Elsevier

Carbon

Volume 124, November 2017, Pages 429-472
Carbon

Review article
Long-wavelength, multicolor, and white-light emitting carbon-based dots: Achievements made, challenges remaining, and applications

https://doi.org/10.1016/j.carbon.2017.08.072Get rights and content

Abstract

The use of carbon-based dots (C-bDs), involving carbon dots, carbon quantum dots, and graphene quantum dots, as a new class of photoluminescent nanomaterials is rapidly expanding. Their many advantages including water solubility, high stability, low toxicity, ease of functionalization, and cost-efficient and simple synthetic routes have introduced them as potential alternatives to conventional semiconductor-based quantum dots. However, difficulty in preparing long-wavelength and multicolor-emitting C-bDs has caused some major disadvantages for these nanomaterials and limited their application in fields such as bioimaging and multicolor patterning. Although different emission colors from C-bDs can be observed by varying their excitation wavelength, this is not identified as real photoluminescence tuning, and in fact, preparing C-bDs with such special photoluminescence properties has proven to be a challenging task.

This review summarizes to date successes in preparing long-wavelength, multicolor, and white-light-emitting C-bDs along with their potential applications. We discuss the developments in using specific precursors, synthetic methods, heteroatom doping, and post treatments such as separation and surface modification methods that have led to C-bDs with unique emission colors.

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.

References (256)

  • P. Reiss et al.

    Core/shell semiconductor nanocrystals

    Small

    (2009)
  • J. Tang et al.

    Infrared colloidal quantum dots for photovoltaics: fundamentals and recent progress

    Adv. Mater

    (2011)
  • S. Silvi et al.

    Luminescent sensors based on quantum dot–molecule conjugates

    Chem. Soc. Rev.

    (2015)
  • N. Lewinski et al.

    Cytotoxicity of nanoparticles

    small

    (2008)
  • A. Shiohara et al.

    On the cyto-toxicity caused by quantum dots

    Microbiol. Immunol.

    (2004)
  • S.N. Baker et al.

    Luminescent carbon nanodots: emergent nanolights

    Angew. Chem. Int. Ed.

    (2010)
  • O. Kozak et al.

    Photoluminescent carbon nanostructures

    Chem. Mater

    (2016)
  • X.T. Zheng et al.

    Glowing graphene quantum dots and carbon dots: properties, syntheses, and biological applications

    Small

    (2015)
  • X. Xu et al.

    Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments

    J. Am. Chem. Soc.

    (2004)
  • L. Li et al.

    Focusing on luminescent graphene quantum dots: current status and future perspectives

    Nanoscale

    (2013)
  • C. Ding et al.

    Functional surface engineering of C-dots for fluorescent biosensing and in vivo bioimaging

    Acc. Chem. Res.

    (2013)
  • Y. Dong et al.

    Carbon-based dots Co-doped with nitrogen and sulfur for high quantum yield and excitation-independent emission

    Angew. Chem. Int. Ed.

    (2013)
  • Z.L. Wu et al.

    Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission

    J. Mater. Chem. B

    (2017)
  • G. Eda et al.

    Blue photoluminescence from chemically derived graphene oxide

    Adv. Mater

    (2010)
  • A. Cayuela et al.

    Semiconductor and carbon-based fluorescent nanodots: the need for consistency

    Chem. Commun.

    (2016)
  • S. Zhu et al.

    Strongly green-photoluminescent graphene quantum dots for bioimaging applications

    Chem. Commun.

    (2011)
  • B. De et al.

    A green and facile approach for the synthesis of water soluble fluorescent carbon dots from banana juice

    RSC Adv.

    (2013)
  • L. Zhou et al.

    Graphene quantum dots from polycyclic aromatic hydrocarbon for bioimaging and sensing of Fe3+ and hydrogen peroxide

    Part. Part. Syst. Charact.

    (2013)
  • S. Ghosh et al.

    Photoluminescence of carbon nanodots: dipole emission centers and electron–phonon coupling

    Nano Lett.

    (2014)
  • F. Liu et al.

    Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence

    Adv. Mater

    (2013)
  • S. Hu

    Tuning optical properties and photocatalytic activities of carbon-based “quantum dots” through their surface groups

    Chem. Rec.

    (2016)
  • V. Gude et al.

    Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission

    Phys. Chem. Chem. Phys.

    (2016)
  • S. Lu et al.

    Near-infrared photoluminescent polymer–carbon nanodots with two-photon fluorescence

    Adv. Mater

    (2017)
  • M.J. Krysmann et al.

    Formation mechanism of carbogenic nanoparticles with dual photoluminescence emission

    J. Am. Chem. Soc.

    (2011)
  • B. Li et al.

    Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface

    RSC Adv.

    (2016)
  • S.K. Das et al.

    Single-particle fluorescence intensity fluctuations of carbon nanodots

    Nano Lett.

    (2014)
  • N. Dhenadhayalan et al.

    Unravelling the multiple emissive states in citric-acid-derived carbon dots

    Phys. Chem. C

    (2016)
  • Y.-P. Sun et al.

    Quantum-sized carbon dots for bright and colorful photoluminescence

    J. Am. Chem. Soc.

    (2006)
  • S. Zhu et al.

    The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

    Nano Res.

    (2015)
  • Y. Li et al.

    Chemical nature of redox-controlled photoluminescence of graphene quantum dots by post-synthesis treatment

    Phys. Chem. C

    (2016)
  • A. Sciortino et al.

    Solvatochromism unravels the emission mechanism of carbon nanodots

    J. Phys. Chem. Lett.

    (2016)
  • Y. Song et al.

    Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine

    J. Mater. Chem. C

    (2015)
  • Z.-C. Yang et al.

    Intrinsically fluorescent carbon dots with tunable emission derived from hydrothermal treatment of glucose in the presence of monopotassium phosphate

    Chem. Commun.

    (2011)
  • Z. Chu et al.

    Graphene oxide based BCNO hybrid nanostructures: tunable band gaps for full colour white emission

    RSC Adv.

    (2014)
  • W. Wei et al.

    Non-enzymatic-browning-reaction: a versatile route for production of nitrogen-doped carbon dots with tunable multicolor luminescent display

    Sci. Rep.

    (2014)
  • S.K. Bhunia et al.

    Tuneable light-emitting carbon-dot/polymer flexible films prepared through one-pot synthesis

    Nanoscale

    (2016)
  • S. Qu et al.

    Amplified spontaneous green emission and lasing emission from carbon nanoparticles

    Adv. Funct. Mater

    (2014)
  • J. Zhou et al.

    A low-temperature solid-phase method to synthesize highly fluorescent carbon nitride dots with tunable emission

    Chem. Commun.

    (2013)
  • M. Xu et al.

    A green heterogeneous synthesis of N-doped carbon dots and their photoluminescence applications in solid and aqueous states

    Nanoscale

    (2014)
  • X. Ren et al.

    Graphitic carbon nitride nanosheets with tunable optical properties and their superoxide dismutase mimetic ability

    RSC Adv.

    (2016)
  • Cited by (0)

    View full text