1. Introduction and Literature Review 1
1.4. Reaction Kinetics and Dynamics of Higher Ordered Assembly 21
Understanding the mechanism of formation of nanoscale metal particles starting from atomic scale through various intermediate stage is important to have better control over their size and hence properties. Similarly, studying kinetic of reaction of nanoscale metal particles towards ensemble formation and hence understanding their dynamics allows us to gain insight of their assembly mechanism and hence stability in a system as a whole for rational utilisation in various applications.
With NPs of Au/Ag and their aggregates finding application as transport materials in natural environment, biomedical field etc.125,126 colloidal stability of the same in natural environment and in solution of variable ionic strength is also important. The influence of solution chemistry on the aggregation kinetics of unmodified AgNPs was investigated by Li et al. where a critical coagulation concentration (CCCs) of the NP reaction was obtained through time-resolved
dynamic light scattering (DLS).127 In another report by Chen et al. the influence of NaCl, MgCl2 and CaCl2 on the colloidal stability and aggregation kinetics of AgNPs was delineated.128 Further, Chen et al. adopted a single molecule approach for studying quantitative kinetics of nanoparticle catalysis with real-time monitoring of single-turnover resolution.129
As coupling of surface plasmons results in shift of SPR wavelength, therefore, Alivisatos and co-workers utilised this principle - as “plasmonic rulers” to follow the directed assembly of Au and Ag NPs dimers in real time and studied the kinetics of single hybridization events.130 A significant blue-shift in absorption spectrum was observed upon addition of complementary DNA to an Au pair connected with single stranded DNA. Further, the plasmon rulers were also used to study dynamics of biophysical processes – i.e., bending and cleavage of DNA by the restriction enzyme EcoRV using ensemble kinetic measurements etc. which are important for biotechnology and nanoscience applications.131
Pelton et al. demonstrated a solvent mediated end-to-end assembly of Au nanorods by monitoring the assembly kinetics of the assembly process at different solvent composition and concentration of three different thiol ligands – undecanethiol, 11-mecpatoundecanoic acid and ɑ, ω-undecanethiol to elucidate the mechanism responsible for assembly and optimize condition necessary to obtain desired product.132 Aggregation rate study of neurological proteins is of importance for understanding the progress of many disease like Alzeimer’s, disease, type II diabetes, Parkinson’s disease (PDs) etc. Amyloid aggregates of ɑ- synuclein (AS) in dopaminergic neurons of the brain are the hallmarks of PD.
Therefore, understanding the mechanism of amyloid aggregation using various methodology and reagent that may provide control over the aggregation kinetics is essential. Stefani and co-workers have employed citrate capped AuNPs to investigate their effect on aggregation kinetics of AS using a fluorescent probe.133 They found that gold NPs had a pronounced effect in accelerating protein aggregation with kinetic rate three-fold higher at concentration as low as 20 nM for NPs with 10 nm diameter. In another study, Giorgio et al. studied the kinetics of molecular recognition mediated NP assembly involving Ab conjugated QD with angiopoietin-2 antigen.134 Such understanding of the aggregation kinetics and
Chapter 1 molecular mechanisms of interactions will be particularly important in developing
the sensitivity and specificity of diagnostic methods in biomedical field.
1.5. Importance and Potential Applications of Higher Ordered Assembled Nanostructures
This section describes the various techniques that exploit the unique properties of NPs, NCs and the their higher order self-assembled nanostructures for utilization in the field of photocatalysis, sensing, bio-imaging, surface enhanced spectroscopy, drug delivery, cancer detection, optoelectronic devices, photonic crystals etc.4,17,39,126,135-140
AuNPs in general are sensitive to selective metal ions, anions, peptides, specific biomolecules etc. to undergo aggregation resulting in significant red-shift or broadening in the SP band accompanied by change in the solution color from red to blue. This phenomenon makes AuNP an attractive candidate as colorimetric sensors.17 Geddes et al. have demonstrated a colorimetric glucose sensing assay via the dissociation of Con A-aggregated dextran coated AuNPs. The Con A cross- links the dextran coated AuNPs, which results in concomitant blue-shift in SPR.
The addition of glucose diminishes the Con A-AuNP interaction releasing the individual dextran-coated AuNPs.141-142 Kim and co-workers on the other hand, designed a pH sensitive surface ligand, which can switch its charge from negative to positive under acidic environment.143 Further, they decorated as synthesized AuNPs with the pH sensitive ligand that responded to cellular acidic environment to undergo aggregation along with shift in the plasmon band to longer wavelength.
The hence-formed Au plasmon aggregates with absorption in the NIR range was then utilised for photo thermal cancer therapy. Ensembles of NPs and NCs have also been used as platform for catalysis, nanoscale thermometers, pH sensors, as
‘plasmon rulers’ and as bio- and chemical sensors for disease or analyte detection.57,62,99,144-146
One of the striking features of plasmonic particles and their assemblies is that they support SERS. Assembled plasmonic structures can exhibit up to ten-fold enhancement in Raman scattering signal due to enhanced electromagnetic field at
the hot-junction as compared to individual plasmonic particles.147,148 Literature is therefore replete with engineered self-assembled plasmonic nanostructures for SERS based molecule and cancer bio-marker detection,149 bio-imaging,150 label free monitoring of catalysis151 and single molecule studies.152
A self-assembled structure of NPs also serves as sensitive electrochemical biosensors.153 Assembled NPs deposited on electrodes tends to increase the surface area of the electrode by generating a porous surface and also provided an intimate contact with redox analyte owing to the nanoscale curvature of nanoparticles thereby serving as an efficient bridge for the electron transfer between redox analyte and the surface of the electrode.63 The electronic coupling between NPs in 3D-assemblies cross linked with organic matrix is highly sensitive to external chemical functionality. This property therefore opens up new possibilities of 3D assemblies of NPs to be used as chemiresistor type sensing application.145 Also, 3D semiconductor NP assemblies possess unique electrical properties which make them attractive for the fabrication of photoconductive photodetectors.154 Additionally, DNA directed assembly of NPs enables unprecedented control over the intrinsic photonic crystal property like – lattice parameter, crystal symmetry etc. by taking advantage of the programmability of DNA base pairing and hence finds utility in photonic research like metamaterials, quantum optics etc.155 Self-assembled structures of metal NPs and NCs have also been proven to be excellent multifunctional carriers for delivery of drugs, sensors for cancer detection and superior one/two photon imaging agent.126,156-158
One of most striking features of NCs is the emergence of their strong tunable photoluminescence which makes them suitable for applications such as the color conversion layer for fabricating light-emitting devices (LEDs). For example, a WLED prototype with color coordinate at (0.32, 0.36) is fabricated by combining CuNC ribbons (blue-green), CuNC sheets (yellow), and AuNCs sheets (red) with the feed ratio of 1.2/1/ 1.5, which is the first of its kind that employs metal NCs to produce WLEDs.87 Additionally, the unique AIE/AIEE property exhibited by Au NCs and Cu NCs makes them suitable for the design of various optoelectronic devices,159 pH sensing62 and fluorescent thermometers.57 Further assembly of Au NCs stabilised with appropriate ligands supports modulation of electron transport
Chapter 1 by sorption of vapors with exceptional selectivity and sensitivity making it
suitable for chemiresistor sensing.146 On the other hand, metal mediated crystalline assembly of AuNCs have also been explored for hydrogen storage and sensing application.160
1.6. Objective of the Thesis
The current state of the art development of colloidal nanoscale metal particles and the scientific interest in the manipulation of the same at the nanoscale to design and fabricate assembled nanostructures are vast. However, there is plenty of room to explore newer techniques to pursue the field of nanoscale assembly for their utilisation as advanced nanomaterials. The present thesis initiates a step forward to design and explore newer techniques to generate advanced nanomaterials and understanding their reaction pathway for their utilization in the field ranging from spectroscopy to biology.
The salient features of the current issues concerning the present thesis are as follows:
How do nanoscale metal particles react? Studying the reaction pathway and kinetics of nanoscale particles, employing Au NPs and NCs of Cu?
Can we achieve control over the reaction of these nanoscale particles in terms of dimers, trimers and higher order nanostructures?
How do pre-designed molecules, inorganic metal ions and their complexes allow us to achieve higher nanostructures of NPs and NCs and what advantageous physicochemical properties are induced therein?
How will the new collective properties – induced thereby in the higher ordered nanostructures – be utilised for their application in biology or spectroscopy study?
Therefore, we have attempted to address the above issues through the experiments carried out and hence their results in the following chapter of the thesis.