**2. Methods**

## *2.1. Fabrication of Mesoporous Silicon Surfaces*

A detailed scheme of the fabrication of the Au-functionalized substrates is reported in Figure 1. Silicon substrates were electrochemically etched to obtain porous silicon. Porous silicon is a form of silicon with arrays of pores penetrating through its structure [31]. The average pore size (PS) determines the class of the porous silicon material: substrates with PS < 2 nm, 2 < PS < 50 nm, or PS > 50 nm are classified as nanoporous, mesoporous, macroporous silicon substrates, respectively [31]. In this work we produced mesoporous silicon substrates with two di fferent non-overlapping values of pore size: MeP1 silicon substrates with PS1 ≈ 11 nm and MeP2 silicon substrates with PS2 ≈ 21 nm. We used P-type, 100 silicon wafers as a substrate. We cut the originating silicon wafers into regular square chips with a side of ≈1 cm. We then positioned the chips in an impermeable electrolytic cell where samples were exposed to a solution of hydrofluoric acid (HF) and ethanol or methanol, under the action of an external electric field [30]. As a result, hydrogen ions in solution were accelerated towards the silicon substrate etching the pores. Substrates with a di fferent pore size were obtained by tuning the parameters of the technique: the intensity of etching current, the concentration of HF in solution, the type of neutral component in solution (ethanol or methanol), and the time of the process. MeP1 silicon with an average pore size of PS ≈ 11 nm was obtained using a mixture of HF, D.I. (de-ionized) water and ethanol in a proportion of 1:1:1 in volume. In the process, a value of current density of I = 20 mA/cm<sup>2</sup> was applied for 5 min at 25 ◦C. MeP2 silicon with an average pore size of PS ≈ 21 nm was obtained using a mixture of HF, D.I. water, and methanol in a proportion of 5:3:2 in volume. In the process, a value of current density of I = 4 mA/cm<sup>2</sup> was applied for 5 min at 25 ◦C. In all cases, the thickness of the porous layer is of some tens of micrometers. Since porous silicon is intrinsically hydrophobic [32], samples were oxidized in an oven at 200 ◦C for 2 h before use. The photoluminescence of mesoporous silicon was verified by imaging the light emission of the samples under UV radiation (365 nm).

## *2.2. Electroless Deposition of Gold Nanoparticles Clusters*

Clusters of gold nanoparticles were deposited on the porous sample surface using electroless deposition techniques. In the technique, metal ions in solution are reduced on an autocatalytic surface to form solid deposits of that metal [33]. Following the methods reported in [34], we treated the porous silicon samples in a solution of HF and gold (III) chloride (AuCl3) in a concentration of 0.15 M (HF) and 1 mM (AuCl3) for 3 min at 50 ◦C. In solution, the ions of gold react with the exposed silicon surface yielding gold nanoparticles with an average particle size d ≈ 20 nm. Samples were then rinsed in D.I. water at room temperature for 30 s.

**Figure 1.** (**a**) An initial silicon chip of approximately 1 × 1 cm is electrochemical etched using a Teflon cell containing a solution of hydrofluoric acid (HF), D.I. (de-ionized) water, and methanol/ethanol in different ratios. (**b**) Upon activation of an external controlled voltage, positive ions in solution are accelerated towards the silicon substrate, creating pores within its structure. (**c**) Depending on the parameters of the process, including etching time, current and voltage intensity, and the concentration of the reagents in solution, one can obtain porous silicon surfaces with a tailored morphology. (**d**) The porous silicon sample is then placed in a baker along with a solution of hydrofluoric acid (HF) and gold(III) chloride (AuCl3). (**e**) The resulting electroless process enables the deposition of gold ions in solution on the autocatalytic porous-silicon surface. (**f**) By varying the parameters of the electroless process, including temperature, concentration, and time, one can produce substrates with controlled gold-nanoparticles shape, size, and density.
