Luminol (99%)

Luminol

Other Names: 3-Aminophthalhydrazide, 3-Aminophthalic Hydrazide, 5-Amino-2,3-dihydro-1,4-phthalazinedione, 5-Amino-2,3-dihydrophthalazine-1,4-dione
Chemical Formula: C₈H₇N₃O₂
Molar Mass: 177.16 g/mol
CAS Number: 521-31-3
SMILES: C1=CC2=C(C(=C1)N)C(=O)NNC2=O
Appearance: Pale yellow to yellow crystalline solid or powder

Luminol is a nitrogen-containing heterocyclic compound best known for its intense chemiluminescence during oxidative reactions. Structurally, it is a cyclic hydrazide derived from 3-aminophthalic acid and contains a fused aromatic ring together with a phthalazine-1,4-dione system. NIST reports the molecular formula C₈H₇N₃O₂, molecular mass 177.1601 g/mol and CAS Registry Number 521-31-3. Its systematic name is 5-amino-2,3-dihydrophthalazine-1,4-dione.

Chemical and physical properties

Luminol is a crystalline compound with relatively low solubility in neutral water but substantially improved solubility under alkaline conditions as a result of deprotonation. Published studies describe luminol as a diprotic acid with pK_a values of approximately 6.7 and 15.1. Depending on pH, it can therefore occur in neutral, monoanionic or dianionic forms. The acid-base state strongly influences its oxidation chemistry and chemiluminescent efficiency.

Reported thermal data place its melting or decomposition region at approximately 319–320 °C. Strong heating is therefore associated with thermal transformation rather than ordinary low-temperature volatilization. Luminol solutions are sensitive to reaction conditions, particularly pH, oxidant concentration and the presence of catalytic metal ions or enzymes.

The characteristic property of luminol is its ability to produce visible light during oxidation. In alkaline solution, oxidation generates reactive intermediates that ultimately form an electronically excited 3-aminophthalate species. Relaxation of this excited product to the ground state releases a photon, typically producing blue emission with a maximum near 425 nm, although the exact wavelength depends on solvent and reaction conditions.

Chemiluminescence

Luminol chemiluminescence is commonly initiated using an oxidizing system such as hydrogen peroxide in an alkaline medium. Transition-metal ions, heme-containing compounds and peroxidase enzymes can markedly accelerate the oxidation process. The reaction involves formation of oxidized luminol intermediates followed by nitrogen elimination and generation of electronically excited 3-aminophthalate. The subsequent return of this product to its ground state produces the characteristic blue light emission.

The intensity and duration of luminol emission depend on numerous variables, including pH, oxidant concentration, catalyst identity, solvent composition, temperature and luminol concentration. Chemiluminescence can therefore be adapted for highly sensitive analytical measurements in which the amount of emitted light is correlated with the concentration or activity of a target substance.

Applications

Luminol is extensively used in analytical chemistry as a chemiluminescent reagent. Luminol-based systems are employed for detection and quantification of hydrogen peroxide, reactive oxygen species, metal ions and numerous analytes that influence oxidation or catalytic activity. Because chemiluminescence does not require an external excitation light source, luminol assays can provide very low optical background and high analytical sensitivity.

One of the best-known applications of luminol is the presumptive detection of latent blood traces in forensic analysis. Hemoglobin-associated iron catalyzes luminol oxidation, allowing very small or visually undetectable blood residues to generate visible chemiluminescence. The method is highly sensitive but is considered presumptive because other oxidizing substances and certain metal-containing materials can also produce or enhance light emission.

Luminol is also widely used in biochemical assays involving peroxidase enzymes. Horseradish peroxidase and related enzymes catalyze oxidation reactions that generate luminol chemiluminescence, enabling highly sensitive detection in immunoassays, blotting methods, enzyme studies and other bioanalytical procedures. Luminol-based chemiluminescence has additionally been investigated for detection of proteins, nucleic acids and cellular oxidative processes.

In electroanalytical chemistry, luminol is an important electrochemiluminescent probe. Electrochemical oxidation at an electrode surface can generate reactive luminol intermediates that interact with reactive oxygen species and produce excited 3-aminophthalate. Luminol electrochemiluminescence has consequently been developed for biosensors, immunosensors and other highly sensitive analytical platforms.

Luminol and structurally modified luminol derivatives are also investigated in photochemistry and materials science. Modification of the aromatic ring or hydrazide framework can alter solubility, emission intensity, emission wavelength and compatibility with aqueous or biological systems, making these compounds useful model systems for studying chemiexcitation and molecular light-emission processes.

Handling and safety

Luminol should be handled using standard laboratory protective measures, including suitable gloves and eye protection, while minimizing formation and inhalation of dust. Solutions should be protected from unnecessary exposure to light and incompatible reactive chemicals. Strong oxidizing agents can initiate rapid oxidation and chemiluminescent reactions, while strongly acidic or alkaline conditions alter the protonation state and chemical behavior of the molecule. Published reviews note that complete human toxicological characterization is limited, so appropriate laboratory precautions should be maintained.

Scientific references

NIST Chemistry WebBook, SRD 69. Luminol. CAS Registry Number 521-31-3.

Barni, F.; Lewis, S. W.; Berti, A.; Miskelly, G. M.; Lago, G. Forensic application of the luminol reaction as a presumptive test for latent blood detection. Talanta, 2007, 72, 896–913. DOI: 10.1016/j.talanta.2006.12.045.

Merényi, G.; Lind, J.; Eriksen, T. E. Luminol chemiluminescence: chemistry, excitation, emitter. Journal of Bioluminescence and Chemiluminescence, 1990, 5. DOI: 10.1002/bio.1170050111.

Chandel, A. L. S.; Khan, S. A.; Kher, R. S.; Tiwari, A. Investigation on the chemiluminescence reaction of the phenylhydrazine-luminol-peroxide system. Luminescence, 2012, 27, 455–458. DOI: 10.1002/bio.1374.

Fang, Y. et al. Progress of the Electrochemiluminescence Biosensing Strategy for Clinical Diagnosis with Luminol as the Sensing Probe. ChemElectroChem, 2017. DOI: 10.1002/celc.201700465.

Safety
Signal Word: Warning

First Aid Measures
Eye Contact: Rinse with water for at least 15 minutes. Seek medical attention if irritation occurs.
Skin Contact: Wash with soap and water. Remove contaminated clothing. Seek medical advice if irritation develops.
Inhalation: Move to fresh air. Seek medical attention if respiratory symptoms occur.
Ingestion: Rinse mouth with water. Do not induce vomiting. Seek medical advice if feeling unwell.

Handling and Storage
Storage Conditions: Store in a cool, dry, well-ventilated area in tightly sealed containers. Keep away from strong oxidizers and direct sunlight.
Handling Precautions: Avoid formation of dust. Use appropriate protective equipment. Ensure good ventilation during handling.