Evan Spruijt

Alkyl-Functionalized Oxide-Free Silicon Nanoparticles: Synthesis and Optical Properties†

Highly monodisperse silicon nanoparticles (1.57 +/- 0.21 nm) are synthesized with a covalently attached alkyl monolayer on a gram scale. Infrared spectroscopy shows that these silicon nanoparticles contain only a few oxygen atoms per nanoparticle. XPS spectra clearly show the presence of unoxidized Si and attached alkyl chains. Owing to the relatively efficient synthesis (yields approximately 100-fold higher than of those previously reported) the molar extinction coefficient epsilon can be measured: epsilon(max) = 1.7 x 10(-4) M(-1)cm(-1), only a factor of 4 lower than that of CdS and CdSe nanoparticles of that size. The quantum yield of emission ranges from 0.12 (C(10)H(21)-capping) to 0.23 (C(16)H(33)-capping). UV/Vis absorption and emission spectroscopy show clear vibrational progressions (974 +/- 14 cm(-1); up to five vibrational bands visible at room temperature), resembling bulk SiC phonons, which support the monodispersity observed by TEM. This was also confirmed by time-resolved fluorescence anisotropy measurements, which display a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles.

Enhanced transcription rates in membrane-free protocells formed by coacervation of cell lysate

Liquid–liquid phase transitions in complex mixtures of proteins and other molecules produce crowded compartments supporting in vitro transcription and translation. We developed a method based on picoliter water-in-oil droplets to induce coacervation in Escherichia coli cell lysate and follow gene expression under crowded and noncrowded conditions. Coacervation creates an artificial cell-like environment in which the rate of mRNA production is increased significantly. Fits to the measured transcription rates show a two orders of magnitude larger binding constant between DNA and T7 RNA polymerase, and five to six times larger rate constant for transcription in crowded environments, strikingly similar to in vivo rates. The effect of crowding on interactions and kinetics of the fundamental machinery of gene expression has a direct impact on our understanding of biochemical networks in vivo. Moreover, our results show the intrinsic potential of cellular components to facilitate macromolecular organization into membrane-free compartments by phase separation.

Interfacial tension between a complex coacervate phase and its coexisting aqueous phase

Complex coacervation is the associative phase separation in a solution of positively and negatively charged macroions. Despite the widespread use of coacervation in e.g. micellar assemblies (complex coacervate core micelles), drug carriers and thin films, there is virtually no experimental data on the interfacial tension between such coacervate phases (polyelectrolyte complexes) and their coexisting aqueous phases or on the influence of salt thereon. In this paper we use colloidal probe AFM measurements of capillary adhesion forces to obtain the interfacial tension between a complex coacervate phase of two polyelectrolytes with high charge density and its coexisting aqueous phase. We find that the interfacial tension is of order 100 µN/m, decreases with increasing salt concentration and vanishes at the critical point. Interestingly, we find that the critical scaling exponent for the interfacial tension found in segregative demixing also applies here.

Reversible Electrochemical Switching of Polyelectrolyte Brush Surface Energy Using Electroactive Counterions

Polyelectrolyte brushes with electroactive counterions provide an effective platform for surfaces with electrochemically switchable wetting properties. Polycationic poly(2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) (PMETAC) brushes with ferricyanide ions ([Fe(CN)6] 3-) were used as the electrochemically addressable surface. After a negative potential of -0.5 V was applied to the [Fe(CN)6](3-)-coordinated PMETAC brushes, the [Fe(CN)6](3-) species were reduced to [Fe(CN)6](4-), and the surface became more hydrophilic. By application of alternating negative and positive potentials, PMETAC brushes were switched reversibly between the reduced state ([Fe(CN)6]4-) and oxidized state ([Fe(CN)6]3-), resulting in reversible changes in water contact angles. The time required for a complete contact angle change can be tuned from 1 to 20 s, by changing the brush thickness and the concentration of supporting electrolyte. We present an electrochemical brush transport model that includes the electrochemical reaction at the charged electrode and describes ion transport through the brush phase covering the electrode. The model quantitatively describes the response of the contact angle (hydrophilicity) to the applied voltage as a function of background ionic strength and brush thickness, supporting the proposed mechanism of ion transport through the brush and electrochemical reaction at the electrode. A typical diffusion constant for ferricyanide in a PMETAC brush of any thickness in 5 mM KCl supporting electrolyte was found to be 2 x 10(-15) m2 s(-1), 5 to 6 orders of magnitude smaller than its bulk solution value.

Ultralow adhesion and friction of fluoro-hydro alkyne-derived self-assembled monolayers on H-terminated Si(111)

New fluorine-containing terminal alkynes were synthesized and self-assembled onto Si(111) substrates to obtain fluorine-containing organic monolayers. The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), static water contact angle measurements (CA), and atomic force microscopy (AFM). The SAMs exhibit excellent hydrophobicity, with static water contact angles of up to 119° and low critical surface tensions of 5-20 mN/m depending on the number of F atoms per molecule. IRRAS confirmed the formation of highly ordered monolayers, as indicated by the antisymmetric and symmetric stretching vibrations of the CH(2) moieties at 2918-2920 and 2850-2851 cm(-1), respectively. Upon increasing the number of fluorine atoms in the alkyne chains from 0 to 17, the adhesion of bare silica probes to the SAMs in air decreases from 11.6 ± 0.20 mJ/m(2) for fluorine-free (F0) alkyne monolayers to as low as 3.2 ± 0.03 mJ/m(2) for a heptadecafluoro-hexadecyne (F17)-based monolayer. Likewise, the friction coefficient decreases from 5.7 × 10(-2) to 1.2 × 10(-2). The combination of high ordering, excellent hydrophobicity, low adhesion, and low friction makes these fluoro-hydro alkyne-derived monolayers highly promising candidates for use in high-performance microelectronic devices.

On the stability and morphology of complex coacervate core micelles : from spherical to wormlike micelles

We present a systematic study of the stability and morphology of complex coacervate core micelles (C3Ms) formed from poly(acrylic acid) (PAA) and poly(N-methyl-2-vinylpyridinium)-b-poly(ethylene oxide) (PM2VP-b-PEO). We use polarized and depolarized dynamic and static light scattering, combined with small-angle X-ray scattering, to investigate how the polymer chain length and salt concentration affect the stability, size, and shape of these micelles. We show that C3Ms are formed in aqueous solution below a critical salt concentration, which increases considerably with increasing PAA and PM2VP length and levels off for long chains. This trend is in good agreement with a mean-field model of polyelectrolyte complexation based on the Voorn-Overbeek theory. In addition, we find that salt induces morphological changes in C3Ms when the PAA homopolymer is sufficiently short: from spherical micelles with a diameter of several tens of nanometers at low salt concentration to wormlike micelles with a contour length of several hundreds of nanometers just before the critical salt concentration. By contrast, C3Ms of long PAA homopolymers remain spherical upon addition of salt and shrink slightly. A critical review of existing literature on other C3Ms reveals that the transition from spherical to wormlike micelles is probably a general phenomenon, which can be rationalized in terms of a classical packing parameter for amphiphiles.

Macromolecular crowding creates heterogeneous environments of gene expression in picolitre droplets

Understanding the dynamics of complex enzymatic reactions in highly crowded small volumes is crucial for the development of synthetic minimal cells. Compartmentalised biochemical reactions in cell-sized containers exhibit a degree of randomness due to the small number of molecules involved. However, it is unknown how the physical environment contributes to the stochastic nature of multistep enzymatic processes. Here, we present a robust method to quantify gene expression noise in vitro using droplet microfluidics. We study the changes in stochasticity in cell-free gene expression of two genes compartmentalised within droplets as a function of DNA copy number and macromolecular crowding. We find that decreased diffusion caused by a crowded environment leads to the spontaneous formation of heterogeneous micro-environments of mRNA as local production rates exceed diffusion rates of macromolecules. This heterogeneity leads to a higher probability of the molecular machinery to stay in the same microenvironment, directly increasing the system’s stochasticity.

Direct measurement of the strength of single ionic bonds between hydrated charges.

The strength of ionic bonds is essentially unknown, despite their widespread occurrence in natural and man-made assemblies. Here, we use single-molecule force spectroscopy to measure their strength directly. We disrupt a complex between two oppositely charged polyelectrolyte chains and find two modes of rupture: one ionic bond at a time, or cooperative rupture of many bonds at once. For both modes, disruption of the ionic bonds can be described quantitatively as an activated process. The height of the energy barrier is not only lowered by added salt, but also by the applied force. We extract unperturbed ionic bond lifetimes that range from milliseconds for single ionic bonds at high salt concentration to tens of years for small complexes of five ionic bonds at low salt concentration.

Reversible assembly of oppositely charged hairy colloids in water

We present an experimental study of the fully reversible assembly of oppositely charged colloidal particles in aqueous solutions. Our polystyrene colloids are charged by a grafted polyelectrolyte brush on their surface and stabilized at all salt concentrations by a neutral adsorbed polymer layer. Below a critical salt concentration oppositely charged colloids form clusters and gels with a fractal nature. The fractal dimension of those aggregates increases with increasing salt concentration. Above the critical salt concentration no aggregation takes place, due to the stabilizing neutral adsorbed polymer. Moreover, the aggregated structures are fully reversible and can be redispersed by simply increasing the salt concentration above the critical concentration. We confirm that time-dependent interaction forces are at the basis of the formation of clusters in the present system by atomic force microscopy measurements as a function of salt concentration and contact time. The force measurements show that the attraction between particles strengthens in time due to interpenetration of the polymer brushes, driven by polyelectrolyte complexation. These particles are a promising step toward a reversible and controlled self-assembling system in water, using colloidal particles as building blocks.

Sedimentation dynamics and equilibrium profiles in multicomponent mixtures of colloidal particles

In this paper we give a general theoretical framework that describes the sedimentation of multicomponent mixtures of particles with sizes ranging from molecules to macroscopic bodies. Both equilibrium sedimentation profiles and the dynamic process of settling, or its converse, creaming, are modeled. Equilibrium profiles are found to be in perfect agreement with experiments. Our model reconciles two apparently contradicting points of view about buoyancy, thereby resolving a long-lived paradox about the correct choice of the buoyant density. On the one hand, the buoyancy force follows necessarily from the suspension density, as it relates to the hydrostatic pressure gradient. On the other hand, sedimentation profiles of colloidal suspensions can be calculated directly using the fluid density as apparent buoyant density in colloidal systems in sedimentation-diffusion equilibrium (SDE) as a result of balancing gravitational and thermodynamic forces. Surprisingly, this balance also holds in multicomponent mixtures. This analysis resolves the ongoing debate of the correct choice of buoyant density (fluid or suspension): both approaches can be used in their own domain. We present calculations of equilibrium sedimentation profiles and dynamic sedimentation that show the consequences of these insights. In bidisperse mixtures of colloids, particles with a lower mass density than the homogeneous suspension will first cream and then settle, whereas particles with a suspension-matched mass density form transient, bimodal particle distributions during sedimentation, which disappear when equilibrium is reached. In all these cases, the centers of the distributions of the particles with the lowest mass density of the two, regardless of their actual mass, will be located in equilibrium above the so-called isopycnic point, a natural consequence of their hard-sphere interactions. We include these interactions using the Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of state. Finally, we demonstrate that our model is not limited to hard spheres, by extending it to charged spherical particles, and to dumbbells, trimers and short chains of connected beads.