Nitrogen, sulfur, boron and flavonoid moiety co-incorporated carbon dots for sensitive fluorescence detection of pesticides

Abstract

Multiple dopants contained carbon dots (CDs) with enhanced selectivity have aroused extensive attentions. In this work, three heteroatoms (nitrogen, sulfur, boron) and flavonoid moiety derived from the extracts of Ginkgo biloba leaves co-incorporated CDs were synthesized based on the hydrothermal strategy for novel carbon materials investigation and sensing applications. Nitrogen and flavonoid moiety co-doped CD (N-CD-Fla), nitrogen, sulfur, and flavonoid moiety co-doped CD (NS-CD-Fla), nitrogen, sulfur, boron and flavonoid moiety co-doped CD (NSB-CD-Fla) were prepared, respectively. Three CDs had diversities in nanoparticle sizes, chemical structures, and luminescence properties. NS-CD-Fla exhibited blue luminescence and had higher quantum yield (QY) than N-CD-Fla (yellow-green) and NSB-CD-Fla (blue-green), which can be attributed to its higher N content and N, S synergistic enhancement effect. The luminescence of the three CDs was speculated to be related with the electron transition of CN bonds derived from grafting amines groups onto the flavonoid-related moieties. N-CD-Fla, NS-CD-Fla, and NSB-CD-Fla were further proved to be sensitive to fenitrothion (Fen), dithianon (Dit), dinoseb (Din) and their structural analogues, they were employed as superior chemosensors for the three pesticides sensitive and selective detection. The application potentials of the novel chemosensors based analytical methods were further evaluated by real sample analysis.

Introduction

As a class of newly emerging promising photoluminescent carbon nanomaterials, carbon dots (CDs) have received increasing attentions in various optical sensing applications such as bioimaging, chemical sensing, medical diagnose, electrocatalysis, and photovoltaic devices due to their superior merits of easy preparation, water solubility, low biological toxicity, outstanding biocompatibility and unique physical-chemical properties [[1], [2], [3]]. Various CDs have been exploited based on the top-down and bottom-up strategies [2]. The top-down strategy generally employs the large molecular carbon compounds as the carbon source for producing nano-carbon based on a serious of physical or chemical technologies such as laser ablation, arcdischarge, electrooxidation, and oxidative acid treatment of carbon raw materials, whereas the bottom-up approach generally employs the smaller molecular carbon compounds as the precursors based on their dehydration, polymerization, carbonization, and passivation for producing CDs, such as the explored strategies of microwave or thermal assisted pyrolysis, hydrothermal, electrochemical or ultrasonic assisted synthesis, which adopted cheap raw materials and simple synthetic steps, and produced CDs with negligible defects and superior homogeneity in terms of chemical structures [4]. Among various bottom-up strategies, hydrothermal approach has proved to be facile and effective, and has become the most commonly applied strategy for CDs exploration [3,5]. Many types of precursors have been explored for hydrothermal producing CDs, including natural carbon precursors such as trees leaf [6], corn juice [7], lemon and onion juice [8], pomelo juice [9], and organic compounds such as α-lipoic acid [2], citric acid and ethylenediamine [10], sorbic acid and l-proline [11], sodium thiosulfate and sodium citrate [12], lactose and vitamin B₁₂ [13], ethanolamine [14], and triethylamine [15].

Generally, doping CD with heteroatom such as nitrogen, sulfur, and boron can effectively enhance its photoluminescence quantum yield and intrinsically improve its sensing performance by introducing localized state and tuning its composition, structure, electronic density, electron transfer and other intrinsic properties [16,17]. Various doped or co-doped strategies have been reported for tacking the sensing applications, such as nitrogen doped CDs [18,19], sulfur doped CDs [17], nitrogen and sulfur co-doped CDs [[20], [21], [22]], nitrogen and phosphorus co-doped CDs [23,24], nitrogen and boron co-doped CDs [25,26], nitrogen, sulfur and boron co-doped CDs [27,28]. Different dopants contained CDs generally exhibit different photoluminescent phenomena, therefore suitable controlling the functional groups, compositions and structures of CDs are indispensable for expanding their sensing applications, as well as excavating their complicated luminescence mechanisms. Recently, multiple dopants contained CDs with enhanced selective recognition ability have emerged as a new hotspot in CDs sensing research.

CDs can inherit functional groups from the precursors, which will lay the basis for their binding with metal ions based on the excited-state electron transfer [29], or interaction with organic analytes based on the effective energy flow between the CD fluorophore and analytes [30]. Therefore, CDs have been widely applied for both inorganic ions and organic analytes sensing detection, such as sensing of Fe³⁺ [2], Pb²⁺ [29], ClO⁻ [31], glucose [4] and insulin [32]. CDs have also been explored as promising fluorescence sensors for pesticides detection, such as chlorophyll derived CD for organophosphate pesticides detection [33], nitrogen doped CD for atrazine detection [34], aptamer-mediated CD for acetamiprid detection [35], quaternized CD for dichlorvos detection [36], and so on [30].

Recently, carbon materials based novel sensors have been continually explored for pesticides sensitive and selective sensing detection, such as graphene based electrochemical sensors [[37], [38], [39], [40]], and CDs based fluorescence sensors [30,33,41]. Generally, the sensors based fluorescence detection methods provide low cost, convenience, high-efficiency, on-site operation, and simple sample pre-treatment as compared to other conventional analytical methods such as chromatographic analysis, mass spectrometric analysis and electrochemical analysis [41], which have potential applications for pesticides detection from various real sample matrices.

In the present work, nitrogen and flavonoid moiety co-incorporated CD (N-CD-Fla), nitrogen, sulfur and flavonoid moiety co-incorporated CD (NS-CD-Fla), nitrogen, sulfur, boron and flavonoid moiety co-incorporated CD (NSB-CD-Fla) were separately prepared for novel carbon materials investigation and sensing applications based on the one-pot bottom-up hydrothermal strategy by employing the flavonoid extracts of Ginkgo biloba leaves as the carbon source. Flavonoid moiety incorporated CD has been explored as a selective fluorescent probe for Pb²⁺ detection [2]. However, its optical characteristics, surface and local chemical features were considerably tuned with the addition of heteroatoms. Three formulations of addition non-metallic elements were explored for preparation of CDs with diversities in structures and properties. N-CD-Fla, NS-CD-Fla, and NSB-CD-Fla respectively exhibited yellowish green, blue and blue-green luminescence, and were respectively sensitive to fenitrothion (Fen), dithianon (Dit), dinoseb (Din) and their structural analogues. The chemical structure, luminescence properties and sensing selectivity of the three CDs were further investigated, and their application potentials in sensing detection of pesticides were deeply excavated and explored.