Research Article |
Corresponding author: J. Ruud van Ommen ( j.r.vanommen@tudelft.nl ) Academic editor: DJ Monsma
© 2023 Albert Santoso, Bart J. van den Berg, Saeed Saedy, Eden Goodwin, Volkert van Steijn, J. Ruud van Ommen.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Santoso A, van den Berg BJ, Saedy S, Goodwin E, van Steijn V, van Ommen JR (2023) Robust surface functionalization of PDMS through atmospheric pressure atomic layer deposition. Atomic Layer Deposition 1: 1-13. https://doi.org/10.3897/aldj.1.105146
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Polydimethylsiloxane (PDMS) has been widely employed as a material for microreactors and lab-on-a-chip devices. However, in its applications, PDMS suffers from two major problems: its weak resistance against common organic solvents and its chemically non-functional surface. To overcome both issues, atmospheric pressure atomic layer deposition (AP-ALD) can be used to deposit an inorganic nanolayer (TiOx) on PDMS that, in turn, can be further functionalized. The inorganic nano layer is previously communicated to durably increase the organic solvent resistance of PDMS. In this study, we investigate the possibility of this TiOx nano layer providing surface anchoring groups on PDMS surfaces, enabling further functionalization. We treat PDMS samples cured at three different temperatures with AP-ALD and measure the hydrophilicity of the treated samples as an indicator of the presence of surface anchoring groups. We find that all the treated PDMS samples become hydrophilic right after the AP-ALD treatment. We further find that the AP-ALD-treated PDMS samples cured at 150 °C and 200 °C maintain their hydrophilicity, while the samples cured at 70 °C become less hydrophilic over time. The presence of surface anchoring groups through TiOx nano layer deposition on PDMS is further demonstrated and utilized by depositing gold nanoparticles (AuNPs) on the AP-ALD-treated samples. The samples exhibit visible light absorbance at 530 nm, a typical absorbance peak for AuNPs. In conclusion, this study demonstrates the use of nano layers grown by AP-ALD to solve the two major problems of PDMS simultaneously, widening its applicability, especially for use in high-end applications such as catalysis and bio-sensing.
PDMS, Atmospheric ALD, Titanium dioxide, wettability, functionalization
In the fields of medicine, biology, and chemistry, there is a rising trend of using polydimethylsiloxane (PDMS) as a material for microfluidics and protective coatings [
To overcome the limitations of PDMS, researchers explored two main directions: bulk modification and surface treatment [
Among all vapor deposition methods, atomic layer deposition is renowned as it faithfully coats complex nanostructures with a wide variety of coating materials properties [
In this paper, we follow up on our previous work by extending the use of the atmospheric pressure ALD (AP-ALD) of titanium oxide (TiOx) to functionalize the PDMS substrate. While TiOx has been demonstrated to provide a protective layer against organic solvents [
Illustration of modification of PDMS through deposition of a TiOx nano layer and further functionalization through the deposition of gold nanoparticles (AuNPs). The deposition of a TiOx nano layer both increases the organic solvent resistance of PDMS and renders the surface hydrophilic through hydroxyl groups. These groups act as anchoring points for further functionalization, here shown for AuNPs.
PDMS samples were fabricated by mixing the elastomer and curing agent (Sylgard 184 Elastomer Kit, Dow Corning) in a ratio of 10:1. After manual stirring of the mixture for 2 min, trapped air bubbles were removed using a vacuum desiccator for 30 min. The degassed mixture was poured into a Petri dish and placed in the vacuum desiccator again for another 30 min. PDMS was then cured in an oven at 70 °C, 150 °C, or 200 °C for at least 10 hours. Samples of 25 mm × 30 mm (0.5 mm thick) were cut from the cured PDMS and bonded on a glass slide after plasma treatment (oxygen in air, Harrick, PDC-002) at 0.2–0.4 mbar for 140 s.
TiOx was deposited using a home-built atmospheric-pressure setup with a tubular flat-substrate reactor, where the precursor was delivered parallel to the substrate, as described previously [
To demonstrate the ability to functionalize the TiOx-treated PDMS surface, we performed a second AP-ALD treatment in which we deposited gold nanoparticles (AuNPs) on TiOx-treated PDMS samples cured at 150 °C. AuNPs were deposited using trimethylphosphino-trimethyl gold(III) (6-Me, prepared according to literature procedure [
In order to characterize the surface morphology, field emission scanning electron microscopy (FE-SEM, Hitachi Regulus SU8230) at a beam current of 1–5 µA and electron energy of 1–10 keV was conducted. To obtain the surface elemental information, X-ray photoelectron spectroscopy (XPS) (ThermoFisher Scientific Nexsa) equipped with a monochromatic Al Kα radiation source and a pass energy of 30 and 100 eV for the survey scan and ion-beam etching unit was used. Depth profiling was conducted by etching the surface using Ar+ ions (2 keV with a raster size of 2 mm) while a flood gun was used to compensate for the differential charging. Thermo Avantage 5.913 and CASA-XPS software were used to post-process the XPS peak profile, where the spectra were charge-corrected with the adventitious carbon peak at 284.8 eV. The thickness of the TiOx layer was approximated using an etch rate obtained in our previous study [
To obtain the particle size distribution of AuNPs, ALD-coated PDMS samples were sonicated in HNO3 1 M (Merck Sigma) for 15 min and left immersed for 48 h to dissolve the TiOx layer under the AuNPs and disperse them into the solution. The solution was then centrifuged (MicroCL 21/21R, Thermoscientific) at 14800 rpm for 10 min and the supernatant was decanted as much as possible prior to being washed three times with ethanol (96%) and transferred onto Quantifoil copper TEM grids (coated with carbon). Transmission electron microscopy (TEM) images of the AuNPs were acquired using a JEOL JEM1400 microscope operating at a voltage of 120 kV working in bright-field mode. The average particle size and particle size distribution curves were obtained using the diameter of more than 350 individual particles analyzed using ImageJ.
To quantify the surface wetting property, the dynamic contact angles were measured using a Krüss drop shape analyzer (Suppl. material
To examine our hypothesis about the presence of uncured monomer in PDMS that diffuses out and compromises the AP-ALD coating, we pre-washed the PDMS samples to remove non-crosslinked molecules from the PDMS [
To measure the photonic effect of the TiOx-treated PDMS surface functionalized with AuNPs, light absorption was measured with a wide scan reading (300–800 nm) using a NanoDrop™ 2000/2000c spectrophotometer. The reported value was an average of 3 measurements via the typical uncertainty (2 s.d.).
Before examining the treated samples, we point out that deposition of TiOx on PDMS may result in both surface and subsurface growth, and therefore is more ambiguous to characterize [
Of note, XPS milling may influence the XPS reading. In case this artifact is significant, we expect it to influence the profiles obtained for different numbers of cycles in a similar way. Yet, when inspecting, for example, the profiles obtained with 50 and 100 cycles in Suppl. material
We use hydrophilicity as an indicator for the presence of surface anchoring groups (in this case hydroxyl groups) on the ALD-coated PDMS surfaces. More specifically, we use the contact angle of a droplet of water brought into contact with the coated surfaces. Although the observed angle depends on the way of contacting, it is bounded by the contact angle hysteresis range, with an upper and lower bound intrinsic to the surface. The upper bound, called the advancing contact angle, is taken as the relevant characteristic in this work. While the range is known to depend on the roughness of the surfaces, we confirm the roughness of the samples’ surfaces to be similar [
A well-known phenomenon in the coating of PDMS samples is the gradual temporal change in surface composition leading to the recovery of the hydrophobicity [
The AP-ALD coating on PDMS samples cured at 70 °C renders the surface hydrophilic over prolonged times, yet it is subject to change due to the out-diffusion of uncured monomers. Earlier studies have suggested that curing of PDMS at higher temperatures changes the internal crosslinking of PDMS, leading to a tighter network [
Since treated PDMS surfaces may usually not be stored in a nitrogen environment in a glove box, we performed an additional experiment where AP-ALD-coated samples (cured at 70 °C, 150 °C, and 200 °C) were kept in the air at ambient conditions in clean storage. The advancing contact angle after 800 h is 83°±4°, 72°±2°, and 71°±2° respectively, comparable to the samples stored in the nitrogen environment in the glove box. This also shows that both curing at temperature 150 °C and 200 °C and deposition of an (intact) surface-subsurface layer through AP-ALD are essential in obtaining a stable hydrophilic TiOx coating on PDMS. This stable hydrophilicity, along with a significant percentage of TiOx on the surface, also indicates a high surface coverage of hydroxyl groups (estimated to be in the order of 1014 molecules per cm2, [
To demonstrate the ability to functionalize the PDMS surface, we deposit gold nanoparticles (AuNPs) on AP-ALD-coated PDMS samples. Figure
Scanning electron microscopy image of PDMS samples coated with 5 cycles of Au precursors preceded by (a) 100 cycles and (b) 0 cycles of Ti precursors on PDMS samples cured at 150 °C. Both AP-ALD processes were conducted at 100 °C. The insets show the corresponding XPS spectra at the typical Au4f binding energy range. (c) UV-Vis spectra of PDMS, PDMS coated with TiOx, and PDMS coated with TiOx and AuNPs. The insets show a photograph of the three samples, a TEM image of the TiOx-AuNPs-coated PDMS sample, and the corresponding particle size distribution of the AuNPs.
In conclusion, we show that a nano layer of metal oxide can be deposited on the transparent soft elastomer PDMS, providing it with the necessary hydroxyl groups to allow further functionalization of the surface. We find that the surface-subsurface TiOx layer obtained through atmospheric pressure ALD is critical in providing a surface that remains hydrophilic and stable over prolonged periods of time, with reduced out-diffusion of uncured monomer. For stable hydrophilic TiOx layers that do not display hydrophobic recovery, curing of PDMS at a temperature of at least 150 °C is found to be of key importance. In comparison with PE-ALD and Th-ALD, AP-ALD offers a robust nano layer that can be further functionalized. We illustrate the further functionalization of the TiOx-coated PDMS surfaces by depositing gold nanoparticles, also through ALD, opening the opportunity window to high-end applications.
There are no conflicts of interest to declare.
This publication is part of the Open Technology programme (with project number 16913) financed by the Dutch Research Council (NWO). We thank Mojgan Talebi, Joost Middelkoop, and Bart Boshuizen for their technical support. We acknowledge that Figure
Supplementary images
Data type: figures (PDF file)
Explanation note: fig. S1: Measurement of the advancing (left) and receding contact angle (right) on AP-ALD-treated PDMS samples. In the main manuscript, we report the advancing contact angles. fig. S2: (a) Approximated TiOx layer thickness obtained from XPS depth profiling with the etch rate determined using a TiOx layer on a silicon wafer as described previously [