Absorption spectra of benzoic acid in water at different pH and in the presence of salts: insights from the integration of experimental data and theoretical cluster models

Enhanced HONO Formation from Aqueous Nitrate Photochemistry in the Presence of Marine Relevant Organics: Impact of Marine-Dissolved Organic Matter (m-DOM) Concentration on HONO Yields and Potential Synergistic Effects of Compounds within m-DOM

Nitrous acid (HONO) is a key molecule in the reactive nitrogen cycle. However, sources and sinks for HONO are not fully understood. Particulate nitrate photochemistry has been suggested to play a role in the formation of HONO in the marine boundary layer (MBL). Here we investigate the impact of marine relevant organic compounds on HONO formation from aqueous nitrate photochemistry. In particular, steady-state, gas-phase HONO yields were measured from irradiated nitrate solutions at low pH containing marine-dissolved organic matter (m-DOM). m-DOM induces a nonlinear increase in HONO yield across all concentrations compared to that for pure nitrate solutions, with rates of HONO formation increasing by up to 3-fold when m-DOM is present. Furthermore, to understand the potential synergistic effects that may occur within complex samples such as m-DOM, mixtures of chromophoric (light-absorbing) and aliphatic (non-light-absorbing) molecular proxies were utilized. In particular, mixtures of 4-benzoylbenzoic acid (4-BBA) and ethylene glycol (EG) in acidic aqueous solutions containing nitrate showed more HONO upon irradiation compared to solutions containing only one of the molecular proxies. This suggests that synergistic effects in the HONO formation can occur in complex organic samples. Atmospheric implications of the results presented here are discussed.

Tuning the Fluorometric Sensing of Phosphate on UiO-66-NH2(Zr, Ce, Hf) Metal Nodes

Metal-organic frameworks (MOFs) with intrinsic luminescent properties, modular structure, and tunable electronic properties, provide unique opportunities for designing target-specific molecular sensors by systematically choosing their constituent building blocks. We report a simple one-step MOF-based sensing platform for phosphate (P) detection that combines the luminescent properties of 2-aminoterephthalic acid (ATA) with the affinity of rationally selected nodes in UiO-66-NH2 to bind with P. This MOF possesses an electron-donating amine group that controls the light-harvesting characteristics of the linkers. Substituting Zr6 node with Ce6 or Hf6 results in a series of isostructural MOFs with distinct optical properties that are nonexistent in the unsubstituted MOF. We have utilized these MOFs to quantitatively measure P, using its ability to bind strongly to metal nodes inhibiting the LMCT process and altering the linker's photon emission. Using this system, detection limits of 4.5, 7.2 and 10.5 μM were obtained for the UiO-66-NH2(Ce), UiO-66-NH2, and UiO-66-NH2(Hf) respectively, adopting a straightforward single step procedure. These results demonstrate that the selection of metal nodes in a series of isostructural MOFs can be used to modulate their electronic properties and create sensing probes possessing the desired characteristics needed for the detection of environmental contaminants.

Improving the Selectivity of the C-C Coupled Product Electrosynthesis by Using Molecularly Imprinted Polymer─An Enhanced Route from Phenol to Biphenol

We developed a procedure for selective 2,4-dimethylphenol, DMPh, direct electro-oxidation to 3,3',5,5'-tetramethyl-2,2'-biphenol, TMBh, a C-C coupled product. For that, we used an electrode coated with a product-selective molecularly imprinted polymer (MIP). The procedure is reasonably selective toward TMBh without requiring harmful additives or elevated temperatures. The TMBh product itself was used as a template for imprinting. We followed the template interaction with various functional monomers (FMs) using density functional theory (DFT) simulations to select optimal FM. On this basis, we used a prepolymerization complex of TMBh with carboxyl-containing FM at a 1:2 TMBh-to-FM molar ratio for MIP fabrication. The template-FM interaction was also followed by using different spectroscopic techniques. Then, we prepared the MIP on the electrode surface in the form of a thin film by the potentiodynamic electropolymerization of the chosen complex and extracted the template. Afterward, we characterized the fabricated films by using electrochemistry, FTIR spectroscopy, and AFM, elucidating their composition and morphology. Ultimately, the DMPh electro-oxidation was performed on the MIP film-coated electrode to obtain the desired TMBh product. The electrosynthesis selectivity was much higher at the electrode coated with MIP film in comparison with the reference nonimprinted polymer (NIP) film-coated or bare electrodes, reaching 39% under optimized conditions. MIP film thickness and electrosynthesis parameters significantly affected the electrosynthesis yield and selectivity. At thicker films, the yield was higher at the expense of selectivity, while the electrosynthesis potential increase enhanced the TMBh product yield. Computer simulations of the imprinted cavity interaction with the substrate molecule demonstrated that the MIP cavity promoted direct coupling of the substrate to form the desired TMBh product.

First mesomorphic and DFT characterizations for 3- (or 4-) n-alkanoyloxy benzoic acids and their optical applications

New liquid crystalline hydrogen bonded 3- (or 4)-n-alkanoyloxy benzoic acids were synthesized and probed theoretically and experimentally. The molecular structures of these compounds were elucidated by proton NMR, carbon-13 NMR and elemental analyses. Differential scanning calorimetry (DSC) was used to investigate the thermal and mesomorphic properties of all the symmetrical dimers that bearing identical alkanoyloxy chains. Moreover, polarized optical microscopy (POM) was used to determine their mesophases. The findings show that all the designed symmetrical dimers exhibit the smectic mesophase with relative thermal stability that depends on the length of their terminal side chain. Additionally, the experimental findings of the mesomorphic behavior are further supported by DFT calculations. The alkanoyloxy benzoic acid para-derivatives (In) were shown to be more stable than their meta-substituted (IIn) analogues due to stronger hydrogen bonding interactions. The computed reactivity parameters showed that the position of ester moiety has a significant impact on the acids reactivity. The absorbance spectra of both the 3- (or 4)-n-alkanoyloxy benzoic acids revealed a blue shift with the increment of the of alkyl chain size; however, the energy band gaps of 3-n-alkanoyloxy benzoic derivatives were found to be slightly higher than those of the 4-n-alkanoyloxy benzoic acids. Moreover, the photoluminescence spectrum of the prepared materials is rather broad, and exhibited a red shift as the alkyl chain length increases. The fluorescence lifetime shown to rise as alkyl chain length grows longer, and 3-n-alkanoyloxy benzoic acids have slightly longer lifetime compared to their 4-n-alkanoyloxy benzoic analogues.

Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NO X , and Organic Nitrogen

Atmospheric nitrous acid (HONO), a trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its daytime sources and sinks. HONO is known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive OH radical, making it important to have accurate atmospheric HONO estimations. Despite its rapid photolysis, recent field observations have found quasi-steady-state concentrations of HONO at midday, suggesting photolytic HONO formation pathways to replenish daytime atmospheric HONO. Recent studies suggest that the presence of complex organic photosensitizers in atmospheric aerosols converts atmospheric NO₂ into HONO. To better understand the effect of environmental photosensitizers in daytime mechanisms of HONO formation, we present here laboratory studies on the heterogeneous photolytic reduction of NO₂ by humic acid films, a proxy for organic chromophoric compounds. The effect of pH and Cl^- in the photosensitized formation of HONO and other nitrogen-containing gases is also investigated. A dual Fourier transform infrared (FTIR) system is utilized to simultaneously perform in situ analysis of condensed-phase reactants and gas-phase products. We find that the rate of HONO formation is faster at lower pHs. Nitrogen incorporation in the complex organic chromophore is observed, suggesting a competing pathway that results in suppressed daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to the organic-mediated photolytic formation of nitrosyl chloride (ClNO), a known precursor of HONO. Overall, this work shows that organic acid photosensitizers can reduce adsorbed NO₂ to form HONO, ClNO, and NO while simultaneously incorporating nitrogen into the organic chromophores present in aerosol.

Changes in light absorption and composition of chromophoric marine-dissolved organic matter across a microbial bloom

Marine chromophoric dissolved organic matter (m-CDOM) mediates many vital photochemical processes at the ocean's surface. Isolating m-CDOM within the chemical complexity of marine dissolved organic matter has remained an analytical challenge. The SeaSCAPE campaign, a large-scale mesocosm experiment, provided a unique opportunity to probe the in situ production of m-CDOM across phytoplankton and microbial blooms. Results from mass spectrometry coupled with UV-VIS spectroscopy reveal production of a chemodiverse set of compounds well-correlated with increases in absorbance after a bacterial bloom, indicative of autochthonous m-CDOM production. Notably, many of the absorbing compounds were found to be enriched in nitrogen, which may be essential to chromophore function. From these results, quinoids, porphyrins, flavones, and amide-like compounds were identified via structural analysis and may serve as important photosensitizers in the marine boundary layer. Overall, this study demonstrates a step forward in identifying and characterizing m-CDOM using temporal mesocosm data and integrated UV-VIS spectroscopy and mass spectrometry analyses.

Biosynthesis of Ag Nanoparticles Using Caralluma acutangula Extract and Its Catalytic Functionality towards Degradation of Hazardous Dye Pollutants

Nanomaterials, today, are an integral part of our everyday lives, industrial processes and appliances. Biosynthesis, because of its environmental sustainability, is now becoming a hot topic. The biosynthesis of nanomaterials using plant phytochemicals enhances the nanomaterial’s biocompatibility and its compatibility with the environment too. Hence, forthe first time, this study uses Caralluma acutangula (CA) plant extracts to synthesize silver nanoparticles (CA-AgNPs) and characterize them using UV–visible spectroscopy, FTIR, Raman spectroscopy, XRD, TEM, TGA, SEM, EDX, zeta potential, and bandgap analysis. The particle size distributions of CA-AgNPs were observed to fall in the range of 2–6 nm predominantly using TEM images. High crystallinity % was calculated as 86.01 using XRD data. Extracted phytochemicals from CA were characterized and analyzed using GC-MS. The bandgap (Eg) of CA-AgNPs was calculated as 3.01 eV and zeta potential was found to be −16.1 mV. The biosynthesized CA-AgNPs were confirmed for their degradation efficiency of two toxic water pollutant dyes: Congo red, CR (95.24% degradation within 36 min), and methylene blue, MB (96.72% degradation within 32 min), in the presence of NaBH4. Different doses of CA-AgNPs and NaBH4 were checked for their chemical kinetics and rate constant analysis. The chemical kinetics were explored on the basis of integrated rate law model equations and confirmed as pseudo-zero-order reactionsfor CR and MB dyes. The rate constant ‘k’ for CR and MB was calculated as 0.0311 and 0.0431 mol.L−1.min−1, respectively.

Sensitization of lanthanide complexes through direct spin-forbidden singlet → triplet excitation

Ln^III complexes may display luminescence within the ultraviolet-visible-near-infrared spectral window and although they render bright emissions mainly due to the classical singlet-triplet-state-assisted ligand sensitization, which would be the photophysical parameters if they could be excited through direct spin-forbidden singlet → triplet transitions? Herein, we report the sensitization of Ln complexes through spin-forbidden S₀ → T transitions in a series of homobimetallic Eu^III, Tb^III, Er^III, and Yb^III complexes with halogen-substituted benzoate ligands. As halogens and Ln^III atomic numbers increase, intense singlet → triplet absorption/excitation bands and relative quantum yields up to 18% were achieved due to an increased spin-orbit coupling effect. Moreover, the near-UV-shifted excitation may enable application in luminescent solar concentrators where Yb^III near-infrared luminescence matches the maximum efficiency of the crystalline Si photovoltaic cell. Therefore, the spin-relaxed excitation channel provides new opportunities to improve the Ln^III complex luminescence and potential within the energy conversion field.

Photo-persulfate oxidation and mineralization of benzoic acid: Kinetics and optimization under UVC irradiation

The strong oxidant, persulfate (PS, S₂O₈^2-), was applied to treat the synthetic wastewater of benzoic acid (BA) under UV irradiation. UVC light initiated a chain reaction that derived the sulfate radical (SO₄•^-) and hydroxyl radical (HO•) from S₂O₈^2- ion. The experiment parameters, including light irradiation (UVA and UVC), pH, dose ratio ([PS]₀/[BA]₀), initial concentration ([BA]₀, mg/L), was optimized based on degradation efficiency and total organic carbon (TOC) removal of BA, which reached up to 100% and 96%, respectively, under pH 3.0. The best dose ratio was close to equivalent stoichiometry (and [PS]₀/[BA]₀ = 15) for the treatment of 100 mg-BA/L, suggesting that UV/S₂O₈^2- was able to completely convert BA to carbon dioxide and water. The scavenging test showed that SO₄•^- contributed to about 60% of degradation rate, which the HO• predominated the mineralization rate, i.e., TOC removal. A consecutive kinetic model was proposed to clarify the reaction sequence and rate-determining factor of photo-persulfate oxidation for benzoic acid.

Absorption Spectra and the Electronic Structure of Gallic Acid in Water at Different pH: Experimental Data and Theoretical Cluster Models

Gallic acid (GA) has been characterized in terms of its optical properties in aqueous solutions at varying pH in experiments and in theoretical calculations by analyzing the protonated and deprotonated forms of GA. This work is part of a series of studies of the optical properties of different carboxylic acids in aqueous media. The experimental electronic spectra of GA exhibit two strong well-separated absorption peaks (B- and C-bands), which agree with previous studies. However, in the current study, an additional well-defined low-energy shoulder band (A-band) in the optical spectra of GA was identified. It is likely that the A-band occurs for other carboxylic acids in solution, but because it can overlap with the B-band, it is difficult to discern. The theoretical calculations based on density functional theory were used to simulate the optical absorption spectra of GA in water at different pH to prove the existence of this newly found shoulder band and to describe and characterize the full experimental optical spectra of GA. Different cluster models were tested: (i) all water molecules are coordinated near the carboxy-group and (ii) additional water molecules near the hydroxy-groups of the phenyl ring were included. In this study, we found that both the polarizable continuum model (dielectric property of a medium) and neighboring water molecules (hydrogen-bonding) play significant roles in the optical spectrum. The results showed that only an extended cluster model with water molecules near carboxy- and hydroxy-groups together with the polarizable continuum model allowed us to fully reproduce the experimental data and capture all three absorption bands (A, B, and C). The oscillator strengths of the absorption bands were obtained from the experimental data and compared with theoretical results. Additionally, our work provides a detailed interpretation of the pH effects observed in the experimental absorption spectra.

Water-Soluble Melanoidin Pigment as a New Antioxidant Component of Fermented Willowherb Leaves (Epilobium angustifolium)

Willowherb (Epilobium angustifolium L., family Onagraceae) is a well-known food and medicinal plant used after fermentation as a source of beverages with high antioxidant potential. Despite this long history of use, only a few papers have described the chemical profile and bioactivity of fermented willowherb tea in general. To understand the basic metabolic differences of non-fermented and fermented E. angustifolium leaves, we used general chemical analysis, high-performance liquid chromatography with photodiode array detection and electrospray ionization triple quadrupole mass spectrometric detection assay, and an isolation technique. As a result, the content of 14 chemical groups of compounds was compared in the two plant materials; 59 compounds were detected, including 36 new metabolites; and a new water-soluble phenolic polymer of melanoidin nature was isolated and characterized. The fundamental chemical shifts in fermented E. angustifolium leaves relate mainly to the decrease of ellagitannin content, while there is an increase of melanoidin percentage and saving of the antioxidant potential, despite the significant changes detected. The strong antioxidative properties of the new melanoidin were revealed in a series of in vitro bioassays, and a simulated gastrointestinal and colonic digestion model demonstrated the stability of melanoidin and its antioxidant activity. Finally, we concluded that the new melanoidin is a basic antioxidant of the fermented leaves of E. angustifolium, and it can be recommended for additional study as a promising food and medicinal antioxidant agent.

Highly selective fluorescent turn-on-off sensing of OH-, Al3+ and Fe3+ ions by tuning ESIPT in metal organic frameworks and mitochondria targeted bio-imaging

Herein we report a multifunctional high performance metal organic framework (Zn-DHNDC MOF) based chemosensor that displays an exceptional excited state intramolecular proton transfer (ESIPT) tuned fluorescence turn-on–off response for OH⁻, Al³⁺ and Fe³⁺ ions along with mitochondria targeted bio-imaging. Properly tuning ESIPT as well as the hydroxyl group (–OH) allows Zn-DHNDC MOF to optimize and establish chelation enhanced fluorescence (CHEF) and chelation enhanced quenching (CHEQ) based sensing mechanisms. The MOF benefits from acid-base interactions with the ions which generate a turn-on bluish green fluorescence (λ_Em 492 nm) for OH⁻, an intense turn-on green fluorescence (λ_Em 528 nm) for Al³⁺ and a turn-off fluorescence quenching response for Fe³⁺ ions. The aromatic –OH group indeed plays its part in triggering CHEF and CHEQ processes responsible for the turn-on-off events. Low limits of detection (48 nM of OH⁻, 95 nM for Al³⁺, 33 nM for Fe³⁺ ions), high recyclability and fast response time (8 seconds) further assist the MOF to implement an accurate quantitative sensing strategy for OH⁻, Al³⁺ and Fe³⁺ ions. The study further demonstrates the MOF's behaviour in cellular medium by subjecting it to live cell confocal microscopy. Along with a bio-compatible nature the MOF exhibited successful accumulation inside the mitochondria of MCF7 cancer cells, which defines it as a significant bio-marker. Therefore the present work successfully represents the multidisciplinary nature of Zn-DHNDC MOFs, primarily in sensing and biomedical studies.

ESIPT tuned fluorescence sensing of OH⁻, Al³⁺ and Fe³⁺ ions and mitochondria targeted bio-imaging by a Zn-DHNDC MOF.

Toward a microscopic model of light absorbing dissolved organic compounds in aqueous environments: theoretical and experimental study

Water systems often contain complex macromolecular systems that absorb light. In marine environments, these light absorbing components are often at the air-water interface and can participate in the chemistry of the atmosphere in ways that are poorly understood. Understanding the photochemistry and photophysics of these systems represents a major challenge since their composition and structures are not unique. In this study, we present a successful microscopic model of this light absorbing macromolecular species termed "marine derived chromophoric dissolved organic matter" or "m-CDOM" in water. The approach taken involves molecular dynamics simulations in the ground state using on the fly Density Functional Tight-Binding (DFTB) electronic structure theory; Time Dependent DFTB (TD-DFTB) calculations of excited states, and experimental measurements of the optical absorption spectra in aqueous solution. The theoretical hydrated model shows key features seen in the experimental data for a collected m-CDOM sample. As will be discussed, insights from the model are: (i) the low-energy A-band (at 410 nm) is due to the carbon chains combined with the diol- and the oxy-groups present in the structure; (ii) the weak B-band (at 320-360 nm) appears due to the contribution of the ionized speciated form of m-CDOM; and (iii) the higher-energy C-band (at 280 nm) is due to the two fused ring system. Thus, this is a two-speciated formed model. Although a relatively simple system, these calculations represent an important step in understanding light absorbing compounds found in nature and the search for other microscopic models of related materials remains of major interest.

Quantum Chemical Microsolvation by Automated Water Placement

We developed a quantitative approach to quantum chemical microsolvation. Key in our methodology is the automatic placement of individual solvent molecules based on the free energy solvation thermodynamics derived from molecular dynamics (MD) simulations and grid inhomogeneous solvation theory (GIST). This protocol enabled us to rigorously define the number, position, and orientation of individual solvent molecules and to determine their interaction with the solute based on physical quantities. The generated solute-solvent clusters served as an input for subsequent quantum chemical investigations. We showcased the applicability, scope, and limitations of this computational approach for a number of small molecules, including urea, 2-aminobenzothiazole, (+)-syn-benzotriborneol, benzoic acid, and helicene. Our results show excellent agreement with the available ab initio molecular dynamics data and experimental results.

Absorption spectra of pyruvic acid in water: insights from calculations for small hydrates and comparison to experiment

This study shows that small hydrate models including the roles of both neutral and deprotonated speciated forms provide a good quantitative description and a microscopic interpretation of the experimental spectrum of pyruvic acid in aqueous solution.