Optical and Physicochemical Characterizations of a Cellulosic/CdSe-QDs@S-DAB5 Film

CdSe quantum dots nanoparticles were coated with the thiolated (DiAminoButane based dendrimer) DAB dendrimer of fifth generation (S-DAB5) and embedded in a highly hydrophilic regenerated cellulose (RC) film by simple dip-coating method (immersion in QD-dendrimer aqueous solution) as a way to get a flexible nano-engineered film (RC-4/CdSe-QDs@S-DAB5) with high transparency and photoluminescence properties for different applications. Optical changes in the RC film associated with QDs inclusion were determined by spectroscopic ellipsometry (SE) measurements, which provide information on changes caused in the refraction index and the extinction coefficients of the film, as well as by light transmittance/reflectance curves and photoluminescence (PL) spectra. Impedance spectroscopy (IS) and other typical physicochemical techniques for material characterization (TEM, SEM and XPS) have also been used in order to have more complete information on film characteristics. A comparison of RC-4/CdSe-QDs@S-DAB5 film optical characteristics with those exhibited by other RC-modified films depending on the type of dendrimer was also carried out.


Introduction
Colloidal quantum dot (QD) nanoparticles are semiconductor nanocrystals, with a core-shell structure and a diameter ranging ideally between 2 nm and 10 nm, which display unique electronic and optical properties (between bulk semiconductors and discrete molecules) based on both size and chemical composition [1][2][3][4][5]. QDs can emit light at wavelengths ranging from the UV to the IR, a photon emission at a longer wavelength than the one absorbed (electron-hole recombination process) [6]. Their properties include highest photostability, high extinction coefficient and brightness, magnetic, thermal and antibacterial characteristics as well as small size, this latter being a great advantage over other nanoparticles (NPs) used for multifunctional probes (polymeric or silica NPs), due to their large surface area [7].
In fact, QDs are nowadays of significant interest in different nanotechnology fields such as biomedical, electronics and optoelectronic devices due to their pure color emission, wide color gamut and high quantum efficiency [8]. Moreover, QDs also exhibit narrow band gap, sharp emission, and excellent spectral properties [9][10][11], but they are unprotected from external agents (oxidation, water, heat, or harsh environments) which significantly limit their long-term stability [12][13][14]. In this context, the inclusion of QDs

Preparation of the Cellulosic Films Modified with CdSe-QDs@ S-DAB5 Nanoparticles
A highly swelling (>80%), elastic, and transparent film of regenerated cellulose (RC) from Cellophane Española S.A. (Burgos, Spain was selected for easy inclusion of CdSe-QDs@S-DAB5 by depth-coating method) [43]. Pieces of the RC-4 film were immersed in an aqueous solution of CdSe-QDs@S-DAB5 for 2 h at room temperature, then they were taken off and their surfaces gently dried with paper. These samples will be named RC-4/CdSe-QDs@S-DAB5, and a scheme of this flexible and easily handle film is shown in Figure 2.

Surface Analysis: FE-SEM Microscopy and XPS Spectroscopy
The surface morphology of the cellulosic film coated with CdSe-G5 QDs nanoparticles was analyzed using an FEI Talos F200X field-emission scanning electron microscope (FE-SEM) with a double beam (Helios Nanolab 650 de FEI Company, Oxford, UK). Surface chemical characterization of the studied film was carried out by X-ray photoelectron spectroscopy (XPS, Physical Electronics ULVAC-PHI Lake Drive East, Chanhassen, MN,

Preparation of the Cellulosic Films Modified with CdSe-QDs@ S-DAB 5 Nanoparticles
A highly swelling (>80%), elastic, and transparent film of regenerated cellulose (RC) from Cellophane Española S.A. (Burgos, Spain was selected for easy inclusion of CdSe-QDs@S-DAB 5 by depth-coating method) [43]. Pieces of the RC-4 film were immersed in an aqueous solution of CdSe-QDs@S-DAB 5 for 2 h at room temperature, then they were taken off and their surfaces gently dried with paper. These samples will be named RC-4/CdSe-QDs@S-DAB 5 , and a scheme of this flexible and easily handle film is shown in Figure 2.
dendritic polyamine DAB-AM and triethylamine in CH2Cl2 was used for S-DAB5 drimer synthesis; the obtained dendrimer (formulae: (NHCH2CH2SH)64, mass: 12,808. u.a.m and size of around 6.6 nm) is soluble in aqueous solutions but insoluble in org solvents. Figure 1a shows a schematic of the S-DAB5 dendrimer, while Figure 1b sh the CdSe-QDs@S-DAB5 nanoparticles

Preparation of the Cellulosic Films Modified with CdSe-QDs@ S-DAB5 Nanoparticles
A highly swelling (>80%), elastic, and transparent film of regenerated cellulose from Cellophane Española S.A. (Burgos, Spain was selected for easy inclusion of C QDs@S-DAB5 by depth-coating method) [43]. Pieces of the RC-4 film were immerse an aqueous solution of CdSe-QDs@S-DAB5 for 2 h at room temperature, then they w taken off and their surfaces gently dried with paper. These samples will be named 4/CdSe-QDs@S-DAB5, and a scheme of this flexible and easily handle film is show Figure 2.

Surface Analysis: FE-SEM Microscopy and XPS Spectroscopy
The surface morphology of the cellulosic film coated with CdSe-G5 QDs nanop cles was analyzed using an FEI Talos F200X field-emission scanning electron micros (FE-SEM) with a double beam (Helios Nanolab 650 de FEI Company, Oxford, UK). Su chemical characterization of the studied film was carried out by X-ray photoelectron s troscopy (XPS, Physical Electronics ULVAC-PHI Lake Drive East, Chanhassen,

Surface Analysis: FE-SEM Microscopy and XPS Spectroscopy
The surface morphology of the cellulosic film coated with CdSe-G 5 QDs nanoparticles was analyzed using an FEI Talos F200X field-emission scanning electron microscope (FE-SEM) with a double beam (Helios Nanolab 650 de FEI Company, Oxford, UK). Surface chemical characterization of the studied film was carried out by X-ray photoelectron spectroscopy (XPS, Physical Electronics ULVAC-PHI Lake Drive East, Chanhassen, MN, USA). XPS spectra were recorded with a Physical Electronics PHI 5700 spectrometer with X-ray MgK α radiation as the excitation source (300 W, 15 kV, 1253.6 eV). High-resolution spectra were recorded at two take off angles, 45 • (standard analysis angle) and 70 • , by a concentric hemispherical analyzer operating in the constant pass energy mode at 29.35 eV and using a diameter analysis area of 720 µm. The residual pressure in the analysis chamber was maintained below 10 −9 Torr during data acquisition. Accurate ±0.1 eV binding energies were determined with respect to the position of the adventitious C 1s peak at 284.8 eV. A PHI ACCESS ESCA-V6.0F software package was used for acquisition and data analysis. Atomic concentration percentages (A.C. %) of the sample elements were determined after subtraction of a Shirley-type background considering the corresponding area sensitivity factor for the different measured spectral regions [45].

Optical Characterization
An Edinburgh Instruments FLS920 (Livingston, UK), equipped with a Xe lamp (450 W) as the excitation source and monochromatic LEDs (PicoQuant PLS), controlled by a PDL880-B system, was used for steady-state fluorescence measurements.
Transmittance/reflection measurements were performed with a Varian Cary 5000 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) provided with an integrating sphere of Spectralon for wavelength ranging between 250-2000 nm.
Spectroscopic Ellipsometry (SE) measurements were carried out with a spectroscopic ellipsometer (Sopra-Semilab GES-5E) using wavelengths in the range from 200 nm to 1000 nm after striped the back surface of the sample for reduction of interference fringes associated with multiple light reflections at the back interface (see Supplementary Figure S1). WinElli software v. 2.2 (Sopra-Semilab, Paris, France) was used for data fit. SE is a nondestructive technique that allows the determination of optical/morphological parameters of inorganic and polymeric thin films (refraction index, extinction coefficient or thickness) [46][47][48]. Two characteristic parameters, angles Ψ and ∆, are measured to determine changes associated to film surface or bulk phase [46,49], which are related with differential changes in amplitude and phase between the incident and reflected light waves (through the Fresnel reflection coefficients ratio of polarized light) by [46]: tan(Ψ)e i∆ = r p /r s (1) where r s and r p indicate the amount of light in perpendicular (s) and parallel (p) planes, as it is schematically shown in Figure 3. Because SE is based on the ratio of two measured values, it is very accurate and reproducible, and no standard sample is required [46]. and using a diameter analysis area of 720 µm. The residual pressure in the analysis c ber was maintained below 10 −9 Torr during data acquisition. Accurate ±0.1 eV bin energies were determined with respect to the position of the adventitious C 1s pe 284.8 eV. A PHI ACCESS ESCA-V6.0F software package was used for acquisition and analysis. Atomic concentration percentages (A.C. %) of the sample elements were d mined after subtraction of a Shirley-type background considering the corresponding sensitivity factor for the different measured spectral regions [45].

Optical Characterization
An Edinburgh Instruments FLS920 (Livingston, UK), equipped with a Xe lamp W) as the excitation source and monochromatic LEDs (PicoQuant PLS), controlled PDL880-B system, was used for steady-state fluorescence measurements.
Transmittance/reflection measurements were performed with a Varian Cary spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) provided with an grating sphere of Spectralon for wavelength ranging between 250-2000 nm.
Spectroscopic Ellipsometry (SE) measurements were carried out with a spectrosc ellipsometer (Sopra-Semilab GES-5E) using wavelengths in the range from 200 nm to nm after striped the back surface of the sample for reduction of interference fringes ciated with multiple light reflections at the back interface (see Supplementary Figure  WinElli software v. 2.2 (Sopra-Semilab, Paris, France) was used for data fit. SE is a destructive technique that allows the determination of optical/morphological param of inorganic and polymeric thin films (refraction index, extinction coefficient or thick [46][47][48]. Two characteristic parameters, angles Ψ and Δ, are measured to deter changes associated to film surface or bulk phase [46,49], which are related with differe changes in amplitude and phase between the incident and reflected light waves (thr the Fresnel reflection coefficients ratio of polarized light) by [46]: tan(Ψ)e iΔ = rp/rs where rs and rp indicate the amount of light in perpendicular (s) and parallel (p) pl as it is schematically shown in Figure 3. Because SE is based on the ratio of two meas values, it is very accurate and reproducible, and no standard sample is required [46]  SE measurements were performed at three different incident angles (Φ o = 65 • , 70 • and 75 • ) since it can also give information on sample homogeneity, the presence of surface impurities and roughness [49,50].

Impedance Spectroscopy Measurements
Impedance spectroscopy (IS) measurements were performed with dry samples in an electrode/sample/electrode test cell [51]. The electrodes were connected to an Impedance Analyzer (Solartron 1260, Solartron Analytical, Wokingham, UK) and measurements were recorded for 100 data points with frequency (f) ranging between 10 Hz and 10 7 Hz, at a maximum voltage of 0.01 V. The impedance, Z, is a complex number, Z = Z real + j Z img , which can be separated into real and imaginary parts by algebra rules. Electrical parameters (resistance (R) and capacitance (C)) can be determined by analyzing the impedance plot (Z real versus −Z img ) by considering equivalent circuits. The simplest case, for homogeneous systems, corresponds to a semi-circle and it is due to a parallel association of resistance (R) and capacitor (C) [46], which are related to Z real and Z img by the following expressions: where ω represents the angular frequency (ω = 2πf). However, complex systems usually present distribution of relaxation times and the resulting plot is a depressed semi-circle, which is associated with a non-ideal capacitor or constant phase element (CPE), and its impedance is expressed by [52]: Q(ω) = Y o (jω) −m , where Y o represents the admittance and m is an experimental parameter (0 ≤ m ≤ 1) and in these cases an equivalent capacitance (C eq ) can be determined. This kind of analysis allows us to determine the presence/effect of different polymer-modifying elements [53,54].  On the other hand, when the surface analysis is carried out by XPS, it is possible to detect the presence of other atoms such as nitrogen, by the N 1s signal from the dendritic structure. Here the surface chemical concentrations (in A.C.%) at the standard analysis angle (45°) are: C 1s (77.5%); O 1s (20.5%); N 1s (0.4%); Cd 3d (0.02%) and Se 5d (0.02%); On the other hand, when the surface analysis is carried out by XPS, it is possible to detect the presence of other atoms such as nitrogen, by the N 1s signal from the dendritic structure. Here the surface chemical concentrations (in A.C.%) at the standard analysis angle (45 • ) are: C 1s (77.5%); O 1s (20.5%); N 1s (0.4%); Cd 3d (0.02%) and Se 5d (0.02%); moreover, a slight increase in Cd 3d and Se 5d values (0.03%) was obtained from measurement performed at a higher incident angle (70 • ), which corresponds to deeper analysis, which seems to confirm the presence of CdSe QDs-DAB 5 into the film structure (A.C. % of the different elements at 70 • incident angle are indicated in Supplementary  Information (Table S1).

Surface and Bulk Analysis
The XPS study of the surface provides the chemical oxidation state of the characteristic QDs ions ( Figure 5). The analysis of the C 1s reveals the presence of three main contributions assigned to: (i) adventitious carbon (sp 2 hybridized carbon) -C-C-/-C-H at 284.8 (69.53%); (ii) -C-OH/-C-N at 286.39 (24.1%) and (iii) -C=O at 288.2 eV (6.5%) functional groups (Figure 5a), related mainly to the presence of dendrimer molecule [55]. On the other hand, the N 1s spectra (Figure 5b) exhibits a major peak at around 400.0 eV and it was attributed to N atoms bonded with sp 2 -hybridized C atoms [56]; Figure 5b also shows the Cd 3d signal from the QDs, at 405.1 and 412.1 eV, assigned to the 3d 3/2 and 3 d 5/2 orbitals. Figure 5c shows the O 1s signal, a peak at 532.5 eV, assigned to the C-O and C-O-C signal. It should be indicated that the Se signal was not observed due to its low sensitivity factor and dendrimer coverage. On the other hand, when the surface analysis is carried out by XPS, it is possible to detect the presence of other atoms such as nitrogen, by the N 1s signal from the dendritic structure. Here the surface chemical concentrations (in A.C.%) at the standard analysis angle (45°) are: C 1s (77.5%); O 1s (20.5%); N 1s (0.4%); Cd 3d (0.02%) and Se 5d (0.02%); moreover, a slight increase in Cd 3d and Se 5d values (0.03%) was obtained from measurement performed at a higher incident angle (70°), which corresponds to deeper analysis, which seems to confirm the presence of CdSe QDs-DAB5 into the film structure (A.C. % of the different elements at 70° incident angle are indicated in Supplementary Information (Table S1).
The XPS study of the surface provides the chemical oxidation state of the characteristic QDs ions ( Figure 5). The analysis of the C 1s reveals the presence of three main contributions assigned to: (i) adventitious carbon (sp 2 hybridized carbon) -C-C-/-C-H at 284.8 (69.53%); (ii) -C-OH/-C-N at 286.39 (24.1%) and (iii) -C=O at 288.2 eV (6.5%) functional groups (Figure 5a), related mainly to the presence of dendrimer molecule [55]. On the other hand, the N 1s spectra (Figure 5b) exhibits a major peak at around 400.0 eV and it was attributed to N atoms bonded with sp 2 -hybridized C atoms [56]; Figure 5b also shows the Cd 3d signal from the QDs, at 405.1 and 412.1 eV, assigned to the 3d3/2 and 3 d5/2 orbitals. Figure 5c shows the O 1s signal, a peak at 532.5 eV, assigned to the C-O and C-O-C signal. It should be indicated that the Se signal was not observed due to its low sensitivity factor and dendrimer coverage. Another technique able to give information on bulk material and interface (electrode/sample surface) contributions is impedance spectroscopy (IS). IS is an alternating current (a.c.) technique commonly used for electrical characterization of homogeneous and heterogeneous materials as well as for composite systems (such as electrolyte/sample commonly named Electrochemical Impedance Spectroscopy or EIS) using equivalent circuits as models [53,54,57,58]. IS provides quantitative and/or qualitative information related to charge movement/adsorption for the analyzed samples by means of the electrical resistance or capacitance (equivalent capacitance for non-homogeneous systems, C eq = (RY o ) (1/m) /R [52]) respectively, which can be significantly affected by the structure of the analyzed system and material characteristics. Figure 6a shows the Nyquist plot (Z real vs. −Z img ) obtained for dry samples of the original RC-4 film and the modified RC-4/CdSe-QDs@S-DAB 5 one, where differences in both bulk sample and electrode/sample interface can be observed. For comparison reason, the Nyquist plot for another nano-engineered film, RC-4/CdSe-QDs@S-DAB 2 , modified with CdSe QDs covered by S-DAB 2 dendrimer (formula and mass: NHCH 2 CH 2 SH) 8 and 1478.3187 u.m.a.) is shown in Figure 6b, and in this case, practically no differences were obtained for the electrode/sample region, although slight differences in the bulk films contribution can be observed. Figure 6c,d show the Bode plots (Z real vs. f and −Z img vs. f, respectively) which permit seeing in a clearer way the differences between RC-4 and RC-4/CdSe-QDs@S-DAB 5 films, where the slight shift to lower frequency obtained for this latter film is an indication of its more compact structure associated with the nanoparticles inclusion.
film, RC-4/CdSe-QDs@S-DAB2, modified with CdSe QDs covered by S-DAB2 dendrimer (formula and mass: NHCH2CH2SH)8 and 1478.3187 u.m.a.) is shown in Figure 6b, and in this case, practically no differences were obtained for the electrode/sample region, although slight differences in the bulk films contribution can be observed. Figure 6c,d show the Bode plots (Zreal vs. f and −Zimg vs. f, respectively) which permit seeing in a clearer way the differences between RC-4 and RC-4/CdSe-QDs@S-DAB5 films, where the slight shift to lower frequency obtained for this latter film is an indication of its more compact structure associated with the nanoparticles inclusion. The fit of the two slightly depressed semicircles shown in Figure 6a by a non-linear program allows us the estimation of the electrical resistance (R) and equivalent capacitance (C eq ) of the films, which permits the determination of their electrical conductivity (σ The fit of the two slightly depressed semicircles shown in Figure 6a by a non-linear program allows us the estimation of the electrical resistance (R) and equivalent capacitance (C eq ) of the films, which permits the determination of their electrical conductivity (σ = d/S·R) and dielectric constant (ε = C·d/S), where d and S represent the thickness and surface of the films, respectively. The values obtained for the RC-4/CdSe-QDs@S-DAB 5 film are: σ = 3.1 × 10 −7 (Ω·m) −1 and ε = 8.1, which represent a reduction of around 18% in the conductivity and an increase of 7% in the dielectric constant with respect to the support film. Therefore, IS results seem to confirm the presence of CdSe-QDs@S-DAB 5 into the structure of the cellulosic film as well as on its surface.

Optical Analysis
Optical characterization techniques, photoluminescence (PL), light transmission and spectroscopic ellipsometry (SE) are of great interest in the analysis of thin films due to their non-invasive/non-destructive character, providing important characteristics of the analyzed samples.
One of the most significant properties of CdSe QDs is their fluorescent emission, with a maximum intensity of 534 nm; consequently, the luminescence character of the RC-4/CdSe-QDs@S-DAB 5 film shown in Figure 7, obtained at an excitation source of 475 nm, was expected. The emission spectra of the QDs-dendrimer modified film show an intensity maximum at 536 nm and exhibits higher intensity than that presented by the CdSe QDs. This fact is associated with the presence of the dendrimer on the surface of the QDs, since the thiol terminal functional groups increase the QDs emission process, as was already indicated in a previous study [59], but they do not change the emission wavelength. was expected. The emission spectra of the QDs-dendrimer modified film show an intensity maximum at 536 nm and exhibits higher intensity than that presented by the CdSe QDs. This fact is associated with the presence of the dendrimer on the surface of the QDs, since the thiol terminal functional groups increase the QDs emission process, as was already indicated in a previous study [59], but they do not change the emission wavelength. Spectroscopic ellipsometry (SE) results are presented in Figure 8. In particular, Figure  8a,b show a comparison of the wavelength dependence of the experimental parameters, tan(Ψ) and cos(Δ), measured at different light incident angles (Φo = 65°, 70° or 75°) for the RC-4/CdSe-QDs@S-DAB5 film and the RC-4 support, where differences depending on both incident angle and film can be observed. Optical characteristic parameters such as the refractive index (n) and the extinction coefficient (k) can be determined from SE measurements using the ellipsometer software, and their dependence on wavelength at the different light incident angles is shown in Figure 8c,d, respectively, for both films. Spectroscopic ellipsometry (SE) results are presented in Figure 8. In particular, Figure 8a,b show a comparison of the wavelength dependence of the experimental parameters, tan(Ψ) and cos(∆), measured at different light incident angles (Φ o = 65 • , 70 • or 75 • ) for the RC-4/CdSe-QDs@S-DAB 5 film and the RC-4 support, where differences depending on both incident angle and film can be observed. Optical characteristic parameters such as the refractive index (n) and the extinction coefficient (k) can be determined from SE measurements using the ellipsometer software, and their dependence on wavelength at the different light incident angles is shown in Figure 8c,d, respectively, for both films. As it can be observed in Figure 8c rather similar values for both refraction index and extinction coefficient were obtained at the different light incident angles for the RC-4 support, which is an indication of film smooth surface and homogeneity, and the small differences are attributed to surface impurities associated to environmental contamination; the following average values were determined: <n> = 1.56 ± 0.03 and <k> = 0.06 ± 0.03, As it can be observed in Figure 8c rather similar values for both refraction index and extinction coefficient were obtained at the different light incident angles for the RC-4 support, which is an indication of film smooth surface and homogeneity, and the small differences are attributed to surface impurities associated to environmental contamination; the following average values were determined: <n> = 1.56 ± 0.03 and <k> = 0.06 ± 0.03, which do not differ significantly from those reported for regenerated cellulose (1.48) and other polymers (between 1.53 and 1.58) measured at a unique wavelength of 589 nm [60], and zero in the case of extinction coefficient (no light absorption). However, higher values variability depending on the light incident angle was obtained for the RC-4/CdSe-QDs@S-DAB 5 film, which is attributed to sample inhomogeneity caused by inclusion of the CdSe-QDs@S-DAB 5 nanoparticles, as well as the surface roughness associated to CdSe QDs presence already The effect of the type of QDs (bare silicon dots, carbon dots, or nitrogen-doped carbon dots) on optical characteristics of the RC-QDs modified films was already studied in a previous paper [28] by comparing wavelength dependence of n and k parameters, and differences in both curve shape and values depending on the selected QDs nanoparticles were obtained. Therefore, the effect on optical parameters associated with dendrimers coverage of the CdSe QDs was considered in this work; then, three different generations of S-DAB dendrimers (2, 3 and 5 generations) were considered: (i) the RC-4/CdSe-QDs@S-DAB 5 film, (ii) the RC-4/CdSe-QDs@S-DAB 2 film (CdSe QDs covered with the generation 2 S-DAB dendrimer indicated above), and (iii) the RC-4/CdSe-QDs@S-DAB 3 film (CdSe QDs covered with the generation 3 dendrimer; formulae and mass: (NHCH 2 CH 2 SH) 16 and 3096.8635 u.a.m.), and the wavelength dependence of n and k for these three RC-4/CdSe-QDs@S-DAB x modified films, at a light incident angle of 70 • , is shown in Figure 9. As it can be observed, n and k parameters determined for the three films exhibit a similar type of wavelength dependence, but differences in the corresponding values depending on dendrimer generation were obtained, with the following sequence: RC-4/CdSe-QDs@S-DAB 5 film > RC-4/CdSe-QDs@S-DAB 3 film > RC-4/CdSe-QDs@S-DAB 2 , being this fact more significant for the extinction coefficient. Consequently, the choice of both the type of QDs and their coating allows us to obtain films with differentiated optical properties. On the other hand, it should be indicated that SE results also provide information on the real and the imaginary parts of the dielectric constant (ε r and ε i , respectively) considering that ε = (n + i k) 2 [46], which could be of interest in some applications.
The effect of CdSe-QDs inclusion on light transmission and reflection characteristics of the support film was also investigated. Figure 10a shows the transmittance spectra for the RC-4 and the RC-4/CdSe-QDs@S-DAB 5 films, and practically no effect due to the presence of the nanoparticles is observed in the visible region, but slight differences in the nearinfrared (NIR) region were obtained (1% average reduction); this fact seems to be related to the dendrimer coverage since similar dependence for the whole wavelength range was obtained for the RC-4/CdSe-QDs@S-DAB 5 and the RC-4/CdSe-QDs@S-DAB 3 films (see the insert in Figure 10a). High transmittance values are of interest for optical applications; in fact, the value obtained (>90%) is like that reported for carboxymethyl cellulose films modified with carbon dots with application in agriculture (sunlight conversion) [21]. DAB5 film > RC-4/CdSe-QDs@S-DAB3 film > RC-4/CdSe-QDs@S-DAB2, being this fact more significant for the extinction coefficient. Consequently, the choice of both the type of QDs and their coating allows us to obtain films with differentiated optical properties. On the other hand, it should be indicated that SE results also provide information on the real and the imaginary parts of the dielectric constant (εr and εi, respectively) considering that ε = (n + i k) 2 [46], which could be of interest in some applications. The effect of CdSe-QDs inclusion on light transmission and reflection characteristics of the support film was also investigated. Figure 10a shows the transmittance spectra for the RC-4 and the RC-4/CdSe-QDs@S-DAB5 films, and practically no effect due to the presence of the nanoparticles is observed in the visible region, but slight differences in the near-infrared (NIR) region were obtained (1% average reduction); this fact seems to be related to the dendrimer coverage since similar dependence for the whole wavelength range was obtained for the RC-4/CdSe-QDs@S-DAB5 and the RC-4/CdSe-QDs@S-DAB3 films (see the insert in Figure 10a). High transmittance values are of interest for optical applications; in fact, the value obtained (>90%) is like that reported for carboxymethyl cellulose films modified with carbon dots with application in agriculture (sunlight conversion) [21].
The comparison of light reflectance spectra obtained for the RC-4 and the RC-4/CdSe-QDs@S-DAB5 films presented in Figure 10b also shows similar dependence, but an average reflectance % reduction in the NIR region of ~5% due to QDs inclusion. However, the type of dendrimer seems to affect light refection since a more significant reduction (around 20% for the whole range of wavelength) was obtained for the RC-4/CdSe-QDs@S-DAB3 film.

Conclusions
A photoluminescence film was obtained by the inclusion of CdSe QDs covered by thiolate DAB5 dendrimer nanoparticles (CdSe-QDs@S-DAB5) into a highly swollen and elastic regenerated cellulose (RC) film support by depth-coating into an aqueous solution of these nanoparticles. The nano-engineering RC-4/CdSe-QDs@S-DAB5 film exhibits modified optical characteristics when compared with the RC-4 support, with a significant increase in the values of the extinction coefficient but only around 4% in the refraction index. Moreover, the influence of the cover-dendrimer generation (S-DAB2, S-DAB3 or S-DAB5) on n and k parameters was also demonstrated, indicating that both the types of QDs and the coating (in case) influence both optical parameters. The RC-4/CdSe-QDs@S-DAB5 film also shows very high transmittance (>90%), with only small changes at the near-infrared region for both light transmission and reflection (around 3%), when compared with the support RC-4 film, and only small changes associated with the dendrimer generation were obtained.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Comparison of wavelength dependence (Ψ) for a non-striped RC-4 sample (dashed line) and a The comparison of light reflectance spectra obtained for the RC-4 and the RC-4/CdSe-QDs@S-DAB 5 films presented in Figure 10b also shows similar dependence, but an average reflectance % reduction in the NIR region of~5% due to QDs inclusion. However, the type of dendrimer seems to affect light refection since a more significant reduction (around 20% for the whole range of wavelength) was obtained for the RC-4/CdSe-QDs@S-DAB 3 film.

Conclusions
A photoluminescence film was obtained by the inclusion of CdSe QDs covered by thiolate DAB 5 dendrimer nanoparticles (CdSe-QDs@S-DAB 5 ) into a highly swollen and elastic regenerated cellulose (RC) film support by depth-coating into an aqueous solution of these nanoparticles. The nano-engineering RC-4/CdSe-QDs@S-DAB 5 film exhibits modified optical characteristics when compared with the RC-4 support, with a significant increase in the values of the extinction coefficient but only around 4% in the refraction index. Moreover, the influence of the cover-dendrimer generation (S-DAB 2 , S-DAB 3 or S-DAB 5 ) on n and k parameters was also demonstrated, indicating that both the types of QDs and the coating (in case) influence both optical parameters. The RC-4/CdSe-QDs@S-DAB 5 film also shows very high transmittance (>90%), with only small changes at the near-infrared region for both light transmission and reflection (around 3%), when compared with the support RC-4 film, and only small changes associated with the dendrimer generation were obtained.