Specifically, as the excitation wavelength changes from 300 to 500 nm in a 20-nm increment, the PL peak shifted from 450 to 550 nm, while the intensity increases before the excitation wavelength reaches 380 nm

and then gradually decreases followed by increase of excitation wavelength. However, NVP-BSK805 research buy in the PL spectra of C-dots (Additional file 1: Figure S2b), we cannot find that there is no a typical λ ex dependence character. When the excitation wavelength changes from 280 to 440 nm, the PL intensity at around 480 nm varies and hits its maximum at an excitation wavelength of 380 nm. But the emission wavelength does not change its location. Moreover, before the excitation wavelength reaches 380 nm, there is more than one emission peak in the PL spectra with only one peak around 480 nm remaining when excited at 390 nm and longer wavelength. Furthermore, photoluminescence excitation (PLE) spectra this website of RNase A@C-dots (Figure 2b) have only one peak located at around 390 nm, while the PLE spectra of C-dots (Additional file 1: Figure S2b) owns two with an additional one around 290 nm. The existence of RNase A has not only changed the features and locations of PL spectra but also enhanced the intensity of photoluminescence. When excited at 360 nm, the intensity of

RNase A@C-dots is about 30 times the intensity of C-dots (Additional file 1: Figure S2c). As to quantum yield, Table 1 shows that the quantum yield of the RNase A@C-dots is 24.20% which is dramatically higher than the 0.87% yield of C-dots. Even after having been passivated with PEG2000 which is widely accepted as an efficient way to improve the quantum yield of C-dots [8], the quantum yield of C-dots is 4.33%, still much lower than that of the RNase A@C-dots. Table 1 Related photoluminescent quantum yield (PLQY) of RNase A@C-dots, C-dots, and C-dots-PEG 2000 (C-dots passivated by PEG 2000 ) Sample RNase A@C-dots C-dots C-dots-PEG 2000 PLQY [%] 24.20 0.87 4.33 Luminescence decay (Figure 2c) has an average excited-state lifetime

of 3.3 ns for emission at 450 nm with an excitation wavelength of 380 nm which during is comparable to those reported [2, 23]. The relatively short lifetime might as well suggest the radioactive recombination of the excitation contributing to the fluorescence [23]. The FTIR spectrum (Figure 3d) shows the presence of (C = O) (1,719 cm−1), (O-H) (3,425 cm−1), (C-N) (1,209 cm−1), and (N-H) (2,994 cm−1) which directly indicates Rnase A coated C-dot surface. This can also be confirmed by the X-ray photoelectron spectroscopy (XPS) of RNase A@C-dots (as shown in Figure 3a,b,c). Moreover, the high-resolution N 1 s spectrum of the RNase A@C-dots (Figure 3c) has clear signs of both amide N (399.3 eV, C-N) and doping N (400.4 eV, O = C-NH-) atoms. The XPS (Additional file 1: Figure S3) of the C-dots only shows the signals of -COOH and -OH, and neither amide N nor doping N is detected.