Electrochemical and optical modulation of selenide and telluride ternary alloy quantum dots genosensors
Ndangili, Peter Munyao
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Electroanalytical and optical properties of nanoscale materials are very important for biosensing applications as well as for understanding the unique one-dimensional carrier transport mechanism. One-dimensional semiconductor nanomaterials such as semiconductor quantum dots are extremely attractive for designing high-density protein arrays. Because of their high surfaceto-volume ratio, electro-catalytic activity as well as good biocompatibility and novel electron transport properties make them highly attractive materials for ultra-sensitive detection of biological macromolecules via bio-electronic or bio-optic devices. A genosensor or gene based biosensor is an analytical device that employs immobilized deoxyribonucleic acid (DNA) probes as the recognition element and measures specific binding processes such as the formation of deoxyribonucleic acid-deoxyribonucleic acid (DNA-DNA), deoxyribonucleic acid- ribonucleic acid (DNA-RNA) hybrids, or the interactions between proteins or ligand molecules with DNA at the sensor surface.In this thesis, I present four binary and two ternary-electrochemically and optically modulated selenide and telluride quantum dots, all synthesised at room temperature in aqueous media. Cationic gallium (Ga3+) synthesized in form of hydrated gallium perchlorate salt[Ga(ClO4)3.6H2O] from the reaction of hot perchloric acid and gallium metal was used to tailor the optical and electrochemical properties of the selenide and telluride quantum dots. The synthesized cationic gallium also allowed successful synthesis of novel water soluble and biocompatible capped gallium selenide nanocrystals and gallium telluride quantum dots. Cyclic voltammetric studies inferred that presence of gallium in a ZnSe-3MPA quantum dot lattice improved its conductivity and significantly increased the electron transfer rate in ZnTe-3MPA.Utraviolet-visible (UV-vis) studies showed that incorporation of gallium into a ZnSe-3MPA lattice resulted in a blue shift in the absorption edge of ZnSe-3MPA from 350 nm to 325 nm accompanied by decrease in particle size. An amphiphilic bifunctional molecule, 3-Mercaptopropionic acid (3-MPA) was used as a capping agent for all quantum dots. It was found that 3-MPA fully solubilised the quantum dots, made them stable, biocompatible, non agglomerated and improved their electron transfer kinetics when immobilized on gold electrodes.Retention of the capping agent on the quantum dot surface was confirmed by Fourier transform infrared spectroscopy (FTIR) which gave scissor type bending vibrations of C-H groups in the region 1365 cm-1 to 1475 cm-1, stretching vibrations of C=O at 1640 cm-1, symmetric and asymmetric vibrations of the C-H in the region 2850 cm-1 to 3000 cm-1 as well as stretching vibrations of –O-H group at 3435 cm-1. The particle size and level of non-agglomeration of the quantum dots was studied by high resolution transmission electron microscopy (HRTEM). The optical properties of the quantum dots were studied using UV-vis and fluorescence spectroscopic techniques.Quantum dot/nanocrystal modified gold electrodes were prepared by immersing thoroughly cleaned electrodes in the quantum dot/nanocrystal solution, in dark conditions for specific periods of time. The electrochemical properties of the modified electrodes were characterized by cyclic voltammetry (CV), square wave voltammetry (SWV), electrochemical impedance and spectroscopy (EIS). Six sensing platforms were then prepared using quantum dot/nanocrystal, one of which was used for detection of dopamine while the rest were used for detection of a DNA sequence related to 5-enolpyruvylshikimate-3-phosphate synthase, a common vector gene in glyphosate resistant transgenic plants.The first sensing platform, consisting of ZnSe-3MPA modified gold electrode (Au|ZnSe-3MPA) gave rise to a novel method of detecting dopamine in presence of excess uric acid and ascorbic acid. Using a potential window of 0 to 400 mV, the ZnSe-3MPA masked the potential for oxidation of uric and ascorbic acids, allowing detection of dopamine with a detection limit of 2.43 x 10-10 M (for SWV) and 5.65 x 10-10 M (for steady state amperometry), all in presence of excess uric acid (>6500 higher) and ascorbic acid (>16,000 times higher). The detection limit obtained in this sensor was much lower than the concentration of dopamine in human blood(1.31 x 10-9 M), a property that makes this sensor a potential device for detection of levels of dopamine in human blood.The other sensing platforms were prepared by bioconjugation of amine-terminated 20 base oligonucleotide probe DNA (NH2-5′-CCC ACC GGT CCT TCA TGT TC-3′) onto quantum dot modified electrodes with the aid of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). The prepared DNA electrodes were electrostatically hybridized with different sequences which included 5′-GAA CAT GAA GGA CCG GTG GG-3′ (complementary target), 5′-CATAGTTGCAGCTGCCACTG-3′ (non complementary target) and 5′-GATCATGAAGCACCGGAGGG-3′ (3-base mismatched target).The hybridization events were monitored using differential pulse voltammetry (DPV) and SWV by monitoring the guanine oxidation signal or using EIS by monitoring changes in the charge transfer resistance. The quantum dot genosensors were characterized by low detection limits (in the nanomolar range), long linear range (40 - 150 nM) and were able to discriminate among complementary, non-complementary and 3-base mismatched target sequences.