Provided is a method that utilises linear dichroism (LD) to identify the presence of a target molecule (L) in a sample. The method comprises providing an alignable scaffold (20), preferably biomolecular fibre M13, comprising a first binding region and having a high aspect ratio of greater than 5:1, providing a substrate (e.g. a substantially spherical non-alignable moiety (12)) comprising a second binding region which binds the first binding region in the absence of the target molecule in such a way that the LD signal of the alignable scaffold is reduced or minimised relative to the unbound and aligned scaffold, wherein one of the first and second binding regions is a receptor capable of binding the target molecule, exposing the substrate-bound scaffold to the sample such that binding of the target molecule, if present, to the receptor releases the scaffold from the substrate, and measuring the LD signal of the scaffold before and after exposure to the sample. A reagent and an apparatus for use in the method are also provided. A reagent (10) and an apparatus for use in the method are also disclosed.