Stavros Stavrakis

Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping

Prior to this work, fully inorganic nanocrystals of cesium lead halide perovskite (CsPbX3, X = Br, I, Cl and Cl/Br and Br/I mixed halide systems), exhibiting bright and tunable photoluminescence, have been synthesized using conventional batch (flask-based) reactions. Unfortunately, our understanding of the parameters governing the formation of these nanocrystals is still very limited due to extremely fast reaction kinetics and multiple variables involved in ion-metathesis-based synthesis of such multinary halide systems. Herein, we report the use of a droplet-based microfluidic platform for the synthesis of CsPbX3 nanocrystals. The combination of online photoluminescence and absorption measurements and the fast mixing of reagents within such a platform allows the rigorous and rapid mapping of the reaction parameters, including molar ratios of Cs, Pb, and halide precursors, reaction temperatures, and reaction times. This translates into enormous savings in reagent usage and screening times when compared to analogous batch synthetic approaches. The early-stage insight into the mechanism of nucleation of metal halide nanocrystals suggests similarities with multinary metal chalcogenide systems, albeit with much faster reaction kinetics in the case of halides. Furthermore, we show that microfluidics-optimized synthesis parameters are also directly transferrable to the conventional flask-based reaction.

Observation of the equilibrium CuB-CO complex and functional implications of the transient heme a3 propionates in cytochrome ba3-CO from Thermus thermophilus. Fourier transform infrared (FTIR) and time-resolved step-scan FTIR studies.

We report the first evidence for the existence of the equilibrium CuB 1+-CO species of CO-bound reduced cytochrome ba 3 fromThermus thermophilus at room temperature. The frequency of the C-O stretching mode of CuB 1+-CO is located at 2053 cm−1 and remains unchanged in H2O/D2O exchanges and, between pD 5.5 and 9.7, indicating that the chemical environment does not alter the protonation state of the CuB histidine ligands. The data and conclusions reported here are in contrast to the changes in protonation state of CuB-His-290, reported recently (Das, T. K., Tomson, F. K., Gennis, R. B., Gordon, M., and Rousseau, D. L. (2001)Biophys. J. 80, 2039–2045 and Das, T. P., Gomes, C. M., Teixeira, M., and Rousseau, D. L. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 9591–9596). The time-resolved step-scan FTIR difference spectra indicate that the rate of decay of the transient CuB 1+-CO complex is 34.5 s−1 and rebinding to heme a 3 occurs withk 2 = 28.6 s−1. The rate of decay of the transient CuB 1+-CO complex displays a similar time constant as the absorption changes at 1694(+)/1706(−), attributed to perturbation of the heme a 3propionates (COOH). The ν(C-O) of the transient CuB 1+-CO species is the same as that of the equilibrium CuB 1+-CO species and remains unchanged in the pD range 5.5–9.7 indicating that no structural change takes place at CuB between these states. The implications of these results with respect to proton pathways in heme-copper oxidases are discussed.

Millisecond‐Timescale Monitoring of PbS Nanoparticle Nucleation and Growth Using Droplet‐Based Microfluidics

The early-time kinetics (<1 s) of lead sulfide (PbS) quantum dot formation are probed using a novel droplet-based microfluidic platform, which allows for high-throughput and real-time optical analysis of the reactive process with millisecond time resolution. The reaction platform enables the concurrent investigation of the emission characteristics of PbS quantum dots and a real-time estimation of their size and concentration during nucleation and growth. These investigations reveal a two-stage mechanism for PbS nanoparticle formation. The first stage corresponds to the fast conversion of precursor species to PbS crystals, followed by the growth of the formed particles. The growth kinetics of the PbS nanoparticles follow the Lifshitz-Slyozov-Wagner model for Ostwald ripening, allowing direct estimation of the rate constants for the process. In addition, the extraction of absorption spectra of ultrasmall quantum dots is demonstrated for first time in an online manner. The droplet-based microfluidic platform integrated with online spectroscopic analysis provides a new tool for the quantitative extraction of high temperature kinetics for systems with rapid nucleation and growth stages.

Scalable production of CuInS2/ZnS quantum dots in a two-step droplet-based microfluidic platform

We report the scalable formation of CuInS2/ZnS nanocrystals using a two-stage microfluidic reactor integrated with a real-time optical detection system, which is able to monitor reaction parameters prior and subsequent to the addition of the shell material. By injecting a ZnS single source precursor in droplets containing CuInS2 cores and without the need of purification steps, we are able to obtain core–shell nanocrystal populations emitting between 580 and 760 nm with significant narrower size distributions (90–95 nm) than for the same material systems synthesized on the macroscale. In-line monitoring allowed for rapid assessment of optimum reaction parameters (Cu/In, S/(Cu + In), Zn/(Cu + In) molar ratios, temperatures and reaction time) and enabled the formation of CuInS2/ZnS nanocrystals with high photoluminescence quantum yields (∼55%) within a few seconds. We believe that this synthetic methodology will be of significant utility in controllable production of ternary and quaternary metal chalcogenides, complex core–shell and doped nanostructures.

Elasto-Inertial Focusing of Mammalian Cells and Bacteria Using Low Molecular, Low Viscosity PEO Solutions

The ability to manipulate biological cells is critical in a diversity of biomedical and industrial applications. Microfluidic-based cell manipulations provide unique opportunities for sophisticated and high-throughput biological assays such as cell sorting, rare cell detection, and imaging flow cytometry. In this respect, cell focusing is an extremely useful functional operation preceding downstream biological analysis, since it allows the accurate lateral and axial positioning of cells moving through microfluidic channels, and thus enables sophisticated cell manipulations in a passive manner. Herein, we explore the utility of viscoelastic carrier fluids for enhanced elasto-inertial focusing of biological species within straight, rectangular cross section microfluidic channels. Since the investigated polymer solutions possess viscosities close to that of water and exhibit negligible shear thinning, focusing occurs over a wide range of elasticity numbers and a large range of Reynolds numbers. With a view to applications in the robust focusing of cells and bacteria, we assess and characterize the influence of accessible focusing parameters, including blockage ratio, volumetric flow rate, cell concentration, and polymer chain length.

High-throughput microfluidic imaging flow cytometry

Recently, microfluidic-based flow cytometry platforms have been shown to be powerful tools for the manipulation and analysis of single cells and micron-sized particles in flow. That said, current microfluidic flow cytometers are limited in both their analytical throughput and spatial resolution, due to their reliance on single point interrogation schemes. Conversely, high-speed imaging techniques can be applied to a wide variety of problems in which analyte molecules are manipulated at high linear velocities. Such an approach allows a detailed visualization of dynamic events through acquisition of a series of image frames captured with high temporal and spatial resolution. Herein, we describe some of the most significant recent advances in the development of multi-parametric, optofluidic imaging flow cytometry for the enumeration of complex cellular populations.

Fluoropolymer-Coated PDMS Microfluidic Devices for Application in Organic Synthesis

In recent years there has been huge interest in the development of microfluidic reactors for the synthesis of small molecules and nanomaterials. Such reaction platforms represent a powerful and versatile alternative to traditional formats since they allow for the precise, controlled, and flexible management of reactive processes. To date, the majority of microfluidic reactors used in small-molecule synthesis have been manufactured using conventional lithographic techniques from materials such as glasses, ceramics, stainless steel, and silicon. Surprisingly, the fabrication of microfluidic devices from such rigid materials remains ill-defined, complex, and expensive. Accordingly, the microfluidic toolkit for chemical synthesis would significantly benefit from the development of solvent-resistant microfluidic devices that can be manufactured using soft-lithographic prototyping methods. Whilst significant advances in the development of solvent-resistant polymers have been made, only modest steps have been taken towards simplifying their use as microfluidic reactors. Herein, we emphasize the benefits of using a commercially available, amorphous perfluorinated polymer, CYTOP, as a coating with which to transform PDMS into a chemically inert material for use in organic synthesis applications. Its efficacy is demonstrated through the subsequent performance of photooxidation reactions and reactions under extremely acidic or basic conditions.

Colloidal Lenses Enable High Temperature Single Molecule Imaging and Improve Fluorophore Photostability

Although single molecule fluorescence spectroscopy was first demonstrated at near-absolute zero temperatures (1.8 K), the field has since advanced to include room temperature observations largely due to the use of high numerical aperture objective lenses, brighter fluorophores, and more sensitive detectors. This has opened the door for many chemical and biological systems to be studied at native temperatures at the single molecule level both in vitro and in vivo. However, systems and phenomena that operate at temperatures above 37 °C remain difficult to study at the single molecule level due to the need for index matching fluids with high numerical aperture (NA) objective lenses. These fluids act as a thermal conductor between the sample and the objective and sustained exposure to high temperature can cause the objective to fail. This has prevented the single molecule study of thermophilic organisms, the interactions of their protein repertoire, and the temperature-dependent unfolding kinetics of nucleic acids and proteins. Here we report that high index of refraction micron-sized colloidal lenses are capable of achieving single molecule imaging at 70 °C by incorporating a focusing element in immediate proximity to an emitting molecule; the optical system is completed by a low numerical aperture optic which can have a long working distance and an air interface. TiO2 colloidal lenses were used for parallel imaging of surface-immobilized single fluorophores and to measure real-time single molecule mesophilic and thermophilic DNA polymerase strand displacement replication through an immobilized template at 23 °C and 70 °C, respectively. Fluorophores in close proximity to TiO2 also exhibited a ∼40% increase in photostability due to a reduction of the excited-state lifetime.