Fabrication of dual emission carbon dots and its use in highly sensitive thioamide detection

Highlights

• Highly fluorescent carbon dots (CDs) was synthesized using l-tryptophan and ethylenediaminetetraacetic acid.

• The as-synthesized CDs exhibit dual emission bands that correspond to the tryptophanyl cores and functional groups.

• The as-synthesized CDs show very low detection limit and wide linear range in the thioacetamide sensing.

Abstract

Highly sensitive thioamide sensor was realized by the fluorescent carbon dots (CDs) synthesized using l-tryptophan and ethylenediaminetetraacetic acid as novel precursors via hydrothermal synthesis. The as-synthesized CDs exhibit dual emission bands at 360 and 450 nm, which corresponded to the tryptophanyl cores, and defects and/or functional group related emission sites. The reaction conditions including reaction time, temperature, and the precursor ratio were systematically studied to obtain the high quantum yield and quenching efficiency. This is the first report to use organic CDs in thioacetamide (TAA) detection. Although workers in leather, textile, paper industries suffer from exposure of TAA well-known as liver carcinogen to human, there was no method which facilitate non-toxic, simple and rapid detection. When the as-synthesized CDs were used as thioacetamide (TAA) sensing, an extremely low detection limit (~70 nM) and a very wide linear range (0–10 mM) were achieved by the different TAA induced quenching properties of each emission band. Moreover, their properties were much better than those of amperometry or voltammetry based TAA sensing.

Introduction

Many attempts have been made to develop effective sensing devices by using carbon allotrope families such as carbon nanotubes (CNT), graphene-like 2D materials, graphene quantum dot (GQD), carbon dot (CDs) for the industrial and biological applications [[1], [2], [3], [4], [5], [6]]. Among them, CDs draw a great attention for the fluorescence sensor because of their high sensitivity, excellent selectivity, simplicity and rapid response, which are originated from their unique molecular system that physicochemical properties can be changed by the interaction with specific chemical species [7,8].

Generally, the synthesis of CDs can be classified into top-down [[9], [10], [11]] and bottom-up approaches [[12], [13], [14]]. Recently, bottom-up approaches have been widely studied because properties of CDs such as doping of heteroatoms and structures can be precisely controlled by the precursors and synthetic process [[15], [16], [17], [18]]. Krysmann et al. synthesized CDs based on citric acid and ethanolamine through pyrolysis, which showed the presence of amide intermolecular reaction and formation of carbogenic cores by cross-linking reactions in the interchain of CDs [15]. Song et al. suggested the possible intermediate in synthesis process and formation of carbon core from assembly of fluorophore and crosslinking of polymer clusters [16]. Sarkar et al. presented the effect on optical properties of nitrogen doping in CDs through computational modeling based on time-dependent density functional theory (TD-DFT). They revealed that the graphitic nitrogen had an important role of long-wavelength emission [17]. Hola et al. clarified the role of graphitic nitrogen by the theoretical calculation and experimental result together. They showed that graphitic nitrogen created mid-gap states in original gaps, which showed great effects on the photoluminescence properties of CDs [18].

Thioacetamide (TAA) is well-known as liver carcinogen to human. It can lead to necrosis and liver injury by inhibiting protein synthesis, RNA and DNA activity. Nonetheless, people can be exposed to it in various workplaces including leather, textile, and paper industries [19,20]. Even though the monitoring of TAA level is very important in view of health care, only limited studies have been reported so far. D. Cinghiţă et al. reported the used a glass carbon (GC) electrode to detect TAA by the electro-catalytic system, especially by the anodic oxidation [21]. D. Saha et al. synthesize the ZnO quantum dot (QD) to detect the TAA via interaction between surface of QD and –NH and sulphur groups of TAA [22].

Herein, we report a very sensitive photoluminescence (PL) based TAA sensor. Photoluminescent CDs are successfully synthesized from EDTA (ethylenediaminetetraacetic acid) and l-tryptophan via hydrothermal synthesis which is one of the bottom-up approaches and has several advantages in view of freedom of precursor choice as well as easiness in the controlling of reaction conditions. In our knowledge, this is first time to show the thioacetamide sensor based on organic CDs, which is more advantageous than metal-based QDs in view of low toxicity of organic precursors [23]. The reaction conditions are systematically investigated to control the nitrogen and oxygen functionalities precisely, which resulted in high quantum yields (QYs). The CDs fabricated in this study exhibit excellent sensitivity and selectivity towards TAA with wide linear range and low detection limit, which can be due to photo-induced electron transfer (PET) between tryptophan structure of CDs and TAA. Especially, dual emission with different quenching characteristics can realize highly sensitive thioacetamide sensor with wide linear range and low detection limit. We have used fluorescence spectroscopy for the detection to achieve more sensitivity. The as-prepared probe shows dual emission when excited at 240 nm. The dual emission peaks increased the accuracy of the assay method. In this way, the influence by external agents on the test results can also be avoided [24]. By using dual emission probe, we have achieved detection of TAA over two different ranges.