Study on the Assembly Structure Variation of Cetyltrimethylammonium Bromide on the Surface of Gold Nanoparticles

Abstract

In this work, the self-assembly behavior of cetyltrimethylammonium bromide (CTAB) on the surface of citrate-capped gold nanoparticles (AuNPs) in solution has been studied by UV-vis absorption spectroscopy, fluorescence probe techniques, ζ potentiometric methods, transmission electron microscopy, etc. The UV-vis spectra show that the color with the increase of CTAB for the mixture containing CTAB and a given amount of AuNPs changes from red to blue and then to red. The absolute value of ζ potential corresponding to this color change decreases initially and then increases. Specially, the reversible color change, from red to blue and then to red, could be observed only in the case of a gradual addition of a AuNP solution to a CTAB solution; however, this reversible change is not suitable for the mixture formed in a reverse order of mixing. The results from pyrene used as the fluorescence probe indicate that the features in the fluorescence spectrum (including fluorescence quenching, I ₁/I ₃, and the excimer) well correspond to those from the UV-vis spectrum mentioned above. Based on the experimental results, the mechanism of the assembly structure variation of CTAB on the surface of negatively charged AuNPs was proposed. For a given amount of AuNPs, the assembly structure of CTAB on the surface of AuNPs undergoes the transformation from a monolayer to a bilayer with the increase of CTAB. In the case of the concentration of CTAB far beyond its critical micelle concentration (CMC) and the higher ratio of CTAB and AuNPs, there is a possibility of the formation of an extra micellar structure only after the formation of a double-layer structure.

Figure 1

(a) Change in the UV–vis spectrum with different CTAB concentrations for the mixture of CTAB and a given amount of AuNPs (1.50 × 10¹² NPs/mL), and photos (top) of the corresponding samples. (b) Change in the ζ potential of citrate-capped AuNPs (1.50 × 10¹² NPs/mL) with different concentrations of CTAB. (c) Transmission electron microscopy (TEM) images of AuNPs in the presence of different CTAB contents.

Figure 2

Change in the UV–vis spectra with different concentrations of CTAB (10 NPs/mL).

Figure 3

Proposed possible mechanism for the variation in the assembly structure of CTAB on the citrate-capped AuNPs.

Figure 4

(a) Fluorescence spectra of pyrene (1.629 × 10^–8 M) in a series of mixtures with different ratios of AuNPs to CTAB as given in the mixture C (AuNPs: 1.50 × 10¹² NPs/mL). (b) Change in the pyrene (0.036 mM) fluorescence spectrum and intensity (inset) with time for the mixture containing CTAB and AuNPs. CTAB: 1 mM, AuNPs: 1.50 × 10¹² NPs/mL.

Figure 5

UV–vis spectra of the supernatant of aggregated AuNPs induced by CTAB and the aggregates suspended in different concentrations of CTAB. The aggregated AuNPs obtained from the experiment in Figure 4b.

Figure 6

Proposed mechanism for the effect of the interaction between CTAB micelles and AuNPs on the microenvironment of the fluorescence probe pyrene.

Figure 7

Photos showing the color change in the solutions formed by (A) CTAB and (B) AuNPs in different mixing orders and their corresponding resulted precipitates dispersed in CTAB solutions.

Figure 8

TEM images of the precipitations from the mixtures made in different mixing orders. (A) Addition of CTAB into AuNPs and (B) addition of AuNPs into CTAB.