Materials Today Chemistry
Flexible superhydrophobic gold film for magnetical manipulation of droplets
Graphical abstract
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 V
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)
- et al.
Superhydrophobic surfaces
Curr. Opin. Colloid Interface Sci.
(2006) - et al.
Bio-inspired design of multiscale structures for function integration
Nano Today
(2011) - et al.
Durable superhydrophobic surface with highly antireflective and self-cleaning properties for the glass covers of solar cells
Appl. Surf. Sci.
(2018) - et al.
Endurable water-repellent glass for automobiles
Thin Solid Films
(1999) - et al.
Simple treatment of cotton textile to impart high water repellent properties
Appl. Surf. Sci.
(2010) - et al.
Flexible 3d porous superhydrophobic composites for oil-water separation and organic solvent detection
Mater. Des.
(2020) - et al.
Droplet-based gene expression analysis using a device with magnetic force-based-droplet-handling system
J. Biosci. Bioeng.
(2010) - et al.
Fabrication of micro-/nano-structured superhydrophobic ceramic coating with reversible wettability via a novel solution precursor vacuum plasma spray process
Mater. Des.
(2018) - et al.
Hierarchical micro- and nanostructures induced by nanosecond laser on copper for superhydrophobicity, ultralow water adhesion and frost resistance
Mater. Des.
(2018) - et al.
Polymer mediated layer-by-layer assembly of different shaped gold nanoparticles
J. Colloid Interface Sci.
(2017)
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.
Surface enhanced Raman scattering properties of dynamically tunable nanogaps between au nanoparticles self-assembled on hydrogel microspheres controlled by ph
J. Colloid Interface Sci.
Fabrications of nanocomposite gold-polymer metamaterials consisting of periodic microcavities with tunable optical properties
Optik
Fabrication of robust gold superhydrophobic surface on iron substrate with properties of corrosion resistance, self-cleaning and mechanical durability
J. Alloys Compd.
Direct deposition of gold nanoparticles onto polymer beads and glucose oxidation with H2O2
J. Colloid Interface Sci.
Low-energy electron irradiation assisted diffusion of gold nanoparticles in polymer matrix
Radiat. Phys. Chem.
Effect of the contact angle on the morphology, residence time distribution and mass transfer into liquid rivulets: a CFD study
Chem. Eng. Sci.
Water contact angles on quartz surfaces under supercritical co2 sequestration conditions: experimental and molecular dynamics simulation studies
Int. J. Greenh.
Kinetics and mechanism of the reduction of tetrachloroaurate(iii) by formate in acidic aqueous solution
J. Inorg. Nucl. Chem.
Facile preparation of gold-coated polydimethylsiloxane particles by in situ reduction without pre-synthesized seed
Colloids Surf., A
In situ synthesis of directional gold nanoparticle arrays along ridge cracks of PDMS wrinkles
Colloids Surf., A
Magnetite nanoparticles synthesized by co-precipitation method: the effects of various iron anions on specifications
Mater. Chem. Phys.
Controllable adhesive superhydrophobic surfaces based on PDMS microwell arrays
Langmuir
Biomimetics: lessons from nature–an overview
Philos. Trans. R. Soc. A
What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces
Chem. Soc. Rev.
Robust and stable transparent superhydrophobic polydimethylsiloxane films by duplicating via a femtosecond laser-ablated template
ACS Appl. Mater. Interfaces
Applications of bio-inspired special wettable surfaces
Adv. Mater.
Transparent alumina based superhydrophobic self–cleaning coatings for solar cell cover glass applications
Sol. Energy Mater. Sol. Cells
Cited by (3)
- e
These authors contributed equally to this article.