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Abstract
The authors developed plasmonic structures of zinc (Zn) and zinc oxide (ZnO) on the surface of a tapered optical fiber using optical tweezing. The powdered Zn or ZnO was added to gold nanorods (GNRs) solution and sonicated before tweezing. The concentration of the mixture was 0.5 µg/µl. The GNRs were present in the cetyltrimethylammonium bromide (CTAB) solution. The authors investigated the plasmonic structures and explored the effect of the tweezing conditions on the distribution of the Zn or ZnO on the tapered fiber surface.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nanomaterials based on a Zinc Oxide (ZnO) semiconductor have attracted worldwide attention for biomedical and industrial applications due to their unique optical, electronic, and chemical properties [1–5]. The distinctive attributes of nanoscale ZnO materials such as its biocompatibility, biodegradability, and lower toxicity makes it a good candidate in cancer treatment, such as Colon cancer, Hepatocarcinoma, Lung cancer, and Ovarian cancer [1]. Several techniques have been proposed and demonstrated to develop ZnO nanostructure with different morphologies for various applications [6–11]. It was found that one-dimensional nanostructure (e.g., nanorods) has good chemical and thermal stability. Several researchers have reported the process of developing ZnO nanorods (ZnO-NRs) using surfactants (e.g., Cetyltrimethylammonium bromide - CTAB) [12,13]. Researchers demonstrated that the temperature variation during the growing process could change the ZnO nanostructure's morphologies which thus affect the sensitivity [6,12,13]. Further, the Zn and ZnO based plasmonic structures were successful as the Surface-Enhanced Raman Spectroscopy (SERS) substrate in sensing [14,15]. Recently, a Surface Plasmon Resonance (SPR) based sensor using Zn Nanoparticles has been proposed [16]. Also, a Localized Surface Plasmon Resonance (LSPR) based sensor using ZnO Nanorods and silver nanoparticles has been developed [17].
Although some sensors based on ZnO – nanostructures were reported for Biomedical applications, there are still opportunities to develop a more efficient system using optical fiber technology. Several techniques have been reported to deposit Zinc Oxide (ZnO) nanostructures on an optical fiber surface, such as the sol-gel method [18,19], and the Atomic layer deposition (ALD) method [20]. In this article authors proposed and demonstrated a simple optical method (optical tweezing) for the first time in their knowledge, to deposit Zn or ZnO on the surface of a tapered fiber to develop a plasmonic structure, which will find application as a SERS substrate. The authors will present the manufacturing process of the plasmonic structure and its optical characteristics. The authors presented preliminary results on the formation of the plasmonic structure using ZnO at the OSA Optical Sensors and Sensing Congress 2021 [21].
The Optical tweezing method was invented by Arthur Ashkin and received a Nobel Prize in 2018. Ashkin's original work was to trap dielectric nanoparticles using a laser [22]. In the present case the laser was used to tweeze Zn or ZnO and GNRs on a tapered optical fiber's surface. The description of the optical tweezing process can be found in the Ref. [23].
2. Experimental results and discussions
A mixture of GNRs and ZnO with a concentration of 0.5 µg/µl (300 µg of ZnO powder + 600 µl of GNRs) was used to develop the plasmonic structure. The GNRs were in a CTAB solution (Nanopartz – A12-10-1064-CTAB-DIH-1, OD at 1064 nm > 1.0).
Figure 1 shows the scanning electron microscope (SEM) image of the prepared sample. A drop of solution (0.5 µg/µl ZnO powder in GNRs) was placed on the carbon tape for 2 hours to dry completely before scanning in SEM. Figure 1 (a) shows the ZnO nanostructure of different lengths with the GNRs scattered in the medium. Figure 1 (b) shows a magnified image of the ZnO nanostructure of size ∽ 400 nm. In general, the hexagonal Wurtzite crystal structure is the most stable in ZnO. Further, ZnO-NRs can grow along the (0001) or the c-axis, which is the most favorable growth direction [4].
Fig. 1. SEM image of Gold nanorods (GNRs) and Zinc Oxide (ZnO) nanostructure distribution on a carbon tape. (a) ZnO (10’s of nm in length and Hexagonal in shape) with GNRs (67 nm in length and 10 nm in diameter, Aspect ratio: 6.7), and (b) magnified image of the ZnO nanostructure.
The tapered optical fiber was manufactured by etching a multimode fiber (MMF) of core and cladding diameters 110 µm and 125 µm, respectively, using a dynamic etching process [23–25]. For tweezing, a laser beam was coupled at the untapered end of the tapered fiber, and the light propagated along the length to the tip of the fiber. The evanescent field in the tapered region interacted with the surrounding medium, containing the solution of ZnO and GNRs, and produced enough gradient force compared to the scattering force to tweeze nanomaterials (ZnO + GNRs) on the surface of the tapered fiber [24]. Figure 2 is the schematic of the manufacturing process of the plasmonic structure.
Figure 3 shows the SEM image of the plasmonic structure obtained using double tweezing (used lasers at 1064 nm with 8.5 mW power, and 632 nm with 10 mW power, consecutively), which is a distribution of GNRs and ZnO on the tapered fiber along the tapered length. The obtained plasmonic structure consists of a series of rings of ZnO + GNRs with an approximate width and separation of 5.0 µm and 7.5 µm, respectively. The formation of rings depends on the number of modes supported by the fiber at a particular location and how they interfere in the surrounding space [24].
Fig. 3. SEM image of ZnO and GNRs distribution on the tapered fiber surface obtained using optical tweezing.
Figure 4 shows the magnified SEM image of one of the rings in Fig. 3. It is clear from the image that the ZnO nanostructure shape remains intact during the tweezing process and distributes itself in a ring configuration on the tapered fiber's surface. The experiment was repeated with different ZnO concentrations in GNRs solution (e.g., 1.0 µg/µl). However, the ring structure was not formed when higher concentration of ZnO was used, this is due to the gradient force being insufficient to overcome the scattering force beyond a certain concentration. Figure 5 was obtained using optical tweezing with a 1064 nm laser, where a series of rings of GNRs and ZnO with an average separation of 7.22 µm and width 9 µm are visible along the tapered fiber. The variation in the separation and width of the rings compared to the double tweezing case is due to the excitation of fiber modes at different locations along the tapered fiber at 632 nm wavelength compared to 1064 nm and depends on the coupling efficiency of laser to the untapered end of the fiber [23]. Therefore, using different wavelength or tweezing conditions, the distribution of ZnO + GNRs can be changed on the tapered fiber surface depending on the applications.
Fig. 5. SEM image of GNRs and ZnO forming ring-like structure on the tapered fiber obtained using 1064 nm laser (single tweezing).
Figure 6 shows the UV-Visible spectrum of Zinc Oxide (ZnO). The spectrum shows the characteristic peak of ZnO at wavelength of 379 nm which is due to the electronic transition from the valence band to conduction band [26].
Figure 7 (a and b) shows the SEM image of the tapered fiber after it was dipped into the GNRs + ZnO solution for more than 2 hours without laser. The authors observed patches of GNRs + ZnO nanostructure distribution on the tapered fiber's surface but no ring structure, and there was very little ZnO compared to the results previously obtained with laser. The presence of Zn and O was confirmed using energy dispersive spectroscopy (EDS) (Fig. 7 (c)). The ZnO and GNRs were visible. On the other hand, when the experiment was performed only with GNRs solution and without laser, no deposition of GNRs were observed [24].
Fig. 7. (a) SEM Image shows the patches of GNRs and ZnO on the tapered fiber surface obtained without a laser. (b) Magnified SEM image of GNRs and ZnO distribution on the tapered fiber surface in (a) and (c) shows the EDS results which confirms the presence of ZnO on the tapered fiber surface.
Further, the ZnO's surface charge in the aqueous solution can be positive or negative, depending on the solution's pH. In the present case, the GNRs solution's pH is 7.0, which makes the ZnO surface positively charged [27]. Thus, the electrostatic force between the polar ZnO and silica molecule brings them to the tapered fiber's surface. Further, the growth of ZnO and GNRs in aqueous media is governed by the adsorption of CTAB to different facets [12,28,29]. The ionized CTAB forms a bilayer surface. The CTAB capped the growth axis of the hexagonal ZnO nanostructure and stopped further growth.
The authors repeated the experiment as described in the above paragraph to obtain Fig. 7, with ZnO in CTAB solution (concentration 0.5 µg/µl) without GNRs. The SEM image (Fig. 8) shows non-uniform distribution of ZnO on the tapered fiber surface. Further, it was reported that the amount of CTAB at a particular temperature during the formation of ZnO plays a vital role in determining the morphology of the ZnO nanostructure [12]. The absence of a laser beam kept the temperature of the solution constant.
Figure 9 shows the transmission electron microscope (TEM) image of the ZnO used for tweezing purpose. It was found that the ZnO powder contained ZnO nanostructures with non-uniform size.
To investigate the interaction of GNRs and Zn, the authors used pure Zn dust and mixed it with the GNRs solution to obtain a concentration of 0.5 µg/µl. Figure 10 shows the SEM image of the distribution after tweezing with a laser at 1064 nm wavelength and 8.5 mW power. A periodic ring structure of Zn and GNRs on the tapered fiber was produced. The inset of Fig. 10 is a magnified image of a ring, which shows the spherical Zn particle and GNRs. It is clear that one can tweeze element Zn (or ZnO) after mixing with the GNRs solution and obtain a plasmonic structure. Figure 11 shows the SEM image of pure Zn dust, where spherical Zn particles of different sizes are visible [30].
Fig. 10. SEM image of GNRs and Zn dust distributed on the tapered fiber surface tweezed with a laser at 1064 nm wavelength. Inset shows the enlarged image of a ring where spherical Zn and GNRs are visible.
Figure 12 (b) represents optical image of the tapered regions of the fiber obtained using a camera after double tweezing (at 1064 nm and 632 nm, consecutively) under the microscope (Fig. 12 (a)). The periodic ring structure with a light distribution of the ZnO + GNRs between two rings is visible.
Fig. 12. (a) Microscope set-up (b) Optical image of a tapered fiber showing the ring structure produced by GNRs and ZnO.
In this study, the authors discussed the procedure of preparing plasmonic structure of ZnO or Zn and GNRs on the tapered fiber surface. They also reported the distribution of ZnO along the tapered fiber length with and without laser. The plasmonic structure can be modified by changing the laser wavelength and power.
3. Conclusions
The distribution of Zn or ZnO and GNRs on the fiber surface can be manipulated by choosing different tweezing wavelengths and utilizing single or double tweezing. The developed plasmonic structure can be used as a SERS substrate. The authors are investigating the efficiency of the plasmonic structure as a SERS substrate and will report the results in the future.
Funding
Canada Foundation for Innovation; Natural Sciences and Engineering Research Council of Canada.
Acknowledgement
The authors would also like to acknowledge all present and past postdoctoral fellow, graduate, and undergraduate students in the research group for their assistance in the regular lab activities.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request.
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