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Title: Co-precipitation synthesis of lanthanide-doped nanoparticles for sensing applications
Authors: Wong, Yuen Ting
Degree: M.Phil.
Issue Date: 2019
Abstract: The development of lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) has been an exciting topic in sensing and biomedical fields. In addition to the merits of high signal-to-noise ratio and excellent resistance against photo-bleaching, UCNPs exhibit low toxicity while allowing low energy excitation to minimize damages to bio-tissues. High quality of UCNPs with controllable sizes and shapes can be prepared by co-precipitation method. Moreover, the surface of UCNPs can be modified easily for enhanced water-dispersibility and further conjugations, which are important for specific detections in aqueous medium. Luminescence resonance energy transfer (LRET) is a well-recognized strategy to achieve upconversion luminescent (UCL) sensing of heavy metal ions. However, these typical LRET-based sensors are dependent on the properties of molecular quenchers; while the necessary spectral overlap between quencher absorbance and UCL for LRET may limit the flexibility of probe design. In this work, an electrochemical (E-chem) approach was firstly adopted to develop a facile UCL sensor for copper (II) (Cu²⁺) ions without using any dye quenchers. A composite probe was successfully fabricated by accommodating ligand free NaGdF₄:Yb³⁺/Tm³⁺@ NaGdF₄:Eu³⁺ core@shell UCNPs into a porous carbon fiber cloth (CFC). Its E-chem performance and optical properties were studied in response to Cu²⁺ ions, revealing a remarkable improvement in the sensitivity and selectivity due to E-chem assisted accumulation of analyte ions on the composite probe. Quenching mechanisms involved in the assays were also analyzed by Stern-Volmer (SV) relationships and lifetime measurements. Limit of detection (LOD) of the composite probe under E-chem assistance was down to 82 ppb while a comparable LOD was also found from E-chem signals, a value about 20-fold lower than the international safety guidelines. Hence, it is suggested that the composite probe is suitable for monitoring Cu²⁺ ion contamination in drinking water and maybe developed into an E-chem/UCL dual modal sensor. Apart from heavy metal ion sensing, a sensor for rapid, sensitive and specific virus detection is also of great importance for early diagnosis and treatment to reduce death rate. UCNPs have raised much attention for virus detection owing to their unique optical properties. In this work, a sandwich assay was also explored to detect different subtypes of flu virus genes based on LRET between UCNPs and gold nanoparticles (AuNPs). Poly(acrylic acid) (PAA)-capped NaGdF₄:Yb³⁺/Er³⁺@ NaGdF₄:Yb³⁺/Nd³⁺ core@shell UCNPs were prepared by co-precipitation, followed by two-step surface modification. They presented outstanding water-dispersibility and were efficiently conjugated with a segment of oligonucleotide (oligo) probe onto the UCNPs. AuNPs quenchers were conjugated with another segment of the oligo probe via acid-assisted treatment. Accordingly, the addition of target oligos enabled LRET to occur and give a signal-off homogeneous assay with a LOD at 34 pM. Considering the enhanced detection performance present in heterogeneous assays, a composite probe consisting of nanoporous polystyrene spheres (nPS) loaded with the core@shell UCNPs was also proposed.
Subjects: Hong Kong Polytechnic University -- Dissertations
Pages: xx, 126 pages : color illustrations
Appears in Collections:Thesis

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