Mechanistic Analysis and Control of Electro‐Fabrication with Soft Matter: Polysaccharide Self‐Assembly with Electrochemically Produced Metal Ions

Electro‐fabrication is an emerging additive manufacturing approach that includes a variety of mechanisms and crosslinking chemistries to induce the self‐assembly‐ of soft matter. Within electro‐fabrication techniques, anodic electro‐fabrication is of great interest, because it can use the electrode as a sacrificial source of metal ions for the controlled formation of chelated hydrogels. However, it remains challenging to understand how the various solution conditions (e.g., buffer capacity, ligands) and imposed voltages must be controlled to achieve the chelation and sol‐gel transition. Herein, a comprehensive thermodynamics‐based framework is built to guide the selection of conditions for the self ‐assembly of polysaccharide hydrogels by crosslinking with transition metal ions generated via corrosion. This methodology is demonstrated through real‐time spectro‐electrochemical monitoring and employed for the electro‐fabrication of chitosan hydrogels chelated with Au3+, Ag+, Cu2+, and Pd2+, and alginate hydrogels chelated with Cu2+. The presented guidelines are intended to extend the capabilities of the controlled fabrication of assemblies crosslinked with metal ions, opening new horizons for unprecedented materials design with precisely tailored structures and functions.


Introduction
Chelated biomacromolecules form part of biology's time-testedmaterial's toolbox. [1]These play a fundamental role in the DOI: 10.1002/admi.202300543outstanding properties of hard protective shells, [2] fangs, [3] and bio-adhesives, [4] to name a few.These properties stem from the interplay between the metal ion's coordination sphere and the ligand strength of the polymer's anchor points, e.g., functional groups.This is defined as supramolecular complexation or chelation, and it provides a versatile means for noncovalent crosslinking, endowed with functionalities that are derived from the presence of organometallic crosslinking sites.This has been of interest to the field of soft chemistry, as the physicochemical properties of chelated hydrogels can be tailored to suit applications in self healing, [2,[5][6][7][8][9] antibiotic, [6,[10][11][12] wound healing, [12][13][14][15] drug delivery, [16,17] mechanical actuation, [6,[18][19][20][21][22][23] stimuli responsive materials by redox [24] or homogeneous reactions, [7,[25][26][27][28] catalysis, [29][30][31] sensing, [32,33] bioinspired materials with fluorescent, [34] adhesive properties, [4] among others.Seeking to harness the capabilities of these materials, several publications have evaluated the interaction of metal ions and biomacromolecules, most notably, histidine-containing proteins, [3,7,35] alginate, [29,36] chitosan, [13,25,[37][38][39] and derivatives. [6,11,12,30,40]However, the controlled synthesis of chelated hydrogels remains challenging. [3,11,12,25]lectrochemistry with sacrificial metal electrodes provides an exciting opportunity for the generation of metallic centers for the fabrication of chelated hydrogels.Particularly, it offers unprecedented abilities for the spatio-temporal synthesis of these materials because it is possible to achieve in situ loading of metal-ions into a hydrogel, [18][19][20][21][22] or polymeric solution to trigger chelation and crosslinking. [32,33,41]Naturally, this approach requires high control of metal ion oxidation state and speciation.From this frame of reference, we identified an opportunity of expanding the theory of controlled synthesis of chelated hydrogels by formally introducing corrosion thermodynamics.Herein, we propose a framework to translate these complex systems into comprehensive overviews of the interplay between a given metal, polysaccharide, electrolyte, imposed voltages, pH, buffer capacity and ligand chemistry.We employ the seminal work on corrosion by Marcel Pourbaix, the potential-pH diagrams, also known as Pourbaix Diagrams. [42]These diagrams intuitively describe the thermodynamics of an extensive set of competing reactions within the chemical environment. [43]Therefore, we believe that they can describe the experimental requirements to attain the electro-fabrication of chelated hydrogels.Here we introduce, for the first time, Pourbaix theory as a guiding tool to drive hydrogel electro-fabrication via metalion release, that is, corrosion.
We successfully demonstrated our method with materials known for possessing significant levels of corrosion resistance, particularly noble metals such as Au, Ag, Pd, and Cu, which are electrochemically noble.These materials are historically perceived as having low reactivity and hold an important biomedical relevance as mentioned in Table S1 (Supporting Information).We have opted to work with noble metals due to their distinct corrosion properties.This deliberate choice facilitated the creation of discrete case studies, each exploring a distinct parameter associated with the electrofabrication of chelated hydrogels within an electrochemical framework.
This manuscript outlines five specific objectives to guide our investigation into electro-fabrication processes employing Pourbaix diagrams: 1. Electro-fabrication of Chelated Hydrogels via Corrosion: We explore the electro-fabrication of chelated hydrogels using Pourbaix diagrams as a guiding tool.In pursuit of these objectives, we conducted a comprehensive parametric analysis involving the four metals, two polysaccharides, two acid species, and two salts.Each parameter combination serves as a controlled experimental setting, allowing precise exploration of the specified objectives.
Our study begins with gold and chitosan due to their transcendence as the most used electrode and polysaccharide for electrofabrication applications. [44,45]Subsequently, we introduce the use of either a weak and a strong acid, HOAc and HCl, to probe initial pH and buffer capacity effects.Next, we parametrize electrofabrication with gold and silver with two different salts, KCl and KNO 3 , to investigate the effect of salt selection.Then, we employ palladium to address the kinetic limitation of the framework.Finally, we use copper and alginate to showcase the second electrofabrication route encoded in the Pourbaix diagrams. [46]This selection of parameters and objectives not only contributes to an enhanced understanding of electro-fabrication processes but also underscores the utility and versatility of Pourbaix diagrams as a powerful tool in chelated hydrogel synthesis.
To the best of our knowledge, the electrochemical synthesis of chelated hydrogels with noble metals, in our case, gold and palladium, has not been previously reported.We believe the use of noble metals in this context has been precluded by a missing understanding of their corrosion mechanisms.A plausible root for the elusive link may have spurred from the common misconception that noble metals are electrochemically immune to corrosion.With this framework, we expect to evidence the strengths of understanding corrosion thermodynamic theory and its capabilities for the controlled synthesis of metallo-supramolecular structures and chelated hydrogels.

Electro-Fabrication of Chelated Hydrogels via Corrosion
Gold is often referred to as the most noble among noble metals because of its excellent stability and low reactivity under standard conditions.This property makes gold the most used electrode material in many electrochemical applications, including electro-fabrication. [47] However, gold is not electrochemically inert, it can undergo controlled dissolution in chlorinated media by the application of an external oxidative potential.This is visualized in the Pourbaix diagram for the Au-Cl-H 2 O system displayed in Figure 1a.Au-Cl-H 2 O Pourbaix diagram depicts which gold species is more thermodynamically favorable in a coordinate system of applied potential and pH.For example, at a potential of 0 V versus Ag/AgCl, Au 0 is thermodynamically favorable across the whole pH window.At a potential of 1.2 V versus Ag/AgCl we have two domains.At pH < 8.3, AuCl 4 − is the favored species, whereas at pH > 8.3 is Au(OH) 3 .Each boundary in these phase diagrams represents the theoretical equilibrium between species.Pourbaix diagrams portray three regions of interest, immunity, passivation, and corrosion, while considering an extensive set of possible and competing reactions.From the frame of reference of electro-fabrication of chelated hydrogels, it is desired to know how to dissolve metal ions from our metal source, in other words, how to achieve corrosion.In this section we will leverage the Au-Cl-H 2 O Pourbaix diagram to derive an unambiguous methodology for the electro-fabrication of a gold-chitosan complex, [Au(Ch)] 3+ .
We employed cyclic voltammetry to evaluate the effect of chitosan over the thermodynamics of gold corrosion.The cyclic voltammogram in the control solution (HOAc-KCl @pH 2.9) is displayed in Figure S3a (Supporting Information), and in chitosan (Ch-HOAcKCl @pH 4.4) is presented in Figure 1b.For both electrolytes, during the forward scan, an anodic current rise at E 0 > 0.8 V versus Ag/AgCl due to the formation of AuCl 4 − .Conversely, during the reverse scan, a cathodic peak resolve at a E 0 < 0.7 V versus Ag/AgCl due to the reduction of AuCl 4 − .These cyclic voltammograms accurately map the potential axis of the Pourbaix diagram at a pH of 2.9 and 4.4, as shown in Figure S3a (Supporting Information) and Figure 1b, respectively.
The voltammograms show that the anodic process is unaffected by chitosan, however, the monitored cathodic currents suggest the occurrence of an irreversible process in the chitosan solution.To evaluate this, we studied the reversibility of the system by analyzing the percentage of Au recovery for each cycle − ligand exchange with chitosan occurs (cyan dotted line) and chitosan's pK a (black dashed line).b) Cyclic voltammogram for gold in a chitosan solution (Ch-HOAc-KCl @pH 4.4).c) Au recovery plot in the control (HO-Ac-KCl @pH 2.9) and chitosan solution (Ch-HOAc-KCl @pH 4.4).d) Spectral trace of gold corrosion in the control solution (HO-Ac-KCl @pH 2.9).Inset: Schematic illustrating the electrochemical cell and optical detection of AuCl 4 − (aq).e) Correlation curve for the electrochemical and optical data at A 313 .The color of the data points indicates t referenced to the color bar in (d).f) Spectral trace of gold corrosion in a chitosan solution (Ch-HOAc-KCl @pH 4.4).Inset: Schematic that the formation of [Au(Ch)] 3+ (gel) interrupts the mass transport of aqueous Au 3+ species to the spectrometer's beam path.g) Photograph of the electro-fabricated [Au(Ch)] 3+ (gel).h) Absorbance spectra of [Au(Ch)] 3+ (gel) and control solutions.i) [Au(Ch)] 3+ (gel) coordination schematic.
of the cyclic voltammogram (Figure 1c).We defined recovery as percentage of AuCl 4 − reduced into Au 0 normalized over the produced AuCl 4 − for each cycle of the cyclic voltammogram, that is: where q C e and q A e are the cathodic and anodic charge transfer per cycle, respectively.For the control solution (HOAc-KCl @pH 2.9) Au recovery remains constant for all cycles, which is expected from a reversible electrochemical system.Conversely, for the chitosan solution, during the initial cycles, Au recovery is lower but at each subsequent cycle it increases.This is accompanied by the formation of a gel over the gold electrode, suggesting that the AuCl 4 − produced at the anodic cycle reacts with chitosan, forming the gel, deemed [Au(Ch)] 3+ (gel), which decreases the amount of available AuCl 4 − to be reduced during the firsts cathodic cycles.
Thus, the formation of the gel follows a reversible electrochemical reaction (E r ) followed by an irreversible chemical reaction (C i ), The gradual increment of Au recovery indicates that that C i is kinetically slower than the forward reaction E r , thus [Au(Ch)] 3+ (gel) encapsulates the subsequently produced AuCl 4 − , increasing its concentration-and availability to be reduced-near the electrode.
The Pourbaix analysis and cyclic voltammetries results can be used to forecast the corrosion behavior of gold at a fixed potential.To verify the products of gold corrosion, we perform real-time spectrophotometry coupled to chronoamperometry (realtime monitoring of current at a constant applied potential).Figure 1d shows the spectra obtained during a chronoamperometry at 1.2 V versus Ag/AgCl in the control solution (HOAc-KCl @pH 2.9).The strong absorbance peak at 313 nm is well identified as AuCl 4 − ligand charge-transfer band. [48]From the calibration curves (Figure S3b, Supporting Information), we found ɛ 313 = 5.41 ± 0.052 mM −1 cm −1 , value comparable to previous works, where AuCl 4 − 's ɛ 312.5 and ɛ 314.5 in HCl are reported as 5.6 and 5.56 mM −1 cm −1 , respectively. [49,50]The spectroelectrochemical correlation (Figure 1e) confirms the linear relationship (R 2 = 0.9943) of n Ox (t) and n e (t), yielding a calculated charge transfer of z Exp = 3.022 and corrosion efficiency K = 99.25%.These results demonstrate that, under these experimental conditions, AuCl 4 − formation is thermodynamically favored with no competition, i.e., neither oxygen evolution (OER) nor electrochlorination reactions are significant.By performing the same experiment in the chitosan solution (Ch-HOAc-KCl @pH 4.4), 0.63 μmol of AuCl 4 − were dissolved into the solution but no spectral peak of aqueous Au 3+ complexes was detected by the spectrometer, as displayed in Figure 1f.This is attributed to AuCl 4 − chelation with chitosan at the vicinity of the electrode's surface, crosslinking the polysaccharide and producing the gel [Au(Ch)] 3+ as seen in Figure 1g.This sol-gel transition at the surface of the working electrode results in what has been deemed as chitosan's anodic electrodeposition, [41] or coordinated electrodeposition. [32]he sol-gel transition of chitosan triggered by AuCl 4 − is attributed to a ligand exchange of Au 3+ coordination sphere with chitosan functional groups. [48]To evaluate this, we studied the UV-vis absorption of AuCl 4 − solutions versus the electrofabricated [Au(Ch)] 3+ (gel).Figure 1h shows that the electrofabricated [Au(Ch)] 3+ (gel) does not exhibit the characteristic AuCl 4 − ligand charge-transfer band.Notably, the spectrum of AuCl 4 − is invariant at pH between 2.9 and 4.4 (blue dotted line in Figure 1h), ruling out changes due to the basicity of chitosan.The lack of AuCl 4 − charge transfer band in the hydrogel spectrum might suggest that it does not contain gold ions, however, after two days, the gel develops a surface plasmon resonance absorption band, characteristic of gold nanoparticles (AuNPs), [48] confirming the initial presence of Au 3+ in the gel.This is expected, as chitosan is a well-known reducing agent for AuNPs. [51,52]igure 1i shows the hypothesized coordination of Au 3+ and chitosan.This coordination was proposed by Vo, K. et al., [48] and Pestov, A. et al., [51] and, although it was derived for aqueous solutions of [Au(Ch)] 3+ (aq) complexes, it accurately illustrates chitosan gelation with Au 3+ , where Au 3+ functions as bridging ligand of chitosan chains, crosslinking it.

Understanding Initial pH and Buffer Impact
Chitosan chelation with Au 3+ occurs at pH >3.5, [48] we illustrate this in the Au-Cl-H 2 O Pourbaix diagram as a vertical cyan dotted line inside AuCl 4 − stability region.The formation of [Au(Ch)] 3+ complex is thermodynamically favorable to the right of this line.The rightmost limit of this area is defined at chitosan's pK a (≈6.3), because, above this pH, chitosan undergoes its pHactuated sol-gel transition, we draw this limit on the Pourbaix diagram as a black dashed vertical line.The area enclosed by these limits is the theoretical region where the formation of [Au(Ch)] 3+ complex is thermodynamically favorable (depicted as the purple region in Figure 1a).Therefore, to achieve the electro-fabrication of [Au(Ch)] 3+ (gel), we must induce Au corrosion and we must control the pH in two ways: (i) ensuring that the initial pH of the experiment lies within the pH window of [Au(Ch)] 3+ complex formation, and (ii) preventing pH excursion during corrosion.The latter requirement is non-trivial because, during gold corrosion, local pH changes are bound to occur, not from OER, but from Au 3+ Lewis acid activity, as described by the following reactions: Therefore, buffer capacity is required to confer control over chitosan sol-gel transition.To evaluate this, we conducted [Au(Ch)] 3+ (gel) electro-fabrication with chitosan solutions prepared with either HOAc or HCl, pristine or titrated-that is, without and with pH adjustment, respectively.As expected, chitosan gelation does not take place in Ch-HCl-KCl @pH 1.9 (Figure S4, Supporting Information), because this pH falls outside the region of [Au(Ch)] 3+ .Conversely, the comparison between the titrated solutions, Ch-HOAc-KCl @pH 5.3 versus Ch-HCl-KCl @pH 5.3, highlights the importance of buffer capacity.For both solutions, [Au(Ch)] 3+ gelation was observed (Figure S4f,I, Supporting Information) but, in the solution prepared with HCl, Au 3+ species are detected in the aqueous phase.Metal corrosion inevitably leads to the release of Lewis acids at the electrodeelectrolyte interface.In the solution prepared with HCl, the pH changes derived from the release of AuCl 4 − caused a drift in the pH near the electrode, outside of [Au(Ch)] 3+ window, preventing proper crosslinking.Thus, buffer capacity is required to maintain the condition at which gelation is thermodynamically favored during corrosion.

Evaluating Salt Effects
The previous sections relied on Cl − ability to promote gold corrosion, however, this should not be generalized.Corrosion and passivation mechanisms can vary significantly in multiligand (M-L-H 2 O) systems. [53]To evaluate this, we conducted experiments aimed at electro-fabricating [M(Ch)] n+ (gel) using gold and silver corrosion in chlorinated or unchlorinated electrolytes.
The Pourbaix diagram for the Au-H 2 O system (Figure 2a) shows that the electro-fabrication of [Au(Ch)] 3+ (gel) is not thermodynamically favorable in unchlorinated electrolytes.Figure 2b,c shows the comparison between cyclic voltammograms for gold in Ch-HOAc @pH 5.3 solutions with either KCl or KNO 3 .Au-H 2 O Pourbaix diagram accurately maps the redox processes in the voltammogram for the nitrated solution (Ch-HOAc-KNO 3 @pH 5.3), where the half wave potential (ii) matches with the theoretical boundary of Au/Au(OH) 3 redox equilibrium.However, the same diagram fails to correlate with the cyclic voltammogram for the chlorinated solution (Ch-HOAc-KCl @pH 5.3), where the half wave potential (i) is shifted to a less anodic potential as it corresponds to the theoretical equilibrium of Au/AuCl 4 − (red dashed line).As expected, [Au(Ch)] 3+ (gel) formation did not occur in the unchlorinated electrolyte.Silver is the opposite case, as depicted in the Pourbaix diagrams for the Ag-H 2 O and Ag-Cl-H 2 O systems (Figure 2d,f).In an unchlorinated electrolyte, Ag + is thermodynamically stable under conditions suitable for [Ag(Ch)] + (gel) electro-fabrication (at potentials above 0.23 V versus Ag/AgCl, and at pH between 3.74 [25] and 6.3).We demonstrated this by attempting to deposit chitosan onto silver in either an unchlorinated (Ch-HOAc-KNO 3 @pH 5.3) or a chlorinated (Ch-HOAc-KCl @pH 5.3) solution.For the former solution, chitosan solgel transition takes place (Figure 2e), whereas in the latter it does not (Figure 2g).These findings underscore two key points, the Pourbaix diagram enable predictions regarding the thermodynamic favorability of electro-fabricating [M(Ch)] n+ (gel), and it emphasizes the importance of accurately constructing the diagrams, as a M-H 2 O diagram does not necessarily describes a M-L-H 2 O system and vice versa. [53]For the sake of completeness, we also evaluated the methodology with copper for the electro-fabrication of [Cu(Ch)] 2+ .The results are explained in detail in Supporting Information, including Figure S5 (Supporting Information).

Uncovering Kinetic Limitations
While Pourbaix diagrams can be employed as "maps" for tracing electro-fabrication pathways, one must not forget that these are built solely from a thermodynamic point of view and no kinetic data is conveyed.Thus, some corrosion products might not be practically achieved.For this section, we present palladium as model metal, which its corrosion to PdCl 4 2− is impeded by chitosan.The Pd-Cl-H 2 O Pourbaix diagram (Figure 3a) suggests that [Pd(Ch)] 2+ electro-fabrication is theoretically possible.Figure 3b,c present the cyclic voltammograms of palladium corrosion in the control (HOAc-KCl @pH 2.9) and chitosan solutions (Ch-HOAc-KCl @pH 4.4), respectively.Mapping the cyclic voltammogram of the control solution onto the Pourbaix diagram indicates that the anodic peak (j) at E 0 > 0.4 V versus Ag/AgCl corresponds to the formation of PdCl 4 2− .Conversely, for the chitosan solution this peak (jj) is notably absent.These results suggest that chitosan prevents palladium corrosion into PdCl 4 2− , a critical step required for the electro-fabrication of the [Pd(Ch)] 2+ hydrogel.
The spectro-electrochemical analysis shows palladium corrosion to PdCl 4 2− at 0.6 V versus Ag/AgCl in the control solution (Figure 3d).58] Our results demonstrated the linear correlation (R 2 = 0.9992) between n Ox (t) and n e (t), we note that the corrosion efficiency K = 86.5% (Figure S6c, Supporting Information), indicating that more charge (2.31 e − ) was needed to produce each Pd 2+ , compared the theoretical expectation (2e − ).This discrepancy in current stem from unaccounted reactions or a cumulative error from the calibration curve, where K = 100% falls within ɛ 279 standard deviation.For the chitosan solution, the spectro-electrochemical analysis for [Pd(Ch)] 2+ electro-fabrication (Figure 3e) shows that no Pd 2+ species are detected in the aqueous phase.During this experiment, a gel formed around the palladium electrode, as shown in Figure 3f, however, the deposition was not as successful as the previous cases, as chitosan prevented palladium corrosion.
To assess the impact of chitosan on PdCl 4 2− production, we conducted a comparative analysis of the corrosion rates of gold, copper and palladium in their control and chitosan solutions.Coulometries (cumulative charge transfer as a function of time at an applied constant potential) of gold, copper, and palladium, in either a control or chitosan solution, are portrayed in Figure 3g-i, respectively.For the timescale of [Au(Ch)] 3+ (gel) electro-fabrication (1 min), gold corrosion rates do not change significantly in the control (HOAc-KCl @pH 2.9) versus chitosan solution (Ch-HOAc-KCl @pH 4.4).In the case of copper, the electro-fabrication was carried out in 4 min, and its corrosion rate in the chitosan solution (Ch-HOAc-KCl @pH 5.3) is approximately half of the control (HOAc-KCl @pH 2.9).Conversely, for palladium, its corrosion rate is ≈50 times smaller in the chitosan solution (Ch-HOAc-KCl @pH 4.4) as compared to the control (HOAc-KCl @pH 2.9), for a 9 min experiment.These results indicate that, although we can define a theoretical region in the Pourbaix diagrams at which [M(Ch)] n+ electrofabrication is thermodynamically favorable, it does not mean that the product is kinetically viable.This kinetic consideration becomes apparent when comparing cyclic voltammograms, a crucial element of our framework-Recall the contrast of palladium voltammograms in the control (Figure 3b) and chitosan solution (Figure 3c).
In this section, we described kinetic limitation of the Pourbaix diagram, by exhibiting that [Pd(Ch)] 2+ (gel) electro-fabrication at an applied constant potential might not be practical in terms of time.However, the electrochemical dissolution of metals that are kinetically less susceptible to corrosion can be achieved at practical corrosion rates by employing cyclic techniques. [59]In fact, [Pd(Ch)] 2+ (gel) electro-fabrication was remarkably effective during palladium's cyclic voltammetry in the chitosan solution (Figure S6e, Supporting Information).This is discussed further in Supporting Information, including Figure S6 (Supporting Information).

Speciation-Driven Electro-fabrication
We have demonstrated how to control the local electro-fabrication of chitosan by driving a reaction along the vertical axis of the Pourbaix diagram (i.e., corrosion).Herein, we demonstrate that a horizontal pathway to electro-fabrication is also possible by the electrochemical control of metal oxides speciation.We employ alginate as model polysaccharide to trace electrofabrication routes along the horizontal axis of a copper Pourbaix map.We note that this approach of electro-fabrication is not suitable for chitosan due to the requirement of an initial basic pH.
Conventionally, alginate hydrogels are synthesized by complexing alginate with divalent cations, most commonly Ca 2+ .Its electro-fabrication has been achieved by electrochemically controlling Ca 2+ release by CaCO 3 speciation. [46,60]Given that Cu 2+ serves as a crosslinker Alg, [36,61] we can translate the principle of Ca 2+ speciation technique with Cu 2+ system, from a colloidal suspension of Cu(OH) 2 .Briefly, two sodium alginate (3% m/) solutions were prepared: 1) for the corrosion method, the solution contained 100 mM HOAc, and 2) for the speciation method, the solution contained a colloidal suspension of 25 mM Cu(OH) 2 .The first solution was paired to a copper electrode, and the electro-fabrication was carried out by applying a potential of 0.4 V versus Ag/AgCl for 4 min.The second was paired to a Platinum electrode, and the electro-fabrication was carried out by applying a potential of 2 V versus Ag/AgCl for 4 min.Both methods electrochemically control the release of Cu 2+ .However, the former is an EC reaction mechanism (Corrosion, Chelation/Crosslinking; Figure 4b), while the latter follows an ECC (Electrolysis, Speciation, Chelation/Crosslinking; Figure 4c).
We can describe both systems in terms of the Pourbaix diagram as shown in Figure 4a, where we condense the theory of copper corrosion and Cu(OH) 2 speciation.The corrosion approach is depicted as a vertical arrow, indicating that, by manipulating the applied potential axis, we drive the required heterogeneous reaction Cu ⇄ Cu 2+ + 2e − .The speciation method is depicted as a horizontal arrow, showing that it is necessary to lower the pH to drive the homogeneous reaction Cu(OH) 2 + 2H + ⇄ Cu 2+ + 2H 2 O.As discussed in the Buffer capacity and pH section, the corrosion pathway requires pH control from a buffered media, if not, the system could drift towards lower pH values-depicted as a dotted curved arrow in Figure 4a.On the contrary, the speciation method benefits from an unbuffered media, because the metal ion precursor would not compete for the hydronium ions generated by water electrolysis.This conception showcases the strength of building electrofabrication pathways of chelated hydrogels through the Pourbaix diagram, as it encodes the ingredients of the experimental setup and the electrochemical reaction mechanisms involved in the process.

Conclusion
In this study, we applied the Pourbaix analysis to guide the electro-fabrication of chitosan and alginate chelated hydrogels via self-assembly.The introduction of corrosion thermodynamic theory provides a missing piece of the puzzle toward understanding the conditions to attain and control the electrochemical synthesis of these materials, that is, specific metallic complexes are required, and controlling their formation is essential to success.Through parametric testing, we demonstrated that this thermodynamic framework provides a more indepth understanding of the electrochemistry involved in the electro-fabrication mechanism, as well as the significance of the components in the system.We anticipate that our conceptual framework can be extrapolated to other metals and biomacromolecules.Thus, we believe that these foundations can strengthen the toolbox of chelated softmaterial synthesis.
Preparation of Solutions: Chitosan (Ch) solutions of 1% mass by volume (m/) were prepared by dissolving Ch in either 100 mM acetic acid (HOAc) or 100 mM HCl solutions under stirred conditions.Undissolved residues were filtered out.The pH of these solutions was measured to be 4.4 ± 0.1 and 1.9 ± 0.1, respectively.From these, another set of solutions was prepared by titrating with stochiometric amounts of NaOH to obtain a pH of 5.3 ± 0.1.To each solution, appropriate amounts of either KCl or KNO 3 were added to reach a concentration of either 150 mM Cl − or NO 3 − .Another set of solutions with equal contents of acids and salts were prepared without Ch.
To facilitate clear and consistent identification of these solutions throughout the study, a naming convention based on their constituents and pH values were adopted, e.g., a solution of Ch (1% m/), 100 mM HOAc, and 150 mM KCl at pH of 4.4 was referred to as Ch-HOAc-KCl @pH 4.4, the same solution without Ch was HOAc-KCl @pH 2.9-the pH difference was due to Ch amine groups basicity-and the titrated version was Ch-HOAc-KCl @pH 5.3.
Experimental Setups and Experiments: Spectro-electrochemical(SEC) studies were carried out with a modular UV-vis spectroscopy coupled to a mobile electrochemical workstation (PalmSens4, PalmSens), as displayed in Figure S1a (Supporting Information).The spectroscopy setup consists of a deuterium tungsten-halogen light source (DH-3P-CAL, Ocean Insight), a temperature-controlled cuvette holder (qpod 2e, Quantum Northwest; BATH 10/US, Quantum Northwest) and a high-performance UVvis spectrometer (QE Pro-ABS, Ocean Optics; FX, Ocean Insight) with a 200 μm slit.The light was transmitted through solarization resistant optic fibers (QP6002-SR, Ocean Insight).A three-electrode electrochemical cell was built inside a cuvette with either a gold, silver, copper, or palladium working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode (MF-2052, BASi), as shown in Figure S1b (Supporting Information).All measurements were conducted under stirred conditions at 298.15 K.For every measurement, a new disposable cuvette was used (UV-Cuvette macro 759 170, BRAND).Cyclic voltammetries (CV) were conducted under different potential ranges depending on the working electrode material: 0-1.2, −0.8-0.4,and 0-1.4 V versus Ag/AgCl for Au, Cu, and Pd, respectively.All cyclic voltammetries were performed with an initial anodic scan, starting from the open-circuit potential (OCP) of the cell, for 20 cycles at a scan rate of 50 mV s −1 .Cyclic voltammograms were reported following the IUPAC convention and, for all figures, the initial cycles were not plotted.Chronoamperometries (CA) were conducted by applying an external potential (E 0 ) of 1.2, 0.8, 0.4, and 0.6 V versus Ag/AgCl for Au, Ag, Cu, and Pd, respectively.
][64][65] By convention, the activities of solid species and metal ions were assumed to be 1 M and 1 μM, respectively.From the experimental conditions, Cl − activity was set to 110.715 mM-which corresponds to a Cl − concentration of 150 mM at 298.15 K. [66] The equilibrium potential of Ag/AgCl (3 M NaCl) was set as 209 mV versus standard hydrogen electrode (SHE).Following the Pourbaix diagram's axes reference frame, from low to high potentials on the Yaxis and, from acid to basic media on the Xaxis, the thermodynamic equations were written according to the following conventions: HA where Red and Ox were the reduced and oxidized species, respectively, z, the number of electrons (e − ) transferred, E 0 Ox and E Ox were the standard and adjusted oxidation potentials, respectively, R, the molar gas constant, T, the temperature, F, the Faraday's constant,  Red and  Ox were the activities of Red and Ox, respectively, and HA was an acid that dissociates to its conjugate base (A − ) and a proton (H + ) with an acid dissociation constant K a = 10 −pK a .All equations were computed at 298.15 K. Intermediary speciation transitions, for example AuCl })] 3+ (12)   where Ch{NH 2 }, Ch{OH C3 }, and Ch{O Glycosidic } refer to the amine, hydroxide at C3 position, and glycosidic oxygen of Ch, respectively.However, for clarity, the complex as [Au(Ch)] 3+ was referred.This notation was generalize to the other metals as: M n+ + Ch ⇄ [M (Ch)] n+ (13)   where, M n+ was a metal ion M with n+ oxidation state.Data analysis and Interpretation: Molar extinction coefficients (ɛ), of the corrosion products were derived from the calibration curves of the absorbance (A) of known concentrations (c) of the species.Following the Beer-Lambert law, A  =   ⋅  ⋅ c, where A  was the absorbance measured at a wavelength , ɛ  the molar absorptivity at , and  the optical pathlength-which for all the experiments was 1 cm.For AuCl 4 − , 5 μl of a stock solution of 10 mM KAuCl 4 , 100 mM HOAc, and 110 mM KCl were added stepwise to a solution of 3 ml of 100 mM HOAc and 150 mM KCl.At each step, the characteristic absorbance of the ligand charge transfer band was recorded, for AuCl 4 − this peak was observed at 313 nm (A 313 ).A similar procedure was conducted for CuCl 2 and PdCl 2 .The absorbance of Ag + was out of the range of the spectrometer (<230 nm), [67] therefore, its corresponding spectroscopic data were could not acquire.Table S3 (Supporting Information) specifies the content of the solutions of each experiment and their respective reference.
Spectro-electrochemical correlation graphs were developed by associating amperometric and spectroscopic data.From the former, the moles of electrons (n e ) transferred during the experiment were calculated as where I(t) the current measured at time t, and t f was the time instance where the E 0 was reverted to OCP, i.e., the duration of the chronoamperometry.Naturally, ∫ t f 0 I(t)dt = q e | t f 0 , which was the amount of charge transferred during the chronoamperometry.From the latter, the moles of the corrosion product (n Ox ) was calculated as where  was the volume of the cell.Following the half-cell reaction, Red⇌Ox + ze − , this can define that, under ideal conditions, for every mol of Ox produced, z moles of e − were transferred, thus, Experimentally, n Ox (t) carries a time difference (Δt), due to mass transport of Ox from the electrode-electrolyte interface to the spectrometer's beam path, which leads to n Ox (t + Δt) = n e (t) ⋅ z −1 Exp .From these equations, the corrosion efficiency (K) was calculated as where z Th and z Exp were the theoretical and experimentally obtained values of z, respectively.

2 .
Understanding Initial pH and Buffer Impact: We analyze how initial pH conditions and buffer capacity influence electrofabrication.3. Evaluating Salt Effects: We assess the impact of salt selection on electro-fabrication outcomes.4. Uncovering Kinetic Limitations: We investigate kinetic constraints within Pourbaix diagrams and their practical implications.5. Speciation-Driven Electro-fabrication: We demonstrate an alternative electro-fabrication route guided by speciation considerations within Pourbaix diagrams.

Figure 1 .
Figure 1.Thermodynamics-based framework linking gel formation to corrosion for gold as model metal with chitosan.a) Au-Cl-H 2 O Pourbaix diagram.The purple region represents the theoretical area where [Au(Ch)] 3+ anodic electro-fabrication is thermodynamically favorable, it is enclosed between the pH at which AuCl 4− ligand exchange with chitosan occurs (cyan dotted line) and chitosan's pK a (black dashed line).b) Cyclic voltammogram for gold in a chitosan solution (Ch-HOAc-KCl @pH 4.4).c) Au recovery plot in the control (HO-Ac-KCl @pH 2.9) and chitosan solution (Ch-HOAc-KCl @pH 4.4).d) Spectral trace of gold corrosion in the control solution (HO-Ac-KCl @pH 2.9).Inset: Schematic illustrating the electrochemical cell and optical detection of AuCl 4 − (aq).e) Correlation curve for the electrochemical and optical data at A 313 .The color of the data points indicates t referenced to the color bar in (d).f) Spectral trace of gold corrosion in a chitosan solution (Ch-HOAc-KCl @pH 4.4).Inset: Schematic that the formation of [Au(Ch)] 3+ (gel) interrupts the mass transport of aqueous Au 3+ species to the spectrometer's beam path.g) Photograph of the electro-fabricated [Au(Ch)] 3+ (gel).h) Absorbance spectra of [Au(Ch)] 3+ (gel) and control solutions.i) [Au(Ch)] 3+ (gel) coordination schematic.

Figure 4 .
Figure 4. [Cu(Alg)] 2+ anodic electro-fabrication by corrosion vs speciation.a) Pourbaix diagram map for the electro-fabrication of [Cu(Alg)] 2+ via corrosion (red arrows) and speciation (purple arrow).The purple region represents the theoretical area where [Cu(Alg)] 2+ anodic electro-fabrication is thermodynamically favorable.Schematics depicting the differences between Alg electro-fabrication by b) Copper corrosion and c) Cu(OH) 2 speciation, including a photograph of the [Cu(Alg)] 2+ (gel) deposited by each method, respectively.The color of the [Cu(Alg)] 2+ (gel) deposited via speciation is due to the presence of residual colloidal Cu(OH) 2 entrapped in the gel.