A simple and versatile self-assembled monolayer based surface plasmon resonance immunosensor for highly sensitive detection of 2,4-D from natural water resources

In this work, we demonstrate the development of a highly sensitive and selective surface plasmon resonance (SPR) immunosensor for label-free detection of 2,4-dichlorophenoxyacetic acid (2,4-D) as low as 10 ppt (pg ml^-1) direct from aquatic environmental samples. 2,4-D is a systemic herbicide and a potential endocrine disrupting chemical. Primary screening methods for detection of 2,4-D should be easy-to-use, inexpensive and usable for routine analysis of a large number of food and potable water samples. Here, we fabricate the sensing surface of the SPR immunosensor simply by covalent amide binding of a bovine serum albumin conjugate of 2,4-D (hereafter, 2,4-D-BSA) on the Au-thiolate self-assembly of simple and commercially available 3-mercaptopropionic acid (MPA). An indirect competitive immunoassay method that assures the detection of low-molecular-weight analytes was accomplished for the detection of 2,4-D, in which the specific affinity binding of monoclonal anti-2,4-D antibody (hereafter, 2,4-D-mAb) on the 2,4-D-BSA bound SAM surface was examined at various concentrations of 2,4-D. The extent of the 2,4-D-mAb binding on the sensor surface is inhibited by the presence of 2,4-D, and the SPR angle response is inversely proportional to the concentration of the analyte 2,4-D. With the developed SPR sensor, a low-detection-limit of 0.1 ppb (ng ml^-1) 2,4-D is established with a response time of only 4 min. By taking advantage of the controlled immobilization of 2,4-D-BSA on the SAM surface, the immunoaffinity interactions of 2,4-D-mAb with the 2,4-D-BSA sensor surface and 2,4-D in solution could be significantly modulated. As a result, the sensitivity of the SPR immunosensor is enhanced by about 10-fold to 10 ppt without using any high-molecular-weight labels. After an immunoaffinity binding cycle, the active sensor surface was retrieved by the removal of 2,4-D-mAb from the sensor surface using an acidic buffer (glycine·HCl, pH 2.0). The SPR immunosensor showed excellent selectivity for 2,4-D detection with a negligible cross-sensitivity against various closely related environmental pollutants and is found capable of detecting ppt levels of 2,4-D from spiked river water samples.