Analytische Chemie
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The assessment of water quality demands for multi-target compatible analytical methods. On the one hand, the influence of sewage on natural surface waters must be routinely monitored. On the other hand, estimation of removal efficiencies of pollutants, such as drug residues, is in the focus of industrial and public wastewater treatment. Immunoassays, such as ELISA, are often applied in this area, however, it is only possible to measure a single analyte in one measurement.
In contrast, array technology is capable for measuring multiple substances in parallel. Here we present a four-plex bead-based flow cytometric assay for measuring three drugs (carbamazepine, an antiepileptic drug, diclofenac, an antiphlogistic and caffeine, a psychoactive substance) and the bile acid isolithocholic acid, which is proposed to be used as a fecal marker.
In this suspension array core-shell particles, consisting of a polystyrene core and a silica shell, are used. They can be easily encoded with a fluorophore which is introduced to the core while the shell is functionalized with amino groups to which the analytes or their derivates are bound via NHS chemistry. They are the competitor binding sites for the antibodies.
For the determination of the pollutants, the beads are incubated with a mixture of the analyte-specific antibodies. The molecules on the surface of the beads and the free analyte in solution compete for the binding sites of the antibodies. Bead-bound antibodies can then be visualized via dye-labelled secondary antibodies. For read-out a flow cytometer is used to virtually separate the differently encoded beads. In order to obtain highly selective binding of the antibodies, we investigated different types of surface modifications to overcome unspecific binding, finding that a PEG-based surface is suitable to support our immunoassay format. The resulting multiplexing assay is appropriate to detect the marker substances in the low µg/L range.
Monitoring water quality regarding emerging pollutants, such as drug residues, demands for selective, high-throughput and multi-target analytical methods. On the one hand, the influence of sewage on natural surface waters must be routinely monitored. On the other hand, estimation of removal efficiencies of pollutants, such as drug residues, is in the focus of industrial and public wastewater treatment. Immunoassays, such as ELISA, offer the possibility to be highly sensitive and selective due to the high target affinity and specificity of target recognition by antibodies. Batch-wise processing in microtiter plates allows for the necessary high-throughput, but only a single analyte can be determined within one measurement.
To overcome these disadvantages, we developed a four-plex Suspension Array Fluorescence Immunoassay (SAFIA), which is adaptable for the microtiter plate format. The modular and self-prepared bead support consists of polystyrene-core silica-shell microparticles. While the polystyrene core is used for encoding, by introducing different amounts of fluorescent dyes, the silica shell creates a solid-support for the immunoassay: the target analytes, three drugs, carbamazepine, diclofenac and caffeine, and the fecal marker isolithocholic acid are covalently coupled to amino groups on the surface via NHS chemistry. Specific recognition of these haptens is accomplished by further introduction of PEG moieties, suppressing non-specific binding. A competitive immunoassay is subsequently conducted in a simple mix-and-read procedure, eliminating the demand for laborious washing steps and decreasing time-to-result. An automated flow cytometer is used for simultaneous decoding and quantification.
After optimization, the SAFIA showed limits of detection for all analytes in the low µg L 1 range, meeting the sensitivity criteria for wastewater analysis. Then, applicability of the SAFIA was studied on real wastewater samples from three different wastewater treatment plants in Berlin. The results were in good comparability to LC-MS/MS indicating high matrix stability. Moreover, the accuracy of the assay exceeded that of the respective ELISA. Finally, we used SAFIA to assess the influent of treated and untreated wastewater on the Douro river estuary in Portugal. The obtained results of the analysis were comparable to ELISA. However, the measurements by SAFIA could be carried out in one quarter the time of analysis required for ELISA.
Acknowledgements: The authors acknowledge financial support from Deutscher Akademischer Austauschdienst and from Fundação das Universidades Portuguesas under the protocol CRUP-DAAD (Ações Integradas Luso-Alemãs nºE-20/16, DAAD Zuwendungsvertrag 57212899).