New Chain-Extended Analogues of Salacinol and Blintol and Their Glycosidase Inhibitory Activities. Mapping the Active-Site Requirements of Human Maltase Glucoamylase

The synthesis of new chain-extended sulfonium and selenonium salts of 1,4-anhydro-4-thio-(or 4-seleno)-d-arabinitol, analogues of the naturally occurring glycosidase inhibitor salacinol, is described. Nucleophilic attack at the least hindered carbon atom of 4,6-O-benzylidene-2,5-di-O-p-methoxybenzyl-d-mannitol-1,3-cyclic sulfate by 2,3,5-tri-O-p-methoxybenzyl-1,4-anhydro-4-thio-(or 4-seleno)-d-arabinitol gave the sulfonium and selenonium sulfates, respectively. Subsequent deprotection with trifluoroacetic acid yielded the target compounds. In these analogues, an extended polyhydroxylated aliphatic side chain has been incorporated while maintaining the stereochemistry of C-2' and C-3' of salacinol or blintol. These compounds were designed to probe the premise that they would bind with higher affinity to glucosidases than salacinol because the extra hydroxyl groups in the acyclic chain would make favorable polar contacts within the active site. Both target compounds inhibited recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with Ki values in the low micromolar range. Comparison of these values to those of related compounds synthesized in previous studies has provided a better understanding of structure-activity relationships and the optimal stereochemistry at the different stereogenic centers required of an inhibitor of this enzyme. With respect to chain extension, the configurations at C-2' and C-4' are critical for activity, the configuration at C-3', bearing the sulfate moiety, being unimportant. The desired configuration at C-5' is also specified. However, comparison of the activities of the chain-extended analogues with those of salacinol and blintol indicates that there is no particular advantage of the chain-extension relative to salacinol or blintol. These results are similar to those reported earlier for kotalanol, a 7-carbon-extended derivative, versus salacinol against rat intestinal maltase, sucrase, and isomaltase.

Air-Stable PbSe/PbS and PbSe/PbSexS1-xCore−Shell Nanocrystal Quantum Dots and Their Applications†

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

A cationic chalcogenoxanthylium photosensitizer effective in vitro in chemosensitive and multidrug-resistant cells

Pentacyclic thio- (1) and seleno- (2) analogues of tetramethylrosamine (TMR) were prepared with a julolidyl fragment replacing the 3-dimethylamino substituent in the xanthylium core. The pentacylic structure increases the lipophilicity of 1 and 2 relative to TMR-S and TMR-Se and locks the lone-pair of electrons on the julolidyl N atom into conjugation with the xanthylium core. This conformational rigidization leads to longer wavelengths of absorption, but has little impact on other photophysical properties such as quantum yields for fluorescence and singlet-oxygen generation and fluorescence lifetimes in 1 and 2 relative to TMR-S and TMR-Se. Both 1 and 2 are effective photosensitizers against chemosensitive AUXB1 cells in vitro at 1x10(-7)M and compound 2 is an effective photosensitizer against multidrug-resistant CR1R12 cells in vitro at 1x10(-7)M. While the uptake TMR-S into CR1R12 cells as measured by fluorescence is significantly lower than uptake into chemosensitive AUXB1 cells, there is no significant difference in the uptake of 1 into either AUXB1 or CR1R12 cells. The addition of 2x10(-4)M verapamil to the cells prior to treatment with 1 had no significant effect on the uptake of 1 into either AUXB1 or CR1R12 cells. Treating lipid-activated, purified Pgp with 2 and light gave complete inhibition of Pgp ATPase activity.

Time-Resolved Photoconductivity of PbSe Nanocrystal Arrays

We report the sub-picosecond photoconductivity dynamics of chemically treated PbSe nanocrystal arrays utilizing time-resolved terahertz spectroscopy (TRTS). TRTS allows both the degree of interdot electronic coupling and the carrier dynamics to be extracted simultaneously. The following capping ligands bonded to the quantum dot surface were studied: hydrazine, ethylenediamine, butlyamine, and aniline. In addition, the arrays were treated with NaOH. We find that the treatments affect both the degree of electronic coupling and the carrier dynamics.

Synthesis of Primary Arylselenoamides by Reaction of Aryl Nitriles with Woollins' Reagent

[Structure: see text] The reaction of aryl nitriles with Woollins' reagent followed by water affords a variety of primary arylselenoamides in 60-100% yield. The first crystal structures of two primary selenoamides are reported.

Preparation of colloidal CdSe and CdS/CdSe nanoparticles from sodium selenosulfate in aqueous polymers solutions

Cadmium selenide nanoparticles formation at the interaction between CdCl2 and Na2SeSO3 in aqueous solutions of sodium polyphosphate and gelatin has been studied. Structural and optical properties of CdSe nanoparticles have been characterized. It has been shown that the temperature and the ratio of reagents concentrations are the basic parameters, controlling the size of CdSe nanoparticles. Photocatalytic activity of CdS nanoparticles in Na2SeSO3 reduction has been found and investigated; structural and optical properties of binary CdS/CdSe nanoparticles have been characterized. This photoreaction, when carried out in the presence of CdCl2, results in the formation of composite CdS/CdSe nanoparticles. It has been shown that slow interaction of adsorbed selenosulfate with surface-trapped CdS conduction band electrons is the limiting stage of the photocatalytic reaction.

Architectural Control Syntheses of CdS and CdSe Nanoflowers, Branched Nanowires, and Nanotrees via a Solvothermal Approach in a Mixed Solution and Their Photocatalytic Property

Wurtzite CdS and CdSe nanostructures with complex morphologies such as urchin-like CdS nanoflowers, branched nanowires, and fractal nanotrees can be produced via a facile solvothermal approach in a mixed solution made of diethylenetriamine (DETA) and deionized water (DIW). The morphologies of CdS and CdSe nanocrystals can be easily controlled via tuning the volume ratio of DETA and DIW. Urchin-like CdS nanoflowers made of CdS nanorods are in a form of highly ordered hierarchical structures, while the nanowires are branched nanowires, and the fractal CdS nanotrees are a buildup of branched nanopines. The results demonstrated that solvothermal reaction in a mixed amine/water can access a variety of complex morphologies of semiconductor materials. The photocatalytic activity of CdS particles with different morphologies has been tested by the degradation of acid fuchsine under both UV and visible light, showing that the as-prepared branched CdS nanowires exhibit high photocatalytic activity for degradation of acid fuchsine.

Construction of New Heteroselenometallic Clusters: Formation of Crownlike [Et4N]4[(μ5-WSe4)(CuI)5(μ-I)2] and Octahedral Polymeric [(μ6-WSe4)Cu6I4(py)4]n from Planar [Et4N]4[(μ4-WSe4)Cu4I6] with Additional Faces

The coplanar cluster compound [Et4N]4[(mu4-WSe4)Cu4I6] (1) was prepared from reaction of [Et4N]2[WSe4] with 4 equiv of CuI in N,N-dimethylformamide (DMF) solution in the presence of [Et(4)N]I. Treatment of 1 with pyridine (py) in dry MeCN gave the neutral cluster [(mu4-WSe4)Cu4(py)6I2] (2) in good yield. Recrystallization of 1 from py/i-PrOH resulted in the reorganization of the coplanar WSe4Cu4 core and the formation of a neutral polymeric cluster [(mu3-WOSe3)Cu3(py)3(mu-I)]n (3) containing a nest-shaped OWSe3Cu3 core and a terminal W=O bond. The interaction of cluster 1 with excess PPh3 in CH3Cl3 gave [(mu3-WSe4)Cu3(PPh3)3(mu3-I)] (4) which has a cubanelike SeWSe3Cu3I core. Treatment of 1 with 1 equiv of CuI in dimethyl sulfoxide (DMSO) yielded [Et4N]4[(mu5-WSe4)(CuI)5(mu-I)2] (5) which has a crown-like core structure. Treatment of 1 in DMF with 2 equiv of CuI in the presence of py resulted in the formation of a two-dimensional polymeric cluster, [(mu6-WSe4)Cu6I4(py)4]n (6), consisting of an octahedral WSe4Cu6 repeating unit. The solid-state structures of clusters 3, 5, and 6 have been further established by X-ray crystallography. The nonlinear optical properties of 6 have been also investigated. Cluster 6 was found to exhibit good photostability and a large optical limiting effect with the limiting threshold being ca. 0.3 J cm(-2).

A convenient and stereoselective synthesis of (Z)-allyl selenides via Sm/TMSCl system-promoted coupling of Baylis-Hillman adducts with diselenides

A simple and convenient procedure for stereoselective synthesis of (Z)-allyl selenides has been developed by a one-pot reaction of diselenides with Baylis-Hillman adducts in the presence of samarium metal-trimethylsilyl chloride under mild conditions. Presumably, the diselenides are cleaved by Sm/TMSCl system to form selenide anions, which then undergo S(N)2' substitution of Baylis-Hillman adducts to produce the (Z)-allyl selenides.

Preparation of nucleoside H-phosphonoselenoate monoesters via the phosphinate approach

An efficient entry to nucleoside 3'-H-phosphonoselenoate monoesters via phosphinate intermediates was developed. It involves a reaction of suitably protected nucleosides with triethylammonium phosphinate in the presence of pivaloyl chloride, followed by selenization of the intermediate nucleoside phosphinates with triphenylphosphine selenide, to produce the corresponding nucleoside H-phosphonoselenoates in 86-92% yields.

Structural, spectroscopic and magnetic properties of M[R2P(E)NP(E)R′2]2complexes, M = Co, Mn, E = S, Se and R, R′ = Ph oriPr. Covalency of M–S bonds from experimental data and theoretical calculations

The S/Se-containing bidentate ligands LH of the type R2P(E)NHP(E)R'2, E = S, Se and R, R' = Ph or iPr have been employed to synthesize ML2 (M = Mn, Co) complexes which contain the biologically important MS4 core. Theoretical calculations on the LH and L- forms of the ligands probe the geometric and electronic changes induced by the deprotonation of the LH form, which are correlated with structural data from X-ray crystallography. These results reflect the flexibility of the ligands, which enables them to be rather versatile with respect to the formation of ML2 complexes with varied geometries and MEPNPE metallacycle conformations. A series of old and new ML2 complexes have been synthesized and their structural, spectroscopic and magnetic properties characterized in detail. The nephelauxetic ratio beta of the CoL2 complexes provides evidence of covalent interactions, whereas the EPR properties of the MnL2 complexes are interpreted on the basis of predominant ionic interactions, between the metal center and the ligands, respectively. Additional evidence for the existence of covalent interactions in the CoL2 complexes (R = Ph, iPr, or mixed Ph/iPr), is offered by comparisons between their 31P NMR. The aforementioned notations are supported by extensive theoretical calculations on the ML2 (E = S, R = Me) modelled structures, which probe the covalent and ionic character of the M-S bonds when M = Co or Mn. Wider implications of the findings of the present study on the M-S covalency and its importance in the active sites of various metalloenzymes are also discussed.

Group 12 metal monoselenocarboxylates: synthesis, characterization, structure and their transformation to metal selenide (MSe; M = Zn, Cd, Hg) nanoparticles

Reactions of [MCl2(tmeda)] with potassium salts of monoselenocarboxylic acids gave complexes of the general formula [M(SeCOR)2(tmeda)] (M = Zn, Cd; R = Ph, Tol; Tol = C6H4-p-CH3; tmeda = Me2NCH2CH2NMe2). The analogous mercury complexes were unstable at room temperature and afforded HgSe nanoparticles during the course of reaction. All the complexes were characterized by elemental analysis, IR, UV-vis, NMR (1H, 13C, 77Se, 113Cd) data. The X-ray structural analysis of [Cd(SeCOPh)2(tmeda)] revealed that the complex is a discrete monomer having an approximate tetrahedral coordination environment around the central metal atom with monodentate (Se-bonded) selenocarboxylates. Thermal behavior of these complexes was studied by TG analysis. Pyrolysis in a furnace or in HDA (hexadecylamine) gave MSe nanoparticles, which were characterized by XRD, EDAX, SEM and absorption spectroscopy.

Synthesis and Antioxidant Profile of all-rac-α-Selenotocopherol

all-rac-alpha-Selenotocopherol (6c) has been synthesized in 11 steps in 6.6% total yield. Key steps include chloromethylation to approach the persubstituted aromatic 9b and cyclization of alcohol precursor 10 by radical homolytic substitution at selenium to form the selenotocopherol heterocycle. Determination of the OH bond dissociation enthalpy (BDE) of 6c by electron paramagnetic resonance (EPR) equilibration techniques gave a value of 78.1 +/- 0.3 kcal mol(-1), approximately 1 kcal mol(-1) higher than that of alpha-tocopherol. Kinetic studies performed by measuring oxygen uptake of the induced oxidation of styrene in the presence of an antioxidant showed that selenotocopherol (6c) was a slightly poorer inhibitor than alpha-tocopherol, in agreement with the BDE values. In contrast to simpler selenotocopherol analogues, 6c was not regenerable in the presence of a stoichiometric coreductant in a two-phase lipid peroxidation model.

New multi-1,2,3-selenadiazole aromatic derivatives

The aromatic polyketones 3a-d are versatile compounds for the synthesis of the multi-1,2,3-selenadiazole aromatic derivatives 1a-d. The preparation starts with the reaction between the multi-bromomethylene benzene derivatives 2a-d and 4-hydroxy-acetophenone to give compounds 3a-d which are transformed through the reaction with semicarbazide hydrochloride or ethyl hydrazine carboxylate into the corresponding semicarbazones derivatives 4a-d or hydrazones 5a-d. The reaction with selenium dioxide leads to regiospecific ring closure of semicarbazones or hydrazones to give the multi-1,2,3-selenadiazole aromatic derivatives in high yield.

High-Temperature Microfluidic Synthesis of CdSe Nanocrystals in Nanoliter Droplets

The high-temperature synthesis of CdSe nanocrystals in nanoliter-volume droplets flowing in a perfluorinated carrier fluid through a microfabricated reactor is presented. A flow-focusing nanojet structure with a step increase in channel height reproducibly generated octadecene droplets in Fomblin Y 06/6 perfluorinated polyether at capillary numbers up to 0.81 and with a droplet:carrier fluid viscosity ratio of 0.035. Cadmium and selenium precursors flowing in octadecene droplets through a high-temperature (240-300 degrees C) glass microreactor produced high-quality CdSe nanocrystals, as verified by optical spectroscopy and transmission electron microscopy. Isolating the reaction solution in droplets prevented particle deposition and hydrodynamic dispersion, allowing the reproducible synthesis of nanocrystals at three different temperatures and four different residence times in the span of 4 h. Our synthesis of a wide range of nanocrystals at high temperatures, high capillary numbers, and low viscosity ratio illustrates the general utility of droplet-based microfluidic reactors to encapsulate nanoliter volumes of organic or aqueous solutions and to precisely control chemical or biochemical reactions.

Design and synthesis of an expanded porphyrin that has selectivity for the c-MYC G-quadruplex structure

Cationic porphyrins are known to bind to and stabilize different types of G-quadruplexes. Recent studies have shown the biological relevance of the intramolecular parallel G-quadruplex as a transcriptional silencer in the c-MYC promoter. TMPyP4 also binds to this G-quadruplex and most likely converts it to a mixed parallel/antiparallel G-quadruplex with two external lateral loops and one internal propeller loop, suppressing c-MYC transcriptional activation. To achieve therapeutic selectivity by targeting G-quadruplexes, it is necessary to synthesize drugs that can differentiate among the different types of G-quadruplexes. We have designed and synthesized a core-modified expanded porphyrin analogue, 5,10,15,20-[tetra(N-methyl-3-pyridyl)]-26,28-diselenasapphyrin chloride (Se2SAP). Se2SAP converts the parallel c-MYC G-quadruplex into a mixed parallel/antiparallel G-quadruplex with one external lateral loop and two internal propeller loops, resulting in strong and selective binding to this G-quadruplex. A Taq polymerase stop assay was used to evaluate the binding of TMPyP4 and Se2SAP to G-quadruplex DNA. Compared to TMPyP4, Se2SAP shows a greater selectivity for and a 40-fold increase in stabilization of the single lateral-loop hybrid. Surface plasmon resonance and competition experiments with duplex DNA and other G-quadruplexes further confirmed the selectivity of Se2SAP for the c-MYC G-quadruplex. Significantly, Se2SAP was found to be less photoactive and noncytotoxic in comparison to TMPyP4. From this study, we have identified an expanded porphyrin that selectively binds with the c-MYC G-quadruplex in the presence of duplex DNA and other G-quadruplexes.

Characterization of Nanocrystalline CdSe by Size Exclusion Chromatography

High-performance size exclusion chromatography (HPSEC) is a powerful tool for probing the size and size distribution of complex materials. Here we report its application to the analysis of cadmium selenide nanocrystals produced in organic solvents. If nanocrystal-column interactions are minimized, this method provides an accurate measure of nanocrystal hydrodynamic diameter directly in solution; such information is complementary to TEM in that it can measure the thickness of various capping agents. While the resolution of single-pass HPSEC is limited to 1 nm, we show here that recycling size exclusion chromatography can be applied to assess the fine details of a sample's distribution. Finally, semiconductor nanocrystals can be made a variety of shapes whose optical characteristics are difficult to distinguish. HPSEC can be applied to the general problem of shape separations which we demonstrate with a tetrapod material.

Photoactivated luminescent CdSe quantum dots as sensitive cyanide probes in aqueous solutions

Water-soluble luminescent CdSe quantum dots surface-modified with 2-mercaptoethane sulfonate were synthesized for the selective determination of free cyanide in aqueous solution with high sensitivity (detection limit of 1.1 x 10(-6) M), via analyte-induced changes in their photoluminescence after photoactivation.

Enantiomerically pure P-chiral phosphinoselenoic chlorides: inversion of configuration at the P-chirogenic center in the synthesis and reaction of these substances

Reaction of diastereomerically pure phosphinoselenoic acid salts with oxalyl chloride leads to enantiomerically pure P-chiral phosphinoselenoic chlorides with inversion of configuration at phosphorus; one of these chlorides is converted to a phosphinoselenothioic acid salt with a high degree of enantioselectivity.

Synthesis of Selenium-Containing Polyphenolic Acid Esters and Evaluation of Their Effects on Antioxidation and 5-Lipoxygenase Inhibition

Six novel selenium-containing polyphenolic acid esters were synthesized and evaluated as antioxidants and 5-lipoxygenase inhibitors. Synthesis of the title compounds involved the Mitsunobu reaction of polyphenolic acids with 2-phenylselenoethanol. Compounds and were found to be very effective antioxidants and 5-lipoxygenase inhibitors with activity comparable to or better than caffeic acid (3,4-dihydroxycinnamic acid) phenethyl ester (CAPE).

Efficient Synthesis of the Glucosidase Inhibitor Blintol, the Selenium Analogue of the Naturally Occurring Glycosidase Inhibitor Salacinol

An efficient synthesis of blintol, the selenium congener of the naturally occurring glycosidase inhibitor salacinol, and a potent glucosidase inhibitor itself, is described. Unlike our previously reported synthesis, this improved route makes use of p-methoxybenzyl ether protecting groups in the synthesis of one of the two key intermediates, 2,3,5-tri-O-p-methoxybenzyl-1,4-anhydro-4-seleno-D-arabinitol, from L-xylose. The other key intermediate, 2,4-O-benzylidene-L-erythritol-1,3-cyclic sulfate, was successfully prepared from D-glucose instead of the expensive L-glucose. All protecting groups in the resulting adducts were removed with trifluoroacetic acid to yield a mixture of stereoisomers, thereby obviating the problematic deprotection of benzyl ethers by hydrogenolysis. The major stereoisomer, blintol, was then obtained by fractional crystallization.

Effect of reaction media on the growth and photoluminescence of colloidal CdSe nanocrystals

Using cadium oxide (CdO) as the Cd precursor and tri-n-octylphosphine selenide (TOPSe) as the Se source, TOP-capped and TOP/tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals were synthesized without the use of an acid. The synthetic approach involved the addition of a TOPSe/TOP solution into a CdO/TOP solution with or without TOPO at one temperature and subsequent growth at a lower temperature. The temporal evolution of the optical properties, namely, absorption and luminescence, of the growing nanocrystals was monitored in detail. A comprehensive examination on the control of the photoluminescence (PL) properties was performed by systematically varying the TOP/TOPO weight ratio of the reaction media. Surprisingly, a rational choice of 100% TOP or 80% TOP was found to produce "quality" nanocrystals when monitored under the present experimental conditions and growth-time scale. The term "quality" is mainly based on the sharp features and rich substructure exhibited in the absorption spectra of the growing nanocrystals, as well as the sharp features in the emission spectra with narrow full width at half-maximum (fwhm). There are two distinguishable stages of growth: an early stage (<5 min) and a later stage. TOP plays a major role in the control of a slow growth rate in the early stage, while TOPO controls slow growth in the later stage. The optical sensitivity of the growing nanocrystals when dispersed in nonpolar or polar solvents was studied, including two size-dependent parameters, namely, the solvent sensitivity (PL intensity) and nonresonant Stokes shift (NRSS). The insights gained from the present study enable a synthetic approach in which high-quality CdSe nanocrystals are achieved with high synthetic reproducibility.

Isolation of a Se-Nitrososelenol: A New Class of Reactive Nitrogen Species Relevant to Protein Se-Nitrosation

Nitric oxide (NO) is a messenger molecule implicated in a number of physiological processes. Nitrosation of selenoproteins has been suggested as playing an important role in NO-mediated cellular functions such as the inactivation of glutathione peroxidase (GPx), but no chemical information about Se-nitrosated species has been available to date. Here a stable Se-nitrososelenol (RSeNO), a new class of NO derivative, was synthesized and fully characterized by X-ray crystallography and spectroscopic methods. This Se-nitrososelenol can be formed by direct transnitrosation from an S-nitrosothiol to a selenol, as is the case in the proposed mechanism for the NO-mediated inactivation of GPx.

Synthesis of novel ammonium and selenonium ions and their evaluation as inhibitors of UDP-galactopyranose mutase

The syntheses of two ammonium salts of 1,4-dideoxy-1,4-imino-d-galactitol containing erythritol sulfate side chains are described. The parent compound is a known inhibitor of the enzyme UDP-galactopyranose mutase (UDP-galactopyranose furanomutase, E.C. 5.4.99.9), which is responsible for the conversion of UDP-galactopyranose into UDP-galactofuranose and is presumably protonated in its active form. The side chain was chosen because it is present in a known sulfonium ion alpha-glucosidase inhibitor, salacinol. The syntheses of the selenonium analogues derived from 1,4-dideoxy-1,4-seleno-d-galactitol are also described. The synthetic strategy in the syntheses of all four salts involved the nucleophilic attack of a protected derivative of the alditol at the least hindered carbon of 2,4-O-benzylidene d- or l-erythritol-1,3-cyclic sulfate to give adducts that were subsequently deprotected. The importance of different protecting groups used in the various syntheses is also highlighted. Enzyme inhibition assays carried out on these compounds, and the corresponding sulfonium ions synthesized previously, show that they are poor inhibitors of UDP-galactopyranose mutase.

Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system

This paper reports a plug-based, microfluidic method for performing multi-step chemical reactions with millisecond time-control. It builds upon a previously reported method where aqueous reagents were injected into a flow of immiscible fluid (fluorocarbons)(H. Song et al., Angew. Chem. Int. Ed., 2003, 42, 768). The aqueous reagents formed plugs--droplets surrounded and transported by the immiscible fluid. Winding channels rapidly mixed the reagents in droplets. This paper shows that further stages of the reaction could be initiated by flowing additional reagent streams directly into the droplets of initial reaction mixture. The conditions necessary for an aqueous stream to merge with aqueous droplets were characterized. The Capillary number could be used to predict the behavior of the two-phase flow at the merging junction. By transporting solid reaction products in droplets, the products were kept from aggregating on the walls of the microchannels. To demonstrate the utility of this microfluidic method it was used to synthesize colloidal CdS and CdS/CdSe core-shell nanoparticles.