The present invention describes carbamic acid ester-derived and valeric acid ester-derived polyfunctional cross-linker molecules, and methods for their synthesis and use. The inclusion of polymeric moieties such as poly(alkylene oxide) in the cross-linkers of the present invention can provide advantageous solubility properties in aqueous environments. Such cross-linkers may be used to form conjugates for use in a variety of assay formats.