Peptide Mediated Nanoparticle Assembly investigated using Small Angle X-ray Scattering
Creators: name: Gryko, Piotr
Publisher: name: Imperial College London
Year: 2012

description: Nanoparticle systems arranged through bio-chemical self-assembly mechanisms display novel electronic and optical characteristics applicable to biosensing, tissue engineering, therapeutics and drug delivery. Metallic Nanoparticles such as those composed of gold or CdSe are expected to replace standard chemical fluorophore due to their high optical scattering cross-section. The colloidal nature of nanoparticles necessitates the engineering of NPs surface chemistry to control particle stability and assembly order. The high intensity of modern synchrotron sources, combined with the high electron density contrast of metallic NPs, allows for fast, sensitive, in-situ synchrotron measurements. Here Small Angle X-ray scattering is used to observe nanoscale structural changes and relate them to changes in polypeptide mediated interaction potential, particle surface charge and mobility within aggregates. These results enable strategies for optimising the structure and the optical characteristics of NP assemblies, which are vital for their performance as colourimetric sensors. Here three NP systems are investigated and characterised: (1) Gold nanoparticles assembled using de novo designed polypeptides developed by D. Aili (Chapter 3). The assembly is achieved by the folding dependent bridging of the particles induced by the heteroassociation of immobilised helix-loop-helix polypeptides and a complementary non-linear polypeptide found in solution. Changes in the optical properties of the assembly as a function of particle size and linker concentration are observed, with SAXS used to obtain in-situ structural information on the aggregates. The particles are found to assemble into highly ordered close packed structures with an interparticle spacing of 4.8±0.4nm, corresponding to the length of a folded polypeptide. Changes in particle ordering, aggregation dynamics and mass fractal dimension are observed as a function of linker concentration and particle size. These transitions are related to changes in linker mediated interaction potential, particle surface charge and mobility within the aggregates. (2) Interaction of BSA with AuNP aggregates (Chapter 4). Bovine serum albumin (BSA) is a vital component of standard immunological assays and it is often extended for use in de-novo assays based on peptide modified metallic nanoparticles. Whilst a wide range of plasma proteins such as BSA and HSA have been shown to associate with nanoparticles, there is little work done on the effect of BSA on NP assemblies, which are highly dependent on particle stability. Here the effect of BSA on AuNP aggregates is characterised, using stability assays, DLS, UV-VIS and SAXS. Far from being an inert component, the introduction of BSA directly alters the structure and interaction of AuNP aggregates. Temperature is observed to be a factor in these interactions with a specific affinity for hydrophobic systems. (3) A Quantum Dot - Gold Nanoparticle conjugate assay for the detection of urokinase plasminogen activator (uPA) (Chapter 5). Proteolytic enzymes are used as disease biomarkers for several forms of cancer. The surface assembly of gold nanoparticles onto the surface of quantum dots is demonstrated, with the growth in conjugate size characterised using in-situ SAXS. Complete conjugate disassembly is observed upon the addition of the enzyme uPA, however QDs luminescence does not fully recover. This indicates that other effects may result in the permanent quenching of quantum dots. The work presented here is designed to guide in the synthesis of general principles for assembly of NPs by biomolecular mediated bridging, whilst contributing to the fundamental understanding of the assembly process.