Carbon dots based fluorescence methods for the detections of pesticides and veterinary drugs: Response mechanism, selectivity improvement and application

Highlights

• Strategies for detecting pesticides and veterinary drugs with CDs are diverse.

• Response mechanisms of CDs can be divided into direct and indirect manners.

• Selectivity improvement includes enzymatic and non-enzymatic approaches.

• Most CDs based methods focus on organophosphorus pesticides and tetracyclines.

Abstract

Based on the unique properties of carbon dots (CDs), a number of detection methods with high performances have been built through various strategies for different pesticides and veterinary drugs, including organophosphorus pesticides, tetracyclines, quinolones and others. The performances of these detection methods are comparable or even better than conventional approaches as detection limits of some methods can reach pg/mL level. However, these methods have not been systematically reviewed. This article aims to provide a comprehensively review on the main response mechanisms of CDs to such analytes, selectivity improvement approaches and their wide applications in the detections of pesticides and veterinary drugs in water, plant and animal foods, etc. It is our hope that this review will promote the advance of CDs applied in the detections of pesticides and veterinary drugs, and open perspectives toward the construction of new detection methods based on CDs with high sensitivity, selectivity and accuracy.

Introduction

Pesticides are widely used to control weeds and pests as well as regulate plant growths [1], and veterinary drugs are also administered to prevent and treat diseases, and to promote the growths of food animals [2]. However, the pesticide and veterinary drug residues in foods and the environment because of the excessive use or abuse have potential threats to human health [3,4]. Consequently, effective detection approaches to the residues of pesticides and veterinary drugs are highly necessary.

Traditional methods that have been used to detect the residues of pesticides and veterinary drugs with good accuracy include gas chromatography (GC) [5], high performance liquid chromatography (HPLC) [2,6], and chromatographic methods coupled with mass spectrometry (MS) detectors [1]. However, these methods can only be realized in laboratory tests with complicated pre-treatments and expensive equipment performed by well-trained technicians [5]. Fluorescent methods with high performance probes can be achieved through diverse and flexible patterns, not only in the environment, but also in in vivo [7], in vitro [7] and in situ [8] detections. Thus, fluorescence methods have attracted increasing attention in the development of the detections of pesticides and veterinary drugs. Among them, fluorescent dyes [9] and semiconductor quantum dots [10] are the most used fluorescence probes currently. Nevertheless, their potential toxicity and risk of damage to the environment have compromised their practical applications in the detections of pesticides and veterinary drugs [11].

Since the first appearance in 2004 [12], carbon dots (CDs) have shown many advantages of excellent biocompatibility [13], low toxicity [14], environment-friendliness [14], facile preparation and modification [15,16], superior dispersibility and high chemical stability over their counterparts [17,18], such as heavy metal based quantum dots (QDs), metal nanoclusters and organic dyes [19]. Recently, CDs have increasingly aroused interest in the detection area due to their excellent performances. Their responses to analytes can be realized on the basis of diverse mechanisms, such as inner filter effect (IFE) [15,17], Förster resonance energy transfer (FRET) [20,21] and photo-induced electron transfer (PET) [22,23] and so on. Moreover, CDs can be easily achieved by so-called bottom-up [24] and top-down [25] approaches. Versatile raw materials, such as different small molecules [17,[20], [21], [22]], kinds of biomass [15,23], and massive carbon materials [25] have been explored to synthesize CDs. In doing so, various methods have been used, commonly containing electrochemical oxidation [26], laser ablation [27], hydrothermal carbonization [28], supported synthesis [29], combustion/thermal microwave heating [[30], [31], [32]], chemical oxidation [33], and pyrolysis [34]. Further, CDs can be designed to possess plenty of surface groups; thus, their surfaces are likely tailored and modified to meet different requirements in the detection applications [19].

Based on the unique properties of CDs, lots of detection methods for pesticides and veterinary drugs have been built through various strategies and applied in different real samples. Specially, how to produce responses to analytes is the key to build such detection methods. To date, IFE [15,17], FRET [20,21], PET [22,23] and aggregation-induced emission (AIE) [[35], [36], [37]] have been documented as the main response mechanisms of CDs to these analytes. Currently, it is difficult to design detection methods based on the direct interactions between CDs and the analytes of pesticides and veterinary drugs. Therefore, many of these methods are achieved by trials and screenings to obtain the responses of CDs to targets. It is relatively easier to design detection methods based on indirect interactions between CDs and analytes. For example, the inhibition of organophosphorous pesticides (OPs) on enzyme activity of acetylcholinesterase (AChE) or butyrylcholinesterase (BChE) is well known to scientists [38]. Accordingly, methods for the detection of OPs can be designed when the catalytic products of AChE or BChE exert an effect on the fluorescence response of CDs [15,17,[20], [21], [22], [23]]. On the other hand, selectivity and anti-interference play key roles in constructing detection methods for pesticides and veterinary drugs. Through functional groups on their surface, CDs can be modified by some specific recognition moieties, such as aptamers, molecularly imprinted polymers (MIPs) and anti-bodies to improve the selectivity of the detection methods. To date, although many detection methods based on CDs have been identified for pesticides and veterinary drugs, only OPs and tetracyclines (TCs) are mostly targeted.

It is highly necessary to have a comprehensive review of these methods available so that scientists could become well knowledgeable about the existing advance of CDs in the detections of pesticides and veterinary drugs, and new perspectives could be developed toward the construction of CDs based detection methods. Currently, there have been several reviews [[39], [40], [41]] in which CDs based detection methods for pesticides and veterinary drugs are just involved. For example, in the review of biosensors based on fluorescence carbon nanomaterials for the detection of pesticides [39], as a part, CDs based sensors only for pesticides including enzyme, antibody, aptamer, MIPs-based methods and non-recognition unit-based methods are contained. The applications of fluorescent CDs in food analysis [40] and food safety [41] have been reviewed by Yue et al. and Shi et al., in which, pesticides and veterinary drugs are included as two kinds of important harmful residues in foods detected by the CDs based methods. Therefore, this article is aimed to provide a systematic review of response mechanisms of CDs to the analytes of pesticides and veterinary drugs, approaches to selectivity improvements and the applications in the detections of various specific analytes (see Scheme 1).