Multilayers of well-ordered and close-packed 2D nanostructures of gold nanoparticles (NPs) were fabricated using a layer-by-layer technique. Colloidal spherical Au NPs of 5 and 15 nm diameters were synthesized and, respectively, self-assembled in multilayers. The robustness of these systems was insured by a combination of electrostatic and covalent bonds between nanoparticles and linker molecules. The compacity of the superlattice was characterized by AFM observation and ellipsometry measurements. Evidence of stronger cohesion of multilayers of smaller NPs size was brought by submitting the systems to sonication test. The multilayers have also proved analytical potential when used to detect low concentration methylene blue molecules adsorbed on the Au nanoparticles, by surface-enhanced Raman spectroscopy (SERS). The detection sensitivity of these two sized Au NPs architectures was directly compared to an evaporated “bulk” Au thin film of equivalent thickness. Results have displayed a strong increase of the electromagnetic field enhancement with a decrease of the NPs size, whereas the bulk thin Au film was shown to be inefficient as a SERS substrate. These results bring a nice evidence of size effects on the global performance (SERS, cohesion) and hopefully on the stability of NPs based nanostructures.
In recent years, considerable effort has been devoted to the design and fabrication of structured materials with functional properties. The long-range and large-scale arrangement of nanoparticles (NPs) is an important issue in the development of nanostructured materials with new electronic, magnetic, and photonic properties [
In this field, noble metal nanoparticles, and especially gold nanoparticles, have attracted a great deal of interest. In particular, supported nanostructured films of Au NPs have appeared as promising functional structures for applications as diverse as optical field enhancement [
However, the cooperative effect of these surface plasmons as well as the robustness of the multilayer structure will strongly depend on the interactions between the different building blocks (nanoparticle/nanoparticle, nanoparticle/linker) which determine the overall organization of the network.
For the elaboration of such nanoparticulate multilayers, there are two main approaches: a chemical one via colloids synthesis and assembly and a more conventional one based on chemical vapor deposition (CVD) procedure. One of the advantages of colloidal assembly is the way the particle size, shape, and chemistry can be easily adjusted. For instance, nanocolloidal Au particles can be prepared with diameters ranging from 1 to 120 nm with a relatively high monodispersity. As a result, surfaces made from colloidal Au NPs have a tunable nanometer-scale roughness determined solely by the particle diameter and packing. The ultimate surface and bulk properties of these nanoparticulate surfaces will be directly correlated to these intrinsic parameters of the individual NPs (nature, size, shape, functionalization).
In the present work, the nanoparticulate multilayer superlattice was prepared using the principle of alternating adsorption (layer-by-layer procedure) which was introduced in 1966 by Iler from colloidal particles of opposite charges [
This LbL procedure has therefore inspired our work although we have developed here a less conventional approach based on the use of two alternating and distinct bonds between the consecutive layers to assemble the Au particles in the multilayer. Two sizes of spherical nanoparticles were used (5 and 15 nm diameter) to build two different multilayer systems. The particles were transferred from the nanocolloidal solution onto self-assembled monolayers (SAMs) coated silicon wafers, using dip-coating process. Two consecutive NPs layers were tightly bonded which was insured by the combination of two alternating and distinct interactions. Basically, an electrostatic interaction was used to bind the charge-stabilized (citrates) colloidal gold nanoparticles to the terminal amine of an alkylthiol linker. The second binding is a covalent one (Au/S) that attaches the second layer of Au particles to the terminal sulfur (S) of the alkylthiol linker, as shown in Figure
Schematic representation of the combined electrostatic-covalent layer-by-layer (LbL) formation of Au NPs multilayers (relative size of linker and NPs are not at scale).
Compared to previous routes of structuring NPs into supported multilayers, this approach has several distinguished advantages: (i) aqueous solutions of colloidal Au NPs are easy to prepare and are stable for long periods of time, (ii) colloidal Au NPs can be prepared with a wide range of shape and size, (iii) multilayers of nanoparticles can be assembled easily using a layer-by-layer technique, and (iv) the distance between NPs can be controlled by adjusting the length of the linker molecule.
Finally, these nanoparticulate thin films were assessed for their size-dependent cohesive and optical (SERS) properties. The robustness of both sized nanostructured multilayer systems has been tested by submitting them to mechanical vibrations provided by sonication during several tens of minutes. The optical SERS sensitivity of the multilayer systems versus the size of the constitutive particles was assessed, using blue methylene as a model analyte molecule. These results were discussed for their dependence on nanoparticle size in the supported multilayers, compared to those obtained on a uniform Au film of identical thickness, and deposited by thermal evaporation.
Gold nanoparticles were prepared using a colloidal procedure. Citrate-stabilized Au nanoparticles were synthesized according to a procedure published by Frens [
An aqueous solution (50 mL) of tetrachloroauric (III) acid hydrate HAuCl4. H2O at 3% weight was prepared (solution A). This auric solution was reduced and stabilized with citric acid trisodium, by adding 1.25 mL of 1% weight aqueous solution of citric acid trisodium (solution B) to the previous one, under strong stirring during 15 min.
The result of this reaction is a red suspension of spherical nanogold particles of average size 15 nm (Figure
TEM images of a homogeneous size distribution of spherical Au nanoparticles (scale bar 100 nm): (a) average diameter of 15 ± 3 nm, (b) average diameter of 5 ± 1 nm.
0.75% NaBH4 solution dissolved in a 1% sodium citrate solution was prepared (solution C). 1.25 mL of solution B was added to solution A under strong stirring. Then 1 mL of solution C was added consecutively and stirred for additional 5 min. The resulting colloidal solution is a dark blue solution of nanogold particles of average size of 5 nm (Figure
It is worth mentioning here that the average particle size (mean value and dispersion) is highly sensitive to slight variations in the processing parameters, such as mixing rate, temperature, humidity, and ageing of the reagents as shown in Figures
TEM images of Au nanoparticles of an average diameter of 15 nm (a) with a homogeneous size distribution (scale bar 100 nm), (b) with a heterogeneous size distribution (scale bar 100 nm) due to the use of a lower operating temperature (60°C instead of 80°C) in the synthesis process.
The principle of the elaboration of the NPs multilayers is based on a layer-by-layer (LbL) deposition technique that consists essentially in the following steps
The substrate, a silicon (Si) wafer bearing its native silica (SiO2) layer of
The sample was immersed in the solution of Au nanoparticles of either 15 nm or 5 nm diameter for 15 to 20 minutes (optimal time to reach a compact layer of nanoparticles). Then, it was rinsed and sonicated in water for about 1 minute in an ultrasonic bath. Finally, the sample was rinsed again with water and dried under nitrogen flow.
This first step is aimed at cleaning the Au NPs to obtain a bare gold surface. The organic citrate molecules adsorbed on the Au NPs (Figure
Schematic representation of the charge-stabilized (citrates) colloidal gold nanoparticles (relative size of stabilizing molecules and nanoparticles are not at scale).
The above Au NPs layer bearing NH2 terminal groups is re-immersed in the same solution of colloidal nanoparticles for 15 to 20 minutes, producing, as in the first step, a second citrate-stabilized Au layer, via NH2/citrate (negatively charged COO-) bonds. The following steps for the multilayer construction consist in repeating the above alternate cycle.
Gold colloidal nanoparticles were first characterized by transmission electron microscopy (TEM) and UV-Vis spectrophotometry in order to determine the morphology and size of the gold nanoparticles. Secondly, the Au NPs multilayers were characterized by AFM and ellipsometry to confirm the formation of the layered structures, their compactness, and the spatial organization of the NPs.
As shown in Figure
The 15 nm colloidal dispersions show an absorption band at about 519 nm in the UV-visible spectrum (Figure
UV-Vis absorption spectra of gold colloidal solutions. The thin line corresponds to the spectral response of the nanocolloidal solution of average particles size of 15 nm, as previously determined by TEM in Figure
The gold nanoparticles were deposited into multilayers architecture and characterized by ellipsometry and AFM to verify, respectively, the homogeneity and morphology of the NPs assembly.
For both multilayer systems made respectively, of 15 and 5 nm diameter particles, the thickness of each NPs layer has been measured, using a Null-Ellipsometer (Multiskop, Optrel, Germany), working at 532 nm (Nd-YAG laser), in the “laser-polarizer-compensator-sample-analyzer” arrangement. For the ellipsometric modeling, each layer is represented by a monolayer of Au nanoparticles with its tethered thiolate molecular film (linker). The thickness (
The thickness (
Evaluation of the complex refractive index (
Refractive index | Extinction coefficient | |
---|---|---|
Au evaporated film 30 nm | 0.488 | −2.4 |
5 nm Au colloidal 1 layer equivalent to 5 nm | 3.814 | −0.896 |
5 nm Au colloidal 4 layers equivalent to 20 nm | 3.014 | −1.664 |
15 nm Au colloidal 4 layers equivalent to 60 nm | 3.045 | −1.522 |
Evolution of the multilayer film thickness versus the number of layers. The measurements were done by ellipsometry. One layer is made of a monolayer of gold nanoparticles with its tethered thiolate molecular film (linker). Dotted line (5 nm size NPs), continuous line (15 nm size NPs).
However, it is well known that dielectric constants are independent of film thickness above a critical thickness, which for gold is around 25 nm [
However, we note that our ellipsometry film thicknesses are in average larger by ~1-2 nm than estimated from the diameter of the NPs. This may arise from the presence of a few aggregates averaging the outmost layer.
To compare the effectiveness of our multilayer structure to a standard bulk thin gold film, we have prepared Au thin films by thermal evaporation. Contrary to previous results, the optical constants of Au evaporated thin films are close to those expected for bulk Au material, due to the higher compactness of slowly evaporated thin films.
Tapping mode AFM in ambient conditions has been used to characterize both the particle size and the morphology of the multilayers (packing, spatial organization). Figure
AFM pictures in tapping mode of the last NPs layer at the different (successive) steps of the formation of the complete multilayer structure on the substrate. From (a) to (c), the images correspond to the 15 nm-sized NPs, and from (d) to (f), to the 5 nm NPs. 15 nm NPs: (a) first layer, zoom in:
For the first layer of Au NPs (15 or 5 nm), AFM observations show well-ordered and close-packed surface structures as it appears respectively, in Figures
These functional multilayered structures have been tested for their cohesion and adhesion using mechanical vibrations provided by a sonicator bath (Fisherbrand Ultrasonic cleaner, 35 kHz, 40 W) for several tens of minutes. AFM images have been carried out before and after the mechanical test. The results (displayed in Figure
AFM pictures in tapping mode of the fourth layer of the multilayer structure before and after sonication in water for 20 minutes. (a) and (b) correspond to the AFM images of the 15 nm-sized NPs, and (c) and (d), to the 5 nm Au NPs. 15 nm Au NPs
The cohesion of the multilayer elaborated with small Au NPs thus appears stronger than that prepared with larger ones. This finding is in good agreement with what can be expected from size effects in such nanomaterials. Indeed, the total free volume (tetrahedral and octahedral voids) in a close-packed 2D assembly of monodisperse spherical particles (1 or 2 layers for instance) increases with the size of the particles [
The multilayered structures were then used in SERS to characterize the effect of both the layering (number of layers) and the NPs size on their Raman sensitivity.
To promote the above results towards molecular characterization and detection, the SERS spectra of the bare (as-prepared) nanoparticulate multilayers were recorded, compared to standard thermally evaporated gold films, and presented in Figure
(a) SERS spectra of 15 nm Au NPs multilayer structure and thermally evaporated Au film. (1) (blue curve) a standard Au thermally evaporated film of 30 nm, (2) (violet curve) one monolayer of gold nanoparticles deposited onto an SAMs-coated silicon wafer, (3) (red curve) three layers of nanoparticles hybrid architecture, and (4) (dark green curve) five layers of nanoparticles hybrid architecture. (b) SERS spectra of 5 nm Au NPs multilayer structure (1) (green curve), one monolayer of gold nanoparticles deposited onto an SAMs-coated silicon wafer, (2) (dark blue curve) two layers of nanoparticles hybrid architecture, (3) (blue curve) three layers of nanoparticles hybrid architecture, and (4) (pink curve) four layers of nanoparticles hybrid architecture.
The results of Raman spectra show that there is no signal for the thermally evaporated Au thin film, whose thickness is equivalent to 30 nm. For the nanoparticulate multilayer structure, the Raman analysis shows the emergence of signals, the intensity of which depends on the number of layers.
Basically, for the 15 nm-sized NPs in this experiment, there is no significant band up to two layers (Figure
For the 5 nm-sized NPs system, the same main peak was observed. However, contrary to the larger NPs multilayer, the intensity of the main band at 1540 cm−1 was already significant from the first layer, and increases continuously up to the fourth layer. The enhancement of Raman signal between one and four layers is higher compared to the larger NPs multilayer colloidal system, and of the order of 70.
Moreover, by comparing two equivalent multilayer thicknesses made of the two Au NPs sizes (e.g., 3 layers of 5 nm Au NPs film versus 1 layer of 15 nm NPs film), the intensity enhancement factor is of the order of 100. This result definitively shows the powerful enhancement of SERS sensitivity by the sole size effect of NPs entering the surface structure.
To test the SERS sensitivity of these NPs size-adjusted multilayers structures, a standard dye molecule, the methylene blue (MB), is used as a probe molecule.
We used surface-enhanced Raman spectroscopy (SERS) to detect the adsorption of very small amounts of methylene blue (MB) on the Au NPs multilayer structure, which was compared to the standard, thermally evaporated Au thin film. Prior to SERS measurement, 10-7 M of MB solution was prepared and both the nanoparticulate and thermally evaporated films were immersed for one minute in the diluted solution and then dried in air. A typical SERS spectrum of methylene blue adsorbed on the Au evaporated film and the five layers nanoparticulate structure is shown in Figure
SERS spectra of methylene blue adsorbed on a 15 nm-sized NPs multilayer structure and a thermally evaporated Au film; (a) (blue curve) thermally evaporated Au film, (b) (green curve) five layers of gold nanoparticles multilayer architecture.
The SERS spectra and the characteristic peaks of MB are displayed in Figure
Assignment of the main peaks of the SERS spectra of methylene blue adsorbed on NPs multilayer structure.
Cm−1 | Assignments |
---|---|
1623 | C–C and C–N–C |
1438 | C–C |
1398 | C–C |
C–N–C | |
1190 | COO– |
In the spectra of Figure
In Figure
(a) Zoom in on the main MB peak at 1623 cm−1 of methylene blue adsorbed on a 15 nm diameter NPs multilayer structure and Au thermal evaporated film obtained by SERS. (i) (blue curve) Au evaporated thermally evaporated film, (ii) (red curve) three layers of nanoparticles hybrid architecture, (iii) (green curve) five layers of gold nanoparticles hybrid architecture. (b) Evolution of the intensity of the main MB peak at 1623 cm−1 of methylene blue adsorbed NPs multilayer structure versus the number of layers.
These enhanced Raman signals are mainly due to the enhanced electromagnetic field resulting from the coupling over the localized plasmons of the discrete and close-packed Au NPs in the multilayer structure [
But more interestingly, if we compare the Raman spectrum of an MB molecule adsorbed onto 3 layers of 5 nm-sized Au NPs to the one adsorbed on a monolayer of 15 nm-sized NPs as displayed in Figure
SERS spectra of methylene blue adsorbed 15 nm- and 5 nm-sized Au NPs multilayer structure (A). (black curve) 3 layers of 5 nm-sized gold nanoparticles hybrid architecture, (B). (red curve) monolayer of 15 nm-sized gold nanoparticles hybrid architecture.
Finally, the layer-by-layer technique can be used to create a combined electrostatic-covalent structure of robust Au NPs multilayers. For a given NPs size and shape, the surface coverage of the NPs in a layer, the compactness, and the number of layers in the structure constitute the main parameters, which determine the cohesion and the collective optical properties of the surface structure.
As it has been demonstrated in this paper, the size adjustment of the NPs in multilayered surface structure provides a promising route for elaborating effective SERS-active substrate for the detection of very small amounts of organic molecules in either air or solutions. Moreover, the adjustment of the interlayer distance by the chain length of the linker [
In the present paper, a facile fabrication route of a multilayer architecture, alternating organic molecules and gold colloidal nanoparticles onto silicon wafer, had been successfully developed. The robustness of the superlattice structure has been demonstrated by the strong combined electrostatic and covalent binding between molecules and nanoparticles. These interactions enable to insure the compactness of the superstructure and a high surface coverage of each individual layer. Finally, these multilayer structures of Au nanoparticles have proved to be an effective analytical tool when used to detect very low amount of methylene blue dye adsorbed on the structures, by surface-enhanced Raman spectroscopy (SERS). Furthermore, we showed that both the cohesion and the SERS sensitivity of the multilayer structures were significantly enhanced by decreasing the size of the Au nanoparticles building units, bringing a clear evidence of size effects in nanostructured materials. The combination of this proved size effect with the shape and the organization of nanocolloidal metal particles in general, and of Au nanocolloids in particular, provides a new and effective tuning parameter towards the improvement of the stability and sensitivity of nanostructure-based devices.
A short description of the different characterization tools is provided in this section.
Film thicknesses, including the grafted amine-terminated monolayer and the Au NPs layers, were determined by null ellipsometry (Multiskop, Optrel, Berlin, Germany). All measurements were done under ambient conditions at an incidence angle of 70°, using an Nd-YAG laser ( Surface images were acquired with an atomic force microscope in Tapping mode powered by a Nanoscope IV controller (Digital Instruments, Santa Barbara, CA) using Silicon AFM tips (Nanosensor, Park Scientific) with a spring constant of The UV-Vis spectra of the gold solutions were generated using a Perkin-Elmer Lambda 35 UV-Vis spectrophotometer. The Raman spectra were obtained with a Horiba Labram Raman equipment, using an Olympus BX40 microscope and an excitation source of 632.81 nm. The laser power at the sample was 5 mW, and the resolution was set at 1.6 cm−1. The size and morphology of nanoparticles were analyzed by transmission electron microscopy (TEM) performed on a Philips CM 200 at an accelerating voltage of 200 kV. TEM samples were prepared by dipping 400 mesh carbon-coated copper grid into the colloidal suspension of nanoparticles and air dried for 24 h prior to analysis.
This work was supported by the Centre National de la Recherche Scientifique (CNRS).