Halogen-Doped Carbon Dots: Synthesis, Application, and Prospects: History
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Subjects: Nanoscience & Nanotechnology
Contributor:
Kun Luo
,
Yanmei Wen
,
Xinhuang Kang

Carbon dots (CDs) have many advantages, such as tunable photoluminescence, large two-photon absorption cross-sections, easy functionalization, low toxicity, chemical inertness, good dispersion, and biocompatibility. Halogen doping further improves the optical and physicochemical properties of CDs, extending their applications in fluorescence sensors, biomedicine, photocatalysis, anti-counterfeiting encryption, and light-emitting diodes. This review briefly describes the preparation of CDs via the "top-down" and "bottom-up" approaches and discusses the preparation methods and applications of halogen (fluorine, chlorine, bromine, and iodine)-doped CDs.The main challenges of CDs in the future are the elucidation of the luminescence mechanism, fine doping with elements (proportion, position, etc.), and their incorporation in practical devices. 

  • carbon dots
  • halogen doping
  • fluorescence
  • sensors
  • biomedicine

1. Introduction

The term “carbon dots” (CDs) usually refers to carbon or graphene quantum dots (CQDs or GQDs, respectively), carbon nanodots, or polymer dots [1]. As a new type of carbon nanomaterial, CDs are defined as dispersed spheroidal carbon particles less than 10 nm in diameter with fluorescence properties [2]. In 2004, fluorescent carbon was first isolated by the Scrivens group during the purification of single-walled carbon nanotubes [3]. In 2006, these fluorescent carbon nanoparticles were first referred to as CDs [4].
The CDs core is dominated by sp2-hybridized carbon atoms, and there are abundant surface functional groups [5][6]. The internal electronic structures include σ → σ*, σ → π*, π → π*, n → π*, and n → σ* electron transitions, which affect the physicochemical properties of CDs, especially their optical properties [7][8]. The design and synthesis of CDs have been extensively studied owing to the numerous advantageous characteristics of CDs, such as high luminous efficiency, stability, and much greater optical stability than organic dyes and semiconductor quantum dots [9][10][11]. Moreover, they have low toxicity, good biocompatibility, excellent photostability, and tunable fluorescence [12][13][14][15]. Under different excitations, CDs possessing different particle sizes, elemental doping contents, and surface functional groups can emit different types of fluorescence [16][17]. The preparation methods of CDs are diverse, and they have been prepared using graphene, carbon nanotubes, organic small molecules, oligomers, fruits, and vegetables as raw materials (Figure 1).
Molecules 27 04620 g001 550
Figure 1. Various raw material sources used in the synthesis of CDs including small molecule compounds, polymers, carbon materials, fruits, and vegetables.
Their highly advantageous optical properties endow CDs with considerable research value [18][19]. However, traditional preparation methods yield CDs with several drawbacks, such as low fluorescence quantum yields (QY), low visible-light utilization, and short emission wavelengths, significantly limiting their applications. To overcome these shortcomings, surface passivation and heteroatomic doping have been performed on CDs.However, surface passivation often involves tedious steps and toxic chemicals and changes the location and number of the original functional groups used for sensing and analysis [20][21]. Heteroatomic doping involves introducing non–metallic atoms or metal ions to CDs to change their energy–gap structure and generate different intrinsic properties [22]. The latter is one of the most effective, extensive, and sensitive methods for controlling the optical and electronic transport properties of CDs [23][24]. Recently, non–metallic atom doping of CDs has become common, including nitrogen [25][26], phosphorous [27][28], sulfur [29][30], and silicon [31][32]. Doping changes the distributions and surface chemical structures of the different carbon hybrids, improving the optical properties of CDs and facilitating new applications.
This review primarily discusses halogen–doped CDs preparation methods and performance. Specific applications in anti–counterfeiting encryption, environmental monitoring, biosensing, gene detection, drug delivery, and biological imaging are also summarized. Hence, a theoretical basis for future CDs research and development is established.

This entry is adapted from the peer-reviewed paper 10.3390/molecules27144620

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