The visible light-driven and self-powered photoelectrochemical biosensor for organophosphate pesticides detection based on nitrogen doped carbon quantum dots for the signal amplification
Graphical abstract
A novel visible light driven self-powdered photoelectrochemical (PEC) biosensor was constructed by adopting the NCQDs/TiO2 as the photoactive matrix and AChE as the biological recognition molecule for the sensitive detection of organophosphorus pesticides (OPs) by using chlorpyrifos as the model.
Introduction
In recent decades, Organophosphorus pesticides (OPs), one kind of pesticides with highly toxic and high efficiency, have been extensively applied in agriculture all over the world to prevent the crops and vegetables from being attacked by insects and guarantee the farm production [1]. Nevertheless, with the development of agriculture, the high dosage use and improper disposal of OPs have given rise to some very serious problems, such as environmental pollution and OPs residues in agricultural products. Because the OPs can bind to acetylcholinesterase (AChE), it can lead to the serious and irreversible inhibition the activity of AChE and then disrupt the nervous system of the mammal or even result in death [2]. For the past few years, more than 100 thousand of people had died from OPs poisoning, because the food or drinking water which is contaminated with OPs has been taken in Ref. [3]. With hypertoxicity and fatalness, OPs have been prohibited in the agricultural application based on the provisions of the Food and Agricultural Organization (FAO) and the United Nations Environment Programme (UNEP) [4]. Nowadays, people are paying more and more attention to body health, environmental conservation, and food safety. So, a lot of detection methods have been developed for OPs monitoring [5,6], including liquid chromatography-massspectrometry (LC-MS) [7], (enzyme-based) electrochemical sensor [8], and fluorescence biosensor [9], and so on. All these strategies can achieve the selective and sensitive detection of OPs. However, all methods either need expensive laboratory equipment, complex sample preparation or require skillful experimenter. So, the eco-friendly, energy-saving, easy-to-operate and highly sensitive detection methods are eagerly needed to monitor the OPs.
Photoelectrochemical (PEC) detection, acted as the newly emerged detection method, have been apace developed [10]. Compared with conventional electrochemical and optical method, PEC biosensor has low background noise and high sensitivity owing to the completed separation between the detection signal (photocurrent) and excitation source (light). In addition, PEC biosensor also possesses many other advantages, such as simply instruments, easy operation and low cost [11]. So, PEC biosensors have been broadly applied to the detection of inorganic ions [12], DNA sequence [13], disease or cancer markers [14], and so on. Moreover, the self-powered PEC biosensors had been developed, in which no extra voltage was provided. Without extra voltage, on the hand, the self-powered PEC biosensor can make sufficient use of light, on the other hand, it also can avoid the interference of oxidation/reductive substances and reduces damage to biological molecules on the electrode to improve the sensitivity of the detection [15,16]. The photocurrent response of the self-powered PEC biosensor heavily depends on the photoactive material. Hence, photoactive materials play a very important role in the fabrication of the PEC biosensor. Until now, many photoactive materials have been applied in PEC biosensor, including ZnO [17], TiO2 [18], g-C3N4 [19], and so on. Especially, attributed to the high biological and chemical stability, abundant raw materials, good optical properties and nontoxicity, TiO2 have been regarded as the attractive candidate for applying in the preparation of the PEC biosensor. However, shared with the wide band gap, TiO2 can only absorb ultraviolet (UV) light (λ<400 nm), that does not benefit to make the best use of the solar light, because the UV light only accounts for less than 5% in solar light [20]. And the application of TiO2 for biosensor also is limited, because the UV light can damage the biomolecules. So, many methods have been developed to enable TiO2 to effectively utilize the visible light, for intense, nonmetal atoms doping modification [21], noble metal modification [22], and dye sensitization [23].
Carbon quantum dots (CQDs) are the newly emerged economic environmentally friendly carbon materials, which possesses small sizes below 10 nm and narrow band gap. Compared with semiconductor quantum dots, such as CdTe [24], CdS [25], CQDs are more environmentally friendly. Owing to its preeminent optical properties, size-dependent emission wavelength, quantum confinement, photostability, good water solubility and biocompatibility, CQDs have been applied in many fields, including solar cells [26], fluorescent sensors [27,28] etc., Meanwhile, CQDs have the upconversion photoluminescence (PL) properties that it can absorb long wavelength light with the emission of light at short wavelength, that is different from traditional inorganic semiconductor-based QDs [29]. In fact, CQDs still have some shortcoming, for example, low fluorescence quantum yield and poor electron transfer capability. Nitrogen-doped carbon quantum dots (NCQDs) obtain enormous attention because nitrogen elements doping will cause charge delocalization and provide tunable optical property. The gained NCQDs possess high fluorescence quantum yield, enhanced electron transfer capability, and conductivity. So, NCQDs can be expected to exhibit superexcellence performances in the PEC biosensor.
Herein, the visible light-driven and self-powdered PEC biosensor was fabricated for the OPs detection. As depicted in Scheme 1, in the process of construction, the TiO2 was served as the photoactive substrate material, hydrothermal prepared NCQDs was used as the photosensitizer for signal amplification. Owning to narrow band gap, high fluorescence quantum yield, fast electron transportation and upconversion photoluminescence properties of the NCQDs, the photocurrent response of the obtained NCQD/TiO2/ITO electrode showed about 42 times than that of the TiO2/ITO electrode under visible light. With easy chemical modification, good film-formation ability, and nice adhesion, the chitosan (CS) was modified on the NCQD/TiO2/ITO electrode to prevent the nanomaterials from peeling off from the electrode. Then the AChE was covalently immobilized on the surface of the electrode by using glutaraldehyde as the cross-linking agent. The AChE can catalytic hydrolysis of acetylcholine chloride (ATCl) to generate thiocholine which can serve as the electron donor to strengthen the separation of the photogenerated electron-hole pair, resulting in the enhanced photocurrent output. The prepared PEC biosensor was applied in the detection of OPs by using chlorpyrifos as the model. When the prepared biosensor was exposed to the solution containing chlorpyrifos, the activity of AChE was inhibited by chlorpyrifos, the obvious decrease of photocurrent response was observed. Based on the relationship between the inhibitions of chlorpyrifos and the photocurrent responses, the visible light-driven and self-powdered PEC biosensor was fabricated for rapid and sensitive detection of chlorpyrifos.
Section snippets
Chemical and reagents
Titanium butoxide (C16H36O4Ti), Glutaraldehyde (GLD, 50% aqueous solution) and Acetylcholine chloride (ATCl) were purchased from Macklin Chemical Reagent Co. Ltd. (Shanghai, China). Ammonium citrate and ethylenediamine were obtained from Guangdong Xilong Chemical Co. Ltd. (Shantou, China). Ethanol absolute was available from Damao Chemical Reagent Factory (Tianjin, China). Acetic acid was provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Chitosan (CS) and chlorpyrifos were
The UV–vis absorption spectra of CQDs and NCQDs
As shown in Fig. 1A, CQDs had the strong optical absorption in the UV region and weak absorption near to the visible range. Compared with CQDs, NCQDs exhibited two characteristic peaks at around 238 nm and 340 nm respectively. The representative absorption peak at 238 nm was due to the carbon core which belonged to π-π* electronic transitions of the aromatic sp2 domains. The characteristic absorption peak at 340 nm was originated from the n–π∗ electronic transition of CO and CN bond, which was
Conclusion
In summary, the elaborated visible light-driven and self-powdered PEC biosensor was prepared for OPs detection. With the excellent optical performance and the remarkable electrical conductivity, NCQDs exhibited excellent sensitization performance on the NCQD/TiO2/ITO electrode. Based on the enzyme inhibition, the prepared PEC biosensor can achieve the sensitive detection of OPs by using chlorpyrifos as the model. The obtained PEC biosensor revealed the broad detection range and low detection
Acknowledgements
We are grateful for the financial support from the Natural Science Foundation of Guangdong Province (No. 2014A030313480) and the Science & Technology Project of Guangdong Province (No. 2013B030600001 & No. 2016A020223014).
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