Flexible superhydrophobic gold film for magnetical manipulation of droplets

https://doi.org/10.1016/j.mtchem.2021.100531Get rights and content

Highlights

  • Flexible gold film with non-wetting property was fabricated on polydimethylsiloxane substrate.

  • The synthesis protocol is cost-effective, efficient and environmental friendly.

  • The mechanism of the superhydrophobic gold film was demonstrated.

  • Non-wetting surface for performing magnetically manipulation of droplets.

  • Superhydrophobic surface with interconnected pores as a robust surface enhanced raman spectroscopy (SERS) substrate.

Abstract

Herein, we present a simple, efficient, and economical approach for the preparation of superhydrophobic gold film embedded on polydimethylsiloxane (PDMS) sheets without the requirement of surface pretreatment. The reduction reaction between chloroauric acid (HAuCl4) and sodium formate (HCOONa) at room temperature was performed to generate the aggregated gold microstructures on a PDMS sheet without chemical residuals. Superhydrophobic property was achieved when deposition time was reached to 2 h with water contact angle >160° and low contact angle hysteresis (H = 1.93°). Systematic investigations of the size, morphology, and mechanism of the generated gold films are presented. The generated gold film contains two different layers involving uniform spherical gold particles attached to the PDMS surface with the complex hierarchical structures on top. The complex structures play an important role in the superhydrophobic property, as they strongly promote the roughness to the PDMS surface. The durability of the fabricated gold film was elucidated by dropping ~7,200 waterdrops and external physical forces (e.g. stretch, bend, and twist). The main structures and their superhydrophobic properties have not disoriented after the tests. Moreover, the surface of the gold film demonstrated the potential applications as magnetical manipulation of droplets and a robust Surface enhanced Raman spectroscopy (SERS substrate).

Introduction

Superhydrophobicity is the distinctive property of the surface that affords the value of water contact angle (WCA) over 150°, together with low contact angle hysteresis and low sliding angle [[1], [2], [3], [4], [5]]. This unusual non-wetting surface creates the abilities of water repellent, self-cleaning, antisticking, and antifreezing [6,7]. All beneficial properties have been used in several applications in industrial and biological fields, such as self-cleaning solar cells [8,9], water-repellent glasses [10,11], waterproof textiles [[12], [13], [14]], blood vessel replacement [1], oil-water separation [15,16] and so on. Nevertheless, superhydrophobic surfaces have been used in magnetowetting applications where the contact angle and the morphology of ferrofluid droplets are manipulated under the applied magnetic field [[17], [18], [19], [20]]. Besides, the motion of the magnetic nanoparticle-contained droplets can be precisely controlled to present droplet-based liquid transportation, which benefits the development of the cleaning process, sample preconcentration, and biochemical sensors [[21], [22], [23], [24], [25], [26], [27]].

The lotus leaf is known as a symbol of superhydrophobic surface with the reported contact angle and contact angle hysteresis of 164° and 3°, respectively [3,5,7]. The extreme water repellent property originates from the combination between the branch-like nanostructures (120 nm in diameter) on the tips of micropapillates (5–9 μm in diameter) and the low surface energy of the epicuticular wax coated on the leaf surface [7,28,29]. Inspired by nature, scientists have attempted to develop such superhydrophobic surfaces by following these two principles: generation of suitable surface roughness using various micro- or nano-structures and utilization of low surface energy materials to cover the surface or even construct the roughness structures on a low-energy surface [5]. Several effective methods have been proposed for the preparation of superhydrophobic surfaces, for example, electrohydrodynamics [28], nanocasting [29], femtosecond laser-ablated template [5], porous polymer coating [30], plasma spray process [31], and nanosecond laser [32]. However, the usages of expensive machines and the multistep productions of superhydrophobic are still under consideration.

The deposition method to fabricate the superhydrophobic surface is seemed to be more simple, easier, and low production cost. This approach can be used to use several types of substrates, such as solid, metal, and polymer substrates. Especially, deposition methods of micro- or nano-particles on substrates have been widely studied [[33], [34], [35], [36]]. Abdelsalam et al. [37] proposed the electrochemical deposition of gold with submicrometer sphere template. Cui et al. [38] introduced the superhydrophobic surface using chemical deposition of gold nanoflower on iron foil, which provided the WCA value up to 169°. In 2008, Ishida et al. [39] presented the deposition of gold nanoparticles directly on several types of polymer bead to be used as a catalyst. Deore et al. [40] synthesized and coated gold nanoparticles on polyvinyl alcohol (PVA) sheet using low-energy electron radiation technique. Recently, Ahmed et al. [41] immersed the substrates, such as glass or silicone, into gold solution to deposit the gold nanoparticles on those substrates to monitor the wettability of the generated gold film. However, these methods involved several fabrication steps with many chemical reagents that are extremely difficult to be cleaned and removed to preserve the virgin gold surface.

Herein, we demonstrate a simple, inexpensive, and effective method to in-situ fabricate flexible superhydrophobic gold film (FSAuF) embedded on free-standing PDMS substrate, which provides the WCA value as high as 160° and displays a perfect non-wetting surface. Although there are some alternative materials which provide good hydrophobicity, similar to gold, however, gold has distinctive properties as it is very stable, inert to chemical reaction and biocompatible. Importantly, the surface of gold can be easily functionalized and applied to various applications (e.g. SERS substrate). The method involved the redox reaction between chloroauric acid (HAuCl4) and sodium formate (HCOONa) at room temperature. The developed method provides a very clean surface of gold film, as there are no residual chemicals from the redox reaction. The reaction is prolonged on free-standing polydimethylsiloxane (PDMS), allowing the deposition of the generated gold microparticles on the substrate without assisting air plasma. The deposition and the interaction between the gold microparticles on a PDMS surface are investigated via time-dependent scanning electron microscope (SEM) observations and Fourier-transform infrared (FT-IR) spectroscopy. Superhydrophobic properties of the gold film are monitored by both static and dynamic WCA measurements. We offer a new potential application of FSAuF to be used as a magnetofluidic substrate. On the substrate, the water drops could be freely moved on the FSAuF surface by applying only a magnetic field. The application can extend as a powerful microfluidic device for chemical sensing and medical sensors because the vacuum pump will not be required in the case. Moreover, the possibility of FSAuF to be used as a potential Surface enhanced Raman spectroscopy (SERS) substrate is also demonstrated.

Section snippets

Chemicals and materials

Tetrachloroauric(III) acid trihydrate (HAuCl4·3H2O) and HCOONa were purchased from Sigma–Aldrich (Singapore). Sylgard 184 silicone elastomer base and Sylgard 184 silicone elastomer curing agent were obtained from DOW Corning Corporation (Midland, USA). All reagents were analytical grade and were further used without any additional purification. Maghemite (γ-Fe2O3) nanoparticles were used as magnetic particles to transport a droplet [42]. All glassware and magnetic bars were cleaned with liquid

Formation of flexible superhydrophobic gold film

In the study, FSAuF was fabricated by one-pot synthesis using a chemical reaction between HAuCl4 and HCOONa, which acts as metal ion sources and reducing agent, respectively, at room temperature. The generation of gold particles is spontaneous because of the positive value of cell potential as follows:[AuCl4] + 3e → Au0 + 2Cl    E0 = +1.002 V[AuCl4] + 2e → [AuCl2] + 2Cl    E0 = +0.926 V[AuCl2] + e → Au0 + 2Cl    E0 = +1.154 VCO2 + 2H+ + 2e → HCOOH    E0 = −0.199 V2[AuCl4]+3HCOOH2Au0

Conclusion

Superhydrophobic gold film was successfully fabricated by a spontaneous reduction reaction between HAuCl4 and HCOONa under ambient condition on a PDMS substrate without any surface pretreatment. The relationship between deposition time and wettability of the fabricated gold films was revealed. It was found that superhydrophobic property was obtained when deposition time reached 2 h with WCA >160° and low contact angle hysteresis (H = 1.93°). By the reaction, a free pattern of non-wetting gold

Credit author statement

S. Nootchanat: Methodology, Visualization, Writing – original draft; S. Boonmeewiriya: Project administration, Investigation, Writing – original draft; A. Parnsubsakul: Validation, Visualizationl; N. Insin: Methodology, Resources; S. Ekgasit: Resources, Funding acquisition; K. Wongravee: Conceptualization, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research has been supported by the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its Research Network NANOTEC (RNN) program. S.N. is a postdoctoral fellow supported by Rachadapisek Sompote Endowment Fund, Chulalongkorn University. S.B. would like to thank Faculty of science, Chulalongkorn University, for a research assistant scholarship.

References (57)

  • B. Kaboudin et al.

    Polymer supported gold nanoparticles: synthesis and characterization of functionalized polystyrene-supported gold nanoparticles and their application in catalytic oxidation of alcohols in water

    Appl. Surf. Sci.

    (2017)
  • H. Li et al.

    Surface enhanced Raman scattering properties of dynamically tunable nanogaps between au nanoparticles self-assembled on hydrogel microspheres controlled by ph

    J. Colloid Interface Sci.

    (2017)
  • K.H. Tan et al.

    Fabrications of nanocomposite gold-polymer metamaterials consisting of periodic microcavities with tunable optical properties

    Optik

    (2017)
  • S. Cui et al.

    Fabrication of robust gold superhydrophobic surface on iron substrate with properties of corrosion resistance, self-cleaning and mechanical durability

    J. Alloys Compd.

    (2017)
  • T. Ishida et al.

    Direct deposition of gold nanoparticles onto polymer beads and glucose oxidation with H2O2

    J. Colloid Interface Sci.

    (2008)
  • A.V. Deore et al.

    Low-energy electron irradiation assisted diffusion of gold nanoparticles in polymer matrix

    Radiat. Phys. Chem.

    (2014)
  • D. Sebastia-Saez et al.

    Effect of the contact angle on the morphology, residence time distribution and mass transfer into liquid rivulets: a CFD study

    Chem. Eng. Sci.

    (2018)
  • C. Chen et al.

    Water contact angles on quartz surfaces under supercritical co2 sequestration conditions: experimental and molecular dynamics simulation studies

    Int. J. Greenh.

    (2015)
  • B.S. Maritz et al.

    Kinetics and mechanism of the reduction of tetrachloroaurate(iii) by formate in acidic aqueous solution

    J. Inorg. Nucl. Chem.

    (1976)
  • M.C. Lim et al.

    Facile preparation of gold-coated polydimethylsiloxane particles by in situ reduction without pre-synthesized seed

    Colloids Surf., A

    (2017)
  • K. Park et al.

    In situ synthesis of directional gold nanoparticle arrays along ridge cracks of PDMS wrinkles

    Colloids Surf., A

    (2018)
  • F. Yazdani et al.

    Magnetite nanoparticles synthesized by co-precipitation method: the effects of various iron anions on specifications

    Mater. Chem. Phys.

    (2016)
  • J. Yong et al.

    Controllable adhesive superhydrophobic surfaces based on PDMS microwell arrays

    Langmuir

    (2013)
  • B. Bhushan

    Biomimetics: lessons from nature–an overview

    Philos. Trans. R. Soc. A

    (2009)
  • X.-M. Li et al.

    What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces

    Chem. Soc. Rev.

    (2007)
  • D. Gong et al.

    Robust and stable transparent superhydrophobic polydimethylsiloxane films by duplicating via a femtosecond laser-ablated template

    ACS Appl. Mater. Interfaces

    (2016)
  • X. Yao et al.

    Applications of bio-inspired special wettable surfaces

    Adv. Mater.

    (2011)
  • S. Kumar et al.

    Transparent alumina based superhydrophobic self–cleaning coatings for solar cell cover glass applications

    Sol. Energy Mater. Sol. Cells

    (2017)
  • Cited by (3)

    e

    These authors contributed equally to this article.

    View full text