Mild Microfluidic Approaches to Oxide Nanoparticles Synthesis.

Oxide nanoparticles (oxide NPs) are advanced materials with a wide variety of applications in different fields. The use of continuous flow methods is particularly appealing for their synthesis due to the high control achieved over the reaction conditions and the easy process scalability. The present review focuses on the preparation of oxide NPs using microfluidic setups at low temperature (≤80 °C), since the employment of mild reaction conditions is crucial for developing sustainable and cost-effective processes. A particular emphasis will be put on the improvement over the final product features (e. g., size, shape, and size distribution) given by flow methods with respect to conventional batch procedures. The main issues that arise by treating NPs suspensions in microfluidic systems are product deposition or channel clogging; mitigation strategies to overcome these drawbacks will also be presented and discussed.

Microfluidic Crystallization of Surfactant-Free Doped Zinc Sulfide Nanoparticles for Optical Bioimaging Applications.

The room-temperature controlled crystallization of monodispersed ZnS nanoparticles (average size of 5 nm) doped with luminescent ions (such as Mn2+, Eu3+, Sm3+, Nd3+, and Yb3+) was achieved via a microfluidic approach. The preparation did not require any stabilizing ligands or surfactants, minimizing potential sources of impurities. The synthesized nanomaterials were characterized from a structural (XRD and XAS at lanthanide L3 edges), morphological (TEM), and compositional (XPS, ICP-MS) perspective, giving complementary information on the materials' features. In view of potential applications in the field of optical bioimaging, the optical emission properties of the doped nanoparticles were assessed, and samples showed strong luminescent properties while being less affected by self-quenching mechanisms. Furthermore, in vitro cytotoxicity experiments were conducted, showing no negative effects and evidencing the appeal of the synthesized materials for potential applications in the field of optical bioimaging.

Ligand-free ZnS nanoparticles: as easy and green as it gets.

The controlled nucleation and crystallization of small pure sphalerite ZnS nanoparticles was achieved under batch and continuous flow conditions at low temperature, in water and without the use of any stabilizing ligand. The obtained nanoparticles displayed a narrow size distribution and high specific surface area. Moreover, the synthesis was suitable to directly obtain stable water-based suspensions and the products were found to be active photocatalysts for the hydrogen evolution reaction.

Microfluidic light-driven synthesis of tetracyclic molecular architectures.

Herein we report an effective synthetic method for the direct assembly of highly functionalized tetracyclic pharmacophoric cores. Coumarins and chromones undergo diastereoselective [4 + 2] cycloaddition reactions with light-generated photoenol intermediates. The reactions occur by aid of a microfluidic photoreactor (MFP) in high yield (up to >98%) and virtually complete diastereocontrol (>20:1 dr). The method is easily scaled-up to a parallel setup, furnishing 948 mg of product over a 14 h reaction time. Finally, a series of manipulations of the tetracyclic scaffold obtained gave access to valuable precursors of biologically active molecules.

A microfluidic photoreactor enables 2-methylbenzophenone light-driven reactions with superior performance.

Light-driven reactions of 2-methylbenzophenones (2-MBPs) occur with improved yields (up to >98%) and reaction rates (up to 0.240 mmol h-1) by using a tailored microfluidic photoreactor (MFP). For the first time, coumarins were converted into 4-benzylated chromanones in high yields (50-93%) and diastereoselectivity (up to >20 : 1 dr), thus by-passing their photo-dimerisation end-reaction.

Stereoselective photopolymerization of tetraphenylporphyrin derivatives on Ag(110) at the sub-monolayer level.

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.

A nanocellulose-dye conjugate for multi-format optical pH-sensing.

A pH-sensitive azo-dye covalently grafted onto cellulose nanocrystals yields nanostructured optodes in the form of membranes, sticks, and water-based inks for optical pH detection.

Nanocrystalline cellulose-porphyrin hybrids: synthesis, supramolecular properties, and singlet-oxygen production.

An amino-trisulfonate tetraphenylporphyrin was conjugated to carboxylate cellulose nanocrystals giving rise to a new nanomaterial with interesting binding properties.

Shape-selective growth of silver nanoparticles under continuous flow photochemical conditions.

A microfluidic setup for the photochemical nucleation and growth of silver nanoparticles with controlled morphologies is described. The combination of microstructured reactors and efficient LED illumination speeds the growth process up and enhances the shape-wise homogeneity of the produced nanostructures.

Tailoring the wetting properties of thiolene microfluidic materials.

A post functionalization method for the control of the wettability of thiolene resins of the NOA family is presented. Treatment of open model surfaces or closed microchannels with chlorosilane derivatives resulted in dramatic changes in the behaviour of droplets and streams contacting the surfaces. The experimental findings are confirmed by the fabrication of a Y-junction device that works as a passive valve for water streams.

Bulky melamine-based Zn-porphyrin tweezer as a CD probe of molecular chirality.

The transfer of chirality from a guest molecule to an achiral host is the subject of significant interest especially when, upon chiral induction, the chiroptical response of the host/guest complex can effectively report the absolute configuration (AC) of the guest. For more than a decade, dimeric metalloporphyrin hosts (tweezers) have been successfully applied as chirality probes for determination of the AC for a wide variety of chiral synthetic compounds and natural products. The objective of this study is to investigate the utility of a new class of melamine-bridged Zn-porphyrin tweezers as sensitive AC reporters. A combined approach based on an experimental CD analysis and a theoretical prediction of the prevailing interporphyrin helicity demonstrates that these tweezers display favorable properties for chiral recognition. Herein, we discuss the application of the melamine-bridged tweezer to the chiral recognition of a diverse set of chiral guests, such as 1,2-diamines, α-amino-esters and amides, secondary alcohols, and 1,2-amino-alcohols. The bulky periphery and the presence of a rigid porphyrin linkage lead, in some cases, to a more enhanced CD sensitivity than that reported earlier with other tweezers.

On-line monitoring and active control of dye uptake in dye-sensitised solar cells.

Real-time monitoring of dye loading (N3 and N719) under continuous flow conditions on TiO(2) photoanodes for dye-sensitized solar cells has been applied to quantitatively investigate dye uptake kinetics, demonstrating that static impregnation provides in all cases higher dye loading and, as a consequence, better working devices.

The continuous-flow cycloaddition of azomethine ylides to carbon nanotubes.

This communication demonstrates a straightforward continuous-flow method for efficient exohedral functionalisation of carbon nanotubes which affords soluble samples in a much shorter time over conventional batch processing.

Surface-driven porphyrin self-assembly on pre-activated Si substrates.

Self-assembled molecular thin films of melamine-bridged bis-porphyrin dyad, both in free form, P(H2)P(H2), or as Zn(II) metallated complex, P(Zn)P(Zn), were deposited on crystalline Si(100) by soaking or drop-casting techniques. The influence of the adopted preparation methodology, the substrate surface pre-activation procedure and the used solvent (THF or CHCl3) on the morphology and surface coverage of the resulting films was investigated by FE-SEM (Field Emission-Scanning Electron Microscopy) and AFM (Atomic Force Microscopy). The chemical composition and electronic structure of the most promising systems were also addressed by XPS (X-ray Photoelectron Spectroscopy). The results pointed out that an accurate tuning of porphyrin-porphyrin, porphyrin-substrate and porphyrin-solvent molecular interactions allow to tailor the morphogenesis and chemico-physical properties of the final self-assembled films. In addition, preliminary gas sensing tests evidenced the potential of the present porphyrin-based films for the development of new molecular sensing devices.

Fullerene/porphyrin multicomponent nanostructures on Ag(110): from supramolecular self-assembly to extended copolymers.

A novel two-step bottom-up approach to construct a 2D long-range ordered, covalently bonded fullerene/porphyrin binary nanostructure is presented: in the first place, reversible supramolecular interactions between C60 and 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin are exploited to obtain large domains of an ordered binary network, subsequently a reaction between fullerene molecules and the amino-groups residing on porphyrin units, triggered by thermal treatment, is used to freeze the supramolecular nanostructure with covalent bonds. The resulting nanostructure resists high temperature treatments as expected for an extended covalent network, whereas very similar fullerene/porphyrin nanostructures held together only by weak interactions are disrupted upon annealing at the same or at lower temperatures.

Solid-supported Zn(II) porphyrin tweezers as optical sensors for diamines.

A melamine-bridged bis-Zn(II) porphyrin dyad covalently attached onto amino-functionalized TentaGel polymer beads or controlled pore glass (CPG) undergoes a reversible and marked colour change from purple to green upon exposure to a biogenic diamine such as cadaverine both in organic and aqueous media.

Melamine-bridged bis(porphyrin-Zn(II)) receptors: molecular recognition properties.

Dimeric metalloporphyrin hosts with tweezer-like structures have been synthesized by reacting the cyanuric chloride scaffold, CC, with 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, P, and 5-(4-aminophenyl)-10,15,20-trimesitylporphyrin, M, to yield the homoconjugates free bases PP and MM and the heterodyad PM. Metalation with Zn(II), gives three structurally related ditopic receptors P(Zn)P(Zn), P(Zn)M(Zn), and M(Zn)M(Zn) with differential steric hindrance and conformational rigidity. The solution structure and supramolecular properties of these porphyrin dimers have been investigated as isolated molecules and in the presence of aliphatic alpha,omega-diamines of general formula H(2)N-(CH(2))(n)-NH(2) (n = 4-8) by spectroscopic and theoretical studies including multidimensional NMR, UV-vis, molecular modeling, and computational NMR methods. Binding constants in the range 4.2 x 10(6) to 3.4 x 10(7) M(-1) are observed in dichloromethane at 298 K, with a 3 orders of magnitude increase as compared to monodentate nBuNH(2), thus indicating the occurrence of a host-guest ditopic interaction. Linear correlation graphs are obtained by plotting the Soret band shift (Delta nu, cm(-1)) of the complex as a function of the diamine chain length. Combined NMR evidence and OPLS 2005 Force Field conformational analysis point to a mutual adaptation of both the binding partners in the host-guest complex, whose geometry is mainly dictated by the steric impact of the bulky substituents at the porphyrin periphery.

Design of acidochromic dyes for facile preparation of pH sensor layers.

Eight new acidochromic dyes have been synthesised that can be used for optically monitoring pH in the range from 3 to 12. Their corresponding pK(a) values have been both measured and calculated theoretically by means of density functional theory. The synthesis of these new dyes is facile without the need for chromatographic purification. The dyes can be covalently linked to polymers containing hydroxyl functions such as cellulose, polyurethane hydrogel, and hydroxyalkyl methacrylate. The resulting sensor layers exhibit significant colour changes both in the UV and in the visible spectral range.

Diacylglycerolipids isolated from a thermophile cyanobacterium from the Euganean hot springs.

The Phormidium sp. ETS-05 thermophile blue-green alga is one of the most typical and widespread species of cyanobacteria of the thermal muds of the Euganean hot springs, the therapeutic properties of which have been known since ancient times. The polar diacylglycerolipids of this cyanobacterium consists of monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. We have isolated and purified these four diacylglycerolipids from ETS-05, and then analysed them for their quantitative and structural features and fatty acid contents. The monogalactosyldiacylglycerol and digalactosyldiacylglycerol show a marked presence of polyunsaturated fatty acids, of which C18 : 4 is the most common. We propose that these glycoglycerolipids can be used as markers for monitoring the thermal mud colonisation process.

Fast catalytic hydroxylation of hydrocarbons with ruthenium porphyrins.

Ruthenium porphyrin complexes such as carbonylruthenium(II) tetrakispentafluorophenylporphyrin [Ru(II)(TPFPP)(CO)] were found to be efficient catalysts for the hydroxylation of alkanes in the presence of 2,6-dichloropyridine N-oxide as the oxidant under mild, nonacidic conditions. Up to 14 800 turnovers (TO) and rates of 800 TO/min were obtained for the hydroxylation of adamantane. The hydroxylation of cis-decalin afforded cis-9-decalol and cis-decalin-9,10-diol, exclusively, thus, excluding a long-lived radicals mechanism. The kinetics of product evolution in a typical catalytic oxygenation showed an initial induction period followed by a fast, apparently zero-order phase with maximum rates and high efficiencies. Deuterium isotope effects (kH/kD) in the range of 4.2-6.4 were found for the hydroxylation of alkanes. A Hammett treatment of the data for the oxidation of para-substituted toluene derivatives showed a linear correlation with a highly negative rho+ value of -2.0. On the basis of kinetic and spectroscopic evidence, Ru(VI)(TPFPP)(O)2, Ru(II)(TPFPP)(CO), and Ru(IV)(TPFPP)Cl2 observed during catalysis were ruled out as candidates for the active catalyst responsible for the high efficiencies and turnover rates in the oxidation reactions. The fastest rates of adamantane hydroxylation with 2,6-dichloropyridine N-oxide were achieved by the reductive activation of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. This redox activation is consistent with the formation of an active Ru(III) intermediate in situ by a one-electron reduction of the Ru(IV) porphyrin. EPR spectra characteristic of Ru(III) have been observed upon the reduction of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. In the adamantane oxidation mediated with Ru(III)(TPFPP)(OEt), it was found that, during this process, the Ru(III) porphyrin was gradually converted to a dioxoRu(VI) porphyrin. Concomitant with this conversion, the reaction rates decreased. Catalyst activation was also stimulated by autoxidation of the solvent CH2Cl2. On the basis of these data, a mechanism is proposed that incorporates a "fast" cycle involving metastable Ru(III) and oxoRu(V) intermediates and a "slow" oxidation cycle, mediated by oxoRu(IV) and trans-dioxoRu(VI) porphyrin complexes.