Photocurrent generation mechanism of self-powered ZnO/graphite CD enhanced Schottky diode. Photographs of the CD solution in the presence and absence of UV light are shown in the inset.
Contents
Introduction
Chapter 3: N-doped carbon dots from cigarette-tobacco: Picric acid sensing in real water sample and synthesis of CD-MWCNT
Chapter 4: Carbon dots and ZnO based flexible and self-powered Schottky diode for UV-photodetection
Chapter 5: Carbon dots decorated SWCNT/ZnO based hybrid electrode for optically responsive supercapacitor
Summary and future scopes
Introduction
Abstract
Overview
CDs can be defined as quantum dots that have the source of carbonaceous materials. In addition to PL properties and low toxicity, a more important characteristic of CDs is high water solubility [9, 10].
Types of Carbon dots
The presence of functional groups on the surfaces, such as −COOH and/or −OH, is responsible for the high solubility of CDs in water [10]. The PL mechanism in CQDs is due to the quantum confinement and intrinsic luminescence of nanomaterials.
![Figure 1.1. Types of CDs: CQDs, GQDs, and CPDs [12].](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10342655.0/39.892.168.736.482.924/figure-types-cds-cqds-gqds-cpds.webp)
Synthesis techniques of Carbon dots
They used EtOH/H2SO4 as a solvent and the QY of their synthesized CDs was 75%. 22], on the other hand, explored the ultrasound technique using acid/alkali-assisted glucose dissolution for the synthesis of fluorescent carbon nanoparticles.
Sources for precursor materials
The research regarding the synthesis techniques of CDs is not currently limited to the above individual processes. 32] showed the synthesis of CDs from potato starch by acid-assisted ultrasonication method and used the CDs to detect Zn2+.
Applications of Carbon dots
- Sensors
- Photoluminescence-based (PL-based) Sensors
- Chemiluminescence-based (CL-based) Sensors
- Electrochemiluminescence-based (ECL-based) Sensors
- Optoelectronics
- Light-emitting diodes (LEDs)
- Solar cells
- Photodetectors
- Energy storage
- Supercapacitors
- Batteries
- Catalysis
- Biological applications
However, further addition of GQDs to the mixture showed a detrimental effect due to CD aggregation. A large number of cell lines have been used to test the bioimaging capabilities of CDs.
Knowledge gap
Objective and outline of the thesis
This technical chapter includes sections A and B (sensing and bioimaging)
Here we have not focused on the synthesis of CDs from some new precursor materials. We have used citric acid and ethylenediamine (EDA) to synthesize the CDs by hydrothermal method.
The main focus of this chapter is to fabricate a photoresponsive supercapacitor using CD and then explore the working mechanism of the supercapacitor using electrochemical analysis, which we have successfully presented. Further, this chapter discusses future goals based on the work reported in previous technical chapters.
Synthesis of non-toxic Carbon dots from potato for the detection of Cr 6+
Sensing and bioimaging
Introduction
There are various methods to synthesize CDs as mentioned in the previous chapter, such as electrochemical method [17], hydrothermal method [15], microwave synthesis technique [18-20], simple heating technique [21], etc. Elements such as O and H present in the carbohydrate (apart from C element) help to synthesize CDs. In this work, the authors used cane sugar, a type of locally available bread, and sugar as precursors for the synthesis of CDs with QY of 0.63%, respectively.
Although a considerable amount of research on the synthesis of CDs from biosources has been successfully carried out, there are many more biosources to be discovered for synthesizing CDs with significant properties. We show the selective and sensitive detection of these two ions in the presence of other cations as well as anions.
Experimental section
- Materials
- Synthesis of fluorescent CDs
- Characterizations
- Quantum yield calculation
- Stability of the CDs
- Cell culture
- Cytotoxicity test
- Bioimaging
- Detection of chromium and iron
UV-vis spectra of the synthesized CDs were obtained with a UV-vis spectrophotometer (PerkinElmer, Lambda 35). The quantum yield (QY) of the prepared CDs was calculated according to the method used by Goswami et al. 33] As in the typical method, the QY of the CDs was determined using a quinine sulfate reference fluorescence sample.
To study the cytotoxicity of the prepared CDs, an MTT (3-[4,5-Dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) assay was performed on HeLa (human cervical cancer) cells exported. Then 40 μl of a specific concentration of chromium in the form of Cr6+ was added to 5 ml of the CD solution.
Results and discussion
- Optical property of CDs
- Characteristics, particle size, and stability of the CDs
- Detection of chromium and iron in water sample
- Selectivity of the detection method for chromium and iron
- Feasibility with real samples
- Cytotoxicity and bioimaging
UV spectra (black line) and PL spectra (blue line) of CDs in aqueous solution. shows the dependence of the PL intensities of the CD solution on the excitation wavelength. We used the PL property of the CDs to detect chromium in the form of Cr6+ and iron in the form of Fe3+ ions. From the two graphs of Figure 2.6 it can be seen that the PL intensity of the CD solution noticeably decreases when the metal ions are added to the solution.
The PL quenching data follow the Stern−Volmer equation. a) PL spectra of the CD solution in the presence of different Cr6+. We also verified the toxicity of the as-synthesized CDs and this is shown in figure 2.13.
Conclusions
The blue fluorescence coming from the cytoplasm of the cells shows that the CDs are localized in the cytoplasm, whereas no fluorescence was seen in the nucleus. In our study, the entry of CDs into cells is attributed to their small size, which makes it easier to pass through the cell membrane. The luminescent property of the synthesized CDs was used to detect heavy metals in the water sample, and the CDs showed a good selectivity and sensitivity towards Cr6+ and Fe3+ ions with a detection limit of 0.012 µM and 0.000 549 µM for Cr6+ and Fe3+, respectively.
This non-toxic nature, easy uptake and high QY of the synthesized CDs have been used for biological imaging of cells. The success of this work lies in achieving high stability, PL tunability, LOD for Cr6+ and Fe3+, and nontoxicity and bioimaging for HeLa cells from the synthesized CDs.
Sensing and optoelectronics
Introduction
Photoluminescence-based (PL-based) sensing of heavy metals (Cr6+, Fe3+) by carbon droplets (CDs) has been reported in detail in the previous chapter. PL quenching of CDs in the presence of several quenchers has been further explored by scientists to detect numerous explosive materials such as trinitrophenol (TNP), nitrobenzene (NB), trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP ), etc. 17] reported on the non-covalent bonding of CNTs with CdSe nanoparticles to show the change in the morphology of CdSe nanoparticles in the presence of CNTs.
The aforementioned discussion shows that there is a very limited amount of work that has reported the use of CDs synthesized from the same precursor material in photodetection as well as PL-based sensing. On the other hand, in the case of the synthesis of the UV-responsive nanocomposite, the CDs were attached with MWCNTs.
Experimental section
- Synthesis of fluorescent CDs
One of them is the PL-based sensing of PA in water bodies, and the other is to use the CDs to synthesize UV-responsive CD-multiwalled carbon nanotubes (MWCNT) nanocomposite. This PA sensing ability of the CDs in water bodies has been further investigated to detect PA in real water samples (tap water and Brahmaputra water). In a typical method, the nicotine-containing tobacco material of the cigarette was ground using a mortar, then 1.14 g of the ground material was added to 22 ml of DI water.
The picture of the CD solution under visible and UV light is shown in the inset of scheme 3.1. The only additional part was the addition of a different amount of EDA and 300 µl) to the water-tobacco mixture before heating in the microwave.
Schematic representation of the synthesis steps of the CDs. The photographs of the CD solution in the presence and absence of UV light are shown in the inset
- Synthesis of CD-MWCNT nanocomposite
- Device fabrication and photoresponse measurement
- Characterization of the CDs and the CD-MWCNT nanocomposite
- Calculation of Quantum yield (QY)
- Detection method of PA
- Stability tests of the CDs
- Results and discussion
- Optical properties of CDs
- Surface characteristics and stability of the CDs
- Sensing of PA
- Characteristics of CD-MWCNT nanocomposite
- Photoresponse characteristics of the nanocomposite
- Conclusions
This indicates that the PL property of the CDs dominated in the UV domain. Stability analysis of the synthesized CDs with (a) storage time, (b) pH of the solvent, and (c) ionic strength of the solvent (different concentrations of NaCl were used to control the ionic strength). However, Figure 3.8c shows no change in the TRPL diagrams of the CDs at different PA concentrations.
Fixation of the CDs on the MWCNT introduced the PL property into the nanocomposite, which is shown in Figure 3.14d. Raman spectra, shown in Figure 3.15a, give a clearer picture of the confirmation of the CDs with MWCNT.
Carbon dots and ZnO based flexible and self-powered Schottky diode for UV-photodetection
Optoelectronics
Introduction
The authors reported detectability and responsivity of the UV detector down to Hz1/2/W and 36 A/W, respectively. On the other hand, another photoresponsive characteristic of ZnO nanoparticles is the PL property. However, these articles do not report the fabrication of flexible photodetectors, although they readily indicate the development of self-powered photodiodes.
Thus, the production of flexible and self-powered photodetectors has attracted the attention of researchers and has also become a challenging task. Here, a novel contribution is the growth of ZnO nanorods on a graphite-coated paper substrate.
Experimental section
- Materials
- ZnO nanorod fabrication
- CD synthesis and attachment with ZnO
- Electrical measurement
- Characterizations
The ZnO seed-deposited graphite-coated paper substrate was then treated in a hydrothermal reaction at 90 °C for 6 h to grow the ZnO nanorods. Then, the ZnO-grown substrate was kept at 50 °C overnight to improve the crystallinity of the nanorods. The CDs were attached to the ZnO nanorods by immersing the nanorods in a CD solution for 2 hours.
Two tungsten probes were connected to the graphite end and the ZnO end of the device for measuring the electrical response. To prepare the samples for FETEM and PL characterizations, the ZnO-grown paper was immersed in 5 mL of DI water inside a culture tube and sonicated for 10 min.
Results and discussion
- ZnO characteristics
- Schottky diode analysis
- Photoresponsive behavior of the diode
- Band diagram
- Self-powered (zero biased) photodetection and flexibility of the diode
This statement can be confirmed by the XRD pattern of the synthesized nanorods, which is shown in Figure 4.3a. The Williamson–Hall (W-H) technique [32] was used to calculate the average crystallite size (𝐿) and stress (𝜀) of the nanorods. Information about the defects in the synthesized ZnO was obtained from the PL spectra of the ZnO nanorods.
The transient response characteristics at zero bias are represented in the graph shown in Figure 4.10b. Therefore, the desorption of the oxygen molecule (neutral) in the atmosphere increases, leaving the CD in a charge-neutral state [56].
Conclusions
The flexibility of the fabricated Schottky diode has also been tested and the result is shown in Figure 4.17. The ON/OFF current ratio at different bending cycles is shown in this figure and the corresponding transient response of the device is also shown in the inset. The responsiveness and specific detectivity of the fabricated UV detector have been calculated as 9.57 mA/W and 4.27 × 108 Jones at a wavelength of 330 nm, respectively.
It is observed that the total capacitance of the hybrid electrode is a combination of both electrical double layer capacitance (EDLC) and pseudocapacitance. Furthermore, the reaction mechanism of the system has also been investigated and the occurrence of intercalation of K+ ions in the defects of ZnO has been confirmed with reasoned explanations.
