Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols.

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Impact of chlorination and ozonation of dissolved organic matter on its photo-induced production of long-lived photooxidants and excited triplet states.

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (3CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA254), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]0 = 0.1 and 5.0 µM). Linear inter-correlations were observed for the relative changes in SUVA254, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., k0.1obs/rCDOMabsand k5.0obs/rCDOMabs, for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated k0.1obs/rCDOMabsratio decreased with increasing oxidant dose and with decreasing SUVA254 and EDC, while the 3CDOM*-dominated k5.0obs/rCDOMabsratio positively correlated with E2:E3. Finally, it was concluded that precursors of 3CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water.

Oxidizing triplet excited states of organic matter (3C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because 3C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (1O2*), which can interfere with 3C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to 1O2*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with 1O2*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

A Novel Approach of Combining Methylene Blue Photodynamic Inactivation, Photobiomodulation and Oral Ingested Methylene Blue in COVID-19 Management: A Pilot Clinical Study with 12-Month Follow-Up.

Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 virus was first recognized in late 2019 and remains a significant threat. We therefore assessed the use of local methylene blue photodynamic viral inactivation (MB-PDI) in the oral and nasal cavities, in combination with the systemic anti-viral, anti-inflammatory and antioxidant actions of orally ingested methylene blue (MB) and photobiomodulation (PBM) for COVID-19 disease. The proposed protocol leverages the separate and combined effects of MB and 660nm red light emitted diode (LED) to comprehensively address the pathophysiological sequelae of COVID-19. A total of eight pilot subjects with COVID-19 disease were treated in the Bahamas over the period June 2021-August 2021, using a remote care program that was developed for this purpose. Although not a pre-requisite for inclusion, none of the subjects had received any COVID-19 vaccination prior to commencing the study. Clinical outcome assessment tools included serial cycle threshold measurements as a surrogate estimate of viral load; serial online questionnaires to document symptom response and adverse effects; and a one-year follow-up survey to assess long-term outcomes. All subjects received MB-PDI to target the main sites of viral entry in the nose and mouth. This was the central component of the treatment protocol with the addition of orally ingested MB and/or PBM based on clinical requirements. The mucosal surfaces were irradiated with 660 nm LED in a continuous emission mode at energy density of 49 J/cm2 for PDI and 4.9 J/cm2 for PBM. Although our pilot subjects had significant co-morbidities, extremely high viral loads and moderately severe symptoms during the Delta phase of the pandemic, the response to treatment was highly encouraging. Rapid reductions in viral loads were observed and negative PCR tests were documented within a median of 4 days. These laboratory findings occurred in parallel with significant clinical improvement, mostly within 12-24 h of commencing the treatment protocol. There were no significant adverse effects and none of the subjects who completed the protocol required in-patient hospitalization. The outcomes were similarly encouraging at one-year follow-up with virtual absence of "long COVID" symptoms or of COVID-19 re-infection. Our results indicate that the protocols may be a safe and promising approach to challenging COVID-19 disease. Moreover, due its broad spectrum of activity, this approach has the potential to address the prevailing and future COVID-19 variants and other infections transmitted via the upper respiratory tract. Extensive studies with a large cohort are warranted to validate our results.

Photochemical oxidation of phenols and anilines mediated by phenoxyl radicals in aqueous solution.

Reactive intermediates formed upon irradiation of chromophoric dissolved organic matter (CDOM) contribute to the degradation of various organic contaminants in surface waters. Besides well-studied "short-lived" photooxidants, such as triplet state CDOM (3CDOM*) or singlet oxygen, CDOM-derived "long-lived" photooxidants (LLPO) have been suggested as key players in the transformation of electron-rich contaminants. LLPO were hypothesized to mainly consist of phenoxyl radicals derived from phenolic moieties in the CDOM. To test this hypothesis and to better characterize LLPO, the transformation kinetics of selected target compounds (phenols and anilines) induced by a suite of electron-poor model phenoxyl radicals was studied in aerated aqueous solution at pH 8. The phenoxyl radicals were generated by photosensitized oxidation of the parent phenols using aromatic ketones as photosensitizers. Under steady-state irradiation, the presence of any of the electron-poor phenols lead to an enhanced abatement of the phenolic target compounds (at an initial concentration of 1.0 × 10-7 M) compared to solutions containing the photosensitizer but no electron-poor phenol. A trend of increasing reactivity with increasing one-electron reduction potential of the electron-poor phenoxyl radical (range: 0.85‒1.12 V vs. standard hydrogen electrode) was observed. Using the excited triplet state of 2-acetonaphthone as a selective oxidant for phenols, it was observed that the reactivity correlated with the concentration of electron-poor phenoxide present in solution. The rates of transformation of anilines induced by the 4-cyanophenoxyl radical were an order of magnitude smaller than for the phenolic target compounds. This was interpreted as a reduction of the radical intermediates back to the parent compound by the superoxide radical anion. Laser flash photolysis measurements confirmed the formation of the 4-cyanophenoxyl radical in solutions containing 2-acetonaphthone and 4-cyanophenol, and yielded values of (2.6 - 5.3) × 108 M-1 s-1 for the second-order rate constant for the reaction of this radical with 2,4,6-trimethylphenol. These and further results indicate that electron-poor model phenoxyl radicals generated through photosensitized oxidation are useful models to understand the photoreactivity of LLPO as part of the CDOM.

Experimental and computational electrochemistry of quinazolinespirohexadienone molecular switches - differential electrochromic vs photochromic behavior.

Our undergraduate research group has long focused on the preparation and investigation of electron-deficient analogs of the perimidinespirohexadienone (PSHD) family of photochromic molecular switches for potential application as "photochromic photooxidants" for gating sensitivity to photoinduced charge transfer. We previously reported the photochemistry of two closely related and more reducible quinazolinespirohexadienones (QSHDs), wherein the naphthalene of the PSHD is replaced with a quinoline. In the present work, we report our investigation of the electrochemistry of these asymmetric QSHDs. In addition to the short wavelength and photochromic long-wavelength isomers, we have found that a second, distinct long-wavelength isomer is produced electrochemically. This different long-wavelength isomer arises from a difference in the regiochemistry of spirocyclic ring-opening. The structures of both long-wavelength isomers were ascertained by cyclic voltammetry and 1H NMR analyses, in concert with computational modeling. These results are compared to those for the symmetric parent PSHD, which due to symmetry possesses only a single possible regioisomer upon either electrochemical or photochemical ring-opening. Density functional theory calculations of bond lengths, bond orders, and molecular orbitals allow the rationalization of this differential photochromic vs electrochromic behavior of the QSHDs.

Near-Infrared Activated Black Phosphorus as a Nontoxic Photo-Oxidant for Alzheimer's Amyloid-ß Peptide.

The inhibition of amyloid-ß (Aß) aggregation by photo-oxygenation has become an effective way of treating Alzheimer's disease (AD). New near-infrared (NIR) activated treatment agents, which not only possess high photo-oxygenation efficiency, but also show low biotoxicity, are urgently needed. Herein, for the first time, it is demonstrated that NIR activated black phosphorus (BP) could serve as an effective nontoxic photo-oxidant for amyloid-ß peptide in vitro and in vivo. The nanoplatform BP@BTA (BTA: one of thioflavin-T derivatives) possesses high affinity to the Aß peptide due to specific amyloid selectivity of BTA. Importantly, under NIR light, BP@BTA can significantly generate a high quantum yield of singlet oxygen (1 O2 ) to oxygenate Aß, thereby resulting in inhibiting the aggregation and attenuating Aß-induced cytotoxicity. In addition, BP could finally degrade into nontoxic phosphate, which guarantees the biosafety. Using transgenic Caenorhabditis elegans CL2006 as AD model, the results demonstrate that the 1 O2 -generation system could dramatically promote life-span extension of CL2006 strain by decreasing the neurotoxicity of Aß.