**1. Introduction**

Optically active materials doped with lanthanide (RE) ions dispersed in polymer are appropriate for use in a diversity of optoelectronic applications such as amplifiers, fibers, and

waveguides due to their stumpy expenditure and simplicity of processing and lofty performance [3, 13]. RE ions offer intense narrow spectral emissions from the long-lived excitedstates arising from partially filled 4fn orbitals [19]. When supplemented with the polymer host, these favorable radiative properties of RE ions have a tendency to get diminished by the intrinsic soaring vibrational frequencies of different bonds of polymers. Moreover, RE ion salts exhibit limited solubility in polymers and tend to aggregate, resulting in enhanced scattering and luminescence quenching even at low concentrations. Consequently, RE ions are united with an organic ligand before being dispersed in polymeric hosts to circumvent this. These organic ligands absorb the incident light energy and transfer it to the RE ion and can act as antenna ligands as shown in **Figure 1**.

The encapsulation of the RE ion is done with an appropriate organic ligand that is capable of transferring the input energy absorbed by the ligand to the RE ion. The triply ionized RE ions can be complexed with diverse types of strongly absorbing chelating ligands, e.g., β-diketones bipyridines, cryptands, calixarenes, cyclodextrins, and crown ethers [23]. Such complexed systems have large solubility in a lot of common organic solvents, such as chloroform, benzene, toluene, and are also soluble in several polymer matrices. One such ligand is salicylic acid (here after Sal), which is naturally prevalent as its glucosides in willow leaves and poplar, and birch. It is commonly recognized as ortho hydroxybenzoic acid or 2-hydroxybenzoic acid. Major component from the manufacture of salicylic acid is used in the production of aspirin (acetylsalicylic acid) nowadays. Molecular structure of salicylic acid is shown in **Figure 2**. The crystal structure of salicylic acid is monoclinic and has been resolved by Cochran. The crystal structure of salicylic acid is centro-symmetric carboxylic acid dimers. The hydroxyl group is hydrogen bonded intra-molecularly to the carbonyl oxygen. This leads to a less flexible molecule and dimer and a reduced intermolecular hydrogen bonding. The sensitized luminescence of lanthanide ions (especially Tb3+ ion by salicylic acid (Sal) has been found to play an important role in the analytical chemistry for the analysis of trace amount of Sal and its derivatives in biological systems. Kaur et al. [20] have prepared terbium complexes with salicylic acid and observed enhanced luminescence properties of terbium ion. RE(III) ions are chelated with ligands that have broad intense absorption bands. In these systems, intense ion luminescence originates from the intramolecular energy transfer

**Figure 1.** Pictorial representation of antenna effect.

**Figure 2.** The structure of salicylic acid.

waveguides due to their stumpy expenditure and simplicity of processing and lofty performance [3, 13]. RE ions offer intense narrow spectral emissions from the long-lived excitedstates arising from partially filled 4fn orbitals [19]. When supplemented with the polymer host, these favorable radiative properties of RE ions have a tendency to get diminished by the intrinsic soaring vibrational frequencies of different bonds of polymers. Moreover, RE ion salts exhibit limited solubility in polymers and tend to aggregate, resulting in enhanced scattering and luminescence quenching even at low concentrations. Consequently, RE ions are united with an organic ligand before being dispersed in polymeric hosts to circumvent this. These organic ligands absorb the incident light energy and transfer it to the RE ion and

The encapsulation of the RE ion is done with an appropriate organic ligand that is capable of transferring the input energy absorbed by the ligand to the RE ion. The triply ionized RE ions can be complexed with diverse types of strongly absorbing chelating ligands, e.g., β-diketones bipyridines, cryptands, calixarenes, cyclodextrins, and crown ethers [23]. Such complexed systems have large solubility in a lot of common organic solvents, such as chloroform, benzene, toluene, and are also soluble in several polymer matrices. One such ligand is salicylic acid (here after Sal), which is naturally prevalent as its glucosides in willow leaves and poplar, and birch. It is commonly recognized as ortho hydroxybenzoic acid or 2-hydroxybenzoic acid. Major component from the manufacture of salicylic acid is used in the production of aspirin (acetylsalicylic acid) nowadays. Molecular structure of salicylic acid is shown in **Figure 2**. The crystal structure of salicylic acid is monoclinic and has been resolved by Cochran. The crystal structure of salicylic acid is centro-symmetric carboxylic acid dimers. The hydroxyl group is hydrogen bonded intra-molecularly to the carbonyl oxygen. This leads to a less flexible molecule and dimer and a reduced intermolecular hydrogen bonding. The sensitized luminescence of lanthanide ions (especially Tb3+ ion by salicylic acid (Sal) has been found to play an important role in the analytical chemistry for the analysis of trace amount of Sal and its derivatives in biological systems. Kaur et al. [20] have prepared terbium complexes with salicylic acid and observed enhanced luminescence properties of terbium ion. RE(III) ions are chelated with ligands that have broad intense absorption bands. In these systems, intense ion luminescence originates from the intramolecular energy transfer

can act as antenna ligands as shown in **Figure 1**.

128 Acrylic Polymers in Healthcare

**Figure 1.** Pictorial representation of antenna effect.

through the excited state of the ligand to the emitting level of the RE(III) ion. Salicylic acid is an effective aromatic carboxylic acid, also known as ligand transferring energy to the lanthanide ions. The sensitization of Tb(III) luminescence by the salicylic acid and its derivatives were also used for determination of salicylic acid in biological applications.

Thus, RE coordinated complexes in polymer hosts are used for numerous practical applications and their luminescence efficiency may be improved by a variety of methods [3, 4, 13, 20, 22]. Besides the sensitization by organic ligands, the fluorescence of the lanthanides in the complexes can be further enhanced by the use of synergistic agents, which provide an insulating layer around the lanthanide complex, reducing the probability of radiationless energy transfer from the complex to the solvent. In addition, these are usually adopted to expel adsorbed water from the first coordination sphere and thus enhance luminescence as water molecules quench the luminescence through radiationless deactivation. Recently, synergetic ligands have been used to control the supermolecular structure of rare earth complexes and grafted the RE complex in the host matrix to form homogeneous stable functional materials and different synergetic ligands such as trioctylphosphine oxide (TOPO), thenoyltrifluoroacetonatone (TTA), tripenylphosphine oxide (TPPO), 1,10-phenanthroline (Phen), 2,2-bipy (Dipy), trioctylphosphine (TOPO), etc. We have used 1,10-phenanthroline (Phen) for our investigations—10-phenanthroline (Phen) is a heterocyclic organic compound and a bidentate ligand. It forms a strong complex with most metal ions. The molecular formula of Phen is C12H<sup>8</sup> N2 , and the chemical structures of some such ligands are shown in **Figure 3**. Any effort to improve the efficiency by increasing the RE concentration does not succeed, as aggregation of ions takes place at higher concentrations and these acts as quenching centers.

Nanoparticles (NPs) are emerging as interesting luminescent nanoscale materials not only for basic research but also for numerous applications in varied devices such as optical amplifiers, color displays, solid-state lasers, etc. The NPs affect the luminescence and dynamics of optically energetic lanthanide ions. Occurrence of enhanced intensity of emission is correlated

**Figure 3.** The structure of 1,10-phenanthroline ligand.

by means of energy transfer amid nanoparticles and the active rare earth ions. The second available option for the augmentation effect is via local energy field of NPs acting on the lanthanide ions positioned in their propinquity. This effect is prominent in particular when there is a matching of the surface plasmon resonance wavelength of the NPs and the incident light beam wavelength or luminescence wavelength [7, 26]. High surface-to-volume ratio and local phenomena such as absorption or changes in the surface electronic state may contribute significantly to special properties.

Metallic and semimetallic NPs are striking examples to be explored nowadays. The phenomenon of quantum size effect has a great influence on the physical properties that are very different from those of the bulk ones. Bismuth is a semimetal with a small energy overlap between the conduction and the valence bands. The size-induced semimetal to semiconductor transition and the related quantum confinement effects are potentially useful for optical and electro-optical device applications [2, 10, 16, 36, 37]. It has a high carrier mobility, a highly anisotropic Fermi surface, and miniature effective mass [2, 24, 46]. Below the size of 30 nm, the Bi NPs behave as semiconductors [44]. The formations of semimetal to semiconductor materials are being used in different scientific devices such as in optic and electro optic devices [39]. Bi NPs exhibit absorption in the UV region [27–29, 43]. The absorption peak shifts toward lower wavelengths in the case of smaller Bi NPs and, vice versa. The stability of Bi NPs decreases with a decrease in its particle size (due to enhanced surface to volume ratio). Nevertheless, these demonstrate strong reactive morphology. There are various methods to produce NPs. Among the diverse techniques used to fabricate NPs, laser ablation synthesis in solution (LASiS) generates pure NPs free from any type of contamination and thus, is matchless. In this technique, numerous factors, namely, laser power and wavelength, repetition rate of pulsed laser, spot size of laser beam, and most importantly the medium (solvent) used, etc., control the fabrication of NPs. The medium used for the preparation of NPs in LASiS decides the nature and steadiness of the colloidal NPs that can be further altered/improved by means of varying the pH of the solution. In numerous cases, laser-induced colloidal NPs are extremely reactive as they may react with the media and/or amidst themselves heading for the formation of multifarious configurations and varied agglomerated structures at room temperature provided there exists suitable liquid environment. But one can overcome this agglomeration by the inclusion of certain smart comparable ionic materials that could errand Coulomb repulsion [1], or else the NPs can be enclosed through dissimilar polarity layer that should favor Coulomb repulsion. The tuning of the structure of NPs from core shell to hollow nano structures is also achievable by pH variation. The researchers ought to use metallic nanoparticles for tuning the optical properties of the chosen host or activator (here the lanthanide ions), and for this intention, the sensitivity of NPs is crucial, which predominantly depends on the surface or volume plasmon frequency of NPs along with very negligible involvement of the local field effect. Thus, a deep knowledge and a clear understanding of the comprehensive mechanism of formation of NPs under diverse pH environments is a must. Bismuth-induced nanomaterials encompass exhaustive investigations to become the focal point of further research owing to their new applications (due to its semiconducting properties).

The inorganic luminescent rare earth ions and their stable complexes with varied compatible organic ligands enhance the emission of the RE ions in different media. Further, the incorporation of metallic or oxide NPs to the stable RE organic complexes unlocks the prospects for escalating their photoluminescence emission intensity by several orders of magnitude. Numerous researches and deep studies on the improvement of optical properties of RE ions and their complexes in diverse matrices have been approved and are still ongoing [34, 35, 38]. Polyvinyl alcohol (PVA) is a semicrystalline and nontoxic polymer possessing three isomers. Due to the different isomers, it shows a range of absorption and commensurate emission. The emissions are not significant from an application point of view and thus need to be enhanced. The incorporation of NPs comes to rescue and is the one best-suited option. This inclusion of NPs is achievable for the reason that the PVA molecules have large number of voids in their chains. Also, the occurrence of Bi NPs in the PVA molecule can transform/enhance the electronic emission from PVA by means of energy transfer from them and/or their local field effect. The augmentation is realistic for the reason that there exists the buffet of energy level of the PVA molecule as well as the Bi NPs absorbs immensely in the UV region. In the present chapter, differently shaped Bi NPs were prepared by laser ablation in solvents changing the pH of the solvents and characterized by transmission electron microscopy (TEM), SEM, and Raman studies. Laser ablation was used for preparation of Bi NPs in water, sodium hydroxide, and hydrochloric acid. The colloidal solution of as-prepared NPs were separately added to the Tb3+ ions and also to (Tb(Sal)<sup>3</sup> (Phen)) complex in the PVA host polymer to attain thin films. The photoluminescence spectrum of the synthesized polymer films were scrutinized with 266 and 355 nm excitations. The reason for choosing these wavelengths being that the 266 nm excitation wavelength is resonant with the SPR band of Bi NPs, and the 355 radiation for excitation is off-resonant exciting only the Tb ions. Observed augmentation in the photoluminescence emission intensity of the activator terbium ion and its complex in the existence of bismuth NPs entrenched in the PVA host are explained.
