*2.1. All-Dielectric Metasurface Biosensors*

The all-dielectric metasurface substrates were fabricated through the nanolithography process for silicon-on-insulator (SOI) substrates. The SOI substrates comprised a top layer of crystalline silicon of 200 nm thickness, a middle layer of buried oxide (or SiO2) of 375 nm thickness, and a base silicon wafer of 675 *μ*m thickness. Following the nanopatterns written on an electron-beam resist, only the top layer was normally etched down to the middle layer, which resulted in silicon nanopellet arrays (Figure 1c). The top-down nanolithography process has been previously reported in detail [18,23]. Originally, the all-dielectric metasurfaces were conceived with being stimulated from the finding of large FL-enhancing effects in plasmon–photon hybrid metasurfaces [24–26]. We note that the metasurface substrates are reusable after the washdown of the adhered proteins in experiments. The washdown was conducted in two steps: (i) the substrates were first washed for 5 min in a neutral ultrasonic cleaning liquid (7-5337-02, As-One, Osaka, Japan), which was 10-times diluted in advance using purified water, were rinsed four times in the purified water under applying ultrasonic waves (5 min each), and were made dry blowing N2 gas; (ii) the substrates were immersed in so-called piranha solution, which consists of 96% sulfuric acid

and 30% H2O2 liquid (the volume ratio 3:1), for 15 min, and were rinsed with distillated water for 20 min. The validity of the piranha solution to remove organic deposits on silicon nanostructures was previously reported [27]. After the washdown, we did not find any residue emitting FL on the substrates. This reusability has the potential to reduce running costs significantly.

To control the flow of liquid reagents including human serums, we combined the metasurface substrate with a microfluidic (MF) chip and prepared a metasurface biosensor. The MF chip was made of polydimethylsiloxane (PDMS), which was transparent to visible light and was designed to have six MF paths in accordance with the six metasurface areas (Figure 1c). Inlet and outlet holes were introduced at both ends of each MF path to facilitate connecting from the outside; this can be seen in previous reports [20,21]. The PDMS was naturally absorbed on the metasurface substrate, enabling us to flow the liquid reagents.
