Colloidally stable and surfactant-free protein-coated gold nanorods in biological media

Abstract

In this work, we investigate the ligand exchange of cetyltrimethylammonium bromide (CTAB) with bovine serum albumin for gold nanorods. We demonstrate by surface-enhanced Raman scattering measurements that CTAB, which is used as a shape-directing agent in the particle synthesis, is completely removed from solution and particle surface. Thus, the protein-coated nanorods are suitable for bioapplications, where cationic surfactants must be avoided. At the same time, the colloidal stability of the system is significantly increased, as evidenced by spectroscopic investigation of the particle longitudinal surface plasmon resonance, which is sensitive to aggregation. Particles are stable at very high concentrations (cAu 20 mg/mL) in biological media such as phosphate buffer saline or Dulbecco's Modified Eagle's Medium and over a large pH range (2-12). Particles can even be freeze-dried (lyophilized) and redispersed. The protocol was applied to gold nanoparticles with a large range of aspect ratios and sizes with main absorption frequencies covering the visible and the near-IR spectral range from 600 to 1100 nm. Thus, these colloidally stable and surfactant-free protein-coated nanoparticles are of great interest for various plasmonic and biomedical applications.

Keywords: CTAB replacement; biocompatible; colloidal stability; ligand exchange; lyophilized; protein coating.

Figure 1

(a) UV–vis–NIR spectra of all AuNR@CTAB. (b) L-LSPRs of AuNR before and after BSA coating. (c) UV–vis–NIR spectra of three selected AuNR samples (Nos. 5, 11, and 15) before and after BSA coating. (d–f) TEM images of the AuNR@BSA from (c).

Figure 2

UV–vis–NIR spectra and photographs of three selected AuNRs, namely, (a) No. 5 at 760 nm, (b) No. 11 at 900 nm, and (c) No. 15 at 1050 nm dispersed in different media: AuNR@BSA samples (c_Au = 0.2 mM) were dissolved at different pH (12 and 2), in PBS buffer (150 mM, pH 7.5), in DMEM+10% NCS (pH 7.5), and highly concentrated (1 mM). AuNR@CTAB samples are included for comparison.

Figure 3

Redispersion behavior of different freeze-dried AuNR@BSA powders: (a) No. 5 with 1 mg/mL sucrose (c_Au = 0.34 mM); (b) No. 11 from DMEM+10% NCS (c_Au = 0.12 mM); and (c) No. 15 with 1 mg/mL BSA (c_Au = 0.16 mM). All powders spontaneously redispersed in water at pH 12 (cuvettes). (d) UV–vis–NIR spectra of AuNR@BSA samples before (dashed line) and after freeze-drying (full line).

Figure 4

SERS of AuNPs (AuNS: spheres; AuNR: rods) with CTAB (black, upper) and BSA (red, lower) coating dispersed in water and compared with conventional Raman spectra of crystalline CTAB and dry BSA: (a) Counterion signals; (b) ammonium signals; (c) skeletal chain vibrations (upper) and amide bands (lower); (d) methyl/methylene “fingerprint”. The spectra are offset, scaled for clarity, and show raw data without background correction.