GOLD BASED NANOMATERIALS: BIOMOLECULE DETECTION, DRUG RELEASE AND CATALYTIC APPLICATIONS

Abstract

The thesis entitled, “Gold based nanomaterials: biomolecule detection, drug release and catalytic applications” has been divided into four chapters. Chapter 1 presents a general introduction for the gold nanoparticles (AuNP) and their properties. The origin of color in the AuNP solution due to surface plasmon resonance (SPR) and its utility in the detection of biomolecules were discussed. The unique properties displayed by gold nanomaterials are determined by their morphologies. Various methods to control the morphologies of the nanoparticles includes the use of different shapes of seed and different capping agents like DNA, amino acids and proteins for controlling the shape of gold nanoarchitectures. Materials containing two or more different nanoscale functionalities are attractive candidates for advanced nanomaterials. Bimetallic gold-iron nanomaterials being magnetic in nature possess hybrid properties were discussed for their uses in the magnetic resonance imaging, drug delivery process and in the catalytic process of organic or inorganic reactions. Scheme A. Gold based nanomaterials for biomolecule detection, drug release and catalytic application. The last part of this chapter discusses the scope of the present work. Different gold nanoarchitectures can be formed by growth controlled reactions using seed mediated methods. The growth reactions in presence of DNAs or natural amino acids and their combination can be utilized for the development of different gold nanoarchitectures, which might be suitable for the detection of biomolecule such as targeted DNA or targeted peptide sequence. Facile methods for the synthesis of gold based nanocomposites can be developed for different catalytic and drug release applications. ii Chapter 2 describes the formation of the gold based architectures after the growth reactions from the separate treatment of amine modified DNAs and natural amino acids. This chapter has been divided into two sections. Section A describes the role of reducing agent for formation of nanoflowers and nanospheres with single strand amine modified DNA Gold nanoseeds (AuNP seed) were synthesized by the citrate reduction method. Growth mediated formation of gold nanoflowers from AuNP seed was studied in the presence of 600 nM DNA and reducing agents hydroxylamine and hydroquinone.PMR(H2N-C6-5’- ACATCAGT-3’) resulted in nanoflower formation only with hydroxylamine, PML (H2N-C6- 5’-GATAAGCT-3’) or no DNA, resulted in nanospheres with both reducing agents. The growth reactions by variation of seed concentration (0.15 nM to 0.45 nM) were examined with both reducing agents. With hydroxylamine nanoflowers were formed for PMR only at high seed amount. In the case of hydroquinone reduction, nanoflowers were formed only at low seed concentration. Control experiments with MMR (H2N-C6-5’-TCTTCTGT-3’), MML (H2N-C6- 5’-GTTTTGCT-3’) and other thymine modified DNAs did not show any nanoflower formation. The role of 5’-ACA and amine terminal for the development of nanoflowers was confirmed with other mutated sequences, both end modified and non modified PMR. The requirement of 8-mer sequences for the gold nanoflowers formation was monitored by decreasing length of PMR. The nanoflowers and nanosphere obtained from PMR and PML were used for detection of DNA sequence of miR-21with hydroquinone. Section B describes the selective formation of coral shaped nanomaterials with methionine (Met). AuNP were incubated with all natural amino acids (9 mM) followed by addition of hydroxylamine and gold salt. Arg, His, Ser, Phe, Met, Trp developed blue colored and others developed red colored solutions or got precipitated. Transmission electron microscopy (TEM) confirmed formation of coral shaped nanostructures with Met and aggregated or non-aggregated with other amino acid. Controlled reactions with other molecules and inorganic salts containing sulfur resulted in aggregated or non aggregated nanoparticles. On decreasing amino acid concentration to 50 M aggregation was observed only for His and Met and coral formation from Met was inhibited. Seed variation methodology with three representative amino acids Arg, His and Met at 50 M were performed within the range 0.03 nM to 0.45 nM. In the case of Arg, the intensity of absorption increased with increasing seed amount and other two amino acids showed blue shift in the absorption peak with increasing seed amount. Decreasing trend of particle size with increasing seed amount was observed from TEM images. In the case of Met, network structure was observed at low seed amount. iii Chapter 3 describes the growth reaction of AuNP after the combined treatment of amine modified DNAs and selected amino acids. This chapter has been divided into two sections. Section A describes the role of amine modified DNA for detection of single Arg over Lys substitution in peptide. In order to develop a tool for the detection of Arg, the earlier growth reactions with all natural amino acids were further explored in presence of PMR. The nanoseeds were incubated with 600 nM PMR prior to addition of 9 mM amino acid. Arg resulted in the formation of nanoflowers whereas aggregated or non-aggregated spherical nanoparticles were observed for other amino acids. This confirmed the selectivity of Arg among the amino acids for the nanoflowers formation which is due to strong hydrogen bonding between Arg and DNA. For further improvement in the selective detection process of Arg, the amino acid concentration was lowered to 50 M, interestingly, at this concentration, Arg selectively formed nanoflowers, Met and His did not form aggregation. The selective detection of Arg to form gold nanoflowers was further applied for detection of Arg in a peptide sequence Ac-(AAAAR)3A-NH2 (RRR) and Ac-(AAAA)3K2RA-NH2 (KKR) containing single arginine. This was compared with Lys substituted peptides Ac-(AAAAK)3A-NH2 (KKK) Section B describes the time dependent growth reactions of AuNPs with PMR and MMR in presence of amino acids containing polar side chains Ser, Thr, Asn and Gln. Gold nanoseeds were incubated with these amino acids for 30 minutes followed by the addition of hydroxylamine and gold salt. Nanoparticles synthesized with Ser changed red color to blue immediately while others resulted in red color and developed blue color gradually with time. The growth reactions were again performed in presence of 600 nM PMR and MMR before addition of amino acids. PMR selectively inhibited aggregation for Ser based reaction whereas both PMR and MMR inhibited the aggregation for Thr, Asn and Gln based reactions, confirmed from TEM images and DLS measurements. The sensitivity of the DNA for interactions with Thr, Asn and Gln was performed by lowering the concentration of DNA for aggregate formation observed after 12 h by appearance of dual peaks in the absorption spectra. The sensitivity was rationalized as a combined effect of specific van der Waals interaction, hydrogen bonding and water mediated bonds between DNA and these amino acids. Chapter 4 describes the seed mediated approach for the formation of gold-iron oxide nanocomposites for doxorubicin release and catalysts for oxygen evolution reaction. This chapter has been divided into two sections. iv Section A describes the benign and straight forward one-pot syntheses for Au-FexOy(19) using sodium citrate as a stabilizing agent and iron powder as a reducing agent and metal source. The Au:Fe ratios in the nanocomposites were modulated by changing the seed concentration, amount of stabilizing agent and reducing agent. The exchange bias (EB), which arises from the interaction between antiferromagnetic hematite (Fe2O3) and ferromagnetic magnetite (Fe3O4) present within the nanocomposites, was found to increase progressively from 7→6→9 at 5K. EB of 5 was coincidental with 7 at 5K, while it showed anomalous behavior at high temperature due to presence of unblocked Fe3O4, γ-Fe2O3 and α-Fe2O3 in nanocomposite5. The coercivity at 5K increased progressively from samples 5→7→6→9.These citrate stabilized Au-FexOy nanocomposites were utilized for catalytic applications of organic nitroarene reduction process and host-guest chemistry with doxorubicin drug. Section B describes the syntheses of tryptophan stabilized Au-FexOy nanocomposites as electro catalysts for oxygen evolution reaction (OER). Among all natural amino acids, only the solution containing tryptophan developed a violet color, indicating the formation of gold nanomaterials. Nanocomposites (1025) were synthesized by varying the seed concentration and Fe amount. The representative nanocomposite10, 15, 21 and 25 were used as electro catalysts in OER reaction. The polarization curves demonstrated the better OER activity of the catalysts compared to the mixture of seeds and commercially available Fe2O3 and Fe3O4. Chronoamperometry test was carried out for the nanocomposites to check the stability at their corresponding over potentials for 5 h indicating a long-term viability of these materials. Slight loss in the iron content was observed after the post electrochemical behavior indicating materials are sufficiently stable. The thesis ends with an overall conclusion and provides scopes for further research in this area.