Copper (I) oxide-based nanoparticles for bioimaging applications

Zohora, N 2019, Copper (I) oxide-based nanoparticles for bioimaging applications, Doctor of Philosophy (PhD), Science, RMIT University.

Document type: Thesis
Collection: Theses

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Title Copper (I) oxide-based nanoparticles for bioimaging applications
Author(s) Zohora, N
Year 2019
Abstract This thesis aims to rationally design semiconductor nanoparticles and metal-semiconductor core-shell nanomaterials and investigate their potential as fluorescent probes for bio-imaging applications. Inorganic semiconductor nanoparticles are considered promising fluorescent probes for biological imaging, as they offer various advantages over their conventional organic fluorescent molecules including bright emission, photostability and low power excitation. To use these materials for biological imaging, these nanomaterials need to be biocompatible and emit in the near-infrared (NIR) region, where the auto-fluorescence contributions from the biological samples are minimum. Among the various semiconductor nanomaterials, Cu2O nanomaterials are chosen for the present study. This is a p-type direct bandgap semiconductor material (2.17 eV in bulk form), which can be synthesized in a wide range of morphologies such as nanocubes, nanospheres, nanorods and nano octahedrons. To date, these nanoparticles have not been greatly studied as fluorophores and they have never been optimised for biological imaging studies. Therefore, the first major objective of the thesis was to obtain Cu2O nanocubes with uniform size and high yield.  The nanocubes were synthesized using the seed-mediated approach where the presence of capping agent, sodium dodecyl sulfate (SDS) during synthesis plays a key role to stabilise these nanocubes and two major modifications during the synthesis led to the formation of uniform sized nanocubes. Controlling the ageing time of the seeds and the concentration of the precursors were the key parameters that enabled nanoparticles to be fabricated with a uniform shape and a high yield.

Subsequently, the fluorescent properties of these uniform size Cu2O nanocubes were investigated to study their suitability for bioimaging applications. The nanocubes are used in this study are 293 ± 18 nm along one side on average. These nanocubes exhibit strong emission in the NIR region, which is highly desirable for bioimaging applications due to the reduced autofluorescence from the biological samples in this spectral region. This strong NIR emission was observed to shift at lower temperatures and their emission wavelength and intensity can also be tuned as a function of temperature. Oxygen vacancies and their defect structures were found to be responsible for their emission in the NIR region. Their brightness and photostability were found to be extremely suitable for biological applications. Individual Cu2O nanocubes were studied using a marked substrate which was milled with a focused ion beam to locate and collect optical data from 19 individual particles. This study reveals that single Cu2O nanocube can emit light with counts up to 487K counts/s for at least 120 seconds with only 11 µW (1.7 W/cm2) laser excitation. Highly bright and photostable intrinsic fluorescence from Cu2O nanocubes at low excitation powers suggests that the nanocubes are suitable for long time bioimaging experiments. Lifetime measurements of individual nanocubes were estimated and found to have two component lifetimes. This property could also be advantageous for lifetime-based imaging applications using these nanoparticles.

For testing these materials for bio-imaging applications, cell viability of these nanomaterials was carried out on HEK293 cells and BV2 cells and it was observed that cell viability was not significantly influenced by nanoparticle incubation regardless of the incubation period, cell type or nanoparticle concentration.

An LDH (Lactate dehydrogenase) activity test was also carried out on both HEK293 cells and BV2 cells. For this study, HEK293 cells did not exhibit any significant concentration-dependent increase in cytotoxicity at any time point. For BV2 cells, LDH activity has not elevated after 2 hours of incubations with Cu2O nanocubes regardless of the Cu2O concentration for BV2 cells. However, after 24 hours of incubation with Cu2O nanocubes for BV2 cells, for 4 µg/mL and 2µg/mL, LDH activity largely increased, although cells that are treated for 2 hours then washed and kept for 22 hours for regular incubation showed much lower LDH activity for these two concentrations compared to 24 hours incubations. This study suggests that concentration-based cytotoxicity highly depends on cell types and not all cell types show cytotoxicity towards Cu2O nanocubes. For phagocytic cells, short-term incubation such as for 2 hours, nanoparticles concentrations up to 4 µg/mL can be used. In the case of a long-term incubation, such as 24 hours, less than 500 ng/mL is suggested for this cell type.

The attractive optical properties from Cu2O nanomaterials can be combined with NIR absorbing plasmonic gold nanorods in the form of their AuNR and Cu2O core-shell nanoparticles, which are another potential candidate nanomaterial for the bioimaging applications. The synthesis method for these particles involved the formation of a Cu2O shell on the surface of gold nanorods which enables control of the thickness of the shell. The capping agent, SDS was used during synthesis for the stabilisation of these nanoparticles. SEM and TEM analysis of these materials clearly demonstrated the formation of core-shell nanoparticles where the average length of this Au(rods)@Cu2O core/shell nanoparticles is of 80.6±10 nm with 25.6 ± 3.5 nm average shell thickness, while the optical properties show the combination of fluorescent and plasmonic characteristics.  This data shows that the fluorescence properties of Au nanorods can be enhanced remarkably due to plasmon-exciton interactions after Cu2O shell formation on the surface.

Overall, synthesised Cu2O nanocubes were demonstrated to have highly bright and photostable emission, with temperature dependent tunability of emission spectra and brightness.  In addition, possessing an ability to enhance fluorescence properties through plasmon-exciton interactions, combined with the positive biocompatibility results, enable these materials as desirable candidates for future bioimaging applications in areas such as angiogenesis.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Science
Subjects Condensed Matter Physics not elsewhere classified
Keyword(s) Nanoparticles
Copper oxide
Life time
BV2 cells
LDH activity
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Created: Wed, 03 Jul 2019, 14:14:28 EST by Adam Rivett
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