INTRODUCTION
Background of pesticides detection
Pesticides are widely used in agricultural production, and the pesticides demand is increasing due to population growth leading to a priority in promoting high agricultural productivity The ingredients of pesticides are chemicals that can control pests or plant growth have become effective tools in boosting agricultural productivity However, pesticides are also the world's leading concern in food safety, which are the main cause of more than 200,000 deaths each year [1] Furthermore, the health concerns associated with the consumption of products containing pesticides are becoming more noticeable in developing countries due to the lack of knowledge and understanding of farmers using pesticides in cultivation
At present, many pesticides with nitro-aromatic compounds (NACs) such as parathion, nitrofen, fenitrothion and mesotrione have been commonly used to protect crops NACs pesticides are one of the leading causes of environmental pollution due to their high biological activity and toxicity [2] Most pesticides do not distinguish between plants and pests, they have the potential to harm humans, animals and other living organisms if used incorrectly One of the most important aspects of minimizing potential risks to humans and the environment is the monitoring of these pesticide residues Up to now, the methods of detecting pesticides have made remarkable progress Typical are the traditional chromatographic detection methods, including gas chromatography (GC) [3], high-performance liquid chromatography (HPLC) [4] and gas chromatography- mass spectrometry (GC ± MS) [5], besides other methods such as electrochemical analysis [6], biosensors based on AChE [7], etc All of these methods have their own advantages and disadvantages, in which the chromatographic method shows accurate and high sensitivity but requires expensive equipment investment, time-consuming, complicated operation and poor portability There is a reason why developing a new method to detect pesticide residues with advantages: fast, sensitive, reliability and low- cost for timely handling is one of the important issues that need to be focused on research
No Names Structure WHO Acute Hazard
Ia = Extremely hazardous; II = Moderately hazardous; III = slightly hazardous; U Unlikely to present acute hazard in normal use; O = Obsolete as pesticide, not classified.
Fluorescence sensor for the detection of nitroaromatic pesticides
In recent years, a fluorescence-based method has been approached due to better efficiency, simplicity, fast, and reliability This technique is based on colorimetric and fluorimetric responses which are mainly focused on by vast fluorescence sensors Optical sensors based on photoluminescence (PL) quenching have attracted great interest due to their sensitivity, low cost and ease of operation [9]
Fig 1-1: Schematic illustration of the working principle of the fluorescence quenching based sensor for NACs pesticide detection
In 2009, Mallard-Favier et al [10] synthesized a new peracetylated cyclodextrin trimer bearing three 1,2,3-triazole linkers that the fluorescent tripod exhibits a very good variation of emission fluorescence by the addition of pendimethalin with extremely low detection limits (0.8-4 àM)
In 2014, Kumar et al [11] synthesized a luminescent nanocrystal metal-organic framework (NMOF1) for chemosensing of the nitroaromatic-containing organophosphate pesticides such as parathion, methyl parathion, paraoxon and fenitrothion
In 2018, Hergert et al [12] reported the molecular chain effect in phenylene ethynylene oligomers to detect insecticides The results show a significant increase in Stern-Volmer quenching - beyond the molecular wire effect
In 2018, Zhao et al made a significant contribution by developing a highly sensitive and selective pyranine-EDVHGIOXRUHVFHQW³WXUQ-RII´PHWKRG sensor for paraquat detection The optimized conditions employed in the design of this sensor enabled efficient paraquat detection in real-world samples, showcasing its potential for practical applications.
In 2019, Sun et al developed a fluorescent probe employing molecularly imprinted polymers (MIPs) encapsulating biomass-derived carbon quantum dots (CQDs) This probe leverages the capture capacity of CQDs@MIPs to selectively bind to mesotrione, resulting in fluorescence quenching This innovative approach enables rapid and sensitive detection of mesotrione.
In 2019, Hu et al [15] also reported a colorimetric chemosensor for the simple and rapid detection of dimethoate pesticides in agricultural products based on the inhibition of the peroxidase-like activity of gold nanoparticles (AuNPs) Optical chemosensing techniques based on functional materials/nanomaterials such as conjugated molecules/oligomers/polymers, macrocycle and luminescent metal-organic frameworks have shown great potential for sensitive and selective detection of pesticide
In 2020, Zhang et al [16] reported that a carbazole-containing polymer with significant inherent porosity exhibited apparent fluorescence quenching upon raising the concentration of trifluralin, and the fluorescence quenching with the increase of concentration is up to 84%.
Thesis Objective
Functionalization of conjugated fluorophore and understanding of molecular interaction of electron donors and electron acceptors are very important to produce efficient fluorescence sensors The main objective of this thesis is to investigate the applicability of fluorescence conjugated molecular as a fluorescent chemosensor to detect nitroaromatic pesticides The design of conjugated molecular using in this thesis is based on pyrene derivative combined with either 4-(2-ethylhexyl)-4h-dithieno[3,2- b:2',3'-d]pyrrole or 9,9-dioctyl-9H-fluorene These fluorescent chemosensors are expected to provide critical insight of the detection mechanism, thus contributing to the future development of fluorescent chemosensors that will detect more efficiently.
LITERATURE REVIEW
Fluorescence quenching theory
The applications of the fluorescence sensor were based on the principle that the molecular contact between analytes and fluorophore either decreases the fluorescence by quenching, or increases fluorescence by suppressing the quenching effect This contact can be the result of the complex formation (static quenching), the diffusive encounter (dynamic quenching), energy transfer and photoinduced electron transfer (PET)
Fig 2-1: Quenching mechanisms of fluorescent sensor which is used in the process of detecting analytes And the effect of temperature on the effectiveness of dynamic (a) and static (b) quenching [17]
2.1.1 Static and Dynamic quenching mechanism
Static quenching occurs when a non-emissive ground-state is formed through the interaction between the sensor and the quencher The complex immediately returns to the ground state without emission of the photon when absorbing light leading to quenching of the initial fluorescence intensity However, an excited-state electron transfer takes place from the excited state of the sensor to the quencher through collision in the dynamic quenching with the mechanism of energy transfer or charge transfer [18]
Fluorescence decay lifetime measurements distinguish static and dynamic quenching mechanisms In static quenching, the decay lifetime remains constant, as non-emissive complexes form Dynamic quenching, a diffusion-controlled process, exhibits a reduced lifetime with increasing quencher concentration due to brief interactions between excited sensor and colliding quencher molecules Measuring fluorescence lifetime changes provides an effective means of determining the nature of quenching processes.
Dynamic quenching of fluorescence is described by the Stern±Volmer equation and the correlation of lifetime with quencher concentration can be expressed as: ߬ ߬ ൌ ͳ ܭ ሾܳሿሺͳሻ ܨ ܨ ൌ߬ ߬ ሺʹሻ
Where ܨ and ܨ are the fluorescence intensities in the absence and presence of a quencher, respectively; ߬ and ߬ are the fluorescence life times of the sensors before and after addition of quencher at a given concentration [Q], respectively; ܭ are the dynamic Stern±Volmer quenching constant
The dependence of fluorescence intensity (F) on quencher concentration ([Q]) for static quenching can be derived using the association constant for complex formation This relationship is described by the Stern-Volmer equation: F0/F = 1 + KSV[Q], where KSV represents the Stern-Volmer constant for static quenching This equation highlights the inverse relationship between fluorescence intensity and quencher concentration in static quenching processes.
Now, for steady-state fluorescence quenching involving both collisional and static quenching, the quenching process is examined by Equation (4): ܨ ܨ ൌ ሺͳ ܭ ሾܳሿሻሺͳ ܭ ௌ ሾܳሿሻሺͶሻ ൌ ͳ ሺܭ ܭ ௌ ሻሾܳሿ ܭ ܭ ௌ ሾܳሿ ଶ
For very low concentrations of the analyte, the quadratic term has less contribution and Eqn (4) would yield a linear plot However, at higher concentrations, the plot deviates from linearity and bends upwardly This equation greatly explains an upward bending curvature at high quencher concentrations or an exponential fitting of the Stern± Volmer plot under some circumstances [20]
Static and dynamic quenching can also be distinguished by their different dependence on temperature or viscosity [18] As shown in Fig 2-1a and Fig 2-1b, higher temperatures result in faster diffusion and hence larger amounts of dynamic quenching While for static quenching, higher temperatures will typically promote the dissociation of weakly bound complexes, and hence result in lower amounts of non- emissive fluorophore±quencher complexes One additional method to distinguish static and dynamic quenching is by careful examination of the absorption spectra of the sensor Dynamic quenching only affects the excited states of the sensor, and thus no changes in the absorption spectra are expected In contrast, the ground-state complex formation will frequently lead to perturbation of the absorption spectrum of the fluorophore Furthermore, the bimodular quenching constant k q is also used for discriminating between static and dynamic quenching, and k q is calculated using the ratio of the Stern± Volmer quenching constant (K SV ) to unquenched fluorescence lifetime (W 0 ) For dynamic quenching, diffusion-controlled quenching typically results in values of k q near 10 10 M -
1 s -1 ; while for static quenching, the k q value is generally several orders larger than 10 10
While both ways can be operative, static quenching prevails in pesticides detection due to its larger K SV for quencher binding to many fluorophore indicators and higher sensitivity in general Although the fluorescent sensors of dynamic quenching have the potential to lead faster and more reversible detection, they possess much smaller K SV and demonstrate a lower sensitivity [22]
The energy transfer mechanism has also been used to develop a number of sensors, and can dramatically enhance the fluorescence-quenching efficiency and improve sensitivity The energy transfer is divided into Fửrster resonance energy transfer (FRET), Dexter energy transfer (DET) and Surface energy transfer (SET)
The term FRET is the acronym for Fửrster resonance energy transfer, named after a German scientist who discovered it in 1948 In FRET, the photonic energy of a first fluorophore (the donor) is acquired by a second fluorophore (the acceptor), and then emitted by the second fluorophore FRET occurs without the appearance of a photon due to long range dipole±dipole interactions between fluorophore and quencher The distance between the fluorophore and quencher was in the range of 10 ű100 Å [17]
FRET is an electrodynamic phenomenon that can be explained by using classical physics FRET occurs between fluorophore in the excited state and quencher in the ground state when the emission spectrum of fluorophore overlaps with the absorption spectrum of the quencher FRET occurs without the appearance of a photon due to long range dipole±dipole interactions between fluorophore and quencher [18, 23-25] According to FRET theory [18, 26, 27], the rate of energy transfer depends on (1) the relative orientation of the donor and acceptor dipoles, (2) the extent of overlap of the fluorescence emission spectrum of the donor (the fluorophore) and the absorption spectrum of the acceptor (the quencher), and (3) the distance between the donor and the acceptor The probability of resonance energy transfer depends upon the extent of overlap between these molecules
The effect of DET is based on electron transfer, not photon transfer and therefore requires a match between the redox potentials of donor and acceptor [17]
SET is a rather new process It is most often observed with (metal) nanoparticles and involves a metallic surface (such as on gold NPs) and a molecular (organic) dipole SET was theoretically predicted in 1978 by R Chance et al and experimentally proven in the 2000s [28, 29]
Photoinduced electron transfer (PET) can be explained that between the sensor (electron donor or electron receptor) and the quencher (electron receptor or electron donor) occurred the electron transfer, formed the cation radical and the anion radical respectively In this process, a complex was formed between the electron donor and electron acceptor that can return to the ground state without emission of a photon, but in some cases exciplex emission was observed PET plays an important role in the fluorescence quenching process, and provides useful insights into the development of fluorescence sensors [19]
PET contained reductive PET and oxidative PET Reductive PET was that sensor as an electron receptor got electron from the electron donor The driving force for reductive electron transfer was the energy gap between the lowest unoccupied molecular orbitals (LUMO) of quencher and the highest occupied molecular orbitals (HOMO) sensor Oxidative PET was contrary to reductive PET The driving force for oxidative electron transfer was the energy gap between the LUMO of the sensor and the LUMO of the quencher [18, 30] Therefore, the energy gap of the LUMO and HOMO or the LUMO and LUMO between the sensor and the quencher existed would demonstrate that the quenching mechanism was PET.
Fluorescent materials for chemosensor
With high sensitivity and simplification, fluorescence-based sensors as one of the most commonly used sensing candidates, have been widely applied in broad fascinating fields, ranging from biomedical diagnosis [31-34] to environmental monitoring [35], food safety and quality control [36], as the signal change can be collected via spectrofluorometer and observed by naked eye on-site [37, 38] In recent years, great efforts have been devoted to developing new fluorescent materials in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time With the development of advancing technologies, various kinds of materials have been widely employed for the fabrication of fluorescence sensing platforms, such as small fluorophores, conjugated polymers, supramolecular systems, aggregation-induced emission-active materials and bio-inspired materials [19]
Small fluorophores are the main focus in the early development of fluorescence- based sensors because they provide a variety of benefits such as simple synthesis and varied pathways of fluorescence quenching The main difference between conjugated polymeric systems and small molecule-based detection lies in the mechanism of fluorophore quenching and the absence of excitonic migration in the small molecules Polymer-based sensors frequently detect analytes through static quenching; in contrast, well-resolved small molecules typically work through dynamic quenching A further difference is that conducting polymers exhibit increased quenching efficiency as several excitons within a polymer can be quenched by one molecule of bound analyte; small molecule fluorophores are quenched in a stoichiometric fashion of one analyte per fluorophore (as shown in Fig 2-2) Besides simple small molecules, functional small molecule fluorophores have been appearing and attracting great attention in recent years, and presumably show greater potential in fluorescence sensing by the form of oligofluorophores, self-assembling small fluorophores or doping of these molecules into a matrix, because they can enhance binding and introduce exciton migration [19]
Fig 2-2: The schematic illustration of molecular wire theory
In 2010, Lee et al reported the dipyrene appended calix [4]arene with the two pyrene substitutes oriented to the same side of the ensemble Both the monomer (Oem 375 nm) and excimer (Oem = 470 nm) emission were quenched upon the addition of TNT ± a member of NACs family - in acetonitrile with a detection limit down to 1.1 nM [39]
In 2016, researchers developed a new fluorescent sensor called benzimidazo[2,1-a]benz[de] isoquinoline-7-one-13-(N-butylthioamide) for detecting mercury ions (Hg2+) This sensor exhibited remarkable selectivity and sensitivity for Hg2+ through a specific desulfurization reaction triggered by the metal ion Upon exposure to Hg2+, the sensor's fluorescence intensity dramatically increased by up to 40-fold, providing a clear and robust signal for the presence of Hg2+.
Fig 2-3: Fluorescence enhancement of sensor sensor benzimidazo[2,1-a]benz[de] isoquinoline-7-one-13-(N-butylthioamide) by reaction with Hg 2+
In 2018, the synthesis and quenching behavior of a series of water-soluble, carboxylate-carrying phenylene ethynylene oligomers²monomer to tetramer²and their polymers were reported by Hergert et al Their quenching behavior with different test analytes such as paraquat, mercury salts, picric acid, etc in water was investigated, and the results were compared to that of the conjugated polymer For monovalent quenchers, only the molecular wire effect applies But for divalent quenchers, multivalency effects are also important [12]
In this thesis, we choose a functional small molecule as a material to detect NACs pesticides, more specifically triad molecules It is well known that the fluorescence quenching increases with the molecular weight for conjugated monomers, which is attributed to the increased diffusion length of the exciton through molecular wire theory
In order to enhance the exciton diffusion in the case of simple small molecules, triad- system fluorophores have been synthesized and investigated Generally, the triad-systems are defined as three units of small molecule fluorophores or the previous basic bones Due to their molecular structures, they possess some unique properties
Conjugated polymers (CPs) have recently been used successfully in nitrated fluorescence detection Compared with small molecule fluorophores, they have an extended exciton migration pathway and efficient electronic communication between quenchers along the polymer backbone CPs are excellent electron donors, and their donor ability is enhanced by the delocalized S* excited state, which facilitates exciton migration and hence increases the electrostatic interaction between the polymer and electron- deficient nitroaromatic analytes For CPs fluorescent sensors, Swager et al proposed that binding one receptor site resulted in an efficient quenching of all emitting units in an entire conjugated polymeric molecule relative to single molecule systems This DPSOLILFDWLRQLVNQRZQDVWKHààPROHFXODUZLUHảảHIIHFWRUWKHààRQHSRLQWFRQWDFWPXOWL- SRLQWUHVSRQVHảảHIIHFWZKLFKLVLOOXVWUDWHGLQFig 2-2 [41, 42]
In 2007, Aditya Narayanan and co-workers reported that two conjugated polymers P1 and P2 (Fig 1-5) which show good multiphoton absorption properties This is combined with the excellent sensitivity of the multiphoton excited fluorescence to the presence of nitroaromatic compound [43]
Fig 2-4: Two conjugated polymers P1 and P2 with three-dimensional iptycene units
In 2013, F Chu, G Tsiminis, N.A Spooner, T.M Monro demonstrate a suspended on the fluorescence quenching of a surface-attached conjugated polymer poly[2- methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) This is the first demonstration of a fluorescent conjugated polymer sensor capable of measuring liquid nitroaromatic compound samples loaded within an optical fiber This technique is used to identify 1,4-dinitrobenzene (DNB), a member of the nitroaromatics family of explosives, in acetone for concentrations as low as 6.3 ppm in a total sampling volume of 27 nL and to quantify its concentration using the fluorescence decay lifetime, requiring an analysis time of only a few minutes [44]
In 2016, Talbert, Levine and co-workers reported the significant fluorescence enhancement of conjugated polymer PFBO-derived nanoparticles in the presence of aromatic organochlorine pesticides This pesticide-mediated fluorescence enhancement leads to reversible pesticide detection systems with KLJKVHQVLWLYLW\DVORZDVȝ0DV well as significant generality and straightforward reversibility [45]
Fig 2-5: 2,1,3-benzooxadiazole-alt-fluorene (PFBO) structure
The application of CPs for fluorescence detection has the great advantage of high sensitivity CPs based chemosensor can be effective under a variety of conditions, including in aqueous, solid-sate and especially in vapour phase However, the complicated synthesis and purification process of CPs materials are their significant drawbacks, which make them difficult to utilize in real life
The field of supramolecular systems focuses on the non-covalent interactions between molecules that give rise to molecular recognition and self-assembly processes Supermolecular systems include macrocycle, dendrimer, supramolecular polymer, and metal-organic framework In particular, dendrimer and metal-organic framework
(MOFs) are especially interesting and have a great potential to be applied in fluorescence sensors [19, 46]
In 2011, Shaw and co-workers reported on three generations of fluorescent carbazole dendrimers with spirobifluorene cores which interacted with NACs compound primarily via fluorescence quenching (Fig 1-7) [47]
Fig 2-6: Structures of dendrimers for three generations defined as G1±G3
2.2.4 Aggregation induced emission (AIE)- active materials and bio-inspired fluorescent materials
Aggregation induced emission (AIE) is a phenomenon where certain fluorescent materials, such as tetraphenylethene (TPE), triphenylbenzene (TPB), and siloles, emit efficiently when aggregated, in contrast to the aggregation caused quenching effect observed in other materials These AIE-active polymers remain virtually non-emissive in good solvents, but exhibit intense fluorescence upon aggregation in poor solvents or formation of solid films This unique property makes AIE materials effective as fluorescence sensors.
Fig 2-7: Small molecules for AIE materials such as tetraphenylethene (TPE), triphenylbenzene (TPB) and siloles
In recent years, bio-based materials such as peptides, protein, and DNA have been employed for fluorescence-based detection The bio-materials can be functionalized in two different roles In the first role, the biomaterial is only used as a support or a functional layer [48] Secondly, researchers are harnessing fluorescent properties of biomaterials such as fluorescent proteins to serve as pesticide sensors Generally, the fluorescence of protein is caused by three intrinsic fluorophores present in the protein, such as tryptophan, tyrosine, and phenyl alanine residues The intrinsic fluorescence of many proteins is mainly contributed by tryptophan alone The fluorescent properties of proteins were applied for pesticide quenching, taking advantage of the fact that tryptophan excitation can be quenched by numerous agents [18, 49]
Molecular design of fluorophores
Fully understanding the relationship between structure and property is critical for the design of high-performance chemosensors and benefits for recognizing the sensorảV nature and envisaging the ideal materials for the application Some organic compounds like a pyrene, fluorene, dithienopyrrole, carbazole are remarkable for their unique electroconducting and fluorescence properties, thus they are suitable for many applications ,WLVNQRZQWKDWHQODUJLQJWKHHIIHFWLYHFRQMXJDWLRQGHJUHHRIDʌ-system and enhancing its binding ability to an analyte is still an effective way to enhance its performance in sensing the analyte [50-52]
Among the fluorescent probes, pyrene derivatives are widely used as efficient fluorophores due to their high quantum yield with a large electron-rich conjugated plane, chemical stability on molecular labeling and high fluorescent sensing behaviors [53, 54]
Pyrene exhibits distinct fluorescence properties, shifting from ultraviolet to visible wavelengths with increasing concentration This bathochromic shift indicates the formation of pyrene excimers Pyrene's fluorescence can be quenched by electron-deficient nitroaromatic compounds (NACs) due to S±S stacking interactions A proposed mechanism involves the formation of excited fluorophore-quencher ion pairs, facilitating electron transfer from pyrene to NACs.
Nevertheless, the major problem of pyrene is low fluorescence efficiency caused by the strong intramolecular aggregating tendency of the fluorophore at high concentration or pure crystal [58-62] To solve this problem, the incorporation of different peripheral attachments into the suitable positions of pyrene core has been introduced This can cause twisting in the structure and prevent excimer formation resulting in dramatic improvement in the photoluminescence properties, especially in the fluorescence quantum yield [63, 64] However, a few studies have been reported on developing novel pyrene-based compounds for detection of nitroaromatic compounds as well as theoretical studies of the reaction mechanism
In 2009, Chen and co-workers carried out a study to examine the application of pyrene-functionalized ruthenium (Ru) nanoparticles for the detection of NACs The metallic Ru core served as a conducting medium for an extended intraparticle charge delocalization, leading to a quenching efficiency more than 1 order of enhanced magnitude than the equivalent concentration of 1-bromopyrene in DMF solution [65]
In 2011, Zhiqiang Wang and co-workers achieved two novel pyrene derivatives 1,6-bis(3,5-diphenylphenyl)pyrene (BDPP) and 1,6-bis(2-naphthyl)pyrene (BNP) Both quantum yields And prove that the high efficiency and stability of the device indicate that BDPP is a promising emitting material for non-doped deep-blue OLEDs [66]
In 2012, Nakorn Niamnont and co-workers found that the synthesis of a novel, tunable star-shaped triphenylamine fluorophores containing pyrene (TEP, TAP) or corannulene (TEC, TAC) show variable fluorescence quenching sensitivity toward nitro explosives The most sensitive fluorophore is capable of detecting TNT on the ng cm -2 scale; the array is useful for identifying NACs [67]
Fig 2-8: (top) Structures of fluorophores; (bottom) HOMO-LUMO of TAP and TEP
In 2016, Sharad Chandrakant Deshmukh and his group designed selective sensing of metal ions and nitro explosives by efficient switching of excimer-to-monomer emission of an amphiphilic pyrene derivative A simple and smart amphiphilic pyrene compound was developed for strong and stable excimer formation in aqueous media
Sensing applications of the excimer in the aqueous media were demonstrated in the present work through detections of metal ions and nitro explosives [68]
In 2021, WZRDSSURDFKHVWRZDUGV³WXUQRII´IOXRUHVFHQFHGHWHFWLRQRIQLWURDQDO\WHV were reported by Kovalev, I S and co-workers Tuning the chemosensors structure of pyrene-based lipophilic/biphilic or by changing the environment was achieved with Stern-Volmer quenching constant as high as 2.28±3.14 × 10 4 M í1 (for structure modification approach) and 4.67 × 10 5 M í1 (for changing of envi ronment approach) [69]
Reccently, fluorene and dithienopyrrole derivative show interesting spectroscopic, photophysical and thermal stability properties which should be favorable for fluorescence sensors Among the various class of organic fluorophores studied, these are quite popular, because of its structural planarity, extended conjugation, highest occupied molecular, lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, and strong intermolecular interaction [70, 71]
To detect nitroaromatic pesticides, this study synthesizes two pyrene-based conjugated compounds These compounds comprise pyrene coupled with fluorene or dithienopyrrole groups, following established literature protocols.
Overview of thesis
The research content of my thesis is divided into three parts namely Chapter 3, Chapter 4 and Chapter 5
Chapter 3 mainly focuses on the stages involved in the synthesis of novel conjugated monomers as well as the analytical methods used to analyze the chemical and optical properties of materials including: proton nuclear magnetic resonance spectroscopy ( 1 H NMR), infrared spectroscopy (FTIR), UV-visible absorption spectroscopy, and photoluminescence spectroscopy (PL) The synthesized triads 2PDTP and 2PDOF based on pyrene derivative combined with either 4-(2-ethylhexyl)-4h- dithieno[3,2-b:2',3'-d]pyrrole or 9,9-dioctyl-9H-fluorene, and the structure of monomers is depicted in Fig 2-9 below:
Fig 2-9: Structure of 2PDTP and 2PDOF
Chapter 4 will discuss the issue of synthesis and the results from chemical and optical properties of news monomers: x The triads 2PDTP and 2PDOF, after being synthesized and purified, will be characterized for their chemical structure using FTIR and NMR spectroscopy to demonstrate that the synthetic reactions were performed successfully x The optical properties of 2PDTP and 2PDOF will be analyzed and investigated through UV-Vis absorption and PL spectroscopy x Finally, the applicability of the monomers as a fluorescent sensor for detecting NACs pesticides will be evaluated through the PL spectroscopy of the materials mixed with analytes Based on the analytical results, which will contribute to the study of the detection mechanism as well as the effect of functional groups on the material's applicability as a fluorescent sensor Conclusion is the main topic of Chapter 5 which will summarize, give clearly state answers WRWKHWKHVLVảVUHVHDUFKLVVXHs and recommendations for further research.
EXPERIMENTAL
Materials and Reagents
N-bomosuccinimide (NBS, 99%), pyrene (98%), 3,3'-dibromo-2,2'-bithiophene
(98%) and 2-ethylhexylamine (98%) were purchased from Acros Organic Mesotrione (98%), ả-(9,9-dioctyl-9H-fluorene-2,7-dityl)bis(4,4,5,5-tetramethyl-1,3,2- dioxaborolane) (DOF, 99%), tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3, ả-bis(diphenylphosphino)-ả-binaphthyl (BINAP, 98%), sodium tert- butoxide (t-BuONa, 97%), palladium(II) acetate (Pd(OAc)2, 98%), tricyclohexylphosphine tetrafluoroborate (P(Cy)3.HBF4, 97%), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4, 99%), cesium carbonate (Cs2O3, 99%) and pivalic acid (PivOH, 99%) were purchased from Sigma-Aldrich (St Louis,
MO, USA) Potassium carbonate (K2CO3, 99%) was purchased from Acros and used as received
Chloroform (CHCl3, 99.5%), dimethylfomamide (DMF, 99%) and toluene (99.5%) were purchased from Fisher/Acros and dried using molecular sieves under N2 Dicloromethane (DCM, 99.8%), absolute ethanol (99%) and hexane (99%) were purchased from Fisher/Acros and used as received.
Analysis and measurement methods
Column chromatography is a chromatography method used to isolate a single chemical compound from a mixture Chromatography is able to separate substances based on differential adsorption of compounds to the adsorbent; compounds move through the column at different rates, allowing them to be separated into fractions The technique is widely applicable, as many different adsorbents (normal phase, reversed phase, or otherwise) can be used with a wide range of solvents The technique can be used on scales from micrograms up to kilograms The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process The latter prevents cross-contamination and stationary phase degradation due to recycling Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column [72]
Thin-layer chromatography (TLC) plays a vital role in optimizing the purification process using column chromatography It provides insights into the behavior of compound mixtures during separation By analyzing the results obtained through TLC, researchers can determine the ideal conditions for column chromatography, ensuring efficient and targeted purification of compounds.
Nuclear magnetic resonance spectroscopy (NMR)
Nuclear magnetic resonance NMR spectroscopy is routinely used by chemists to elucidate chemical structures using simple one-dimensional techniques The techniques allow detection of the nuclei spins in intense magnetic field Interpretation of a simple first order NMR spectrum usually involves evaluation of five principal features; a) the number of signals, b) WKHFKHPLFDOVKLIWįYDOXHLQSSPSDUWSHUPLOOLRQFWKHVLJQDO multiplicity quantifying the number of neighboring non-equivalent magnetically active nuclei, d) the coupling constant J corresponds to the spacing in Hz between two couplings and e) the signal integral
The NMR spectra were recorded on a Bruker Avance AM500 FT-NMR spectrometer at Viet Nam Academy of Science and Technology, Ha Noi For all the molecules, chloroform-d (CDCl3) containing tetramethylsilane (TMS) as an internal standard (0 ppm) was used as solvent The chemical shifts values are calibrated to the solvent values: CDCl3 = 7.26 ppm for proton The following abbreviations have been used for the NMR assignment of signal multiplicity: s for singlet, d for doublet, t for triplet, m for multiplet and br for broad
All synthesized monomers were characterized by 1 H NMR to confirm their structures In a typical sample, the compound is dissolved in deuterated chloroform The weight concentration of the samples is at 10-20 mg/ml for 1 H NMR measurements
Infrared spectra were recorded from KBr by FTIR Bruker Tensor 27 at National Key Laboratory for Polymer and Composite Materials The infrared spectroscopy allows detection of characteristic vibration frequencies of molecules according to respective vibrations modes Infrared spectroscopy measurement allows detection of different functional groups and can be useful as complementary information to confirm proposed molecules structures
Ultraviolet and visible radiation interacts with matter which causes electronic transitions (promotion of electrons from the ground state to a high energy state) The ultraviolet region falls in the range between 100-400 nm, the visible region fall between 380-780 nm
In photochemistry, the terms "bathochromic shift" and "hypsochromic shift" describe changes in spectral band position A bathochromic shift occurs when the spectral band shifts to a longer wavelength (lower energy), also known as a "red-shift." Conversely, a hypsochromic shift occurs when the spectral band shifts to a shorter wavelength (higher energy), also known as a "blue-shift."
UV-Visible absorption spectroscopy measurements were conducted on an ASEQ LR1 spectrophotometer within a wavelength range of 250-800 nm at the National Key Laboratory for Polymer and Composite Materials These measurements were recorded in solutions, and the absorption spectra were acquired as a function of absorbance and wavelength (in nm).
Photoluminescence spectroscopy is a useful technique for studying the optical and electronic properties of the excited state of materials providing information about recombination and relaxation processes Two modes of photoluminescence can be studied by this measurement, excitation mode and emission mode Photoluminescence excitation is studied by fixing one emission wavelength and varying the excitation wavelength to give an excitation spectrum For our study, we will focus on the photoluminescence emission This mode allows studying the emission spectrum at fixed excitation wavelength The picture Fig 3-1 can allow better understanding of the photoluminescence emission
Fig 3-1 : Jablonski diagram of electronic transitions, absorption spectrum and emission spectrum
Photons absorbed by molecules excite electrons, resulting in a peak in the absorption spectrum The excitation energy then undergoes internal conversion, dissipating energy Subsequently, electrons return to the ground state, releasing energy as fluorescence, creating a peak in the emission spectrum The difference between excitation and emission energy is known as Stokes shift.
Photoluminescence spectroscopy measurement also allows the study of the charge transfer complex (in our oligomers) and the photo-induced charge transfer in the blend oligomer/polymer with fullerenes (PCBM) Charge transfer and energy transfer are nonradiative processes therefore the quenching or absence of emission can be detected by photoluminescence measurement
Photo luminescence spectra were recorded on an Ocean optics SF-2000 spectrometer over the wavelength range of 350900 nm at National Key Laboratory for Polymer and Composite Materials.
Synthesis of 4-(2-ethylhexyl)-4h-dithieno[3,2-b:2',3'-d]pyrrole (DTP) [73]
To a solution of Pd2(dba)3 (94.25 mg, 0.10 mmol), BINAP (256.36 mg, 0.41 mmol) and t-BuONa (725.36 mg, 7.55 mmol) in toluene (8 mL) in a two neck round bottom flask were added 3,3'-dibromo-2,2'-bithiophene (1111.78 mg, 3.43 mmol) and 2- ethylhexylamine (443.44 mg, 3.43 mmol) under nitrogen The reaction mixture was degassed by three freeze-pump-thaw cycles and purged with nitrogen Then the reaction was carried out at 110 °C under N2 for 24 h After the reaction mixture had cooled to room temperature, the mixture was extracted with chloroform three times (3 × 20 ml), and the combined organic layers were washed with 10% NaCl (150 ml), dried over
K2CO3 and isolated by filtration The solvent was removed by rotary evaporation, and the product was purificated by column chromatography on a silica gel column using the eluent hexane Finally, the product was dried in a vacuum oven at 50 o C to obtain the yellow viscous liquid (Yield = 56%)
Synthesis of 1-Bromopyrene [74]
Pyrene (1.00 g, 4.94 mmol) was added to 20 ml of anhydrous DMF in a 100 ml two neck round bottom flask After cooling the solution to 0°C, N-bromosuccinimide (NBS) (0.97 g, 5.43 mmol) was added slowly to the reaction mixture Then, the reaction mixture was warmed to room temperature and stirred for overnight in the absence of light After completion, the reaction was quenched by addition of ice water and extracted with chloroform (60 ml) The organic layer was washed with brine (150ml), dried over anhydrous K2CO3, and finally isolated by filtration The solvent was rotary evaporated, and the product was precipitated with cold hexane to obtain the pure white powder The product was subsequently dried in a vacuum oven at 50 o C to obtain the white powder (Yield = 94%)
Synthesis of 4-(2-ethylhexyl)-2,6-di(pyren-1-yl)-4h-dithieno[3,2-b:2',3'- d]pyrrole (2PDTP)
To a solution of Pd(OAc)2 (6.74 mg, 0.03 mmol), P(Cy)3.HBF4 (22.09 mg, 0.06 mmol), Cs2CO3 (283.46 mg, 0.87 mmol) and PivOH (17.36 mg, 0.17 mmol) in toluene (8 mL) in a two neck round bottom flask were added 1-bromopyrene (202.43 mg, 0.72 mmol) and DTP (84.53 mg, 0.29 mmol) under nitrogen The reaction mixture was degassed by three freeze-pump-thaw cycles and purged with nitrogen Then the reaction was carried out at 110 °C under N2 for 48h After the reaction mixture had cooled to room temperature, the mixture was extracted with chloroform three times (3 × 20 ml), and the combined organic layers were washed with 10% NaCl (150 ml), dried over
K2CO3 and isolated by filtration The solvent was rotary evaporated, and the products were purification by column chromatography on a silica gel column using a mixture of hexane and DCM (V/V = 40:1) as the eluent Finally, the product was dried in a vacuum oven at 50 o C to obtain the orange powder (2PDTP, Yield = 52%)
3.6 Synthesis of ả-(9,9-dioctyl-9H-fluorene-2,7-diyl)dipyrene (2PDOF)
The synthesis of 2PDOF involved the reaction of Pd(PPh3)4 and K2CO3 in a toluene/ethanol/H2O mixture The reaction proceeded under nitrogen at 80°C for 19 hours, after which the product was extracted with chloroform and purified via column chromatography using a hexane and DCM eluent The isolated light yellow powder (2PDOF) exhibited a 48% yield.
Nitroaromatic pesticide detection
For NACs detection experiments, different concentrations (0, 0.5, 1, 5, 7.5, 10, 25,
50, 75 àM) of Mesotrione are added into 1 àM 2PDTP and 2PDOF solutions in CHCl3 After reaction for 5 min at room temperature, the corresponding fluorescence spectra were recorded at the exciting wavelength of 365 nm.
RESULTS AND DISCUSSION
Synthesis and structure characterization
4.1.1 Direct Arylation synthesis of 4-(2-ethylhexyl)-2,6-di(pyren-1-yl)-4h- dithieno[3,2-b:2',3'-d]pyrrole (2PDTP) a) Synthesis results
The synthetic route of 2PDTP is schematically described in Fig 4-1 below
Fig 4-1: Synthetic routes for the conjugated molecular 2PDTP Reagents and conditions: (i) Pd 2 (dba) 3 , BINAP, t-BuONa, toluene, 110 °C; (ii) DMF, r.t.; (iii)
Pd(OAc) 2 , P(Cy) 3 HBF 4 , Cs 2 CO 3 , PivOH, toluene, 110 °C
DTP (1), 1-bromopyrene (2) were synthesized according to the reported procedure
As shown in Fig 4-1, the Buchwald±Hartwig reaction between 3,3'-dibromo-2,2'- bithiophene and 2-ethylhexylamine catalyzed by Pd2(dba)3 at 110 o C for 24 h afforded compound 1 in a yield of 56%
The preparation of 2PDTP involved the D±bromination of pyrene using N- bromosuccinimide at room temperature in polar solvent dimethylformamide (DMF) in the absence of light This reaction affords selectively compound (2) with a high yield of 94%
Then, the direct reaction of compounds 1 and 2 was performed in the presence of the Pd(OAc)2 as the catalyst in anhydrous toluene at 110 o C under N2 for 48h, which was transformed into 2PDTP in yield of 52% This direct arylation mechanism based on the reaction of 1-bromopyrene and DTP begins with the oxidative addition of 1- bromopyrene (R) to the Pd(0) complex in the presence of ligand (P) P(Cy)3.HBF4 to form aryl-halo complex P-Pd(Br)-R Carboxylate ion coordinates with the aryl-halo complex to deprotonate DTP 5ả while simultaneously forming the 5ả-Pd bond, hence leading to the formation of the transition state The final product 2PDTP is formed by the reductive elimination of R-5ảIURPSDlladium Reductive elimination of the cross- coupled product regenerates palladium (0) and deprotonation of the resulting carboxylic acid by stoichiometric base regenerates the carboxylate co-catalyst (Fig 4-2) [75, 76]
Fig 4-2: Direct arylation mechanism of 2PDTP synthesis reaction
After being purified by column chromatography, all the synthesis of compounds were characterized by 1 H NMR and FTIR spectrometries to confirm the structure b) Chemical structure characterization
The 1 H NMR spectra of the intermediate and final products are shown and analyzed as follows [77]
Fig 4-3: The 1 H NMR spectrum (500 MHz, CDCl 3 ) of 1-Bromopyrene
Fig 4-3 shows the 1 H NMR spectra of 1-bromopyrene Peak a at 8.44 ppm was assigned WRWKHDURPDWLFSURWRQDWSRVLWLRQ³´ in the pyrene ring Those peaks b, c, e and h were observed from 8.27 to 8.17 ppm were attributed to the protons at positions ³´³´³´DQG³´LQWKHS\UHQHXQLW,QDJUHHPHQWZLWKWKLVthe broad multiplets in the range from 8.11 to 8.01 ppm were corresponding to the protons of other positions of pyrene unit However, the synthesized 1-bromopyrene may contain an unexpected product, 1,3-dibromopyrene, which remained in the product after purification This unexpected product revealed some resonance peaks that were not related to 1- bromopyrene, and interfered with the analysis shown in Fig 4-3 above
Fig 4-4: The 1 H NMR spectrum (500 MHz, CDCl 3 ) of DTP
The 1 H NMR spectrum of DTP revealed characteristic peaks corresponding to the monomer structure of the synthesized compound shown in Fig 4-4 The signal was observed at 7.18 ppm, 6.91 ppm assignable to the aromatic protons of thiophene ring In the aliphatic region, the broad multiplets in the range from 3.98 to 0.82 ppm were assigned to the protons of the alkyl side chains
Fig 4-5: The FTIR spectrum of 2PDTP Table 4-1: IR absorption wavenumbers (cm -1 ) and their functional group for 2PDTP
In the case of 2PDTP, the functional groups of 2PDTP were characterized by FTIR spectroscopy [78] (Fig 4-5) The FTIR spectrum of 2PDTP was scanned in the frequency region of 400-4000 cm -1 using an infrared spectrometer by employing the KBr pellet technique As shown in Fig 4-5, the FTIR spectrum of triad 2PDTP exhibited bands from 2868 to 3044 cm -1 attributed to the C-H stretching vibrations of ethyl -hexyl group and C-H linkage in aromatic structure The characteristic band from 1516 to 1600 cm -1 is corresponding to aromatic C=C stretching and C-H deformation vibrations The peaks from 1020 to 1103 cm -1 indicate the existence of C-N of pyrrole structures The band from 719 to 824 cm -1 is ascribed to the C-S stretching vibrations of thiophene structures To further evaluate the chemical structure of 2PDTP, the 1 H NMR spectroscopy has been used
Fig 4-6: The 1 H NMR spectrum (500 MHz, CDCl 3 )of 2PDTP
The 1 H NMR spectrum in Fig 4-6 exhibited all characteristic peaks corresponding to the chemical structure of 2PDTP The peaks from 0.89 to 2.13 ppm are presented for the alkyl side chains The peak at 4.24 SSPSHDN³N´) is attributed to the proton of methylen linking to dithieno[3,2-b:2',3'-d]pyrrole moieties It is interesting that the peaks at 6.91 and 7.18 ppm of DTP (Fig 4-4) was disappeared, this result suggested that the dithieno[3,2-b:2',3'-d]pyrrole units have coupled completely with 1-bromopyrene to form 2PDTP7KHSHDNDWSSPSHDN³M´LVFRUUHVSRQGLQJWRWKHSURWRQVDWSRVLWLRQ ³´LQWKHWKLRSKHQHXnit The peaks from 8.03 to 8.7 ppm are assigned to the protons of pyrene moieties However, the final product may contain some impurity compounds such as dithieno[3,2-b:2',3'-d]pyrrole that is appeared at 6.9 ppm to 7.2 ppm, and the peaks at 5.25 ppm that is corresponding to the proton of CH2Cl2 solvents which remains in the final product after purification
The FTIR and 1 H NMR results indicated that the direct arylation reaction has been performed successfully to obtain the fluorophore 2PDTP
4.1.2 Suzuki cross-coupling s\QWKHVLVRIả-(9,9-dioctyl-9H-fluorene-2,7- diyl)dipyrene (2PDOF) a) Synthesis results
The synthetic route of 2PDOF is schematically described in Fig 4-7 below
Fig 4-7: Synthetic routes for the conjugated molecular 2PDOF Reagents and conditions: (i) DMF, r.t.; (ii) Pd(PPh 3 ) 4 , K 2 CO 3 , toluene/ethanol/H 2 O (10:2:1.5, v/v),
Synthesis of 1-bromopyrene was achieved by a reported method As depicted in Figures 4-7, the reaction of pyrene and NBS under ambient conditions without light exposure for an extended duration yielded 1-bromopyrene in 94% yield through a dibromination process.
Then, the Suzuki cross-coupling reaction of 1-bromopyrene and DOF was toluene/ethanol/H2O at 80 o C under N2 for 19h, which was transformed into 2PDOF in yield of 48% This Suzuki cross-coupling mechanism based on the reaction of 1- bromopyrene and DOF begins with the oxidative addition of 1-bromopyrene (R) to the Pd(0) to form R-Pd(II) complex A molecule of carbontrioxide base then replaces the bromide on the palladium complex, while another adds to the dioxaborolane group of DOF to form a borate regent making 9,9-dioctyl-9H-fluorene JURXS 5ả PRUH QXFOHRSKLOLF 7UDQVPHWDODWLRQ ZLWK WKH ERUDWH WKHQ IROORZV ZKHUH 5ả UHSODFHs the carbontrioxide anion on the Pd(II) complex Reductive elimination then gives the final product, regenerates the palladium catalyst, and the catalytic cycle can begin again (Fig 4-8) [79]
Fig 4-8: Suzuki cross-coupling mechanism of 2PDOF synthesis reaction
After being purified by column chromatography, all the synthesis compounds were characterized by FTIR and 1 H NMR spectrometries to confirm the structure b) Chemical structure characterization
In the preceding section, the 1 H NMR spectrum of 1-bromopyrene was shown and analyzed The chemical structure of the final product 2PDOF will be characterized using FTIR [78] and 1 H NMR spectrometries [77] in the following
First, the 2PDOF was structurally elucidated by FTIR analysis The FTIR spectrum of 2PDOF was scanned in the frequency region of 400-4000 cm -1 using an infrared spectrometer by employing the KBr pellet technique In Fig 4-9, the FTIR spectrum indicated bands in the range of 2840-2975 cm -1 relating to the ±CH2 and ±CH3 stretching and peak at 1461 cm -1 relating to the ±CH3 bending of octyl brand chain, while the C=C stretching mode of aromatic ring exhibited in the range of 1620-1570 cm -1 and the band at 837 cm -1 attributed to the C-H bending formation The FTIR spectrometer has determined chemical functional groups present within 2PDOF To investigate more about 2PDOFảVFKHPLFDOVWUXFWXUH, 1 H NMR spectrometry has been used
Fig 4-9: The FTIR spectrum of 2PDOF
Table 4-2: IR absorption wavenumbers (cm -1 ) and their functional group for 2PDOF
Fig 4-10 depicts the 1 H NMR spectrum of the obtained fluorophore, which revealed characteristic peaks corresponding to the fluorophore structure In the 1 H NMR spectrum of 2PDOF, the signals were observed at 0.8 ppm to 2.1 ppm assignable to alkyl VLGH FKDLQV 7KH SHDN DW SSP LV DWWULEXWHG WR WKH SURWRQ DW SRVLWLRQ ³[´ LQ WKH IOXRUHQH XQLW DQG WKH SURWRQ DW SRVLWLRQ ³D´ LQ WKH S\UHQH XQLW +RZHYHU WKH EURDG multiplets in the ranges from 7.98 to 8.09 ppm and from 8.18 to 8.3 ppm in the aromatic region are corresponding to the protons of the pyrene ring In addition, the multiplets peak from 8.1 to 8.15 ppm which is described as the protons of pyrene unit at position ³F´DQGRIIOXRUHQHXQLWDWSRVLWLRQ³\´DQG³M´
Fig 4-10: The 1 H NMR spectrum of 2PDOF
The results acquired from 1 H NMR and FTIR spectrometry were analyzed to provide useful information about the structure and ultimately confirm a successful synthesis of conjugated oligomer 2PDOF.
Optical properties of 2PDTP and 2PDOF
UV-Vis spectroscopy has been used to investigate the absorption properties of fluorophores 2PDTP and 2PDOF
Fig 4-11: UV-Vis absorption spectra of 2PDTP (A) and 2PDOF (B) at different concentrations in CHCl 3 with a path length of 1 cm
Fig 4-11 showed the UV-Vis absorption spectra of 2PDTP and 2PDOF in dilute CHCl3 VROXWLRQDWGLIIHUHQWFRQFHQWUDWLRQVȝ0ȝ0ȝ0ȝ0,QWKHFDVH
% of 2PDTP, absorption bands emerged at 261 nm with a shoulder of 325 nm and 410 nm which are corresponding to the absorption of 4-(2-ethylhexyl)-4h-dithieno[3,2-b:2',3'- d]pyrrole and pyrene moieties The absorption band at 261 nm and shoulder of 325 nm was assigned to the electronic transition of dithienopyrrole, and the other at 410 nm attributed to the electronic transition of pyrene The fluorophore 2PDOF showed the absorption bands emerged at 340 nm which was assigned to the electronic transition of fluorene and pyrene, while the wavelength from 400 nm onwards has no absorption
Based on the Lambert-Beer law, the maximum molar absorption coefficients ³İ´ of 2PDTP at 410 nm and 2PDOF at 340 nm were estimated with the value of 23800 M -
1.cm -1 and 33525 M -1 cm -1 , respectively The fact that 2PDOF has a higher molar absorption coefficient than 2PDTP indicates that 2PDOF has a larger conjugated system than 2PDTP
Fig 4-12: Estimate the molar absorption coefficients Hof triad 2PDTP and
2PDOFaccording to Lambert-Beer law at the maximum wavelength of absorption
Table 4-3: Optical properties of triad 2PDTP and 2PDOF
Fluorescence quenching studies with Mesotrione in solution
The fluorophores 2PDTP and 2PDOF had been explored the sensory properties to apply for tracing of nitroaromatic pesticides via fluorescence quenching mechanism where the fluorophores 2PDTP and 2PDOF as the donor chromophores which are associated with nitroaromatic as acceptor moieties via dipole±dipole coupling The fluorescence of 2PDTP and 2PDOF in solution was excited at the wavelength of 365 nm The photoluminescence of 2PDTP and 2PDOF had been investigated upon adding different amounts of herbicide mesotrione into the solution The solutions of 2PDTP and 2PDOF in CHCl3 were prepared with CM of 1.0 àM as the host stock solution, followed by the addition of different amounts (0 ± 75PM) of mesotrione Interestingly, the results exhibited the fluorescence intensities decreased gradually with increasing the concentration of mesotrione compound This phenomenon suggests that the binding between mesotrione with 2PDTP and 2PDOF causes fluorescence quenching Fig 4- 13A and 4-13B exhibit the fluorescence quenching of 2PDTP and 2PDOF in the presence of mesotrione compound At the concentration of 75 àM of mesotrione, the fluorescence intensities of 2PDTP and 2PDOF decrease significantly, causing the color to change from bright green to dark green in the case of 2PDTP, and from bright blue to dark blue in the case of 2PDOF It can be clearly seen that the color changed of 2PDTP and 2PDOF can be easily observed with naked eyes In addition, the fluorescence intensity of 2PDOF is higher than those of 2PDTP due to the expansion RIʌFRQMXJDWHG systems, whereas the pyrene compound did not make sense for detecting Mesotrione EHFDXVHRIWKHODFNRIH[SDQGHGʌFRQMXJDWHGV\VWHPV
Fig 4-13: Fluorescence quenching of 2PDTP (A) and 2PDOF (B) in CHCl 3 ȝ0 upon addition of Mesotrione (0-75 PM)
According to the World Health Organization ± Food and Agriculture Organization (WHO ± FAO) [8], the concentration of mesotrione used in agriculture and several orders is an acceptable daily intake (