Purification, Selection, and Partition Coefficient of Highly Oxidized

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Purification, selection and partition coefficient of highly oxidized carbon dots in aqueous two-phase systems based on polymer-salt pairs Edgar Ronny Delgado, Larissa Almeida Alves, Rodrigo Moreira Verly, Leandro Rodrigues de Lemos, and João Paulo de Mesquita Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02361 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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Purification, selection and partition coefficient of highly oxidized carbon dots in aqueous two-phase systems based on polymer-salt pairs

Edgard R. Delgado, Larissa A. Alves, Rodrigo M. Verly, Leandro R. De Lemos, and João P. de Mesquita*

Department of Chemistry, Federal University of the Jequitinhonha and Mucuri Valleys, Rodovia MGT 367 - Km 583, nº 5000, Alto da Jacuba CEP 39100-000, Diamantina-MG, Brazil. Tel: +55 38 3532 1200.

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ABSTRACT In general, the methodologies for the preparation of carbon dots lead to the formation of nanostructures with size and surface chemistry heterogeneity. Since the electronic and optical properties of these nanoparticles are directly associated with these properties, the development of purification and selection strategies is essential. Herein, we report a systematic study of the spontaneous partition and separation of highly oxidized carbon dots prepared by the dehydration and oxidation reactions of cotton cellulose in aqueous two-phase systems (ATPS) based on polymer-salt pairs. The partition of the carbon dots was investigated in different ATPS in which were evaluated the effects of the cations and anions of the salts, molecular mass and nature of the polymer, tie-line length, initial pH and surface modification of the nanoparticles on the partition coefficient (K). The results showed that the best separation occurred with a system consisting of PEO 1500 + lithium sulfate + water using reduced carbon dots with hydrazine. Alternatively, the lowest value of K, 0.79, was obtained for a poly(ethylene oxide) PEO 1500 + sodium tartrate + water system with pH = 6 using oxidized carbon dots. The detailed analyzes of the top and bottom phases of the systems with fluorescence and UV-Vis spectroscopy showed that ATPS are capable, in addition to partitioning, of separating nanoparticles with different optical properties, which are directly associated with the surface properties and particle size. We believe that the presented methodology is an alternative, practical, fast and potentially scalable technique for the separation of carbon nanostructures with different optical properties.

KEYWORDS: Carbon dots, aqueous two-phase systems, partition, optical and electronic properties


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1. INTRODUCTION Since their discovery in 2004,1 carbon dots (CDs) have attracted significant interest because of their optical properties, low toxicity and facile preparation.2 Currently, several methodologies for the preparation of CDs have been developed.3 However, the vast majority of the synthetic routes, with a few exceptions, lead to size and structure non-uniformity or superficial chemical heterogeneity. At first, these small differences between nanoparticles do not appear to be significant, however, studies conducted in the last decade have shown that their interesting electronic properties are complex and are directly related to the size and structure of the surface.1,4,5 In addition, studies have shown the dependence of photoluminescence with the excitation wavelength,6,7 which many authors have attributed to an optical selection of particles with different physicochemical properties.8 Due to their interesting properties and the fact that these materials present enormous potential for replacing traditional metallic semiconductors in various areas with economic, environmental and performance advantages, several studies have reported the use of these nanoparticles in the development of sensors,9,10 catalysis, photocatalysis,11–13 biomarkers14–17 and new devices related to energy conversion, such as light-emitting diodes (LEDs).18,19 While LED development requires a material with well-defined and narrow emission characteristics, a biomarker needs to present a high quantum yield for the use of a lower amount of substances with a high image resolution. This means that the heterogeneity and processability of the as-synthesized materials may limited their implementation in electronics devices and photonics applications. In this context, it is clear that current research regarding CDs must present either new synthetic routes that produce nanostructures with well-defined electronic and optical properties, or develop new


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methodologies of separation, isolation and purification that are more efficient, inexpensive and feasible for large-scale applications. In the literature, several chromatographic methodologies have been used with very promising results.20–24 Lu et al. reported the separation of carbon quantum dots with high performance liquid chromatography using a C18 column and binary eluents of water-acetonitrile or methanolacetonitrile, after several optimization experiments. In their work, carbon quantum dots were prepared using resorcinol as a precursor and it was found that several of the fractions obtained emitted radiation in different regions of the electromagnetic spectrum with quantum yields higher than 70%.21 Ding et al. hydrothermally prepared CDs using phenylenediamine and urea as precursors. The suspension obtained after 10 h of reaction was passed through a silica chromatographic column and the authors were able to isolate different fractions of CDs with different photoluminescent (PL) properties. According to the authors, the separation process occurs due to the different degrees of oxidation of the nanostructures, which are responsible for the different optical properties of the CDs.22 However, its large-scale use can be complicated and in many cases, organic solvents are required, which are no longer economically sustainable. Aqueous two-phase systems (ATPS) offer an excellent alternative for the separation, isolation and purification of CDs, since they are environmentally safe, economically viable, quick and scalable for large-scale applications.25 ATPS can be obtained from the mixture of aqueous solutions of two polymers, a polymer and an electrolyte, or between two salts,26 which under specific thermodynamic conditions (temperature, pressure and concentration), at equilibrium, two phases are formed, each enriched in one of the ATPS constituents. In these systems, the major component in each phase is water and its constituents are low cost, meaning these systems fit within the principles of green chemistry. The first report of ATPS was made in 1896,27 and


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since the 1950s, this technique has been applied to the separation of several strategic analytes, such as bioparticles,28–30 dyes,25 metals31,32 and others.33 It has recently been shown that the presence of nanoparticles in ATPS can have a significant effect on the purification/partitioning of biomolecules.34 The possibility of fine modulation of the properties of the ATPS phases, which can be obtained by changing the concentration or nature of their constituents, opens a significant field of study on the partitioning of nanoparticles in these systems. However, although it is an interesting platform for separation of these analytes, the number of partition studies of nanoparticles in ATPS is still limited. Among the few published studies are the partitions of Ag and Au nanoparticles.35 More recently, studies have reported the partitioning of carbon nanotubes in these systems with separation yields over 70% with a high efficiency with respect to the structural characteristics of the nanoparticles.36–39 In these studies, the partition of the nanoparticles was performed on ATPS formed by two polymers. We believe that the polymer-polymer systems used for the partitioning of carbon nanotubes have been correctly chosen, since the differences in chemical environments in these systems are smaller and less hydrophilic when compared to polymer-salt systems, as well as the hydrophobic/hydrophilic properties of carbon nanostructures, which are predominantly formed by a network of aromatic rings. In this work, we chose ATPS formed by a polymer and an electrolyte (salt) because of the highly hydrophilic characteristics of the CDs obtained after oxidation with HNO3.40 Normally, the ATPS formed by polymers and electrolytes present more applications due to the significant difference between the chemical environments of the top phase (TP) and bottom phase (BP), and the greater difference between the physicochemical properties of the phases. Thus, the separation


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processes are more efficient in these systems.41 To the best of our knowledge, this is the first work that uses these partitioning systems for the optical selection of CDs. We have completed an extensive study of the partitioning/separation of CDs obtained from the dehydration/oxidation reaction of cotton cellulose carried out in ATPS prepared with aqueous solutions of polymer, alkaline and alkaline earth metal salts. We have also evaluated the effect of the parameters that influence the partitioning of analytes in ATPS and, in addition, the surface of the nanostructures was reduced with hydrazine and its effect on the partition coefficient evaluated.

2. MATERIALS AND METHODS 2.1. Preparation of oxidized carbon dots (OCD) The OCD were prepared from the dehydration and oxidation reactions of cotton cellulose.40 First, 5 g of cotton cellulose (Apolo) was dehydrated in 20 ml of H2SO4 (Isofar, Brazil) at 80 °C for the production of a carbonaceous material. The reaction was interrupted by adding 100 ml of water. Subsequently, the carbon material obtained was washed and re-dispersed in HNO3 (2 M) (Synth, Brazil), and the resulting mixture was refluxed for 12 h for total oxidation of the carbon material. Finally, the suspension was neutralized with Na2CO3 (Dinâmica, Brazil) and dialyzed (Sigma membrane).

2.2. Preparation of reduced carbon nanostructures with hydrazine (HCD) The reduction reaction with hydrazine was conducted based on the work of Ren et al. with some modifications.42 Here, 0.2 g of hydrazine sulfate was added to a 1 gL-1 OCD solution. Then, the pH was adjusted to ten with ammonium hydroxide and the solution


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was refluxed for 6 h. Finally, the pH of the solution was decreased with HCl and the suspension dialyzed (Sigma membrane) against distilled water.

2.3. Characterization Transmission electron microscopy images of the CDs were taken using a FEI Tecnai G2Spirit with 200 kV acceleration voltage. FTIR spectra were obtained using a Shimadzu spectrometer, model IR Prestige 21 with attenuated total reflectance. Potentiometric titration curves were performed at 25 °C under a nitrogen atmosphere using a SCHOTT automatic titrator (Titroline 7000), as previously reported.43–45 Solid-state

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C NMR (300

MHz) spectra were collected using a Bruker Avance III HD 300 spectrometer equipped with a pneumatic accessory for solid sample analysis. Zeta potentials (ζ-potential) were measured using a Zetasizer Nano-ZS (Malvern Instruments, UK). UV-vis spectroscopy was performed with a Cary 50 UV-vis (Varian) spectrometer. PL emission measurements were performed using a Molecular Devices/SpectraMax.

2.4. Partitioning studies of CDs in ATPS The partitioning studies of CDs in ATPS were evaluating by the partition coefficient (K) (Eq. (1)). The K is a value that expresses how much of each species of interest (CDs) is transferred in the TP and BP in both.

=

(1)

For this, 0.300 g of a CDs solution of 1.08 g L-1 was added to the system formed by polymer (PEO1500 or PEO4000 or L64) + salt (Li2SO4 or Na2SO4 or MgSO4 or Na2C4H4O6 or Na3C6H5O7) + H2O, already prepared in several values of tie-line length (TLL) according to the


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composition of the system data obtained from literature data.46–49 The ATPS were shaken and left to stand in a thermostatic bath at 298.15 K. After a period of 12 h, phase separation occurred and the thermodynamic equilibrium was reached. The top and bottom phases were collected separately and diluted by a factor of 6 and 1.2, respectively, in order to determine the CDs concentration. The absorbance at 280 nm was used to determine the CDs concentration. All analytical signals were obtained with a UV-Vis spectrophotometer (Varian, Cary 50).

3. RESULTS AND DISCUSSION The CDs obtained using this methodology present sizes between 1.5 and 4.0 nm with a quasi-spherical morphology, significant amounts of oxygenated functional groups and strong emission in the green region of the electromagnetic spectrum.40,45,50

3.1. Partitioning coefficient and purification of CDs in ATPS The purity and heterogeneity of the as-synthesized carbon nanoparticles have limited applications in electronics, photonics devices and bioimaging. To rapidly and simply obtain nanoparticles with homogeneous electronic properties, we use ATPS based on polymer-salt pairs. However, the partitioning of the analytes in these systems is highly complex and is influenced by different factors, such as hydrophobic interactions, surface charges, electrical potential differences between phases, viscosity and density differences, the nature of system constituents and the characteristics of the molecule or nanoparticles to be partitioned.51 Here, the effects of all the variables that interfere in the phase transfer process in ATPS, i.e., the cation and anion of the salt, the hydrophobicity of the macromolecule, the TLL and the pH, were evaluated in order to understand the behavior of the carbon nanostructures in the different systems. Thus,


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we aimed to find the best polymer-salt pair, not only the one with the largest partition coefficient, but also the system that selects CDs by size or surface properties. First, the effects of three ATPS-forming polymeric structures on the partition coefficient of the OCD in systems prepared with lithium sulfate were evaluated. L64 ((poly(ethylene oxide))13−(poly(propylene oxide))30−(poly(ethylene oxide))13) is a copolymer with a central hydrophobic block of poly(propylene oxide) and two hydrophilic blocks at the ends composed of poly(ethylene oxide) that form micelles in aqueous solution. PEO1500 and PEO4000 are linear polymers formed by poly(ethylene oxide) with molar mass esof 1500 and 4000 g mol-1, respectively. The images of the three systems (Figure S1) clearly show the TP of the PEO1500/Li2SO4 system enriched with the OCD and a similar coloration for the two phases in the L64/Li2SO4 system. The partition coefficients of the nanoparticles were determined and the results are shown in Figure 1. Except for the ATPS formed by L64, the partition coefficients were greater than one, showing that the OCD are predominantly concentrated in the polymer phase. The order observed for the K values was PEO1500 > PEO4000 > L64, suggesting that OCD interact better in less hydrophobic environments. Although the OCD has a core composed primarily of graphene sheets, which are typically more hydrophobic, the edges of the basal planes of these nanostructures are composed of significant amounts of highly hydrophilic oxygenated functional groups. In a study of the partitioning of malachite green dye, which has many hydrophilic functional groups, the authors obtained K values for PEO1500, PEO4000 and PEO6000 of 4.10 × 104, 3.26 × 104 and 1.97 × 104, respectively.52 Upon evaluating the hydrophobicity order of the polymers in this work, the results are clearly similar to those obtained previously.


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Figure 1. Partition coefficient of the OCD sample in the ATPS prepared with PEO1500, PEO4000 or L64 + Li2SO4 + H2O.

The effect of the cations and anions on the partition coefficient was evaluated, as shown in Figure 2.

Figure 2. Effect of the anions (a) and cations (b) on the partition coefficient of the OCD sample in the ATPS formed by PEO1500 + salts + H2O.


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According to Figure 2(a), there is a preferential partitioning of the nanoparticles for the TP in the ATPS formed by SO42-. Alternatively, they are concentrated in the BP of the system prepared with C4H4O62-. Unlike the organic anions, C4H4O62- and C6H5O73-, which may interact more strongly with the analyte, for example, through hydrogen bonding, retaining it in the BP, the sulfate anion favors a greater transfer of the OCD to the TP due to a smaller interaction with the nanoparticles. As shown in Figure 2(a), an increment of the TLL causes an increase in the value of K for the sulfate, whereas this behavior is the opposite for the tartrate. The TLL is a thermodynamic parameter, which at constant pressure and temperature expresses the difference in intensive thermodynamic functions between the TP and BP. The increase in the TLL causes a higher segregation between the polymer and salt between the phases, promoting the growth of the salt mass fraction in the BP and a decrease in the TP. For ATPS prepared with tartrate, the analyte has a higher affinity for the BP. Thus, with increasing TLL, the tartrate concentration increases in the BP, enhancing the number of molecules to interact with the carbon nanostructures, further decreasing the values of K. For the sulfate, the behavior is reversed, because in this system the CDs have a higher affinity for the TP. The K for the ATPS formed by citrate is not affected by the TLL, since the analyte does not present a preferential partition for one of the phases (K ≈ 1). The salting-out effect on the CD partition in sulfate ATPS is also important, since high concentrations of salts can decrease the nanoparticle solubility due to the high number of salt ions competing for water molecules, thereby increasing the transfer of the analyte to the TP.53 The effect of the cation on the analyte partition behavior in ATPS is discussed based on the model proposed by da Silva and Loh.54 According to this theory, the cations forming the ATPS can interact with the ethylene oxide segment of the macromolecule. Through calorimetric


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measurements, they observed that the Li+ ion interacts more strongly with the polyethylene chains compared to others cations. Thus, the macromolecule interacting with Li+ is more charged, making the ATPS formed by it more hydrophilic and positively charged. As oxidized CDs have a predominantly negative surface at pH 6, they interact more strongly with the more hydrophilic TP and positively charged with lithium sulfate (Figure 2(b)). So far, our discussion has focused on the substances forming the ATPS. However, the modification of the surface characteristics of the nanoparticles, either by altering the pH of the medium or by eliminating functional groups, should have a great impact on the partition coefficient of the CDs. The pH is an important variable because the carbon nanoparticles used in this work have a significant amount of weak Brønsted acids with different ionization constants (see supplementary information). Therefore, at different pH values, these groups may be charged or not.

Figure 3. Effect of pH on the partition coefficient of the OCD sample in the ATPS formed by PEO1500 + Li2SO4 + H2O.


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As can be seen in Figure 3, the decrease in pH significantly increases the affinity of the analyte by the TP. For a TLL of 35, the K increases from ~5, at pH 6, to a value close to 15, at pH 3. If, at pH 3, most acid functional groups are protonated, the carbon dots are relatively less hydrophilic compared to nanoparticles at pH 9. In this condition, the surface acids are almost totally discharged, and relatively more hydrophilic. As a consequence, at acidic pH values, the affinity of the nanoparticles by the TP increases significantly, which is less hydrophilic. Structural modification on the surface of the carbon nanoparticles was carried out after the typical reduction reaction with hydrazine sulfate in an alkaline medium. The FTIR and 13C NMR spectra of carbon nanoparticles before and after the reduction reaction are shown in Figures S2 and S3, respectively. Both analyses reveal remarkable changes in the functional groups present on the OCD and HCD nanoparticle surfaces. Notably, after the reaction with hydrazine sulfate, the band located at 1713 cm-1 (νC=O stretching) disappears, whereas an intense decrease of the relative absorption of the band centered at 1100 cm-1 (νC-O stretching) occurs. In addition, an abrupt narrowing in the profile of the band located at around 3214 cm-1 (νO-H) is observed in the HCD FTIR spectrum in comparison with the OCD. These results suggest the elimination of carbonyl, carboxylic and other functional groups with the C-O bond of the CD structure. In addition, there is a reversal of the relative intensities of the bands centered at 1575 and 1365 cm1

, which may be associated with the presence of the C=N bonds of hydrazones (Figure S2). The

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C NMR spectra show the typical signals for oxygenated functional groups of carbon

materials.55,56 The most intense signals in the OCD sample are associated with the presence of sp2 carbon (peak at 127 ppm) and carbonyl and carboxylic groups (peaks at 150 and 174 ppm, respectively). Nevertheless, a significant decrease in the intensity of peaks corresponding to the C=O and C-O groups in relation to the sp2 carbon peak is observed after the reduction reaction,


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reasserting the elimination of some oxygenated functional groups. The quantitative information of the acidic functional groups was obtained from the experimental data adjustments of the potentiometric titration curves (Figure S4). The results are shown in Table S1. While the OCD sample shows a total of 2.58 mmolg-1 of acidic functional groups with a carboxylic acid characteristic pKa, the HCD sample has 1.17 mmolg-1. In addition, the reduced sample has an amount of functional groups with pKa > 9, almost three times greater than the oxidized sample. This increase is attributed mainly to the presence of -NH2 groups of hydrazones, introduced on the surface of nanomaterials. In general, the results of the characterizations are in agreement with the studies carried out by other authors,57,58 that is, the elimination of epoxy, carbonyl and some carboxylic groups. The CDs structures idealized before and after the reaction of surface modifications with hydrazine sulfate are shown in Figure 4.

Figure 4. Idealized surface modification for CDs: before (OCD) and after reactions with hydrazine sulfate (HCD).

The effect of the surface modification of the CDs on its photoluminescent properties are shown in Figure S5. The investigation showed a blue shift of the emission maximum and a small narrowing of the emission band after the reduction reaction. Biroju et al. have shown that green emissions are related to COOH and CO groups, while blue emissions are associated to the sp2/sp3


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domains and functional epoxy groups.59 Alternatively, Tian et al. reported that the oxygenated functional groups deform the bands markedly, promoting their widening, making it difficult for electron-hole recombination and emission energy reduction.60 In addition, it has been observed that the emission of both samples presents a strong dependence on the wavelength of excitation, due to the optical selection differently sized nanoparticles and/or different emissive traps (functional groups) on the CDs surface.2 Once the surface modification of the nanoparticles was confirmed, i.e., the elimination and substitution of some oxygenated functional groups, we conducted the partition studies of the different nanostructures in aqueous systems prepared with PEO1500/Li2SO4 at pH 6 (Figure 5). As expected, reduced nanoparticles have a higher partition coefficient due to the increased hydrophobic character, caused by the elimination of some highly hydrophilic oxygenated functional groups. As a consequence, there is an increase in their affinity for the top phase of the ATPS, which is known to be less hydrophilic.

Figure 5. Effect of the surface modification of CDs on partition behavior in the ATPS formed by PEO1500 + Li2SO4 + H2O at pH 6.


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Overall, the partition coefficient results suggest that CDs exhibit a higher affinity for the TP, mainly in the presence of more hydrophilic polymers. However, the versatility provided by the different possibilities for surface modification allows the tuning characteristics of the nanoparticles to a wide variety of ATPS.

3.2. CD selection in ATPS The optical and electronic properties of the CDs are dependent on the characteristics of the surface functional groups and particle size.8 On the one hand, particle size dependent PL is similar to the quantum effect observed for metal semiconductors,4 on the other, our fluorescence results have shown that the elimination and modification of some oxygenated functional groups increases the photoemission energy (blue shift). Thus, a detailed investigation of the fluorescent properties of the BP and TP of the ATPS with the partitioned carbon nanoparticles can provide important information regarding the potential of these systems in the separation/selection of carbon nanoparticles with different optical properties. The emission spectra obtained for the TP and BP of the PEO1500 + salt (Li2SO4 or Na2SO4 or Na3C6H5O7 or Na2C4O6H4) + H2O systems with OCD are shown in Figure S6. Figure 6 shows the maximum emission versus excitation wavelengths of the TP and BP of the evaluated ATPS.


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Figure 6. Wavelength of maximum emission versus excitation wavelength for TP and BP of the ATPS formed by PEO1500 + salt (Li2SO4 (a) or Na2SO4 (b) or Na3C6H5O7 (c) or Na2C4O6H4 (d)).

In the system formed by Li2SO4, the differences in the emission wavelengths of the TP relative to the BP are evident (Figure 6(a)). The emission wavelength of the bottom phase, in general, is approximately 10 nm lower, meaning the nanoparticles concentrated in the BP present photoemissions with higher energies. Alternatively, the maximum TP and BP emission wavelengths of the Na3C6H5O7 system are practically the same (Figure 6(c)). Intermediate results


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were observed for the Na2SO4 and Na2C4O6H4 systems (Figures 6(b) and (d)). The contour plots (Figure S7) clearly show a lower area of emission of the BP for the ATPS prepared with Li2SO4, with a maximum emission centered at 450 nm. The UV-Vis spectra of the top and bottom phases of these systems were also obtained (Figure S8). In general the spectra profiles in all systems are similar with a better defined shoulder at wavelengths less than 300 nm in the top phases. However, for the system prepared with Li2SO4 it is possible to verify a more significant difference between the normalized absorbances of the top and bottom phases at wavelengths lower than 550 nm. All these results are directly related to the partition coefficients observed for these ATPS and suggest that the PEO1500 + Li2SO4 + H2O system presents the appropriate thermodynamic properties to partition and select/purify the highly OCD prepared in this work with different optical properties. Here, the CDs have an amphiphilic structure formed by a hydrophobic core, basically composed of graphene sheets that are functionalized with different oxygenated functional groups that are highly hydrophilic (shell). Thus, the increase in particle size enhances the predominance of graphene sheets (core), thereby decreasing the hydrophilic character (Figure 7). We believe that the smaller particles clearly have a higher polarity and are concentrated in the BP, while the larger, less hydrophilic, particles are predominantly concentrated in the TP, in agreement with the photoemission energies observed (Figure 7(b)).


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(a)

(b)

Figure 7. a) Illustration of decreased hydrophilic character with increasing particle size. b) Schematic representation of the CDs separation process in the aqueous two-phase system based on particle size and polarization.


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To show the effectiveness of the ATPS and the efficiency of the BP to select smaller and more polarized nanoparticles, the hydrazine reduced CDs were added to a ATPS formed by PEO1500 + Li2SO4 + H2O, containing the OCD sample and vice versa (Figure S9). The fluorescence spectra obtained with different excitation wavelengths for the TP and BP of the system containing the OCD + HCD samples are shown in Figure 8.

Figure 8. Fluorescence spectra of the (a) BP and (b) TP of the ATPS PEO1500 + Li2SO4 + H2O containing OCD + HCD at pH 6.

The difference in the profile of the TP and BP spectra of the system containing the oxidized and reduced CDs samples is evident. The profile of the TP curve is much wider as a consequence of the presence of HCD and OCD nanoparticles, which emit at shorter and higher wavelengths, respectively. Alternatively, in addition to being narrower, the emission maxima of the bottom phase spectra are very similar to those obtained for the BP of the ATPS prepared with the presence of the OCD sample (Figure 9). Similar results were obtained for the ATPS containing HCD + OCD (see Figure S10).


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Figure 9. Wavelength of maximum emission versus excitation wavelength for the BP of the ATPS formed by PEO1500 + Li2SO4 + H2O containing OCD and OCD + HCD.

All these results demonstrate the potential of ATPS, especially the ATPS formed by PEO1500 + Li2SO4 + H2O, which has a highly selective chemical environment capable of selecting, purifying or isolating smaller, more polarized carbon nanoparticles with better defined optical properties.

4. CONCLUSIONS The partitioning of CDs was investigated in different ATPS and the effects of the cations and anions of the salts, the polymers and the initial pH of the system in the partition coefficient were evaluated. The results showed that the best separation occurred with the PEO1500 + lithium sulfate system. The lowest partition coefficient value was obtained for the PEO1500 + sodium tartrate system. This study showed that pH has a significant effect on the separation process due to the presence of oxygenated functional groups on the surface of nanoparticles with different


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acidity constants. Alternatively, the surface modification of the nanoparticles significantly alters their partitioning, that is, the elimination of some oxygenated functional groups increases their hydrophobic character and their affinity for the TP of the studied systems, which are known to be less hydrophilic. The detailed analysis of the TP and BP of some of these systems with fluorescence spectroscopy showed that the ATPS are capable, in addition to partitioning, of separating nanoparticles with different optical properties, which are associated directly to the surface properties and particle size. We believe that ATPS are a viable platform for the rapid purification and selection of CDs with well-defined optical properties or as a preparatory step for other methodologies of purification or separation of carbon nanoparticles. Currently, our research group has studied multiphase aqueous systems for a more efficient separation of nanoparticles with different optical properties. In addition, the introduction of CDs into the ATPS as additives can significantly improve the partitioning of biomolecules, as well as metals, by complexation reactions of the functional groups present on their surface. Finally, the results presented here can have a significant impact on future industrial processes of purification/selection of different carbon nanostructures.

■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications ------. Digital image of the CD separation in the ATPS formed by a) PEO1500 + Li2SO4 + H2O, b) PEO4000 + Li2SO4 + H2O and c) L64 + Li2SO4 + H2O. Physico-chemical and optical characterization of the surface modification of CDs with hydrazine sulfate: Potentiometric titration, FTIR,

13

C NMR and fluorescence spectra. Fluorescence and UV-Vis spectra, contour

plots of the excitation-emission fluorescent matrix of the OCD in the top and bottom phases of


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different ATPS. Digital image of the ATPS PEO1500 + Li2SO4 + H2O containing OCD, HCD and mixtures: OCD + HCD and HCD + OCD.

■ AUTHOR INFORMATION Corresponding author *E-mail: [email protected] or [email protected]. Tel./Fax.: 55-38-3532-1200. The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors are grateful to the LMMA and associated projects FAPEMIG (CEX-112-10), SECTES/MG, RQ-MG (CEX-RED-00010-14), (APQ-02790-14) and CNPq (grant no. 478228/2013-9) for the financial support.

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