We report in this paper the synthesis, characterisation and spin-crossover (SCO) properties of a series of nanocomposite materials containing very small (∼3 nm) Hofmann clathrate [Fe(pz){M(CN)₄}] (M = Ni²⁺, Pt²⁺, Pd²⁺) nanoparticles embedded into different biopolymer polysaccharide matrices: chitosan and alginate. With the objective to understand the influence of the matrix on the SCO properties in these systems, we have modified the internal structure of the matrix, leading to aerogel, xerogel, beads or thin film samples. We show that the cooperativity associated with the spin crossover behaviour depends not only on the cyanometallate bivalent metal, but also on the nature and morphology of the polysaccharide matrix. Moreover, we confirm that for a specific morphology of the matrix, the bistability is maintained in such 3 nm nanoparticles.