Gold nanoparticle size controlled by polymeric Au(I) thiolate precursor size

Abstract

We developed a method in preparing size-controllable gold nanoparticles (Au NPs, 2-6 nm) capped with glutathione by varying the pH (between 5.5 and 8.0) of the solution before reduction. This method is based on the formation of polymeric nanoparticle precursors, Au(I)-glutathione polymers, which change size and density depending on the pH. Dynamic light scattering, size exclusion chromatography, and UV-vis spectroscopy results suggest that lower pH values favor larger and denser polymeric precursors and higher pH values favor smaller and less dense precursors. Consequently, the larger precursors led to the formation of larger Au NPs, whereas smaller precursors led to the formation of smaller Au NPs. Using this strategy, Au NPs functionalized with nickel(II) nitriloacetate (Ni-NTA) group were prepared by a mixed-ligand approach. These Ni-NTA functionalized Au NPs exhibited specific binding to 6x-histidine-tagged Adenovirus serotype 12 knob proteins, demonstrating their utility in biomolecular labeling applications.

Figure 1

Crystal structure of Ad12 knob protein showing the top view (A) and the side view (B) orientations (Reference 24). The arrows in B indicate the N-termini where His-tags were placed.

Figure 2

TEM micrographs of the Au NP 1 synthesized at different pH values; in parentheses are the average sizes in nm. (a) 5.5 (6.3), (b) 6.1 (5.4), (c) 6.3 (4.7), (d) 6.5 (6.0), (e) 6.8 (3.6), (f) 7.0 (3.3), (g) 7.5 (2.6), (h) 8 (2.2). All of the micrographs are in the same scale.

Figure 3

Size dependence of the AuNP 1 with pH as measured by EM. Solid squares correspond to the average sizes of Au NP capped with GSH, while open squares represent the average sizes of Au NP 2. The error bars indicate the standard deviations. About one hundred particles were analyzed for each point.

Figure 4

Studies on the dependence of polymeric Au(I)SG precursor size on pH. A. Hydrodynamic radius at different pH values of the polymeric Au(I)SG precursors measured using DLS. Error bars are standard deviation of three trials. B. The size exclusion chromatograms of the polymeric Au(I)SG precursors at pH 4.8, 6.4, and 7.9 with retention times of 22.9, 23.4, and 23.7 min, respectively. The precursors were separated using Superose 6 column with 10 mM phosphate or acetate buffered saline as the elution buffer.

Figure 5

UV-Vis spectra of the polymeric Au(I)SG precursor at different pH values.

Figure 6

A: TEM micrograph of His-tagged Ad12 knob proteins labeled with Au NP coated with GSH/Lys-Ni-NTA-SH (negatively stained). Inset: Higher magnification image of the core-shell structure formed between the knob proteins and Au NP. Arrows indicate the position of the knob proteins around the black dot (Au NP). B: TEM micrograph of the mixture of non-His-tagged Ad12 knob proteins and Au NPs. Both images are shown in the same scale.

Scheme 1

Synthesis of Lys-NTA-SH.

Scheme 2

Synthesis of Au NPs. A. Au NP coated with GSH only; B. Au NP coated with 3:1 mixture of GSH and Lys-NTA-SH. Reaction conditions: i. HAuCl₄, ii. Adjust pH, iii. NaBH₄.

Scheme 3

Proposed mechanism for the formation of Au NP from Au³⁺ and GSH.

Scheme 4

An illustration of the dependence of the Au-S-Au angles and the corresponding intramolecular forces of the NP precursor on pH.

Scheme 5

The ionization of GSH. The pK₁, pK₂, pK₃, and pK₄ values are 2.12, 3.53, 8.66, and 9.62, respectively (Reference 40).

Scheme 6

A schematic representation of the proposed mechanism of the formation of size-controllable Au NPs capped with GSH.

Scheme 7

The binding reaction between a His-tagged protein and the Au NP functionalized with NiNTA.