Autocatalytic Metallization of Fabrics Using Si Ink, for Biosensors, Batteries and Energy Harvesting

Abstract Commercially available metal inks are mainly designed for planar substrates (for example, polyethylene terephthalate foils or ceramics), and they contain hydrophobic polymer binders that fill the pores in fabrics when printed, thus resulting in hydrophobic electrodes. Here, a low‐cost binder‐free method for the metallization of woven and nonwoven fabrics is presented that preserves the 3D structure and hydrophilicity of the substrate. Metals such as Au, Ag, and Pt are grown autocatalytically, using metal salts, inside the fibrous network of fabrics at room temperature in a two‐step process, with a water‐based silicon particle ink acting as precursor. Using this method, (patterned) metallized fabrics are being enabled to be produced with low electrical resistance (less than 3.5 Ω sq−1). In addition to fabrics, the method is also compatible with other 3D hydrophilic substrates such as nitrocellulose membranes. The versatility of this method is demonstrated by producing coil antennas for wireless energy harvesting, Ag–Zn batteries for energy storage, electrochemical biosensors for the detection of DNA/proteins, and as a substrate for optical sensing by surface enhanced Raman spectroscopy. In the future, this method of metallization may pave the way for new classes of high‐performance devices using low‐cost fabrics.

: SEM image of Si ink drop-casted onto cellulose paper. Self-aggregation of Si particles is apparent amidst the partial coverage of cellulose fibers, which is inhomogeneous at micron scale. Figure S2: Energy-dispersive X-ray spectroscopy spectra of Ag, Au and Pt deposited with the SIAM method inside paper, taken with a SEM at 5keV beam energy. Figure S3: The maximum tensile stress withstood by paper before breaking decreases after 20 min. soaking in water, and after soaking in HF (5%). The maximum strain before breaking observed for each sample was similar (0.05±0.01). Samples measured 30×5 mm 2 and 5 repeats were done for each case, with errors corresponding to the standard deviation.
Mechanical tests were performed using an Instron 5543 tensile tester (1 kN load cell, Bluehill 3 software). The Ag layer is approximately 20 µm thickness and therefore we approximate a deposition rate of 2 m min -1 .

Figure S5
: Energy-dispersive X-ray spectroscopy spectrum of Ag deposited by SIAM on paper, taken in a SEM at 20 keV beam energy, demonstrate the presence of residual Si.

Figure S6:
In the bottom photograph the left half of a strip of paper is printed with orange wax and the right half with Si ink. Ag is then electrolessly deposited on the Si ink (by SIAM method). On the top photograph, Ag is also deposited over the wax in the middle section. The optical microscope image of the cross-section shows Ag is deposited above the wax on the left side, whereas on the right side it is deposited through the full cross-section of the paper. Figure S7: Metallized paper substrates have been bent by attaching between two clamps, which were then drawn closer together (S3 inset diagram). The length was measured before (initial length) and after bending (final length). Bending distance is the difference between these lengths. The corresponding change in resistance R, normalized to the sample resistance before bending R 0 , shows a gradual linear increase approaching 12 mm bending distance, at which point the conductive pathways began to separate, and higher variance is obtained. There were n = 6 repeats for samples from 2 to 14 mm bending distance, and n = 2 repeats for 16 mm bending distance as the samples began to break.

Calculation of Electrocatalytically Active Area:
The dependence of the peak current on the scan rate was evaluated by cyclic voltammetry sweeping the potential from -0.8 to +1.0 V vs Ag/AgCl at 10, 25, 50, 75, 100, 150 and 200 mV s -1 . As the inset in Figure S4 shows, a standard Ag/AgCl reference electrode (Fisherbrand Accumet model from FisherScientific, NL) and a graphite rod (Sigma Aldrich), as counter electrode, were placed close to the surface of the Ag-Si paper electrode (2 cm 2 of geometric area) which acts as working electrode. The electrochemical cell was connected to the potentiostat (PalmSens3 model from PalmSens, UK) with crocodile clamps. Then, 1 mL of 1 mM K 4 Fe(CN) 6 solution in 0.1 M KCl was slowly added on the paper electrode, wetting the whole electrochemical cell and avoiding solvent evaporation during the experiment. According to the Randles-Sevcick equation for a flat electrode and for diffusion-controlled processes at 25 °C. [1][2][3][4] = (2.69×10 5 ) 3/2 1/2 1/2 Where is the peak current (A), is the number of electrons transferred (n = 1 for ferrocyanide), the effective area of the electrode (cm 2 ), is the diffusion coefficient of ferrocyanide in aqueous solutions (6.50 10 -6 cm 2 s -1 ), is the concentration (1 mol cm -3 ) and is the scan rate (V s -1 ).
Cyclic voltammograms, as those shown in Figure S4      Cost of SIAM: The amount of Ag metal deposited by SIAM of paper is 0.004 g cm -2 . This was calculated by removing Ag from SIAM in a HNO 3 bath, and then weighing the difference. The amount of Ag deposited from commercial ink is 0.053 g cm -2 (Ag/AgCl 50:50 Paste from Gwent Group, UK). The SIAM process requires, therefore, an order of magnitude less Ag per unit area.
Using SIAM, a square centimeter of paper may be metallized with 0.1 ml 1M AgNO 3 salt, costing $0.40 g -1 , therefore, the cost of SIAM of paper is $0.007 cm -2 . With commercial ink costing $2 g -1 , the cost of metallization of paper is $0.1 cm -2 , which is more than an order of magnitude greater.