In this work, we report a qualitative approach for detecting the adsorption of C-reactive protein on phosphocholine-terminated self-assembled monolayers without the use of any labels. An amplified plasmon of concentration-induced silver nanoparticle aggregates located ∼4.0 nm away from the C-reactive protein via the phosphocholine-terminated self-assembled monolayer linker is considered to be the source of the robust electromagnetic enhancement. The high level (109 to 1010 M-1) of apparent binding constant (KA) of C-reactive protein suggests that the immobilized surface was well-oriented without extreme random stacking. A Raman sensitivity toward the C-reactive protein around 2800-3000 cm-1 was noted, which gradually increased upon the addition of successive layers up to approximately 6-7 layers of phosphocholine-coated silver nanoparticle aggregates, with minimum detection amounts of ∼0.01 ng mL-1 in buffer and ∼0.1 ng mL-1 in 1% serum. A cross-reactivity test confirmed the excellent selectivity and specificity of the measured signals. A computational study based on the finite-difference-time-domain method successfully demonstrated the enhanced (∼1.1 × 106) electromagnetic field of the 2-D silver nanoparticle aggregates as compared with that of isolated particles, and was congruent with the analytical enhancement factor (1.7 × 105). © The Royal Society of Chemistry 2015.
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