Carbon dots in sample preparation and chromatographic separation: Recent advances and future prospects

https://doi.org/10.1016/j.trac.2020.116135Get rights and content

Highlights

  • Carbon dots (CDs) have emerged as a new carbon-based fluorescent nanomaterials.

  • Different types of synthesis strategies of CDs are introduced.

  • Applications of CDs in sample preparation are summarized in details.

  • Applications of CDs in chromatographic separation are also described.

  • The limit and future perspectives for CDs in chromatographic science are indicated.

Abstract

Carbon dots (CDs) as a new class of zero-dimensional carbon-based nanomaterials that have gained growing interest and attention in many research areas. In addition to their comparable optical properties, CDs have the desired advantages of economy, low toxicity, excellent biocompatibility, simple synthetic routes and abundant carbon source. Since their discovery, CDs have been widely used in sensors, nanomedicine, bioimaging, and photo/electrocatalysis fields. To better understand the significant role of CDs in separation science, it becomes necessary to summarize the applications of CDs in sample preparation and chromatographic separation systematically. In this review, we aim to demonstrate the up-to-date advances of CDs with an emphasis on their properties, classification, synthetic methods and applications in sample preparation and chromatographic separation along with some perspectives on the current challenges and opportunities in this exciting and promising field. This review may open up some insights and inspiration for future research work.

Introduction

As a type of “zero-dimensional” quasi-spherical, nanocrystalline, or amorphous carbon-based fluorescent nanomaterials, carbon dots (CDs) have recently exhibited tremendous application potential in areas including sensors, bioimaging, catalysis, drug delivery, phototherapy [[1], [2], [3]]. CDs mainly including carbon nanodots (CNDs), graphene quantum dots (GQDs), and polymer dots (PDs), as illustrated in Fig. 1 [4]. CNDs are always divided into carbon nanoparticles without a crystal lattice and carbon quantum dots (CQDs) with an obvious crystal lattice. Among them, CQDs were accidentally discovered during the processing of single-walled carbon nanotubes by Xu et al. in 2004 [5]. GQDs are tiny graphene fragments with a lateral size less than 100 nm in single or few layers, and diameters ranging from 3 to 20 nm mainly [6]. PDs are cross-linked or aggregated polymers that are synthesized from linear monomers or polymers. Besides, PDs can also be assembled from a carbon core and connected polymer chains [7].

Compared to conventional semiconductor quantum dots or organic fluorophores, CDs have unique photoelectronic and fluorescent properties, chemical inertness, rich surface functional groups, facile functionalization, ease of synthesis, low or nontoxicity and environmental compatibility, and an abundance of raw materials in nature, placing them in an advantageous position for achieving unprecedented performance [[8], [9], [10]]. Check out the number of CDs-related scientific publications on the Web of Science, we can see the number of articles and reviews published in the field of CDs rapidly raised starting from 2005. In order to promote the development of CDs further, a detailed summary of the key areas involving CDs is urgently desired.

Over the past few decades, a variety of advanced carbon-based nanomaterials, including graphene, carbon nanotubes (CNTs), oxidized nanodiamond, fullerene, and their chemically modified analogs have been widely used in chromatographic separation and preconcentration [[11], [12], [13], [14], [15]]. These nanomaterials usually exhibit many unique advantages. For example, their large specific surface area can enhance the sorption performance or retention capacity of the analyte, their unique surface chemical properties, including hydrogen-bonding interactions, coordination interactions, hydrophilic/hydrophobic interactions, π-π stacking, etc., can provide distinctly different selectivity towards the separation of various analytes compared to other types of adsorption materials or chromatographic stationary phases. In addition, they have high thermal/mechanical stabilities. Although nanomaterials possess all the above-mentioned advantages and achieves significant progress, there are still some inherent problems that cannot be avoided, such as a tedious and time-consuming preparation procedure. The over strong adsorption capacity of nanomaterial may bring about the chromatographic peak tailing or sample resolution difficult. The anomalous morphology and easy agglomeration of nanomaterial would affect the homogeneity of the packings, which cause low column efficiency [15].

Recently, the application of CDs in separation science field has gradually attracted attention, due to their advantages that are different from traditional carbon-based nanomaterials, such as simple preparation, nanoscale size, high surface-to-volume ratio, moderate adsorption performance, rich surface functional groups, strong design ability, and excellent dispersion capability in aqueous solutions. These characteristics make them suitable for extraction applications and chromatographic separation.

In view of this, the present review aims to enhance researchers’ understanding and awareness of the increased contribution of CDs in separation science, especially in sample pretreatment and chromatographic separation. Looking back at the relevant reviews of nanomaterials in this field over the past five years, it can be found that although the existing literature has covered the application of various nanomaterials in separation science, there is almost no discussion with respect to the application of CDs in separation science[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]. In fact, CDs have enormous potential to prepare advanced materials with remarkable performance in separation science. Several studies on the use of modified CDs for sample preparation or chromatographic separation were reported from 2012. Taking into account all the above considerations, this review focuses specifically on the latest advances (from 2012 to the present) of CDs in this exciting and promising field along with some perspectives on the current challenges and opportunities.

Section snippets

Synthesis strategy of CDs

The choice of starting material and synthesis method plays a key role in the properties of CDs and associated applications. Up to now, a range of starting materials and synthesis strategies have been applied for the design and preparation of CDs. In brief, the fabrication approaches of CDs can be broadly divided into two categories: “top-down” carbon source cracking methods and “bottom-up” organic carbonization approaches [4]. Generally speaking, “top-down” methods are helpful to yield CDs with

Applications

Currently, CDs have been widely applied in the separation science field, including sample preparation and chromatographic separation.

Conclusion and perspectives

This review mainly focuses on the latest advances of CDs in sample preparation and chromatographic separation. On the one hand, the application of CDs to sample preparation can improve the dispersibility of the material, thereby enhance the extraction performance, reproducibility, and precision of the method. On the other hand, CDs, as modifiers of chromatographic separation materials, can increase the interaction sites between the analyte and the chromatographic stationary phase, allowing

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.

Acknowledgments

This study work was supported by the National Key R&D Program of China (2019YFC1905501), the National Natural Science Foundation of China (No. 21822407) and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT: Ministry of Science and ICT) (No. NRF-2019R1A2C1010032).

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