Recent studies have highlighted the biological significance of 8-hydroxyquinoline-2-carboxylic acid (8-HQA) as a metal chelator. In this work, several techniques were applied for the study of the interaction of 8-HQA with vanadium(IV/V) oxidometal ions at a temperature of T = 298.2 K and an ionic strength of I = 0.20 mol dm(-3) in KCl(aq). The redox behavior of the chemical system was defined, and the stability constants of the formed complexes were determined by H+-ion selective electrode potentiometry and UV-vis spectrophotometric titrations, while nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopies and mass spectrometry provided stoichiometric and structural information. The formation of both oxidovanadium(IV) and dioxidovanadium(V) ML complexes with 8-HQA was observed in aqueous solution, with both complexes being particularly stable in acidic conditions. The dioxidovanadium(V) compound is stable even at neutral pH, but dimeric or tetrameric hydrolytic species are predominant under alkaline conditions. In contrast, oxidovanadium(IV) complexes undergo oxidation as the pH increases. Nevertheless, under strictly anaerobic conditions, the complexation of oxidovanadium(IV) by 8-HQA can also occur at pH > 6.0. The nature of the oxidovanadium(IV) and dioxidovanadium(V) ML complexes, representing the major species formed in solution, was further investigated by DFT calculations

The growing demand for efficient and sustainable wastewater treatment technologies has intensified interest in alternatives to conventional methods. Performic acid (PFA) has lately emerged as an innovative disinfectant and oxidant that can improve traditional wastewater treatment methods (e.g., chlorination) while complying with stricter regulations on disinfection by-products formation. Although use of PFA as a disinfectant is growing, scientific knowledge about its physicochemical properties remains limited and insufficiently detailed. This study aims to fill this gap through comprehensive experimental assessments of the PFA thermal and acid-base stability, including the evaluation of the protonation constant logKPFA at different temperatures (T) and ionic strengths (I). Experiments on thermal self-decomposition showed that PFA degradation follows Arrhenius kinetics, with an activation energy of 59.7 +/- 3.9 kJ & sdot;mol- 1. Despite its intrinsic instability, storing PFA at T <= 0 degrees C allows for sufficient preservation for short- to medium-term applications after synthesis. PFA decomposes quickly converting entirely to H2O2 in alkaline conditions. While it undergoes partial spontaneous degradation at pH 7, decreasing the total oxidant titer. Spontaneous degradation is strictly linked to the PFA protonation constant value logKPFA, which was determined at various temperatures (10 degrees C <= T <= 30 degrees C) and ionic strengths (0.05 M <= I <= 1.00 M) in NaClO4(aq) aqueous solution. Extrapolation at infinite dilution resulted in a thermodynamic protonation constant logTKPFA = 7.58 +/- 0.01. Overall, the findings offer valuable kinetic and thermodynamic data crucial for predicting the behaviour of PFA in real-world applications and especially in advanced water treatments.

The development of chelating agents capable of selectively binding UO22+ in aqueous media remains a major challenge in uranium recovery and remediation, largely due to competitive complexation by vanadium and iron ions. Here, we introduce two water-soluble Girard-T-based acylhydrazone ligands, a planar pentadentate 2,2'-(((1E,1'E)-pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(hydrazin-1-yl-2-ylidene))bis(N,N,N-trimethyl-2-oxoethan-1-aminium) (H2dapGT2+) and tetradentate 2,2'-(((1E,2E)-ethane-1,2-diylidene)bis(hydrazin-1-yl-2-ylidene))bis(N,N,N-trimethyl-2-oxoethan-1-aminium) (H2glyxGT2+), designed to maximize equatorial coordination of the uranyl ion. The reaction of UVIO22+ and FeIII with H2dapGT2+ and H2glyxGT2+ led to the synthesis of [UVIO2(dapGT)(H2O)][ClO4]2 (1), [UVIO2(dapGT)MeOH][SbF6]2 (1'), [{UVIO2(dapGT)}2(μ-VV4O12)]·8H2O (2·8H2O), [UVIO2(glyxGT)(CH3COO)]ClO4 (3), [FeIII(HdapGT2+)Cl2][ClO4]2 (4) and [{FeIII(dapGT2+)}2(μ-O)(CH3OH)(H2O)][ClO4]4 (5). Single-crystal X-ray diffraction reveals that dapGT2+ occupies five from the six available sites of the uranyl equatorial plane to form highly stable hexagonal bipyramidal complexes, while the corresponding FeIII complexes adopt pentagonal bipyramidal geometries. Combined spectroscopic, thermodynamic, and theoretical studies show that dapGT2+ exhibits exceptional stability and pronounced selectivity for UVIO22+ over VVO2+, outperforming amidoxime-based chelators under competitive conditions. In contrast, the more flexible glyxGT2+ ligand forms weaker but still selective uranyl complexes. These findings establish ligand denticity and equatorial plane multidentate ligation as key design principles for achieving uranyl selectivity in water and provide a viable framework for next-generation actinide chelators relevant to uranium extraction, waste management, and actinide sequestration.

Since aquaculture is playing an increasingly important role in food supply, ensuring the healthiness of aquatic environments is a fundamental issue. Anthropogenic activities are often a source of contamination for water and, among the pollutants released, potentially toxic elements (PTEs) represent a well-known hazard to human health and ecosystem safety. In this study, carboxymethylcellulose-alginate (HY-ACMC), methacrylated-chitosan (HY-MCHI), and methacrylated-gelatine (HY-MGEL) hydrogels are synthesized, characterized, and tested as sorbents for PTEs removal. The removal efficiency is significantly affected by pH and contaminants' concentration. Furthermore, experiments in real aquaculture samples from Italy and Denmark farms are carried out to evaluate the matrix effect. Finally, hydrogel regeneration is optimized and sorbent efficiency for multiple cycles of water remediation treatment is investigated. All tested materials show promising removal capabilities even in real water. HY-ACMC has proven to be the most effective for single-cycle remediation treatments thanks to its high performance in all the studied conditions, although it is unstable to regeneration. On the other hand, regeneration with Na2EDTA improves HY-MCHI efficiency, granting its prolonged employment over time. Considering both performances in real water samples and reusability results, HY-MGEL seems to be the most reliable material for multiple cycles of water remediation treatments.

The definition of reliable equilibrium constants is an essential step in speciation studies, as the uncertainty and the consistency of the estimated values concur to define the reliability of the speciation model. It is therefore necessary to evaluate the uncertainty contribution of all factors involved at each step of the data acquisition procedure and to be aware about the consequences of systematic errors on the best-estimated values of experimentally measured equilibrium constants. In this work, a series of computer-generated H+-ion selective electrode titration curves simulating the alkalimetric titration of equimolar mixtures of Zn2+-EDTA in aqueous solutions have been processed by different equilibrium data fitting software. Hence, the uncertainties of the refined stability constants of the Zn2+-EDTA complexes could be derived, while excluding the experimental variability that intrinsically affects all practical experiments. The sensitivity of a given chemical system to different data processing strategies and to possible errors in the input data was evaluated. The systematic errors considered relate to the potential reading, the concentration of the titrant and solution components, and ionic strength variations during titrations. The outcomes highlight how unsuitable decisions taken at the stage of nonlinear least squares fitting of the data can affect the results and underline that the main error contribution is related to the measurement of the glass-electrode potential. The processing of simulated data sets can be a useful tool to alert the researchers to the sensitivity of a given chemical system to different strategies and to possible errors in the input data.

The Dissolved Organic matter (DOM) has been demonstrated to contribute to water self-cleaning and waste-derived substances similar to DOM have already been tested as Fe(III) complexants to degrade compounds of emerging concern (CECs) via photo-Fenton process at pH 5-6. On the other sideDOM has to be removed during natural water potabilization because it negatively affects taste and smell and yields toxic disinfection by-products. In the present work, the DOM exploitation before removal has been explored for the first time. In particular, DOM has been developed as an iron complexing agent for the photo-Fenton degradation of caffeine (taken as representative CEC) in water matrix. The effect of pH, DOM concentration and Fe(II) concentration on the process has been studied. The formation of DOM-Fe(III) complexes has been demonstrated through potentiometric and fluorescence measurements. Promising results for caffeine degradation have been obtained, also when applying the method to a riverine water sample.

Defining the distribution of the chemical species in a multicomponent system is a task of great importance with applications in many fields. To clarify the identity and the abundance of the species that can be formed by the interaction of the components of a solution, it is fundamental to know the formation constants of those species. The determination of equilibrium constants is mainly performed through the analysis of experimental data obtained by different instrumental techniques. Among them, potentiometry is the elective technique for this purpose. As such, a survey was run within the NECTAR COST Action - Network for Equilibria and Chemical Thermodynamics Advanced Research, to identify the most used software for the analysis of potentiometric data and to highlight their strengths and weaknesses. The features and the calculation processes of each software were analyzed and rationalized, and a simulated titration dataset of a hypothetic hexaprotic acid was processed by each software to compare and discuss the optimized protonation constants. Moreover, further data analysis was also carried out on the original dataset including some systematic errors from different sources, as some calibration parameters, the total analytical concentration of reagents and ionic strength variations during titrations, to evaluate their impact on the refined parameters. Results showed that differences on the protonation constants estimated by the tested software are not significant, while some of the considered systematic errors affect results. Overall, it emerged that software commonly used suffer from many limitations, highlighting the urgency of new dedicated and modern tools. In this context, some guidelines for data generation and treatment are also given.

Biosurfactants account for about 12% of the global value of the surfactant market, which is currently dominated by synthetic surfactants obtained from fossil sources. Yet, the production of biosurfactants from renewable feedstock is bound to increase, driven by the increasing pressure from both society and governments for chemistry-based industries to become more ecofriendly and economically sustainable. A photo-chemical oxidation process is reported here, yielding new biosurfactants from urban biowaste in water that perform as a solvent and terminal oxidant reagent at room temperature without the addition of conventional oxidants and catalysts. Products with 200–500 kDa molecular weight are obtained. They lower the surface tension of water down to 34 mN/m at 0.5–2 g/L concentration. The estimated cost is rather low (0.1–1.5 EUR/kg), which is competitive with the cost of synthetic surfactants but much lower than the cost of the best-performing bacterial surfactants. For the implementation of the photo-chemical oxidation process at the industrial level, the results suggest that the new biosurfactants obtained in the present work may not reach the performance level of the best-performing bacterial surfactants capable of lowering the surface tension of water down to 28 mN/m. Yet, the biosurfactants produced by the photo-chemical process have a greater chance of being marketed on large scales.

The chemical composition of the soluble fraction of atmospheric particulate matter (PM) and how these components can combine with each other to form different species affect the chemistry of the aqueous phase dispersed in the atmosphere: raindrops, clouds, fog, and ice particles. The study was focused on the analysis of the soluble fraction of Arctic PM 10 samples collected at Ny-Ålesund (Svalbard Islands, Norwegian Arctic) during the year 2012. The concentration values of Na + , K + , NH 4 + , Ca 2+ , Mg 2+ , Mn 2+ , Cu 2+ , Zn 2+ , Fe 3+ , Al 3+ , Cl − , NO 2 − , NO 3 − , SO 4 2− , PO 4 3− , formate, acetate, malonate, and oxalate in the water-soluble fraction of PM 10 were determined by atomic spectroscopy and ion chromatography. Speciation models were applied to define the major species that would occur in aqueous solution as a function of pH (2–10). The model highlights that (i) the main cations such as Na + , K + , Mg 2+ , and Ca 2+ occur in the form of aquoions in the whole investigated pH range; (ii) Cu 2+ , Zn 2+ , and, in particular, Fe 3+ and Al 3+ are mostly present in their hydrolytic forms; and (iii) Al 3+ , Fe 3+ , and Cu 2+ form solid hydrolytic species that precipitate at pH values slightly higher than neutrality. These latter metals show interesting interactions with oxalate and sulfate ions, too. The speciation models were also calculated considering the seasonal variability of the concentration of the components and at higher concentration levels than those found in water PM extracts, to better simulate concentrations actually found in the atmospheric aqueous phase. The results highlight the role of oxalate as the main organic ligand in solution. Graphical Abstract

Archaeology is the discipline that studies past human civilizations by collecting, documenting and analysing the traces left by the ancestors. The cross-contamination of different scientific fields such as geology, chemistry and physics endorsed a more accurate analysis of the archaeological sites. This work analysed samples from the archaeological site of Org & egrave;res, located near La Thuile (Valle d'Aosta) through the following methodologies: ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy), and CHNS analyser. Inside the site artefacts from the Roman era and structures from the 17th century were found. Results of four different areas were compared with samples taken outside the site. The main soil constituents were analysed through principal component analysis that allowed an in-depth study of the relationships between the samples and highlighted the variables that most influenced the observed relationships. By evaluating the data set, it has been possible to date the site and the activities that were carried out at the archaeological site.

ZVI-Fenton, which is the combination of zero-valent iron (metallic Fe) and H2O2 is a relatively cheap advanced oxidation process for the elimination of contaminants from wastewater. Here we experimentally tested the ZVI-Fenton reaction at pH 4 towards two crucial goals in the treatment of secondary (partially treated) urban wastewater: (i) degradation of pharmaceuticals such as anti-inflammatory drugs (ibuprofen) and antibiotics (cefazolin, sulfamethoxazole), and (ii) elimination of a considerable fraction of bacteria through a combination of acidic pH and strongly oxidising conditions. In detail, ZVI-Fenton at pH 4 achieved degradation of both primary contaminants and potentially problematic transformation intermediates. The latter include toxic 4-isobutylacetophenone from ibuprofen and compounds potentially retaining antibiotic properties, namely cefazolin products with an intact β-lactam ring and sulfamethoxazole products retaining the p-amino sulfonic acid moiety. Furthermore, the ZVI-Fenton process significantly lowered the total abundance of bacteria, greatly aiding the final disinfection stage. Overall, both objectives were successfully achieved demonstrating that ZVI-Fenton at pH 4 is an efficient treatment against chemical and microbiological contaminants.

Evidence is here provided that irradiation of some lake water samples can trigger the formation of fluorophores with humic-like properties, at the same time increasing water absorbance. This phenomenon is the opposite of photo -bleaching, which is often observed when natural waters are irradiated. The photoproduced humic-like fluorophores observed here would be of autochthonous rather than allochthonous origin, which marks a difference with the fraction of humic substances that derives from terrestrial sources. Photogeneration of humic-like compounds can be highlighted in water samples where the fluorescence signal of initially occurring humic substances is low, so that their photobleaching is minimised. Samples that are most likely to show photoinduced formation of humic-like fluorophores are in fact characterised by high values of protein-like vs. humic-like contribution ratios to fluores-cence, as evidenced by parallel factor (PARAFAC) analysis. Mountain lakes in late summer appear to be suitable candidates to highlight the described phenomenon. In some cases, lake-water irradiation caused a decrease in the spectral slope of the absorbance that, together with increasing absorbance values, is consistent with an increase in molecular mass and aromaticity of organic matter. The absorbance increase triggered by irradiation might play a role in screening biologically harmful UV radiation, in mountain environments that would otherwise be characterised by very clear water that allows for easy transmission of UV light along the water column.

A new, open-source, practical, modern and multi-platform Python application for concentrations calculation (PyES - Python Equilibrium Species) was developed by re-writing and implementing the ES4, a freeware computer program originally written using the BASIC programming language. Currently PyES has two working modes, i.e., titration simulation and species distribution, and it can handle both precipitation and solution equilibria. Noteworthy, PyES is actually the only open-source software able to perform calculations at variable ionic strength, taking into account the dependence of the stability constants on it, and to take into account the error propagation to estimate the uncertainties in the calculated concentrations using those of the formation constants, solubility products and components concentrations. Various tests were performed to verify the reliability of PyES with very satisfying results. Moreover, PyES is user friendly and compatible with existing operative systems. The data analysis results can be visualized in a graphical presentation and can be easily exported as .xlsx or .csv files. PyES and the corresponding source code are available for download at htt ps://github.com/Kastakin/PyES.

Both inorganic and organic complexation of metal cations in clouds or rainwater is essential to describe the global biogeochemical cycles of metals, because complexation can increase metal solubility and stabilize some of their oxidation states. Within a Project of the National Research Program in the Antarctica, atmospheric depositions were collected during the Antarctic summer 2017–2018 in eight sampling sites. The main ionic components occurring in water extracts of these atmospheric depositions were quantified, and a chemical model was applied, in order to identify the main species occurring in the samples. The speciation study showed that most cations were present as aquoions, except for Fe, which occurred predominantly in hydrolytic forms. The model allowed us to foresee the effect of an increase in the concentration levels of all the solution components, by simulating what could happen when the original particles act as cloud condensation nuclei. The role of inorganic anions as complexing agents becomes important when increasing total concentrations of all the solutes by a factor >100 compared to the water extracts, while the presence of organic acids acquires significance for samples having organic acid concentration higher than 10−5 mol L−1. Moreover, it was possible to pinpoint the formation constants that mostly affect the chemical system, and to gain insight into the behavior of metals in wet depositions, which is fundamental knowledge in atmospheric photochemistry studies and in the modeling of the biogeochemical cycles of metal cations.

Diclofenac (DCF) is a nonsteroidal anti-inflammatory drug to treat pain and inflammatory diseases. The high consumption of the drug leads to a significant change in the ecosystem. With the aim of optimizing a fast screening analysis for DCF detection on many samples with a sensitive and cheap procedure, we considered electrochemical methods using carbon-based electrodes as sensors. The electrochemical behavior of the DCF was studied on glassy carbon electrodes (GCE) and on screen-printed carbon electrodes (SPCEs) from two different suppliers after an anodic activation. The surface of the SPCEs was analyzed by scanning electron microscope (SEM) and Energy Dispersive Spectrometry (EDS). On all the activated electrodes, the voltammetric procedure (Differential Pulse Voltammetry) for the determination of DCF was optimized by the Experimental Design method, and the linearity range of the response, as well as the calibration and limit parameters (limits of detection—LoD; limit of quantification—LoQ), were defined. Analyses on SPCEs were performed both by immersing the electrode in the solution and by deposing a drop of solution on the electrode. DCF signals are stabilized by the polishing process and enhanced by the anodic activation and acid pH. The electrochemical response of DCF is not reversible, and its by-products tend to be adsorbed on the surfaces, particularly on GCE. The lowest limit parameters were obtained using the GCE (LoD = 1.6 µg L−1) and the SPCE, having the smallest surface, immersed in solution (LoD = 7 µg L−1).

The total antioxidant capacity (TAC) of human plasma is an index of the redox buffer capacity of this biological fluid and could be a biomarker for those disorders affecting redox status. Distinguishing physiological from pathological conditions needs a reference. Therefore, this work aims to define the reference intervals for TAC of human plasma of apparently healthy adult individuals. TAC was measured using the CUPRAC-BCS (CUPric reducing antioxidant capacity-bathocuproinedisulfonic acid) method previously optimized and tested in a clinical laboratory. A population of 500 blood donors was selected, plus an additional 222 pathological patients carrying specific defective metabolisms, namely, hyperuricemia, hyperbilirubinemia, and type 2 diabetic mellitus. The reference intervals of TAC were calculated according to international guidelines. Due to the response of a partitioning test, the reference intervals for healthy population were separately defined for male (258) and female (151) groups. The reference intervals (µmol L−1) resulted: 727–1248 for the male subgroup and 637–1048 for the female subgroup. The absence of an age effect on TAC values was verified. The reference intervals evaluated allow a discussion on some pathological conditions overloading the plasma with redox-active waste substances.

Background: Metabolic and physicochemical evaluation is recommended to manage the condition of patients with nephrolithiasis. The estimation of the saturation state (β values) is often included in the diagnostic work-up, and it is preferably performed through calculations. The free concentrations of constituent ions are estimated by considering the main ionic soluble complexes. It is contended that this approach is liable to an overestimation of β values because some complexes may be overlooked. A recent report found that β values could be significantly lowered upon the addition of new and so far neglected complexes, [Ca(PO4)Cit]4− and [Ca2H2(PO4)2]. The aim of this work was to assess whether these complexes can be relevant to explaining the chemistry of urine. Methods: The Ca–phosphate–citrate aqueous system was investigated by potentiometric titrations. The stability constants of the parent binary complexes [Cacit]− and [CaPO4]−, and the coordination tendency of PO43− toward [Ca(cit)]− to form the ternary complex, were estimated. βCaOx and βCaHPO4 were then calculated on 5 natural urines by chemical models, including or not including the [CaPO4]− and [Ca(PO4)cit]4− species. Results: Species distribution diagrams show that the [Ca(PO4)cit]4− species was only noticeable at pH > 8.5 and below 10% of the total calcium. β values estimated on natural urine were slightly lowered by the formation of [CaPO4]− species, whereas [Ca(PO4)cit]4− results were irrelevant. Conclusions: While [CaPO4]− species have an impact on saturation levels at higher pHs, the existence of ternary complex and of the dimer is rejected.

Ambroxol hydrochloride (AMB), used as a broncho secretolytic and an expectorant drug, is a semi-synthetic derivative of vasicine obtained from the Indian shrub Adhatoda vasica. It is a metabolic product of bromhexine. The paper provides comprehensive and detailed research on ambroxol hydrochloride, gives information on thermal stability, the mechanism of AMB degradation, and data of practical interest for optimization of formulation that contains AMB as an active compound. Investigation on pure AMB and in commercial formulation Flavamed® tablet (FT), which contains AMB as an active compound, was performed systematically using thermal and spectroscopic methods, along with a sophisticated and practical statistical approach. AMB proved to be a heat-stable and humidity-sensitive drug. For its successful formulation, special attention should be addressed to excipients since it was found that polyvinyl pyrrolidone and Mg stearate affect the thermal stability of AMB. At the same time, lactose monohydrate contributes to faster degradation of AMB and change in decomposition mechanism. It was found that the n-th order kinetic model mechanistically best describes the decomposition process of pure AMB and in Flavamed® tablets.

(1) Background: Much effort has been expended to investigate the antioxidant capacity of human plasma, attempting to clarify the roles of both metabolic and food substances in determining defenses against oxidative stress. The relationship between the total antioxidant capacity (TAC) and the concentrations of redox-active biomolecules in the human plasma of healthy and cardiopathic individuals was investigated in the present study to develop a chemical speciation model. (2) Methods: Plasma was collected from 85 blood donors and from 25 cardiovascular surgery patients. The TAC was measured using the CUPRAC-BCS (CUPric Reducing Antioxidant Capacity - Bathocuproinedisulfonic acid) method. Biomolecule concentrations were determined via visible spectrophotometry or HPLC/RP techniques. The relationship between the TAC and the concentrations was defined by applying a multiple regression analysis. The significance of the variables was first tested, and chemical models were proposed for the two datasets. The model equation is βTAC=∑iβi·Ai, where βi and [Ai] are the electronic exchange and the molar concentrations of the ith antioxidant component, respectively. (3) Results: The major contributions to the TAC, ~80%, come from endogenous compounds in both healthy and cardiopathic individuals, whereas the contributions from exogenous compounds were different between the two datasets. In particular, γ-tocopherol showed a different role in the chemical models developed for the two groups.

The photophysics and photochemistry of 4-hydroxybenzophenone (4HOBP) are interesting because they can give some insight into the behavior of humic material. Here we show that 4HOBP has a number of fluorescence peaks: (i) an intense one at excitation/emission wavelengths Ex/Em ~ 200–230/280–370 nm, likely due to an excitation transition from S 0 to S 5 or S 6 , followed by S 2 → S 0 in emission (S n denotes the singlet states of 4HOBP); (ii) a minor peak at Ex/Em ~ 270–300/320–360 nm (S 0 → S 2 in absorption and S 2 → S 0 in emission), and (iii) very interesting signals in the typical emission region of humic substances, most notably at Ex/Em ~ 200–220/400–500 nm and Ex/Em ~ 260–280/400–470 nm (in both cases the emission corresponded to an S 1 → S 0 transition). The peak (i) (Ex/Em ~ 200–230/280–370 nm) is quite intense at low 4HOBP concentration values, but it undergoes an effective inner-filter phenomenon. Remarkably, 4HOBP shows fluorescence peaks that arise from S 2 → S 0 transitions and that do not follow Kasha’s rule. Fluorescence is observed in aprotic or poorly protic solvents, and to a lesser extent in aqueous solution. The excited states of 4HOBP, and most notably 4HOBP-S 1 , are much stronger acids than 4HOBP-S 0 . Therefore, excited 4HOBP is quickly deprotonated to 4OBP − -S 0 in ~neutral solution, with a considerable loss of the fluorescence properties. Higher fluorescence intensity can be observed under acidic conditions, where excited-state deprotonation is less effective, and in basic solution where the dissociated 4OBP − -S 0 form prevails as the ground state. The excited states of 4OBP − are formed directly upon radiation absorption, and being weak bases they do not undergo important acid–base equilibria. Therefore, they can undergo radiational deactivation to produce significant fluorescence emission.