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

• Published in: RSC advances 2015-01, Vol.5 (44), p.34720-34729
• Main Authors: Kim, Hyunmin, Kim, Eunjoo, Choi, Eunsook, Baek, Chul Su, Song, Bokyung, Cho, Chang-Hee, Jeong, Sang Won
• Format: Article
• Language: English
• Subjects: Aggregates; Binding; Constants; Nanostructure; Proteins; Self-assembled monolayers; Silver; Stacking
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C5RA00040H

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 (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 ∼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 × 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 × 10 5 ).