Alkali Cation Doping for Improving the Structural Stability of 2D Perovskite in 3D/2D PSCs.

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K+ and Rb+, are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to a higher crystallinity of 2D perovskite and a better film morphology with fewer defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced nonradiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in an ambient atmosphere for 1200 h.

Azide photochemistry for facile modification of graphitic surfaces: preparation of DNA-coated carbon nanotubes for biosensing.

A facile, two-step method for chemically attaching single-stranded DNA to graphitic surfaces, represented here by carbon nanotubes, is reported. In the first step, an azide-containing compound, N-5-azido-nitrobenzoyloxy succinimide (ANB-NOS), is used to form photo-adducts on the graphitic surfaces in a solid-state photochemical reaction, resulting in active ester groups being oriented for the subsequent reactions. In the second step, pre-synthesized DNA strands bearing a terminal amine group are coupled in an aqueous solution with the active esters on the photo-adducts. The versatility of the method is demonstrated by attaching pre-synthesized DNA to surfaces of carbon nanotubes in two platforms-as vertically-aligned multi-walled carbon nanotubes on a solid support and as tangled single-walled carbon nanotubes in mats. The reaction products at various stages were characterized by x-ray photoelectron spectroscopy. Two different assays were used to check that the DNA strands attached to the carbon nanotubes were able to bind their partner strands with complementary base sequences. The first assay, using partner DNA strands tethered to gold nanoparticles, enabled the sites of DNA attachment to the carbon nanotubes to be identified in TEM images. The second assay, using radioactively labelled partner DNA strands, quantified the density of functional DNA strands attached to the carbon nanotubes. The diversity of potential applications for these DNA-modified carbon-nanotube platforms is exemplified here by the successful use of a DNA-modified single-walled carbon-nanotube mat as an electrode for the specific detection of metal ions.

Fabrication of a Reusable Carbon Dot/Gold Nanoparticle/Metal-Organic Framework Film for Fluorescence Detection of Lead Ions in Water.

Solid-state sensing platforms are desirable for the development of reusable sensors to promote public health measures such as testing for drinking water contamination. A bioinspired metal-organic framework (MOF)-based material has been developed by imitating metal-protein interactions in biological systems to attain high sensitivity and selectivity to Pb2+ through fluorescence sensing. A zirconium terephthalate-type framework (also known as NH2-UiO-66) was modified with both gold nanoparticles and thiol-functionalized carbon dots to give HS-C/Au(x)/UiO-66 composites with different Au content (x) and were subsequently adapted into films that show extraordinary sensitivity to Pb2+. The HS-C/Au(1.4)/UiO-66 film that consists of 1.4 wt % Au shows a quenching response with the limit of detection of 80 parts per trillion and sustained performance for five cycles. Moreover, the fluorescence response of the HS-C/Au(x)/UiO-66 film to Pb2+ can be reversed from emission quenching to enrichment of fluorescence by increasing the Au content. The performance of the HS-C/Au(x)/UiO-66 film as a solid-state sensor demonstrates its potential for application in reusable sensing devices to ensure public safety from Pb2+ contamination in drinking water.

Green ammonia synthesis using CeO2/RuO2 nanolayers on vertical graphene catalyst via electrochemical route in alkaline electrolyte.

The electrochemical synthesis of ammonia at ambient temperature and pressure has the potential to replace the conventional process for the production of ammonia. However, the low ammonia yield and poor long-term stability of catalysts for the synthesis of ammonia hinders the application of this technology. Herein, we endeavored to tackle this challenge by synthesizing 3-D vertical graphene (VG) on Ni foam via a one-step, low-temperature plasma process, which offered high conductivity and large surface area. Subsequently, the vertical graphene on Ni foam was loaded with nanolayers of ruthenium oxide (RuO2, ∼2 nm) and cerium oxide (CeO2, <20 nm) nanoparticles via magnetron sputtering. The incorporation of nanoparticle layers (RuO2 and CeO2/RuO2) on VG significantly increased the NH3 yield in KOH electrolyte. Finally, the performance and long-term stability of this composite material were successfully demonstrated by the addition of CeO2/RuO2 nanolayers on the VG electrocatalyst. The catalyst achieved an excellent performance with a high ammonia synthesis yield of 50.56 µg mgtotal cat.-1 h-1 (1.11 × 10-10 mol cm-2 s-1) during the performance evaluation period of 36 h. This observation was also verified by density functional theory calculation, where CeO2 exhibited the best catalytic performance compared to RuO2 and pristine graphene.

One-step method for generating PEG-like plasma polymer gradients: chemical characterization and analysis of protein interactions.

In this work we report a one-step method for the fabrication of poly(ethylene glycol) PEG-like chemical gradients, which were deposited via continuous wave radio frequency glow discharge plasma polymerization of diethylene glycol dimethyl ether (DG). A knife edge top electrode was used to produce the gradient coatings at plasma load powers of 5 and 30 W. The chemistry across the gradients was analyzed using a number of complementary techniques including spatially resolved synchrotron source grazing incidence FTIR microspectroscopy, X-ray photoelectron spectroscopy (XPS) and synchrotron source near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Gradients deposited at lower load power retained a higher degree of monomer like functionality as did the central region directly underneath the knife edge electrode of each gradient film. Surface derivatization experiments were employed to investigate the concentration of residual ether units in the films. In addition, surface derivatization was used to investigate the reactivity of the gradient films toward primary amine groups in a graft copolymer of poly (L-lysine) and poly(ethylene glycol) (PLL-g-PEG copolymer) which was correlated to residual aldehyde, ketone and carboxylic acid functionalities within the films. The protein adsorption characteristics of the gradients were analyzed using three proteins of varying size and charge. Protein adsorption varied and was dependent on the chemistry and the physical properties (such as size and charge) of the proteins. A correlation between the concentration of ether functionality and the protein fouling characteristics along the gradient films was observed. The gradient coating technique developed in this work allows for the efficient and high-throughput study of biomaterial gradient coating interactions.

Fluorocarbon Plasma Gas Passivation Enhances Performance of Porous Silicon for Desorption/Ionization Mass Spectrometry.

Desorption/ionization on porous silicon mass spectrometry (DIOS-MS) is shown to be a powerful technique for the sensing of low-molecular-weight compounds, including drugs and their metabolites. Surface modification of DIOS surfaces is required to increase analytical performance and ensure stability. However, common wet chemical modification techniques use fluorosilanes, which are less suitable for high-throughput manufacturing and analytical repeatability. Here, we report an alternative, rapid functionalization technique for DIOS surfaces using plasma polymerization (ppDIOS). We demonstrate the detection of drugs, metabolites, pesticides, and doping agents, directly from biological matrices, with molecular confirmation performed using the fragmentation capabilities of a tandem MS instrument. Furthermore, the ppDIOS surfaces were found to be stable over a 162 day period with no loss of reproducibility and sensitivity. This alternative functionalization technique is cost-effective and amenable to upscaling, ensuring avenues for the high-throughput manufacture and detection of hundreds of analytes across various applications while still maintaining the gold-standard clinical technique using mass spectrometry.

Differential functionalisation of the internal and external surfaces of carbon-stabilised nanoporous silicon.

We report the first method to introduce differential functionalities in the interior pore walls and exterior surface of highly stable thermally hydrocarbonised porous silicon (THCpSi) films. The approach exploits the hydrophobicity of the hydrosilylated THCpSi to, first, selectively functionalise the external surface, and subsequently derivatise the hydrophobic internal pore walls.

Morphology and surface properties of high strength siloxane poly(urethane-urea)s developed for heart valve application.

A series of siloxane poly(urethane-urea) (SiPUU) were developed by incorporating a macrodiol linked with a diisocyanate to enhance mixing of hard and soft segments (SS). The effect of this modification on morphology, surface properties, surface elemental composition, and creep resistance was investigated. The linked macrodiol was prepared by reacting α,ω-bis(6-hydroxyethoxypropyl) poly(dimethylsiloxane)(PDMS) or poly(hexamethylene oxide) (PHMO) with either 4,4'-methylenediphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), or isophorone diisocyanate (IPDI). SiPUU with PHMO-MDI-PHMO and PHMO-IPDI-PHMO linked macrodiols showed enhanced creep resistance and recovery when compared with a commercial biostable polyurethane, Elast-Eon™ 2A. Small and wide-angle X-ray scattering data were consistent with significant increase of hydrogen bonding between hard and SS with linked-macrodiols, which improved SiPUU's tensile stress and tear strengths. These SiPUU were hydrophobic with contact angle higher than 101° and they had low water uptake (0.7%·w/w of dry mass). They also had much higher siloxane concentration on the surface compared to that in the bulk. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 112-121, 2019.

Fiber with Butterfly Wings: Creating Colored Carbon Fibers with Increased Strength, Adhesion, and Reversible Malleability.

Colored and color-changing materials are central to perception and interaction in nature and have been exploited in an array of modern technologies such as sensors, visual displays, and smart materials. Attempts to introduce color into carbon fiber materials have been limited by deleterious impacts on fiber properties, and the extension of colored fibers toward "smart composites" remains in its infancy. We present carbon fibers incorporating structural color, similar to that observed on the surface of soap bubbles and various insects and birds, by modifying the fiber surface through in situ polymerization grafting. When dry, the treated fibers exhibit a striking blue color, but when exposed to a volatile solvent, a cascade of colors across the visible light region is observed as the film first swells and then shrinks as the solvent evaporates. The treated fibers not only possess a unique color and color-changing ability but also can be reversibly formed into complex shapes and bear significant loads even without being encased in a supporting polymer. The tensile strength of treated fibers shows a statistically significant increase (+12%), and evaluation of the fiber-to-matrix adhesion of these polymers to an epoxy resin shows more than 300% improvement over control fibers. This approach creates a new platform for the multifaceted advance of smart composites.

Layer-by-layer deposition of chitosan nanoparticles as drug-release coatings for PCL nanofibers.

Nanogels were prepared by ionotropic gelation of chitosan (CS) with tripolyphosphate (TPP). The use of such nanogels to prepare coatings by layer-by-layer deposition (LbL) was studied. The nanogels were characterized in terms of particle size, zeta-potential and stability. Nanogel suspensions were used to build polyelectrolyte multilayers on silicon wafers and on PCL fiber mats by LbL-deposition. Three different polysaccharides were used as polyanions, namely chondroitin sulfate, alginate and hyaluronic acid. The ellipsometric thickness was demonstrated to depend significantly on the type of polyanion. XPS analysis with depth profiling further substantiated the differences in the chemical composition of the films with the different polyanions. Furthermore, XPS data clearly indicated a strong penetration of the polyanions into the CS-TPP layer, resulting in a complete exchange and release of the TPP ions. The LbL-deposition also was studied with PCL fiber mats, which were modified with a chitosan-PCL-graft polymer and alginate. The possibility to create graded coatings on the fiber mats was shown employing fluorescently labelled CS-TPP nanoparticles. The potential of the coatings as drug delivery system for therapeutic proteins was exemplified with the release of Transforming Growth Factor ß3 (TGF-ß3). The CS-TPP nanogels were shown to encapsulate and release therapeutic proteins. In combination with the layer-by-layer deposition they will allow the creation of PCL fiber mat implants having with drug gradients for applications at tissue transitions.

Silicon as a ubiquitous contaminant in graphene derivatives with significant impact on device performance.

Silicon-based impurities are ubiquitous in natural graphite. However, their role as a contaminant in exfoliated graphene and their influence on devices have been overlooked. Herein atomic resolution microscopy is used to highlight the existence of silicon-based contamination on various solution-processed graphene. We found these impurities are extremely persistent and thus utilising high purity graphite as a precursor is the only route to produce silicon-free graphene. These impurities are found to hamper the effective utilisation of graphene in whereby surface area is of paramount importance. When non-contaminated graphene is used to fabricate supercapacitor microelectrodes, a capacitance value closest to the predicted theoretical capacitance for graphene is obtained. We also demonstrate a versatile humidity sensor made from pure graphene oxide which achieves the highest sensitivity and the lowest limit of detection ever reported. Our findings constitute a vital milestone to achieve commercially viable and high performance graphene-based devices.

Thiol surface functionalization via continuous phase plasma polymerization of allyl mercaptan, with subsequent maleimide-linked conjugation of collagen.

Thiol groups can undergo a large variety of chemical reactions and are used in solution phase to conjugate many bioactive molecules. Previous research on solid substrates with continuous phase glow discharge polymerization of thiol-containing monomers may have been compromised by oxidation. Thiol surface functionalization via glow discharge polymerization has been reported as requiring pulsing. Herein, continuous phase glow discharge polymerization of allyl mercaptan (2-propene-1-thiol) was used to generate significant densities of thiol groups on a mixed macrodiol polyurethane and tantalum. Three general classes of chemistry are used to conjugate proteins to thiol groups, with maleimide linkers being used most commonly. Here the pH specificity of maleimide reactions was used effectively to conjugate surface-bound thiol groups to amine groups in collagen. XPS demonstrated surface-bound thiol groups without evidence of oxidation, along with the subsequent presence of maleimide and collagen. Glow discharge reactor parameters were optimized by testing the resistance of bound collagen to degradation by 8 M urea. The nature of the chemical bonding of collagen to surface thiol groups was effectively assessed by colorimetric assay (ELISA) of residual collagen after incubation in 8 M urea over 8 days and after incubation with keratinocytes over 15 days. The facile creation of useable solid-supported thiol groups via continuous phase glow discharge polymerization of allyl mercaptan opens a route for attaching a vast array of bioactive molecules. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1940-1948, 2017.

Extremely Low Density and Super-Compressible Graphene Cellular Materials.

Development of extremely low density graphene elastomer (GE) holds the potential to enable new properties that traditional cellular materials cannot offer, which are promising for a range of emerging applications, ranging from flexible electronics to multifunctional scaffolds. However, existing graphene foams with extremely low density are generally found to have very poor mechanical resilience. It is scientifically intriguing but remains unresolved whether and how the density limit of this class of cellular materials can be further pushed down while their mechanical resilience is being retained. In this work, a simple annealing strategy is developed to investigate the role of intersheet interactions in the formation of extreme-low-density of graphene-based cellular materials. It is discovered that the density limit of mechanically resilient cellular GEs can be further pushed down as low as 0.16 mg cm-3 through thermal annealing. The resultant extremely low density GEs reveal a range of unprecedented properties, including complete recovery from 98% compression in both of liquid and air, ultrahigh solvent adsorption capacity, ultrahigh pressure sensitivity, and light transmittance.

Effects of Surface Composition on the Aerosolisation and Dissolution of Inhaled Antibiotic Combination Powders Consisting of Colistin and Rifampicin.

Colistin is often the only effective antibiotic against the respiratory infections caused by multidrug-resistant Gram-negative bacteria. However, colistin-resistant multidrug-resistant isolates have been increasingly reported and combination therapy is preferred to combat resistance. In this study, five combination formulations containing colistin (COL) and rifampicin (RIF) were prepared by spray drying. The lowest minimum inhibitory concentration (MIC) value against Pseudomonas aeruginosa PAO1 was measured for the formulation of COL/RIF = 4:1 with relatively high emitted doses (over 80%) and satisfactory fine particle fractions (over 60%). Data from X-ray photoelectron spectroscopy (XPS) and nano-time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed the surfaces of particles were mainly covered by rifampicin even for the formulation with a mass ratio of COL/RIF = 4:1. Because colistin is hygroscopic and rifampicin is hydrophobic, moisture absorption of combination formulations was significantly lower than the pure colistin formulation in the dynamic vapour sorption results. To investigate the dissolution characteristics, four dissolution test methods (diffusion Franz cell, modified Franz cell, flow-through and beaker methods) were employed and compared. The modified Franz cell method was selected to test the dissolution behaviour of aerosolised powder formulations to eliminate the effect of membrane on dissolution. The results showed that surface enrichment of hydrophobic rifampicin neither affected aerosolisation nor retarded dissolution rate of colistin in the combination formulations. For the first time, advanced surface characterisation techniques of XPS and ToF-SIMS have shown their capability to understand the effect of surface composition on the aerosolisation and dissolution of combination powders.

Immobilisation of a thrombopoietin peptidic mimic by self-assembled monolayers for culture of CD34+ cells.

Compared to soluble cytokines, surface-tethered ligands can deliver biological signalling with precise control of spatial positioning and concentration. A strategy that immobilises ligand molecules on a surface in a uniform orientation using non-cleavable linkages under physiological conditions would enhance the specific and systemic delivery of signalling in the local environment. We used mixed self-assembled monolayers (SAMs) of oxyamine- and oligo(ethylene glycol)-terminated thiols on gold to covalently install aldehyde- or ketone-functionalised ligands via oxime conjugation. Characterisation by electrochemistry and X-ray photoelectron spectroscopy showed quantitative immobilisation of the ligands on SAM surfaces. The thrombopoietin mimetic peptide, RILL, was immobilised on SAMs and the bioactivity of the substrate was demonstrated by culturing factor-dependent cells. We also optimised the immobilisation and wash conditions so that the peptide was not released into the culture medium and the immobilised RILL could be re-used for consecutive cell cultures. The surface also supported the growth of haematopoietic CD34+ cells comparable to the standard thrombopoietin-supplemented culture. Furthermore, the RILL-immobilised SAM surface was as effective in expanding uncommitted CD34+ cells as standard culture. The stimulatory effect of surface-tethered ligands in haematopoietic stem cell expansion supports the use of ligand immobilisation strategies to replicate the haematopoietic stem cell niche.

Water-dispersible magnetic carbon nanotubes as T2-weighted MRI contrast agents.

An efficient MRI T2-weighted contrast agent incorporating a potential liver targeting functionality was synthesized via the combination of superparamagnetic iron oxide (SPIO) nanoparticles with multiwalled carbon nanotubes (MWCNTs). Poly(diallyldimethylammonium chloride) (PDDA) was coated on the surface of acid treated MWCNTs via electrostatic interactions and SPIO nanoparticles modified with a potential targeting agent, lactose-glycine adduct (Lac-Gly), were subsequently immobilized on the surface of the PDDA-MWCNTs. A narrow magnetic hysteresis loop indicated that the product displayed superparamagnetism at room temperature which was further confirmed by ZFC (zero field cooling)/FC (field cooling) curves measured by SQUID. The multifunctional MWCNT-based magnetic nanocomposites showed low cytotoxicity in vitro to HEK293 and Huh7 cell lines. Enhanced T2 relaxivities were observed for the hybrid material (186 mM(-1) s(-1)) in comparison with the pure magnetic nanoparticles (92 mM(-1) s(-1)) due to the capacity of the MWCNTs to "carry" more nanoparticles as clusters. More importantly, after administration of the composite material to an in vivo liver cancer model in mice, a significant increase in tumor to liver contrast ratio (277%) was observed in T2 weighted magnetic resonance images.

SU-8 photolithography on reactive plasma thin-films: coated microwells for peptide display.

We have developed a technique to create 50µm-deep microwells coated with a reactive and robust thin film, which withstands photolithographic processing, and allows for subsequent chemical functionalisation with biological cues (i.e. peptides). First, plasma polymerisation of 1-bromopropane was used to generate a bromine-functionalised thin film (BrPP) on a substrate of silicon wafer. Second, an epoxy functionalised polymer UV photoresist, SU-8, was deposited and developed to create 50µm-deep patterned microwells that display the BrPP coating at their base. Third, amino acids or peptides were selectively attached to the bottom of the microwells through bromine displacement by an amine or thiol nucleophile. Each surface functionalisation step was monitored by XPS, AFM, and contact angle measurements. These functionalities were then used as linkers to immobilise enzymes (e.g. HRP), which retain activity at the end of the process as shown by a biochemical activity assay. Peptide promoters of cell attachment were also immobilised and their functionality was evaluated using an L929 fibroblast adhesion assay. In conclusion, this work describes an innovative combination of plasma thin film deposition and photolithography to create 50µm-deep functionalised microwells for peptide display in biological applications.

Dissolution of a poorly water-soluble drug dry coated with magnesium and sodium stearate.

The purpose of this research was to investigate the influence of dry coating micronized cohesive powders of a poorly water-soluble drug, indomethacin with force control agents, on its dissolution performance. A dry mechanical fusion method (mechanofusion) was used to coat indomethacin powders with magnesium stearate (0.25%,1%,5%) and sodium stearate (5%). After mechanofusion, significantly increased bulk and tapped densities and decreased intrinsic cohesion were observed for all samples. X-ray photoelectron spectroscopy analysis confirmed that a thicker magnesium stearate surface coating was achieved with increasing concentrations of the material. Dissolution was studied using the USP paddle method in buffer pH 5.0; several modelling approaches were used to explore the dissolution mechanisms. Whilst the bi-exponential equation represented dissolution of mechanofused indomethacin powders occurring from dispersed and agglomerated particles, it provided unrealistic parameter estimates for the two coating materials of contrasting properties. Initial increases in indomethacin dissolution were dependent on the concentration of magnesium stearate mechanofused onto the drug powders. The dissolution enhancing effect of indomethacin powders mechanofused with 5% sodium stearate was attributed to its surfactant properties that increased dispersion of indomethacin agglomerates. Initial drug release from the coated powders was described by a matrix-diffusion system according to the Higuchi model.

Peptides grafted from solids for the control of interfacial properties.

We have used solid-phase peptide synthesis to graft a peptide monolayer from a solid in order to modify the interfacial properties. We grafted a 15-residue peptide, EKEKEKEKEKEKEGG, containing a zwitterionic sequence of alternating lysine and glutamic acid residues from the surface of an aminosilanized silicon wafer by placing the silicon wafer within a commercial microwave peptide synthesizer. Such synthesizers are routinely used to make peptides on porous beads, but the peptides are subsequently cleaved and used independently of the solid support; our aim is to utilize the covalently bound peptide to control the surface properties without the need for cleavage and reattachment. We confirmed the presence of this peptide layer on the surface by X-ray photoelectron spectroscopy and ellipsometry. Atomic force microscopy was then used to study the forces between the peptide-modified surface and a borosilicate glass sphere as a function of the solution pH. The adsorbed peptide makes the silicon wafer pH responsive: at high pH the glass particle is repelled from the wafer, and at low pH it is attracted. Previous studies with synthetic polymers have shown that the "grafting from" method allows a much higher film density than "grafting to". We propose that the application of grafting from strategies to peptide layers may offer three additional benefits: (1) the film density can be controlled independently of the primary sequence of the peptide, (2) the sequence constraints for spontaneous adsorption are removed, and (3) the procedure is fast and efficient, which may lead to lower costs and the ability for high-throughput surface biofunctionalization. Moreover, peptide layers offer increased sequence diversity, control, and functionality compared to conventional polymer brushes.