Ultrastable and Biofunctionalizable Gold Nanoparticles

Abstract

Gold nanoparticles provide an excellent platform for biological and material applications due to their unique physical and chemical properties. However, decreased colloidal stability and formation of irreversible aggregates while freeze-drying nanomaterials limit their use in real world applications. Here, we report a new generation of surface ligands based on a combination of short oligo (ethylene glycol) chains and zwitterions capable of providing nonfouling characteristics while maintaining colloidal stability and functionalization capabilities. Additionally, conjugation of these gold nanoparticles with avidin can help the development of a universal toolkit for further functionalization of nanomaterials.

Keywords: DLS; UV; biofluids; freeze-drying; gold nanoparticles; ion strength; stability; surface plasmon resonance.

Figure 1

a) Chemical structure of the AuNP ligand, composed of an oligo (ethylene glycol) chain followed by a carboxybetaine zwitterionic group. b) TEM of the 15 nm AuNPs after functionalization with the engineered ligand (AuNP-Zwit). c) DLS profiles of AuNP-Zwit and AuNP-Citr (without ligand functionalization) in phosphates buffer (PB).

Figure 2

a) UV spectra of AuNP-Citr, b) 15 nm AuNP-PEG, c) 15 nm AuNP-Zwit and d) 25 nm AuNP-Zwit at different ion strengths, evidencing the enhanced stability conferred by the zwitterionic ligand. e) UV spectrum of 15 nm and f) 25 nm AuNP-Zwit, after successive cycles of freeze-drying.

Figure 3

Comparison of NP size after 24 hr incubation in serum a) 15 nm AuNPs b) 25 nm AuNPs. Particle size distribution for NPs in the presence and absence of serum proteins (50%) evidencing the stability of AuNP-Zwit c) DLS profiles of 15 nm AuNP-Citr, AuNP-PEG and AuNP-Zwit and d) DLS profiles of 25 nm AuNP-Citr, AuNP-PEG and AuNP-Zwit respectively.

Figure 4

a) Scheme of the conjugation of AuNP-Zwit/avidin with biotin-functionalized HRP. b) UV spectrum of AuNPs alone and conjugated with HRP after the addition of ABTS (substrate). c) Normalized substrate transformation by the AuNP-Zwit/avidin-HRP conjugates before and after freeze-drying. As evidenced by the initial rates of reaction (Vi), the AuNP-Zwit/avidin conjugates are capable of enduring freeze-drying stress without losing activity.