## 4 Conclusions  
This work complements two recent studies. One investigated the preparation and utilization of photoluminescent cellulose nanocrystals covalently equipped with GEA-type CDs as cytocompatible  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \(E\) & \(R_{p0.2}\) & \(\sigma_{max}\) & \(\varepsilon_{max}\) \\  & [GPa] & [MPa] & [MPa] & [\%] \\ \hline TOBC & 6.93 \(\pm\) 0.43 & 60.4 \(\pm\) 1.24 & 127 \(\pm\) 14.0 & 4.15 \(\pm\) 0.78 \\ TOBC/CD & 6.92 \(\pm\) 0.16 & 55.7 \(\pm\) 2.35 & 138 \(\pm\) 9.14 & 5.53 \(\pm\) 0.74 \\ \hline \end{tabular}
\end{table}
Table 1: Mechanical properties of TOBC nanopaper (3 mm wide, clamping length 20 mm) as obtained by tensile testing.  
Figure 8: Response of TOBC reference and TOBC/CD hybrid nanopaper towards tensile stress (**a**). Water contact angle 10 s after a droplet had been placed on the surface of TOBC reference (**b**) and TOBC/CD hybrid nanopaper (**c**).  
Figure 7: FEG-SEM images of TOBC/CD hybrid nanopaper at different magnification.  
probes for in vitro bio-imaging of macrophage cells [45]. The second one compared the pros and cons of chemical versus physical grafting of photoluminescence amino-functional carbon dots onto nanofibrillated TEMPO-oxidized pulp cellulose for subsequent preparation of transparent photoluminescence gels and aerogels. The results of this study confirm that medically pure, biocompatible bacterial cellulose can be easily equipped with covalently immobilized photoluminescent carbon dots. Following TEMPO-mediated oxidation, optional nanofibrillation and EDC/NHS mediated grafting of aminofunctional carbon dots, the obtained dispersion of BC/CD hybrid fibrils in aqueous medium can be processed to functional nanopaper for biosensing applications. Considering the comparatively low porosity and opaqueness of the formed paper, EDC/NHS-mediated dip-grafting of aminofunctional CDs onto paper formed beforehand from TEMPO-oxidized BC is considered somewhat more efficient than grafting of the CDs onto BC fibers in aqueous suspension state prior to sheet formation. At sufficiently high degree of oxidation, it is expected that nanofibrillation in combination with flattening agents and mechanical \"surface smoothing\" as reported elsewhere [48] can produce transparent nanopaper. In this case, covalent grafting of CDs onto BC nanofibrils prior to paper formation would be meaningful since higher amounts of CDs could be incorporated contributing to a higher PL response and, hence, sensitivity towards a respective target analyte. Even though a comparatively high relative QD of ca. 32% was obtained for the CDs prepared from urea and citric acid, sensitivity can be probably further increased by optimizing CD synthesis in terms of precursor materials, hydrothermal conditions and purification.  
Conceptualization, F.L.; methodology, S.Q., S.P. and F.L.; validation, S.Q., S.P. and F.L.; formal analysis, S.Q. and S.P.; investigation, S.Q., S.P., B.U., P.T.; resources, F.L. and W.G.-A.; data curation, S.Q. and S.P.; writing--original draft preparation, S.Q. and S.P.; writing--review and editing, F.L. and S.P.; visualization, S.P.; supervision, F.L.; project administration, F.L.; funding acquisition, F.L.  
This research was funded by the Austrian BMLFUW Ministry (WoodWisdom Net+ project AeroWood 2014-2017).  
The authors cordially thank [PERSON] and [PERSON] for their support with mechanical testing and AFM analysis (University of Natural Resources and Life Sciences Vienna, Institute of Wood Technology and Renewable Materials) as well as [PERSON] (University of Natural Resources and Life Sciences Vienna, Division of Biochemistry) for fluorescence measurements. The support by BOKU Vienna Open Access Publishing Fund is thankfully acknowledged.  
The authors declare no conflicts of interest in this work.