Morphological transformation of calcium phenylphosphonate microspheres induced by micellization of γ-polyglutamic acid

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Abstract

The γ-polyglutamic acid (γ-PGA), an anionic homo-polyamide similar to naturally occurring polyaspartic acid in mollusk shells, is selected for morphosynthesis of calcium phenylphosphonates. It has been found that they exhibit a series of interesting morphological transformation with changing γ-PGA amount and initial pH. A rare, reversible transformation between solid and hollow microspheres, in particular, is observed. Systematic investigation suggests, for the first time, that γ-PGA exhibits the “schizophrenic” micellization behavior in response to structural and morphological changes caused by spontaneous hydrophobic modification with phenylphosphonic acids and calcium ions. The present work introduces a new paradigm for biomimetic synthesis. The results not only bring insight into the template mechanism of biomacromolecules from spontaneous hydrophobic modification, but also provide a universally applicable strategy for morphosynthesis of metal phosphonates containing hydrophobic groups. In addition, solid and hollow calcium phenylphosphonate microspheres with hierarchical structures are excellent adsorbents in the removal of aqueous lead ions. They consistently exhibit 100% Pb2+ removal efficiency when Pb2+ concentration is not more than 600 mg L−1 and even after four recycles, making them promising candidates as efficient and reusable adsorbents.

Graphical abstract

It has been found for the first time that γ-PGA exhibits the “schizophrenic” micellization behavior, in that it induces and controls a series of interesting morphology transformations of hierarchical calcium phenylphosphonates. In particular, a rare, reversible morphology transformation between solid microspheres and hollow microspheres is observed with increasing pH.

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Introduction

In nature, biomacromolecules play important roles in directing and controlling the formation of biominerals with various morphologies and sophisticated structures. Biomimetic synthesis using natural biomacromolecules not only brings insights into their structure-directing roles and templating mechanism in the biomineralization process, but also inspires the design and synthesis of new functional materials [1], [2], [3].

Many natural biomolecules, including amino acids [4], peptides [5], proteins [5], [6] and DNA [7], have been consequently used in controlling the morphology and structure of calcium phosphate materials, and the area has seen a lot of encouraging results during the past two decades. It is commonly recognized that the electron-rich functional groups in biomolecules, for example, the extensively studied carboxyl groups, are able to interact with divalent calcium ions to provide nucleation sites for the formation of crystallites, and can control the subsequent growth of crystallites through interactions with certain crystal faces of the crystallites. Moreover, the strength of the interactions can be modulated by changing the pH value of the solution so as to control the length and aspect ratio of the crystallites [8]. However, questions remain on how the structural and morphological changes of biomacromolecules take place during biomineralization and their roles in directing and controlling the formation of calcium phosphates. In order to simulate biomacromolecules naturally existed in the biominerals, various biopolymers and synthetic polymers containing carboxyl groups, such as polyamino acids [9], [10], [11], polypeptides [12], [13] and block copolymers [14], [15], have been used in biomimetic synthesis of calcium phosphates. Furthermore, the double-hydrophilic block copolymers (DHBCs) known as “schizophrenic” polymers were also utilized [15], [16], [17], [18], [19], [20]. In general, DHBCs consist of two or multiple water-soluble blocks, of which one or two hydrophilic blocks can be transformed into hydrophobic ones upon changing the external environmental conditions (such as temperature, pH value, ionic strength, light, and the complexation with oppositely charged metal ions, small molecules or polymer chains etc.), while the other hydrophilic blocks promote the dissolution and dispersion of copolymers. Such a transformation occurs in respond to external physical or chemical stimuli, which cause DHBCs to exhibit “schizophrenic” micellization behavior and form various different self-assembled aggregates in solution so that DHBCs are known as “schizophrenic” polymers [20], [21]. In the biomimetic synthesis of calcium phosphates using DHBCs, DHBCs are also likely to exhibit the “schizophrenic” micellization behavior in the presence of a large number of calcium ions or/and by changing temperature and pH, which in turn might affect the morphology and structure of calcium phosphates. Despite its importance, the role of this unique behavior of DHBCs on the formation of calcium phosphates has not attracted much attention.

Herein, γ-polyglutamic acid (γ-PGA), a natural anionic homo-polyamide similar to naturally occurring polyaspartic acid in the mollusk shells, was selected and used in the morphosynthesis of calcium phenylphosphonates. It has been found for the first time that γ-PGA exhibits the “schizophrenic” micellization behavior to induce and control the morphology transformation of calcium phenylphosphonates. As far as we know, polyglutamic acid generally presents itself as a hydrophilic block in DHBCs that are able to respond to external environmental changes, however, itself alone cannot exhibit the “schizophrenic” micellization behavior in aqueous solution [21], [22], [23]. The “schizophrenic” micellization of γ-PGA occurs in this synthetic system only if γ-PGA experiences structural and morphological changes in the co-participation of calcium ions and phenylphosphonic acids, which is somewhat similar to the phosphorylation of proteins accompanied by proteins’ hydrophility/hydrophobicity transformation and conformational change. As a result, calcium phenylphosphonates exhibit a series of interesting morphology transformations with the increase of γ-PGA addition amount and initial pH value. It should be noted that mutual morphology transformation between solid microspheres and hollow microspheres has rarely been encountered before. To further understand the role of γ-PGA and its “schizophrenic” micellization behavior in the formation of calcium phenylphosphonates, the effect and mechanism of γ-PGA in the morphology transformation and crystal phase transition of calcium phenylphosphonates was investigated and discussed in detail. Considering that phosphonates themselves are effective flocculants for heavy metal ions, calcium ions are environment friendly and biocompatible, and the calcium phenylphosphonate products exhibit solid or hollow microsphere morphology and hierarchical structure, such calcium phenylphosphonate microspheres thus possess the characteristics of an excellent adsorbent for efficient, eco-friendly and reusable removal of very toxic aqueous lead ions.

Section snippets

Materials and characterization

All the chemicals were obtained commercially and directly used without further purification. Phenylphosphonic acid (PPA, 98%) was purchased from Acros. Calcium chloride anhydrous (CaCl2), and NaOH were analytical grade and purchased from Tianjin Kemiou Chemical Reagent Co. The γ-polyglutamic acid (γ-PGA) (MW 700–1000 kDa) was purchased from Xi'an Tongze Biotechnology Co. Ltd.

Scanning electron microscopy (SEM) was performed on a SUPRA55 SAPPHIRE electron microscope operating at an accelerating

The effect of γ-PGA addition amount on the morphology and crystal phase composition of calcium phenylphosphonates

The facile synthesis of calcium phenylphosphonates was implemented by a solution method at 50 °C when the initial pH value of reaction system was 8. As shown in Fig. 1A, the calcium phenylphosphonates synthesized with different added amount of γ-PGA were characterized by SEM, in comparison with that in the absence of γ-PGA. It is clear that the morphology of calcium phenylphosphonates changes from huge thin sheets to microspheres when the amount of γ-PGA was increased from 0 to 80 mg. In Fig. 1

Conclusions

In summary, it has been found for the first time that γ-PGA exhibits the “schizophrenic” micellization behavior, in that it induces and controls a series of interesting morphology transformations of hierarchical calcium phenylphosphonates. In particular, a rare, reversible morphology transformation between solid microspheres and hollow microspheres is observed with increasing pH. Whereas γ-PGA alone does not exhibit the “schizophrenic” micellization behavior in the aqueous solution, in this

Acknowledgments

Financial support of this work was provided by the Opening Foundation of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China (N-17-02 and N-18-10) and Program for Innovative Talents of Higher Learning Institutions of Liaoning, China (2018).

Declaration of Competing Interest

The authors declare no completing financial interest.

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