Semiconductor nanomaterials can be synthesized in situ, by reaction of chalcogenide nanomaterials (e.g. S, Se, Te) with metal cations. In situ here is defined as changing the chemical composition of an existing nanostructure while that structure remains in-place. In situ synthesis is attractive for the purpose of producing hollow semiconductor nanostructures, for flow-reactor based syntheses (in which the parent nanostructure acts as a solid phase), and for biological applications such as imaging markers compatible with electron and fluorescence microscopies (EM/FM). In the case of Se, several approaches of in situ conversion of Se nanostructures to metal selenide nanostructures are previously described. Se nanomaterials can either be crystalline, comprised of monoclinic (m-Se) or trigonal (t-Se, black) crystal phases, or amorphous (α-Se, red). Black t-Se nanowires react with Cd 2+ , Pt 2+ , Ti 2+ , and Zn 2+ ions at 100C to create Se@MSe (core@shell) nanowires. The same t-Se nanowires can react with Ag + at 25C to make AgSe nanowires. Similarly, 90nm diameter red a-SeNPs react with Pt 2+ , forming PtSe@Se core-shell structures at 50C. Mechanistically the conversion from Se to metal selenide is distinct from well-studied approaches such as cation exchange. In the case of t-Se (chains, hexagonal) and α-Se (rings), the materials are speculated to undergo either thermally driven or spontaneous disproportionation, wherein polymeric Se breaks to transient Se 2+ and Se 2-species. Experimental evidence for this is so far lacking, however, as is a more detailed mechanistic analysis. The Se 2-species from t-Se could react with metal cations to form metal selenide semiconductor shells.