Fmoc-PEG8-NHS ester is a heterobifunctional linker with an Fmoc-protected (9-fluorenylmethyloxycarbonyl) amine on one end and an NHS ester on the other, connected by an 8-unit PEG spacer. Used extensively in bioconjugation and peptide synthesis, it enables sequential attachment of peptides or proteins to other molecules, surfaces, or nanoparticles. The NHS ester reacts selectively with primary amines to form stable amide bonds under mild aqueous conditions. The PEG spacer enhances aqueous solubility, reduces steric hindrance, and improves reaction efficiency. Post-conjugation, the Fmoc group can be orthogonally deprotected using piperidine or mild base, revealing a free amine for further modification. Commonly applied in drug delivery systems to PEGylate therapeutic agents for better targeting, circulation time, and biocompatibility. Also utilized in surface functionalization for biosensors, diagnostic tools, and nanoparticle coatings, promoting precise molecular interactions. Essential for creating PEGylated biomolecules that exhibit improved stability and reduced immunogenicity in biopharmaceutical applications.