Jurriaan Huskens

Cyclodextrin-based supramolecular nanoparticles for biomedical applications

Supramolecular host-guest interactions are ideal for engineering supramolecular nanoparticles (SNPs), because their modular character offers the possibility of using the same basic SNPs made of very similar building blocks in a variety of applications. The most widely used host is cyclodextrin (CD), therefore, this review will focus on SNPs involving CD as the host entity. In the first part, particle formation and size control are described, and the forces that induce the assembly between the different components and, therefore, result in the formation of stable and controllable nanoparticles. In the second part, the use of CD-based SNPs for diagnostics and therapeutics is described. Here, the emphasis is on how the therapeutic agent/imaging component is included in the system and how it is released at the target site. CD-based SNPs provide great possibilities for the formulation of nanoparticles for biomedical applications because of their high flexibility, stability, modular character, and biocompatibility.

Metal-Organic Polyhedra-Coated Si Nanowires for the Sensitive Detection of Trace Explosives

Surface-modified silicon nanowire-based field-effect transistors (SiNW-FETs) have proven to be a promising platform for molecular recognition in miniature sensors. In this work, we present a novel nanoFET device for the sensitive and selective detection of explosives based on affinity layers of metal-organic polyhedra (MOPs). The judicious selection of the geometric and electronic characteristics of the assembly units (organic ligands and unsaturated metal site) embedded within the MOP cage allowed for the formation of multiple charge-transfer (CT) interactions to facilitate the selective explosive inclusion. Meanwhile, the host-stabilized CT complex inside the cage acted as an effective molecular gating element to strongly modulate the electrical conductance of the silicon nanowires. By grafting the MOP cages onto a SiNW-FET device, the resulting sensor showed a good electrical sensing capability to various explosives, especially 2,4,6-trinitrotoluene (TNT), with a detection limit below the nanomolar level. Importantly, coupling MOPs-which have tunable structures and properties-to SiNW-based devices may open up new avenues for a wide range of sensing applications, addressing various target analytes.

Molecular photoswitches mediating the strain-driven disassembly of supramolecular tubules

Significance Developing molecular machines has been a leading goal for scientists, but to be practically valuable their mechanically relevant motion must be decoupled from the Brownian storm that dominates in solution. The disassembly of cellular tubules operates by the switching of the shapes of the building blocks, ultimately pulling sets of chromosomes apart. Here, we show that artificial photoswitches can be incorporated within supramolecular microtubules and that individual switching events lead to conversion of light into elastic energy that can be stored, accumulated, and subsequently released to produce a mechanical effect. The work paves the way toward fully artificial supramolecular machines that convert molecular motion into sophisticated operation modes, at length scales that are typically the realm of living matter. Chemists have created molecular machines and switches with specific mechanical responses that were typically demonstrated in solution, where mechanically relevant motion is dissipated in the Brownian storm. The next challenge consists of designing specific mechanisms through which the action of individual molecules is transmitted to a supramolecular architecture, with a sense of directionality. Cellular microtubules are capable of meeting such a challenge. While their capacity to generate pushing forces by ratcheting growth is well known, conversely these versatile machines can also pull microscopic objects apart through a burst of their rigid tubular structure. One essential feature of this disassembling mechanism is the accumulation of strain in the tubules, which develops when tubulin dimers change shape, triggered by a hydrolysis event. We envision a strategy toward supramolecular machines generating directional pulling forces by harnessing the mechanically purposeful motion of molecular switches in supramolecular tubules. Here, we report on wholly synthetic, water-soluble, and chiral tubules that incorporate photoswitchable building blocks in their supramolecular architecture. Under illumination, these tubules display a nonlinear operation mode, by which light is transformed into units of strain by the shape changes of individual switches, until a threshold is reached and the tubules unleash the strain energy. The operation of this wholly synthetic and stripped-down system compares to the conformational wave by which cellular microtubules disassemble. Additionally, atomistic simulations provide molecular insight into how strain accumulates to induce destabilization. Our findings pave the way toward supramolecular machines that would photogenerate pulling forces, at the nanoscale and beyond.

Doping of semiconductors by molecular monolayers: monolayer formation, dopant diffusion and applications

The continuous miniaturization in the semiconductor industry brings electronic devices with higher performance at lower cost. The doping of semiconductor materials plays a crucial role in tuning the electrical properties of the materials. Ion implantation is currently widely used. Yet, this technique faces challenges meeting the requirements for smaller devices. Monolayer doping (MLD) has been proposed as one of the alternative techniques for doping semiconductors. It utilizes dopant-containing organic molecules and grafts them onto semiconductor surfaces. The dopant atoms are subsequently driven into the substrate by high temperature annealing. MLD has shown the capability for ultra-shallow doping and the doping of 3-D structures without causing crystal damage. These features make this technique a promising candidate to dope future electronic devices. In this review the processes for monolayer formation and dopant incorporation by annealing will be discussed, as well as the applications of MLD in device fabrication.

Assessment of Cooperativity in Ternary Peptide-Cucurbit[8]uril Complexes

Abstract Evaluating cooperativity for cucurbit[8]uril (CB[8])‐mediated ternary complexation is required for understanding and advancing designs of such ternary self‐assembled systems. A key issue is to dissect the contributions of the binding steps of the first and second guest molecules to the overall ternary complex formation energy. This is addressed by performing concentration‐dependent titrations between CB[8] and guests by means of concentration‐dependent calorimetric and 1H‐NMR titrations. The sensitivity of the fitting of the cumulative heat of complexation of the calorimetric titrations is evaluated in terms of fitting error and enthalpy–entropy compensation and, together with the NMR spectroscopic analysis of the separate species, non‐cooperative binding is conceived to be the most probable binding scenario. The binding behavior of CB[8] homoternary complexes is similar to CB[8] heteroternary complexes, with an enthalpy‐driven tight fit of the guests in the CB[8] cavity overcoming the entropic penalty. Also for these types of complexes, a non‐cooperative binding is the most probable.

Molecular Monolayers for Electrical Passivation and Functionalization of Silicon-Based Solar Energy Devices

Silicon-based solar fuel devices require passivation for optimal performance yet at the same time need functionalization with (photo)catalysts for efficient solar fuel production. Here, we use molecular monolayers to enable electrical passivation and simultaneous functionalization of silicon-based solar cells. Organic monolayers were coupled to silicon surfaces by hydrosilylation in order to avoid an insulating silicon oxide layer at the surface. Monolayers of 1-tetradecyne were shown to passivate silicon micropillar-based solar cells with radial junctions, by which the efficiency increased from 8.7% to 9.9% for n+/p junctions and from 7.8% to 8.8% for p+/n junctions. This electrical passivation of the surface, most likely by removal of dangling bonds, is reflected in a higher shunt resistance in the J-V measurements. Monolayers of 1,8-nonadiyne were still reactive for click chemistry with a model catalyst, thus enabling simultaneous passivation and future catalyst coupling.

Spatial decoupling of light absorption and catalytic activity of Ni–Mo-loaded high-aspect-ratio silicon microwire photocathodes

A solar-driven photoelectrochemical cell provides a promising approach to enable the large-scale conversion and storage of solar energy, but requires the use of Earth-abundant materials. Earth-abundant catalysts for the hydrogen evolution reaction, for example nickel–molybdenum (Ni–Mo), are generally opaque and require high mass loading to obtain high catalytic activity, which in turn leads to parasitic light absorption for the underlying photoabsorber (for example silicon), thus limiting production of hydrogen. Here, we show the fabrication of a highly efficient photocathode by spatially and functionally decoupling light absorption and catalytic activity. Varying the fraction of catalyst coverage over the microwires, and the pitch between the microwires, makes it possible to deconvolute the contributions of catalytic activity and light absorption to the overall device performance. This approach provided a silicon microwire photocathode that exhibited a near-ideal short-circuit photocurrent density of 35.5u2009mAu2009cm−2, a photovoltage of 495u2009mV and a fill factor of 62% under AM 1.5G illumination, resulting in an ideal regenerative cell efficiency of 10.8%.Catalysts are required to increase the rate of H2 evolution over silicon photocathodes, but their presence can lead to parasitic light absorption. Here, the authors explore the contributions of catalysts and light absorption to the overall performance of Si microwires, depositing Ni–Mo catalysts spatioselectively to optimize efficiency.

Controlling protein surface orientation by strategic placement of oligo-histidine tags

We report oriented immobilization of proteins using the standard hexahistidine (His6)-Ni2+:NTA (nitrilotriacetic acid) methodology, which we systematically tuned to give control of surface coverage. Fluorescence microscopy and surface plasmon resonance measurements of self-assembled monolayers (SAMs) of red fluorescent proteins (TagRFP) showed that binding strength increased by 1 order of magnitude for each additional His6-tag on the TagRFP proteins. All TagRFP variants with His6-tags located on only one side of the barrel-shaped protein yielded a 1.5 times higher surface coverage compared to variants with His6-tags on opposite sides of the so-called β-barrel. Time-resolved fluorescence anisotropy measurements supported by polarized infrared spectroscopy verified that the orientation (and thus coverage and functionality) of proteins on surfaces can be controlled by strategic placement of a His6-tag on the protein. Molecular dynamics simulations show how the differently tagged proteins reside at the surface in “end-on” and “side-on” orientations with each His6-tag contributing to binding. Also, not every dihistidine subunit in a given His6-tag forms a full coordination bond with the Ni2+:NTA SAMs, which varied with the position of the His6-tag on the protein. At equal valency but different tag positions on the protein, differences in binding were caused by probing for Ni2+:NTA moieties and by additional electrostatic interactions between different fractions of the β-barrel structure and charged NTA moieties. Potential of mean force calculations indicate there is no specific single-protein interaction mode that provides a clear preferential surface orientation, suggesting that the experimentally measured preference for the end-on orientation is a supra-protein, not a single-protein, effect.

Pathways to electrochemical solar-hydrogen technologies

Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

Electron Transfer Mediated by Surface-Tethered Redox Groups in Nanofluidic Devices

Electrochemistry provides a powerful sensor transduction and amplification mechanism that is highly suited for use in integrated, massively parallelized assays. Here, the cyclic voltammetric detection of flexible, linear poly(ethylene glycol) polymers is demonstrated, which have been functionalized with redox-active ferrocene (Fc) moieties and surface-tethered inside a nanofluidic device consisting of two microscale electrodes separated by a gap of <100 nm. Diffusion of the surface-bound polymer chains in the aqueous electrolyte allows the redox groups to repeatedly shuttle electrons from one electrode to the other, resulting in a greatly amplified steady-state electrical current. Variation of the polymer length provides control over the current, as the activity per Fc moiety appears to depend on the extent to which the polymer layers of the opposing electrodes can interpenetrate each other and thus exchange electrons. These results outline the design rules for sensing devices that are based on changing the polymer length, flexibility, and/or diffusivity by binding an analyte to the polymer chain. Such a nanofluidic enabled configuration provides an amplified and highly sensitive alternative to other electrochemical detection mechanisms.