Biotin-PEG8-TCO is a PEG linker containing a biotin group and a trans-cyclooctene (TCO) functional group. Biotin is useful for affinity-based applications such as pull-down assays or for ligating with streptavidin proteins while TCO is a click chemistry handle that quickly reacts with tetrazine groups on target molecules. The inclusion of a PEG linker in this molecule improves its aqueous solubility.