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Review
. 2021 Jan 15;12(1):84.
doi: 10.3390/mi12010084.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Smita Das  1 , Lightson Ngashangva  1 , Pranab Goswami  1
Affiliations
Review

Carbon Dots: An Emerging Smart Material for Analytical Applications

Smita Das et al. Micromachines (Basel). .

Abstract

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.

Keywords: analytical; carbon dots; chemiluminescence; electrochemiluminescence; optical; photoluminescence; smart materials.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The band gap and transition of various colour exhibited by CDs (near infrared CD: NIR-CD, red-CD: r-CD, yellow CD: y-CD, green CD: g-CD, blue CD: b-CD, white CD: w-CD). (i) Proposed mechanism of up-conversion PL in Near IR (NIR) CDs [76] (Reproduced with permission), (ii) Proposed mechanism of CDs with O-defects, P-defects and N-defects states [24] (Reproduced with permission). (iii) Energy diagram of the CDs indicating the role of intersystem, charge transfer [41] (Reproduced with permission). (iv) Band gap transitions of CDs by introducing CTC, TC, and OTC [17] (Reproduced with permission).
Figure 2
Figure 2
(i) Energy levels (HOMO and LUMO) of G-CDs and R-CDs (left side); Luminescence mechanism (right side) [92] (Reproduced with permission). (ii) Excited-State for Doped C-Dots at Core State and Surface State, [51] (Reproduced with permission).
Figure 3
Figure 3
(i) Phosphorescent spectra of TA-CDs powder against time, illustration of the CDs with water-stimuli-responsive production and the encryption and decryption process [118] (Reproduced with permission). (ii) The photoenergy transformation in the IA-CDs/TiO2 nanocomposite system [119]. (Reproduced with permission). (iii) Proposed mechanism of the pressure triggered fluorescence [19] (Reproduced with permission). (iv) PL peaks when pressure is released or increased (left) and proposed mechanism of piezochromic behaviour of N-CDs (right) [54] (Reproduced with permission). (v) Illustration of PL enhancement under high pressure [120] (Reproduced with permission).
Figure 4
Figure 4
(i) Multi-stimuli responsive behaviour of the CDs, (ii) Proposed energy-transfer mechanism in white-light-emitting hydrogel [125] (Reproduced with permission). (iiia) Formation of H-CD monomers and their aggregates (the disulphide bond in dithiosalicylic acid molecular is highlighted with yellow). (iiib) Photographs of the H-CD’s two-switch-mode luminescence principle. (iiic) Fluorescence principle and proposed structure of H-CD’s core and surface [129] (Reproduced with permission).
Figure 5
Figure 5
The reversible phase transfer of the amidine-modified CDs and the reversible luminescence change between blue and cyan-green luminescence change in the presence of CO2 and N2 [131] (Reproduced with permission).
See this image and copyright information in PMC

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