Facile, green and clean one-step synthesis of carbon dots from wool: Application as a sensor for glyphosate detection based on the inner filter effect

Highlights

• Green synthesis of carbon dots from wool.

• The synthetic method was clean and need not any additives, such as acids, bases, salts, or other toxic reagents.

• Detection of glyphosate based on the inner filter effect.

• This sensor exhibited superior accuracy and sensitivity for glyphosate.

• The established method were employed to detect glyphosate in cereal samples.

Abstract

In this work, we reported a green route for the fabrication of fluorescent carbon dots (CDs). Wool, a kind of nontoxic and natural raw material, was chosen as the precursor to prepare CDs via a one-step microwave-assisted pyrolysis process. Compared with previously reported methods for preparation of CDs based on biomass materials, this method was simple, facile and free of any additives, such as acids, bases, or salts, which avoid the complicated post-treatment process to purify the CDs. The CDs have a high quantum yield (16.3%) and their fluorescence could be quenched by silver nanoparticles (AgNPs) based on inner filter effect (IFE). The presence of glyphosate could induce the aggregation of AgNPs and thus result in the fluorescence recovery of the quenched CDs. Based on this phenomenon, we constructed a fluorescence system (CDs/AgNPs) for determination of glyphosate. Under the optimized conditions, the fluorescence intensity of the CDs/AgNPs system was proportional to the concentration of glyphosate in the range of 0.025–2.5 μg mL⁻¹, with a detection limit of 12 ng mL⁻¹. Furthermore, the established method has been successfully used for glyphosate detection in the cereal samples with satisfactory results.

Introduction

Glyphosate (N-[phosphonmethyl]glycine) is a postemergence, broad-spectrum, and non-selective herbicide that exhibits excellent performance and effect in weed control [1]. Due to its relatively low toxicity to mammals, glyphosate has become the most frequently used herbicide all around the world [2]. However, the indiscriminate application of glyphosate may easily lead to high-level residues in agricultural products, which can create a potential threat to human health [3]. The median lethal dose (LD50) of glyphosate in rats is around 5000 mg kg⁻¹ [4]. Although the acute toxicity of glyphosate for rats is relatively low, the additives and surfactants in commercial formulations can improve the absorbance of glyphosate and thus increase the toxicity [4]. Recently, the glyphosate has been classified as probably carcinogenic to humans [5]. Therefore, it is of a significant importance to establish a reliable method for the determination of glyphosate in agricultural samples.

Up to now, numerous analytical methods have been developed for glyphosate detection, such as gas chromatography (GC) [6], [7], [8], capillary electrophoresis (CE) [9], [10], and high performance liquid chromatography (HPLC) [11], [12]. However, derivatization procedures have to be performed to improve the detection sensitivity of glyphosate, due to the low volatility, high water solubility, high polarity and absence of chromophore or fluorophore groups in the molecular structure. Other detection methods for glyphosate have been reported, such as enzyme-linked immunosorbent assay (ELISA) [13], ion chromatography (IC) [14] and inductively coupled plasma-mass spectrometry (ICP-MS) [15]. These methods show high sensitivity and good accuracy, but they often require time-consuming and complicated steps, expensive instrumentations and specific skills for operation, which limit their practical application. Therefore, developing simple, low-cost, rapid and sensitive method for the determination of glyphosate has become a pressing demand.

Among the various detection techniques, fluorescence detection provides many advantages such as rapid response, high sensitivity, simplicity, and ease of operation. A fluorescent method has also been established for glyphosate determination [2] based on the fluorescence resonance energy transfer (FRET) between CdTe quantum dots and gold nanoparticles. However, the raw materials for preparation of CdTe quantum dots were expensive and the synthetic process was relatively complex, which may limit their wider applications.

Recently, photoluminescent carbon nanodots (CDs) have received great attention, due to their high resistance to photobleaching, high aqueous solubility, excellent biocompatibility, low toxicity and robust chemical inertness [16], [17], [18]. Up to now, tremendous effort has been made to synthesize CDs with excellent properties through different ways or materials. Compared with conventional chemicals, biomass materials exhibit amazing advances in the fabrication of CDs, as they are easy to obtain, inexpensive and nontoxic. More recently, many biomass materials have been used to prepare CDs, such as hair fiber [19], [20], spider silk [21], silk [22], apple juice [23], potato [24], protein [25], etc. However, these methods suffered from various drawbacks including time-consuming procedures, low quantum yields, addition of strong acid or alkali. In this work, we reported a simple, facile and green route to synthesize photoluminescent CDs from wool by a one-step microwave method without any additives, such as acids, bases, or salts. The resultant CDs can be directly applied as fluorescent probes only through simple centrifugation and filter.

The inner filter effect (IFE) refers to the absorption of the excitation and/or emission light of fluorophores by absorbers in the detection system [26]. The recent studies have explored the application of IFE in developing novel fluorescent methods for analyte detections [26], [27], [28], [29], [30], [31], [32]. In this study, we designed a new fluorescent probe for glyphosate detection based on the IFE of silver nanoparticles (AgNPs) for the fluorescence of CDs. To construct an effective fluorescent probe for the detection of glyphosate, high dispersion and uniform size distribution of AgNPs were important for this work. The principle of our experimental design is illustrated in Scheme 1. The fluorescence intensity of CDs can be quenched by AgNPs based on inner-filter effect (IFE). However, in the presence of glyphosate, negatively charged glyphosate can induce the aggregation of positively charged AgNPs by electrostatic interaction. The absorbance decreases of AgNPs led to the fluorescence recovery of the quenched CDs. Therefore, a highly sensitive fluorescence sensing system for glyphosate detection was developed. And this sensing system has been successfully applied to detect glyphosate in cereal samples with good precision and accuracy.