- Email: [email protected]
Fluorescence mechanism of xylan-derived carbon dots: Toward investigation on excitation-related emission behaviors
Journal of Luminescence 223 (2020) 117199
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Fluorescence mechanism of xylan-derived carbon dots: Toward investigation on excitation-related emission behaviors Pei Yang a, b, Ziqi Zhu a, b, Wei Zhang a, b, Tao Zhang a, b, Xinghui Li a, b, Min Luo a, b, Weimin Chen a, b, Minzhi Chen a, b, **, Xiaoyan Zhou a, b, * a b
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China Fast-growing Tree & Agro-fibre Materials Engineering Center, Nanjing 210037, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Xylan Carbon dots Fluorescence mechanism Surface states
The thoroughly understanding of photoluminescence (PL) featuring with excitation-dependent emission (EDE) and excitation-independent emission (EIE) are critical for designing carbohydrate-derived carbon dots (CDs) with excellent PL characteristics. Herein, two types of xylan-derived CDs with EDE and EIE were facilely synthesized under the doping effect of NH4OH and polyethyleneimine, respectively. The investigation on optical properties, morphology, and chemical structure of these synthesized CDs revealed that both the EDE and EIE were heavily dependent on the surface states rather than the size and structure of carbogenic cores. Furthermore, the multiple energy levels induced by surface-contained C–OH, C–O–C, and C–O functionalities were responsible for the EDE, while the formation of plentiful C–N or C– –N like groups, which together with the decreased O-containing groups resulted in a uniform surface state that contained concentrated energy levels, thus generating the EIE behavior of xylan-derived CDs.
1. Introduction In the past few years, fluorescent carbon dots (CDs) have aroused increasing research interest because of their many unique properties, including nano-sized structure, good water solubility, excellent biocompatibility, low toxicity and unique photoluminescence (PL). Furthermore, they have been widely adapted in diverse technologies, such as lighting, bioimaging, sensing, photocatalysis, and so forth [1–4]. Thus far, over the hundreds of precursor materials have been developed to synthesize CDs with many attractive photoluminescence (PL) prop erties. Compared with commonly used benzene series [5,6], citric acid [7,8] and amino acid [9], carbohydrate serving as carbon source for the preparation of CDs have attracted tremendous attention by their virtues of environmental friendliness, excellent water solubility, low prepara tion cost and typically inherently lack toxicity. Given these outstanding characteristics, many types of CDs with excellent PL performances yielding from carbohydrate have been developed via a wide range of synthesis strategies, and also bestowed with various applications [10–14]. Thus far, there are great efforts have been made to probe the
luminescence mechanism of CDs, but the conclusions regarding the intrinsic mechanism are difficult to formulate a unified theory. In fact, it is most likely that carbon precursor with distinct chemical structure could dominate the PL properties of resultant CDs significantly even the same synthesis route and heteroatom doping are performed. For instance, extensive researches on the synthesis of citric acid-derived CDs and phenylenediamine-derived CDs have pointed out that the conju gated sp2-domain, particle size, graphitization degree and heteroatom doping effect are the key factors for tailoring the PL behaviors [1,7,15, 16], while the different fluorophores are responsible for the PL emis sions arising from carbonized polymer dots that are commonly synthe sized using polymer molecule as a component of precursor [17,18]. Comparatively, the PL emissions of carbohydrate-derived CDs were simply ascribed to the size effect or surface defect states, while the systematic explorations on the PL mechanism of carbohydrate-derived CDs are still seldom reported. Therefore, in addition to exploring a facile and high-output method to synthesize carbohydrate-derived CDs, the thoroughly understanding of distinct PL behaviors, particularly for typical excitation-dependent emission (EDE) and excitation-independent emission (EIE), is also highly critical for their
* Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. ** Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail addresses: [email protected] (M. Chen), [email protected] (X. Zhou). https://doi.org/10.1016/j.jlumin.2020.117199 Received 22 December 2019; Received in revised form 17 February 2020; Accepted 5 March 2020 Available online 6 March 2020 0022-2313/© 2020 Published by Elsevier B.V.
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
value-added utilizations. In the present research, we demonstrated the use of renewable and low-cost xylan as carbon source for microwave-assisted hydrothermal synthesis of CDs within 15 min. For better understanding functionalization-controlled PL performances, the xylan-derived CDs were endowed with EDE and EIE behaviors through nitrogen doping from the NH4OH and polyethyleneimine (PEI), respectively. Structural and chemical characterizations evidenced that the different surface states induced by various oxygen-containing groups and nitrogenrelated moieties within the nanostructure of CDs played a critical role in determining EDE and EIE features. These findings will open up the possibility of designing carbohydrate-derived CDs with desirable PL performances.
doped with NH4OH is given below as a typical example, 0.5 g of xylan was dissolved in 10 mL ultrapure water, and followed by adding 300 μL NH4OH with magnetic stirring to obtain a homogeneous solution. Sub sequently, the as-formed starting materials was heated at 180 � C for 15 min in the microwave reactor. After cooling naturally, the obtained mixture was firstly centrifuged for 10 min at 10,000 rpm to dislodge nonfluorescent deposit, and then filtered through 0.22 μm PTFE syringe filter to remove large particles, the obtained suspension was further dialyzed with a dialysis membrane (MWCO: 1000 Da) for 48 h to exclude unreacted small molecular fragments, the resultant xylan-derived CDs by employing NH4OH as nitrogen source was denoted as 1-CDs. To probe the doping effect of nitrogen source, 0.8 g of PEI was used to instead of NH4OH for preparing another typical nitrogen-doped CDs, while the synthesis procedure was otherwise the same as the preparation of 1-CDs, this type of CDs was labelled as 2-CDs. Similarly, another type of PEI-doped CDs, which was denoted as 3-CDs, was syn thesized in AC solution with mass fraction of 2%. As a control experi ment, a facile two-step synthesis process was performed. Firstly, 300 μL of NH4OH was adopted as nitrogen dopant to participate in the CDs formation with a duration of 10 min, after a few minutes of cooling down, 0.8 g of PEI was dispersed into the obtained black product, then the mixture was further heated for 5 min under microwave irradiation to achieve control sample, the resulting CDs sample was labelled as 4-CDs, the reaction temperature and other synthetic processes were the same as that for the preparation of other types of xylan-derived CDs. All the solid CDs samples were collected as a light powder by freeze-drying after strict purification.
2. Experimental section 2.1. Materials Xylan (85%), quinine sulfate (99.0%) and PEI (M.W. 600, 99%) were purchased from Aladdin Biochemical Co., Ltd (Shanghai, China). So dium hydroxide (96%), ethyl alcohol (99.7%), NH4OH (25–28%), and acetic acid (AC, 99.5%) were obtained from Nanjing Chemical Reagent Co., Ltd. Prior to use as carbon source, the xylan underwent purification by using sodium hydroxide and ethyl alcohol. All other reagents used were commercially available and of analysis-grade purity without further purification. The ultrapure water was used throughout this experiment.
3. Results and discussion
2.2. Equipment and characterization
3.1. Optical properties of xylan-derived CDs
The microwave synthesis system was commercially available and produced by Chemistry Electronic Microwave Company, (DISCOVER SP, USA). Transmission electron microscopy (TEM) observations were per formed on a JEM-2100 UHR microscope (JEOL, Japan) with an accel erating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an AXIS UltraDLD spectrometer (Shimadzu, UK) using Mg as the exciting source. Fourier transformed transform infrared (FTIR) spectra was obtained on a Vertex 80V FTIR spectrometer (Bruker, Germany). Lambda 950 ultraviolet–visible (UV–vis) spectro photometer (PerkinElmer, USA) was used to record UV–vis absorption spectra and light transmittances. PL spectra were collected using an F7000 spectrophotometer (Hitachi, Japan), the band pass for excitation and emission was set as 10 nm. Time-resolved fluorescence decay curves were measured using a FLS 980 time-correlated single-photon counting system (EI, UK).
The weight of each freeze-dried CDs sample was recorded carefully, and the product yield was determined to be 36.1% for 1-CDs, 61.3% for 2-CDs, 73.4% for 3-CDs, and 43.9% for 4-CDs, respectively. All of these solid CDs samples can be well re-dispersed in water with ultrasound processing within several seconds and emitted bright fluorescence under 365 nm UV light, implying the high hydrophilcity and stability of xylanderived CDs. To understand the emission behaviors of these different types of xylan-derived CDs, the PL spectra of each sample obtained at excitation wavelength ranging from 300 to 380 nm were investigated. As seen in Fig. 2a, the 1-CDs displayed typical EDE feature, that is, the emission peaks shifted to longer wavelength, accompanied by the vari ation of PL intensities. Fig. 2b and c illustrate that the 2-CDs and 3-CDs were highly similar in terms of emission behaviors, their emission peaks were insusceptible to the variation of excitation wavelengths, although the PL intensity was highly sensitive to excitation light of different wavelengths, thus manifesting typical EIE performance. Particularly, both the EDE and EIE were observed in the PL spectra of 4-CDs (Fig. 2d), the emission peaks exhibited red-shift phenomenon when the excitation ranged from 280 to 340 nm, whereas the peak position remained un changed as excitation wavelength was further increased. To study the PL
2.3. Synthesis of xylan-derived CDs The xylan-derived CDs featuring with EDE and EIE behaviors were synthesized by a facile microwave method as previously reported with some modification [19], the illustration of synthetic routes for each typical CDs is shown in Fig. 1. The detailed synthesis procedure of CDs
Fig. 1. Schematic illustration for the synthetic route of (a) 1-CDs, (b) 2-CDs, (c) 3-CDs, and (d) 4-CDs. 2
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
Fig. 2. Emission spectra of 1-, 2-, 3- and 4-CDs under various excitation wavelengths.
Fig. 3. UV–vis absorption spectra of 1-, 2-, 3- and 4-CDs.
property of CDs deeply, amount of 0.8 g PEI was used as a single raw material to synthesize CDs under the same condition as the sample of xylan-derived CDs. The resulting product were colorless transparent solution without fluorescence under UV light, suggesting that the PEI can be hardly carbonized alone to form CDs in the microwave-assisted
hydrothermal environment. Moreover, the correlation function curves of excitation and emission obtained from these xylan-derived CDs samples further clarified the distinction between EDE and EIE features (Fig. S1). As seen in Figs. S2 and 3D fluorescence plots show that the maximum emission center of 2-, 3-, and 4-CDs was located at 466 nm,
Fig. 4. Represent TEM observations (a–d), and HRTEM images (e–h) of different CDs samples from 1-CDs to 4-CDs. 3
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
while the 1-CDs exhibited optimal emission at 443 nm, thus revealing that energy gaps can be largely affected by different nitrogen dopants. Furthermore, the relative quantum yields (QYs) of all CDs samples were measured in reference to quinine sulfate (QY ¼ 54% at 360 nm excita tion) under 360 nm excitation. As a result, the QYs were determined to be 3.3% for 1-CDs, 7.9% for 2-CDs, 13.7% for 3-CDs, and 5.4% for 4-CDs, evidencing that the doping effect of PEI contributed more to the PL ef ficiency of xylan-derived CDs. Unlike graphene quantum dots showing quantum confinement effect and edge effect [20], the PL performances of CDs synthesized following bottom-up route mainly depends on their morphological structure and chemical composition. Hence, the EDE and EIE arising from xylan-derived CDs occurs possibly due to the difference in centric carbon core structure and/or surface chemical forms. UV–vis spectroscopies were also analyzed to probe the multiple electron transition channels within CDs, as shown in Fig. 3 there were two bands located at 270 and 310 nm in the absorption spectrum of 1CDs, the peak position around 270 nm was generally explained by the – C or C– – N bonds in carbon cores, whereas the π–π* transitions of C– wide band with lower intensity centered at 310 nm was assigned to the – O, which is a common behavior for CDs synthe n–π* transition of C– sized following bottom-up route [21,22]. The PEI passivated 2-CDs and 3-CDs showed single absorption band at 350 nm, which could be induced by surface states in CDs due to the attachment of multiple functional groups on the edge of carbon cores. For the 4-CDs, the intrinsic absorption with peak wavelength at 270 nm, and a wide ab sorption band ranging from 310 to 350 nm can be observed, which could be resulted from the co-doping effect of NH4OH and PEI. The dual ab sorption behaviors for the NH4OH and PEI co-doped CDs could be an important factor for the co-existed EDE and EIE performances. Fig. S3 illustrates the fluorescence dynamic decay of each synthesized emissive nanostructures, the average fluorescence lifetimes for xylan-derived CDs samples from 1-CDs to 4-CDs were determined to be 4.80, 4.84, 4.58 and 3.25 ns. Clearly, all these emission behaviors generating from different CDs samples were on nanosecond scale with small difference, evidencing that the EDE and EIE from xylan-derived CDs had only fluorescent properties and similar luminescent process.
the intrinsic reason for the EDE and EIE performances. Furthermore, it is noteworthy that the 3-CDs showed more regular structure and uniform size distribution, which was mainly attributed to the participation of AC in the synthesis reaction, the precursor containing acid could be bene ficial for the homogeneous nucleation and growth of nanodots via an accelerated carbonization process [9], which resulted in the complete formation of carbon spheres within 3-CDs. Fig. 5 shows the particle size distribution of each CDs sample by measuring about 100 individual particles, it is apparent that the 2-CDs displayed wide size distribution with respect to the 1-, 3- and 4-CDs, suggesting that the difference in particle size was not responsible for EDE and EIE. Moreover, the average diameter of approximately 3.27 nm, 8.58 nm, 11.28 nm and 4.75 nm were determined for the 1-CDs, 2-CDs, 3-CDs and 4-CDs, respectively. Despite their significant difference in average diameters, the different optimal emissions of the four typical CDs was independent on the variation of size dimension, implying that the surface states rather than the size effect or sp2-domain gave rise to the PL emission of xylan-derived CDs regardless of their EDE or EIE behavior. Based on the above results, we proposed a possible formation mechanism for explaining the difference in morphology and carbonized structure among the synthesized xylan-derived CDs. By comparing the 1CDs and 4-CDs, we speculated that the xylan molecules under the doping effect of NH4OH could first formed small carbogenic cores at the initial stage due to the high reaction rate induced by microwave irradiation, and then the small CDs as cores reacted with PEI accompanied with a certain degree of carbonization. Ultimately, the xylan-derived CDs with relatively large size were generated. It should be mentioned that the freshly formed crosslinking molecules resulting from dehydration re action of xylan were more likely to react with PEI chains by amidation process with respect to the passivation of carbon cores using PEI. As a
3.2. Morphologies and structures of xylan-derived CDs The morphological observations for these CDs samples were char acterized to understand the origin of EDE and EIE, as shown in Fig. 4 (a–d), it is clear that these nanoparticles were quasi-spherical and showed uniform dispersion. The high-resolution TEM (HRTEM) images (Fig. 4(e–h)) illustrate that the 1-CDs, 3-CDs and 4-CDs had similar wellresolved lattice fringes with a spacing of 0.21 nm, corresponding to the d-spacing of the graphene (100) planes. Thus, it is reasonable to deduce that these three typical CDs were predominantly of graphitic nature. Compared with the graphite-like carbonic cores within 1-, 3- and 4-CDs, the 2-CDs, which was only passivated or doped by PEI during the CDs formation, demonstrated their nature of amorphous carbon form. The above finding, that is the distinct difference in the structure of carbo genic cores (crystalline and amorphous characteristics), should not be
Fig. 6. FTIR characterization for the synthesized CDs samples.
Fig. 5. Size distribution histogram of (a) 1-CDs, (b) 2-CDs, (c) 3-CDs, and (d) 4-CDs. 4
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
consequence, the 2-CDs and 3-CDs synthesized by one-step using PEI as passivant/dopant were possessed of large size in comparison to the 4CDs. According to the previous research, the graphitization degree of CDs is closely correlated with dehydration and carbonization conditions. In our case, using industrial NH4OH reagent as dopant in the aqueous medium can increase the vapor pressure of the reactants, and thus accelerating the dehydration and carbonization process, which could be a primary reason for the high graphitization degree of 1-CDs and 4-CDs. Although covalent bonding can be formed, PEI, as a branched chain polymer, could also increase the interlacements/entanglements of crosslinked molecules generating from the dehydration of xylan, which was adverse to the condensation reaction of intermediate polymer. Eventu ally, the efficient carbonization was incomplete within the short reac tion time. Therefore, the 2-CDs with limited carbonization degree showed no obvious crystal lattices structure. For the 3-CDs with crys talline nature, the added AC could contribute to the dehydration and carbonization process of xylan and PEI due to the catalysis of acid, – C bonds were ordered as the resulting in that a huge number of C– intense reaction proceed, and further took the dominant place in the formation of carbon cores.
3.3. Chemical composition of xylan-derived CDs Fig. 6 shows the FTIR spectrum of as-obtained CDs. The absorption peaks located at 3425 and 3275 cm 1 were ascribed to the stretching vibrations of O–H and N–H, respectively. It is notable that the 1-CDs presented an obviously broad peak of stretching of N–H, suggesting that 1-CDs were possessed of more abundant amino groups than 2-CDs and 3-CDs. The peaks at 2934 and 2820 cm 1 corresponded to the C–H bond, while the peaks at around 1638 and 1588 cm 1 were assigned – O and C– – C or C– – N, respectively. Very to the stretching vibration of C– distinct peak absorbing at 1407 cm 1 was only observed in 1-CDs, which – O in carboxyl groups was attributed to the stretching vibration of C– [23,24]. Absorption band peaked at 1463 and 1352 cm 1 were detected in the spectra of 2-CDs, 3-CDs and 4-CDs, but showed no diagnostic signal from 1-CDs, these two bands were commonly related to the stretching and bending vibration of C–N [25,26], the formed C–N like groups could be originated from intermolecular dehydration and further amidation reactions between the carboxyl- and amino-groups, indi cating that PEI acted as not only an N-dopant, but also a surface passivation agent that can functionalize CDs surfaces with abundant amide groups. Moreover, the C–O–C groups with a broad band at 1050 cm 1 can be also observed for all CDs samples. Although different
Fig. 7. XPS survey spectra and high-resolution XPS of the C1s, O1s, and N1s spectra of the (a–d) 1-CDs, (e–h) 2-CDs, (i–l) 3-CDs and (m–p) 4-CDs. 5
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
nitrogen source participated in the synthesis of xylan-derived CDs, all resulted nanostructures were rich in multiple oxygen- and nitrogen-containing functional groups that contributed to their good water solubility. To gain insight into the intrinsic mechanism behind the EDE and EIE from xylan-derived CDs, XPS investigation was performed, and the analysis results were shown in Fig. 7. The survey spectra of all CDs samples presented four representative peaks of C1s (285 eV), N1s (400 eV), Na1s (496 eV), and O1s (532 eV), and the relative content of each element was displayed in the corresponding panel of survey spectrum. These findings suggest that the different CDs samples from 1-CDs to 4CDs were composed of the same elements but had different element content. Note that the detected signal of Na1s was generated from the purification of xylan using NaOH solution prior to synthesize CDs. Moreover, the xylan-derived CDs with PEI passivation contained much more nitrogen element than that their NH4OH doped counterparts, which mainly associated with the fact that the nitrogen content of PEI was higher than that of NH4OH; also, the easy decomposition of NH4OH during the heating process could be another reason for the lower ni trogen content of 1-CDs. In the high-resolution spectra, the C1s band of 1-CDs and 4-CDs can be resolved into four peaks, which were assigned to – C/C–C (284.7 eV), C–N (285.6–285.9 eV), C–O (286.6–286.7 eV), C– – O (287.7–288.1 eV). Unlike the 1-CDs and 4-CDs, the decon and C– volution of C1s spectra obtained from PEI passivated 2-CDs and 3-CDs demonstrated the lack of C–O bond, thus evidencing that the one-step PEI passivation will reduce C–O groups on the xylan-derived CDs sur faces. The decline of this oxygen-containing groups could lower non radiative recombination within CDs, and thus contribute to the enhancement of QY. The high-resolution XPS N1s spectra of 1-CDs and 4-CDs both exhibited four distinct peaks at around 398.8, 399.8, 400.9, and 402.2 eV, which associated with pyridinic N, pyrrolic N, graphitic N, and amino N, respectively [27], while for 2-CDs and 3-CDs, the analysis on the N1s spectra revealed two types of nitrogen including pyridinic N and pyrrolic N. The O1s band of 1-CDs and 4-CDs contained three typical – O, C–OH/C–O–C, peaks at 531.1, 532.4, and 533.5 eV, representing C– and C–O, respectively [28]. For the PEI passivated 2-CDs and 3-CDs, the – O and C–OH/C–O–C moieties were also confirmed from presence of C– the deconvolution of O1s band, but the C–O was not observed. Since extensive researches have confirmed that the surface-contained nitro gen- and oxygen-related groups play a vital role in tailoring spectral characteristics, the amounts of chemical bonds within these different CDs samples were further extracted for comparative analysis, as seen in – C/C–C component, which is considered as the main Table 1. The C– skeleton of carbogenic cores, demonstrated their higher level in 2-CDs and 3-CDs. Such phenomenon could be highly related to the PEI doping or passivation, the PEI with high content of carbon and nitrogen elements not only functionalize surface domains of CDs by chemical bonding, but also took part in the formation of carbogenic cores, accompanied by a certain degree of carbonization during the intense hydrothermal reaction. Among the synthesized xylan-derived CDs samples, it is apparent that the 3-CDs, which were synthesized in acidic – C/C–C bonds and a low condition, possessed the highest content of C– level of O-related moieties, evidencing that adding AC in precursor so lution could facilitate the dehydration, polymerization and carboniza tion of xylan, as well as contributed to efficient PEI doping or passivation, such behaviors could play an important role in enhancing
the product yield of CDs. Moreover, 2-CDs and 3-CDs contained the most amount of N atoms as forms of pyridine, which could be beneficial for the formation and introduction of electron traps within their surface band structures [29]. Based on the first-principles calculations, the ox ygen in carbon-based nanostructures as the form of C–OH (hydroxy) and C–O–C (epoxy) functional groups have a large possibility to create or tune band gap, and the formation of energy gaps depends on the con tents of C–OH/C–O–C groups [22,30,31]. Therefore, it is reasonable to believe that the large amount of C–O, C–OH, and C–O–C moieties within the 1-CDs and 4-CDs can introduce surface trapping states with a series of energy levels, resulting in that the CDs emit photons that vary with excitation energy. In general, the surface states are not created by single chemical group but the hybridization of the carbon backbones and connected functional groups. According to the findings above, we speculated that the hydroxy- and epoxide-related energy levels can be induced since the insufficient nitrogen doping in xylan-derived CDs structure, as a result of this characteristic, the different emissive trap sites led to multiple electron transition channels, and gave rise to PL emission with EDE behavior. Specifically, although the graphitic N has been confirmed to be prone to generate midgap states within π–π* gap of N-doped CDs and play an important role of the PL emission [32], but due to the fact that the graphitic N were in tiny amount in 1-, and 4-CDs, it is reasonable to deduce that the effect of graphitic N was far less than that of O-containing groups in dominating EDE performance of xylan-derived CDs. In the case of 2-, and 3-CDs, the efficient dehydration and amidation reaction in the synthesis system led to the decrease of – N and C–N C–OH, C–O–C, and C–O groups, and generated abundant C– bonds simultaneously, the surface-contained similar chemical species bonding with nano-sized carbon cores resulted in a uniform surface state that emit fluorescence with EIE performance. The above-mentioned intrinsic reasons for EIE of xylan-derived CDs are inconsistent with several previous reports in which the EIE performance of citric acid-derived CDs was considered to associate with amino groups-induced single energy level [33,34]. 3.4. Fluorescence mechanism of xylan-derived CDs Based on the above evidences, an energy-level model for the surface states within xylan-derived CDs was presented to visualize the distinc tion between EDE and EIE performances, as shown in Fig. 8. Extensive researches have manifested that the π–π* and n–π* transitions coexist in the CDs structures; however, the n–π* transition is more likely to pre – O bonds dominate emission behaviors due to the formation of C– [35–37]. In our case, both the FTIR arrangement and XPS analysis – O bonds within xylan-derived CDs demonstrated the existence of C– regardless of EDE and EIE behaviors, thus the n–π* transition was the main electron transition channel from ground state to excited state. Due to the 1-CDs contained plentiful C–OH, C–O–C, and C–O groups, the induced energy levels (O-related states) exhibited discrete distribution within the n–π* gap. The excited electrons transferred from ground state (n) to the highest excited state (π*), and then suffered from non-radiative relaxation from the highest excited state to the lowest excited state. After that, most of electrons were trapped by the multiple energy levels and finally returned to the ground state and gave rise to the fluorescence –N with EDE property. By contrast, the formation of plentiful C–N and C– moieties and the decrease of O-containing groups on CDs surfaces
Table 1 Relative contents of different chemical bonds in CDs samples as determined by XPS. Sample 1-CDs 2-CDs 3-CDs 4-CDs
C1s
N1s
O1s
C¼C/C–C
C–N
C–O
C¼O
pyridinic N
pyrrolic N
graphitic N
amino N
C¼O
C–OH/C–O–C
C–O
0.110 0.34 0.43 0.30
0.14 0.19 0.31 0.15
0.18 – – 0.22
0.11 0.11 0.05 0.11
0.007 0.064 0.068 0.009
0.011 0.049 0.037 0.008
0.010 – – 0.008
0.010 – – 0.006
0.27 0.14 0.08 0.10
0.10 0.04 0.02 0.06
0.05 – – 0.04
6
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
Fig. 8. Energy-level diagram of xylan-derived CDs with EDE and EIE characteristics.
induced the generation of N-related state that contained concentrated energy levels within the n–π* gap. As a result, efficient electron transi tions can occur from N-related state to the ground state, thus contrib uting to CDs emitted PL featuring with EIE performance.
Appendix A. Supplementary data
4. Conclusions
References
In summary, by employing facile microwave-assisted hydrothermal method, the xylan-derived CDs featuring with EDE and EIE were syn thesized under the doping effect of NH4OH and PEI, respectively. Investigation on the PL mechanism of xylan-derived CDs revealed that surface states rather than the structure and size of carbon core mainly dominated the emission behaviors. Further, the large number of C–OH, C–O–C, and C–O groups on the surfaces of xylan-derived CDs were highly responsible for the EDE performance by introducing multiple energy levels within n–π* gap. Comparatively, the formation of plentiful nitrogen-related chemical bonds, and the decrease of oxygen-containing groups made the surface states became more uniform, the generated identical electronic transition channel resulted in the EIE behavior of xylan-derived CDs.
[1] F. Yuan, Z. Wang, X. Li, Y. Li, Z. Tan, L. Fan, S. Yang, Bright multicolor bandgap fluorescent carbon quantum dots for electroluminescent light-emitting diodes, Adv. Mater. 29 (2017) 1604436. [2] H. Ali, S.K. Bhunia, C. Dalal, N.R. Jana, Red fluorescent carbon nanoparticle-based cell imaging probe, ACS Appl. Mater. Interfaces 8 (2016) 9305–9313. [3] J. Wang, F. Qiu, X. Li, H. Wu, J. Xu, X. Niu, J. Pan, T. Zhang, D. Yang, A facile onepot synthesis of fluorescent carbon dots from degrease cotton for the selective determination of chromium ions in water and soil samples, J. Lumin. 188 (2017) 230–237. [4] J. Zhang, X. Liu, X. Wang, L. Mu, M. Yuan, B. Liu, H. Shi, Carbon dots-decorated Na2W4O13 composite with WO3 for highly efficient photocatalytic antibacterial activity, J. Hazard. Mater. 359 (2018) 1–8. [5] J. Chen, J.S. Wei, P. Zhang, X.Q. Niu, W. Zhao, Z.Y. Zhu, H. Ding, H.M. Xiong, Redemissive carbon dots for fingerprints detection by spray method: coffee ring effect and unquenched fluorescence in drying process, ACS Appl. Mater. Interfaces 9 (2017) 18429–18433. [6] K. Jiang, S. Sun, L. Zhang, Y. Wang, C. Cai, H. Lin, Bright-yellow-emissive N-doped carbon dots: preparation, cellular imaging, and bifunctional sensing, ACS Appl. Mater. Interfaces 7 (2015) 23231–23238. [7] X. Miao, D. Qu, D. Yang, B. Nie, Y. Zhao, H. Fan, Z. Sun, Synthesis of carbon dots with multiple color emission by controlled graphitization and surface functionalization, Adv. Mater. 30 (2018) 1704740. [8] X. Miao, X. Yan, D. Qu, D. Li, F.F. Tao, Z. Sun, Red emissive sulfur, nitrogen codoped carbon dots and their application in ion detection and theraonostics, ACS Appl. Mater. Interfaces 9 (2017) 18549–18556. [9] H. Ding, J.S. Wei, P. Zhang, Z.Y. Zhou, Q.Y. Gao, H.M. Xiong, Solvent-controlled synthesis of highly luminescent carbon dots with a wide color gamut and narrowed emission peak widths, Small 14 (2018) 1800612. [10] Z. Xu, C. Wang, K. Jiang, H. Lin, Y. Huang, C. Zhang, Microwave-assisted rapid synthesis of amphibious yellow fluorescent carbon dots as a colorimetric nanosensor for Cr(VI), Part. Part, Syst. Char. 32 (2015) 1058–1062. [11] X. Wang, K. Qu, B. Xu, J. Ren, X. Qu, Microwave assisted one-step green synthesis of cell-permeable multicolor photoluminescent carbon dots without surface passivation reagents, J. Mater. Chem. 21 (2011) 2445–2450. [12] Y.Y. Zhang, M. Wu, Y.Q. Wang, X.W. He, W.Y. Li, X.Z. Feng, A new hydrothermal refluxing route to strong fluorescent carbon dots and its application as fluorescent imaging agent, Talanta 117 (2013) 196–202. [13] S.A. Hill, D. Benito-Alifonso, D.J. Morgan, S.A. Davis, M. Berry, M.C. Galan, Threeminute synthesis of sp(3) nanocrystalline carbon dots as non-toxic fluorescent platforms for intracellular delivery, Nanoscale 8 (2016) 18630–18634. [14] Z.Q. Zhu, P. Yang, M.Z. Chen, T. Zhang, Y.Z. Cao, W. Zhang, X.Y. Zhou, W.M. Chen, Microwave synthesis of amphiphilic carbon dots from xylose and construction of luminescent composites with shape recovery performance, J. Lumin. 213 (2019) 474–481. [15] J. Liu, D. Li, K. Zhang, M. Yang, H. Sun, B. Yang, One-step hydrothermal synthesis of nitrogen-doped conjugated carbonized polymer dots with 31% efficient red emission for in vivo imaging, Small 14 (2018) 1703919. [16] S. Qu, D. Zhou, D. Li, W. Ji, P. Jing, D. Han, L. Liu, H. Zeng, D. Shen, Toward efficient orange emissive carbon nanodots through conjugated sp2-domain controlling and surface charges engineering, Adv. Mater. 28 (2016) 3516–3521. [17] H. Zhang, X. Dong, J. Wang, R. Guan, D. Cao, Q. Chen, Fluorescence emission of polyethylenimine-derived polymer dots and its application to detect copper and hypochlorite ions, ACS Appl. Mater. Interfaces 11 (2019) 32489–32499.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2020.117199.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Pei Yang: Conceptualization, Methodology, Formal analysis, Writing - original draft. Ziqi Zhu: Investigation. Wei Zhang: Investi gation. Tao Zhang: Investigation. Xinghui Li: Investigation. Min Luo: Investigation. Weimin Chen: Investigation. Minzhi Chen: Conceptu alization. Xiaoyan Zhou: Supervision, Writing - review & editing, Project administration. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 31870549], the Jiangsu Nature Science Foun dation [grant number BK20161524]; the Program for 333 Talents Project in Jiangsu Province [grant Number BRA2016381]; the Post graduate Research & Practice Innovation Program of Jiangsu Province [grant number KYCX17_839]; and the Advanced Analysis and Testing Center of Nanjing Forestry University.
7
P. Yang et al.
Journal of Luminescence 223 (2020) 117199 [29] Y. Liang, Y.F. Shen, C.L. Liu, X.Y. Ren, Effects of chemical bonds between nitrogen and its neighbor carbon atoms on fluorescence properties of carbon quantum, J. Lumin. 197 (2018) 285–290. [30] C.T. Chien, S.S. Li, W.J. Lai, Y.C. Yeh, H.A. Chen, I.S. Chen, L.C. Chen, K.H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, C. W. Chen, Tunable photoluminescence from graphene oxide, Angew. Chem. Int. Ed. 51 (2012) 6662–6666. [31] Y. Zhang, D.Q. Fang, S.L. Zhang, Y.H. Wen, Z.Z. Zhu, Structures and electronic properties of oxidized graphene from first-principles study, Europhys. Lett. 105 (2014) 37005. [32] K. Hola, M. Sudolska, S. Kalytchuk, D. Nachtigallova, A.L. Rogach, M. Otyepka, R. Zboril, Graphitic nitrogen triggers red fluorescence in carbon dots, ACS Nano 11 (2017) 12402–12410. [33] Y.H. Yuan, Z.X. Liu, R.S. Li, H.Y. Zou, M. Lin, H. Liu, C.Z. Huang, Synthesis of nitrogen-doping carbon dots with different photoluminescence properties by controlling the surface states, Nanoscale 8 (2016) 6770–6776. [34] B. Li, Y. Guo, A. Iqbal, Y. Dong, W. Li, W. Liu, W. Qin, Y. Wang, Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface, RSC Adv. 6 (2016) 107263–107269. [35] Y. Zhang, Y. Hu, J. Lin, Y. Fan, Y. Li, Y. Lv, X. Liu, Excitation wavelength independence: toward low-threshold amplified spontaneous emission from carbon nanodots, ACS Appl. Mater. Interfaces 8 (2016) 25454–25460. [36] Y. Zhang, P. Zhuo, H. Yin, Y. Fan, J. Zhang, X. Liu, Z. Chen, Solid-state fluorescent carbon dots with aggregation-induced yellow emission for white light-emitting diodes with high luminous efficiencies, ACS Appl. Mater. Interfaces 11 (2019) 24395–24403. [37] S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: from fluorescence mechanism to up-conversion bioimaging applications, Adv. Funct. Mater. 22 (2012) 4732–4740.
[18] S.G. Liu, T. Liu, N. Li, S. Geng, J.L. Lei, N.B. Li, H.Q. Luo, Polyethylenimine-derived fluorescent nonconjugated polymer dots with reversible dual-signal pH response and logic gate operation, J. Phys. Chem. C 121 (2017) 6874–6883. [19] P. Yang, Z.Q. Zhu, M.Z. Chen, W.M. Chen, X.Y. Zhou, Microwave-assisted synthesis of xylan-derived carbon quantum dots for tetracycline sensing, Opt. Mater. 85 (2018) 329–336. [20] C. Xia, S. Zhu, T. Feng, M. Yang, B. Yang, Evolution and synthesis of carbon dots: from carbon dots to carbonized polymer dots, Adv. Sci. (2019) 1901316. [21] H. Liu, R.S. Li, J. Zhou, C.Z. Huang, Branched polyethylenimine-functionalized carbon dots as sensitive and selective fluorescent probes for N-acetylcysteine via an off-on mechanism, Analyst 142 (2017) 4221–4227. [22] S. Hu, A. Trinchi, P. Atkin, I. Cole, Tunable photoluminescence across the entire visible spectrum from carbon dots excited by white light, Angew. Chem. 54 (2015) 2970–2974. [23] F. Li, C. Liu, J. Yang, Z. Wang, W. Liu, F. Tian, Mg/N double doping strategy to fabricate extremely high luminescent carbon dots for bioimaging, RSC Adv. 4 (2014) 3201–3205. [24] Y. Dong, J. Shao, C. Chen, H. Li, R. Wang, Y. Chi, X. Lin, G. Chen, Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid, Carbon 50 (2012) 4738–4743. [25] H. Ding, S.B. Yu, J.S. Wei, H.M. Xiong, Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism, ACS Nano 10 (2016) 484–491. [26] M. Zheng, S. Liu, J. Li, D. Qu, H. Zhao, X. Guan, X. Hu, Z. Xie, X. Jing, Z. Sun, Integrating oxaliplatin with highly luminescent carbon dots: an unprecedented theranostic agent for personalized medicine, Adv. Mater. 26 (2014) 3554–3560. [27] S. Lu, L. Sui, J. Liu, S. Zhu, A. Chen, M. Jin, B. Yang, Near-infrared photoluminescent polymer-carbon nanodots with two-photon fluorescence, Adv. Mater. 29 (2017) 1603443. [28] W. Li, S. Wu, H. Zhang, X. Zhang, J. Zhuang, C. Hu, Y. Liu, B. Lei, L. Ma, X. Wang, Enhanced biological photosynthetic efficiency using light-harvesting engineering with dual-emissive carbon dots, Adv. Funct. Mater. 28 (44) (2018) 1804004.
8
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Fluorescence mechanism of xylan-derived carbon dots: Toward investigation on excitation-related emission behaviors Pei Yang a, b, Ziqi Zhu a, b, Wei Zhang a, b, Tao Zhang a, b, Xinghui Li a, b, Min Luo a, b, Weimin Chen a, b, Minzhi Chen a, b, **, Xiaoyan Zhou a, b, * a b
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China Fast-growing Tree & Agro-fibre Materials Engineering Center, Nanjing 210037, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Xylan Carbon dots Fluorescence mechanism Surface states
The thoroughly understanding of photoluminescence (PL) featuring with excitation-dependent emission (EDE) and excitation-independent emission (EIE) are critical for designing carbohydrate-derived carbon dots (CDs) with excellent PL characteristics. Herein, two types of xylan-derived CDs with EDE and EIE were facilely synthesized under the doping effect of NH4OH and polyethyleneimine, respectively. The investigation on optical properties, morphology, and chemical structure of these synthesized CDs revealed that both the EDE and EIE were heavily dependent on the surface states rather than the size and structure of carbogenic cores. Furthermore, the multiple energy levels induced by surface-contained C–OH, C–O–C, and C–O functionalities were responsible for the EDE, while the formation of plentiful C–N or C– –N like groups, which together with the decreased O-containing groups resulted in a uniform surface state that contained concentrated energy levels, thus generating the EIE behavior of xylan-derived CDs.
1. Introduction In the past few years, fluorescent carbon dots (CDs) have aroused increasing research interest because of their many unique properties, including nano-sized structure, good water solubility, excellent biocompatibility, low toxicity and unique photoluminescence (PL). Furthermore, they have been widely adapted in diverse technologies, such as lighting, bioimaging, sensing, photocatalysis, and so forth [1–4]. Thus far, over the hundreds of precursor materials have been developed to synthesize CDs with many attractive photoluminescence (PL) prop erties. Compared with commonly used benzene series [5,6], citric acid [7,8] and amino acid [9], carbohydrate serving as carbon source for the preparation of CDs have attracted tremendous attention by their virtues of environmental friendliness, excellent water solubility, low prepara tion cost and typically inherently lack toxicity. Given these outstanding characteristics, many types of CDs with excellent PL performances yielding from carbohydrate have been developed via a wide range of synthesis strategies, and also bestowed with various applications [10–14]. Thus far, there are great efforts have been made to probe the
luminescence mechanism of CDs, but the conclusions regarding the intrinsic mechanism are difficult to formulate a unified theory. In fact, it is most likely that carbon precursor with distinct chemical structure could dominate the PL properties of resultant CDs significantly even the same synthesis route and heteroatom doping are performed. For instance, extensive researches on the synthesis of citric acid-derived CDs and phenylenediamine-derived CDs have pointed out that the conju gated sp2-domain, particle size, graphitization degree and heteroatom doping effect are the key factors for tailoring the PL behaviors [1,7,15, 16], while the different fluorophores are responsible for the PL emis sions arising from carbonized polymer dots that are commonly synthe sized using polymer molecule as a component of precursor [17,18]. Comparatively, the PL emissions of carbohydrate-derived CDs were simply ascribed to the size effect or surface defect states, while the systematic explorations on the PL mechanism of carbohydrate-derived CDs are still seldom reported. Therefore, in addition to exploring a facile and high-output method to synthesize carbohydrate-derived CDs, the thoroughly understanding of distinct PL behaviors, particularly for typical excitation-dependent emission (EDE) and excitation-independent emission (EIE), is also highly critical for their
* Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. ** Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail addresses: [email protected] (M. Chen), [email protected] (X. Zhou). https://doi.org/10.1016/j.jlumin.2020.117199 Received 22 December 2019; Received in revised form 17 February 2020; Accepted 5 March 2020 Available online 6 March 2020 0022-2313/© 2020 Published by Elsevier B.V.
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
value-added utilizations. In the present research, we demonstrated the use of renewable and low-cost xylan as carbon source for microwave-assisted hydrothermal synthesis of CDs within 15 min. For better understanding functionalization-controlled PL performances, the xylan-derived CDs were endowed with EDE and EIE behaviors through nitrogen doping from the NH4OH and polyethyleneimine (PEI), respectively. Structural and chemical characterizations evidenced that the different surface states induced by various oxygen-containing groups and nitrogenrelated moieties within the nanostructure of CDs played a critical role in determining EDE and EIE features. These findings will open up the possibility of designing carbohydrate-derived CDs with desirable PL performances.
doped with NH4OH is given below as a typical example, 0.5 g of xylan was dissolved in 10 mL ultrapure water, and followed by adding 300 μL NH4OH with magnetic stirring to obtain a homogeneous solution. Sub sequently, the as-formed starting materials was heated at 180 � C for 15 min in the microwave reactor. After cooling naturally, the obtained mixture was firstly centrifuged for 10 min at 10,000 rpm to dislodge nonfluorescent deposit, and then filtered through 0.22 μm PTFE syringe filter to remove large particles, the obtained suspension was further dialyzed with a dialysis membrane (MWCO: 1000 Da) for 48 h to exclude unreacted small molecular fragments, the resultant xylan-derived CDs by employing NH4OH as nitrogen source was denoted as 1-CDs. To probe the doping effect of nitrogen source, 0.8 g of PEI was used to instead of NH4OH for preparing another typical nitrogen-doped CDs, while the synthesis procedure was otherwise the same as the preparation of 1-CDs, this type of CDs was labelled as 2-CDs. Similarly, another type of PEI-doped CDs, which was denoted as 3-CDs, was syn thesized in AC solution with mass fraction of 2%. As a control experi ment, a facile two-step synthesis process was performed. Firstly, 300 μL of NH4OH was adopted as nitrogen dopant to participate in the CDs formation with a duration of 10 min, after a few minutes of cooling down, 0.8 g of PEI was dispersed into the obtained black product, then the mixture was further heated for 5 min under microwave irradiation to achieve control sample, the resulting CDs sample was labelled as 4-CDs, the reaction temperature and other synthetic processes were the same as that for the preparation of other types of xylan-derived CDs. All the solid CDs samples were collected as a light powder by freeze-drying after strict purification.
2. Experimental section 2.1. Materials Xylan (85%), quinine sulfate (99.0%) and PEI (M.W. 600, 99%) were purchased from Aladdin Biochemical Co., Ltd (Shanghai, China). So dium hydroxide (96%), ethyl alcohol (99.7%), NH4OH (25–28%), and acetic acid (AC, 99.5%) were obtained from Nanjing Chemical Reagent Co., Ltd. Prior to use as carbon source, the xylan underwent purification by using sodium hydroxide and ethyl alcohol. All other reagents used were commercially available and of analysis-grade purity without further purification. The ultrapure water was used throughout this experiment.
3. Results and discussion
2.2. Equipment and characterization
3.1. Optical properties of xylan-derived CDs
The microwave synthesis system was commercially available and produced by Chemistry Electronic Microwave Company, (DISCOVER SP, USA). Transmission electron microscopy (TEM) observations were per formed on a JEM-2100 UHR microscope (JEOL, Japan) with an accel erating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an AXIS UltraDLD spectrometer (Shimadzu, UK) using Mg as the exciting source. Fourier transformed transform infrared (FTIR) spectra was obtained on a Vertex 80V FTIR spectrometer (Bruker, Germany). Lambda 950 ultraviolet–visible (UV–vis) spectro photometer (PerkinElmer, USA) was used to record UV–vis absorption spectra and light transmittances. PL spectra were collected using an F7000 spectrophotometer (Hitachi, Japan), the band pass for excitation and emission was set as 10 nm. Time-resolved fluorescence decay curves were measured using a FLS 980 time-correlated single-photon counting system (EI, UK).
The weight of each freeze-dried CDs sample was recorded carefully, and the product yield was determined to be 36.1% for 1-CDs, 61.3% for 2-CDs, 73.4% for 3-CDs, and 43.9% for 4-CDs, respectively. All of these solid CDs samples can be well re-dispersed in water with ultrasound processing within several seconds and emitted bright fluorescence under 365 nm UV light, implying the high hydrophilcity and stability of xylanderived CDs. To understand the emission behaviors of these different types of xylan-derived CDs, the PL spectra of each sample obtained at excitation wavelength ranging from 300 to 380 nm were investigated. As seen in Fig. 2a, the 1-CDs displayed typical EDE feature, that is, the emission peaks shifted to longer wavelength, accompanied by the vari ation of PL intensities. Fig. 2b and c illustrate that the 2-CDs and 3-CDs were highly similar in terms of emission behaviors, their emission peaks were insusceptible to the variation of excitation wavelengths, although the PL intensity was highly sensitive to excitation light of different wavelengths, thus manifesting typical EIE performance. Particularly, both the EDE and EIE were observed in the PL spectra of 4-CDs (Fig. 2d), the emission peaks exhibited red-shift phenomenon when the excitation ranged from 280 to 340 nm, whereas the peak position remained un changed as excitation wavelength was further increased. To study the PL
2.3. Synthesis of xylan-derived CDs The xylan-derived CDs featuring with EDE and EIE behaviors were synthesized by a facile microwave method as previously reported with some modification [19], the illustration of synthetic routes for each typical CDs is shown in Fig. 1. The detailed synthesis procedure of CDs
Fig. 1. Schematic illustration for the synthetic route of (a) 1-CDs, (b) 2-CDs, (c) 3-CDs, and (d) 4-CDs. 2
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
Fig. 2. Emission spectra of 1-, 2-, 3- and 4-CDs under various excitation wavelengths.
Fig. 3. UV–vis absorption spectra of 1-, 2-, 3- and 4-CDs.
property of CDs deeply, amount of 0.8 g PEI was used as a single raw material to synthesize CDs under the same condition as the sample of xylan-derived CDs. The resulting product were colorless transparent solution without fluorescence under UV light, suggesting that the PEI can be hardly carbonized alone to form CDs in the microwave-assisted
hydrothermal environment. Moreover, the correlation function curves of excitation and emission obtained from these xylan-derived CDs samples further clarified the distinction between EDE and EIE features (Fig. S1). As seen in Figs. S2 and 3D fluorescence plots show that the maximum emission center of 2-, 3-, and 4-CDs was located at 466 nm,
Fig. 4. Represent TEM observations (a–d), and HRTEM images (e–h) of different CDs samples from 1-CDs to 4-CDs. 3
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
while the 1-CDs exhibited optimal emission at 443 nm, thus revealing that energy gaps can be largely affected by different nitrogen dopants. Furthermore, the relative quantum yields (QYs) of all CDs samples were measured in reference to quinine sulfate (QY ¼ 54% at 360 nm excita tion) under 360 nm excitation. As a result, the QYs were determined to be 3.3% for 1-CDs, 7.9% for 2-CDs, 13.7% for 3-CDs, and 5.4% for 4-CDs, evidencing that the doping effect of PEI contributed more to the PL ef ficiency of xylan-derived CDs. Unlike graphene quantum dots showing quantum confinement effect and edge effect [20], the PL performances of CDs synthesized following bottom-up route mainly depends on their morphological structure and chemical composition. Hence, the EDE and EIE arising from xylan-derived CDs occurs possibly due to the difference in centric carbon core structure and/or surface chemical forms. UV–vis spectroscopies were also analyzed to probe the multiple electron transition channels within CDs, as shown in Fig. 3 there were two bands located at 270 and 310 nm in the absorption spectrum of 1CDs, the peak position around 270 nm was generally explained by the – C or C– – N bonds in carbon cores, whereas the π–π* transitions of C– wide band with lower intensity centered at 310 nm was assigned to the – O, which is a common behavior for CDs synthe n–π* transition of C– sized following bottom-up route [21,22]. The PEI passivated 2-CDs and 3-CDs showed single absorption band at 350 nm, which could be induced by surface states in CDs due to the attachment of multiple functional groups on the edge of carbon cores. For the 4-CDs, the intrinsic absorption with peak wavelength at 270 nm, and a wide ab sorption band ranging from 310 to 350 nm can be observed, which could be resulted from the co-doping effect of NH4OH and PEI. The dual ab sorption behaviors for the NH4OH and PEI co-doped CDs could be an important factor for the co-existed EDE and EIE performances. Fig. S3 illustrates the fluorescence dynamic decay of each synthesized emissive nanostructures, the average fluorescence lifetimes for xylan-derived CDs samples from 1-CDs to 4-CDs were determined to be 4.80, 4.84, 4.58 and 3.25 ns. Clearly, all these emission behaviors generating from different CDs samples were on nanosecond scale with small difference, evidencing that the EDE and EIE from xylan-derived CDs had only fluorescent properties and similar luminescent process.
the intrinsic reason for the EDE and EIE performances. Furthermore, it is noteworthy that the 3-CDs showed more regular structure and uniform size distribution, which was mainly attributed to the participation of AC in the synthesis reaction, the precursor containing acid could be bene ficial for the homogeneous nucleation and growth of nanodots via an accelerated carbonization process [9], which resulted in the complete formation of carbon spheres within 3-CDs. Fig. 5 shows the particle size distribution of each CDs sample by measuring about 100 individual particles, it is apparent that the 2-CDs displayed wide size distribution with respect to the 1-, 3- and 4-CDs, suggesting that the difference in particle size was not responsible for EDE and EIE. Moreover, the average diameter of approximately 3.27 nm, 8.58 nm, 11.28 nm and 4.75 nm were determined for the 1-CDs, 2-CDs, 3-CDs and 4-CDs, respectively. Despite their significant difference in average diameters, the different optimal emissions of the four typical CDs was independent on the variation of size dimension, implying that the surface states rather than the size effect or sp2-domain gave rise to the PL emission of xylan-derived CDs regardless of their EDE or EIE behavior. Based on the above results, we proposed a possible formation mechanism for explaining the difference in morphology and carbonized structure among the synthesized xylan-derived CDs. By comparing the 1CDs and 4-CDs, we speculated that the xylan molecules under the doping effect of NH4OH could first formed small carbogenic cores at the initial stage due to the high reaction rate induced by microwave irradiation, and then the small CDs as cores reacted with PEI accompanied with a certain degree of carbonization. Ultimately, the xylan-derived CDs with relatively large size were generated. It should be mentioned that the freshly formed crosslinking molecules resulting from dehydration re action of xylan were more likely to react with PEI chains by amidation process with respect to the passivation of carbon cores using PEI. As a
3.2. Morphologies and structures of xylan-derived CDs The morphological observations for these CDs samples were char acterized to understand the origin of EDE and EIE, as shown in Fig. 4 (a–d), it is clear that these nanoparticles were quasi-spherical and showed uniform dispersion. The high-resolution TEM (HRTEM) images (Fig. 4(e–h)) illustrate that the 1-CDs, 3-CDs and 4-CDs had similar wellresolved lattice fringes with a spacing of 0.21 nm, corresponding to the d-spacing of the graphene (100) planes. Thus, it is reasonable to deduce that these three typical CDs were predominantly of graphitic nature. Compared with the graphite-like carbonic cores within 1-, 3- and 4-CDs, the 2-CDs, which was only passivated or doped by PEI during the CDs formation, demonstrated their nature of amorphous carbon form. The above finding, that is the distinct difference in the structure of carbo genic cores (crystalline and amorphous characteristics), should not be
Fig. 6. FTIR characterization for the synthesized CDs samples.
Fig. 5. Size distribution histogram of (a) 1-CDs, (b) 2-CDs, (c) 3-CDs, and (d) 4-CDs. 4
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
consequence, the 2-CDs and 3-CDs synthesized by one-step using PEI as passivant/dopant were possessed of large size in comparison to the 4CDs. According to the previous research, the graphitization degree of CDs is closely correlated with dehydration and carbonization conditions. In our case, using industrial NH4OH reagent as dopant in the aqueous medium can increase the vapor pressure of the reactants, and thus accelerating the dehydration and carbonization process, which could be a primary reason for the high graphitization degree of 1-CDs and 4-CDs. Although covalent bonding can be formed, PEI, as a branched chain polymer, could also increase the interlacements/entanglements of crosslinked molecules generating from the dehydration of xylan, which was adverse to the condensation reaction of intermediate polymer. Eventu ally, the efficient carbonization was incomplete within the short reac tion time. Therefore, the 2-CDs with limited carbonization degree showed no obvious crystal lattices structure. For the 3-CDs with crys talline nature, the added AC could contribute to the dehydration and carbonization process of xylan and PEI due to the catalysis of acid, – C bonds were ordered as the resulting in that a huge number of C– intense reaction proceed, and further took the dominant place in the formation of carbon cores.
3.3. Chemical composition of xylan-derived CDs Fig. 6 shows the FTIR spectrum of as-obtained CDs. The absorption peaks located at 3425 and 3275 cm 1 were ascribed to the stretching vibrations of O–H and N–H, respectively. It is notable that the 1-CDs presented an obviously broad peak of stretching of N–H, suggesting that 1-CDs were possessed of more abundant amino groups than 2-CDs and 3-CDs. The peaks at 2934 and 2820 cm 1 corresponded to the C–H bond, while the peaks at around 1638 and 1588 cm 1 were assigned – O and C– – C or C– – N, respectively. Very to the stretching vibration of C– distinct peak absorbing at 1407 cm 1 was only observed in 1-CDs, which – O in carboxyl groups was attributed to the stretching vibration of C– [23,24]. Absorption band peaked at 1463 and 1352 cm 1 were detected in the spectra of 2-CDs, 3-CDs and 4-CDs, but showed no diagnostic signal from 1-CDs, these two bands were commonly related to the stretching and bending vibration of C–N [25,26], the formed C–N like groups could be originated from intermolecular dehydration and further amidation reactions between the carboxyl- and amino-groups, indi cating that PEI acted as not only an N-dopant, but also a surface passivation agent that can functionalize CDs surfaces with abundant amide groups. Moreover, the C–O–C groups with a broad band at 1050 cm 1 can be also observed for all CDs samples. Although different
Fig. 7. XPS survey spectra and high-resolution XPS of the C1s, O1s, and N1s spectra of the (a–d) 1-CDs, (e–h) 2-CDs, (i–l) 3-CDs and (m–p) 4-CDs. 5
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
nitrogen source participated in the synthesis of xylan-derived CDs, all resulted nanostructures were rich in multiple oxygen- and nitrogen-containing functional groups that contributed to their good water solubility. To gain insight into the intrinsic mechanism behind the EDE and EIE from xylan-derived CDs, XPS investigation was performed, and the analysis results were shown in Fig. 7. The survey spectra of all CDs samples presented four representative peaks of C1s (285 eV), N1s (400 eV), Na1s (496 eV), and O1s (532 eV), and the relative content of each element was displayed in the corresponding panel of survey spectrum. These findings suggest that the different CDs samples from 1-CDs to 4CDs were composed of the same elements but had different element content. Note that the detected signal of Na1s was generated from the purification of xylan using NaOH solution prior to synthesize CDs. Moreover, the xylan-derived CDs with PEI passivation contained much more nitrogen element than that their NH4OH doped counterparts, which mainly associated with the fact that the nitrogen content of PEI was higher than that of NH4OH; also, the easy decomposition of NH4OH during the heating process could be another reason for the lower ni trogen content of 1-CDs. In the high-resolution spectra, the C1s band of 1-CDs and 4-CDs can be resolved into four peaks, which were assigned to – C/C–C (284.7 eV), C–N (285.6–285.9 eV), C–O (286.6–286.7 eV), C– – O (287.7–288.1 eV). Unlike the 1-CDs and 4-CDs, the decon and C– volution of C1s spectra obtained from PEI passivated 2-CDs and 3-CDs demonstrated the lack of C–O bond, thus evidencing that the one-step PEI passivation will reduce C–O groups on the xylan-derived CDs sur faces. The decline of this oxygen-containing groups could lower non radiative recombination within CDs, and thus contribute to the enhancement of QY. The high-resolution XPS N1s spectra of 1-CDs and 4-CDs both exhibited four distinct peaks at around 398.8, 399.8, 400.9, and 402.2 eV, which associated with pyridinic N, pyrrolic N, graphitic N, and amino N, respectively [27], while for 2-CDs and 3-CDs, the analysis on the N1s spectra revealed two types of nitrogen including pyridinic N and pyrrolic N. The O1s band of 1-CDs and 4-CDs contained three typical – O, C–OH/C–O–C, peaks at 531.1, 532.4, and 533.5 eV, representing C– and C–O, respectively [28]. For the PEI passivated 2-CDs and 3-CDs, the – O and C–OH/C–O–C moieties were also confirmed from presence of C– the deconvolution of O1s band, but the C–O was not observed. Since extensive researches have confirmed that the surface-contained nitro gen- and oxygen-related groups play a vital role in tailoring spectral characteristics, the amounts of chemical bonds within these different CDs samples were further extracted for comparative analysis, as seen in – C/C–C component, which is considered as the main Table 1. The C– skeleton of carbogenic cores, demonstrated their higher level in 2-CDs and 3-CDs. Such phenomenon could be highly related to the PEI doping or passivation, the PEI with high content of carbon and nitrogen elements not only functionalize surface domains of CDs by chemical bonding, but also took part in the formation of carbogenic cores, accompanied by a certain degree of carbonization during the intense hydrothermal reaction. Among the synthesized xylan-derived CDs samples, it is apparent that the 3-CDs, which were synthesized in acidic – C/C–C bonds and a low condition, possessed the highest content of C– level of O-related moieties, evidencing that adding AC in precursor so lution could facilitate the dehydration, polymerization and carboniza tion of xylan, as well as contributed to efficient PEI doping or passivation, such behaviors could play an important role in enhancing
the product yield of CDs. Moreover, 2-CDs and 3-CDs contained the most amount of N atoms as forms of pyridine, which could be beneficial for the formation and introduction of electron traps within their surface band structures [29]. Based on the first-principles calculations, the ox ygen in carbon-based nanostructures as the form of C–OH (hydroxy) and C–O–C (epoxy) functional groups have a large possibility to create or tune band gap, and the formation of energy gaps depends on the con tents of C–OH/C–O–C groups [22,30,31]. Therefore, it is reasonable to believe that the large amount of C–O, C–OH, and C–O–C moieties within the 1-CDs and 4-CDs can introduce surface trapping states with a series of energy levels, resulting in that the CDs emit photons that vary with excitation energy. In general, the surface states are not created by single chemical group but the hybridization of the carbon backbones and connected functional groups. According to the findings above, we speculated that the hydroxy- and epoxide-related energy levels can be induced since the insufficient nitrogen doping in xylan-derived CDs structure, as a result of this characteristic, the different emissive trap sites led to multiple electron transition channels, and gave rise to PL emission with EDE behavior. Specifically, although the graphitic N has been confirmed to be prone to generate midgap states within π–π* gap of N-doped CDs and play an important role of the PL emission [32], but due to the fact that the graphitic N were in tiny amount in 1-, and 4-CDs, it is reasonable to deduce that the effect of graphitic N was far less than that of O-containing groups in dominating EDE performance of xylan-derived CDs. In the case of 2-, and 3-CDs, the efficient dehydration and amidation reaction in the synthesis system led to the decrease of – N and C–N C–OH, C–O–C, and C–O groups, and generated abundant C– bonds simultaneously, the surface-contained similar chemical species bonding with nano-sized carbon cores resulted in a uniform surface state that emit fluorescence with EIE performance. The above-mentioned intrinsic reasons for EIE of xylan-derived CDs are inconsistent with several previous reports in which the EIE performance of citric acid-derived CDs was considered to associate with amino groups-induced single energy level [33,34]. 3.4. Fluorescence mechanism of xylan-derived CDs Based on the above evidences, an energy-level model for the surface states within xylan-derived CDs was presented to visualize the distinc tion between EDE and EIE performances, as shown in Fig. 8. Extensive researches have manifested that the π–π* and n–π* transitions coexist in the CDs structures; however, the n–π* transition is more likely to pre – O bonds dominate emission behaviors due to the formation of C– [35–37]. In our case, both the FTIR arrangement and XPS analysis – O bonds within xylan-derived CDs demonstrated the existence of C– regardless of EDE and EIE behaviors, thus the n–π* transition was the main electron transition channel from ground state to excited state. Due to the 1-CDs contained plentiful C–OH, C–O–C, and C–O groups, the induced energy levels (O-related states) exhibited discrete distribution within the n–π* gap. The excited electrons transferred from ground state (n) to the highest excited state (π*), and then suffered from non-radiative relaxation from the highest excited state to the lowest excited state. After that, most of electrons were trapped by the multiple energy levels and finally returned to the ground state and gave rise to the fluorescence –N with EDE property. By contrast, the formation of plentiful C–N and C– moieties and the decrease of O-containing groups on CDs surfaces
Table 1 Relative contents of different chemical bonds in CDs samples as determined by XPS. Sample 1-CDs 2-CDs 3-CDs 4-CDs
C1s
N1s
O1s
C¼C/C–C
C–N
C–O
C¼O
pyridinic N
pyrrolic N
graphitic N
amino N
C¼O
C–OH/C–O–C
C–O
0.110 0.34 0.43 0.30
0.14 0.19 0.31 0.15
0.18 – – 0.22
0.11 0.11 0.05 0.11
0.007 0.064 0.068 0.009
0.011 0.049 0.037 0.008
0.010 – – 0.008
0.010 – – 0.006
0.27 0.14 0.08 0.10
0.10 0.04 0.02 0.06
0.05 – – 0.04
6
P. Yang et al.
Journal of Luminescence 223 (2020) 117199
Fig. 8. Energy-level diagram of xylan-derived CDs with EDE and EIE characteristics.
induced the generation of N-related state that contained concentrated energy levels within the n–π* gap. As a result, efficient electron transi tions can occur from N-related state to the ground state, thus contrib uting to CDs emitted PL featuring with EIE performance.
Appendix A. Supplementary data
4. Conclusions
References
In summary, by employing facile microwave-assisted hydrothermal method, the xylan-derived CDs featuring with EDE and EIE were syn thesized under the doping effect of NH4OH and PEI, respectively. Investigation on the PL mechanism of xylan-derived CDs revealed that surface states rather than the structure and size of carbon core mainly dominated the emission behaviors. Further, the large number of C–OH, C–O–C, and C–O groups on the surfaces of xylan-derived CDs were highly responsible for the EDE performance by introducing multiple energy levels within n–π* gap. Comparatively, the formation of plentiful nitrogen-related chemical bonds, and the decrease of oxygen-containing groups made the surface states became more uniform, the generated identical electronic transition channel resulted in the EIE behavior of xylan-derived CDs.
[1] F. Yuan, Z. Wang, X. Li, Y. Li, Z. Tan, L. Fan, S. Yang, Bright multicolor bandgap fluorescent carbon quantum dots for electroluminescent light-emitting diodes, Adv. Mater. 29 (2017) 1604436. [2] H. Ali, S.K. Bhunia, C. Dalal, N.R. Jana, Red fluorescent carbon nanoparticle-based cell imaging probe, ACS Appl. Mater. Interfaces 8 (2016) 9305–9313. [3] J. Wang, F. Qiu, X. Li, H. Wu, J. Xu, X. Niu, J. Pan, T. Zhang, D. Yang, A facile onepot synthesis of fluorescent carbon dots from degrease cotton for the selective determination of chromium ions in water and soil samples, J. Lumin. 188 (2017) 230–237. [4] J. Zhang, X. Liu, X. Wang, L. Mu, M. Yuan, B. Liu, H. Shi, Carbon dots-decorated Na2W4O13 composite with WO3 for highly efficient photocatalytic antibacterial activity, J. Hazard. Mater. 359 (2018) 1–8. [5] J. Chen, J.S. Wei, P. Zhang, X.Q. Niu, W. Zhao, Z.Y. Zhu, H. Ding, H.M. Xiong, Redemissive carbon dots for fingerprints detection by spray method: coffee ring effect and unquenched fluorescence in drying process, ACS Appl. Mater. Interfaces 9 (2017) 18429–18433. [6] K. Jiang, S. Sun, L. Zhang, Y. Wang, C. Cai, H. Lin, Bright-yellow-emissive N-doped carbon dots: preparation, cellular imaging, and bifunctional sensing, ACS Appl. Mater. Interfaces 7 (2015) 23231–23238. [7] X. Miao, D. Qu, D. Yang, B. Nie, Y. Zhao, H. Fan, Z. Sun, Synthesis of carbon dots with multiple color emission by controlled graphitization and surface functionalization, Adv. Mater. 30 (2018) 1704740. [8] X. Miao, X. Yan, D. Qu, D. Li, F.F. Tao, Z. Sun, Red emissive sulfur, nitrogen codoped carbon dots and their application in ion detection and theraonostics, ACS Appl. Mater. Interfaces 9 (2017) 18549–18556. [9] H. Ding, J.S. Wei, P. Zhang, Z.Y. Zhou, Q.Y. Gao, H.M. Xiong, Solvent-controlled synthesis of highly luminescent carbon dots with a wide color gamut and narrowed emission peak widths, Small 14 (2018) 1800612. [10] Z. Xu, C. Wang, K. Jiang, H. Lin, Y. Huang, C. Zhang, Microwave-assisted rapid synthesis of amphibious yellow fluorescent carbon dots as a colorimetric nanosensor for Cr(VI), Part. Part, Syst. Char. 32 (2015) 1058–1062. [11] X. Wang, K. Qu, B. Xu, J. Ren, X. Qu, Microwave assisted one-step green synthesis of cell-permeable multicolor photoluminescent carbon dots without surface passivation reagents, J. Mater. Chem. 21 (2011) 2445–2450. [12] Y.Y. Zhang, M. Wu, Y.Q. Wang, X.W. He, W.Y. Li, X.Z. Feng, A new hydrothermal refluxing route to strong fluorescent carbon dots and its application as fluorescent imaging agent, Talanta 117 (2013) 196–202. [13] S.A. Hill, D. Benito-Alifonso, D.J. Morgan, S.A. Davis, M. Berry, M.C. Galan, Threeminute synthesis of sp(3) nanocrystalline carbon dots as non-toxic fluorescent platforms for intracellular delivery, Nanoscale 8 (2016) 18630–18634. [14] Z.Q. Zhu, P. Yang, M.Z. Chen, T. Zhang, Y.Z. Cao, W. Zhang, X.Y. Zhou, W.M. Chen, Microwave synthesis of amphiphilic carbon dots from xylose and construction of luminescent composites with shape recovery performance, J. Lumin. 213 (2019) 474–481. [15] J. Liu, D. Li, K. Zhang, M. Yang, H. Sun, B. Yang, One-step hydrothermal synthesis of nitrogen-doped conjugated carbonized polymer dots with 31% efficient red emission for in vivo imaging, Small 14 (2018) 1703919. [16] S. Qu, D. Zhou, D. Li, W. Ji, P. Jing, D. Han, L. Liu, H. Zeng, D. Shen, Toward efficient orange emissive carbon nanodots through conjugated sp2-domain controlling and surface charges engineering, Adv. Mater. 28 (2016) 3516–3521. [17] H. Zhang, X. Dong, J. Wang, R. Guan, D. Cao, Q. Chen, Fluorescence emission of polyethylenimine-derived polymer dots and its application to detect copper and hypochlorite ions, ACS Appl. Mater. Interfaces 11 (2019) 32489–32499.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2020.117199.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Pei Yang: Conceptualization, Methodology, Formal analysis, Writing - original draft. Ziqi Zhu: Investigation. Wei Zhang: Investi gation. Tao Zhang: Investigation. Xinghui Li: Investigation. Min Luo: Investigation. Weimin Chen: Investigation. Minzhi Chen: Conceptu alization. Xiaoyan Zhou: Supervision, Writing - review & editing, Project administration. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 31870549], the Jiangsu Nature Science Foun dation [grant number BK20161524]; the Program for 333 Talents Project in Jiangsu Province [grant Number BRA2016381]; the Post graduate Research & Practice Innovation Program of Jiangsu Province [grant number KYCX17_839]; and the Advanced Analysis and Testing Center of Nanjing Forestry University.
7
P. Yang et al.
Journal of Luminescence 223 (2020) 117199 [29] Y. Liang, Y.F. Shen, C.L. Liu, X.Y. Ren, Effects of chemical bonds between nitrogen and its neighbor carbon atoms on fluorescence properties of carbon quantum, J. Lumin. 197 (2018) 285–290. [30] C.T. Chien, S.S. Li, W.J. Lai, Y.C. Yeh, H.A. Chen, I.S. Chen, L.C. Chen, K.H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, C. W. Chen, Tunable photoluminescence from graphene oxide, Angew. Chem. Int. Ed. 51 (2012) 6662–6666. [31] Y. Zhang, D.Q. Fang, S.L. Zhang, Y.H. Wen, Z.Z. Zhu, Structures and electronic properties of oxidized graphene from first-principles study, Europhys. Lett. 105 (2014) 37005. [32] K. Hola, M. Sudolska, S. Kalytchuk, D. Nachtigallova, A.L. Rogach, M. Otyepka, R. Zboril, Graphitic nitrogen triggers red fluorescence in carbon dots, ACS Nano 11 (2017) 12402–12410. [33] Y.H. Yuan, Z.X. Liu, R.S. Li, H.Y. Zou, M. Lin, H. Liu, C.Z. Huang, Synthesis of nitrogen-doping carbon dots with different photoluminescence properties by controlling the surface states, Nanoscale 8 (2016) 6770–6776. [34] B. Li, Y. Guo, A. Iqbal, Y. Dong, W. Li, W. Liu, W. Qin, Y. Wang, Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface, RSC Adv. 6 (2016) 107263–107269. [35] Y. Zhang, Y. Hu, J. Lin, Y. Fan, Y. Li, Y. Lv, X. Liu, Excitation wavelength independence: toward low-threshold amplified spontaneous emission from carbon nanodots, ACS Appl. Mater. Interfaces 8 (2016) 25454–25460. [36] Y. Zhang, P. Zhuo, H. Yin, Y. Fan, J. Zhang, X. Liu, Z. Chen, Solid-state fluorescent carbon dots with aggregation-induced yellow emission for white light-emitting diodes with high luminous efficiencies, ACS Appl. Mater. Interfaces 11 (2019) 24395–24403. [37] S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: from fluorescence mechanism to up-conversion bioimaging applications, Adv. Funct. Mater. 22 (2012) 4732–4740.
[18] S.G. Liu, T. Liu, N. Li, S. Geng, J.L. Lei, N.B. Li, H.Q. Luo, Polyethylenimine-derived fluorescent nonconjugated polymer dots with reversible dual-signal pH response and logic gate operation, J. Phys. Chem. C 121 (2017) 6874–6883. [19] P. Yang, Z.Q. Zhu, M.Z. Chen, W.M. Chen, X.Y. Zhou, Microwave-assisted synthesis of xylan-derived carbon quantum dots for tetracycline sensing, Opt. Mater. 85 (2018) 329–336. [20] C. Xia, S. Zhu, T. Feng, M. Yang, B. Yang, Evolution and synthesis of carbon dots: from carbon dots to carbonized polymer dots, Adv. Sci. (2019) 1901316. [21] H. Liu, R.S. Li, J. Zhou, C.Z. Huang, Branched polyethylenimine-functionalized carbon dots as sensitive and selective fluorescent probes for N-acetylcysteine via an off-on mechanism, Analyst 142 (2017) 4221–4227. [22] S. Hu, A. Trinchi, P. Atkin, I. Cole, Tunable photoluminescence across the entire visible spectrum from carbon dots excited by white light, Angew. Chem. 54 (2015) 2970–2974. [23] F. Li, C. Liu, J. Yang, Z. Wang, W. Liu, F. Tian, Mg/N double doping strategy to fabricate extremely high luminescent carbon dots for bioimaging, RSC Adv. 4 (2014) 3201–3205. [24] Y. Dong, J. Shao, C. Chen, H. Li, R. Wang, Y. Chi, X. Lin, G. Chen, Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid, Carbon 50 (2012) 4738–4743. [25] H. Ding, S.B. Yu, J.S. Wei, H.M. Xiong, Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism, ACS Nano 10 (2016) 484–491. [26] M. Zheng, S. Liu, J. Li, D. Qu, H. Zhao, X. Guan, X. Hu, Z. Xie, X. Jing, Z. Sun, Integrating oxaliplatin with highly luminescent carbon dots: an unprecedented theranostic agent for personalized medicine, Adv. Mater. 26 (2014) 3554–3560. [27] S. Lu, L. Sui, J. Liu, S. Zhu, A. Chen, M. Jin, B. Yang, Near-infrared photoluminescent polymer-carbon nanodots with two-photon fluorescence, Adv. Mater. 29 (2017) 1603443. [28] W. Li, S. Wu, H. Zhang, X. Zhang, J. Zhuang, C. Hu, Y. Liu, B. Lei, L. Ma, X. Wang, Enhanced biological photosynthetic efficiency using light-harvesting engineering with dual-emissive carbon dots, Adv. Funct. Mater. 28 (44) (2018) 1804004.
8