A conjugated carbon chain is a sequence of alternating single and double bonds, with highly delocalized, polarized, and electron-dense π bonds driving its electrical and optical activity. Polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTH), poly(para-phenylene) (PPP), poly(-phenylenevinylene) (PPV), and polyfuran are examples of common conducting polymers (CPs) (Figure 10). Synthetic conducting polymeric materials are widely used in a variety of applications, including packaging, adhesives and lubricants, microelectronic electrical insulators, and implanted biomedical devices ^[90][91]. Shirakawa et al. ^[91][92] discovered the first conducting polymer, a halogenated derivative of poly(acetylene), opened the door to a burgeoning area of fascinating new uses. The charge carrier mobility (which can be increased by doping) and significant light absorption in the UV–visible range are attributed to the delocalized (conjugated) electronic structure of poly(acetylene). Unfortunately, poly(acetylene) is difficult to manufacture and unstable in the presence of oxygen or water, making it unsuitable for a variety of applications. ^[92][93] Moreover, polyacetylene doped with bromine has a million times the conductivity of unadulterated polyacetylene, and this research was recognized with a Nobel Prize in 2000. CPs have recently been developed for use as roll-up displays for computers and mobile phones, flexible solar panels to power portable equipment, and organic light-emitting diodes in displays, in which television screens, luminous traffic, information signs, and light-emitting wallpaper in homes are expected to broaden the use of conjugated polymers as light-emitting polymers ^[15][16]. In this context, the insertion of inorganic atoms into the polymer chain is a powerful strategy for changing the characteristics of conjugated polymers. ^[93][94] In this context, Rupar et al. ^[94][95] reported the preparation of poly(9-borafluorene) vinylene (P9BFV, Figure 11) that showed simultaneous turn-off/turn-on fluorescence responses to fluoride in solution and NH₃ (Figure 12) in the gas phase due to the presence of three-coordinate boron. It is important to emphasize that only a small number of conjugated polymers act as optical ammonia sensors ^[95][96][97][96,97,98].

Figure 10.

 Classic examples of conducting polymers.

Figure 11. Synthesis of the polymer P9BFV.

 Synthesis of the polymer P9BFV. Reproduced with permission from reference [95]. Copyright 2015, John Wiley and Sons.

Figure 12. (a) Emission spectra of a P9BFV film before exposure to NH₃ vapors, (b) emission spectra of a P9BFV film while being exposed to NH₃ from aqueous ammonium hydroxide, (c) emission spectra of a P9BFV film 5 min after removal of the NH₃ source and NH₃  vapors. Reproduced with permission from reference [95]. Copyright 2015, John Wiley and Sons.

The synthesis of the polymer P9BF was conducted via the Yamamoto reductive coupling ^[98][99] of Tip (Br). The polymerization is performed with small amounts of bromobenzene, which acts as a capping agent for the step-growth polymerization. On the other hand, the copolymer P9BFV was synthesized via Stille coupling ^[99][100].

Compared to the homopolymer P9BF, the prolonged conjugation of P9BFV, owing to the addition of the vinylene group, results in a lower optical bandgap (2.12 eV) and LUMO (4.0 eV, calculated by CV).

P9BFV has a strong solid-state fluorescence with an almost identical spectrum to that observed in the solution. The fluorescence changed from yellow to blue when the film was irradiated with UV light and exposed to ammonia. Furthermore, these alterations are reversible: removing the ammonia atmosphere within 5 min restores P9BFV’s fluorescence spectrum to its normal condition. When thin films of P9BF were exposed to ammonia, they behaved almost identically to P9BFV.

Along with CPs FPs materials, the Bielawski group reported the ring-opening metathesis polymerization of (bicyclo[2.2.2]octa-2,5,7-triene) (barrelene) to prepare copolymers with norbornene, producing robust films ^[100][101]. The copolymers prepared underwent spontaneous dehydrogenation in the presence of air or when exposed to laser pulses (phenylene vinylene), affording precisely defined fluorescent patterns with micrometer-sized dimensions. Direct laser writing (DLW) ^[101][102] on films of poly(barrelene-co-norbornene) produced a succession of well-defined patterns with micrometer dimensions, which were observed by the fluorescence of the conjugated polymer that formed after irradiation.

As demonstrated by various spectroscopic methods, PPV was produced by spontaneous dehydrogenation of the homo- and copolymers in the presence of air. According to a series of thermal studies, the copolymer displayed an exothermic response when heated to 100 °C, most likely due to oxidation (dehydrogenation), but did not undergo significant mass loss until around 400 °C (under a N2 atmosphere). Thermal aromatization was achieved in seconds after direct laser writing of the barrelene-containing copolymers by using a 400 nm diameter laser beam at a wavelength of 355 nm to write on the substrates that consisted of the copolymer at speeds of approximately 350 μm s⁻¹. Raman spectroscopy was used to look at the dark and luminous portions of the copolymer to see if the laser aided in the oxidation process. The dark parts of Figure 13 showed signals consistent with poly(barrelene-co-norbornene). In contrast, the fluorescent sections showed signals compatible with PPV and consistent with the observations reported by the authors. This chemistry has the inherent advantage of allowing the monomer to be integrated into various macromolecular scaffolds and at different compositions, resulting in a diverse range of materials suitable for laser machining and current lithography applications.

Figure 13. (a) Laser-induced oxidation of poly(barrelene) to polyphenylene vinylene (PPV). (b) Examples of patterns that were written. Photographs were taken under visible light (left) and UV light (λex = 488 nm; right). (c,d) Raman spectra were recorded for the areas indicated in blue or red (see insets) of a film subjected to DLW. The films were prepared by drop-casting a solution of poly(barrelene-co-norbornene) (200 mg) in CH₂Cl₂  (4 mL) into a PTFE mold. Reproduced from reference [101] with permission from the Royal Society of Chemistry (Licensed under CC-BY 3.0).

Because of its increased detection sensitivity in the detection of a wide range of environmental contaminants and bioactive chemicals, conjugated polymer-based sensors have received a lot of interest ^{[102][103][104][105]}[103,104,105,106]. Since the first conjugated polymer sensor was reported in the 1990s, this sensing platform has advanced more than two decades, with an explosion of research in this sector noted in the previous 10 years ^[106][107]. For instance, water-soluble conjugated polymers with ionic side chains are known as conjugated polyelectrolytes (CPEs) ^{[107][108][109][110][111]}[108,109,110,111,112], and they have garnered attention in the past decade mainly because of their application as sensors, energy converters, and antimicrobials ^{[112][113][114][115]}[113,114,115,116]. For example, Tan et al. ^[116][117] developed and synthesized four anionic CPEs with a poly(paraphenylene ethynylene) (PPE, Figure 14a) backbone but varied pendant ionic side chains. These CPEs were shown to attach to metal ions with varying selectivities via polymer–metal ion interactions, resulting in a wide range of fluorescence responses. Four CPEs were structurally and photophysically characterized and used as PPE sensor arrays. The sensor array’s fluorescence intensity responses were tested after the injection of eight different metal ions separately.

Figure 14. (a) Structures of the four conjugated polyelectrolytes, PPE1, PPE2, PPE−IDA, and PPE−HPA, (b) response patterns constructed based on fluorescence quenching of the four polymers by eight metal ions at 5 μM each. The response patterns were generated from the ratios of the polymers’ initial to final emission intensities. Error bars represent the standard deviations of six replicates for each PPE−metal ion pair. Polymers are PPE1, PPE2, PPE−IDA, and PPE−HPA, and metal ions are Pb²⁺, Hg²⁺, Fe³⁺, Cr³⁺, Cu²⁺, Mn²⁺, Ni²⁺, and Co²⁺. Adapted with permission from reference [117]. Copyright 2015, American Chemical Society.

This approach generated environmentally friendly fluorescent materials, with easy sample preparation and measurements and convenient data processing with a simple pattern analysis procedure that distinguishes from other existing methods. The authors improved the methodology for detecting combinations of harmful metal ions, with the possibility of expanding to other metal ions in the future.

The development of new detecting methodologies based on FPs has also been extended to organic molecules. Tetracycline (Tc), for example, is a type of antibiotic that is commonly used in veterinary medicine, human treatment, and agriculture, and it is critical to measure in water. The development of simple and efficient methods for detecting and removing TC from water is highly desirable but continues to be a challenge. Because of major environmental concerns, including ecological hazards and human health consequences, tetracyclines are a special case among the many antibiotics used. Most data suggest that tetracycline antibiotics are ubiquitous substances found in many ecological compartments due to their widespread use ^[117][118][118,119]. More than 70% of tetracycline antibiotics are excreted and released inactive form into the environment after administration via urine and feces from people and animals. Because of their very hydrophilic nature and low volatility, they have demonstrated great endurance in the aquatic environment ^[119][120]. Iyer et al. ^[120][121] synthesized a CPE following a palladium-catalyzed Suzuki cross-coupling polymerization, poly[5,5′-(((2-phenyl-9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(oxy))diisophthalate] sodium (PFPT), and used it for highly sensitive detection of tetracycline in water (Figure 15).

Figure 15. (a) Fluorescence spectra of PFPT (6.6 μM) with increasing concentration of Tc (6.6 μM) in aqueous media (HEPES 10 mM, pH 7.4). Inset: color change of PFPT under UV light (lamp excited at 365 nm) before and after adding Tc. (b) Stern–Volmer plot of PFPT upon the addition of Tc in aqueous media. Effect of Tc (6.6 μM) and various other antibiotics (6.6 μM) on the (c) emission spectra of PFPT and their corresponding (d) bar diagram depicting the quenching percentage. Reproduced with permission from reference [121]. Copyright 2017, American Chemical Society.

Due to the electron-rich nature of the polymer PFPT, cyclic voltammetry analysis revealed only the oxidation peak, from which the HOMO level was determined to be 5.95 eV. From the beginning of the UV–vis absorption spectrum, the optical band gap was calculated to be 3.35 eV, whereas the LUMO level was determined to be 2.6 eV. The HOMO (7.55 eV) and LUMO (4.53 eV) energies of tetracycline were also determined using cyclic voltammetry and onset UV–vis measurements. These results demonstrate that photoinduced electron transfer from the polymer’s LUMO (2.6 eV) to the quencher Tc’s LUMO (4.53 eV) is the sole mechanism by which Tc selectively quenches PFPT fluorescence.

In 100% aqueous media, the detection limit of PFPT toward Tc is reported to be 14.35 nM/6.80 ppb (6.8 ng/mL). The effective electron transport from PFPT to Tc via electrostatic/hydrogen bonding interactions resulted in a high quenching efficiency with a K_sv value of 1.57 × 10⁵ M⁻¹.

As well as sensing materials ^[54][55], CPs have been intensively researched for biosensing and bioimaging applications due to their great light-harvesting capacity, outstanding photostability, and easily surface-modifiable features ^{[121][122][123]}[122,123,124]. For instance, photodynamic therapy (PDT) ^[124][125][125,126] has become an important cancer treatment that uses reactive oxygen species (ROS) (such ¹O₂ and hydroxyl radicals) created by exposing photosensitizers (PS) to light irradiation in an oxygen-rich environment to destroy cancer cells. Zhang and coworkers ^[126][127] prepared a positively charged water-soluble polythiophene polymer (PT0, Figure 16). This exhibited high photo- and pH-stability, a sizeable two-photon absorption cross-section, and the capability to generate ¹O₂ (g).

Figure 16. Illustration of PT0 for simultaneous two-photon excitation fluorescence imaging and photodynamic therapy.

 Illustration of PT0 for simultaneous two-photon excitation fluorescence imaging and photodynamic therapy. Reproduced with permission from reference [127]. Copyright 2017, American Chemical Society.

Irradiation using 780 and 900 nm lasers of 406, and 473 mW of laser power, respectively, with prolonged irradiation (4 to 5 min) was needed to kill HeLa cells efficiently. Although PT0 does not appear to distinguish cancer cells from noncancerous cells, such selectivity might be achieved via a method such as combining PT0 with a cancer cell targeting probe, which will require additional investigation.