DOI: 10.14457/tu.the.2020.518
title: Carbon dots derived from chemical and natural materials for chemical sensing applications

description: In this work, high sensitivity, selectivity, stable sensing probe prepared from carbon dots was developed for the detection of various important analytes. Carbon dots were synthesized from chemical and natural materials using different methods. All as-prepared carbon dots were characterized to examine their properties and chemical structures using several characterization techniques, such as transmission electron microscopy, UV-vis spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It was found that all carbon dots demonstrated size with less than 10 nm, exhibited blue fluorescence emission under UV irradiation, and showed hydrophilic functional groups on their surface but they have some differences depending on the precursors and synthetic conditions. Firstly, the carbon dots were synthesized from maleic acid and triethylenetetramine via a facile one-pot pyrolysis with nitric acid. The synthesis conditions provided high fluorescent carbon dots with a high quantum yield of 6.3%. The nitrogen-rich carbon dots were used as fluorescent probes for the detection of pyrimethanil fungicides via the inner filter effect and strong π-π interaction. The carbon dots were sensitive to pyrimethanil across a linear range from 0.5 to 75 µM with a low detection limit of 14 nM. The practical, portable, and reusable paper-based sensor was developed. It was capable of detecting pyrimethanil at the detection limit of 0.8 µM and could be reused for at least 8 cycles. The cytotoxicity of carbon dots was also tested to demonstrate that they are sufficiently safe to use. Natural materials such as black sesame seeds, corncobs, and iota-carrageenan were also applied as precursors for the synthesis of carbon dots. Secondly, the carbon dots were synthesized from black sesame seeds via the hydrothermal method and used as a dual-mode probe for the detection of ammonia in vapor and solution phases. They exhibited blue-color emission under UV excitation in solution with a quantum yield of 2%. The obtained carbon dots in optical electronic nose could sense and differentiate ammonia vapor in various volatile organic compounds. For detecting ammonia in aqueous solutions, the limit of detection was found to be 0.8%v/v. The density functional theory calculations were also used to confirm the change in optical absorbance of carbon dots when exposed to solvent vapors. The computational results supported the experimental findings well in which the carbon dots and ammonia formed the most stable complex among all vapor molecules computed. A paper-based device was also constructed to prove the practicality and versatility of our carbon dot-based ammonia sensors. Thirdly, corncobs were used to synthesize the carbon dots with a quantum yield of 2%. The fluorescent carbon dots-based sensing solution with tunable excitation and emission properties was performed through the inner filter effect for the detection of methyl nicotinate. The effects of excitation wavelength and carbon dot concentrations were studied. It was found that the use of 0.05 mg·mL-1 carbon dots under 260 nm-excitation provided a high sensitivity with a low detection limit of 3 nM in a wide range 5-500 µM methyl nicotinate concentrations. The portable paper-based sensor showed good results with a limit detection of 14.8 µM. The carbon dots-based thin film in optical electronic nose was also performed, which could sense and differentiate methyl nicotinate vapor in the water samples as a human breath model. Another natural precursor used for the carbon dot synthesis was iota-carrageenan. The iota-carrageenan-derived carbon dots were applied as electrochemical probes for the detection of capsaicin and 17β-estradiol using cyclic voltammetry and linear sweep voltammetry. The study of scan rate, pH, electron transfer process, and adsorption/diffusion processes was discussed. Herein, the carbon dots were grafted onto an indium tin oxide (ITO) substrate via a novel one-step thermal annealing process. This was used as an electrode for the electrochemical detection of capsaicin. The carbon dot-grafted ITO electrode was sensitive across a wide linear range from 0.05 to 500 µM, with a remarkably low detection limit of 5.4 nM. The electrode was electrochemically stable and can be reused for several hundred times due to the excellent recyclability over 450 cycles. In addition to high sensitivity, stability, and selectivity, the sensor could produce the detection results within 5 minutes. For the electrochemical detecting 17β-estradiol, The sensor was simply prepared by drop-casting carbon dots (CDs)/polyaniline (PANI) composite on a glassy carbon electrode (GCE) using poly(vinylidene fluoride) as a binder. The CDs-PANI/GCE was sensitive to 17β-estradiol across a linear range from 0.001 to 100 µM with a detection limit of 43 nM. The electrochemical measurement can be performed in 2 minutes and the probe can be reused for several hundred times. the oxidation reaction of 17β-estradiol occurred on the surface of CDs-PANI/GCE via favorable energy levels and dominantly surface adsorption process through π-π stacking and hydrogen bonding between 17β-estradiol and CDs/PANI. The carbon dots synthesized from different chemical and natural sources showed similar small nanoparticles with various hydrophilic functional groups on the surface including oxygen- and nitrogen-containing groups, endowing them with unique optical, electronic, and sensing properties. Based on all results, the carbon dots have the potential for the detection of various analytes in solution and vapor phases using different techniques including fluorescence, electrochemical, and optical electronic nose systems. The chemical sensors based on carbon dots developed in this work will be useful as potential tools in various fields.