Controllable fabrication of polymeric nanowires by NIL technique and self-assembled AAO template for SERS application

A controllable strategy to fabricate the polymeric nanowires with high throughput and low cost is developed by using the thermal nanoimprint lithography (NIL) technique and self-assembled anodic aluminum oxide (AAO) template. The length of polymeric nanowires can be controlled by adjusting the duration of thermal NIL. A fill mechanism of thermoplastic intermediate polymer stamp (IPS) polymer pressed into the AAO nanopores is closely studied. The as-prepared IPS polymeric nanowire-based Surface-Enhanced Raman Scattering (SERS)-active substrate exhibits a remarkable reproducibility. The effective adsorption of the R6G as probe molecule near to hotspots generated at 3D vertically aligned polymeric nanowire SERS active substrates shows extraordinary enhancement of Raman signal with an enhancement factor (EF) of 105–106. The present strategy is of great guiding significance to broaden the use of thermal NIL technique and AAO template for the fabrication of other nanomaterials, especially for the flexible and transparent polymer-based nanomaterials.


Scientific Reports
| (2021) 11:14929 | https://doi.org/10.1038/s41598-021-94513-w www.nature.com/scientificreports/ about 90 nm. Each cylindrical hole and the alumina area around it form a hexagonal cell, which contains a cylindrical central nanohole perpendicular to the surface of the Al foil below as shown in Fig. 2b. The pore diameters within the cell size can be adjusted through pore-widening with acid solutions, and the thickness of the AAO layer can be adjusted by changing the duration of anodization [16][17][18] . After a short time of thermal NIL (3 Min), the polymer cannot be pressed into the inside of the AAO pore, but on the concave surface of the AAO pore, as shown in Fig. 2a, where the regularity of the AAO surface is inherited by the surface of the polymeric film (see Fig. 2c). But the polymer pressed into the inside of the AAO pore comes to form the polymeric nanowires, under a long duration of thermal NIL (40 Min), as shown in Fig. 2d.

Controllable fabrication of polymeric nanowires.
It is necessary to select appropriate times, the applied pressure, and appropriate temperature of thermal NIL that determine the viscosity of the polymer, and a pressure-assisted template technique was used to ensure that the polymer completely fills the pores of the template during the thermal NIL 24 . In this experiment, the polymeric nanowires (see Fig. 3) were fabricated by   www.nature.com/scientificreports/ pressing a thermoplastic IPS film into the initial AAO template at the temperature of 155 • C and the pressure of 40 bar respectively for different times. At 3 Min, the polymer cannot be pressed into the inside of the AAO pore, the surface of the polymer inherits the regularity of the AAO surface and formed well as shown in Fig. 3a. When the time of thermal NIL increases to 6 Min, the polymeric nanowires are separated from each other and formed nanowires (see Fig. 3b). As the time of thermal NIL increases to 30 Min, the IPS polymeric nanowires grow significantly (see Fig. 3c), and the diameter of the polymeric nanowire is about 80 nm. The further increase of the length of polymer nanowires leads to polymer nanowires from vertical (see Fig. 3c) to inclined morphology (see Fig. 3d), and finally collapsed, as shown in Fig. 3e,f. These results show that the length of polymeric nanowires can be controlled by adjusting the duration of thermal NIL. A filling mechanism is observed in this thermal NIL process as shown in Fig. 4. The thermoplastic IPS film is shaped by pressing into the AAO template after it is heated above glass transition temperature (T g ). When the thermoplastic IPS film is compressed between applied pressure and the AAO template, the viscous polymer undergoes a thermal and mechanical compression and is forced to flow into the pores of the AAO, so that it fully conforms to the surface relief of the template. In the case of large-area periodic AAO structures, the specific area which contributes to the filling of a single pore is limited by the structure density or the distances between the pores. The nanostructured region is surrounded by an unstructured area and the majority of material flows from the borders. For this case, the squeeze flow in the interpore regions can be neglected and the effective template area is the area between the edge of the structured region and the template border.   www.nature.com/scientificreports/ ires SERS substrate prepared shows a well-ordered distribution. A large number of Au nanoparticles gather on the top of the polymeric nanowires, and a small amount of Au nanoparticles gather on the upper rim of the nanowires by sputtering technology. The hexagonally arranged Au nanoparticles on the polymeric nanowires inherit the periodic hexagonal arrangement of the AAO template, which results in highly uniform shape and size distribution of the coated Au nanoparticles. It is worth mentioning that the vertically aligned polymeric nanowires decorated with Au have an enhanced surface to volume ratio, which will lead to a good enrichment of the analyte molecules. These nanostructures can be used as plasmonic template for surface enhanced Raman scattering (SERS).

SERS performance: LSPR behavior.
As well-known, the SERS activity of nanostructured samples strongly depends on their optical properties being related to the strong amplification of the local EM field in the proximity of the nanostructured metallic surface. This amplification is caused by excitation of localized surface plasmonic resonances (LSPR) of individual nanowires or nearby optical coupling structures. Figure 6 shows the transmittance spectra of as-prepared 20 nm Au-coated polymeric nanowire sample and Au-free polymeric nanowire sample as comparison (polymeric nanowire with the diameter of ~ 80 nm and length of ~ 300 nm), in the wavelength range of 300-900 nm. Compared with the Au-free polymeric nanowire sample, the spectrum of the Au-coated polymeric nanowire sample shows a minimum transmittance signal with the center at 550 nm, which can be recognized as a plasmon absorption peak 25 . Such an Au-coated polymeric nanowire sample appear to be optically coupled 25 , leading to the formation of active hot spots, which is then fixed at the excitation wavelength of 550 nm to match the optimal SERS efficiency conditions. SERS performance: reproducibility of Raman signals. Considering the significance of the reproducibility of Raman signals on SERS substrate in practical applications, we randomly selected five collection points on the as-prepared substrate to collect the SERS spectra of R6G molecules with a concentration of 10 −6 M from a    26 . In addition, the Raman spectra of R6G on nanowires without Au shows weak Raman signals that is indistinguishable due to the noise, which can be ignored.
SERS performance: SERS EF calculation. Among these Raman peaks, the maximum enhancement is observed at 615 cm −1 , which was chosen to estimate the enhancement factor (EF) of SERS substrate according to the method previously reported 27 . According the method described in previous reports 28 Fig. 7b). For the reference sample with R6G at 10 −1 M on a plane glass substrate, N Raman will be ~ 7.29 × 10 7 . Considering N SERS ≈ 10 4 -10 5 , the enhancement factor (EF) is therefore estimated to be about 10 5 -10 6 . In addition, the R6G absorption band and laser excitation wavelength (532 nm) in the SERS experiment are in resonant conditions, the resulted resonant effect between excitation and analytes molecules absorption gives rise to a resonant Raman effect that provide a further boost to the Raman scattering. Considering the resonant Raman effect, the EF value estimated above is a conservative, lower bound of the real enhancement factor.

Conclusions
In conclusion, the present research shows that good control of the fabrication of polymeric nanowires can be achieved through a combination of thermal NIL techniques and self-assembled AAO template. The length of polymeric nanowires can be controlled by adjusting the duration of thermal NIL. A fill mechanism of thermoplastic IPS pressed into the AAO nanopores is closely studied. The present method is a facile, cost-effective, reproducible, and low-cost strategy to fabricate SERS polymeric nanowire substrates on a large scale. The asprepared IPS polymeric nanowire-based SERS-active substrate exhibits a remarkable reproducibility. The effective adsorption of the R6G as probe molecule near to hotspots generated at 3D vertically aligned polymeric nanowire SERS active substrates shows extraordinary enhancement of Raman signal with an EF of 10 5 -10 6 . The present strategy is of great guiding significance to broaden the use of thermal NIL technique and AAO template for the fabrication of other nanomaterials, especially for the flexible and transparent polymer-based nanomaterials.

Methods
Synthesis of AAO template. A two-step anodization process is performed for the self-assembled synthesis of the AAO membrane, which results in well-ordered hexagonally arranged nanopore arrays, as shown in Fig. 1a. First, a 0  www.nature.com/scientificreports/ irregular alumina in a mixture of 1.8 wt % chromic acid and phosphoric acid at 60 °C for 8 h. The second anodizing was performed under the same experimental conditions as the first anodization, in which appropriate times were set to obtain AAO membrane of desired thicknesses. To widen the nanopores, the chemical etching of as-prepared AAO membrane was performed in 5 wt % phosphoric acid solution at 40 °C, in which appropriate etching times were set to obtain the desired diameter of nanopores. The detailed synthesized process has been described in the previous works 21,22 .
Fabrication of polymeric nanowires. The schematic in Fig. 1 shows the fabrication process of polymeric nanowires. An anti-adhesion pretreatment for the initial NIL template is necessary for the subsequent NIL process. The as-prepared AAO membrane as an initial NIL template immersed in a 0.6 mmol/mL 1H,1H,2H,2H perflurodecyltrichlorosilane (FDTS) /isooctane solution for 10 min to prevent adhesion of the polymeric film to the AAO template 23 . Then it is cleaned with isooctane, acetone, IPA, and water successively, followed by blowdrying. After AAO pore-widening and anti-adhesion pretreatment, the polymeric film is transferred onto AAO template, as shown in Fig. 1b. The polymeric nanowires are fabricated by pressing the relatively soft IPS film ((a thermoplastic polyolefin polymeric film, developed by Obducat AB) against the hard pre-treated AAO template through thermal NIL at the temperature of 155 °C and the pressure of 40 bar respectively as shown in Fig. 1c, in which specific times were set to obtain polymeric nanowires of different length. Finally, when the IPS polymeric nanowires are cooled to a temperature below the glass transition temperature (T g ), it is obtained after mechanically demolding and residue removal of AAO, as shown in Fig. 1d. An Ar + laser (λ = 532 nm) with a power of 5 mW served as the excitation source is used for Raman and SERS measurement. The integration time was 10 s for each spectrum at room temperature. Commercial software (Labspec, Horiba-Jovin Yvon) was used to fit the baseline of all Raman spectra. Raman signals were recorded at five different points of the substrate, and the standard deviation (SD) and relative standard deviation (RSD) of the spectra were calculated.

Characterization.
SERS enhancement factors: evaluation of the number of probed molecules. According the method described in previous reports 28 , the number of molecules probed in the Raman can be estimated from N Raman = N Avo × c Raman × V laser , where N Avo of 6.022 × 10 23 is the Avogadro number, c Raman is the molar concentration and V laser the diffraction limited volume probed by the microscope objective. V laser can be estimated from V laser = (4π/3) × b 1 b 2 b 3 , where the semi-axes b 1 = b 2 = 0.61 × λ/NA, b 3 = 2 × λ/NA 2 , λ = 633 nm and NA (numerical aperture) = 0.9. Consequently, V laser will be ~ 1.21 μm 3 , N Raman will be ~ 7.29 × 10 8 c Raman . According the method described in previous reports 28 , the number of probed molecules in SERS (N SERS ) can be calculated from N SERS = N mol × N nano , where N mol is the number of molecules surrounding each nanostructure, N nano is the number of nanostructures present in the laser spot. N mol is calculated through the information on the nanowires' hydrodynamic radius while nanowires are mixed with R6G. The value of N mol is estimated to be the orders of 10 3 . The number of nanowires (N nano ) in the laser focus can be estimated in the ideal situation where the nanowires totally fill the focal laser spot semi-volume (V laser ≈ 1.21 μ m 3 ). The value of N nano is estimated to be the orders of 10 3 , yielding a number of probed molecules N SERS ~ 10 6 . For 80 × 270 nm nanowires, less than 10% of the molecules located and absorbed at edge of the nanowires surface and the hot spots will experience the SERS enhancement, yielding N SERS ≈ 10 4 -10 5 .

Data availability
The data used to support the findings of this study are available from the corresponding author upon request.