Overview
ICG-PEG-MAL is a trifunctional molecular conjugate combining the optical properties of indocyanine green (ICG) with the biocompatibility of polyethylene glycol (PEG) and the reactivity of maleimide (MAL). The ICG component provides strong near-infrared fluorescence ideal for deep-tissue imaging, while the PEG spacer enhances water solubility and reduces nonspecific binding. The maleimide terminus enables covalent conjugation to thiol (-SH) groups present in cysteine residues of proteins or other biomolecules. This reagent has become essential in bioconjugation chemistry, particularly for creating fluorescently labeled antibodies, peptides, and nanoparticles. Its 780/810 nm excitation/emission wavelengths minimize tissue autofluorescence and absorption, making it superior to visible-range fluorophores for in vivo applications. The modular design allows researchers to precisely control labeling density and maintain biological activity of conjugated molecules.
Physical and Chemical Properties
ICG-PEG-MAL exhibits unique photophysical characteristics with a molar extinction coefficient of approximately 120,000 cm-1M-1 at 780 nm and quantum yield around 0.12 in aqueous solutions. The PEG component (typically 2k-5k Da) provides excellent hydrophilicity, with logP values ranging from -1.5 to -3.0 depending on PEG length. Maleimide groups remain stable at pH 6.5-7.5 but hydrolyze gradually at higher pH. The conjugate demonstrates good thermal stability below 40°C but undergoes photodegradation under prolonged light exposure. In phosphate-buffered saline (PBS), the compound remains stable for 24-48 hours at room temperature when protected from light. The fluorescence intensity shows minimal pH dependence between pH 5-9, making it suitable for diverse biological environments.
Main Applications
In biomedical research, ICG-PEG-MAL serves three primary functions: as a fluorescent tag for biomolecules, a tracking agent for drug delivery systems, and a targeting probe for molecular imaging. Researchers frequently conjugate it to antibodies for sensitive detection of tumor markers or to peptides for receptor-specific imaging. The maleimide-thiol reaction enables site-specific labeling of engineered proteins containing cysteine residues. The pharmaceutical industry utilizes this conjugate to monitor nanoparticle biodistribution and drug release kinetics. When attached to liposomes or polymer nanoparticles, ICG-PEG-MAL allows real-time tracking of carrier systems using fluorescence imaging or photoacoustic tomography. Recent advances include its use in fluorescence-guided surgery, where antibody-ICG-PEG-MAL conjugates help surgeons identify tumor margins with high precision.
Safety and Storage
As a light-sensitive compound, ICG-PEG-MAL requires handling under amber light or in low-light conditions. The powder form should be stored in sealed, desiccated containers at -20°C with desiccant packs. Prepared solutions are best used immediately but may be stored at 4°C for short-term use (≤72 hours) if protected from light. Safety precautions include wearing nitrile gloves, safety goggles, and working in a fume hood when handling powder. Although not classified as highly toxic, the compound may cause eye and skin irritation. Spills should be cleaned with ethanol followed by water. Disposal must comply with local regulations for organic dyes and PEG compounds.
B2B Procurement Guide
When sourcing ICG-PEG-MAL, buyers should specify four critical parameters: PEG molecular weight (typically 2k, 5k, or 10k Da), maleimide content (≥90% active), ICG purity (usually >95% by HPLC), and endotoxin levels (<1 EU/mg for in vivo use). Reputable suppliers provide certificates of analysis including MALDI-TOF mass spectrometry and fluorescence spectroscopy data. Bulk purchases (1g+) often qualify for 15-30% discounts, with lead times of 2-4 weeks for custom PEG lengths. For GMP-grade material required in clinical applications, expect 3-6 month production cycles and 3-5x higher costs. Always verify supplier capabilities for cold chain shipping and proper packaging with desiccant and oxygen absorbers.
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