Introduction

Considerable attention has been focused on AuNPs due to their unique optical, catalytic and electronic properties. AuNPs are known to have a strong plasmon resonance absorption in the visible region (called surface plasmon resonance, SPR) due to the collective oscillation of free electrons in the conduction band, and therefore they are being used for optical sensors, Surface-Enhanced Raman Scattering (SERS) [1]. Generally speaking, more is understood about the properties and the mechanism of formation of AuNPs, a better control of their sizes, shapes and applications can be achieved.

A number of synthetic methods have been developed to prepare monodispersed AuNPs, such as thermal reduction of Au(III)-containing salts (like AuCl₄^-) including traditional citrate reduction [1] and the widely employed Brust-Schiffrin method of two-phase synthesis with thiol stabilization [2], and photochemical reduction methods [3]. The photochemical synthesis of AuNPs offers the possibility of controlling the rate of NP formation as well as the spatial and temporal control that characterized many photochemical reactions [3-5]. Various agents and approaches have been used in photochemical reductions, such as photosensitizers, dendrimers (as stabilizers), and the placement of metal salts in polymer films [3]. The most common precursor (HAuCl₄) shows a strong absorption at ~ 320 nm and significant absorptions at shorter ultraviolet wavelengths, which matches the emissions of common UV lamps (center around 350 nm), so it provides a useful region for photochemical synthesis of AuNPs.

Reactive intermediates can be photochemically generated in intermolecular reactions involving triplet ketones [3, 5]. Ketyl radicals, one of powerful reducing agents, normally are involved in metal ion reduction as reducing species, in that the redox potentials of ketyl radicals are thermodynamically favorable in gold ion reduction process [3]. Zhao et al. attributed formation of NPs to a multiphoton process of 800 nm fs irradiation light to form electrons capable of reducing AuCl₄^- to Au(0) [6]. Kurihara et al. generated AuNPs with γ-irradiation and laser irradiation to study transient absorption spectrum and the presence of intermediates [7]. Malone et al. discussed possible mechanisms of formation inside polymer gels by photochemical reduction [8].

It has also been suggested that the surfactants play an important role in the formation of NPs through micelle formation, and preformed NPs are able to catalyze disproportionation [4]. Anisotropic AuNPs have been prepared by the photochemical reduction in the room-temperature ionic liquid without any additional capping agents, in which the ionic liquid can act as a reaction medium, template, and capping agent [9]. Sporadic researches showed that different solvents (like acetone and methanol) affect the AuNP synthesis, too [10].

However, the effect of solvents on the AuNP synthesis has not been studied yet, so this paper aims to point out the significance of solvent effects on photochemical reduction-based on synthesis method. Here we found that continuous wave (cw) UV irradiation was able to reduce Au(III) at room temperature in most of common solvents, and a mechanism was presented in which AuNPs formed from Au(0), which are proposed to be produced from the disproportionation of the gold ions in their higher oxidation states.

Experimental Section

Results and Discussion

Previous researchers proposed the mechanism for the formation of AuNPs using UV irradiation, in which they suggested that Au(III) is firstly excited by UV radiation, and then reduced into Au(II) which is unstable, so Au(II) further reacted disproportionately to form Au(I) and Au(III) (the starting state) [4]. The Au(I) can be photo-reduced into Au(0) by absorbing photons. The Au(0) atoms are formed homogeneously throughout the solution, in that the binding energy between two metal atoms is much stronger than the atom-solvent binding energy, and followed by a multi-step process, these atoms progressively coalesce into clusters, and further become AuNPs [3, 5]. The photochemical reduction of Au(III) to Au(0) has already been studied by many researchers, but almost none considered the effect of solvents on the formation of AuNPs. In our experiment, there were not any reducing or protective agents added in solutions at all, it is clear that different solvents have significant effects on the formation of AuNPs, which can be seen from both absorption spectra and the color changing of reaction solutions and TEM observation as well.

After the Au(III) solutions were irradiated under UV light for an hour, their absorption spectra were recorded (Figure 1). Except for DMSO and acetic acid (HAc), all of HAuCl₄ solutions mixed with other organic solvents exhibited the reduction of Au(III), since the absorption spectrum of Au(III) disappeared progressively. And in the meanwhile, the HAuCl₄ solutions mixed with acetone (ACTN), isopropyl alcohol (IPA), ethanol (EA) and methanol (Me) showed a typical surface plasmon resonance (SPR) peak around 520 nm, which indicated the formation of AuNPs. And the absorbance peak intensity increased with the prolonged reaction time (Figure S1). For a control experiment, the aqueous solution of HAuCl₄ also forms AuNPs. The HAuCl₄ solution mixed with acetonitrile (ACN) and tetrahydrofuran (THF) didn’t show any apparent specific absorption peaks, but they have large absorption from 500 nm to 1000 nm, some nanostructures with longer dimensions or large aspect-ratio formed [11], and also some aggregation of AuNPs was observed, which might be due to the absence of surfactants in solutions, a compromise to avoid any new effects on morphological modulation of gold nanostructures. For DMSO and HAc solutions, there was a very weak peak at around 520 nm at 20 mins, but the peak disappeared at 40 mins and 1 hour.

Figure 1Full-screen View

(Color online). Absorption spectra of chloroauric acid solutions mixed with different organic solvents after 1-hour UV irradiation.

The photos of solutions after UV irradiation in Fig. 2 gave us a more intuitional understanding on the formation of AuNPs. All the colors of these solutions matched with the absorption spectra showed in Figure 1. Those red solutions represent gold nanospheres and the blue solutions represent other kind of gold nanostructures which normally have at least one longer dimension or a large aspect-ratio (such as nanorods, triangles and nanosheets, Figure 2A ). ACTN, IPA, EA and Me solution color changed from colorless to red and deep red with the increasing time, but for ACN and THF solutions color changed from colorless to light blue to deep blue, and no red color was observed during 1 hour (Figure S2). After leaving these solutions under sunlight for another hour, gold nanospheres in the red solution remained stable, but aggregation and precipitation began to appear in the blue solution (Figure 2B ).

Figure 2Full-screen View

(Color online) Photos of Au(III) reaction solutions mixed with different solvents after 1-hour UV irradiation (A) and again 1-hour sitting under sunlight (B).

Ketones are good photosensitizers for NP synthesis not only because of the energy they can absorb or deliver but rather because of the reducing free radicals they can generate [3]. Under UV irradiation, acetone will dissociate into CH₃• and CH₃CO• which could have been used for the reduction of HAuCl₄ [5]. The concentration of HAuCl₄ in the initial solution of all reactions is the same and the reaction time is also the same, but from the spectra in Figure 1, it can be seen that the final yield of gold nanospheres in acetone are obviously more than those in others, which is consistent with those reported from previous experiments. Typical gold nanospheres were also produced in solutions containing Me, EA, and IPA, indicating that hydroxyl groups also have an effect on the reduction of HAuCl₄. It can be inferred that carbonyl has a stronger effect than hydroxyl. For ACN and THF, we thought they are not reductive, but they can affect the shape of the gold nanostructures, also they can affect the degree of AuNP aggregation. Furthermore, ketones and alcohols maybe also served as protective agents to stabilize the gold nanospheres.

TEM tests were taken to characterize the structure of the AuNPs (Figure 3). From the TEM results, we could see that more longer-dimension AuNPs were observed, such as triangle, nanorod rhombic and irregular shape of AuNPs appeared in the TEM images. TEM results was consistent with the color of the solution and the UV-vis absorbance. And the TEM results could be as the persuasive and supplementary data to the UV-vis absorbance and color change of the solution. According these results and the TEM data, we deduced that the blue color was the combination result of the longer dimension and the aggregation of AuNPs.

Figure 3Full-screen View

(Color online) The UV absorption spectra and transmission electron microscope of gold nanostructures. The UV absorption spectra of acetone, ethanol, isopropanol and acetonitrile were A1 to D1 respectively. A2 to D3 are corresponding TEM images.

Conclusion

UV irradiation is a facile photochemical method for synthesis of gold nanostructures. In order to control both sizes and shapes, most studies introduced different solvents or protective agents in the synthesis process. Our study focused on the mere solvent effects on gold nanostructures synthesis. Apparently, both ketone and alcohol solvents tend to form gold nanospheres. Solvents like ACN and THF can induce the formation of nanostructures with other shapes (like rods or triangles), and the AuNPs are more tend to aggregate. So they cannot be used as protective agents, otherwise the products are unstable in solution. Therefore, adding protective agents could be a good choice when synthesizing nanostructures in these solvents. The solvent effects on AuNP synthesis are significant, we hope this article can help to choose right solvents for AuNP synthesis using UV irradiation.