Deposition of silver nanoparticles on organically-modified silica in the presence of lignosulfonate

Abstract

It is shown that the chemical reduction of silver ions by lignosulfonate (LS) in a mixed aqueous–organic solvent produces silver nanoparticles (AgNPs). If, additionally, spherical silica (SiO₂) (surface-functionalized with various organic groups) is introduced to the reaction mixture, the LS-stabilized Ag-NPs are deposited on the surface of the silica spheres, forming a SiO₂–LS–AgNPs hybrid material. The efficiency of the process is found to depend significantly on both the polarity of the organic solvent and the hydrophobicity of the SiO₂-grafted functionalities. The most effective synthesis was in a mixed dimethylformamide–water solvent with octadecylsilane-functionalized SiO₂. It is concluded that hydrophobic forces are essential for the successful coupling of LS–AgNPs with the surface of the modified silica. The formation of LS–AgNPs was monitored by UV-Vis spectroscopy, and the properties of the final hybrid materials were determined by EDS, elemental analysis, NIBS, TGA colorimetric analysis and HRTEM techniques. The resulting SiO₂–LS–AgNPs hybrids were also used as SERS substrates with Rhodamine 6G as a test molecule.

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Introduction

In the last decade there has been growing interest in nanostructures of precious metals, particularly silver and gold, owing to their exceptional physical, chemical and biological as well as optical properties.^1–3 Moreover, combining metal nanoparticles with the universality, non-toxicity and functionality of the silica matrix makes it possible to obtain new types of surfaces with interesting properties and new potential applications, ranging from catalysis, biodetection and optoelectronics to energy storage. For example, Wang's group¹ applied the SiO₂–Ag system as a catalyst for the decomposition of methyl orange, while Rameshkumar's group used silica core – nanosilver in the colorimetric detection of mercury ions.² In turn, Ojeda-Martinez tested the biocidal properties of SiO₂–Ag against the bacteria Staphylococcus aureus and Escherichia coli.³ The silica-silver system has also been used in a plasmonic material. The aim of that study was to detect antibodies using surface-enhanced Raman spectroscopy (SERS).^4,5

According to numerous reports in the literature, silver nanostructures can be obtained by chemical reduction of the salt in an aqueous or non-aqueous environment, using for instance hydrazine, citrate, or sodium borohydride,^6–8 and/or by selecting natural polymers such as gelatin, gum arabic and lignin for biochemical reduction of ionic silver.^9–12 Gelatin is a mixture of proteins and peptides derived after hydrolysis of animal collagens, while gum arabic is a well-known polysaccharide harvested from the branches and trunks of the plant Acacia senegal. Lignin, the derivate of phenylpropane, is present in trees and is responsible for the compactness of the cell structure of wood. Lignosulfonates (LS) are water-soluble lignin derivatives which are formed as by-products of the paper industry in the digestion of wood, and depending on the composition of the bath (usually HSO₃⁻), are obtained as calcium, sodium or magnesium lignosulfonates. Due to their stabilizing, reducing and binding properties, lignosulfonates can be used in a variety of fields, including in the rubber, plastics, paper and cosmetics industries and in medicine.^13–19

Many renewable solutions have proved to be useful in the preparation of silver nanoparticles (AgNPs), including extract of banana peel in acetone, extract of geranium leaves, and alfalfa sprout and mushroom extracts.^20,21

The aim of this work is to obtain a novel silica–lignosulfonate–silver systems wherein the lignin derivative is a reducer and stabilizer of silver nanoparticles. The use of lignosulfonates in the process leads to the formation of unique composites, while allowing the utilization of a waste product of the cellulose industry. The rationale behind the work is that technical lignins, including lignosulfonates, undergo strong adsorption on many surfaces, especially on high-surface sorbents, and that lignosulfonates have proved to be effective reducing/stabilizing agents for silver nanoparticles.¹¹ We decided for a first time to investigate this process in the presence of organically-modified silica. It is expected that SiO₂–LS–AgNPs will find applications as functionalized fillers of biologically active polymer composites, bioactive bead filters, and similar.

Experimental

Materials

Sodium salt of lignosulfonic acid (Sigma-Aldrich, Germany) was used as the reducing and stabilizing agent in the synthesis of AgNPs. An ammoniacal solution of silver(I) nitrate was used as the silver source in the reaction mixture. It was obtained by combining aqueous solution of AgNO₃ and ammonia (both reagent grade, POCh, Poland). Silicon dioxide (SiO₂) was obtained by the modified Stöber method^22–24 according to reactions (1) and (2):

Si(OC₂H₅)₄ + 4H₂O → Si(OH)₄ + 4C₂H₅OH (1)

Si(OH)₄ → SiO₂ + 2H₂O (2)

Detailed physicochemical and structural properties of silica are presented in Fig. S1 and Table S1 (see ESI†). Then, silica was modified by 5 wt/wt of selected organofunctional compounds with different degrees of hydrophilicity. The surface modification of the inorganic matrix promoted the binding of SiO₂ with LS and LS–AgNPs. For this purpose, N-2-(aminoethyl)-3-aminopropylotrimethoxysilane, phenyltrimethoxysilane and octadecylsilane (Sigma-Aldrich, Germany) were used as functionalizing agents.²⁵ Due to the poor wetting of the obtained silicas by water, the SiO₂–LS–AgNPs synthesis was carried out in aqueous–organic solutions. The organic solvents of high grade purity used were 1,4-dioxane and acetone (POCh, Poland) or N,N-dimethylformamide (Fluka, Germany).

Preparation of SiO₂–LS–AgNPs hybrids

Typical synthesis used 0.2 g of unmodified or organosilane-modified silica, to which 1.5 mL of organic solvent was added. To this solution was added 1 mL of sodium lignosulfonate at a concentration of 12.5 g L⁻¹, and 2.5 mL of the ammoniacal solution of AgNO₃ at a concentration of 1 g L⁻¹. After stirring for 10 min, the mixtures were allowed to undergo sedimentation, and the supernatant was removed. The solid was washed with aqueous–organic solution in a ratio of 2 : 3. Finally, the dried samples were subjected to physicochemical analysis. In this publication are the following designations samples: A – unmodified SiO₂, B – aminosilane-grafted silica, C – phenylsilane-functionalized SiO₂, D – octadecylsilane-modified silica. Effect of solvent: 1 – dioxane, 2 – acetone, 3 – DMF.

Characterization

The size of particles was determined by a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., UK) using the NIBS method. The contributions of colors, saturation and shade of color were determined in colorimetric analysis based on the CIE L*a*b* color space system (Spectrophotometer Specbos 4000, JETI Technische Instrumente GmbH, Germany). The morphology and microstructure of the products were analyzed using a Zeiss EVO 40 electron scanning microscope and a Jeol 1200 EX II (Japan) transmission electron microscope. Thermogravimetric analysis was conducted using a TG Jupiter STA449F3 (Netzsch, Germany). Samples weighing approximately 10.0 mg were placed in an Al₂O₃ crucible and heated at a rate of 10 °C min⁻¹ from 30 °C to 1000 °C in a nitrogen atmosphere. Optical absorption spectra were determined with an Ocean Optics UV-Vis spectrophotometer (model USB4000, USA). The content of elemental silver in the final hybrid materials was determined by ICP techniques after treatment of the solid samples with hot concentrated nitric acid. A Plasmaquant 100 emission spectrometer (Carl Zeiss, Germany) operating at 2450 MHz was used for the determination.

High resolution transmission electron microscopy (HRTEM) was used to analyze the hybrid materials in terms of chemical composition, type of microstructure defects, size and shape of nanocrystallites and their local orientation, using a JEM2200FS transmission electron microscope (JEOL, Japan) equipped with a Cs-corrected illumination system, ultra-high resolution (UHR) objective lens (Cs = 0.5) and in-column filter. The chemical analysis of the samples was performed by the energy dispersive X-ray (EDX) method using an energy dispersive X-ray analyzer and JED Series AnalyseProgram analysis software (JEOL, Japan). HRTEM micrographs were recorded using a 2K × 2K CCD camera (Gatan Inc., Japan) and evaluated by means of Fast Fourier Transformation (FFT) using the DigitalMicrograph software package (Gatan Inc., Japan) to obtain the orientation of individual nanocrystallites. The HRTEM results were completed by the simulation of HRTEM contrasts using JEMS software (Stadelman).

Raman spectra were recorded with an Ocean Optics modular Raman spectrometer (model QE65000, USA), equipped with a 785 nm laser. To prepare the SERS substrate, an aliquot of solid material dispersed in acetone was cast on gold-coated silicon wafer and allowed to dry at room temperature.

Results and discussion

UV-Vis spectroscopy

We have shown previously that aqueous solutions of lignosulfonates are effective reducing and stabilizing agents for AgNPs in the reaction with complexed silver(I) ions (Milczarek et al. 2013).¹¹ Unfortunately, the direct introduction of organically modified silica to such a reacting mixture was impossible due to its low wettability. It required the presence of an organic solvent to disperse the silica and to make it ready to acquire AgNPs formed in the liquid phase. Therefore, as explained in the experimental section, all forms of silica were first dispersed in an organic solvent, and then mixed with an aqueous solution of LS, and finally a silver salt in the form of its ammonia complex was added. The first question that arose was whether the organic solvent had any effect on the formation of LS–AgNPs. To answer this question, experiments were carried out in water and in mixed aqueous–organic solution in the absence of silica for the three selected solvents. After 24 h, UV-Vis spectra were recorded for all mixtures.

As shown in Fig. 1, all of the solutions displayed a characteristic absorption band at ca. 420 nm, attributed to the presence of plasmonic nanoparticles of silver, but its magnitude was influenced by the presence of organic solvent. Generally, for all organic solvents the absorption peak heights were lower compared with the case of water as the sole solvent, indicating the inhibiting effect of organic solvents on the formation of LS–AgNPs. The strongest inhibiting effect was observed for acetone, for which the level of absorption was slightly over half of that recorded for pure aqueous solution. It should be noted, however, that the absorption peak position, which provides a rough estimate of the nanoparticle size, was virtually constant for all of the mixtures, meaning that the organic solvents impeded only the kinetics of formation of LS–AgNPs, and not their size.

Fig. 1 UV-Vis spectra of the reaction mixture consisting of lignosulfonate and silver–ammonia complex, in aqueous and mixed aqueous–organic environment after 24 h reaction time. Legend indicates co-solvents mixed with water.

When the reaction was carried out in the presence of silica (pure or organically modified), the intensity of the plasmonic band of Ag-NPs in the liquid phase was further reduced, indicating the capture of some fraction of the nanoparticles by the dispersed solid. Again, the observed phenomenon was an effect of the organic solvent, but it was also strongly related to the surface structure of silica. To explain this, we suggest a tentative reaction mechanism comprising four factors that may be responsible for the effectiveness of the deposition (adsorption) of silver nanoparticles onto the surface of silica, which include (Scheme 1): (i) the reaction kinetics between solution phase LS and the Ag–ammonia complex; (ii) the reaction kinetics between LS adsorbed on the surface of silica and the Ag–ammonia complex; (iii) the adsorption equilibrium between dissolved LS and surface-adsorbed LS; and (iv) the adsorption equilibrium between solution phase LS–AgNPs and surface-adsorbed LS–AgNPs. We believe that the presence of organic solvent influenced mainly the reaction kinetic rates (k₁ and k₂), while the ionic character and hydrophobicity of the organic groups grafted on the surface of silica contributed mostly to the adsorption equilibrium constants (k₁ and k₂). Hence, depending on the solvent chosen and the hydrophobicity of the silica surface, the reaction sequences leading to the deposition of silver nanoparticles on the silica surface may proceed through two different routes (I and II). The first assumes that AgNPs are formed in the liquid phase and then adsorbed on the surface of solid silica. The second assumes that LS is adsorbed on the silica surface before it reacts with the dissolved complexed silver ions.

Scheme 1 Tentative reaction mechanism leading to the modification of the silica surface with silver nanoparticles using lignosulfonates as reducing/stabilizing agent.

At this stage of our investigations, we speculate that the modification proceeds through route I except in the case of amino-functionalized silica, where coulombic interactions between protonated amino-functionalities (positively charged) and the negatively charged lignosulfonates favour route II. As will be shown later, route I seems to be more effective in terms of the achievable AgNPs grafting (see the EDS analysis below).

The reaction mixtures containing all of the studied silicas were allowed to sediment, and the supernatants were analysed by UV-Vis spectroscopy to monitor the presence of silver colloid (Fig. 2). The strongest effect was observed for octadecyl-functionalized silica in DMF as co-solvent. Under these conditions, UV-Vis spectroscopy revealed no silver colloid in the liquid phase, indicating that either the formation of silver nanoparticles was for some reason inhibited in the presence of this type of modified silica, or that the silver nanoparticles (formed in the liquid phase) were strongly adsorbed onto the silica (route I).

Fig. 2 UV-Vis spectra of the reaction mixture consisting of lignosulfonate and silver–ammonia complex in the presence of various silica samples in aqueous/DMF solution, performed after a reaction time of 24 h.

The EDS analysis of silica samples after contact with the liquid reaction mixture also returned the highest content of silver for octadecyl-functionalized silica with the reaction taking place in H₂O–DMF solvent, as described below.

EDS and elemental analysis as well as the content of silver analyzed by ICP spectroscopy

Fig. 3 shows the atomic content of silver in the samples synthesized with different surface-functionalized silicas in mixed aqueous–organic solvents.

Fig. 3 Silver content analyzed by EDS, expressed as atomic content. A: unmodified silica, B: aminosilane-grafted silica, C: phenylsilane-functionalized silica, D: octadecylsilane-modified silica.

As can be seen, the solvent has a very strong effect on the grafting efficiency of silver in the analyzed samples. In the dioxane–water mixture the content of silver analyzed by EDS was at most 0.12 wt%. Much higher silver grafting were obtained in acetone–water and in DMF–water solvents, especially for octadecyl-functionalized silica. In this case, the highest silver concentration (ca. 0.97 wt%) was measured for the DMF–water mixture.

The main question remaining unanswered at this stage was the content of lignosulfonate and silver in the final SiO₂–LS–AgNPs composite materials. The EDS analysis described above provided only semi-quantitative information on the silver content, due to the limited penetration depth of the electron beam. In other words, the silver content estimated by EDS was presumably overestimated with respect to its real content in the bulk sample.

Therefore the content of silver was additionally analyzed by ICP spectroscopy, after stripping it from the silica spheres with hot concentrated nitric acid. On the other hand, the quantity of lignosulfonate adsorbed on the silica could be estimated from the elemental analysis, bearing in mind that lignosulfonate is the only component containing sulphur. The elementary analysis results and the calculated contents of Ag and LS are summarized in Table 1.

Table 1 Elemental analysis, with silver content and calculated lignosulfonate content, for samples synthesized in an H₂O–DMF mixture

Sample	Elemental analysis/wt%				CAg/wt%	CLS/wt%
	N	C	H	S
LS	0.12	41.44	5	5.8
A	0.09	0.77	1.18	—
A3	0.08	1.94	1.20	0.20	0.15	3.45
B	0.31	1.24	1.28	—
B3	0.27	2.95	1.31	0.25	0.13	4.31
C	0.02	1.30	1.27	—
C3	0.02	1.89	1.36	0.22	0.30	3.79
D	0.02	2.67	1.44	—
D3	0.01	3.05	1.58	0.28	0.78	4.83

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As can be seen, the content of silver ranges from 0.13 to 0.78 wt%, and the highest graftings were recorded for samples based on silica modified with highly hydrophobic surfaces, i.e. phenyl- and octadecyl-functionalized. By contrast, the content of lignosulfonate is less dependent on silica modification, and ranged from 3.45 wt% for amine-modified silica to 4.85 wt% for octadecyl-functionalized silica.

Colorimetric analysis

The effect of surface modification for all of the studied silica samples could be seen with the naked eye, and the modified samples, depending on silica type (surface functionalization) and the organic co-solvent present in the reaction mixture, exhibited colours ranging from white to pale pink, beige, light yellow and brown (Fig. 4).

Fig. 4 Color appearance of various silica samples after contact with a reaction medium consisting of aqueous–organic solvent, lignosulfonate and silver–ammonia complex. Effect of SiO₂ functionalization: A: unmodified silica, B: aminosilane-grafted silica, C: phenylsilane-functionalized silica, D: octadecylsilane-modified silica. Effect of solvent: 1 – dioxane, 2 – acetone, 3 – DMF.

In order to make a more accurate assessment of the colour of the samples obtained, colorimetric analysis was performed. The results are presented in Table 2.

Table 2 Colorimetric characteristics of SiO₂–LS–Ag

Sample	Colorimetric data
	L*	a*	b*	C*	h*	dE
LS	32.42	9.66	21.88	23.92	66.19	65.03
A1	98.16	6.56	−1.98	6.76	343.11	8.87
A2	96.42	4.68	3.09	5.60	33.43	5.17
A3	55.74	3.34	2.53	4.19	37.12	38.2
B1	98.94	6.56	17.06	18.28	68.97	16.48
B2	88.36	5.26	20.94	21.59	75.91	19.68
B3	47.07	2.65	8.73	9.12	73.11	47.19
C1	99.77	0.31	1.16	1.21	75.02	6.14
C2	94.31	2.73	9.28	9.67	73.60	7.03
C3	48.49	1.35	1.15	1.77	40.36	45.37
D1	90.44	3.07	7.03	7.67	66.39	6.18
D2	53.73	7.24	8.18	10.92	48.49	41.09
D3	30.89	2.17	3.87	4.43	60.73	62.97

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The parameter L* defines the brightness of the sample, and the components a* and b* refer to the transition of colors from green to red and from blue to yellow respectively. Also determined for the SiO₂–LS–Ag systems were the saturation (C*) and hue (h*), and the color difference being the result of changes in its individual components (dE*). Based on the results, it was found that the highest brightness is exhibited by SiO₂–LS–Ag systems obtained in an aqueous–organic system using an organic dioxane solvent, for which the brightness parameter took values between 90.44 and 99.77. The composites obtained using DMF as the organic solvent had the lowest brightness, ranging from 30.89 to 55.74. In turn, the greatest change in the dE* parameter, describing the total color change, was observed for the systems obtained in the presence of DMF. Differences in color may result from the different polarities of the organic solvents used, which are ordered as follows: dioxane < acetone < DMF.

Thermogravimetric analysis

Fig. 5 shows the thermogravimetric curves for silica modified with octadecylsilane (sample D), silica modified with octadecylsilane with the addition of silver nanoparticles (sample D3), and sodium lignosulfonate (LS). The thermogravimetric curve recorded for lignosulfonate exhibits two characteristic stages. The first, below 200 °C, results from endothermic water desorption from the sample. The second and more severe mass loss (∼40%), starting at a temperature of about 200 °C, reflects the gradual degradation and consequent fragmentation of the compound. The results are in agreement with data given by Brebu et al.²⁶ and Lemes et al.²⁷

Fig. 5 Thermogravimetric analysis of lignosulfonate (LS), silica modified with octadecylsilane (sample D), and SiO₂–LS–AgNPs composite prepared in DMF (sample D3).

A very interesting relationship was observed by comparing samples D3 and D, which respectively contain octadecylsilane-modified silica with silver nanoparticles bound to the surface, and the same form of modified silica without silver. Analysis of the TG curves (Fig. 5) shows that the silver nanoparticles present on the surface of the SiO₂–LS hybrid cause a slight improvement in the thermal stability of the sample, which also provides indirect confirmation of the effectiveness of the method used to obtain SiO₂–LS–Ag composites.

Hydrodynamic particle size distribution

The particle size distribution of the SiO₂–LS–AgNPs composites was analyzed (see Fig. 6). The results show that the systems contain a very wide range of particle diameters (from 35 nm up to 6000 nm), irrespective of the starting materials used (A: unmodified silica, B: aminosilane-grafted silica, C: phenylsilane-functionalized silica, D: octadecylsilane-modified silica).

Fig. 6 Particle size distributions of SiO₂–LS–AgNPs composites prepared in organic solvent: (a) dioxane; (b) acetone; (c) DMF.

Each of the graphs shows the volume fraction of small particles with diameters in the range 35–150 nm (except for samples A1, A3 and D2), and a dominant broad band of larger particles with diameters in the range 400–6000 nm, forming aggregate and agglomerate structures. The graphs also indicate the dominant particle diameter and the volume fraction of the respective systems. The smallest particles in terms of dominant diameter were obtained in a dioxane environment (A1 = 164 nm, B1 = 955 nm, C1 = 712 nm, D1 = 825 nm). The use of acetone or DMF as organic solvent resulted in an increase in the dominant particle diameters (except for sample C3), which were respectively A2 = 1280 nm, B2 = 1990 nm, C2 = 1480 nm, D2 = 1480 nm for acetone, and A3 = 342 nm, B3 = 1720 nm, D3 = 1990 nm for DMF. The increase in particle size may be also due to the formation of complex heterogenous structures consisting of multiple silica particles interconnected via lignin fibers (see next paragraph). It should be noted that the volume fraction of particles having the dominant diameter in each case does not exceed 24%.

High resolution transmission electron microscopy

Generally the investigated samples contain large spherical particles of Stöber SiO₂, which are connected partially via lignin fibers (Fig. 7a). More detailed investigation of the surface of such large particles (diameter approximately 0.5–1 μm), as well as of the lignin fibers, showed the presence of spherical nanocrystalline inclusions of 5–10 nm in diameter (Fig. 7b).

Fig. 7 Bright-field image of Ag–lignosulfonate composite (a), areas of lignosulfonate fibers (b), and high-resolute silver nanoparticle with corresponding indexed FFT (c), with associated results of EDX analysis (d), and simulated HRTEM contrasts and corresponding SAED pattern of the Ag nanocrystallite (e).

In addition to the carbon and silicon of lignosulfonates, the chemical analysis of these particles by the EDX method shows a considerable content of silver (Fig. 7d). To estimate the phase composition of these nanoparticles, HRTEM micrographs were made and analyzed using local FFT. The analysis of interplanar distances and angles calculated by FFT confirmed that these are nanoparticles of pure silver. The major fraction of these nanoparticles contains defects – twins – which are typical for silver due to the low stacking-fault energy. Fig. 7c shows exactly such a nanoparticle, with the [110]-direction parallel to the primary beam. The HRTEM micrograph shows two crystallographically equivalent types of twins: twin A with the (1−11) twinning plane, and twin B with the (−111) twinning plane. The arrows mark these twinning planes on the HRTEM micrograph. As a result of twinning and appropriate rotation of the crystallite regions, additional reflexes are formed by twin A, labeled * on the FFT, and by twin B, labeled **. The coincident (1−11) and (1−1−1)* reflections, and the (−111) and (−11−1)** reflections, are a result of mutual planes of the twin and matrix.

To verify whether the HRTEM contrasts really are caused by the twins, the indexed FFT was used to create an atomic model of a nanoparticle with twins, using CrystalMaker software. The model contained about 72 000 atoms. The simulation of HRTEM contrasts and diffraction pattern, based on this model, was performed by the multislice method using the JEMS software package (Stadelman). The following instrumental parameters for the calculation were used: accelerating voltage 200 kV, spherical aberration coefficient C_s = 0.5 mm, defocus 60 nm. The thickness of the simulated supercell was 9.8 nm. The result of the HRTEM contrast simulation (Fig. 7e) conforms to the contrasts observed in the TEM (Fig. 7c).

The simultaneously simulated SAED pattern (only the matrix-based reflexes are marked) is completely in accordance with the FFT of the investigated silver nanoparticle (Fig. 7c).

SERS properties of SiO₂–LS–AgNPs composite

There are several reports in the literature dealing with the SERS properties of AgNPs grafted onto silica spheres, the observed SERS signal amplification being attributed to the presence of “hot spots” from nanocontacts between neighboring nanoparticles. Hence it should be expected that the materials described here can serve as detecting substrates for selected analytes.^28–30

Rhodamine 6G is a cationic dye routinely used for preliminary testing of the SERS properties of various materials. We assumed that Rhodamine 6G would be preconcentrated on lignosulfonate-stabilized AgNPs, due to its poly-anionic character. Fig. 8 compares the recorded Raman signals for a 10⁻⁵ M solution dropped on a flat Au surface and on the same surface with a cast deposit of sample D3.

Fig. 8 Raman spectra of Rhodamine 6G on a flat Au surface and on the same substrate modified with a cast deposit of sample D3.

As can be seen, in the former case a featureless signal is recorded, which does not provide any information that could be attributed to the presence of the dye. By contrast, the Raman spectra recorded on sample D3 show a characteristic set of peaks that are attributed to C–H in-plane bending, C–O–C stretching and C–C stretching of the aromatic ring. The most intensive peak was recorded at 1512 cm⁻¹, and its height was a linear function of the logarithm of dye concentration in the solution dropped on the substrate.

The SERS properties of the SiO₂–LS–AgNPs composites were again dependent on the co-solvent and the surface modification of the silica, as depicted in Fig. 9. The most active samples were those obtained in an H₂O–DMF mixture. The samples prepared in H₂O–dioxane and H₂O–acetone mixtures where much less active.

Fig. 9 SERS activity of SiO₂–LS–AgNPs composites. The intensity of the signal at 1512 cm⁻¹ was normalized against the most active sample, D3.

The calibration curve was constructed by plotting the most intensive Raman peak as a function of Rhodamine 6G concentration and it is presented in Fig. 10. As seen, a linear correlation between the two variables is observed down to micromolar concentrations. Similar sensitivity was observed for AgNPs/SiO₂ obtained by other synthetic route.³¹

Fig. 10 Calibration curve plotted as a function of peak intensity at 1512 cm⁻¹ versus R6G concentration.

Conclusions

We have demonstrated that lignosulfonate can be used as a reducing agent for silver ions to form silver nanoparticles. Simultaneously it acts as a “glue” immobilizing the resulting nanoparticles on organically modified silica. The progress of the reduction reaction and the grafting of silver nanoparticles were dependent on both the polarity of the solvent and the polarity of the silica surface groups. The main conclusion drawn is that the highest silver grafting are obtained in a mixed aqueous–organic solvent containing polar co-solvent (e.g. DMF) for silica particles surface-modified with non-polar groups, e.g. long alkyl chains. This general conclusion opens up the possibility of further optimization studies, including solvents not tested in this work and silica particles modified with various organic moieties. Plausible applications of the obtained SiO₂–LS–AgNPs hybrids include the preparation of solid substrates for SERS analysis of selected analytes, especially those with positive ionic charge, due to pre-concentration on the negatively charged lignosulfonate.

Acknowledgements

The study was financed within the Polish National Centre of Science funds according to decision no. DEC-2013/09/B/ST8/00159. Additionally, this work was also partially supported by the following research grants DFG Grant EH 394/3-1, CryPhys Concept Program (BMBF) and DAAD (Section 323, Project no. 50015537).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08418g

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