Single-step ambient-air synthesis of graphene from renewable precursors as electrochemical genosensor

Thermal chemical vapour deposition techniques for graphene fabrication, while promising, are thus far limited by resource-consuming and energy-intensive principles. In particular, purified gases and extensive vacuum processing are necessary for creating a highly controlled environment, isolated from ambient air, to enable the growth of graphene films. Here we exploit the ambient-air environment to enable the growth of graphene films, without the need for compressed gases. A renewable natural precursor, soybean oil, is transformed into continuous graphene films, composed of single-to-few layers, in a single step. The enabling parameters for controlled synthesis and tailored properties of the graphene film are discussed, and a mechanism for the ambient-air growth is proposed. Furthermore, the functionality of the graphene is demonstrated through direct utilization as an electrode to realize an effective electrochemical genosensor. Our method is applicable to other types of renewable precursors and may open a new avenue for low-cost synthesis of graphene films.

thermal CVD approaches for the production of graphene films.

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Ruoff et al. S1 Kim et al. S2 Tour et al. S3 Bae et al. S4 This method Activated GO/Graphite interface 5.6 x 10 -12 M ssDNA [S7] Supplementary Note 1. Calculation of oxygen consumption in the reactor during the growth using soybean oil.

Carbon precursor
(i) Using the dimensions of the quartz tube, the volume of the growth chamber was calculated (0.00196 m 3 ).
(ii) Providing the dimensions of the Ni foils (4 cm x 2 cm), the surface area of the Ni foils was calculated (as double sided, giving a total of 16 cm 2 ).
(iii) We clarify as previously provided that the amount of carbon source is 0.14 mL of soybean oil, which is a liquid under ambient conditions.
(iv) Calculations to demonstrate that the amount of solid carbon sources is sufficient to consume all the O2 in the growth chamber.
In considering consumption of O2 by the carbon in the growth chamber, we emphasise that this will be a complex process due to the decomposition of soybean oil yielding numerous molecular fragments which consume O2 through different reaction pathways. This is clear from our results presented in Supplementary Fig. 1 which shows the variety of products (e.g. H2, C, CH3, C2H2, C2H5, C2H6 etc.) from the soybean oil precursor at different temperatures, from 300 to 600 °C. The likely combustions reactions include: Using the growth chamber dimensions and STP conditions, it is calculated that 0.0168 mol O2 (g) is present. Also, it is noted that at the temperatures involved in the ambient-air process, CO2 does not undergo further decomposition.
Using the average density of soybean oil (0.917 g mL -1 ) and an average chemical composition (linoleic acid -52%, oleic acid -25%, palmitic acid -12%, linolenic acid -6%, stearic acid -5%), it is calculated that ~0.0081 mol of C and ~0.0151 mol of H were present in the growth chamber (n.b. an additional ~0.0001 mol of O from soybean oil is also present, which we do not consider further).
If O2 was only consumed through the reaction of C (reaction (1) above), then O2 would be slightly in excess with a remainder of 0.0087 mol.
However, all other reaction pathways have a greater consumption rate of O2. For instance, if O2 was solely consumed through the reaction of C2H5 (reaction (3) above), then all the O2 will be expended and C will be in excess with a remainder of 0.0035 mol.
We recognise that all these reaction pathways will likely proceed, and so the combined consumption of O2 will yield an excess of C in the chamber. Furthermore, we noted that the presence of O2 could be non-uniform in the growth chamber, given that the temperature inside and outside the hot-walled furnace were vastly different. This could lead to the local environment in the immediate vicinity of the soybean oil precursor and Ni foils to have a significantly lower concentration of O2. The calculations thus present an upper limit in estimating the amount of O2 to be consumed by the soybean oil precursor.
We therefore can conclude that the amount of carbon source we use in the experiment-0.14 mL of soybean oil-is sufficient to consume the O2 in the growth chamber, yielding an excess of C from which our graphene can form.

Supplementary Note 2. Estimation of carrier mobility for the graphene film.
The carrier mobility of the graphene film is estimated from the defect density in the film, (iii) Substituting these variables into La, and calculating for the defect density, yields (1/La) 2 ranging from 8.26x10 9 to 2.27x10 10 cm -2 , respectively, for the lower and upper bounds of the ID/IG ratios.
(iv) Consequently, by reference to the work by Hwang et al., [S8] which correlates the defect density to the carrier mobility, we may provide an estimate for our film mobility, in the order of 500 -750 cm 2 V -1 s -1 . In addition, such mobility is in accordance with Salehi-Khojin et al. [S9] and Chen et al., [S10] , where a similar morphology, grain size, and defect level in the graphene films were seen.

Supplementary Note 3. Competitive advantages of the present ambient-air graphene synthesis method.
Graphene production inherits high costs and complexities. This impedes its commercial viability.
However, this ambient-air technique provides a significantly cheaper, greener, simpler and safer approach for the synthesis of graphene, as compared to the conventional thermal CVD methods (Supplementary Table 1 and Table 2). We attribute this to a key feature unique to this single-step thermal process, the growth of graphene in an ambient-air environment. Consequently, purified gases (e.g., argon, hydrogen, methane) that are expensive and hazardous are not required.
Instead, a safe, minimally-processed renewable precursor (soybean oil) functions as the source of carbon, and the ambient-air environment is tailored to enable the growth of graphene films.
In the conventional thermal CVD methods, the processing chamber is firstly evacuated to remove the ambient air. Next, the processing chamber is brought up to atmospheric pressure by filling the processing volume with purified gases. Finally, these purified gases are constantly circulated with extensive vacuum operation over a prolonged duration. These processes maintain an optimal flow of purified gases to enable the growth of graphene.
In the ambient-air process for graphene synthesis, these conventional steps are not necessary.
Instead, graphene growth is promoted by direct control of the precursor content, process parameters (e.g., cooling rate, temperature, etc.), and ambient-air environment, without the use of any purified gases, in a single-stepped approach. As such, this ambient-air process has the potential to be easily integrated into existing graphene manufacturing infrastructures.