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Enhancement of photoluminescence emission of Gd3+ activated borate phosphors for phototherapy lamps: A review

In this review paper, we discuss about photoluminescence emission of Gd3+ in various inorganic phosphors. Narrow band UVB (NB-UVB) emission of Gd 3+ in 312-315 nm is well sought after in medical field to treat photo-responsive diseases like atopic dermatitis, vitiligo, psoriasis, etc. However, this 4f-4f, 6Pj → 8S7/2 intra-configurational transition of Gd3+ is weak and hence needs to be sensitized. Suitable sensitizers like Bi3+, Pr3+ are found to enhance the PL emission of Gd3+. Such phosphors with sensitized Gd3+ emission is highly suitable for phototherapy lamps.


Introduction
Sunlight therapy or heliotherapy, since ancient times, is used to treat various cutaneous diseases in humans. The sacred Indian book, 'Atharva Veda' has alluded to the treatment of leucoderma by sunlight, along with ingestion of seeds of Bavachee plants [1]. Dried mature seeds of this plant, 'Psoralea corylifolia' contain furocoumarinsphotosensitizing psoralen compounds found beneficial for curing skin disorders [2]. The paramount benefit of sunlight is its ability to enhance the body's vitamin D supply.
Modern Phototherapy started in 19th century, with the use of artificial light source for phototherapy by Niels Rydberg Finsen. He (1896) developed a chemical ray's lamp and could successfully use it for the treatment of lupus vulgaris. At the Phototherapy Institute in Copenhagen, he cured over 800 patients from lupus vulgaris using carbon-arc. Finsen also called as 'the father of ultraviolet therapy' received Nobel Prize in medicine -the only Nobel Prize in dermatology. The award was conferred, "in recognition of treatment of lupus vulgaris with concentrated light rays" [1]. Filtered sunlight has also been reported by Slusher et. al. as safe and effective phototherapy treatment of hyperbilirubinaemia [3].
Phototherapy of hyperbilirubinaemia was first reported in 'Lancet', one of the oldest, medical journals in 1958 by Rochford Hospital, Essex, England. Sister Jean Ward, working as a nurse at Rochford General Hospital at Essex in England, paved the way to its discovery. Consultant paediatrician R.H. Dobbs and resident registrar Richard Cremer, described that Sister Jean,'on warm summer days would wheel the more delicate infants out into the porch courtyard, sincerely convinced that the combination of fresh air and warm sunshine would do them much more good than the stuffy overheated atmosphere of an incubator' [4]. Rochford Hospital continued with these experimental treatments and later, Cremer together with P.W. Perryman, a biochemist, revealed that exposure to sunlight or artificial blue light from 40W fluorescent tubes would reduce the bilirubin levels present during jaundice in newborn infants [5]. Jaundiced newborn infants in Brazil, Uruguay, Chile, Italy, France and England were treated by sunlight after Cremer's demonstration. However, the word 'Phototherapy' was first used by Ferreira et al. in September 1960, on experimental treatment of 77 jaundiced infants [4,6].

Phototherapy
Phototherapy is the use of light to cure skin diseases, upon exposure to natural sunlight or man-made lamps [7]. Radiation from sun is a mixture of electromagnetic radiations differentiated into several regions according to wavelength.
Ultraviolet (UV) wavelength band of the electromagnetic spectrum with wavelength in the range of 200-400 nm is further on the basis of its effect on the human derma:. UVA (320-400 nm) or long wave UV, UVB (280-320 nm) termed as the erythemal region, and UVC (200-280 nm) as the germicidal region. UVA region is classified as UVA1 with wavelength in the region 340-400 nm and UVA2 with 320-340 nm wavelengths. These UV radiations (UVR) are absorbed by the biological molecules called chromophores. They are biological molecules like DNA, RNA, protein or drugs [8].
UVA1 radiation has gained widespread recognition in the field of dermatology and rheumatology [9], to treat diseases like extragenital lichen sclerosus, Scleroderma, T-cell lymphoma, Lupus erythematosus etc [10]. Psoralen photochemotherapy combines the use of Psoralen -a photosensitizer and long wave UVA (near 361 nm) radiation for the treatment of multiple skin disorders [11].
Broadband UVB explained by Goeckerman in 1925 was used for many cases of psoriasis, until replaced by super-efficient Narrowband UVB which could greatly clear psoriatic lesions and reduce inflammation in the skin. This cost effective, non-immuno-suppressive treatment is ascertained as a phototherapeutic modality for the management of psoriasis. [12]. With proper selection of doses, UVC promotes wound healing. It's lethal and mutagenic consequences causes harm to the microorganisms and at the same time preserves viability of mammalian cells [13] Chief medical use of phototherapy also is in the treatment of hyperbilirubinemia or jaundice. Absorbed light breaks down the accumulated bilirubin into its water soluble and excretable products. Biliblankets and fibreoptic or Compact fluorescent light tubes and blue LEDs, do not produce heat and are termed medically safe for the treatment of Neonatal Jaundice. Phototherapy is being recently used for diabetic retinopathy and treatment of hair loss.
Phototherapy used in diagnosis and therapy prognosis assessment of cancer is known as Photodynamic therapy and imaging. It is also used to control or prevent photo dermatoses like polymorphus light eruption or solar utricaria. [14]. Excimer laser and Targeted phototherapy are efficient treatment modalities with lesser side-effects as they directly focus the light on targeted lesions in complex areas of body like ear, nose etc [7].
Phototherapy and Photochemotherapy are administered depending on the action spectrum of the disease. For ex: NB-UVB (311 nm) is used mostly to treat atopic dermatitis, vitiligo, psoriasis, chronic actinic dermatitis etc. Bath-PUVA is used for treating contact dermatitis, scleroderma, graft versus host disease, lichen sclerosus, etc. UVA1 is used to cure Necrobiosis lipoidica, Scleroderma, Chronic actinic dermatitis etc. Blue light spectrum is used for the treatment of hyperbilirubinemia [8].

Work done in phototherapy
Absorption of ultraviolet radiations by chromophores present in the skin causes production of DNA photoproducts. As a result, photochemical reactions occur that subsequently bring about biological changes essential for the repair of tissue structures. [15]. UV light from phototherapy lamps is focussed on the diseased body part. Phosphor materials activated with Eu 2+ , Pb 2+ , Gd 3+ and Pr 3+ are considered efficient for utilization in phototherapy lamps. Many commercial phototherapy phosphors use Gd 3+ and Eu 2+ as activators [16].
Trivalent Gd 3+ has 8 S7/2 as its ground level and 6Pj, 6Ij, 6Dj, 6Gj as the excited levels. Excitation of the Gd 3+ ions from the ground to higher energy level results in 6 P7/2 → 8 S7/2 transition in NB-UVB region near 312-315 nm. Hence, Gd 3+ is one of the suitable activators much sought after by researchers in their quest for advanced phosphors applicable in new technologies. This emission is highly suitable in medical field for the treatment of photo-responsive diseases. So, variety of host lattices are studied for Gd activator to get good Gd activated phosphors that show remarkable NB-UVB emission. Such phosphors are in great demand for phototherapy lamps to treat skin diseases like atopic dermatitis, vitiligo, psoriasis, seborrehic dermatitis, etc [17].
Borate atom has planar sp 2 alongwith three dimensional sp 3 hybridized orbitals and hence can easily hold three to four oxygen atoms to shape various anionic groups. As, inorganic borates can be easily synthesized, are structurally feasible for doping on multiple cationic sites available in the host lattice, they are extensively used in research and for commercial purpose. Diversity of structure types, supreme optical quality, and low temperature of synthesis, transparent for a large range of wavelength, good chemical stability and wide band gap make borates a desired choice in phosphors for phototherapy, optical and laser devices, plasma display panels, LED etc. LaB3O6:Gd 3+ , Pr 3+ and CeMgB5O10:Gd3+ are the well-recognized NB-UVB phosphors used for commercial purpose, mostly in phototherapy lamps [7,18]. Here, we discuss some of the Gd activated phosphors with their potential in phototherapy. Efficacy and application of a phosphor depends on its synthesis and also on the choice of host material. Oxides are easy to synthesize, possess good thermal and chemical stability and hence are the preferred choice as the host for a phosphor. Rare-earth activated barium aluminates show high quantum efficiency, long persistent behaviour and good stability and finds enormous applications in luminescent devices like fluorescent screens, display panels etc.

Gd
Singh and co-workers [17] prepared Ba0.97Al2O4:Gd0.03 3+ phosphor by combustion synthesis technique to study the emission of Gd 3+ . Though the synthesis temperature required for BaAl2O4 is between 900-1300 o C, XRD confirmed the hexagonal phase of BaAl2O4, obtained by combustion synthesis at 500 o C furnace temperature. Using Scherrer formula the crystallite size was calculated to be 44.21 nm. An insight was gained into the electronic structure of gadolinium ions through the EPR study. g factor presents a picture of the symmetry that exists among the dopant and the corresponding host lattice and is calculated using the EPR equation hν = gβB. Here, h represents the Planck's constant, ν is the value of microwave frequency, β stands for the Bohr magneton, and the magnetic field is given by B. The g factor corresponds to symmetry between the dopant and the host lattice. Gd being a paramagnetic substance gets magnetized under the influence of external magnetic field. The spin Hamiltonian given by the equation H = g0βB S + D[S 2 z − 1/3S(S + 1)] + E(S 2 X − S 2 Y) very well explains the energy levels occupied by such Gd 3+ ions. g0βB S is the Zeeman term, ligand-field effects are given by second-order terms with D being the axial constant for energylevel splitting and E is the corresponding rhombic constant for a zero magnetic field. For BaAl2O4:Gd 3+ the resonance signals of EPR spectrum have the effective g values as 2.14, 4.56 and 6.75. Due to spin-orbit interactions of rare earth ions and the phosphor-lattice environment, the g factors get deviated from 2.0023 -the free electron value. Paramagnetic ions present at the sites with weak ligand fields, create resonance signal with g factor of 2.14. When the ions are located at the sites of strong ligand field, resonance signals with g value greater than 2.0 is obtained. 4.56 as a g factor is obtained for a resonance signal of Gd. This indicates that a strong ligand field is present, having a coordination number less than 6 with orthorhombic symmetry.
PL excitation spectrum monitored at 314 nm emission wavelengths, depicts the characteristic Gd 3+ peaks at 244, 246, 252, 275 and 279 nm owing to intra-configurational transitions of Gd 3+ ions. Transitions from 8 S7/2 which is the ground state of the Gd 3+ ions to the excited states 6Ij and 6Dj are detected. PL emission spectrum is recorded for the prominent 273 nm ( 8 S7/2→6Ij) excitation wavelength. Emission spectrum depicts an intense narrow peak at 314 nm. This peak is attributable to Gd 3+ ions' electric dipole 6 P7/2 → 8 S7/2 transition. The present NB-UVB emission finds great importance in the medical field for the treatment of cutaneous diseases by means of phototherapy lamps. [17].

LaPO4: Gd 3+
Chauhan et. al [19] successfully prepared Gd 3+ activated inorganic lanthanum phosphate (LaPO4) by recrystallization method. PL excitation spectra of LaPO4:Gd 3+ phosphor was recorded at room temperature by monitoring the 312 nm emission wavelength. The spectra depicts strong absorption peak at 275 nm. Upon excitation at 275 nm wavelength, the phosphor emits magnificently at 312 on account of 6 Pj → 8 S7/2 Gd 3+ ions transition. Maximum concentration of Gd 3+ ions that produced intense emission in LaPO4:Gd 3+ phosphor was found to be 0.04 mol%. Critical distance R0 for LaPO4:Gd 3+ phosphor was estimated to be 15.37 A 0 and the Stokes shifts was 4312cm -1 . This narrow band UVB emitting, LaPO4: Gd 3+ luminescent material has good potential as a phototherapy lamp phosphor [19].

SrY2O4:Gd 3+
Recently, Singh et. al [20] synthesized Gadolinium ion-activated SrY2O4 phosphors using sol-gel method. The photoluminescence spectra of the phosphor for different concentrations of Gd 3+ ions were recorded in order to get insight into the luminescence behaviour of SrY2O4 phosphor. Excitation of SrY2O4:Gd 3+ at 275 nm portrayed two bands in the NB-UVB spectral regions: one at 309 nm originated due to 6 P5/2 → 8 S7/2 electric dipole transition and another at 315 nm by 6 P7/2 to 8 S7/2 transition of Gd 3+ ions. The strong emission of the phosphor at 315 nm could be exploited for a NB-UV light. EPR spectra disclosed that Gd 3+ ions show variation in crystal field strength. The results confirm the utility of SrY2O4:Gd 3+ phosphor for phototherapy lamp applications which renders UVB light [20].
[21] was synthesized using sol-gel process with Gd concentration varying between 0.01 to 0.11 mol%. On excitation at 273 nm, the developed Na3Y1−xSi2O7:Gdx sample exhibited an ultraviolet emission when Gd ions underwent transition from 6 P7/2 to 8 S7/2 energy state resulting in a sharp and dominant peak at 313 nm [21].

CaMgP2O7: Gd 3+
Low sintering temperatures, excellent thermal as well as charge stabilities have gained importance for phosphate materials as luminescent materials. Kunghatkar et. al [22]have successfully synthesized CaMgP2O7 phosphor with gadolinium as activator by different synthesis techniques like solution combustion, sol-gel, solid-state reaction and wet chemical. XRD, SEM, EPR, and PL were made use of to study the phosphor briefly. XRD confirms that Gd 3+ doping does not lead to any significant variation in the crystal structure. CaMgP2O7 forms a monoclinic crystal structure with P21/n as the space group. In CaMgP2O7, Ca 2+ ion shows coordination with eight oxygen ions and Mg 2+ with six oxygen ions supplied by P2O7 4-. Gd 3+ ions may possibly substitute Mg 2+ , P 5+ or Ca 2+ ion site in CaMgP2O7 host lattice. Solid state method created irregular rod like particles with an average size of 5μm, sol-gel formed spherically shaped particles of 2μm size. Agglomerated particles of 2 μm were formed on adoption of solution combustion as well as wet chemical synthesis. CaMgP2O7:Gd³⁺ presented effective g value near to 5.88, 2.88, and 1.91 due to location of Gd 3+ ions at sites with cubic symmetry. Photoluminescence emission at 314 nm wavelength when excited at 274 nm excitation proved proper incorporation of Gd 3+ ions in the present host lattice. Different characterization methods revealed that good results are achieved with sol-gel and solid state and are better for the preparation of phosphate group than the remaining two methods. The phosphor prepared by all the four methods exhibited intense excitation peak at 274 nm on account of 8 S7/2→ 6 Ij transition of Gd 3+ ions and weak peaks at 247 nm and 253 nm due to 8 S7/2→ 6 Dj transitions, when monitored by its emission at 314 nm. 6 P7/2→ 8 S7/2 , the 4f-4f electric dipole transition is seen in the form of emission peak at 314 nm. Emission intensity was witnessed to increase along with dopant concentration till 7 mol% without concentration quenching. Highest intensity was witnessed for the phosphor obtained by sol gel method with lowest intensity for the one obtained by sol-gel method. The emission in narrow band ultra violet region is applicable in phototherapy treatment and can be made intense by the use of sensitizing material for Gd 3+ [22].
The phosphor with Gd 3+ as the activator show weak intensity of the NB-UVB emission near 313 nm as the 4f7 transitions of Gd 3+ has a very low absorption strength. This emission can be enhanced by adding a co-activator to the compound. This co-activator functions like a sensitizer; absorbs the energy that is incident on it and transfers to Gd ions, which subsequently emit with enhanced intensity. A suitable sensitiser, sensitizes gadolinium emission and gives a luminescent material with high efficiency [23]. Such luminescent materials with enhanced emission are discussed below.
To further increase the PL intensity of 314 nm emission peaks in BaB8O13:Gd 3+ phosphor, Pr 3+ is used as a sensitizer. Sensitizer traps the excitation energy and gives it to the activator that produces characteristic emission with enhanced intensity. BaB8O13: Pr 3+ , Gd 3+ was prepared with constant Gd 3+ concentration at 2 mol % and varying the Pr 3+ concentration from 0.05 to 05 mol