Key Takeaways & Executive Findings
- •• Two-dimensional ice-like water layers form on self-assembled monolayers supporting Cytochrome C under ambient conditions without nanoconfinement. • Direct characterization of morphology, composition, melting, and crystallization of 2D ice-like water using AFM and nano-AFM-IR. • The formation is attributed to the activation energy for water desorption from Cyt C being nearly twice that from SAMs. • Provides a new model system to study water–protein interactions and ice phase transitions under normal conditions.
Abstract
Directly correlating the morphology and composition of interfacial water is vital not only for studying water icing under critical conditions but also for understanding the role of protein–water interactions in bio-relevant systems. In this study, we present a model system to study two-dimensional (2D) water layers under ambient conditions by using self-assembled monolayers (SAMs) supporting the physisorption of the Cytochrome C (Cyt C) protein layer. We observed that the 2D island-like water layers were uniformly distributed on the SAMs as characterized by atomic force microscopy, and their composition was confirmed by nano-atomic force microscopy-infrared spectroscopy and Raman spectroscopy. In addition, these 2D flakes could grow under high-humidity conditions or melt upon the introduction of a heat source. The formation of these flakes is attributed to the activation energy for water desorption from the Cyt C being nearly twofold high than that from the SAMs. Our results provide a new and effective method for further understanding the water–protein interactions.
1. Introduction
The interfacial water on proteins plays key roles in proteins’ stabilities, dynamics and functionalities in a wide variety of biosystems [1–6]. These roles range from preventing protein collapse [7] to participating in processes such as ligand binding [8] and folding [9], lubrication between proteins and lipids [10], and exchange of hydrogen atoms of the amide backbone with the surrounded solvent [11]. Although it has been demonstrated that the hydration layer on proteins can resemble ice-like water [12], the precise structure of this ice-like water and the mechanisms by which interactions that proteins can induce ice formation remain poorly understood. One of the main challenges is the simultaneous characterization of the topography and composition of protein–water complexes, particularly under normal pressure and temperature (NPT) conditions without nanoscale confinement. Therefore, there is an urgent need to develop methods for stabilizing ice-like water on proteins under these conditions to address outstanding questions regarding protein–water interactions.
Here, we developed a method to utilize self-assembled monolayers (SAMs) of sodium 11-mercaptoundecane-1-sulfonate (HSC11SO3Na) assembled on an ultra-flat template-stripped gold (AuTS) surface as the substrate, to achieve physisorption and then partially desorption of Cytochrome C (Cyt C) [13–16], at ambient condition. Using atomic force microscopy (AFM), we directly imaged island-like ice plateaus on the surfaces, revealing their unexpected growth under high humidity or melting upon heating via continuous AFM scanning. Complementary characterization techniques, including nano-atomic force microscopy-infrared spectroscopy (AFM-IR), Fourier transform infrared (FTIR) reflectance measurements, and confocal Raman spectroscopy, confirmed the composition of these ice-like plateaus. Temperature-programmed infrared reflection absorption spectroscopy (IRRAS) measurements revealed a nearly 2 times stronger interaction between water and Cyt C than the SAMs. This work sheds new light on the formation of the 2D ice-like flakes under NPT conditions without nanoscale confinement, contributing to the further understanding of the unique interactions between interfacial water and proteins.
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Wuxian Peng, Linbo Li, Xiyue Bai, Ping Yi, Yu Xie, Lejia Wang, Wei Du, Tao Wang, Jian-Qiang Zhong, Yuan Li (2025). Observation of Ice-Like Two-Dimensional Flakes on Self-Assembled Protein Monolayer without Nanoconfinement under Ambient Conditions. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01689-1
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Frequently Asked Questions
What is the main finding of this paper?
The paper reports the observation of two-dimensional ice-like water flakes on self-assembled monolayers supporting a Cytochrome C protein layer under ambient conditions, without nanoconfinement. The formation is attributed to the higher activation energy for water desorption from Cyt C compared to the SAMs.
How was the ice-like water characterized?
The ice-like water was characterized using atomic force microscopy (AFM) for morphology, and nano-atomic force microscopy-infrared spectroscopy (AFM-IR), Raman spectroscopy, and FTIR for composition. Temperature-programmed IRRAS was used to measure interaction strengths.
What is the significance of this study?
This study provides a new model system to study water–protein interactions and ice phase transitions under normal conditions, which is crucial for understanding biological processes and developing new materials.
What are the key techniques used?
Key techniques include self-assembled monolayers (SAMs) on template-stripped gold, atomic force microscopy (AFM), nano-AFM-IR, Raman spectroscopy, and temperature-programmed infrared reflection absorption spectroscopy (IRRAS).
What are the potential applications of this research?
The findings could have implications in cryopreservation, understanding protein hydration, and designing surfaces that control ice formation, which is relevant in fields like food science, medicine, and materials engineering.
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