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18 November 2021

An overview of Structured Biosensors for Metal Ions Determination

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,
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and
1
Center on Natural Sciences and Humanities, Federal University of ABC, Santo André 09210-580, Brazil
2
Department of Applied Chemistry, LAQV-REQUIMTE, Faculty of Pharmacy, University of Porto, 4050-313 Porto, Portugal
*
Author to whom correspondence should be addressed.
Chemosensors2021, 9(11), 324;https://doi.org/10.3390/chemosensors9110324
This article belongs to the Special Issue Modern Directions in Ion Electroanalysis for Real World Applications
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Abstract

The determination of metal ions is important for nutritional and toxicological assessment. Atomic spectrometric techniques are highly efficient for the determination of these species, but the high costs of acquisition and maintenance hinder the application of these techniques. Inexpensive alternatives for metallic element determination are based on dedicated biosensors. These devices mimic biological systems and convert biochemical processes into physical outputs and can be used for the sensitive and selective determination of chemical species such as cations. In this work, an overview of the proposed biosensors for metal ions determination was carried out considering the last 15 years of publications. Statistical data on the applications, response mechanisms, instrumentation designs, applications of nanomaterials, and multielement analysis are herein discussed.

1. Introduction

According to the International Union of Applied Chemists (IUPAC), biosensors are devices that mimic biological conditions by mediating biochemical reactions between the analytes and the biological recognition element (enzymes, immunosystems, tissues, organelles, and whole cells) kept in direct contact with a transducer, yielding an optical, electrical or thermal signal [1]. They are constructed by coupling biomolecule(s) to a transducer aiming at the conversion of a biochemical process into a physical output. This definition is still following a biotechnological glossary published in 1992 [2]. Definition updates are desired once other mechanisms that yield biological responses are nowadays exploited with biosensors [3]. A wider definition was proposed by a review article that described biosensors as compact analytical devices with incorporated biological or biologically derived recognition systems for analyte quantification [4], which is suitable considering the design of the developed sensor and other types of responses (e.g., piezoelectric and acoustic detection).
The interaction of metal ions with some biorecognition molecules allows the quantification of these elements using biosensors. In some cases, speciation [5,6] and multielement analysis [7,8,9,10,11,12,13,14,15,16,17,18,19] are reached by exploiting several biochemical processes in the same system or the same type of interaction for different analytes. Possible biochemical responses result from the conformational changes of proteins, the quenching or emission of fluorophores, spectroscopic shift, and the enhancement or hindrance of the combination of DNA strands [20]. Determinations carried out with biosensors overcome some drawbacks of other strategies for element analysis based on techniques such as atomic absorption spectrophotometry (AAS) [21], atomic fluorescence spectrometry (AFS) [22], inductively coupled plasma atomic emission spectrometry (ICP OES) [23], inductively coupled plasma mass spectrometry (ICP-MS) [24], and X-ray fluorescence spectrometry [25]. Mainly, biosensors show lower costs of acquisition and maintenance, lower detection limits without prolonged sample preparation steps, and adequate selectivity [26]. In many cases, biosensors have been applied to the accurate determination of elements at trace and ultra-trace levels [25]. Despite the usual low detection limits of electrochemical techniques, interferences among metal cations have been generally reported and the addition of masking agents, for example, is sometimes necessary to circumvent drawbacks [27]. By modifying electrodes with biocomponents, selectivity is enhanced in a simple and environmentally friendly way [25]. Additionally, biosensors are generally easily miniaturized, contributing to the development of in situ and automated procedures [3,28] with minimum sample preparation.
Inorganic element analysis is important because it comprises the determination of essential ionic nutrients for the maintenance of life. Biological systems depend on compounds with metal ions as determinants of their structure (e.g., cytochrome) or as essential participants of biochemical reactions. Manganese, copper, and iron compose the chemical structure of important biological compounds, such as superoxide dismutases [29] and hemoglobin [30]. Relevant examples of the essential participants of biochemical reactions are nickel, zinc, and magnesium. The latter acts as a charge shielding ion in ATP reactions [31]. Inorganic salts with low solubility also show their importance in biological systems. Calcium phosphate is the main constituent of bone and teeth structures [32] and provides a primary source of phosphorus in plant cells [33]. Due to the variable concentrations of these nutrients in the biological system of humans, daily maximum ingestion is recommended, which varies from thousands of milligrams, e.g., of calcium [32], to a few micrograms, such as in the case of chromium(III) [34]. However, the excess of essential metal cations can lead to system disorders such as renal and neurological diseases that might be a consequence of the excessive ingestion of calcium [32] and manganese [29], respectively.
Harmful metal ions such as mercury, cadmium, lead, and chromium(VI), do not show nutritional values and pose a major public health concern according to the World Health Organization (WHO) [35]. Thus, legal limits have been established for these elements in environmental and food samples. According to WHO guidelines [35], the safe recommended limits for Cd, Hg, Pb and total Cr in drinking waters are 3.0, 6.0, 10, and 50 µg L−1, respectively.
Considering the abovementioned highlights of metal cations acting as essential and toxic agents in biological systems at low concentrations, the development of reliable and sensitive analytical procedures with adequate selectivity can be achieved with biosensors. Thus, this article aims to provide a critical overview on the development of biosensors for metal ions analysis, with electrochemical, optical, and piezoelectric transductions proposed in the last 15 years. Procedures based on the immobilization or entrapment of biorecognition species on solid platforms were included in this review, excluding, however, strategies that considered suspensions of nanomaterials as biosensors. Previous reviews on enzyme-based electrochemical (bio)sensors [20,25] were focused on heavy metals analysis. Therefore, interesting works not mentioned elsewhere, including those discussing sensors with non-electrochemical responses, are also herein discussed.

2. General Aspects and Statistics

Chemical analyses are generally a high demand task in analytical laboratories. Thus, sensitive, cheap, fast procedures that require simple instrumentation are sought. In this context, biosensors meet these requirements compared to other strategies for the detection of metallic elements. In the past 10–15 years, over 150 articles focusing on biosensors for metal ions analysis have been published, according to a survey of the ISI Web of Knowledge database. These data were displayed in Figure 1, which shows that despite the decrease in the number of publications since 2015, there were over 3400 citations, with an average of 25 citations per article and an h-index of 35. Before 2013, less than 25 articles had been published with this aim. Since 2014, the number of citations showed a ca. 550% increase and increments between 18 and 112% were observed yearly until 2020. These data demonstrate the trends in the studies of biosensors for metal ions analysis. Despite 2020 being an atypical year, in which the attention of biological research was redirected to resolving other emerging issues, the numbers of publications and citations still increased.
Figure 1. Number of publications (-○-) and citations (-•-) of procedures with metal ions biosensing since 2013. Data were retrieved from ISI Web of Knowledge.
The most frequent metal ion determinations were Pb(II), Cu(II), Hg(II), and Cd(II), comprising 98% of the publications considered herein (Figure 2a). Some of the developed biosensors were applied to the determination of multiple analytes, yielding a sum higher than 100%, as shown in Figure 2a. The interest of each metal ion is also demonstrated by each percentage in the considered database. Among the metals indicated in Figure 2a, Cu(II) has been a target ion for the development of biosensors because cupric ions easily coordinate with biomolecules, such as proteins, with multiple amino groups in their structure. Likewise, biosensors for Cd(II), Hg(II), and Pb(II) were developed based on their bonding to the nitrogen bases of DNA strands. Along with other analytes, such as Ni(II) and K(I), DNA-based sensors comprise over 50% of the proposed procedures for metal ion biosensing due to the use of variable strategies.
Figure 2. Overview of the applications of biosensors for metal ions determination. (a) analytes, (b) samples, (c) analytical techniques, and (d) types of platforms. The fractions were calculated based on the selected articles.
Environmental analysis requires analytical procedures with low limits of quantification due to the threshold limits of hazardous metal ions [35], which can be achieved with biosensors, especially with electrochemical transduction. The analysis of tap, drinking, river, well, and lake waters comprised 75% of the proposed procedures (Figure 2b). This is a consequence of the problematic presence of toxic metal ions in environmental samples and their bioaccumulation. Biological fluids, food, soil, and wastewater analysis have also been exploited, comprising 34% of the surveyed data. Most of applications were directed to liquid samples, such as plasma/serum [7,9,36,37,38,39,40,41,42,43,44,45], juices [9], and wastewaters [46,47,48,49,50,51,52,53,54,55,56].
It is important to point out that, according to Figure 2b, almost a quarter of the proposed biosensors were not applied to real samples in view of their lack of selectivity. The significance of the studies carried out regarding the detection of metal ions justifies, nonetheless, the argument that the real application of procedures should still be sought. In that scope, sample preparation can be further evaluated to enhance important analytical features such as selectivity. In those cases where trace analysis was feasible, even microdigestion [57], i.e., decomposition using micro amounts of sample and reagents, could be employed as a fast and clean step for sample preparation.
A variety of transducers have been exploited for biosensing. According to Figure 2c, 85% of the applications were based on the differential pulse (DPV), square wave (SWV), cyclic (CV), anodic (ASV), or cathodic stripping (CSV), and alternate current voltammetry (ACV), and amperometry (AP), electrochemical impedance spectroscopy (EIS), potentiometry (PT), conductometry (CD), and chrono-amperometry (CA). The development of selective procedures with low detection limits using affordable and simple instrumentation makes bioelectrochemical sensors extremely attractive. Spectrochemical biosensors might require higher costs of equipment acquisition and maintenance such as for surface plasmon resonance (SPR), surface-enhanced reflectance spectrometry (SERS), and infra-red reflectance spectrometry (IRS). Nevertheless, inexpensive lab-made photometers and fluorimeters have not been exploited for biosensing. Fluorescence (FL) has been more frequently employed [12,58,59,60,61,62] due to the higher sensitivity and the ease of collecting fluorescence emissions rather than transmitted radiation using solid-phase supports.
The widespread use of electrochemical transducers reflects on the types of material frequently employed as supports of biocompounds. According to Figure 2d, gold (GE) and glassy carbon electrodes (GCE) have been the most employed ones, comprising 61% of the applications. Their electrical and chemical properties and ease for the adherence of biomolecules and nanoparticles make them unique materials for the construction of biosensors. Glass, platinum, screen-printed (SPE), carbon paste (CPE), carbon film, pencil graphite (PGE), and carbon/epoxy mixture electrodes have also been used for immobilizing biomolecules for the electrochemical sensing of metal ions. Gold platforms were also applied for piezoelectric [63] and light scattering [48] measurements. As for other optical detection, glass [64], biopolymers [58], and paper [59] have served as adequate supports for the construction of biosensors.
In general, some common experimental conditions were observed for the use of biosensors. Temperature and pH are critical parameters in enzymatic reactions, as well as for favoring complexation. As these reactions generally simulate living systems, physiological pH (between 7.0 and 7.5) and temperature (from 25 to 40 °C) were employed in most applications. These conditions also contributed to improve the lifetime of biosensors by avoiding significant changes in the folding of the sensing component somehow. When complexation is the main principle involved, the temperature was generally not controlled. In rare exceptions, an acidic pH between 1 and 2 was exploited for biosensing [54,65,66,67,68]. An alkaline pH was not evaluated due to the decrease in biochemical responses and to the hydrolysis of metal ions. In time, 2-amino-2-hydroxymethyl-propane-1,3-diol (TRIS) and phosphate solutions were mainly employed as buffers.
The short and long-term stabilities of biosensors are important features. Apart from disposable devices, biosensors lasted from a few determinations up to three months without significant losses of analytical response. Mainly, the conditions of storage, the immobilization strategy, and the number of analyses per day have a direct influence on their lifetime. In most cases, the preservation of the devices was carried out under refrigeration at 4 °C and preferably immersed in a buffer solution, which enabled their use for more than a workday. In a few cases, biosensors were stored at an ambient temperature and under drying conditions, such as in the determination of Cd(II) [46,69]. Alternatively, a mix of reagents can be stocked and used for the daily preparation of the biosensor. In general, these mixtures showed longer lifetimes. Despite the valuable importance of this parameter, many works have not developed a systematic evaluation of the stability of their respective biosensors. This is one of the critical pieces of information that must be explicitly discussed.
Another poorly discussed topic in many articles was the cycles of reusage of the sensors. Most of the works did not make clear if their biosensors were reusable after determinations and if so, the limiting number of replicates that were carried out for that device. In general, reusable biosensors based on a metal complexation with DNA and other biomolecules were submitted to cleanup with EDTA. The removal of the analyte from the biosensor was achieved within 5–30 min, being ready for reuse after rinsing with a buffer solution [13,53,70,71]. Rarely has a simple cleaning with a buffer without additional reagents proven to be efficient for recycling. Nevertheless, the regeneration with EDTA must not be taken as a general approach because it might affect the performance of the sensor by removing the metal ion from enzymes, for example. Aminoacids were also employed for the regeneration of biosensors, especially when the bonding with DNA strands was exploited. In this regard, cysteine was employed for the regeneration of sensors for Hg(II) determination [41,72,73], for the ability to form a more stable complex with the analyte. This simple strategy increased the lifetime of the biosensors up to 15-fold.

3. Biorecognition Mechanisms and Features

The mechanism involved in a biosensor response depends on the employed bio-recognition system. Despite several biochemical reactions that have been exploited for biosensing metal ions, the bottom line is the ability of analytes and biomolecules to form coordinated adducts. This bonding is generally exploited to adsorb/accumulate metal ions upon the sensor surface or to alter biochemical reactions. The former does not always mimic naturally occurring processes. For example, the determination of Fe(III) was based on its coordination with deferrioxamine [66], which normally does not occur in living organisms. On the other hand, enzymatic reactions and the hybridization of DNA strands were promoted or hindered after the reaction of specific sites of enzymes and nitrogen bases with cations.

3.1. Metal Ion Accumulation

The accumulation of metal ions on the surface of the sensors has been used to enhance analytical responses. Biomolecules or whole cells were generally immobilized or entrapped at the surface of the transducer to promote the approximation of the analyte. The association of this strategy with electrochemical transducers significantly lowered the detection limits, especially when stripping voltammetry was employed for the determination of elements [17,46,47]. The coordination of metal ions with proteins [74], aminoacids [17,75,76,77,78,79], synthetized peptides [70], hormones (melatonine [80] and oxytocin [7]), DNA strands [17,46,81,82,83], and a neurotransmitter (neurokinin B [38]) were exploited to achieve detection limits as low as 0.80 ag L−1 [81]. The selected articles regarding this mechanism are displayed in Table 1.
Table 1. Selected applications based on an accumulation of ions on the biosensor surface.
Natural extracts were sources of biomolecules for the construction of biosensors. Chlorophyll was obtained from spinach extract before immobilization for the potentiometric determination of Mg(II) [100]. After the purification of the extract, chlorophyll was mixed with polyvinyl chloride (PVC) and nitrophenyloctylether to yield a plastic membrane that was used to implement the potentiometric ion-selective electrode. Despite the low detection limit (78 µg L−1) and suitable response time (up to 120 s), further selectivity refinements must be carried out for the sample application because of Ca(II) interference. Nevertheless, this type of electrode could be employed, e.g., for Mg(II) and Ca(II) determination using ion chromatography with potentiometric detection [103].
The accumulation of cations on biosensors can hinder the electroactivity of chemical mediators. The determination of Fe(III) in plants, alloys and pharmaceutical formulations as low as 5.6 ng L−1 using the EIS technique was achieved after the immobilization of Deferrioxamine B on a GE [66]. The complex increased the impedance response due to the constrained electron transfer with the ferrocene/methanol probe.
In the case of optical biosensors, immobilized biomolecules were generally deri-vatizing reagents that yielded shifts of fluorescence [58,59,61,94], molecular absorption [104], Raman effect by light scattering [48,105], and surface plasmon resonance [64,65]. Optosensors have been exploited to a lesser extent compared to electrochemical ones because of the direct relationship between the instrumentation costs and the performance achieved. However, the possibility of exploring low-cost laboratory devices for photometric and fluorimetric measurements for specific applications may be a future challenge in the field of optical sensors. Determinations at ng L−1 levels based on the complexation of cations were achieved in half of the surveyed works that employed optosensors [48,61,104,105], indicating that strategies for enhancing sensitivity must be exploited.
Lyophilized cells have also been employed to act as pre-concentration probes of biosensors. Cu(II) [89,90] and Pb(II) [43] determinations using adsorptive stripping voltammetry were performed with immobilized microalgae and yeasts, which are prone to the construction of biosensors because of their high resistance, low cost, and ready availability. Biosensors based on Rhodotorula mucilaginosa and Tetraselmis chuii were used for Cu(II) detection as low as 29 ng L−1. The former was suitable for this application because of its well-known Cu(II) bioaccumulation [89]. On the other hand, the biosorption of the analyte by Tetraselmis chuii [90] and other microalgae is not well understood, but it is probably promoted by biopolymer components of their cell walls. Besides the natural accumulation of cations, modifications of cell walls were also explored. The detection of Pb(II) was estimated at 3.4 × 10−6 ng L−1 by using a biosensor with immobilized yeast cells coated with Co3O4 and Au nanoparticles, which were responsible for the adsorption of the analyte [43].

3.2. Catalysis and Inhibition

Biosensors for metal ions have also been constructed based on rate changes in particular enzymatic activity due to the bonding of cations or due to participation in the intermediate steps of the process. Biosensors based on metabolic and enzymatic reactions are presented in Table 2.
Table 2. Selected applications of biosensors based on catalysis and inhibition.
The spectrophotometric detection of Cu(II) in drinking water and milk [114] was carried out after the immobilization of tyrosinase apoenzyme on polyacrylamide sol-gel. Extracts from Agaricus bisporus were purified and treated with EDTA to remove the prosthetic Cu(II) ions from the quaternary structure of the enzyme before immobilization. In this way, the enzyme was active only in the presence of the analyte. Spectral shifts were observed due to the catalyzed conversion of dopamine into dopachrome, which showed an absorption maximum at 475 nm. As the substrate was also immobilized with the enzyme, spectrophotometric measurements were carried out directly on the solid phase, which contributed to a detection limit estimated at 63 ng L−1 for Cu(II).
Full-enzyme biosensors are less tedious to implement but usually raise selectivity concerns. They are generally based on the effect of the analyte on an enzymatic reaction. Enhancement of the immobilized catalase, for instance, was exploited for the determination of Ca(II) in milk samples [106] by mimicking the enzyme actuation in plant cells. The evolution of oxygen originated from the decomposition of H2O2 increased the amperometric signal due to the activation of catalase by Ca(II). In this way, it was possible to quantify the analyte from 40 to 400 mg L−1 with a common oximeter by coating the gold cathode with a PTFE membrane embedded with a gelatin/enzyme mixture. Nonetheless, high concentrations of concomitants such as Mg(II) must be carefully considered to avoid inaccuracy. Another example regards Mn(II) determination based on the enhanced activity of the horseradish peroxidase (HPR) in the presence of O2 and a 1,2-naphtoquinone mediator [116]. In the presence of the analyte, the reduction of the mediator was enhanced after oxidation by the HPR. Despite the demonstrated potential, further studies are required for this biosensor to improve selectivity to pave the way to application in real samples.
The inhibition of enzymatic activity was generally based on the affinity of analytes for sites of the enzyme (sulfhydryl group of cysteine) and replacement of the hydrogen atoms by making a covalent bond with sulphur. The consequent conformational shift of the enzyme hindered its affinity for the substrate [118]. Meanwhile, electrochemical transducers have been mainly used to follow up the consumption of substrates with several approaches. The inhibition of the catalytic decomposition of H2O2 on the electrode surface due to the activity of catalase [115] or HRP [19,107,113] was successfully applied for the determination of Hg(II), Cd(II), Cu(II), and Pb(II). The response to the substrate increased with the concentration of the analytes because less H2O2 was decomposed due to lower enzymatic activity.
Chemical mediators were employed in the sample solution [69] or immobilized on the same platform of the biosensing system [8,13,15,18]. The latter strategy is advantageous because it minimizes reagent consumption and the number of steps for analysis. Additionally, the detection limits are lowered because the immobilization facilitates the charge transfer to the electroactive sites of enzymes, such as those reported for sensors with immobilized polyaniline [16] and poly(neutral red) [113]. The response due to the electrochemical regeneration of these mediators yielded signal outputs that were up to 2.5-fold higher than bare electrodes. The incorporation of glucose oxidase on the platform coated with polypyrrole for Cd(II) determination [13] yielded a detection limit of 450 µg L−1, which was 3.5-fold lower than a proposed similar procedure using the same mediator and enzyme in the solution. In this case, the mediator was essential for the electrochemical transduction because the substrate and the products of the enzymatic reaction were electrochemically inactive.
The deposition of the oxidized substrate after the enzymatic reaction was exploited during the development of an optical biosensing platform. The determination of Cu(II) in natural waters by IRS was based on the catalyzed oxidation of 4-chloro-1-naphtol (4-CN) by immobilized HRP [108]. This process led to the deposition of solid 4-chloro-1-naphton (4-CNP) on the optode, thus altering the infrared spectrum of the support. Therefore, the presence of Cu(II) ions hindered the formation of 4-CNP because of the inactivation of the enzyme. The estimated detection limit (80 µg L−1) was not as low as the ones obtained using electroanalytical techniques, but it was suitable for Cu(II) determination in waters.
The inhibitory effect of urease was also exploited for toxic metals detection using an unusual strategy. As the enzyme and polyaniline were immobilized on a GE with Nafion® [10], after the enzymatic decomposition of urea, the ammonium ions formed a tertiary complex by bonding with the amino groups from polyaniline and the sulphone groups from Nafion®. In the presence of Hg(II) and Pb(II), the enzymatic reaction was inhibited, which avoided the formation of the tertiary complex and altered the profile of cyclic voltammograms.
The inhibition of naturally occurring enzymatic reactions of whole cells has also been exploited for analytical purposes. The determinations of Hg(II) [55] and Ni(II) [56] were based on hindering the activity of phosphatase (in Chlorella sp.) and urease (in Bacillus sphaericus), respectively. The low cost of commercially available enzymes makes the use of a living cells culture unnecessary for sensors based on the same monitoring of substrates [111,119] and products [56]. Nevertheless, for enzymes in which extraction procedures are laborious and generate high amounts of waste, the use of whole cells can pave the way for simpler and cleaner approaches.
Some biochemical processes, such as complex metabolic reactions, are hard to mimic with synthetic molecules, thus the use of whole cells comes to hand in these situations. The quantification of metal cations was achieved using the inhibition of the metabolism of fluorescent micro-organisms [12,60,109]. The quenching of the fluorescence of genetically modified Escherichia coli [60,109] and yeasts [109] was exploited for Cu II) determinations. On the other hand, the fluorescence of immobilized microalgae cells was enhanced in the presence of Cu(II), Cd(II) and Pb(II) [12] because of the inhibition of photosynthetic electron transport pathways. Therefore, the accumulated energy was released as electromagnetic radiation. Despite the low selectivity and the need for detailed accuracy studies, these approaches are suitable for the toxicity evaluation of waters.
The inhibitory effects on metabolic pathways have also been applied to determination of metal cations with microbial fuel cells (MFC) [50,52] by hindering the decomposition of organic matter due to competitive reactions and, thus, affecting the generation of electrical energy. Shewanella putrefaciens and Ochrobacterium anthropic biofilms were used to coat the anode of MFC for Cu(II) [50] and Cr(VI) [52] detection. In the former, despite the occurrence of side reactions in the presence of the cation, there was evidence that the lithotrophic behavior of Schiwanella putrefaciens and Cu(II) adsorption on the anode also contributed to the analytical response. Due to the portability and simplicity of MFC, these devices are suitable for an in situ evaluation of toxicity of industrial wastewaters. However, the biofilm must be resistant enough to overcome aggressive media, such as a high salinity and a low temperature. Other approaches have also been proposed for the detection of metal ions in environmental samples with MFC, but a detailed evaluation of the analytical features was neglected, thus allowing qualitative applications [120,121,122,123].
The acceleration of metabolic reactions was exploited for the determination and speciation analysis of Fe with Thiobacillus ferooxidans [54], Leptospirillium ferrooxidans [67], and Acidithiobacillus ferrooxidans [68] by monitoring the increase in O2 consumption. These microorganisms are resistant to acidic media, which allowed direct determinations of Fe in acidic samples, such as mine waste and mineral extracts [54]. Additionally, a rare behavior was shown by simply participating in a direct reaction involving the metal cation without the need for laborious electrode modifications or the use of mediators. Additionally, the determination of Cu(II) was carried out in waters as low as 13 µg L−1 by an optical sensor based on the immobilization of genetically modified Saccharomyces cerevisiae cells [109]. The presence of the cation induced the production of 5′-phosphoribosylaminoimidazole in the purine synthesis pathway that was oxidized and accumulated due to oxidative stress, and which gives the cells a red color.

3.3. Affinity

Selective interactions of biorecognition systems have been used for metal ions biosensing (as displayed in Table 3). An antigen-based sensor was constructed for the determination of Hg(II) in cosmetics by exploiting a competitive complexation. It consisted of methylmercury bonded to ovalbumin firstly adsorbed on GCE. The subsequent reaction with a monoclonal antibody labeled with quantum dots yielded an electrochemiluminescent (ELC) blank signal [124]. In the presence of the analyte, a competitive reaction occurred, which kept the antibodies tagged with quantum dots away from the GCE, thus decreasing the ELC signal.
Table 3. Selected applications of biosensors based on affinity.
As an alternative to immunoaffinity systems, single and double-stranded nucleic acids, often encompassing those based on simple nucleotides, have been used as recognizing elements in biosensors, generally referred to as aptasensors. Different features make these biomolecules attractive, particularly their ability to be synthesized outside living organisms, yield complexes with formation constants in the nanomolar concentration region, enabling sensor reuse, and their high chemical and thermal inertness [24] compared, for example, to antibodies. Over 90% of aptasensors for Cd(II), Hg(II), and Pb(II) exploited the ligand ability of nitrogen to achieve low detection limits (as low as 8.0 × 10−10 ng L−1) [81,105]. The bonding of cations to the thymine base from DNA strands was exploited for selective determinations.
The mechanisms were based on conformational shifts and the cleavage or hybridization of DNA strands mediated by metal ions. In addition, DNAzymes were also exploited for detection due to their selective and sensitive reactions with cations. As an example, the determination of Hg(II) was based on the complexation of the analyte with a thymine (THY) base from an immobilized single-stranded DNA, resulting in the union of a pair of strands [73]. The formed complex interacted with the mediator disodium-anthraquinone-2,6-disulfonate (AQDS) via an anthracene ring. Thus, the electrode was covered with AQDS, which enhanced electron transfer and led to an increase in the current signal.
The effects on the conductivity of biosensors due to the conformational shifts of immobilized DNA strands after bonding with metallic ions were also used for biosensing [9,72,96,98,99,102,130,141,149,150,181]. In general, the charge transfer was enhanced after the DNA binding to Cu(II) [130], Pb(II) [102,181], and Hg(II) [9,72,96,98,99,141,149,150]. On the other hand, the formation of a G-quadruplex structure with a K(I) hindered charge transfer [155,156] and led to detection limits as low as 5.0 pg L−1 for application in urine samples. In this case, the high dilution of the sample contributed to minimizing interferences from concomitants that were present at low concentrations in the sample.
The selective binding of DNA bases to Hg(II) and Pb(II) was also used for the development of procedures based on responses due to the cleavage or the hybridization of DNA strands. These reactions promote the (dis)assembly of DNA strands and aptamers labeled with a chemical mediator, such as methylene blue, ethyl green, ferrocene, and gold nanoparticles [9,36,37,41,137,159,175] for bioelectrochemical sensors. Competitive reactions between analyte and chemical mediators with immobilized oligonucleotides were also reported [71,140,153,160,162,164,165].
Compared to proteins or RNA molecules, DNAzymes are an excellent choice for metal ion detection because of their relatively low cost and high stability toward hydrolysis. All of these features make the metal-dependent DNAzymes particularly attractive as a biosensor platform for selective metal ion detection. The mechanism is based on the immobilization of double-stranded DNA comprising a substrate and an enzyme strand that can hybridize to the substrate through two base-pairing regions. The 5′-portion of the enzyme binds the substrate and the 3′-region through the formation of a DNA triplex. In the presence of the cation, the substrate is cut at the cleavage site (deoxyguanosine) and the fragments are released, leaving a part of the substrate strand immobilized on the sensor. In this regard, DNAzymes were exploited for fluorescent and electrochemical sensors for the determination of Pb(II) [9,28,45,63,158,166,167,172,173,174,175,179,180], Cu(II) [133], Cd(II) [126], Hg(II) [40,135,138], Mg(II) [42], and Ni(II) [157]. In these procedures, substrate strands were labeled with electrochemical and fluorescent probes that showed responses after their disassembly from the platform. Alternatively, strands labeled with enzymes, such as peroxidase, were used [44,119,142] to monitor the enzymatic reactions.
In modern approaches, the labeling of DNAzymes with porphyrins [143,168,169,170], composites [45], and metal-organic frameworks (MOF) [131,161,177] were also proposed due to their enzyme-like activities. The bonding of hemin to DNA strands catalyzed the decomposition of H2O2 [187]. On the other hand, iron-based MOF [131,161,177] acted on the decomposition of H2O2 via a Fenton reaction [188]; thus, the consumption of hydrogen peroxide and the generation of hydroxyl radicals were exploited to yield analytical signals. As an additional advantage, these replacing catalysts were more stable than enzymes, which minimized the susceptibility of the biosensors to degradation.
The mechanisms discussed are schematically summarized in Figure 3.
Figure 3. Scheme of the main mechanisms of biosensors for metal ions determinations. (A): Metal coordination; (B): bioaccumulation; (C): biosorption; (D): hybridization of DNA strands; (E): DNAzymes reactions; (F): Enhancement of enzymatic activity; (G): inhibition of enzymatic activity; (H): metabolism alteration of cells; S: substrate; P: product; X: generic element; Mn+: generic cation.

4. Biosensors Platforms

The immobilization or entrapment of biosensing structures varied according to the transducing strategy, which was usually carried out with electrochemical, optical, and vibrational approaches. The best stability and performance are the main features that must be considered for the construction of biosensors. Figure 4 shows the main components of platforms used for the construction of biosensors.
Figure 4. Schematic representation of possible compositions and treatments of layers of biosensors platforms.

4.1. Electrochemical Sensors

Electrochemical sensors are most often constructed using an electrode as a physical support for the biorecognition element. GE and electrochemically gold-coated GCE were the commonly employed platforms due to their chemical inertness, high conductivity, and the simple steps required for the immobilization of biomolecules. The interaction of gold with thiol groups via a redox reaction led to a self-assembled monolayer on the metallic surface, resulting in the deposition of gold thiolates [189]. The execution of simple steps such as dip and drop coatings allowed for an easy immobilization of the naturally or synthetically thiolated molecules, such as peptides and DNA strands, after 2 to 12 h of contact with the surface [9,26,36,37,42,44,59,73,98,119,124,127,133,140,143,144,153,156,158,164,165,167,168,169,172,176]. Alternatively, the gold layer was modified with mercaptohexanol [134,168], hexanethiol [159], and aminothiophenol [154,155] to immobilize the target molecules and nanoparticles. Special attention is needed when executing this approach to avoid the sensor lacking conductivity due to the deposition of high quantities of organic compounds.
Amino and carbonyl groups were used to form peptide bonds with proteins and enzymes, whereas the presence of hydrophobic groups enabled the immobilization of non-polar molecules via π-π stacking. The modification of gold surfaces with 3-mercaptopropionic acid (MPA) followed by the coupling reaction with N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) enabled the formation of active ester sites (amino polymerization method) [66,93]. Thiolated molecules such as mercaptohexanol [134] were also added after immobilization to prevent the bonding and adsorption of concomitants in the sample, mainly proteins.
The amino polymerization method has also been employed for the modification of GCE without a metallic coating. In this case, carboxyl groups were generated on the carbon surface after electrochemical oxidation to enable the polymerization reaction [91,137]. Alternatively, the modification of an electrode was carried out with carboxylated carbon nanotubes (CNT), which was applied for the immobilization of glucose oxidase, aiming at Cd(II) determination [69].
The electrochemical deposition of DNA on modified PGE was an ingenious and fast approach exploited for Hg(II) determination in seawaters and seafood [81]. The preparation of the sensor was faster compared to the chemical modifications that generally required 30–60 min to assemble the sensing biomolecule.
A common modification of biosensing platforms has been carried out with reduced graphene derivatives [9,28,40,46,69,98,99,135,141,161,167,173], which show high thermal, chemical, and mechanical stability [64]; good biocompatibility [135]; and high conductivity, especially reduced graphene oxide. As an example, the current signals of cyclic voltammograms obtained with a ferrocyanide solution increased two-fold after coating the GCE electrode with reduced graphene oxide, aiming at the immobilization of polynucleotides for Pb(II) determination [167]. The adherence of graphene on carbon-based surfaces required drop coating as a simple and fast preparation step. Due to the few oxygenated functional groups, the chemical modifications of biomolecules were not often exploited for immobilization on graphene-modified surfaces [40]. Except for single strand DNA, which interacted with reduced graphene oxide via π-π stacking [173], biomolecules were generally retained by polymeric nets or immobilized after the deposition of gold nanoparticles (GNP) [167].
As an alternative to immobilization, the entrapment of biomolecules by surface coating with polymers [19,47,69,107,139] and sol-gel materials [14,53] has been stated. The chosen materials must produce adequate layers for containment, presenting low solubility and high stability in the measurement medium (generally water). In addition, adequate porosity must be sought to enable the diffusion of analytes and mediators. Artificial polymers such as Nafion® were used for the determination of Cd(II) [69,107], Cu(II) [19,47], Hg(II) [139], and Pb(II) [19]. In general, a suspension of the polymer (mixed or not with the sensing system) was dropped onto the electrode and left to dry prior to analytical applications.
Polymers from natural sources such as chitin [95], gelatin [106], and chitosan [11,72,160,177] layers were employed with simple steps for electrode modification. Despite the greenness of this alternative, the solubility of biopolymers in water is generally higher than artificial ones, which hinder the lifetime of the sensors. Furthermore, they are more susceptible to dissolution due to pH and ionic strength variations. Nevertheless, their simplicity and environmentally friendly feature must be highlighted as adequate choices for the construction of biosensors.
Biosensors for metal ions analysis have also been constructed by mixing the components of the electrodes with the sensing biomolecule. The amperometric determination of Mn(II) [116] was carried out by mixing a carbon paste electrode with HRP to give rise to an electrode. Despite the simplicity, the lifetime of the electrode is compromised due to the leaching of the enzyme. Additionally, the repeatability and reproducibility might be hindered as well. Nevertheless, a discussion on electrode lifetime and precision was neglected in this work, indicating that further studies must be carried out before the application of this biosensor. Lifetime drawbacks can be attenuated by retaining the sensing biomolecule in a plastic membrane, as it was proposed for the determination of Mg(II) [100].
A modified Fe3O4 magnetic solid was used for the simultaneous determination of Cd(II) and Pb(II) in natural waters [17]. After synthesis, the magnetic particles were bonded to glutathione (via sulfhydryl groups) that acted as a complexing agent of the analytes. The biosensor was prepared by ingeniously coupling a GCE on the tip of a PTFE tube filled with a magnet. In this way, the sensing magnetic particles were retained at the outer surface of the GCE due to the electromagnetic attraction to the inner magnet. This simple strategy showed good performance for the analysis of natural waters, paving the way for the construction of other sensors using the same principle of immobilization.
Additional platforms used for the construction of biosensors include indium tin oxide glass (ITO) electrodes and field-effect transistors (FET). The former, known for its high conductivity and transparency, was employed for the determination of Cu(II) in tap water [92] after the electrodeposition of GNP modified with amino acids. Modifications of FET sensors were carried out at the SiO2 gate of the device and used for Cd(II) [129], Cu(II) [39], Fe(III) [30], and Ca(II) [84,125] determinations in biological fluids and drinks. The silanization of the gate was required using amino-terminated silane, such as aminopropyltriethoxisilane [39]. Afterward, glutaraldehyde was added to form a bridge with a carboxyl group terminal that allowed biomolecules to bond to amino groups. Graphene-based FET was also employed for the determination of Hg(II) in mussel digests [99] via the immobilization of aptamers on the sensor. An additional feature of FET biosensors was the miniaturization that required low volumes of sample for analysis, especially for those applications on biological materials [39,84]. Other approaches include modified electrodes of commercial oximeters [49,54,67,68,106] based on an electrode coating with biofilms and the use of MFC for element analysis [50,52].

4.2. Optical and Piezoelectric Sensors

To a lesser extent than electrochemical biosensors, optodes have been applied to element analysis exploiting molecular absorption spectrophotometry (MAS) [104,114], SERS [48,105,178], SPR [64,65,147,179], IRS [108], digital imaging (DI) [109,171], and FL [58,59,60,61,62,150]. The latter was frequently used due to the simplicity of the instrumentation and the ease of detection. In general, the construction of optosensors was based on the entrapment of the sensing biomolecule or whole cells with polyacrylamide [60,114], tetramethoxysilane sol-gels [58], and calcium alginate beads [109]. In this case, the porosity must be adequate to favor the diffusion of the analyte.
Polyaniline was used to immobilize sensing biomolecules via an interaction with amino groups. The devices were constructed with a solid piece of the polymer [62] or with an electrochemically coated optical fiber [104] aiming at Cu(II) determinations. Silicon-based platforms were also employed for the quantification of Cu(II) [108] and Pb(II) [48] in natural waters. For enzyme immobilization, the surface of the platform was silanized with aminopropyl(triethoxyl)silane and diisopropylethylamine before the enzyme’s assembly. Alternatively, the immobilization of biomolecules was carried out after chemically depositing silver nanoparticles.
Biosensors constructed with gold platforms were also used for optical detection by SERS [65] and SPR [64]. In this case, the immobilization of biomolecules was carried out as previously discussed for electrodes exploiting reactions with gold. In addition, gold platforms were also used for biosensors with piezoelectric responses [63,163]. In this case, quartz microbalances were coated with a gold film, which allowed the attachment of polynucleotides via thiol groups bonding, allowing for the determination of Pb(II) in waters.

5. Nanomaterials in Biosensing Detection

Nanomaterials have unambiguously set their place in many areas. They have been employed both as a recognition and support agent in biosensing detection due to their chemical, physical, electrical, and optical properties compared to bulk materials [92]. Furthermore, the modification of sensors with nanoparticles, especially electrodes, yielded higher stability, sensitivity, and repeatability [30]. Coating platforms with nanomaterials yields higher chemical and mechanical stability, thus the lifetime of the biosensors is elongated. Better precision is related to stability because lower signal deviations are observed when degradation of the device is avoided. In addition, immobilized enzymes show an enhanced stability and activity, hence coating electrodes with nanomaterials can be easily carried out to achieve the biomolecules’ immobilization [190]. The usual nanomaterials employed in biosensors and their benefits are schematically shown in Figure 5.
Figure 5. Types of nanomaterials commonly used in structured biosensors and their main contributions to the devices’ performance.
Over 30% of the proposed procedures for metal determinations employed biosensors modified with nanoparticles. Modifications were carried out with GNP [7,28,36,40,44,48,63,71,73,76,81,131,133,135,139,142,155,158,161,163,167,171,172,175,176,177], CNTs [11,19,30,69,107,117,162], silver nanoparticles (SNP) [51,138,159], mesoporous carbon [62,73], fullerenes [175], and quantum dots [124,157,184,186]. Coating GE biosensors with GNPs [135,155] aimed to increase the surface area to enhance its sensitivity. The response of the determination of Hg(II) increased up to 60% with a GNP-coated electrode compared to a bare GE [135]. The electrochemical and adsorptive coatings of GCE [98,124,133,139,158,159,167,175], SPCE [96], paper [59,76], and glass [28,92,131] sensors with GNP allowed for the immobilization of compounds via thiol bonding and higher conductivity to achieve low detection limits. The deposition of reduced graphene oxide was generally used to modify carbon-based sensors before the deposition of GNP to enhance the sensitivity due to the synergistic effect of these materials [40,135]. In addition to the coating of sensors, DNA strands were also bonded to GNPs to act as recognizing factors, such as that proposed for Hg(II) determination in natural waters [136]. In this work, labeled DNA strands with nanoparticles adhered to the biosensor surface after hybridization with the immobilized complementary strands, which improved the conductivity and, consequently, the electron transfer.
The use of SNP was also exploited for lowering detection limits by enhancing the conductivity of the sensor. The low stability of metallic silver nanoparticles is the main limitation to its use; however, it was explored as a support for biomolecules aiming to determine Hg(II) [138], Cu(II) [51], and Pb(II) [159,173].
The use of CNT in biosensors has been proposed to obtain high stability, an improved conductivity, and a faster electron transfer rate of redox reactions [38]. The interaction with carbon-based electrodes via π-π stacking allowed an efficient dip coating of the surface with suspensions of CNT [162]. On the other hand, the limited binding sites of CNTs required (electro)chemical modifications to enhance the performance of the biosensors. The affinity of modified CNTs with amino groups was used to immobilize biomolecules, such as antibodies [30], and mediators (e.g., polydiallyldimethylammonium) [38]. Modification with carboxyl groups (obtained by mixing CNT and HNO3) paved the way for cross-linking enzymes in the presence of glutaraldehyde [117] and the polymerization method [69]. In both cases, the stable bonding of biomolecules with the surface was achieved.
Other carbon-based materials were employed in biosensors for metal analysis, leading to procedures with low detection limits [73,160,175,176]. The determination of Pb(II) was carried out with a biosensor modified with entrapped carbon dots doped with nitrogen and phosphorus to enhance conductivity [160]. In this case, one of the lowest detection limits was achieved with a GCE for Pb(II) determination in natural waters, estimated at 0.79 ng L−1. In this sense, ordered mesoporous carbon was also assessed for enhancing the performance of aptasensors that showed detection limits as low as 41 ng L−1 for Pb(II) [176] and 0.12 pg L−1 for Hg(II) [73].
To a small extent, nanomaterials based on palladium [45], Fe3O4 [17], polypyrrol [47,70], ZnO [191], and CdSe quantum dots [124] were also used for the construction of biosensors for metal ions analysis aiming at the enhancement of conductivity, but the analytical features were not improved compared to biosensors conceived with other nanomaterials. Additionally, the electrochemical deposition of bulk polypyrrole yielded detection limits twice as low as that of the sensors prepared with nanomaterials of the same compound.

6. Multielement Analysis

The analysis of multiple metal ions with biosensors has been proposed in few articles [7,8,9,10,11,12,13,14,15,16,17,18,19,186]. Despite the adequate response to various cations, most procedures were not selective enough for simultaneous or sequential determinations. This is a difficult achievement because the responses derived from similar biochemical mechanisms, based on interactions with DNA strands [9,186], whole cells [12], proteins [7,17], and enzyme sites [8,10,11,13,14,15,16,18,19].
In general, the response intensities tend to be different for each cation due to variable reaction mechanisms, kinetics, and stability constants [11]. Therefore, an order of reactivity can be assessed to avoid interferences. For example, the determination of Cd(II), Co(II), Cu(II), and Ni(II) was carried out based on bonding with glucose oxidase sites that hindered the coordination of glucose with the enzyme [8]. Despite the same mechanism being observed for the assessed cations, the inhibition of enzymatic activity was more pronounced after the addition of Cd(II), followed by Co(II), Ni(II), and Cu(II). The biosensor showed versatility for the determination of the analytes, but mutual determinations without sample preparation were unlikely. For this reason, 30% of the biosensors proposed for multielement determination were not applied to the analysis of real samples without additional strategies for differentiation [8,10,12,14].
A standard addition method was one alternative to solve the problem and was used for Cu(II) and Pb(II) determination in tap waters [19], and Hg(II), Cd(II), Pb(II), and Cr(VI) in milk [11]. Therefore, the interference of other metal ions was considered as matrix effects, which was minimized by this type of calibration. However, this strategy can be ineffective for some concentration ranges of the interfering ions. Additionally, tiring and laborious steps were usually attributed to this strategy with solution handling that can lead to systematic errors.
For multielement quantification, masking agents were also used to enhance accuracies, such as for the sequential determination of Cu(II) and Zn(II) in human serum using a urease-based biosensor [7]. At first, Cu(II) was quantified after masking Zn(II) with pyrophosphate, thus only Cu(II) was bonded with enzyme sites. Afterward, Zn(II) was determined using another portion of the sample containing thiourea as a masking agent for Cu(II). Despite the two-step determination of cations, simple approaches for sequential multi-element quantification enabled the exploitation of a versatile biosensor. As the response was obtained after 5 min, the procedure was suitable for applications in clinical laboratories.
Speciation analysis of chromium in tap and wastewaters was achieved with a biosensors array [5]. Cr(III) and Cr(VI) determinations were based on the inhibition of tyrosinase and glucose oxidase, respectively. Each enzyme was immobilized on a GCE platform that allowed selective and simultaneous responses for each oxidation state of chromium. This ingenious alternative for speciation analysis has scarcely been exploited and could be further applied for the differentiation of chemical species simultaneously.

Author Contributions

D.L.R.: Conceptualization, methodology, formal analysis, investigation, resources, data curation, writing—original draft preparation, writing—review and editing, visualization, supervision, project administration, funding acquisition. V.M.: Investigation, data curation, writing—original draft preparation, visualization. C.M.P.G.A.: Conceptualization, formal analysis, investigation, resources, writing—review and editing, visualization, supervision. A.N.A.: Resources, writing—review and editing, visualization. M.d.C.B.S.M.M.: Conceptualization, formal analysis, investigation, resources, writing—review and editing, visualization, supervision, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by São Paulo Research Foundation (FAPESP), grant number [2018/131579-0], in part by the Coordenação de Aperfeiçoamento de Pes-soal de Nível Superior-Brasil (CAPES)-Finance Code 001, and through the project UIDB/50006/2020, funded by FCT/MCTES through national funds.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data will be furnished under request.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

ABTS2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)
ACValternate current voltammetry
AFMatomic force microscopy
APamperometry
APMamino polymerization method
APTS3-aminopropyltriethoxysilane
ASVanodic stripping voltammetry
BSAbovine serum albumine
CAchronoamperometry
CCchronocoulometry
CDconductometry
CHcobalt(II) hexacyanoferrate
CNTcarbon nanotubes
CPEcarbon paste electrode
CSVcathodic stripping voltammetry
CVcyclic voltammetry
DIdigital imaging
DLdetection limit
DNAdesoxyribonucleic acid
DPVdifferential pulse voltammetry
ECLelectrochemiluminescence
EDCN-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide
EDTAethylenediaminotetracetic acid
EGethyl green
EISelectrochemical impedance spectroscopy
FETfield-effect transistor
FLfluorescence
GCEglassy carbon electrode
GEgold electrode
GNPgold nanoparticles
HRPhorseradish peroxidase
IRSinfrared reflectance spectrometry
ITCBE1-(4-Isothiocyanobenzyl)ethylenediamine-N,N,N′,N′-tetraacetic acid
ITOindium tin oxide
LSVlinear sweep voltammetry
MAmonoclonal antibody
MASmolecular absorption spectrometry
MBmethylene blue
MFCmicrobial fuel cell
MGmethyl green
MOFmetal-organic frameworks
NADHreduced nicotinamide adenine dinucleotide
NHSN-Hydroxysuccinimide
NPOEnitrophenyloctylether
NRneutral red
PDDApolydiallyldimethylammonium
PECphotoelectrochemical detection
PGEpencil graphite electrode
PTpotentiometry
PTFEpolytetrafluorethylene
PVCpolyvinyl chloride
QMquartz microbalance
SERSsurface-enhanced Raman spectrometry
SPDPsuccinimydil-3-(2-pyridyldithiol) propionate
SPEscreen-printed electrode
SPRsurface plasmon resonance
SWVsquare wave voltammetry

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