Sampling, TiO2–Ag–NP handling and data management

Samples were collected by swab methods³⁶. Each swabbing was performed far from the routine cleaning settings (≥ 9 h from chemical cleansing) by one of ours (Luca Berto) alongside with one assistant (Antonio Vella).

Figure 1 summarizes the process of sampling by swab method.

Brief summary of the process of swab test for RLU performance. (A) The co-author LB while swabbing on the inside part of a vertical surface (a window glass, for example), (B) Way by which the swab must be handled and used on the surface; (C) Inserting the swab in the RLU detection device; (D) an example of RLU read out; (E) and (F) Different examples of the WIWELL TiO₂–Ag–NP application (a job container and an office); (G) Microbiology output on a TSA plate (37 °C, 48 h) before and (H) after the application for 12 h of the WIWELL TiO₂–Ag–NP adhesive film.

Initially, the operator leaves UltraSnap swab be equilibrated to room temperature (about 21–25 °C) before any intended use, then he holds the swab firmly and, by twisting and pulling top of the swab out of its tube, makes it ready to use it. Subsequently, he thoroughly swabs a standard area (Fig. 1A) following the producer’s instruction and swabbing a 4 × 4 inches or 10 × 10 cm area, following a zig zag path (Fig. 1B). Areas (surfaces) are usually inner part of windows (glass) or indoor objects far from human contacts. The operator replaces the swab into the swab tube, then he uses thumb and forefinger to break the Snap-Valve, holding firmly the tube and bending the bulb backward and forward for some seconds. He therefore squeezes the bulb at least twice to allow the liquid expulsion down the swab shaft, shakes the swab bud in the liquid for 5–10 s and then he will read the sample in the bio-luminometer (Fig. 1C) within 30 s, holding luminometer upright. Then he presses the starting button and will read the RLU value (Fig. 1D).

Sampling was performed in quadruplicate, i.e., for each evaluated site at least 4 swabs were performed in the same sampling collection (each sample = 4 swabs) and all performed starting at 3.00 p.m. ± 15 min, far from routine cleansing time (6.00 a.m.). Each swab was carried out in an empty space without persons, except for testing performers, where the complete absence of crowding people was induced at least 30 min before collection, in order to standardize the indoor climate parameters and prevent statistical confounders. Each tested indoor space was endowed with one simple type of WIWELL TiO₂–Ag–NP photocatalytic film, attached on different walls, either on a supporting frame or directly, 20 h before the sampling. The TiO₂–Ag–NP photocatalyst is an adhesive sheet with two dimensional typologies, as reported above: (a) a Type-1 (60 × 90 cm) sheet (used for experiments in schools and work indoor places), (b) a Type-2 (30 × 50) cm sheet in farm housing cabs. Each photocatalyst, appearing as an adhesive panel on the wall, was set at a distance from the center of the room ≤ 3.00 m, from the floor ≥ 1.5 m, from the top ≥ 1.0 m. The photocatalyst was left on its place for all the experimental time.

Sampling was performed at 3.00 p.m. in the day before the experiment and before attaching the WIWELL TiO₂–Ag–NP photocatalytic film (control samples) and in the day/days of experiment, with the previously attached WIWELL TiO₂–Ag–NP photocatalytic film (data samples). The number of sampling data for each indoor investigated area was calculated by sample size statistics in order to achieve a margin of error close to 5%. Therefore, a number of at least 70 samples was considered sufficient to reach a 95% of a population proportion undergoing the effect with a 95% confidence interval (error = 5.11%). Data from time 0 and the end of data collection were independently elaborated. The Cohen k for the accordance (89.74%) was 0.6533.

Sampling spots were designed in order to include at least four indoor sub-areas and eight sampling options, depending on the study rationale, from the photocatalyst source and ventilation, within 1.5 m (near), i.e., near/far the photocatalyst, near/far a window or a door, near/far the center of a room, near/far the walls of a room. As data regarded the complessive indoor volume of the investigated room, spots did not differ each other significantly, whereas outliers (≥ 3 SD) removed from the final data managing.