Elektrohemijska detekcija i određivanje jona As3+ u uzorcima vode pomoću elektroda na bazi legura i nanokompozita zlata

Abstract

Prisustvo teških metala u vodi predstavlja aktuelan problem već dugi niz godina, kako u svetu, tako i kod nas. Za razliku od organskih zagaĊivaĉa koji su biorazgradivi, teški metali nisu biorazgradivi, pa mogu biti prisutni u lancu ishrane više godina. Pri tome su male koliĉine esencijalnih metala, kao što su gvoţĊe, bakar i cink, neophodne za zdrav ţivot; meĊutim, postaju kobni po ţive organizme kada preĊu dozvoljenu koncentracijsku vrednost. Nasuprot esencijalnim metalima, neesencijalni metali, kao što su arsen, olovo, kadmijum i ţiva, su toksiĉni ĉak i u tragovima. Iz tog razloga neophodno je poznavati dozvoljene i prisutne koliĉine obe pomenute vrste elemenata u vodi, ali i u namirnicama koje predstavljaju sastavni deo svakodnevnog ţivota. Arsen je jedan od najštetnijih elemenata za zdravlje ljudi zbog ĉega se akcenat u sve većoj meri stavlja na njegovu detekciju primenom relativno jednostavnih, a jeftinih, elektrohemijskih senzora. Elektrohemijska metoda, anodna ―stripping‖ voltametrija, pokazala se kao dobra metoda za odreĊivanje jona arsena pomoću elektroda na bazi zlata. Ove elektrode su pokazale visoku aktivnost za elektroanalitiĉko odreĊivanje jona arsena u vodi; no, visoka cena ovog plemenitog metala diktira dalju potragu za elektrodnim materijalima visoke aktivnosti, a niţe cene u odnosu na elektrode od zlata. Shodno tome, u okviru ove doktorske disertacije sintetisani su novi elektrodni materijali na bazi legura i nanokompozita zlata u kombinaciji sa drugim metalima, što u velikoj meri smanjuje cenu tih elektroda u odnosu na elektrode od ĉistog plemenitog metala. Elektrodni materijali su podeljeni u tri grupe: u prvoj grupi se nalaze legure zlata sa elementima retkih zemalja (samarijum, Au-Sm; disprozijum, Au-Dy; holmijum, Au-Ho i itrijum, Au-Y), u drugoj elektrode prevuĉene kobaltom i zlatom galvanostatskim taloţenjem, CoAu elektrode, a u trećoj grupi su elektrokatalizatori dobijeni nanošenjem nanoĉestica zlata i kobalta na redukovani grafen oksid (Co/rGo, Au/rGO i CoAu/rGO). Nakon sinteze materijala, izvršena je njihova fiziĉko-hemijska analiza u cilju dobijanja detaljnih informacija o koliĉini prisutnog metala u datom uzorku, sastavu i morfologiji uzorka. Posle karakterizacije, ispitan je mehanizam elektrooksidacije arsena na datim elektrodnim materijalima i optimizovani su merni parametri anodne „stripping― voltametrije (vreme i potencijal depozicije i brzina polarizacije) kako bi se dobile što niţe vrednosti granice detekcije. Naime, maksimalna dozvoljena koncentracija jona arsena u vodi prema Svetskoj zdravstvenoj organizaciji iznosi samo 10 ppb te elektrode moraju biti dovoljno osetljve da detektuju i tako male koliĉine jona arsena. Au-Ho legura je pokazala najniţu vrednost granice detekcije od 0,8±0,01 ppb u opsegu linearnosti 2 - 12 ppb. Ipak, Au-Y legura je uzeta kao bolja jer je pokazala nisku granicu detekcije od 1,6±0,2 ppb sa širim opsegom linearnosti (2 - 20 ppb). CoAu elektroda je pokaza granicu detekcije od 2,2±0,5 ppb u opsegu linearnosti 4 - 20 ppb. Najviša granica detekcija je dobijena sa CoAu/rGO elektrodom (4,5±0,1 ppb). CoAu/rGO je u spomenutoj trećoj grupi elektroda dala najbolji elektroanalitiĉki odgovor u realnim uzorcima voda (reĉnim uzorcima i uzorcima iz gradskog vodovoda). Elektrode drugih grupa su takoĊe pokazale zapaţenu aktivnost u realnim uzorcima; npr. Au-Y elektroda je dala jasan elektrohemijski odgovor u prisustvu jona arsena kako u reĉnim uzorcima (iz reke Drine u Loznici, i reke Begej u Zrenjaninu), tako i u uzorcima vode iz gradskog vodovoda. Svi ovi rezultati pokazuju da se pomenute elektrode mogu koristiti u elektrohemijskim senzorima za detekciju jona arsena.

The presence of heavy metals in water and food is a persistant problem for many years, both in the world and in our country. Unlike organic pollutants that are biodegradable, heavy metals are not biodegradable, so they can be present in the food chain for many years. Small amounts of essential metals, such as iron, copper and zinc, are necessary for a healthy life; however, they become fatal to living organisms when they exceed the permissible concentration value. Contrary to essential metals, non-essential metals such as arsenic, lead, cadmium and mercury are toxic even in trace amounts. For this reason, it is necessary to know the permitted and present amounts of both mentioned types of elements in water, but also in food as an integral part of everyday life. Arsenic is one of the most harmful elements for human health, which is why the increasining attention is focused on its detection using relatively simple and cheap electrochemical sensors. The electrochemical method, anodic stripping voltammetry, proved to be a good method for the detection and determination of arsenic ions using gold electrodes. These electrodes showed high activity for the electroanalytical determination of arsenic ions in water; however, the high price of this precious metal dictates the further search for electrode materials with high activity, and lower prices compared to the gold electrodes. Accordingly, within the framework of this doctoral dissertation, new electrode materials based on alloys and nanocomposites of gold with other metals were synthesized, thus greatly reducing the price of those electrodes compared to electrodes made of pure precious metal. Studied electrode materials are divided into three groups: the first group comprises alloys of gold with rare earth elements (samarium, Au-Sm; dysprosium, Au-Dy; holmium, Au-Ho and yttrium, Au-Y), the second electrodes coated with cobalt and gold by galvanostatic deposition, CoAu electrodes, and the third electrocatalysts obtained by grafting gold and cobalt nanoparticles on reduced graphene oxide (Co/rGo, Au/rGO and CoAu/rGO). After the synthesis of the mentioned materials, their physico-chemical analysis was carried out to obtain detailed information on the amount of metal present in the given sample, composition and morphology of the sample. Subsequently, the mechanisam of arsenic electrooxidation on the studied electrode materials was investigated and operating parameters of anodic stripping voltammetry (deposition time and potential, and polarization rate) were optimized in order to obtain the lowest possible detection limit values. Namely, the maximum allowed value of arsenic in water according to the World Health Organization is as low as 10 ppb. Au-Ho alloy showed the lowest value of the detection limit of 0,8±0,01 ppb in the linear range of 2 - 12 ppb. Still, Au-Y alloy was considered as better one due to low limit of detection of 1,6±0,2 ppb in a wider linear range (2-20 ppb). The CoAu electrode showed a detection limit of 2,2±0,5 ppb in the linear range of 4-20 ppb. The highest limit of detection was obtained with the CoAu/rGO electrode (4,5±0,1 ppb). Within the mentioned third group of materials, CoAu/rGO gave the best electroanalytical response in real water samples (river samples and city water supply samples). Electrodes of other groups also showed noticeable activity in real samples; for example, Au-Y electrode gave a clear and well-defined electrochemcial response in the presence of arsenic ions both in river samples (from the Drina river in Loznica and the Begej river in Zrenjanin), as well as in water samples from the city water supply. All these results show that the mentioned electrodes can be used in electrochemical sensors for the detection of arsenic ions.