Tuning the size and the photocatalytic performance of gold nanoparticles in situ generated in photopolymerizable glycomonomers

Abstract

Polymer nanocomposites containing gold nanoparticles (Au NPs) in situ photogenerated during the UV-curing process were prepared starting from methacrylated glycomonomers with α-D-glucofuranose or D-mannitol structural units, other mono(di)methacrylates and AuCl₃. The formation of Au NPs inside the polymer matrix was evidenced through UV-vis, TEM and XRD analyses, the mean size of the spherical particles being around 33 nm. By adding methacrylates functionalized with carboxylic, quaternary ammonium or thiol groups (MA-COOH, MA-N⁺, MA-SH) in each formulation, the mean size of gold nanoparticles in the hybrid films (purple/dark-blue, red, green) decreased from 6.4 nm to 1.5 nm (for those stabilized by a gold–thiol bond). The catalytic activity was tested for the photodegradation of methylene blue (MB) and methyl orange (MO) in aqueous phase under visible irradiation conditions, and the catalytic performance of the resulting hybrid films was improved by the size decrease of the Au NPs. The hybrid films prepared with 30 wt% MA-SH were more active than the samples with 10 wt% thiol methacrylate. Thus, the percentage degradation with nanosized Au NPs and 30 wt% photopolymerized MA-SH was about 98% in MB and 94% for MO (after 100 min of visible irradiation). Finally it was proved that the hybrid polymeric films can be reused as photocatalysts with high performance for the removal of dyes from contaminated wastewater.

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Introduction

Metal nanoparticles (NPs)/polymer composites are considered an innovative class of multifunctional materials that combine a unique set of characteristics which could not be encountered just in organic polymers or inorganic components.¹ Due to their remarkable electrical, optical or mechanical properties, these types of hybrid composites are very interesting for applications in nanoscience and nanotechnology.^2–5 In the literature, many studies have focused on the production and characterization of these systems, particularly on those containing gold nanoparticles (Au NPs)^6–8 that exhibit unusual physical, chemical, and biocompatibility properties, of interest for a wide range of applications (optics, photonics, catalysis, and biomedical fields). It was found that the properties of such metal nanocomposites strongly depend on the particle size, shape, assembly state and surrounding dielectric environment.^9,10 In the last two decades, the global awareness on the environmental management directed the research related to the materials based on Au NPs, including the polymer nanocomposites^11–14 towards the understanding of their catalytic performance.^15–17 In chemical terms, gold is a catalytically inert material in bulk state, but Au NPs with sizes under 100 nm present two essential features: (i) catalyze several reactions of the organic compounds at temperatures below 200 °C ¹⁸ and (ii), strongly absorb visible light due to the surface plasmon resonance (SPR) effect,¹⁹ which offers the opportunity to exploit the catalytic activity by performing some reactions in visible light even at ambient temperature. Taking in consideration that solar energy is a clean abundant energy source and the energy of sunlight incident on the earth is about 10 000 times more than the current energy consumption in the world,²⁰ photocatalysis seems to be a promising approach for solving some of the environmental problems of the world, today. Up to now, the photocatalysts developed and explored are mainly the materials incorporating titanium dioxide, which can access only UV light (<4% of sunlight).²¹ Since the visible light (400–800 nm in wavelength) constitutes approximately 43% of the solar energy,²² it had become necessary to create efficient photocatalysts that can work well under visible light and consequently, plasmonic photocatalysts (e.g. materials with gold nanoparticles) are intensively studied. Most of the routes for the synthesis of such materials involved the use of chemical reducing agents (sodium borohydride, N,N-dimethyl formamide, trisodium citrate, and other compounds) or a high pressure technique. Generally, these usual methods proved to be too expensive, require harsh chemicals and toxic solvents, consume much time, and generate low yield and hazardous by-products.^23,24 Owing to environmental concerns, some attempts have been made to substitute the mentioned fabrication methods with non toxic, clean and eco-friendly processes in order to accomplish the principles of green chemistry.²⁵ Actually, gold/silver nanoparticles were realized through biosynthesis using fungus Verticillium,²⁶ edible mushroom extract²⁷ or egg shell of Anas platyrhynchos.²⁸ Among the polymer hosts, chitosan polysaccharide^29–31 and other polycarbohydrates like xanthan gum and starch^32,33 have been taken in study for the preparation of plasmonic gold nanoparticles. Also, using green chemical decoration of multiwalled carbon nanotubes with polyoxometalate-encapsulated gold nanoparticles, tri-component nanohybrids with enhanced photocatalytic activities under visible light irradiation were prepared.³⁴

Another concept approached by several authors^35–40 furnished polymer – Au NPs composites through the employing of the photocuring process (UV irradiation), which allows simultaneous photopolymerization of the monomers and photoinduced formation of gold nanoparticles via the reduction of metal salts. Indeed, the reactions induced through UV radiation occur in an eco-friendly manner without pollutant emission (volatile organic compounds) and can be conducted in very short time (seconds/minutes) at room temperature with no degradation of sensitive molecules.^41,42 Hence, the utilization of this attractive technique on bio-monomers derived from biomass will extend both the fundamentals of green chemistry and especially, applications that could replace petroleum-based polymers. Biomass deposits (plants, microorganisms, fungi, marine organisms and animals) represent an easily accessible and a cheap renewable source of carbohydrate compounds as a mixture of polysaccharides (starch, cellulose, chitosan), oligosaccharides (sucrose, maltose, lactose), and monosaccharide (fructose, glucose, mannose, galactose). In a previous paper, we reported the synthesis of photoactive carbohydrates bearing a monosaccharide unit in the structure and their behaviour in the photopolymerization/copolymerization reactions under the exposure to UV light or the two-photon polymerization induced by femtosecond laser pulses (wavelength of 775 nm).⁴³ In the present work, a series of hybrid nanocomposites based on the photopolymerizable mono(di)methacrylates containing monosaccharide (α-D-glucofuranose, D-mannitol) and gold nanoparticle precursors have been synthesized, characterized and tested in catalysis applications under the action of visible light at ambient temperature. The effect of the addition of comonomers with different functionalities (–COOH, –N⁺, –SH) on the nanocomposite properties and their photocatalytic performance in degrading methylene blue and methyl orange dyes, was investigated using hybrid composite films in aqueous solutions.

Results and discussion

UV-vis absorption and TEM studies

Two composite films (F1, F2) were prepared via the photopolymerization of 3-O-methacryloyloxyethylcarbamoyl-1,2 : 5,6-di-O-isopropylidene-α-D-glucofuranose (CMA-1) or 3,4-di-O-methacryloyloxyethylcarbamoyl-1,2 : 5,6-di-O-isopropylidene-D-mannitol (CMA-3) and acid urethane oligodimethacrylate with a photosensitive benzophenone moiety (BP-UDMA) (Scheme 1) in the presence of Irg2959 as photoinitiator, as summarized in Table 1. By the incorporation of AuCl₃ (2 wt%) in the monomer mixtures, the particles of gold precursor from each formulation are photochemically converted in gold nanoparticles (Au NPs) in the absence of stabilizers, considered often responsible for the appearance of nanoseeds and the further growth of the particles.⁴⁴ A first evidence for the generation of Au NPs in the resultant composites was the change in colour of the photopolymerized films after 600 s of UV irradiation, which becomes intense purple. The formation of gold nanoparticles in F1 or F2 take place through the reduction of Au³⁺ to Au⁰ and this outcome can be proved by the appearance of the characteristic plasmon in the UV-vis absorption spectra (Fig. 1a). Generally, metallic NPs with sizes smaller than the wavelength of light show strong dipolar excitations in the form of localized surface plasmon resonances (LSPR).⁴⁵ In our composites, the characteristic band of Au NPs is located at 542 nm for F1 and at 545 nm for F2 (Fig. 1a), and the peaks have symmetrical shape, indicating a uniform size distribution of the gold nanoparticles in the organic phase.⁴⁶

Scheme 1 Structure of the photopolymerizable glycomonomers (CMA-1, CMA-3) and benzophenone oligomer (BP-UDMA).

Table 1 Composition (wt%) of the experimental formulations based on glycomonomers, benzophenone urethane macromer and functional comonomers with gold precursor (2 wt%)

Sample	CMA-1 (wt%)	CMA-3 (wt%)	BP-UDMA (wt%)	MA-COOH (wt%)	MA-N^+ (wt%)	MA-SH (wt%)
F1	50	—	50	—	—	—
F2	—	50	50	—	—	—
F1 + 10% MA-COOH	45	—	45	10	—	—
F1 + 30% MA-COOH	35	—	35	30	—	—
F2 + 10% MA-COOH	—	45	45	10	—	—
F2 + 30% MA-COOH	—	35	35	30	—	—
F1 + 10% MA-N^+	45	—	45	—	10	—
F1 + 30% MA-N^+	35	—	35	—	30	—
F2 + 10% MA-N^+	—	45	45	—	10	—
F2 + 30% MA-N^+	—	35	35	—	30	—
F1 + 10% MA-SH	45	—	45	—	—	10
F1 + 30% MA-SH	35	—	35	—	—	30
F2 + 10% MA-SH	—	45	45	—	—	10
F2 + 30% MA-SH	—	35	35	—	—	30

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Fig. 1 UV-vis absorption spectra for Au NPs in the F1/F2 formulations (a) and TEM bright field image (b) and the statistical distribution (c) of the gold nanoparticles into F1 film after 600 s of UV irradiation.

Through measuring the full-width at half maximum (FWHM) values of the UV-vis spectra, values of about 64 nm (F1) and 67 nm (F2) were acquired. From these findings it can be concluded that in the photocrosslinked systems, a quite narrow size distribution of the Au NPs is noticed.

More information on the size, morphology and distribution of Au NPs in the polymeric matrix was obtained by transmission electron microscopy (TEM) analysis. The TEM images revealed that the particles are spherical in morphology, highly stable and uniformly dispersed in the films, as can be seen for the F1 hybrid composite in Fig. 1b. In addition, the particle-size-distribution histogram of the Au NPs (Fig. 1c) confirmed their relatively uniform sizes, the majority of the particles having dimensions between 20 and 50 nm, and the average particle size was around 33 nm. These data argue that the nature of monosaccharide unit (α-D-glucofuranose/D-mannitol) or the number of acrylic functionalities of the glycomonomers doesn't have a major influence in the process of photoinduced formation of gold nanoparticles.

As mentioned before, the achieving of hybrid materials with tunable dimension/shape of gold NPs is a great challenge of our days due to the recognition of the new and changing properties on the nanometer scale. Therefore, to control the size of gold nanoparticles in situ photogenerated in the above formulations, functional co-monomers (structures given in Scheme 2) like 10-(methacryloyloxyethylcarbamoyloxy)-decylmonosuccinate (MA-COOH), 2,2-bis-(methacryloyloxyethyl)-N-dodecylmethyl ammonium bromide (MA-N⁺) or 2-[N-methacryloyloxyethyl-(N′-2-thioethyl)] (urea) (MA-SH) were added in different proportions (10 wt% and 30 wt%) in F1 and F2 (see Table 1). After the inclusion of AuCl₃ (2 wt%) and Irg2959, the compositions were subjected to UV irradiation for the same time period (600 s). The first visible sign was the change in the colour of the resulting films: the films F1 and F2 containing 10 wt% of MA-COOH methacrylate became purple-blue, and those with 30 wt% MA-COOH are dark blue. The addition of 10 wt% or 30 wt% MA-N⁺ led to the formation of hybrid polymeric films with red colour, meanwhile by adding a methacrylate with SH group (MA-SH, 10 and 30 wt%) green films can be visualized. In the UV-vis spectra of the composite films a red shift of the characteristic plasmon of gold nanoparticles accompanied by an increase of the broadness of the absorption band were observed. These modifications of LSPR peak positions and of the width of the absorption band are determined by the variation in the oscillation frequency of the electrons of gold nanoparticles which can be caused through the changing of size and shape, or surrounding medium of the Au NPs.¹⁶ From Fig. 2a and b it can be remarked that the curve profiles of the hybrid polymeric composites containing MA-COOH are similar with those displayed by F1 and F2 films, but the insertion of 10 wt% MA-COOH in the formulations induced a shift with about 11 nm (F1 + 10% MA-COOH) and 32 nm (F2 + 10% MA-COOH), correspondingly, towards larger wavelengths. With 30 wt% MA-COOH, a higher bathochromic-shift with 27 and 41 nm, respectively was found in the absorption maxima of both photopolymerized compositions.

Scheme 2 Structure of the functional comonomers MA-COOH, MA-N⁺ and MA-SH.

Fig. 2 Effect of the addition in the F1/F2 formulations of 10 wt%/30 wt% MA-COOH (a and b), MA-N⁺ (c and d) and MA-SH (e and f) on the UV-vis absorption spectra for Au NPs.

Besides, by determining the full-width at half maximum values of the UV-vis spectra for the composite films incorporating MA-COOH and Au NPs it can be noticed that this parameter increased with increasing content of MA-COOH (F1 + 10% MA-COOH: 84 nm; F1 + 30% MA-COOH: 124 nm) compared to F1 (64 nm). The replacement of MA-COOH monomer with the polymerizable cationic surfactant (MA-N⁺) in the F1/F2 formulations has reflected in similar UV-vis absorption spectra (Fig. 2c and d). Furthermore, for F1/F2 + 10% MA-SH and F1/F2 + 30% MA-SH, it appeared a little difference in the UV-vis absorption spectra (Fig. 2e and f) and quite unexpected, the values of FWHM were around 155 nm for the polymeric films.

The in situ formation of Au NPs in the hybrid composites during the UV irradiation was also verified using TEM investigation. From the TEM micrographs (Fig. 3a and b), the incorporation of 30 wt% of MA-COOH or MA-N⁺ methacrylate in the F1/F2 formulations led to gold spherical NPs which are uniformly dispersed within the polymer matrix. The particle-size distribution histograms of the Au NPs (Fig. 3a and b—right side) indicated that the dimensions of the Au NPs generated in the presence of these monomers dramatically decreased comparatively to the base F1 film.

Fig. 3 TEM bright field images and statistical distribution of the gold nanoparticles into F1 + 30% MA-COOH (a) and F1 + 30% MA-N⁺ (b) composites after 600 s of UV irradiation.

Consequently, the nanoparticles have the diameter in the range of 1.0–14 nm and the mean size in both cases (F1 + 30% MA-COOH and F1 + 30% MA-N⁺) is around 6.4 nm. The diameter diminution of Au NPs can be explained by the fact that the flexible carboxylic group from MA-COOH or the cationic function of the surfactant methacrylate act as stabilizers for nascent gold nanoparticles, amplifying thus the effect of stabilization produced by the –COOH groups linked to the aromatic rings of BP-UDMA. It seems that the MA-COOH and MA-N⁺ comonomers affect in the same manner the photogeneration of gold nanoparticles, but a higher concentration of Au NPs is yielded in the presence of MA-COOH. At this point we can mention that the variations appeared in the UV-vis spectra are caused by the Au NPs sizes and by the changes in the surrounding area.

Visualizing then the TEM images of the composite films derived from the formulations with MA-SH monomer, known as a typical stabilizer for gold nanoparticles (Fig. 4a), the formation of small clusters of Au NPs with globular shape and dimensions between 15 and 70 nm was confirmed. Examination of the clusters in the hybrid film based on F1 + 30% MA-SH (Fig. 4b) suggested that these are able to develop a monodisperse system, in which the gold nanoparticles have spherical shape and diameters around 1–2 nm (Fig. 4c), the mean size being 1.5 nm. Therefore, the use of MA-SH methacrylate in the formulations offers an excellent control on the Au NPs dimension compared to the flexible carboxylic methacrylate (MA-COOH) and the cationic methacrylate (MA-N⁺).

Fig. 4 TEM bright field images at 200 nm (a) and 20 nm (b) and statistical distribution (c) of the gold nanoparticles into F1 + 30% MA-SH hybrid film after 600 s of UV irradiation.

EDX, XPS and XRD analyses

All hybrid nanocomposites with Au NPs formed in situ were explored by means of energy-dispersive X-ray spectroscopy (EDX), technique that permits the quantification of the amount as well as the spatial distribution of the elements through the X-ray elemental mapping images. In the EDX patterns (Fig. 5) the characteristic peaks for C, O, N and S (for the formulations with MA-SH comonomer) along with the signal specific for elemental gold indicate that the reduction of gold ions to Au⁰ took place, thus supporting the results presented before. Also, the elemental mapping image of gold atoms (Fig. 5, inset) sustains a uniform distribution of the Au atoms within the polymer matrix. The results obtained during the quantitative analysis revealed that the quantity of elemental gold in the samples varied between 1.2–1.6 wt% in harmony with the initial feeding ratio.

Fig. 5 EDX patterns for F2 (a) and F2 + 30% MA-SH (b) hybrid films and the corresponding elemental mapping images for Au NPs.

Moreover, the synthesized composites were also investigated by X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique, probing the outmost 5–10 nm of the samples.⁴⁷ The XPS analysis (Fig. 6a) shows clearly the signals attributed to C, N and O from the polymer environment, and also it can be observed a weak signature for S in the composites containing the thiol compound. However, the presence of gold atoms could not be evidenced by this analysis, probably because the polymer layer around the noble metal particles was deeper than 10 nm, in agreement with the literature data reported for other polymer–Au NPs composites.⁴⁸ The results obtained from XPS can lead to the idea that the Au NPs in situ generated are spread inside the crosslinked polymer matrix.

Fig. 6 XPS spectra for F1 + 30% MA-COOH and F1 + 30% MA-SH composites (a); XRD pattern for the polymeric hybrid film F2 + 30% MA-SH (b).

In order to attest that the Au nanoparticles exist only in their metallic state, X-ray diffraction (XRD) analysis was further employed. The noble metal NPs proved to have tunable growth in a preferential (111) facet, because the peak corresponding to the (111) plane is more intense than those corresponding to the others planes. For all the photopolymer nanocomposites containing gold nanoparticles, the X-ray diffraction pattern put in evidence the existence of crystalline gold, the characteristic peaks to the diffraction planes of face centered cubic structure being located at around 2θ = 38°, 44.2°, 64.8° and 77.6°, as is exemplified in Fig. 6b for the F2 + 30% MA-SH. These results derived from our experiments are in good accord with the literature data^49,50 and demonstrated that Au NPs were formed by the in situ reduction of Au³⁺ ions using UV light. Also, it was observed that the XRD patterns show a broad peak around 2 theta value of 20.5° corresponding to the amorphous structure of polymer matrix.

Influence of Au NPs size on photocatalytic activity

The photocatalytic activity of the synthesized hybrid materials was evaluated in the degradation of an organic dye, namely methylene blue (MB) at first, in ambient conditions and under visible irradiation, which imitates the solar radiation. To the best knowledge of the authors, this type of photocatalyst has not been tested in the past. The photobleaching reaction of MB was performed in aqueous solution (3.33 × 10⁻⁵ M) in the presence of hybrid composite films. The dye degradation process was monitored by measuring the decrease of the UV-vis absorption band characteristic to MB as a function of the irradiation time.

Fig. 7a and b illustrate the UV-vis absorption spectra of MB solution obtained during visible irradiation in the presence of F1 or F1 + 10% MA-SH films with Au NPs, where it can be observed that the characteristic band of the methylene blue dye positioned at 671 nm is reduced under visible light irradiation. Under these conditions, it should be pointed out that the MB solution is decolorized after ca. 130 min of irradiation (F1), and 100 min in the case of F1 + 10% MA-SH (Fig. 7a and b inset). To confirm the photocatalytic activity of the polymeric films with Au NPs, two control experiments were carried out: the MB solution was irradiated with visible light for 120 minutes in the absence of polymer composite and another solution was kept in dark in the presence of a film comprising gold nanoparticles. In both cases the dye solutions showed negligible degradation. Moreover, we used the Lambert–Beer law (A = εlc) for determining the MB concentration as a function of the irradiation time, and the plots are given in Fig. 7c. In the described experiments, it was found that the degradation efficiency of MB is dependent on the hybrid film employed, and the photocatalytic performance is improved with decreasing the size of Au NPs embedded in the polymer matrix. So, after 100 min of visible light irradiation, the F1 composite comprising Au NPs with mean size around 33 nm decayed 64% of dye, while the F1 + 30% MA-COOH and F1 + 30% MA-N⁺ composites induced a photolysis degree of 78% and 74%, respectively. Although the Au NPs from the last two composites have almost the same diameter (6.4 nm), a small difference in catalytic performance appears, the F1 + 30% MA-COOH being a better catalyst. This behaviour can be ascribed to the higher concentration of Au NPs generated in situ in the presence of the flexible carboxylic methacrylate. Besides, the preparation of hybrid films with Au NPs via a photoassisted process by a methacrylate with SH group improved efficiently the photoactivity of the final nanostructured materials. As shown in Fig. 7c, about 90% (F1 + 10% MA-SH) and 98% (F1 + 30% MA-SH) methylene blue was decomposed after 100 min of visible irradiation, the observed difference being related to the Au NPs from each sample. It may be assume that the photocatalysts containing MA-SH have a higher performance due to the reduced dimension of Au NPs (∼1.5 nm) and the existence of the small clusters of nanoparticles which emphasizes the LSPR effect.⁵¹ The photodegradation of MB dye using the hybrid polymeric films with Au NPs in the visible light can be explained on the basis of the excitation of localized surface plasmon resonance of gold nanoparticles, promoting the intraband excitation of 6sp electrons to higher energy level (conduction band) and creating positive charges (holes) in the ground level, which can capture electrons from reactant molecules to oxidize them, and then returning to their metallic state.^52,53 Since the photodegradation process takes place in water, a more correct explanation could be connected to the hydrophilic nature of the polymer network which adsorbed the dye aqueous solution driving thus the H₂O molecules to the active centre of the catalyst. The holes of Au NPs interact with hydroxyl groups from water furnishing ions-radicals. Then, the dissolved oxygen molecules react with conduction band electrons creating superoxide radical anions O₂⁻, which in the next step are protonated with the formation of hydroxyl radicals HO₂ and at the end, the dye degradation is accomplished through the activity of superoxide anions.^28,54 Furthermore, the rate constants (k) for the MB degradation process in the presence of the hybrid films were determined according to equation:

ln(C₀/C_t) = kt

where C₀ and C_t are the values of the concentration at times t₀ and t, respectively, and k is the rate constant.

Fig. 7 Changes in the UV-vis absorption spectra of an aqueous methylene blue solution in the presence of F1 (a) and F1 + 10% MA-SH (b) composite films used as catalysts monitored as a function of visible irradiation time; temporal evolution of the MB concentration (c) and fitted curves plot for the kinetic estimation of MB photodegradation (d) in the presence of all hybrid films.

The experimental data of the photodegradation reactions fitted well to the first-order kinetic (Fig. 7d), and the calculated rate constants (k) were found to be 10.8 × 10⁻³ min⁻¹ for F1 composite, 17.9 × 10⁻³ min⁻¹ for F1 + 30 wt% MA-COOH, 16.8 × 10⁻³ min⁻¹ for F1 + 30 wt% MA-N⁺, 24.2 × 10⁻³ min⁻¹ for F1 + 10 wt% MA-SH and 47.0 × 10⁻³ min⁻¹ for F1 + 30 wt% MA-SH. It should be noticed that this parameter also depends mostly on the Au NPs dimension and the speed of degradation increases as they shrink in size, when there is a raise in the number of low-coordinated Au atoms which favours the interaction with the reactant and facilitates the degradation process of MB.¹⁵

Another confirmation of the catalytic activity of the hybrid films could be given by the photocatalytic degradation of methyl orange (MO) in aqueous solution (6.66 × 10⁻⁵ M). Fig. 8 presents the changes in the absorbance spectrum of the MO solution in the presence of photocatalyst based on F1 + 30 wt% MA-SH and Au NPs under visible irradiation, where the reductions of the absorbance peak at 507 nm are a clear indicator of the degradation of the dye molecule with no formation of new bands in the UV-vis region during the photocatalytic reactions.

Fig. 8 Changes in the UV-vis absorption spectra of an aqueous methyl orange solution in the presence of F1 + 30% MA-SH hybrid film used as catalysts monitored as a function of visible irradiation time and fitted curve plot for the kinetic estimation of MO photodegradation (inset).

As shown in this figure, the degradation efficiency of MO dye within 100 min of irradiation was 94% (in set). For this one, the colour of the reaction solution changed from intensive orange to slightly pink, and after 120 min, the solution becomes colourless. The rate constant calculated for this experiment was 31.8 × 10⁻³ min⁻¹, suggesting that the bleaching process is slower for MO than for MB.

Further, the F1 + 30 wt% MA-SH film was again tested to see if this can be reused in the photodegradation of methylene blue. As shown in Fig. 9a, in the second usage as catalyst the hybrid polymeric film with Au NPs has kept more or less the same performance of the catalytic activity and after 100 min of visible irradiation induced a photolysis degree of 95% for MB dye (Fig. 9b). The experimental data of the photodegradation reaction also fitted to the first-order kinetic (Fig. 9c), and the calculated rate constant (k) is 34.9 × 10⁻³ min⁻¹.

Fig. 9 Changes in the UV-vis absorption spectra of an aqueous methylene blue solution in the presence of F1 + 30 wt% MA-SH composite reused as catalysts monitored as a function of visible irradiation time (a); temporal evolution of the MB concentration (b) and fitted curves plot for the kinetic estimation of MB photodegradation (c) in the presence of F1 + 30 wt% MA-SH at first and second utilization as catalyst.

This result seems to indicate a slow-down of the photodegradation process, which can be due to the diminished adsorption capacity of the active surface area of Au NPs for dye, adsorption being a major condition for a heterogeneous catalytic reaction.

So, it can affirm that the hybrid polymeric films with gold nanoparticles could be employed as visible light-driven catalyst with high performance, but their reuse in successive cycles should be exercised.

Conclusions

Hybrid composites containing in situ-photogenerated gold nanoparticles were prepared via photopolymerization process using mono(di)methacrylates and gold particle precursor. The UV-vis and TEM investigations demonstrated that the size of the metallic nanoparticles can be controlled through the addition of a suitable functional comonomer to the formulations before UV irradiation, while X-ray diffraction confirmed the existence of Au NPs in the polymer matrix. The method used for the synthesis of composites is simple, efficient, cheap, and environmentally friendly, and the hybrid polymeric films have photocatalytic activity for methylene blue and methyl orange dyes under visible light irradiation. This feature that recommends the use of composite films in water purification under sunlight opens new perspectives for the obtaining of hybrid materials with desired characteristics.

Experimental

Materials

2-Isocyanatoethyl methacrylate (IEMA), 1,2 : 5,6-di-O-isopropylidene-α-D-glucofuranose, 1,2 : 5,6-di-O-isopropylidene-D-mannitol, benzophenone-3,3′,4,4′-tetracarboxylic dianhydride, poly(ethylene glycol) (PEG, Mw = 400 g mol⁻¹), 1,10-decanediol, succinic anhydride, N-methyl diethanolamine, 1-bromododecane, cysteamine, 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone (Irgacure 2959), gold chloride (AuCl₃) were used as received (from Sigma Aldrich Chemical).

Synthesis of monomers

Synthesis of 10-(methacryloyloxyethylcarbamoyloxy)-decylmonosuccinate (MA-COOH). First, it was yielded the carboxylic intermediate (I-COOH) as follows: to a well-stirred solution of 1,10-decanediol (5 g, 28.7 mmol) in 30 mL DMF, 28.7 mmol succinic anhydride (2.87 g) were added and the mixture was kept at 65 °C for 8 h in the presence of pyridine as catalyst. After the removal of solvent, the product was dissolved in CH₂Cl₂ and the organic phase was washed with water. I–COOH was collected after removing solvent under reduced pressure.

I-COOH: yield: 7.4 g (94.9%). ¹H NMR (CDCl₃, δ ppm): 4.07 (COO–CH̲₂); 3.75 (HO–CH̲₂); 2.64 (OCO–CH̲₂–CH̲₂–COOH); 1.6 (COO–CH₂–CH̲₂); 1.27 (CH₂ from alkyl). FTIR (KBr, cm⁻¹): 3426 (OH); 2990–2820 (C–H); 1737 (CO from COOH); 1668 (CO from ester); 1260–1030 (C–O–C).

Next, the carboxylic methacrylate (MA-COOH) was obtained from reaction of a solution of 2-isocyanatoethyl methacrylate (3.7 mL, 25.54 mmol) in 10 mL anhydrous THF added dropwise to a solution of I–COOH (7 g, 25.54 mmol) in 25 mL THF, the mixture being kept under stirring at 36 °C for 24 h. The course of the reaction was followed through the infrared absorption of the isocyanate stretching band (around of 2260 cm⁻¹), the reaction being considered completed after the disappearance of the NCO band from the FTIR spectrum. The solvent was removed under reduced pressure and the final compound was collected.

MA-COOH: yield: 10.2 g (92.7%). ¹H NMR (CDCl₃, δ ppm): 6.12 (CH̲₂=C in trans position relative to CH₃ unit); 5.58 (CH̲₂=C in cis position relative to CH₃ unit); 4.20 (COO–CH̲₂–CH₂–NH); 4.06 (OCO–CH̲₂–(CH₂)₈–CH̲₂–OCO); 3.48 (CH̲₂–NH–COO); 2.63 (HOOC–CH̲₂–CH̲₂–COO); 1.93 (CH₃); 1.6 (COO–CH₂–CH̲₂); 1.27 (CH₂ from alkyl chain). FTIR (KBr, cm⁻¹): 3376 (OH); 2950–2820 (C–H); 1720 and 1670 (CO from ester/COOH); 1640 and 815 (methacrylate double bond CH₂=C); 1539 (amide II); 1455 (amide I) 1297–1033 (C–O–C).

Synthesis of 2,2-bis-(methacryloyloxyethylcarbamoyloxyethyl)-N-dodecylmethyl ammonium bromide (MA-N⁺). For the preparation of photopolymerizable quaternary ammonium derivative MA-N⁺, 3 g (8 mmol) of N,N,N,N-dodecyl-di-β-hydroxyethylmethyl ammonium bromide (synthesized as previously described⁵⁵) were dissolved in 25 mL anhydrous THF and 2.3 mL (16 mmol) 2-isocyanatoethyl methacrylate were added, the mixture being stirred at 40 °C for 24 h in the presence of dibutyltin dilaurate. The course of the reaction was followed through the infrared absorption of the isocyanate stretching band (2260 cm⁻¹). After the evaporation of the solvent, the urethane dimethacrylate MA-N⁺ was collected as colorless viscous liquid.

Yield: 4.9 g (88%). ¹H NMR (CDCl₃, δ ppm): 6.7 (NH); 6.14 (CH₂=C in trans position relative to CH₃ unit); 5.59 (CH₂=C in cis position relative to CH₃ unit); 4.56 (NH–COO–CH̲₂–CH₂); 4.24 (COO–CH̲₂–CH₂–NH); 3.96 (COO–CH₂–C̲H₂–NH); 3.6 (COO–CH₂–CH₂–N⁺); 3.44 (CH̲₃–N⁺ and CH₂–CH̲₂–N⁺); 1.93 (CH₂=C–CH₃); 1.75 (CH̲₂–CH₂–N⁺); 1.25 (–CH₂); 0.88 (–CH₂–CH̲₃). FTIR (KBr, cm⁻¹): 3256 (NH); 2854–2956 (C–H); 1720 (CO); 1637 and 813 (CH₂=C–); 1536 (amide II); 1256 and 1167 (C–O).

Samples preparation

For the preparation of hybrid composites with gold nanoparticles we employed as photopolymerizable monomers the glycomonomers 3-O-methacryloyloxyethylcarbamoyl-1,2 : 5,6-Di-O-isopropylidene-α-D-glucofuranose (CMA-1) and 3,4-Di-O-methacryloyloxyethylcarbamoyl-1,2 : 5,6-Di-O-isopropylidene-D-mannitol (CMA-3),⁴³ the benzophenone urethane macromer (BP-UDMA),³⁹ 10-(methacryloyloxyethylcarbamoyloxy)-decylmonosuccinate (MA-COOH), 2,2-bis-(methacryloyloxyethyl)-N-dodecylmethyl ammonium bromide (MA-N⁺), and 2-[N-methacryloyloxyethyl-(N′-2-thioethyl)] (urea) (MA-SH).⁴⁰ The hybrid composites were obtained through photopolymerization technique, as described below: the monomer mixtures (CMA-1/CMA-3 and BP-UDMA) containing gold salt (2 wt% AuCl₃) were photopolymerized by UV irradiation for a period of 600 s, using Irgacure 2959 (1 wt%) as photoinitiator, finally thin films (thickness around 0.2 mm) being produced. Additionally, various amounts of functional comonomers (10 or 30 wt% MA-COOH, MA-N⁺, MA-SH) were introduced before photopolymerization in the formulations according to data given in Table 1. For achieving a homogeneous composition of the formulations, few drops of ethanol were added, and then the mixtures were applied on glass/teflon plate and photopolymerized. The UV irradiation was performed using an Hg–Xe lamp (Hamamatsu Lightningcure Type LC8, Model L9588) with a light intensity of 25 mW cm⁻².

Methods

The UV-vis spectra of all samples were recorded using a Perkin Elmer Lambda 2 spectrophotometer. Transmission electron microscopy (TEM) analyses were performed using a HITACHI T7700 microscope operating at 120 kV in high-resolution mode. For these measurements, the compositions were directly deposited onto the copper grid and subjected to UV irradiation, further being dried at 50 °C into a vacuum oven for 24 h. TEM micrographs were analyzed with Image J software, which permit measuring the Au NPs dimensions and creating the size distribution histograms. Microscopic investigations were performed on an environmental scanning electron microscope QUANTA200 coupled with an energy dispersive X-ray spectroscope (ESEM/EDX). The dried samples were examined in low vacuum mode operating at 20 kV using an LFD detector. For the X-ray photoelectron spectroscopy investigations, a Physical Electronics PHI-5000 VersaProbe XPS system with a monochromatized Al K radiation (1486.6 eV) was used. The take-off angle of the photoelectrons was 45°. All the XPS peak positions in the survey spectra were calibrated with respect to the C 1s peak at 284.6 eV. To identify the gold nanoparticles into the photopolymerized films, the X-ray diffraction analysis was performed by wide angle X-ray diffraction (WXRD) using a Shimadzu X-ray diffractometer Lab X XRD-6000 with CuKα radiation (λ = 0.15406 nm), running at an operating voltage of 40 kV and a current of 30 mA.

Photocatalytic activity measurements

Photocatalytic activity of the synthesized hybrid materials was evaluated following the degradation of the methylene blue (MB) or methyl orange (MO) aqueous solutions in ambient condition under visible irradiation. A piece of each composite film (1 g) was added into 50 mL of MB (3.33 × 10⁻⁵ M) or MO (6.66 × 10⁻⁵ M) aqueous solutions. The mixture was then continuously irradiated with a visible light source, employing Xe lamp (λ = 400–800 nm, Hamamatsu Lightningcure Type LC8, Model L9588). 1.5 mL of the solution was collected at given time intervals and further analyzed using an UV-vis spectrophotometer (Perkin Elmer Lambda 2). For comparison, a MB solution without any composite film inside was irradiated for 120 min and also we added F1 + 30% M-SH in the MB solution and kept in the dark for the same period of time. The extinction coefficient of MB and MO was independently measured to quantify the evolution of the concentration of the dye.

Acknowledgements

A. L. Chibac is thankful for the financial support offered by European Social Fond—“Doctoral and postdoctoral programs—support for increasing research competitiveness in the field of Exact Sciences”—ID POSDRU/159/1.5/S/137750, Sectorial Operational Programme Human Resources Development. The authors E. C. Buruiana, T. Buruiana and V. Melinte are thankful for the financial support offered by CNCSIS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0164 (40/5.10.2011).

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