Deciphering the Mechanism of Base-Triggered Conversion of Ammonia to Molecular Nitrogen and Methylamine to Cyanide.

We report an in-depth investigation into the ammonia oxidation mechanism by the catalyst [RuIII(tpy)(dmabpy)NH3]3+ ([Ru(NH3)]3+). Stoichiometric reactions of [Ru(NH3)]3+ were carried out with exogenous noncoordinating bases to trigger a proposed redox disproportionation reaction, which was followed using variable-temperature NMR spectroscopy. An intermediate species was identified as a dinitrogen-bridged complex using 15N NMR and Raman spectroscopy on isotopically labeled complexes. This intermediate is proposed to derive from coupling of nitridyl species formed upon sequential redox disproportion reactions. Acetonitrile displaces the dinitrogen bridge to yield free N2. DFT calculations support this lower-energy pathway versus that previously reported for ammonia oxidation by the parent [RuIII(tpy)(bpy)NH3]3+ complex. These experimental and computational results are consistent with the interpretation of redox disproportionation involving sequential hydrogen atom transfer reactions by an amide/aminyl intermediate, [Ru(NH2)-]+ ⇔ [Ru(NH2)•]+, formed upon deprotonation of the parent complex. Control experiments employing a large excess of ammonia as a base indicate this new proposed lower-energy pathway contributes to the oxidation of ammonia to dinitrogen in conditions relevant to electrocatalysis. In addition, analogous methylamine complexes, [Ru(NH2CH3)]2+/3+, were prepared to further test the proposed mechanism. Treating [Ru(NH2CH3)]3+ with a base cleanly yields two products [Ru(NH2CH3)]2+ and [Ru(CN)]+ in an ∼3:1 ratio, fully consistent with the proposed cascade of hydrogen atom transfer reactions by an intermediate.

Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia.

Global ammonia production reached 175 million metric tons in 2016, 90% of which is produced from high purity N2 and H2 gases at high temperatures and pressures via the Haber-Bosch process. Reliance on natural gas for H2 production results in large energy consumption and CO2 emissions. Concerns of human-induced climate change are spurring an international scientific effort to explore new approaches to ammonia production and reduce its carbon footprint. Electrocatalytic N2 reduction to ammonia is an attractive alternative that can potentially enable ammonia synthesis under milder conditions in small-scale, distributed, and on-site electrolysis cells powered by renewable electricity generated from solar or wind sources. This review provides a comprehensive account of theoretical and experimental studies on electrochemical nitrogen fixation with a focus on the low selectivity for reduction of N2 to ammonia versus protons to H2. A detailed introduction to ammonia detection methods and the execution of control experiments is given as they are crucial to the accurate reporting of experimental findings. The main part of this review focuses on theoretical and experimental progress that has been achieved under a range of conditions. Finally, comments on current challenges and potential opportunities in this field are provided.

Cyanine-Based Polymer Dots with Long-Wavelength Excitation and Near-Infrared Fluorescence beyond 900 nm for In Vivo Biological Imaging.

Bioimaging in the near-infrared window is of great importance to study the dynamic processes in vivo with deep penetration, high spatiotemporal resolution, and minimal tissue absorption, scattering, and autofluorescence. In spite of the huge progress on the synthesis of small organic fluorophores and inorganic nanomaterials with emissions beyond 900 nm, it remains a tough challenge to synthesize semiconducting polymers with fluorescence over this region. Here, we synthesized a series of heptamethine cyanine-based polymers with both absorption and emission in the near-infrared region. We prepared these polymers as semiconducting polymer dots (Pdots) in pure water with great biocompatibility. The fluorescence quantum yield of the Pdots can be as high as 14% with a full width at half-maximum of 53 nm, and their single-particle brightness is more than 20 times higher than commercial quantum dots or ∼300 times brighter than Food and Drug Administration (FDA)-approved indocyanine green (ICG) dyes. We further demonstrated the use of cyanine-based Pdots for specific cellular labeling and long-term tumor targeting in mice. We anticipate that these cyanine-based ultrabright Pdots could open up an avenue for next generations of near-infrared fluorescent agents.

Molecular Engineering and Design of Semiconducting Polymer Dots with Narrow-Band, Near-Infrared Emission for in Vivo Biological Imaging.

This article describes the design and synthesis of donor-bridge-acceptor-based semiconducting polymer dots (Pdots) that exhibit narrow-band emissions, ultrahigh brightness, and large Stokes shifts in the near-infrared (NIR) region. We systematically investigated the effect of π-bridges on the fluorescence quantum yields of the donor-bridge-acceptor-based Pdots. The Pdots could be excited by a 488 or 532 nm laser and have a high fluorescence quantum yield of 33% with a Stokes shift of more than 200 nm. The emission full width at half-maximum of the Pdots can be as narrow as 29 nm, about 2.5 times narrower than that of inorganic quantum dots at the same emission wavelength region. The average per-particle brightness of the Pdots is at least 3 times larger than that of the commercially available quantum dots. The excellent biocompatibility of these Pdots was demonstrated in vivo, and their specific cellular labeling capability was also approved by different cell lines. By taking advantage of the durable brightness and remarkable stability of these NIR fluorescent Pdots, we performed in vivo microangiography imaging on living zebrafish embryos and long-term tumor monitoring on mice. We anticipate these donor-bridge-acceptor-based NIR-fluorescent Pdots with narrow-band emissions to find broad use in a variety of multiplexed biological applications.

Dual Colorimetric and Fluorescent Imaging of Latent Fingerprints on Both Porous and Nonporous Surfaces with Near-Infrared Fluorescent Semiconducting Polymer Dots.

Semiconducting polymer dots (Pdots) have recently been proven as a novel type of ultrabright fluorescent probes that can be extensively used in analytical detection. Here, we developed a dual visual sensor based on Pdots for fingerprint imaging. We first designed and synthesized two types of near-infrared (NIR) fluorescent polymers and then embedded ninhydrin into the Pdot matrix. The resulting Pdot assays showed the colorimetric and fluorescent dual-readout abilities to detect latent fingerprints on both porous and nonporous surfaces. The developed fingerprints clearly revealed first-, second-, and third-level details with high contrast, high selectivity, and low background interference. We also grafted the chemical groups on the nanoparticle surface to investigate the mechanisms involved in the fingerprint development processes. We further utilized this assay in note paper and checks for latent fingerprint imaging. We believe that this dual-readout method based on Pdots will create a new avenue for research in fingerprint detection and anticounterfeiting technology.

Quinoxaline-Based Polymer Dots with Ultrabright Red to Near-Infrared Fluorescence for In Vivo Biological Imaging.

This article describes the design and synthesis of quinoxaline-based semiconducting polymer dots (Pdots) that exhibit near-infrared fluorescence, ultrahigh brightness, large Stokes shifts, and excellent cellular targeting capability. We also introduced fluorine atoms and long alkyl chains into polymer backbones and systematically investigated their effect on the fluorescence quantum yields of Pdots. These new series of quinoxaline-based Pdots have a fluorescence quantum yield as high as 47% with a Stokes shift larger than 150 nm. Single-particle analysis reveals that the average per-particle brightness of the Pdots is at least 6 times higher than that of the commercially available quantum dots. We further demonstrated the use of this new class of quinoxaline-based Pdots for effective and specific cellular and subcellular labeling without any noticeable nonspecific binding. Moreover, the cytotoxicity of Pdots were evaluated on HeLa cells and zebrafish embryos to demonstrate their great biocompatibility. By taking advantage of their extreme brightness and minimal cytotoxicity, we performed, for the first time, in vivo microangiography imaging on living zebrafish embryos using Pdots. These quinoxaline-based NIR-fluorescent Pdots are anticipated to find broad use in a variety of in vitro and in vivo biological research.

Dual colorimetric and fluorescent sensor based on semiconducting polymer dots for ratiometric detection of lead ions in living cells.

Recently, semiconducting polymer dots (Pdots) have become a novel type of ultrabright fluorescent probes which hold great promise in biological imaging and analytical detection. Here we developed a visual sensor based on Pdots for Pb(2+) detection. We first embedded near-infrared (NIR) dyes into the matrix of poly[(9,9-dioctylfluorene)-co-2,1,3-benzothiadiazole-co-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PFBT-DBT) polymer and then capped the Pdots with polydiacetylenes (PDAs), in which parts of the PDAs were prefunctionalized with 15-crown-5 moieties to form Pdots. The high selectivity of these Pdots for lead ions is attributed to the formation of 2:1 15-crown-5-Pb(2+)-carboxylate sandwich complex on the Pdot surface. After Pb(2+) chelation, the conjugation system of the PDA was perturbed and strained, causing a chromatic change of the PDA from blue to red. At the same time, the encapsulated NIR dyes were liable to leach out that resulted in an emission variation of the Pdots. Accordingly, lead ions can be recognized by either color change or emission variation of the Pdots. We also loaded these nanoprobes into live HeLa cells through endocytosis, and then monitored changes in Pb(2+) levels within cells, demonstrating their utility for use in cellular and bioimaging applications. In addition, we fabricated easy-to-prepare test strips impregnated with Pdot-poly(vinyl alcohol) films to identify Pb(2+) in real samples, which proved their applicability for in situ on-site detection. Our results suggest that this Pdot-based visual sensor shows promising potential for advanced environmental and biological applications.

Near-infrared fluorescent semiconducting polymer dots with high brightness and pronounced effect of positioning alkyl chains on the comonomers.

In recent years, semiconducting polymer dots (Pdots) have emerged as a novel class of extraordinarily bright fluorescent probes with burgeoning applications in bioimaging and sensing. While the desire for near-infrared (NIR)-emitting agents for in vivo biological applications increases drastically, the direct synthesis of semiconducting polymers that can form Pdots with ultrahigh fluorescence brightness is extremely lacking due to the severe aggregation-caused quenching of the NIR chromophores in Pdots. Here we describe the synthesis of dithienylbenzoselenadiazole (DBS)-based NIR-fluorescing Pdots with ultrahigh brightness and excellent photostability. More importantly, the fluorescence quantum yields of these Pdots could be effectively increased by the introduction of long alkyl chains into the thiophene rings of DBS to significantly inhibit the aggregation-caused emission quenching. Additionally, these new series of DBS-based Pdots can be excited by a commonly used 488 nm laser and show a fluorescence quantum yield as high as 36% with a Stokes shift larger than 200 nm. Single-particle analysis indicates that the per-particle brightness of the Pdots is at least 2 times higher than that of the commercial quantum dot (Qdot705) under identical laser excitation and acquisition conditions. We also functionalized the Pdots with carboxylic acid groups and then linked biomolecules to Pdot surfaces to demonstrate their capability for specific cellular labeling without any noticeable nonspecific binding. Our results suggest that these DBS-based NIR-fluorescing Pdots will be very practical in various biological imaging and analytical applications.

Polydiacetylene-enclosed near-infrared fluorescent semiconducting polymer dots for bioimaging and sensing.

Semiconducting polymer dots (P-dots) recently have emerged as a new type of ultrabright fluorescent probe with promising applications in biological imaging and detection. With the increasing desire for near-infrared (NIR) fluorescing probes for in vivo biological measurements, the currently available NIR-emitting P-dots are very limited and the leaching of the encapsulated dyes/polymers has usually been a concern. To address this challenge, we first embedded the NIR dyes into the matrix of poly[(9,9-dioctylfluorene)-co-2,1,3-benzothiadiazole-co-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PF-BT-DBT) polymer and then enclosed the doped P-dots with polydiacetylenes (PDAs) to avoid potential leakage of the entrapped NIR dyes from the P-dot matrix. These PDA-enclosed NIR-emitting P-dots not only emitted much stronger NIR fluorescence than conventional organic molecules but also exhibited enhanced photostability over CdTe quantum dots, free NIR dyes, and gold nanoclusters. We next conjugated biomolecules onto the surface of the resulting P-dots and demonstrated their capability for specific cellular labeling without any noticeable nonspecific binding. To employ this new class of material as a facile sensing platform, an easy-to-prepare test paper, obtained by soaking the paper into the PDA-enclosed NIR-emitting P-dot solution, was used to sense external stimuli such as ions, temperature, or pH, depending on the surface functionalization of PDAs. We believe these PDA-coated NIR-fluorescing P-dots will be very useful in a variety of bioimaging and analytical applications.

Coumarin dye-embedded semiconducting polymer dots for ratiometric sensing of fluoride ions in aqueous solution and bio-imaging in cells.

This paper describes a simple platform that employs coumarin dye-encapsulated semiconducting polymer dots as a fluorescent probe for ratiometric and sensitive fluoride anion detection, in which the sensing mechanism is based on the deprotection of the tert-butyldimethylsilyl group on coumarin to induce Förster resonance energy transfer.

Photoactivated ratiometric copper(II) ion sensing with semiconducting polymer dots.

This paper describes a simple platform that employs spiropyran-functionalized semiconducting polymer dots as a fluorescent probe for photoactivated ratiometric and sensitive Cu(2+) detection, in which the sensing mechanism is based on photogenerated merocyanine that can selectively bind Cu(2+) to induce Förster resonance energy transfer.