Molecular Plasmonics

Introduction

Unstoppable scientific progress in the development and preservation of human civilization is the solution to many of the challenges facing humankind in the third millennium. Humanity is constantly opening new opportunities for itself, but it also faces new challenges, among which are the challenges in the fight against new diseases, in preserving the environment, and in confronting terrorist threats. According to this, research in biochemistry, chemosensory, and materials science is of considerable scientific and practical interest. Surface plasmon resonance (SPR) takes a special place among the modern scientific methods, which combine the possibilities for research in the aforementioned directions. Surface oscillations of free electrons in high-conductive materials (surface plasmons) upon their resonance excitation with light create a sensitive electromagnetic field that penetrates the adjacent medium and can be used as an active supersensitive probe to changes in its refractive index. The non-destructive nature of the surface plasmon field allows investigating biomolecules in their natural state without the use of different types of labels in real time. The SPR method is not limited to biomolecular studies, which are mostly nanoscale. The ability to measure the physical parameters of a substance in a real-time mode is also necessary in the world of macromolecules,

Molecular Plasmonics: Theory and Applications Volodymyr I. Chegel and Andrii M. Lopatynskyi Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-65-5 (Hardcover), 978-0-429-29511-9 (eBook) www.jennystanford.com for example, while studying conformational transformations in polymers during the polymerization process or under the influence of external factors. In this case, an important additional advantage of the SPR method is the possibility of macromolecular studies using electrochemical approaches. The process kinetics is one of the major factors in the study of materials that exhibit redox properties, and electrochemical SPR provides an opportunity for real-time study of redox transformations in materials with electrorefractive, electrochromic, and conductive functions.

Research is being conducted with the use of SPR to enhance optical transitions—in particular, in the directions of surface- enhanced fluorescence and surface-enhanced infrared spectroscopy. Among the number of urgent problems of the SPR method, one can distinguish the need for a detailed explanation of the nature of the optical response in multilayer structures with pronounced heterogeneity, the study of processes at the media boundary under the external factors influence, and the determination of mechanisms for the interaction of biomolecules of various types and shapes with an electromagnetic field on the surface of a solid.

In contrast to the plasma wave that occurs at the surface plasmon resonance, localized surface plasmon is a collective oscillation of conduction electrons excited by the electromagnetic field of an incident light, which is confined in three-dimensional space. When the size of the metal particles decreases to the nanometer range, their optical properties change sharply with the appearance of localized surface plasmon resonance (LSPR) and their behavior differs significantly from the bulk material. At the same time, a significant dependence of the optical parameters of nanoparticles on their size and shape arises. That is why it is necessary to study the interaction mechanisms of nanosized metal particles with molecules and to explain the optical response of sensor structures on their basis, which operate due to the LSPR phenomenon. Because of the small size of individual biomolecules, in order to achieve an effective interaction between a molecule and a nanoparticle, the LSPR method requires precise control of the localized surface plasmon electromagnetic field spatial profile for the placement of molecules within this field. The use of LSPR method in nanomedicine imposes additional conditions on the shape, size, LSPR wavelength, and surface functionalization of metallic nanoparticles when used for targeted delivery, visualization, and plasmonic excitation inside the human body. At present, LSPR has already proven itself as a promising scientific method, and with the growth of production capabilities of nanomaterials, its role is rapidly increasing, and research into its use and search for new applications are becoming more and more relevant.

As a result of the scientific community's efforts in SPR and LSPR research, during the last decades a new promising scientific trend has been formed: plasmonics. This book presents generalized research results of its authors using the SPR and LSPR methods related to the studies of a variety of molecules and molecular complexes, as well as their interaction with external factors and objects of influence. This new direction of plasmonics was called the molecular plasmonics.

Overview of Current Research Progress in Molecular Plasmonics

The development of plasmonics has led to the appearance of advanced methods and analysis tools for applications in molecular research and to the formation of a scientific field known as molecular plasmonics. Nanosized electromagnetic fields of surface plasmons can interact with polymeric macromolecules, cells, and biomolecules of different sizes via optical, thermal, and mechanical influences. The control of the interaction between these objects and surface plasmons allowed the development of approaches for the effective detection, analysis, capture, transport, and manipulation of these objects. For example, since the electromagnetic field concentrated near the particles is sensitive to molecular interactions, metal nanoparticles can function as sensors for understanding biological processes of the molecular level [1]. Due to the increased light scattering and absorption cross section, metal nanoparticles are extremely sensitive markers for immunological analysis and molecular spectroscopy [2]. The high intensity and large gradient of electromagnetic (EM) fields of LSPR in the near field of a particle lead to the appearance of significant optical forces, which are the basis for the plasmonic tweezers used for the study of individual molecules [3].

Due to the small size of single molecules, to achieve effective interaction between the molecule and the surface plasmon, the molecular plasmonics requires precise control of the plasmons' spatial profile and the molecules' position within the limits of the metallic nanoparticles near field. The use of metal nanoparticles in vivo imposes additional conditions related to their shape, size, wavelength, and surface functionalization for directed delivery, visualization, and plasmonic excitation inside the human body. Progress in the nanomanufacturing, measurement methods, tools and calculations provided the opportunity to accurately simulate and control the profile of the near and far fields of LSPR in a wide range of wavelengths. This approach made possible the necessary time, spatial, and spectral control of the interactions between the plasmon and the molecule.