Author

Xiao Liu

Date of Award

January 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Julia Xiaojun Zhao

Abstract

Graphene-based nanomaterials are novel emerging materials with their superior physical, chemical and mechanical features that have applications in a broad diversity of fields. Their excellent properties include: 1) Superior biocompatibility compared to the traditional QDs because they are free from toxic heavy metals. 2) Large surface area and easy covalent/chemical modification which makes them convenient as drug delivery carriers, in comparison with liposomes. 3) High light absorption in the near infrared (NIR) region. Compared to metal nanoparticles, which gives them better photothermal properties. 4) Strong quenching efficiency; graphene-based nanomaterials were able to quench various fluorophores through fluorescence resonance energy transfer (FRET). 5) Biofunctionalization; graphene-based materials display excellent capabilities for direct binding with biomolecules. Considering these significant properties, this dissertation work focuses on the development of graphene-based nanomaterials and applications of these nanomaterials in different fields. A total of four projects are presented.

In the first project, a multi-modal therapeutic drug delivery system was developed for cancer therapy. A reduced graphene oxide coated with mesoporous silica (rGO@msilica) nanocarrier was designed for chemo-photothermal therapeutic capacity. The inner layer of the rGO served as a photothermal agent while the outer layer of the mesoporous silica acted as a pH-trigged drug carrier. Doxorubicin (DOX) was loaded into the mesoporous silica vehicle was a chemotherapy agent. In the acidic environment, DOX was gradually released from the nanocarrier to fulfill the chemotherapeutic function. Meanwhile, rGO@msilica showed a strong photothermal (PTT) effect under a NIR irradiation, generating synergistic therapeutic efficiency for destroying cancer cells.

In the second project, a “turn-on” fluorescence method was developed for monitoring exonuclease III enzymatic activity based on the interaction between graphene oxide (GO) and a DNA hairpin probe (HP). In the absence of Exo III, the strong π-π interaction between the fluorophore-tagged DNA and GO caused efficient fluorescence quenching via FRET. In the presence of Exo III, the fluorophore tagged 3’-hydroxyl termini of DNA probe was digested to set the fluorophore free from adsorption by GO, causing less fluorescence quenching. This simple GO-based probe showed highly sensitive and selective linear response in the low detection range from 0.01 U/mL to 0.5 U/mL and with a limit of detection (LOD) of 0.001 U/mL. Compared with other fluorescent probes, this assay exhibited superior sensitivity and selectivity in both buffer and fetal bovine serum samples. The assay was also low cost and easy to set up.

In the third project, humic acid, a new carbon source was used to fabricate graphene quantum dots (GQDs) by an easy one pot hydrothermal reaction. The morphology of the blue emission graphene quantum dots was characterized by high resolution transmission electron microscopy (HRTEM) and dynamic light scattering (DLS). The results showed crystalline lattice spacing of 0.286 nm and a hydrodynamic diameter of 6.6 nm. X-ray photoelectron spectroscopy (XPS) and FT-IR have demonstrated diverse functional groups on the GQDs, which yielded strong chelating interactions with Cu2+. The optical properties of GQDs were characterized by photoluminescence (PL) spectra and ultraviolet-visible (UV-Vis) spectroscopy. Both showed that the GQDs had an excitation-dependent fluorescence behavior and a large stoke shift with maximum excitation/emission wavelength at 340/470 nm. These properties will provide a useful signal for fluorometric methods of analysis. Furthermore, the GQDs showed a good photostability and a relative high quantum yield of 20%, which is adequate to serve as a bioimaging probe. Metal ion selectivity studies indicated that the GQDs could be applied as Cu2+ sensing nanoprobe for drinking water in the linear range from 1 M to 40 M and a low LOD of 0.44 M. Overall, this work provided a simple method to produce GQDs as well as a Cu2+ sensing nanoprobe with low cost raw materials humic acid, which would be a great improvement in biomedical fields.

In the fourth project, Eu3+ coordinated polymer nanoparticles based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH PPV-Eu) were developed and used for Cu2+ detection. The designed MEH PPV-Eu probe has both fluorescence signal from the conjugated polymer and the Eu signal detected by spICP-MS. Cu2+ were proven to have efficient chelating interactions with carboxylic groups, which resulted in a significant fluorescence quenching due to aggregation. The aggregation increased the Eu intensity per particle and was also quantified by single-particle inductively coupled plasma mass spectrometry (spICP-MS). For the fluorometric method, it obtained a linear range from 2 M to 50 M and a LOD of 0.29 M. For the spICP-MS method, it achieved a lower LOD of 0.42 pM and a wider linear range from 1.0×10-3 nM to 1.0×104 nM. This combination assay will provide a rapid and fast approach with an extremely low detection limit, indicating its superior potential in environmental analysis.

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