Biosynthesis of the Metalloclusters of Nitrogenases.

Nitrogenase is a versatile metalloenzyme that is capable of catalyzing two important reactions under ambient conditions: the reduction of nitrogen (N2) to ammonia (NH3), a key step in the global nitrogen cycle; and the reduction of carbon monoxide (CO) and carbon dioxide (CO2) to hydrocarbons, two reactions useful for recycling carbon waste into carbon fuel. The molybdenum (Mo)- and vanadium (V)-nitrogenases are two homologous members of this enzyme family. Each of them contains a P-cluster and a cofactor, two high-nuclearity metalloclusters that have crucial roles in catalysis. This review summarizes the progress that has been made in elucidating the biosynthetic mechanisms of the P-cluster and cofactor species of nitrogenase, focusing on what is known about the assembly mechanisms of the two metalloclusters in Mo-nitrogenase and giving a brief account of the possible assembly schemes of their counterparts in V-nitrogenase, which are derived from the homology between the two nitrogenases.

Membraneless channels sieve cations in ammonia-oxidizing marine archaea.

Nitrosopumilus maritimus is an ammonia-oxidizing archaeon that is crucial to the global nitrogen cycle1,2. A critical step for nitrogen oxidation is the entrapment of ammonium ions from a dilute marine environment at the cell surface and their subsequent channelling to the cell membrane of N. maritimus. Here we elucidate the structure of the molecular machinery responsible for this process, comprising the surface layer (S-layer), using electron cryotomography and subtomogram averaging from cells. We supplemented our in situ structure of the ammonium-binding S-layer array with a single-particle electron cryomicroscopy structure, revealing detailed features of this immunoglobulin-rich and glycan-decorated S-layer. Biochemical analyses showed strong ammonium binding by the cell surface, which was lost after S-layer disassembly. Sensitive bioinformatic analyses identified similar S-layers in many ammonia-oxidizing archaea, with conserved sequence and structural characteristics. Moreover, molecular simulations and structure determination of ammonium-enriched specimens enabled us to examine the cation-binding properties of the S-layer, revealing how it concentrates ammonium ions on its cell-facing side, effectively acting as a multichannel sieve on the cell membrane. This in situ structural study illuminates the biogeochemically essential process of ammonium binding and channelling, common to many marine microorganisms that are fundamental to the nitrogen cycle.

Catalytic asymmetric addition of an amine N-H bond across internal alkenes.

Hydroamination of alkenes, the addition of the N-H bond of an amine across an alkene, is a fundamental, yet challenging, organic transformation that creates an alkylamine from two abundant chemical feedstocks, alkenes and amines, with full atom economy1-3. The reaction is particularly important because amines, especially chiral amines, are prevalent substructures in a wide range of natural products and drugs. Although extensive efforts have been dedicated to developing catalysts for hydroamination, the vast majority of alkenes that undergo intermolecular hydroamination have been limited to conjugated, strained, or terminal alkenes2-4; only a few examples occur by the direct addition of the N-H bond of amines across unactivated internal alkenes5-7, including photocatalytic hydroamination8,9, and no asymmetric intermolecular additions to such alkenes are known. In fact, current examples of direct, enantioselective intermolecular hydroamination of any type of unactivated alkene lacking a directing group occur with only moderate enantioselectivity10-13. Here we report a cationic iridium system that catalyses intermolecular hydroamination of a range of unactivated, internal alkenes, including those in both acyclic and cyclic alkenes, to afford chiral amines with high enantioselectivity. The catalyst contains a phosphine ligand bearing trimethylsilyl-substituted aryl groups and a triflimide counteranion, and the reaction design includes 2-amino-6-methylpyridine as the amine to enhance the rates of multiple steps within the catalytic cycle while serving as an ammonia surrogate. These design principles point the way to the addition of N-H bonds of other reagents, as well as O-H and C-H bonds, across unactivated internal alkenes to streamline the synthesis of functional molecules from basic feedstocks.

The Biocatalytic Potential of Aromatic Ammonia-Lyase from Loktanella atrilutea.

Characterization of the aromatic ammonia-lyase from Loktanella atrilutea (LaAAL) revealed reduced activity towards canonical AAL substrates: l-Phe, l-Tyr, and l-His, contrasted by its pronounced efficiency towards 3,4-dimethoxy-l-phenylalanine. Assessing the optimal conditions, LaAAL exhibited maximal activity at pH 9.5 in the ammonia elimination reaction route, distinct from the typical pH ranges of most PALs and TALs. Within the exploration of the ammonia source for the opposite, synthetically valuable ammonia addition reaction, the stability of LaAAL exhibited a positive correlation with the ammonia concentration, with the highest stability in 4 M ammonium carbamate of unadjusted pH of ~9.5. While the enzyme activity increased with rising temperatures yet, the highest operational stability and highest stationary conversions of LaAAL were observed at 30 °C. The substrate scope analysis highlighted the catalytic adaptability of LaAAL in the hydroamination of diverse cinnamic acids, especially of meta-substituted and di-/multi-substituted analogues, with structural modelling exposing steric clashes between the substrates' ortho-substituents and catalytic site residues. LaAAL showed a predilection for ammonia elimination, while classifying as a tyrosine ammonia-lyase (TAL) among the natural AAL classes. However, its distinctive attributes, such as genomic context, unique substrate specificity and catalytic fingerprint, suggest a potential natural role beyond those of known AAL classes.

In Situ S-Doping Engineering for Highly Efficient NH3-SCR over Metal-Free Carbon Catalysts: A Novel Synergetic Promotional Mechanism.

Cyclic desulfurization-regeneration-denitrification based on metal-free carbon materials is one of the most promising ways to remove NOx and SO2 simultaneously. However, the impact of S-doping induced by the cyclic desulfurization and regeneration (C-S-R) process on the selective catalytic reduction (SCR) is not well understood. Herein, it is demonstrated that the C-S-R process at 500 °C induces in situ S-doping with a significant accumulation of C-S-C structures. NOx conversion was dramatically enhanced from 18.95% of the original sample to 84.55% of the S-doped sample. Density functional theory calculations revealed that the C-S-C structure significantly regulates the electronic structure of the C atom adjacent to the ketonic carbonyl group, thereby significantly altering the NH3 adsorption configuration with superior adsorption capacity. Moreover, S-doping induces an extra electron transfer between the N atom of the NH3 molecule and the C atom of the carbon plane, thereby promoting the activation of NH3 over the ketonic carbonyl group with a reduced energy barrier. This study elucidates a synergetic promotional mechanism between the ketonic carbonyl group and the C-S-C structure for SCR, offering a novel design strategy for high-performance heteroatom-doped carbon catalysts in industrial applications.

Regulating Electron Metal-Support Interaction to Suppress N2O Formation in the Selective Catalytic Oxidation of Ammonia.

N2O is a common byproduct in the selective catalytic oxidation of ammonia, and its generation often needs to be inhibited due to its strong greenhouse effect. In this paper, using Ag/ZSO-Y as a model catalyst, the N2O selectivity was reduced by 30% through modulation of the electron metal-support interaction. The results demonstrate that the work function of the support can be regulated by the content of the doping element. As the Zr content increases in SnO2, the work function of the support decreases. Moreover, there is a positive correlation between the charge transfer amount and the work function of the support. A series of in situ DRIFTS and density functional theory calculations revealed that the -NO and -N reactions are the primary pathways for N2O formation. By adjustment of the work function of the support through varying the Zr doping level, the electronic structure of Ag NPs was further tuned, resulting in an increased reaction energy barrier for -NO and -N reactions, effectively suppressing N2O formation.

Asymmetric Coordination of Single-Atom Ru Sites Achieves Efficient N(sp3)-H Dehydrogenation Catalysis for Ammonia Oxidation.

Ruthenium single-atom catalysts have great potential in ammonia-selective catalytic oxidation (NH3-SCO); however, the stable sp3 hybrid orbital of NH3 molecules makes N(sp3)-H dissociation a challenge for conventional symmetrical metallic oxide catalysts. Herein, we propose a heterogeneous interface reverse atom capture strategy to construct Ru with unique asymmetric Ru1N2O1 coordination. Ru1N2O1/CeO2 exhibits intrinsic low-temperature conversion (T100 at 160 °C) compared to symmetric coordinated Ru-based (280 °C), Ir-based (220 °C), and Pt-based (200 °C) catalysts, and the TOF is 65.4 times that of Ag-based catalysts. The experimental and theoretical studies show that there is a strong d-p orbital interaction between Ru and N atoms, which not only enhances the adsorption of ammonia at the Ru1N2O1 position but also optimizes the electronic configuration of Ru. Furthermore, the affinity of Ru1N2O1/CeO2 to water is significantly weaker than that of conventional catalysts (the binding energy of the Pd3Au1 catalyst is -1.19 eV, but it is -0.39 eV for our material), so it has excellent water resistance. Finally, the N(sp3)-H activation of NH3 requires the assistance of surface reactive oxygen species, but we found that asymmetric Ru1N2O1 can directly activate the N(sp3)-H bond without the involvement of surface reactive oxygen species. This study provides a novel principle for the rational design of the proximal coordination of active sites to achieve its optimal catalytic activity in single-atom catalysis.

Membraneless Electrochemical Synthesis Strategy toward Nitrate-to-Ammonia Conversion.

Electroreduction of nitrate (NO3RR) to ammonia in membraneless electrolyzers is of great significance for reducing the cost and saving energy consumption. However, severe chemical crossover with side reactions makes it challenging to achieve ideal electrolysis. Herein, we propose a general strategy for efficient membraneless ammonia synthesis by screening NO3RR catalysts with inferior oxygen reduction activity and matching the counter electrode (CE) with good oxygen evolution activity while blocking anodic ammonia oxidation. Consequently, screening the available Co-Co system, the membraneless NO3--to-NH3 conversion performance was significantly higher than H-type cells using costly proton-exchange membranes. At 200 mA cm-2, the full-cell voltage of the membraneless system (∼2.5 V) is 4 V lower than that of the membrane system (∼6.5 V), and the savings are 61.4 kW h (or 56.9%) per 1 kg NH3 produced. A well-designed pulse process, inducing reversible surface reconstruction that in situ generates and restores the active Co(III) species at the working electrode and forms favorable Co3O4/CoOOH at the CE, further significantly improves NO3--to-NH3 conversion and blocks side reactions. A maximum NH3 yield rate of 1500.9 µmol cm-2 h-1 was achieved at -0.9 V (Faraday efficiency 92.6%). This pulse-coupled membraneless strategy provides new insights into design complex electrochemical synthesis.

Regulating the Selectivity of Nitrate Photoreduction for Purification or Ammonia Production by Cooperating Oxidative Half-Reactions.

The removal and conversion of nitrate (NO3-) from wastewater has become an important environmental and health topic. The NO3- can be reduced to nontoxic nitrogen (N2) for environmental remediation or ammonia (NH3) for recovery, in which the tailoring of the selectivity is greatly challenging. Here, by construction of the CuOx@TiO2 photocatalyst, the NO3- conversion efficiency is enhanced to ∼100%. Moreover, the precise regulation of selectivity to NH3 (∼100%) or N2 (92.67%) is accomplished by the synergy of cooperative redox reactions. It is identified that the selectivity of the NO3- photoreduction is determined by the combination of different oxidative reactions. The key roles of intermediates and reactive radicals are revealed by comprehensive in situ characterizations, providing direct evidence for the regulated selectivity of the NO3- photoreduction. Different active radicals are produced by the interaction of oxidative reactants and light-generated holes. Specifically, the introduction of CH3CHO as the oxidative reactant results in the generation of formate radicals, which drives selective NO3- reduction into N2 for its remediation. The alkyl radicals, contributed to by the (CH2OH)2 oxidation, facilitate the deep reduction of NO3- to NH3 for its upcycling. This work provides a technological basis for radical-directed NO3- reduction for its purification and resource recovery.

Modulating the Activity and SO2 Resistance of α-Fe2O3 Catalysts for NH3-SCR of NOx via Crystal Facet Engineering.

The development of Fe-based catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) has garnered significant attention due to their exceptional SO2 resistance. However, the influence of different sulfur-containing species (e.g., ferric sulfates and ammonium sulfates) on the NH3-SCR activity of Fe-based catalysts as well as its dependence on exposed crystal facets of Fe2O3 has not been revealed. This work disclosed that nanorod-like α-Fe2O3 (Fe2O3-NR) predominantly exposing (110) facet performed better than nanosheet-like α-Fe2O3 (Fe2O3-NS) predominantly exposing (001) facet in NH3-SCR reaction, due to the advantages of Fe2O3-NR in redox properties and surface acidity. Furthermore, the results of the SO2/H2O resistance test at a critical temperature of 250 °C, catalytic performance evaluations on Fe2O3-NR and Fe2O3-NS sulfated by SO2 + O2 or deposited with NH4HSO4 (ABS), and systematic characterization revealed that the reactivity of ammonium sulfates on Fe2O3 catalysts to NO(+O2) contributed to their improved catalytic performance, while ferric sulfates showed enhancing and inhibiting effects on NH3-SCR activity on Fe2O3-NR and Fe2O3-NS, respectively; despite this, Fe2O3-NR showed higher affinity for SO2 + O2. This work set a milestone in understanding the NH3-SCR reaction on Fe2O3 catalysts in the presence of SO2 from the aspect of crystal facet engineering.

Unraveling the Mechanistic Origin of High N2 Selectivity in Ammonia Selective Catalytic Oxidation on CuO-Based Catalyst.

NH3 emissions from industrial sources and possibly future energy production constitute a threat to human health because of their toxicity and participation in PM2.5 formation. Ammonia selective catalytic oxidation to N2 (NH3-SCO) is a promising route for NH3 emission control, but the mechanistic origin of achieving high N2 selectivity remains elusive. Here we constructed a highly N2-selective CuO/TiO2 catalyst and proposed a CuOx dimer active site based on the observation of a quadratic dependence of NH3-SCO reaction rate on CuOx loading, ac-STEM, and ab initio thermodynamic analysis. Combining this with the identification of a critical N2H4 intermediate by in situ DRIFTS characterization, a comprehensive N2H4-mediated reaction pathway was proposed by DFT calculations. The high N2 selectivity originated from the preference for NH2 coupling to generate N2H4 over NH2 dehydrogenation on the CuOx dimer active site. This work could pave the way for the rational design of efficient NH3-SCO catalysts.

Unlocking Mixed-Metal Oxides Active Centers via Acidity Regulation for K&SO2 Poisoning Resistance: Self-Detoxification Mechanism of Zeolite-Confined deNOx Catalysts.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient NOx reduction strategy, while the denitrification (deNOx) catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances, such as alkali metal (e.g., K), SO2, etc., in industrial flue gases. It is essential to understand the interaction among various poisons and their effects on the deNOx process. Herein, the ZSM-5 zeolite-confined MnSmOx mixed (MnSmOx@ZSM-5) catalyst exhibited better deNOx performance after the poisoning of K, SO2, and/or K&SO2 than the MnSmOx and MnSmOx/ZSM-5 catalysts, the deNOx activity of which at high temperature (H-T) increased significantly (>90% NOx conversion in the range of 220-480 °C). It has been demonstrated that K would occupy both redox and acidic sites, which severely reduced the reactivity of MnSmOx/ZSM-5 catalysts. The most important, K element is preferentially deposited at -OH on the surface of ZSM-5 carrier due to the electrostatic attraction (-O-K). As for the K&SO2 poisoning catalyst, SO2 preferred to be combined with the surface-deposited K (-O-K-SO2ads) according to XPS and density functional theory (DFT) results, the poisoned active sites by K would be released. The K migration behavior was induced by SO2 over K-poisoned MnSmOx@ZSM-5 catalysts, and the balance of surface redox and acidic site was regulated, like a synergistic promoter, which led to K-poisoning buffering and activity recovery. This work contributes to the understanding of the self-detoxification interaction between alkali metals (e.g., K) and SO2 on deNOx catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NOx reduction.

Selective Nitrate Electroreduction to Ammonia on CNT Electrodes with Controllable Interfacial Wettability.

The development of electrocatalysts that can efficiently reduce nitrate (NO3-) to ammonia (NH3) has garnered increasing attention due to their potential to reduce carbon emissions and promote environmental protection. Intensive efforts have focused on catalyst development, but a thorough understanding of the effect of the microenvironment around the reactive sites of the catalyst is also crucial to maximize the performance of the electrocatalysts. This study explored an electrocatalytic system that utilized quaternary ammonium surfactants with a range of alkyl chain lengths to modify an electrode made of carbon nanotubes (CNT), with the goal of regulating interfacial wettability toward NO3- reduction. Trimethyltetradecylammonium bromide with a moderate alkyl chain length created a very hydrophobic interface, which led to a high selectivity in the production of NH3 (∼87%). Detailed mechanistic investigations that used operando Fourier-transform infrared (FTIR) spectroscopy and online differential electrochemical mass spectrometry (DEMS) revealed that the construction of a hydrophobic modified CNT played a synergistic role in suppressing a side reaction involving the generation of hydrogen, which would compete with the reduction of NO3-. This electrocatalytic system led to a favorable process for the reduction of NO3- to NH3 through a direct electron transfer pathway. Our findings underscore the significance of controlling the hydrophobic surface of electrocatalysts as an effective means to enhance electrochemical performance in aqueous media.

Closing Dichloramine Decomposition Nitrogen and Oxygen Mass Balances: Relative Importance of End-Products from the Reactive Nitrogen Species Pathway.

In drinking water chloramination, monochloramine autodecomposition occurs in the presence of excess free ammonia through dichloramine, the decay of which was implicated in N-nitrosodimethylamine (NDMA) formation by (i) dichloramine hydrolysis to nitroxyl which reacts with itself to nitrous oxide (N2O), (ii) nitroxyl reaction with dissolved oxygen (DO) to peroxynitrite or mono/dichloramine to nitrogen gas (N2), and (iii) peroxynitrite reaction with total dimethylamine (TOTDMA) to NDMA or decomposition to nitrite/nitrate. Here, the yields of nitrogen and oxygen-containing end-products were quantified at pH 9 from NHCl2 decomposition at 200, 400, or 800 µeq Cl2·L-1 with and without 10 µM-N TOTDMA under ambient DO (∼500 µM-O) and, to limit peroxynitrite formation, low DO (≤40 µM-O). Without TOTDMA, the sum of free ammonia, monochloramine, dichloramine, N2, N2O, nitrite, and nitrate indicated nitrogen recoveries ±95% confidence intervals were not significantly different under ambient (90 ± 6%) and low (93 ± 7%) DO. With TOTDMA, nitrogen recoveries were less under ambient (82 ± 5%) than low (97 ± 7%) DO. Oxygen recoveries under ambient DO were 88-97%, and the so-called unidentified product of dichloramine decomposition formed at about three-fold greater concentration under ambient compared to low DO, like NDMA, consistent with a DO limitation. Unidentified product formation stemmed from peroxynitrite decomposition products reacting with mono/dichloramine. For a 2:2:1 nitrogen/oxygen/chlorine atom ratio and its estimated molar absorptivity, unidentified product inclusion with uncertainty may close oxygen recoveries and increase nitrogen recoveries to 98% (ambient DO) and 100% (low DO).

Precisely Controlled Polymerization of Two-Dimensional Conducting Polymers in Quasi-Liquid Layer Enables Ultrahigh Sensing Performance.

Gas sensors based on conducting polymers offer great potential for high-performance room temperature applications due to their cost-effectiveness, high-sensitivity, and operational advantage. However, their current performance is limited by the deficiency of control in conventional polymerization methods, leading to poor crystallinity and inconsistent material properties. Here, the quasi-liquid layer (QLL) on the ice surface acts as a self-regulating nano-reactor for precise control of thermodynamics and kinetics in the polymerization, resulting in a 7.62 nm thick two-dimensional (2D) polyaniline (PANI) film matching the QLL thickness. The ultra-thin film optimizes the exposure of active sites, enhancing the detection of analyte gases at low concentrations. It is validated by fabricating a chemiresistive gas sensor with the 2D PANI film, demonstrating stable room-temperature detection of ammonia down to 10 ppt in ambient air with an impressive 10% response. This achievement represents the highest sensitivity among sensors of this kind while maintaining excellent selectivity and repeatability. Moreover, the QLL-controlled polymerization strategy offers an alternative route for precise control of the polymerization process for conducting polymers, enabling the creation of advanced materials with enhanced properties.

Synthesis of amorphous-MnO2/Clinoptilolite and its utilization for NH4+-N oxidation in an anoxic environment.

Mediating the anoxic ammonia oxidation with manganese oxide (MnOx) can reduce the requirements of dissolved oxygen (DO) concentrations in constructed wetlands (CWs) and improve the removal of ammonium nitrogen (NH4+-N). Recent studies that employed natural manganese ore and/or mine waste as substrates in CWs may develop potentially negative environmental effects due to leachates. However, removing NH4+-N by anoxic ammonia oxidation is influenced by the crystal form of MnOx. In this study, a novel clinoptilolite-based amorphous-MnO2 (amorphous-MnO2/clinoptilolite) was synthesized by the sol-gel method as an alternative substrate to improve the efficiency of anoxic ammonia oxidation and reduce the impact of Mn ion leaching. According to the anoxic ammonia oxidation experiment of clinoptilolite, amorphous-MnO2/clinoptilolite, and manganese ore on NH4+-N, the amounts of NH4+-N removed were 24.55 mg/L/d, 44.55 mg/L/d, and 11.04 mg/L/d, respectively, and the initial NH4+-N concentration was 49.53 mg/L. These results indicated that the amorphous-MnO2/clinoptilolite had both the adsorption and the anoxic ammonia oxidation performance. The recycling experiment demonstrated that the effect of anoxic ammonia oxygen mediated by amorphous-MnO2 would not diminish with the gradual saturation of clinoptilolite for NH4+-N. Furthermore, the anoxic ammonia oxidation consumed NH4+-N in the clinoptilolite, which restored the adsorption capacity of the clinoptilolite and simultaneously decreased the leakage of manganese ions in the process, making it environmentally friendly. Therefore, the amorphous-MnO2/clinoptilolite provided an excellent substrate material for the constructed wetland under an anoxic environment, which greatly improved the nitrogen removal capacity compared to existing substrate materials.

A novel method for production of nitrogen fertilizer with low energy consumption by efficiently adsorbing and separating waste ammonia.

Recovering waste NH3 to be used as a source of nitrogen fertilizer or liquid fuel has recently attracted much attention. Current methods mainly utilize activated carbon or metal-organic frameworks to capture NH3, but are limited due to low NH3 adsorption capacity and high cost, respectively. In this study, novel porous materials that are low cost and easy to synthesize were prepared as NH3 adsorbents by precipitation polymerization with acid optimization. The results showed that adsorption sites (‒COOH, -OH, and lactone) which form chemical adsorption or hydrogen bonds with NH3 were successfully regulated by response surface methods. Correspondingly, the dynamic NH3 adsorption capacity increased from 5.45 mg g-1 to 129 mg g-1, which is higher than most known activated carbon and metal-organic frameworks. Separation performance tests showed that NH3 could also be separated from CO2 and CH4. The findings in this study will advance the industrialization of NH3 polymer adsorbents and provide technical support for the recycling of waste NH3.

Short hydrogen bonds enhance nonaromatic protein-related fluorescence.

Fluorescence in biological systems is usually associated with the presence of aromatic groups. Here, by employing a combined experimental and computational approach, we show that specific hydrogen bond networks can significantly affect fluorescence. In particular, we reveal that the single amino acid L-glutamine, by undergoing a chemical transformation leading to the formation of a short hydrogen bond, displays optical properties that are significantly enhanced compared with L-glutamine itself. Ab initio molecular dynamics simulations highlight that these short hydrogen bonds prevent the appearance of a conical intersection between the excited and the ground states and thereby significantly decrease nonradiative transition probabilities. Our findings open the door to the design of new photoactive materials with biophotonic applications.

Rapid sulfuric acid-dimethylamine nucleation enhanced by nitric acid in polluted regions.

Recent research [Wang et al., Nature 581, 184-189 (2020)] indicates nitric acid (NA) can participate in sulfuric acid (SA)-ammonia (NH3) nucleation in the clean and cold upper free troposphere, whereas NA exhibits no obvious effects at the boundary layer with relatively high temperatures. Herein, considering that an SA-dimethylamine (DMA) nucleation mechanism was detected in megacities [Yao et al., Science 361, 278-281 (2018)], the roles of NA in SA-DMA nucleation are investigated. Different from SA-NH3 nucleation, we found that NA can enhance SA-DMA-based particle formation rates in the polluted atmospheric boundary layer, such as Beijing in winter, with the enhancement up to 80-fold. Moreover, we found that NA can promote the number concentrations of nucleation clusters (up to 27-fold) and contribute 76% of cluster formation pathways at 280 K. The enhancements on particle formation by NA are critical for particulate pollution in the polluted boundary layer with relatively high NA and DMA concentrations.

Development of a Sensitive Colorimetric Indicator for Detecting Beef Spoilage in Smart Packaging.

This study aimed to fabricate and characterize a novel colorimetric indicator designed to detect ammonia (NH3) and monitor meat freshness. The sensing platform was constructed using electrospun nanofibers made from polylactic acid (PLA), which were then impregnated with anthocyanins as a natural pH-sensitive dye, extracted from red cabbage. This research involved investigating the relationship between the various concentrations of anthocyanins and the colorimetric platform's efficiency when exposed to ammonia vapor. Scanning electron microscope (SEM) results were used to examine the morphology and structure of the nanofiber mats before and after the dip-coating process. The study also delved into the selectivity of the indicator when exposed to various volatile organic compounds (VOCs) and their stability under extreme humidity levels. Furthermore, the platform's sensitivity was evaluated as it encountered ammonia (NH3) in concentrations ranging from 1 to 100 ppm, with varying dye concentrations. The developed indicator demonstrated an exceptional detection limit of 1 ppm of MH3 within just 30 min, making it highly sensitive to subtle changes in gas concentration. The indicator proved effective in assessing meat freshness by detecting spoilage levels in beef over time. It reliably identified spoilage after 10 h and 7 days, corresponding to bacterial growth thresholds (107 CFU/mL), both at room temperature and in refrigerated environments, respectively. With its simple visual detection mechanism, the platform offered a straightforward and user-friendly solution for consumers and industry professionals alike to monitor packaged beef freshness, enhancing food safety and quality assurance.