Review articleLong-wavelength, multicolor, and white-light emitting carbon-based dots: Achievements made, challenges remaining, and applications
Graphical abstract
Introduction
Photoluminescent nanomaterials have drawn considerable scientific attention for a variety of purposes and applications. For many years, researchers focused on semiconductor nanocrystals, known as quantum dots (QDs), because of their superior performance over conventional organic dyes, especially their adjustable emission color that could be simply tuned by varying the size of nanocrystals [1], [2], [3], [4]. However, high-performance QDs are usually composed of toxic heavy metal elements that have raised serious concerns about their applications in the medical field [5], [6]. Therefore, the search for developing alternative photoluminescent nanomaterials with lower toxicity is of increasing interest.
In this regard, photoluminescent carbon-based dots (C-bDs) including graphene quantum dots (GQDs), carbon quantum dots (CQDs), and carbon dots (CDs) have emerged as potential platforms in the development of new photoluminescent nanomaterials [7], [8], [9], [10]. Since the first report on the discovery of CDs in 2004 [11], many studies have been directed to these nanomaterials to achieve better synthesis routes, improve their photoluminescence (PL) quantum yields (QY), explore their physico-chemical characteristics, and develop their application. Today, remarkable progresses have been achieved in developing new synthesis approaches for large-scale and inexpensive production of C-bDs from either bulky carbon structures through “top-down” methods or by small molecules using “bottom-up” methods [12]. Although the QY of C-bDs in the early years of discovering barely reached few percent, engineering the surface functionality and doping appropriate heteroatoms have resulted in significant improvement in their PL QYs [13], [14]. Several fascinating properties such as high resistance to photobleaching, low-cost and easy preparation, ease of bioconjugation, and, more importantly, lack of intrinsic toxicity and incorporated toxic elements have revealed the high potential of C-bDs as appropriate replacements for organic dyes and heavy metal-based QDs in various fields such as electronics, sensing, and biology.
However, the emission color variation in C-bDs is less pronounced compared with that found in traditional semiconducting QDs, and C-bDs mostly emit a blue/green color [15]. Unlike QDs, the multicolor emission of C-bDs cannot be achieved by simply varying the particle size alone [16]; in fact, in most cases, the PL color is related to the surface groups rather than the size. The critical importance of long-wavelength, multicolor, and white-light-emitting phosphors in bioimaging, multicolor patterning, and white-light-emitting diodes (WLEDs), respectively, together with applications such as sensor arrays and full-color displays have recently directed substantial attention to preparing C-bDs with such exceptional optical properties. Although different emission colors can be observed by varying the excitation wavelength of C-bDs, this is not identified as real PL tuning. What is truly required is C-bDs with multiple emission colors under the same excitation wavelength, similar to semiconducting QDs that can be excited by a broadband light without compromising the peak emission intensity. Preparation of C-bDs with such features has proven to be a challenging task, mainly because of their complicated PL mechanism.
The purpose of this comprehensive literature review is to summarize successes to date in preparing long-wavelength, multicolor, and white-light-emitting C-bDs. We focus on the developments in using specific precursors, synthetic methods, heteroatom doping, and post treatments such as surface passivation, separation, and purification methods to achieve this goal. The related reports are discussed in detail in the categorized sections. Although some reports might correspond to more than one special section, we have tried to discuss them in the most fitting parts. In this review, C-bDs refer to various nanosized photoluminescent carbon materials including CDs as amorphous quasi-spherical nanodots without crystal lattices and quantum confinement (also referred in some published papers as carbon nanoclusters; carbon nanodots, CNDs; and carbon nanoparticles, CNPs) and nanodots with quantum confinement and crystalline structure including spherical QDs, referred as CQDs, and the π-conjugated single-layer or few-layer graphene QDs, also referred as GQDs [17].
Section snippets
PL mechanism in C-bDs
A common fascinating feature of all types of C-bDs is their PL property, which appears similar if not identical. However, the diversity and complexity of C-bDs make the PL of these nanomaterials complicated, and despite many literature reports discussing the origin of PL emissions, it remains a matter of debate. Generally, quantum size effect [18], [19], [20], surface state (surface defects) [21], [22], [23], [24], molecular and molecule-like states [25], [26], [27], and even synergistic
Carbon source
Indeed, C-bDs are special PL nanomaterials that have been prepared from a wide variety of initial sources. As C-bDs produced from different sources show almost similar Sp2 conjugated carbon structures in their cores and similar nitrogenated/oxygenated functional groups on their surface, it might be expected that they must exhibit similar PL properties. However, a literature search reveals that a regular change in the initial source in some cases can result in C-bDs with obviously different
Solvent
Many reports investigating the effect of solvents on the synthesis of C-bDs reveal a negligible effect of this factor on the PL color of CNPs [82], [83], [84], [85]. For example, refluxing of the candle soot [82] or graphite powder [85] in different organic solvents such as ethylene glycol, ethanol, 1-butanol, cyclohexane, toluene, and dimethylbenzene or laser irradiation of a carbon powder suspension in various organic solvents including PEG200N, diamine hydrate, and diethanolamine resulted in
Surface modification
The PL color of C-bDs is mostly related to surface groups rather than size. While the way by which these surface groups affect the PL remains poorly understood, conventional technologies for PL enhancing, PL modulation, sensing application of C-bDs based on engineering surface states, chemical doping, and surface passivation have been reported. Many studies show that PL centers are located at surface states, which are hybrid structures formed by oxygen/nitrogen-containing functional groups at
Heteroatom doping
Similar to surface functionalization, many researcher reports support the importance of heteroatom doping, especially nitrogen doping, on improving the PL QYs and tuning the PL color of C-bDs [14], [168], [169]. In a work by Niu’s group [87], it was theoretically demonstrated that even different N-doping types and positions led to different absorption and emission behaviors. The center N-doping created non-fluorescent mid-states, while edge N-doping modulated the energy levels of excited states
Separation methods
Purification and size-based separation of nanoparticles is an essential step in the preparation of well-defined nanoparticles for both applications and fundamental studies. In this regard, both size and charge separation methods have turned out as attractive approaches in the purification and separation of C-bDs with different PL properties. Gel electrophoresis, silica column chromatography, column chromatography, size-exclusion chromatography, dialysis, ultrafiltration, and centrifugal methods
Excitation-dependent PL behavior in C-bDs
The excitation-dependent PL behavior is one of the most interesting properties of C-bDs because different emission colors from these nanomaterials can be observed by simply changing their excitation wavelength, without varying the chemical composition or size. Despite the differences in the size, shape, chemical surface, and synthesis method, most reported C-bDs still exhibit more and less similar excitation-dependent PL behavior [92], [203], [204], [205], [206], [207], [208], [209], [210],
Size-dependent PL in C-bDs
Generally speaking, despite some reports [123], [189], in most situations, PL of CDs and CQDs is related to their surface groups rather than their size. On the other hand, GQDs exhibit a highly size-dependent PL emission [227]. Although graphene is an extended π-network known as a zero-band gap material, reduction of GO sheets or cutting a graphene sheet into small pieces can create isolated nanosized sp2 islands with conjugated π-domains typically immersed in sp3 carbon/oxygen matrix, named
White-light-emitting C-bDs
Over the past decade, white light phosphors have triggered intense interests to fabricate WLEDs and organic WLEDs (OLEDs) with better color stability, better reproducibility, and simpler fabrication process [237]. In this regard, some researchers have considered white light generation from C-bDs by chemical modification of their surface or even from unmodified C-bDs [37], [136], [203], [216], [218], [238], [239], [240], [241], [242], [243], [244], [245], [246].
It is interesting to know that
Applications
Considering the unique properties and advantages of C-bDs, many applications in fields such as chemical sensing, biosensing, bioimaging, nanomedicine, and photocatalysis have been reported for these nanomaterials [9], [256]. Although we mostly aimed to summarize the progresses in the synthesis and development of multicolor, long-wavelength, and white-light-emitting C-bDs in this review, the reported applications of the discussed papers, which can be categorized in three parts, biorelated
Summary and challenges remaining
In this review, we summarized all efforts in preparing C-bDs with long-wavelength, multicolor, and white-light emission and mentioned their applications. For the ease of readers, a summary of some distinctive achievements along with their maximum emission wavelength, PL QY, and utilized precursors and methods are presented in Table 1.
Although the PL wavelength of GQDs showed high dependency on the quantum confinement effect, in most cases, the main agent in the determination of the PL behavior
Acknowledgements
The authors acknowledge the Iran NanotechnologyInitiative Council (INIC) for the partial support of this project.
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