Conjugated polymers (CPs) offer a myriad of opportunities to couple analyte receptor interactions, as well as nonspecific interactions, into observable (transducible) responses. A key advantage of CP-based sensors over devices using small molecule (chemosensor) elements is the potential of the CP to exhibit collective properties that are sensitive to very minor perturbations. In particular, the CP’s transport properties, electrical conductivity or rate of energy migration, provide amplified sensitivity.23 CP-based sensors have been formulated in a variety of schemes, which includes conductometric, potentiometric, colorimetric and fluorescence sensors.
Conductometric sensors display changes in electrical conductivity in response to an analyte interaction. Potentiometric sensors rely on analyte-induced changes in the system’s chemical potential. Colorimetric sensors refer to changes in a material’s absorption properties. Fluorescence is a widely used and rapidly expanding method in chemical sensing. Aside from inherent sensitivity, this method offers diverse transduction schemes based upon changes in intensity, energy transfer, wavelength (excitation and emission), and lifetime. There are advantages to using CPs in
fluorescent sensory schemes due to amplification resulting from efficient energy migration. The combination of amplification and sensitivity in CP-based sensors is evolving to produce new systems of unparalleled sensitivity.76,77
Figure 1.1.5 Typical CPs used for detecting alkali or alkaline-earth metal ions
Analyte specificity in CP-based sensors results from the covalent or physical integration of receptors, imprinting, and/or the CP’s overall electrostatic and chemical characteristics. CPs functionalized with polyalkyl ether chains, crown ether, and aza crown ether moieties have been the most thoroughly studied covalently modified systems.78 In 1989, Roncali and co-workers reported the synthesis of poly[3-(3,6-dioxaheptyl)thiophene] (1) and examined its voltammetric properties in
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the presence of Bu4N+ and Li+ electrolytes.79,80 This was said to be the first conjugated polymer system with a covalently attached functional group for ion complexation.
After this report, a lot of CPs (polythiophene and polypyrrol) with crown ether were synthesized to selectively detect alkali or alkaline-earth metal ions.81-92 (Figure 1.1.5)
Figure 1.1.6 Pyiridyl-based conjugated polymers as chemosensors
The ability of pyridyl-based ligands to coordinate a large array of transition metal ions makes them an attractive functionality to be incorporated into CP sensors. Ligands of this general class can be placed in direct π-communication with the polymeric and/or backbone tethered by extended alkyl chains (Figure 1.1.6). In both cases, chelation of transition metal ions planarizes the pyridyl recognition sites to increase the conjugation and reduce the local band gap and thus lead to conformational, optical, or electrochemical changes in the CP. A significant amount of research has been devoted
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to the study of bipyridine-based conjugated polymers due to their photophysical and electrochemical properties.93-97
Those CPs with crower ether or pyridyl groups were the most widely used materials as conductometric, potentiometric and colorimetric sensors for detecting metal ions. Now fluorescence quenching used in chemical or biochemical sensing has been paid much more attention because of its real-time and amplified response. The utility of CPs for fluorescence-based sensing was first demonstrated by Zhou and Swager.98,99 A general finding of these studies is that the act of “wiring receptors in series” creates superior sensitivity over a small molecule indicator. The observed amplification is a result of the ability of the CP’s delocalized electronic structure (i.e., energy bands) to facilitate efficient energy migration over large distances (Figure 1.1.7).
Figure 1.1.7 Band diagram illustrating the mechanism of quenching behavior for conjugated polymers
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receptor analyte conjugated backbone
+
To demonstrate this principle, studies were conducted in parallel on a small molecule indicator containing a fluorescent monomeric cyclophane receptor. The cyclophane receptors were chosen to bind paraquat and related compounds that are very effective electron-transfer quenching agents. (Figure 1.1.8 A) By conducting detailed photophysical studies, these investigators were able to determine that both the monomer and polymer displayed quenching resulting from the binding of the paraquat by the cyclophane to form a rotaxane complex. Comparisons in solution of the quenching demonstrated a greatly enhanced sensitivity of the polymer over the monomeric compound. The proposed origin of this effect is facile energy migration along the polymer backbone to the occupied receptor sites (Figure 1.1.7). The signal amplification resulting from energy migration in CPs was also applied in 1998 by Yang and Swager for the detection of explosives, specifically 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT).76,77 (Figure 1.1.8 B)
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Figure 1.1.8 A: The first reported molecular structure of conjugated polymer and quencher used as fluorescence chemosensor. B: the structure of PPE derivatives used for detecting TNT