|Marpongahtun||1. Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Kampus USU, Padang Bulan, 20215, Sumatera Utara, Indonesia. 2. Cellulosic|
|Andriayani||1. Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Kampus USU, Padang Bulan, 20215, Sumatera Utara, Indonesia. 2. Cellulosic|
|Yugia Muis||Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Kampus USU, Padang Bulan, 20215, Sumatera Utara, Indonesia|
|Saharman Gea||1. Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Kampus USU, Padang Bulan, 20215, Sumatera Utara, Indonesia. 2. Cellulosic|
|Suci Aisyah Amaturrahim|
|Amru Daulay||Postgraduate School, Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Jl Bioteknologi No.1, Medan, 20155, Indonesia|
The synthesis of modified carbon dots (N-CDs) from nanocrystalline
cellulose, as the carbon source, with the combination of urea and
ethylenediamine, as nitrogen dopant agents, was successfully carried out by
pyrolysis at 300°C. The N-CDs dispersed in both ethanol and distilled water
generated bright blue fluorescent color under a UV lamp at 365 nm wavelength
with a 29% quantum yield value. FTIR analysis confirmed that the surfaces of
carbon dots were modified by amine and N-doped groups on the carbon ring
structure and the data from the UV-VIS spectrum also showed that assumption.
Produced N-CDs had a size distribution of 2-5 nm with an average diameter of
around 3.4 nm. The ability of N-CDs as a detector was explored from the
fluorescence quenching by Hg2+ ions, in which it reached 40%. The
determination of Hg2+ could be completed in 10 min with a wide
linearity range from 0-100 µM and a detection limit of 59 µM by the static
Carbon dots; Metal ion sensor; Nanocrystalline cellulose; N-dopant agent; Static quenching
Mercury (II) ion (Hg2+) is one of the heavy metals commonly utilized in industrial processes related to pesticides, catalysts, batteries, and gold mining. Due to its high toxicity, the presence of this metal has caused serious environmental pollution that badly affected human health (Liu et al., 2013). Instrumental methods, such as atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), are generally more preferred to detect metal ions. They have high selectivity and sensitivity, even at a very low or trace concentration (Zhou et al., 2019). However, these techniques have some drawbacks, where their analysis requires skillful operators, is time-consuming and high-cost, and with equipment that is difficult to carry (Ju et al., 2011). Meanwhile, fluorescent chemosensors are promising techniques, providing qualitative and quantitative and quantitative metal ion detection that are cost-efficient with high selectivity and sensitivity, simple sample preparation, and no requirement of reference solutions, real-time monitoring, and fast response time (Kaur, Kaur, and Kumar, 2018; Ju et al., 2011).
Carbon dots (CDs), one of the carbon materials with attractive optical properties and sizes smaller than 10 nm, have drawn great attention into nanotechnology owing to their wide range of applications in chemical biosensors, biomedical applications – including biomedical imaging, drug delivery, clinical diagnosis, and optoelectronics (Kusrini et al.,2020; Devi et al., 2018; Wang et al., 2017). Quantum dots were studied to be used as fluorescent labels to substitute toxic synthetic dyes in biomedical imaging and it turned out that they were more stable than organic dyes (Lim et al., 2021; Goreham et al., 2019). The attractive optical properties, such as adjustable photoluminescence and multiple emission with dependent excitation, were obtained from the effect of quantum confinement or the existence of conjugated ?-domain (Zu et al., 2017). CDs are superior to the other conventional fluorescent detectors with their excess photobleaching resistance, neglectable toxicity, chemical inertness, good biocompatibility, inexpensive charge, water-soluble, and facile synthesis (Zuo et al., 2015). They can be used as detectors based on their interaction with the substances and confirmed by the decreasing fluorescence intensity, also known as the quenching effect. High quenching effect shows that CDs have excellent detection capability of various metal ions, such as Fe(III) (Cheng, Xing, and Wu 2019), Cu(II) (Liu et al., 2017), Pb(II) (Wang et al., 2015), Cr(VI) (Zhang et al., 2017), As(III) (Gupta et al., 2016), Ag(I) (Ahmed, Kumar, and Veerappan, 2016), and Hg(II) (Yan et al., 2016). The sensing mechanism based on fluor escence quenching consists of static and dynamic types. Static quenching occurs when a non-fluorescent complex is formed from the interaction between the surface groups in carbon dots, such as –COOH, –OH, and –NH2, and metal ions as quenchers (Sagbas and Sahiner, 2019), while dynamic quenching is dependent on the diffusive collision of excited fluorescence molecule with the quencher (Wu et al., 2017).
The carbon sources for CDs have a great influence on their quantum yields and optical properties. Therefore, screening carbon sources with non-toxic characteristics, stability in composition, and extensively available is still the center of focus in the preparation of CDs. Cellulose is the most abundant biopolymer and accounts for fifty percent of the natural biomass. Cellulose consists of several hundred of 1,4-anhydrous-D-glucopyranose units arrange into a long linear chain (Trache et al., 2017). Nanocrystal cellulose is a nanosized crystalline region in cellulose fibers obtained by acid hydrolysis. It has many advantageous properties, such as good biocompatibility, large surface area, unique morphology, and stable chemical properties. Moreover, it is renewable, environment-friendly, and non-toxic (Rochardjo et al., 2021; Souza et al., 2016). Furthermore, the molecules contain a lot of hydroxyl and ether groups, which could provide the elemental and basic structure in the formation of CDs.
The pyrolysis method is commonly used in CDs synthesis. This method is advantageous due to the directness, repeatability, and practicality of producing CDs (Wang et al., 2017). However, most CDs prepared via biomass pyrolysis method usually have low fluorescence quantum yields, hence limiting their utilization. Souza et al. (2016) synthesized CDs from nanocrystal cellulose by pyrolytic process at 300-600°C and produced a 1.64% quantum yield (Souza et al., 2016). Fluorescence quantum yield is greatly associated with the surface condition, especially with the presence of functional groups or heteroatom-doping that contain of O, N, and S elements. Nitrogen atoms can generate a new surface state (N-state). The electrons trapped in the N-state can produce a high yield of radiative recombination and depress non-radiative recombination. Therefore, improving the quantum yield of carbon dots can be achieved by modifying the surfaces. The preferred choices of dopants to improve QY in CDs in the respective order are as follows: primary amine > secondary amine (while tertiary amine is hardly applied to produce CDs) and diamine > monoamine (Zhai et al., 2012). Shen et al. (2016) improved the quantum yield of cellulose by up to 21.7% by using urea as the N-doped source via the hydrothermal method (Shen et al., 2016).
In this study, modified CDs with high quantum yield, low toxicity, good water-solubility and high photostability were prepared by the pyrolysis of nanocrystal cellulose as the carbon source, with the combination of urea and ethylenediamine as modified agents. Nanocrystal cellulose was isolated from the fibers of oil palm empty fruit bunches as an approach to utilize the waste from PT. Perkebunan Nusantara IV, Indonesia. The selectivity and sensitivity of Nitrogen-doped carbon dots (N-CDs) in various metal ions were determined. Produced carbon dots were applied as fluorescent probes to detect Hg2+ metal ions in water.
Ethylenediamine, ethanol absolute, urea, standard solution of Cd(NO3)2, Pb(NO3)2, Cu(NO3)2, Cr(NO3)2, Mg(NO3)2, Fe(NO3)2, Mn(NO3)2, Ni(NO3)2, Co(NO3)2, Hg(NO3)2, Zn(NO3)2, NaNO3, and H3AsO4 were purchased from Merck, Germany. Oil palm empty fruit bunches (OPEFB) were collected from the oil palm plantation developed by Adolina PT. Perkebunan Nusantara IV, Indonesia. Dialysis membrane was obtained from Biodesign Inc., New York. All chemical reagents were pro-analysis grade, and they were directly used untreated. Deionized water was used throughout the entire experiment.
2.2. Carbon dots synthesis
UV-VIS absorption spectrum was analyzed by using Jenway 7305 spectrophotometers, and Photoluminescence PL Matrixes (EEM) were obtained from Duetta Horiba Fluorescence and Absorbance Spectrometer with Excitation Emission method. The morphology and mean diameter of N-CDs were characterized by using H9500 high-resolution TEM (HRTEM)-EDX operating at 200 kV. Fourier transform infrared spectroscopy (FTIR) was measured at wavenumbers ranging from 500-4000 cm-1 using a Shimadzu IR Prestige-21 spectrometer with a KBR disc. The fluorescent lifetime was analyzed by using Horiba FluoroMax Hybrid Fluorescence Spectroscopy steady State and Life Time System.
Figure 1 Schematic illustration of N-CDs preparation and formation mechanism.
2.4. Quantum yield measurement
The quantum yield of N-CDs was calculated based on the literature (Zhang and Chen, 2013) by using the following equation (1):
In brief, quinine sulfate (0.5 M H2SO4 as solvent) with QY of 54%, was chosen as a reference standard. The subscripts u and s indicated the values for N-CDs and quinine sulfate respectively, ? is the QY, Y is the integrated area of the fluorescence emission peak, A is the absorbance at 370 nm, and ? is the refractive index (s= 1.369, u =1.332).
2.5. Metal ion detection
The selectivity detection of Hg2+ was performed based on N-CDs fluorescence quenching with various metal ions. N-CDs dispersion (1µg/mL) was mixed with 100 µM concentration of each metal ion, such as Cd2+, Pb2+, Cu2+, Cr2+, Mg2+, Fe2+, Mn2+, Ni2+, Co2+, Hg2+, and Zn2+ in an acidic solution. The sensitivity of Hg2+ ion detection was investigated by the addition N-CDs solution into different concentrations of Hg2+ ranging from 0.1µM to 100 µM. All the experiments were carried out by 1:1 solution ratio (not only N-CDs: various metal ions, but also N-CDs: various concentrations of Hg2+), and each mixture was gently stirred for 15 minutes at room temperature before the fluorescence measurement was recorded at 370 nm.
3.1. Characterization of N-CDs
modification by nitrogen-containing functional groups in carbon dots was
confirmed by FTIR spectra. Figure 2 shows the FTIR spectra of nanocrystalline
cellulose and N-CDs. Nanocrystalline cellulose (NCC) had a broad band at 3387
cm-1 that could be assigned to O-H stretching vibrations and the
peak at 2916 cm-1 corresponded to C-H stretching vibrations. These
were aligned with the previous study (Lu and Hsieh, 2010). The interaction between the hydrogen bond and absorbed
water was detected at 1635 cm-1, and the band at 1319 cm-1
was related to the presence of the crystalline structure of cellulose (Yun-Fei, Maimaiti, and Bo, 2017). The absorption peaks at 1210 cm-1 and 1056 cm-1
were associated with C-O-C stretching of aromatic ether linkages, the peak at
1056 cm-1 exhibited -CH2- stretching vibration band, and
the peak at 894 cm-1 indicated ?-glycosidic bond that connected
glucose monomers in cellulose (Shanmugarajah et al., 2015).
The absorption peaks of N-CDs at 3370 cm-1, 1680 cm-1, and 1635 cm-1 corresponded to –OH/–NH, C=O and C=C stretching vibrations in respective orders (Nie et al., 2014; Hsu and Chang, 2012). The presence of nitrogen-containing functional groups in CDs was proven by the appearance of specific peaks at 1404 cm-1 and 1270 cm-1, which were assigned to typical stretching modes of C-N-C heterocyclic and C-N bonds according to the previous research (Wu et al., 2017). These results confirmed that nitrogen-containing functional groups were formed during the pyrolysis process with the combination of ethylenediamine, urea, and NCC. Nitrogen species were efficiently doped into the framework of CDs.
Figure 2 FTIR spectra of NCC and N-CDs
Figure 3 (a) HRTEM of N-CDs (b) Magnification image of HRTEM (c) Histogram of particle size distribution
3.2. Optical properties of N-CDs
The UV-VIS absorption spectrum of N-CDs showed two typical absorption peaks at 270 nm (?-?* transition of C=C aromatic) and 340 nm (n-?* transition of C=O/C=N bonds) (Kumar et al., 2017), as shown in Figure 4. The presence of nitrogen-containing functional groups in the surfaces of CDs was proven by the shift of absorption peaks to longer wavelengths, such as from 270 nm to 340 nm. Therefore, it was reasonable to assume that nitrogen-containing groups unshared electron pairs to become auxochromes and reacted with C=C aromatic as chromophores. From the photographic image (shown as an inset in Figure 4), the N-CDs dissolved in ethanol were yellowish under daylight. But, under UV lamp irradiation (365 nm), the modified carbon dots produced bright blue fluorescence. The strong fluorescence caused nitrogen atom in functional groups to not only generate chromophores on the surface but also to act as energy trap sites to trap excitons and induce transitions, thus emitting fluorescence (Yun-Fei, Maimaiti, and Bo, 2017). By using quinine sulfate as a reference, the quantum yield of N-CDs was 29% at 390 nm excitation wavelength. This result indicated that the amine functional group from the modified agent increased the QY to become much higher than in the previous study.
this study, we used two polar solvents (distilled water and ethanol) to
extract/dissolve the N-CDs. The use of different solvents turned out to have
affected the intensity of photoluminescence and the maximum emission wavelength
of N-CDs. Carbon dots dissolved/extracted by ethanol had a maximum emission
intensity at 470 nm wavelength when 390 nm excitation wavelength was given
(Figure 5a). Meanwhile, carbon dots dissolved/extracted with distilled water
had a maximum emission intensity at 450 nm wavelength when 370 nm excitation
wavelength was given (Figure 5b). The carbon dots dispersed in ethanol had
higher photoluminescence intensity than CDs in distilled water. This occurred
because the presence of oxygenated species in
carbon dots increased the density and electron transfer on the CDs, which
affected the optical properties and luminescence produced. Good dispersibility of
carbon dots in protic solvents could happen due to hydrogen bond interaction
between the oxygenated functional groups and
solvent. No precipitation indicated complete dispersion in carbon dots.
Although carbon was hydrophobic, because the particle size was below 10 nm and
it was supported by a plentiful of hydrophilic functional group, it could
stabilize the carbon nanoparticles in an aqueous solution.
3.3. Mechanism fluorescence of N-CDs and their quenching process for Hg2+ detection
As depicted in Figure 1, nanocrystalline cellulose was used as the carbon source, while ethylenediamine and urea were used as the modification agents or N-dopant agents. The N-CDs obtained had strong fluorescence and high stability. The surface conditions, which were related to the functional groups and the degree of surface oxidation, determined the strong fluorescence emission of N-CDs (Liu et al, 2019). During pyrolysis at 300°C under a nitrogen (N2) atmosphere, NCC (precursor) underwent a degradation process involving several steps, such as decarboxylation, dehydration, condensation, and also aromatization (Supriadi et al., 2017). When ethylenediamine and urea were presented to the reaction system, not only were some nitrogen-containing functional groups formed but N atoms were also introduced to the precursor carbon nuclear lattice through the high-temperature pyrolysis process. If the degree of surface oxidation increased, the surface defect would form greatly. The surface could yield defect sites when nitrogen with functional groups was introduced to it. These defect sites resulted in trapped fluorescence emission that emerged from the excitons radiative recombination (Yang et al., 2012).
Figure 7 shows a schematic illustration of the fluorescence quenching mechanism of N-CDs in the presence of Hg2+. The strong quenching fluorescence intensity of N-CDs by Hg2+ was related to the large affinity of nitrogen and oxygen on the surface of N-CDs (Atchudan et al., 2017). The fluorescence quenching was caused by the complex compounds formed from the electron-rich functional groups in CDs to the vacant d-orbitals in Hg2+. Higher binding affinity in the d-orbitals in Hg2+ ions on the surface of the carbon dots might be the main reason for the selective characteristics. Meanwhile, Fe2+ ions had only enough fluorescence intensity quenching. Even though Fe2+ could react with electron donor groups in carbon dots, their electrophilic ability/electron capture was weaker than that of Hg2+.
High fluorescent N-CDs were successfully synthesized from cellulose in a
one-step synthesis by pyrolysis; the ethylenediamine/urea was used as the
nitrogen source, and ethanol was the solvent. The resulting N-CDs emitted
strong blue fluorescence with QY value of 29% and could be considered as
hydrophilic carbon dots owing to good dispersion in water and ethanol.
Moreover, the N-CDs were spherical without having any agglomeration. The
average diameter was 3.4 nm, and the lattice spacing was 0.206 nm. Functional
groups in N-CDs were confirmed by FTIR analysis. Based on its excellent water
solubility and persistently high fluorescence intensity, the N-CDs could be
utilized as a fluorescent probe for sensitive and selective direct detection of
Hg2+ in a concentration range of 0-100 µM and detection limit of 59
µM. The fluorescence quenching of N-CDs by Hg2+ was a static
quenching type, where the Stern-Volmer diagram plot showed an upward curvature
and the average lifetime of N-CDs was measured to be 5.14 ns.
We would like to express our gratitude to the Ministry of Research and
Technology for the funding support through DRPM 2019 program via the PDUPT
scheme with contract number 154/UN126.96.36.199/KP-DRPM/2019.
|R2-CE-4863-20210712020026.jpg||Figure 4 revision|
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