Applications Of Quantum Dots And Nano-SiO₂ In Analytical Chemistry

Our focus is on the application of quantum dots and nano- SiO₂ in analysis of ions, organic molecules and biological molecules. Quantum dots(QDs) have captivated scientists and engineers over the past two decades owing to their unique size-dependent, symmetric, narrow and stable emission. Nanosized silicon dioxide (SiO₂) has been widely used in many fields, such as catalyst and semi-conductive material. The nanostructure materials play an important role in adsorption of biomolecules due to their large specific surface area and high surface free energy. The paper includes eight chapters. Chapter 1 is the review about QDs and nano-SiO₂. Chapter2- 8 is the experimental section.Chapter 1, the review about quantum dots, including structure, energy state, luminescence and energy flow, mainly focuses on the synthesis and modification of QDs. The use of luminescence quantum dot for optical is discussed too. The review also refers to the structure, character, application, synthesis of nanosized silicon dioxide, as well as the interaction between organic molecules, inorganic ion and silicon dioxide.Chapter 2, CdS quantum dots (QDs) modified with bismuthiol II potassium salt is prepared by one step. Based on the characteristic fluorescence enhancement of CdS QDs at 480 nm by silver ions, simultaneously, a red shift of fluorescence emission bands of CdS QDs from 460 to 480 nm is observed. A simple, rapid, sensitive and specific detection method for silver ion is proposed. Under optimum conditions, the fluorescence intensity of CdS QDs was linearly proportional to silver ion concentration from 0.01 to 5.0μmol L^-1 with a detection limit of 1.6 nmol L^-1. Possible fluorescence enhancement mechanism is also studied.Chapter 3, a novel assay of DNA is proposed based on the measurement of enhanced resonance light-scattering (RLS) signals resulting from the interaction of nano-CdS-Lcysteine with DNA. The RLS signals of nano-CdS-Lcysteine are greatly enhanced by DNA at the peak around 460 nm. Linear relationships are in range of 20— 700 ng mL^-1 for calf thymus DNA, 30-800 ng mL^-1 for fish sperm DNA. Their detection limits (S/N = 3) are 1.5 and 1.9 ng mL^-1, respectively. Based on this, a new direct quantitative determination method for DNA is established.Chapter 4, we discovered that there was FRET between CdS QDs (λ_em=480 nm) and acridine orange (λ_ex=490 nm), meanwhile, to our surprise, CdS quenched the fluorescence of AO. And DNA enhanced fluorescent signals of the system of AO and CdS. The enhanced extents were in good proportion to DNA concentrations. Based on this, a novel sensitive method was employed to determine DNA with good selectivity and sensitivity. The calibration curve was linear over 60-4000 ng mL^-1. The determination limit (3a) was 4.39 ng mL^-1. The method was applied to the determination of DNA in synthetic samples with satisfied results. The fluorescence of AO is quenched by forming AO -CdS ground state complex.Chapter 5, we have investigated the reaction mechanism between CdS and organic dyes. Pyronine B adheres to on the surface of CdS and prevents CdS aggregating, which leads to the decrease of fluorescence quantum yield of CdS. Fluorescence resonance energy transfer (FRET) happens from CdS to Rhodamine B, Butyl rhodamine B and Rhodamine 6G, respectively. There is a higher FRET efficiency between CdS and Neutral red with the value being 0.282. Moreover, coupling reaction between -NH₂ of Neutral red and -COOH of CdS -TGA leads to red shift of CdS fluorescence spectrum. The reaction also occurs between Safranine T and CdS. At the same time, the Fluorescence of ST is quenched by forming ST -CdS ground state complex.Chapter 6, CdS QDs modified by chitosan and polyethyleneimine can be quenched by Cu²⁺. According to the experimental result, we develop the novel sensor for Cu²⁺. By fluorescent spectrum, absorbance spectrum and TEM, the mechanism between Cu²⁺ and CdS modified by chitosan and polyethyleneimine is also discussed. The interaction between Cu²⁺ and the QDs surface should be of the ion-binding type.Chapter 7, a novel sensitive fluorescence method is employed to determine ctDNA (calf thymus DNA) with good selectivity and sensitivity, using nano-SiO₂ particles as an effective dispersant and stabilizer for acridine orange (AO). Compared with resonance light scattering (RLS) technique and the conventional method that used organic dyes as fluorescence probe, the developed method is not only of more tolerance with coexisting foreign substances, but also of longer stability for determination. The interaction of AO with naon-SiO₂ and ctDNA results in fluorescent signal enhancement. The enhanced extents were in good proportion to the concentration of ctDNA. The calibration curve was linear over 0.66—55.60μg mL^-1. The determination limit (3a) was 0.015μg mL^-1. The method is applied to the determination of ctDNA in synthetic samples with the satisfactory result.Chapter 8, organic dyes doped SiO₂ gels films were prepared by Sol-Gel method. The extent of hydrolysis of tetraethylorthosilicate can effect the changes of hydrogen bond between Rhodamine B and solvent, as well as the type of excite state of acridine orange in sol. By the dye fluorescent analysis among aqueous phase, ethanol phase, sol, gel film, we presume that the three-dimensional matrix of SiO₂ is the main reason, which results in blue shift of film embedded dyes.