Design and optimization of bifunctional peptides for controlled core–shell nanocapsule formation

Abstract

Nanocapsules with core–shell structures hold significant potential across diverse applications. Biomimetic templating offers a benign approach for synthesizing inorganic nanostructures using biomolecules, leveraging amino acid sequences from natural sources and combinatorial biology in a process known as biomineralization. This study investigates the design and functionality of bifunctional peptides for controlled interfacial biosilicification. Five bifunctional peptides were designed and compared for their surface activity, structural behavior, and biosilicification capability under benign conditions. AM1 and SurSi-G1 demonstrate rapid adsorption, lower interfacial tension, and higher surface activity. In contrast, SurSi and its variants show slower adsorption due to higher molecular charge, resulting in high interfacial tension. Biosilicification assays confirmed that peptide charge strongly influences particle morphology, with SurSi and SurSi-R3 yielding well-dispersed silica nanoparticles, while AM1, SurSi-R2, and SurSi-G1 formed larger aggregates. Low ionic strength and sufficient surface charge enhance electrostatic interaction between positively charged bifunctional peptides and negatively charged hydrolyzed silicic acid, facilitating controlled biosilicification at interface and enabling the precise formation of core–shell nanocapsules. These findings highlight the pivotal role of peptide sequence and charge distribution in determining surface activity and interfacial biosilicification, providing insights for optimizing nanocapsule synthesis.

Introduction

The development of nanomaterials has led to significant advancements across various fields such as medicine, energy, catalysis, and more. For more advanced applications, the ability to construct structures with greater complexity beyond solid particles is crucial [1]. Core-shell nanomaterials offer superior properties compared to solid nanoparticles, with the shell providing a protective layer that ensures chemical and mechanical stability, and the core for loading different cargoes [2]. Core-shell silica nanocapsules have attracted considerable interest due to their excellent biocompatibility and versatile surface functionalization[3]. For example, silica shell can be surface-modified with optical, magnetic, or biological properties, further improving their performance in applications like drug delivery and controlled release [4], [5], [6].

The synthesis of core–shell silica nanocapsules typically relies on templating methods, utilizing either hard or soft templates as the core. For example, hard templates can include polystyrene or iron oxide, while soft templates may involve gas bubbles and emulsion droplets [7]. Then the template surface can be functionalized to promote silicification and condensation on the surface to form a dense shell. To enhance loading capabilities, the template needs to be selectively removed. However, removing these templates can be challenging, often requiring high temperatures or toxic solvents [8], [9], [10]. Therefore, there is a growing interest in developing a benign, environmentally friendly, and efficient preparation process for making core–shell silica nanocapsules [11], [12].

Biomineralization meets these criteria effectively, as it is a natural process where organisms produce inorganic minerals through the use of biomolecules [13]. Among them, biosilicification has been extensively studied, revealing the mechanism underpinning peptide-mediated silica formation, for example, using polycationic peptides like silaffins [14], [15], [16]. These peptides exhibit unique sequence features, such as a high density of hydroxyl-containing amino acids including serine, tyrosine, and/or cationic amino acids, such as lysine, arginine, histidine. These amino acid residues play a critical role in facilitating silica nucleation and subsequent growth by interacting with silica precursors via hydrogen bonding and/or electrostatic interaction [16]. By functioning as catalysts, templates, or scaffolds, these biomolecules enable biosilification to create complex structures and patterns [15], [16], [17], [18]. Studies have demonstrated that designed peptides can facilitate the formation of novel silica-based materials under mild conditions [3], [12], [19], [20].

Our group has previously reported a dual-functional peptide for fabricating oil-core-silica-shell materials [3], [12]. This peptide features two key modules for dual functions − a surface-active module and a biomineralization module. Together, these components enable the simultaneous formation of a nanoemulsion template and the surrounding silica shell under neutral pH and room temperature conditions. However, despite the successful fabrication of nanocapsules using this method, the underlying formation mechanism and design principles of the peptides have yet to be fully elucidated.

This study aims to elucidate the relationship between the design of bifunctional peptide sequences and their efficacy in fabricating silica nanocapsules. The focus is on balancing the surface activity and biosilicification activity of the peptides to improve control over the fabrication of nanoemulsion templates and the silica shell. We began by evaluating their molecular charge and surface activity and identified a key trade-off that governs their ability to stabilize nanoemulsions and mediate biomineralization. By examining how varying ionic strengths and different silica precursors affect nanoparticle growth and aggregation, we gained detailed insights into the formation of silica core–shell structures. A combination of molecular dynamics (MD) simulations, circular dichroism (CD) spectroscopy, and Drop Shape Analysis (DSA) was used to characterize the structural behavior of the peptides in bulk solutions or at oil–water (O/W) interfaces. Importantly, we established a clear link between the molecular charge of the peptides and their interfacial packing density, which directly influences their performance in nanocapsule formation. These findings offer valuable insights for designing biomineralizing peptides capable of mediating the biomimetic formation of functional nanomaterials, especially core–shell nanocapsules, for future applications.

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Materials

All peptides were synthesized by GenScript (Piscataway, NJ) with a purity higher than 95.0 %. Water was obtained from a Milli-Q system (Merck Millipore, Germany) with a 0.22 μm filter. Miglyol® 812 N oil was obtained from IOI Oleo GmbH (Germany). Tetraethoxysilane (TEOS) and Triethoxyvinylsilane (TEVS) with a purity ≥ 99.0 % were of analytical grade (Sigma-Aldrich). Other chemicals were all analytical grade from Sigma-Aldrich, and were used as received unless otherwise specified.

Zichao Guo, Yang Li, Letao Xu, Jiaqi Wang, Jitong Lyu, Guangze Yang, Yun Liu, Yue Hui, Chun-Xia Zhao, Design and optimization of bifunctional peptides for controlled core–shell nanocapsule formation, Journal of Colloid and Interface Science, 2025, ISSN 0021-9797, https://doi.org/10.1016/j.jcis.2025.01.250.

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