Transition Metal Sensing with Nitrogenated Holey Graphene: A First-Principles Investigation

The toxicity of transition metals, including copper(II), manganese(II), iron(II), zinc(II), hexavalent chromium, and cobalt(II), at elevated concentrations presents a significant threat to living organisms. Thus, the development of efficient sensors capable of detecting these metals is of utmost importance. This study explores the utilization of two-dimensional nitrogenated holey graphene (C2N) nanosheet as a sensor for toxic transition metals. The C2N nanosheet’s periodic shape and standard pore size render it well suited for adsorbing transition metals. The interaction energies between transition metals and C2N nanosheets were calculated in both gas and solvent phases and were found to primarily result from physisorption, except for manganese and iron which exhibited chemisorption. To assess the interactions, we employed NCI, SAPT0, and QTAIM analyses, as well as FMO and NBO analysis, to examine the electronic properties of the TM@C2N system. Our results indicated that the adsorption of copper and chromium significantly reduced the HOMO–LUMO energy gap of C2N and significantly increased its electrical conductivity, confirming the high sensitivity of C2N towards copper and chromium. The sensitivity test further confirmed the superior sensitivity and selectivity of C2N towards copper. These findings offer valuable insight into the design and development of sensors for the detection of toxic transition metals.


Introduction
First row transition metals (Sc, Ti, Vn, Cr, Mn, Fe, Co, Ni, Cu and Zn) play undeniable role in various disciplines which include medicine, construction, catalysis, nuclear processes, engineering, and numerous medical applications [1][2][3]. One of the major application accounts for their catalytic properties in isomerization, hydrogenation, oxidation, polymerization and building small molecules, etc. [4,5] In a biological system, transition metals are associated with most of proteins to perform number of enzymatic processes and transportation to their target [6]. However, high concentration causes toxicity and is hazardous to life [7]. All elements of the series have some level of toxicity, but some are highly toxic while others are moderate [8,9]. The first row transition metals have oxidation state dependent toxicity [10]. Manganese has an interesting chemistry of multiple oxidation states. Being essential nutrition, manganese helps in the production of glucose and the feeding of mitochondria during its maintenance [11]. However, Mn in +2 oxidation state predominates in cellular toxicity [12]. Immoderate exposure of Mn 2+ causes "Manganism", which is neurodegenerative disorder which leads to neuronal death [13]. Some general effects of manganese toxicity are memory loss, insomnia, headache, and speech disturbances [14]. Similarly, iron is a good biocatalyst but it also has harmful effects [15]. A high

Geometric Optimization
The structure of the cluster model of the nitrogenated holey graphene (C2N) nanosheet is shown in Figure 1. A single unit of a C 2 N nanosheet comprises nitrogen atoms which are arranged in a periodical manner to set up a ring diameter of 8.30 Å. The pyrazine ring of C 2 N provided us with highly electronegative nitrogen which acts as an extremely powerful part, cohering and adsorbing toxic transition metals. The single monolayer of C 2 N reflects four possible sites for the adsorption of analytes: (a) in the middle of C 2 N surface (A), (b) in the middle of nitrogen atoms (B), (c) above the pyrazine ring (C), and (d) above the benzene ring (D). To find the most stable geometry with the lowest energy for each TM@C 2 N complex, all possible orientations of each metal ion over carbon nitride surface were explored. All the complexes (Cu 2+ @C 2 N, Mn 2+ @C 2 N, Co 2+ @C 2 N, Fe 2+ @C 2 N, Cr 6+ @C 2 N, Zn 2+ @C 2 N) exhibited the best performance in the central position, i.e., position (A), as it was the most stable and had the lowest energy ( Figure 2).
Molecules 2023, 28, x FOR PEER REVIEW 3 of 16 on human health and the environment. It is important to handle and use C2N nanosheets with caution and follow the appropriate safety guidelines to minimize the potential risks.

Geometric Optimization
The structure of the cluster model of the nitrogenated holey graphene (C2N) nanosheet is shown in Figure 1. A single unit of a C2N nanosheet comprises nitrogen atoms which are arranged in a periodical manner to set up a ring diameter of 8.30 Å. The pyrazine ring of C2N provided us with highly electronegative nitrogen which acts as an extremely powerful part, cohering and adsorbing toxic transition metals. The single monolayer of C2N reflects four possible sites for the adsorption of analytes: (a) in the middle of C2N surface (A), (b) in the middle of nitrogen atoms (B), (c) above the pyrazine ring (C), and (d) above the benzene ring (D). To find the most stable geometry with the lowest energy for each TM@C2N complex, all possible orientations of each metal ion over carbon nitride surface were explored. All the complexes (Cu 2+ @C2N, Mn 2+ @C2N, Co 2+ @C2N, Fe 2+ @C2N, Cr 6+ @C2N, Zn 2+ @C2N) exhibited the best performance in the central position, i.e., position (A), as it was the most stable and had the lowest energy ( Figure 2).  The most stable configuration of TM@C2N complexes (Cr 6+ @C2N, Co 2+ @C2N, Zn 2+ @C2N, Fe 2+ @C2N, Mn 2+ @C2N and Cu 2+ @C2N).

Most Stable Spin State of TM@C 2 N Complexes
In this study, the analytes selected for sensing studies were six metals of a firstrow transition series (Cu 2+ , Mn 2+ , Fe 2+ , Zn 2+ , Cr 6+ and Co 2+ ). First, it was essential to determine the most stable spin state of the TM@C 2 N complexes. The optimization of Cu@C 2 N, Mn@C 2 N and Co@C 2 N was carried out in doublet, quartet, sextet, and octet spin states. The relative energies of different spin states for all the complexes are given in Table 1. The most stable spin states obtained for Cu@C 2 N, Mn@C 2 N and Co@C 2 N were doublet, sextet, and quartet spin states, respectively. Similarly, the rest of metal complexes, such as Fe@C 2 N, Cr@C 2 N and Zn@C 2 N, showed quintet, triplet and singlet spin states to the most stable states, respectively.

Interaction Energies
The interaction energies and interaction distances of all the complexes are given in Table 2. In all the complexes, the transition metal interacts with N-atoms of the C 2 N cavity ( Figure 3). The value of interaction energy of the Cu 2+ @C 2 N complex is the lowest among all the studied complexes, i.e., −6.6 kcal mol −1 along with 2.2 Å of interaction distance between closest interacting (Cu-62 . . . .N-47) atoms. The E int and D int (Cr-61 . . . N-34) in the Cr 6+ @C 2 N complex are −9.2 kcal mol −1 and 2.09 Å, respectively. In the case of Zn 2+ @C 2 N, Fe 2+ @C 2 N and Co 2+ @C 2 N complexes, the E int is −15.9, −25.9 and −20.7 kcal mol −1 , respectively. The interaction energy of Mn 2+ @C 2 N (−43.1 kcal mol −1 ) is greater than the rest of the five complexes showing chemisorption; it interacts at site A of the C 2 N cavity with an interaction distance of 2.34 Å.

Natural Bond Orbital Analysis (NBO)
The analysis of natural bond orbital reveals the ability of the sensor to detect the toxic transition metals. The transfer of charge, as well as the direction of charge transfer, was determined using NBO analysis. The calculated charge transfer is listed in Table 3. The transfer of charge may occur from C2N to metal or metal to C2N. In this study, the charge values on adsorbed metals are 0.871 | | (Cu), 1.640 | | (Fe), 1.672 | | (Mn), 1.652 | | (Zn), 0.861 | | (Co) and 1.79 | | (Cr). The QNBO values of TM@C2N complexes show that charges shifted from analytes (transition metals) to the C2N nanosheet, as evidenced by the highly electron-rich cavity of C2N (due to the presence of electronegative nitrogen) and the positive charge of the metals. The following order of charge transfer was observed in the complexes: Cr 6+ @C2N ˃ Mn 2+ @C2N ˃ Zn 2+ @C2N ˃ Fe 2+ @C2N > Cu 2+ @C2N > Co 2+ @C2N. The interaction energy results of the TM@C 2 N complexes support the existence of a physisorption mechanism, except for Mn@C 2 N and Fe 2+ @C 2 N which show chemisorption. The interaction energy trends observed for TM@C 2 N complexes are Cu@C 2 N > Cr@C 2 N > Zn@C 2 N > Co@C 2 N > Fe@C 2 N > Mn@C 2 N, respectively. The results of the interaction energy indicate that C 2 N can accommodate transition metals on its surface, but the highest interaction energy was seen for the Mn@C 2 N complex (−43.1 kcal mol −1 ).

Natural Bond Orbital Analysis (NBO)
The analysis of natural bond orbital reveals the ability of the sensor to detect the toxic transition metals. The transfer of charge, as well as the direction of charge transfer, was determined using NBO analysis. The calculated charge transfer is listed in Table 3. The transfer of charge may occur from C 2 N to metal or metal to C 2 N. In this study, the charge values on adsorbed metals are 0.871 |e| (Cu), 1.640 |e| (Fe), 1.672 |e| (Mn), 1.652 |e| (Zn), 0.861 |e| (Co) and 1.79 |e| (Cr). The Q NBO values of TM@C 2 N complexes show that charges shifted from analytes (transition metals) to the C 2 N nanosheet, as evidenced by the highly electron-rich cavity of C 2 N (due to the presence of electronegative nitrogen) and the positive charge of the metals. The following order of charge transfer was observed in the complexes: Cr 6+ @C 2 N >Mn 2+ @C 2 N >Zn 2+ @C 2 N >Fe 2+ @C 2 N > Cu 2+ @C 2 N > Co 2+ @C 2 N.

Frontier Molecular Orbital Analysis (FMO)
The reactivity of interacting substances is significantly defined by frontier molecular orbital analysis (FMO). The energy gap obtained by taking the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) greatly influences the conductivity as well as the stability of complexes. The E HOMO and E LUMO of C 2 N are −7.87 eV and −2.17 eV, respectively. The H-L energy gap in C 2 N is 5.71 eV. In TM@C 2 N complexes, the H-L energy gap reduced to 2.24 eV (Cu), 4.71 eV (Fe), 4.80 eV (Mn), 4.44 eV (Zn), 2.70 (Cr) and 4.56 eV (Co) as graphically represented in Figure 4. In the Cu@C 2 N complex, HOMO and LUMO energies were −13.3 eV and −11.06 eV, respectively. A moderate reduction in H-L energy gap was observed for the complexes of Fe, Mn, and Co, i.e., 4.71, 4.80 and 4.56 eV, respectively. However, a remarkable decrease was observed in the E H-L energy gap of Cu 2+ @C 2 N (2.24 eV), which indicates the increased conductivity and sensitivity of C 2 N towards copper. Similarly, the LUMO (−21.35 eV) and HOMO (−24.05) of Cr 6+ @C 2 N were more highly stabilized compared to the bare C 2 N unit (−2.17 eV, −7.870 eV), which caused a notable decrease in the H-L energy gap (2.70 eV). The notable decrease in the E H-L gap evidences the greater sensitivity of C 2 N towards copper and chromium. The appreciable downfall in the HOMO-LUMO energy gap of any substances represents its appreciable sensitivity and selectivity towards toxic transition metals. Moreover, the orbital densities are also analyzed to visualize the interaction behavior of transition metals and the C 2 N surface ( Figure 5). All the complexes show totally different orbital densities for LUMO. Except for manganese, the densities of LUMO for the rest of complexes were distributed at different half portions of the C 2 N cavity. In case of Mn@C 2 N, the LUMO was entirely located on manganese metal at the center of cavity, whereas the density of HOMO was present as seen in the C 2 N unit. The highest decline in the H-L energy gap was observed in the Cu@C 2 N having the maximum conductivity. Therefore, Cu tends to enhance the conductivity of the carbon nitride surface as compared to the other selected metals.

Frontier Molecular Orbital Analysis (FMO)
The reactivity of interacting substances is significantly defined by frontier molecular orbital analysis (FMO). The energy gap obtained by taking the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) greatly influences the conductivity as well as the stability of complexes. The . The notable decrease in the EH-L gap evidences the greater sensitivity of C2N towards copper and chromium. The appreciable downfall in the HOMO-LUMO energy gap of any substances represents its appreciable sensitivity and selectivity towards toxic transition metals. Moreover, the orbital densities are also analyzed to visualize the interaction behavior of transition metals and the C2N surface ( Figure 5). All the complexes show totally different orbital densities for LUMO. Except for manganese, the densities of LUMO for the rest of complexes were distributed at different half portions of the C2N cavity. In case of Mn@C2N, the LUMO was entirely located on manganese metal at the center of cavity, whereas the density of HOMO was present as seen in the C2N unit. The highest decline in the H-L energy gap was observed in the Cu@C2N having the maximum conductivity. Therefore, Cu tends to enhance the conductivity of the carbon nitride surface as compared to the other selected metals.

Non-Covalent Interactions (NCI)
NCI analysis reveals the nature of interactions between the analyte and the surface through RDG graphs and 3D isosurfaces. The RDG graph is based on the following equation:

Non-Covalent Interactions (NCI)
NCI analysis reveals the nature of interactions between the analyte and the surface through RDG graphs and 3D isosurfaces. The RDG graph is based on the following equation: where ∇ρ and ρ are electronic density gradient and electronic density, respectively. The color scheme of NCI graphs comprises different types of interactions; the steric repulsion is reported in the red color, while weak and strong interactions are represented by the green and blue colors, respectively. In the 3D isosurfaces of all the TM@C 2 N complexes, the patches of interactions are mainly shown in the middle part, i.e., the center of the ring; the dotted patches reflect weak interactions, whereas the thicker patches show the strong interactions. In all TM@C 2 N complexes, the appearance of green surfaces of different intensities between the metals and C 2 N surface indicates the existence of strong and weak van der Waals interactions. In all the complexes, steric clashes are observed from the presence of thicker patches of red color; these repulsive forces are observed due to the presence of the delocalized electrons present in the metals as well as the nitrogen atom of the pyrazine rings. In RDG plots as shown in the Figure 6, the TM@C 2 N presents a variety of greenish peaks between −0.02 and 0.01 a.u. A dispersive light-bluish and the greenish spike (nearly −0.02 a.u.) implies the presence of strong non-bonding interactions.

RDG s 1 2 3
∇ where ∇ and are electronic density gradient and electronic density, respectively. The color scheme of NCI graphs comprises different types of interactions; the steric repulsion is reported in the red color, while weak and strong interactions are represented by the green and blue colors, respectively. In the 3D isosurfaces of all the TM@C2N complexes, the patches of interactions are mainly shown in the middle part, i.e., the center of the ring; the dotted patches reflect weak interactions, whereas the thicker patches show the strong interactions. In all TM@C2N complexes, the appearance of green surfaces of different intensities between the metals and C2N surface indicates the existence of strong and weak van der Waals interactions. In all the complexes, steric clashes are observed from the presence of thicker patches of red color; these repulsive forces are observed due to the presence of the delocalized electrons present in the metals as well as the nitrogen atom of the pyrazine rings. In RDG plots as shown in the Figure 6, the TM@C2N presents a variety of greenish peaks between −0.02 and 0.01 a.u. A dispersive light-bluish and the greenish spike (nearly −0.02 a.u.) implies the presence of strong non-bonding interactions. Figure 6. 2D RDG graphs of TM@C2N complexes (Cr 6+ @C2N, Co 2+ @C2N, Zn 2+ @C2N, Fe 2+ @C2N, Mn 2+ @C2N and Cu 2+ @C2N). Figure 6. 2D RDG graphs of TM@C 2 N complexes (Cr 6+ @C 2 N, Co 2+ @C 2 N, Zn 2+ @C 2 N, Fe 2+ @C 2 N, Mn 2+ @C 2 N and Cu 2+ @C 2 N).

QTAIM Analysis
In QTAIM analysis, the bond nature between analyte and complex depends on bondcritical point (BCP). BCP's are further classified into five components, i.e., electronic density (ρ), potential energy density V(r), energy density H(r), Laplacian of electron density (∇ 2 ρ) and kinetic energy density G(r). The bond-critical point (BCP) can be more distantly elucidated by the following equation: which shows that the sum of the potential energy and kinetic energy density is equal to the electron density. The value of H(r) > 0 and H(r) < 0 indicates the presence of closed-shell and shared-shell interactions, respectively. The bond-critical point (BCP) results of electronic density (ρ) and Laplacian (∇ 2 ρ) are given in Table 4 and the BCPs are depicted in Figure 7. The geometry of Cu@C 2 N complexes consists of four BCPs. The bond-critical point values of ρ ranges from 0.02 to 0.05 a.u. and ∇ 2 ρ from 0.07 to 0.19 a.u. Among four BCPs values, two interactions (N23 -Cu30 and N36 -Cu30 of C 2 N and Cu) contain the highest value of electronic density (ρ), 0.05 a.u. In Cu@C 2 N, the values of electronic density (ρ) are less than 0.1, which indicate the presence of weak van der Waals interactions, as confirmed by a 3D isosurface of an NCI plot. The highest number of BCPs obtained among the studied systems was six in the cases of Fe 2+ @C 2 N and Mn 2+ @C 2 N. The values of 0.02 a.u to 0.35 a.u for ρ, and −0.10 a.u to −0.97 a.u for ∇ 2 ρ, were obtained for Fe 2+ @C 2 N. In case of Fe 2+ @C 2 N, one of the values of electronic density (ρ) was less than zero, which indicates the existence of electrostatic interactions, as confirmed by SAPT0 analysis. Similarly, the electronic density (ρ) of Zn 2+ @C 2 N (0.2 to 0.4 a.u) is greater than 0.1, which confirms the electrostatic interactions between zinc and the C 2 N nanosheet. For Cr 6+ @C 2 N and Co 2+ @C 2 N, the values of electronic density confirm the presence of van der Waals interactions, i.e., ρ < 0.1 as confirmed by the greenish patches in the RDG graph. The outcomes of QTAIM analysis were in great accordance with the NCI and SAPT0 analyses.   7. The BCPs obtained using a QTAIM analysis of TM@C 2 N complexes.

SAPT0 Analysis
Symmetry-adapted perturbation theory (SAPT) provides a quantitative analysis of the noncovalent interaction between two entities through the perturbative approach by directly computing the interaction energy as a perturbation to the Hamiltonian of the individual monomers instead of the supermolecular approach. The division of interaction energy into different components such as dispersion, exchange, electrostatic and induction has been performed using SAPT (E int SAPT = E ele + E ind + E disp + E exch ). The interpretation of SAPT0 analysis is helpful for providing an explanation of the nature of interactions between the analytes (metals) and the C 2 N unit, and for quantifying the chemical bonds. The interactions of SAPT0 for six complexes are shown in Table 5. The components of SAPT0, which are negative, reveal the presence of attractive interactions between the C 2 N units and the transition metals. The exchange part contains the positive energies, which indicates the presence of repulsive interactions between the C 2 N unit and the analytes. As shown in Table 5, the negative energies of SAPT0 among all the complexes (TM@C 2 N) denote the presence of attractive interactions. The interaction energies obtained in SAPT0 studies of metal complexes were −255.09 kcal/mol (Cu 2+ @C 2 N), −131.88 kcal/mol (Fe 2+ @C 2 N), −283.35 kcal/mol (Mn 2+ @C 2 N), −306.20 kcal/mol (Zn 2+ @C 2 N), −174 kcal/mol (Co 2+ @C 2 N) and −5145.7 kcal/mol (Cr 6+ @C 2 N). The highest contribution towards the total SAPT0 was observed for E elec. Hence, electrostatic interactions dominate and stabilize the complexes. The findings of the SAPT0 analysis are consistent with the NCI and QTAIM analyses. The trend in supermoleculor interaction energy (without solvent) is exactly followed by the E SAPT0 . The trend for E SAPT0 is Cr@C 2 N > Zn@C 2 N > Mn@C 2 N > Cu@C 2 N > Co@C 2 N < Fe@C 2 N.

Electrical Conductivity and Sensitivity Analysis
Electrical conductivity (σ) is calculated for pristine C 2 N and TM@C 2 N complexes at 300 K and the σ values are given in Table 6. The conductivity of TM@C 2 N complexes show a marked increase as compared to pristine C 2 N. In particular, Cu 2+ @C 2 N and Cr 6+ @C 2 N conductivities increase largely when compared to other TM@C 2 N complexes. This large increase in conductivity can be converted to an electrical signal. Therefore, it can be concluded that the nitrogenated holey graphene C2N may be a promising electronic sensor for the detection of Cu and Cr. To further confirm the higher sensitivity of C 2 N towards toxic transition metals, the sensing characteristics of C 2 N were quantitatively analyzed using a sensitivity (S) test. Sensitivity is the response of a sensor towards analyte exposure. A high value of S means the material is an excellent sensor for the particular analyte. The sensitivity of the C 2 N nanosheet is found to be 142.63, 10.79, 0.09, 0.69, 107.30, and 51.90 towards Cu, Fe, Mn, Zn, Cr, and Co, respectively. Cu adsorbed on the C 2 N complex shows the highest S value, which indicates that among the mixture of analytes, C 2 N is the most sensitive and selective towards Cu.

Computational Methodology
The level of theory employed for geometry optimization is M05-2X/6-31+G (d, p) [37,38]. In our investigation of C 2 N-transition metal complexes, non-covalent interactions are involved, and therefore, we have employed the hybrid meta-exchange-correlation functional M05-2X. The choice of M05-2X functional is based on its accuracy in describing non-covalent interactions, as demonstrated by benchmark studies such as the one by Burns et al. [39] and other studies reported in the literature on the non-covalent interactions [40][41][42][43]. Additionally, the choice of an appropriate basis set is a crucial factor in computational simulations. To this end, we have utilized the double zeta basis set 6-31+G(d,p), which includes diffuse and polarized functions and strikes a balance between accuracy and computational efficiency [44]. The interaction energies between C 2 N and transition metals were calculated by using the following equation: where E TM@C 2 N , E C 2 N and E TM are the interaction energies of C 2 N-transition metal complexes, pristine C 2 N surface, and isolated transition metal, respectively. To find the lowest energy structure, each metal is placed in different possible orientations and the results are visualized using Gaussview 5.0 [45]. To validate the true minima's nature of the TM@C 2 N complexes, vibrational frequencies were examined for the presence of imaginary frequencies. An NCI analysis was carried out using VMD and Multiwfn software for an improved evaluation of the interactive sites between C 2 N and the selected transition metals [46,47]. The electronic properties, such as the natural bond orbital (NBO) and frontier molecular orbital (FMO) analyses, were performed at the same level of theory used for optimization. The non-covalent interactions between C 2 N and transition metals were quantified by bond-critical point using QTAIM analysis [48]. The interaction energies between transition metals and the C 2 N nanosheet were also analyzed using SAPT0 (symmetry-adaptation perturbation theory). SAPT0 analysis illustrates four types of interactions: electrostatic (∆E elst ), exchange (∆E exh ), induction (∆E ind ) and dispersion (∆E dis ) [49]. The equation for ∆E int through SAPT0 is as follows: ∆E int = ∆E elst + ∆E exh + ∆E ind + ∆E dis All the SAPT calculations were performed using PSI4 1.6 software [50].

Conclusions
This study aimed to systematically investigate the ability of a two-dimensional nitrogenated holey graphene (C 2 N) nanosheet to detect toxic transition metals (Cu 2+ , Mn 2+ , Fe 2+ , Zn 2+ , Cr 6+ , and Co 2+ ) using DFT calculations. The interaction energies between C 2 N and each of the transition metals were calculated, with values of −6.6, −43.1, −20.7, −25.9, −9.2, and −15.9 kcal mol −1 for Cu 2+ @C 2 N, Mn 2+ @C 2 N, Co 2+ @C 2 N, Fe 2+ @C 2 N, Cr 6+ @C 2 N, and Zn 2+ @C 2 N, respectively. These results suggest that the adsorption of the transition metals on C 2 N is mainly due to physisorption, except for Mn and Fe, which are chemisorbed. The dominant interaction between C 2 N and the metals was found to be the electrostatic force of attraction, which stabilizes the TM@C 2 N complexes. The computed E H-L gap of the C 2 N nanosheet was found to decrease significantly upon the adsorption of Cu 2+ and Cr 6+ . The NCI plots and QTAIM analysis showed the presence of strong and weak van der Waals interactions between C 2 N and the metals. The increase in the electrical conductivity of Cu 2+ @C 2 N and Cr 6+ @C 2 N, as compared to the pristine C 2 N, indicates the superior sensitivity of C 2 N towards these metals. The sensitivity (S) test also confirms the higher sensitivity and selectivity of C 2 N towards Cu 2+ . In conclusion, the results of this study suggest that the two-dimensional nitrogenated holey graphene (C 2 N) nanosheet could be an effective sensor for detecting copper (II) and hexavalent chromium. The findings may also provide valuable insights into the design and development of sensors for detecting other toxic metals.