Dobijanje nanofosfora na bazi fluorapatita dopirani Pr3+ jonima za bio-medicinske primene

Abstract

Luminescentni nanokristali (nanofosfori) na bazi fluorapatita (FAP-a) dopirani elementima retkih zemalja idealni su kontrastni agenti za bio-medicinske primene, kao što su detekcije, snimanja, praćenja i terapije ćelija kancera. Kancer je jedna od najčešćih bolesti modernog doba čiji uspeh lečenja zavisi od rane dijagnostike i neinvazivnog tretmana. Luminescentne nanočestice mogu uneti inovativnu paradigmu u lečenje kancera kombinovanjem biosnimanja, dijagnostike i tretmana. Za studije fluorescentnih biosnimanja nanokristali fluorapatita dopirani retkim zemljama kao kontrastni agenti pružaju značajne prednosti u vidu velikih kontrasta i dugotrajnosti luminescencije, i što je još važnije visoke biokompatibilnosti, netoksičnosti i bioaktivnosti. Glavni ciljevi ove doktorske disertacije su sinteza novih luminescentnih multifotonskih bionanomaterijala na bazi fluorapatita dopiranih jonima prazeodimijuma (Pr3+), njihova karakterizacija i evaluacija primene za fluorescentna biosnimanja kancera. Sintezom nanoprahova u umerenim uslovima metodom ko-precipitacije, a potom sušenjem na 110 oC i kalcinacijom na temperaturama od 700 i 1000 oC očekuje se pronalaženje najboljih uslova za dobijanje novih nanofosfora koji bi našli i različite bio-medicinske primene u oblasti fluorescentnih biosnimanja. Proučavane su tri vrste PrFAP nanokristala, sa 0,1%, 0,5% i 1% atomskih procenta Pr3+, zajedno sa nedopiranim FAP kontrolnim uzorkom. Nivoi energije aktivator jona Pr3+ sadrže metastabilna multipletna stanja koja nude mogućnosti efikasnih emisionih linija u više boja u FAP nanokristalima, kao i u infracrvenoj i ultravioletnoj oblasti spektra. Metodom ko-precipitacije na sobnoj temperaturi (25 oC), a potom sušenjem na 110 oC, sintetisani su monofazni heksagonalni nanokristali PrFAPs nepravilnog sfernog oblika. Termičkom analizom sintetisanih uzoraka, na osnovu detektovanih temperaturnih opsega procesa dekarbonacije i dehidroksilacije, utvrđene su temperature kalcinacije od 700 i 1000 oC. Termička analiza i karakterizacija uzoraka su pokazale da Pr3+ joni dovode do stabilizacije FAP strukture na višim temperaturama, što je pripisano unosu lantanoidnih jona sa specifičnim magnetnim osobinama u sistem i stvaranju jačih privlačnih sila sa O2- anjonima. Nanokristali sušeni na 100 oC i kalcinisani na 1000 oC, zbog prisustva defekata kristalne rešetke koji zadržavaju emisiju Pr3+ jona, nisu pokazali luminescentne karakteristike od značaja za primene u medicinskim fluorescentnim biosnimanjima. Kalcinacijom uzoraka na 700 oC izrađen je novi tip aktiviranih fluorapatitnih nanokristala dopiranih prazeodimijumom (PrFAPa) sa ekscitaciono-emisionim profilima u vidljivom delu spektra. Fizičko-hemijska karakterizacija potvrdila je sferne kristale heksagonalne strukture do nanometrske veličine od oko 20 nm. Kvantno-hemijske kalkulacije predvidele su da se joni Pr3+ ugrađuju u kristalnu rešetku FAP nanokristala na položaju Ca2 (6h), što je praćeno deformacijama pozicije F- jona. Pretpostavljeni mehanizam supstitucije je jedan jon Pr3+ za jedan Ca2+, s delimičnom supstitucijom anjona F– sa O2– i OH– i stvaranjem vakansi usled postizanja neutralnosti sistema. Rezultati in vitro biokompatibilnosti i hemokompatibilnosti pokazali su da nanokristali PrFAPa nisu toksični za žive ćelije. Pored toga, internalizacija PrFAPa nanokristala od strane ćelija kancera kože (A431) i pluća (A549) je proučavana korišćenjem konfokalne mikroskopije i mikroskopije širokog polja zasnovane na fluorescenciji. Nanokristali pokazuju karakterističnu zelenu emisiju na 545 nm (3P0→3H5 tranzicija Pr3+ jona) i narandžastu emisiju na 600 nm (1D2→3H4), koje su korišćene za razlikovanje od pozadinske autofluorescencije ćelije. Studije dobijenih slika konfokalnom mikroskopijom u plavom, zelenom i crvenom kanalu su otkrile da nanokristali mogu da prepoznaju ćelijsku površinu i da se lepe za nju, ali nisu potvrdile ulazak nanokristala u ćelije. Mikroskopija širokog polja je detektovala emisione prelaze u zelenoj i narandžastoj boji i potvrdila da luminescentni signal dolazi iz unutrašnjosti ćelija. Korišćenjem rezonantne ekscitacije od 488 nm i emisije od 600 nm PrFAPa nanokristala, konfokalnom mikroskopijom ekstrahovan je signal fluorescencije iz unutrašnjosti ćelija kancera. Ortogonalne projekcije u 3D konfokalnim ravnima pokazuju da su nanokristali u stanju da uđu u ćelije kancera i da se raspoređuju po citoplazmi. Sveukupno, ovako dobijeni nanokristali PrFAPa su biokompatibilni i od testiranih uzoraka, aktivirani nanokristali dopirani sa 0,5% Pr3+ pokazuju najviše potencijala za primenu u medicinskim fluorescentnim biosnimanjima kao kontrastni agenti.

Luminescent nanocrystals (nanophosphorus) based on fluorapatite (FAP) doped with rare earth elements are ideal contrast agents for biomedical applications such as cancer cell detection, imaging, tracking and therapy. Cancer is one of the most common diseases of the modern times whose success of the cure depends on early diagnosis and non-invasive treatment. Luminescent nanoparticles can bring an innovative paradigm into the treatment of cancer by combining bioimaging, diagnostics and treatment. Rare earth doped fluorapatite nanocrystals as contrast agents for studies of fluorescence bioimaging, offer significant advantages in terms of high contrasts and long-term luminescence, and more importantly high biocompatibility, non-toxicity and bioactivity. The main objectives of this doctoral dissertation are the synthesis of novel luminescent multiphoton bionanomaterials based on fluorapatites doped with praseodymium ions (Pr3+), their characterization and evaluation of their application for cancer fluorescence bioimaging. Synthesis of nanopowders under moderate conditions by the co-precipitation method, followed by dried at 110 °C and calcination at 700 and 1000 °C, is expected to find the best conditions for obtaining new nanophosphors that would find different bio- medical applications in the field of fluorescence bioimaging. Three types of PrFAP nanocrystals were studied, with 0,1%, 0,5%, and 1% atomic percentages of Pr3+, together with an undoped FAP control sample. Energy levels of the Pr3+ ion activator contain metastable multiplet states that offer the possibility of efficient multi-color emission lines in FAP nanocrystals as well as in the infrared and ultraviolet regions of the spectrum. Single-phase hexagonal nanocrystals PrFAPs of irregular spherical shape were synthesized by the method of co-precipitation at room temperature (25 oC) and then drying at 110 oC. Thermal analysis of the synthesized samples, based on the detected temperature ranges of the decarbonation and dehydroxylation processes, determined calcination temperatures of 700 and 1000 oC. Thermal analysis with characterization showed that Pr3+ ions lead to stabilization of the FAP structure at higher temperatures, which was attributed to the entry of lanthanoid ions with specific magnetic properties into the system and the creation of stronger attractive forces with O2- anions. Nanocrystals dried at 100 oC and calcined at 1000 oC, due to the presence of crystal lattice defects that quench the emission of Pr3+ ions, did not show luminescent characteristics of significance for applications in medical fluorescence imaging. Calcination of the samples at 700 oC produced a new type of activated praseodymium doped fluorapatite nanocrystals (PrFAPa) with excitation-emission profiles in the visible part of the spectrum. Physicochemical characterization confirmed spherical crystals of hexagonal structure up to a nanometer size of about 20 nm. Quantum-chemical calculations predicted that Pr3+ ions would be embedded in the crystal lattice of FAP nanocrystals at the Ca2 position (6h), which was followed by deformations of the F- ion position. The assumed substitution mechanism is one Pr3+ ion for one Ca2+, with partial substitution of F– anions with O2– and OH– and creation of vacancies due to achieving system neutrality. The results of in vitro biocompatibility and hemocompatibility showed that PrFAP nanocrystals were not toxic to living cells. In addition, the internalization of PrFAPa nanocrystals by skin (A431) and lung (A549) cancer cells was studied using fluorescence-based confocal microscopy and wide-field microscopy. The nanocrystals show characteristic green emission at 545 nm (3P0→3H5 transition of Pr3+ ion) and orange emission at 600 nm (1D2→3H4), which we use to discriminate from cell autofluorescence. Studies of the images obtained by confocal microscopy in the blue, green, and red channels revealed that nanocrystals could recognize the cell surface and adhere to it, but they did not confirm the entry of nanocrystals into the cells. The wide-field microscopy detected emission transitions in green and orange color, and confirmed that the luminescent signal was coming from inside the cells. Using resonant excitation of PrFAP nanocrystals at 488 nm and emission of 600 nm, confocal microscopy extracted the fluorescence signal from inside the cancer cells. Orthogonal projections across 3D confocal stacks show that the nanocrystals are able to enter the cells positioning themselves within the cytoplasm. Overall, the obtained PrFAPa nanocrystals are biocompatible and of the tested types, the 0,5% Pr3+ doped nanocrystals show the highest promise as a tracking nanoparticle probe for bioimaging applications.