## 3 Results and Discussion  
Aiming to develop a facile method for the preparation of fully biobased and biocompatible optical dipstick sensing systems for wound fluids, covalent immobilization of carbon dots on oxidatively modified bacterial cellulose and subsequent processing to BC nanopaper was investigated. In a first step different types of carbon dots were synthesized by microwave-assisted co-hydrothermolysis of appropriate precursor compounds in buffered aqueous solution. Following isolation and purification by dialysis the photoluminescence properties of the obtained carbon dots in terms of emission wavelength, pH stability and quantum yield were investigated. Atomic force microscopy in combination with Gwyddion data visualization and analysis confirmed that microwave-assisted co-hydrothermolysis of the various mixtures of natural and synthetic precursor materials followed by dialysis resulted in spherical nanoparticles in the desired single-digit nanoscale range (Figure 1). As a matter of course, a minor impact of the different precursor compounds and materials is evident from the slightly different size of the obtained CDs which increases in the order LCA (1-3 nm) < GEA (2-5 nm) < LU (4-7 nm) < CAU (5-10 nm). The small size of the obtained CDs has been also confirmed by dynamic light scattering on the example of GEA- (2.7 nm) and LEA-type CDs (1.5 nm). All CD types prepared in this work could be easily dispersed in water. The emission maxima of CDs in aqueous dispersion state were reached within a narrow excitation wavelength range of 340-360 nm. As a compromise and in favor of using a uniform excitation wavelength, all photoluminescence experiments performed to study the impact of pH and of the EDEA/glycerol ratio (for GEA-type CDs) on the photoluminescence intensity were conducted at \(\lambda_{exc}\) = 350 nm. This is virtually identical with the optimum \(\lambda_{exc}\) of GEA-type CDs (351 nm) exemplarily used for the preparation of TOBC/CD hybrid nanopaper. The emission maxima (PL\({}_{\text{max}}\)) of all types of CDs at \(\lambda_{exc}\) = 350 nm ranged from 450 to 480 nm with CAU-type CDs (450 nm) having the lowest and LEA-type CDs the highest values (476 nm). Accordingly, all CD dispersions had an intense light blue appearance under near UV light (Figure 2a). The greenish color of some crude products (cf. Figure 2a, left) is most likely caused by lower-molecular by-products which is concluded from the observation that the color changed to light blue after dialysis. Size-dependent variation of PL\({}_{\text{max}}\) as reported  
Figure 1: Atomic force microscopy (AFM) images of LEA CDs (**a**), GEA CDs (**b**), CAU CDs (**c**) and LU CDs (**d**). for two different samples of carbon dots (\(\O\) 3.8 nm versus \(\O\) 13 nm) generated by laser ablation photoluminescence [43] was not observed for our microwave-assisted hydrothermal approach and set of samples, respectively. This supports the common assumption that PL in CDs is rather governed by the size and conformation of conjugated \(\pi\) electron domains next to surface effects triggered by surface defects or hetero-element surface moieties than by quantum confinement effects [44]. This work investigated the optimum ratio of GEA precursor compounds aiming to increase the photoluminescence properties and quantum yield. It has been shown that by variation of the glycerol-to-EDEA ratio from 80:20 to 55:45 (w/w) corresponding to a molar C/N ratio of 12.91 to 6.71 the PL intensity (\(\lambda_{exc}\) = 350 nm) could be tuned to a great extent. The highest values were obtained for molar C/N ratios of about 8-9 while the PLmax peak position remained virtually unaffected by variation of the glycerol-to-EDEA ratio. Therefore, all GEA type CDs used in this study were prepared at glycerol-to-EDEA ratio of 70:30.