They are responsible for the enhanced PL intensity of RNase A@C-dots [33]. Figure 3 XPS and FTIR spectra and zeta potential. (a) XPS C 1 s spectrum. (b) XPS O 1 s spectrum. (c) XPS N 1 s of RNase A@C-dots. (d) FTIR spectra of RNase A@C-dots. (e) Zeta potential of RNase A@C-dots. The average zeta potential of C-dots (Figure 3e) is 0.02 mV, slightly beyond zero. Considering the fact that cells are with positive charges, a zeta potential of no less than zero is definitely favorable in cell labeling and imaging. (The

influence of microwave condition on PL of carbon dots was also investigated, as shown in Additional file 1: Figure S5). Effects of pH on PL properties of RNase A@C-dots Although the mechanism of PL properties of C-dots is still unclear and debatable, there is solid evidence of lower quantum efficiency of C-dots that is caused by the fast recombination of excitations located at surface energy traps [8]. LY294002 supplier Therefore, after modifying the surface of C-dots using different Selleckchem Daporinad surface passivation reagents, the PL properties of the C-dots

can be significantly improved [7, 8, 34]. In this work, we firstly introduce the bioactive enzyme RNase A to synthesize C-dots by one-step micro-assisted synthesis method. The mechanism of the PL enhancement could be explained by following two reasons: Firstly, we propose that the electron-donating effect which resulted from the abundant amino acid groups on the surface of RNase A, especially those amino acids with benzene rings, might contribute a lot to the much enhanced Ketotifen PL intensity of the C-dots. To test our assumption, we select tryptophan and thenylalanine as replacements of RNase A to synthesize C-dots in the same conditions. As shown in Additional file 1: Figure S5b, both tryptophan and thenylalanine can greatly enhance the PL intensity. Secondly, we think that in the microware heating reaction, RNase A acts as a N doping reagent that causes the PL enhancement of the C-dots. The data of IR and XPS can also support the point. In the biological application, pH is a very important factor that we

firstly take into consideration. Herein, the influence of pH values over the PL of the RNase A@C-dot clusters is indicated in Figure 2d. The fact that pH values could affect the PL intensity has been seen in quite a few studies [10, 21, 32, 35]. Generally, PL intensity reaches its maximum at a certain pH values, 4.5 [35] or 7 [21]. At the same time, a slight redshift in the emission peak was identified with the increase of pH value [35]. Interestingly, the pH value played a unique role upon the PL of RNase A@C-dots. There was a noticeable redshift in the emission peak when the pH went from 2.98 to 11.36. However, the PL intensity decreases continuously as pH values increase. Specifically, the C-dots lost about 25% of its PL intensity when the pH increases from 2.98 to 7.32 and retain only 40% of its intensity when the pH value comes to 11.36.