Materials
Sodium sulfide nonahydrate (98%), sodium thiomethoxide (95%), ethanethiol (99.7%), ethylmethyl sulfide (96%), potassium metabisulfite (98%) were obtained from Sigma-Aldrich (Castle Hill, NSW, Australia). Tartaric acid and sodium chloride were obtained from Merck (Frenchs Forest, NSW, Australia); absolute ethanol from Rowe Scientific (Lonsdale, SA, Australia); and copper (II) sulfate pentahydrate was obtained from Ajax Chemicals (Sydney, NSW, Australia). Water was obtained from a Milli-Q purification system (Millipore, North Ryde, NSW, Australia).
Allylamine (AA) (reagent grade, 98%) and 2-methyl-2-oxazoline (POx) (98%) were obtained from Sigma-Aldrich (Australia) and used as provided. Microscope slides and 100-mesh stainless steel sheets were used as substrates for plasma deposition.
Analysis of volatile sulfur compounds
Volatile sulfur compounds were analyzed using gas chromatography with sulfur chemiluminescence detection (GC-SCD) as described in Siebert et al.7. Tropical sulfhydryls were measured using LCMS according to Capone et al.9.
Flowering experiments
Model wine solutions were used to evaluate the effectiveness of surfaces in removing VSCs, to determine the optimal treatment time to remove VSCs, as well as to evaluate whether SO2 interferes with the smart surface’s ability to remove unwanted VSC from the wine. Oxygen free model wine (<1 ppb oxygen) was prepared in an anaerobic hood by adding degassed ethanol (<1 ppb oxygen) to degassed MilliQ water (<1 ppb oxygen) previously pH adjusted to pH 3.6 using tartaric acid . Oxygen-free model wine (10 mL) was added to 22 mL amber vials inside an anaerobic hood (<1 ppb oxygen). Stock solutions of hydrogen sulfide, methanethiol, and ethanethiol were prepared in an anaerobic hood using degassed MilliQ water and the stock solutions were added to the model wine to give final concentrations of approximately 25 μg/L for each VSC. Intelligent surfaces were then added to the vials containing the wine model with VSCs inside the anaerobic hood and the vials sealed with solid PTFE caps. To evaluate the different coating materials, VSC analysis was performed after 24 h. To determine the best treatment time, VSC analysis was performed after 3 h, 6 h, and 24 h.
To assess the effect of SO2 of VSC binding to the nanoengineered surface, the oxygen-free SO2 Solutions were prepared in an anaerobic hood using degassed Milli Q water and then added to oxygen-free model wine to give the final SO2 concentrations of 10, 20, and 30 mg/L. The smart surfaces were inserted into an oxygen-free model wine with hydrogen sulfide and sulfur dioxide, the vials sealed with solid PTFE caps, and stored in an anaerobic hood for 24 hours. Treated wine samples were removed from the anaerobic hood after 24 h and VSC analysis was performed.
Chardonnay, Sauvignon Blanc and Shiraz wines from the 2020 and 2021 vintages produced in South Australia are sourced from local wineries. Wines were prescreened for VSC concentrations and wines with naturally high levels of hydrogen sulfide, methanethiol, and ethanethiol were selected for this test. These wines were used to determine the effectiveness of the surface in removing VSCs naturally present in the wine, to compare the effectiveness of the smart surface compared to copper painting, and to determine whether removal are of wise contents the preferred tropical sulfhydryls. To evaluate the surface’s effectiveness in removing VSCs naturally present in wine, smart surfaces were placed inside 42 mL vials inside an anaerobic hood, 40 mL of each wine was added, and the vials sealed with solid PTFE caps and stored indoors. anaerobic hood for 24 hours.
To compare the effectiveness of VSC removal between copper fining and remediation using a smart surface, oxygen-free copper solutions were prepared in an anaerobic hood using degassed MilliQ water and added to a subset of wine (40 mL) to give the final concentration of 0.1 mg. /L copper. The vials were sealed with solid PTFE caps and stored in an anaerobic hood for 24 hours.
To evaluate if the smart surfaces remove desirable tropical sulfhydryls, the smart surfaces were placed in 150 mL Schott bottles inside an anaerobic hood, wine (120 mL) with a natural concentration of 4-MSP , 3-SH and 3-SHA were added, the containers were sealed with solid PTFE caps and stored in an anaerobic hood for 24 h.
All treated wines were removed from the anaerobic hood after 24 h and VSC analysis was performed as described by Siebert et al. (2010). All samples were prepared in triplicate.
Plasma polymerization
Allylamine (AA) (reagent grade, 98%) and 2-methyl-2-oxazoline (POx) (98%) were obtained from Sigma-Aldrich (Australia) and used as supplied. Glass microscope slides and 100-mesh stainless steel sheets were used as substrates for plasma deposition. Plasma polymerization was performed in a custom-built reactor equipped with a 13.56 MHz plasma generator.10. Allylamine was deposited at a precursor pressure of 0.13 mbar, and 2-methyl-2-oxazoline at 0.08 mbar. The power used for placing the two monomers is 40 W and 50 W, respectively. In both cases, the plasma deposition time is two minutes. Before deposition, all surfaces were cleaned by using air plasma for 2 min at 50 W.
Synthesis of gold nanoparticles (AuNPs)
Gold nanoparticles were synthesized by reducing hydrogen tetrachloroaurate (HAuCl4) with trisodium citrate. A 50 mL solution of 0.01% HAuCl4 was brought to boiling temperature with vigorous stirring. Under vigorous stirring, a 1% aqueous solution of trisodium citrate (TSC) was added. To achieve particle sizes of 38 and 68 nm in diameter, 0.5 mL and 0.3 mL of TSC were added, respectively. After the addition of trisodium citrate, the color of the solution changed from light yellow to wine red within minutes. The solution was kept for an additional 20 minutes at boiling temperature and then cooled to room temperature.11.
Immobilization of gold nanoparticles
Plasma-polymerized allylamine and 2-methyl-2-oxazoline coated surfaces were immersed for 24 hours in 38 and 68 nm AuNPs solution. Allylamine has a positive charge when placed in an aqueous solution, while the carboxylic acid groups functionalizing the AuNPs have a net negative charge. Immersion of AA-coated surfaces in AuNPs solution leads to strong electrostatic binding of nanoparticles to the surface. After binding the gold nanoparticles, the surfaces were washed with water to remove the bound nanoparticles and dried in a vacuum. In the case of POx, these polymer coatings in plasma are known to retain a population of intact oxazoline rings that bind covalently to nanoparticles and other entities that carry COOH functionalities.12,13.
X-ray photoelectron spectroscopy (XPS)
XPS spectra were obtained using a Kratos Axis Ultra XPS spectrometer (Kratos Analytical Ltd, UK) with a monochromatic Al source and operated at 15 keV and 15 mA to obtain a survey spectrum from 0 eV to 1100 eV for all surface coatings. To compensate for the surface charge effects, all binding energies are focused on the C1s neutral carbon peak at 285 eV. CasaXPS software was used for processing and curve fitting.
Thickness measurements
The thickness of the deposited plasma polymers was determined using a variable angle ellipsometer (VASE, JA Woolam Co. USA). Experimental data were analyzed with the software WVASE32 (JA Woolam). The optical properties of the silicon wafer and the native oxide layer are extracted from the software. A refractive index of 1.5514 It is assumed that for all plasma polymer layers.
Contact angle
The contact angle was measured using the sessile drop method with a custom-made contact angle goniometer. A drop of water is placed on top. Images of the droplet were taken using a horizontal digital microscope. Contact angles were determined by drawing a tangent near the edge of the drop using the drop shape analysis software ImageJ with the DropSnake plugin. Experiments were conducted at room temperature in a clean room.
Fourier transform infrared spectroscopy (FTIR)
An IRTracer-100 FTIR spectrometer (Shimadzu) equipped with a liquid nitrogen cooled MCT detector was used for all measurements. Measurements were performed using a Quest Single Reflection ATR Accessory (Specac), equipped with a diamond ATR crystal. In all cases, 128 scans with a resolution of 4 cm-1 taken to obtain a satisfactory signal-to-noise ratio. The ATR effect and atmospheric contributions from carbon dioxide and water vapor are corrected by the background produced by an empty ATR device.
Scanning electron microscopy (SEM)
SEM was used to determine the morphology and density of gold nanoparticles immobilized on the surface. An FEI Quanta 450 FEG-ESEM equipped with an EDAX Apollo X Energy Dispersive X-Ray (EDX) spectrometer was used for the analysis. SEM images were analyzed using Image J software. For calculating the number of nanoparticles per μm2,% surface coverage, and interparticle distance we prepared three samples per nanoparticle size. These samples were analyzed by taking three images per sample.
Statistical analysis
The significance of the data was evaluated by the Student t attempt. Data are presented as means ± (SD). Q<0.05 was considered statistically significant. All experiments were repeated at least three times. Figures were prepared using Origin 6.0 and CorelDRAW 11 software.
Reporting summary
Additional information on research design is available in the Nature Research Reporting Summary linked to this article.
