Mak165

Hydrogen peroxide release was measured using a fluorometric hydrogen peroxide assay kit (Sigma Aldrich, MAK 165). First, square-shaped cryogel samples (4 mm × 4 mm × 1 mm) containing 0%, 0.1% and 0.2% CaO₂ (n = 5) were placed in 200 µl of PBS per cryogel in a 24-well plate at room temperature and 200 rpm. At 15 min, 1 h, 2 h, 3 h, 4 h and 5 h, the cryogels were transferred to fresh PBS in a new well, 50 µl of the sample was removed, and the amount of H₂O₂ released was measured fluorometrically. Assays were performed according to the manufacturer’s instructions. The readings were normalized with the readings observed with control (0% CaO₂) cryogels and were recorded until the fluorescence vanished.

A modified protocol^{33 (link)} was applied for the H₂O₂ measurement using a fluorometric H₂O₂ assay
kit (MAK165, Sigma-Aldrich). Detailed procedures of assay preparation
can be found in our previous study.^{23 (link)} The
H₂O₂ formation was quantified within 2 h from
the preparation of working solutions due to the instability of the
probe. A calibration was performed using H₂O₂ standard solutions with concentrations ranging from 0.05 to 1.5
μM in PBS to maintain pH at 7.4 (Figure S1). The reaction vials consisted of 2.94 mL sample (Milli-Q
water + filter extracts + PBS) and 60 μL working solution. The
dilution factors were adjusted for different SOA samples so that the
final H₂O₂ concentrations would be below 1.5
μM. All H₂O₂ measurements were conducted
with a filter blank with the same dilution factor as the samples.
The addition of working solution to the samples was considered as
the start of the reaction, and the measurement was conducted after
the reaction vials were incubated at the room temperature of 298 K
for 15 min. A spectrofluorophotometer (RF-6000, Shimadzu) was used
to measure the fluorescence of the reagents at excitation and emission
wavelengths of 540 and 590 nm, respectively.