Ispitivanje primene neorganskih supstrata na bazi nanokristala TiO2 za detekciju i kvantifikaciju malih molekula SALDI TOF masenom spektrometrijom

Abstract

Izvod: MALDI TOF (Matrix Assisted Laser Desorption and Ionization Time-Of-Flight) masena spektrometrija je meka jonizaciona tehnika koja se prvenstveno koristi za analizu biomolekula (proteina i nukleinskih kiselina) i omogućava detekciju molekula uz veoma nizak stepen fragmentacije. Kako bi se sprečila/ublažila fragmentacija izazvana direktnom ekscitacijom laserom, uzorak se meša sa matricama koje imaju visok apsorpcioni koeficijent u oblasti u kojoj laser emituje. Primarno su se koristile organske, „tradicionalne“ matrice. Pored brojnih prednosti, organske matrice pokazuju i značajne mane koje onemogućavaju primenu MALDI masene spektrometrije za kvantitativnu analizu molekula i sprečavaju detekciju molekula malih masa manjih od 1000 Da. Kako se u ovoj grupi molekula nalaze i različiti biološki značajni molekuli (metaboliti) postoji potreba za prevazilaženjem nedostataka organskih matrica. Jedan od najranije primenjenih načina je zamena organskih matrica neorganskim jedinjenjima i/ili nanočesticama (supstratima). Koriste se različiti supstrati na bazi ugljeničnih polimera i sol-gel polimerne strukture na bazi silicijum dioksida. Upotreba nanostrukturnih supstrata u metodi koja je nazvana SALDI (Surface Assisted Laser Desorption and Ionization) u poslednjoj deceniji je predmet velikog broja publikovanih radova. Međutim, nanočestice od metala i oksida metala, kao što su Au, Pt, Ag, ZnO, Fe i MnO2/MnO3, nude prednost u odnosu na ostale materijale - stabilnije su u vazduhu i poseduju visoku provodljivost. Prednost nanočestica je apsorpcija energije lasera i efikasan prenos energije do uzorka. Istakle su se nanočestice od titan(IV)-oksida zbog svoje dostupnosti, netoksičnosti i niske cene, što je takođe predmet mnogih publikovanih radova. Osim navedenih osobina, titan(IV)-oksid je poluprovodnik sa dobrom UV apsorpcijom. Generalno, TiO2 snažno apsorbuje UV svetlost (ima znatnu širinu energetske barijere, 3,2 eV) azotnog lasera koji se koristi u MALDI TOF masenoj spektrometriji, ali proces pripreme nanočestica TiO2 verovatno ima snažan uticaj na apsorpciju jer ona zavisi od veličine, oblika i sastava čestica. Površina, a posredno i veličina i oblik kristala utiču na analitičke performanse i jonizacionu efikasnost u SALDI masenoj spektrometriji. U ovoj doktorskoj disertaciji korišćeni su nanokristali TiO2 različite veličine i oblika: koloidne nanočestice TiO2 (TiO2 NČ) prosečnog dijametra 5 nm, elipsoidne nanočestice TiO2 (TiO2 ENČ) dužine 40-50 nm, lateralne dimenzije 14-16 nm i nanotube TiO2 (TiO2 NT) dužine od 100 do 150 nm i prosečnog dijametra od 11 nm. Molekuli na kojima je bila testirana potencijalna primena nanokristala su biološki aktivni molekuli malih masa: amino-kiseline (L-cistein, L-alanin, DL-metionin), tripeptid glutation, polni steroidni hormoni (estradiol, testosteron, progesteron), ugljeni hidrati (D-(+)-glukoza, D-(+)-maltoza, rafinoza, arabinoza, β-ciklodekstrin), limunska kiselina, deksametazon (deksazon), vitamini (vitamin A i vitamin E).

Summary: MALDI TOF (Matrix-Assisted Laser Desorption and Ionization Time-Of-Flight) mass spectrometry is soft ionization technique. Primarily, this technique was used for the analysis of biomolecules such as proteins and nucleic acids with low level of their fragmentation. In order to prevent/reduce fragmentation of analyzed molecules, induced with direct excitation with laser, samples are mixed with matrix molecules. Matrix has high absorption coefficient in the range of the laser emission. In spite of numerous advantages, there are serious drawbacks of the matrices, and because of that they cannot be used for quantitative MALDI mass spectrometric analysis and for the detection of small molecules (molecular mass less than 1000 Da). In this group of molecules (mass less than 1000 Da) there are various biologically active molecules (metabolites), so there is a great need to overcome disadvantages of the application of organic matrices. Considerable efforts have been made to overcome the above mention problems and several alternative approaches have been developed: an organic-matrix-free approach in which the substrates, usually nanoparticles act as a matrix. Many are in use: graphite, silica gel, carbon powder, activated carbon, graphene, porous silicon, and many more. The term SALDI (Surface-Assisted Laser Desorption and Ionization) was coined to designate the techniques that use nanostructured substrates. The use of various nanoparticles as substrates in SALDI MS has attracted a lot of attention in the last decade. Nanoparticles absorb the laser energy and then rapidly transfer to analyzed molecule. However, the substrates prepared from materials based on metal-oxides (Аu, Pt, Ag, ZnO, Fe and MnO2/MnO3) are more stable in the air and have a high conductivity. Titanium(IV)-oxide (TiO2) is considered to be a good candidate for SALDI substrate since it is readily available, chemically stable, nontoxic and inexpensive material. Titanium(IV)-oxide is a semiconductor with high absorptivity of UV light of nitrogen laser (have a large band gap 3.2 eV) which is used in MALDI TOF mass spectrometry, but the method of synthesis of TiO2 nanocrystal have a great impact on absorption because this phenomena depends on size, shape and composition of nanoparticles. In this doctoral thesis, the applicability of TiO2 nanocrystals of different size and shape was tested. Colloidal TiO2 nanoparticles (NPs, average diameter ~ 5 nm), TiO2 prolate nanospheroids (PNSs, length: 40–50 nm, the lateral dimension: 14–16 nm) and TiO2 nanotubes (NTs, length: 100-150 nm, average diameter 11 nm) were used as substrates for potential SALDI TOF MS quantitative analysis of low mass molecules. The analyzed molecules were biologically active molecules with small molecule mass (Mm less than 1000 Da): amino acids (L-cysteine, L-alanine, DL-methionine), tripeptide glutathione, sex steroid hormones (estradiol, testosterone, progesterone), carbohydrates (D-(+)-glucose, D-(+)-maltose, raffinose, arabinose, β-cyclodextrin), citric acid, dexamethasone (dexasone), vitamins (vitamin А and vitamin Е).