Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite

This chapter deals with the effect of alternating electrical current on hydroxyapatite [HAp, Ca 10 (PO 4 ) 6 (OH) 2 ] and doped HAp along with their optical response and the processes involved. The dielectric constant, permittivity and ac conductivity were analyzed to have an insight into the surface charge polarization phenomenon. Further, the magnitude and the polarity of the surface charges, microstructure, and phases also play significant role in the cell proliferation and growth on the implants. Besides, the mechanism behind the electrical properties and the healing of bone fracture are discussed. The influence of various dopants on the optical properties of HAp viz., absorbance, transmission, band gaps and defects energy levels are analyzed along with the photoluminescence and excitation independent emission. In the future outlook, the analysis of effect of doping is summarized and its impact on the next generation biomaterials are elucidated.


Introduction
Hydroxyapatite (HAp) is one of the phases of calcium phosphate having excellent biological properties. Human bone contains 75% of inorganic materials (HAp) and remaining organic contents (predominantly collagen) and water. The HAp is analogous to the inorganic compositions of bone. The drawbacks of HAp to be used as an implant (bone and dental replacement material) are its weak mechanical strength, resorbability and to an extent this could be overcome by doping with metal ions. However, there were no adequate new bone formation between living bone and the implants due to the slow osteoconductivity [1]. Even variations in shape, roughness of the implants did not enhance osteoconduction [2]. Some other routes say tissue engineering, even growth factors [bone morphogenic protein (BMP)] [3] etc., also possess some treatment issues. Further, these parameters could not improve the osteoconduction. So, the application of the electric field on the HAp nanoparticles to induce strongly or weakly oriented dipoles, depending on the polarization of the dopants and band gap of the HAp nanoparticles to improve its biocompatibility. Besides, the dopants modify the ionic and space charge polarizations which vary with the ionic size of the dopants. permittivity and ac conductivity of HAp. In the case of polymer/HAp composites, the dielectric behavior is distinct due to the presence of carbon and hydrogen chains and the type of bonding. On annealing, the dielectric response of the composites changes significantly due to the formation of voids and porous structures. Yamashita et al. reported the chemical effects of the electrically polarized HAp in simulated body fluid [23] and in addition enhanced the in vitro bioactivity and in vivo osteoconductivity [24,25]. Similarly, the bioactivity of HAp has been improved by doping of metal ions viz., iron [26,27], silver [28,29], magnesium [30,31], strontium [32,33], etc., in HAp. Co-dopants like magnesium-silver ions [34], ironzinc [35], etc., have also been used for augmenting physical and biological properties. Electrical and dielectric properties of HAp are crucial for understanding the dipole polarization and surface charge. Depending on the radius of metal ions, substitution or doping takes place in the matrix of HAp which in turn alters the local dielectric polarization leading to micro and macro polarization. Further, these types of polarization modify the surface charges to either positive or negative. In the case of co/tri doping, the basis of lattice has higher atomic and electronic polarization which alter local bonding of the atoms. When electromagnetic field is applied to the co-doped lattice, the wave dispersion is modified when compared to the pristine dielectric material. The ex situ electromagnetic fields enhance the bone fracture healing and bone mineralization [36]. Bowen et al. reported that an enhancement in the barium titanate incorporation in HAp lattice increases the permittivity and ac conductivity [37]. Nakamura et al. revealed the migration of protons along the columnar structure of HAp under electric field at high temperature [38]. Enhanced bone growth was noticed on the negatively charged surface whereas it was reduced on positively charged surface [39]. Osteogenesis was promoted in the dog's jawbones using HA-BaTiO 3 composites without any change in phase [40]. Nakamura et al. augmented the osteoconduction of the canine bone owing to the bioactive HAp surface as well as accelerated surface charge by electrical polarization [41]. In the lattice of HAp, the hydroxyl atoms were projected along c-axis and charges are modified on application of the electric field and with an increase in temperature. The projection of hydroxyl groups alters to either abundant negative or positive charge that would aid the cell growth and cell proliferation.
The interaction of light with the HAp nanoparticles is another interesting phenomenon, though an insulating material, the presence of phosphate ions aids the emission of photons of different wavelengths. Addition of metal ions in the lattice of HAp tunes the defect energy levels and alters the chemical potential. The dopants can affect the optical and photoluminescence (PL) of HAp. These properties depend on the absorption, transmission, and band gap as well. Further, these properties favor the use of the HAp nanostructure for bio-imaging and biosensing applications. The doped HAp facilitates the trapping of photons at defect sites or ease transmission which enables them for opto-electronic device applications. Popa and Ciobanu reported the enhanced PL by the cerium ions doped HAp without any structural change [42]. The band gap of erbium-doped HAp was reduced displaying red and green emissions [43]. The various emissions were possible due to the formation of defect energy levels coupled with the HAp energy levels which alters the rate of recombination of electron and hole pairs. Further, the doping concentration varies the active centers of recombination. Feng et al. reported that the Eu 3+ /Gd 3+ dualdoped HAp nanorods show enhanced PL with a sustained ibuprofen (IBU) release. Further, the nanorods were injected into mice and demonstrated that these nanorods could be used for in vivo imaging [44]. Europium-doped HAp revealed high photoluminescence intensity at 900°C due to the migration of europium from Ca1 to Ca2 site [45]. Fluorescent europium-doped HAp nanowires were prepared by hydrothermal technique and in situ dispersion of them for dental applications [46]. Chang et al.
reported dual emission from nitrogen-doped carbon dots (N-CDs)/HAp:europium, gadolinium composite due to aggregation of N-CDs [47]. The nanostructures either one or zero dimensions significantly affect the electronic motion forming many energy levels due to the quantum confinement effect. Moreover, the optical properties of HAp vary on doping of quantum dots, nanowires, etc. When light is shone on the materials, various local emissions occurred that would be absorbed in the matrix subsequently leading to the partial emission of photons. In the case of cerium-doped HAp, the luminescence quenching occurred between the nearest cerium ions in the matrix as reported by Kolesnikov et al. [48]. Various fluoridated HAp doped with Eu 3+ ion nanoparticles were prepared by hydrothermal technique and the fluorine ions could influence the crystal field environment for luminescence conversion [49]. Reddy et al. observed a strong green emission from Ni 2+ -doped Ca-Li HAp (CLHA) [50]. Erbium-ytterbium-molybdenum tri-doped HA/β-TCP phosphor was synthesized using solid-state reaction by Van et al. They demonstrated 650 times higher green emission in the presence of molybdenum [51]. In the case of tri-doped HAp, each dopant generates its energy levels and active centers for recombination leading to different emissions. Further, the emission depends on the dopant concentration as well. The praseodymium-doped fluorapatite illustrated green emission at 545 nm and orange emission at 600 nm [52]. Europium/barium co-doped and F-substituted nHAp (HA@nFAp:Eu/Ba) showed high sensitivity in both computed tomography and fluorescence imaging [53]. Carbon dots/HAp/PVA (CDs/HA/PVA) dual-network (DN) hydrogel scaffold showed an excellent fluorescence in non-invasive monitoring field for the in vivo evaluation [54]. In vivo investigation is a challenging task; these advanced materials help in monitoring the cell viability and cell proliferation on implants without following supplementary way to understand cell interaction. Xing et al. developed heparin-coated Eu 3+ -doped HAp nanoparticles (SH-Eu:nHAP) for bioimaging [55]. HAp nanorods tune color from blue to red and also emit white colors depending on the temperature, excitation wavelengths, etc. [56]. Optical responses, band gap, and emission behavior of materials vary on thermal treatments; further, thermal defects and production of active centers were responsible for the rapid/slow recombination of the electron and hole pairs. Hence, thermal-based luminescence is a fascinating process of emission with respect to various temperatures. However, thermally induced defects are not stable and returned to the ground state by emitting photons. Further, doping modifies the interatomic and intra-atomic transition states of basis during thermoluminescence. For instance, thermoluminescence of HAp doped with different percentages of lanthanum (La), Eu, gadolinium (Gd), and dysprosium (Dy) has been examined under the gamma radiation and (1 m%) Dy-doped samples demonstrated its capability to be used for gamma radiation dosimetry [57]. Cathodoluminescence and thermoluminescence nature of calcium phosphates were also investigated in the UV-IR range with the wavebands due to hydroxyl groups, nonbridging oxygen, etc. [58]. HAp and YAG:Ce ceramics were synthesized by Huang et al. at 850°C to fabricate white light-emitting diodes with phosphor in ceramics (PiCs) color converters in transmission mode [59]. So, these green diodes are useful in reducing the consumption of toxic materials (europium, cadmium etc.) and will guide researchers to develop various green-based diodes. Gd-dependent blue emission was observed on selenium (Se) and gadolinium (Gd) dual ions doped HAp nanoparticles under ultraviolet (UV) irradiation [60]. Copper-doped Sr-HAp was prepared by solid-state reaction Sr 10 (VO 4 ) 6-x (PO 4 ) x O 2 Cu y H 2-y-δ (x = 0-6; y = 0-0.24) as the x value decreases; copper-doped samples show intense blue emission [61]. In most of the cases, the PL emission was reduced due to the concentration quenching and non-radiative recombination of electron and hole pairs which evidently showed the variation of active centers in the matrix. Europium-doped calcium phosphate apatite-based colloids possess narrow emission with long luminescence lifetimes suitable for nanoprobe as reported by Kattan et al. [62]. These colloids were stable over time and excite near visible or visible light domains. The excited states in the colloids were highly stable and electron decay time was slow. Further, non-radiative recombination centers might be far away from each other to reduce the bleaching effect. Mesoporous strontium ions doped HAp samples exhibited blue emission and sustained drug release [63]. The PL intensity of doped HAp was correlated to the amount of ibuprofen released [63]. Similarly, Yang et al. reported the red emission of europium-doped HAp and its PL intensity altered with the release of drug molecules [64]. This idea has many practical difficulties at in vivo level and could be addressed by developing advanced analysis. Thus, doped HAp nanostructures can be an ideal candidate for both bio-imaging and drug delivery. Still, there is a lack of fundamental and basic understanding of electrical and optical nature of doped HAp. The main focus of this chapter is to discuss few instances of metal ions doped HAp and its electrical and optical behavior. Further, the path that lead to the development of next generation advanced biomedical materials will also be examined.

Types of calcium phosphates
Calcium phosphates are found in the bone and teeth as the major inorganic constituent. Depending on the Ca/P ratio and structure, the phases of calcium phosphate are classified as tabulated in Table 1. Although these phases of calcium phosphate are different from each other by their chemical composition, but they are biocompatible, non-toxic, and osteoconductive in nature which have ability to stimulate tissue regeneration. The HAp and tricalcium phosphate (TCP) are the mostly used ceramics in biomedical applications among the phases of calcium phosphate. The combination of these two phases result in a new phase formation which was recognized as a biphasic calcium phosphate used in bone repair and replacement applications. The phases of calcium phosphate have their individual adsorption and resorption capacities.  [65].
The solubility of calcium phosphate (CaPs) varies from each other and the in vivo degradation was foreseen in the order MCPM>TTCP>α-TCP > DCPD>DCP > OCP > β-TCP > CDHA>HAp [29]. Monocalcium phosphate monohydrate (MCPM) is soluble in water and is acidic in nature. Therefore, it cannot be used in the bone repair applications. The MCPM combined with α-TCP or β-TCP, makes it less soluble [65,66]. Usually the MCPM has been used as fertilizer and commonly known as superphosphate fertilizer. The Monocalcium phosphate (MCP) was prepared by heating the MCPM at 100-110°C and is similar to the MCPM. The MCP has been rarely used due to its highly hygroscopic nature [67].
Dicalcium phosphate (DCP) is biocompatible and bioresorbable and has been used as bone cement [68,69]. Dicalcium phosphate dihydrate (DCPD) also known as brushite can be easily synthesized and has osteoconductive property. It is a metastable material and can be converted to other phases DCP, OCP, CDHA, and TCP by varying the pH of the reactant solutions. The octacalcium phosphate (OCP) is responsible for the formation of teeth and bone. It crystallizes very slowly and is not suitable for bone cement application. The α-TCP and β-TCP are polymorphs having same chemical composition with different structures [70,71]. The α-TCP's solubility is better than that of β-TCP but both are biocompatible, resorbable, and have been used as bone cement. Among the phases of CaPs, the HAp is more stable and highly crystalline in nature. When the HAp is heated above 900°C, it is converted to oxyapatite (OXA) in an inert atmosphere. The tetracalcium phosphate (TTCP) was prepared by heating the DCP and calcium carbonate at 1400°C with rapid cooling [72].

Electrical properties
HAp is a bioceramic having a high dielectric constant. Researchers have studied the impact of electric field on the HAp to understand the real part and imaginary part of the permittivity [37,38]. These properties provide us an insight into the dielectric polarization of electric dipoles with respect to electric field. Further, the dielectric studies aid to understand in vitro electrical polarization. The respective real and imaginary part of permittivity grants dispersion of dipoles and energy dissipation. Some other parameters say temperature, experimental condition and dopants can influence and alter the dielectric properties. Doping sites and dopant's radius alter the electronic and orientation polarization, whereas co or tri-doping significantly affects the structure and phase, further, the dielectric properties of these materials might vary due to the rate of dipole relaxation. Generally, at low frequency alternating field, the dielectric constant is high and varies with increase in frequency. Nakamura et al. studied the polarization of HAp using direct current and showed that proton migrates along the columnar OH channel [38]. The polarized charge was high enough to improve biological activity. Usage of the electrical fields enabled the accelerated fracture healing in bones; however, it was slow in the case of long bone [72][73][74]. There are some reports on electrical stimulation which has been used to enhance the bone growth in spinal fusion [75,76]. Further, it has been employed to treat osteonecrosis as well as osteoarthritis [77]. The dielectric constant of fluids and tissues in the bone and HAp is crucial for healing. The porosity of HAp plays a vital role for the local electrical field strength [78,79]. Human bones also contain many elements such as magnesium, zinc, and strontium etc., so, the impact of the elements in the bone also play a vital role in the dielectric constant of bone. Further, it as well depends on the elemental size, concentration, and interrelated porosity. For instance, doping of magnesium ions on HAp enhances the dielectric constant ( Figure 1A) [30]. At low frequency, the dielectric constant was high due to ionic polarization; however, it was reduced by lagging of dipoles with respect to electric field. At low magnesium ions doping, the dielectric constant was low and almost equal to the dielectric constant of HAp. So, the low level doping did not highly polarize the ions in the direction of electric field and difficult to locate the dopant site position either at calcium site 1 or calcium site 2. As the Mg concentration increases, both the sites might be occupied and at low frequency the dipoles were strongly oriented, whereas it was disturbed at high frequency. Similarly, the cadmium-doped HAp revealed a higher dielectric constant in comparison with pure HAp. However, 40% of cadmium-doped HAp shows a decrease in the dielectric property. At 1 kHz frequency, the dielectric constant of HAp, doped samples 10 at.% Cd, 20 at.% Cd, 25 at.% Cd, 30 at.% Cd, and 40 at.% Cd were 6.75, 7.12, 8.16, 7.13, 7.26, and 6.24 respectively. At 40 at.% Cd, the structural change from hexagonal to monoclinic with varying crystallinity was responsible for the reduction in the dielectric constant [80]. In the case of silver-doped HAp, the dielectric constant was enhanced due to a high dipole polarization [81]. The dopant size varies the phase of HAp leading to amorphous or partial amorphization which completely modifies the local coordination and chemical potential. Horiuchi et al. revealed the two step relaxations of fluorine substituted HAp (F-HAp) [82]. They depicted that one step from 600 to 60 Hz and other 60 to 20 Hz in HAp which were analogous to the F-HAp. At low frequency, the electric dipoles are largely (L) relaxed, whereas at high frequency, the relaxation was small (S). Further, they calculated the activation energy of HAp and F-HAp as 0.63 and 0.62 eV, respectively, for one type of relaxation and it was independent of fluorine substitution. In case of other relaxation, the activation energy was significantly influenced by fluorine substitution due to the change in orientation of hydroxyl groups [82].
In the case of tungsten (W)-doped HAp, the relative permittivity was reduced up to 1 MHz and from 1 to 5 MHz the permittivity was constant [83]. The dielectric constant of W-doped HAp was high at low frequency due to the polarization of electronic, ionic, dipolar, and space charge. As the doping concentration increases, the dielectric constant was enhanced [83]. Thus, the dopants enhance spreading of the electromagnetic fields in the damaged bone sites or fractured area for rapid healing. Similarly, the other metal ions strontium [21], iron [26] etc., have been used as dopants in HAp to enhance dielectric property. A crucial and impressive part is how dielectric constant varies with concentration and with various dopants and the mechanism involved. The co-doping in HAp leads to the creation of many defects and alters the ionic polarization with respect to electric field. For instance, iron and zinc ions were co-doped in HAp at various concentrations (0.01, 0.05,  [35] with permission from Elsevier) and (C) The real part of relative permittivity as a function of frequency for (a) Pristine, (b) 1HAp (1×10 15 ions/cm 2 ), (c) 2HAp (1×10 16 ions/cm 2 ) and (d) 3HAp (1×10 17 ions/cm 2 ) (reproduced from [84] with permission from Elsevier). and 0.1 M) by ultrasonication. At higher concentration (FZH3) (0.1M), dielectric constant was greater ( Figure 1B) compared to the other samples owing to the enhanced ionic polarization and strong orientation of dipoles at low frequency. As the frequency increases, the dielectric constant was reduced in all the samples due to lagging of the dipoles [35]. Generally, at low frequency, the value of dielectric is high due to slow relaxation of dipoles with high orientation. At high frequency, the dielectric constant was reduced due to strong re-alignment of dipoles with field [30,83]. Further, it depends on the size of dopants, crystal structure, and concentration providing atomic site variations which enable the variations in the unit cell and crystal structure. Moreover, the dopants provide vacancies which affect space charge localization/delocalization. This facilitates the alterations in the dielectric constant. In case of the HAp structure, the hydroxyl ions aligned along c-axis parallel to calcium and phosphate(PO 4 ) 3− ions favor high polarization of electric dipoles thereby increasing the ionic conduction [38]. Surface charge of HAp is another important parameter which assists the bone cell growth. The polarized surface of HAp had improved the osteobonding in canine bone tissues [39]. So, the electric properties have been employed to understand the cellular behavior on bone and to develop bone prostheses [38]. The surface charge of HAp depends on the porosity and dielectric strength. It can vary on low energy ion implantation which are capable of altering the surface properties without affecting the bulk properties of the materials. Nitrogen ions (N +1 ) were implanted in HAp matrix with varying ion fluences 1 × 10 15 (1HAp), 1 × 10 16 (2HAp), and 1 × 10 17 (3HAp) ions/cm 2 [84]. At higher ion fluence (3HAp), relative permittivity was significantly enhanced (Figure 1C) in comparison with other samples because of the higher ionic and space charge polarizations. The surface and subsurface of HAp were modulated on ion implantation altering the dipole polarization thereby leading to the variation in the surface charge of the samples. The polarization varies locally leading to dielectric strength modification at the surface, subsurface and at macro level. Hence, the surface charge can be tuned by different dopants, temperature, and frequency.
The variation of dielectric constant with frequency of pure and Fe-doped HAp, Mn-doped HAp, and Co-doped HAP sample at room temperature was examined by Panneerselvam et.al. [7]. The dielectric constant of the doped samples decreased sharply up to 10 kHz, and then reduced gradually with further increase in frequency. This difference in dielectric constant is due to the four types of polarization such as electronic polarization (frequencies up to 10 16 Hz), ionic polarization (10 13 Hz), orientation polarization (up to 10 10 Hz), and space charge polarization or interfacial (up to 10 3 Hz) [7]. In all the samples, as the doping concentration increases, the dielectric constant was enhanced due to ionic polarization. The highest dielectric constant was exhibited by 5% MnHAp and the lowest by 5% CoHAp [7]. The ac conductivity measurements are used to study the mechanism of hopping and it is a complex phenomenon. Further, it depends on frequency and temperature [30]. The ac conductivity of Mg-doped HAp was enhanced compared to HAp (Figure 2A). There was low ac conductivity at low frequency due to the weak turnaround of ions with the electric field. However, it was increased at higher frequency on Mg-doped samples due to the complex array of ions and proton segregation along c-axis [30,85]. Similar trend also noticed in W-doped HAp samples which linearly depend on the frequency range. Further, it obeyed the universal frequency power law. Towards understanding the mechanism of ac conductivity of the samples, the proverbial Jonscher equation (σ ac = σ dc + Bω s ) was used [86], where B-constant, ω-angular frequency and s-frequency exponent. Using the slope of lnσ vs. lnω ac and the s values were calculated as 1.0622, 1.0600, 1.0623, 1.0501, 1.0518, 1.0629, and 1.0534 for pure and 1, 5, 10, 20, 30, and 40% of W in HAp, respectively. Here, s is almost ≤1, revealing no measureable direct current conductivity and displays prompt hopping with a translational motion [83]. In the case of the Cd-doped HAp samples, the alternating current conductivity was enhanced from 10 −10 to 10 −5 s cm −1 with an increase in frequency. The frequency exponent(s) was determined from the slope of logσ vs. logω and its value equal to 1 for the samples [80]. Eventually, it displays a lack of direct current conductivity [87]. Similar, s value was realized for the Te-doped HAp samples however, their ac conductivity enhanced at low concentration due to s ≥ 1 leading to localized hopping. At higher concentration, it was reduced due to s ≤ 1 leading to translation motion with a rapid hopping motion [88]. Ion implantation could also be employed to alter the ac conductivity, without using expensive and toxic chemicals. It can precisely modulate the ac conductivity up to a particular depth. Nitrogen ions implanted samples demonstrated higher ac conductivity at higher frequency owing to the strong arrangement of complex ions compared to pristine ( Figure 2B). However, it was less at low frequency due to the weak turnover of ions [84].

Optical properties
The interaction of light on materials leads to transmission, absorptions and reflection. These parameters depend on refractive index, wavelength, dielectric constant, and dopants. Dopants play a vital role in creating the abundant defects/vacancies in the lattice of materials. When light interacts on the bound charges of the materials, they either transmit or reflect back. The structure and phase of HAp varied depending on the type of dopant [80] which also alters the optical properties of HAp. The optical properties of HAp were enhanced by developing smaller grains and low porosity in the HAp matrix. It is an optically anisotropic material and possesses a refractive index in the range 1.644 and 1.651 depending on light polarization and direction of propagation [89]. A highly transparent HAp nanosized grains were used to observe in vivo interactions with proteins/cells [90,91]. For a proper insight of the optical properties, intrinsic correlated parameters/quantities say crystalline structure, dispersion of phonon, band structure, dielectric response, band gap, etc. are required. Density functional theory displays a local or semi local exchange-correlation interaction of HAp providing a band gap between 4.5 and 5.4 eV depending on the type of valence states [92,93]. However, the (semi) local exchange-correlation underestimates the band gap of HAp, and thus its value is above 5.5 eV [94]. So, the defect free HAp is transparent to visible light under middle or far ultraviolet illumination having a band gap >6 eV. Interesting point to note is that the dopants produce defects level in between conduction band and valence band.
For instance, the thenoyltrifluoroacetonate (TTA)-and europium (Eu)-doped HAp were used to enhance transmission spectra by 20% compared to PMMA. The TTA shows a red shift and the transmission edge from 275 to 375 nm. Both the TTA and HAp mixture restrict the formation of PMMA clusters improving the transparency. In this case, the absorption and scattering losses were introduced by the TTA and the HAp nanoparticles [95]. The other example, Cd-doped HAp showed enhanced reflectivity at the Cd (40 at.%) due to the structural transformation to the monoclinic phase. Further, it alters the refractive index of the samples as well [80]. At a particular atomic percentage, the structural transformation occurred affecting the electronic polarization thereby, altering the reflectivity. In the case of erbium doping in HAp, seven absorption bands were noticed [43] interactions of Er 3+ ions [96]. As the erbium doping concentration increases, the intensity of the peaks increased due to the replacement of Ca 2+ by Er 3+ ions. Further, there was a blue shift observed on the doped HAp due to the reduced particle size [97]. The Er-HAp samples possess direct band gap (n = 1/2) so band to band transitions was allowed. As the erbium doping concentration increased, the band gap was enhanced to 4.46 eV from 4.02 eV. Hence the structure, phase, defects and particle size of HAp are the key parameters to alter either the absorption or reflection. Further, the symmetry of the HAp also plays a predominant role in modifying the electronic polarization and dielectric constant.
Apurba et al. studied the optical response of HAp films of various thicknesses on amorphous SiO 2 substrates [98]. All the films showed 75-96% transparency in the visible region. There are two regions noticed, one is absorption and other a transparent oscillating region. Below 300 nm, the transmittance of the HAp films displayed robust absorption. When the film thickness enhances, the absorption edge was shifted to higher wavelength in turn altering electronic polarization Thus, these films revealed a strong absorption in UV region and transparency in the visible regions as well. The band gap of the varying thickness of HAp thin films was in the range 5.25-4.67 eV due to an increase in crystallinity and grain size. The band gap of thin films is similar to the band gap of bulk HAp (in the range of 5.4 to 4.51) [99]. For annealed films, the refractive indices were augmented with an increase in the film thickness [98]. Flores et al. reported that using the Kubelka-Munk function through the maximum of the first derivative, the absorption edge values were 5.62, 4.74 and 4.6 eV which correspond respectively to TbW0 [calcium-deficient (CD) HAp], TbW10 (terbium 10%-doped CDHA), and TbW12 (terbium 20%-doped CDHA) [100]. The optical band gap values of the TbW0, TbW10 and TbW12 samples were 5.41, 4.49 and 4.38 eV respectively. This demonstrates that as the Tb increases, the band gap was reduced. Here the host matrix having calcium vacancies was considered as n-type, whereas the Tb acts as a p-type dopant. When the Tb ions substitute at that calcium site then the electrons were delocalized to conduction band. Further, in the case of the TbW10 and TbW12 samples, the band gap was reduced. From the UV photoelectron spectra, the valence band energies of Ev0 (TbW0), Ev1 (TbW10) and Ev2 (TbW12) were 3.33, 2.55 and 2.40 eV, respectively [100]. It exhibits a decrease in the valence band at the higher concentration of Tb and varies Fermi level of the samples.
Feng et al. studied the drug adsorption capacity of HAp using absorption spectra [44,101]. The respective drug adsorption capacity of the undoped HAp nanorod and the Eu 3+ /Gd 3+ HAp was 653.5 and 841.4 mg/g. The interaction of the dopants with drug molecules might be strong due to the presence of oxygen vacancies and atomic defects which pave a path for sustained or rapid release of drug molecules. This revealed that the doped nanorods displayed a high drug adsorption compared to undoped sample with a reduced absorbance. The size of the dopant and its doping concentration play a predominant role in modifying the crystal structure thereby the Fermi level position gets altered leading to the enhancement or decrease of the band gap. When photon interacts with the material, either high absorbance or transmission of photons occurs due to the variation in energy levels of defects or vacancies. The optical response of the doped HAp is not only dependent on material property and type of dopant (drug or organic molecules) but also on the type of wavelength used. Moreover, different wavelengths have distinct capability of light reflection, absorbance, and transmission as well.
The carbon-based HAp showed enhanced optical absorption. N-doped carbon dots (N-CDs) revealed absorption at about 240 and 340 nm, due to the Π▬Π* transition of C〓C and Π▬Π* transition of C〓O and C▬N/C〓N, respectively [56].
In the case of HAp:Eu,Gd, the absorption at 200-280 nm due to the wide band gap of HAp with a peak (395 nm), was assigned to the intrinsic 4f-4f transition absorption of Eu [56] confirming the incorporation of Eu into HAp. The absorption of N-CDs/HAp:Eu,Gd was superior than single N-CDs and HAp:Eu,Gd and its band edge shifted to 550 nm due to the porous structure and multiple reflection [47]. The CDs based system could be used as a stable bioimaging candidate. Ni 2+ -doped calcium-lithium hydroxyapatite (CLHA) nanopowders were synthesized by mechanochemical synthesis by Reddy et al. [50]. The spectra of these powder possess four absorption band at 420, 718, 794, and 1189 nm attributed to the transitions of 3 A 2g (F) → 3 T 1g (P), 3 T 1g (F), 1 E 1g (D) (spin-forbidden transition), and 3 T 2g (F), respectively [50]. The chemical potential of the CLHA is drastically affected leading to the modification of conductivity and dielectric properties. Iron, manganese, and cobalt ions doped HAp possess direct band gap (index number n = 1/2). The band gap of 5% Co-doped HAp was higher than Fe, Mn ions doped samples [7]. The band gaps were modified due to the presence of defect energy levels which shift Fermi level toward the valence or conduction band.

Photoluminescence
The PL of HAp depends on the defect energy level formation, structure, and size of the nanoparticle. Moreover, its band gap is above 5.5 eV, when dopants added to HAp matrix, the band gap perhaps vary due to the formation of many defects energy levels in between valence and conduction band. When the HAp nanostructure was excited by different wavelengths, electrons were accordingly excited to the higher states sometimes, these electrons recombine with holes promptly to produce radiative emission. However, in some cases, the emission might be non-radiative. Proper understanding of the effect of dopant is required to create either light active or inactive centers which enable emission at different wavelengths. However, it also depends on the excitation wavelength as well. The defects energy level formation is a complex phenomenon containing the associated band position and the band bending process. It as well depends on the size, phase and atomic arrangements. In order to reduce non-radiative emission, the doping concentration must be properly tuned to avoid the occurrence of concentration quenching; otherwise, abundant non-radiative centers are facilitated [34].
Addition of sodium hydroxide (NaOH) varies PL of Sr-HAp samples [102]. By varying NaOH content, the shape of the spectra did not change drastically; however, the intensities vary significantly. The Sr-HAp sample (0.15 g NaOH) displays the strongest emission and the weakest intensity was at 0.40 g NaOH due to the variation in luminescent centers. Here, neither Sr 2+ nor PO 4 3− is responsible for the luminescence but it was due to the carbon and oxygen related active electronic defect centers on the host matrix [102]. Some reports are available for the self-activated luminescence due to the addition of trisodium citrate which enables the creation of carbon monoxide impurities [103]. In the case of the Tb-doped samples, the excitation peaks of TbW10 (terbium 10% CDHA) and TbW12 (terbium 20% CDHA) were matched with that of CDHA. Further, the vacancies and carbonate radicals act as acceptor chromophores and terbium ions as donor chromophores. The interaction of chromophores leads to a non-radiative relaxation through defects and impurities in each sample [100]. Two different wavelengths (255 and 264 nm) were used for excitation of the samples. The emission peaks were associated with 5 D 4 → 7 F 6 (489 nm), 5 D 4 → 7 F 6 (544 nm), 5 D 4 → 7 F 4 (585 nm), and 5 D 4 → 7 F 3 (622 nm) due to intraconfigurational 4f-5d transitions of the Tb 3+ ion. At the 544 nm, a high intense green light emission was noticed in the doped samples  [104,105]. There was a weak contribution of the PL emission from the host matrix of both samples and displayed a deformation of the peak shape with a small shift in the peaks 5 D 4 → 7 F 6 and 5 D 4 → 7 F 4 transitions. The emission from TbW10 was high due to chemical composition, self-activated CDHA, whereas at higher Tb doping (TbW12), the intensity was reduced due to concentration quenching leading to non-radiative relaxation [100].
Yang et al. reported the broad excitation spectrum at 250 nm due to the charge transfer between europium and oxygen ions and further, small peaks were noticed at longer wavelength due to f-f transitions of Eu 3+ [28]. It was monitored by 5 D 0 → 7 F 2 transition at 612 nm. The two peaks were prominently noticed at 5 D 0 → 7 F 1 (590 nm) and 5 D 0 → 7 F 2 (612 nm). Even the IBU-loaded europium-doped HAp samples display a clear PL spectrum and it can be tracked by the luminescence intensity. The fascinating part is how the PL intensity varies with release rate of drug molecules. As the cumulative release rate increases, the PL intensity enhances and reached maximum when all drug molecules were released completely. The organic groups of the IBU highly quenched the emission due to europium in the IBU-Eu:HAp. The IBU creates high non-radiative centers in the matrix. However, the release of IBU augments with a weakening of quenching effect which was confirmed by enhanced PL intensity. Thus, it was used as a potential probe for monitoring the drug release [28].
In the case of dual ions doped samples, the europium/gadolinium dual-doped HAp nanorods showed a maximum excitation peak at 394 nm [44]. There was no significant emission on the co-doping of europium (Eu 3+ )/gadolinium (Gd 3+ ) compared to the lone europium doping. However, the PL intensity was altered and four emission peaks were noticed at 590, 615, 650, and 699 nm [44]. The prominent peak was identified at 615 nm due to 5 D 0 → 7 F 2 transition within Eu 3+ ions. The other PL peaks were observed at 590, 650, and 699 nm owing to their respective 5 D 0 → 7 F 1 , 5 D 0 → 7 F 3 , and 5 D 0 → 7 F 4 transitions. As the doping concentration of europium increases, the PL emission intensity was enhanced. The highest PL intensity occurred in 5 mol% Eu 3+ but with an enhanced intensity compared to the dual doped samples. The PL intensity was higher at the doping ratio of Eu 3+ to Gd 3+ (1:2) than the ratio 1:1. On UV irradiation, the dual doped samples displayed a robust red emission from powder as well as from powder dispersed solution [44]. So, the PL intensity depends on the phase, particle size and defect based active centers which absolutely alter either the inter-or intra band transition and emission.
In the case of Mg ions doping on HAp, the PL intensity was enhanced accompanied by a modification of the spectra. At higher Mg concentration, the peak was reduced due to weak radiative recombination [30]. Further, the defects create many active centers for non-radiative recombination and the centers might be very close to each other for a rapid decrease in the PL intensity. As the Mg and Ag ions in HAp augments, the PL intensity was enhanced, however, as the concentration raises, the intensity was reduced (Figure 3A) [34]. With further increase in co-doping, the PL intensity was enhanced but lesser than the lowest concentration of co-doping [34]. A similar emission was also seen in the iron and zinc co-incorporated HAp samples, but as the co-doping increases, the PL intensity was reduced ( Figure 3B) [35]. So, the PL intensities were tuned by varying co-doping concentration which might alter the radiative recombination active center in the HAp matrix. Ion implantation is one of the effective routes to dope a particular element at the desired depth and varying defects energy level to tune the PL intensity. Nitrogen ion implantation enhanced the PL emission of HAp [84]. As the ion fluence increases, the PL intensity was enhanced due to the radiative recombination of electron-hole pair without the influence of concentration quenching effect ( Figure 3C) [84]. There are reports on different synthesis routes for doping of various elements on HAp viz., microwave-assisted route followed by ion implantation which is similar to dual ions doping [106]. However, the implantation was used to create/dope ions on the surface and subsurface with associated defects/vacancies without disturbing bulk properties of HAp. For instance, nitrogen ion implantation on magnesium ion incorporated HAp samples demonstrated that at low fluence of nitrogen, the PL intensity was enhanced [106]. However, at high fluence, the intensity was reduced due to concentration quenching and high non-radiative centers created on the surface. The PL intensity of unmodified Mg-doped HAp decreased drastically compared to the surface modified samples [106]. In the case of polymer (polyvinyl alcohol) doping in HAp, the PL intensity was raised [107] and the highest PL intensity was noticed at higher concentration due to the formation of abundant active sites created by the carbon and oxygen radicals in the polymer chains. So, the PL signal could be enhanced by optimizing the doping concentration, varying the band gap and creating active recombination defect sites and vacancies.
Machado et al. studied the photoluminescence of HAp nanorods at different excitation wavelengths (380 to 680 nm) [56]. They demonstrated a shift of emission from blue to red owing to the absence of the short wavelength emission with an increase in the excitation. Near-UV excitation, the intensity was higher with blue/green emissions. Eventually, the best emission intensity was in the range 330 to 430 nm [56]. Erbium-ytterbium (Yb)-molybdenum (Mo) tri-doped HA/β-TCP phosphor prepared by solid-state reaction was examined for intense green upconversion (UC) emission [51]. M5 ((0.5% mol) Er-(10% mol) Yb-(8% mol) Mo tri-doped HA/β-TCP) showed green emission intensity 650 times than that of the M1 (0.5% mol Er-doped HA/β-TCP), M2 ((0.5% mol) Er-(10% mol) Yb co-doped HA/β-TCP) due to energy transfer from 2 F 7/2 , 3 T 2 > state of Yb 3+ − MoO 4 2− dimer to the 4 F 7/2 of Er 3+ ions. Further, red emission was weak because of feeble absorption cross-section of 4 I 13/2 and the green emission band can be controlled by the Mo 6+ doping concentrations. It aids to use the material for bioimaging application [51]. N-doped carbon dots/HAp:Eu,Gd (N-CDs/HAp:Eu,Gd) composite were studied [47] and the PL spectra of N-CDs/HAp:Eu,Gd simultaneously revealed the emission from N-CDs and HAp:Eu,Gd on single wavelength excitation. The PL emission depicted no shift under different wavelengths revealing excitation independent emission. It clearly shows that the electronic structure of N-CDs and Eu ions were not affected during the synthesis. So, both the N-CDs and HAp:Eu,Gd were preserved. The PL lifetime of both N-CDs/HAp:Eu,Gd and N-CDs illustrated a single exponential decay but the lifetime decreased from 11.7 ns (N-CDs) to 5.6 ns (N-CDs/HAp:Eu,Gd) owing to the photogenerated charges [47]. Further, it could be used for bioimaging with distinct differentiation of colors due to different emission from carbon dots and doped HAp samples. Polymer/inorganic based structures can also affect the PL. Karthikeyan et al.  [34] with permission from Elsevier), (B) Photoluminescence spectra of (a) FZH0, (b) FZH1, (c) FZH2 and (d) FZH3 (reproduced from [35] with permission from Elsevier) and (C) Photoluminescence of (a) pristine, (b) 1HAp, (c) 2HAp and (d) 3HAp (reproduced from [84] with permission from Elsevier). Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092 described the formation of core-shell structures on argon ion implanted polymerbased zinc ions incorporated HAp and reported that the zinc ions and the polymer lead to formation of many defects which in turn produces additional energy states. The PL intensity at the lower fluence samples (1 × 10 14 ions/cm 2 ) showed enhancement in the PL intensity due to the presence of higher radiative active centers. On increasing the fluence viz., 1 × 10 15 ions/cm 2 and 1 × 10 16 ions/cm 2 , the PL intensity was decreased owing to the formation of more non-radiative centers [108]. The incorporation of metal ions such as iron not only play a major role in tailoring the optical properties but also when exposed to magnetic field, they tend to change their morphologies depending on the magnetic field strength and have potential biological application such as protein absorption [109]. It could also lead to the development of magnetic based prolonged PL emission. Hence, the structure and morphology play a prominent role in altering the radiative/non-radiative recombination sites which can be correlated to the PL intensity decay.

Summary and future outlook
The effect of electric field, electromagnetic dispersion, dielectric relaxation and correlated structural variation of the doped HAp samples was discussed. The role of hydroxyl ions, frequency exponent and the mechanism of ac conductivity by translation and hopping motion in the doped HAp samples were elaborated. Further, the electrical studies showed that the polarized surface charge either positive or negative, could alter the growth of bone cells on HAp. The surface charge, microstructure, densification, porosity, phase, electrical dipole reversal and its strong orientation were responsible for the enhancement in the dielectric constant of the doped HAp samples. The optical response depends on the dopant size and its concentration and associated phase transformation. The formation of different energy levels and electronic level transitions on doping enabled the tuning of the PL intensity and multi-color emissions which are independent of the excitation wavelength. The drug-loaded doped HAp samples and their PL intensity were correlated to the rate of drug release. Thus, the europium ion-doped HAp behaves as a one-dimensional nanoprobe for tracking the drug release. Further, different synthesis routes, doping, and co-doping are capable of generating new energy levels between conduction and valence band. Moreover, it was understood that the respective radiative and non-radiative emission were due to the activated luminescent centers and the concentration quenching of the doped samples.
Electro-opto biomaterials could be developed for the rapid healing of bone defects and fracture owing to the enhanced bone cell proliferation and growth. Current, in vitro and in vivo analysis routes must be improved further to control infection and to enhance the cell growth after implantation. To have an exact biomimicking of the bone, multielemental incorporation of the HAp is essential. Still, there is a lack of advanced analysis technique to have an insight of the surface charge interaction with the bone cells and how these charges assist the precipitation, mineralization, and growth of bone tissues. Solar/photon and magneto/lightbased advanced biomaterials need to be developed for rapid healing and recovery.