The stability of gold nanoparticles is a major issue which decides their impending usage in nanobiotechnological applications. Often biomimetically synthesized nanoparticles are deemed useless owing to their instability in aqueous medium. So, surfactants are used to stabilize the nanoparticles. But does the surfactant only stabilize by being adsorbed to the surface of the nanoparticles and not play significantly in moulding the size and shape of the nanoparticles? Keeping this idea in mind, gold nanoparticles (GNPs) synthesized by l-tryptophan (Trp) mediated reduction of chloroauric acid (HAuCl4) were stabilized by anionic surfactant, sodium dodecyl sulphate (SDS), and its effect on the moulding of size and properties of the GNPs was studied. Interestingly, unlike most of the gold nanoparticles synthesis mechanism showing saturation growth mechanism, inclusion of SDS in the reaction mixture for GNPs synthesis resulted in a bimodal mechanism which was studied by UV-Vis spectroscopy. The mechanism was further substantiated with transmission electron microscopy. Zeta potential of GNPs solutions was measured to corroborate stability observations recorded visually.
Gold nanoparticles are important to bionanotechnology, owing to their easy synthesis, inert nature, good biocompatibility, and unique optical properties [
In recent years, many biomimetic synthetic [
Chloroauric acid (Sigma-Aldrich), l-tryptophan (Fluka AG), and sodium dodecyl sulphate (Sigma, USA) were used as received. All experiments were performed in Millipore purified water.
Gold nanoparticles are produced in absence of any stabilizer by the reaction of chloroauric acid (HAuCl4) with l-tryptophan (Trp) mixed in different ratio. The UV-Vis spectra of the produced nanoparticles were then recorded in JASCO V-650 spectrophotometer. The same experiment was performed in presence of 1 mM sodium dodecyl sulphate (SDS). The stable gold sols were then characterised by scanning electron microscopy (SERON INC, South Korea, AIS 2100), energy dispersive X-ray spectroscopy (Oxford Instruments, UK, INCA E350), transmission electron microscopy (Zeiss, Libra 12), and zeta potential measurements (Zetasizer, Malvern, Nano-Z).
Again, the particle size determines the peak position. At nanoregime, the variation in surface to volume ratio with increasing size shows a gradually decreasing trend. As a result, on increasing size the fraction of surface free electrons decreases. So less energy is required to polarize them. Thus, red shift (shift to longer wavelength) is observed on increasing size. Moreover, plasmonic dipolar coupling leads to red shift of the plasmon band on agglomeration.
GNPs were synthesized by the reaction of HAuCl4 and Trp in absence of any stabilizing agent. UV-Vis spectrums of the GNP solutions were recorded, as shown in Figure
UV-Vis spectrum of gold nanoparticles in absence of any stabilizer in aqueous medium formed by the reaction of 0.5 mM HAuCl4 with (a) 0.5 mM l-Trp and (b) 1 mM Trp and that of 1 mM HAuCl4 with (c) 0.5 mM l-Trp and (d) 1 mM Trp.
Another set of similar reactions where the ratios of HAuCl4 and Trp were the same as above, was carried out in presence of 1 mM SDS as stabilizer. The UV-Vis spectrums of the gold nanoparticles solutions were recorded as shown in Figure
UV-Vis spectrum of gold nanoparticles in 1 mM SDS solution formed by the reaction of 0.5 mM HAuCl4 with (a) 0.5 mM Trp and (b) 1 mM Trp and that of 1 mM HAuCl4 with (c) 0.5 mM Trp and (d) 1 mM Trp.
Transmission electron microscope images of GNPs formed in the above four reaction sets prepared in presence of 1 mM SDS were recorded as shown in Figures
Transmission electron microscope image of gold nanoparticles in 1 mM SDS solution formed by the reaction of 0.5 mM HAuCl4 with (a) 0.5 mM Trp and (b) 1 mM Trp and that of 1 mM HAuCl4 with (c) 0.5 mM Trp and (d) 1 mM Trp.
Zeta potential measurements were carried out in an electrophoretic cell to determine the long term stability and to provide a basis to the earlier mentioned visual observations about the stability of different gold nanoparticles solutions. Zeta potential values recorded after formation of GNPs shown in Figures
Studying of the kinetics involves plotting of UV-Vis
UV-Vis spectrum recorded at 530 nm with time (in minutes) for the reaction of 0.5 mM Trp with 1 mM HAuCl4 in absence of any stabilizer.
All the solution sets of HAuCl4 and Trp in presence of SDS were subjected to fixed wavelength UV-Vis absorption at their respective
UV-Vis spectrum recorded at 543 nm with time (in minutes) for the reaction of 1 mM Trp with 1 mM HAuCl4 in 1 mM SDS solution.
The absorbance versus time plot (Figure
UV-Vis spectrums recorded during the reaction of 1 mM Trp with 1 mM HAuCl4 in 1 mM SDS solution.
In region A (Figure
Finney et al. [
Schematic representation of bimodal mechanism of formation of gold nanoparticles.
First two steps, namely, nucleation and autocatalytic growth, are difficult to characterize by optical method as magnitude of
It can be seen that the variation in wavelength in region B is very less. It reflects that though bimolecular agglomeration has started but the rate is not that high. The rate law for the last two steps, namely, bimolecular agglomeration and autocatalytic agglomeration, may be given as follows:
In region C, there is a drastic increase in the wavelength as well as absorbance. This indicates that the formation of C has increased and as a result the size distribution shifts toward the higher wavelength. Now the rate equation can be given by
In region D, trace resembles a saturation growth type of profile. In this region, the change in wavelength is very less. It may be said that, in this region, the predominant process occurring is autocatalytic agglomeration.
This bimodal mechanism of formation of GNPs is further confirmed by transmission electron microscopy by recording the image of the GNPs formed after 5 minutes of initiation of reaction, that is, in the plateau region shown in Figure
Transmission electron microscope image of GNPs formed after 5 minutes of initiation of reaction between 1 mM HAuCl4 and 1 mM Trp in presence of 1 mM SDS.
It is assumed that UV-Vis absorbance value is proportional to the concentration and approximate concentration of gold nanoparticles determined by taking the approximate size of gold nanoparticles as obtained from TEM image shown in Figures
UV-Vis spectrum recorded at 543 nm with time (in minutes) for the reaction of 1 mM Trp with 1 mM HAuCl4 in 1 mM SDS solution, showing fitted curve using (
L-tryptophan reduced gold nanoparticles are stabilized in aqueous solution by SDS. Results revealed that SDS plays an important role in the formation mechanism of the gold nanoparticles. Moreover, the reaction kinetics of GNPs in this case is easily observable by a steady-state spectroscopic method such as UV-Vis spectroscopy unlike NaBH4 where the reaction is too fast and citrate method where heating is required to initiate the reaction. The kinetic details of the formation processes of GNPs reveal the bimodal mechanism. The observation of formation of gold nanoparticles of two different sizes, spaced over a time gap of about 30 minutes during the process of synthesis, opens up the prospect of utilizing the same reaction to tune nanoparticles size. The GNPs prepared by this method are quite stable, which suggests that their usage in biological application can be safely extrapolated in future. This study guides our attention towards the potential usage of anionic surfactants in tuning the size of GNPs along with their stabilization.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to Dr. B. N. Jagatap, Director of Chemistry Group, and Dr. D. K. Palit, Head of Radiation & Photochemistry Division, for their encouragement during the course of this study.