Hotplate

The sizes of NiO_x NPs were estimated using transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan) images. The surface morphology of the sensor electrodes was characterized by scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and atomic force microscopy (AFM, Park System XE100, Suwon, Korea). Energy-dispersive X-ray spectrometry (EDS, Hitachi S-4800, Tokyo, Japan) analysis was conducted to investigate the elemental composition of the electrode. X-ray diffraction (XRD, Bruker D8 Advance, Billerica, MA, USA) patterns were recorded for phase identification. The resistance was measured using a multimeter (Agilent U1251B, Santa Clara, CA, USA) while the temperature was precisely controlled by a hot plate (Fisher Scientific, Hampton, NH, USA). The temperature of the hot plate also was monitored by a commercial thermocouple (Type K, EA11A). The resistivity (ρ) of the electrode was calculated using the equation: ρ = R·(A/l), where R, A, and l are the resistance, cross-sectional area, and length of the electrode, respectively.

Wires soldered to an IDEA chip were connected to the function generator (Stanford Research Systems, Sunnyvale, CA, USA) to apply a constant 5V peak-to-peak (Vpp) voltage at various frequencies (see Figure 1). A polymer cage cut out of double-sided stick tape (3M, St. Paul, MN, USA) was used to contain the suspension of CNTs atop of the IDEAs. The CNT suspension was deposited in a series of 10 μL drops. A microscope glass slide (Thermo Fisher Scientific, Fisherbrand, Waltham, MA, USA) was placed atop of the IDEA chip/polymer cage assembly to decrease the evaporation rate of the solution. The resistance between the fingers was measured with a 3320 Innova multimeter (Innova, Irvine, CA, USA). In order to facilitate solvent evaporation after deposition, the IDEA chips were placed on a hot plate (Fisher Scientific, Hampton, NH, USA) at 200 °C for 20 min. The resistance of the CNT bridges was measured again after the heat treatment. This was done to understand the extent to which the evaporation of the residual solution induces closer contact between the nanotubes.

This was based on our previously published methods [10 (link),21 (link)]. In brief, calcium hydroxide (Sigma Aldrich, Dorset, UK) (50 mmol) was suspended in 500 mL deionized water (dH₂O) into which 0, 1, 2.5, or 5 mmol silver nitrate (Sigma Aldrich, UK) (corresponding to 0, 2, 5, or 10 mol%, respectively) was added and stirred at 400 rpm for 1 h on a hotplate (Fisher Scientific, Loughborough, UK) at 90 °C. Phosphoric acid (Sigma Aldrich, UK) (30 mmol) was dissolved in 250 mL dH₂O, poured into the calcium and silver preparation, and stirred for a further hour. The suspension was left to settle overnight, after which the supernatant was poured off and the silver-doped nHA suspension was washed with dH₂O (3 × 500 mL). The silver-doped nHA suspensions were dried at 60 °C and ground in an agate mortar and pestle.

Microfluidic moulds were fabricated using Ordyl^® SY 300 dry film negative photoresist (55 μm thickness, ElgaEurope s.r.l., Milan, Italy) on 75 mm × 50 mm borosilicate glass slides (Corning Inc., Corning, NY, USA). After cleaning with acetone and isopropanol and dehydration of the glass slide on a hotplate (Thermo Fisher Scientific, Waltham, MA, US) for 5 min at 150 °C, two sheets of photoresist were laminated onto the slide using a thermal laminator (325R6, FalconK, France) at 120 °C and roller speed 4. Using an exposure masking UV LED lamp (UV-KUB 2, Kloé, Montpellier, France) the photoresist was then exposed to UV light (365 nm, 23.3 mW/cm²), for 7 s with a film photomask (Selba S.A., Versoix, Switzerland) and subsequently developed with a solvent blend (Ordyl^® SY Developer, ElgaEurope s.r.l., Milan, Italy) for approximately 10 min to remove unexposed sections of the photoresist. The mould fabrication process was finished with a hard bake of 30 min at 120 °C on a hotplate. This mould can be used for both sTPE hot embossing as well as PDMS soft lithography. Moulds with thicker features can be achieved by laminating successive layers of the photoresist before the exposure masking step.