Used in bioconjugation to link molecules such as peptides, proteins, or dyes to primary amines on biomolecules. The NHS ester reacts efficiently with lysine residues or N-termini of proteins, forming stable amide bonds. The Ald-Ph group provides a benzaldehyde handle for further modification, often used in click chemistry or hydrazone ligation. The PEG4 spacer improves solubility, reduces aggregation, and increases flexibility between linked components, making it ideal for constructing complex biomolecular probes, antibody-drug conjugates, or diagnostic reagents. Commonly applied in assay development, targeted delivery systems, and surface immobilization in biosensors.