Photoluminescent carbon nanodots: synthesis ...

Materials Today Volume 18, Number 8 October 2015

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Photoluminescent carbon nanodots: synthesis, physicochemical properties and analytical applications Prathik Roy, Po-Cheng Chen, Arun Prakash Periasamy, Ya-Na Chen and Huan-Tsung Chang* Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan

Carbon nanodots (C-dots) possessing photoluminescence (PL) properties have become interesting materials for sensing and imaging, with the advantages of water-dispersibility, biocompatibility, chemical and photo stability. They can be prepared from organic matter such as tea, grass, coffee, and small organic molecules like glycine and glucose through hydrothermal routes. In this review, we focus on the recent advances in the synthesis and characterization of C-dots along with their optical (absorption, PL, upconverted PL) properties and analytical applications. Having bright PL, biocompatibility, chemical and photo stability, as well as low toxicity, C-dots have been used for the detection of metal ions and for cell imaging. C-dots prepared from organic matter such as used tea and ginger possess a great inhibitory effect on the growth of cancer cells, showing their excellent potential as new drugs. Introduction Carbon nanodots (C-dots) have become interesting materials for sensing of various analytes and for imaging cells, mainly because of their strong photoluminescence (PL), biocompatibility, chemical and photo stability [1–8]. Furthermore, carbon precursors such as ground coffee [9], used tea [10], candle soot [11] and grass [12] which can be used to prepare C-dots are abundant and costeffective. C-dots usually comprise of discrete, quasispherical nanoparticles (NPs) with sizes below 10 nm [13]. C-dots have apparent sp2 characters and possess many functional groups such as amino, epoxy, ether, carbonyl, hydroxyl, and carboxylic acid on their surface [14]. The rich surface-functional groups of C-dots result in their high hydrophilicity and readiness for functionalization with various organic, polymeric, inorganic, or biological species [15– 18]. C-dots have interesting PL properties that are dependent on their size, edge shape, surface ligands (passivation agents), and defects [12,14,19–24]. The excitation-wavelength (lex) dependent PL is an interesting feature of many C-dots; when the C-dots are excited from ultra violet (UV) to visible light, their emission *Corresponding author:. Chang, H.-T. ([email protected])

wavelengths vary from UV to the near-infrared (NIR) region [25–27]. On the other hand, lex-independent PL properties of highly uniform sizes of C-dots result from their homogeneous surface states of sp2 clusters have been reported [4,11,14,21]. This review briefly summarizes the preparation of C-dots, their physicochemical properties and the most recent advances of Cdots over the last five years, to shed light on their great potential in analytical applications. While C-dots have been used in catalysis, photovoltaic devices, optoelectronic devices, and energy storage [28–33], these topics are beyond the purview of this paper.

Synthesis Synthetic methods for C-dots can be classified into two main categories, namely, top-down and bottom-up synthetic approaches [28,32]. Top-down methods for the preparation of C-dots from starting materials such as graphite powder or multi-walled carbon nanotubes (MWCNTs) are usually conducted under harsh physical or chemical conditions [1,11,34]. On the other hand, small molecules such as glucose and fructose are used to prepare C-dots through bottom-up approaches by applying external energy such as ultrasonication, microwave pyrolysis, and heating [35,36].

1369-7021/ß 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://dx.doi.org/10.1016/ j.mattod.2015.04.005

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Top-down approaches Arc-discharge method The arc-discharge method has been applied to prepare C-dots with a quantum yield (QY) of 1.6% from crude carbon nanotube soot [8]. After oxidation of the crude material (soot) with 3.3 M HNO3, the oxidized materials were extracted with alkaline solution (pH 8.4). The extracted materials were then purified by conducting gel electrophoresis. RESEARCH: Review

Laser ablation Using a laser ablation method with a Nd:YAG laser (1064 nm, 10 Hz), C-dots were synthesized separately from graphite powder and cement in a flow of argon gas carrying water vapor [1]. After oxidation of the crude products with 2.6 M HNO3 for 12 h and surface passivation with polymers such as diamine-terminated poly(ethylene glycol) (PEG1500N) and poly(propionylethyleneimine-co-ethyleneimine) (PPEI-EI), bright PL C-dots were obtained both in solution and solid states, with QYs ranging from 4.0% to 10%. The QY of as-prepared C-dots could be further improved through doping with inorganic salts (e.g. zinc acetate and Na2S or NaOH), in which the dopants (e.g. ZnS and ZnO) likely functioned as a secondary passivating agent for the C-dots [37]. The resulting doped C-dots showed strong PL (QY 45%) when excited at 450 nm. Different colors of PL C-dots were formed in various solvents and aqueous solutions, in which the organic molecules served as passivation ligands. The QY values of the C-dots prepared were 3.0–8.0% [38].

Electrochemical methods Electrochemical treatment of MWCNTs grown on a carbon paper in a degassed acetonitrile solution containing tetrabutylammonium perchlorate as a supporting electrolyte was conducted to prepare C-dots by applying cycling potentials ranging from 2.0 to 2.0 V [39]. An intense blue PL at 410 nm was observed when the as-prepared C-dots were excited at 365 nm, with a QY of 6.4%. By adjusting the ionic liquid/water ratios, different carbon nanomaterials including C-dots were formed (Fig. 1) [40]. The anodic oxidation of water formed hydroxyl and oxygen radicals that subsequently oxidized the graphitic electrode, leading to the release of graphene sheets (GSs) that were stabilized by the ionic liquid anions. The QY values of the produced C-dots were in the range of 2.8–5.2%. Using graphite rods as both the anode and the cathode, water-dispersible C-dots were prepared in alkaline solution (NaOH/EtOH) [21]. By varying the current density in the range of 20–180 mA cm 2, the size of the C-dots was controlled; a lower current density led to the formation of larger C-dots with longer emission wavelengths. The highest QY of the C-dots was around 12%.

Bottom-up approaches Thermal routes The combustion soot of candles has been used as a starting material for C-dots. C-dots were obtained after being treated with an oxidant such as HNO3 and H2O2/AcOH [11]. As-prepared Cdots were separated by polyacrylamide gel electrophoresis, showing that C-dots with higher mobility had PL at shorter emission wavelengths. The QY values of C-dots ranged from 0.8% to 1.9%. Treatment of the soot from natural gas with HNO3, followed by 448

FIGURE 1

Ionic liquid-assisted electrochemical exfoliation of the graphite anode. Reprinted with permission from Ref. [40]. Copyright 2012 American Chemical Society.

neutralization with sodium bicarbonate and purified by dialysis led to the formation of PL C-dots [34]. By separately adding metal salts, including AgNO3, Cu(NO3)2, and PdCl2, to the C-dots solutions, metal nanostructures (NSs) were formed on the surfaces of C-dots in the presence of a reducing agent (ascorbic acid) [34]. The QYs of C–Ag, C–Pd, and C–Cu nanocomposites were 36.7%, 33.4%, and 60.1%, respectively.

Microwave-assisted methods Transparent aqueous solutions of saccharides and PEG200 at various concentrations were heated in a microwave oven operating at 500 W for 2–10 min to prepare C-dots [35]. As-prepared C-dots showed interesting lex-dependent PL properties when excited at the wavelengths ranging from 330 to 460 nm. The QYs of C-dots ranged from 3.1% to 6.3%. C-dots were also prepared by subjecting glucose to either a strong acidic or alkaline solution under ultrasonication for 4 h [36]. The PL emission of the C-dots obtained covered the entire visible to near infrared (NIR) spectral range. The C-dots possessed up-conversion PL properties when excited at 700–1000 nm, showing emission in the wavelength range of 450–750 nm, with QY values up to 7%.

Hydrothermal and aqueous based methods Hydrothermal treatment of used coffee ground has been used to prepare PL C-dots [41]. The used coffee grounds were dried in an oven prior to being ground into fine powder, which was then placed in an autoclave and calcined in air at 300 8C for 2 h. The Cdots (QY 3.8%) were formed through four consecutive stages: dehydration, polymerization, carbonization, and passivation. A similar green approach was applied to prepare C-dots from used green tea at 300 8C for 2 h. The resulting black carbonized powder was re-suspended in ultrapure water and then subjected to dialysis to further purify the C-dots. The QY of as-prepared highly watersoluble PL C-dots was 4.3% [10]. The abundant catechins in the

green tea likely played an important role in the formation of Cdots and passivation of their surfaces. Four different molecules (glycine, cadaverine, 2-amino-2-hydroxymethyl-propane-1,3-diol (TRIS), and ethylene diamine-tetraacetic acid (EDTA)) containing either an amino group or a carboxyl group or both in aqueous solutions were separately calcined hydrothermally at 300 8C for 2 h (Fig. 2) [9]. The result revealed that precursors possessing both amino and carboxyl groups were superior for the preparation of highly water-dispersible and PL C-dots. The QY values of C-dots synthesized from glycine, EDTA, TRIS, and cadaverine were 30.6%, 26.6%, 26.0%, and 5.4%, respectively. (3-Aminopropyl)trimethoxysilane (APTMS) as a precursor was used for the synthesis of organosilane-functionalized C-dots at 300 8C for 2 h, without the introduction of an additional passivating agent [42]. The QY value of APTMS SiC-dots was determined to be 42.6%. Similarly, ammonium citrate and 4-aminoantipyrine as the carbon sources were used to prepare C-dots in air at 300 8C for 2 h [43]. Different organic ammonium species, serving as covalently attached surface modifiers, altered the hydrophilicity and QY of the C-dots. For example, the QY of C-dots prepared from 2-(2-aminoethoxy)-ethanol citrate salt was 3%. Furthermore, EDTA was used to prepare C-dots at 400 8C for 2 h in a nitrogen atmosphere [44]. Some of the EDTA precursors were incompletely decomposed, which were incorporated onto the C-dots, leading to improved hydrophilicity. The QYs of C-dots were measured to be up to 40.6%. Carbohydrates such as glucose, sucrose, and starch as starting materials in the presence of a strong acid such as sulfuric acid have also been used to prepare C-dots [3]. Successive treatment of these solutions with nitric acid broke down the carbonaceous materials to form carbon nanomaterials including C-dots and introduced carboxyl groups on their surfaces. Surface passivation with polymers or

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organic molecules was required to enhance their PL intensity. For example, 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) was used to passivate as-prepared C-dots, with a QY value of 13%. Surfactant-modified silica spheres were used to prepare C-dots [2]. Colloidal silica spheres were first functionalized with an amphiphilic triblock copolymer F127 ((ethylene oxide)106(propylene oxide)70(ethylene oxide)106). Phenol/formaldehyde resins as the carbon source of the C-dots reacted with F127 functionalized silica spheres under basic conditions at 66 8C for 2 days, leading to the formation of resol/F127/SiO2. The dried satellite-like resol/ F127/SiO2 composites were then calcined to remove F127, and SiO2 spheres were etched away with NaOH. After surface passivation of the C-dots with PEG1500N, strong PL (QY 14.7%) was observed when excited at 365 nm. Citric acid in the presence of mesoporous silica nanospheres as supports was used to prepare Cdots [45]. Because C-dots were confined in the supports, particle aggregation greatly reduced, leading to strong PL (QY 23%). The as-prepared C-dots emitted bright blue PL and possessed up-conversion PL properties. Alternatively, magnesium-substituted microporous aluminophosphate molecular sieves were used for the preparation of C-dots [46]. By varying synthetic conditions such as isothermal heating time (5 min to 2 h) and choosing different carbon sources (acetone, ethanol, acetic acid, or n-hexane), different colors of C-dots were obtained, with QYs up to 40%.

Physicochemical properties Absorption C-dots usually have an apparent optical absorption in the UV region, with a tail extending out into the visible region. A band centered on 250–300 nm that is known as a p–p* transition peak is common in most C-dots (Fig. 3). C-dots prepared via top-down methods usually reveal size-dependent absorption properties,

FIGURE 2

The four stages of C-dots formation, using glycine as a model precursor molecule: dehydration, polymerization, carbonization, and passivation. Reprinted with permission from Ref. [9]. Copyright 2010 John Wiley & Sons. 449

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RESEARCH: Review FIGURE 4 FIGURE 3

Absorption and PL emission spectra, with increasingly longer excitation wavelengths (in 15 nm increments starting from 350 nm) of C-dots. Inset: PL photograph of a C-dots solution under a hand-held UV lamp (365 nm). The normalized PL intensities are plotted in arbitrary units. Reprinted with permission from Ref. [41]. Copyright 2012 Royal Society of Chemistry.

ranging from 6.20 to 4.92 eV (200–252 nm) as the size is increased from 12 to 22 nm [47,48]. Moreover, the surface functional groups also play some roles in determining the absorption wavelength of C-dots. For example, the absorption band of C-dots exhibits a red shift after functionalization with amino groups (ammonia or TTDDA) [3,49]. Interestingly, C-dots prepared via different methods also reveal different optical absorption features. For example, glucose-based C-dots prepared through hydrothermal, microwave thermal, and ultrasonic methods possess different absorption band shapes around 250–300 nm [36,50–52].

Photoluminescence (PL) The two main PL mechanisms that have been suggested for C-dots are defect state emission (surface energy traps) and intrinsic state emission (electron–hole recombination, quantum size effect/zigzag sites) [24]. The blue emission (shorter wavelength) is from the intrinsic state emission and the green emission (longer wavelength) is from the defect states, which were confirmed by the PL colors of C-dots grafted with amino functional groups (m-Cdots) and C-dots reduced by NaBH4 (r-C-dots) as shown in Fig. 4. The emission of r-C-dots is at the UV region because their surface carboxylic functional groups were eliminated. As a result, their surface state decreased and the dominant PL was from the intrinsic state, which was further supported by the PL of pristine C-dots that had almost no surface oxygen groups (defects) [53]. Luminescence decays of C-dots revealed multiexponential PL decays with an average excited-state lifetime of 5 ns for emission at 450 nm. The multiexponential nature of the lifetime suggested the presence of different emissive sites of PL process. It was suggested that PL from intrinsic state decayed (t < 5 ns) faster than that of the defect states (10 ns > t > 5 ns) [24,53]. It is easier to govern PL properties of C-dots via controlling their surface functional groups (defect states) rather than changing their 450

Scheme of bandgap changing of C-dots, m-C-dots and r-C-dots. Reprinted with permission from Ref. [24]. Copyright 2012 John Wiley & Sons.

core composition (intrinsic states). In other words, reduction and passivation of the surfaces of C-dots are efficient to remove their non-radiative electron-hole recombination centers such as epoxy/ carboxylic acid groups and to increase their QY [54]. Passivating agents are usually amino-containing molecules or polymers including TTDDA [3], 1-hexadecylamine [55], octadecylamine [56], PEG1500N, N-(b-aminoethyl)-g-aminopropyl methyldimethoxy silane [57]. The QYs of C-dots before and after passivation with TTDDA were 1% and 13%, respectively. To simplify the passivation process from two steps to one step, co-pyrolysis or one-pot hydrothermal route of carbon precursors and passivation agents such as ammonia solution [49], branched polyethylenimine [58], dimethylformamide [59] and ethylenediamine [60] have been proposed. The C-dots prepared from citric acid and ethylenediamine exhibited blue PL with high QY value of 80.6% [60]. Furthermore, the precursors with amino groups such as glycine [9], chitosan [61] and APTMS [42] were demonstrated to prepare Cdots without passivation that possessed high QY values (30.6%, 43%, 42.6%, respectively). Moreover, chemical reduction by NaBH4 [62], hydrazine (N2H4) [20] and photochemical reduction by UV light [63] were demonstrated to increase the QY value of Cdots. C-dots also show fascinating lex-dependent PL properties (Fig. 3) that are quite different from popular PL nanomaterials such as Au nanodots, Ag nanoclusters (NCs), and semiconductor quantum dots [42,64–69]. Various sizes of C-dots with different PL emission wavelengths ranging from UV to NIR, have been prepared via different synthetic routes. Though much effort has been dedicated to the research on C-dots, the exact PL mechanism remains unsettled and requires further investigation. The PL of C-dots has been suggested to relate to their surface energy traps/ states (some defects on the surface of C-dots), recombination of electron/hole pairs of carbon, quantum confinement effect, polyaromatic structures, edge effects (zigzag and armchair type), and oxygen-containing groups [1,20,22,24,49,62]. The size of C-dots should be small enough to support quantum confinement of emissive energy traps to the particle surface. The role of surface

passivation by the organic moieties makes the surface sites more stable in order to facilitate more effective radiative recombination [70,71]. The formation of excitons (electron/hole pairs) on the surface of C-dots is explained through photoinduced charge separation, which is further supported by PL quenching in the presence of electron/hole scavenger [19]. The C-dots prepared from hydrothermal cutting of GSs also reveal size-dependent PL properties; PL wavelength shifts from 450 nm (2.75 eV) to 486 nm (2.55 eV) as the size of C-dots is increased from 5 to 17 nm [48]. Their PL may originate from free zigzag sites with a carbene-like triplet ground state, based on their pH dependent PL properties [75]. As shown in Fig. 5, the carbenelike zigzag sites are protonated at low pH, resulting in the breakdown of emissive triple carbene state and thus PL quenching. It is worth pointing out the carbene-like structures are mostly present in the zigzag sites. When the shape of C-dots changes from circular/elliptical, which is mixed edges of zigzag and armchair, to polygonal with mostly armchair edge, their size increases from 5 to 35 nm, leading to increased PL energy that is in contradiction to the quantum size effect of C-dots [48]. The edge effect on the PL of C-dots was further supported with similar PL profiles of C-dots and graphitic structures, which possess similar edge types, but different sizes and shapes [23,76]. The PL originates from the transition of the lowest unoccupied molecular orbital (LUMO) to the highest

FIGURE 5

Mechanism for the hydrothermal cutting of oxidized GSs into GQDs: (a) mixed epoxy chain composed of epoxy and carbonyl pair groups (left) is converted into a complete cut (right) under the hydrothermal treatment. (b) Models of the GQDs in acidic (right) and alkali (left) media. The two models can be converted reversibly depending on pH. The pairing of s(*) and p(*) localized electrons at carbene-like zigzag sites and the presence of triple bonds at the carbene-like armchair sites are represented. Reprinted with permission from Ref. [22]. Copyright 2010 John Wiley & Sons.

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occupied molecular orbital (HOMO). The HOMO–LUMO gap is dependent on the size of the fragments. If the gap increases inversely proportional to the increases in the size of C-dots, various sizes of C-dots would have different excitation and emission spectra [21]. In addition, smaller C-dots usually have stronger PL intensity. Most reported C-dots, no matter prepared via top-down or bottom up methods, possess oxygen containing groups, including carbonyl, epoxy/ether, and carboxylic acid, on their surfaces. These functional groups form surface states of C-dots, with energy levels between p and p* states [50]. The maximum emission wavelength red-shifted as the oxygen-related surface state increased when C-dots were oxidized by supplying a high voltage [14]. C-dots reduced with NaBH4 showed that the number of oxygen containing groups affected the emission wavelength and QY of C-dots [62]. Recently, doping other atoms such as nitrogen and sulfur to create new surface states has also been demonstrated as an efficient way to control PL emission wavelength and to increase QY [77]. The unique lex dependence of the emission wavelength and intensity of C-dots is likely due to inhomogeneous size distribution of C-dots and/or emissive traps [1]. Size-dependence was demonstrated as a plausible reason for lex-dependent PL properties [21,59]. Gel electrophoresis and high-performance liquid chromatography have been applied for the fractionation of C-dots to resolve C-dots with different sizes/shapes and charges that show lex-independent PL properties [4,11,21,72–74]. C-dots with sizes of 1.2, 1.5–3, and 3.8 nm emit at the UV (350 nm), visible (400– 700 nm), and NIR regions, respectively. The separated C-dots possessed different maximum emission wavelengths and QYs, revealing important roles of the sizes/shapes and charges of Cdots playing in determining their PL properties. On the other hand, the lex-dependent PL properties of various C-dots having same sizes were mainly due to the number of surface oxidation degrees [62]. C-dots with higher oxidation degrees revealed lexdependent PL properties over the wavelength range from 280 to 380 nm, while C-dots with lower oxidation degrees did not reveal such PL properties. Oxidation facilitated to generate oxide defects on the surface of the C-dots, which would introduce complex surface states to trap the excitons under excitations. The radiative recombination of those surface-trapped excitons would give the PL emission with corresponding energy. Consequently, C-dots with higher oxidation degrees possessed more surface states, resulting in PL emissions with varied energies at different excitations [78]. The two plausible mechanisms seem to be contradictory to each other and thus more detailed study of the PL of C-dots is required. Besides typical PL properties, C-dots possess up-conversion properties [79–81], which is a process that the sequential absorption of two or more photons leads to the emission of light at a shorter wavelength than the excitation wavelength. C-dots showed up-conversion emissions located in the range from 325 to 425 nm when excited at a longer wavelength (from 500 to 1000 nm), which was attributed to the multiphoton active process [1,21]. C-dots prepared by hydrazine hydrate reduction of graphene oxide (GO) exhibited up-conversion emissions peaks shifted from 390 to 468 nm when the excitation wavelength was changed from 600 to 800 nm [82]. The spectrum was regarded as an anti-Stokes transition, where the energy levels of p and s 451

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orbitals were provided by the carbene ground-state multiplicity. When a bunch of low-energy photons excited the electrons of the p orbital, the p electrons were transited to a high-energy state such as the lowest unoccupied molecular orbital (LUMO), and then the electrons transited back to a low-energy state. Thus, an up-conversion PL emitted when the electrons transited back to the s-orbital. More recently, the up-conversion PL of C-dots has been suggested to originate from the excitation of second-order diffraction light (l/2) [82]. RESEARCH: Review

Morphologies and surface properties C-dots are quasispherical or spherical shaped particles [83], with sizes in the range of 1–10 nm, depending on their preparation conditions and precursors [84,92–96]. C-dots produced from the combustion soot of a natural gas burner were 4.8 0.6 nm in size, with lattice spacing similar to that of graphite (Fig. 6a and inset). The C-dots synthesized through the electrochemical oxidization of carbon fibers were spherical in shape, regardless of the applied potentials used (Fig. 6b). Smaller sized C-dots (