Perylene Diimide-Based Optical Chemosensors for pH Probing: Comparison
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Please note this is a comparison between Version 2 by Wendy Huang and Version 1 by Shuai Chen.

The precise control and monitoring of pH values remain critical for many chemical, physiological and biological processes. Perylene diimide (PDI)-based molecules and materials exhibit excellent thermal, chemical and photochemical stability, unique UV-vis absorption and fluorescent emission properties, low cytotoxicity, as well as intrinsic electron-withdrawing (n-type semiconductor) nature and impressive molecular assembly capability. These features combined enable promising applications of PDIs in chemosensors via optical signal modulations (e.g., fluorescent or colorimetric). One of the typical applications lies in the probing of pH under various conditions, which in turn helps monitor the extracellular (environmental) and intracellular pH change and pH-relying molecular recognition of inorganic or organic ions, as well as biological species, and so on. 

  • perylene diimide
  • chemosensor
  • pH probing
  • colorimetric sensor
  • fluorescent sensor

1. Introduction

The molecules of perylene 3,4,9,10-tetracarboxyl diimide (PTCDI or PDI) form a series of unique functional semiconductors and conjugated organic dyes or pigments. PDIs have exceptional photo absorption and emission characteristics, excellent thermal, chemical, optical and photochemical stability, as well as outstanding n-type (strong electron affinity) semiconductor properties. These features can be tuned via molecular structure modification, molecular assembly or aggregation, or exogenous chemical action, etc. [1,2,3,4][1][2][3][4]. Such structural and property tunability makes PDIs ideal candidates for development as chemosensors for detection of various chemicals in both liquid and gas phases, especially organic and inorganic ions, and biological species [3,5,6,7,8][3][5][6][7][8]. Very often, the sensitivity and selectivity of the chemosensors are dependent on the pH of the analyte solution, meaning that monitoring and control of the pH are crucial for precise detection of target analytes. Such pH dependence of sensing performance has been reported and reviewed in some previous publications [3,7,8][3][7][8]. Nonetheless, there is a lack of systematic review of quick and precise detection of the pH value itself with PDI-based optical sensors, although such pH probing would be crucial for practical application in the health, pharmaceutic, biological, food and environmental analysis scenarios, wherein the pH may change frequently under various conditions. In 2020, Borisov et al. gave an excellent and quite comprehensive review of optical pH sensors, including a brief introduction of PDI sensors for pH probing [9]. In 2021, Singh et al. summarized the progress in PDI-based chemosensors, describing briefly their application in the detection of H+ and OH− ions (equivalent of pH) [8].
Compared to the most popular and widely used commercial pH glass electrodes based on the electrochemical principle, the use of optical (colorimetric, fluorescent) chemosensors removes the technical limits due to expensive, complex and bulky electrical equipment, which usually requires sophisticated control and maintenance [9,10][9][10]. Optical sensors can also be visualizable but inert and robust to the interference of extraneous electrical, magnetic and microwave fields. These sensors are more cost effective and could even be used in disposable form, just as the widely used pH test papers, despite some limitation in continuous real-time measurement [10]. Moreover, the small size and aqueous solubility of chromophore and fluorophore make them attractive for use in cell and other biofluid (e.g., blood, urine, etc.) imaging, where it is difficult or impossible to employ pH electrodes. Thus, molecular colorimetric and fluorescent sensors have been recognized as the most competitive techniques for pH probing beyond the conventional pH glass electrodes [9].

2. Colorimetric Chemosensors

The colorimetric pH response of PDI-based chemosensors benefits from their strong absorptivity in the mid-visible spectral region, with molar absorption coefficient as high as 30,000–90,000 cm−1 M−1 [2,10][2][10] in the broad spectra (color) variation range [5]. However, due to the rigid perylene skeleton and the unique π-conjugation with the imide nitrogen as the node, structural substitution at the imide position generally has little influence on the spectral absorption property of PDI molecules [1]. At present, only few studies have been reported, such as those on hydrochromism [11] and pH-mediated color change [12], in this aspect, mainly due to the considerable challenge of the intense red background and poor water solubility of hydrophobic PDI core. Satisfyingly, modification at its bay area could result in considerable influence on its absorption spectra and improvement in solubility [1].

2.1. Hydrochromism for pH and Humidity Sensing

Upon functionalization of PDI scaffold at its 1,7-bay position by electron-donating 3-hydroxycyclobutenedione moieties, the derivative (namely PDI−1) thus obtained showed pronounced acidity and solubility in polar solvents [11]. The intramolecular electron transfer (IET) between cyclobutene moiety and PDI backbone can be modulated significantly by the protonation/deprotonation of the -OH groups, thus resulting in bathochromic shift of the absorption peak from 570 nm (acidic form itself) to 612~667 nm (conjugate base form PDI−12− in different organic solvents). The solvent effect of the colorimetric response can also be utilized for probing the solvent polarity and humidity, which can be applied in monitoring trace water in organic solvents, such as tetrahydrofuran (THF), as well as film sensing of humidity in the gas phase. The sensor film, fabricated by immobilization of PDI−1 in polyethylene glycol (PEG) matrix, showed drastic color change from red-purple to blue-green accompanied by distinct absorption spectra change upon exposure to water moisture. Such halochromic response is instantaneous, and reversible, for which the sensor material can be regenerated after drying.

2.2. Synergistic CO2 and pH Sensing

Similar to the solid-state sensing described above, a PDI molecule substituted at the bay area with the 2-phenylimidazole group, namely PDI−2, was developed into a CO2 sensor through the colorimetric response toward pH change [12]. The solid-state CO2 sensor was fabricated by blending PDI−2 with tetraoctylammonium hydroxide (TOA) as the lipophilic base, tributyl phosphate (TBP) as the plasticizer and ethyl cellulose (EC) as the matrix into hydrophilic poly(ethylene terephthalate) as the support. In its pristine state under nitrogen, PDI−2 exists in the deprotonated format by forming a salt with TOA ions, exhibiting a blue color. Upon a decrease in pH, PDI−2 becomes protonated at the imidazole moiety, thus changing its color to red. Such pH response can be adapted to the sensing of CO2, which is a typical weak acid when dissolved in water. By measuring the colorimetric response of the PDI−2 composite sensor immersed in water, the gas phase CO2 in contact with water can be monitored in a broad pressure range from 0.5 to 1000 hPa. Taking advantage of the structure and composition tunability, the composite sensor of PDI−2 may find wide application in quality inspection of beverage and food packaging, health monitoring and others, where constant monitoring of the pH or CO2 level is crucial.

3. Fluorescent Chemosensors

In comparison to the colorimetric sensors (including the most commonly used pH test paper), fluorometric pH probing has intrinsic advantages of fluorophores and the corresponding sensing mechanisms, such as high sensitivity, rapid response and much improved selectivity enabled by the specific molecular design [9,13][9][13]. Unlike other common pH-responsive fluorophores, PDIs have been considered as some of the most attractive candidates in chemosensor design because of their excellent electron-accepting ability, strong fluorescence emission (high quantum yield, close to 100% in the monomeric and small oligomeric states) and high photostability [3,4,10][3][4][10]. In general, a PDI-based pH sensor consists of PDI backbone modified with an electron donor group, such as amine, which can interact with protons to initiate the protonation/deprotonation process. Before protonation, the free base state of amines functions as an effective electron donor, causing quenching of the fluorescence of PDI through PET [8,14][8][14]. Upon protonation, the energy level of the lone pair of electrons in amine becomes lower than the highest occupied molecular orbital (HOMO) of PDI, thus blocking the PET for PDI. As a result, the strong fluorescence of PDI is resumed. Such fluorescence turn-on response can be utilized to quantize the concentration of protons, as well as probe the change of pH. The substitution with amine in PDI can be either at the imide position or the bay area, and the amine substituents can be primary, secondary, tertiary or aromatic amines. Depending on the substitution position and the type of amines, the fluorescence color (wavelength) and intensity modulation efficiency upon protonation could vary significantly. Since the nitrogen at the imide position is a node in the π-conjugation of PDI, substitution at this position does not change the electronic property of PDIs [1].
Deprotonation of the -OH group generates an anionic state of the cyclobutene moiety, thus increasing its electron-donating power. This results in significant red shift of the absorption peak due to the enhancement of the IET transition [11]. The increased electron-donating power would also enhance the PET process, and thus, fluorescence quenching [8]. For the PDIs without or with minimal bay substitution, the limited solubility in some solvents often leads to molecular assembly (aggregation) primarily driven by the strong π–π stacking interaction, resulting in significant quenching of fluorescence [1]. To mitigate the aggregation induced fluorescence quenching, the substitution at the imide or bay area is often modified with steric groups, so as to enhance the molecular dispersion of PDIs in solution [2]. If the substituents are protonatable or deprotonatable, the molecular assembly may become significantly dependent on the pH, which in turn changes the electrostatic interaction between molecules. Depending on the molecular structure design, pH-triggered fluorescence response could be due to various chemistry processes, such as PET [10,12[10][12][13][14][15],13,14,15], supramolecular (de)aggregation [16[16][17],17], fluorescence resonance energy transfer (FRET) [18], tunable lateral dimension of one-dimensional (1D) nanostructure [19] or volume phase transition of unimolecular micelle [20], and so on. These optical pH chemosensors have been intensively utilized for environmental or health analyses and cell imaging [21,22[21][22][23],23], etc. In addition to fluorescence emission intensity, fluorescence lifetime can also be used as a sensing signal given the excellent photochemical stability of PDIs [24,25][24][25].

3.1. pH Sensing Based on Photoinduced Electron Transfer (PET) Mechanism

As discussed above, PDI-based pH sensors are commonly reliant on the PET mechanism, for which the electron transfer process occurs between the PDI backbone and electron donor group covalently modified at the imide or bay positions, and the pH response is due to the protonation or deprotonation of the electron donor moiety [14,26][14][26]. Despite the highly efficient PET sensing response, PDI fluorescent sensors have been much less used in aqueous media for pH probing in comparison to the more common applications in organic or mixed media for detection of other ionic species [3]. This is likely due to the challenge in tunning the aqueous solubility of PDIs in a wide range of pH. One of the early PDI-based pH sensors, namely PDI−3, was composed of an asymmetric structure with one imide modified with nonyldecyl and the other with aniline [14]. The aniline acts as an electron donor and organic base, which can be protonated, and the nonyldecyl substitution helps prevent molecular aggregation due to the bulky steric hindrance. Thanks to the high molecular dispersibility, PDI−3 was successfully developed as a single-molecule probe for pH, metal ions and other organic functional groups, such as ketone, on surface via protonation and other reactions at the aniline site [14]. In the unbound state, the energy level of free base aniline is higher than the HOMO of PDI, and the efficient PET process causes almost complete fluorescence quenching of PDI, resulting in zero emission background. Upon protonation, the energy level of aniline is lowered, thus blocking the PET process. As a result, the fluorescence of PDI is resumed depending on the yield of protonation, which in turn depends on the concentration of protons (or pH). However, such fluorescence turn-on sensing response can also be realized through other chemical reactions of aniline, such as coordination interaction with metal ions, such as Zn2+, Pt2+, or Schiff’s base reaction with ketones or aldehydes. This poses a challenge for detection selectivity when used in the bulk solution phase, although the different bounding interactions can potentially be distinguished at single-molecule level on the surface by measuring the different “blinking” kinetics of fluorescence [14]. Taking advantage of the quick reversible protonation/deprotonation of amines, PDI sensors based on the PET mechanism have become increasingly popular for pH probing depending on the pKa of the amine moieties [10,11,12,13,14,15,26,27][10][11][12][13][14][15][26][27].

3.2. pH Sensing Based on Supramolecular (De)Aggregation Mechanism

Supramolecules, as one type of the most investigated biomimetic architectures, have been widely used in chemosensors. On account of the planar molecular skeleton of PDI, PDI molecules have a high tendency to stack, forming supramolecular assemblies via weak interactions, such as π–π interaction, and hydrogen bonding, etc. [1]. Thus, PDIs have been recognized as one of the extensively studied artificial building blocks in supramolecular chemistry, including chemosensing [2,30][2][28]. Given the fact that pH control is usually critical for the optimized aggregation behavior and shape-defined assembly morphology of PDIs molecules, as well as their uses for ion sensing in the liquid phase [3,31[3][29][30],32], PDI−based fluorescent sensors have demonstrated great potential for use in pH probing or detection of pH-related ions [3,16,17][3][16][17].
Unlike the PET-based fluorescence response upon protonation with H+, the fluorescence and other photophysical properties of PDI supramolecules can be modulated by the stimulation of H+/OH− variation in external aqueous environment, which may cause dramatic structural and morphological changes to the supramolecular assembly. For example, pH stimulus can drive the reversible conversion from quenched assemblies to the fluorescent molecule of PDI−12 [16]. Such (de)aggregation induced pH response takes advantage from its considerable water solubility due to the two ionized amino-imidazole groups grafted on the π-conjugated PDI core. The amino group acts as a protonation site, showing response to pH simulation. PDI−12 was successfully used in detecting pH change due to the formation of gluconic acid from the reaction between glucose and glucose oxide. This implies the potential of using PDI−12 in monitoring in situ the presence of glucose, as well as other bio-species in biofluids or intracellular microenvironments. In another example [17], the solubility of PDI−13/14/15/16 was adjusted by bay substitution, while the imide positions were grafted with different protonated tertiary amine groups possessing variable electron-donating characteristics and alkylcarbonyl linkers with different lengths. Such series of PDI-based pH probes were investigated in HCl/THF solutions with different concentrations. In addition to π–π stacking, hydrogen bonding and hydrophobic interactions, the controlled charge interactions between large rigid π-core and the PDI core in relation to the tuned protonation degree have a significant influence on the pH induced self-assembly morphology of PDI molecules. Upon protonation, a large Stokes shift of the emission spectra occurred, accompanied by a fluorescence color change from red to blue of PDI−13, PDI−14 and PDI−15.

3.3. pH Sensing Based on Fluorescence Resonance Energy Transfer (FRET) Mechanism

In addition to the above-mentioned shape-defined supramolecular assemblies (e.g., nanofibers), some other supramolecular architectures can also be utilized in pH sensing. Inspired by the natural amphiphiles forming biological bilayer membranes in living vesicle systems, some amphiphilic building blocks of PDIs have been synthesized and used for chemosensors [18,33][18][31]. By functionalizing the hydrophobic skeleton of PDI with asymmetric hydrophilic and hydrophobic groups at its imide positions, amphiphilic derivatives PDI−17 and PDI−18 (Figure 1) and the sensing performance were reported [18]. Such water-soluble molecules can be co-assembled into a bilayer nanoscopic vesicle (acting as an energy acceptor) in aqueous solution, for which water-soluble energy donors can be enclosed inside the vesicle. By adjusting the spectral overlap of donors and acceptors, a controlled pH-sensitive FRET process from the “core” (encapsulated donor) to the “shell” (bilayer dye membrane) can be realized, so as to drive the optical sensing application of such nanocapsule vesicle in aqueous solutions. Ultrasensitive pH response was obtained for the nanocapsule sensor as tested in a wide range of pH 3.0–11.0; the sensing response was revealed as dramatic fluorescence color change, covering the whole visible light range under UV (366 nm) illumination.
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Figure 1.
Molecular structures of PDI−17 and PDI−18.

3.4. pH Sensing Based on Tunable Lateral Dimensions of 1D Nanostructures

Unlike the pH-triggered aggregation–deaggregation process, an alternative way to realize the pH sensing response in water using PDIs is through tuning the lateral morphology of 1D assembly, as evidenced with a histidine-modified PDI−19 molecule [19]. With histidine auxiliaries, PDI−19 demonstrates bio-inspired bolaamphiphility in the zwitterionic form under physiological pH, which plays with the two pKa values of 4.1 (carboxylic acid) and 7.3 (imidazole ring). The 1D self-assembly of PDIs is highly dependent on the molecular structure of substituents at the imide position in association with the various non-covalent intermolecular interactions (e.g., electrostatic attraction/repulsion, hydrogen bonding, etc.), which in turn can be initiated and tuned by pH change through the protonation−deprotonation process. It was found that the lateral dimension of the assembly of PDI−19 was reversibly transformed from thick fiber (at pH = 7) to thin fiber (pH = 10) and belt (pH = 2), and this resulted in a corresponding change in fluorescence accompanied by supramolecular chiroptical switching (left-handed to right-handed helical self-assembly). Taking advantage of the reversibility of the pH induced structural and property change, the reported PDI system may help develop new application scenarios for pH probing in water, capitalizing on the unique features of chiroptics and biomedicine.

3.5. pH Sensing Based on Volume Phase Transition of Unimolecular Micelle

Compared to the dimensional change of 1D molecular assembly, structure-controllable unimolecular micelles can also be developed into optical chemosensors reliant on the pH induced structure change. Upon substitution with sufficiently large dendrimers, PDI−20 and PDI−21, can behave as a unimolecular micelle with the core being hydrophobic and the shell hydrophobic. The globular “core–shell” morphology (average diameters around 20 nm) makes the micelles a unique platform to be used in pH probing in complete aqueous media (pH = 7) [20]. These micelles displayed reversible volume phase transition around their pKa values (6.34 and 8.05, respectively) due to the ionization (fully charged state; size increasing) or deionization (totally uncharged state; size decreasing) of the polymer chains induced by pH variation. Accompanying size change, the significant change of fluorescence can be used as sensor signals for probing the pH. It is the flexible cationic or anionic polyelectrolyte side chains as the outer shell, which contributes to high water solubility (>10 g/L) and high fluorescence quantum yields in water (0.11 for PDI−20 and 0.13 for PDI−21) by preventing the central PDI chromophore from aggregation, for which the electrostatic repulsion capability (in relation to the polymer chain stretching or collapsing) is highly dependent on pH.

3.6. pH Imaging in Cells

External pH stimuli or an acid-base microenvironment are extremely crucial for cell survival as well as its internal and external physiological/pathological activities in vivo, and therefore, accurate in situ probing of the pH would help in health monitoring and disease diagnosis [9,24,34][9][24][32]. PDIs as fluorophores for pH monitoring in cells have gained particular interest mainly because of the advantages in the following aspects [13,21,22,23,24][13][21][22][23][24]: versatile molecular design, robust structure with combined chemical/photochemical/thermal stability, lower biological toxicity, high fluorescent emission brightness (intensity) under lower excitation energy, lower auto-emission and light scattering interference from intracellular microenvironments, desired cell penetration capability, etc. Further support comes from the essential light penetration across tissues and multidimensional analysis ability of the fluorescence technique itself. The key challenge remaining for PDI-based sensors lies in their hydrophobic skeletons and aggregation-induced weak emission intensity in aqueous solution. To date, amphiphilic PDIs with good solubility and biodistribution in water are the most preferable architectures for intracellular pH probing via fluorescence intensity or lifetime modulation as the signal.

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