Label-free C-reactive protein SERS detection with silver nanoparticle aggregates†
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).