In addition to the dry parts of a lateral-flow assay, there are also the biological components that allow the visualization of the results. Because of the relatively small size of a drug-of-abuse molecule, a competitive immunoassay with an antibody molecule conjugated to a colloidal gold particle is used. Antibodies can be polyclonal or monoclonal. In all cases, the antibodies have to be purified before use. Gold colloids are formed by the reduction of gold tetrachloric acid through a "nucleation" process. The size and shape of the colloids depend on the type and amount of reducer used. An accurate and reproducible lateral-flow assay requires the use of high-quality gold conjugates. The most common size of colloidal gold particle used is 40 nm. Conjugation of colloidal gold particles and antibodies depends on the availability and accessibility of three amino acid residues—lysine, tryptophan, and cysteine. Once a high-quality antibody-gold conjugate is formed, it can be applied to the conjugate pad either by soaking or by spraying. The drying process that follows is essential. It is affected by temperature, humidity, air flow, and pad thickness. Typically, forced-air systems are employed in conjunction with elevated temperature in the drying process. Finally, the proper functioning of a lateral-flow assay also depends on other nonbiological components, such as surfactants, blocking reagents, and buffers.
From: Forensic Science and Medicine: Drugs of Abuse: Body Fluid Testing Edited by R. C. Wong and H. Y. Tse © Humana Press Inc., Totowa, NJ
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