Recent development of carbon quantum dots regarding their optical properties, photoluminescence mechanism, and core structure

Abstract

Carbon quantum dots (CDs) are a relatively new class of carbon nanomaterials which have been studied very much in the last fifteen years to improve their already favorable properties. The optical properties of CDs have drawn particular interest as they display the unusual trait of excitation-dependent emission, as well as high fluorescence quantum yields (QY), long photoluminescence (PL) decay lifetimes, and photostability. These qualities naturally lead researchers to apply CDs in the field of imaging (particularly bio-imaging) and sensing. Since the amount of publications regarding CDs has been growing nearly exponentially in the last ten years, many improvements have been made in the optical properties of CDs such as QY and PL lifetime. However, a great deal of confusion remains regarding the PL mechanism of CDs as well as their structural properties. Therefore, presented in this review is a summary and discussion of the QYs and PL lifetimes reported in recent years. The effect of method as well as precursor has been evaluated and discussed appropriately. The current theories regarding the PL mechanism of CDs are discussed, with special attention to the concept of surface state-controlled PL. With this knowledge, the improvement of preparation and applications of CDs related to their optical properties will be easily accomplished. Further improvements can be made to CDs through the understanding of their structural and optical properties.

Figure 1.

Bright field and fluorescence images of: (A) H1299 cells. (B) H1299 cells incubated with CDs. (C) H1299 cells incubated with nanocomplexes of CDs and hyaluronic acid. Scale bar is 20 μm. Reproduced from Ref. with permissions from The Royal Society of Chemistry.

Figure 2.

CHO cells imaged by CDs. (A) Bright field image. (B) Confocal image excited at 405 nm. (C) Confocal image excited at 488 nm. (D) Confocal image recorded at 561 nm. Scale bar is 10 μm. Ref. - Published by The Royal Society of Chemistry.

Figure 3.

Confocal image of CDs labeling the U87 cells. Adapted with permission from Ref. . Copyright 2014 American Chemical Society.

Figure 4.

HaCaT cell confocal images without (a-c) and with (d-f) 500 μg/mL CDs. Images a and d are excited at 405 nm, b and e are excited at 488 nm, c and f are excited at 543 nm. Figure adapted from Ref. with permission Korean Carbon Society.

Figure 5.

(a)Normalized PL emission spectrum with excitation wavelength increasing from 310 nm in 20 nm increment (inset: UV/vis absorption spectrum of CDs obtained); (b) Confocal microscopy image of Hela cell incubated with CDs (λex =488 nm). Figure adapted from Ref. with permissions from Elsevier.

Figure 6.

(a) The schematic illustration of PL emission of hydroxyls-coated CDs better than the regular carboxyls-coated CDs (e^–: electrons, h⁺: holes); (b) The PL quenching effect of CDs by different metal ions (λex =310 nm). Figure adapted from Ref. with permissions from Springer.

Figure 7.

PL emission spectrum of CDs excited from 245 to 395 nm (a); PL decay lifetime profile of the CDs (b). Figure adapted from Ref. with permission The Royal Society of Chemistry.

Figure 8.

PL decay lifetime and quantum yield of CDs made at 130, 160, 200 and 240 °C (a); The PL emission spectra of CDs made at 160 and 240 °C (b); The quenching effect of different ions on the CDs. Figure adapted from Ref. with permissions from Nature Research.

Figure 9.

(a) PL emission decays of HeLa cells cultured in CDs aqueous solution with a concentration of 500 μg/mL at different temperatures; (b) PL decay lifetimes extracted from the PL transients recorded every 15 min for 24 h of HeLa cells cultured in CDs aqueous solution (500 μg/mL); (c) Temperatures obtained using the calibration curve based on a calibration curve described by an equation: T = 330.59–94.99τ+11.87τ²-0.54τ³, R_adj²=0.998, T, temperature; τ, lifetime; (d) Temperatures measured by a reference thermometer; (e) Histogram revealing the deviation between (c) and (d), the solid line is the distribution curve. Figure adapted from Ref. with permissions from the American Chemical Society.

Figure 10:

Image of different CDs fractions under UV light and the modeling of their bandgap based on surface oxidation. Figure adapted from Ref. with permissions from the American Chemical Society.

Figure 11:

(A) Summary of preparation of CDs. (B) Proposed representative structure for each CDs. (C) Proposed energy level diagram for surface states of CDs. (D) Nitrogen percentages in CDs samples. Figures adapted from Ref. with permissions from The Royal Society of Chemistry.

Figure 12:

A scheme of energy band structure and possible PL process for CDs. Figure adapted from Ref. with permission from MDPI.

Scheme 1.

(a) Histogram showing number of references related to CDs and QY per year. (b) Histogram showing number of references related to CDs and fluorescence lifetime per year (Note: while not the correct technical term, fluorescence lifetime is the term used most often in the literature to describe the PL decay lifetime of CDs).