Micro cover glass no 5

The photomasks were fabricated by photolithography as follows. The photomask design was created using the software Rhinoceros 4.0 (Robert McNeel & Associates). The slide glass (micro cover glass No.5, 0.45–0.60 mm thickness, Matsunami Glass) was cleaned for 90 s with a hydrophilic treatment device (Ion Bomberder, Vacuum Device Co., Ltd.). Then, the glass slide was enclosed in an aluminum container with 100 μl of HMDS. The container was sealed and heated to 90 °C for 1 h in order to improve photoresist adhesion. The photoresist S1818 was spin-coated on the treated glass slide. The spin coating device (Opticoat SpinCoater, Mikasa Co., Ltd.) was used for spin coating at a maximum spin frequency of 3 000 rpm for 30 s. After the resist material was coated on the slide glass, the glass slide with photoresist was pre-baked for 1 min at 115 °C and left to cool to down to room temperature (approximately 25 °C). To expose the photoresist, we used a mask-less pattern generator μPG 101 (laser wavelength 375 nm, Heidelberg Instruments Mikrotechnik GmbH). After exposure, the photoresist was developed using the developer NMD and cleaned with IPA. Thereafter, a metal deposition device (VE2012 TMP vacuum evaporator, Vacuum Device Co., Ltd.) was used for the deposition of chromium and gold, in this order. The remaining photoresist was removed using an acetone bath in an ultrasonic cleaner.

Interdigitated microelectrodes with sawtooth edges and a 70 μm gap between the teeth (Fig. 1A and B) were prepared using the lift-off method.^{35 (link)} Briefly, a S1818G photoresist (Microchem) was spin coated onto a micro cover glass No. 5 (Matsunami) treated with plasma oxidation (90 s on an ion bombarder, Vacuum Device Co., Ltd) and silylated with hexamethyldisilazane (HMDS, Wako AWK3814) via vapour phase deposition.^{36 (link)} The photoresist was spin coated at a maximum spin frequency of 3000 rpm for 30 s (Opticoat SpinCoater, Mikasa). Subsequently, the slide was pre-baked for 1 min at 115 °C, cooled to room temperature, and exposed using a maskless pattern generator with resolution of 3 μm (μPG 101, Heidelberg Instruments; laser wavelength, 375 nm). Afterwards, the photoresist was developed with tetramethylammonium hydroxide solution 2.38% (OFPR-NMD-3, Tokyo Ohka Kogyo Co., Ltd.) and cleaned with isopropyl alcohol (IPA, Kanto Chemical Co. Inc. JIS K8839). The developed slide was then coated sequentially with chromium and gold using a metal evaporator (VE2012 TMP vacuum evaporator, Vacuum Device Co., Ltd). Finally, the undeveloped photoresist was removed with acetone (Wako, DSG4138), revealing a sawtooth pattern. ESI Method 1† contains additional details regarding the sample preparation.