Abstract
Among several detection techniques, fluorescence based optical sensors are regarded as superior owing to distinct features like simplicity, remarkable sensitivity and prompt signal response time. Signal amplification remains most effective strategy to further boost the performance of such sensors. Thanks to the unique light–harvesting and energy transfer properties of conjugated polymers (CPs) which make them as promising and key candidates for achieving proficient sensing through amplified fluorescence signal. Owing to such remarkable properties, past decade has witnessed numerous CPs based optical sensors and devices for monitoring of various species. In this review, we first introduce CPs and highlight their exclusive characteristics. Then, the superiority of CPs over small molecule based sensors has been demonstrated along with thorough discussion on underlying sensing mechanism. Afterwards, it is described how solid state sensing using CPs dominates over solution based sensing in terms of sensitivity. Later on, CP-amplified fluorescence detection of some explosives, pollutants, biomarker etc. on solid support has been summarized by shedding light on some recent representative papers. Finally, the current challenges and future prospective for advancement of the respective research area has been discussed.
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Conjugated polymers (CPs) are organic macromolecules consisting of alternating σ and π bonds along the hydrophobic backbone. The π-bonds are delocalized throughout the polymer chain and are responsible for the specific photophysical and electrical properties of the CPs. Three eminent scientists, Alan MacDiarmid, Alan Heeger and Hideki Shirakawa first discovered the conducting property of conjugated polymer in halogen doped polyacetylene in 1977 that led them to win Nobel Prize in Chemistry 2000. Since then, CPs have been widely used in the field of optoelectronics because of their remarkable semiconducting and optical properties. 1 Moreover, they have also appeared as preferred materials in the arena of sensing due to their remarkable detection efficiency, good chemical, thermal and photo stability and excellent thin film device fabrication capability compared with small molecule probes. During the designing of sensing materials, the main aim is to achieve the effective transfer of charge. Fortunately, CPs relies on planar and rigid aromatic repeating units to form an extended macromolecule, which endorse them with excellent film forming ability and promote efficient charge transfer and ultimately signal amplification 2 unlike in devices based on small molecules. Diverse CPs with signal amplification properties have been used to fabricate various types of sensor devices according to the requirement. CPs that exhibit low band–gap, good solution processibility, smooth film forming ability and absorption/emission in higher wavelengths are considered better for achieving high performance state-of-the-art sensor devices. Moreover, CPs can also be used as high capacity adsorbents via functional group modifications 3 and regarded as prominent materials for fabricating organic electronic devices. 4 Owing to their various characteristics, currently they are employed globally for designing 5–15 photovoltaics, light emitting diodes (LEDs), organic field–effect transistors (OFETs) and sensors. Although, OFET sensors based on CPs exhibit some drawbacks such as lack of selectivity, baseline drift issue and air instability due to undesired reactions of sensing functionalities with oxygen and moisture, yet, they are considered superior over inorganic metal oxide sensors which require high operational temperature. In addition, various methodologies like structural modifications, 16 double–layer strategy, 17 have been emerging to overcome the issue of stability and attain air–stable CPs based gas sensors. Additionally, optical sensors employing fluorescence properties of CPs exhibit better air stability under ambient conditions.
Types of Conjugated Polymers and Current Synthetic Strategies
CPs can be acquired with variable backbone chains (Figs 1 and 2), viz. poly (p-phenylene vinylene) (PPV), poly(p-phenylene) (PPP), polyfluorene (PF), polydiacetylene (PDA), poly(fluorene-co-phenylene) (PFP), poly (p-phenylene ethynylene) (PPE), polypyrrole (PPy), polythiophene (PT), etc. Synthesis of such CPs usually involves the choice of suitable π–conjugated backbone and side chain modifications with alkyl groups for tuning the overall absorption/emission and attaining desirable solubility in suitable solvents. For developing colorimetric sensors, the wavelength of CPs can be tuned via strategic designing of CP with donor–acceptor strategy and observing respective colorimetric change in signal after interaction with particular sensing analyte which induced signal transduction processes. Although various CP derivatives have been synthesized in the past, the main chain structure and associated terminal charged group remains nearly same. The conventional methods for the synthesis of diverse range of CPs with various backbones are listed in Fig. 2. 8 Most common synthetic techniques include palladium–catalyzed coupling reactions (Suzuki, 18 Heck, 19 and Sonogashira 20 ), wessling reaction, 21 photopolymerization reaction, 22 FeCl3 based oxidative polymerization and so on.
Figure 1. Chemical structure of some established fluorescent CPs.
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Figure 2. Common organic reactions typically used for synthesis of CPs. The figure is reproduced with permission from Ref. 8. Copyright 2012 American Chemical Society.
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Standard image High-resolution imageConjugated Polymers as Sensory Material
Sensor is a kind of device which consists of the sensing element and conversion components which can be used to sense particular information and provides the output as a signal. To promote the development in the important areas such as environmental protection, homeland security and medical diagnosis etc. scientists across the globe have been working to develop probes with the multiple advantages of high sensitivity, low cost and high adaptability. 23–34 There are many criteria that need to be fulfilled for any sensor device to categorize it as an efficient or "Ideal" sensor. These parameters include high selectivity, adequate sensitivity, water solubility, low toxicity, and so on (Fig. 3). In the diverse disciplines of chemical science, biochemical and materials engineering, the use of CPs as a highly sensitive material represents a novel and promising future. In accordance with the signal response, CP based optical sensors can be divided into two groups, viz. fluorometric and colorimetric. Colorimetric detection is mainly based on the variation of the CP absorption spectra or differences in the color of the polymer solution, while fluorometric detection is based on the change of fluorescence intensity and emission color. 35 Fluorometric detection is considered superior to colorimetric in terms of sensitivity.
Figure 3. Illustration of an ideal sensor system based on conjugated polymer.
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Standard image High-resolution imageVariations in emission intensity and fluorescence lifetime are generally monitored in fluorometric detection. By different signal transfer processes, 36–38 such as intra or inter chain polymer aggregation, photo-induced electron transfer (PET), Förster resonance energy transfer (FRET), it is easy to vary optical properties of CPs's. These signal transduction processes are usually operational by conducive non-covalent reciprocity between the probe and the sensing analyte, which brings them in near proximity to accomplish a change in signal. Some monomers of synthetic polymers also have sensing function to detect analytes, but the sensitivity of monomers is much lower than that of CPs. 39 Several investigators have proved that solid state sensors based on fluorescence have attracted wider attention at home and abroad because of their excellent detection performance. 40 Developing light harvesting CP based fluorescence sensing platforms are more beneficial and advantageous for effective and amplified identification of chemical and biological species. Swager's, 10–12 Wang's, 8,9 Liu's 7 etc. pioneering research groups has made significance contribution in the area of CPs and discovered several noteworthy probes for sensing applications.
Signal Amplification in Conjugated Polymers
The strong absorption and emission properties of the CPs are attributed to the migration of excitons along the backbone chain. These excitons transfer to low energy/electron acceptor sites at specific distances, resulting in amplification of the receiver signal and enhanced fluorescence quenching. At the organic interface, the CP buffer layer can promote the absorption and transfer of charge by forming a dipole moment, thus improving the efficiency of the sensor. 41 The photophysical characteristics of CP are determined by the effective conjugation length, stable chemical characteristics, specific intermolecular arrangements and intermolecular packaging, providing an adequate mechanism for sensor applications. Due to their ability to intensify the signal from a recognition case, sensors based on CPs are considered more sensitive than small molecule. The conducting properties of the whole backbone or multiple absorbing units are simultaneously impacted by the binding of an analyte to a receptor connected to the CP. This recognition process eventually leads to signal amplification or super quenching of fluorescence, whereas fluorescence changes in small molecule sensors usually occur due to single chromophore following the binding event, which has lower sensitivity and slower response than CPs based sensors (Fig. 4). The signal amplification process in CPs was first revealed by Swager in 1995 and is known as the "molecular-wire effect." This remarkable feature of CPs supports its wide application as a chemical and biosensor due to the need for extremely low concentrations of probe.
Figure 4. Illustration of "molecular-wire effect" phenomenon observed exclusively in CPs.
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Standard image High-resolution imageFluorescence Quenching and Dequenching Mechanism
The optical signal response of CPs observed with the occurrence of fluorescence "turn-off" (quenching) or "turn-on" (dequenching) process. The emission of CP is efficiently quenched during the quenching process by non-radiative relaxation mechanisms or polymer chain aggregation. In such a phase, the signal amplification depends on the duration of the exciton diffusion and the type of material (Fig. 5 ABC). The level of signal gain is finite when using solution-based quenching studies because exciton diffusion is confined to a 1-D random movable system. Since, it is indiscriminately diffused along an independent polymer chain, the exciton can revisit the same sites many times. However, by allowing the excitons to diffuse more naturally in three dimensions, the level of signal gain will significantly increase. Continuous polymer films thus demonstrate superior properties of signal amplification compared to the same polymers in the solution state. According to the above discussion, the solid state sensors exhibit higher sensitivity which has attracted extensive attention of researchers. However, CPs are easy to quench in the aggregation state, which affects the sensing ability of polymers in the solid state. 42 The analyte binding disrupts the electron density in the polymer backbone or changes the polymer chain conformation in a dequenching phase, which results in the appearance of emission. In 2020, Singh and co–workers reported NaYF4:Eu@TiO2:Er core/shell nanomaterial to maximize the photoluminescence quantum yield and minimizing the concentration quenching effect. 43 In the same year, Arya's group pointed that nanomaterials reduce the possibility of voids between crystals due to their high packing density. Therefore, nanomaterials also possess great potential in the field of fluorescent sensing applications. 44 Note that the dequenching procedure is typically less sensitive than the quenching system (Fig. 5 DEF). This is due to the fact that some of the quencher molecules will remain located within the exciton diffusion length in order to reach a strongly quenched state, and it will be impossible to retrieve all the possible fluorescence during the dequenching phase by eliminating those quenchers.
Figure 5. Effect of dimensionality on the sensitive nature of CPs via quenching and de-quenching mechanism. Higher dimension materials exhibit greater sensitivity than 1−D or isolated polymer chains. The figure is reproduced with permission from Ref. 10. Copyright 2007 American Chemical Society.
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Standard image High-resolution imagePotential Application of Conjugated Polymers
Explosive and pollutant detectors
Picric acid (PA) is a kind of explosive with high detonation velocity and explosive power. In recent years, it has been widely used in industry and medical treatment as a raw material. Because of its high solubility, PA can easily dissolve in water to become the main culprit of water pollution. Even low levels of PA in water can have a significant impact on the environment and human health. 45 Owing to this, the sensitive detection of PA becomes the focus of research in recent years considering environmental protection and homeland security. In this regard, several solid state sensor based on CPs has been developed. 46,47 In 2015, Malik and co-workers synthesized a new cationic conjugated polyelectrolyte PFMI by Suzuki coupling polymerization without employing any complex purification method and demonstrate the fluorescence based detection of nitroexplosive PA in intricate environment. 48 The solid state sensor demonstrate extraordinary fluorescence response towards PA in both solution as well as vapor phase using cheap, portable, lightweight fluorescent paper strips and two terminal thin film device formed by dip-coating/fabrication of conjugated polymer nanoparticles (Fig. 6). In addition, the two terminal sensor devices fabricated with PFMI nanoparticles provide an unprecedented and technical platform for detecting of PA on solid support. The ultra-sensitive performance of the probe in the detection of PA is attributed to the "molecular- wire effect" as well as the potential photoinduced electron (PET) transfer.
Figure 6. Structure of conjugated polymer PFMI and multimodal sensing of nitroexplosive and pollutant picric acid on solid support using fluorescent paper strips and conjugated polymer nanoparticles fabricated two terminal thin-film devices. The figure is reproduced with permission from Ref. 48. Copyright 2015 American Chemical Society.
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Standard image High-resolution imageOwing to lack of specific receptors and similar electron deficient affinity of other nitroaromatics, PA detection is highly susceptible to interference from other nitro explosives. 49 To solve this problem, in 2015, Hussain and co-workers strategically engineered conjugated polymer PMI to detect PA at levels of parts per trillion (ppt). 47 By the usage of simple paper strips and polymer doped chitosan (CS) films, the conjugated polyelectrolyte PMI can realize the ultra-sensitive detection of PA on a solid foundation and in 100% aqueous media (Fig. 7). Unparalleled selectivity was accomplished by the association of resonance energy transfer (RET) and ground state charge transfer as confirmed by theoretical and experimental evidences. Imidazolium functional groups attached on the PMI side chains facilitate solubility of sensor molecule in water and promote electrostatic attraction with PA. Thus, the existence of imidazolium groups is conducive for the rapid and specific recognition of PA. In order to analyze the selectivity and interference by other species, different kinds of nitro explosive analytes were introduced to PMI. According to the experimental results, PA can cause obvious quenching of PMI at very low concentrations, while other analytes displayed insignificant or no quenching effect on PMI. 50,51 The exclusive response of PMI towards PA was attributed to the anion exchange between the polymer and the strongly acidic phenolic hydroxyl group after deprotonation. 52
Figure 7. Demonstration of picric acid detection on solid platform using conjugated polymer PMI. (A) Fluorescent paper strips with various concentration of PA and (B), (C) PMI doped chitosan film with thumb impression of PA residue under (B) daylight and (C) UV–light. The figure is reproduced with permission from Ref. 47. Copyright 2015 Royal Society of Chemistry.
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Standard image High-resolution imageLater on, Iyer's research group 53 reported polyfluorene derivative, PFAM for picogram level detection of PA. The fluorescent paper strips has been utilized again to achieve the fast and precise identification of nitro explosive picric acid on solid support with extremely low detection limits. In general, the fluorescence quenching behavior could be due inner filter effect (IFE) and/or FRET that has been intensely studied and investigated. Among them, the IFE is considered more flexible and simpler because it does not require modification of the sensor molecule. 54 The amplified signal response of PFAM exclusively for PA was accomplished by the powerful inner filter effect (IFE), a mechanism different from the commonly existing process for nitroaromatic detection. Earlier, some amine-substituted molecules have been explored as efficient materials for detecting PA at very low concentrations due to the protonation of amine groups, formation of hydrogen bonds, and electrostatic interactions. 55–57 Thus, the extremely high sensitivity and unique selectivity of PFAM for PA was attributed to the pendant amine groups on the side chain that promote protonation-assisted photoinduced electron transfer. PFAM displayed high selectivity could achieve the specific detection of PA even in the presence of complex natural water environment and other common nitro explosives. Consequently, the PFAM based solid platform offers superior trace detection capability, even in the complex environments.
In 2018, the same group developed a new conjugated polymer, PFTPBZ with high yield via Suzuki coupling polymerization. 58 Compare with previous reported polymer; PFTPBZ does not contain any receptors and displayed strong emission intensity in solid state. The conventional fluorophores often exhibit good fluorescence characteristics only in the solvent state as the phenomenon of aggregation-caused quenching (ACQ) occurs in the aggregate state. 59 However, PFTPBZ demonstrated aggregation-induced enhanced emission (AIEE) property and exhibit better luminescence performance in the solid state than in the solvent state, resulting in the improved and rapid detection of PA using portable and sensitive paper strips. The portable test strips for solid mode detection was achieved by simply soaking the certain size filter paper strips into the solution of PFTPBZ followed by air drying. The sensor based on conjugated polymer achieves ultra-low detection limits for PA in solid, liquid and vapor states. Under the irradiation of a handheld UV lamp, different concentrations of PA interact with the paper strips to produce different degrees of quenching, and complete quenching can be achieved in a short time. Washing the used paper strip can restore the original state of sensor, which proves the reusability of the solid sensor (Fig. 8).
Figure 8. (a) Various concentration of picric acid on PFTPBZ coated fluorescent test strips demonstrating sensing on solid support. (b) Change in PFTPBZ coated fluorescent test strips after introducing PA vapor demonstrating vapor mode detection on solid support. (c) Reusability characteristics of solid state sensor strips. The figure is reproduced with permission from Ref. 58. Copyright 2018 American Chemical Society.
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Standard image High-resolution imageThe nanostructures of CPs with huge surface areas facilitate efficient sensing on solid state. One method of fabricating such polymer based highly sensitive nano-sensors is construction of interconnecting network of nanowires with excellent photoelectric properties and large specific surface area. In 2018, Sakaguchi and co-workers reported the preparation of nanowires with uniform length and controlled number density via solid-state polymerization of 9,9'-spirobi[9H-fluorene] (SBF) molecules through high-energy charged particle irradiation. 60 For the preparation process (Fig. 9), SBF was chosen as the conjugated molecule owing to its reasonable solubility and uniform structures which promote easy fabrication through solution processing technique. The prepared nanostructures were comprehensively characterized and emission spectra of SBF nanowires were found variable from typical SBF molecules because of extended bond formation due to polymerization. The resultant smart nanofibers displayed significant fluorescence quenching with the addition of pollutant nitrobenzene, confirming the prospective of developed solid state system for sensing applications.
Figure 9. Demonstration of nanowire fabrication processes using organic thin films of SBF derivatives for solid state sensing applications. The figure is reproduced with permission from Ref. 60. Copyright 2018 American Chemical Society.
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Standard image High-resolution imageDetection of biomarkers
Based on the metal-enhanced fluorescence effect of silver nanostructures, Wang's team designed an optical nanoruler based solid sensor that can be used for label free detection proteins. 61 The core of the nanotube is the combination of conjugated polyelectrolyte PFVCN, non-conjugated polymers PEI, PAA and silver nanoprism. The interactions between antibody and antigen induce change in the metal-fluorophore distance, resulting in the fluorescent signal response by conjugated polyelectrolyte (Fig. 10). The system can be used for detecting target antigens in a sensitive and rapid way. The interaction between the metal and the fluorophore only occurs within a certain range of the metal layer, 62,63 and the condition for the two to achieve effective and rapid binding is the presence of protein. The metal-enhanced fluorescence (MEF) effect imply that the fluorescence intensity of the fluorophore will be enhanced attributed to amplified near-fields of metal. 64 This effect was controlled by the antigen-antibody affinity pair, which provides a potential optical nanomanipulator for typical biological detection.
Figure 10. Structure of the polymer materials used and respective mechanism for antigen detection assay developed using nanoruler detection platform. The figure is reproduced with permission from Ref. 61. Copyright 2015 John Wiley & Sons.
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Standard image High-resolution imageBy using cationic polyfluorene derivative PFP, the same group constructed a new photoelectrochemical (PEC) cytosensor in 2018. 65 Blue emitting positive charged PFP can produce stable photocurrent because of its excellent photoelectric conversion efficiency under light illumination. Indium tin oxide (ITO), a kind of semiconductor with excellent photoelectric properties was used to form the electrode and combined with 3-phosphonopropionic acid. The antibody of EpCAM was then modified by amide condensation reaction. Through specific antibody-antigen interaction, EpCAM overexpressed SKBR-3 cells were trapped on the electrode. The detection signal for target SKBR-3 cells was turned-on by the Coulombic interaction between positively charged PFP and anionic cell membrane (Fig. 11). The new PEC cytosensor could be used to replace traditional semiconductor materials, and combines the advantages of electrical sensing and fluorescent sensing for detection. At the same time, it does not need high voltage like ordinary electric sensor, and can effectively avoid the background interference that often exists in fluorescence detection. Due to the strong photoelectric efficiency of the conjugated polymer, this innovative mode of cell sensor shows prominent sensitivity and exhibit good effect in selecting target cells using the unique detection of antibody-antigen.
Figure 11. Fabrication of PEC cytosensor and its working principle for target cell detection using fluorescent conjugated polymer PFP. The figure is reproduced with permission from Ref. 65. Copyright 2018 American Chemical Society.
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Standard image High-resolution imageThe key domain of organic bioelectronics is the development of innovative techniques and high-precision device for comprehensive analysis. Such bio-electronic devices could be useful for the early and immediate diagnosis of diseases to reduce mortality, especially most problematic cardiovascular diseases and cancer. Hussain and co-workers developed an advanced, multi-modal and portable sensor device to achieve precise monitoring of biological processes and cardiac biomarker creatine kinase (CK). 66 Taking advantage of the unique conformation of cationic polythiophene PMNT, identification of CK under multiple environments was effectively performed via change in optical and electrical activity through excellent naked-eye visualization and remarkable fluorescence turn-on detection capability without using conventional methods of antibody or enzyme coupling reaction (Fig. 12). In addition, using PMNT's optical properties at the interface of microelectronics and biology, a cost-efficient and compact platform was also presented to simply and conveniently track CK. This method illustrates the promising future of conjugate-polymer-based portable biomedical devices in the early diagnosis of disease and monitoring biological activities, which remains highly desirable topic in the field of organic bioelectronics.
Figure 12. Pictorial representation of the multimodal assay developed for device based detection of cardiac biomarker creatine kinase activity using cationic conjugated polythiophene PMNT. The figure is reproduced with permission from Ref. 66. Copyright 2019 John Wiley & Sons.
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Standard image High-resolution imageThe identification of disease-related biomarkers in the early phase has an important role in preventing the further deterioration of the disease. In recent years, several enzyme-based assays have been developed to trace specific biomarker. However, most of such assays are infrastructure-dependent, time-consuming and resource intensive that makes them unviable in resource-constrained areas. Thus, it is necessary to design a highly biocompatible widespread sensor that can detect cancer biomarkers at ultra-low concentrations conveniently. Dye to high photoluminescence quantum yield (PLQY) up to 55%, conjugated polythiophenes can directly and effectively produce enzyme free signals, thus exhibit a good future in the biological and medical fields. In 2020, Salah et al. developed an enzyme-free, cost-effective, and convenient-to-use assay for cancer biomarkers, which entails fluorescent molecularly imprinting conjugated polythiophenes (FMICPs) 67 using two conjugated polymers CP1 and CP2 (Fig. 13). Thorough investigations revealed FMICP nanofibers (FMICP NFs) can achieve superior sensitivity compared to the original FMICP because of their unique surface and quantum size effects. This nanostructure enables extremely sensitive and accurate detection of alpha-fetoprotein (AFP) and carcinoembryonic than the standard enzyme method. Moreover, the solid sensor was successfully utilized in the rapid detection of AFP at very low concentrations in patients of liver cancer. Moreover, the research findings of FMICP and FMICP NFs were consistent with those of clinical ELISA.
Figure 13. Structure of conjugated polythiophenes CP1 and CP2, molecular imprinting strategy and fabrication of nanofibers via cost effective electrospinning method for the detection of cancer biomarker alpha-fetoprotein (AFP.) The figure is reproduced with permission from Ref. 67. Copyright 2020 Elsevier B.V.
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Standard image High-resolution imageNanowires have been intensely explored and extensively studied in the fields of electronic, optoelectronic and nano electronic sensing because of its high flexibility, excellent electrical properties and outstanding optical properties. 68 The interaction of nanowires with unique sensing ability and biomarkers can cause changes in electrical or optical signals, which can transform the unmeasurable and unobservable signals into measurable photoelectric signals, providing a basis for the construction of new biosensor. At present, the research direction of nanowires mainly includes electron-nanowire and fluorophore-nanowire. The former refers to the signal transmission of nanowire through the combination of electron and a field gating effect. 69 The latter is to transfer the resonance energy between the excited conjugated polymer (donor) and the molecular fluorophore (acceptor) by fluorescence induction after conjugated polymer binds to labeled fluorophore. In 2016, Wasin et al. designed "clickable" nanowires with extremely high surface area to volume ratio, ultra-high sensitivity by single particle nanofabrication technique (SPNT). 70 In this study, tetramethylrhodamine (TAMRA) with high stability and low pH–dependence was used as a fluorophore. The specific binding of TAMRA–modified probe to oligonucleotides and guanosine in DNA resulted in obvious fluorescence quenching phenomenon, which was caused by charge transfer between TAMRA and the analyte. Under the action of light, the conjugated polymer cross-linking reaction forms the clear structure of the nanostructure, fluorescence molecules and nanowires interact to cause effective resonance energy transfer to generate observable signals in order to realize the rapid and sensitive detection of biomarkers. Figure 14 shows the single particle nanofabrication technique (SPNT) employed to attain TAMRA-labeled polyfluorene nanowire system. Firstly, the charged particles with acceleration potential induced cross-linking reaction according to the particle trajectory in the polymer film and then converted it into nanowires. Then, TIPS-acetylene groups were deprotected by TBAF to attain alkyne (−C≡CH) moieties, and finally the surface of nanowires was functionalized TAMRA through copper−catalyzed alkyne−azide click reaction.
Figure 14. Illustration of (a) single particle nanofabrication technique (SPNT) to attain TAMRA-labeled polyfluorene nanowire system on solid support and (b) its working principle based on FRET for biosensing applications. The figure is reproduced with permission from Ref. 70. Copyright 2016 American Chemical Society.
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Standard image High-resolution imageDetection of toxic food adulterants
Sudan dyes are widely used in industry as the main raw materials for the production of cosmetics, coatings, paints and other chemical products because of their bright colors, easy availability and low cost. Since many years, Sudan dyes are widely circulated in the food industry as a food additive. However, later studies have found that Sudan dyes contained phenylazo groups, which release aniline during the decomposition process. 71,72 Moreover, the solubility of Sudan dyes in water is extremely low and they are responsible for the deposition of toxic substances after entering the digestive system and cause irreversible damage to human health. In addition, it is also a potential carcinogen and mutagen due to which the food standards agency and the European Union have banned the use of Sudan dye as a food additive. Though, due to the low cost of Sudanese dyes, illegal trades associated with them are still in practice, thus, it is necessary to establish a fast and accurate Sudan dye detection method. Upto date, most detection methods are based on chromatography and mass spectrometry which not only need pretreatment but also require expensive equipment conditions. Therefore, development of cost-effective and reliable method for rapid detection of Sudan dyes remains highly demandable.
In 2014, Huang's group reported a novel self-assembled nanomiceller system based on fluorescent conjugated polymer PF-PE and amphiphilic surfactant pluronic F127 for sensitive and rapid detection of Sudan dyes. 73 In order to investigate the specific detection of Sudan dyes by this system, capsaicin and β-carotene were used as interfering dyes. The results showed that the addition of the Sudan dye to fluorescent nanomicelles lead to significant quenching via photoinduced electron transfer process, while the addition of control dyes did not cause any substantial quenching. Subsequently in 2018, Fan's research team developed a fluorescence sensing method based on inner filter effect (IFE), by utilizing polyphenylene vinylene (PPV) as a material on solid state to optimize the selective detection of Sudanese dyes. 74 With PPV as fluorophore and polyvinyl alcohol (PVA) as the substrate, two fluorescent solid materials with good moisture stability were fabricated as electrospinning fiber membrane and drip film (Fig. 15). These two cross-linked materials displayed reversible sensing properties and maintain the original sensing performance and detection accuracy even after ten regenerations. Sudan dyes can remarkable quenched the fluorescence of the membrane and film, while other food pigments and possible food additives showed no significant effect on membrane quenching. By analyzing
Figure 15. (a) Protocol to prepare reusable conjugated polymer fibrous membrane and film for (b) inner-filter effect (IFE) based selective detection of Sudan dyes. The figure is reproduced with permission from Ref. 74. Copyright 2018 American Chemical Society.
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Standard image High-resolution imagethe sensing process, it was found that the shape of the components and the concentration of PPV have little effect on IFE mechanism-compliant high selectivity and sensitivity and further illustrating the advantage simple preparative process.
Metal ion detection
In order to restrain interpolymer π–π interactions, insulated π-conjugated polymers (ICPs) become an excellent choice of materials. 75 Under normal circumstances, CP adjusts the luminescence color by changing the conjugated skeleton through monomer reconstruction, which is time-consuming, irreversible and not favorable for recycling. Current approach to regulate the luminescent color of polymers is to use external stimulation to induce non-covalent bonding without changing the skeleton structure, 76–78 which can be reversible, rapid and convenient. In 2016, Terao's group designed an ICP containing 2,2'-bipyridine moieties as metal coordination sites. 79 In this solid sensor, the metallized polymer formed by the combination of 2,2'-bipyridine with polymer can adjust the luminous color reversibly without causing any changes in the main backbone chain. This sensor exhibit ability able to produce efficient emission in solid state with the same intensity as in dilute solution because of the insulation structure based on permethylated α-cyclodextrin (PM α-CD). The design of ICPs redistributes the conjugated chain electron cloud through the external stimulus of metal salt and conjugated polymer coordination, and realizes the sensitive, reversible and quick response to different metal ions. Such distinct change of the luminescence intensity of ICPs (Fig. 16) indicates an indispensable platform for the discrimination of metal ions using ICP based solid-state sensor.
Figure 16. Change in emission of insulated polymer film under UV-light on introducing different metal ions. The figure is reproduced with permission from Ref. 79. Copyright 2016 John Wiley & Sons.
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Standard image High-resolution imageRecognition of latent fingerprints
Conjugated polymers play an important role in the field of biosensors because of brilliant biocompatibility, high emission intensity and low toxicity. 8,80,81 The strong interaction of the CPs with the fingerprint components reveals the fluorescent images from different non-porous smooth surfaces that are highly vital for criminal and forensic investigations. In 2017, Iyer et al. developed an extremely efficient latent fingerprint (LFP) technology. 82 The recognition of latent fingerprints using CPs exhibit four advantages. First, CPs can effectively reduce the influence of background interference on detection through strong emission. Second, the strong interaction between the polymer and LFP enhances the contrast of fingerprints on different substrates. Third, the use of LFP for fingerprint detection does not need complicated additional treatment, because the presence of AIEE-active moiety along the polymer backbone and polar head groups, which is conducive to better visualize the LFP. Finally, the fingerprint obtained by LFP technology exhibit extraordinary abrasion resistance. After a certain degree of physical abrasion or chemical treatment, the fingerprints still possess high resolution. Therefore, the newly prepared conjugated polyelectrolyte PFTPEBT-MI can visually reveal the details of the fingerprint with high contrast that makes the system very convenient (Fig. 17). Moreover, the probe can effectively resist the interference of external factors, which is important in order to establish well-preserved latent fingerprints rapidly and efficiently. 83 The strong interaction exists between PFTPEBT-MI molecules and fingerprint components, partly from electrostatic and another from hydrophobic. These interactions allow the PFTPEBT-MI to be easily transferred to the LFP to reveal the fluorescent images. This method can be applied to metal, plastic, glass, aluminum foil and other substrates, and is easy to operate without cumbersome handling. The LFP production manufacture was achieved by steeping the fingerprint-loaded substrate in the CPE solution for around 1 min. This conjugated polymer sensors with solid substrates effectively block background interference by intense emission.
Figure 17. Demonstration of high resolution latent fingerprints developed using conjugated polymer PFTPEBT-MI. The figure is reproduced with permission from Ref. 82. Copyright 2017 American Chemical Society.
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Based on above discussion, it can be said that CPs are among excellent class of fluorescent probes widely employed in the domain of sensing as films, paper strips, portable optoelectronic devices, etc. Table I shows comparison and compilation of the conjugated polymer-based solid-state sensors discussed in this review.
Table I. The comparison and compilation of the conjugated polymer-based sensors discussed in this review.
| Conjugated Polymer | Sensing Platform | Analyte | Category | References |
|---|---|---|---|---|
| Poly(3,3'-((2-phenyl-9H-fluorene-9,9-diyl)bis(hexane-6,1diyl))bis(1-methyl-1H-imidazol-3-ium)bromide) (PFMI) | Paper and Device | Picric Acid | Explosive and Pollutant | 48 |
| Poly(1,4-bis(6-(1-methylimidazolium)-hexyloxy)-benzenebromide) (PMI) | Paper | Picric Acid | Explosive and Pollutant | 47 |
| Poly[4,4'-(((2-phenyl-9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(oxy))-dianiline)] (PFAM) | Paper | Picric Acid | Explosive and Pollutant | 53 |
| Poly(2,7-(9,9-dioctylfluorene)-co−3,4-diphenyl-2,5-distyrylthiophene) (PFTPBZ) | Paper | Picric Acid | Explosive and Pollutant | 58 |
| 9,9'-Spirobi[9H-fluorene] (SBF) | Nanowires | Nitrobenzene | Pollutant | 60 |
| Poly[9,9'-bis(6,6'-( N, N, N -trimethylaminium)fluorene-2,7-ylenevinylene- co -alt-2,5-di-cyano-1,4-phenylene] (PFVCN) | Device | Protein | Biomarker | 61 |
| Poly(9,9-bis(6'-(N,N,N,-trimethylammonium)hexyl)fluorene-co-alt-1,4-phenylene)bromide (PFP) | Device | SKBR-3 Cells | Biomarker | 65 |
| Cationic poly(3-alkoxy-4-methylthiophene) (PMNT) | Device | Creatine Kinase | Biomarker | 66 |
| Conjugated Polythiophenes (FMICPs) | Paper and Nanofibers | α-fetoprotein | Biomarker | 67 |
| Poly[(9,9-dihex-5-yn-1-ylfluorenyl-2,7-diyl)-co-(9,9'-di-n-octylfluorenyl-2,7-diyl)] (F6E8) and poly[(9,9-dihex-5-yn-1-ylfluor-enyl-2,7-diyl)-co-(2,2'-bithiophene)] (F6E2T) | Nanowires | Biomolecules | Biomarker | 70 |
| Poly(phenylenevinylene) and cross-linked poly (vinyl alcohol) (PPV/CPVA) | Film | Sudan Dyes | Food Adulterants | 74 |
| Permethylated α-cyclodextrin based insulated π-conjugated polymer (PM α-CD based ICP) | Paper | Cd, In, Sn etc. | Metal ions | 79 |
Summary and Perspectives
Conjugated polymers with excellent light harvesting and optoelectronic properties represent one of the most favorable signal amplified materials in the field of detection. In this review, we comprehensively highlighted the dynamic properties of CPs which brand them as preeminent materials and systematically discussed some representative papers to show potential of CP as solid-state sensors. Several pioneering methods, protocols, portable and cost-effective sensing devices has been established in past decade based on CPs for proficient detection of environmental pollutants, toxic chemicals, explosives, biomarkers and so on. Though, small molecule probes are easy to prepare, yet CPs exhibit high superiority over them in the arena of sensing due to ultra-sensitivity and rapid signal response towards particular analyte. In spite of several features, longstanding issue of aggregation caused quenching (ACQ) in CP based fluorescence sensors still exists which drastically affects its optical properties as well as sensing performance. One prospective solution to this issue is to incorporate recently emerged aggregation induced emission (AIE) active moieties along the backbone. However, one has to face typical designing strategy and deal with additional synthetic steps to attain such kind of AIE active conjugated polymer system. Suitable receptors are usually attached onto the pendant chains of CPs to accomplish selectivity for desired sensing analyte, but undesirable signals due to non-specific interaction with interfering species remain one of the biggest challenges. Such undesired signals could be avoided through preparation of CP based intelligent nanomicelles as fluorescent probe using co-doping of amphiphilic surfactants like pluronic F-127. This would allow hydrophobic CPs to remain protected inside the core and targeting receptors would remain outside the surface for binding. The fluorescence of CPs can be directly observed with simple and portable instruments and even directly through naked eye for achieving rapid and visual detection. The future development of CPs should focus on further improving their accuracy and specificity i.e. reducing the detection limit and achieving selective detection of analytes. At the same time, more attention is required to improve the portability and viability of CP based probes to meet the healthcare demand in the remote areas. Moreover, presently many types of CP based probes exists, but fewer studies focused on the underlying sensing mechanism for particular analyte, therefore, the future research should also pay attention to the principle of sensing and the interaction between molecules and CPs. In addition, with the growing mobile health (mhealth) technology, the future research on CPs cannot be limited to simply detection and also need much emphasis on how to develop CP based state-of-the-art, high-precision, sophisticated, smartphone based devices with real time function. Furthermore, the domain of organic bioelectronics and point of care (PoC) devices direly need smart and multifunctional optoelectronic materials like CPs in order to fabricate intelligent sensor devices to fulfill the need of healthcare sector. Thus, CP based sensors exhibit indispensable role in multidisciplinary areas of sciences and remain ever popular contender in the development of high performance sensors irrespective of the some challenges.
Acknowledgments
Financial support from National Natural Science Foundation of China (Nos. 21305107, 81701830), the Natural Science Foundation of Shaanxi Province (Nos. 2020JM-066, 2020JQ-019), the Fundamental Research Funds for the Central Universities (Nos. xjh012020001, xjj2017028) and China Postdoctoral Science Foundation (No. 2020M673368) is gratefully acknowledged.