Koloidni hemijski nanosenzori zasnovani na luminiscenciji trovalentnih jona europijuma

Abstract

Hemijski senzori privlaĉe mnogo paţnje zbog raznovrsnih primena u svakodnevnom ţivotu. Jedna od podvrsta senzora su i optiĉki hemijski senzori zasnovani na korišćenju optiĉkih metoda kao što su apsorpcija, rasejavanje svetlosti, fluorescencija, itd. U okviru ove doktorske disertacije istraţivanje je zapoĉeto sintezom serije uzoraka na bazi lantan fosfata (LaPO4) koji su aktivirani jonima europijuma (Eu3+) u širokom opsegu koncentracija (aktiviranje je izvršeno u opsegu 0 ≤ x ≤ 1 za sistem La1-xEuxPO4, gde je x udeo Eu3+ jona). Materijali su sintetisani koloidnom sintezom uz upotrebu citratnih jona za dobijanje kompleksnih jedinjenja retkih zemalja (La3+ (Eu3+)-Cit3- ). Iz serije sintetisanih uzoraka odabrana su dva sistema za dalje eksperimente: La0,5Eu0,5PO4 i jedinjenje u kom su joni La3+ u potpunosti zamenjeni jonima Eu3+ – stehiometrijski EuPO4. Sintetisana su još dva uzorka u kojima su: (a) joni lantana (La3+) zamenjeni jonima disprozijuma (Dy3+) i (b) joni lantana (La3+) zamenjeni jonima erbijuma (Er3+), pri ĉemu je koncentracija europijuma bila x = 0,5 (Dy1-xEuxPO4 i Er1-xEuxPO4). Svi uzorci su monokliniĉne monazitne kristalne strukture sa P121/m1 prostornom grupom. Dobijene su ĉestice nanodimenzija, preteţno sfernog oblika ĉije su optiĉke osobine detaljno ispitane. Sve ispitivane uzorke karakteriše intenzivna emisija u crvenom delu elektromagnetnog spektra na oko 592 nm i 618 nm koja potiĉe od jona europijuma. Mogućnost sintetisanih uzoraka da se u koloidnoj formi ponašaju kao hemijski nanosenzori zasniva se na praćenju promena intenziteta luminescencije jona europijuma. Do smanjenja intenziteta luminescencije dolazi u prisustvu razliĉitih supstanci koje se ponašaju kao gasitelji intenziteta luminescencije – kvenĉeri (engl. Quencher). Lista analita za hemijski senzing je opseţna i obuhvata katjone, anjone, toksiĉne gasove, pesticide, isparenja, produkte aktivnosti enzima kao sto je H2O2, itd. U okviru ove disertacije ispitivana je osetljivost sintetisanih uzoraka na prisustvo metala (Cu2+, Hg2+, Cd2+, Pb2+, Zn2+) i pesticida (2,4- dihlorfenoksisirćetna kiselina (2,4-D) i 2-metil-4-hlorfenoksisirćetna kiselina (MCPA)). Navedeni kvenĉeri luminescencije, korišćeni u okviru disertacije, predstavljaju ĉeste zagaĊivaĉe ţivotne sredine koji mogu imati neposredan uticaj na ljudsko zdravlje. Upotreba fluorescencije kao metode za detekciju koja je u odnosu na postojeće metode u upotrebi, jednostavnija, brţa i manje zahtevna kako prilikom snimanja uzoraka, tako i prilikom pripreme uzorka, predstavlja veliku prednost. Osim fotoluminescentnih (emisioni i eksitacioni spektri, odreĊivanje vremena ţivota) i UV -VIS (engl. Ultra violet-visible) merenja za svaki sistem u prisustvu razliĉitih gasitelja emisije odreĊene su konstante kvenĉovanja na osnovu Stern-Volmer jednaĉine kao i limiti detekcije. Najveća osetljivost sintetisanih sistema kao i najniţi limit detekcije dobijen je kada su kao kvenĉer intenziteta emisije Eu3+ korišćeni joni Cu2+ . Intenzitet emisije jona Eu3+ koji opada dodatkom kvenĉera (eksperiment sa jonima Cu2+) ponovo je uspostavljen dodatkom vodenog rastvora EDTA (kompleksirajuće sredstvo). Znaĉaj ovog eksperimenta je u ĉinjenici da ukazuje na mogućnost ponovne upotrebe nanosenzora. Rezultati dobijeni u okviru ove disertacije otvaraju mogućnost za razvoj novih sistema koji bi se potencijalno koristili kao hemijski senzori u mnogobrojnim oblastima kao što su kontrola kvaliteta hrane, aerodinimika, biohemija ili zaštita ţivotne sredine.

Chemical sensors attract a lot of attention due to a variety of applications in everyday life. One of the sensor subtypes are optical chemical sensors which are based on the usage of optical methods such as absorption, light scattering, fluorescence, etc. The investigations throughout this doctoral dissertation started with the synthesis of a series of samples based on LaPO4 systems that have been activated with europium ions (Eu3+ ) in wide concentration range (activation is performed in the range 0 ≤ x ≤ 1 for the La1- xEuxPO4 system, where x is the fraction of Eu3+ ions). Materials were obtained by colloidal synthesis using citrate ions for the production of complex rare earth compounds (La3+ (Eu3+ )-Cit3- ). From the series of synthesized samples, two samples were selected for further experiments: La0.5Eu0.5PO4 and the one in which the La3+ ions are completely replaced by the Eu3+ ions – stoichiometric EuPO4. Two more samples were synthesized: (a) one where the lanthanum ions (La3+) were replaced by dysprosium ions (Dy3+), and (b) one where the lanthanum ions (La3+) was replaced by erbium ions (Er3+); the concentration of europium ions was x = 0.5 (Dy1-xEuxPO4 and Er1-xEuxPO4). All samples have pure monoclinic monazite crystal structure with P121/n1 space group. Ultrasmall round shaped nanoparticles were obtained and their optical properties were thoroughly examined. All the tested samples are characterized by the dominant red luminescence in the electromagnetic spectrum with two intense emission lines: at about 592 nm and 618 nm, originating from the europium ions. The ability of synthesized materials to act as chemical nanosensors in the colloidal form is based on monitoring the changes in the intensity of luminescence of the europium ions. A decrease in the intensity of luminescence occurs in the presence of various ranges of substances that act as quenchers. The list of analytes for chemical sensing is very extensive and it includes cations, anions, toxic gases, vapors, products of the enzymatic action such as H2O2, etc. Within this dissertation, the sensitivity of the synthesized systems to the presence of heavy metal ions (Cu2+, Hg2+, Cd2+, Pb2+ , Zn2+) and pesticides (2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4- chlorophenoxyacetic acid (MCPA)) was investigated. Quenchers of luminescence mentioned above represent common environmental pollutants that may have an indirect impact on human health. The usage of a method such as fluorescence which is compared to existing methods in use simple, less time consuming and demanding when preparing a sample, is a great advantage. In addition to photoluminescent (excitation, emission, lifetime) and UV-VIS measurements quenching constants based on the Stern-Volmer equation were determined for each system in the presence of various quenchers, as well as the limit of detections. The highest sensitivity of synthesized systems to presence of quenchers and the lowest detection limit was obtained for Cu2+ ions. The emission intensity of the Eu3+ ions which decreases with the addition of the quencher (an experiment with Cu2+ ions as quencher) was recovered by the addition of an aqueous EDTA solution (complexing agent). The experiment with the recovery of the initial intensity of the emission is important because it points to the possibility of the fluorescent probe reusabillity. The results obtained in this dissertation open up the possibility of developing new systems that could potentially be used as chemical sensors in many areas such as food quality control, aerodynamics, biochemistry or environmental protection.