LDI-TOF-MS Analysis of ZnO Nanocrystals

The LDI-TOF-MS analysis was performed using ultrafleXtreme mass spectrometer (Bruker Daltonics, Hamburg, Germany) and ZnO NCs suspended in water were spotted on the ground steel target (Bruker Daltonik, Bremen, Germany) according to the previously described protocol [10 (link)] with the required modification (without the α-Cyano-4-hydroxycinnamic acid (HCCA) matrix). Protein Calibration Standards I (Bruker Daltoniks, Bremen, Germany), Peptide Calibration Standard and two signals characteristic for the matrix [M-H]⁺ and [2M-H]⁺ were used for the external calibration, according to the standardized Bruker sample preparation procedure. Molecular fingerprint (MF) spectra of ZnO NCs were recorded in reflectron positive mode, within a m/z range of 100–3500, and we applied an acceleration voltage of 25 kV. Fragment spectra were recorded using the LIFT default method (Bruker Daltonics, Hamburg, Germany) at 100% of laser power, global attenuator 50% with calibration on the immonium ions [29 (link),30 (link)]. The voltage on the LIFT electrodes was 19.0 and 2.7 kV, respectively. MS spectra were registered in FlexControl (Bruker Daltonics, Hamburg, Germany), while the FlexAnalysis (Bruker Daltonics, Hamburg, Germany) was used for data analysis.