Active plasmonic nanostructures for biomedical applications

Abstract

Real-time and label-free detection of protein molecules at ultralow concentration in their natural state is considered the “Holy-Grail” in biomedical research. Protein molecules pop up in the bodily fluids such as saliva, blood serum, at early stage of any infection or disease and circulate throughout the body. Therefore, the emergence of that particular infection or disease can be envisioned through the detection of the signature protein markers. The early detection of the disease would help to start the treatment early, and thus ensure therapeutic success which will eventually increase the survival rates and quality of life. The early detection of protein molecules is necessary for the diagnostics as well as for environmental monitoring, emergency response and homeland security. But the desired detection of protein molecules in the early stage is extremely challenging because of the ultralow concentration of the protein markers in the bodily fluid at the early stage and their acutely small size (< 3 nm). One way to overcome this hurdle is to use the extraordinary electromagnetic responses of noble metal nanoparticles (MNPs). Here stable gold nanostars (AuNS) have been synthesized to use their property for sensing. A surfactant-free, simple, one step wet-chemistry method was used to synthesize these spiky nanoparticles, which were stable in aqueous media for more than five months. Based on their characterization and the numerical analysis, it has been realized that these nanoantennas could be an efficient agent for the early detection of disease. Furthermore, for the marker detection, the heterodimeric nanogap, created between a nanostar antenna tip and a gold nanosphere, was seen to be more effective than those single nanostar antennas because of their higher intensity enhancement capability and also the optimum electric field map at the hot-spots which acts as the binding site for molecule. Both the AuNS antenna and the hybrid one could be easily converted to a biosensor, by anchoring suitable anti-bodies on the surface of the nanoantenna. Surprisingly, these gold nanostar antennas were seen to have both the non-edge breathing modes and the well-known edge dipolar mode. The optically active edge dipolar mode will be useful for the detection of protein molecules by using their localized surface plasmon resonance (LSPR) effect which is same as any conventional plasmonic biosensor. But the non-edge breathing modes of nanostar antenna will be helpful to determine the mass of adsorbed analyte based on the cantilever principle. The mass estimation (having the information about the polarizability and the size) of the markers is very crucial because it would provide the information about the number of amino acids present in that molecule which will help for better understanding of its molecular structure and thus will be useful for designing its anti-agent. This efficient acousto-plasmonic nanoantenna therefore could become a key element at a point of care. To go one step forward in this research area, Fano-lineshape based sensing was thought to be a promising idea. Here the reported Fano line-shape arises from the coupling of the gold nanorods dipped in thermo-responsive polymer matrix and a silver thin film. The Fano system was seen to respond to both the change in external temperature and the refractive index. This kind of Fano system will be helpful for the label-free detection of the foreign protein molecule with high efficiency and also for identifying the marker’s thermodynamic state and reactions of the molecule which is crucial for protein engineering. All these constitute the base of the discussion of part I of the thesis which is about the light harvesting plasmonic nanoantennas. In the 2nd part of the thesis, AZO metasurfaces and their optical activities are discussed. Aluminium doped Zinc Oxide (AZO) is a low-loss material and popular as an alternate plasmonic material. The highly ordered AZO nanotubes array system has seen to have gas sensing capability. The reported H2 gas detection within a very short time can make this system suitable for industrial application. The detection of H2 gas of lower concentration with the help of these nanostructures is also useful to detect the presence of bacteria by tasting their exhaled H2 gas. On the other hand, the AZO solid nanopillars arrays are seen to have generalized Brewster angle phenomena which can be useful for many applications including the optical switching. Lastly, some additional works have been described in a brief way. In this section, photonic nanojet related theoretical study, asymmetric sound transmission behaviour shown in 3D printed acoustics metamaterials, focal-length tunability of metalens and plasmon assisted cancer therapy has been reported. As per my belief and understanding, all these studies reported in this thesis will enrich the related research areas.