Professor Stephen Evans
- Position: Professor
- Areas of expertise: lipid membranes; microbubbles; drug delivery nanoparticles; microfluidics; single cell phenotpying; surface characterisation; raman spectroscopy; XPS; ellipsometery
- Email: S.D.Evans@leeds.ac.uk
- Phone: +44(0)113 343 3852
- Location: 8.34 EC Stoner
- Website: Personal home page | MNP Group
- Director of Research and Innovation
- Head of Molecular and Nanoscale Physics Group
Self-Organising molecular systems and their behaviour at interfaces have provided a focus to the research carried out in my group for the past 20 years. The majority of studies have centred on three classes of systems:
I) Supported lipid membranes and Biofunctionalised Inorganic Surfaces
II) Liquid crystal self-assembled monolayer interactions
III) Inorganic nanostructured materials
I) Supported lipid membranes and Biofunctionalised Inorganic Surfaces
The cytoplasmic membrane separates the environment inside a cell from that outside. Such membrane structures are highly dynamic and yet display long-term stability and are effective barriers against the transport of water-soluble ions and large molecules, thus providing the ability to generate the electrical and chemical gradients necessary for cell function.
The transduction of signals (chemical or electrical), metabolites and nutrients across such membranes are governed by proteins that sit within the membrane. The wide variety of roles played by these proteins has made them important targets in drug discovery as well for understanding aspects of cellular biology and disease. In spite of their importance, the structures of only a relatively small number of membrane proteins (<100) are known from the many thousands that exist. Furthermore such proteins are generally unstable outside of their membranous environment, thus methods that permit their manipulation and interrogation in artificial cellular mimics could play a useful role in determining structure-function relations as well as offer interesting possibilities for diagnostic screening, semi-synthetic sensing systems.
Bio-sensing and Electron Transport
In Leeds, we have developed a range of methodologies for attaching such membranes to solid supports and for creating membrane arrays or complex membrane patterns. We have incorporated a number of functional membrane proteins, including the acetylcholine receptor and the G-coupled protein bovine rhodopsin, to demonstrate their applicability for bio-sensing applications based on impedance spectroscopy, ion conductance or optical spectroscopic techniques and the work has been supported by EPSRC, BBSRC, Qinetiq and Philips (William, Jenkins, Cheng, Jeuken, Krzeminski).
Since such proteins play critical roles in smell, taste, sight and so on, a long term goal has been the development of integrated hybrid organicâ€“inorganic systems utilizing the highly evolved protein receptors in conjunction with silicon or carbon based electronics.
Synthetic/Minimal Systems and Hierarchical Assembly
To the formation systems of increasing complexity we have been developing two minimal systems, both on surfaces and within giant unilamellar vesicles.
The simplest of these is to create actin-like cortices on planar membranes and within vesicles. (Johnson, Barfoot) Such cortices are expected to provide simple mimics of the real actin systems and are the first steps toward creating dynamic, enclosed scaffolds. Current work on these systems has been instigating the role of membrane domains for localizing the actin nucleating protein ponticulin and controlling actin nucleation /formation. The use of such systems for controlling the mechanical properties of gas-filled, lipid-coated microbubbles is currently being explored and these systems have potential for creating artificially villi (proposal with Banting).
A more complex system is that of bacterial cell wall formation. The bacterial cell wall is constructed of peptidoglycan, a combination of short peptide chains that are cross-linked through transglycosylated and transpeptidation steps at the bilayer interface in a process involving a number of membrane proteins. Both the membrane proteins as well as the lipopetide pre-cursors need to be incorporated into the lipid membrane in a functional form. Our work has demonstrated that this has been possible either through purely synthetic routes or by using the e. coli inner membrane to form the supported lipid bilayer (Spencelayh, Dodds, Vinatier). This offers new routes for the production of simple screening platforms for testing novel antibiotics, as well as for generating a better understanding of the process of bacterial cell wall formation.
Membrane Protein Manipulation and Brownian Ratchets
A great challenge for studying structure-function relations of membrane proteins is their instability outside of their membranous environment. Further, reconstituting membrane proteins at high concentration in liposomes or supported bilayers has proved difficult. Methods permitting the manipulation of such proteins could therefore play a significant role for their in-membrane purification, concentration and functional studies. We are currently looking to see how the combination of electric fields and asymmetric patterns can be used to move, concentrate and hopefully crystallize membrane proteins. These studies will allow multi-modal studies to be simultaneously performed on membranes containing low protein levels and are aslo likely to lead to application in screening and diagnostics (Cheetham 2011, 2012). We also envisage this approach will allow the separation of membrane species for example separations of lipid species and or membranes protein whilst still within their native membrane.
Microbubbles for Therapeutic Delivery
Lipid and polymer stabilised microbubbles (MBs) have been used as contrast enhancers in ultrasound (US) imaging for nearly 30 years. The air liquid interface and the compressibility of the MBs reflects and scatters sound waves more effectively than tissue interfaces alone, enhancing US images of internal organs. Recently, there has been much interest in combining the imaging properties of MBs with the possibility of therapeutic delivery, such as for the delivery of drugs for chemotherapy or DNA/siRNA vectors for gene therapy.
The Leeds project brings together a multidisciplinary team, including scientists, engineers and clinicians to develop therapeutic microbubbles for the treatment of colorectal cancer. However the approach is quite generic and is also likely to find application for the treatment of cardiovascular and musculoskeletal disease.
The key aspects of the project are:
i) The development of microfluidics for the formation of microbubbles with complex architectures.
ii) The development of novel surface coatings and their mechanical characterization.
iii) The development of ultrasound for triggered microbubble disruption and drug release.
iv) The in-vitro and in-vivo testing of drug microbubble architectures and efficacy of drug targeting.
In addition to coordinating the overall project the Evans group is responsible for part i) and ii). In part i) we are using a combination of self-assembly and microfluidics to drive the formation of microbubbles containing a lipid coating plus a payload (liposome) and finally an antibody layer for targeting the bubbles plus payload (Peyman). In the future we will extend this system to deliver hydrophobic drugs and gene therapy vectors. In part ii) we are investing the mechanical and physic-chemical properties of microbubbles with lipid coatings and seeking to develop novel coatings including for example creating microbubbles with actin scaffolds. We are also seeking to develop bubbles hydrophobic materials in their core (Grant, McKendry).
II) Liquid crystal/self-assembled monolayer interactions
Self-assembled monolayers (SAMs) offer a versatile way of controlling surface free energy and can be readily patterned to provide spatial control over the chemical groups presented at the SAM/ambient interface. We have used evanescent wave ellipsometry and optical microscopy to study the anchoring and alignment of liquid crystals at such SAM modified substrates (Allison). Interestingly, on patterned surfaces, once the feature size was reduced below ~10 um we found it was possible to obtain excellent uniform LC alignment over large areas and to spatially control the assembly of focal conic defects (Bramble 2007). Most recently we extended this work from calamitic systems to discotic ones where we have found similar control could be extended over a variety of columnar phase forming materials, including phthalocyanines. This may offer interesting possibilities for extension to light harvesting systems; our structures have strong similarities with the alignment found in chlorosomes (Bramble 2010).
III) Inorganic nanostructured materials (Nanoparticles and Nanowires)
Nanoparticles and Nanowires are of interest for a wide variety of potential applications from energy harvesting to medical treatment. Our work started on the application SAM functionalised nanoparticles for chemical vapour vapour sensing (Johnson, Zhang). More recently however our interest has focussed on metallic and to a lesser degree semiconductor nanowires. Projects currently being pursued are:
i) conductivity in ultrathin metallic nanowires (<20 nm) here the interest is in obtaining low resistivity samples in which electron scattering is dominated by surface scattering as opposed to scattering from grain boundaries. Under such conditions the binding of charged species to the surface, eg a bio molecule, will lead to new scattering centres and hence changes in conductivity. We are undertaking single nanowire electronic measurements using 4-probe STM and have achieved resistance values approaching the theoretical predictions for diameters >20 nm and are now seeking to reduce these to enter the highly surface sensitive regime (Critchley).
ii) The use of biological (viral and peptide) templates for high active surface area electrodes with potential for the efficient catalytic oxidation of methanol (Gorzny).
iii) Development of nanomaterials for medical applications; a) use of Au/Ag hollow tubes for disease specific therapeutic delivery and b) use of polymer nanoparticles incorporating Fe3O4 and Qdots for in-cell tracking and multimodal imaging.
Key Papers relevant to Current Research Interests
The key papers relevant to the ongoing portfolio of research, and as referred to above, are given in Appendix I.
Research groups and institutes
- Molecular and Nanoscale Physics
Current postgraduate research students
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