## 3 Results and Discussion
However, even at higher degree of oxidation, nanofibrillation of BC using a high-pressure homogenizer did not yield individualized nanofibrils as we obtained for never-dried wood pulp for  
Figure 3: Impact of varying pH in the range from 2 to 12 on photoluminescence properties (\(\lambda_{exc}\) = 350 nm) of the different types of CDs. PL: photoluminescence. example. AFM analysis revealed that both BC samples were partially disintegrated only, regardless whether high pressure homogenization was applied or not. While the diameters of the fibrils obtained from the stronger oxidized and mechanically delaminated BC sample ranged from about 2-10 nm with an average of 5.48 \(\pm\) 0.28 nm (AFM, n = 45), those of the solely oxidized and disintegrated (kitchen mixer) material had an average fibril diameter of 9.05 \(\pm\) 2.84 nm (AFM, n = 45; Figure 5a,b). The average length of the fibrils--which appeared to be twisted in longitudinal direction (Figure 5b)--was somewhat below 10 \(\upmu\)m for both types of BC samples (Figure 5a). Considering aspects like cost-benefit ratio or gain of CD particle density on the final surface of the opaque BC nanopaper, we decided to omit the nanofibrillation step and used therefore the BC batch oxidized at near-neutral conditions (TOBC) for covalent coupling of CDs as described below. Aiming to develop a green approach for facile manufacture of paper-based optical sensors, we employed classic aqueous EDC/NHS coupling chemistry to graft the largely bio-based and good water-dispersible CDs onto the surface of the oxidatively activated BC. For demonstration purposes we selected GEA-type CDs since we had recently comprehensively studied their EDC/NHS-mediated bonding to the surface of TEMPO-oxidized cellulose nanocrystals [45]. While X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) had confirmed the presence of amino groups on the surface of the respective CDs, polyelectrolyte titration, and zeta potential measurements revealed a pH dependent charge density varying between zero (pH > 8.5) and +500 \(\upmu\)mol\(\cdot\)g\({}^{-1}\) (pH 4.5). Surface sensitive analytical techniques using a quartz crystal microbalance with dissipation monitoring and Surface Plasmon Resonance complementing XPS and FT-IR measurements confirmed the establishment of covalent bonds between surface amino groups of the GEA-type CD nanoparticles and carboxyl groups on the surface of CNC. Since this study employed  
Figure 4: Relative quantum yields of the different types of carbon dots in comparison with the reference quinine sulfate (QS; **a**) and conductometric titration curve to determine the degree of oxidation for bacterial cellulose (BC) subjected to TEMPO-mediated oxidation at near pH neutral conditions (**b**). Figure 5: AFM pictures of bacterial cellulose subjected to TEMPO-mediated oxidation in aqueous near-neutral- (pH 6.8; **a,b**) and alkaline conditions (**c**) at different magnification. not only the same type (GEA) but also material of the same batch of carbon dots and a comparable carboxylated cellulosic substrate, successful covalent bonding of GEA CDs onto the surface of the TOBC materials can be assumed. This is also evident from the photoluminescence properties of the final TOBC/CD hybrid nanopaper (Figure 6) and the aqueous phase obtained when forming the sheet from respective TOBC slurries that were subjected to EDC/NHS-mediated coupling with CDs prior to paper formation. While the hybrid paper exhibited strong bluish photoluminescence, the drained water was significantly depleted by fluorescent CDs increasing with residence time. This was similar for prefabricated TOBC nanopaper dip-grafted with GEA-type CDs (Figure 6). Also here, EDC/NHS-mediated dip-coupling imparted the paper surface blue photoluminescence increasing with residence time in the coupling bath. Water contact angle of the nanopaper is virtually also not affected by introduction of carbon dots as evident from the very similar value compared to the CD-free reference paper (Figure 8b,c).