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Vol. 8. Issue 3.
Pages 2721-2731 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2721-2731 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.008
Open Access
Evaluation of the physical properties of fluorescent carbon nanodots synthesized using Nerium oleander extracts by microwave-assisted synthesis methods
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Sinem Simseka, Melis Ozge Alasb, Belma Ozbeka,
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bozbek@yildiz.edu.tr

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, Rukan Gencb,
Corresponding author
rukangnc@gmail.com

Corresponding authors.
a Department of Chemical Engineering, Faculty of Chemical and Metallurgical Engineering, Yildiz Technical University, 34210 Esenler, Istanbul, Turkey
b Department of Chemical Engineering, Faculty of Engineering, Mersin University, 33343 Yenisehir, Mersin, Turkey
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Table 1. Formulation of reaction media.
Table 2. Hydrodynamic particle size and surface zeta potential data of CDs.
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Abstract

Rapid and one-step green synthesis of carbon nanodots (CDs) from Nerium oleander leaves was accomplished via domestic microwave oven and microwave-assisted hydrothermal synthesizer under various physical conditions. Effects of the synthesizer system, extract type depending on the solvents used in plant extraction, and synthesis conditions; such as reaction time, reaction temperature, surface passivation agent inclusion into the reaction medium, on the physicochemical properties and optical feature of CDs were investigated. The impacts of relevant conditions on CDs feature were determined clearly via UV–visible spectrophotometry, fluorescence spectrophotometry, dynamic light scattering analysis, and Fourier transform infrared spectroscopy. According to the results, while the most effective parameter on the fluorescence feature of CDs was determined as the presence of surface passivation agent (polyethylene glycol, PEG), the alteration in the fluorescence intensity depending on the reaction time and the reaction temperature was also observed. It was reported that the synthesis system and PEG existence in the reaction medium were more effective than the other inspected parameters on the hydrodynamic particle size, in general. Under optimum conditions, nanoparticles with a hydrodynamic size of less than 5nm were obtained. The surface zeta potential charges of all particles were found as negative. While the extract type was significantly effective on the surface zeta potential of CDs synthesized by domestic microwave oven, the reaction temperature was found to be significant for the ones synthesized via microwave-assisted hydrothermal synthesis.

Keywords:
Carbon nanodots
Fluorescent nanoparticles
Nerium oleander
Microwave synthesis
Microwave-assisted hydrothermal synthesis
Surface passivation
Full Text
1Introduction

Carbon nanoparticle studies have been gaining popularity since their accidental exploration in 2004 [1]. Carbon nanodots (CDs) applications in bio-imaging, drug delivery, solar cells, photodetectors and etc. become prevalent because of their unique properties such as optic features, mechanical strength, thermal and electrical conductivity, and etc. [2–11]. Even though CDs can be synthesized by various methods; in the early stage of this scientific invention, laser ablation [12,13], electro-oxidation [14–16] or oxidative acid treatment [17,18] had been used for the synthesis. Fortunately, easier, cheaper, fast and also proper for large-scale production methodologies have been developed as hydrothermal [19–21], microwave/microwave assisted [22–32] and ultrasound [33] synthesis. As the microwave-assisted synthesis of CDs can be executed by using domestic microwave oven (MWO), it can be also performed by microwave-assisted hydrothermal (MAH) synthesis which is the combination of hydrothermal and microwave methods in a high pressure-sealed reaction vessel [24]. By virtue of these novel techniques, utilization of natural products as carbon source became popular. Although nanoparticles arising from natural materials were reported widely for years [34–36], plant-based carbon nanoparticle synthesis has been studying recently [19,37–43].

Nerium oleander (Oleander), an evergreen, toxic shrub plant is one of the members of the Apocynaceae family. Long narrow green leaves are characteristic for this species, and its flowers are commonly white or pink. Nerium oleander has a wide range of chemical constituents such as cardiac glycosides, cardenolides, pregnanes, triterpenes, polyphenols, flavonoid glycosides and etc. [44–49]. For the synthesis of CDs from natural sources, the saccharide content of the reaction media is important due to the utilization of saccharides and their derivatives as carbon sources. Therefore, the chemical composition of natural extracts is also has a crucial significance. As is known, the solvents used in the extraction step can change the chemical contents of the extracts dramatically. For Oleander, while aqueous extracts of their leaves contain sterols, flavonoids, terpenoids and etc., these compounds are not situated in ethanol extracts. But, in contrast, cardiac glycosides and saponins appear in ethanol extracts [50]. Besides, 2.3% of Oleander aqueous leaf extract is crude polysaccharides like arabinose, galacturonic acid, galactose and rhamnose [45]. Therefore, Oleander can be an appropriate candidate for CDs synthesis, and the investigation of the effects of the different solvent extracts of its leaves can give remarkable results.

Even, microwave/microwave assisted methods are used for CDs synthesis, also using these techniques combining with the other synthesis methods for photoluminescence (PL) enhancement of CDs have been reported [51–53]. In this original paper, the effects of the type of microwave synthesis methods (either conventional or hydrothermal) and synthesis conditions on the physicochemical properties of CDs were investigated. The results of the present study showed that the method of microwave-based synthesis reveals a significant change in particle size, surface properties, and PL characteristics.

2Experimental studies2.1Materials and methods2.1.1Plant material

Nerium oleander leaves were collected from Esenler, İstanbul (41°01′37.7″N, 28°53′32.1″E) at 82m in May 2016 was booked in Izmir, Ege University Faculty of Pharmacy Herbarium (IZEF) with number 6056.

2.1.2Chemicals and equipment

Ethanol and polyethylene glycol (PEG) 10000N were purchased from Sigma-Aldrich. Elma, TI-H 5 ultrasonic bath was used during the extraction process. Microwave syntheses were accomplished at Arcelik MD 565 S domestic microwave oven and Mars (CEM) microwave assisted hydrothermal synthesizer. Characterization studies of CDs were performed using Shimadzu UV-1800 UV-Vis spectrophotometer, Agilent Cary Eclipse fluorescence spectrophotometer, Malvern Zeta Sizer Nano ZS, and Perkin Elmer Frontier FT-IR.

2.2Preparation of plant extracts

Oleander fresh leaves were collected from nature and cleaned up by distilled water three times. An oven at 70°C was used to dry the leaves and after two days from the drying period, leaves were ground to enhance their surface areas. Two different solvent extracts of Oleander leaves were prepared in an ultrasonic bath at room temperature for 5h with ultra-pure water and ethanol at the concentration of 12.5g leaves/100mL solvent for each. At the final step of the extraction procedure, extracts were centrifuged at 5000rpm for 15min to remove solid parts and clear extracts were kept at +4°C.

2.3Microwave-assisted synthesis of CDs2.3.1Synthesis using microwave oven

For the synthesis of CDs via domestic microwave oven, a kitchen type microwave oven with 800W output power was used. Ethanol and aqueous extracts of N. oleander were utilized as carbon sources for CDs synthesis, separately. For the synthesis of CDs by using Oleander ethanol extract as the carbon source, 1mL ethanol extract and 1mL ultra-pure water were added to the ceramic crucibles. Similarly, for the synthesis of CDs by using aqueous extract, 1mL aqueous extract and 1mL ethanol were added to the ceramic crucibles. To investigate the effect of surface passivation agent existence in the reaction medium on CDs feature, 1g PEG 10000N was added to the some of the formulations and blended in the ceramic crucibles containing 1mL extract and 1mL ethanol or ultra-pure water until all ingredients completely dissolved (Table 1). According to the extract types and PEG existence in the reaction medium, subscripts E, W and PEG were used after ‘CD’ term for ethanol, water, and polyethylene glycol 10000N, respectively.

Table 1.

Formulation of reaction media.

Carbon nanodots  EtOH extract of Oleander  H2O extract of Oleander  Ethanol  Ultra-pure water  PEG 10000N 
CDE  1mL  –  –  2mL  – 
CDE@PEG  1mL  –  –  2mL  1
CDW  –  1mL  1mL  1mL  – 
CDW@PEG  –  1mL  1mL  1mL  1

For the CDs synthesis via MWO, ceramic crucibles containing reaction medium were placed at MWO with 800W output power for 5, 15 and 40min, respectively. After the synthesis step, each residue in the crucibles was scratch and dissolved in 6mL ultra-pure water and centrifuged for 20min at 6000rpm and 13,500rpm, respectively, to obtain clear solutions. The supernatants were collected and stored at +4°C. For the characterization studies, 50μL of these clear solutions diluted in 3mL ultra-pure water. A brief schematic representation of the experimental part is placed in Fig. 1.

Fig. 1.

Schematic view of the experimental procedure.

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2.3.2Synthesis using microwave-assisted hydrothermal synthesizer

The MAH synthesis of CDs was fulfilled in a Mars (CEM) microwave synthesizer at 200 and 250°C with a temperature ramp change period for 15min and reaction time for 15min. Reaction media for CDs synthesis were prepared as mentioned in Section 2.3.1, and the formulations were transferred to the hydrothermal reactor made up of Teflon. After the synthesis step, each residue was dissolved in 6mL ultra-pure water and centrifuged for 20min at 6000rpm and 13,500rpm, respectively, to obtain clear solutions. The supernatants were collected and stored at +4°C. For the characterization studies, 50μL of these clear solutions diluted in 3mL ultra-pure water.

2.4Characterization studies

Photoluminescence spectroscopy (Agilent Cary Eclipse) was conducted for optical characterization and the UV absorption spectra of CDs were recorded on a UV-spectrophotometer (Shimadzu UV-1800) using a 1cm path length cuvette [54]. Additionally, fluorescence emitted from CDs was visualized with a UV lamp at 365nm wavelength. Particle size distribution and surface zeta potential of CDs synthesized were measured with dynamic light scattering (DLS) technique (Malvern Zetasizer Nano-ZS). A clear disposable zeta cell was used for particle size distribution, and approximately 1mL of CD solution was taken in it. Measurements were made in three replicates each with 12 zeta runs and mean particle size and standard deviations were determined. Approximately 1mL of CD solution was taken in a clear disposable zeta cell in the same way as the size measurement for surface zeta potential measurement. Three rounds of measurements each with 20 zeta runs were performed for analysis with dip cell. The Fourier-transform infrared (FT-IR) spectrum of CDs with a resolution of 4cm−1 was collected over the range of 450–4000cm−1 on an FT-IR spectrometer (Perkin Elmer Frontier FT-IR). All measurements were conducted in ambient atmosphere and room temperature.

3Results and discussion3.1Photoluminescence of CDs

Fluorescence feature of CDs was observed and photographed under 365nm UV light (Fig. 2), and the fluorescence intensity values were investigated at 365nm excitation by fluorescence spectroscopy (Fig. 3). The PL enhancement due to polyethylene glycol as a surface passivation agent was verified with the clear observation of higher fluorescence emission obtained from PEG included formulations of CDs (Fig. 3) [6,55–57]. In MWO based synthesis, the highest PL intensities for CDE@PEG and CDW@PEG was achieved once after 40min of reaction at 800W output power. Furthermore, it was observed that the maximum emission peaks (λm) of these CDE@PEG and CDW@PEG were red-shifted (Figs. 2a and 3b). This red-shifted PL emission of CDs could be attributed to a oxygenated surface revealing narrower energy levels and increased surface defects, thereof [58]. At shorter synthesis periods as 5 and 15min in MWO, no PL was observed from CDs synthesized (Figs. 2a, 3a and b) which shows that the shorter time operation times below 40min, carbonization was not sufficient and contribution of PEG as surface passivation agent was not occurred yet [59].

Fig. 2.

Fluorescence feature of particles under 365nm UV light. (a) Domestic microwave oven and (b) microwave-assisted hydrothermal system.

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Fig. 3.

fluorescence spectra of CDs. (a and b) Domestic microwave synthesis and (c and d) microwave-assisted hydrothermal system (*The intensity peak of CDw in Fig. 1a situates under the peak of CDW@PEG).

(0.58MB).

For CDs synthesized via MAH system, the highest PL intensities were achieved at 250°C after 15min (Figs. 2b and 3d). While the PL values for CDW@PEG at both 200 and 250°C were similar, a dramatic increase was observed at 250°C for CDE@PEG (Fig. 3c and d). This case could be explained by the insufficient carbonization ratio of the contents of ethanol extract of Oleander at this temperature [59]. Eventually, comparision of both microwave-based systems by the final PL intensities of the CDs clearly showed that MWO-based synthesis is a better alternative to generate CDs with enhanced PL.

3.2UV–vis spectra of CDs

Fig. 4 presents the absorbance spectra of carbon dots at certain concentrations (12mg/mL). The peaks observed in the range of 300–400nm can be associated with the n–π* transition of CO [60–62]. The peaks between 240 and 300nm can be related to the π–π* transition of aromatic units [6,63,64]. These peaks at the range of 240–300nm and 300–400nm were observed at all CDs studied. Strong absorption peaks at ∼230nm and ∼290nm were connected with the π–π* transition of the non-bonding electrons [54]. Absorbance increases in CDs synthesized by MWO were observed at CDE@PEG and CDW@PEG after 40min while the same behavior was observed in MAH synthesis. These increases at around 370nm can be associated with the increase of surface energy levels in CDs [65].

Fig. 4.

Absorbance spectra of CDs. (a and b) Domestic microwave synthesis and (c and d) microwave-assisted hydrothermal system.

(0.54MB).
3.3Measurement of hydrodynamic size and surface zeta potential

Hydrodynamic particle size (Rh) and surface zeta potential (ʐ-Pot) data presented by means of changing parameters were shown in Table 2.

Table 2.

Hydrodynamic particle size and surface zeta potential data of CDs.

Method  Sample  Hydrodynamic particle size (Rh)±Std. Dev. (nm)  Average surface zeta potential (ʐ-Pot)±Std. Dev. (mV)  Conductivity (mS/cm)  Mob (μmcm/Vs) 
Synthesis via domestic microwaveCDE Synt. period: 5min  64.77±9.735  −28.3±5.8  0.0112  −2.217 
CDE@PEG Synt. period: 5min  108.12±5.603  −12.3±0.473  0.0268  −0.962 
CDW Synt. period: 5min  15.46±2.253  −20.6±6.09  0.0256  −1.611 
CDW@PEG Synt. period: 5min  2.27±0.226  −14±3.06  0.0375  −1.099 
CDE@PEG Synt. period: 15min  172.28±7.651  −16.1±1.153  0.0234  −1.2637 
CDW@PEG Synt. period: 15min  2.66±0.227  −22.37±1.358  0.0338  −1.752 
CDE@PEG Synt. period: 40min  47.4±3.39  −11.4±3.2  0.0023  −0.8936 
CDW@PEG Synt. period: 40min  3.38±0.338  −16.7±4.63  0.0342  −1.307 
Synthesis via microwave-assisted hydrothermal synthesizerCDE Synt. temp.: 200°C  288.41±29.637  −11±2.32  0.0864  −0.8656 
CDE@PEG Synt. temp.: 200°C  5.7±0.587  −13.4±1.04  0.0622  −1.053 
CDW Synt. temp.: 200°C  615.99±47.218  −14.8±3.04  0.101  −1.163 
CDW@PEG Synt. temp.: 200°C  2.8±0.256  −14.63±3.5  0.0909  −1.146 
CDE Synt. temp.: 250°C  370.45±45.117  −8.9±1.598  0.033  −0.698 
CDE@PEG Synt. temp.: 250°C  2.66±0.25  −9.67±0.749  0.0984  −0.7567 
CDW Synt. temp.: 250°C  825.01±72.125  −13.87±1.168  0.0466  −1.087 
CDW@PEG Synt. temp.: 250°C  4.9±0.589  −8.48±2.185  0.0875  −0.664 

For CDs synthesized via MWO method, Rh of most of the particles was reported as smaller than 100nm while aqueous extract based CDs were stated as the smallest ones with 2.27±0.226nm (Fig. 5a). This case may be related to the chemical composition of aqueous extract. In which extract expected to be richer in water-soluble polysaccharide and/or its derivatives as compared to the ethanol extract due to polarity difference of the both solvents. Besides, presence of polysaccharides may enhance the surface passivation and may induce the formation of smaller particles.

Fig. 5.

Hydrodynamic size of CDs in nanometer. (a) Domestic microwave synthesis and (b) microwave-assisted hydrothermal system.

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As for CDs synthesized via MAH method, Rh of the particles were reported in the range of 2.66–825nm (Fig. 5b). When the data evaluated, the impact of PEG existence in the reaction medium was clearly observed. CDs synthesized from PEG included formulations were found as extremely smaller than PEG-free formulations (Fig. 5b). The effect of surface passivation agent on the size of particles was found compatible with the previous report performed by Sonthanasamy et al. [55]. However, as can be seen from Fig. 5b, no significant relationship between synthesis temperature and size was determined.

ʐ-Pot measurements showed that all CDs synthesized had negatively charged surface regardless of the operation parameters evaluated and changed in the range of −8.9 and −28.3mV (Table 2). There is no clear relation between surface potential and synthesis conditions for MWO synthesis of CDs. The only noticeable difference between surface potential data is PEG included CDs synthesized from aqueous extract showing more negative charges than the ethanol extract based ones which also supports the narrow size distribution of those CDs. When the passivation agent was not included, a more negative surface net charge was obtained due to a more acidic COOH rich particle surface (Fig. 6a). For MAH synthesis of CDs, a recognizable decrease on the negativity of the particles’ surface potential was observed with ascending synthesis temperature as from −11 to −8.9, −14.8 to −13.9, −13.4 to −9.7 and −14.6 to −8.5mV for CDE, CDW, CDE@PEG, and CDW@PEG, respectively (Fig. 6b).

Fig. 6.

Surface zeta potential values of CDs in mV. (a) Domestic microwave synthesis and (b) microwave-assisted hydrothermal system.

(0.24MB).
3.4Surface properties by FT-IR

The changes in the surface functional groups depending on the reaction conditions were investigated by FT-IR spectrometer. For CDs synthesized by MWO, while the general profiles of FT-IR spectra were quite similar to each other, only CDs synthesized for 5min showed different profiles (Fig. 7). While for CDE@PEG and CDW@PEG with 5min synthesis period in MWO, OH/NH stretching peaks were identified at ∼3660cm−1, these peaks were disappeared with the increased operation times (Fig. 7b). The disappearance of free OH peak around 3600cm−1 for CDs with 15 and 40min synthesis period in MWO, can be explained by decreased number of OH groups by domination of other functional groups instead. Moreover, broad H bonded OH stretching vibrations were specified at around 3260cm−1 for CDE@PEG and CDW@PEG synthesized via MWO for 15 and 40min, and these peaks may be attributed to intermolecular hydrogen bonding in CDs (Fig. 7b) [2,39,66–68]. Aliphatic CH stretching bands between 2880 and 2980cm−1 were observed for most of the CDs [2,66,69]. Nevertheless, the peaks at around 1383 and 1370cm−1 were only observed at CDs with 5min synthesis period in MWO, and these peaks were attributed to CH2 bending vibrations [69]. Unlike CDs obtained via MAH synthesis, sharp peaks observed at ∼1740cm−1 and ∼1630cm−1 were indicated with CO stretching and CC bending, respectively, for all of the CDs synthesized by MWO (Fig. 7a and b) [2,39,66,68–70]. While strong CO stretching at around 1740cm−1 can be observed in CDs with 5min synthesis period, this peak disappeared with the ascending synthesis period and CC bendings were observed in place of this peak. Additionally, CC bending at 1630cm−1 may be associated with the aromatic structures in CDs by combining the UV–vis data which was indicated with the π–π* transition at around 260nm (Figs. 4 and 7b) for CDE@PEG and CDW@PEG synthesized via MWO for 15 and 40min [39,71]. Also, the stretching vibrations at between 3000 and 3020cm−1 were related to the aromatic CH stretching bands [2,66].

Fig. 7.

FT-IR spectra of CDs. (a and b) Domestic microwave synthesis and (c and d) microwave-assisted hydrothermal system.

(0.85MB).

For CDs synthesized via MAH, OH/NH stretching peak was identified at ∼3660cm−1 for only CDW@PEG synthesized at 250°C (Fig. 7d) [2,39,66–68]. Similarly, broad H bonded OH stretching vibrations at around 3260cm−1 which were attributed to intermolecular hydrogen bonding in CDs were observed for only CDW with 250°C synthesis temperature (Fig. 7d) [2,39,66–68]. As in CDs synthesized via MWO, aliphatic CH stretching bands between 2880 and 2980cm−1 were observed for most of CDs synthesized by MAH synthesizer (Fig. 7c and d) [2,66,69]. Weak CH2 bending vibrations were identified at around 1383 and 1370cm−1 for CDE and CDW@PEG with synthesis temperature 250 and 200°C, respectively [69]. Weak stretching bands observed for CDs synthesized at 250°C at around 1040, 1160 and 1250cm−1 were attributed to CO vibrations (Fig. 7d). Also, vibrations at ∼1040cm−1 may be related with CO stretch of carboxylic (COOH) group [2,39,66,69,71]. As for the stretching vibrations at 957cm−1, they may be associated with COH stretchings in epoxy groups (Fig. 7c and d) [66]. These results showed that in both systems the final surface functional groups were affected by the time-period of energy given to the system and in each system resulting functional groups differentiated depending on the extract type used.

4Conclusions

Herein, the application of two microwave based methods for one-step synthesis of CDs were demonstrated; and the impacts of the system, extract type, reaction medium, and reaction conditions were determined clearly. The main findings were summarized as follows:

  • Reaction period was found as critically significant on the fluorescence feature of CDs synthesized using MWO. Both CDE@PEG and CDW@PEG, the optimum time for synthesis process was found to be 40min which resulted in almost 10 times higher PL values.

  • For CDE@PEG synthesized via MWO, the hydrodynamic sizes of the particles were varied from 108nm to 47nm for 5 and 40min, respectively.

  • The dramatic effect of the surface passivation agent on the particle size was reported for MAH synthesis while the hydrodynamic size range of CDE and CDW was between 288 and 825nm, this range decreased to 2.6 and 5.7nm for CDE@PEG and CDW@PEG.

  • For MAH synthesis of CDs, a recognizable decrease on the negativity of the particles’ surface potential was observed with ascending synthesis temperature as from −11 to −8.9, −14.8 to −13.9, −13.4 to −9.7 and −14.6 to −8.5mV for CDE, CDW, CDE@PEG, and CDW@PEG, respectively.

  • Although, the chemistry and fluorescence features of CDs synthesized by MWO were similar in general for both solvent extracts of Oleander, the hydrodynamic particle size and the surface potential of CDs were varied depending on the extract types due to variations of the extract contents.

  • When two microwave-based synthesis methods were compared, MWO synthesis was determined as a better alternative to generating photoluminescence enhancement on CDs. However, the opportunity of CDs synthesis at really short reaction times via MAH synthesis is undeniable.

The results of the present study add piece of new insight to the PL mechanism of CDs prepared from two different extracts (aqueous and ethanol) of N. oleander and provide an effective strategy for preparation CDs with two microwave based synthesis routes for their further use in different applications including, catalysis, drug delivery and imaging.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

Sinem Simsek gratefully acknowledges TUBITAK (The Scientific and Technological Research Council of Turkey), and Melis O. Alas thanks the Council of Higher Education of Turkey for the post-graduate scholarship.

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