Time-Resolved Fluorescence Spectra of Untreated and Sodium Borohydride-Reduced Chromophoric Dissolved Organic Matter

High selectivity fluorescence-based CQDs@cellulose membrane sensor for detection of hazardous gas

Toluene, a harmful gas, is extremely harmful to living organisms, and its distribution and concentration in the environment have always attracted much attention. However, traditional detection methods have poor selectivity, cannot detect low concentrations, and the equipment is bulky, expensive, and time-consuming. In this study, a fluorescence spectroscopy method based on carbon quantum dots (CQDs) composite cellulose membrane (CQDs@CMC membrane) was developed, which addressed the limitations of CQDs, such as excessive operational expenses, intricate detection procedures, and prolonged analysis duration, enabling highly sensitive and specifically targeted detection of toluene gas. Firstly, hydrophobic propylene glycol/betaine (Gly/Bet) CQDs was prepared by hydrothermal method, and Gly/Bet CQDs composite membranes were prepared by combining them with CMC membrane, and they were characterized separately. It was found that Gly/Bet CQDs had a slight blue shift compared to the optimal excitation wavelength of the Gly/Bet CQDs @CMC membrane. Gly/Bet CQDs were uniformly loaded on the CMC membrane, and the fluorescence lifetime and fluorescence quantum yield (FQY) increased to 5.3 ns and 18.34 ns, respectively. The detection of harmful gas toluene was carried out by Gly/Bet CQDs composite membrane, and the toluene concentration had a great influence on the fluorescence intensity of Gly/Bet CQDs@CMC membrane, and when the toluene concentration was in the range of 200-1400 ppm, the fluorescence intensity was in proportion to the toluene concentration. The detection limit for p-toluene is 0.452 ppm, which is very sensitive. According to the results, the Gly/Bet CQDs@CMC membrane is capable of effectively detecting toluene gas, which offers a solid theoretical foundation for the later use of toluene emission in real industry and toluene gas detection in daily life. In addition, it detects toluene and interfering gases with high selectivity.

Time-resolved fluorescence measurements of dissolved organic matter (DOM) as a function of environmental parameters in estuarine waters

Fluorescent lifetimes of dissolved organic matter (DOM) and associated physicochemical parameters were measured over 14 months in an estuary in Southern California, USA. Measurements were made on 77 samples from sites near the inlet, mid-estuary, and outlet to maximize the range of physicochemical variables. Time-resolved fluorescence data were well fit to a triexponential model with an intermediate lifetime component (τ1: 1 to 5 ns), a long lifetime component (τ2: 2 to 15 ns), and a short lifetime component (τ3: < 1 ns). The amplitude of the short-lived component dominated all measurements (60-70%). However, fractional contributions to steady-state fluorescence were dominated by the intermediate and long-lived components at most wavelengths. Lifetimes varied as a function of both excitation and emission wavelength suggesting structural differences in DOM fluorophores. Lifetimes decreased from the estuary inlet to the outlet and were positively correlated with absorbance and DOC concentrations and negatively correlated with salinity and spectral slope. Quenching experiments with halide ions demonstrated that fluorophores are quenched by heavy ions and that different fluorophores are quenched at different rates. However, concentrations of ions in seawater are not high enough for quenching to completely account for observed lifetime changes across the estuary. The observed variation in lifetimes between sites is instead primarily attributed to structural changes associated with DOM processing. Higher lifetimes are associated with less processed material at the inlet site.

Time-resolved fluorescence of oils and oil distillates in artificial seawater at low excitation wavelengths: Assessing the use of lifetimes to decouple oil and dissolved organic matter (DOM) fluorophores in natural waters

It is challenging to decouple typical protein-like chromophoric dissolved organic matter (CDOM) fluorophores from oil-related fluorophores in natural waters using standard steady-state fluorescence techniques. In the present work, time-resolved florescence spectroscopy was explored as a means of differentiating between these two types of fluorophores. Fluorescence lifetimes of oil products were measured as a function of excitation and emission wavelength in artificial seawater. A triexponential model gave τ1 = ~1-10 ns, τ2 = ~3-30 ns, and τ3 = ~0.2-2 ns. Time-resolved fluorescence amplitudes were dominated by τ3 (τ3 55-65 %; τ1 25-29 %; τ2 11-16 %) and contributions to steady-state fluorescence were dominated by τ1 and τ2. Lifetimes increased with decreasing λex. Fluorescence was quenched by halide ions, but ion concentrations in natural waters are insufficient for quenching to significantly change lifetimes. Heavier, denser oils had red-shifted emission and lower lifetimes. Results suggest that time-resolved spectroscopy can decouple oil and protein-like CDOM fluorophores at λex < 300 nm.

Participation of strong charge-assisted hydrogen bonds in interactions of dissolved organic matter represented by Suwannee River Humic Acid

Terrestrial dissolved organic matter (DOM) plays critical roles in many biotic and abiotic environmental reactions as well as in water treatment. Its structure is therefore of great interest. We examined dissolved Suwannee River Humic Acid (HA) to probe the potential participation of exceptionally strong, negative charge-assisted hydrogen bonds, (-)CAHB, in DOM cohesion and interaction with small weak acids using high performance size exclusion chromatography (HPSEC), transmission electron microscopy, zeta-pH curves, and pH drift experiments. The results support a previously proposed two-tier state of aggregation, in which tightly-knit primary particles (≤ ∼10 kDa) form larger secondary aggregates (up to micrometer in size). Evidence for (-)CAHB is gained through zeta potential changes and pH drift experiments. The primary particles interact with (-)CAHB-capable solutes (simple carboxylic acids and phosphate) but not (-)CAHB-incapable solutes. We identified disruption of intra-segmental and inter-molecular (-)CAHB leading to swelling and disaggregation, as well as formation of nouveau (-)CAHB with free groups on HA. The effects were solute-concentration dependent and greater at pH 5 than pH 6, consistent with CAHB theory. Phosphate induced the greatest shifts in the HPSEC molecular size distribution curves. The shifts were unaffected by prior stripping of innate polyvalent metals. We conclude that the (-)CAHB contributes to the cohesion of DOM, affecting its size and charge, and provides a means by which weak acid pollutants, nutrients, and natural compounds can interact with DOM. Such interactions have implications for the behavior of DOM in the environment, the fate and transport of anthropogenic pollutants, and the roles DOM play in water treatment technologies.

Critical review of fluorescence and absorbance measurements as surrogates for the molecular weight and aromaticity of dissolved organic matter

Dissolved organic matter (DOM) is ubiquitous in aquatic environments and challenging to characterize due to its heterogeneity. Optical measurements (i.e., absorbance and fluorescence spectroscopy) are popular characterization tools, because they are non-destructive, require small sample volumes, and are relatively inexpensive and more accessible compared to other techniques (e.g., high resolution mass spectrometry). To make inferences about DOM chemistry, optical surrogates have been derived from absorbance and fluorescence spectra to describe differences in spectral shape (e.g., E2:E3 ratio, spectral slope, fluorescence indices) or quantify carbon-normalized optical responses (e.g., specific absorbance (SUVA) or specific fluorescence intensity (SFI)). The most common interpretations relate these optical surrogates to DOM molecular weight or aromaticity. This critical review traces the genesis of each of these interpretations and, to the extent possible, discusses additional lines of evidence that have been developed since their inception using datasets comparing diverse DOM sources or strategic endmembers. This review draws several conclusions. More caution is needed to avoid presenting surrogates as specific to either molecular weight or aromaticity, as these physicochemical characteristics are often correlated or interdependent. Many surrogates are proposed using narrow contexts, such as fractionation of a limited number of samples or dependence on isolates. Further study is needed to determine if interpretations are generalizable to whole-waters. Lastly, there is a broad opportunity to identify why endmembers with low abundance of aromatic carbon (e.g., effluent organic matter, Antarctic lakes) often do not follow systematic trends with molecular weight or aromaticity as observed in endmembers from terrestrial environments with higher plant inputs.

Phototransformation of Lignin-related Compounds in Chromophoric Dissolved Organic Matter Solutions

Lignin is a major terrestrial source of chromophoric dissolved organic matter (CDOM), and studying the phototransformation of lignin monomers and their related compounds can enhance our understanding of CDOM intramolecular interactions. Coniferyl aldehyde (Coni) and sinapaldehyde (Sina) form ground-state complexes with CDOM, with equilibrium constants of 7,800 (± 1,800) and 20,000 (± 2,000) M-1, respectively. In comparison, vanillin (Van) exhibits minimal affinity for CDOM complexation. The bimolecular reaction rate constants between singlet oxygen (1O2) and these phenolic carbonyl compounds ranged from 0.46 (± 0.02) to 1.8 (± 0.1) × 107 M-1s-1, which is approximately one order of magnitude lower than their reaction rate constants (0.51 (± 0.02)-1.25 (± 0.02) × 108 M-1s-1) with the triplet excited state of CDOM (3CDOM*). In acidic CDOM solutions (pH 5.0), 1O2, H2O2, and organic peroxyl radicals had negligible impact on the transformation. Comparing the initial transformation rate in the presence and in the absence of NaN3 or furfuryl alcohol led to an overestimation of the contribution of 1O2 to the transformation of Van, Coni, or Sina. 3CDOM* scavengers could not fully inhibit the transformation of Coni or Sina. The remaining transformation is considered to arise from either the unquenched intra-CDOM phase 3CDOM* or a fraction of Coni⊂CDOM or Sina⊂CDOM complex, which underwent intramolecular photoinduced chemical reactions.

Fluorescence Quenching of Humic Substances and Natural Organic Matter by Nitroxide Free Radicals

Fluorescence spectroscopy is one of the most frequently used techniques for studying dissolved organic matter (DOM) in natural and engineered systems. However, the spatial distribution and fluorophores, including local and interacting states, within DOM's larger structure remains poorly understood. In this study, we used two nitroxide fluorescence quenchers to evaluate the chemical and spatial heterogeneity of DOM fluorophores. Several results from quenching experiments with cationic 4-amino-TEMPO (tempamine), including downward-curving Stern-Volmer plots and spectral dependent quenching, show that multiple emitting species contribute to the observed emission even at a single excitation wavelength. Furthermore, for DOM isolates of diverse geographic origins (soil vs aquatic) and isolation procedures (reverse osmosis vs humic substances), the maximum extent of quenching occurs on the red edge of the emission spectra. For soil humic substance isolates, the spectral dependent quenching was significant enough to affect a blue shift in the average emission wavelength. The same soil humic substance isolates whose emission spectra were blue shifted by tempamine quenching were also blue shifted by decreasing solution pH and decreasing solvent polarity, which suggests a role for anionic fluorophores (e.g., hydroxybenzoic acids) in long wavelength fluorescence. Finally, curvature in Stern-Volmer plots indicate that between 10 and 50% of emitting species detected by steady-state fluorescence are inaccessible to quenching by tempamine, suggesting that this fraction of fluorophores may be inaccessible to water solvent. Results from this study provide an assessment of the spatial distribution of fluorophores within DOM and help to reconcile prior studies on the role of solvent polarity and pH on DOM fluorescence.

Computational Calculation of Dissolved Organic Matter Absorption Spectra

The absorption spectrum of dissolved organic matter (DOM) is a topic of interest to environmental scientists and engineers as it can be used to assess both the concentration and physicochemical properties of DOM. In this study, the UV-vis spectra for DOM model compounds were calculated using time-dependent density functional theory. Summing these individual spectra, it was possible to re-create the observed exponential shape of the DOM absorption spectra. Additionally, by predicting the effects of sodium borohydride reduction on the model compounds and then calculating the UV-vis absorbance spectra of the reduced compounds, it was also possible to correctly predict the effects of borohydride reduction on DOM absorbance spectra with a relatively larger decrease in absorbance at longer wavelengths. The contribution of charge-transfer (CT) interactions to DOM absorption was also evaluated, and the calculations showed that intra-molecular CT interactions could take place, while inter-molecular CT interactions were proposed to be less likely to contribute.

Fluorophores in surface freshwaters: importance, likely structures, and possible impacts of climate change

Fluorescence spectroscopy is one of the most useful techniques currently available for the characterisation of organic matter in natural water samples, because it combines easy availability of instrumentation, high sensitivity and limited requirements for sample treatment. The main fluorophores that can be found in natural waters are usually proteins (and/or free amino acids) and humic substances (humic and fulvic acids). The identification of these fluorescent compounds in water samples helps to obtain information about, among others, biological activity in the water body, possible transport of organic matter from soil, and the phenomenon of photobleaching that decreases both the absorbance and (usually) the fluorescence of natural organic matter. Interestingly, all these phenomena can be affected by climate change, which could alter to different extents the ratio between aquagenic and pedogenic fluorophores. Several events induced by warming in natural waters (and especially lake water) could enhance algal growth, thereby also enhancing the production of aquagenic organic matter. Intense precipitation events could increase the export of pedogenic material to surface waters, while photobleaching would be enhanced in the epilimnion of lakes when summer stratification becomes longer and more stable because of higher temperatures. Interestingly, photobleaching affects humic substances to a higher extent compared to protein-like material, thus protein fluorescence signals could be more preserved in stratified waters.

Ultrafast Energy Transfer Determines the Formation of Fluorescence in DOM and Humic Substances

Humification is a ubiquitous natural process of biomass degradation that creates multicomponent systems of nonliving organic matter, including dissolved organic matter (DOM) and humic substances (HS) in water environments, soils, and organic rocks. Despite significant differences in molecular composition, the optical properties of DOM and HS are remarkably similar, and the reason for this remains largely unknown. Here, we employed fluorescence spectroscopy with (sub)picosecond resolution to elucidate the role of electronic interactions within DOM and HS. We revealed an ultrafast decay component with a characteristic decay lifetime of 0.5-1.5 ps and spectral diffusion originating from excitation energy transfer (EET) in the system. The rate of EET was positively correlated to the fraction of aromatic species and tightness of aromatic species packing. Diminishing the number of EET donor-acceptor pairs by reduction with NaBH₄ (decrease of the acceptor number), decrease of pH (decrease of the electron-donating ability), or decrease of the average particle size by filtration (less donor-acceptor pairs within a particle) resulted in a lower impact of the ultrafast component on fluorescence decay. Our results uncover the role of electronic coupling among fluorophores in the formation of DOM and HS optical properties and provide a framework for studying photophysical processes in heterogeneous systems of natural fluorophores.