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634a5084de2a2174b2a60019 | 5 | The free energy for the formation of the sandwich complex (∆G -see Fig. above), involving the aromatic additive, was determined for a variety of different cases.The results, compiled in Table , indicate that for many cases, such a stabilization would be substantially favourable.It may be argued that the additive could directly attack the catalyst and take part in protonation chemistry, without the reactants being involved, but this is unlikely. Previous computational investigations by Wang and co-workers have indicated that the direct attack of the naphthol additive on the catalyst (quinine-squaramide, in their case) was not favourable. Nevertheless, |
634a5084de2a2174b2a60019 | 6 | we have investigated the possibility of proton transfer from the naphthol additive to the quinuclidine nitrogenfor theBzCPD catalyst case. The deprotonation of naphthol with the concomitant protonation of the quinuclidine nitrogen was seen to be not possible, as the quinuclidine nitrogen gave back the proton to the oxygen of the naphthol (see Fig. in SI).It is likely that other additives such as R-BINOL, which have a similar structure, will also not be able to directly react with the catalyst or substrate. Therefore, such minor side reactions have been ignored in the current work. Moreover, the dual complexation indicated here (hydrogen bonding and π stacking) would be significantly preferred to the additive solely interacting with one of the substrates through hydrogen bonding (as has been suggested in all the experimental reports considered in the current study). It is, however, noteworthy, that for two cases (entries 6 and 9 in Table ), the complexation was seen to be unfavourable, i.e., such a sandwiched complex would not be formed. This result indicates that while such a possibility is present in the majority of the additives that have been experimentally employed, there do exist cases where the aromatic additive would not function in this fashion. In these cases, any observed improvement in yield or enantioselectivitywould be due to the other reasons mentioned in the literature: improvement in the pKa of the reaction, or because of the aromatic additive hydrogen bonding with one of the substrates. In order to investigate the mechanism of Chen's thiocyanation reaction with 2-naphthol as additive, we considered the system comprising of the catalyst, BzCPD, the electrophile, N- compared to the Wynberg ion-pair hydrogen bonding pathway. Therefore, such a transfer thiocyanation pathway has not been studied in the current work. It is also to be noted that, due to the available conformational space between the C6'-OH and the quinuclidine NH of the catalyst, the nucleophile can approach in both the syn or anti fashions to give different syn-S, syn-R and anti-S, anti-R products (see Fig. in SI). We considered all the approaches to find the best approach. It was seen the syn approach was favoured over the anti by 6.3 kcal/mol(see Fig. in SI). Therefore the complete pathway for the thiocyanation was then studied for the syn approach pathway.The energy profile for the Wynberg ion-pair hydrogen bonding model, which is the most favorable pathway for the thiocyanation reaction with the 2-naphthol additive, is presented in Fig. . In the dual activation mechanism shown in Fig. above, the formation of the hydrogen-bonded complex Intrm1 between the oxindole and catalyst OH is energetically favoured by 5.7 kcal/mol. The proton transfer from the oxindole carbon to the quinuclidine nitrogen takes place after surmounting a barrier of 5.1 kcal/mol. This leads to the ion-pair complex Intrm2, which is 7.1 kcal/mol more stable than the totally separated reactant species. |
634a5084de2a2174b2a60019 | 7 | We note that the 2-naphthol and the thiocyanating reagent have not been considered from the starting point of the calculation, i.e., from Intrm1, as the enantioselective step in the complete cycle is the formation of TSwyn2A. This reduces the computational cost without hampering the reliability. After the formation of Intrm2, the 2-naphthol(Ad) additive and N- in the SI). The energy differences between the enantio-controlling TSs are somewhat overestimated by the DFT calculations. This is not an uncommon situation in quantum mechanical investigations of catalysis, as has been pointed out earlier, for instance, by Houk and coworkers. 41 |
634a5084de2a2174b2a60019 | 8 | After establishing the preferred dual activation mode (i.e., the new Wynberg ion-pair-hydrogen bonding model with 2-naphthol as additive), we subsequently explored the origin of stereoinduction in this reaction. In order to shed light on the source of energy differences between TSwyn2A(S) and TSwyn2A'(R), we performed a form of energy decomposition analysis (EDA) for the two transition state (TS) structures (see Fig. below). We note here that this same approach for EDA analysis has recently been adopted for the electrophilic trifuloromethylthiolation of β-keto ester with N Trifluoromethylthiophthalimide by Xue and coworkers. The activation energy ΔE # of the TSs can be written as ΔE # = ΔEdef + ΔEint, where the terms ΔEdef and ΔEint are the deformation and interaction energies, respectively. The deformation energy ΔEdef is the energy difference that arises from structural changes toward the TS formation. The interaction energy ΔEint corresponds to the energy difference between the sum of the cationic catalyst, the anionic oxinidole substrate, the neutral N-thiocyanatophthalimide substrate and the 2-naphthol additive, minus the complex at the TS structure. The results are presented in Table . Both ΔΔEint and ΔΔEdef are positive, meaning that the two components contribute synergistically to the overall energy difference between TSwyn2A(S) and TSwyn2A'(R). |
634a5084de2a2174b2a60019 | 9 | In other words, the substrates in TSwyn2A(S) are not only more stable but also interact more favorably with the catalyst, than do those in TSwyn2A'(R). The results of the EDA analysis are shown in Table , and indicate clearly that the large energy separation between TSwyn2A(S) and TSwyn2A'(R) mainly arises from differences in the interaction energy (ΔΔEint = 5.0 kcal/mol). |
634a5084de2a2174b2a60019 | 10 | In order to reveal the difference in interaction energies between the substrates and the catalyst (ΔΔEint), the non-covalent interaction (NCI) analysis of the TS structures was conducted, which enabled the visualization of the non-covalent interactions. The NCI analyses of TSwyn2A(S) and TSwyn2A'(R) reveal that, in addition to the strong conventional N + -H and C-H•••π 48 interactions are present. Most interestingly, there is a favorable π•••π stacking interaction between the additive and the electrophile, which also contributes to the stabilization of both TSwyn2A(S) and TSwyn2A'(R) (see Fig. and S11 in SI). As the substrates and additive interact with the catalyst through a network of hydrogen bonding interactions, as well as through a π•••π stacking interaction, it is still not easy to figure out which factors are the major ones contributing to the ΔΔEint. In order to understand the factors involved, it is therefore also necessary to quantify the weak interactions in the transition state structures. |
634a5084de2a2174b2a60019 | 11 | For a better understanding of the contributions of individual non-covalent interactions to the ∆∆Eint, we have applied Espinosa's equation ("equation( )") to quantify the strength of hydrogen bonds in the hydrogen-bond network. The equation implies the correlation of the hydrogen bond energy (EHB) to the pressure exerted on the electrons around the critical point (V(rcp)), both related to the strength of the hydrogen bonding (HB) interaction. |
634a5084de2a2174b2a60019 | 12 | It is seen that the strong conventional O-H in SI). The π•••π stacking energy for the TSwyn2A(S), which is the major transition state, is -11.6 kcal/mol, whereas the π•••π stacking energy for the minor transition state TSwyn2A'(R) is -9.9 kcal/mol (see Fig. in SI). Therefore, the π•••π interaction is seen to be 1.7 kcal/mol stronger in TSwyn2A(S) than in TSwyn2A'(R) (see Fig. in SI). Adding all the non-covalent interactions for the major TS, i.e. TSwyn2A(S), yields the value -70.0 kcal/mol, while the corresponding value for the minor TS, i.eTSwyn2A'(R), is -63.6 kcal/mol, i.e., unfavourable by 6.4 kcal/mol. This gives us a clear idea as to why one enantioselective product is favoured over the other.For details of the calculations ofthe NCI, please see Section 6in the SI |
634a5084de2a2174b2a60019 | 13 | In order to further verify whether the sandwich complex stabilized by hydrogen bonding and π•••π stacking, shown in Fig. , would lead to enhancement of the stereoselectivity, we investigated another organocatalytic system: L-pipecolic acid with a backbone of (R,R)-1,2-diphenylethylenediamine, which catalyses the asymmetric nitroaldol reaction reaction.This has been experimentally investigated by Feng and coworkers. The reason for choosing this particular case, as explained in the Introduction, was because different nitrophenol substituted additiveswere investigated in the experimental report, and different enantioselectivities were obtained forthe different cases.2,4-dinitrophenol was experimentally found to be the best aromatic additive,leading to the highest enantioselectivity (entry 1 in Table ), followed by 4nitrophenol, phenol and 2,4,6-trinitrophenol.These experimental results provide us with an opportunity to evaluate the robustness of our proposed model: if the trend obtained for the enantioselectivity from our proposed model matches the reported experimental trend, this experimental report would serve as a validation of the newly proposed model. |
634a5084de2a2174b2a60019 | 14 | . The lowest barrierfor the stereochemical transition state (2.9 kcal/mol) was obtained for the2,4-dinitrophenoladditive (see Fig. and Table below). The highest barrier(10.4 kcal/mol) was obtained when 2,4,6-trinitrophenol was employed as the additive(see Fig. and Table below). The barriers for all the four cases matches the trend of the experimentally observed enantioselectivity. We note here that, in order to reduce the computational expense, we have focused on the favoured enantioselective pathway. |
634a5084de2a2174b2a60019 | 15 | ). (This latter possibility had been highlighted in the experimental report of this reaction system by Feng and co-workers. ) What this shows is that the newly proposed sandwich conformation of the aromatic additive is the more correctpreferred conformation of the system, both prior to and during the transition state formation, as the reaction proceeds. We have also below). This further highlights the role of the aromatic additive as a second catalyst, and shows how it increases the enantioselectivity of the desired product during the reaction. There have been a vast number of organocatalytic reactions where the addition of aromatic additives such as acids or alcohols, has been seen to have a beneficial effect on the yield and the enantioselectivity. The conventional explanation for this has been their ability to increase the pKa, or in some cases, create a proton shuttle mechanism, or act as a hydrogen bonding agent with one of the substrates. However, till date, there has been no systematic investigation of the role that the additive may be playing. It is easy to see that such a study is essential, because without understanding the exact role of the additive vis-à-vis asymmetric organocatalysis, it is not possible to systematically improve the catalysts systems, other than by trial and error. The current computational study with density functional theory (DFT) is the first systematic study of the role of additives, focusing on aromatic additives, and investigating their role in a wide variety of experimentally reported systems. What the investigations demonstrate is that aromatic additives can form a stable sandwich complex, stabilizing both the electrophile and the nucleophile through non-covalent interactions, while maintaining the correct relative orientation of the two substrates relative to each other. This has been demonstrated for a wide range of organocatalyst systems employing different aromatic additives. We note here that, while the majority of the cases studied showed this favourable interaction, there were two cases out of nine where such sandwich complex formation was seen to be unfavourable. For such systems, the beneficial role of the additive would be due to other factors, such as increasing the pKa, acting as a proton shuttle or hydrogen bonding with one of the substrates. This result also points at a simple manner in which the role of the aromatic additive in new asymmetric organocatalysis could be evaluated: a few optimization calculations involving the proposed aromatic additive and the organocatalyst system would quickly reveal whether the role uncovered in the current work would be operational in the system that would be experimentally investigated. |
634a5084de2a2174b2a60019 | 16 | Subsequent to the finding of the stable sandwich complexes in the majority of the cases, rigorous investigations have been done to demonstrate that such a sandwich complex would serve as the precursor to favourable reaction pathways that would lead to enhanced enantioselectivity. This (phenol, 4-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol) as the aromatic additive, was also explored. The purpose of exploring this second case was to exploit data reported experimentally that showed that slight changesinthe electronic nature of the additive changed the enantioselectivity of the reaction. Calculations with our newly proposed sandwich modelgave the exact same trend forthe enantioselectivity as observed experimentally. |
634a5084de2a2174b2a60019 | 17 | These results point at an important general principle: when an aromatic additive is employed in asymmetric organocatalysis, it can act as a second catalyst by sandwiching itself between the organocatalyst coordinated substrates, and thus serve to improve the enantioselectivity. In otherwords, thecurrentworkwouldenablethe rational design oforganocatalyticsystems. Given the rapid upsurge in interest in organocatalysis in recent years, the insights gained about the role of aromatic additives from the current work can help in the design of more efficient organocatalytic systems in the near future. |
634a5084de2a2174b2a60019 | 18 | All the calculations for the structures reported in this work have been done using density functional theory (DFT). The calculations have been carried out with Turbomole 7.4 56 using the TZVP 57 basis set. Geometry optimizations were performed using the Perdew, Burke, and Ernzerhof (PBE) functional. Dispersion corrections (D3) have been included in all the calculations. Solvent corrections have also been included using the COSMO model, with ε=8.93 and ε=10.90 employed to model dichloromethane, CH2Cl2, when the nucleophile is oxindole and dichloroethane, CH3CH2Cl2, when the nucleophile is the β-keto ester. Therefore, the level of theory employed is PBE-D3/TZVP+COSMO(CH2Cl2) and PBE-D3/TZVP+COSMO(CH3CH2Cl2) respectively. Furthermore, in order to underscore the reliability of the enantiomeric calculations, all the enantioselective transition states have also been further optimized at the M06-2X/TZVP+COSMO(CH2Cl2) level of theory for the case when the nucleophile is oxindole. The values calculated at the M06-2X/TZVP+COSMO(CH2Cl2) level of theory are provided in the Tables . The results (∆∆G # values) show that the trends obtained with the PBE functional are replicated with the M06-2X functional. It is worth mentioning here that the basis set superposition error(BSSE) correction is not included in the current calculations.Rzepa and coworkers have shown that the error due to BSSE becomes significantly less (~0.2 kcal/mol) when they computationally investigated different stereochemical transition states for organocatalytic reactions at the TZVP level. Since the current calculations have been done at the TZVP basis set level, BSSE corrections were therefore unnecessary. The resolution of identity (RI) along with the multipole accelerated RI (marij) approximations have been employed for an accurate and efficient treatment of the electronic Coulomb term in the DFT calculations. Necessary care was taken to ensure that the obtained transition state structures possessed only one imaginary frequency corresponding to the correct normal mode, in order to obtain more reliable energy values for the investigated potential energy surface. In addition, intrinsic reaction coordinate (IRC) calculations were done with all the transition states in order to further confirm that they were the correct transition states, Here, Vmolec is the molecular volume, [X] is the concentration of molecules (mol/L) in solution and N0 is the Avogadro number. The translational entropy can be obtained after considering the free volume correction, and after inserting the value of Vfree in the Sackur-Tetrode equation. The total entropy is then calculated by adding the corrected translational entropy and the entropic contributions from the rotational and vibrational components. Multiwfn were employed for topology analysis. Structures with NCI plots were generated using VMD. |
634a5084de2a2174b2a60019 | 19 | According to transition state theory, the rate constant for the formation of R and S enantiomeric products can be expressed as kRand kSby "equation ( 3) where∆𝐺𝐺 ≠ is the free energy of activation for the formation of R or S respectively, kb is the Boltzmann constant, h is the Planck constant, T is the temperature and R is the universal gas constant. Stereoselectivity of the chemical reactions can be represented as an enantiomeric ratio (er). The er (for the enantioselective reaction) is directly proportional to the relative rates of formation of the enantiomeric products; that is, the ratio of products depends on the relative free energy barrier (ΔΔG # ) , which can be represented by "equation ( )": The enantioselectivity in a reaction is commonly reported by enantiomeric excess (ee), which can be calculated by "equation ( )" as: |
60cc97a326161182158b07f2 | 0 | Enzymatic electrochemistry has rapidly expanded into research fields that encompass applications across disciplines from fundamental understanding of enzymology to biosensors, biofuel cells and semi-artificial photosynthesis. Hydrogenases (H2ases) combine protons and electrons to reversibly produce H2 at the thermodynamic potential using Fe or NiFe active sites and have therefore been extensively studied as a model system for reversible electrocatalysis. Formate dehydrogenases (FDH) have similarly garnered attention due to their ability to reversibly reduce CO2 to formate, with high selectivity when immobilised on an electrode. Bioelectrochemistry on thin-film electrodes has long provided mechanistic and analytical insight into enzyme function, as well as establishing bioelectrolysis as a potential method of product synthesis. Porous electrodes enable higher enzyme loading, and hence higher current densities, improving the overall performance by increasing the consumption or production of desired chemicals. Solution assays provide insight into the influence of bulk pH and ionic strength on enzyme activity and the conclusions have informed the choice of electrolyte in electrochemical studies. However, the high current densities achievable using the latest generation of high surface area electrode scaffolds, usually used as stationary electrodes, presents a significantly different local chemical environment during turnover to that of bulk solution assays. With enzymes operating at a high rate of catalysis, for example D. vulgaris Hildenborough (DvH) [NiFeSe]-H2ase has a turnover frequency (TOF) of ~8300 s -1 for H2 evolution, the local proton concentration is likely to change significantly if the enzyme is placed in a porous environment. Therefore, consideration of local concentration gradients, as opposed to the bulk electrolyte solution, appears to be critical to understand and optimise enzymatic activity in such porous electrodes. |
60cc97a326161182158b07f2 | 1 | The precise conditions of the local chemical environment are decisive for enzyme activity and the local environment must therefore be tailored to the nature of the immobilised enzyme for optimal catalysis. Both the enzymatic H2 evolution reaction (HER) (eq. 1) and formate production by the enzymatic CO2 reduction reaction (CO2RR) (eq. 2) involves the net consumption of protons as part of their mechanism, leading to an increased local pH at the electrode surface. Due to their high activity and selectivity, H2ases can provide a simple, illustrative model system of how catalysis causes local pH changes within porous architectures, whilst FDHs can demonstrate the effect for the CO2RR. |
60cc97a326161182158b07f2 | 2 | The local CO2RR environment has been extensively modelled and conclusions applied to improve the selectivity of heterogeneous, and heterogenised molecular catalysts. Due to competition between HER and CO2RR when using synthetic catalysts, compromises, such as basicity, are often made to minimise the HER even at the cost of CO2RR activity due to the overall benefit of selectivity. The complexity of these interfacial interactions is considerably reduced when considering enzymatic catalysis due to the inherent specificity, selectivity and low overpotential requirements of the protein to drive catalysis. Without the competing HER, the local environment in enzymatic systems can be controlled without compromise to facilitate the CO2RR. Therefore, enzymatic CO2 electroreduction provides a paragon for the development of more 'ideal' synthetic molecular catalysts in the future. |
60cc97a326161182158b07f2 | 3 | Here, the local chemical environment of enzymatic HER and CO2RR systems were studied by bioelectrochemistry and computational methods using a finite element model (FEM). Firstly for the HER, and then for the CO2RR, the change in local pH on mesoporous electrodes were studied bioelectrochemically by the current density obtained in solutions with buffers of different pKa. For this purpose, DvH [NiFeSe] H2ase and DvH W-FDH were chosen as enzymes with high activity for their respective reduction reactions and low product inhibition (Figure ). The pH-activity solution assays for H2ase and FDH had previously been undertaken, showing an optimum pH of 6 and 7.1 respectively (Figure ). Independently, a FEM was developed from basic physical principles and enzyme-activity dependence studies for pH, buffer concentration and substrate concentration. The accuracy of the model was then confirmed by predicting the current density outcome of each experiment. The excellent match between the experimental and simulated results validated the FEM and hence the model could be used to simulate the changes in local pH, and other species concentrations. The model was then used to iterate the optimum conditions for each system and the resulting increase in current density was demonstrated experimentally. Finally, the conclusions were experimentally tested on a different electrode architecture, an inverse opal electrode, to demonstrate the wide-ranging applicability of this study. |
60cc97a326161182158b07f2 | 4 | To demonstrate the effect of local chemical environment changes in confined electrodes, first a mesoporous indium-tin-oxide (mesoITO) film on fluorine-doped tin oxide-coated glass (synthesised 50 nm ITO particles, film thickness of 8.5 μm, Figures ) was chosen as the electrode. The mesoporous structure could be simulated as a randomly close packed structure and hence did not require too computationally expensive modelling. ITO was chosen as the electrode material due to its conductivity and high affinity for enzyme binding and versatility to prepare porous materials. The enzymes, H2ase for the HER and FDH for the CO2RR, were then immobilised onto the electrode by dropcasting. A three-electrode set-up was used in the chosen electrolyte solution to measure the resultant current density and the solution was stirred to maximise mass transport to the electrode. To demonstrate the reproducibility of the findings at least three repeats were measured for each experiment, each with a newly assembled bioelectrode, in a fresh electrolyte solution. |
60cc97a326161182158b07f2 | 5 | For the electrolyte, two zwitterionic Good's buffers were selected for this study: 2-(Nmorpholino)ethanesulfonic acid (MES) (pKa = 6.27) and 3-(N-morpholino)-propanesulfonic acid (MOPS) (pKa = 7.18) (Figure ). These buffers were chosen due to their similar chemical composition to ensure that any changes in enzyme productivity could be attributed to the different pKa as opposed to physical or chemical interactions with the electrode or enzyme. These buffers are also known to not coordinate with the main metals present in enzymes and reference electrodes. The effect of the specific buffer concentration on the enzyme activity was measured for this study and the results presented in the Supporting Information, Enzyme Current Density, and the buffer concentration has been kept below inhibitory levels throughout this study. The electrolyte solution for the CO2RR also contains CO2 and its pH-dependent equilibrium with carbonic acid (H2CO3), bicarbonate (HCO 3 & ) and carbonate (CO 3 2& ), which has been further discussed in the Supporting Information, Equilibria. |
60cc97a326161182158b07f2 | 6 | To accurately prepare the electrolyte solutions, taking into account the influence of ionic strength, equations to calculate the activity coefficients of all species were applied using the freeware chemical equilibrium model Visual MINTEQ (Supporting Information, Ionic Strength). In initially compared electrolytes, the ionic strength has been kept constant by the addition of KCl. The buffers were prepared by combination of the correct concentration of acid and base buffer species to give the desired bulk pH. The pH at which MES and MOPS have identical buffer capacity is 6.54 and so this was selected as the bulk pH (Figure and Supporting Information, Buffer Capacity). The grey dotted vertical line demonstrates the pH at which the buffer capacity of the two buffers is equal. |
60cc97a326161182158b07f2 | 7 | The FEM was constructed to simulate the effect of local environment changes on enzymatic HER (DvH [NiFeSe] H2ase) and CO2RR (DvH W-FDH) in the mesoporous electrode, using COMSOL Multiphysics 5.5. Similar models have been applied to follow the concentration of H2 as substrate for H2 oxidation and O2 inhibition in redox hydrogel films or carbon nanotubes, though these models did not consider the concomitant change in local pH or other electrolyte species. The model, and its governing parameters and equations, is described in detail in the Supporting Information, Finite Element Model, and is summarised in Figure . |
60cc97a326161182158b07f2 | 8 | The model uses the Nernst-Planck equation to describe the potential distribution within the electrode, alongside the kinetics of the enzyme which is assumed to be homogenously distributed within the porous electrode. Furthermore, known modifiers on enzyme activity such as pH and substrate concentration that are described using common analytical expressions such as Michaelis-Menten kinetics are included where appropriate. Finally, this is combined with a mass transport model that incorporates the kinetics and thermodynamics of solution reactions. This approach gives a model that is predictive of electrode currents and performance and, critically, is completely independent of the experimental data. This prediction of experimental currents across a range of conditions and the excellent agreement of the predicted and experimental values provides validation of the model. Furthermore, it is indicative of the absence of additional unconsidered interactions, demonstrating that a more comprehensive analysis than that presented was not required. |
60cc97a326161182158b07f2 | 9 | Once the model was validated, it has been used to simulate a quantified understanding of the previously inaccessible local environment, demonstrating the time-dependent changes in the local pH and other species concentrations. The predictive nature of this FEM could then allow the solution composition to be iterated for the best experimental system performance and highest simulated current densities, which were then validated experimentally. Once the local environment had been optimised for the enzymatic HER and CO2RR on mesoporous electrodes, a novel electrode architecture was introduced to demonstrate the applicability of the conclusions on electrolyte composition to a different system. Hierarchical inverse opal (IO) electrodes were chosen due to their macro and mesoporous structure which enables increased enzyme loading compared to purely mesoporous electrodes, as has been previously demonstrated. A shift to TiO2 also allowed the application of more negative reduction potentials (-0.8 V vs. SHE) than that being used with mesoITO (-0.6 V vs. SHE) due to the degradation of ITO at more reducing potentials. The IO-TiO2 electrodes were constructed with co-assembled ~21 nm anatase TiO2 particles and 750 nm diameter polystyrene spheres and then annealed to give IO macropores with a total film thickness of 40 μm (Figure ). With these electrodes the principles of local chemical environment adjustment for enzymatic electrochemistry were applied and hence significant improvements in the current density were again achieved. same buffer capacity at a bulk pH 6.54 (Figure ). The mesoITO|H2ase electrode in the MOPS electrolyte solution displayed 60% higher catalytic activity for proton reduction than the MES solution at an applied bias of -0.6 V vs. SHE (Figure ). The H2 produced was measured by gas chromatography of a sample of the reaction headspace and a Faradaic efficiency (FE) of (88 ± 5)% was determined (Figure ). The average current density of the repeats, with a new bioelectrode in new electrolyte for each repeat, has been shown as the solid line with the standard deviation (N ≥ 3) as the shaded area/error bars. The reproducibility of the system is demonstrated by the narrow standard deviation of both the current density and the product quantification. Voltammetry of the mesoITO|H2ase and mesoITO electrodes are also shown in Figure , showing minimal current density for the enzyme-free control electrodes. |
60cc97a326161182158b07f2 | 10 | As described in Methodology and Simulation, the experimental system was also simulated using a FEM and the predicted current density shown as a dotted line over the experimental results (Figure ). Assuming a FE of 100%, the predicted H2 evolution is shown as separate bars in Figure . The simulated current densities were in close agreement to the experimental data for both the MES and MOPS electrolytes, supporting the validity of the model and its description of the enzyme kinetics, activity and solution mass transport. With the FEM matching the experimental conditions and outcomes, the model was then used to simulate the changes in local pH and buffer capacity within the electrode over time (Figure ), and as a function of distance from the electrode (Figure ). |
60cc97a326161182158b07f2 | 11 | Due to the change in local proton concentration as the catalysis occurs, the pH at the electrode instantly increases (Figure ). The peak activity of proton reduction by DvH [NiFeSe] H2ase in solution assays was shown to be at pH 6, with a reasonably sharp decrease in activity on deviation from the optimum (Figure ). Therefore as the local pH increases away from the optimum, a decrease in the rate of catalysis of the enzyme will occur. However, the change in local proton concentration is reduced when using an electrolyte solution with a high buffer capacity at that local pH (Figure ). Therefore, we observe a lower local pH change in the electrodes for the MOPS buffer (pKa = 7.2) compared with MES (pKa = 6.3) at a local pH of >7.5 (ΔpH = 1.24 and 1.76 for MOPS and MES, respectively). Consequently, the enzyme activity is higher with the MOPS electrolyte solution and therefore higher experimental current densities were observed than using MES solution (Figure ). This can additionally be observed in the decrease in current density between the first and second scan of the initial voltammetry scans (Figure ). The combination of experimentation and computation was therefore able to demonstrate that porous electrodes with confined enzymes exhibit a significant local pH change, thereby affecting the current and product output. Consequently, the bulk pH should not be the optimum pH for the enzyme, but instead should be aligned as to give a local pH at the optimum. It can also be concluded that the pKa of the electrolyte buffer should be matched to the local pH, not the bulk pH. Qualitatively, for DvH H2ase HER the optimum pH is 6 (Figure ), and therefore a lower bulk pH of about 4-5 would likely be appropriate with a buffer pKa of 6 (for example MES). |
60cc97a326161182158b07f2 | 12 | for DvH H2ase H2 evolution on the mesoITO electrode. By iteration of the solution properties in the FEM, the system could be optimised to give the highest predicted current densities, taking into consideration the contrasting effects of MES concentration on enzyme activity (Supporting Information, Enzyme Current Density) and buffer capacity for an optimised local pH. The optimal solution determined by the FEM was a MES concentration of 240 mM and a bulk pH of 4.24, which predicted a resulting local pH of 5.94, closely matching the optimum (pH 6) for HER with DvH H2ase, and a simulated current density of 4.4 mA cm -2 . |
60cc97a326161182158b07f2 | 13 | However, the optimised system exhibited deterioration of current densities over the hour. The decrease can be attributed to the build-up of hydrogen gas in the pores and on the electrode as demonstrated by the rejuvenation of the activity after the bubbles were removed from the electrode (Figure ). Whilst the simulation does account for the mass transport effects of bubble formation (Supporting Information, Solution Domain), it does not account for bubble blocking or other effects. For example, H2 bubble formation may cause increased film loss, through enzyme displacement, re-orientation and degradation. H2 also affects the thermodynamic potential according to the Nernst equation, an effect even more pronounced at the low overpotentials applied here. Though usually H2 is a key inhibitor of H2ases, DvH |
60cc97a326161182158b07f2 | 14 | [NiFeSe] H2ase is less susceptible to H2 inhibition as other H2ases and is hence an ideal candidate for application in confined architectures. However, if the local concentration was high enough it is likely that inhibition would still occur. Thus the simulation does not show the same decline in current density over time and predicts a greater amount of H2 (77.5 μmol cm -2 ). Therefore, the optimised electrolyte has effectively reached the highest current densities achievable in this electrode system due to the limitations imposed by the production of H2, and other accelerated sources of enzyme-film loss. |
60cc97a326161182158b07f2 | 15 | CO2, the substrate for FDH, forms the CO2/HCO 3 & (pK1= 6.34) and HCO 3 & /CO 3 2& (pK2 = 10.32) buffer equilibria when dissolved in H2O (Supporting Information, Equilibria). However, on the relatively short timescales within the diffusion layer at the electrode, the slow kinetics of CO2 hydration result in minimal interconversion of CO2 and HCO 3 & in response to the shifting equilibria. Therefore the CO2 concentration is almost unchanged by differing pH within the diffusion layer. As such the buffering effect of CO2/HCO 3 & , and similarly the formation of aqueous CO2 from HCO 3 & , is limited in its mitigation of the local concentration gradients. In heterogeneous and heterogenised molecular catalyst systems, the poor buffering of the CO2/HCO 3 & equilibrium benefits the system by enhancing the local pH change, which slows down the competing HER, improving selectivity for the CO2RR. However, in enzymatic CO2RR, the HER is negligible due to high selectivity and low overpotential requirements preventing the HER from occurring directly at the metal oxide electrode. Therefore, increasing the buffer capacity would be beneficial, and so an additional, faster-buffering species can be introduced. Three electrolyte solutions were therefore chosen, consisting firstly of a 'bicarbonate only' electrolyte (BC) and then two alternative electrolytes which include an additional 100 mM buffer as follows: BC-MES and BC-MOPS. A pH of 6.54 was again selected as at this pH the buffer capacity of electrolyte BC-MES and BC-MOPS is equal (Figure ), and also the CO2 equilibria species concentrations do not change between all the electrolytes (Figure ). |
60cc97a326161182158b07f2 | 16 | From the electrochemical experiments on a mesoITO scaffold with FDH (52 pmol) at an applied bias of -0.6 V vs. SHE, we observe significant activity in the BC-MES and further enhanced current in the BC-MOPS electrolyte solution (Figure ). As before, repeats of each solution were made and a narrow standard deviation (N ≥ 3) demonstrated the reproducibility of the system. The FEM current density prediction was shown to closely match the experimental results, again confirming the accuracy of the model. The formate production was measured by ion chromatography and displayed a FE of (97 ± 7)% (Figure ) and negligible average H2 evolution with a FE of (0.2 ± 0.2)%, thus demonstrating the excellent selectivity of FDH. The mesoITO electrode without FDH showed minimal current (Figure ). |
60cc97a326161182158b07f2 | 17 | From the FEM evaluation of the local electrode environment, it is apparent that the local pH is significantly reduced at the electrode with electrolyte BC-MES and BC-MOPS compared to BC (Figure ). The highest buffer capacity at the electrode (BC-MOPS) again causes the lowest local pH change (Figure ). A pH of 7.1 has been shown in solution assays to be the optimum pH of the FDH for CO2 reduction, and therefore increasing the pH beyond this optimum decreases enzyme activity (Figure ). Whilst the increased initial buffer capacity of BC-MES improves upon the BC system, the higher pKa of MOPS enables a high buffer The experimental data shows continued functionality of the FDH in electrolyte BC despite the local pH increase to 8.4 at which the solution assay suggests <20% activity. However the CO2 concentration would have changed dramatically above pH 7.5 in the assay as the addition of 50 mM of NaHCO3 was used to provide CO2, rather than saturated CO2. Therefore, the enzyme may in fact function better at a higher bulk pH than the assay suggests, as supported by the slightly lower predicted current densities from the FEM model in electrolyte BC. The influence of low CO2 concentration as opposed to the pH effect on the solution assay is further corroborated by the continued oxidation activity of the enzyme at high pH. These results further demonstrate the importance of considering the effect of pH on CO2 when designing experiments and suggest that solution assays may be more accurate with saturated CO2. |
60cc97a326161182158b07f2 | 18 | The FEM was then used to iterate the optimum electrolyte for enzymatic CO2RR to provide the greatest predicted currents. The resultant conditions consisted of MOPS (86 mM) and NaHCO3 (50 mM) at a bulk pH of 6.39. Experimentally, this optimum electrolyte (BC-MOPSopt) gave 20% higher current densities (Figure ). Thus showing again that a high buffer capacity and matching the pKa of the additional buffer to the local pH, as opposed to the bulk pH, is critical to stabilising the local pH and hence maximising product formation. |
60cc97a326161182158b07f2 | 19 | The above local environment optimisation conclusions were then applied to hierarchical inverse opal (IO) electrodes to demonstrate their applicability to alternative electrode architectures (Figure ). The mesoporous scaffold enables high loading of the enzyme whilst the macropores, formed by a polystyrene sphere co-assembly procedure, increase the diffusion of electrolyte species, such as substrates and products, into and out of the electrode. |
60cc97a326161182158b07f2 | 20 | With the IO-TiO2 electrodes the loading could be doubled from that used with mesoITO and more negative reduction potentials (-0.8 V vs. SHE) were able to be applied. The less-confined IO structure should also increase the mass transport of protons to the local environment of the enzyme, decreasing the local pH increase caused by catalysis. |
60cc97a326161182158b07f2 | 21 | First, the IO electrode IO-TiO2|H2ase25 was run with the same loading (25 pmol) and at the same applied potential (-0.6 V vs. SHE) with 240 mM MES and a bulk pH of 4.24 to demonstrate the increased mass transport of the macroporous structure (Figure ). As the local pH increased less than in the confined mesoITO structure, the local pH remained below the optimum and therefore the current density for IO-TiO2 was significantly lower (-2.2 mA cm -2 ) than for mesoITO (-4.8 mA cm -2 ). If the local proton consumption is then increased beyond the mass transport limit by doubling the enzyme loading and applying a more reductive potential (IO-TiO2|H2ase50), the local pH again increases as in the mesoporous electrode. |
60cc97a326161182158b07f2 | 22 | Therefore, the current density increased to -8.0 mA cm -2 , producing 83 μmol cm -2 of H2 at a FE of 92% with a H2ase-based turnover number (TON) for H2 of 3.2×10 5 and turnover frequency (TOF) of 9 s -1 . The TON and TOF was calculated from the product quantification and the enzyme loading, and since the number of active enzymes in direct electron transfer with the surface is unknown, the TON and TOF of these systems therefore represent the lower limits of the actual values. Due to the significantly higher current densities obtained for this system, even greater bubble effects are expected, which can be observed in the decrease in current density over the one hour-long experiment. The stability of CO2 reduction by the mesoITO|FDH52 electrode, with the same loading (52 pmol), was demonstrated over 10 hours producing 22 μmol cm -2 h -1 of formate with a FE of 103% (Figure ). As with H2ase, the increased mass transport of the macroporous IO structure can be observed by the lower current density of IO-TiO2|FDH52 at -0.6 V vs. SHE. On increasing the local proton consumption by again doubling the enzyme loading (104 pmol -IO-TiO2|FDH104) and applying a more reductive potential (-0.8 V vs. SHE), the local pH increases, raising the current density to -3.0 mA cm -2 . However, the local pH then increases beyond the optimum due to the significantly higher current densities, therefore the bulk pH must again be lowered to 4.59 to further increase the enzyme activity. As the increase in local pH is slower in the less-confined IO structure, the local pH change can be observed in the chronoamperometry as an initial increase in current density, peaking at 1.2 h. The final electrolyte solution demonstrates current densities of -3.6 mA cm -2 , producing 0.55 ± 0.043 mmol cm -2 of formate (55 ± 4.3 μmol cm -2 h -1 ) at a FE of (96 ± 8.7)% with a FDH-based TON for formate of 1.0×10 and TOF of 28 s -1 . Thus demonstrating the significant 18-fold improvement that can be achieved compared to previously reported systems (IO-TiO2|FDH38, 38 pmol, 0.2 mA cm -2 ) by simple adjustment of the experimental parameters. |
60cc97a326161182158b07f2 | 23 | The critical importance of understanding the local chemical environment of highly active confined enzymes in porous electrodes has been experimentally demonstrated. An independent FEM was also applied to predict the experimental currents, thus verifying the model and allowing the simulation and understanding of local concentration changes, an experimentally challenging feat in these complex multicomponent enzyme systems on porous electrodes. For the HER and the CO2RR, the local pH was shown to increase around 2 pH units due to enzyme catalysis in a confined environment. Dramatic differences in activity were achieved without inhibitively large buffer concentrations, but instead through careful buffer selection and manipulation of the electrolyte pH. In mesoporous electrodes, a 9-fold increase in the HER current density from -0.5 mA cm -2 to -4.8 mA cm -2 produced 50 ± 3 μmol cm -2 h -1 of H2 and a 2-fold current density increase for CO2 reduction, produced 21 ± 0.1 μmol cm -2 h -1 of formate. Finally, in an alternative architecture of inverse opal electrodes, even greater current densities were achieved through the combination of increased mass transport due to macroporous structuring, higher loading and more reductive potentials with an optimised electrolyte. For the HER current densities of -8.0 mA cm -2 produced 80 μmol cm -2 h -1 of H2 with a FE of 92% and for the CO2RR current densities of -3.6 mA cm -2 produced 55 ± 4.3 μmol cm -2 h -1 of formate, over 10 hours giving a total formate production of 0.55 ± 0.043 mmol cm -2 with a FE of (96 ± 8.7)%. These figures demonstrate a 3-fold increase for the HER and an 18-fold increase for the CO2RR on analogous reported electrochemical systems in which the local environment was not considered. Through this work we have shown the ability to use experimental data and computational methods in tandem to analyze and optimise biological-artificial hybrid electrodes. |
60cc97a326161182158b07f2 | 24 | Electrolyte adjustment to mitigate the local pH changes of enzymes on a porous electrode has been shown as a simple method to maximise the product formation, important for fuel-forming applications. Whilst the precise conditions for each system will depend on the enzyme, current density and electrode architecture, we have demonstrated with the IO electrodes that the following guidelines can be applied to optimise any porous bioelectrochemical system. To achieve this the bulk pH should not be chosen for the 'ideal' enzyme activity pH (determined by solution assay), but so that the local pH changes lead the solution into the optimum range. |
60cc97a326161182158b07f2 | 25 | Whereas for oxidation reactions the local pH decreases, and a higher pH is appropriate. Then a buffer should be chosen to have good buffering at the local pH optimum, which in a perfect system will be the 'idea' enzyme activity pH. Further significant developments can be achieved with the utilisation of a less densely structured porous electrode to facilitate the mass transport of species. Moreover, as high current density bioelectrochemical systems reach the mass transport limits of current electrode architectures, a shift towards more dynamic flow technology, such as gas diffusion electrodes, may be required to make advances in the future. |
60cc97a326161182158b07f2 | 26 | In enzymatic fuel cells the optimisation of the local environment could similarly improve the current performance of these bioelectrochemical devices. Low power output remains one of the key issues which hinders the wider application of enzymatic fuel cells. The densely porous electrodes employed in these systems, such as multi-walled carbon nanotubes, alongside the redox polymers often used to mediate electron transfer will lead to the development of large concentration gradients between the electrodes and the bulk. Whilst consideration of the substrate concentration across these electrodes has been made, the local pH and electrolyte composition has not been accounted for and therefore has the potential to significantly increase power output as shown here for bioelectrochemical fuel synthesis. The negative effect of the local pH increase in enzymatic CO2 reduction is enhanced compared to systems utilising synthetic catalysts as there is no concomitant beneficial change in selectivity against HER. This work demonstrates the increasing importance of studying enzymes as exemplary selective and specific molecular catalysts. As artificial molecular catalysts become more sophisticated, and hence more selective, the techniques and conclusions of this enzymatic research will become ever more applicable. |
60cc97a326161182158b07f2 | 27 | This study has demonstrated that the local chemical environment is distinctly different when highly active enzymes are confined at high loading under electrocatalytic conditions. This highlights the importance of distinguishing electrochemical techniques into two groups. Firstly, those which easily allow for mechanistic and fundamental enzyme studies, such as the use of unconfined, flat rotating disk electrodes which minimise mass transport effects and prevent the establishment of local environments. And secondly, stationary, porous electrodes which allow for significantly higher loading and current densities for product application purposes, but also establish local chemical environments distinct from the bulk solution. Whilst high loading porous electrodes can be used in enzymology studies, the local environment changes must be accounted for to prevent misinterpretation of activity changes. A FEM has been shown to be a powerful technique for understanding fundamental experimental observations in highly complex and convoluted applications. This research prompts a variety of other implications not specifically addressed here, but of equal importance to the field, including the local concentration of salt species and electron mediators, the confinement effect of redox polymers, the local pH decrease in oxidation reactions, and local pH changes in live cell biofilms. Supporting Information. |
67c460aa81d2151a02b8d0b4 | 0 | Nanotechnology has revolutionized agriculture with the development of nanoparticles with unique properties. Ranging from 1-100 nanometers, these NPs exhibit distinct physicochemical characteristics, such as high surface area and tunable reactivity. Among these, CuO NPs stand out as ideal candidates for various agricultural applications, including their use as pesticides, herbicides, and targeted nutrient delivery systems . However, there are toxicological and environmental issues with using chemically generated CuO NPs. To address this issue, sustainable approaches have been reported to obtain CuO. |
67c460aa81d2151a02b8d0b4 | 1 | Further studies demonstrated that CuO NPs synthesized from Zizyphusspina leaf extract were highly effective in controlling Fusarium solani, the pathogen responsible for root rot in tomatoes, surpassing the efficacy of chemical fungicides and Trichoderma biocide treatments. These CuO NPs also promoted more vigorous tomato seedling growth . Additionally, Khaldari et al. used green tea (Camellia sinensis) and lavender (Lavandula angustifolia) leaf extracts to synthesize CuO NPs and demonstrated that low concentrations (as low as 4 μg/mL), the NPs significantly promoted root and shoot growth in lettuce and tomato seedlings. |
67c460aa81d2151a02b8d0b4 | 2 | Although several studies have examined the synthesis of CuO NPs using plant extracts, the use of yerba mate (Ilex paraguariensis) in this process has not yet been explored. Rich in bioactive compounds, including flavonoids and polyphenols, yerba mate exhibits strong antioxidant capacity, which can aid in stabilizing NPs and influencing their physicochemical properties. Furthermore, while CuO NPs have been extensively studied for their antimicrobial properties and role as a micronutrient source, their direct effects on seed germination and early plant growth remain poorly understood, particularly when synthesized using a green approach. Moreover, the impact of synthesis conditions on NPs formation and properties remains insufficiently explored, underscoring the need for further investigation into how reaction parameters influence nanoparticle characteristics. Another gap in the field is the investigation of the mechanism of formation of NPs from green sources to understand the interaction of the bioactive in the extract and their influence on the properties of the NPs. In light of this, we propose an innovative approach utilizing yerba mate extract for the green synthesis of CuO NPs, eliminating the need for aggressive chemical agents and promoting a method aligned with sustainable chemistry principles. |
67c460aa81d2151a02b8d0b4 | 3 | Herein, CuO NPs were synthesized using an eco-friendly approach, characterized physicochemical and morphologically, and evaluated for their effects on agricultural seeds. To predict the response and identify the optimal synthesis conditions, a factorial design was employed to examine the effects of temperature and metal precursor concentration on CuO yield. Additionally, we propose a reaction mechanism to elucidate the formation of NPs and their interaction with the bioactive compounds in the extract. Finally, the CuO NPs obtained under the most favorable conditions were applied to Lactuca sativa (lettuce) and Solanum lycopersicum (tomato) seeds at varying concentrations, assessing their effects on physiological parameters related to seed germination and early seedling growth. Our findings may contribute to the advancement of more sustainable agricultural strategies by reducing reliance on synthetic fertilizers and promoting the valorization of plant waste in nanomaterial production. |
67c460aa81d2151a02b8d0b4 | 4 | To synthesize CuO, copper nitrate trihydrate [Cu(NO₃)₂.•3H₂O, 241.60 g/mol] (Neon reagents, Brazil) was used as Cu source. The extract was prepared with distilled water and commercial yerba mate. The germination and physiological parameters tests were carried out using tomato (Solanum lycopersicum) and lettuce (Lactuca sativa) seeds. The yerba mate and the seeds of tomato and lettuce were purchased at a local market in Florianópolis, Santa Catarina, Brazil (27°35'43.4"S,48°32'52.8"W). |
67c460aa81d2151a02b8d0b4 | 5 | To synthesize the CuO NPs, the extract was first prepared using a ratio of 1 g of yerba mate to 10 mL of distilled water (1:10) under constant stirring for 30 min at 70 °C. The extract was then centrifuged for 30 min at 5000 rpm, vacuum filtered, and refrigerated at 4 °C . The synthesis of CuO NPs was based on the methodology of Koteeswari et al. with few modifications. To this end, 40 mL of the yerba mate extract was mixed with 1, 1.5, and 2 g of Cu(NO3)2•3H2O under constant stirring for 2 h, varying the reaction temperature by 40, 60, and 80 °C. After the synthesis reaction, the next steps were centrifugation for 30 min at 5000 rpm, washing with distilled water to remove residual nitrates, and drying in an oven for 24 h at 60 °C. The material was then calcined at 400 °C for 2 h and characterized . The condition that had the highest conversion yield of CuO NPs was selected, and the produced CuO NPs were applied to lettuce and tomato seeds (Figure ). |
67c460aa81d2151a02b8d0b4 | 6 | This study applied a factorial experimental design to study the influence and interaction of the variables for the synthesis of CuO NPs. A 3 k full factorial planning matrix was defined using the values and coded levels provided in Table . Thus, based on the preparation procedure, the independent variables selected were reaction temperature and mass of Cu(II) nitrate trihydrate, Cu(NO3)2•3H2O. The response factor in the factorial design was the product conversion yield (PCY). |
67c460aa81d2151a02b8d0b4 | 7 | The number of experimental runs generated from a factorial design for the two variables consisted of nine factorial points and three repetitions at the central point, indicating that a total of twelve experiments were required. Statistica ® software (Statistica ® v.10.0, TIBCO, USA) was used to calculate the model coefficients, evaluate significant variables, and create graphs for interpretation. |
67c460aa81d2151a02b8d0b4 | 8 | The CuO NPs were characterized by X-ray diffraction (XRD) (Rigaku MiniFlex600 Diffractometer) with Cu Kα radiation (λ = 1.5418 Å), and with nickel filter in secondary optics, power of 40 kV, current of 30 mA and 1° divergence slit. 2 was scanned from 5º to 50º, with a step of 0.02°.The Scherrer equation was used to calculate the crystallite size from XRD patterns according to Equation 1 : 𝐷 = 𝐾𝜆 𝛽 𝑐𝑜𝑠 𝜃 where D is the crystallite size, K is the Scherrer constant (0.9), 𝜆 is the wavelength of the X-rays used (0.154 nm), 𝛽 is the full or full width at half maximum (𝐹𝑊𝐻𝑀, radians), and 𝜃 is the peak position (radians) . |
67c460aa81d2151a02b8d0b4 | 9 | XRD data analysis was performed using X'PertHighScore Plus 5.1 software (Malvern Panalytical, Worcestershire, UK). Fourier Transform Infrared Spectroscopy (FTIR) was used to identify functional groups in an Infrared Spectrophotometer Agilent Technologies -Cary 660 FTIR model. The sample was prepared in KBr tablets with 2% (w/w). Subsequently, the sample was pressed to obtain a circular tablet. For the analysis of characteristic XRD peaks and characteristic bands of FTIR spectra, Origin Pro 8.5 software (OriginLab, Northampton, Massachusetts, USA) was used. |
67c460aa81d2151a02b8d0b4 | 10 | The optical property of the synthesized CuO NPs was determined by measuring the absorbance of light over a range of 200 nm to 800 nm using a UV-Vis spectrophotometer (Infinite M200 multi-reader). A zeta potential analysis was carried out to investigate the particles' surface charge and colloidal stability (Malvern® Zetasizer Nano, ZS90). The particle size distribution (Malvern® Zetasizer Nano, ZS90) was realized using dynamic light scattering (DLS) to determine the size of particles in suspension by analyzing the intensity of the light scattered by these particles. |
67c460aa81d2151a02b8d0b4 | 11 | To verify the ability of CuO NPs to improve seed germination, four replicates of 25 and 100 seeds of tomato and lettuce were used in each treatment, respectively. First, seeds were disinfested in 1% sodium hypochlorite solution for 10 min, rinsed thoroughly with distilled water, and superficially dried with paper. Next, seeds were immersed in different concentrations of CuO NPs for 5 h in the dark: 25 mg/L, 50 mg/L, and 100 mg/L . Distilled water was used as a control (0 mg/L). After imbibition, seeds were transferred to Germitest roll papers moistened with distilled water and stored in a cultivation room at 25 ºC and 12h photoperiod. Seeds were evaluated daily until no seed germinated for three consecutive days. Seeds were considered as germinated when they presented 2 mm of radicle protrusion. At the end of the test, germination rate (%) and vigor, based on the germination speed index (GSI) were analyzed. GSI was calculated as the sum of the ratio between the number of germinated seeds per day and the respective day of observation: GSI = G1/N1 + G2/N2 + G3/N3 +•••+ Gn/Nn, where G1, G2, G3,..., Gn = number of germinated seeds on the day of observation, and N1, N2, N3,..., Nn = the number of days elapsed from sowing . At the end of the germination test, 15 seedlings from each of the four replicates were used to measure seedling growth. These measurements were made 10 days after sowing for lettuce seeds and 15 days for tomato seeds. Seedlings were randomly taken from the replicates without looking to avoid seedling selection . For seedling dry weight, the four samples were dried in an oven at 60 ºC for 48 h. Physiological data were subjected to homoscedasticity (Bartlett test) and normality (Shapiro-Wilk test) assessments. One-way ANOVA was performed, and the means were compared by Tukey's test (p ≤ 0.05). Data was expressed as the mean ± SD of the replicates. Statistical analysis was performed in R core team (2024). |
67c460aa81d2151a02b8d0b4 | 12 | The conversion yield of the product ranged from 5.99% to 26.74%. Run 3, with a mass of 2 g of Cu(NO3)2•3H2O and a temperature of 40 °C, showed the lowest mass yield (5.99%). On the other hand, test 9 showed the highest product yield (26.75%) using 2 g of precursor and 80ºC. The differences are clear when comparing tests 3, 6, and 9, which used the same mass of Cu(NO3)2•3H2O (2 g) and varied the temperature by 40 ºC, 60 ºC, and 80°C. When the temperature increases from 40°C to 80°C, the yield increases by approximately 4.5 times. The model was evaluated using Analysis of Variance (ANOVA), with 95% reliability, where each interaction and the values of the terms are shown in Table . |
67c460aa81d2151a02b8d0b4 | 13 | Insert Table The data in Table shows that the reaction temperature (linear) is highly significant (X1), exerting a positive influence on the response variable analyzed, as evidenced by its high quadratic sum value. In addition, the product of the linear and quadratic interactions of reaction temperature and mass of copper nitrate trihydrate were also significant, as shown by the p-value of less than 0.05. |
67c460aa81d2151a02b8d0b4 | 14 | The values of the coefficients of determination (R 2 ) and adjusted R 2 were 96.2% and 94.77% respectively. This high value of the coefficient of determination suggests that the proposed model is suitable for estimating the value of the product's conversion yield. Another significant factor observed in the ANOVA is the F value for Lack of Fit (2.833) concerning Pure Error. This value was not significant, so the model was adequate for predicting the response. |
67c460aa81d2151a02b8d0b4 | 15 | The response surface is represented in Figure , b, where high-temperature values are favorable to the conversion yield of the product, CuO. Figure shows the Pareto graph resulting from the experimental design. The linear temperature and the linear and quadratic interactions of the mass of Cu(NO3)2•3H2O (g) and temperature had a significant effect (p<0.05) on the response variable (PCY). |
67c460aa81d2151a02b8d0b4 | 16 | Temperature was the variable that had the greatest influence on the copper oxide conversion yield, with a positive effect according to the Pareto chart (Figure ) and ANOVA (Table ). This means that high synthesis temperatures favor the conversion of copper oxide and may be related to various factors. According to Arrhenius's theory, the speed of a chemical reaction generally increases with increasing temperature . Therefore, high temperatures increase the average kinetic energy of molecules, and this increases the probability of collision, which will eventually favor the reaction rate and conversion of the desired product . |
67c460aa81d2151a02b8d0b4 | 17 | In addition, the temperature can influence the solubility of certain reagents. In aqueous solutions, the solubility of many compounds tends to increase with temperature, which may facilitate the availability of copper ions to react with the components of the yerba mate extract . Noubigh et al. studied the effects of temperature on the solubility of some phenolic compounds in water and found that solubility increases with increasing temperature. An increase of 25 ºC caused a 2.4fold increase in the solubility of vanillin . |
67c460aa81d2151a02b8d0b4 | 18 | Another relevant factor is the rate of molecular diffusion, which is directly correlated with temperature . With greater mobility of the molecules in the solution, the reactants interact more quickly, increasing the rate of product formation. Finally, it is crucial to consider the organic compounds present in yerba mate, such as polyphenols and alkaloids, which can act as reducing, dispersion, or stabilizing agents in the formation of copper oxide . The effectiveness of these compounds can be enhanced at higher temperatures, favoring the complete conversion of the precursors into copper oxide Finally, the experimental design proposed a polynomial model that statistically describes the CuO NPs synthesis process Eq. ( ). The model represented by Eq. ( ) has coded variables X1 and X2 corresponding to the reaction temperature and mass of copper nitrate trihydrate respectively. |
67c460aa81d2151a02b8d0b4 | 19 | The synthesis of copper oxide often uses metal salts such as copper nitrate (Cu(NO₃)₂) or copper chlorides precursors, combined with a strong base such as sodium hydroxide (NaOH), in a process called chemical precipitation .In this method, the metal precursor is dissolved in an aqueous solution, and the pH of the reaction medium is increased to between 8 and 12 by adding a strong base, creating an alkaline environment . Under these conditions, the copper ions (Cu²⁺) |
67c460aa81d2151a02b8d0b4 | 20 | After the formation of copper hydroxide, it is converted into copper oxide through calcination or heat treatment . This process involves dehydrating the hydroxide, resulting in the formation of the corresponding metal oxide . However, sodium hydroxide is a corrosive reagent that requires proper handling and disposal. In this study, a green approach was used to synthesize copper oxide, using only yerba mate extract and the metal precursor salt, to reduce environmental impact and ensure a safe route. In this methodology, the reaction between the copper nitrate and the bioactive compounds presents in the yerba mate extract results in an acidic solution (pH=4). |
67c460aa81d2151a02b8d0b4 | 21 | Under these conditions, rather than forming copper hydroxide initially, a metal complex is formed between copper and the organic constituents of the extract . The acidity of the medium, influenced by the extract, facilitates this complexation of copper ions with the bioactive compounds present in yerba mate, leading to the formation of a reaction intermediate . Copper oxide nanoparticles are then formed after calcination at 400 °C. X-ray diffraction (XRD) analysis of the metal complex (Figure ) reveals amorphous behavior, corroborating the presence of the reaction intermediate. |
67c460aa81d2151a02b8d0b4 | 22 | Figure shows the FTIR spectra of the intermediate copper complex formed during the reaction and the yerba mate extract used in the synthesis. The FTIR analysis was performed to provide a more detailed understanding of the functional groups present in both the metal complex formed between copper and the bioactive compounds in the yerba mate and the extract used in the synthesis. |
67c460aa81d2151a02b8d0b4 | 23 | According to the transmittance peak at 2929 cm -1 is associated with C-H stretching, common in organic compounds, corroborating with the most intense stretching peak for the extract at this wavelength. This peak may be related to the methyl or methylenic groups present in organic compounds, such as fatty acids, terpenes, or other organic components of yerba mate . |
67c460aa81d2151a02b8d0b4 | 24 | The absorption peaks around 1625 cm⁻¹ and 1392 cm⁻¹ can be attributed to the bending vibrations of the O-H bond, along with vibrations from methylene (-CH₂) or methyl (-CH₃) groups present in compounds such as fatty acids and polyphenols within the extract . In the intermediate copper complex, the band at 1625 cm⁻¹ corresponds to the C=C aromatic bending vibration , while the band at 1392 cm⁻¹ is attributed to the stretching interactions between copper and oxygen atoms . The stretching peak at 1272 cm⁻¹ is characteristic of both symmetrical and functional groups, such as ethers or phenolic acids . Additionally, the absorption peak at 1072 cm⁻¹ in the yerba mate extract is assigned to the stretching vibration of the C-OH side group . |
67c460aa81d2151a02b8d0b4 | 25 | Figure shows the proposed mechanism for the formation of copper oxide nanoparticles from yerba mate extract. Initially, the precursor salt is mixed with yerba mate extract under heated conditions (step I). Subsequently, a reaction intermediate is formed through the complexation of copper ions with the bioactive compounds found in yerba mate (step II). |
67c460aa81d2151a02b8d0b4 | 26 | Additionally, other significant phenolic compounds identified in the literature include gallic acid, caffeic acid, syringic acid, ferulic acid, p-coumaric acid, and rutin . Yerba mate is also rich in flavonoids, compounds known for their antioxidant properties, which protect cells against damage caused by free radicals, and for their role in promoting cardiovascular health by reducing inflammation and improving vascular function. Among the main flavonoids present in yerba mate are rutin, luteolin, and quercetin, the latter being one of the majority of flavonoids in the composition . In Figure , it can be seen that, in step II, the complex formed involves the sharing of electrons between the surface of the nanoparticles and the organic components of the extract, represented by "R" . Although "R" represents the majority of constituents of the yerba mate extract, other components may also be present . Subsequently, high-temperature calcination promotes the formation of CuO-NPs (step III). |
67c460aa81d2151a02b8d0b4 | 27 | Arun et al. reported the formation of a metal complex between copper and the bioactive in Bombax ceiba extract, where the polyphenolic rings, such as vitexin, attract copper ions and, when decomposed at 400 °C, generate CuO NPs. Plant extracts are rich in low molecular weight primary and secondary metabolites, including bioactive compounds such as flavonoids, phenols, alkaloids, |
67c460aa81d2151a02b8d0b4 | 28 | Polyphenols, in particular, are fundamental in the synthesis of nanoparticles due to the ability of their phenolic hydroxyl groups and oxygen atoms to form metal-phenolate complexes through chelating interactions . When these complexes are subjected to calcination at 400 °C, they decompose, resulting in the formation of nanoparticles. This behavior was also observed in the study by Sharma et al. , where the facility of polyphenols to donate electrons promoted the formation of the nickel-phenolate complex, which, after calcination, led to the production of NiO nanoparticles. |
67c460aa81d2151a02b8d0b4 | 29 | Ellagic, gallic, and tannic acids, along with other polyphenols and phenolic derivatives such as dihydroxybenzene, are often mentioned in the literature for their ability to form stable complexes with metal ions . The presence of hydroxyl groups in these plant polyphenolic compounds is fundamental for stabilizing these complexes with metal ions . All the peaks are sharp and strong, revealing the good crystallinity of the CuO nanostructures synthesized. |
67c460aa81d2151a02b8d0b4 | 30 | an amorphous pattern before this step. The crystallite size was calculated using the Sherer equation, resulting in 22.9 nm. Ahamed et al. synthesized copper oxide with a size of 23.43 nm using the precipitation method. Therefore, the result obtained is consistent with the literature. Furthermore, the presence of well-defined, sharp structural peaks in the XRD patterns and the crystallite size of less than 100 nm confirms the nanocrystalline nature of the synthesized oxide. |
67c460aa81d2151a02b8d0b4 | 31 | Particles can be made up of one or more crystallites with different orientations. If the crystallite is the same size as the particle, the particle is called monocrystalline. If the crystallite is smaller than the particle, i.e. when the particle is made up of more than one crystallite, it is classified as polycrystalline. For this reason, there can be differences between the size of the crystallite and the size of the particle in a material . |
67c460aa81d2151a02b8d0b4 | 32 | Figure shows the FTIR spectra of the CuO NPs. The band identified at 3410 cm -1 is attributed to the O-H stretching of the water absorbed by the sample during the test . According to Benhadria et al. , nanocrystalline materials have a high surface area concerning volume and, therefore, absorb moisture, which confirms the band in this region of the spectrum. |
67c460aa81d2151a02b8d0b4 | 33 | The absorption peaks around 1630 cm -1 and 1058 cm -1 can be attributed to the bending vibrations of the O-H bond, combined with the presence of copper atoms . Finally, according to Sagadevan et al. , the frequency peak identified at 520 cm -1 confirmed the presence of CuO NPs, characteristic of Cu-O bond vibrational frequencies. Thus, the IR spectra results suggest the presence of Cu-O bonds, with very characteristic bands and some constitutional water adsorbed on the copper oxide structure, consistent with studies in the literature . |
67c460aa81d2151a02b8d0b4 | 34 | According to Prakash et al. , the band in the 400-650 cm -1 range is widely reported in works in the literature that synthesize copper oxide. However, some studies have observed an additional peak at 1383 cm -1 , attributed to the stretching of the Cu +2 -O -2 bonds . This variation |
67c460aa81d2151a02b8d0b4 | 35 | According to the work of Kayani et al. , the band at 624 cm -1 is attributed to copper oxide with oxidation number +1, copper (I) oxide, and Cu2O, commonly known as cuprite. This band was not identified in the spectrum of the nanoparticles synthesized in this work. Instead, the presence of the band in the 520 cm -1 region indicates that the oxide is in the form of CuO, i.e., copper (II) oxide, with oxidation number +2, known as tenorite . |
67c460aa81d2151a02b8d0b4 | 36 | The zeta potential of the CuO NPs measured was -13.5 mV (Figure ). As previously described, visual observation of the plant extract after mixing with the metal precursor was the first signal for the biosynthesis of CuO NPs . The color change to dark greenish can be attributed to the vibration and resonance of charged ions on the surface of the NPs . A negative zeta potential indicates that the particles have a net negative charge on the surface, possibly due to the ionization of functional groups or the adsorption of negative ions. A zeta potential of -13.5 mV suggests moderate colloidal stability, leading to a tendency for agglomeration between the particles . In general, high zeta potential values (above ±30-40 mV) are indicative of more significant electrostatic repulsion between the particles, which helps prevent aggregation, keeps the particles more dispersed, and indicates good physical stability of the nanosuspension . |
67c460aa81d2151a02b8d0b4 | 37 | This stability of NPs from plant extracts can be attributed to the protective agents in the plant extract . According to Suárez-Cerda et al. , electrostatic mechanisms promote the stabilization of NPs in colloidal solution. The presence of carbonyl groups, coming from the plant extract, on the surface of the CuO NPs suggests the formation of an electrostatic sphere, like an electrical charge, consequently causing repulsion between charged NPs during their growth phase. |
67c460aa81d2151a02b8d0b4 | 38 | The repulsion of free electrons on the surface of CuO NPs results in the formation of the maximum peak shown in Figure , with light intensity at the wavelength observed at its maximum absorbance . According to El-Trass et al. and Velsankar et al. , there is a proportional relationship between the content of CuO NPs present in solution with the wavelength at the maximum observed absorbance, where a value greater than 300 nm suggests a high amount of CuO NPs formed per volume of solution. Furthermore, the shift in the band of the UV-Vis spectrum between 250-800 nm is a result of the morphology, size, and state of aggregation of the NPs . |
67c460aa81d2151a02b8d0b4 | 39 | Morales-Lozoya et al. suggest that the broader spectrum of the UV-vis band indicates greater polydispersity of the NPs. Fouda et al. report that the presence of a single SPR peak in the UV-vis spectrum at a wavelength between 300-400 nm provides an additional indication of the shape and size of the CuO NPs, indicating that they are spherical with small size since as their dimensions decrease to the nanoscale the wavelength of the light becomes large with the size of the NPs . Saif et al. reported a similar absorption peak around 383 nm, whereas Narayanan et al. observed a peak near 400 nm. Therefore, the results obtained confirm the successful formation of CuO NPs, in agreement with studies reported in the literature . |
67c460aa81d2151a02b8d0b4 | 40 | As shown in Figure , the hydrodynamic diameter of the suspension containing CuO NPs, determined by the DLS, was 63.41 nm. This value is consistent with studies in the literature (Table ), where a larger cumulative distribution is observed between 40 nm and 70 nm. Particle size is a significant factor, since smaller particles have a larger surface area concerning their volume, contributing to the agglomeration and repulsion effects of NPs. It is important to mention that the average particle size using DLS analysis is affected by several factors, such as percentages of homogeneity (sizes increased with the inhomogeneous distribution of CuO NPs in the solution) and plant metabolites that cover the surface of the NPs, interfering with the size calculation . |
67c460aa81d2151a02b8d0b4 | 41 | The X-ray Fluorescence Energy Dispersive Spectroscopy (EDX) results indicated that the structures formed by the nanoparticles are composed of oxygen and copper, strongly suggesting the formation of CuO NPs, as illustrated in Figure . Clear peaks in the spectrum at approximately 0.5 keV correspond to the electronic transitions of oxygen, while the peaks at 1.0 keV, 8.1 keV, and 8.9 |
67c460aa81d2151a02b8d0b4 | 42 | The EDX analysis confirmed the presence of CuO NPs, giving a ratio of 34.42% oxygen and 65.58% copper, corroborating that the main components of the biosynthesized sample are copper and oxygen. The absence of additional elements in the EDX spectrum suggests a high purity of the nanoparticles. Overall, the EDX results validated the chemical composition of the CuO NPs and the effectiveness of the synthesis method employed . |
67c460aa81d2151a02b8d0b4 | 43 | The tendency for NPs to agglomerate may be due to factors such as van der Waals intermolecular attractions, which can lead to particle aggregation when they are very close to each other, as well as the small size of CuO NPs providing a larger surface area regarding its volume, thus increasing the probability of agglomeration . |
67c460aa81d2151a02b8d0b4 | 44 | After characterizing the CuO NPs, we aimed to verify their effects on seeds of agronomical interest, such as tomatoes and lettuce. The effects of CuO NPs in plants are still poorly understood and the data available is inconsistent, largely due to the variety of experimental conditions of the different studies. The mechanisms by which CuO NPs act in plants are also unknown, but there is a strong possibility that they interact with reactive oxygen species (ROS) and may interfere with or improve cell metabolism . |
67c460aa81d2151a02b8d0b4 | 45 | In seeds treated with CuO NPs, 30-50% of seeds germinated on the second day of the experiment, while control seeds started germinating only on the third day, a result that was also reflected in the 2.26-fold increase observed in the GSI. In the case of lettuce seeds, the treatment of 25 mg/L presented a significantly higher germination percentage than the control on the first day (p < 0.05). About 80% of seeds treated with 25 mg/L of CuO NPs germinated, while 57% of seeds germinated in the control (Figure ). |
67c460aa81d2151a02b8d0b4 | 46 | CuO NPs application has either a neutral or inhibitory effect on seed germination , with few showing positive effects . However, in all cases, the application method was adding the CuO NPs to the substrate (usually filter paper). In this work, seeds were immersed in CuO NP solutions for 5h only and then transferred to Germitest papers moistened with distilled water. We speculate that this treatment was sufficient to raise the ROS levels to the "oxidative window for germination" while avoiding the detrimental effects of excessive ROS. Excessive amounts of Cu can interfere with photosynthetic and respiratory processes, enzyme synthesis and plant ultrastructural development . This may also explain why the lowest CuO NPs concentrations improved seed vigor while higher concentrations presented no such effects. |
67c460aa81d2151a02b8d0b4 | 47 | Post-germination growth showed that CuO NPs resulted in longer roots and shoots for both species, especially in the concentrations of 25 mg/L and 50 mg/L (Figure ). Cu is an essential cofactor for many enzymes, playing crucial roles in several processes, such as photosynthesis, respiration, nucleotide metabolism, cell wall formation, reproductive development, and stress tolerance . This can explain the increased growth in shoot and root observed in seeds treated |
67c460aa81d2151a02b8d0b4 | 48 | with 25 and 50 mg/L of CuO NPs, as illustrated by lettuce seedlings (Figure ). An increase in dry mass was also detected in lettuce seeds treated with 25 mg/L CuO NPs (Figure ). Root and shoot growth show various responses to CuO NPs, having positive, neutral, and negative effects depending on the concentration and CuO NPs production. Essa et al. also reported an increase in the seedling growth of wheat after the application of CuO NPs in concentrations of 0.03 and 0.22mg/L, which is a similar concentration to what was used in our experiment. As pointed out by Feigl , |
67c460aa81d2151a02b8d0b4 | 49 | There were only a few studies showing the effects of CuO NPs on seeds. Tomato seeds presented an increased root and shoot growth in lower concentrations of CuO NPs (20-200 mg/L), while higher concentrations caused a negative effect, possibly due to oxidative stress . Both shoot and root growth in lettuce appear to be negatively affected by CuO NPs if provided by foliar application . The data presented here mainly showed a positive effect for both species, which may be related to the application method and, perhaps, the consequence of faster germination. |
67c460aa81d2151a02b8d0b4 | 50 | The synthesis of copper oxide nanoparticles (CuO NPs) using yerba mate extract proved to be a sustainable and efficient approach, eliminating the need for aggressive precipitating agents and reducing environmental impact. Characterization confirmed that crystalline CuO NPs were obtained, with an average crystallite size of 22.9 nm and a hydrodynamic diameter of 63.41 nm. The conversion yield was strongly influenced by the reaction temperature, reaching a maximum of 26.75% at 80°C and 2 g of copper nitrate. The proposed synthesis mechanism indicated the formation of an intermediate complex between copper and yerba mate bioactives, the calcination of which resulted in the formation of nanoparticles. FTIR, XRD and UV-Vis spectroscopy analyses corroborated the presence of pure CuO, while zeta potential analysis (-13.5 mV) indicated moderate colloidal stability. |
67c460aa81d2151a02b8d0b4 | 51 | The effect of CuO NPs on the germination and growth of Lactuca sativa and Solanum lycopersicum revealed significant benefits at the initial stage of seedling development. Soaking the seeds in a 25 mg/L solution of CuO NPs accelerated germination, with a 2.26-fold increase in the germination speed index (GVI) of tomato seeds and a 40% higher germination rate on the first day for lettuce seeds compared to the control. In addition, the application of NPs promoted superior root and shoot growth, indicating a promising potential for their use in agriculture. |
67c460aa81d2151a02b8d0b4 | 52 | The findings suggest that CuO NPs may promote germination without causing phytotoxic effects, possibly by modulating reactive oxygen species (ROS) levels within the optimal oxidative range for germination. The development and dry mass of the seedlings were significantly increased by the lowest concentrations of CuO NPs (25 mg/L and 50 mg/L), whereas greater concentrations did not exhibit the same advantages. |
67c460aa81d2151a02b8d0b4 | 53 | These results highlight the potential of CuO NPs to stimulate field research and help reduce the use of conventional fertilizers and promote more sustainable agricultural management. However, long-term studies and trials under real field conditions are needed to assess the viability of this technology in agricultural systems. It is also essential to study the interaction of CuO NPs with conventional fertilizers and to carry out ecotoxicological studies to ensure the environmental safety of their application. In this sense, we suggest that future work should investigate the optimization of CuO NPs concentrations for different soil types and crops, as well as the assessment of long-term ecological effects, in order to better understand the applicability of NPs in the field. |
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