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 similar to 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 (10(9) to 10(10) M-1) of apparent binding constant (K-A) 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 similar to 0.01 ng mL(-1) in buffer and similar to 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 (similar to 1.1 x 10(6)) 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 x 10(5)).