Tailoring the Meso-Structure of Gold Nanoparticles in Keratin-Based Activated Carbon Toward High-Performance Flexible Sensor

Highlights Materials with contradictory performance were prepared using wool keratin (WK). WK used for in situ preparation of AuNPs and N-doped carbon precursor as well. Two- and three-electrode flexible strip sensors designed for pH and UA detection. Electronic supplementary material The online version of this article (10.1007/s40820-020-00459-5) contains supplementary material, which is available to authorized users.


S1.2 Preparation of Regenerated WK Solution
The raw wool fibers were treated to extract WK via reduction method 1 . Accurately weighted 12.5 g pre-treated wool fibers were added in a 250 mL aqueous solution containing urea (4 M), Na2S (0.1 M), and SDS (0.02 M). The above mixture was stirred at 50 °C for 12 h with subsequent filtration. The filtrated stock was dialyzed against ultrapure water (18 MΩ) using a dialysis bag (Solarbio, molecular cut-off about 3500 Da) for 3 d. The dialyzed solution was concentrated at 60 °C for 8 h, and the concentration of WK solutions was 5 wt% on average. The obtained WK solutions were stored at 4 °C before further usage.

S1.3 Preparation of AuNCs@WK
In a typical experiment, 5 mL aqueous solution of HAuCl4 (10×10 −3 M) was added to 5 mL WK solution (2.5 wt%) under vigorous stirring. The pH of reaction mass is adjusted by addition of a 0.5 mL NaOH solution (1 M) 10-12, which was then retained at 45 °C for 12 h. The acquired AuNCs@WK were preserved at 4 °C for future application.

S1.4 Preparation of AuNCs@WK Composite Sponges
The composite mass is subjected to the freeze-drying method to sponge formation. The obtained suspension of AuNCs@WK (without dialysis) was transferred into a mold and frozen at −20 °C for 24 h. Followed to this the hybrid solidified mass was dried by a freeze-dryer (-106 °C ) for 48 h to fabricate 3D porous composite sponges.

S1.5 Preparation of AuNPs@NPWC
The solid mass of AuNCs@WK is heated initially to 150 °C for 60 min in a tube furnace with N2 atmosphere to take out water content, followed by heating to 350 °C (2 °C min −1 ) for 60 min to develop the incipient conjugated carbon skeleton. It transformed into the polyaromatic N-doped porous carbon matrix with temperature rise from 350 to 700 °C at a rate of 2 °C min −1 and calcined for 2 h. The carbonized material was washed several times with DI water to remove salt residues. The black solid composite material was dried at 60 °C overnight.

S1.6 Characterization of AuNPs/NPWC
The crystallographic phase of the as-carbonized composite materials was characterized by X-ray diffraction (XRD, Bruker D8 AVANCE). The surface morphology, porosity and distribution of AuNPs in the composite materials were examined with a field emission gun scanning electron microscope (FEGSEM, Hitachi SU70) with an accelerating voltage of 5 kV. The carbon matrix, size and morphology of nanoparticles were characterized by transmission electron microscopy (TEM) through a JEM-2100F (JEOL, Japan). Nitrogen adsorption/desorption isotherms at 77.3 K were obtained using a Micromeritics TriStar II 3020 static volumetric analyzer. Prior to adsorption measurements, the samples were degassed for 12 h at 100 º C ensuring that the residual pressure fell below 10 mbar. The Brunauer-Emmett-Teller surface area was calculated within the relative pressure range 0.05 to 0.2. Total volume was calculated at P/P0 = 0.99. The surface element state and the constituent of N-functional groups were detected by X-ray photoelectron spectroscopy (Quantum 2000, PHI, USA), whereas, Raman spectra were taken using (HORIBA LabRAM HR Evolution) with a 532 nm excitation laser. Commercial handy UA meter of Sinocare made was used for UA comparative study, whereas pH comparison study has been performed by commercial pH meter of Thermo scientific made.