Preface

The conversion of low-energy photons into high-energy photons, known as "frequency upconversion," using advanced optical materials has become an emerging research field with wide consequence and impact in various scientific areas ranging from healthcare to energy and security. The materials showing frequency upconversion properties are known as upconversion (UC) materials. UC materials reveal variety of applications in different fields, viz. color display, two-photon imaging in confocal microscopy, WLEDs, high-density optical data storage, upconvertors, under sea communications, solid-state lighting, sensors, photovoltaics, photocatalysis, food industry, indicators, anti-counterfeiting, bioimaging, cancer therapy and other biological fields. It is known that in comparison to ultraviolet (UV) and visible light the near-infrared (NIR) light is abundant and non-destructive in nature. It has deep penetration in the organisms and less harmful quality. UC luminescent materials in nanosize range are known as UC nanomaterials or UC nanoparticles (UCNPs). UCNPs excited with non-destructive NIR light are a better choice than the conventional downconversion nanoparticles because they are free from autofluorescence, have low light penetration, and cause less severe photo-damage to living organisms. It is notable to mention that the low efficiency of UC materials definitely becomes a major barrier for their application in a wide range. For researchers, it is a top priority to overcome this problem. Several engineered UCNPs, e.g. organic, inorganic, hybrids, and thin films, have been explored widely to obtain highly efficient UC luminescent materials. Usually, organic luminescent materials suffer poor stability under harsh conditions and have poor long-term reliability, but have a greater ductility than inorganic materials. The inorganic luminescent materials are more durable and possess high thermal stability. So, the hybrid materials consisting of both inorganic and organic components, namely, metal organic frameworks (MOFs), have attracted researchers with enhanced luminescence properties as compared to the bare organic and inorganic materials. To enhance the upconversion efficiency, spherical metal nanoparticles showing plasmon resonance in close proximity of the UCNPs are utilized. The plasmonic nanostructures are widely used to evolve the UCNPs with improved electronic, metallic, and optical properties. When the surface plasmon resonance wavelength of the metallic nanostructure matches with the excitation wavelength of upconversion mechanism, signal enhancement occurs. Usually, the coating of gold (Au) and silver (Ag) nanoparticles is used to tune the luminescence properties of UCNPs, though the nanoparticles exhibit plasmon absorption in 400-600?nm range.

The upconversion emission efficiency can be enhanced by several ways, including doping with sensitizer, non-lanthanides, and coating with inorganic shell. The non-lanthanide co-doping in UCNPs has also been used frequently in order to get enhanced luminescence intensity along with the use of sensitizer ion. The co-doping of activator and sensitizer ions with proper concentration in an appropriate host matrix is essential to achieve highly efficient UC emission as the concentration quenching has a prejudicial effect on the luminescence intensity. The phonon frequency, stability, cost effectiveness, non-hygroscopic, and non-toxic nature of the UC materials are of utmost importance. The security of any important data, currency, etc. has become very crucial to prevent counterfeiting. UCNPs with high luminescence intensity can be validated in anti-counterfeiting applications. These materials are also utilized for visual exposure of fungicides, thiram, etc., which can be broadly applied in soybeans, apples, wine farming, etc., to avoid crop diseases and excessive use of pesticides. Rare-earth-ions-based UC emission has tremendous advantages in terms of long excited lifetime, sharp emission bandwidth, low autofluorescence, high photostability, high resolution, low toxicity, etc. Rare-earth ions are found to be very sensitive to even small changes in chemical surroundings. Therefore, it becomes essential to get information about the symmetry, bonding of the probe ion, and how they change their optical properties with chemical composition of the host materials. For getting the high quantum efficiency, concentration of the dopants should be high, but it may cause concentration quenching due to the interaction between the excited and unexcited neighbors. Therefore, the nano-structured materials containing metallic nanoparticles are of particular interest because the large local field around the rare-earth ions positioned near the nanoparticles may increase the luminescence efficiency. Among several strategies, the coating of upconversion nanoparticles with inorganic materials shell is an effective method to get enhanced UC luminescence. The core@shell approach offers shielding to the surface particles and thus reduces the surface defects and possibility of quenching. This core@shell architecture is very much beneficial in biomolecule conjugation and thus suitable for many biological applications. Different coating strategies have been employed according to the required application purposes. UCNPs probes can function as multiple contrast agents for concurrent use in altered medicinal imaging modalities by providing corresponding diagnostic information (i.e. MRI and CT). Bio-conjugation on the surface of the UCNPs shows a much enhanced imaging performance in comparison to the clinically used fluorescent dyes. Innovative bio-imaging methods are being established by combining the conventional medical imaging modalities using core-shell structured UCNPs.

The book entitled Upconverting Nanoparticles: From Fundamentals to Applications is completely different from the previously published books in all respects, including the basics, scientific and technological demands. It is divided into eighteen chapters. Chapter 1, authored by Mondal and Rai, introduces the basic concepts of upconversion, and upconversion of nano-particles. The introduction to frequency upconversion and its various mechanisms, excitation and de-excitation processes in hosts containing rare-earth ions along with the spectroscopic properties of rare-earth ions/transition metals are described in this chapter. The rate equations relevant to excited-state absorption and energy transfer processes with an overview of the UCNPs have been introduced. Chapter 2, authored by Mukhopadhyay and Rai, describes the synthesis protocol of upconversion nanoparticles. In this chapter introduction to host materials and synthesis strategies of UC nanomaterials like solid-state reaction, co-precipitation, sol-gel, hydrothermal, combustion, thermolysis, microwave-assisted synthesis, core@shell synthesis techniques, etc. have been described. Chapters 3 and 4, authored by Jain et al.; Ojha and Ojha, refer to characterization techniques and analysis; Raman and FTIR spectroscopic techniques and their applications, respectively. Various structural and optical techniques for the characterization of UCNPs, viz. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectroscopy, dynamic light scattering (DLS), photoluminescence, Fourier transform infrared (FTIR), have been reported. Chapters 5, 6 and 7, authored by Ranjan et al.; Prasad and Rai; and Pattnaik and Rai, summarize the fundamental aspects of UCNPs based on their properties, frequency upconversion in UCNPs containing transition metal ions, and frequency upconversion in UCNPs containing rare-earth ions, respectively . Along with introduction the dynamics of UCNPs on the basis of fluorescence decay times, quantum yield measurement of UCNPs, frequency upconversion and its various mechanisms have also been interpreted. The various routes to enhance the upconversion luminescence along with the technological applications of UCNPs have been described.

Chapters 8, 9, and 10, authored by Singh; Dwivedi; and Ningthoujam et al., are devoted to the smart and new type of upconverting nanoparticles; surface modification and (bio) functionalization of upconverting nanoparticles, and frequency upconversion in core@shell nanoparticles, respectively. These chapters outline the upconverting core@shell nanostructures, hybrid upconverting nanoparticles, magnetic-upconverting nanoparticles, UC-based metal-organic frameworks, surface modification, bio-functionalization of upconverting materials, synthesis of core@shell and core@shell@shell UCNPs, and use of UCNPs for security, biological, and sensing applications. Chapters 11, 12, 13, 14, and 15, authored by Kumar, Mishra and Shwetabh; Singh et al.; Dey; Mahata, De and Lee; and Shahi and Rai, deal with the UCNPs in solar, forensic, security ink, and...