Silica-coated metallic nanoparticle-based hierarchical super-hydrophobic surfaces fabricated by spin-coating and inverse nanotransfer printing

Superhydrophobic surfaces covered with micro- or nanostructures (such as posts, grooves, or holes) can effectively trap bubbles.^1,2 If a liquid surface contact is restricted to the top of the roughness, known as the Cassie state, the liquid will be in contact with the solid only over a fraction of the surface, resulting in drag reduction, facilitation of heat transfer, self-cleaning, anticorrosion, antisticking, and anticontamination. They have opened many exciting opportunities in marine technology, automotive, biosensing, and lab-on-a-chip technologies.^3–6 

Experiments suggested that superhydrophobic surfaces with metallic nanostructures can break the diffusion limit and detect molecules at femto- or attomolar solutions,^5,7 if droplets over superhydrophobic surfaces maintain quasispheres during evaporation and do not wet the surface, because this way can effectively localize molecules and decrease the detection limit of plasmonic nanosensors. Hence, the integration of metallic nanostructures and superhydrophobic surfaces can pave the way toward developing high-sensitive, label-free, and miniaturized biosensors with potential applications ranging from early disease diagnosis to a fast sequence of genomes.⁸ Currently, metallic nanoparticles were decorated over superhydrophobic surfaces by electron-beam lithography which is expensive and not suitable for the large-scale fabrication, limiting its applications.

In addition, for droplets to remain quasispherical, the surface needs to remain in the Cassie state. However, it is recognized that the transition from the Cassie state to the Wenzel state (a fully wetting state) during evaporation can be triggered under certain conditions including high pressure.⁹ Once the surface is fully wetted, the advantages of the superhydrophobicity like self-cleaning will lose. In other words, a strategy to delay this transition is highly desirable to fully exploit superhydrophobic surfaces. Recent studies revealed that superhydrophobic surfaces with hierarchical structures consisting of both the microstructure and the nanostructure can increase the contact angle and improve the stability and robustness of such surfaces.^10–13 Interestingly, patterned nanoparticles can play a dual role. On one hand, they serve as the plasmonic sensors; on the other hand, they promote the superhydrophobicity and improve the sensitivity of the detection. Hence, it is important to develop a cost-effective and scale-up method to pattern metallic nanostructures onto superhydrophobic surfaces and assure that such surfaces can maintain the Cassie state as long as possible during evaporation.

In this letter, we describe a simple fabrication technique to transfer nanoparticles onto a superhydrophobic surface. Our method consists of two key steps. First, we apply a spin-coating method developed by Jiang et al.,^14–17 where they generate a monolayer or multilayer of colloidal latex particles by spin-coating. Here, we spin-coat gold nanoparticles onto the silicon wafer. Next, we modify the nanotransfer printing technique,^18–22 which transfers metal from the raised regions of the rigid stamp onto a flat polydimethylsiloxane (PDMS) surface. Instead, we transfer gold nanoparticles from the flat rigid stamp onto the raised regions of a PDMS structured surface by bringing them into intimate physical contact. We coin our method as inverse nanotransfer printing.

Our method is simple, low-cost, and ready to scale up. In this letter, we pattern nanoparticles onto superhydrophobic surfaces and demonstrate that the hierarchical surface with nanoparticles enhances superhydrophobicity by both effectively increasing the contact angle and delaying the Cassie-Wenzel transition.

First, we spin-coated a layer of gold nanoparticles onto the silicon wafer. 70 nm gold nanoparticles coated with a 10 nm hydrophobic silica shell were purchased from Ted Pella, Inc. The silica shell is intended to prevent the aggregation of colloidal gold nanoparticles. In order to spin-coat a uniform layer of nanoparticles, the silicon substrate was treated with 3-acryloxypropyl trichlorosilane (APTCS).²³ 

Figure 1(a) presents the procedures for uniformly patterning a layer of gold nanoparticles with spin coating. In order to generate a high-quality layer of nanoparticles, we used the spin-coating approach developed by Jiang et al.¹⁵ Briefly, the process has two different acceleration speeds. At the low acceleration speed of 200 rpm/s, the spin coating remained at 200 rpm for 120 s, 300 rpm for 120 s, 1000 rpm for 60 s, and 3000 rpm for 20 s. At the high acceleration speed of 1000 rpm/s, the spin coating continued at 6000 rpm for 20 s and finally at 8000 rpm for 360 s. After being soft baked around $60 °C$ for 15 min, the wafer slowly cooled down to room temperature and is ready to serve as the stamp in the subsequent inverse nanotransfer printing.

Structured superhydrophobic surfaces in PDMS were fabricated by soft lithography techniques.^24,25 The soft lithography technique can be applied to produce patterns on the micrometer- and nanometer-length scales. Here, we used a DVD disk to create patterned surfaces in PDMS. The DVD disk has a patterned structure with holes and serves as the mold. The PDMS solution was poured over the DVD disk under vacuum to remove bubbles and then baked at $60 °C$ for 1 h. In our practice, the degree of curing was 30% lower than that of a typical soft lithographic curing procedure. In other words, our PDMS surface was not fully cured. We found out that 70% curing can effectively help to reach the conformal adhesion between the PDMS surface and gold nanoparticles for better nanotransfer printing.

With the PDMS structured surface, we can begin the process of the inverse nanotransfer printing [Fig. 1(b)]. We placed the PDMS surface on top of the silicon stamp under high vacuum (⁠$9.5×10−4 Pa$⁠) for 3 h to achieve an intimate, conformal contact and adhesion between the raised regions of the PDMS surface and the silicon stamp. Then, we baked them for 6 h at $60 °C$ to further promote the adhesion of nanoparticles to the raised regions of the PDMS surface. Finally, we peeled the structured PDMS surface and the stamp apart. Nanoparticles were transferred from the stamp to the PDMS surfaces. Figures 2(a) and 2(b) show the AFM images of the PDMS structured surfaces without and with nanoparticles.

It was demonstrated that hierarchical surfaces with nanoroughness can effectively suppress the Cassie-Wenzel transition and improve the superhydrophobicity.^10–13 To examine whether our fabricated hierarchical structure can increase the superhydrophobicity, we carried out experiments on the evaporation of sessile droplets over structured surfaces with and without nanoparticles. The process of a freely evaporating droplet on a superhydrophobic surface is intriguing.^26,27 Initially, the droplet over a structured surface is in the Cassie state where air bubbles are trapped inside the rough surface underneath the droplet,¹ leading to a high contact angle. The decrease in the droplet size, due to the natural evaporation, results in a larger Laplace inner pressure, which can drive the droplet into a fully wetting (Wenzel) state.⁹ 

The evaporation process was conducted at room temperature (⁠$21±1 °C$⁠) with a relative humidity of $22 ± 2%$⁠. The used water was ultrapurified with a specific resistivity of 18.2 $MΩ cm$ (PURELAB Classic, ELGA LabWater, LLC). The initial droplet volume was 2 $μl$⁠. The side-view images of the evaporating droplet were recorded. Figure 3 plots the snapshots of time-dependent evaporating droplets over the hierarchical superhydrophobic surface. The volume of the droplet and the contact angle were measured against the time by a plug-in for ImageJ that uses low-bond axisymmetric drop shape analysis.^28,29

Figure 4(a) plots the macroscopic contact angle $θ$ as a function of time during the process of the natural evaporation for the superhydrophobic surfaces with nanoparticles (the solid line) and without nanoparticles (the dashed line). Figure 4(b) shows the normalized droplet volume V/V₀, where V₀ is the initial volume as a function of the normalized time $(tf−t)/tf$ in which $tf$ is the total droplet life time in the natural evaporation. The solid line is the power-law relation [$V(t)∼(tf−t)3/2$]. This law is the result of a pure diffusive evaporation,³⁰ which corresponds to the evaporation of the droplet with a constant contact angle where the contact line recedes and the droplet self-similarly reduces its volume. The droplet with the Cassie state maintains its quasispherical shape during the evaportation.⁵ Once the transition to the Wenzel state occurs, the contact line is pinned and the data deviate from this power-law relation. Evidently, Fig. 4(b) shows that nanoparticles on the superhydrophobic surface effectively delay the Cassie-Wenzel transition which is also confirmed by the time-dependence contact-angle dynamics in Fig. 4(a). The contact angle over a hierarchical surface maintains a constant value for a longer period time compared to that over a standard superhydrophobic surface.

In addition, we also examined the contact angle hysteresis by depositing a droplet over a tilted surface and observing the droplet's motion. Figure 5 shows that the droplet slides on the superhydrophobic surface with nanoparticles at a low roll-off angle (⁠$α=3.0°$⁠). Our designed hierarchical superhydrophobic surface with nanoparticles demonstrates a very low contact angle hysteresis $Δθ<2.5°$⁠. The low contact angle hysteresis and low sliding angle indicate that the superhydrophobic surface with nanoparticles has extraordinary water repellency.

Finally, to prove the concept of biosensing, we deposited droplets with different concentrations of Rhodamine 6G over both the glass substrate and the silica-coated gold nanoparticle-based superhydrophobic surface. After the droplets completely dried out, we measured the Raman signals using a LabRAM HR Raman microscope (HORIBA Jobin Yvon) with a 514.32 nm excitation laser (Fig. 6) and examined the surface-enhanced Raman scattering (SERS) sensitivity of our designed superhydrophobic surface with nanoparticles. For the glass substrate, the Raman scattering cannot be detected for 10⁻⁹ M of Rhodamine 6G. In contrast, even for 10⁻¹² M, we still can capture the Raman intensity peak for the superhydrophobic surface with nanoparticles, which is the orders of magnitude improvement. In other words, our fabricated surface has the potential for high-sensitive molecule detection.

In summary, a simple fabrication technique combining both spin coating and inverse nanotransfer printing provides an inexpensive means of transferring silica-coated metallic nanoparticles onto superhydrophobic surfaces made of PDMS over large areas. More specifically, we transferred silica-coated gold nanoparticles to fabricate the hierarchical superhydrophobic surface. Measuring the contact angle and observing the natural evaporation of a sessile droplet demonstrated that the fabricated nanoparticle-based superhydrophobic surface effectively improves the superhydrophobicity. In addition, the SERS measurements suggested that the fabricated nanoparticle-based superhydrophobic surface can significantly enhance the Raman signals. This approach can find important technological applications in biosensing.

This work was, in part, supported by the National Science Foundation under Grant No. ECCS-1509866.