Golfer
Muedas-Taipe
*a,
Michael
Badawi
b,
Angélica María
Baena-Moncada
a and
Miguel
Ponce-Vargas
*bc
aLaboratorio de Investigación de Electroquímica Aplicada, Facultad de Ciencias de la Universidad Nacional de Ingeniería, Av. Túpac Amaru 210, Rímac, Lima, Peru. E-mail: gmuedast@uni.edu.pe
bLaboratoire Lorrain de Chimie Moléculaire (L2CM) UMR CNRS 7053, Université de Lorraine. Faculté des Sciences et Techniques, 54500 Vandœuvre-lès-Nancy, France. E-mail: miguel-armando.ponce-vargas@univ-lorraine.fr
cUniversité de Reims Champagne-Ardenne, Moulin de la Housse 51687, Reims Cedex 02 BP39, France
First published on 15th April 2025
Although cyanide is essential in mining operations, its high toxicity to both human health and the environment makes it an extraction agent that requires continuous in situ monitoring. This can be achieved through electrochemical sensors, which enable optimal detection of cyanide and related species without the need for time-consuming sample preparation steps. Graphene-based electrochemical sensors can be enhanced through non-covalent functionalization, involving the adsorption of a modifier onto the substrate surface via π–π interactions. In this study, we explored the effect of incorporating quinone derivatives onto a graphene substrate using a density functional theory (DFT) approach, coupled with a methodology based on the variation of the electronic density gradient (igmh). This approach aims to identify novel materials for the electrochemical detection of the tetracyanocadmate ion, [Cd(CN)4]2−, from WAD-CN (weak acid dissociable cyanide). First, we quantify the noncovalent contacts between the quinone and the graphene support through a fragment-based calculation. Subsequently, we focus on the coordination bond strength involving Cd2+ and the quinones attached to graphene. Then, we evaluate the effect of incorporating electron-donating substituents, which would directly lead to stronger coordination bonds with the metal center. The results reveal that an optimal balance between the modifier's anchoring on the substrate and its coordination strength toward Cd2+ can be achieved by functionalizing the graphene surface with 3-hydroxy-o-benzoquinones substituted at the 4-position with electron-donating groups. This suggests that experimental efforts conducted in this direction could lead to the development of electrochemical sensors with lower detection limits.
Among the materials used for the manufacture of sensing devices, graphene has been widely employed due to its high electrical conductivity, great stability and extensive surface area.9–15 The performance of graphene sensors can be enhanced through covalent and non-covalent modifications.16 Covalent modification entails the formation of chemical bonds providing a strong link between graphene and organic molecules. However, most covalent functionalization methods result in a reduction in electrical conductivity. Additionally, some of these methods require complex synthesis protocols, which also results in increased costs.17 In contrast, non-covalent modification relies on the adsorption of the modifier onto the graphene surface via π–π interactions, which preserves the graphene π-conjugated system, and thus its electrical conductivity.18–21 Among the vastness of reagents proposed for noncovalent functionalization of graphene, quinones stands out due to their high redox reactivity, excellent electrochemical reversibility and the facile tuning of electrochemical properties achieved by modifying their molecular scaffolds and functional groups.22–26
To understand the effects of the non-covalent quinone attachment to graphene in sensing applications, it is mandatory to obtain an in-depth comprehension of the nature and strength of the interactions involved. Theoretical interpretations based on high-level ab initio calculations have been instrumental in this respect, enabling a better understanding of the interplay between attractive (electrostatic, dispersive, and inductive interactions) and repulsive (exchange repulsion) forces governing the formation of graphene-quinone assemblies at the electrode/solution interphase.27 The proposed models are often supported by computational methods based on quantum mechanics, which explicitly account for the electronic structure of the molecular systems. Recent work conducted by our group concerning the development of electrochemical sensors based on carbonaceous materials decorated with quinone derivatives for the detection of cyanide complexes from WAD-CN (weak acid dissociable cyanide), includes the rationalization of the adsorption process occurring at the electrode/solution interphase by calculations based on density functional theory (DFT).28–30
Herein, the effect of incorporating a panel of quinone derivatives to graphene is explored through a DFT approach complemented by a methodology based on the electronic density gradient variation,31 looking for the rational design of voltammetric sensors oriented to the detection of the tetracyanocadmate ion, [Cd(CN)4]2−, from WAD-CN. It should be noted that [Cd(CN)4]2− is the complex most commonly found in wastewater from mineral extraction activities, and these wastewaters can have pH values as high as 13. However, the optimal performance of some of the sensing platforms developed by our team is achieved at pH 5. Therefore, our current focus is on enhancing the sensing capacity of quinone-functionalized sensors in basic media. To this end, in the first stage, we evaluate the adsorption energies of the quinone-graphene assembly to assess the anchoring of the quinone over the support. We propose a model consisting of a Cd(II) center coordinated to two cyanide ions and two quinone ligands interacting with the graphene sheet, all immersed in an aqueous solvent medium, aiming to replicate the electrode/electrolyte interface under operando conditions. The noncovalent contacts between the complex and the graphene support were quantified, as well as the strength of the coordinative bonds involving the metal center and the surrounding quinone ligands. Next, the effect of incorporating electron-donating substituents into a benzoquinone scaffold, which would directly enhance the coordination bonds with the metal center, was evaluated. Finally, variations in the electronic structure of the functionalized graphene resulting from Cd2+ incorporation were evaluated through a total density of states (TDOS) analysis. This study is part of an ongoing research program focused on developing enhanced electrochemical sensing platforms for detecting cyanide complexes in water. It is anticipated that the computational protocol proposed herein could be extended to related systems aimed at the development of novel electrochemical sensors for detecting water pollutants.
The interaction energy of the ligand–graphene and complex–graphene assemblies has been calculated according to the following scheme:
Eadsorption = Eligand(complex)–graphene − Eligand(complex) − Egraphene |
Additionally, the total density of states (TDOS) according to the Hirshfeld partition has been calculated with the Multiwfn code,41 to evaluate variations in the electronic structure of the functionalized graphene as a result of the Cd2+ capture.
The structures of the quinones were fully optimized considering solvent effects, then placed over graphene and relaxed. The 2D representation of all the considered structures is depicted in Scheme 1. To evaluate the extent of the ligand attachment to the support, the interaction between the moieties is quantified through a fragment calculation, and by an igmh analysis. The igmh approach provides a binding score associated with the attenuation of the electronic density gradient when two molecular fragments approach each other.28 It also provides a graphical representation of the noncovalent surface associated with the graphene–ligand interplay. The optimized geometries of the graphene–ligand systems (1a–7a) are depicted in Fig. 1.
Some relevant geometrical parameters, along with the associated interaction energies and igmh scores, are presented in Table 1. The graphene–ligand distance has been measured from the center of the aromatic ring of the ligand to the closest carbon of the graphene sheet. The graphene–complex distance, in turn, has been calculated as the average of the distances between the aromatic rings of the ligand coordinating the Cd(II) center and the closest carbon of the graphene sheet. The igmh isosurfaces representing the region where noncovalent interactions arise, and the corresponding binding scores for 1a–7a are depicted in Fig. 2.
Graphene–ligand assembly | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12a | 13a | 14a |
Graphene–ligand distance (Å) | 3.22 | 3.24 | 3.25 | 3.20 | 3.19 | 3.19 | 3.19 | 3.13 | 3.17 | 3.21 | 3.16 | 3.17 | 3.19 | 3.17 |
Interaction energy (kcal mol−1) | −25.7 | −25.3 | −22.7 | −24.0 | −18.4 | −11.8 | −13.4 | −15.7 | −15.2 | −15.3 | −13.8 | −15.9 | −16.3 | −17.4 |
igmh binding score (a.u.) | 0.94 | 0.91 | 0.81 | 0.89 | 0.70 | 0.20 | 0.16 | 0.60 | 0.57 | 0.58 | 0.52 | 0.59 | 0.61 | 0.62 |
Graphene–complex assembly | 1b | 2b | 3b | 4b | 5b | 6b | 7b | 8b | 9b | 10b | 11b | 12b | 13b | 14b |
Graphene–complex average distance (Å) | 3.26 | 3.29 | 3.23 | 3.29 | 3.25 | 3.17 | 3.12 | 3.21 | 3.22 | 3.23 | 3.22 | 3.20 | 3.21 | 3.18 |
Interaction energy (kcal mol−1) | −50.6 | −47.1 | −43.4 | −41.3 | −31.4 | −21.8 | −30.2 | −25.6 | −27.4 | −27.2 | −25.2 | −28.3 | −28.4 | −30.9 |
Dihedral angle (°) | 115 | 109 | 107 | 104 | 123 | 123 | 130 | 133 | 124 | 128 | 127 | 130 | 129 | 127 |
igmh binding score (a.u.) | 1.79 | 1.73 | 1.57 | 1.54 | 1.31 | 0.96 | 1.02 | 1.15 | 1.15 | 1.14 | 1.02 | 1.36 | 1.13 | 1.14 |
igmh bonding score (a.u.) | 0.24 | 0.25 | 0.16 | 0.24 | 0.21 | 0.20 | 0.16 | 0.15 | 0.17 | 0.17 | 0.18 | 0.19 | 0.19 | 0.20 |
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Fig. 2
igmh analysis of the graphene–ligand systems involving quinones 1–7 (1a–7a), with an isosurface of 0.01 a.u., and a BGR color code in the range −0.04 a.u. < ρ![]() |
The first graphene–ligand assembly herein evaluated, involves the 1,8-dihydroxyanthraquinone (1a, Fig. 1). It exhibits (quasi-)complete planarity of the quinone over the graphene surface, with only the hydroxyl hydrogen atoms lying slightly outside of the anthraquinone plane. An equilibrium distance of 3.22 Å is found between the moieties, which remains almost unaltered along the series. Remarkably, similar distances are obtained by Young-Kyu et al., for a panel of quinone molecules interacting with pristine graphene, through periodic DFT calculations.43 In turn, the interaction energy obtained from the fragment calculations is −25.7 kcal mol−1. In the 1,4-dihydroxyanthraquinone system (2a), two coordinative sites are placed on opposite sides of the anthraquinone scaffold, which in principle could lead to a broader distribution of captured Cd2+ ions, as the steric effects associated with the close binding sites in 1a are ruled out. A graphene–ligand distance of 3.23 Å and an interaction energy of −25.3 kcal mol−1 are found, very similar to results obtained in 1a.
While the interfragment distance (3.23 Å) remains unchanged, a slightly lower interaction energy (−22.7 kcal mol−1) is found in the graphene–ligand assembly involving 9,10-phenanthroquinone (3a). This can be attributed to the absence of –H⋯π contacts due to the lack of hydroxyl groups. To clarify this point, an –OH was introduced at the 1 position of 3a, leading to the 1-hydroxy-9,10-phenanthroquinone structure (4a). As expected, an increase in the magnitude of the graphene–ligand interaction energy occurs (−24.0 kcal mol−1) relative to the parent 3a.
Moving to ligands featuring a naphthoquinone scaffold, the assembly involving 2-hydroxy-1,4-naphthoquinone (5a) yields a fragment energy of −18.4 kcal mol−1, consistent with a smaller extent of π-stacking. This trend becomes more pronounced when moving to the benzoquinone derivatives: 2-hydroxy-p-benzoquinone (6a) and 4-hydroxy-o-benzoquinone (7a). The interaction energies for these systems are lower than those of the previously tested assemblies, specifically −11.8 kcal mol−1 for 6a and −13.4 kcal mol−1 for 7a, as a consequence of their smaller scaffold. The igmh analysis is able to capture the extent of the π-stacking along the series, with larger igmh scores for the anthraquinone-based systems (0.94 for 1a, and 0.91 for 2a), followed by the phenanthroquinones (0.81 for 3a, and 0.89 for 4a), naphthoquinone (0.70 for 5a), and benzoquinones (0.20 for 6a, and 0.16 for 7a) graphene–ligand assemblies.
The results obtained in this first part suggest that the recovery of the graphene support by the quinone modifiers is plausible in terms of the interaction energy involved, which is approximately −10 kcal mol−1 per molecule ring. Furthermore, the graphene–ligand interfragment distance remains almost constant throughout the series (∼3.2 Å), irrespective of the number of cycles featured by the modifier. It is also noteworthy that the igmh approach is able to quantify the π–π interactions along the considered series.
The noncovalent interaction energy between the complex and graphene is quantified using a fragment calculation (considering graphene-quinone, and [Cd(CN)2] as interacting moieties) and the igmh binding score. The metal–ligand coordination strength, in turn, is quantified by the sum of the four igmh scores associated with the bonds involving the quinone oxygen atoms and the metal center, which is hereafter referred to as the igmh coordinative bonding score.
The optimized geometries for the graphene–complex assemblies are shown in Fig. 3, and the igmh isosurfaces, along with the binding scores, are depicted in Fig. 4.
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Fig. 4
igmh analysis of the graphene–complex systems involving quinones 1–7 (1b–7b), with an isosurface of 0.01 a.u., and a BGR color code in the range −0.04 < ρ![]() |
In all optimized structures a distorted octahedral sphere for the metal center is observed, caused by the π–π interactions between the ligand aromatic rings and graphene. In the graphene–complex assembly featuring two 1,8-dihydroxyanthraquinone ligands (1b), an interaction energy of −50.6 kcal mol−1 is found, which is twice the value obtained with the single ligand over graphene (−25.7 kcal mol−1 in 1a). This suggests that the central cation does not significantly contribute to stabilizing the graphene–complex array. Remarkably, the graphene–complex distances (∼3.2 Å) do not vary with respect to the graphene–ligand series, confirming that π–π interactions rule the formation of the assemblies. Lower interaction energies are obtained for 2b (−47.1 kcal mol−1), 3b (−43.4 kcal mol−1), and 4b (−41.3 kcal mol−1), suggesting less stable structures compared to 1b, despite the same number of rings in the ligands.
Moving to 5b, generated with two 2-hydroxy-1,4-naphthoquinones, the binding energy (−31.4 kcal mol−1) is significantly lower compared to the previous systems, consistent with the presence of two rings in the involved ligands, which result in a less pronounced π-stacking. Following this trend, 6b and 7b, involving benzoquinone ligands, exhibit the lowest interaction energies, i.e., −21.8 kcal mol−1 and −30.2 kcal mol−1, respectively. The difference between their binding energies can be attributed to the absence of –H⋯π interactions in 6b, as the deprotonated hydroxyl groups of the ligands are already involved in Cd2+ coordination.
The igmh approach is applied to evaluate the extent of π-stacking along the graphene–complex series. This approach proves capable of capturing the two-fold increase in the interacting ligand surface, as well as the decrease in π-stacking, ranging from 1.79 for 1b, to 0.96/1.02 for 6b/7b.
The detection of [Cd(CN)4]2− implies the oxidation of the complex at high potentials, resulting in the production of cyanate and the release of Cd2+ ions.28–30 According to the proposed model, the Cd2+ ions are chelated by the adsorbed quinones, forming four coordinative bonds. The igmh score bonding associated with the sum of these bonds (see Table 1) reveals a stronger coordination in the anthraquinone complexes (0.25 for 2b and 0.24 for 1b), and 1-hydroxy-9,10-phenanthroquinone (4b, 0.24), followed by those generated with 2-hydroxy-1,4-naphthoquinone (5b, 0.21) and 2-hydroxy-p-benzoquinone (6b, 0.20). The lower values correspond to 9,10-phenanthroquine (3b, 0.16) and 4-hydroxy-o-benzoquinone (7b, 0.16) complexes.
An appealing result from the igmh bonding analysis concerns the deprotonated hydroxyl group in 1-hydroxy-9,10-phenanthroquinone (4b), which is involved in the chelation of the metal center along with the keto group. This group, in tandem with a carbonyl, leads to a stronger coordination (bonding score of 0.24) relative to 9,10-phenanthroquinone (3b, 0.16), where two carbonyl groups coordinate the metal center. This can be attributed to the more localized electronic density of –O− compared to the CO oxygen. Similarly, the presence of a coordinated deprotonated hydroxyl group in 2-hydroxy-p-benzoquinone (6b, 0.20) enhances the bonding score in comparison to 4-hydroxy-o-benzoquinone (7b, 0.16).
Charge capacity/μC | LOD/mg L−1 | LOQ/mg L−1 | ||
---|---|---|---|---|
Glassy carbon (GC) | Functionalized electrode | |||
1 | 5.31 | 9.28 | 0.70 | 2.13 |
2 | 1.37 | 2.59 | 0.41 | 1.24 |
3 | 14.1 | 20.7 | 0.30 | 0.89 |
5 | 23.1 | 35.3 | 0.16 | 0.47 |
In light of these considerations, a series of 3-hydroxy-o-benzoquinone derivatives (8–14) is proposed, with an average graphene–ligand interaction energy of approximately −16 kcal mol−1 (see graphene–ligand structures and the corresponding igmh analysis in Fig. S3 and S4 of ESI†). In the graphene–complex panel of benzoquinones 8b–14b, we take advantage of the stronger coordination toward Cd2+ provided by a deprotonated –OH group compared to a keto group, as previously demonstrated in the 1-hydroxy-9,10-phenanthroquinone (4b) and 2-hydroxy-p-benzoquinone (6b) graphene–complex assemblies. In addition, the 3-hydroxy-o-benzoquinone moiety permits the incorporation of electron donor substituents at the 4-position, able to electron density donation by π-conjugation toward the adjacent deprotonated hydroxyl, thereby enhancing the coordinative strength of the ligands. The optimized structures for the graphene–complex systems involving the benzoquinone derivatives 8–14 (8b–14b) are presented in Fig. 5, along with the corresponding igmh analysis. The substituents attached to the 4-position of 3-hydroxy-o-benzoquinone (11) range from the electron-releasing –CN (8), –Cl (9) and –Br (10) to the electron-donating –CH3 (12), –O–CH3 (13), and –NH–CH3 (14). The bonding score associated with the metal–ligand coordination varies according to the electron releasing/donating behavior, as illustrated in the following order: –CN (8b, 0.15) < –Cl (9b, 0.17) ∼ –Br (10b, 0.17) < –H (11b, 0.18) < –CH3 (12b, 0.19) ∼ –O–CH3 (13b, 0.19) < –NH–CH3 (14b, 0.20). This demonstrates the impact caused in the Cd2+ coordination by the appending groups, that is well-captured by the igmh bonding score. This is verified by comparing the igmh bonding scores and the interaction energies with the Hammett parameters of the considered substituents (see Table S1 and Fig. S5 in ESI†). It is observed that as the Hammett values become more negative (indicating a stronger electron-donating behavior of the substituent), the igmh bonding score (which reflects the coordinative strength of the quinone ligands) increases. Additionally, the interaction energy becomes more negative (indicating stronger binding) as the electron-donating behavior of the substituents intensifies. For those assemblies where a good balance is found between the metal–ligand interaction and binding energy (12a–14a), a total density of states (TDOS) analysis was performed to evaluate the effect of cadmium complex capture on their electronic properties. We compared the TDOS of the graphene–ligand systems (12a–14a) with those resulting from the incorporation of the cadmium complex (graphene–complex systems 12b–14b). This graphical comparison is presented in Fig. 6, where it can be observed in all cases the Fermi level at −4.3 eV, and a significant variation in the TDOS profile upon cadmium complex capture, particularly at −17 eV, −13 eV, −10 eV (HOMO), and 2.7 eV (LUMO).
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Fig. 6 Total density of states analysis (TDOS) of graphene–ligand (12a–14a) and graphene–complex (12b–14b) assemblies. |
Although the most notable variation occurs in the HOMO region, which can be attributed to the coordination of the metal center by the oxygen atoms, the variation of the LUMO at 2.7 eV can, in turn, be attributed to back-donation from the Cd(II) center to the p orbitals of the carbon atoms bonded to the ketone groups of the quinone ligand upon complex formation. These results demonstrate the sensitivity of the proposed functionalized materials towards WAD-cyanide, making them promising candidates for sensing applications. Moreover, when the TDOS of the graphene support without modifiers is compared to that of quinone-functionalized graphene (see Fig. S6 in ESI†), the profiles are very similar, ensuring that the conductivity of the support is maintained after modification through quinone incorporation.
In summary, the results obtained through the fragment calculations and the igmh approach suggest that a good compromise can be achieved between the number of ligands adsorbed on the graphene support and the coordination strength (which is associated with more effective Cd2+ capture).
This balance can be attained by functionalizing the graphene surface with 4-substituted 3-hydroxy-o-benzoquinones bearing electron-donor groups (12–14), taking advantage of their small size among quinone derivatives and the strong coordination toward the cadmium center, which is enhanced by incorporating electron-donor groups at the 4-position of the benzoquinone ring.
Additionally, the TDOS results reveal that the capture of the cadmium complex generates a variation in the electronic profile, which demonstrates the sensing capability of the proposed functionalized materials.
Footnote |
† Electronic supplementary information (ESI) available: Experimental chronocoulometry, LOD and LOQ results for electrodes based on 1, 2, 3 and 5, optimized structures and igmh analysis of graphene–ligand systems involving quinones 8–14, and total density of states analysis (TDOS) of graphene–ligand and graphene–complex assemblies involving quinones 12–14b. See DOI: https://doi.org/10.1039/d5cp00218d |
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