A small chelate constant (lg β) would benefit the combination of F- and Ln3+ ions resulting in the NaLuF4 lattice [28]. According to coordination chemistry, the chelate constants increase for sodium citrate, SDS, DDBAC, and PEG according to priority [27], resulting in gradually increasing size of UCNPs. Another reason may be attributed to the diverse viscosity of interface of dual phase system after adding surfactant [29]. Figure 1 TEM image of (a) ILs-UCNPs, (b,c) Cit-UCNPs. Figure 2 SEM images of (a) SDS-UCNPs, (b) DDBAC-UCNPs, and (c) PEG-UCNPs. To evaluate the ligand stability in each sample, TGA was performed (Additional file 1: Figures S1c, S2c,

ABT-199 molecular weight S3c, S4c, S5c). TGA curves showed two weight loss stages in the range of 20°C to 900°C. The first weight loss stage in the temperature range of 20°C to 200°C was due to the loss of absorbed water. The second stage from 200°C to 900°C was attributed to the combustion of the organic groups in the samples. A common feature was that weight

of each sample decreased rapidly at 600°C to 700°C. Additionally, when temperature reached 600°C, the weight loss was still less than 10% of the total weight, indicating good stability of each ligand linking. Notably, Cit-Na had shown priority in chelate ability, whose weight loss was only 1.82% until temperature risen up to 900°C. Based on EDX spectrums (Additional file 1: Figures S1d, S2d, S3d, S4d, S5d), fluorine had occupied majority RG7204 ic50 weight of UCNPs, demonstrating that the lead role of capping agent was still ILs, and other surfactants worked as cooperative assistants to develop functional surface. The successful ligand links between surfactants and surface of UCNPs were further verified by FTIR spectroscopy. Figure 3 showed the FTIR spectra of the five UCNP samples. The transmission band peaks at approximately 2,930 this website and 2,854 cm-1 can be assigned to the asymmetric and symmetric stretching vibrations, respectively. However, these features were lost in the spectrum of the Cit-UCNPs sample, suggesting

the disappearance of the –CH2-CH2– groups. What is more, bands peaks at 1,641 and 1,520 cm-1 belonged to the C = O vibrations, indicating the presence of carboxylic groups in Cit-UCNPs. Band peak at 1,206 cm-1 in Figure 3 (c) suggested that the sulfonic acid groups have been attached in the surface. In Figure 3 (d), band peaks at 2,924, 1,532, and 749 cm-1 indicate the presence of phenyl group. Peak at 1,524 cm-1 in Figure 3 (e) could indicate new groups had been attached. On the basis of the above described FTIR results, it can be deduced that the active groups of surfactants capped successfully onto UCNP surface during the synthetic process though part of surface still linked with long alkyl chains from ILs. As a consequence, ILs and surfactants participate synthesis process together as capping agents, competing with each other to cap for UCNPs.