Rapid Determination of Methamphetamine and Cocaine in Saliva by Portable Surface Plasmon Resonance (SPR)

Methamphetamine (MA) and cocaine (COC) are among the most abused drugs worldwide. In this study, a portable dual-channel surface plasmon resonance (SPR) biosensor based on an indirect inhibitive immunoassay was developed for the rapid and simultaneous determination of MA and COC in saliva. MA-bovine serum albumin (MA-BSA) and COC-BSA conjugates were prepared as antigens and immobilized on the sensor chip in each channel. Upon contact with the surface-immobilized antigens, MA monoclonal antibody (MA-Ab) or COC monoclonal antibody (COC-Ab) enhanced the SPR signal. The bonding of the antibodies to antigens was inhibited by the presence of MA or COC in the samples. The concentrations of the antigens and antibodies were optimized, and the resulting SPR inhibitive immunoassay exhibited good sensitivity, with limits of detection for MA and COC equal to 0.95 and 3.14 ng·mL−1, respectively. These values are below the legal limits of these drugs for vehicle drivers in China. Furthermore, the suitability of the method was confirmed using saliva from healthy donors, which demonstrated acceptable specificities with satisfactory recoveries. The results indicate that the portable SPR biosensor is facile and rapid for the sensitive determination of abused drugs.