M-Peg4-phosphonic acid ethyl ester is widely used in the field of bioconjugation and drug delivery. Its primary application lies in modifying biomolecules, such as proteins, peptides, and antibodies, to enhance their solubility, stability, and bioavailability. The compound is particularly valuable in the development of targeted drug delivery systems, where it acts as a linker to attach therapeutic agents to carriers or targeting moieties. Additionally, it is utilized in the synthesis of prodrugs, improving the pharmacokinetic properties of active pharmaceutical ingredients. Its phosphonic acid group allows for strong binding to metal oxides, making it useful in surface modification and nanotechnology applications, such as creating functionalized nanoparticles for medical diagnostics and imaging.