CuO_x nanotubes via an unusual complexation induced block copolymer-like self-assembly of poly(acrylic acid)

Abstract

Polyelectrolyte (PEL) mediated synthesis of functional inorganic nano-materials is attractive due to its versatility and compatibility inside aqueous media. Block copolymers are often employed to facilitate the formation of novel nanostructures. In this work, we report the synthesis of copper oxide (CuO_x) crystals with an unexpected hollow nano-tubular morphology using only poly(acrylic acid) (PAA), i.e., a homopolymer. The quantification of the pH dependent binding limit of Cu²⁺ to PAA, which has often been given little emphasis previously, is found to be critical in understanding the formation mechanism of such nanotubes. Further quantification of the interdependent relationship of pH, α, and [Cu²⁺] : [COOH]₀ using the pK_a-α curves provided important insight. The formation of the CuO_x nanotubes is believed to be due to an unusual copolymer-like self-assembly of PAA as a homopolymer in aqueous solution; caused by interactions between Cu²⁺ and charged PAA chains. Such a unique copolymer-like self-assembly behaviour of the PAA–Cu²⁺ complex manifests within a specific window of [Cu²⁺] : [COOH]₀ ratio around the binding limit. Discussion is also carried out on the possibilities and potential limitations of applying this new concept to other PELs and metal ions.

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Introduction

Nano-sized metallic copper and its oxides possess good potential for catalytic,¹ photo-catalytic,² photovoltaic,³ and sensing⁴ applications. Their usage in bio-related fields including fouling control and nano-toxicology^5,6 is also being explored. CuO_x micro- and nano-crystals have been synthesized via different routes such as reduction under alkali conditions,⁷ double-jet precipitation,⁸ hydrothermal,² micro-emulsion,⁹ and solvothermal¹⁰ methods; usually with the presence of ligands or surfactants to render the desired particle size and dispersion. Although well-defined CuO_x crystallites with sizes of several nanometres can be readily prepared in organic media, aqueous medium synthesis is more desirable due to its biocompatibility and eco-friendliness. The synthesis of a wide range of CuO_x nanostructures and their assemblies in aqueous medium have been attempted.¹¹

Hydrophilic polymers, especially polyelectrolytes (PELs), can be employed as versatile templating agents in the aqueous synthesis of inorganic nano-crystals by utilizing the specific interactions between polymer chains and metal ions. The functions of these polymers are multifold, e.g., (i) as self-assembled nano-reactors;^12,13 (ii) facilitating anisotropic crystal growth via selective adsorption of polymer chains on crystal facets;¹⁴ and (iii) simply providing random confinement for the bio-mimetic growth of nano- or mesocrystals in a concentrated polymer solution or hydrogel.¹⁵

Due to its structural simplicity, biocompatibility, and availability in different molecular weights, poly(acrylic acid) (PAA), either as a homopolymer or in a block copolymer, represents one of the most widely employed PELs in inorganic nano-materials synthesis. The PAA hydrogel has been used as the medium for sol–gel synthesis of nano-particles (PAA in dispersed state)¹⁶ or bio-mineralization of mesocrystals (PAA in semi-solid state).¹⁷ PAA has also been employed for the random capping and confinement of synthesized nano-particles.^15,18 PAA often serves as the model PEL for the investigation of PEL–counterions interactions. Interactions between PAA and alkali, alkali earth, or transition metal ions have been studied through the measurement of viscosity, turbidity, optical absorbance, osmotic pressure, and enthalpy.^19–21

As another research direction, the theoretical and experimental studies of PEL–counterions interactions have been conducted for decades; focusing on counterions fraction, distribution and condensation based on principles of free energy or chemical potential.^22–24 This fundamental knowledge could have been more effectively used as guidelines for PEL mediated synthesis, judging by the critical influence of the PEL–counterions interaction towards the morphologies of synthesized products. However, most reports on PEL mediated synthesis have adopted empirical approaches and focused on correlating the crystallite morphology with synthesis parameters. The reported nano-materials syntheses typically used PELs within a relatively concentrated regime, representing one of the conditions when the theoretical models deviate.^15,25 Furthermore, many reports did not pay attention to the actual binding limits of Cu²⁺ onto available COO⁻ on a charged PAA chain and the variation of pH.^26,27 Therefore, we recognize that better bridging of the fundamental understanding on PEL–counterions interaction and nano-crystal formation mechanism can lead to more intelligent synthesis designs.

As is generally accepted, PAA–Cu²⁺ interaction involves electrostatic attraction, complexation, and counterion condensation; leading to phenomena such as intra- or inter-chain bridging and change of hydration structure.^23,28,29 PAA–Cu²⁺ interaction is known to be distinctly different from interactions between PAA and alkaline earth metal ions, e.g., Ca²⁺.³⁰ The mechanism is affected by the degree of deprotonation (α) that is pH dependent: Cu²⁺ may bind with COO⁻ either in 1 : 1 (low α) or 1 : 2 (high α) ratios²⁸ and form a bridging bidentate structure.²⁹ The chain conformation that changes depending on physicochemical conditions represents a unique characteristic of PELs. The change is reflected as measurable properties such as pK_a, hydrodynamic radius, and solution conductivity. There exist different theories of PEL chain conformation;^31,32 among them the necklace model for flexible PEL is useful in describing PAA. Precise morphological control requires not only the accurate quantification of both the binding limit of PEL–metal ions and pH variation, but also the understanding of their effects towards PEL chain conformation.

In this study, we firstly quantified the relationship among pH, α and [Cu²⁺] : [COOH]₀ ratio. Furthermore, we correlated them to the chain conformation that determined CuO_x nano-crystal morphologies obtained in our aqueous PAA-templated synthesis. The concentration of PAA in the slight semi-dilute regime we employed was critical in minimizing inter-chain interactions but still providing the chance for self-assembly and reasonable nano-crystals yield. In this study, we report a new phenomenon that, under carefully controlled conditions, the nano-crystals formed by the PAA–Cu²⁺ complex bear a striking resemblance to those formed via the self-assembly of an amphiphilic rod–coil–rod triblock copolymer. This has revealed one new dimension of nano-material synthesis by employing PEL as the morphology-directing template.

Experimental section

Materials

CuCl₂·2H₂O (Fisher Scientific) and poly(acrylic acid) (M_n = 124 500 g mol⁻¹, PDI = 1.25, Polymer Source) were purchased and used without further purification. Throughout the experiments of interaction studies and templated synthesis, the concentration of carboxylic acid groups (i.e. [COOH]₀) was fixed at 1.25 mM and an ionic background of 0.01 M NaNO₃ was used for all samples.

Titration experiments

Potentiometric titrations coupled with a cupric ion-selective electrode (denoted as ‘PT + Cu-ISE’ in future appearance) were carried out using a Metrohm Titrando 905 titration system connected with conductivity module 856. Cupric ion concentration (i.e. [Cu²⁺]) was measured using Cu-ISE (Orion 9629BNWP). All the titrations were done at 25 °C; temperature was regulated by linking the jacketed vessel to a water circulator. Two sets of titration experiments were designed and carried out:

Experiment A: Using PT + Cu-ISE to measure the unbound [Cu²⁺], to determine the [Cu²⁺] : [COOH]₀ binding limit of Cu²⁺ onto PAA at different initial pH. Titrating 20 mM CuCl₂·2H₂O (titrant, initial pH = 5) into 1.25 mM PAA of variable initial pH (titrand).

Conditions: pH 6.6, pH 6, pH 5.75, pH 5; only applied for PAA solution.

Experiment B: Studying the dissociation behaviour (pK_aversus α) via pH and conductivity measurements. Titrating 50 mM NaOH (titrant) into 1.25 mM PAA with different [Cu²⁺] : [COOH]₀ ratio (titrand).

Conditions: [Cu²⁺] : [COOH]₀ ratios = 0, 0.05, 0.075, 0.125, 0.175, 0.25, 0.375; pH of titrand was adjusted to ∼3.3 prior to the start of titration.

In all the titration experiments, 30 μL of titrant was titrated into 60 mL of titrand at 180 s interval.

PAA templated synthesis

The aqueous PAA-templated synthesis of Cu based nano-crystals was carried out. The desired amount (i.e. [Cu²⁺] : [COOH]₀ = 0.175) of CuCl₂·2H₂O solution (initial pH = 5) was mixed with PAA solution (initial pH = 6) and the mixture was stirred at room temperature for 14 h. After that, an excess amount of 1 M NaOH (32× higher than [COOH]₀ in mol) was added into the solution to induce hydrolysis and it was stirred further at room temperature for 5 h. The samples were heated subsequently at 55 °C for 3 h.

Characterization

Transmission electron microscopy (TEM) and selective area electron diffraction (SAED) were done using a JEOL 2010 TEM. UV-Vis absorbance was measured using a UV-Vis spectrophotometer (UV 2501PC, Shimadzu).

Results and discussion

Quantification of binding limit of Cu²⁺ onto PAA at different initial pH

In a PEL-templated synthesis, while it is necessary to understand the PEL–metal ions interaction mechanism, the importance of quantifying the binding limit of metal ions onto the PEL cannot be underestimated. Unfortunately, there are very few reports on the quantification of the actual binding limit of weak PEL, which is pH dependent and usually deviates considerably from the deduced binding limit which assumes 100% deprotonation. Therefore, on one hand, often only low nano-particle concentrations were obtained, indicating that the metal ion concentration was far below the binding limits.^15,18 On the other hand, if the metal ion concentration exceeded the binding limits significantly, larger sized crystals with poor size dispersity would be obtained.^26,33

Our synthesis strategy involves employing the PAA homopolymer for complexation with Cu²⁺ and the templated formation of CuO_x nano-crystals. This strategy relies on PAA with small molecular weight polydispersity (PDI). Our hypothesis stems from the notion that the state of metal ions (Cu²⁺ in this case) in the PEL solution (PAA in this case) considerably affects the morphological uniformity of the nano-crystals. This means that preventing the co-existence of bound and unbound Cu²⁺ ions on PAA chains can avoid non-uniform nano-crystal morphology. When Cu²⁺ ions are added into a PAA solution of a certain concentration and initial pH value, they interact with PAA via the well-known mechanism of rapid electrostatic interaction and complexation. The Cu²⁺ complexation onto PAA can be evidenced through changes in the absorbance spectra (see ESI, Fig. S1†). When the [Cu²⁺] : [COOH]₀ binding limit was reached during the titration of Cu²⁺ into the PAA solutions, further Cu²⁺ added became detectable by Cu-ISE. The bound Cu²⁺ ions were non-detectable. Fig. 1(a) plots the [Cu²⁺] detected by the Cu-ISE versus [Cu²⁺] : [COOH]₀ ratio at different initial pH (titration experiment A). It should be mentioned that although it is understood that the binding limit increases with pH of the PAA solution due to increasing α, all analyses in this work were carried out at initial pH values below 6.6. This was to avoid the possibility of Cu(OH)₂ formation due to hydrolysis, which adds complication to the particle morphological uniformity caused by divergence of the PAA–Cu²⁺ interaction manners.³⁴ Within the [Cu²⁺] range used in this study, Cu(OH)₂ is expected to only noticeably form at pH values above 7. The largest [Cu²⁺] : [COOH]₀ binding limit (at initial pH value of 6.6) is only 0.18, far below the theoretical ratio of 0.50. This large shortfall is certainly due to a combination of many factors including incomplete deprotonation and steric effect associated with PAA chain conformation.

Fig. 1 (a) Concentration of unbound Cu²⁺ measured by Cu-ISE during the titration of 20 mM CuCl₂·2H₂O (titrant, initial pH = 5) into 1.25 mM PAA (titrand) of different initial pH (i.e. 5, 5.75, 6, 6.6). The inset in the top-left corner is the 3D diagram showing the relationship among pH, α, and [Cu²⁺] : [COOH]₀; plotted based on the results of titration experiment B. (b) By using the interpolated α, the re-plotted curves of concentration of unbound Cu²⁺versus [Cu²⁺] : [COO⁻] shows that the binding stoichiometry converge towards [Cu²⁺] : [COO⁻] = 0.5.

It needs to be stressed again that the pH values discussed so far are the initial pH values of the PAA solutions prior to the addition of Cu²⁺; the pH is expected to decrease upon titration of Cu²⁺ into the PAA solution and this changes the α. α is a critical parameter in the PAA-templated synthesis because it is not only quantitatively linked to the bound metal ion concentration and the [Cu²⁺] : [COOH]₀ binding limit, it also determines PAA chain conformation. For example, whether PAA chains adopt expanded (at higher pH) or collapsed (at lower pH) conformation affects the accessibility of COOH for the deprotonation and accessibility of COO⁻ for Cu²⁺ complexation. Although these factors discussed above have significant effects on the morphology of the nano-crystals prepared, so far previous works on PEL-mediated synthesis of inorganic nano-materials have largely overlooked this important aspect.

We further approached the binding limit issue by quantifying the relationship among pH, α, and [Cu²⁺] : [COOH]₀. Firstly, by titrating NaOH into a mixture of PAA with different fixed [Cu²⁺] : [COOH]₀ ratios (titration experiments B), pK_a-α curves could be plotted based on the modified Henderson–Hasselbalch equation: pK_a = pH + log[(1 − α)/α] (the curves are included in the ESI, Fig. S3†). Furthermore, based on these pK_a-α curves, we constructed a 3D diagram of pH, α, and [Cu²⁺] : [COOH]₀, shown as the inset in Fig. 1(a) (the analysis details and high resolution 3D diagram are included in the ESI, under the section of ‘Construction of 3D diagram showing the relationship among pH, α, and [Cu²⁺]:[COOH]’†). Based on this 3D diagram, we could employ numerical interpolations of α to re-define the binding limits as real binding stoichiometry by plotting the measured [Cu²⁺] versus [Cu²⁺] : [COO⁻]; the results are shown in Fig. 1(b) (bear in mind that the pH is changing during the titration of Cu²⁺ into the PAA solution). Fig. 1(b) clearly shows that the binding stoichiometry in terms of [Cu²⁺] : [COO⁻] for the PAA solutions of different initial pH values converged towards 0.50. This is an important indication showing that within our experiment window of relatively low pH values, regardless of the α (and its variation) during Cu²⁺ titration, Cu²⁺ ions are quantitatively bound to COO⁻ at the ratio of one Cu²⁺ to two COO⁻ following rule of charge neutrality. This also indicates that Cu²⁺ hydrolysis is negligible.

PAA templated synthesis – a new mechanism

Fig. 2 shows the TEM images of CuO_x nano-crystals obtained via our PAA-templated synthesis, in which the Cu²⁺ loading was fixed at just below the binding limit. At first glance, it seemed that nanorod-like structures were obtained. However, closer examination revealed that they were in fact well-crystallized hollow tubular nanostructures. There are two categories of nanotubes: one is slender with a diameter of 10–30 nm and a length of 80–100 nm, having a typical wall thickness of ∼5 nm, the other is wider with a diameter of 40–60 nm and a length of 30–40 nm, having a typical wall thickness of ∼10 nm. Although the assembled structures are not perfectly mono-dispersed in size, the formation mechanism is same for them. The PAA with slightly larger molecular weight PDI that we used could be one of the reasons affecting the structure regularity. As shown in the inset images, the SAED pattern proved that the nanotubes consist of both Cu and Cu₂O crystal phases, and this fact is further supported by the lattice spacings measured from high resolution TEM images. As in the previously reported synthesis, it is not unusual that both Cu and CuO_x coexist as products.^35,36 We believe that pure Cu nanotubes formed first and they were slowly oxidized into Cu₂O. The absorbance spectra of the synthesized materials were obtained and are included in the ESI, Fig. S2.†

Fig. 2 (a) TEM images of the obtained hollow nano-tubular structures. It is proved by SAED (inset image) that the synthesis product is a mixture of Cu and Cu₂O phases. (b) Lattice fringes at the edge of the nano-tubes (side view) have a d-spacing of ∼0.34 nm, corresponding to Cu₂O (211), while those at the central portion of the nano-tubes have a d-spacing of ∼3.6 nm, corresponding to the Cu (200) plane (top view).

A more intriguing finding is that such nano-tubular structures were not observed in the usual homopolymer mediated synthesis and cannot be simply explained by known mechanisms, which normally involve selective polymer capping or confinement of crystal growth. In fact, the nano-structure as observed in Fig. 2 resembles those obtained via the self-assembly of block copolymers, which is the most plausible scenario.^37,38 This finding prompted us to hypothesize that a unique interaction between Cu²⁺ with PAA is responsible for the obtained regular self-assembled nano-structures. A search of the literature revealed that there are detailed theoretical studies on the charge distribution in PELs based on a single chain using density functional theory (DFT) and Monte Carlo simulations.^39–41 The essence of these works is that the charge distribution is non-uniform along a PEL chain in solution: charges (COO⁻ in our case) are concentrated at end segments of the PEL chain due to charge repulsion along the backbone and energy minimization. The charge distribution was reported to be affected by a range of factors including temperature, solvent condition, charge density, Debye length and chain flexibility. In our study, as all these conditions are fixed, we therefore believe the addition of Cu²⁺ is the critical reason for the change of PAA chain structure. Shown in Fig. 3 are schematic illustrations of the mechanism of how Cu²⁺ affects the PAA chain conformation, together with the self-assembly of the PAA–Cu²⁺ complexes into the nano-tubular structure.

Fig. 3 Schematic illustrations of (a) non-uniform charge distribution on a PAA chain, (b) preferential binding of Cu²⁺ onto the more deprotonated PAA chain ends via electrostatic attraction, (c) formation of a triblock copolymer-like PAA–Cu²⁺ complex due to change in chain stiffness and hydrophilicity (top right inset images show the PAA–Cu complex forms by complexation of one Cu²⁺ with two COO⁻, restricting the rotation), and (d) self-assembly of PAA–Cu²⁺ into a nano-tubular structure.

Fig. 3(a) illustrates the charged chain structure of a PAA chain in solution at a certain pH value. According to earlier theoretical studies, the equilibrium charge distribution is inhomogeneous and the charge density is higher near the chain ends than that in the central segment, i.e., the PAA chain ends are more deprotonated.^39–41 We believe that this non-homogeneous charge distribution and change of the PAA chain conformation during the addition of Cu²⁺ are ultimately responsible for the nano-tubular structure. Fig. 3(a) illustrates an ideal situation with negligible charge distribution at the centre portion of PAA. This is believed to be a good representation of the actual conditions considering (i) the low charge fraction, which is estimated to be ∼0.36 (i.e. 36% of the COOH are deprotonated at pH 6.6), (ii) aggravated charged non-uniformity found on more flexible polyelectrolyte, (iii) fast complexation of the Cu²⁺ with the deprotonated COO⁻ acting to quench the non-uniform structure, (iv) Cu²⁺ lowering the deprotonation energy barrier of its nearby COOH. Furthermore, the negligible Cu²⁺ binding in the centre might not take place in the non-classical crystallization, simply because of low [Cu²⁺] in the vicinity. It can be reasonably deduced that, when Cu²⁺ is added into the PAA solution, preferential binding of Cu²⁺ to the PAA chain ends occurs rapidly via electrostatic attraction (Fig. 3(b)). The distance between two carboxylic acid functionalities (∼0.3 nm) represents the length scale that determines the strong complexation between the Cu²⁺ and COO⁻, which is smaller than the Bjerrum length in aqueous medium (∼0.7 nm). Due to the inter-chain correlated assembly phenomenon, the non-classical crystallization took place within a confined spatial volume defined by the PAA pseudo chain segments lengths. The Cu²⁺ ions are concentrated near the chain ends, while the central segments of the chains would comparatively have less Cu²⁺. This site-preferential Cu²⁺ complexation not only gives rise to a unique structural inhomogeneity of the PAA–Cu²⁺ complex, it also leads to (i) chain stiffening due to steric hindrance of rotation (note that PEL stiffening effect due to metal ions, in term of persistence length, was discussed in earlier theoretical and experimental studies^42,43) and (ii) a change in hydrophilicity (note that the order of hydrophilicity is: COO⁻ > COOH > COO₂Cu). Therefore, such unique interactions of Cu²⁺ with the PAA homopolymer have effectively rendered the homo-PAA chain to possess characteristics of an amphiphilic rod–coil–rod block copolymer, as depicted in Fig. 3(c). As a result of the relatively rigid and more hydrophobic nature of the PAA–Cu²⁺ complex at the chain ends, and based on the knowledge of block copolymer self-assembly, it is logical that such triblock copolymer-like PAA–Cu²⁺ complexes could self-assemble into the nano-tubular structure through the mechanism depicted in Fig. 3(d). The central segments folded and the Cu²⁺ was ‘locked-in’, forming a bi-layered uni-lamellar hollow tubular assembly.⁴⁴ This type of self-assembly behaviour is unlike those reported for DNA molecules possessing complicated molecular architecture.⁴⁵ This is because PAA possess simplistic structures, and such assembly is induced by segmental counterion complexation. During the subsequent hydrolysis and heating steps, the nano-tubular crystals formed via a non-classical crystallization route (similar to biomimetic crystallization) since the diffusion of Cu²⁺ ions is inhibited due to their strong complexation with COO⁻ inside the assembly. The molecular weight of PAA represents an important parameter. When a shorter PAA (10 K g mol⁻¹) was used for the synthesis, spindle-like aggregates formed instead of a nano-tubular CuO_x structure (results not shown). This is believed to be due a much higher thermodynamic penalty for chain folding. Furthermore, a long enough PAA chain is required for the triblock copolymer-like behaviour to manifest.

In fact, the mechanism and concept discussed above are of universal nature in terms of types of (i) PELs and metal ions and (ii) complexed assemblies; in other words, they are not restricted to the PAA–Cu²⁺ system only. However, in order to apply this new mechanism of metal ion binding induced self-assembly, the following prerequisites and conditions need to be considered. Firstly, the molecular weight distribution of the PEL should be sufficiently narrow for the self-assembly of the pseudo block copolymer to occur (PDI of the PAA used in this study is 1.25). Besides that, a suitable PEL concentration needs to be used because non-desirable inter-chain bridging caused by metal ions may take place (before assembly) when the PEL concentration is too high. The initial pH value is another important consideration as it not only determines the metal ion binding limit, high pH also alters or complicates the interaction mechanism due to hydrolysis. Furthermore, the number of metal ions should be sufficient for nano-crystals formation (above the concentration needed for embryo nucleation) while remaining still below the binding limit. While the concept of metal ion induced copolymer-like self-assembly could be extended to other transition metals, each metal ion has its own unique characteristics and interaction behaviour with the PEL.⁴⁶ Finally, we need to bear in mind that copolymer self-assembly itself is highly sensitive to many intrinsic and extrinsic factors. For example, it is not uncommon that products with multiple morphologies are observed in copolymer self-assembly, e.g., rod-like with spherical micelles.⁴⁷ In fact, in this work, a small amount of Cu nanoparticles were observed to coexist with the CuO_x nanotubes (see ESI, Fig. S4†). Therefore, a detailed systematic study is required each time for a different set of PEL and metal ion.

Conclusions

In this work, unique nano-tubular CuO_x crystals were synthesized by employing the PAA homopolymer templated synthesis in aqueous medium. The formation of nano-crystals with this unique morphology is believed to be due to a triblock copolymer-like self-assembly of the PAA–Cu²⁺ complexes. This complexation induced self-assembly behaviour arised from the combination of the following sequential phenomena; non-uniform charge distribution along the PAA chains, preferential Cu²⁺ binding on deprotonated carboxyl groups, which concentrate at the chain ends, and copper complexation induced changes in the chain conformation (via chain-stiffening) and hydrophilicity at the chain ends. The initially flexible coil-like PAA chain essentially became a pseudo rod–coil–rod triblock copolymer with rigid rod-like chain ends, while the central segments remained flexible. This is found to be affected by several factors such as pH and PAA concentration, but critically relied on quantitative control of [Cu²⁺] in the PAA solution in relation to the [Cu²⁺] : [COOH]₀ binding limit. With caution, the application of this proposed new concept of metal ion induced copolymer-like PEL assembly can be extendable to other PELs and transition metal ions.

Acknowledgements

This project is funded by Agency for Science, Technology and Research (A-star, Singapore) under MIMO Program and scholarship from Nanyang Technological University (NTU). One of the authors, Y.N. Liang, would like to thank K.C. Tam for his guidance during 6-month exchange in Canada under Canadian Commonwealth Exchange Program. The electron microscopy works were performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore.

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Footnotes

† Electronic Supplementary Information (ESI) available: Supporting documents of (i) absorbance spectra of PAA–Cu²⁺ complex at different fixed [Cu²⁺] : [COOH]₀; (ii) absorbance spectra of solutions at different stages of synthesis; (iii) pK_a-α curves at different fixed [Cu²⁺] : [COOH]₀; (iv) analysis details on construction of 3D diagram showing relationship among pH, α, and [Cu²⁺] : [COOH]₀; (v) TEM image of spherical Cu nano-crystals formed; had been included. See DOI: 10.1039/c2ra20951a

‡ Author Contributions: Yen Nan Liang is a PhD student who contributed primarily in terms of the experimental works and writing. Jinhua Hu is a PhD student involved in the project. Xiao Hu is the PhD supervisor of above two PhD students. Kam Chiu Tam is the overseas project collaborator.

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