*2.1. Materials*

All reagents (Sigma-Aldrich, Saint Louis, MO, USA) were of high commercial grade and were used without further purification. All solvents used (from Carlo Erba, Val de Reuil, France) were spectrophotometric grade. The free NOPD was synthesized according to our previously published procedure [37].

#### *2.2. Synthetic Procedures*

#### 2.2.1. Synthesis of Amino-Modified MSNs (MSNs-NH2)

MSNs-NH2 were prepared according to literature procedures using cetyltrimethylammonium bromide (CTAB) as template [38]. Briefly, CTAB (1.9 mmoL) was dissolved in 340 mL of water followed by the addition of 2.45 mL of NaOH (2.0 mol <sup>L</sup>−1). The temperature was adjusted at 80 ◦C and tetraethoxy silane (TEOS), 3.5 mL, 18.1 mmoL, and 3-aminopropyl triethoxy silane (APTES), (0.43 mL, 2.04 mmol) were simultaneously added dropwise to the solution during 4 min. The mixture was stirred at 80 ◦C for 2 h to give rise to white precipitate. The solid product was filtered, washed with deionized water and ethanol, and dried at 60 ◦C for 24 h.

#### 2.2.2. Synthesis of the PS-Modified MSNs (PS-MSNs)

500 mg of MSNs-NH2 were added in 15 mL of DMF and sonicated (90 s, 42 KHz). Then PS erythrosine (27 mg, 1 eq), O-(7-Azabenzotriazol-1-yl)-1,1,3,3-tetramethyl uronium hexafluorophosphate (11.1 mg, 1 eq) and N,N-di-isopropiletilamine (10 μL, 2 eq) were added to the suspension. The mixture was stirred at room temperature for 24 h under dark conditions. The solid product was filtered, washed with DMF (4 portions of 5 mL) and dried at 60 ◦C for 24 h. PS-MSNs were prepared with the nominal concentration of ca. 0.03% (*w*/*w*) of PS. The real concentration of the PS was indirectly determined by UV–Vis spectrum of the eluate (post washing) using ε543 = 112.000 M−<sup>1</sup> cm<sup>−</sup><sup>1</sup> in DMF.

#### 2.2.3. Synthesis of NOPD-Modified PS-MSNs (PS-MSNs-NOPD)

One-hundred milligrams of PS-MSNs was added in 10 mL of DMSO and sonicated (90 s, 50 KHz). Then cesium carbonate (10 mg, 0.2 eq) and 4-chloro-1-nitro-2(trifluoromethyl) benzene (30 μL, 2 eq) were added and refluxed overnight. The solid product was filtered and washed with ethyl acetate (to remove the unbounded 4-chloro-1-nitro-2(trifluoromethyl) benzene) and water (to remove the excess of cesium carbonate) and dried under vacuum to obtain PS-MSNs-NOPD. PS-MSNs-NOPD were prepared with the nominal concentration of ca. 0.4% (*w*/*w*) of NOPD. The real concentration of the NOPD was indirectly determined by UV–Vis spectrum of the eluate (post washing) using ε400 = 10.000 M−<sup>1</sup> cm<sup>−</sup><sup>1</sup> in water.

#### 2.2.4. Stability of the PS-MSNs-NOPD

Representative samples of PS-MSNs-NOPD (1 mg mL−1) were either sonicated for 1 h or stirred at 70 ◦C for 2 h in the dark. Afterwards the samples were centrifuged for 25 min and the supernatant was collected and analyzed by UV-VIS absorption. The absence of any relevant absorption between 250 and 800 nm, suggested that no leaching of the chromogenic units from the MSN scaffold occurs.
