Mild fabrication of silica-silver nanocomposites as active platforms for environmental remediation

Abstract

Herein we report a new, simple, low cost and one step way to obtain silica-supported silver nanoparticles (AgNPs) on commercial polyethyleneimine-functionalized silica beads (SiO₂-PEI) under mild experimental conditions. The novel AgNPs/(SiO₂-PEI) material has been thoroughly analyzed using FE-SEM, BET, XRD, XPS and XE-AES analysis. The reduction of Methylene Blue (MB) to Leuco Methylene Blue (LMB) in the presence of NaBH₄ was chosen for testing the catalytic properties of AgNPs/(SiO₂-PEI) towards dyes decoloration. Moreover, the prepared supported nanocatalyst was also found to exhibit excellent catalytic activity towards decoloration of some azo dyes such as E110 and E122.

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1. Introduction

Dyes and pigments that contain heterocyclic aromatic compounds are normally present in the waste waters of textiles, paper, cosmetics and leather industries. They are very dangerous to the environment and human health¹ and, for example, prolonged exposure to them causes irritation of the respiratory system and the gastro-intestinal track. Moreover, most of these dyes (in particular azo dyes) are highly recalcitrant to conventional waste water biological treatment² and pretreatment processes are required to decolorize them efficiently. Many methods are available for this purpose, such as oxidation (with ozone, H₂O₂ or chlorine dioxide), Fenton, photo Fenton, or electrochemical advanced Fenton oxidation, but they are expensive, require harsh working pH ranges or can generate a large volume of ferrous slurry.³ Photocatalytic materials such as TiO₂ have been even largely employed to achieve the thorough degradation of azo dyes, but normally they are active only in the UV range because of their wide band gap.⁴ Thus the development of simple and low cost materials/methods for efficient dye degradation has gained enormous significance. In this context extensive research has been carried out to explore the use of noble metal nanoparticles (NPs i.e. Au or Ag) for catalytic reduction of dyes, due to their enhanced catalytic activity.⁵ Additionally, due to the interest and progress in heterogeneous catalysis, significant attention has been focused on the possibility of stabilizing the NPs onto solid supports such as silica, indium tin oxide, alumina, graphene oxide, etc.⁵ with the purpose of removing and recycling them, after the decoloration treatment, in a simple way. Recently, detailed investigations into the syntheses of silver nanoparticles (AgNPs), with tunable size and shape, have drawn much attention.^6–8

On the basis of our previous results,⁹ in this paper we propose an innovative, simple and low cost procedure for preparing silica-supported AgNPs by using AgNO₃ and commercial polyethyleneimine functionalized silica beads (SiO₂-PEI), under mild conditions.

Polyethyleneimine (PEI) is an interesting material for reducing and stabilizing metal NPs; it is a cationic polymer with a high charge density. PEI fragments containing amino groups can easily chelate with metal ions, and they can also act as both reducing agents and stabilizers in preparation of metal NPs. Indeed, rapid formation of silver nanoparticles occurred within few minutes by heating the solution, indicating that the heat treatment promotes the direct redox reaction between PEI and AgNO₃, without the additional step of introducing other reducing and/or protective agents.¹⁰ Silica beads provide high surface area, give excellent mechanical strength and offer thermal stability. We have investigated the effect of the silver nitrate precursors concentrations and of the reaction times with the aim to optimize the synthesis of AgNPs/(SiO₂-PEI) catalyst. All the samples have been characterized by X-ray Fluorescence (XRF), BET surface area measurements, Field Emission-Scanning Electron Microscopy (FE-SEM), X-ray Diffraction (XRD), X-ray Photoelectron and X-ray Excited Auger Electron Spectroscopy (XPS and XE-AES).

The reduction of Methylene Blue (MB) to Leuco Methylene Blue (LMB) in presence of a reducing agent such as NaBH₄ (ref. 11) was chosen for testing the catalytic properties of AgNPs/(SiO₂-PEI) towards dyes decoloration. Finally, AgNPs/(SiO₂-PEI) have been successfully used as catalysts to decolorize azo-dyes, such as Sunset Yellow and Azorubine.

2. Experimental section

2.1. Materials

AgNO₃ and NaBH₄ were purchased from Sigma-Aldrich and used without further purification; ultrapure water purified with the Milli-Q plus system (Millipore Co., resistivity over 18 MΩ cm) was used in all cases. Commercial silica functionalized with polyethylenimine (SiO₂-PEI) (M_w 75 000−50 000) was purchased from Sigma-Aldrich as 20–60 mesh orange beads. Methylene Blue trihydrate, Sunset Yellow (E110) and Azorubine (E122) were purchased from Sigma Aldrich.

2.2. Preparation of AgNPs/SiO₂-PEI catalyst

The commercial SiO₂-PEI beads were preliminary treated with 1.3 M KNO₃ for 6 h under stirring to eliminate chlorides (present as impurity) via anionic exchange; their presence may in fact cause problems during the AgNPs/SiO₂-PEI preparation, giving rise to silver chloride precipitation as shown by XRD pattern (see ESI, Fig. S1†). Hence 0.500 g of SiO₂-PEI were added to an aqueous solution (50 mL) of 41.2 mM AgNO₃ (PEI/Ag molar ratio equal to 0.025), previously heated at 100 °C. The resulting suspension was kept at 100 °C under stirring at 500 rpm speed for different times (1, 2 or 3 h). In a few minutes, the orange beads turned brown due to the adsorption of silver on SiO₂-PEI and subsequently no changes are observed even after the silver reduction. The AgNPs/(SiO₂-PEI) beads were collected by filtration on a buchner funnel and washed several times with water. This last procedure was carried out until a test with sodium chloride did not show any AgCl precipitate in the washing water. Then AgNPs/(SiO₂-PEI) beads were dried under vacuum for 24 h at room temperature.

2.3. Instruments

FE-SEM measurements were carried out with a Zeiss SUPRA 40VP instrument at a primary beam acceleration voltage of 10 kV; micrographs were collected with an InLens detector. Prior to FE-SEM measurements, samples were coated with 10 nm thick carbon films by a Sputter-Coater (EDWARDS) to suppress charging phenomena leading to image distortions. ImageJ® picture analyzer software was used to estimate the average nanoparticles dimensions.¹² XRD analysis was carried out by means of a PANalytical X'Pert PRO powder diffractometer equipped with a fast X'Celerator detector. Ni-filtered CuKα radiation was used (λ = 0.154 nm, 40 mA, 40 kV). For phase identification the 2θ range was investigated from 15 to 60 2θ degrees with a step size of 0.2° and time per step of 1000 s. The amount of silver on the different samples was determined with a wavelength dispersive XRF instrument (Panalytical Axios Advanced) by comparison with calibration lines, obtained by incipient impregnation adding aqueous solutions of AgNO₃ to 500 mg of SiO₂-PEI matrix. Successively, ca. 145 mg of either standard samples were mixed with about 100 mg of CEREOX Licowax C binding wax micropowder. The resulting solid was ground into a fine powder and pelletized into a 13 mm diameter disk. Each analysis was repeated five times. The specific surface areas of AgNPs/(SiO₂-PEI) samples and of the pristine SiO₂-PEI before and after the heating process at 100 °C and the pre-treatment in 1.3 M KNO₃ (described in paragraph 2.2) were measured by N₂ adsorption–desorption, with BET isotherm equation, using a Sorpty 1750 (Carlo Erba) analyzer. The sample was degassed under vacuum at 90 °C before analysis. XPS and XE-AES analyses were run on a Perkin Elmer Φ 5600ci spectrometer at a working pressure lower than 10⁻⁸ mbar, using a non-monochromatized AlKα excitation source (hν = 1486.6 eV). The spectrometer was calibrated by assigning to the Au4f_7/2 line the Binding Energy (BE) of 84.0 eV with respect to the Fermi level. Charging correction was performed by assigning to the C1s line of adventitious carbon a value of 284.8 eV.^13,14 The estimated standard deviation for BEs was ±0.2 eV. After a Shirley-type background subtraction,¹⁵ raw spectra were fitted by means of a non-linear least-squares deconvolution program, using Gaussian–Lorentzian peak shapes. Atomic percentages were evaluated using sensitivity factors provided by Φ V5.4A software. The samples were introduced directly into the analysis chamber by a fast entry lock system.

2.4. Catalytic activity (decoloration treatment)

The catalytic activity of AgNPs/(SiO₂-PEI) towards the reduction of different dyes in the presence of an excess of NaBH₄ at 25 °C, was followed by UV-vis measurements with a UV-vis single beam Hewlett-Packard 8453 diode array spectrophotometer equipped with a 10 mm quartz cuvette.

2.4.1. Methylene Blue. The reduction of Methylene Blue (MB) to leucomethylene blue (LMB) in the presence of an excess of NaBH₄ was used to investigate the catalytic activity of AgNPs/(SiO₂-PEI) (Scheme 1).

Scheme 1 Reduction reaction of MB to LMB and photographs of the vials at time 0 s (left) and after 300 s (right) in presence of AgNPs/(SiO₂-PEI), MB and NaBH₄.

In a typical experiment, 1 mL of MB solution (9.4 × 10⁻⁵ M) was purged with N₂ gas for about 5 min to remove all dissolved oxygen. Then, the AgNPs/(SiO₂-PEI) and 2 mL of freshly prepared NaBH₄ were added (the following concentrations were tested: 8 × 10⁻² M, 2 × 10⁻² M, 4 × 10⁻³ M). The evolution of the reaction was monitored by UV-vis measurements at 25 °C. No color change was observed when the reaction was performed in the absence of AgNPs/(SiO₂-PEI).

2.4.2. Azo dyes (Sunset Yellow and Azorubine). 2 mL of azo dyes solution (9.4 × 10⁻⁵ M) were purged with N₂ gas for about 5 min; then, the AgNPs/(SiO₂-PEI) and 1 mL of freshly prepared 4 × 10⁻³ M NaBH₄ were added. The decolorization was followed by UV-vis measurements at 25 °C. No color change occurred when the reaction was performed in the absence of AgNPs/(SiO₂-PEI).

3. Results and discussion

3.1. SiO₂-PEI and AgNPs/SiO₂-PEI preparation and characterization

The commercial SiO₂-PEI beads have been characterized and the most relevant data obtained are: 12 wt% of organic material and 0.103 mmol g⁻¹ of amino groups present on the silica surface. SiO₂-PEI beads was pretreated with 1.3 M KNO₃ under stirring for 6 h to allow chlorides removal via anionic exchange. After this pretreatment the specific surface area of commercial SiO₂-PEI is 210 m² g⁻¹ and it resulted stable independently of the heating process (normally at 100 °C in MilliQ water for 2 h).

AgNPs/SiO₂-PEI was obtained by simple addition of 0.500 g of SiO₂-PEI to a stirred aqueous solution (50 mL) of 41.2 mM AgNO₃ (PEI/Ag molar ratio equal to 0.025), previously heated at 100 °C, for a reaction time of 2 h under stirring [sample hereafter termed AgNPs/(SiO₂-PEI-2)]. PEI acts as a non-toxic reducing agent of silver ions due to the reduction properties of the amino functional groups present in the polymer; it allows for controlling both nucleation and growth process at the same time, playing the role of protective agent for the formed NPs.¹⁶ The morphology of AgNPs/(SiO₂-PEI-2) powders (Fig. 1(a)) was investigated by FE-SEM after coating the surface with a 10 nm thick carbon film. FE-SEM images, reported in Fig. 1(b), show the presence of raspberry-like aggregates characterized by an homogeneous coverage of the SiO₂-PEI surface with highly dispersed silver nanoparticles, hence validating the effectiveness of the adopted preparation process in obtaining a controlled Ag aggregate distribution. The average size of silver-containing aggregates was estimated to be 9 ± 2 nm.

Fig. 1 (a) Image of SiO₂-PEI and AgNPs/(SiO₂-PEI-2) powders; (b) FE-SEM micrographs of sample AgNPs/(SiO₂-PEI-2), 2 h reaction time, the particles size distribution is also reported; (c) XRD patterns of AgNPs/(SiO₂-PEI-2) and SiO₂-PEI.

The presence of silver was also confirmed by XRD patterns of AgNPs/(SiO₂-PEI-2) powder as reported in Fig. 1(b). The comparison with the amorphous pattern of bare SiO₂-PEI reveals the diffraction peak at approximately 38.1° associated with the (1 1 1) plane of the silver crystal (ICDD PDF 01-089-3722). The large broadening of this peak is due to the very low dimensions of coherently scattering domains, which is related to the nanometric dimensions of crystal size.

The total amount of silver present on AgNPs/(SiO₂-PEI-2) was estimated to be 1.33 wt% by XRF analysis, as an average of three different prepared samples. Furthermore, a specific surface area of 260 m² g⁻¹ was obtained by BET measurements.

The XPS survey spectrum of AgNPs/(SiO₂-PEI-2) indicates the presence of carbon, oxygen, silver, silicon and nitrogen. No other elements were detected in appreciable amounts (ESI, Fig. S2,† spectrum A). The analysis of the higher resolution surface C1s, O1s, Ag3d, Si2s, N1s and AgMNN regions allowed a more detailed insight into the sample composition (ESI, Fig. S3,† signals A, B and C) and enabled to extract atomic percentage values reported in Table S1 (see ESI†). Furthermore, it also allowed to establish that silver was mainly present in its metallic state, although the minority co-existence of small Ag(I) amounts could be evidenced by calculation of the silver Auger α parameters. More spectroscopic details on XPS and XE-AES investigation are presented and discussed in ESI,† Paragraph 2.

3.2. AgNPs/SiO₂-PEI preparation: reaction time effect

Two different reaction times were even investigated (i.e. 1 and 3 h in respect of the 2 h previously discussed; samples AgNPs/(SiO₂-PEI-1) and AgNPs/(SiO₂-PEI-3), respectively). No significant morphologic differences were observed by FE-SEM images (not reported). In Table 1 the total amount of silver and the specific surface area are reported for the three different samples. Even in this case the differences are quite insignificant and suggest that nucleation and growth of AgNPs occurs during the first hour of reaction.

Table 1 Silver amount wt% and specific surface area for the different investigated samples

Sample name	Experimental conditions	Ag (wt%)	Specific surface area (m^2 g^−1)
AgNPs/(SiO2-PEI-1)	PEI/Ag 0.025, 100 °C, 1 h	1.26	265
AgNPs/(SiO2-PEI-2)	PEI/Ag 0.025, 100 °C, 2 h	1.33	260
AgNPs/(SiO2-PEI-3)	PEI/Ag 0.025, 100 °C, 3 h	1.57	271

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3.3. Catalytic applications

3.3.1. Methylene Blue case. Based on the results reported in the previous section, due to the similar amount of silver presents in all samples we have decided to investigate more in detail the catalytic behavior of AgNPs/(SiO₂-PEI-2) sample. The reduction of MB to LMB in aqueous medium in the presence of large excess of NaBH₄ was used as a model reaction. Whereas the reduction of MB in the absence of silver particles did not occur, upon AgNPs/(SiO₂-PEI-2) introduction complete decoloration was observed within a few minutes. In aqueous medium MB shows an absorption bands at 665 nm with a shoulder at 614 nm, so the kinetics of the reduction reaction can be followed quantitatively by monitoring the change in the intensity of the peak at 665 nm in function of time. Preliminary experiments were carried out by varying the MB/NaBH₄ molar ratio from 1/84 to 1/425 and 1/1700 by keeping constant the amount of MB and varying the NaBH₄ moles with the aim to check the pseudo-first-order conditions. When an excess of reducing agent (NaBH₄) was used, the reduction rate can be assumed to be independent of its concentration, hence a pseudo-first-order kinetic with respect to MB could be employed to calculate the reaction rate.¹⁷ The kinetics of the reaction can be evaluated using the eqn (1):

ln(A_t/A₀) = −kt (1)

where k is pseudo-first-order rate constant, t is the reaction time, A₀ is the absorbance of the peak at 665 nm of MB at time t = 0 and A_t is the absorbance at same wavelength at time t. The k value can be easily calculated by the slope of ln(A_t/A₀) vs. time plot. Fig. 2 reports the plot of the kinetic constants k vs. NaBH₄ moles. A pseudo-first-order condition is observed only when 4.0 × 10⁻⁵ or 1.6 × 10⁻⁴ moles (1/425 or 1/1700 molar ratio, respectively) of NaBH₄ were used; k value was 1.0 × 10⁻² s⁻¹ in both cases, whereas a lower value was obtained with 7.9 × 10⁻⁶ moles (1/84 molar ratio).

Fig. 2 Kinetic constant values at different NaBH₄ moles.

The MB reduction in pseudo-first-order conditions took place very fast using AgNPs as a catalyst. The UV-vis absorption band at 665 nm for MB decreased gradually in time without showing any changes in shape or position of the peak, indicating that MB is reduced without any other side reactions, as it can be seen in Fig. 3(a). In the presence of SiO₂/PEI only, the peak at 665 nm remained almost unaltered even after 2000 s, Fig. 3(b).

Fig. 3 Evolution of absorption spectra of MB by NaBH₄ in presence of: (a) AgNPs/(SiO₂-PEI-2) at MB/Ag/NaBH₄ molar ratio 1/9/1700, inset: the corresponding plot of ln(A_t/A₀) vs. time; (b) SiO₂-PEI at MB/NaBH₄ molar ratio of 1/1700; curve a at t = 0 s and curve b at t = 2000 s.

Moreover, with increasing of the catalyst amount (AgNPs/(SiO₂-PEI-2)), keeping constant the MB/NaBH₄ molar ratio (i.e. 1/1700) it is possible to observe an increase of k. Since the catalytic effect is due to the silver present in AgNPs/(SiO₂-PEI-2) catalyst, in Fig. 4 we show the moles of silver, estimated by XRF analysis. A significant increase in kinetic constant values (around 10 times) occurred when the amount of silver changes from 0.75 to 0.99 μmoles (reaction condition: 1/8/1700 and 1/10.5/1700 MB/Ag/NaBH₄ molar ratio, respectively); a plateau was achieved for higher quantity of silver.

Fig. 4 Kinetic constant values at different Ag moles.

In Table 2 are summarized the kinetic constants k and the R² values obtained for all the catalytic tests carried out.

Table 2 Kinetic parameters of MB reduction for all the catalytic tests

Entry	MB/Ag/NaBH4 (moles/moles/moles)	Reaction time (s)	k^a (10^−2 s^−1)	R^2
a Average ± S.D., n = 3 measures.
1	1/4.4/1700	500	0.80 ± 0.05	0.95
2	1/5.4/1700	500	0.90 ± 0.12	0.97
3	1/8/1700	150	1.03 ± 0.07	0.98
4	1/9/1700	150	2.11 ± 0.15	0.98
5	1/10.5/1700	30	7.67 ± 0.09	0.99
6	1/12/1700	30	7.65 ± 0.11	0.99

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It is worth noting that k values shown in this work are comparable and, in some cases (entries 5 and 6 in Table 2), higher than those usually reported in the literature (see ESI, Table S2†).^18–20

By plotting the absorbance at λ_max (665 nm, A_λmax) vs. time a sigmoidal curve was obtained. As shown in Fig. 5 an induction period, i.e. the time required to start the catalytic reduction and detect an appreciable change in the absorbance, was observed, and it is especially evident for entry 1 in which the lowest silver amount was employed.

Fig. 5 Plots A at λ_max vs. time for AgNPs/(SiO₂-PEI-2) with MB/Ag/NaBH₄ molar ratio: (a) 1/10.5/1700 (b) 1/9/1700 (c) 1/4.4/1700.

Finally, by comparing the plots A_λmax vs. time obtained for 1/8/425 and 1/8/1700 MB/Ag/NaBH₄ molar ratios, an increase of the induction time at about 200 s was also observed when a lower amount of NaBH₄ was used (ESI, Fig. S5†). This behavior is in keeping with previously reported data.^21–23

3.3.2. Azo dyes case (Sunset Yellow and Azorubine). The catalytic efficiency of AgNPs/(SiO₂-PEI-2) was then tested towards the reduction of two synthetic azo dyes: Sunset Yellow (SY) and Azorubine (AZ) (Scheme 2). These dyes are normally added to food and denoted as E110 and E122, respectively.

Scheme 2 Sunset Yellow and Azorubine. Inset: photographs of vials with SY and AZ (a and c) before and (b and d) after decoloration.

Sunset Yellow UV-vis spectrum shows a characteristic strong visible band with λ_max = 485 nm (ref. 24) while Azorubine UV-vis absorption spectrum shows a characteristic band at λ_max = 510 nm,²⁵ in ESI, Fig. S6,† (a) and (b). The SY and AZ reduction reactions in presence of AgNPs/(SiO₂-PEI-2) and under the pseudo-first-order condition employed for MB experiments (1/4/1700 dye/Ag/NaBH₄ molar ratio) took place very rapidly (ca. 1–2 s). No change in color, even after 2000 s, was ever observed using only SiO₂-PEI under similar experimental conditions.

In order to follow the catalytic kinetic via UV-vis (ESI, Fig. S7,† a and b) we decreased the amount of reducing agent and we employed for all the measurements a 1/4/21 dye/Ag/NaBH₄ molar ratio (1.9 × 10⁻⁷ moles dye, 4.0 × 10⁻⁶ moles NaBH₄, 8.0 × 10⁻⁷ moles Ag). Under these conditions, k values of (1.4 ± 0.3) × 10⁻² s⁻¹ and (0.28 ± 0.03) × 10⁻² s⁻¹ for SY and AZ were obtained, respectively.

3.4. Recycling tests

The catalytic performances of recycled samples were tested sequentially three times for MB and SY dyes. Before reusing it, the recycled catalyst was washed three times with copious water and a 0.3 M solution of sodium carbonate in order to favor removal of borates and residual reaction products. As it can be observed in Table 3, a decrease of the kinetic constant values in each of the subsequent cycles occurred. In order to investigate the cause of this behavior further investigations were carried out on the used catalysts. FE-SEM analysis effected after the first catalytic cycle showed an appreciable increase of the mean AgNPs size: i.e. from 9 ± 2 nm to 19 ± 5 nm and 18 ± 5 nm for MB and SY catalysis, respectively (ESI, Fig. S8†).

Table 3 Kinetic constant values for MB and SY in 1st, 2nd, 3rd cycle (k, k₂ and k₃, respectively)

Dye	Experimental condition MB/Ag/NaBH4 (moles/moles/moles)	k (10^−2 s^−1)	k2 (10^−2 s^−1)	k3 (10^−2 s^−1)
MB	1/12/1700	7.65 ± 0.11	0.80 ± 0.09	0.07 ± 0.01
SY	1/4/21	1.40 ± 0.30	0.05 ± 0.02	0.02 ± 0.01

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On the other hand, if we compare XPS analysis on the as-prepared catalyst and on homologues samples after the catalytic tests (ESI, Fig. S2,† spectra A, B, C) no appreciable difference in the silver chemical state took place. Nevertheless, XPS data of AgNPs/(SiO₂-PEI-2) after MB catalysis (ESI, Fig. S4†) evidenced the presence of dye residuals. Taken together, these results seem to suggest that the decreasing of kinetic constant could be attributable both to the AgNPs size variation and to a dye absorption before the catalytic reaction (see ESI, Fig. S10†). Further experimental efforts are needed to produce more stable systems endowed with an enhanced life cycle.

4. Conclusions

Commercial silica PEI can be a convenient, cheap substrate/reagent for preparing, in a very straightforward way, supported AgNPs with an average size of 9 ± 2 nm. The overall process leads to a system capable of both reducing the Ag(I) precursor, stabilizing the resulting AgNPs without the need for any additional agent and preventing aggregation. The XPS analysis of the AgNPs/(SiO₂-PEI) system reveals that silver was mainly present in its metallic state. The resulting materials can behave like heterogeneous catalysts towards the degradation of Methylene Blue in presence of NaBH₄, and show kinetic constants (0.80–7.67 × 10⁻² s⁻¹) comparable or higher than those reported in the literature for similar applications. Finally, AgNPs/(SiO₂-PEI) can be also successfully employed for decoloration of some azo dyes, such as Sunset Yellow and Azorubine, largely used in food industry. FE-SEM and XPS analyses carried out on AgNPs/(SiO₂-PEI) after being used in the catalytic tests showed that an aggregation of Ag nanoparticles occurred and it is responsible for a decrease in the catalytic performances. Further experimental efforts to produce more stable systems endowed with an enhanced life cycles are under way.

Acknowledgements

A.M. thanks the Center for Industrial Research - Advanced Applications in Mechanical Engineering and Materials Technology (CIRI-MAM), University of Bologna for financial support. C.M. and D.B. acknowledge the financial support under the FP7 projects “SOLAROGENIX” (NMP4-SL-2012-310333), Padova University ex-60% 2012–2013, PRAT 2010 (no. CPDA102579) and Regione Lombardia-INSTM ATLANTE projects. The authors are grateful to Dr Simone Bugani for XRF analysis and to Dr Carla Boga for azo dyes supply.

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD, FE-SEM, XPS, XE-AES characterizations for the AgNPs/(SiO₂-PEI-2) samples, UV-Vis spectra for the catalytic reduction of SY and AZ and a comparison of catalytic data. See DOI: 10.1039/c4ra14069a

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