Rapid lateral flow assays based on the quantification of magnetic nanoparticle labels for multiplexed immunodetection of small molecules: application to the determination of drugs of abuse.

A rapid lateral flow immunoassay is presented that uses carboxyl-modified superparamagnetic nanoparticles as labels that can be quantified by highly sensitive multi-channel electronic readers. The approach is generic in that it is likely to be applicable to numerous small molecules. The method permits both single- and multiplex assays at a point-of-need without sample pretreatment. It is user-friendly and offers attractive characteristics demonstrated here for detection of morphine, fentanyl and methamphetamine in urine. The competitive immunoassay uses commercially available reagents that do not require special permissions. After migration of sample, the lateral flow test strips are subjected to an alternating magnetic field at two frequencies. The response from the nanolabels is readout at a combinatorial frequency from the entire volume of a porous immunochromatographic membrane by the magnetic particle quantification technique. Even trace concentrations can be quantified within ≤20 min with the limits of detection (LOD) of 0.20 ng·mL-1, 0.36 ng·mL-1 and 1.30 ng·mL-1 for morphine, fentanyl and methamphetamine, respectively. The second variant presented here features highly sensitive quantification of haptens (LOD for fentanyl - 0.05 ng·mL-1). This is due to high-affinity trapping of magnetic nanolabels in a universal streptavidin-based test strip, which can be also used for detection of virtually any other small molecule. The third variant is of the multiplexed type and intended for rapid and simultaneous detection of the drugs of abuse in human urine with LODs equal to 0.60 ng·mL-1 and 3.0 ng·mL-1 for morphine and methamphetamine, respectively. In addition to the low LODs, the RSDs did not exceed 7%, 9%, and 11% for methamphetamine, morphine and fentanyl, respectively. Graphical abstract Three variants of small molecule detection in competitive format at a point-of-need. Single-plex variants feature antibody and high-affinity streptavidin test lines, while multiplex variant - several antibody test lines. Magnetic nanolabels are quantified from the whole volume of test strip.