Stable and Photothermally Efficient Antibody-Covered Cu3(PO4)2@Polydopamine Nanocomposites for Sensitive and Cost-Effective Immunoassays.

Polydopamine (PDA)-coated or encapsulating Cu3(PO4)2 (Cu3(PO4)2@PDA) nanosheets were synthesized, allowing the C-reaction protein (CRP) antibody to be attached electrostatically for immunosensing of CRP with simple photothermal detection. The antibody-covered Cu3(PO4)2@PDA nanosheets replace the antibody-conjugated enzyme in the enzyme-linked immunosorbant assays. Owing to the high surface area of the 2-D-structured Cu3(PO4)2@PDA nanosheets and the coabsorption of light in the near-IR spectrum by Cu3(PO4)2 and PDA, a small amount of Cu3(PO4)2@PDA confined in the wells of a titer plate generates an easily detectable temperature change after irradiation at 808 nm. The temperature changes, measured by an inexpensive pen-type thermometer, increased linearly with the analyte concentration from 0.42 to 16 pM. We found that the linear relationship can be fitted by the isotherm derived from responses collected from heterogeneous sensors covered with different ligand or antibody densities. The low detection limit (0.11 pM) is largely due to the attachment of a great number of antibodies onto the flat nanosheets. The antibody-covered Cu3(PO4)2@PDA nanosheets are stable and can be used under conditions that are generally unfavorable to enzymatic activities. The excellent agreement between our results and immunoturbidimetric assays of CRP in serum samples from patients and healthy donors demonstrates its utility for disease diagnosis in clinical settings. This cost-effective, biocompatible, and convenient photothermal immunosensor affords a range of possibilities for detecting diverse protein biomarkers.