Fluorescent Polyimide in Sensing Applications

Abstract

Potential advances in sensing can be made by conjugated polymers includes poly(p-phenylene), poly(p-phenylene vinylene), polyfluorene, and poly(thiophene). Among the most important classes of polymers are heterocyclic polymers, such as polyimides, because polyimide nanocomposites possess exceptional mechanical strength as well as chemical, mechanical and temperature resistance. Polyimide offers the potential of providing efficient sensors through its ability to work actively. There is evidence that fluorescent polyimide is efficient at detecting hazardous pollutants. Chemical modifications of the polyimide backbone gave rise to an improved luminescence efficiency of polyimide by incorporating fluorescent chromophores. An overview of recent developments in fluorescent polyimide in sensing applications is presented in this chapter. Some of the fluorescent polyimide materials prepared from different types with surface modification (type-1: perylene tetracarboxylic dianhydride and oxydianiline) (type-2: Tetra (4-aminophenyl) porphyrin and perylenetracarboxylic dianhydride) and (type-3 2-(4,4′-diamino-4′′-triphenylamine)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole) etc. In the following section, the methods and sensing mechanism of fluorescent polyimide are described.

1. Introduction

A chemical sensor converts chemical information into an electrical signal to provide a qualitative or quantitative representation of chemical composition and activity. Chemical sensors create signals by selectively binding a sensing material to an analyte, using a sensing element and a transducer [1]. A sensor’s characteristics are dependent upon chemical species that interact with it. Also, it can be either optical or thermal properties of an analyte, which include conductivity, potential, capacity, heat, mass, or optical constant [2]. The simple, convenient and low-cost features of sensor have caused it to receive a great deal of attention [3]. In the medical, biological and environmental fields, sensors have been developed to detect heavy metal ions at low concentrations, an issue of concern for environmental protection and disease prevention. These devices require a high degree of sensitivity and selectivity [4, 5, 6]. Optically based sensors are particularly appealing due to their wide range of attractive characteristics, such as microfluidic platforms with integration and the ability to monitor environmental hazards [7, 8]. Recent years have seen a surge in popularity for fluorescent sensors due to their flexibility, quick response times, low detection sensitivity, and simplicity of operation. Optical, electrochemical sensors and biosensors can benefit greatly from conjugated polymers once utilized as sensing materials or chemical probes. There has been an incredible increase in the use of polymer materials such as poly(p-phenylene), poly(p-phenylene vinylene), polyfluorene, and poly(thiophene) in large-area displays, promising future advancements in flexible displays [2, 9, 10, 11]. A conjugated polymer backbone or side chain with ionic functional groups produces a conjugated polyelectrolyte that can be combined. They combine the physicochemical properties of polyelectrolytes with those of organic semiconductors, which makes them attractive as materials for sensing, imaging, and device applications [12, 13]. It is possible to synthesize fluorescent polymers by converting fluorescent functional monomers to polymers, using fluorescent compounds as initiators and chain transfer agents, forming chemical bonds between fluorescent groups and polymers, or converting non-fluorescent functional monomers into fluorescent polymers. Fluorescent probes, smart polymers machines, fluorescent chemosensor, fluorescent molecular thermometers, fluorescent imaging, and drug delivery carriers are among their emerging applications in this field. Polymers are advantageous since they can be formed into small particles and thin films that can be coated onto optical fibers, making them ideal for sensors. Several advanced techniques, including electrostatic layer-by-layer assembly and self-assembly of amphiphilic block copolymers with chromophores, have also been employed to manufacture fluorescent systems [14]. The field of thermally stable heteroaromatic polymers is dominated by the polyimides (PI) [15]. Aromatic polyimides are based on aromatic dianhydride and diamine [16]. Among the many different science and engineering fields, that they have been applied in chemistry, physics, electrical and mechanical fields. Aromatic polyimides have received significant attention in recent decades due to their superior physico-chemical, thermal, and mechanical properties [17]. These aromatic polyimide systems are mainly characterized by intramolecular charge transfer (CT) between the diamines and the dianhydrides, and the diamine electrostatic effect dominates their fluorescence properties. As such, it was expected to influence the intensity and strength of charge transfer and donor-acceptor interactions along the polymer backbone [11, 18]. Similarly, a promising type of porous crystalline material known as covalent organic frameworks (COFs) has gained popularity and have displayed promise in various applications, including gas storage, chemosensor, catalysis, and electricity. Meanwhile, COF combined with polyimide (PI) shows great thermal stability, good crystalline structure, large pore sizes, and high surface areas. While synthesis of various PI-COF units with unique properties is still an unsolved problem, there has been no report on PI-COFs that possess fluorescent features, or their application to chemical and biological sensing systems. A novel covalent organic framework of fluorescent polyimide which has been shown to provide enhanced detection of 2,4,6-trinitrophenol (TNP) with excellent sensitivity and selectivity [19]. A method allowing selective chemosensitive Fe³⁺ detection with the COFs has been developed [20]. Recent research has focused mainly on developing and applying polyimide-based fluorescent sensors for food detection. Likewise, polyimide nanocomposite, piezoelectric sensors, and in particular, surface acoustic wave (SAW) biosensors are gaining attention due to their rapid response and non-labeling capabilities for detecting macromolecules in biological systems. Though the mechanism of fluorescent polyimide is not fully understood, it’s believed that charge transfer effect is an important factor. Thus, few researchers have focused on increasing the quantum efficiency of aromatic polyimides by regulating the charge transfer CT effect.

2. Fluorescent polyimides

Polyimides is widely used in the field of sensor, primarily humidity sensor [21], strain sensor [22], gas sensor [23] and bio sensor [24]. Recently aromatic polyimide have gotten incredible consideration because of their predominant physical, chemical, thermal and mechanical properties like resistance to chemical and radiation damages, mechanical durability, temperature tolerance, and mobility. Aromatic polyimides show high-performance engineering polymers, accordingly prompts the use across various science and engineering disciplines. The strong superiority of aromatic polyimides over conventional conjugated polymers, however, has led to a relatively low utilization of these materials in light-emitting devices, as they are typically non-luminous traditional polyimides or luminance with low luminescence efficiency. As a means to enhance the intensity of luminescence of aromatic polyimides. In polyimide backbones or as pendant groups, fluorescent chromophores have been integrated in order to optimize fluorescence properties [11, 17, 25]. Fluorescence quenching, however, occurred because the diamine and dianhydride moieties were undergoing strong intermolecular and intramolecular charge transfer between their occupied and unoccupied molecular orbitals, and there were strong π-π interactions between chromophores. Fluorescence quenching was resolved by using aliphatic monomers. The fluorescence intensity was boosted by hindering the formation of charge-transfer-complexes (CTC) in non-conjugated and low electron-donating groups. Hence, fluorescence has been acquired by the addition of triphenylamine moieties into the polyimide backbone [11]. Also, new kind of polymer developed by the introduction of electroluminescent active organic dye unit into the polyimide backbone. For example, 2,5-distyrylpyrazine, electroluminescent active organic dye unit was incorporated into the polyimide backbone [26]. Polymers made in this way are typically synthesized by reacting dianhydride with a diamine under optimal conditions. To extend the use of polyimide, novel polyimide with different chemical moieties were reported in literature. Like, a novel diphenylfluorene-based Cardo copolyimide containing perylene, synthesized by polycondensation of a diamine 4,4′-(9H-fluoren-9-ylidene)diphenylamine with perylene dianhydride and dianhydride in m-cresol with isoquinoline as catalyst at 200°C [27]. A novel aromatic polyimides containing 4,5-diazafluorene were synthesized using dihydride monomer, 9,9-di[4-(3,4-dicarboxyphenoxy)phenyl]-4,5-diazafluorene dianhydride [28]. A fluorescent polyimide was formulated by reacting perylene tetracarboxylic dianhydride and oxydianiline in N-methyl pyrrolidone solvent under ideal conditions [17]. Developing high-performance flexible light-emitting materials would be greatly enhanced by studies on these materials. As is widely known, polyimides have great optical properties due to their charge-transfer (CT) behavior, especially rigid polyimides with lots of aromatic compounds. Polyimide with high fluorescence efficiency by control of the push-pull relationship via electrostatic interactions between the diamine moieties and the dianhydride moieties [11] developed two different polyimide, they were pyrrole-containing polyimide (PyODPI) and aromatic polyimide containing the triazole group (TzODPI) using two different diamine monomers 4,40 - (1-(4-tritylphenyl)-pyrrole-2,5-diyl)dianiline and 4,40 -(4-(4-tritylphenyl)-1,2,4-triazole-3,5-diyl)dianiline with identical chemical structures that differ in their electronic effects respectively. This aromatic polyimide contains a triazole moiety, as shown by its bright green photoluminescence has a quantum yield as high as 61%, but as a result of the film formation, it has a quantum yield of 13%. The pyrrole-containing polyimide PyODPI, however, completely slowed down the fluorescence. Polymers displayed a completely different fluorescence behavior, since 4,40-(1-(4-tritylphenyl)-pyrrole-2,5-diyl)dianiline monomer contains electron-donating pyrrole groups while 4,40-(4-(4-tritylphenyl)-1,2,4-triazole-3,5-diyl)dianiline monomer carries an electron acceptor 1,2,4-triazole group. A major influence on fluorescence properties was the electronic effect of diamines, which may have controlled the strength of charge transfer and donor-acceptor interaction intensity at polymer backbones. The mechanism for fluorescence has been clarified by computing the orbital distribution and oscillator strength, as well as electron transitions between the ground and excited states of a model molecule. The findings show that a deficiency of electrons in diamine groups improves photoluminescence efficiency by inhibiting the charge-transfer processes that cause fluorescence to be diminished [11].

2,4,6-Trinitrophenol (TNP) is highly explosive and one of the most dangerous nitro-aromatic explosives and is superior to its more conventional counterpart 2,4,6-Trinitrotoluene (TNT). The solvothermal synthesis of a fluorogenic polyimide covalent organic framework (PI-COF) for the detection of 2,4,6-trinitrophenol has been accomplished using tetra(4-aminophenyl) porphyrin and perylenetracarboxylic dianhydride. A key quality of PI-COF is its porous crystalline structure and excellent thermal stability (above 500°C). In addition to providing highly sensitive and selective detection, PI-COFs can be utilized as efficient fluorescent probes for the detection of 2,4,6-trinitrophenol (TNP). The phenomenon might be caused by the combination of electron transfer and inner-filter effects, according to DFT calculations and spectral overlap data. As TNP is detected, a linear response between 0.5 to 10 μM is observed with a detection limit of 0.25 μM [19].

Two new polyimide-based porous covalent organic frameworks has been developed and studied their properties [20]. Those compounds were synthesized by directly heating mixtures of melamine and pyromellitic dianhydride, as well as naphthalenetetracarboxylic dianhydrides in N₂ atmosphere, respectively. The strong fluorescence of these two PI-COFs was due to their high electro-delocalization and inherent rigidity of COF, which resulted from the fluorescence transition π* → n induced by the proper solvents. In response to the strong quenching effects of Fe³⁺ on the fluorescence of the COFs, Fe³⁺-specific chemosensing was achieved. The n − π* transition in N,N-dimethylformamide and the alkaline aqueous solution caused strong fluorescence to be observed in both COF-1 and COF-2 of π-conjugated frameworks. In real samples, a high linear correlation coefficient, a wide linear range, and a low detection limit were achieved. Aggregation effect and π–π reaction could be responsible for fluorescence quenching [20].

An innovative nanofibrous membrane composed of porphyrinated polyimide (PPI) that can detect trace amounts of hydrogen chloride (HCl) gas rapidly. Porphyrin fluorophores can be incorporated covalently into the main chains of polyimide to overcome the porphyrin aggregation and enhance polyimide’s physicochemical stability. The dual chromogenic and fluorogenic properties of the nanofibrous membrane in the presence of HCl gas influence its optical properties through distortions created by the out-of-plane behavior of the macrocyclic porphyrin. Based on calorimetric and fluorimetric analysis, it is readily apparent that the color changes evident are in response to HCl gas. A calculated affinity constant indicates that the nanofibrous membrane sensor has a distinct sensitivity to the presence of HCl, as determined by SPR analysis (1.05 ± 0.23) × 10⁴ L mol⁻¹. The PPI nanofiber membrane sensor is found to be highly sensitive to HCl, based on the apparent binding affinity constant of 1.05 ± 0.23 × 10⁴ L mol⁻¹. A nanofibrous membrane sensor designed using PPI also has improved thermal stability, making it attractive for monitoring emissions of HCl gas from incinerators that burn household, clinical or industrial waste [29].

A large-area and highly porous nanofibrous polymer membrane possesses high sensitivity, and a rapid response time is a key aspect of sensing applications. The study [30] presents a novel zinc porphyrin-containing polyimide (ZPCPI) nanofibrous membrane capable of detecting trace amounts of pyridine vapor rapidly and reversibly. The analyte can be detected at concentrations as low as 0.041 ppm. It is due to the high chemical and thermal resistance of polyimide, as well as its mechanical stability, that pyridine vapor can be detected in harsh environments with zinc porphyrin fluorophore. ZP’s photophysical responses can be attributed to changes in its electronic state as well as geometric distortion caused by strong pyridine coordination. Pyridine possesses both high gas-phase basicity and small molecular size, thus perfectly suited for sensors that select pyridine vapor in comparison with different amines and other gases that could interfere [30].

A large number of porphyrinated nanofibers could be used as sensing materials because of porphyrin’s large versatility in polymer synthesis. A porphyrinated polyimide nanofiber material with unique luminescent properties turned out to be an effective sensory material for detecting TNT vapor (10 ppb) in its trace form. Polyimide nanofibers are improved in terms of physicochemical stability when porphyrin fluorophores are covalently bonded to the main chains: this reduces the aggregation-caused self-quenching of porphyrin fluorescence. Porphyrinated nanofibers have a large surface area-to-volume ratio and excellent gas accessibility, which result in a much more impressive fluorescent quenching behavior towards trace TNT than their spin-coated dense film counterparts. The quenching efficiency of the other compounds, such as 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenol (Picric acid: PA) and nitrobenzene (NB) is much lower than that of TNT. From SPR analysis, porphyrinated nanofibers have an apparent affinity constant of (2.37 ± 0.19) × 10⁷ L/mol, which implies that they are a promising alternative for TNT detection [31].

The synthetic method yields 2-(4,4′-diamino-4′′-triphenylamine)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole, a novel blue fluorescent aromatic diamine. A polyimide and three poly(amide-imide)s consisting of the fluorescent imide-type polymers are produced using polycondensation reactions based on diamine. Aromatic polyimides with amide groups in their polymer chains are classified as aromatic poly(amide-imides), which exhibit higher thermal stability, greater solubility, and lower glass transition temperatures than aromatic polyimides of similar chemical structure. The 1,3,4-oxadiazole rings present in the polymers’ macromolecular chains make them more thermo-oxidatively stable, more hydrolytically resistant, and mechanically strong. As 1,3,4-oxadiazole rings have electron-withdrawing properties, several polymers containing oxadiazole were investigated for use in the production of organic light emitting diodes. Light-emitting diodes are likely to utilize materials with electron-donor modified dimethylamino substituents in para-positions of the pendant chromophoric 2,5-diphenyl-1,3,4-oxadiazole unit. Due to their ability to transport electrons and holes, these substituted diphenyl-1,3,4-oxadiazoles display intense fluorescence. Fluorescence was observed in the blue region with a high quantum yield and a large Stokes shift value. With HCl as a dopant, protonation led to a noticeable decrease in fluorescent intensity, caused by nitrogen atoms that have free electron pairs, derived from 1,3,4-oxadiazole rings and dimethylamino groups [32].

Calorimetric devices can be produced simply, accurately, and eco-friendly by directly using film-based polyimide sensors. Although few studies have been published on film-based PI sensors capable of detecting water in organic solvents via calorimetry. The authors [33] present a synthetic pathway for synthesis of two PIs containing hydroxyl groups that exhibit excellent synergistic effects. The fluoride ion-induced deprotonated species of hydroxyl-containing polyimide can quickly be reprotonated in the presence of water, making it an ideal water-sensing material. In the similar way, deprotonated fluoride ion-induced PIs that contain hydroxyls are easily protonated when exposed to water, which was used as a water sensor [33].

Hybridized local and charge transfer transitions produce white light intrinsically in semi-aliphatic hyperbranched polyimides with epoxide terminal groups (EHBPI). Studies of solution fluorescence indicate the following: 1. Fluorescence is influenced by the backbone structure and the terminal chemical groups at the end of the polymer, 2. Concentration and temperature are key factors influencing solution fluorescence and 3. Copper and iron ions quench solution fluorescence. As well as the increasing drop in quantum yield with higher concentrations or the addition of water, and higher quantum yield found in the solution-state as compared to the solid-state. The semi-aliphatic hyperbranched polyimides, which possess epoxide terminal groups, emits bluish green fluorescence when in solution, and white fluorescence when it is solid. Additionally, a white fluorescence can be detected when the free-standing film is prepared by blending EHBPI with poly(vinyl alcohol). Despite its simplicity, single-component hyperbranched polymers could offer unique benefits in certain applications since they are solid-state white fluorescence. In the UV/Vis absorption spectra and DFT calculations, the hybridized local and charge transfer (HLCT) transition is found to be the lowest electronic transition for semi-aliphatic hyperbranched polyimides, whereas the charge transfer transition is found for aromatic hyperbranched polyimides. Upon excitation, semi-aliphatic hyperbranched polyimides exhibit visible fluorescence due to the hybridized local and charge transfer (HLCT) transition. HBPIs fluoresced more intensely when their terminal phenolic groups were converted to epoxide groups. An intrinsic white-light-emitting hyperbranched polyimide could therefore be used to sense ‘turn-off’ sensors for iron (III). Polymer architecture appears to have complex effects on fluorescence, raising the need for a more targeted molecular design to fully exploit the hyperbranched architecture [34].

3. Polyimide biosensor

Recent research has shown that conducting polymers incorporating nanomaterials can be used as transducing media in biosensing systems [35]. A high surface area to volume ratio of conducting polymer nanocomposites becomes very sensitive and they show outstanding performance combining them with biomarkers yields remarkable selectivity [36]. In addition to the ability of nanoparticles to act as electron connectors, while matrix polymers can also assist with the adsorption of a target substance. The thin film of conducting polymer nanocomposite thus becomes an effective transducing electrode for biosensors [37]. Through electrochemical entrapment, covalent immobilization, or affinity interactions, a thin film of conducting polymers can serve as a medium for immobilizing biomolecules [38]. The immobilized biomolecules bind to analytes through the adsorption process. Those effects may manifest themselves in change in mass, optical activity, electrical conductivity, and temperature around electrode surfaces. This type of change can be detected with the appropriate detector. Piezoelectric biosensors, in particular, surface acoustic wave (SAW) biosensors, are widely used today for sensing biological macromolecules due to their real-time, label-free, and highly sensitive detection capabilities [39]. Also, SAW devices have gained popularity because of their low cost, compact size, and ease of analysis. Especially when it comes to sensitivity, selectivity, and stability, SAW biosensors are greatly impacted by the performance of the bioreceptor surfaces. The main disadvantages of polymer-based SAW devices are their high insertion losses and, therefore, low analytical window. An effective solution can be provided by a conducting polymer nanocomposite system with nanoparticles that act in synergy with the matrix polymer to conduct/transduce acoustic waves. First time, polyimide nanocomposite (PI/AuNP-MoS₂-rGO) was studied as a bioreceptor base used for Carcinoembryonic antigen (CEA) detection. CEA is a tumor-marking protein that is found in several types of cancer. In addition, the thin film of polymer nanocomposite was turned into a bioreceptor by immobilizing CEA antibodies (anti-CEA) through thioglycolic acid bridges and activation with EDC-NHS. Through covalent bonding between the Mercapto part and Au, AuNP can serve as a host for the thioglycolic acid bridge groups. Polyimide nanocomposite showed reliability and stability of the device over time. The biosensor was found to have a limit of detection (LOD) of 0.084 ng/mL. A validation study validated the real-time capabilities of the biosensor by analyzing clinical serum samples and analyzing its selectivity by demonstrating its affinity for other common cancer-marking proteins. Likewise, the biosensor displayed excellent stability, with only 10% reduction in activity recorded until the 80th day of storage [40]. Paraoxon detection was achieved by synthesizing an acetylcholinesterase (AChE) film biosensor based on reduced graphene oxide/polyimide thin films (rGO/PI). By using a modified AuNPs-MoS₂-rGO/PI flexible film biosensor, acetylcholine chloride was hydrolyzed successfully to obtain a large current response at 0.49 V and demonstrate successful immobilization of AChE. The AChE/AuNPs-MoS₂-rGO/PI film biosensor displays a linear response over a concentration range of 0.005–0.150 μg/mL, 4.44 uA/μg/ mL of sensitivity, 0.0014 μg/mL of detection limit, good reproducibility and stability on the paraoxon inhibition on the AChE [41]. There are a number of applications for flexible biosensors in measuring the concentration of target bio-analytes. Additionally, to their flexibility, electrochemical sensors made with 2D materials also have several advantages such as scalability, increased compatibility across a wider area, and greater transparency. Molybdenum disulfide (MoS₂) on a polyimide (PI) substrate was used to fabricate a flexible biosensor that can be used in electrochemistry platforms. Because MoS₂ has a higher electrical conductivity, a flexible MoS₂–Au–PI sensor can provide highly sensitive detection of target proteins with a relatively quick response via cyclic voltammetry. This device can detect hormones such as triiodothyronine (T3), endocrine-related hormones parathyroid hormone (PTH), and thyroxine (T4) with a high degree of sensitivity, as well as locate their position with a high degree of accuracy [24]. Composite membrane sensors made with molybdenum doped reduced graphene oxide and polyimide (Mo-rGO/PI) exhibit good catalytic activity for dopamine (DA), with linear responses from the range of 0.1 to 2000 μM. Furthermore, the assay has good stability and reproducibility, and it is effective at detecting DA in human blood serums [42]. An Au nanoparticle decorated polyimide electrochemical sensor for uric acid has been developed using its precursors, 4,4′(4.4′-isopropylidene-diphenoxy) bis (phthalic anhydride) and aniline tetramer. A nanoparticle of gold was then incorporated into electroactive polyimide. This sensor has the best sensitivity of 1.53 μM, a detection limit of 0.78 μM, and a linear measurement range is about 5 to 50 μM at 310 mV. Au nanoparticles decorated polyimide exhibited the best selectivity for UA, dopamine (DA), and ascorbic acid (AA) [43]. Known for their high surface area, carbon nanotube (CNTs) is also known to have excellent mechanical properties, good electrical properties, and good thermal conductivity. Composite materials with CNT and polymer offer desirable mechanical and electrical properties because CNT can enhance properties of composites. The PI/CNT membrane is superior to the conventional electrodes like indium tin oxide and glassy carbon electrode (GCE) due to its superior conductivity and mechanical properties. GCE is commonly used to construct sensors, however, it has low conductivity. Nanomaterials like Ni(OH)₂ are directly deposited on PI/CNT membranes, and Ni(OH)₂ and PI/CNT membrane work as a sensor without any substrate electrodes. Because of its good conductivity, PI/CNT membranes, which improves Ni(OH)₂ sensing, as well as the electron transfer, therefore PI/CNT-Ni(OH)₂ sensors are most suitable for detecting glucose. It has a number of notable qualities, including a good stability, high selectivity and sensitivity, and rapid amperometric response. PI/CNT–Ni(OH)₂ exhibit 2071.5 μA mM⁻¹ cm⁻² of high sensitivity and 0.36 mM of detection limit at +0.60 V [44]. Ni(OH)₂/MoS_x /CNT/PI sensors advantages included a wide linear range from10 to 1600 μM glucose, quick response, a minimum detection limit of 5.4 μM, high selectivity, reliable reproducibility, and long-term durability up to two weeks. Ni(OH)₂ and MoS_x have a pronounced synergistic effect that explains the superior performance. Ni(OH)₂/MoS_x /CNT/PI sensors for measuring blood glucose [45]. A polyimide (PI)-boron nitride (BN) composite is a dopamine-selective membrane. A polyimide matrix with BN particles showed better porosity, selectivity, and thermal resistance than polyimides without BN particles. A high degree of sensitivity, reversibility, and a low detection threshold (4 × 10⁻⁸ M) were all characteristics of %BN. Polyimide membranes have extremely high R values (0.9904) [46].

4. Conclusions

Its superior physical, chemical, thermal and mechanical properties have made aromatic polyimide a popular material across a wide range of industries. Though there is no complete understanding of the mechanism of fluorescent polyimide, it is thought that charge transfer action plays a crucial role. As a result of intramolecular charge transfer (CT) between diamines and dihydrides, aromatic polyimide systems have strong fluorescence properties, and their properties are largely governed by the diamine electrostatic effect. Fluorescence of aromatic polyimide depends on the intensity, strength, and donor-acceptor interactions along the polymer backbone. Until recently, few researchers have attempted to increase the quantum efficiency of aromatic polyimides by regulating the CT effect. Development of an efficient food detection system using polyimide-based fluorescent sensors is being driven primarily by experimental research. A broad range of polyimide nanocomposite, piezoelectric and surface acoustic wave (SAW) biosensors have gained attention due to their high sensitivities, real-time and label-free functionality.

Acknowledgments

The author, P.R., acknowledges Kalasalingam Academy of Research and Education for providing research fellowship and necessary facilities.

Conflict of interest

The authors declare that they have no conflict of interest.