Templated Nano Electrodeposition: Novel Method for Mass Fabrication of Flexible Plasmonic Metasurfaces

Flexible plasmonic metasurfaces have attracted considerable attention due to the mechanical flexibility of the metasurface that enables new functions and integrated applications, but the costly fabrication often hinders their further employment. Here, this work proposes a novel method to fabricate large‐area, high‐volume, and cost‐effective flexible plasmonic metasurface aspects through the nanotemplated electrodeposition and imprinting transfer processes, which minimize the fabrication cost without sacrificing their nanophotonic performance. The fabricated plasmonic metasurface has comparable performance in biomolecular detection with those fabricated through conventional methods, which shows the potential employment of the templated fabricated flexible metasurface in medical applications such as point to care and in situ biomolecular detection. Moreover, the low‐cost and low processing parameter requirements of this method will also contribute to many other metasurface‐based nano‐optic applications, including surface‐enhanced Raman spectroscopy, fluorescence enhancement, and structural colors.


Introduction
In the past decade, plasmonic metasurfaces, mainly consisted by regular metallic nanostructures, have been identified as one of the most promising techniques in nanoscale photonic modulation [1][2] due to their unique capability of manipulating light in localized volume. Many emerging nanooptic applications have been developed based on plasmonic DOI: 10.1002/adsr.202300008 The fabrication of plasmonic metasurfaces typically consists of multiple steps, including pattern generation (e-beam lithography, [13][14] nanoimprinting, [15] etc.) and material deposition (evaporation, [16] magnetron sputter, [13] etc.).These technologies are widely used in industries such as integrated circuits, but they are often expensive and time-consuming.
Considerable effort has been devoted to investigation of alternative fabrication methods, such as guided selfassembly of metallic nanoparticles, [7] direct laser milling of metal films, [17] and direct nanoimprinting of metals. [18]n the other hand, flexible plasmonic metasurfaces are currently attracting great research interest, [19][20] because of their properties of flexibility, which overcomes the fundamental mismatch between soft, elastic, or curved surfaces in practical applications comparable to typical rigid metasurfaces. [21][24] However, efficient and costeffective approaches to fabricate large-area high-quality flexible metasurfaces still lack investigation for their practical applications.
Herein, we report a novel fabrication method for low-cost, high-volume and large-area flexible plasmonic metasurfaces by combining high-resolution nanoimprint lithography and costeffective templated electrodeposition.A predesigned nanopattern was first prepared on a conductive substrate by nanoimprinting process, then metals were deposited on the nanopattern by an electroplating process and transferred to the final flexible plastic substrate.The obtained patterned plasmonic metasurface displayed excellent tunable plasmonic properties, and can be used in integrated applications in vast fields, such as plasmonic sensing.In the presented method, we only employed commercially available, low-cost materials and instruments, which breaks the cost barrier imposed on developing practical applications.Through this novel method, we demonstrated a proof-of-concept for biosensing.Benefiting from the excellent biocompatibility and stability of the flexible carrier, of the metasurface exhibits comparable performance in sheep antihuman immunoglobulin G (IgG) and human IgG immune complex formation analysis.This novel flexible metasurface fabrication method is expected to be inspiring in many new applications and open new horizons in various fields.

Templated Nano Electrodeposition of the Metasurface
Flexible plasmonic metasurfaces were fabricated through a solution-processed, vacuum-free, and high-throughput nano electrodeposition process and transferred to a flexible thermoplastic by a double-transfer nanoimprint lithography process.we demonstrated a proof of concept for biosensing is an extension of the methodology presented in our previously published work . [20]The nano electrodeposition template is obtained by a thermal nanoimprint lithography process with a flexible replication of the original silica mold using an ultraviolet-curable ormostamp on a polyimide (PI) film.The flexibility of the imprint mold improves the uniformity and fidelity of the imprinted nanostructures on the resist, and waives the requirement of a high-level cleanroom, which could also reduce the fabrication cost of metasurfaces.The total cost of the plasmonic film was 0.16 $ cm −2 , including the amount of gold consumed and the cyclic olefin copolymer (COC) film.
The fabrication process is schematically illustrated in Figure 1A-G.First, a thin layer of imprint resist is spin coated on a cleaned conductive indium tin oxide (ITO) glass substrate (Figure 1a).Second, the nanohole pattern is thermally nanoimprinted in the imprint resist as an electroplating mask using a flexible mold (Figure 1B,C). To fully expose the ITO from the imprint resist and ensure sufficient filling of the resist to avoid void formation in too thin of a resist, the thickness of the resist is carefully adjusted according to the duty cycle of nanostructures on the flex-ible mold.Third, Au is deposited inside the exposed nanoholes using a home-built electrodeposition system, and the thickness of the deposited Au can be tailored by the electroplating time and current (Figure 1D).Fourth, the resist is removed by gently immersing the substrate in acetone (Figure 1E).Finally, gold nanodisks are transferred to a thermoplastic film by a double-transfer nanoimprint lithography process (Figure 1F,G).We have utilized electro-deposition of gold, which is highly stable and gold surfaces can be easily functionalized with a variety of chemical and biological molecules, enabling the development of highly specific and selective sensing platforms.Figure 1H presents a photograph of the metasurface on a COC film displaying uniform visible-light diffraction and excellent flexibility of the fabricated metasurface.
Flexible nanostructures on the resist replica faithfully follow the structures on the flexible molds by scanning electro microscopy (SEM) investigations (Figure 2A), with the ITO surface exposed from the nanoholes for deposition of gold in the following electroplating process (Figure 2B).The lifetime for the flexible mold is investigated by using the same flexible mold for more than ten fabrication cycles.Only a few negligible defects (less than 1%) were found on the mold, which also reduces the fabrication cost of the metasurface.100 nm thick Au nanodisks were deposited inside the nanoholes on the ITO substrate using the nanoimprinted resist as an electroplating mask.The thickness of the gold is calculated by the volume of deposited gold and the exposed area of the ITO.After removal of imprinted resist in acetone, the Au nanodisks were still firmly attached on the ITO substrate and no obvious defects were found during this rinse process under microscopy characterization (Figure 2C).The Au nanodisk were then transferred from the ITO substrate to a thermoplastic COC film through a double-transferred nanoimprint lithography process.The Au nanodisks are fully embedded in the COC film and corresponding to the original nanodisks on the flexible mold.As accounted for by the scanning electron microscopic (SEM) characterizations, negligible fluctuations of geometry or pitch of the gold nanodisks were observed (Figure 2D).

Refractometric Sensing Using the Flexible Metasurface
We experimentally measured and numerically simulated the transmittance spectra of the flexible plasmonic metasurfaces in a medium with a refractive index of 1.333.As shown in Figure 3A, the experimentally measured and simulated transmittance spectra agree well in our interested range (450-900 nm) for n = 1.333 case.The slight shifts in the peak and dip positions might be attributed to the dielectric constant difference between the modeled gold and the electroplated gold, in which impurities and roughness exist.There are three peaks and two dips in the 500-900 nm region in the simulated transmittance spectrum in medium.D1 (560 nm) and D2 (790 nm) are attributed to the (1, 1) and (1, 0) surface plasmon polaritons (SPPs) at the gold-COC interface.P1 arises at 580 nm because of the coupling of SPP (1, 1) at the gold-COC interface with SPP (1, 0) at the gold-liquid interface. [27]The electric field for P1 is mainly localized at the top surface of the gold nanodisk (gold-liquid interface) (Figure 3D-F).The theoretical refractive index sensitivity calculated from the spectra of the array under various refractive indices ranging from 1.333 to 1.447 is 440 nm RIU −1 , which makes it a good candidate for monitoring the changes of local refractive indices.
To examine the performance of the fabricated flexible plasmonic metasurfaces as a refractometric sensor, we collected the transmittance spectra of the flexible metasurfaces by immersing them in liquids with varying refractive indices.In detail, different concentrations of glycerin water mixture liquid with refractive indices of 1.333-1.447were dropped on the metasurfaces.
As displayed in Figure 4D, red shifts of resonances P and T with increasing refractive indices were observed.To clearly present the spectral shift of P and T versus the surrounding environmental refractive index, we plotted their normalized transmittance spectra (Figure 4C,D).The spectral shift of P and T versus refractive index is plotted and linearly fitted in Figure 4B.As expected, refractive index sensitivities of 451.0 nm RIU −1 and 134 nm RIU −1 with a good linear dependence were obtained from P and T, respectively, which also in accordance with previous simulated results.

Anti-IgG and IgG Immune Complex Formation Analysis Using the Flexible Metasurface
The highly localized sensing volume of the gold nanodisks allows to observe the changes in the local refractive index upon biomolecular interactions near the gold surface. [28]To demonstrate the gold nanodisks as a plasmonic biosensor, we have chosen human IgG and sheep antihuman IgG as model proteins and investigated their specific interactions on the plasmonic film.To immobilize IgG, the gold nanodisks surface was modified by 11-mercaptoundecanoic acid (MUA) and N-hydroxysuccinimide (NHS) sequentially. [29]The LSPR displacements after each step are evaluated through transmittance spectroscopy (Figure 5).A shift of 2.7 nm was observed when the sheep antihuman IgG was bonded to MUA, revealing that the substrate could be used in reference to biosensing. Figure 5B presents the transmissive spectral response of the plasmonic film after inhaled in human IgG in phosphate buffered saline (PBS) buffer solution for 30 min with various IgG concentrations ranging from 40 to 640 ng mL −1 .In each measurement, the plasmonic film was immersed in a quartz cuvette and 1 mL human IgG solution was injected and kept for 30 min to allow the interaction with antihuman IgG. Figure 5C shows the detailed spectra around 700 nm.According to the transmittance spectra, it is clearly that IgG binding and their specific interaction with anti-IgG proteins on the gold nanodisks surface were manifested as a red shift in the plasmonic resonance peak, attributed to the binding of IgG and their subsequent interactions with anti-IgG, causing changes of the local refractive index in the vicinity of embedded gold nanodisks.The binding of human IgG of concentrations at 40, 80, 160, 320, and 640 ng mL −1 resulted redshifts of LSPR-peak by 0.7, 1.1, 1.9, 2.0, and 2.6 nm, respectively, which are summarized in Figure 5D.The significant shift of LSPR peak after binding of IgG proteins at higher concentrations is resulting from the larger change in the local refractive index near the gold nanodisks surface.Upon the injection of human IgG, a redshift in transmissive spectra occurs, revealing the binding of the two molecules.The use of a flexible metasurface for IgG sensing enhances sensor flexibility by allowing for a more adapt-able sensor shape, thus expanding the range of potential sensor applications.

Mechanical Stability of the Flexible Metasurface
The embedded nature of disks reduces the risk of delamination from the substrate and enhances their stability under mechanical bending.To verify the mechanical properties of the flexible metasurface, we repeatedly bend the metasurface and observe its stability through the changes of the transmittance spectra during cyclic bending, as shown in Figure 6.After bending for 110 times, the shape of the transmittance spectra and wavelength of the resonances are not significantly altered, which indicates that the flexible metasurface has excellent mechanical stability against repetitive bending without sacrificing its nanophotonic performance, which makes this device a promising candidate for use in flexible applications.

Conclusion
We propose a simple, cost-effective, and efficient method to fabricate flexible plasmonic materials with user-defined patterns.The obtained materials exhibit tunable plasmonic properties and excellent flexibility.We have demonstrated the biosensing capabilities of flexible plasmon metasurfaces.The results showed good sensitivity to anti-IgG and IgG immune complex formation detection.In addition, the flexible plasmonic metasurface has good mechanical capability against repetitive bending.This novel fabrication method is expected to be advantageous for many flexible metasurfaces used in cost-effective point-of-care and on-site detection applications.

Experimental Section
Nanoimprint Mold Fabrication: 2.5 × 2.5 cm 2 silica mold with 500 nm pitch nanodisk array was first fabricated by interference lithography and reactive ion etching processes.Then, 200 μm thick COC films were cut into pieces of the same area with the silica mold.These COC films were cleaned with a cotton swab and rinsed thoroughly in deionized water, and then treated with oxygen plasma (Potentlube, China) for 5 min.The samples were then further cleaned by ultrasonication in acetone and isopropanol for 300 s before being dried under nitrogen flow.Afterward, 0.02 g ormostamp (Micro resist technology, Germany) was then dropped onto the silica mold and covered with a cleaned PI film.A gentle pressure was then applied on the stack to fill the nanoholes on the silica mold with ormostamp and form a uniform layer between the silica mold and the PI film.Thereafter, ormostamp was exposed to UV light (405 nm, 600 mJ cm −2 ) to fully cure the resin Finally, the flexible mold was separated from the ITO glass manually.
Flexible Metasurface Fabrication: ITO glass substrates of 3 × 3 cm 2 area were cleaned with a cotton swab, thoroughly rinsed in deionized water, and then treated with oxygen plasma for 5 min.The samples were then further cleaned by ultrasonication in acetone and isopropanol for 5 min before being dried under nitrogen flow.Then, a 100 nm thick layer of imprint resist dissolved in anisole (glass transition temperature of 70 °C, Dongguan Jincai Materials, China) was spin-coated on the ITO substrate and baked at 130 °C for 1 min.Thereafter, a thermal nanoimprint process was used to pattern the imprint resist on the ITO substrate using a home-built nanoimprint platform consisting of a hydraulic press (Specac Ltd, UK), electrically heated platens with a temperature controller (Specac Ltd, UK), and a chiller (Grant Instruments, UK).A silicone cushion was placed between the flexible mold and the press.The mold/substrate stack was heated to 100 °C for 5 min with an imprinting pressure of approximately 0.4 MPa to press the flexible mold and the ITO substrate.Afterward, the heated stack was cooled down to the room temperature, and the flexible mold was peeled off from the ITO substrate after releasing the pressure, leaving nanoholes transferred on the imprint resist.
Gold was deposited through the nanoholes on the ITO glass substrates using an electrodeposition process.A gold electroplating solution (Caswell, USA) was used for the deposition of gold.A Keithley 2450 SourceMeter was used to supply a 2.5 mA  cm −2 current density during the electrodeposition process.A two-electrode electrodeposition system was set up with nanoimprinted ITO glass as the anode and a 5 × 10 cm 2 platinum-coated titanium grid as the cathode.Afterward, the sample was thoroughly rinsed with deionized water, dried by nitrogen flow, and then immersed in ethanol or acetone for at least 10 min to remove the imprint resist, leaving the electrodeposited gold nanodisks attached on the ITO glass.
The gold nanodisks were then transferred to a 100 μm thick thermoplastic COC film (Grade 8007, TOPAS, Germany) through a hot embossing process using the same home-built nanoimprint platform.The ITO/COC stack was heated to 130 °C for 5 min under an imprinting pressure of 0.5 MPa to press the ITO substrate and the COC film together.Afterward, the heated stack was cooled down to the demolding temperature of 50 °C, and the COC film was manually peeled off from the ITO glass template after releasing the pressure, leaving gold nanodisks transferred to and embedded in the COC film.
Numerical Simulations: Numerical simulations of the transmittance spectra and electrical field distributions of the Au nanorods were carried out using commercial software FDTD solutions (Lumerical Solutions, Canada).The TFSF source has been used in FDTD in order to obtain the near-field along the structure.Periodic boundary conditions were applied to both xand y-directions, perfectly matching layers were used for calculations of the structure arrays.A nonuniform mesh structure with maximum size of 2 nm was used for the interested region.The refractive index of COC was taken as 1.575.The dielectric constants of Au was taken from Palik.
Morphological Characterizations: The morphology of the samples was characterized using a Zeiss (Germany) scanning electron microscope.
Optical Measurements: Transmittance spectra were taken on an ultraviolet/visible/near-infrared spectrometer (USB2000+, Ocean Insights, USA).All transmittance values presented in this paper are normalized to the absolute transmittance through the bare COC film.
Refractive Index Sensitivity Measurements: The index sensitivity was determined by dropping 10 μL of aqueous glycerol solutions with varying calculated concentrations on the same sample and recording the transmittance spectra.After each measurement, the sample was thoroughly rinsed with isopropanol and dried under nitrogen flow.
Immobilization of Sheep Antihuman IgG: The films were cut into 1 × 1 cm 2 pieces and immersed in 1.0 mM ethanolic solution of 11-mercaptoundecanoic acid (MUA, J&K Chemical, 95%, China) at 4 °C for 24 h to form a self-assembled monolayer of MUA.Soon after, the film was rinsed with ethanol and then incubated in a 1:1 aqueous solution of 0.1 m of N-hydroxysuccinimide (NHS, J&K Chemical, 99%, China) and 0.1 m of N-ethyl-N-(3diethylaminopropyl) carbodiimide (EDC, J&K Chemical, 97%, China) for 4 h.Then, the gold nanodisks surface that had been modified with MUA-NHS was immersed in a 0.5 μg mL −1 sheep antihuman IgG in 0.01 m PBSbuffer solution for 24 h to complete the immobilization process.
Biomolecular Detection: sheep antihuman IgG -modified plasmonic films were vertically inserted in a quartz cuvette (5 mm), 1 mL human IgG (Thermo, USA) with concentration ranging from 40 to 640 ng mL −1 in 0.01 M PBS buffer solution was injected into the cuvette and kept for 30 min before collection of the transmittance spectra.
Statistical Analysis: To clearly demonstrate the peak shift response to IgG concentration, the obtained transmittance spectra were normalized to 100% transmittance.Unless otherwise stated, data were expressed in mean ± standard deviation (n = 3).

Figure 1 .
Figure 1.Schematic illustration of the fabrication of a flexible substrate embedded with a gold nanodisk array.A) A thin layer of the imprint resist spin-coated on a conductive ITO glass substrate.B) Replication of the nanohole array in the imprint resist using a flexible thermal imprint stamp.C) A nanohole array formed in the imprint resist, with the ITO surface exposed through the nanoholes.D) Electrodeposition of gold inside the holes to form a uniform gold nanodisk array.E) Removal of the imprint resist to obtain the gold nanodisk array on the conductive substrate.F) Heating and pressing the gold nanodisk array into a thermoplastic film.G) Peeling off the film from the conductive substrate with the gold nanodisk array transferred.H) A photograph of the fabricated flexible metallic nanostructured film, the scale bar represents 1 cm.

Figure 2 .
Figure 2. SEM images of A) nanodisk on flexible template, B) nanohole array on imprint resist, C) electrodeposited Au nanodisk array on ITO substrate, and D) Au nanodisk array embedded on flexible COC substrate.The scale bars in SEM images are 1 μm.

Figure 3 .
Figure 3. A) Experimentally measured and numerically simulated transmittance spectra of the gold nanodisk array with an environmental refractive index of n = 1.333.B) Normalized transmittance spectra of the gold nanodisk array with surrounding refractive indices changes from 1.333 to 1.447.C) Plot of the spectral shift versus surrounding refractive index.The red line is a linear fit, y = 440.7x+ 139.6 with R 2 = 0.98.The refractive index sensitivity of P is 440 nm RIU −1 .D) Y-Z E), X-Z F), and X-Y electrical field distributions of the gold array at P (700 nm) for n = 1.333.The total-field scattered-field (TFSF) source has been used in FDTD in order to obtain the near-field along the structure.

Figure 4 .
Figure 4. Refractive index sensing using the flexible plasmonic metasurface.A) Transmittance spectra of the gold nanodisk array with different concentrations of glycerin solution.B) Plot of the spectral shift versus surrounding refractive index.The refractive index sensitivity of P is 445 nm RIU −1 and that of T is 134 nm RIU −1 .C) Normalized transmittance spectra for P in the spectral region.D) Normalized transmittance spectra for T in the spectral region.

Figure 5 .
Figure 5. Plasmonic biosensing of anti-IgG proteins using IgG-modified flexible metasurface.A) Schematic illustration of anti-IgG surface modification of the gold nanodisk array.B) Transmittance spectra of the metasurface during different modification steps.C) Transmittance spectra of the anti-IgG modified film after immersion in PBS buffer solution of IgG with different concentrations ranging from 40 to 640 ng mL −1 .D) Measured LSPR displacement as a function of IgG concentration.Error bars represent the standard deviation of three replicate measurements (n = 3).

Figure 6 .
Figure 6.Transmittance spectra of the flexible metasurface after repetitive bending to an angle of 15°for 10-110 cycles.