Photoluminescence properties of l-cysteine-derived carbon dots prepared in non-aqueous and aqueous solvents

Highlights

• l-cysteine-derived carbon dots were prepared in non-aqueous and aqueous solvents.

• Photoluminescence properties of CDs prepared in a non-aqueous solvent were enhanced.

• Dehydration between l-cysteine molecules were facilitated in a non-aqueous solvent.

• Oxidation of thiol groups to sulfonic acid groups was promoted in a non-aqueous solvent.

Abstract

Fluorescent carbon dots (CDs) can be produced from amino acids such as l-cysteine by an intermolecular dehydration reaction between NH₂ and COOH groups, followed by carbonization. This dehydration reaction may be hindered in aqueous media because the amino acid molecules are surrounded by water; therefore, we used a non-aqueous reaction media to facilitate the dehydration reaction. In the present study, l-cysteine was heated at 230 °C for 30 min in a non-aqueous solvent, diphenyl ether, which has a high boiling point of 258 °C, to yield CDs-NA. For comparison, we also hydrothermally prepared CDs-A from l-cysteine dissolved in water, with the use of an autoclave and microwave heating system under the same temperature and time conditions. The maximum photoluminescence (PL) intensity of the water dispersion of CDs-NA was 1.5 times as high as that of the CDs-A. The absolute PL quantum yield (QY) of CDs-NA was 4.2%, which was 1.4 times as high as that of CDs-A, 3.0%. Characterization of the elemental compositions, particulate properties, and chemical bonding states confirmed that the non-aqueous solvent not only facilitated CD formation via the dehydration reaction but also contributed to the formation of sulfonic acid groups by oxidation of thiol groups. The rapid particle growth and formation of the additional functional groups might contribute to the increased PL intensity and absolute PLQY of CDs-NA. Through the optimization of the synthesis conditions in a non-aqueous solvent, the absolute PLQY of CDs-NA was increased from 4.2% to 10.2%.

Introduction

Synthesis strategies for fluorescent carbon dots have been explored for more than a decade; however, a need remains for more effective preparative methods. Xu et al. reported fluorescent carbon nanoparticles in 2004 in the process of the electrophoretic purification of single-walled carbon nanotubes prepared by arc discharge [1]. Sun et al. successfully produced fluorescent carbon nanoparticles by laser ablation of a graphite target in argon gas containing water vapor, followed by surface passivation with polyethylene glycol; they named these nanoparticles, carbon dots (CDs) [2]. CDs have since received considerable attention owing to their low toxicity, visible light emission, high photostability, and a characteristic property of excitation-dependent emission [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. CDs have been rapidly fabricated by laser irradiation of graphite powders dispersed in organic solvents [9]. Zhou et al. pioneered electrochemical reactions of multiwalled carbon nanotubes to prepare CDs [10]. These CDs possessed hydrophilic functional groups such as carboxy groups, which have found applications in bioimaging [[11], [12], [13]]. However, the fabrication of CDs by breaking down carbon materials, carbon nanotubes, and graphite requires complex equipment, extended synthesis routes, and difficult purification steps [[14], [15], [16], [17], [18]].

Hydrothermal syntheses of CDs in an autoclave have been widely performed because this represents an eco-friendly and simple synthesis [[19], [20], [21]]. An aqueous solution of citric acid and ethylenediamine has been hydrothermally treated at 150–300 °C for 5 h to yield CDs [19]. An aqueous solution of sodium citrate and ammonium hydrogen carbonate was hydrothermally treated at 180 °C for 4 h to prepare CDs [20]. CDs can be readily prepared by a hydrothermal reaction in a closed reactor fitted with a microwave heating system, in which heating is more uniform and the reaction time is shorter than a conventional electric heating system [[22], [23], [24]]. CDs have been prepared from an aqueous glucose solution through a hydrothermal treatment at 280–700 W for 1–11 min with the use of a microwave heating system [23]. Hydrothermally synthesized CDs have been shown to have applications in bioimaging [19,25], pH sensing [21], thermal sensing [21,26], detection of heavy metals [[27], [28], [29]], and color patterning [19,30].

Numerous works concerning the formation of CDs via an intermolecular dehydration reaction through hydrothermal treatment have been reported [[31], [32], [33], [34], [35]]. CDs were hydrothermally prepared via a dehydration reaction between NH₂ and COOH groups of l-cysteine molecules, followed by carbonization [35]. We have investigated the optimal conditions for the synthesis of CDs through hydrothermal treatment of l-cysteine using a microwave heating system [36]. l-cysteine contains nitrogen and sulfur, as illustrated in Fig. 1 and S1. These elements likely contribute to the formation of new energy levels from heteroatom dopants, resulting in enhanced blue emission [37,38]. We verified that the promotion of the dehydration reaction between l-cysteine molecules is a key factor affecting the photoluminescence (PL) properties of the CDs [36]. On this basis, the dehydration reaction may be inhibited in hydrothermal synthesis because starting materials are surrounded by water. Moreover, water generated by the dehydration reaction remains in an autoclave, which is likely to inhibit further dehydration reactions; therefore, we have focused on non-aqueous systems without the use of an autoclave (a closed reactor) to promote the intermolecular dehydration reaction.

In the present study, we focused on diphenyl ether possessing a high boiling point of 258 °C as a non-aqueous solvent to prepare CDs in an open system; the generated water can be removed from system. l-cysteine was heated at 230 °C in a non-aqueous solvent of diphenyl ether with the use of an oil bath to prepare CDs-NA. For comparison, CDs-A were hydrothermally prepared from l-cysteine in an autoclave with the use of a microwave heating system at the same temperature. To understand the factors determining the differences in the PL properties between CDs-NA and CDs-A, we investigated their elemental compositions, particulate properties, and chemical bonding states.