Preparation and Biomedical Applications of Multicolor Carbon Dots: Recent Advances and Future Challenges

Abstract

as 0D and discrete, quasispherical nanoparticles with the characteristic size of less than 10 nm in shape. In the past few decades, many researchers have been intrigued by overwhelming application of CDs in fields other than solar photovoltaics, [23][24][25] semiconductor photocatalysis, [26] or light-emitting materials, [27][28][29] like in bioimaging, [30][31][32][33] drug delivery/ release, [34][35][36] theranostics, [37] and biosensing/bioassay. [38][39][40] Albeit with biological imaging agents in vitro and in vivo, heavy metals as the essential elements for the core materials of available high-performance semiconductor quantum dots (QDs) exist substantial toxicity, carcinogenic effects, and environmental hazard, even at low levels. [41][42][43][44][45] Historically, boasting the natural virtues of fascinating optical properties (e.g., up/down-conversion and excitation-dependent fluorescence emission, high fluorescence quantum yields (QY), and long fluorescence lifetime), eco-friendliness, high waterdispersible peculiarity, easy surface-functionality, low cost, highly abundant, and inexpensive raw materials, noninvasiveness, low cytotoxicity, and good biocompatibility afford CDs suitable as the elegant alternatives for toxic QDs containing heavy metals. [46][47][48][49] On the other hand, the CDs have excellent photostability against photobleaching and photoblinking as compared with the conventional organic dyes (typically fluorescein and rhodamine B). [50] In particular, multicolor CDs exhibit tunable photoluminescence by exciting the identical CDs, allowing for multimodal biophotosensitizer in biological field without interference from background autofluorescence. In this review, we aim to systematically offer a summary of preparing multicolor CDs and its various biomedical applications, mainly including for bioimaging, drug delivery/release, theranostics, and biosensing/bioassay, highlighting the complete horizons of multiplexed quantitative detection, high-resolution fluorescence imaging, and long-term, real-time monitoring of multicolor CDs.Multicolor carbon dots (CDs) as an emerging subclass of carbonaceous nanomaterials have inspired intensive attention due to the fascinating fluorogenic properties of quantum dots, exhibiting great potential applications in the biomedical field. In some cases, reported CDs with blue or green fluorescence are not desirable for further biological applications owing to the conflicting background autofluorescence, low penetration, and relatively large damage to biological tissue. However, multicolor CDs that can work in the longer wavelength region are being developed to address this issue by overcoming the autofluorescence from the cellular components (usually in the blue and green range). In this review, the development of multicolor CDs is described comprehensively, including for red-emissive or NIR-emissive CDs. Additionally, the preparation methods of multicolor CDs are summarized. Moreover, the factors affecting the luminescence of multicolor CDs are discussed in de...

Applications of Carbon Dots in Optoelectronics

“…It was theoretically shown that the electronic structure of carbon nanoparticles can be governed by size, chemical composition, doping, etc. [ 19 , 46 , 47 ] In particular, in [ 48 ] CDs were considered as nanoparticles with sp 3 -hybridized amorphous carbon cores that contain partially sp 2 -hybridized carbon domains, and it was predicted that the emission of CDs can be redshifted by increase of hybridization factor of those domains within CDs. Another theoretical investigation reported in [ 49 ] showed that covalent-bonded dimers of polyaromatic hydrocarbons at the CDs surface, in contrast to non-interacting monomers, result in a redshift and broadening of PL band, with decrease of its intensity.…”

“…Moreover, the PL quantum yield (QY) of CDs can easily reach very high values, especially in the blue and green spectral range [ 10 , 11 ]. Being a biocompatible and non-toxic alternative to QDs, CDs have been widely used in bioimaging, sensing, and other biomedical applications [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Composite materials based on CDs have been often reported to have phosphorescence observed at room temperature, which is a useful property for data encryption [ 20 , 21 , 22 ].…”

Nonlinear Optics to Glucose Sensing: Multifunctional Nitrogen and Boron Doped Carbon Dots with Solid‐State Fluorescence in Nanoporous Silica Films

“…Photoluminescent carbon quantum dots (CQDs) are taking center stage in many applications such as biomedicine, [ 1 ] optronics, biological imaging, [ 2 ] theranostics, [ 3 ] energy storage, molecular sensing, photocatalysis, [ 4 ] and photoconversion. [ 5 ] This is because of their attractive properties including high photostability, biocompatibility, low toxicity, environmental friendliness, high water solubility, facile synthetic routes, easily tunable photoluminescence (PL), as well as cheap and readily available precursor sources.…”

“…[43] These CQDs also exhibit interesting optical properties. When illuminated by light emitting diode (LED) of λ 405 nm, they emit green PL (2) (Figure 4a) that is absent in precursor solution (1). Insets (1,2) show these two samples under ambient light.…”

Carbon Dots with an Emission in the Near Infrared Produced from Organic Dyes in Porous Silica Microsphere Templates

“…Besides, CDs can be prepared from inexpensive renewable natural sources [ 6 ]. Due to these useful characteristics, CDs and composite materials based thereon have been suggested for use in biomedicine [ 7 , 8 , 9 ], optoelectronics [ 10 , 11 ], sensing [ 6 , 12 ], as luminescent inks [ 13 ], as initiators for polymerization [ 14 ], and even in agriculture to improve plant growth [ 15 ].…”