Ionic bis-nanoparticle networks
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A newly arising challenge in the field of nanoparticle research concerns the control and the understanding of the interparticle interactions and interparticle properties. This should allow the development of materials based on nanoparticle assemblies which represents a great opportunity to exploit nanoparticle collective properties. Although some nanoparticle networks have been reported, few works are addressing the highly exciting problem of forming bis-nanoparticle assemblies in which two different types of nanoparticles are present. In this article we report an original synthesis pathway for the formation of an ionic bis-nanoparticle network, silica/silver, based on the formation of an imidazolium bridging unit. The reaction used for the formation of the bridging imidazolium can be considered as click-like chemistry. The synthesis of the metal/metal oxide hybrid composite material starts from the formation of a metal oxide nanoparticle modified with an imidazole ligand. This composite formation is therefore very general and could be extended to other metal/metal oxide composites.
KeywordsMaterial science Heterocylces Green chemistry Nanostructure
The exploration of nanoparticles as building blocks for functional materials led to a better understanding of how to control the size, shape and composition of these nano-objects. The newly arising challenge in the field of nanoparticle research concerns the control and the understanding of the interparticle interactions and interparticle properties. This should allow the development of materials based on nanoparticle assemblies which represents a great approach to exploit nanoparticle collective properties. To address this issue, material chemists started to investigate molecularly linked nanoparticles in order to obtain nanoparticle networks [1, 2, 3]. In this context, we recently published the synthesis of ionic nanoparticle networks (INNs), in which metal oxide nanoparticles are covalently bridged by imidazolium units . To form this hybrid organic–inorganic material, the formation of the networks was driven by a nucleophilic substitution occurring between an imidazole functional group and a chloroalkyl group present on the surface of the nanoparticles. To this end, one batch of nanoparticles was modified with imidazole functionalities whereas a second one with chloroalkyl groups. The two batches were then mixed in order for the functionalities to react. This reaction is remarkable owing to three main aspects. First, the nucleophilic reaction can be considered as a click-like chemistry meaning that it is performed in environmentally friendly solvents, such as water or ethanol, at low temperature, at 70 °C, without formation of by-products and with high yield, between 84 and 100%. Second, the proposed procedure can be adapted to various metal oxide nanoparticles, e.g. silica or titania [5, 6]. Third, the synthesis pathway proposed to form INNs allows the possibility of forming bis-nanoparticle networks. The term ‘bis-nanoparticle network’ refers to nanoparticle networks in which two different types of nanoparticles are present. Such bis-nanoparticle assemblies show promise in a wide range of applications, e.g. sensors and catalysis [7, 8, 9, 10].
Few works are addressing this highly exciting problem of forming bis-nanoparticle assemblies. Among them, Xu and Ye  reported the formation of organic bis-nanoparticle networks obtained by click reaction of azide and alkyne functional groups grafted on to a surface. Mann and co-workers  recently proposed a very elegant synthesis route to inorganic bis-nanoparticle networks by using a sodium alginate biopolymer for the formation and stabilization of gold nanoparticles before growing a network of ceria nanoparticles in situ.
In some related works, difunctionalized ligands, carboxylic acid-terminated alkanethiols, were used to first modify a gold surface; the pendant carboxylic acid groups then allowed the possibility to attach to other species in a second step [13, 14]. Nevertheless, in this approach the species alternation can hardly be controlled given that both thiol and carboxylic acid groups can anchor to the metal surface. In addition, in a previous work on silica nanoparticles, we showed that use of difunctionalized ligand, 1,3-di(3-trimethoxysilylpropyl)imidazolium iodide, leads to the formation of poorly extended networks [4, 15].
In this article we report an original synthesis pathway for the formation of an ionic bis-nanoparticle network based on the formation of an imidazolium bridging unit.
Results and discussion
The starting unit for the synthesis of the bis-INN is the formation of the imidazole-functionalized nanoparticles, as already reported for the synthesis of INN . The silica nanoparticles are modified with imidazole alkoxysilane. The grafting reaction is performed in ethanol.
The successful use of hydrogen bonds to self-assemble building blocks of various types was recently reviewed . The overall effect of several hydrogen bonds can be very significant and lead to a strong bonding. Concrete examples can be found for the formation of pyrrole-based clusters  but also for the formation of supramolecular layers based on the interaction between imidazole and carboxylic acids [17, 20]. This method will then be adapted in order to design bis-nanoparticle networks.
The FT-IR spectrum obtained after similar proton transfer, by reaction of mercaptopropionic acid on the imidazole-modified silica nanoparticles (SiO2_Im_SH), was measured. Given the very strong absorption band of the Si–O–Si bonds centred at 1,130 cm−1, the presented spectrum (Fig. 4, bottom) was zoomed in order to focus on the relatively lower intensity absorption bands. From this spectrum one may deduce the formation of a strong N–H···O hydrogen bond which gave a broad low frequency band around 2,000 cm−1 (Fig. 4, bottom). The formation of the band at 2,000 cm−1 was reported to be a signature for the N–H···O stretching bands of imidazolium carboxylate and pyridinium carboxylate species . Similar to the results reported in the literature , a very broad adsorption between 1,515 and 1,720 cm−1 reveals the concomitant presence of carboxylate antisymmetric C–O stretching bands, and at relatively higher wavelength, carbonyl stretching bands. In addition, a broad thiol S–H adsorption band appears at 2,460 cm−1 [22, 23].
After obtaining the suspension of thio-imidazolium–carboxylate silica nanoparticles, SiO2_ImHOOC_SH, we added silver nitrate precursor to the suspension before reduction with sodium borohydride. The hybrid material obtained after reduction will be referred to hereafter as SiO2_Ag.
The presence of a sharp so-called pre-peak, at very low diffraction angle, was reported in the X-ray scattering diffractogram of some imidazolium ionic liquids with 6–10 carbon atoms in the alkyl chains and spherical or highly symmetrical anions . The origin of this feature is related to the intermolecular interactions between the imidazolium species and due to the anisotropy of the imidazolium cation. The pre-peak observed refers to the typical distance between charged groups separated by cationic alkyl chains. In the case reported in this article, we assume that the meso-organization of the imidazolium units is forced by the linking on the nanoparticle. Indeed neutral imidazole species are first grafted on to the silica surface. When the imidazole reacts with the carboxylic acid moiety to form charged imidazolium units, even without long mesogen alkyl chains, the charged species are forced, by the grafting, to stay close to one another. This result is in perfect agreement with our previous investigation of short-range order in silica-based nanoparticle networks [26, 27].
We have presented an original synthetic procedure for the preparation of metal/metal oxide composite hybrid materials based on silica and silver nanoparticles. The synthesis starts from imidazole-modified nanoparticles which react with mercaptocarboxylic acid to form an imidazolium carboxylate unit with a pendant thiol group. In a further step silver salts are added to the suspension of thiol-modified nanoparticles and reduced. In this procedure, the two-step formation of the bridging unit ensures a selective linking of the nanoparticles. In this way the metal nanoparticles will be neighboured by metal oxide nanoparticles and vice versa.
As for previously reported INNs, the reaction used for the formation of the bridging imidazolium can be considered as click-like chemistry. This reaction occurs in ethanol or water, at low temperature, with no formation of side products.
By characterizing the final bis-nanoparticle hybrid material, it turned out that starting from neutral imidazole-modified nanoparticles allows one to force the imidazolium self-assembly. The mesoscopic arrangement of the imidazolium units could be determined by X-ray diffraction.
By reaction on the pendant imidazole groups, we assume this procedure can be extended to other imidazole-modified metal oxide nanoparticles, like metal nanoparticles of the previously reported imidazole-modified titanium oxide nanoparticles. These imidazole-modified nanoparticles were obtained by grafting of a phosphonic acid imidazole ligand on to the surface of the titanium oxide nanoparticles.
Chemicals were used without further purification, unless otherwise stated. Imidazole, 3-mercaptopropionic acid, dry methanol, N-methylimidazole, silver nitrate, sodium borohydride, sodium hydride and sodium iodide were obtained from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS) was obtained from Fluka. Aqueous ammonia solution (32%), ethanol and tetrahydrofuran (THF) were obtained from Merck. 3-Chloropropyltrimethoxysilane was obtained from ABCR, acetone from Fisher Scientific and dichloromethane from Riedel-de Haën.
Solid-state NMR spectra were recorded on a Bruker AVANCE 300 (1H at 299.85 MHz, 13C at 75.40 MHz, 11B at 96.21 MHz, 31P at 121.38 MHz and 15N at 30.38 MHz) equipped with a 4 mm broadband MAS probe head. 13C and 15N spectra were recorded with ramped CP MAS experiments. The sample holders were spun at 6 kHz for the 31P, 15N and 13C experiments and at 10 kHz for the 11B measurements. Liquid-state NMR spectra were recorded either on a Bruker AVANCE DPX 300 (1H at 300.13 MHz, 13C at 75.47 MHz), equipped with a 5 mm broadband inverse probe head or on a Bruker AVANCE 250 (1H at 250.13 MHz, 13C at 62.90 MHz, 31P at 101.25 MHz), equipped with a 5 mm QNP probe head. 1H/15N heteronuclear multiple-bond correlation (HMBC) spectra were measured with long-range coupling constants of 5 Hz or 10 Hz.
The samples for TEM measurements were prepared by dispersing the particles in ethanol prior to deposition on a carbon-coated TEM Cu grid. TEM measurements were performed on an analytical TECNAI F20 field emission TEM system connected to an EDX detector (USTEM, Vienna University of Technology). XRD measurements were performed on a Philips X’Pert diffractometer using the Cu–Kα radiation (λ = 1.542 Å).
For the DLS measurements, the solid was dissolved in ethanol. DLS experiments were carried out without previous sonication of the samples. The run time of the measurements was 10 s. Every size distribution curve was obtained by averaging ten measurements. The apparatus was an ALV/CGS-3 compact goniometer system, equipped with an ALV/LSE-5003 light scattering electronics and multiple τ digital correlator, and a 632.8 nm JDSU laser 1145P.
To record the FT-IR spectra, the products were measured using an ATR unit (ZnSe crystal for liquids, diamond for powder samples). The spectrometer was a Bruker Tensor-27-DTGS equipped with an Interferometer RockSolidTM and a DigiTectTM detector system, high sensitivity DLATGS, using the OPUSTM software.
The UV–Vis spectroscopy measurements were carried out on a PerkinElmer Lambda 35 with a scan speed of 480 nm/min and a slit width of 1 nm.
In a small beaker 1 g of N-methylimidazole (12.18 mmol) was mixed with 1.290 g 3-mercaptopropionic acid (12.18 mmol). During the addition an exothermic reaction occurred. The clear solution was stirred overnight.
Modification of SiO 2 nanoparticles with N-imidazolylpropyltrimethoxysilane (SiO2_Im)
60 cm3 of the previously prepared silica nanoparticle suspension, obtained by reacting 15 mmol of tetraethoxysilane, was transferred into a 250-cm3 round-bottom flask and degassed in a vacuum for several minutes to remove excess ammonia. Afterwards 1.152 g of N-imidazolylpropyl trimethoxysilane (5 mmol) were added dropwise. The solution was stirred under argon at room temperature overnight.
Thiol functionalization (SiO2_Im_SH)
To the above prepared solution of imidazole-functionalized silica nanoparticles (imidazole/thiol = 1:1) 0.531 g of 3-mercaptopropionic acid (5 mmol) was added. The solution was stirred overnight.
Formation of the bis-nanoparticle network: growing of the silver nanoparticles (SiO2_Im_Ag)
To 20 cm3 of the above prepared solution of thio-imidazole-functionalized silica nanoparticles 1.032 g silver nitrate (6.1 mmol) dissolved in 50 cm3 water was added (thiol-imidazole/Ag+ = 1:4). A white suspension formed immediately. Afterwards 0.252 g sodium borohydride (6.1 mmol) dissolved in 25 cm3 water was added within 30 s. A violent reaction occurred and a precipitate formed immediately. The suspension was allowed to stir overnight at ambient temperature. Afterwards the product was obtained by centrifugation and washing twice with ethanol, water and acetone successively. Finally the powder was dried under reduced pressure (2 mbar) at 80 °C.
This work was financially supported by the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung (FWF, Project P21190-N17). The authors thank Vinzenz Wolfsgruber.
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