Interaction Mechanisms of Zwitterions with Opposite Dipoles in Aqueous Solutions

Zwitterionic groups have been widely used in antibiofouling surfaces to resist nonspecific adsorption of proteins and other biomolecules. The interactions among zwitterionic groups have attracted considerable attention in bioengineering, whereas the understanding of their nanomechanical mechanism still remains limited. In this work, the interaction mechanisms between two zwitterionic groups with opposite dipoles, i.e., phosphorylcholine (PC) and sulfobetaine (SB), have been investigated via direct force measurements using an atomic force microscope (AFM) and dynamic adsorption tests using the quartz crystal microbalance with dissipation monitoring technique (QCM-D) in aqueous solutions. The AFM force measurements show that the adhesive forces between contacted zwitterionic surfaces during separation in both symmetric and asymmetric configurations were close, mainly due to the enforced alignment of opposing dipole pairs via complementary orientations under confinement. The solution salinity and pH had almost negligible influence on the adhesion measured during surface separation. The QCM-D adsorption tests of PC-headed lipid on PC and SB surfaces showed some degree of adsorption of lipid molecules on the SB surface, whereas not on the PC surface. The different adsorption behaviors indicate that because the outermost negatively charged sulfonic group on the SB faced the aqueous solution, this configuration could facilitate it to form an attractive electrostatic interaction with the PC head of lipid molecules in the solution. This work shows that in addition to hydration and steric interactions, the zwitterionic dipole-induced interactions play an important role in the adhesion and antifouling behaviors of the zwitterionic molecules and surfaces. The improved fundamental understanding provides useful insights into the development of new functional materials and coatings with antifouling applications.

两性离子基团已广泛应用于抗生物污染表面，以抵抗蛋白质和其他生物分子的非特异性吸附。两性离子基团之间的相互作用在生物工程中引起了相当大的关注，然而对其纳米力学机制的理解仍然有限。在这项工作中，通过使用原子力显微镜（AFM）进行直接力测量以及使用带有耗散监测技术的石英晶体微天平（QCM - D）在水溶液中进行动态吸附测试，研究了具有相反偶极的两个两性离子基团，即磷酰胆碱（PC）和磺基甜菜碱（SB）之间的相互作用机制。AFM力测量表明，在对称和非对称配置下分离过程中接触的两性离子表面之间的粘附力相近，这主要是由于在受限条件下通过互补取向强制排列相反的偶极对。溶液的盐度和pH对表面分离过程中测量的粘附力几乎没有影响。PC头部脂质在PC和SB表面的QCM - D吸附测试表明，脂质分子在SB表面有一定程度的吸附，而在PC表面则没有。不同的吸附行为表明，由于SB上最外层带负电的磺酸基团面向水溶液，这种构型可以促进它与溶液中脂质分子的PC头部形成有吸引力的静电相互作用。这项工作表明，除了水合作用和空间位阻相互作用外，两性离子偶极诱导的相互作用在两性离子分子和表面的粘附及抗污染行为中起着重要作用。对基本原理理解的提高为开发具有抗污染应用的新型功能材料和涂层提供了有用的见解。