1 Introduction

Silica materials are a very suitable media to support gold nanoparticles due to its inherent characteristics as chemical inertness, mechanical rigidity, swelling resistance, thermal stability, optical transparency and others. Considering these characteristics several systems silica/gold nanoparticles have been studied in the last decade aiming numerous applications, mainly as catalysts, sensors and biosensors [1–7]. In a general way these systems are obtained by reduction in situ of gold salt solutions or starting from nanoparticle aqueous dispersions used as precursors. In these dispersions, several stabilizing agents were applied; however, their stability, storage and transport are still a subject to be improved. In general, the commercially available nanoparticles are dispersed in liquid medium, and it is very difficult to find nanoparticles in commercial products accessible in powder form. In this context, it is very important the search for new stabilizing agents for nanoparticle that allow the preparation of stable aqueous dispersions and also metal nanopowders, which can be easily storage and transported to be used as nanoparticle vehicles for synthesis of other materials and devices.

The use of stabilizing agents for metal nanoparticles is not only important to avoid the particle aggregation but also in the size and shape control of these materials [8, 9]. Quaternary ammonium salts and thiols are used as nanoparticle stabilizing agents since they may provide both steric and electronic protection to the nanoparticles [10, 11]. In particular, ionic liquids are very versatile stabilizing and template agents for the generation of several nanomaterials [12–15]. It is also possible to use organosilanes containing amine, thiol or imidazolium groups as nanoparticle stabilizer in solid matrices [15–19]. These organosilanes can be used as molecular precursors to prepare new silica based hybrid materials that may combine both organic and inorganic properties.

A particular kind of organosilane precursors are the charged ones, which can be used to prepare ionic hybrid materials. Although the charged silsesquioxanes have been studied by few research groups, they present several important characteristics like water solubility, which enable to form films over metal or oxide surfaces allowing the application in different systems [20–23]. These charged silsesquioxanes were used as molecular precursors in sol–gel and grafting reactions to generate materials: with uniform porosity [24], with anisotropic organization imposed by the charged organic moiety [25], adsorbent for anionic dyes [26, 27], adsorbent for metal complex [28], long chain surfactants for dispersion of multiwall carbon nanotubes in ceramic matrices [29] and also materials for electrochemical sensors [26, 30]. The sol–gel method is very important to prepare silica based hybrid materials due to the slow reactivity of the silicon in sol–gel precursors that allows design the morphological and textural characteristics of the final material. Additionally the sol–gel products can be obtained in the form of powders, monoliths and films, enlarging the possibilities of applications [31, 32]. Thus, these materials show potential to stabilize metal nanoparticles by the sol–gel method, resulting in innovative materials. However, in general charged hybrids were few used as metal nanoparticle stabilizer [7, 15, 33].

In this work the charged silsesquioxane containing the charged 1,4-diazoniabicyclo[2.2.2]octane chloride group bonded in a bridged way (Fig. 1), was used as stabilizing agent of gold nanoparticles in liquid and powder form. The liquid dispersion was applied to prepare new material with catalytic activity, by using the sol–gel method.

Fig. 1
figure 1

Structure of the water soluble silsesquioxane, containing the charged 1,4-diazoniabicyclo[2.2.2]octane chloride group

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2 Experimental section

The colloidal gold nanoparticle dispersions were prepared by reduction of chloriauric acid (HAuCl4) by sodium borohydride (NaBH4) in a water solution of silsesquioxane containing the charged 1,4-diazoniabicyclo[2.2.2]octane chloride group bonded in a bridged way (Fig. 1), assigned as dabcosil. The dabcosil silsesquioxane was prepared according to published procedure [25]. It was used 0.5 mL of 5 10−3 mol L−1 HAuCl4 solution, which was added to 20 mL of dabcosil solution (8 g L−1). In this solution it was added 10 mL of NaBH4 0.02 mol L−1, resulting in a dark red solution assigned as Au-dispersion. The analysis by ultraviolet and visible spectroscopy (UV–Vis) of colloids containing gold nanoparticles were carried out in a Shimadzu UV 1601PC and collected in the range from 300 to 700 nm. The Au-dispersion was evaporated to form the solid assigned as Au-dabcosil. This solid was completely redispersed in water to form Au-redispersion. A part of the Au-dabcosil sample was storage and redispersed after 3 months, to result in Au-redispersion-3-months.

Transmission Electron Microscopy images of dispersions and of the solid were performed in JEOL JEM1200 microscope operating at 120 kV. The samples were dispersed in acetone with the aid of ultrasound bath and deposited on a copper grid coated with carbon. The nanoparticle diameter distribution was obtained by the Quantikov software, using several images.

Solid state NMR spectroscopy was performed on a Bruker 300/P spectrometer using the MAS (Magic angle) with CP (Cross-polarization) for solid Au-dabcosil and dabcosil (pure charged silsesquioxane). The 13C experiments were obtained using pulse length of 1 ms and recycle delay of 2 s while the 29Si measurements were made utilizing pulse length of 2.5 ms and recycle delay of 1 s. X-ray photoelectron spectra (XPS) were also obtained for solid Au-dabcosil and dabcosil samples using a hemispheric Specs VSW HA 100 spectrometer. X-ray diffraction patterns of the Au-dabcosil and dabcosil powdered samples were obtained with a Siemens diffractometer model D500 using CuKα as radiation source.

To apply the Au-dabcosil system as source of gold nanoparticles in the synthesis of new materials, a silica based hybrid xerogel was prepared. In this procedure, 2 mL of Au-dispersion were gelified in presence of 1 mL to tetraethylorthosilicate, using ethylic alcohol as solvent and HF as catalyst. The xerogel was characterized by N2 adsorption–desorption isotherms using Micromeritics 3020 Krypton II equipment. The catalytic activity of xerogel was tested in the reduction of p-nitrophenol, which was conducted in a standard quartz cell with path length of 1 cm and 3 mL of volume. The solution of p-nitrophenol at concentration of 0.015 g L−1 was adjusted at pH 10. This solution was first mixed with NaBH4 0.6 mol L−1 aqueous solution, and after 10 mg of the xerogel catalyst was added and the absorption spectra were recorded by UV–Vis in a Shimadzu UV 1601PC.

3 Results and discussion

The Au-dispersion obtained is very stable for several months, and exhibits a typical UV–Vis spectrum, showing an absorption maximum at 527 nm (Fig. 2), typical of nanoparticles with diameter lower than 20 nm [34]. The Au-dispersion can be evaporated to form the solid assigned as Au-dabcosil, which is showed in the supplementary Fig. 1. This solid can be completely redispersed in water to form Au-redispersion, which displays the same UV–Vis spectrum of the Au-dispersion. The solid Au-dabcosil is very stable, since it can be redispersed after 3 months, to result in Au-redispersion-3-months that presents similar UV spectrum (Fig. 2).

Fig. 2
figure 2

UV–Vis spectra of Au-dispersion, Au-redispersion and Au-redispersion-3-months

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The resulting solid Au-dabcosil, obtained from the Au-dispersion evaporation was analyzed by 13C and 29Si NMR, and the results are presented in Fig. 3a and b, respectively. The 13C NMR spectrum of Au-dabcosil is practically the same obtained for pure charged dabcosil silsesquioxane, indicating that its structure did not undergo changes during the nanoparticle synthesis and drying process [25]. It can be observed that the relative intensity of 13C due to carbon atom of hydrolysable methoxy group decreases its intensity in the Au-dabcosil sample (Fig. 3a), suggesting that this sample is more hydrolyzed and possibly more crosslinked than pure dabcosil charged silsesquioxane. This hypothesis is also supported by the 29Si NMR results, presented in Fig. 3b. The 29Si spectrum of dabcosil silsesquioxane shows three peaks, characteristic of T1 C–Si*(OR)2(OSi), T2 C–Si*(OR)(OSi)2 and T3 C–Si (OSi)3 species at −50.0, −59.3 and −69.0 ppm, respectively [25], while the Au-dabcosil sample shows only the T2 and T3 species, at −58.3 and −67.8 ppm, respectively.

Fig. 3
figure 3

NMR spectra of dabcosil and Au-dabcosil. a 13C and b 29Si

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The Au-dabcosil and pure dabcosil silsesquioxane were characterized by X-ray diffraction (Fig. 4). In the angle region 2θ below 30°, it was observed a large peak for both samples, with angle 2θ near 22º, which is characteristic of silica, and the peaks at 6.2° and 12.5°, which correspond to the interplanar distances of 1.43 and 0.71 nm. These two peaks indicate the existence of organized structure imposed by the presence of the charged organic group, as already reported [25]. This organization was not disturbed by the presence of gold nanoparticles. For the Au-dabcosil sample, diffraction peaks were clearly observed in the high angle region, 2θ above 30º. These peaks correspond to the gold close packed cubic structure according to Joint Committee on Powder Diffraction Standards (JCPDS) card 65-2870. The calculated mean diameter of 14.7 nm for the Au(0) nanoparticles, using the Scherrer equation, is in good agreement to that found by TEM, as discussed in sequence.

Fig. 4
figure 4

X-ray diffraction pattern of the dabcosil and Au-dabcosil

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Typical TEM images of Au-dispersion, Au-redispersion and Au-redispersion-3-months are presented in Fig. 5, simultaneously with the particle size distribution diagrams. The average gold nanoparticle sizes for Au-dispersion, Au-redispersion and Au-redispersion-3-months were, respectively, 9.3, 9.3 and 10.6 nm, with standard deviation lower than 4.0 nm for the three dispersions. These results are in agreement to the UV–Vis spectra confirming that the average gold nanoparticle size is smaller than 20 nm and remains the same after the redispersion.

Fig. 5
figure 5

TEM images and particle size distribution of Au-dispersion, Au-redispersion and Au-redispersion-3-months

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Binding energy values for 4f gold and 2p chlorine peaks, obtained from XPS spectrum of solid Au-dabcosil, are showed in the Table 1. It was observed the presence of metallic gold in major proportion in a relation about 3/1 of A0/Au+ [35, 36]. Regarding the chlorine binding energies, it was detected inorganic chloride for dabcosil and Au-dabcosil. However there is an increasing of 0.8 eV in the Cl 2p1/2 for sample containing gold nanoparticles, suggesting an interaction between chloride ions and gold nanoparticles, contributing to their stabilization [10].

Table 1 XPS analysis
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To explore the stability and possibility of application of this new system, the Au-dispersion was applied as gold nanoparticle vehicle in the synthesis of silica based hybrid material using the sol–gel method. In this method, the precursor components, such as tetraethylorthosilicate, charged silsesquioxane and nanoparticles are dispersed in a nanometric level in an initial liquid, which in sequence undergo polycondensation reactions forming homogeneous and amorphous solid, maintaining the high dispersion of the components. Thus the gold nanoparticles become dispersed in all sample, bulk and surface. The resulting xerogel is transparent and presents reddish colour typical of gold nanoparticles, and a picture is showed in the supplementary Fig. 2. TEM image of the xerogel is presented in Fig. 6, simultaneously with the particle size distribution diagram. It can be observed a higher nanoparticle concentration than in the liquid dispersions; however the average size of gold nanoparticles remains the same, 10.5 nm with a standard deviation of 4 nm, indicating that the coalescence of the nanoparticles did not occur.

Fig. 6
figure 6

TEM image and particle size distribution of gold nanoparticles dispersed in hybrid xerogel

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The N2 adsorption–desorption isotherms of the xerogel are showed in the Fig. 7, together with the BJH pore size distribution curves. It was observed a type IV isotherm, typical of mesoporous materials [32, 37], with predominant pore diameter near 10 nm, characteristic of amorphous xerogels obtained in the presence of HF [32]. From the BET multipoint method, it was obtained a specific surface area of 260 ± 15 m2g−1, making this system very promising as heterogeneous catalyst. Therefore, the catalytic properties of the xerogel were studied and the evolution of the reaction of p-nitrophenol being reduced by sodium borohydride, in the absence and presence of catalyst, is showed in the Fig. 8. The reduction process of p-nitrophenol using sodium borohydride is monitored by measuring the UV–Vis spectra at different time t. The characteristic peak at 400 nm ascribed to p-nitrophenol decreases gradually with time, while a new peak at 310 nm indicates the appearance of a new reduced product, p-aminophenol. This reaction catalyzed by gold nanoparticle was reported for the first time in 2001 [38] and it has been widely applied as model reaction to evaluate the catalytic rate of new catalyst systems containing gold nanoparticles. Different matrices have been used as support for immobilization of gold nanoparticles as polymer, resins [39–42], carbon nanotube [43], natural matrices [44] and inorganic matrices such alumina, silica and clay [45–49]. For these materials the kinetic constant of the reaction are in a range from 3.3 10−5 to 3.6 10−2 s−1. In the present work the reduction reaction of p-nitrophenol using the hybrid xerogel was performed at 298 K, and the kinetic constant of the reaction was determined as 3.5 10−3 s−1. The value found was in the same range of the previous reports. However, it is important to point out that in the present work the amount of gold was very low. Considering the weight of catalyst used, the amount of gold did not exceed 8.3 10−9 mol of gold atoms in the form of nanoparticles, making this system promising.

Fig. 7
figure 7

N2 adsorption–desorption isotherms of xerogel. Inset figure: BJH pore size distribution

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Fig. 8
figure 8

UV–Vis spectra of reaction in solution of p-nitrophenol reduction to p-aminophenol at different times. a in the absence of catalyst; b in the presence of xerogel catalyst

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4 Conclusions

The charged water soluble dabcosil silsesquioxane, containing the charged 1,4-diazoniabicyclo[2.2.2]octane group was successfully applied as stabilizer for gold nanoparticle synthesis, without addition of other components. The gold nanoparticles were smaller than 15 nm. The organic moiety of silsesquioxane does not undergo structural changes in the presence of gold nanoparticles. The gold nanoparticles dispersion can be evaporated, transformed in solid form, stored by several months and completely redispersed in water, without any changes in the optical and morphological properties. Therefore, this system opens the possibility to easy storage and transport of metal nanoparticles in solid form and allows reusing them as vehicle of gold nanoparticles in the preparation of new materials with catalytic activity.