Microfluidic paper-based analytical device for the speciation of inorganic nitrogen species.

A microfluidic paper-based analytical device (µPAD) utilizing gas-diffusion separation and solid-phase reduction was developed for the first time for the determination of both ammonium and nitrate, which are the dominant inorganic nitrogen species in environmental waters. The device consists of 3 filter paper layers accommodating the sample, reagent and detection zones. The reagent zone is separated from the detection zone by a semipermeable hydrophobic membrane and acts as a solid-phase reactor where nitrate is reduced to ammonia by Devarda's alloy microparticles, integrated into a µPAD for the first time. The detection zone incorporates the acid-base indicators bromothymol blue (BTB) or nitrazine yellow (NY) and changes colour in two steps. Initially the colour change is caused by ammonia generated by the reaction of ammonium and sodium hydroxide in the sample zone. This colour change is followed by a subsequent colour change as a result of the ammonia produced by the reduction of nitrate by the Devarda's alloy microparticles. The corresponding reflectance value changes are used for the quantification of the two inorganic nitrogen species in the ranges 6.5-100.0 or 2.1-15.0 mg N L-1 for ammonium and 18.2-100.0 or 4.2-15.0 mg N L-1 for nitrate when BTB or NY are used, respectively. Under optimal conditions the limits of quantification of ammonium and nitrate in the case of BTB were determined as 6.5 and 18.2 mg N L-1, respectively, while the corresponding values in the case of NY were found to be 2.1 and 4.2 mg N L-1. The newly developed µPAD was stable for 62 days when stored in a freezer and 1 day at ambient temperature. It was validated with a certified reference material and successfully applied to the determination of ammonium and nitrate in spiked environmental water samples and soil extracts.

Structurally Precise Two-Transition-Metal Water Oxidation Catalysts: Quantifying Adjacent 3d Metals by Synchrotron X-Radiation Anomalous Dispersion Scattering.

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the locations and percent occupancies of different 3d metals in these heterogeneous catalysts. Without such information, it is hard to quantify catalysis, stability, and other properties of the WOC as a function of the catalyst active site structure. This study combines the site selective synthesis of a homogeneous WOC with two adjacent 3d metals, [Co2Ni2(PW9O34)2]10- (Co2Ni2P2) as a tractable molecular model for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) that quantifies both the location and percent occupancy of Co (∼97% outer-central-belt positions only) and Ni (∼97% inner-central-belt positions only) in Co2Ni2P2. This mixed-3d-metal complex catalyzes water oxidation an order of magnitude faster than its isostructural analogue, [Co4(PW9O34)2]10- (Co4P2). Four independent and complementary lines of evidence confirm that Co2Ni2P2 and Co4P2 are the principal WOCs and that Co2+(aq) is not. Density functional theory (DFT) studies revealed that Co4P2 and Co2Ni2P2 have similar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multistep mechanisms, including likely Co-OOH formation, with the energetics of most steps being lower for Co2Ni2P2 than for Co4P2. Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts.

Competition of Dexter, Förster, and charge transfer pathways for quantum dot sensitized triplet generation.

Quantum dot (QD) sensitized triplet exciton generation has demonstrated promising applications in various fields such as photon up-conversion through triplet-triplet annihilation. However, how direct triplet energy transfer from the QD to the acceptor through Dexter energy transfer (DET) competes with other processes, including Förster resonance energy transfer (FRET) and charge transfer, remains poorly understood. Herein, the competition of these pathways for QD-sensitized triplet excited state generation in CdSe QD-modified boron dipyrromethene (BODIPY) complexes is studied using transient absorption spectroscopy. After excitation of the CdSe QD with 500 nm pulses, the BODIPY triplet excited state is generated through charge recombination in a charge separated intermediate state (QD-·-BODIPY+·). This intermediate state is populated either through FRET from the excited QD to BODIPY followed by electron transfer from the singlet excited state of BODIPY to the QD or through hole transfer from the excited QD to BODIPY. The triplet excited state generation efficiencies from the FRET and hole transfer pathways are estimated to be (6.18 ± 1.39)% and (13.5 ± 3.1)%, respectively. Compared to these indirect pathways, direct DET from the QD to the BODIPY triplet state is kinetically not competitive. These results demonstrate that sequential charge transfer can be an efficient pathway for triplet excited state generation in QD-acceptor complexes.

Enhanced triplet state generation through radical pair intermediates in BODIPY-quantum dot complexes.

Generation of triplet excited states through radical pair intermediates has been extensively studied in molecular complexes. Similar schemes remain rare in hybrid structures of quantum dot-organic molecules, despite intense recent interest of quantum dot sensitized triplet excited state generation. Herein, we demonstrate that the efficiency of the intersystem crossing from the singlet to the triplet state in boron dipyrromethene (BODIPY) can be enhanced in CdSe quantum dot-BODIPY complexes through a radical pair intermediate state consisting of an unpaired electron in the quantum dot conduction band and that in oxidized BODIPY. By transient absorption spectroscopy, we show that the excitation of BODIPY with 650 nm light leads to the formation of a charge separated state by electron transfer from BODIPY to CdSe (with a time constant of 6.33 ± 1.13 ns), competing with internal conversion to the ground state within BODIPY, and the radical pair state decays subsequently by back charge recombination to generate a triplet excited state (with a time constant of 158 ± 28 ns) or the ground state of BODIPY. The overall quantum efficiency of BODIPY triplet excited state generation was determined to be (27.2 ± 3.0)%. The findings of efficient triplet state formation and intermediate radical pair states in this hybrid system suggest that quantum dot-molecule complexes may be a promising platform for spintronics applications.