Since then, researchers have extended their study to other carbon allotropes, and various modification methods to create a range of fluorescent materials have been reported. Because of the impurities in the resultant suspension, further electrophoretic separation occurred, and a fluorescent and fast moving band was isolated these were referred to as fluorescent nanoparticles. CDs were discovered as a byproduct of the synthesis of single-walled nanotubes (SWNTs) prepared by the arc-discharge method. Subsequently, further surface modification is applied to enhance and tune their fluorescence. In the top-down method, carbon macromolecules are cut into smaller pieces using physical forces such as arc-discharge, laser ablation, or electrochemical reactions. Because many synthetic procedures have been summarized elsewhere, here, we briefly describe the development and advances in CD research from the early years of their discovery. Surface modification can be applied after or during CD synthesis via surface passivation, doping, or functionalization. Simply, the former process cleaves bulk carbonaceous materials into CDs via physical, chemical, or electrochemical methods, whereas the latter synthesizes CDs from appropriate precursors from various carbon sources. These methods can be largely categorized into two approaches: top-down and bottom-up. In the past decades, numerous synthetic methods for the preparation of CDs have been explored. Herein, we outline the synthetic methods and physical features of CDs reported in early studies, and we summarize the recent progress in using CDs as probes for heavy metals (Fig.

In addition, the fluorescence quantum yield of CDs can be enhanced by modifying the CD surface by adjusting the intrinsic components and surface groups. In this respect, CDs are good candidate components for metal sensors because they are biocompatible. These toxic metals are easily accumulated in the body and coordinate with biological components, such as enzymes and nucleic acids, hindering normal biological interactions and functions. In contrast, other metals, such as Hg 2+, Pb 2+, and Cd 2+, are detrimental to humans, even in trace amounts. Some heavy metals, such as zinc or iron, are essential for human metabolism and are rarely harmful to human health when in their optimal concentration. Īmong the various possible applications of CDs, here, we summarize how CDs can detect heavy metals, as well as the types of materials that can be utilized. Furthermore, there are several simple methods to modify the surface state of CDs, which allow researchers to tune the solubility and quantum yields of CDs according to their experimental requirements. Compared to other fluorescent raw materials, CDs are synthesized from inexpensive carbon sources that are abundant in nature and are, thus, bio-friendly. Moreover, the main pitfall of QDs is their toxicity, which is due to their heavy metal content, including metals such as cadmium this limits their biological and environmental applications. Additionally, semiconductor quantum dots (QDs) are comparably as good as CDs in terms of photostability, quantum efficiency, and tunable fluorescence, but QDs cannot be used to trace a single molecule for long-term monitoring because of their intrinsic blinking. In contrast, CDs are much more resistant to photobleaching.

For example, organic dyes are inexpensive and effective as fluorescent probes, but they are easily photobleached. CDs have several advantages compared to other conventional fluorescent sensors. All nano-sized fluorescent carbon materials with one dimension less than 10 nm can be classified as CDs, and these can be derived from various carbon materials such as fullerenes, graphite, carbon nanotubes, and graphene. The discovery of fluorescent carbon dots (CDs), also known as carbon quantum dots (CQDs), has attracted tremendous interest from many researchers because of their versatile applications in optoelectronics, biomedical applications, and chemical biosensors.

Carbon metal ion bonding