, screenprinted electrodes, boron-doped diamond electrodes, and pencil graphite electrodes. Both screen-printed and pencil graphite electrodes are single-use electrodes [70].

Mehmandoust et al. (2021) reported gold/silver core–shell nanoparticles (Au@Ag CSNPs) with conducting polymer poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT: PSS) and functionalized multi-carbon nanotubes (F-MWCNTs) on glassy carbon electrodes to create a novel and sensitive voltammetric nanosensor for the first time to monitor trace levels of favipiravir [70]. Under optimized conditions and at a typical working potential of + 1.23 V (vs. Ag/AgCl), Au@Ag CSNPs/ PEDOT: PSS/F-MWCNT/GCE revealed linear quantitative ranges from 0.005 to 0.009 and 0.009 to 1.95 μM with a detection limit of 0.046 nM (S/N = 3) with acceptable relative standard deviations (1.1–4.9 percent) for pharmaceuticals, urine samples, and human plasma without any sample pretreatment (1.12–4.93 percent) [71]. Amino acids, biologics, and antiviral drugs had little or no effect, and the sensing system exhibited exceptional reproducibility, repeatability, stability, and reusability. The results showed that this approach has potential applications in the diagnosis of FAV in clinical samples. The increased surface area and the synergistic interaction between the bimetallic core-shell nanoparticles and the entrapped carbon structures by the conducting polymer resulted in high sensing properties [72]. Ziyatdinova and Gimadutdinova (2021) developed a CeO2.Fe2O3 nanoparticles based sensor for the estimation of lipoic acid which is extensively used in medicine as drug therapy. The SEM analysis (**Figure 4**) indicated the uniform distribution of nanoparticles on the electrode surface resulting in enhanced catalytic oxidation currents, reduced overpotential and improved electron transfer kinetics. The designed optimized sensor *Recent Advances in Voltammetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.108595*

#### **Figure 4.**

*Surface morphology of bare GCE (a) and CeO2.Fe2O3 nanoparticles modified GCE [73].*

**Figure 5.** *SEM images of bare GCE (a) and SeO2-CPB/GCE (b) [74].*

was also successfully tested in pharmaceutical dosages for the lipoid acid [73]. Kavieva and Ziyatdinova (2022) subsequently reported a sensor based on SeO2nanopartciles and surfactant (cetylpyridinium bromide; CPB) for the determination of indigo carmine which is a widely used colorant in pharmaceutical industry. The immobilization of SeO2 nanoparticles on the electrode surface was confirmed with SEM analysis presented in **Figure 5**. The sensor was characterized using CV (**Figure 6**), chronoamperometry and electrochemical impedance spectroscopy (EIS) dictating reduction in charge transfer resistance at the fabricated sensor accompanied with roughly 4 times higher electroactive surface area [74].

#### **2.3 Voltammetric biosensors**

Since the first documented glucose biosensor more than half a century ago, biosensors have developed at an exponential rate. The use of biosensors in many fields such as diagnostics, environment, healthcare, and pharmaceuticals has resulted in the biosensor business growing into a multi-million dollar market that is expected to thrive in the coming years. In this massive biosensor sector, new advances in biosensors in terms of nanobiosensors should expand the range of such technologies in the market.

**Figure 6.** *CVs of 50 μM indigo carmine at SeO2-CPB/GCE at various scan rates in pH 5 phosphate buffer [74].*

Biosensors are generally based on the interaction of target species with the surface of a biomolecular receptor coupled to a transducer to generate a signal. A biosensor system can therefore be categorized based on how the detection was performed, such as electrochemical, mass, or optical. The method of detection can also be used to describe a nanobiosensor; however, the transducer used in such circumstances is usually from the field of nanotechnology, i.e. the use of nanoparticle or nanostructured interfaces. Optical nanoprobes detect various analytes using highly active metal nanoparticles such as gold or silver [75]. Although biosensors include well-established bioanalytical techniques, nanobiosensors have seen significant improvements in their sensitivity, resulting in a significant shift in the research trend from biosensors to the development of nanobiosensors. **Table 3** lists the range of electrochemical biosensors reported for analytes of importance in various sectors with their respective detection limit.

Kusior (2022) has reported a very interesting study for the determination of glucose without an enzyme receptor at copper oxide nanomaterial modified electrode wherein the facet exposition of Cu2O nanoparticles has been linked to the currentpotential profiles. The nanoparticles were synthesized using a wet chemical approach and different facets were exposed to current analysis for glucose determination (**Figures 7** and **8**). It is claimed that the electrochemical sensing or biosensing of Cu2O nanoparticles depended on {100} and {111} facets, with former possessing more neutral states and the latter positive state. The study was performed by synthesizing particles in different sizes by addition of surfactants and the electrochemical performance was tested by CV and amperometry [76].

#### **2.4 Voltammetric sensors for agriculture purposes**

Contamination of pesticides and herbicides in soil, groundwater, rivers, lakes, storm water and air is a major problem. Pesticides are widely used in agriculture around the world and are a key tool for controlling weeds, insects and infections. They were defined as chemicals or mixtures designed to resist, eliminate, prevent or limit


## *Recent Advances in Voltammetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.108595*

**Table 3.**

*Electrochemical biosensor developed for the detection of some analytes.*

**Figure 7.** *SEM images of obtained polyhedral Cu2O particles depending on surfactant used [76].*

**Figure 8.**

*Schematic representation of Au particle adsorption at the polyhedral surface with SEM image of modified Cu2O grains [76].*

the presence or effect of biological organisms capable of causing crop damage [89]. Since 1960, the increased use of pesticides has allowed farmers to significantly increase productivity while avoiding crop losses caused by pests [90]. Herbicides are pesticides that are used to kill or slow the growth of weeds. They were categorized based on their activity (contact or systemic), use (soil, pre-emergent or post-emergent) and mode of action on plant biochemical mechanisms [91, 92]. In addition, their target is classified as non-selective (kills all surrounding plants) or selective (attacks only weeds and leaves the crop alone) [93]. Glyphosate (N-(phosphonomethyl)glycine) was the most commonly used herbicide, followed by atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and 2,4D-dichlorophenoxyacetic acid) [94]. Priscila discussed the detection of herbicides (1990–2018) to obtain certain qualities and increase the detection limit, the matrix can be changed using metals, metal oxides, polymers, clay materials or micro or nanoparticles.

*Recent Advances in Voltammetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.108595*


#### **Table 4.**

*Sensors developed for the detection of some herbicides.*

Electrochemical sensors are good candidates for environmental monitoring and have been successfully fabricated using various types of electrode materials. The applicability and advantages of voltammetric techniques over traditional methods have been well established. Surface modification of the electrode improves its properties, allowing small amounts of a specific analyte to be detected or quantified; for metals or metal oxide electrodes are now able to detect non-electroactive herbicides. Nanostructures have also been shown to have several advantages and are a promising field of study. There are a significant number of biosensor papers that provide exciting new potential to increase their performance for herbicide detection (**Table 4**). Most of the work presented used real samples such as tap and lake water, poisoned soil and food hazardous substances (fruits and vegetables), but there is still a need to incorporate these new devices into commercial analysis or industries.

#### **2.5 Electrochemical energy sensors**

Fuels are an essential part of today's civilization with significant economic and ecological importance. The current search for new sustainable alternative fuels to fossil fuels was based on minimizing the emission of pollutants into the atmosphere such as CO2. Biofuels, as they were widely known, were commonly produced from vegetable oils and their composition can vary depending on their origin, resource types and their matrices. Metals and toxins in fuels, regardless of their origin, such as fossil or alternative, remain a problem that needs to be solved. A large number of metals can be used as catalysts in the oxidation processes of fuels when combined with gasoline, either by contamination or adulteration, and the results can damage automotive engine components [100]. Among many types of current fuels, it is customary to examine and assess the metal contamination in their composition, which exceeds the limitations of regulatory authorities to avoid economic and environmental losses associated with the concentration of specific metals in fuel matrices. There are several variables that affect the maximum amount of metal allowed in gasoline. It was determined by the regulatory body, the type of fuel and even the metal involved. Lead was not permitted in gasoline in Europe and manganese is limited to 2 mg/l [88]; however no metals are permitted in gasoline in the United States [101]. There was extensive literature on the evaluation of metals in fuels using a variety of approaches, most of which have a high material and resource analysis. Due to its mobility, high sensitivity

and low cost compared to other approaches, electrochemical techniques offer a promising alternative route for these kinds of studies.

Dos Santos and Ferreira discussed in a 2020 publication the analysis of Pb2+ and Cu2+ in microemulsified biodiesel using a boron-doped diamond electrode [90]. This process has been used to produce biodiesel from corn oil, according to the researchers [102]. In comparison to other traditional detection methods, the results obtained were good. The analysis was performed despite the fact that these metals are not controlled in biodiesel. Ferreira and co-workers in 2021 used B-doped diamond electrodes in ethanol solution with ASV to measure Fe3+, Cu2+, Zn2+, Pb2+ and Cd2+. Combining interactions and interferences between species, was investigated [103]. A sample of fuel ethanol was tested in a hydroethanolic environment using this approach after the experimental parameters were tuned, with concentrations of investigated ions below the detection limits for the proposed method (**Table 5**).

#### **2.6 Voltammetric environmental sensors**

Some benefits are likely to emerge in the existing water monitoring environment when the mode of analysis moves based on sampling techniques to exclusively Conductivity-Temperature-Depth (CTD) probes have been used underwater since the 1980s, there has been a clear shift towards decentralized techniques approaches in a portfolio of known analytical techniques show clear promise for use in An evaluation of submersible devices in terms of their analytical properties, autonomy, miniaturization, and portability [111]. Plant modified carbon paste electrodes have been reported in literature recently for the estimation of heavy metals. One study was done in 2016 wherein bagasse (waste from sugarcane) was used as a biomaterial in a sensor to detect the hazardous metals Pb and Cd. An electrochemical approach was used to investigate the performance of the bagasse-based carbon paste sensor [112]. SEM, FTIR, CV and BET analysis were used to characterize the modifier's surface which indicated mesoporous pore distribution of bagasse and the specific surface area was calculated to be 89.3 m<sup>2</sup> /g [100]. The accumulation was carried out in pH 6 acetate buffer and stripping analysis was done in HCl at a scan rate of 50 mV/s. The linearity was observed for lead and cadmium in the concentration ranges of 100–600 gL�<sup>1</sup> and 500–1200 gL�<sup>1</sup> with detection limits of 10.1 and 170.64 gL�<sup>1</sup> , respectively for 10 mins of accumulated time.


#### **Table 5.**

*Electrochemical energy sensors for metal analysis.*

Electrochemical sensors have proven ideal for this function, particularly in ion detection. Maria Cuartero reported (2021) an investigation of electrochemical sensors that showed promise for measuring ions in seawater such trace elements, nutrition, and carbon species The preceding five years were chosen as the major era for investigation, but older contributions to the area or goods introduced are included when important technical advancements are highlighted. There was a distinct absence of electrochemical sensors used in in-situ applications, which exacerbated when saltwater was considered: only a few examples have been demonstrated under such difficult conditions.

A glassy carbon electrode (GCE) modified by simultaneous electropolymerization of L-lysine (L-lys) and -cyclodextrin (-CD) film (P-CD-L-lys/GCE) was described as a novel electrochemical platform for the detection of pymetrozine. Using CV in 0.1 M H2SO4, the electrochemical activity of P-CD-L-lys/GCE towards pymetrozine was investigated [113]. The sensor's potential value was shown using LSV to estimate pymetrozine concentration analytically. The linear range is 4.0108 mol/L to 1.0106 mol/L. Pymetrozine's detection limit was determined to be 1.3108 mol/L (S/ N = 3). We were able to acquire satisfactory pymetrozine results in real sample analysis using this approach. The usage of amino acid materials and cyclodextrin in environmental analysis was supported in this study. Chaiyo et al. (2020) reported a simple, low-cost, and highly sensitive voltammetric sensor based on a Nafion, ionic liquid, and graphene composite, modifying a screen-printed carbon electrode for the simultaneous determination of Zn2+, Cd2+, and Pb2+ in drinking water at the nanomolar level [114]. Del Valle and co-workers reported the immobilization of 4 carboxybenzyl-18-crown-6 and 4-carboxybenzo-15-crown-5 on monolayers of aryldiazonium salts anchored on the surface of graphite-epoxy composite electrodes for the simultaneous determination of Cd2+, Pb2+ and Cu2+ in synthetic water samples at to the ppb level (ca. nanomolar) using differential pulse anodic stripping voltammetry (DPASV) [115]. Then, using a comparable electrode modification, Perez-Rafols et al. developed an electronic tongue for the detection of Cd2+, Pb2+, Tl+ and Bi3+ in synthetic water samples [101]. For the detection of trace metals, molecularly imprinted polymers [102–104] and metal-shaped nanoparticles [116, 117] have also been proposed. Voltammetric analysis of Pb2+ in enriched water has been reported using MoS2/rGO flower composite with ultrathin nanosheets [118], Ni/NiO/ MoO3/chitosan 3D foam at the p-n junction interfacial barrier for micromolar level Cu2+ detection [119] and Mn-mediated MoS2nanosheets have been reported recently as a new approach to Pb2+ sensing [120]. A VIP (Voltammetric In-Situ Profiler) for the detection of Cu2+, Pb2+, Cd2+ and Zn2+ using a Hg-based electrode [121] and a kayak equipped with Zn2+voltammetric sensors based on liquid crystal polymer bismuth film [122] are the only two cases reported in the literature that demonstrated in-situ operation in seawater. Single and multiple analyte detection methods have been investigated in the last five years. Maria's (2021) report includes a number of electrochemical techniques for the detection of trace metals, although demonstrated applications in unspiked saline samples are quite limited (**Table 6**). For nitrogenous nutrients, two in-situ solutions were effectively used to produce NO3 and NO2 profiles in saltwater.

After seawater treatment, potentiometric detection of NO3 and NO2 at relatively low doses is possible utilizing multiple electrodes combined in a flow cell. Amperometric NOx and NO2 biosensing is possible using various bacterial chambers in an electrode configuration that can be deployed in any water system using an analyzer or submerged device allowing direct contact of the sensors with the water column at


#### **Table 6.**

*Electrochemical environmental sensors for metal analysis.*

micromolar concentration with separate electrodes. There are no demonstrated in-situ uses of electrochemical devices for in-situ detection of ions in saltwater, according to studies. At the laboratory scale, new advancements are ongoing, with in-situ installation compatibility promised but rarely fulfilled. The few sensors that have been integrated in submersibles appear to be in the early stages of commercialization. As a result, meaningful data is only gathered through the inventors' measurements [135]. Electrochemical sensors have the potential to transform seawater analysis programs, but bringing this vision to fruition would need well-planned phases and targeted research.

#### **2.7 Electrochemical sensing in forensics**

Homicide cases usually involve weapons that release many compounds when fired. These compounds are identified in gunshot residue that may be found at a crime scene, on a suspects' clothing or other items, which then become useful in the investigation of a crime by providing evidence. This gunshot residue (GSR) consists of both inorganic and organic parts that need analysis for a successful investigation. In addition, elements such as antimony, lead, bismuth and copper present in GSR are hazardous to human health and the environment and therefore their determination is necessary [136]. Increasing security needs necessitate the deployment of fielddeployable detectors capable of detecting GSR and nitro-aromatic explosive compounds in real time.

Erden et al. (2011) presented the measurement of antimony and lead in gunshot residues using differential pulsed cathodic adsorptive stripping (DPCAS) and squarewave cathodic adsorption stripping voltammetry (SWCASV) [137]. A hanging drop of mercury was used as a working electrode for GSR samples obtained during test firings at the Police Criminal Laboratory from the shooters' hands. The results demonstrated that both the DPCAdSV and SWCAdSV approaches could be used to successfully

determine metals in GSR, both qualitatively and quantitatively. Vuki et al. (2012) then reported the simultaneous detection of Pb and Sb. along with other organic components of GSR at propellant on a glassy carbon electrode using CV and cyclic-SWV [138]. The thin film Hg GCE was used for Ba analysis because it requires detection of the presence of mercury [139].

Ceto et al. (2012) reported Zn, Pb, Cu, and nickel amalgams on bare and modified SPE by SWV in samples obtained straight from shooters at a nearby shooting range. The fascinating element is that they were able to validate several handling situations including secondary contact with the GSR, loading the handgun, and finally discharging. The subject that fired the firearm was identified other than the ability to distinguish the time of contact with the GSR with respect to different subjects. The use of microelectrodes for lead analysis in GSR was demonstrated by Salles et al. (2012) using the SWV technique [140]. Microelectrodes have substantial benefits for analysis, such as the capacity to deal with small-volume samples, downsizing of equipment, and the removal of mixing in the pre-concentration process. Their study suggested the possibility of linking lead originating from the GSR to discrimination between different types of weapons and ammunition. Wearable, completely stiff finger sensors for quick on-site voltammetric assessment of GSR and explosive surface residues were described by Bandodkar et al. (2013). To build new forensic fingers, they screenprinted the three-electrode arrangement onto a nitrile finger pad and overlay another finger pad with an ionogel electrolyte layer [141]. The novel integrated sampling/ detection system uses "microparticle voltammetry" (VMP) to transmit minute quantities of surface-confined analytes directly onto a fingertip-based electrode array. Voltammetric measurements of sample residues are performed by bringing the working electrode into direct contact with a second finger bed coated in ionogel electrolyte (worn on the thumb), therefore completing the solid-state electrochemical cell [142]. The sampling and screening process took less than four minutes and results in GSR and explosives having distinct voltammetric signatures. The use of a solid, flexible ionogel electrolyte eliminates any liquid handling, reducing leakage, portability, and contamination concerns. The fingertip detection gadget demonstrated great specificity for detecting GSR and nitroaromatic chemical residues. It can tolerate constant mechanical stress without losing its attractiveness, a low-cost way for conducting onsite crime scene investigations in a range of forensic settings [143]. Hashim et al. 2016 reported gold modified screen printed electrode for Cu (II) analysis in GSR using CV (**Figure 9**). It was also compared with ICP-OES analysis with 94% accuracy [144].

The research area was long forgotten, but is now re-emerging as a result of the sensitivity and adaptability that this approach provides. Ott et al. published a paper in 2020 on the analysis of Pb, Sb, and Cu by SWASV employing bare SPE. Two samples were taken in this investigation, one from the hands of volunteers with no direct touch and the other from the hands of those who had recently discharged a weapon. They examined data from 395 genuine shooter samples and 350 background samples, making it the biggest GSR research yet done [145]. Using a simple, quick, and sensitive voltammetric assay, they were able to identify all metals as well as organic bullet residues at the same time. It was detailed how to make a biochar-modified carbon paste electrode. Oliveira et al. used DPAdSV in 2021 to assess lead ions in gunshot residue and hair coloring samples. GSR samples were acquired using an IMBEL 9 mm pistol and 100% cotton fabric, with no hair color specified. To eliminate interference from other metals contained in gunpowder components such as Sb (III), Cu (II), Cr (III), and Fe (II), this work employed the conventional addition technique to measure lead in the acquired samples [146].

**Figure 9.** *CV at gold modified SPE for various Cu(II) concentration with inset showing calibration plot [144].*

Bessa et al. (2021) presented a different approach to lead detection in *Lucilia cuprina*, a necrophagous insect that functioned as a GSR biomarker due to GSR consumption, As a result, an intriguing lead in forensic investigations is provided. Their survival rates were investigated, as well as the influence of lead on their embryonic growth [147]. As a working electrode, a bismuth film was deposited in an SPCE using the SAWASV process. The results showed that lead could be detected on larval samples even in the presence of interference such as Cu, Ba, and Cd. The effect of lead on mortality and larval development time was also successfully investigated [148, 149]. The revival of this area has opened up a whole new avenue for research leading to breakthrough in criminal investigations.

#### **3. Conclusion**

Voltammetry holds the key to future diagnostic tools for fast, real-time and sensitive analysis. It has found use in almost every spectrum of environment, medical care, agriculture, forensic, investigation, food and cosmetic industries for a range of analytes to be determined selectively or simultaneously depending on the requirement. The resurgence of the interest in electrochemical methods for the lucrative metal analysis required for various applications is derived from the advantages it caters to, like sensitivity and versatile application. Nanomaterials and polymers are the advanced materials being explored for their synergistic effect to modify electrode surfaces to overcome the challenges that stand in the way. The research is targeted to develop fast, stable, reproducible, miniaturized real-time sensors for use in various fields.

#### **Acknowledgements**

The authors are thankful to their respective organizations for the facilities provided to the authors to carry out their work. The authors would also like to

acknowledge MDPI publisher for their open access journals which allowed us to reproduce content in form of figures.
