Interfacial Microrheology of Protein Layers using Magnetic Nanowire Probes

蛋白质在空气-水或油水界面吸附并形成刚性界面层的趋势对许多当前和发展中的技术，特别是与食品、生物医学和制药工业有关的技术至关重要。此外，蛋白质层形成的过程为蛋白质变性、蛋白质-蛋白质相互作用和凝胶转变等问题提供了独特的视角。本文描述了一种新的、高灵敏度的方法，通过将磁性纳米线限制在界面上作为主动微流变探针，来表征蛋白质层的界面剪切流变性。智力优势：吸附在界面上的蛋白质与传统小分子表面活性剂的关键区别在于蛋白质倾向于形成具有强粘弹性的层。在许多情况下，这种机械行为可以导致优越的性能，例如稳定乳液和泡沫。因此，了解蛋白质层的流变特性对于理解其形成和稳定性的基本方面以及将其用于技术应用至关重要。基于几何考虑，采用线状探针的微流变学方法自然适合于测量纳米级流体薄膜的剪切流变学，并且纳米线应该比现有的界面剪切流变学技术更敏感。该方法的基础包括表征纳米线在空气-水界面上受到的阻力，当纳米线被精确的磁扭矩旋转时，蛋白质层就形成了。最近的理论预测了各向异性物体的流体动力学行为的根本变化，比如线状粒子，当它被限制在这样一个薄膜上时。通过实验澄清这些预测的有效性及其适用范围将产生深远的影响。这样的澄清也将是必要的，以适当的定量解释所提出的界面流变实验的蛋白质层。磁性纳米线是研究这些理论观点的理想系统，该项目将包括实验来测试这些预测。随着薄膜中纳米线的旋转阻力的理解，该方法将被应用于两种蛋白质层系统的界面剪切流变学研究(i)低浓度溶菌酶溶液，其层形成的特征是诱导期延长；（ii）由乳球蛋白和小分子表面活性剂溶液形成的膜，其中蛋白质层的机械性能对表面活性剂的存在高度敏感。选择这些系统是因为它们所呈现的令人信服的科学问题，以及通过提出的实验方法发现重要新现象的机会。然而，所提出的实验的另一个目标将是建立更广泛的微流变技术与磁性纳米线作为研究界面系统力学性能的重要工具。更广泛的影响：作为该项目的一部分，将从约翰霍普金斯大学的外展倡议“女性科学与工程（WISE）计划”中招募一名女高中生参与研究。该项目更广泛的影响还将包括对研究生和本科生的研究培训和教育。此外，这项工作将对基于界面蛋白质层的研究和技术产生潜在的重大影响，特别是关于目前技术无法进入的具有中尺度非均质性的系统以及小样本量目前是限制因素的材料。刚性界面层对许多当前和发展中的技术至关重要，特别是与食品、生物医学和制药工业有关的技术。

Leheny / Johns Hopkins / 0651666The tendency for proteins to adsorb at air-water or oil-water interfaces and to create stiffinterfacial layers is vital to many current and developing technologies, particularly those related to the food, biomedical, and pharmaceutical industries. Further, the process of protein-layer formation provides a unique perspective on issues of protein denaturation, protein-protein interactions, and the gel transition. This proposal describes experiments to apply a new, high sensitivity approach to characterizing the interfacial shear rheology of protein layers by employing magnetic nanowires confined at the interface as active microrheology probes.Intellectual Merit: A key difference between proteins adsorbed at interfaces and conventional small-molecule surfactants is the propensity of the proteins to form layers that are strongly viscoelastic. In many circumstances, this mechanical behavior can lead to superior properties, such as in stabilizing emulsions and foams. Consequently, knowledge of the rheological properties of protein layers is crucial both for understanding fundamental aspects of their formation and stability as well as for adopting them for technological application. Based on geometric considerations, the proposed microrheology approach using wire-shaped probes is naturally suited for measuring the shear rheology of nanometer-scale fluid films, and the nanowires should be significantly more sensitive than existing interfacial shear rheology techniques. The basis of the approach involves characterizing the drag experienced by nanowires confined to the air-water interface at which protein layers form as the wires are rotated by precise magnetic torques. Recent theory has predicted fundamental changes to the hydrodynamic behavior of an anisotropic object, such as a wire-shaped particle, when it is confined to such a thin film. Clarifying experimentally the validity of these predictions and their range of applicability would have far-reaching implications. Such clarification will also be necessary for a proper quantitative interpretation of the proposed interfacial rheology experiments on protein layers. The magnetic nanowires are an ideal system to investigate these theoretical ideas, and the project will include experiments to test the predictions. With the rotational drag on nanowires in films understood, the approach will then be applied to interfacial shear rheology studies of two protein layer systems (i) lysozyme solutions at low concentration for which layer formation is characterized by an extended induction period and (ii) films formed from solutions of lactoglobulin and small molecule surfactants for which the mechanical properties of the protein layer are highly sensitive to the presence of the surfactant. These systems are selected for thecompelling scientific problems they present and for the opportunities to uncover significant new phenomena through the proposed experimental approach. However, an additional objective of the proposed experiments will be to establish more generally the technique of microrheology with magnetic nanowires as an important tool for studying the mechanical properties of interfacial systems.Broader Impacts: As part of this project, a female high school student will be recruited from the Women in Science and Engineering (WISE) Program, a Johns Hopkins outreach initiative, to participate in the research. The broader impacts of the project will also include research training and education for a graduate student and an undergraduate student. In addition, the work will provide potentially significant impact to both the research on and the technology based on interfacial proteins layers, particularly with regard to systems with mesoscale heterogeneity that cannot be accessed with current techniques and to materials for which small sample quantity is currently a limiting factor. Stiff interfacial layers are vital to many current and developing technologies, particularly those related to the food, biomedical, and pharmaceutical industries.