Liquid magnetic competitive immunoassay of clenbuterol based on surface‐enhanced Raman spectroscopy

The surface-enhanced Raman spectroscopy (SERS)-based immunoassay has attracted considerable interests due to its extremely high sensitivity. However, the involved solid substrate brought significant difficulties in its extensive application. Herein, based on the integration of magnetic nanoparticles and SERS, a facial liquid magnetic competitive immunoassay was developed to detect clenbuterol (CL) with high sensitivity and specificity. The competitive reaction with compatible antibody occurred between target and corresponding antigen attached on nanoparticles. The readout strategy based on the inverse correlation of SERS intensity and concentration was employed to determine the content of target. The SERS intensities were originated from the immunocomplexes in sediment enriched by applying an external magnetic field. The coupling effect between the Au and Fe3O4@Au allowed the increase of SERS intensities for improving the performance of this approach, both for limit of detection (LOD) and sensitivity. The immuno protocol was optimized carefully through screening out the amount ratio of different immunonanoparticles, exchanging the immobilization positions of antibody and coating antigen. It was demonstrated that the application of CL antibody-mercaptobenzoic acid-labeled Au and CL coating antigen-Fe3O4@Au nanoparticles (configuration 1) exhibited higher sensitivity than that of CL coating antigen-mercaptobenzoic acid-labeled Au and CL antibody-Fe3O4@Au nanoparticles (configuration 2). It was determined that the LOD and half maximal inhibitory concentration (IC50) were about 0.22 and 246.31 fg ml−1 for CL target of configuration 1, respectively. It was better notably than the previous reports. Moreover, incompatible target, β-agonist salbutamol (SAL), was also introduced to investigate the effect on CL competitive immunoassay. It revealed that the competitive immunoassay with CL antibody was negligible, indicating the high specificity of the present immunoassay. The SERS-based liquid magnetic sensor is expected to be developed as a promising technique for the detection of micromolecules and to provide a potential application in coupling with other techniques. Copyright © 2017 John Wiley & Sons, Ltd.