Neutron reflectivity at the solid/liquid interface: examples of applications in biophysics

Over the last 20 years, neutron reflection has emerged as a powerful technique for investigating inhomogeneities across an interface, inhomogeneities either in composition (Lu and Thomas 1998 J. Chem. Soc. Faraday Trans. 94 995) or magnetization (Felcher 1981 Phys. Rev. B 24 1995). By measuring the reflected over the incoming intensity of a well collimated beam striking at an interface, as a function of the incident angle and wavelength, the concentration profile giving rise to a reflectivity curve is calculated. The success of neutron reflection arises from the fact that, because of the short wavelengths available, it has a resolution of a fraction of a nanometre, so that information is gained at the molecular level. Unlike x-rays it is not destructive and can be used at buried interfaces, which are not easily accessible to other techniques, such as liquid/liquid or solid/liquid, as well as at solid/air and liquid/air interfaces. It is particularly useful for soft-matter studies since neutrons are strongly scattered by light atoms like H, C, O and N of which most organic and biological materials are formed. Moreover, the nuclei of different isotopes of the same element scatter neutrons with different amplitude and sometimes, as in the case of protons and deuterons, with opposite phase. This allows the use of the method of contrast variation, described below, and different parts of the interface may be highlighted. For biophysics studies, a major advantage of reflectivity over other scattering techniques is that the required sample quantity is very small (<10-6 g) and it is therefore suitable for work with expensive or rare macromolecules.