A method for investigating a target environment to determine whether or in what amount a chemical species may be present therein, which comprises: (a) exposing to said environment an article of manufacture comprising a multiplicity of particles in close-packed orientation, said particles having a core of conductive metal or conductive metal alloy and deposited thereon a ligand which is capable of interacting with said species such that a property of said multiplicity of particles is altered; (b) subjecting said multiplicity of particles to conditions sufficient for said property to be exhibited; and (c) monitoring said property to determine whether there is, or the amount of, any change as an indication of whether, or in what amount, said species is present; a multiplicity of particles suitable for use in such method; and equipment suitable for implementing the method.