Named after the two-faced Roman god, Janus particles (JPs) are defined by their distinct dual chemical compositions on a single particle. Research on micron-sized JPs has yielded remarkable insights, showcasing their unique assembly behaviors both in bulk and at interfaces. However, significant challenges persist, particularly in the synthesis of smaller (<500 nm) JPs, which remains complex and difficult to scale up. To date, there has been no commercial success with JPs. Recently, seeded synthesis methods, such as emulsion polymerization that is already employed in industrial-scale manufacturing, have shown great promise. These methods enable the production of high-quality JPs with different sizes, morphologies, and functionalities. This advancement has inspired more efforts in exploring JP applications across various fields, including emulsion stabilization, drug delivery, electronic devices, and coatings. This review provides a comprehensive overview of the recent progress in the synthesis and application of polymeric JPs, with an emphasis on the seeded synthesis approach. It discusses the underlying reaction mechanisms and explores different strategies for controlling JP morphology. Serving as a roadmap, this review aims to guide the design of novel functional JPs and their potential future applications. The successful implementation of JPs will require careful consideration and a deep understanding of both synthesis and applications, as these are indeed two sides of the same coin.

中文翻译：

---

硬币的两面：Janus 粒子的合成和应用


Janus 粒子 （JP） 以双面罗马神的名字命名，其定义是它们在单个粒子上独特的双重化学成分。对微米级 JP 的研究产生了非凡的见解，展示了它们在块体和界面上的独特组装行为。然而，重大挑战仍然存在，特别是在较小 （<500 nm） JP 的合成中，这仍然很复杂且难以放大。迄今为止，JP 尚未取得商业成功。最近，种子合成方法，例如已经用于工业规模制造的乳液聚合，显示出巨大的前景。这些方法能够生产具有不同尺寸、形态和功能的高质量 JP。这一进步激发了人们更多努力探索 JP 在各个领域的应用，包括乳液稳定、药物输送、电子设备和涂料。本文全面概述了聚合物 JPs 的合成和应用的最新进展，重点介绍了种子合成方法。它讨论了潜在的反应机制并探讨了控制 JP 形态的不同策略。本文以路线图为指导新型功能性 JP 的设计及其潜在的未来应用。JP 的成功实施需要仔细考虑和对综合和应用的深入理解，因为这确实是同一枚硬币的两面。