*3.2. Decontamination Challenges*

Decontamination challenges were employed to investigate the capability imparted by photoactive additives C60 and EO3–C60 onto Tecoflex films to automatically decompose surface-residing contaminants. Two chemical compounds, Demeton-*S* and CEPS, were employed as representative contaminants of organophosphorous and sulfide-based pesticides. The prepared films were first subjected to decontamination challenges consisting of 18 h contaminant residence time.

Figure 6 presents results from a 4 g/m2 Demeton-*S* decontamination challenge in which extracted Demeton-*S* (normalized by the tetralin internal standard) is plotted against additive loading concentration. No correlation was observed between C60 loading concentration in Tecoflex films and Demeton-*S* reduction. Furthermore, reduction of Demeton-*S* on C60 exhibits similar trends in both dark and light conditions. Tecoflex films containing EO3–C60 exhibit increased reduction of Demeton-*S* compared to films containing C60 in dark conditions; however, decomposition of Demeton-*S* did not directly correlate with additive loading. Tecoflex films exposed to light that contained EO3–C60 exhibit direct correlation between reduction of Demeton-*S* and EO3–C60 loading concentration. From this, it is proposed that different modes of action for Demeton-*S* degradation are occurring between Tecoflex films containing C60 and EO3–C60, thus necessitating decomposition byproducts analysis.

**Figure 6.** Demeton-*S* recovered from Tecoflex films after 18 h residence in dark (**a**) and

daylight (**b**) conditions.

Byproduct analysis of the 18 h Demeton-*S* decontamination challenges were performed for further insight into possible modes of action. In addition to reduction in Demeton-*S*, significant byproducts were detected at a retention time of 4.1 min which corresponds to vinyl oxidation product (*S*-vinyl) (**2**). Byproduct concentration, normalized with the internal standard, is plotted against loading concentration in Figure 7. It is apparent that increased C60 loading leads to decreased production of **2** in both dark and light conditions, whereas increasing concentration of EO3–C60 loading results in increasing production of **2**. Unmodified C60 is more reactive than EO3–C60, especially at low concentrations; however, increased concentration of C60 does not result in increased decomposition, most likely due to self-quenching resulting from high C60 proximity from aggregation in the polymer matrix. Qualitative observation indicated that poor solubility of C60 in chloroform facilitates the formation of C60 aggregates, in which the probability of self-quenching between C60 molecules is increased.

For Tecoflex films containing C60, comparable amounts of **2** were detected from decomposition of Demeton-*S* on films that resided in dark and light conditions for 18 h (Figure 7a). From this, it appears that photoactivity against Demeton-*S* is not occurring in the Tecoflex films containing unmodified C60. If photoactivity was occurring, then a greater amount of **2** would be observed on the films exposed to light than in darkness. It has been previously demonstrated that C60 fullerene can behave as an electron acceptor (Lewis acid) toward sulfides [40]. Thus, the electron acceptor behavior of C60 in Tecoflex may facilitate the cleavage of the S–C bond (Figure 8) resulting in the elimination product **2**.

**Figure 7.** Detected *S*-vinyl product (shown as normalized ratio of *S*-vinyl peak area to tetralin peak area) from Tecoflex films loaded with C60 (**a**) and EO3–C60 (**b**) over an 18 h Demeton-*S* challenge.

**Figure 8.** Hypothesized Lewis acid catalyzed sulfide elimination.

$$\underbrace{\ast\_{\mathsf{C}^{\otimes}}}\_{\mathsf{C}^{\otimes}} \xrightarrow{\ast\_{\mathsf{R}^{\otimes}}} \underbrace{\ast\_{\mathsf{R}^{\otimes}}}\_{\mathsf{C}^{\otimes}}$$

On the other hand, photoactivity was apparent from byproduct analysis in the films loaded with EO3–C60 (Figure 7b). In fact, the photoactivity of EO3–C60 continued to increase with increasing concentration indicating that the ethylene oxide moieties, by improving solubility, help to diminish aggregate facilitated self-quenching. In contrast to the films containing C60, an absence of Demeton-*S* decomposition on Tecoflex films containing EO3–C60 in dark conditions was observed. This is likely the result of decreased Lewis acid character upon the covalent attachment of the ethylene oxide moiety.

The combination of photoactivity and oxidation products detected from Tecoflex films containing EO3–C60, and documented capability of fullerene species to photogenerate singlet oxygen [41] has led to the proposed mode of action for Demeton-*S* decomposition on films containing EO3–C60 exposed to visible light presented in Figure 9. The photoactive species embedded in the polymer matrix is first photosensitized upon the absorption of visible light. Subsequent transfer of energy from photosensitized EO3–C60 to ambient atmospheric oxygen results in the formation of singlet oxygen (<sup>1</sup> O2). The photogenerated singlet oxygen, a ROS, then oxidizes the peripheral sulfur of Demeton-*S* that is residing on the coating surface in proximity to the photosensitized additive. The sulfoxide then undergoes Į elimination resulting in the Demeton-*S* vinyl degradation product (**2**) and unstable sulfenic acid, which quickly self-condenses to form the corresponding thiosulfanate.

In addition to decontamination challenges against Demeton-*S*, Tecoflex films loaded with photoactives were also subjected to CEPS decontamination challenges. An 18 h decontamination challenge was initially performed for each sample in both dark and light conditions. Despite minor differences in the amount of CEPS decomposed, significant differences in byproduct formation were observed to be dependent on the conditions in which the sample resided.

**Figure 9.** Proposed oxidation mechanism of Demeton-*S* and the formation of elimination product from photogenerated singlet oxygen.

GC-MS analysis afforded the detection of a notable degradation product of CEPS from the Tecoflex films containing photoactives that resided in daylight conditions. Mass spectra analysis determined that the degradation product was vinyl phenyl sulfoxide (**4**), an oxidation byproduct of CEPS (Figure 1). Furthermore, **4** was not detected from coatings that resided in dark conditions. Figure 10 presents normalized concentrations of **4** detected in the reaction extract from Tecoflex films loaded with C60 and EO3–C60 after an 18 h residence time of CEPS. Production of **4** from Tecoflex films decreases with increasing C60 loading concentration. This is attributed to increasing C60 aggregation with increased C60 loading concentration due to poor solubility and incompatibility with the polyurethane matrix, as previously demonstrated via DSC and XRD. The increase in aggregation effectively limits the available surface area of C60 available for both singlet oxygen generation and contact with the contaminant. Additionally, singlet oxygen quenching is known to occur between proximal C60 molecules in high concentration, such as in aggregates and crystallites [42].

In contrast to the effects observed resulting from C60 loading concentration, the generation of **4** increased with increasing EO3–C60 concentration in Tecoflex from 0.25 to 1.0 wt%. This direct correlation of EO3–C60 loading and generation of **4** can only result from minimized self-quenching due to good dispersion of EO3–C60. These trends support those that were observed in the Demeton-*S* decontamination challenge.

**Figure 10.** CEPS byproduct resulting from residence on Tecoflex films of several additive concentrations following exposure to daylight conditions for 18 h.

From the above 18 h study, 1 wt% loadings of C60 and EO3–C60 were down-selected for an expanded time-dependent CEPS decontamination challenge over the course of 165 h in daylight conditions (Figure 11). Figure 11a displays concentration of CEPS extracted from samples over a 165 h residence time period. Each of the films subjected to the challenge exhibited rapid decrease in CEPS concentration over the first 48 h. This is attributed to inherent attenuation of CEPS as this behavior was observed on the two controls, a Teflon film and unmodified Tecoflex. C60 and EO3–C60 films differentiate from the controls at time points beyond 48 h, after which the degradation rates of CEPS on the controls decrease drastically. In contrast, the C60 and EO3–C60 Tecoflex films exhibit continued linear degradation of CEPS beyond 48 h. This behavior is explained by two separate degradation mechanisms occurring simultaneously. First, the attenuation mechanism, which is initially at a high rate, dominates the first 48 h of degradation. Beyond 48 h, attenuation rate slows to an extent such that the secondary mechanism, photo-oxidation, dominates the overall reaction rate and thus becomes apparent.

Byproduct analysis (Figure 11b) confirms linear increase of oxidation product (**4**) over time on films containing photoactive additives. Tecoflex films containing C60 and EO3–C60 each exhibited a linear relationship between production of oxidation product and residence time, while oxidation was not detected from the controls. The detection of oxidation products only from samples that are exposed to light, that contain either C60 or EO3–C60, and dependent on the concentration of C60 and EO3–C60 indicates that a photo-mediated oxidation process is facilitating the oxidation of CEPS.

In consideration of the data presented herein and the known singlet oxygen generation potential of C60 fullerene, a proposed mode of action for CEPS oxidation is presented in Figure 12. Similar to the mechanism proposed for the oxidation of Demeton-*S*, the photoactive in Tecoflex generates singlet oxygen upon exposure to light. Singlet oxygen oxidizes the *S* atom to sulfoxide, which then through an elimination mechanism produces a vinyl sulfoxide with HCl as a byproduct. In contrast to the mechanism proposed for Demeton-*S*, the oxygen remains on the same byproduct molecule as sulfur. This is due to the stability of the sulfenic acid leaving group in Demeton-*S* decomposition compared to the instability of the potential hypochlorite byproduct in CEPS oxidation.

**Figure 11.** Concentration of CEPS recovered from samples over a 165 h residence (**a**) and normalized vinyl phenyl sulfoxide degradation product (**b**).

**Figure 12.** Potential pathway of sulfide oxidation followed by dehydrohalogenation of CEPS.
