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Design of Mobile Catalytic Nanowires for Targeted Delivery of Therapeutics

Final Report Summary - NANOCOURIERS (Design of Mobile Catalytic Nanowires for Targeted Delivery of Therapeutics)

Nanomaterials have enormous potential in drug targeting, delivery and release, and together with their potential to combine diagnosis and therapy, will emerge as one of the major tools in the field of nanomedicine. Currently, nanomedicine is dominated by nano drug delivery systems which account for >75% of the market. The aim of such systems is to improve the bioavailability and pharmacokinetics of therapeutics, thus improving efficacy and specificity while minimizing side effects. An overarching objective of the Nanocouriers project was to design and fabricate new mobile catalytic nanowires which can act as “nanomotors” for targeted drug delivery applications. Therefore, a primary aim was to fabricate and test new chemically powered catalytic modified nanowires that can be propelled by electrocatalytic decomposition of a suitable fuel.

Specifically, both gold (Au) and platinum (Pt) (and combinations thereof) have been the focus of passionate research both from a chemical and electrochemical perspective due to their potential applications in fields such as imaging, drug delivery and sensor technologies. Furthermore, bimetallic nanostructures, in particular, Pt-based nanostructures have been reported to exhibit good performance as electrocatalysts in liquid fuel cells and are regarded as an alternative to commercial catalysts.

The research strategy involved electro-fabrication of bisegment nanowires of Au and Pt with specific control over their size and shape using a template assisted electrodeposition technique. The exploitation of nanoporous anodic aluminum oxide (AAO) template assisted nanowire (Fig. 1,2) growth has numerous advantages as compared with other methods - simplicity, low cost of processing, easy material handling and the ability to tailor size, chemical composition, and microstructure of nanocomponents with the desired properties. The materials were characterised using electrochemical means together with TEM, AFM, SEM, EDX and Raman spectroscopy (Fig 3,4).

The so-formed nanowires were then used to facilitate guided motion in a liquid fuel (hydrogen peroxide). The synthesised nanowires were functionalised with proteins (glucose oxidase, horseradish peroxidise and urokinase) and their potential for application as nanomotors for biomolecule delivery assessed. The specific focus here was on directed delivery systems for “clot buster” drug cargos i.e. urokinase – a plasminogen activator drug that has revolutionised the treatment of myocardial infarction in recent years. Lysis of cell plasma clots by such plasminogen activators occurs upon binding to the cellular receptor (uPAR), dissolving the clot.

Overall, Au and Pt nanowires (200 nm x 3-4 µm) were successfully prepared by electrochemical deposition using potential sweeping, amperometry and chronocoulometry techniques from metallic salt solutions. Our systematic study revealed that the potential limits, scan rate, the concentration of the metallic salt solutions and charge passed had a significant influence on the nanostructures formed in terms of shape and quantity. Furthermore, we demonstrated that the use of Al sacrificial layer instead of Au/Pd, yielded novel Au and AuPt nanodendrite network structures. The growth of AuPt dendrites was realised by electrodepositing Au initially followed by Pt (from their respective salt solutions). Free standing nanowires and nanodendrites were also obtained by removing the sacrificial layer using mechanical polishing using a cotton tip applicator soaked in 0.5 M CuCl2 in 20% HCl and by dissolving the AAO template in 3 mM NaOH. The nanodendrite synthesis approach usually involves specialist equipment, high temperature, high pressure conditions and noxious additives. The synthesised dendrite systems could find extensive applications as Surface Enhanced Raman Spectroscopy (SERS) sensor. Raman spectroscopy studies on nanodendrites indicated that Au nanodendrites could be used as efficient probes for SERS applications and show great promise as an analytical platform especially in biological systems. The morphology, composition and structure of the synthesized nanowires/nanodentrites were analysed using a range of advanced characterisation tools. Confirmation of the deposition of corresponding Au and Pt nanowires were also reaffirmed by measuring the cyclic voltammetry plots of nanowire deposited AAO template.

In order to examine the ability of the nanostructures to transport common drug carriers, nanoscale propulsion of AuPt nanowires was studied using H2O2 as fuel. Based on video evidence (as depicted in optical image Fig 5.) repeated experiments confirmed that AuPt NWs were mobile in 5 wt% H2O2 fuel with a speed of 10.4 µm/s.

Bioconjugation of the nanowires to Glucose oxidase (GOx), Horseradish Peroxidase (HRP) and Urokinase was achieved via physisorption (Scheme A) as well as covalent attachment using a silane linkage (Scheme B). UV assay data (GOx and HRP) in the presence of 2 mM Tetramethylbenzidine (TMB) and 10 mg/ml Horseradish Peroxidase in PBS buffer (pH7) (monitoring time and protein concentration) is presented in Fig. 6 and Table 1. The yellow colour is due to (TMB)oxidised product and the silane attachment approach indicated higher absorbance signals at 450 nm which could be ascribed to more protein attachment to the nanowires. In the case of Urokinase-AuPt conjugation (Scheme A), Z-Gly-Gly-Arg7-amido-4-methylcoumarin hydrochloride (Z-AMC) and 7-Amino-4-methylcoumarin (AMC) were used as substrate and product respectively which were monitored using fluorescence. The assay data and reaction scheme is shown in Fig. 7 while Fig. 8 presents STEM and TEM images confirming successful bioconjugation for all proteins.

Throughout this project we evaluated new materials for efficient asymmetric nanowires, addressed bioconjugation and mobility testing using urokinase as a “clot buster” model system. The use of such modified nanowires as nanomotors to power nanomachines and nanofactories is one of the most exciting fields of study in nanotechnology. Overall, this project enabled successful fabrication, characterisation and transportation of so-formed heterostructures with an ability to transport important cargos for therapeutic applications. This research has contributed significantly to the optimisation of the electrochemical deposition approach used for nanowire fabrication and has assisted in unravelling the mysteries of practical nanomachines that mimic the function of natural biological nanomotors. Such nanomotors have the potential to act in much the same way that nature does, using biochemistry to power a myriad of biological motors and machines. Synthetic nanomotors have potential applications in nanomachinery, nanomedicine, nanoscale transport and assembly, nanorobotics, fluidic systems and chemical sensing. In addition, the material and physicochemical sciences will also benefit from the outputs of this research which has the potential to open doors to nanosensors, catalytic and light-harvesting devices, supramolecular mediators between electrical and living systems, and other bio- and optoelectronic components.

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