Nanoswelling Structures of Silicone Rubber Under Aluminum Nanoparticles Induced by 193 nm ArF Excimer Laser

Abstract

In metal nanoparticles, localized surface plasmon resonance occurs due to the interaction between electrons on the surface and light. Among them, aluminum (Al) nanoparticles are known to have a resonant absorption wavelength in the ultraviolet light region. In this paper, I found a new phenomenon in which nanoswelling structures are formed on the silicone rubber surface by distributing Al nanoparticles on the surface and irradiating them uniformly with an ArF excimer laser at a wavelength of 193 nm. The formation of the nanoswelling structure was not observed when gold nanoparticles were distributed. Thus, the mechanism of nanoswelling structure formation is considered as follows: localized surface plasmon resonance is induced in the Al nanoparticles by the interaction between the Al nanoparticles and the ArF excimer laser, which causes photodissociation of the Si-O-Si bonds of the silicone rubber underneath, volume expansion due to molecular weight reduction, and swelling to nanometer sizes. The present study provides a new biomimetic method for ensuring the mechano-bactericidal functions of a silicone rubber surface to develop highly functional plastic windows for automobiles.

1. Introduction

In metal nanoparticles, localized surface plasmon resonance occurs due to the interaction between electrons on the surface and light. Due to their unique optical properties, research into their application in biosensing, bioimaging, and photonics has been growing in recent years [1,2,3]. Among metal nanoparticles, the resonant absorption wavelength of gold (Au) is in the visible light region, and when the particle size is approximately 10 nm, it resonates at a wavelength of around 520 nm [4]. In general, light sources in the visible light region are easy to use, so most reports on localized surface plasmon resonance use Au nanoparticles [5]. On the other hand, there are few studies on the synthesis and application of aluminum (Al) nanoparticles, which are metal nanoparticles with a resonant absorption wavelength in the ultraviolet (UV) light region. In the UV light region, far-UV lights with a wavelength of 200 to 240 nm have attracted great attention because they are effective in disinfecting pathogens and are safe for exposure to human eyes and skin [6,7]. In addition, since the optical absorption wavelength of nucleic acid is around 260 nm, it is also thought to be useful in the field of biosensing [8].

In this paper, I synthesized Al nanoparticles that exhibit localized surface plasmon resonance in the UV light region by using femtosecond (fs) laser ablation of an Al plate in liquid and then distributed the Al nanoparticles on silicone ([SiO(CH₃)₂]_n) rubber. By irradiating the sample surface with an ArF excimer laser at a wavelength of 193 nm, I found a new phenomenon in which the silicone rubber beneath the Al nanoparticles swelled to nanometer sizes. The novelties of this paper are as follows: (1) localized surface plasmon resonance of Al nanoparticles induced by 193 nm ArF excimer laser, (2) nanoswelling structures of silicone rubber beneath UV plasmonic Al nanoparticles, and (3) no formation of nanoswelling under Au nanoparticles as a replacement for Al. If a nanoswelling structure can be formed on silicone rubber, it may be possible to confer mechano-bactericidal functions to the rubber [9,10], enabling the development of highly functional plastic windows for automobiles [11,12].

Silicone has a variety of excellent properties, including heat resistance, cold resistance, and chemical resistance, and is used in a wide range of fields in science, technology, and industry. In recent years, molding and additive manufacturing technologies have enabled the creation of a variety of unique silicone microstructures, and it is expected that these new functionalities will be utilized in a wide range of applications [13,14]. However, no research has been conducted to date on forming periodic nanoprotrusion structures on silicone rubber and applying them to plastic automotive window materials with antibacterial properties.

2. Experimental Procedure

In the first step, 2 mL of methanol was placed in a screw bottle, and an Al plate with a size of 5 mm × 5 mm and a thickness of 0.1 mm was placed in it. Then, an fs laser (Spectra-Physics, Milpitas, CA, USA, Spirit One 1040) with a wavelength of 1040 nm and a pulse width of 400 fs was focused and irradiated onto the surface of the Al plate in methanol through a lens with a focal length of 50 mm. The irradiation conditions were as follows: pulse energy of 30 µJ/pulse, a pulse repetition rate of 100 kHz, output power of 3 W, and an irradiation time of 60 min. The colloidal solution in which the Al nanoparticles were dispersed in methanol was dropped onto silicone rubber with a size of 10 mm × 10 mm and a thickness of 2 mm, then dried. The absorption spectrum of the colloidal solution was measured using a UV–visible spectrophotometer (Shimadzu Corporation, Kyoto, Japan, UV-2550).

In the second step, the silicone rubber surface on which the Al nanoparticles were randomly distributed was uniformly irradiated without a lens by using an ArF excimer laser (Coherent, COMPexPro110) with a wavelength of 193 nm and a pulse width of approximately 20 ns. The laser irradiation conditions were as follows: a single-pulse fluence of 20–40 mJ/cm², a pulse repetition rate of 1 Hz, and an irradiation time of 30 min. After ArF excimer laser irradiation, the surface of the silicone rubber was observed using an atomic force microscope (AFM, Hitachi High-Tech, Tokyo, Japan, AFM5100N).

3. Results and Discussion

Figure 1 shows the color change of the methanol solution in the screw bottle after fs laser irradiation. As shown in Figure 1a, a significant color change to a dull yellow was observed. The absorption spectrum was measured using a UV–visible spectrophotometer after diluting the solution 10 times. It was found to exhibit a broad spectrum in the UV light region with a shoulder at a wavelength of approximately 280 nm. For comparison, a Au plate was placed in the methanol instead of Al and the fs laser was focused and the plate irradiated. A colloidal solution specific to Au nanoparticles with a light red color was synthesized (Figure 1b). Also, the plasmon resonance absorption wavelength appeared around 520 nm. Judging from the fact that the present method can also reproduce a colloidal solution of Au nanoparticles, the solution shown in Figure 1a can be inferred to be a colloidal solution of Al nanoparticles.

Figure 2 shows the AFM images of Al particles distributed on the silicone rubber surface before and after ArF excimer laser irradiation. The original surface of the silicone rubber is already wavy, and Al particles with a diameter of approximately 1 µm can be easily observed, but it is difficult to directly observe Al nanoparticles (Figure 2a). For reference, the presence of Al nanoparticles was observed by dropping the colloidal solution onto a flat glass slide. The Al nanoparticles were spherical and had a diameter of 20–40 nm. Furthermore, even when the pulse energy of the fs laser was changed from 30 to 15 μJ/pulse, no significant change in size distribution was observed. In addition, X-ray photoelectron spectroscopy (XPS, Shimadzu, Manchester, UK, KRATOS ULTRA2) revealed signals indicating Al₂O₃ on the surface of the Al nanoparticles. When the silicone rubber surface with Al nanoparticles was irradiated with an ArF excimer laser at a single-pulse fluence of 20 mJ/cm², nanometer-sized protrusion structures were formed on the silicone rubber (Figure 2b). The height of these structures varied widely but was judged to be roughly 500 nm. The formation of similar nanoprotrusion structures was also observed when the fluence was set to 40 mJ/cm². On the other hand, when the silicone rubber surface was directly irradiated with an ArF excimer laser, no nanoprotrusion structures were formed. In the future, in order to demonstrate the reproducibility and uniformity of the nanoprotrusion structure, it will be necessary to establish a method for uniformly distributing Al nanoparticles on silicone rubber, and then perform statistical analysis of the average height, distribution range, and unit density for various nanoprotrusion structures.

To elucidate whether the formation of the nanoprotrusion structure is due to the interaction between Al nanoparticles and the ArF excimer laser, the Au nanoparticles with a plasmon resonance wavelength of 520 nm shown in Figure 1b were randomly distributed on the silicone rubber surface and irradiated with the ArF excimer laser under the same conditions as the Al nanoparticles. As a result, as shown in Figure 3, no formation of nanoprotrusion structures was observed. This is thought to be due to the mismatch in the resonance absorption wavelengths. Thus, it is believed that the formation of the nanoprotrusion structure is due to the interaction between Al nanoparticles and the ArF excimer laser, rather than the effect of local laser heating by metal nanoparticles. To strengthen this conclusion, future studies should evaluate in detail the formed nanoprotrusion structures and the morphology and size distribution of the synthesized Al nanoparticles by using scanning electron microscopy.

I have previously found that when an ArF excimer laser is focused onto the surface of silicone rubber on which silica microspheres are aligned, the main chain structure of the silicone rubber (Si-O-Si bonds) beneath the microspheres is photodissociated, as shown below, and the molecular weight is reduced, causing volume expansion and the formation of micrometer-sized swelling structures [15].

[SiO(CH₃)₂]_n + h ν (193 nm) → [SiO(CH₃)₂]_n−m + [SiO(CH₃)₂]_m

As a result, it was also found that the silicone rubber surface exhibited superhydrophobicity [15]. However, I have not yet been able to form nanometer-sized swelling structures. Since Al generally exhibits optical absorption under ArF excimer lasers, it is thought that the laser light intensity decreases for silicone rubber covered with Al nanoparticles. In this case, the silicone rubber underneath the Al nanoparticles does not swell. Therefore, the mechanism by which the nanoprotrusion structure is formed is considered as follows: the interaction between the Al nanoparticles and the ArF excimer laser induces localized surface plasmon resonance in the Al nanoparticles, which generates an enhanced electric field on the silicone rubber surface below the Al nanoparticles, inducing the nanoswelling phenomenon. The plasmon resonance absorption wavelength in the UV light range of Al nanoparticles has been generally discussed to be between 200 and 400 nm, but it has also been reported that they have resonance absorption wavelengths below 200 nm [16]. Therefore, it is believed that there is no contradiction in the generation of plasmon resonance at a wavelength of 193 nm. In the future, performing electromagnetic field simulations is necessary to quantify the electric field enhancement. To investigate the chemical bonding state of the nanoprotrusion structure, evaluation using XPS, Fourier transform infrared spectroscopy, and Raman spectroscopy is also necessary. In addition, the mechano-bactericidal effect of the formed nanoprotrusion structures is currently still theoretical and requires preliminary in vitro biological testing.

4. Conclusions

I found a new phenomenon in which nanoswelling structures are formed on a silicone rubber surface by distributing Al nanoparticles on the surface and irradiating them uniformly with a 193 nm ArF excimer laser. The nanoswelling structure was not formed when Au nanoparticles were distributed. The mechanism of the nanoswelling is considered as follows: localized surface plasmon resonance is induced in the Al nanoparticles by the interaction between the Al nanoparticles and the ArF excimer laser, which causes the photodissociation of the Si-O-Si bonds of the silicone rubber underneath, volume expansion due to molecular weight reduction, and swelling to nanometer sizes. The present results are expected to expand the applications of biosensing and bioimaging, and also suggest a new nanofabrication method for plasmonics to develop highly functional plastic windows for automobiles. I believe that if we can coat the inside of a plastic window with silicone rubber and form a nanoprotrusion structure on its surface, I can develop a new plastic window with mechano-bactericidal properties.