Specular neutron reflectivity (NR) was used to resolve the conformation of the oligonucleotidepolycationic brush complexes tethered to a solid (Si) support. NR is a powerful technique that provides Ångström-scale information on the structure and composition of brush complexes perpendicular to the solid support. The neutron refractive index (n) is defined as n = 1 - , where the subscript i corresponds to the atomic species i. Unlike the case of X-rays where b is always positive and proportional to the number of electrons, the neutron scattering length b changes in a random fashion throughout the periodic table, and some atoms such as hydrogen even have a negative b value. More importantly, b differs among isotopes of an element (e.g. it is isotope specific), which is the foundation of contrast variations in a neutron experiment. The substitution between hydrogen and deuterium (H/D) has been extensively used in neutron experiments because of their different scattering lengths (-3.74 × 10 -5 Å for 1 H and 6.67 × 10 -5 Å for 2 H). This results in very dissimilar Nb values of H2O and D2O: -0 .56 × 10 -6 Å -2 and 6.35 ×10 -6 Å -2 , respectively. Many commonly used contrasts in solid (silicon)-liquid systems are obtained from the variation in the mixing ratio between H2O and D2O to contrast match the support. The NR profiles from aqueous phases with distinct contrasts are fitted simultaneously to a single model. The constraint of one model, fitting all the contrasts, allows a unique and well-defined structural model to be deduced. NR data were analysed using the exact optical matrix method as previously reported . The analysis of NR data was carried out through the fitting of data to defined models with parameters such as scattering length density, thickness, roughness, and the number of layers, calculating the corresponding reflectivity profiles, and comparing these result to the data obtained from experiments. This process was repeated until the differences between the global fit and the experimental data in different isotopic contrasts was minimised. The optimal fitted parameters were combined to give the scattering length density profile of the sample, which represents the variation in composition perpendicular to the interface. The NR profiles were measured as a function of momentum transfer normal to the interface via the time-of-flight technique Q (𝑄 = &()*+, -) , where θ is the grazing angle of incident neutrons, and λ is the neutron wavelength. The NR profiles were measured at two incident angles (θ = 0.624° and 3.78°) on the FIGARO reflectometer at the Institut Laue-Langevin (ILL), Grenoble, to provide a wide Q-range hence the highest sensitivity to the interfacial structure. The sample was illuminated at a resolution of δQ/Q≈8%.