## 3 Results and Discussion
In view of the planned covalent EDC/NHS-mediated coupling of aminofunctional CDs onto carboxylated surfaces of TEMPO-oxidized cellulose as well as the envisaged usage as biosensor for different types of aqueous media, the impact of varying pH (from 2 to 12) onto photoluminescence properties was studied. The results revealed that GEA-type CDs are virtually insensitive towards pH variation in this range (Figure 3a). LEA and CAU type CDs are largely stable between pH 4 and 12, only at lower pH the photoluminescence intensity decreased significantly in particular for the CAU samples. Carbon dots prepared from lemon juice and urea were most sensitive towards changes of pH as evident from the considerable decrease of PL intensity when lowering the pH from 6 to 2 or rising from 6 to 12. At physiological conditions, i.e., near neutral pH, all types of synthesized CDs however display maximum photoluminescence. Recently, we have reported about the use of GEA-type carbon dots to impart cellulose nanocrystal (CNC) based cytocompatible materials bioimaging capabilities [45]. Besides comprehensive characterization of composition, surface functional groups and fluorescence properties we had determined their relative quantum yields--defined as ratio of emitted and absorbed photons--too. Using this quantum yield value of GEA-type CDs (14.0% at pH 7) as a benchmark, this study confirmed the general observation that replacement of defined CD precursor compounds by natural extracts, such as by lemon juice results in carbon nanoparticles of lower quantum yield (8.4%, LEA). Figure 2: Pictures of aqueous dispersions of LEA (**a**, **left**) and GEA type CDs (**a**, **right**) at \(\lambda_{exc}\) = 366 nm. Impact of the EDEA mass fraction (22–41%, v/v) in the precursor mixture (glycerol + EDEA) on PL intensity of GEA-type CDs (**b**). Comparison of PL characteristics of aqueous dispersions (pH 6) of the different types of CDs prepared (**c**). Similar low values of 5-7% had been reported for CDs prepared from sugarcane juice or strawberry juice [20], for example, and are probably caused by inorganic constituents not removed by centrifugation or filtration, and forming photoluminescence inactive particles upon pyrolysis. Interestingly, replacement of the nitrogen source 2,2\({}^{\prime}\)-(ethylenedioxy)-bis-(ethylamine) (EDEA) by urea considerably increased both photoluminescence intensity (Figure 2c) and quantum yield (Figure 4a) of the obtained particles in aqueous dispersion at comparable concentration. While the highest quantum yield (32%) was obtained for CAU-type particles (citric acid/urea), a somewhat lower but still considerably high quantum yield value of 26.9% was obtained again when substituting citric acid by lemon juice (Figure 4a). It is worth noticing that the use of urea as nitrogen source resulted in materials (LU, CAU) whose PL was more sensitive towards low pH values (Figure 3b,d) than it was the case when EDEA was used. Aiming to develop a facile approach for covalent bonding of carbon nanoparticles onto the surface of bacterial cellulose nanopaper we intended to compare both (i) grafting of CDs prior to and after BC nanopaper formation as well as (ii) the use of TEMPO-oxidized BC without or with subsequent nanofibrillation. To facilitate the latter, TEMPO-mediated oxidation of BC was conducted in alkaline (pH 10-11) [38; 46] instead of near-neutral conditions (pH 6.8) [40; 41] aiming at the introduction of a higher density of (repulsive) surface carboxyl groups which are known to support mechanical delamination using a high-pressure homogenizer for example [47]. Following this approach two types of TOBC were obtained that differed in degree of oxidation (_DO_) as desired and confirmed by conductometric titration (Figure 4b). While the degree of oxidation related to the count of anhydroglucose units was 8.4% (0.52 mmol g\({}^{-1}\)COOH; cf. Figure 4b) for the material treated at milder conditions, it was 12.3% (0.76 mmol g\({}^{-1}\)COOH) when performing the modification in alkaline conditions.