Nanoscopic Properties and Application of Mix-and-Match Plasmonic

Article pubs.acs.org/JPCC

Nanoscopic Properties and Application of Mix-and-Match Plasmonic Surfaces for Microscopic SERS Virginia Joseph,†,‡ Manuel Gensler,§ Stephan Seifert,†,‡ Ulrich Gernert,⊥ Jürgen P. Rabe,§ and Janina Kneipp*,†,‡ †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany Federal Institute for Materials Research and Testing, Richard-Willstätter-Strasse 11, D-12489 Berlin, Germany § Department of Physics, Humboldt-Universität zu Berlin, Newtonstrasse 15, D-12489 Berlin, Germany ⊥ Technical University Berlin, ZELMI, Strasse des 17. Juni 135, D-10623 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Gold and silver nanoparticles can be immobilized on glass slides using aminosilane linkers. Here, we demonstrate that particle monolayer surfaces can also be generated by simultaneous immobilization of both gold and silver nanoparticles with the same organosilane linker. These new surfaces display surface-enhanced Raman scattering (SERS) enhancement typical for gold or silver monolayers, depending on the ratio of the two types of nanoparticles and, at the same time, have the capability to probe complex analytes composed from various molecules which adsorb at only one of the metals. The reported results from scanning electron microscopy, scanning force microscopy, and UV/vis absorbance for surfaces containing one or two types of nanoparticles indicate that an enhancement level above 104 is related to nanoaggregates that form in the 2D plane. High and stable enhancement factors over a wide range of analyte concentrations along with high homogeneity of the enhancement at the microscopic scale make the plasmonic nanoparticle mixand-match surfaces ideal substrates for use in microscopic SERS sensing.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) has become an important analytical tool which enables probing molecules at extremely low concentrations and also complex chemical compositions such as biological structures.1−3 The use of SERS for analytical applications requires stable and reproducible substrates that are easy to prepare so that they can be generated on-demand, directly in the analytical laboratory. Solid SERS sensors show great promise for quantitative detection and characterization.4−7 For the preparation of microscopic and macroscopic plasmonic surfaces, self-assembly of gold and silver nanoparticles on organosilane-functionalized glass can be employed.8,9 In particular, the type of silane linker, such as cyano, mercapto, and amino functional groups, and the use of different solvents have an effect on nanoparticle density and morphology of the resulting substrates.10,11 Different aminosilanes have so far most frequently been used for the successful immobilization of gold10,12,13 and also of silver nanoparticles.14,15 There, surface nanostructure is a function of the deposition process of the silane linker and the nanoparticles, in particular concerning drying and soaking,12 and can include the formation of nanoparticle−polymer multilayer films.10,16 Nanoparticle assemblies on organosilane-functionalized glass have been proposed as SERS substrates almost two decades ago.10−13,15,17,18 © 2012 American Chemical Society

The analytical application of such plasmonic structures in microscopic SERS sensing devices requires a specification of the enhancement and its variation at the microscopic level in the context of the nanoscopic and plasmonic properties of the surfaces. Here, we determine SERS enhancement factors and their distribution at the microscopic level as a function of nanoscopic and plasmonic properties for different substrates that can be generated on-demand for routine analyses using different nanoparticle types. We characterize the surfaces formed by organosilane-immobilized silver and gold nanoparticles using scanning force microscopy (SFM), scanning electron microscopy (SEM), and absorption spectroscopy. Three parameters are of interest when silane-immobilized nanoparticle layers are considered as SERS substrate in a microstructured sensor: (i) the level of enhancement, (ii) the microscopic homogeneity of the enhancement, and (iii) the stability of the enhancement under varying conditions, such as in bioanalytical applications. In principle, the immobilization procedure using organosilanes allows the generation of surfaces containing different types of nanoparticles at the same time. Here, we discuss for Received: December 28, 2011 Revised: March 8, 2012 Published: March 8, 2012 6859

dx.doi.org/10.1021/jp212527h | J. Phys. Chem. C 2012, 116, 6859−6865

The Journal of Physical Chemistry C

Article

the first time the characteristics of such new surfaces comprising both gold and silver nanoparticles immobilized on the same substrate. These surfaces will provide additional functionalities resulting from both gold and silver nanoparticles. Stability and reproducibility of SERS data are investigated using the complex biomolecular mixtures contained in aqueous extracts of pollen samples from different plant species.

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EXPERIMENTAL METHODS A detailed description is provided in the Supporting Information. Plasmonic Nanostructures and Their Properties. Gold and silver nanoparticles were synthesized by reduction according to several different protocols.19−21 Silver nanoparticles were also generated using laser ablation.22 The nanoparticles were immobilized on glass slides using 3aminopropyltrimethoxysilane (APTMS, 97%, Sigma) and 3aminopropyltriethoxysilane (APTES, 98%).10,14,16,17 For the mixed gold/silver surfaces, the silanized substrates were immersed in mixtures of gold and silver nanoparticle solutions with different amounts of gold and silver. UV/vis spectra were recorded from all samples from 300 to 900 nm. The surfaces were imaged by scanning force microscopy (SFM) in intermittent contact mode under ambient conditions. Scanning electron microscopy images were obtained using a Hitachi S-4000 with a cold field emitter. The acceleration voltage was 15 kV. To enhance the conductivity of the sample, a thin carbon film was evaporated. Raman Experiments and Characterization of SERS Performance. Raman spectra were obtained with a Raman microscope (LabRamHR, Horiba, Jobin-Yvon) with a 60× water immersion objective at 632.8 nm excitation (1.5 μm spot diameter, intensity 6.7 × 103 W·cm−2). Data on the distribution of the enhancement factor were generated by raster scanning. Determination of the enhancement factors is described in the Supporting Information. Spectra from aqueous extracts of pollen from different plant species were measured using 785 nm excitation (intensity 6.5 × 105 W·cm−2 in the measurements with nanoparticles in solution, 1.3 × 105 W·cm−2 in the measurements with immobilized nanoparticles). Details on sample preparation are given in the Supporting Information.

Figure 1. UV/vis spectra of nanoparticles immobilized on a glass surface with APTMS (black lines) and APTES (red lines), respectively: (A) citrate-reduced silver nanoparticles, (B) hydroxylamine-reduced silver nanoparticles, (C) laser-ablated silver nanoparticles, and (D) citrate-reduced gold nanoparticles. Please see Supporting Information Figure S3 for spectra of more types of citratereduced gold nanoparticles (last two in Table 1).

For the citrate-reduced silver nanoparticles, the extinction is significantly lower upon immobilization with APTES rather than when APTMS is used (Figure 1A). For identical nanoparticles, the absorbance can be used as indicator of the amount of immobilized particles. The smaller extinction shows that less citrate-reduced silver nanoparticles were immobilized by APTES than by APTMS (Figure 1A). As evidenced by a very broad plasmon band in their extinction spectrum in solution (Figure S1), as well as by TEM data (not shown here), the citrate-reduced silver nanoparticles display a very broad size distribution in solution. Upon immobilization with APTMS, the plasmon band around 400 nm, assigned to individual silver nanoparticles, becomes significantly narrower for the immobilized nanoparticles as compared to the solutions and is located at lower wavelengths (Figure 1A). The lower wavelength suggests that not all nanoparticles sizes may have been immobilized, resulting in a different size distribution on the surface and/or that aggregates have formed, causing absorbance at slightly higher frequency.25 The nanostructure of the surfaces was investigated by scanning force microscopy (SFM). Figure 2 contains the SFM images of all nanoparticle types when immobilized with APTES (Figure 2A, B, C, and E). The images show a homogeneous distribution of the nanoparticles and nanoaggregates on the surfaces. For comparison, also a surface with citrate-reduced silver nanoparticles immobilized with APTMS is shown (Figure 2D). The statistical analysis of the SFM images showed that the measured heights of the nanoparticles in the layers in Figures 2A, B, C, and E (Table 1) are in good agreement with the sizes of the nanoparticles determined by TEM (Supporting Information Table S1). The data indicate that true monolayers of nanoparticles are formed. At the same time, the formation of small aggregates, such as dimers and trimers occurring in the 2D plane can cause plasmonic coupling resulting in extinction in the near infrared (NIR; Figure 2A), in accord with Figure 1 and earlier observations.10 This is in contrast to the observations made for gold nanoparticle monolayers using other linker molecules, where an extension of the plasmon band into the NIR was only found in

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RESULTS AND DISCUSSION Plasmonic Properties and Nanostructure of the Surfaces. The nanostructural and plasmonic properties of the surfaces depend on parameters in the preparation process. Silver nanoparticles of different size distributions and plasmonic properties (Supporting Information Table S1) were produced by bottom-up reduction with citrate19 and hydroxylamine,21 as well as by the top-down process of laser ablation from bulk silver.22 Gold nanoparticles were produced by citrate reduction.19,20 Figure 1 displays the extinction spectra of these four types of nanoparticles immobilized with aminopropyltrimethoxysilane (APTMS) and aminopropyltriethoxysilane (APTES), respectively. They exhibit the plasmon bands of silver nanoparticles around 400 nm (Figure 1A−C) and of gold nanoparticles around 520 nm (Figure 1D), that were also observed before immobilization (see Supporting Information Figure S1). Spectra of immobilized particles can show a pronounced, broad extended plasmon band, typical for nanoaggregates.23,24 The extinction spectra provide evidence that immobilization was successful in all cases. 6860



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Figure 2. SFM images: (A and D) citrate-reduced silver nanoparticles immobilized with APTES and APTMS, respectively, (B) hydroxylaminereduced silver nanoparticles immobilized with APTES, (C) laser-ablated silver nanoparticles immobilized with APTES, (E−G) gold nanoparticles immobilized with APTES.

experiment.29 The enhancement factor was estimated for 100 spots over an area on the surface of 100 × 100 μm2 with a distance of 10 μm between each measurement point in a rectangular grid. Schematic arrays of the enhancement factors at the sampled spots are shown in Figure 3. The highest enhancement factors (∼2 × 106) with good homogeneity of the enhancement were obtained for the substrates with the gold nanoparticles (Figure 3D). Enhancement is lower in the silver nanoparticle assemblies. Those monolayers generated using APTES yield slightly higher enhancement (Figure 3A−C, right panels). The citrate-reduced silver particles show the highest enhancement factors of ∼7 × 105 to 106 with good homogeneity when immobilized with APTES (Figure 3A, right panel). It should be noted that we estimate an average enhancement factor over ∼1 μm2. APTMS as linker for the citrate-reduced silver nanoparticles yields lower signal level and a few spots with high signals (Figure 3A, left panel). This is consistent with the surface’s nanostructure revealing some large aggregates that protrude from the nanoparticle monolayer (Figure 2D). The larger aggregates do not lead to further increase of enhancement30,31 but rather provide a smaller surface for analyte molecules. The hydroxylamine-reduced silver nanoparticles result in surfaces with similar maximum enhancement as the citrate-reduced ones for APTES immobilization but greater variation, depending on the position on the surface (Figure 3B, right panel). The SFM data, interestingly, suggest that for the silver nanoparticles the higher enhancement occurs at lower particle densities: Upon APTES immobilization, the particle density is slightly higher in the hydroxylamine-reduced compared to the citrate-reduced particles at similar particle size (Table 1) and the citrate-capped silver nanoparticles result in surfaces with the higher enhancement. The silver nanoparticles generated by laser ablation show the highest density on the surface (Table 1, Figure 2C), but the lowest enhancement factors of ∼103, characteristic for isolated nanoparticles32,33 (Figure 3C). From the comparison with the extinction spectra (Figure 1), we suggest that, for particles of similar size, the morphology and types of aggregates on the surface must play a major role in the enhancement. The properties of the aggregates are mainly a

Table 1. Height and Particle Density on the Surface Obtained from the SFM Images Displayed in Figure 2 nanoparticles

aminosilane linker

silver, citrate

APTMS

silver, citrate silver, hydroxylamine silver, laser ablation gold, citrate after ref 19 gold, citrate 15 nm after ref 20 gold, citrate 30 nm after ref 20

APTES

height (nm)

particles per μm2

48 ± 12, 195 ± 14a 50 ± 10 45 ± 10 20 ± 3 25 ± 6 12 ± 1

∼60−100 ∼20 ∼70 ∼230 ∼300 ∼1100

25 ± 4

∼350

a

The different values for the citrate-reduced silver nanoparticles that were immobilized with APTMS represent the data for two images on different areas on the substrate.

multilayers.26 In other cases, there occurs a more dense packing (Figure 2C and E), with larger aggregates. In the example of the APTMS-functionalized surface, the citrate-reduced silver nanoparticles form large aggregates, causing a high degree of inhomogeneity at the submicrometer scale (Figure 2D and Table 1). We find that the highest nanoparticle densities are reached with citrate-reduced gold nanoparticles and with ablated silver nanoparticles and that particle density differs by more than a factor of 10 (Table 1). We assume that the differences in nanoparticle density are caused by the different size of the nanoparticles and their different concentrations in solution, both of which will lead to a different interaction with the organosilane linker. The surface charge of the nanoparticles and the amount of positively charged amino groups, both known to cause different electrostatic interaction between nanoparticle surface and the linker,16,27,28 were very similar for all samples. Surface-Enhanced Raman Scattering: Enhancement Factors. We have determined the distribution of SERS enhancement factors at the microscopic level as a function of nanoparticle type and linker molecule. As test analyte molecule we used crystal violet (CV), because it offers the advantage of direct comparison of the normal Raman and the SERS 6861



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Supporting Information (Figure S3E and F). The constant enhancement factor indicates that the organosilane-immobilized nanoparticles are stable on the surface and is a clear advantage over nanoparticle solutions, where analyte-induced aggregation generates variation of enhancement factors.29 Higher reproducibility on surfaces has been reported recently also for other types of immobilized silver nanostructures.38 In particular, the constant SERS enhancement factor due to higher stability enables quantitative SERS measurements. The enhancement factors are within the wide range of enhancement factors (from 2 × 104 to 3 × 108) reported for other solid SERS substrates such as island films,39−41 gold nanoparticle arrays from electrodeposition,42 nanosphere lithography,43 silver films,44 silver films over nanoparticles (Ag FON),45 and silver and gold nanoparticles embedded in the subsurface of a glass matrix.46,47 Surfaces Containing Both Silver and Gold Nanoparticles. The possibility to use the same linker to immobilize both silver and gold nanoparticles can be employed to combine them on one substrate. Figure 4A shows extinction spectra of

Figure 3. Schematic distribution of enhancement factors at positions (x, y) on the different substrates generated by nanoparticle immobilization with APTMS (left column) and APTES (right column): (A) citrate-reduced silver nanoparticles, (B) hydroxylamine-reduced silver nanoparticles, (C) laser-ablated silver nanoparticles, (D) citrate-reduced gold nanoparticles. Spectra were acquired at a step size of 10 μm; diameter of the probed spot: 1.5 μm (not to scale in the schematic). The enhancement factors were determined using 5 × 10−6 M crystal violet as analyte (λ = 633 nm, I = 2.7 × 103 W·cm−2, acquisition time = 1 s).

result of particle capping and the specific interaction of the nanoparticles with the linker molecule. The differences become most obvious when comparing the extinction spectra of the surfaces generated with silver nanoparticles made by laser ablation and those of reduction-made silver nanoparticles (compare Figures 1A−C). The ablation-made particles display almost no signs of aggregation on the surface. In addition to the extinction spectra, the SFM images clearly indicate that, in the case of high enhancement yet low particle densities (e.g., Figure 2A), we are not dealing with isolated nanoparticles of close spacing but with dimers and trimers that are relatively far apart from one another. In such nanoparticle aggregates, high local fields are generated which lead to high enhancement of the SERS signals.1,23,34−37 It should be noted, however, that enhancement is not as high as was observed for larger clusters of gold nanoparticles in solution.34 We think that this can in general be explained by the interaction of the nanoparticles with the linker molecules, which might prevent the formation of aggregates showing extremely small gaps and therefore extremely high level “hot spots” on the aggregates. Within the accuracy of our measurements, no dependence of the enhancement on analyte concentration was found. As an example, the enhancement factor for the 30 nm nanoparticles and for the 15 nm nanoparticles is shown for analyte concentrations between 2 × 10−6 and 1 × 10−5 mol/L in the

Figure 4. (A) Extinction spectra of gold nanoparticles prepared by citrate reduction, immobilized together with citrate-reduced silver nanoparticles at a ratio of ∼10 (green line) and hydroxylaminereduced silver nanoparticles at ratios of ∼3 (blue line), ∼1 (red line), and ∼0.3 (black line), respectively. Scanning electron micrograph of citrate-reduced gold nanoparticles and (B) hydroxylamine-reduced silver nanoparticles at a ratio of ∼0.3, (C) hydroxylamine reduced silver nanoparticles at a ratio of ∼3, and (D) citrate-reduced silver nanoparticles at a ratio of ∼10, immobilized by APTES. Abbreviations: Au(c), citrate-reduced gold nanoparticles; Ag(c), citrate-reduced silver nanoparticles; Ag(h), hydroxylamine-reduced silver nanoparticles.

surfaces that contain citrate-reduced gold nanoparticles together with silver nanoparticles made by citrate or hydroxylamine reduction at different ratios, immobilized using APTES. All spectra display characteristic plasmon bands of both silver and gold, as well as a third plasmon band at longer wavelengths that can be assigned to aggregates. The spectra of the gold and hydroxylamine reduced silver nanoparticles clearly reflect different ratios of gold and silver (Figure 4A): The absorbance assigned to gold nanoparticles around 520 nm rises relative to that of the silver nanoparticles around 420 nm with increasing gold to silver ratio (black over red to blue curve in Figure 4A). It should be noted though that the extinction of the silver and gold plasmon bands cannot be used as a direct measure of 6862



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Figure 5. Schematic distribution of enhancement factors at positions (x, y) on the substrates comprising citrate-reduced gold nanoparticles and hydroxalamine-reduced silver nanoparticles at the ratios: (A) ∼3, (B) ∼1, and (C) ∼0.3. Spectra were acquired at a step size of 10 μm; diameter of the probed spot: 1.5 μm (not to scale in the schematic). The enhancement factors were determined using 5 × 10−6 M crystal violet as analyte (λ = 633 nm, I = 2.7 × 103 W·cm−2, acquisition time = 1 s).

Figure 6. (A) SERS spectra of aqueous extracts of pollen samples from different plant species obtained with gold nanoparticles immobilized using APTES (λ = 785 nm, I = 6.5 × 105 W·cm−2, acquisition time = 1 s; scale bars = 10 000 counts per second). (B and C) Relative standard deviation for the measurements with gold nanoparticles immobilized with APTES and in solution, respectively, determined by averaging 100 spectra.

order of magnitude lower, similar to the surfaces containing only such silver nanoparticles (Figure 3B). This suggests that the properties of the hydroxylamine-reduced silver nanoparticles, which exceed the gold nanoparticles in number, determine the enhancement. At the microscopic scale, the homogeneity of the enhancement for the APTES-immobilized gold and silver nanoparticles is similar to that of the pure gold nanoparticles at the microscopic scale (see Figure 5B and Figure 3D). In the case of the substrates with more silver nanoparticles, homogeneity is higher than in those substrates containing only silver nanoparticles (compare Figures 5C and 3B). The presence of one-third of gold nanoparticles in the silver nanoparticle layer may provide new properties to the surfaces, in particular regarding analyte partitioning in sensing applications. Signal Stability and Application to Analyte Mixtures. High signal fluctuations are often a problem in the analysis of solutions that are complex molecular mixtures. We have used the aqueous extracts of pollen samples to assess the reproducibility of the data obtained from such samples. These samples contain all kinds of biomolecules that can be

particle concentration due to variation in nanoparticle size and varying aggregation. In Figures 4B−D, scanning electron micrographs (SEM) of surfaces are shown that were prepared using a mixture of citrate-reduced gold and hydroxylamine silver nanoparticles (B and C) and citrate-reduced silver nanoparticles (D) in different ratios. As evidenced by the SEM experiments and by energy dispersive X-ray spectroscopy (EDX) data (not shown here) of several selected spots in the sample, the larger nanoparticles, including the rod shaped ones, are silver nanoparticles. They have attached to them the smaller gold nanoparticles which form small gold nanoaggregates, mainly dimers and trimers. We determined the SERS enhancement factors for three types of mixed surfaces with gold nanoparticles and hydroxylamine reduced silver nanoparticles (Figure 5). The enhancement factor is on the order of 106 for the gold to silver ratios of ∼3 and ∼1, respectively (Figure 5A and B). This enhancement is similar to the surfaces containing only gold nanoparticles (compare with Figure 3D). Using the surface with the lower amount of gold nanoparticles, at a gold to silver ratio of ∼0.3 (Figure 4B and Figure 5C), the enhancement factor is about 1 6863



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removed from a pollen grain by dissolution in water. They are useful for the fast classification and identification of pollen and could be employed in the development of fast allergy warning systems, which recently was shown to be feasible with a combination of normal Raman scattering48 or infrared spectroscopy49 and multivariate data analysis. The soluble fraction of pollen from four species, Populus nigra “Italica” (black poplar), Secale cereale (rye), Betula occidentalis (water birch), and Artemisia tridentata (sagebrush) was prepared, and experiments were done with (i) immobilized gold nanoparticles (Figure 6A) and (ii) gold nanoparticles in solution (Supporting Information Figure S4A). The average spectra contain lines that are typical of vibrations from proteins, nucleic acids, amino acids, lipids, and carbohydrates that were also observed in normal Raman spectra of pollen and in SERS spectra of cells (Figure 6A).48,50 Figure 6B and C shows the coefficient of variation for the spectra of Figure 6A and for the data from the control experiments with the nanoparticle solutions, respectively (see Supporting Information Figure S4A for the average spectra and Figure S4B for exemplary spectra from solution). The relative standard deviation with the nanoparticle monolayers is much smaller (Figure 6B) than in the spectra measured with the nanoparticle solutions (Figure 6C). We observed a much higher signal-to-noise ratio in the individual spectra obtained with the immobilized nanoparticles (data not shown), resulting from the higher enhancement factor discussed above. The high signal stability of the spectra obtained with the mixand-match particle monolayer substrates permits application of multivariate analysis methods, which are the prerequisite to fully exploit the multiplex potential of SERS for classification and fast identification of such spectra.

ment at the microscopic scale, especially when APTES is used as organosilane linker. Compared to experiments in solution, where enhancement factors of the same type of monodisperse gold nanoparticles vary with analyte concentration, immobilization leads to stabilization of the substrate and prevents analyte induced aggregation. This is ideal for analyte quantification. Finally, higher signal stability was observed for organosilane-immobilized nanoparticles compared with data obtained with the same nanoparticles in solution as it was demonstrated in experiments with pollen extracts. Such a high signal stability is important if one considers the application of automated data analysis methods for spectral classification in SERS-based analyte detection, identification, and microscopic imaging.

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ASSOCIATED CONTENT

S Supporting Information *

Details on sample preparation and methods, plasmon spectra, and additional information of nanoparticles in solution, SERS spectra of crystal violet, figures on investigations on immobilized monodispers gold nanoparticles, and SERS spectra of soluble pollen extract from control experiments with nanoparticles in solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49-302093-7171. Fax: +49-30-2093-7175. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Simone Rolf and Dr. Franziska Emmerling from BAM Federal Institute for Materials Research and Testing, Berlin, Germany, for providing 15 and 30 nm monodisperse gold nanoparticles and Jonas Schenk from BAM for laserablated silver nanoparticles.

CONCLUSIONS In summary, we have characterized a variety of plasmonic nanoparticle monolayers made by immobilization of three types of silver and three types of gold nanoparticles with aminosilane linkers regarding their SERS enhancement and their potential use as substrates for microstructured sensors. As shown here, silver and gold nanoparticles can be immobilized simultaneously using the same linker molecule. Due to different interaction of gold and silver nanoparticles with analyte molecules, 51 mixing of nanoparticles can provide new functionalities to the monolayers. Since the enhancement is the same as for gold nanoparticle monolayers or silver nanoparticle monolayers, depending on the ratio of both types of nanoparticles, such mixed surfaces enable investigations on analyte interaction with the substrate, measurements with specific binding, or improved partitioning of analyte molecule. The combination of characteristic size and material of the nanoparticles, their density on the surface, and the ability to form aggregates determines the plasmonic properties. Compared to nanoparticle aggregates in solution, enhancement obtained for aggregates on the surfaces is smaller. This could be explained by the formation of aggregates, which on the surfaces is also determined by the interaction of the aminosilane linker molecules with the nanoparticles. This interaction might set limitations and hinder the formation of extremely small gaps between nanoparticles in aggregates which are the prerequisite for achieving high enhancement factors. However, this limitation pays off with a high homogeneity of the enhance-

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