Andrew J. Clulow

Unambiguous detection of nitrated explosive vapours by fluorescence quenching of dendrimer films

Unambiguous and selective standoff (non-contact) infield detection of nitro-containing explosives and taggants is an important goal but difficult to achieve with standard analytical techniques. Oxidative fluorescence quenching is emerging as a high sensitivity method for detecting such materials but is prone to false positives—everyday items such as perfumes elicit similar responses. Here we report thin films of light-emitting dendrimers that detect vapours of explosives and taggants selectively—fluorescence quenching is not observed for a range of common interferents. Using a combination of neutron reflectometry, quartz crystal microbalance and photophysical measurements we show that the origin of the selectivity is primarily electronic and not the diffusion kinetics of the analyte or its distribution in the film. The results are a major advance in the development of sensing materials for the standoff detection of nitro-based explosive vapours, and deliver significant insights into the physical processes that govern the sensing efficacy.

Determination of Fullerene Scattering Length Density: A Critical Parameter for Understanding the Fullerene Distribution in Bulk Heterojunction Organic Photovoltaic Devices

Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6-4.4) × 10(-6) Å(-2)) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.

Engineering dielectric constants in organic semiconductors

The dielectric properties of three pairs of organic semiconductors that contain increasing numbers of cyclopentadithiophene-co-benzothiadiazole moieties (monomer, dimer and polymer) were studied and compared. The materials in each pair differed in the nature of the ‘solubilizing groups’, which are either alkyl- or glycol-based. At low frequencies (<MHz), dielectric constants of up to ∼9 were obtained for the glycolated materials. In addition, the optical- (high-) frequency dielectric constants for the glycolated dimer and polymer were 4.6 and 4.2 respectively, which are the highest values reported thus far for non-ionic organic semiconductors. The external and internal quantum efficiencies (EQE and IQE) of homojunction (i.e., single component) solar cells comprising the dimer and polymer glycolated materials both showed measurable improvements at wavelengths close to their optical gap when compared with the alkylated equivalents. The improvement is suggestive of an increase in the charge generation efficiency, potentially facilitated by the high optical-frequency dielectric constant.

The structural impact of water sorption on device-quality melanin thin films

The melanins are a class of pigmentary bio-macromolecules ubiquitous in the biosphere. They possess an intriguing set of physico-chemical properties and have been shown to exhibit hybrid protonic-electronic electrical conductivity, a feature derived from a process termed chemical self-doping driven by the sorption of water. Although the mechanism underlying the electrical conduction has been established, how the sorbed water interacts with the melanin structure at the physical level has not. Herein we use neutron reflectometry to study changes in the structure of synthetic melanin thin films as a function of H2O and D2O vapour pressure. Water is found to be taken up evenly throughout the films, and by employing the contrast effect, the existence of labile protons through reversible deuterium exchange is demonstrated. Finally, we determine a sorption isotherm to enable quantification of the melanin-water interactions.

Thiophene dendrimer-based low donor content solar cells

Low donor content solar cells containing polymeric and non-polymeric donors blended with fullerenes have been reported to give rise to efficient devices. In this letter, we report that a dendrimeric donor can also be used in solution-processed low donor content devices when blended with a fullerene. A third generation dendrimer containing 42 thiophene units (42T) was found to give power conversion efficiencies of up to 3.5% when blended with PC70BM in optimized devices. The best efficiency was measured with 10 mole percent (mol. %) of 42T in PC70BM and X-ray reflectometry showed that the blends were uniform. Importantly, while 42T comprised 10 mol. % of the film, it made up 31% of the film by volume. Finally, it was found that solvent annealing was required to achieve the largest open circuit voltage and highest device efficiencies.

Relating Structure to Efficiency in Surfactant-free Polymer:Fullerene Nanoparticle-based Organic Solar Cells

Nanoparticle dispersions open up an ecofriendly route toward printable organic solar cells. They can be formed from a variety of organic semiconductors by using miniemulsions that employ surfactants to stabilize the nanoparticles in dispersion and to prevent aggregation. However, whenever surfactant-based nanoparticle dispersions have been used to fabricate solar cells, the reported performances remain moderate. In contrast, solar cells from nanoparticle dispersions formed by precipitation (without surfactants) can exhibit power conversion efficiencies close to those of state-of-the-art solar cells processed from blend solutions using chlorinated solvents. In this work, we use small-angle neutron scattering measurements and transient absorption spectroscopy to investigate why surfactant-free nanoparticles give rise to efficient organic solar cells. We show that surfactant-free nanoparticles comprise a uniform distribution of small semiconductor domains, similar to that of bulk-heterojunction films formed using traditional solvent processing. This observation differs from surfactant-based miniemulsion nanoparticles that typically exhibit core-shell structures. Hence, the surfactant-free nanoparticles already possess the optimum morphology for efficient energy conversion before they are assembled into the photoactive layer of a solar cell. This structural property underpins the superior performance of the solar cells containing surfactant-free nanoparticles and is an important design criterion for future nanoparticle inks.

Deuterated phytantriol – A versatile compound for probing material distribution in liquid crystalline lipid phases using neutron scattering

Phytantriol is an interfacially-active lipid that is chemically robust, non-digestible and forms particles with internal bicontinuous cubic phase structures (cubosomes) when dispersed with non-ionic surfactants at ambient and physiological temperatures. The liquid crystalline internal structure of phytantriol-based cubosomes can be changed to alter the interfacial contact area/topology with the aqueous dispersant to trigger bioactive payload release or to alter the local membrane curvature around bound or embedded proteins. To enable the study of payload distribution, structure and transformation kinetics within phytantriol particles by neutron scattering techniques it is desirable to have access to a deuterated version of this molecule but to date a synthetic route has not been available. The first successful synthesis of phytantriol-d39 is presented here alongside a preliminary physical characterisation of related particle structures when phytantriol-d39 is dispersed using two non-ionic surfactants, Tween® 80 and Pluronic® F127. Synchrotron small angle X-ray scattering (SAXS) was used to confirm that phytantriol-d39-based nanoparticles in D2O form similar liquid crystalline structures to those of their natural isotopic abundance (phytantriol/H2O) counterparts as a function of temperature. Finally, small angle neutron scattering (SANS) with solvent contrast to match out the phytantriol-d39 structuring was used to show that the spatial correlations between the Tween® and Pluronic® non-ionic surfactant molecules are different within dispersed phytantriol-d39 particles with different liquid crystalline structures in D2O. The surfactant molecules in phytantriol-d39/Tween® 80 particles with Im3m cubic structures were found to follow a self-avoiding walk, whereas in phytantriol-d39/Pluronic® F127 particles with Pn3m cubic structures they were found to follow a more rod-like packing arrangement.

Mixed domains enhance charge generation and extraction in small-molecule bulk heterojunction solar cells (Conference Presentation)

It is established that the nanomorphology plays an important role in performance of bulk-heterojunction (BHJ) organic solar cells. From intense research in polymer-fullerene systems, some trends are becoming apparent. For example, small ~10 nm domains, high crystallinity, and low miscibility are typically measured in high-performance systems. However, the generality of these concepts for small-molecule (SM) BHJs is unclear. We present a comprehensive study of performance, charge generation and extraction dynamics, and nanomorphology in SM-fullerene BHJ devices to probe these critical structure-property relationships in this class of materials. In the systems investigated, small domains remain important for performance. However, devices composed of highly mixed domains with modest crystallinity outperform those consisting of pure/highly crystalline domains. Such a result points to an alternative ideal morphology for SM-based devices that involves a predominant mixed phase. This stems from SM aggregation in highly mixed domains that both maximize interface for charge generation and establish continuous pathways for efficient charge extraction. Such a morphological paradigm should be considered in future SM systems in pursuit of high-efficiency large-scale solar power production.

The Curious Case of the OZ439 Mesylate Salt: An Amphiphilic Antimalarial Drug with Diverse Solution and Solid State Structures

Efforts to develop orally administered drugs tend to place an exceptional focus on aqueous solubility as this is an essential criterion for their absorption in the gastrointestinal tract. In this work we examine the solid state behavior and solubility of OZ439, a promising single-dose cure for malaria being developed as the highly water-soluble mesylate salt. The aqueous phase behavior of the OZ439 mesylate salt was determined using a combination of small angle neutron and X-ray scattering (SANS and SAXS, respectively). It was found that this salt has low solubility at low concentrations with the drug largely precipitated in free base aggregates. However, with increasing concentration these crystalline aggregates were observed to dissociate into cationic micelles and lamellar phases, effectively increasing the dissolved drug concentration. It was also found that the dissolved OZ439 spontaneously precipitated in the presence of biologically relevant anions, which we attribute to the high lattice energies of most of the salt forms of the drug. These findings show that aqueous solubility is not always what it seems in the context of amphiphilic drug molecules and highlights that its use as the principal metric in selecting drug candidates for development can be perilous.

The impact of digestion is essential to the understanding of milk as a drug delivery system for poorly water soluble drugs

&NA; Milk has previously been considered as a potential lipid‐based drug delivery system for poorly water soluble drugs but it has never gained significant attention. This is in part because relying on solubility in lipid‐based formulations (in this case milk) does not provide a complete picture of the behavior of such systems upon digestion. Herein, we demonstrate using time resolved X‐ray scattering that the digestion of milk is actually crucial to the solubilisation of a poorly water‐soluble drug, halofantrine. Halofantrine was chosen because its behaviour in lipid‐based formulations has been widely investigated and because of its close structural relationship to lumefantrine, an antimalarial drug of current interest for the treatment of paediatric malaria. The transformation of the drug from a crystalline solid form in suspension in milk, to a solubilised form as a direct consequence of lipolysis highlights that consideration of digestion of the milk lipids as a critical process that influences drug solubilisation and availability for absorption is vital.