Selective implantation of gold nanoparticles onto the surface on one side of a self-standing polymer film

Abstract

A novel method of highly-selective implantation of gold nanoparticles onto one surface of a polymer thin film with sub-micrometer thickness is demonstrated. The poly(methyl methacrylate) (PMMA) film of about 270 nm thickness, where gold nanoparticles (AuNPs) of ∼20 nm diameter were exclusively implanted on one surface, was successfully fabricated. The preparation was carried out by using a water–organic interface at room temperature. The resultant film was self-standing in a loop of as large as about 3 cm diameter. AuNPs were located only at the surface on one side of the PMMA film, which is confirmed using transmission absorption spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. After annealing the PMMA film incorporating AuNPs in hot water, only the surface on one side of the film, where AuNPs were located, became conductive.

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Nanocomposite films comprised of metal nanoparticles (MNPs) and a polymer matrix have attracted broad scientific and technological interest because they have wide potential applications such as in optoelectronic materials, color filters, sensors, catalysis, and so on.^1,2 Especially, asymmetric distribution (alignment) of MNPs in the polymer matrix has been widely investigated to implant polarization properties in the composite films as polarized films.^3–8 However, most of previous fabrication methods rely on evaporation of solvent of the raw-material solution after casting or spin-coating it onto solid substrates, or reduction-induced generation of MNPs in the polymer matrix.

In the meantime, a method of casting the polymer–solvent solution on the water surface is convenient for the quick fabrication of the thin polymer film with the thickness of several tenths of nanometers.⁹ This method has been extended to the fabrication of asymmetric thin films where the one surface is hydrophobic while the other hydrophilic, by asymmetric incorporation of an amphiphilic compound,¹⁰ and furthermore to the fabrication of composite films consisting of metal nanoparticles.^11,12 As to the composite films incorporating metal nanoparticles, Lim et al.¹¹ reported well-ordered structures of AgNPs in the polystylene (PS) matrix, by casting the toluene solution of thiol-capped AgNPs and PS. Pang et al.¹² fabricated a composite film of AuNPs and poly(methyl methacrylate) (PMMA) with ∼50 μm thickness, by casting a small amount of a toluene solution containing hydrophobic AuNPs and PMMA. Those studies used hydrophobic MNPs to be able to disperse them into the mixture of organic solvent (toluene) and polymer. However, the density of MNPs in the polymer film relied on casting (spreading) property of the sample solution. Furthermore, the location of the MNPs along the cross section in the polymer matrix could not yet been controlled.

The liquid–liquid interface may be an alternative interface enabling well-aligned or well-packed structure of MNPs.^13–19 We also found a very rapid and convenient method of fabricating two-dimensional AuNP (sphere and rod) film at the water–hexane interface by the addition of polar solvent such as acetonitrile or methanol as the third solvent;¹⁴ the formation of AuNP film was achieved only within several seconds. Since water-soluble AuNPs can be used in this method, large-scale fabrication of the two-dimensional arrays of foregoing AuNPs is easily achieved. However, the resultant AuNP film could not be free-standing, though free-standing AuNP monolayer film was successfully discovered recently by Xia and Wang, by choosing a capping molecule.²⁰

The aim of the present study is to develop highly-asymmetric implantation of MNPs in the polymer thin film. Here, we report a novel fabrication method of a PMMA thin film ∼270 nm thickness, where water-soluble AuNPs of ∼20 nm diameter were exclusively implanted only on one surface of the film. Interestingly, after annealing the PMMA film incorporating AuNPs on the surface of hot water, only one surface of the film, where AuNPs were located, became conductive.

Materials

The aqueous colloidal solution of AuNPs capped with citric ion was prepared the reduction of HAuCl₄ with sodium citrate by modified Turkevich's method.²¹ The mean diameter of AuNPs evaluated from transmission electron micrograph (TEM) image was 20 ± 2 nm. Poly(methyl methacrylate) (PMMA: Aldrich, MW = 93 000) was used as received. Other reagents were used as received.

Preparation of asymmetric PMMA film incorporating AuNPs

The preparation procedure of our asymmetric PMMA film incorporating AuNPs (AuNP/PMMA) film is illustrated in Fig. 1. First, we prepared the two-phase system comprising of the above-described aqueous colloidal solution (20 mL) of citrate-capped AuNPs (lower phase) and the toluene solution containing PMMA (0.39 mg mL⁻¹, 3 mL) (upper phase) in a 50 mL volume of beaker (inner diameter 5 cm). Upon pouring 10 mL of methanol in the two-phase system, some of AuNPs gathered quickly at the liquid–liquid interface. Possible reason of the gathering of AuNPs at the toluene–water interface is the reduction of interparticle electrostatic repulsion by addition of the polar solvent. Spontaneous vaporization of toluene at room temperature resulted in the floating AuNP/PMMA film at the water surface, as can be shown in Fig. 2a. Then, the floating AuNP/PMMA film could be transferred into an appropriate substrate or a wire loop.

Fig. 1 Schematic illustration for the fabrication of self-standing PMMA film asymmetrically incorporating AuNPs at the water–toluene interface.

Fig. 2 Optical photograph images of the AuNP/PMMA film generated at the water surface: (a) taken from the top of beaker (5 cm diameter), (b) self-standing after transferring in a wire loop (3 cm diameter).

Fig. 2b shows a photograph image of a self-standing AuNP/PMMA film supported by a wire loop (3 cm diameter). The AuNP/PMMA film is transparent, though slight wrinkling appears. It also exhibits the color of pale-pink, showing the presence of AuNPs ascribing from the plasmon oscillation of AuNPs. The corresponding PMMA film without AuNPs was also prepared by similar procedure as a reference.

In order to investigate the conductivity change of the film, we have annealed the AuNP/PMMA film by floating it on the water surface (Fig. 1). Then, water was heated up to 90 °C. After 15 min, the film was withdrawn from the water surface, and cooled to room temperature. Annealed AuNP/PMMA film was denoted as ann-AuNP/PMMA film.

Measurements

Transmission absorption spectra of the films were measured with a JASCO V-670 spectrophotometer attached with an integrating sphere. Scanning electron microscope (SEM) observations were carried out using a HITACHI S-5000 microscope. The thickness of the AuNP/PMMA film was estimated to be ∼270 nm from height analysis of partly scratched the AuNP/PMMA film on the quartz glass plate using atomic force microscopy (AFM; JEOL JSPM-5400). Electronic conductivities of the AuNP/PMMA and ann-AuNP/PMMA film were measured by conductivity meter (HUSO 1116SLD) with 4-probes electrode (distance of two electrodes for potential difference measurement was 1 mm).

The transmission absorption spectra of the AuNP/PMMA film and the corresponding PMMA film are shown in Fig. 3. Normalized absorption spectrum of the aqueous colloidal solution of AuNPs was also shown. A clear plasmon absorption band around 680 nm was observed. The aqueous colloidal solution of AuNPs exhibits the plasmon band at ∼530 nm. Such a remarkable red shift of the peak was attributable to interparticle plasmon coupling of agglomerated AuNPs in the AuNP/PMMA film that was originally formed at the liquid–liquid interface.²² While, the PMMA film without AuNPs shows no appreciable peaks in the 400–1500 nm region. These results confirm that AuNPs are incorporated in the AuNP/PMMA film.

Fig. 3 Transmission absorption spectra of the AuNP/PMMA film (a) and the PMMA film without AuNPs (b). Normalized absorption spectrum of the aqueous colloidal solution of AuNPs (diameter: 20 ± 2 nm) within a range of 300–800 nm (c).

Next, we have compared the contact angle of a water droplet between the water-side (in contact with the aqueous phase during the AuNP/PMMA film preparation) and toluene-side (in contact with the toluene phase) of the AuNP/PMMA film; the results are shown in Fig. 4. It is clear that the contact angle at the water-side surface (61°) is considerably smaller than that at the toluene-side surface (88°). The contact angle at the toluene-side surface was identical to that of the PMMA film prepared by the casting method. Needless to say that the surfaces of AuNPs are capped with citrate ions and thus hydrophilic. These observations clearly show that AuNPs are preferentially located at the surface of water-side while not at the surface of toluene-side.

Fig. 4 Photographs of water droplets for the measurements of contact angles of the both surfaces of AuNP/PMMA film: (a) toluene-side surface, (b) water-side surface (see Fig. 1).

Fig. 5 shows SEM images of the both surfaces of the AuNP/PMMA film. In the water-side surface, the well-packed structure of AuNPs is clearly seen, though small three-dimensional aggregates of AuNPs are found (Fig. 5a). On the contrary, no AuNPs are observed in the toluene-side surface (Fig. 5b). The result is quite consistent with the results of contact angle measurements (Fig. 4), where the hydrophilic AuNPs are exclusively located at the water-side surface. The surface-anchored AuNPs did not peel off the film even though rinsing with water or sonication for some minutes. Thus, the surface-anchored AuNPs seem to be somewhat sintered into the PMMA film, while considerably exposing the hydrophilic surfaces. It is quite interesting that the surface-anchored AuNPs are aligned roughly monoparticle layer level, except some three-dimensional aggregate.

Fig. 5 SEM images of the AuNP/PMMA film and the ann-AuNP/PMMA film: (a) water-side surface and (b) toluene-side surface of the AuNP/PMMA film, (c) water-side surface of the ann-AuNP/PMMA film.

As has been verified in our previous reports,^14,22,23 well-arranged monoparticle layer films of AuNPs are formed at the water–hexane interface on addition of methanol or acetonitrile. Recently, generation of monoparticle layer array of AuNPs was found at the water–toluene interface.^15,18 In the present case, on the other hand, we have further added methanol into the two-phase system of the aqueous colloidal solution of AuNPs and toluene containing PMMA, in order to fabricate nanoparticle layer of AuNPs. Considering those results, it is quite reasonable that the monoparticle layer array of AuNPs are formed at the water-side surface region of PMMA film even in the present case.

XPS analysis of the present AuNP/PMMA film was performed (Fig. 6). A strong peak based on Au was found in the spectrum of water-side surface, while no appreciable peaks based on Au were detected in the spectrum of toluene-side surface. Thus, asymmetric decoration of PMMA film with water-soluble AuNPs was also verified from the XPS results.

Fig. 6 XPS spectra of PMMA film incorporating AuNPs: (a) water-side surface, (b) toluene-side surface.

Before annealing, the toluene-side surface of the AuNP/PMMA film was not conductive (over 200 MΩ) and the resistance of the water-side surface was ∼1.4 kΩ. After annealing, the resistance of the toluene-side surface of the ann-AuNP/PMMA remained over 200 MΩ, while that of the water-side surface dramatically decreased to ∼5 Ω. This result suggests that some of AuNPs in the ann-AuNP/PMMA are attached each other among incorporated AuNPs. The SEM image (Fig. 5c) of the ann-AuNP/PMMA film indicates that AuNPs are still located at the water-side surface. After annealing, considerably larger concave-shaped parts were observed in the AuNP/PMMA film (Fig. 5c), whose structural change is reasonable because glass transition temperature of PMMA is near 90 °C. Nevertheless, apparent holes or breaking of the self-standing films were not observed in the ann-AuNP/PMMA film. Apparent diameters of AuNPs in the ann-AuNP/PMMA film are appreciably smaller than that of the AuNP/PMMA film, which is evaluated from the SEM image (Fig. 5a and c). Distances of gaps between AuNPs in the ann-AuNP/PMMA film are difficult to evaluate because AuNPs seems to be somewhat submerged into inside of film. Alternately, top-to-top distances between AuNPs after annealing are almost same as the apparent diameters of AuNPs in the film before annealing.

The absorption spectrum of the ann-AuNP/PMMA film (Fig. 7) shows that considerable broadening and red-shift the plasmon band above ∼800 nm. This suggests some morphological changes of AuNPs in the ann-AuNP/PMMA film. Accordingly it can be concluded that AuNPs are still attached but somewhat submerged into the PMMA film by annealing process.

Fig. 7 Absorption spectrum of the ann-AuNP/PMMA film.

Conclusions

We have succeeded in the first fabrication of amphiphilic PMMA film where water-soluble AuNPs were exclusively populated only at the one surface of the film. The method is very simple and no sophisticated instruments are necessary. We are trying to fabricate much thinner asymmetric films and the film incorporating another MNPs by using the present method.

Acknowledgements

The present study was partially supported by a Grant-in-Aid for Scientific Research (19023458) on Priority Area (470) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese Government.

Notes and references

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Footnote

† Present address: Department of Materials Science, School of Engineering, The University of Shiga Prefecture, Hikone, Shiga 522-8533, Japan.

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