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. Sec. 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. While specular reflection (angle of incoming beam equal to angle of reflected beam) gives information in the direction perpendicular to the interface, the lateral structure of the interface may be probed by the nonspecular scattering measured at reflection angles different from the specular one (Sinha et al 1998 Phys. Rev. B 38 2297, Pynn 1992 Phys. Rev. B 45 602). This technique is widely used with x-rays while there are far fewer data in the neutron case due to the smaller intensity of neutron beams. An example relevant in biophysics where the neutron technique has been applied is the off-specular scattering from highly oriented multilamellar phospholipid membranes (Munster et al 1999 Europhys. Lett. 46 486). Neutron reflection is now being used for studies of surface chemistry (surfactants, polymers, Lipids, proteins and mixtures adsorbed at liquid/fluid and solid/fluid interfaces), surface magnetism (ultrathin Fe films, magnetic multilayers, superconductors) and solid films (Langmuir-Blodgett films, thin solid films, multilayers, polymer films). The number of reflectometers in the neutron facilities all around the world is increasing although the use of the technique is not yet very common because the availability of beam time is restricted by cost. Since many biological processes occur at interfaces, the possibility of using neutron reflection to study structural and kinetic aspects of model as well as real biological systems is of considerable interest. However, the number of such experiments so far performed is small. The reason for this is probably because it is well known that the most effective use of neutron reflection involves extensive deuterium substitution and this is not usually an available option in biological systems. This problem may be partially solved by deuteriating other parts of the interface as described by Fragneto et al (2000 Phys. Chem. Chem. Phys. 2 5214). In this paper we shall concentrate on the use of specular neutron reflection at the solid/liquid interface, less studied than the solid/air or liquid/air interfaces, although technologically more important. After a brief introduction to the theory and measurement of neutron reflectivity, solid/liquid interfaces both from hydrophilic and hydrophobic solids will be described. Three examples of applications in biophysics will be given: (1) the adsorption of two proteins, beta -casein and beta -lactoglobulin, on hydrophobic silicon; (2) the interaction of the peptide p-Antp43-58 with phospholipid bilayers deposited on silicon; (3) the fluid floating bilayer, a new model for biological membranes.
