Optical Chemosensors: Principles, Chemistry, Strategies, and Applications

Abstract

Chemosensors for anions and cations detections have been extensively used in several disciplines, including pharmacology, environmental science, biology, and chemistry. This field which is a division of supramolecular chemistry has been known for more than 150 years. It deals with chemosensors that recognize and detect anions and cations via optical or electrochemical signals. Today, a sustainable variety of chemosensors are established to detect both anions and cations. Additionally, chemosensors can be used to construct a sensory device and extract, and separate anions and cations. Chemosensors can detect toxic anions such as fluoride and cyanide as well as cations like mercury. Thus, chemosensors have become an attractive area of supramolecular chemistry. This chapter focuses on both colorimetric and fluorometric optical chemosensors and their application for anions and cations detections.

1. Introduction

Czarnick defines the chemosensor as a “molecule of abiotic origin that signals the presence of matter or energy [1].” Furthermore, chemosensors are molecule receptors that can sense and precisely interact with an analyte and generate a response as a detectable signal. The signal can be optical or electrical. It consists of a detection group and a signaling moiety. The detection group is responsible for selectivity and binding efficiency. While converting information into a detectable signal is due to the signaling moiety.

Recently, considerable studies focused on chemosensors because of their susceptible photophysical properties to the environment. The change in the optical signals can provide data on the chemical parameters such as pH and analytes concentration [2]. Chemosensors are extensively investigated to detect heavy metal ions [3, 4]. Optical Chemosensors are classified into colorimetric and Fluorometric chemosensors. The colorimetric receptors display the advantage of naked-eye detection of color changes, making the process simple.

On the other hand, the Fluorometric chemosensors are more chosen for the ratiometric response because of the ratio between the intensity of two emissions which allows for correcting the analytic concentration of the sensor and the effects of environment like temperature and polarity [5]. Today the demand is high for chemosensors that are selective towards specific harmful ions. The toxicity of certain anions and cations for humans as well as animals has motivated researchers to design chromophores that are selective to a specific anion or cation [6–8]. Several international agencies prohibit numerous heavy metal ions because of their toxicity and non-biodegradability, resulting in their accumulation in the environment [9, 10]. Nowadays, chemists and scientists are attempting to design chemosensors that can be used for environmental and industrial sample analysis.

Fluorometric chemosensors have attracted increasing intention for detecting selective anion or cation due to:

• High Sensitivity (single-molecule detection is probable)

• High selectivity

• Short response time

• High spatial and temporal resolution

• Low cost and easily performed instrumentations

In this contribution, both colorimetric and fluorometric chemosensors will be discussed, focusing on fluorometric receptors. The most crucial applications for Fluorimetric chemosensors for detecting Cyanide and Fluoride are also briefly given.

2. Optical chemosensors

Optical sensors are investigation techniques that detect light intensity. They modify the receptor’s photophysical properties upon analyte (guest) binding to the receptor. Depending on the sensor type, these modifications are characterized in UV–visible and fluorescence spectroscopy instruments. The chemosensors can be classified into colorimetric chemosensors, fluorometric chemosensors, or fluorescent chemosensors.

2.1 Colorimetric chemosensors

A colorimetric chemosensor is defined as the color change that occurs after the receptor’s binding with a specific analyte [11]. After binding the receptor and the analyte, the chemosensor’s signaling unit shows a color change [12]. Colorimetric chemosensors have attracted significant interest due to the possibility of obtaining qualitative and quantitative data via the naked eye [13] without referring to any complex techniques [14].

2.2 Fluorometric chemosensors

2.2.1 General principle: design of chemosensor

One of the most valuable response methods for optical readout is fluorescence. The fluorescence chemosensors have attracted considerable interest due to their sensitivity, selectivity, quick response time, on-site and real-time detection, straightforward performance, flexibility, and present low molar estimate of the analyte. In the case of fluorometric chemosensors, binding the analyte to the receptor leads to a change in fluorescence behavior [15]. In 1867, Goppelsroder reported the first fluorescent chemosensor to detect aluminum ions via the formation of a morin chelate which is strongly fluorescent.

The fluorescent chemosensor allows the detection at the picomolar scale; however, the colorimetric sensors can only detect concentration at micromolar levels. The fluorometric chemosensor’s advantages are mainly due to the proportionality of the emitted fluorescence to the analyte concentration. In contrast, in absorbance measurements, the concentration of the analyte and the absorbance are proportional, which is associated with the ratio between intensities measured before and after the beam passes through the sample.

The essential parts of the fluorescent chemosensors are the recognition and signaling moieties, as given in Figure 1. Accordingly, a fluorescent chemosensor is developed by connecting a receptor (ionophore) to a fluorophore responsible for converting the recognition into the photophysical signal (like spectra, fluorescence quantum yield, and lifetime).

Figure 1.

Fluorescence sensing (a) A spaced model (b) An integrated model.

The photophysical properties can alter by binding a particular analyte to the receptor, leading to a fluorescence signal, with either an enhancement or quenching of fluorescence. The binding of the analyte to the receptor leads to an enhanced fluorescence intensity called turn-on chemosensor [16, 17], whereas the analyte bounding to the fluorophore results in a decrease of fluorescent intensity quenching called turn-off chemosensor [18, 19].

Depending on the connection of fluorophore and receptor, two models of fluorescent chemosensors can be defined: spaced (Figure 1(a)) and integrated model (Figure 1(b)). The spaced model offers a design where the fluorophore is connected to the receptor via spacer and signaling moieties that prevent conjugations. In the integrated model, the fluorophore and receptor are conjugatively connected to each other; in this model, the receptor is a part of the π-electron system of the fluorophore.

2.2.2 Photo-induced electron transfer

The fluorescence quenching is due to a photo-induced electron transfer (PET); the normal state returns when a non-luminescent process follows the PET process [10]. The PET process could explain the “on–off” switching of fluorescent chemosensors based on the molecular orbital theory. The basic design of the florescent chemosensors based on PET is a fluorophore- spacer- receptor. This model has a spacer separating the fluorophore from the receptor, consequently electronically disconnecting the π-electron systems from the fluorophore and receptor. The PET process is divided into two types: reductive PET and oxidative PET, depending on the electron acceptor or donor links to the fluorophore and receptor [11]. However, in this study, only the reductive PET will be investigated. The design of the fluorescent chemosensors is given in Figure 2. The reductive PET is defined as the system where the fluorophore is reduced, whereas the receptor is oxidized. In this type of PET process, the fluorophore provides an electron acceptor, and the receptor serves as an electron donor. The electron photo-excitation from HOMO to LUMO occurs through the reductive PET process. Consequently, the fluorophore and the analyte react and cause the occurrence or elimination HOMO and LUMO “adjacent” orbital, resulting in the fluorescence quenching or enhancement, respectively (Figure 3).

Figure 2.

Molecular orbital diagram of the photo-induced electron transfer process (PET).

Figure 3.

Schematic representation of PET chemosensor in the sensing process.

2.2.3 Intra and intermolecular charge transfer

Valeur et al. were the first to introduce the intramolecular charge transfer (ICT) process for cation sensing [12]. ICT process is defined as an excited state where the conjugation of an electron-donating unit (such as –NH₂, –NMe₂, –OCH₃) to an electron-accepting (like >C=O, –CN) unit in one molecule is shown to rise a “push-pull” π- electron system [13], which have been widely applied for cation sensing. A blue shift in the absorption spectrum is observed when the electron donor interacts with the analyte, decreasing the electron-donating character. However, the analyte reacts with the electron acceptor part leading to a red shift due to a developed ICT (Figure 4). Furthermore, alterations in the fluorescence lifetimes and quantum yields are also detected. Thus far, many fluorescent imaging molecules are obtained from the ICT process by modifying either the electron donor, electron acceptor capacity, or π-conjugation degree of the fluorophores to react with the target analyte.

Figure 4.

Schematic representation of ICT chemosensor in the sensing process.

2.2.4 Excited-state intramolecular proton transfer

Excited-state intramolecular proton transfer (ESIPT) has been implemented to design fluorescent chemosensors due to their distinctive and exceptional spectral sensitivity to the environmental medium. ESIPT process is based on a proton transfer from a proton donor (hydroxyl or amino unit) to an acceptor unit (carbonyl oxygen or imine nitrogen) atom in the excited state of a fluorophore which is facilitated by an intramolecular hydrogen bond [20]. A graphical interpretation of the ESIPT process is illustrated in Figure 5.

Figure 5.

Schematic representation of excited-state intramolecular proton transfer system.

Due to the change between the enol and keto form in the system based on ESIPT, the photochemical reactivity of the excited molecules is drastically reduced, leading to improved photostability. Additionally, a significant Stokes shift is observed. Hence, the ESIPT process fits the design of fluorescent chemosensors that necessitates spectral shift for selective detection.

2.2.5 Fluorescence resonance energy transfer

Fluorescence Resonance Energy Transfer (FRET) is another type of fluorescence modulation. The FRET process is a transfer of energy between a pair of fluorophores that operate as energy donors and acceptors, respectively [21, 22]. The FRET depends on the interaction distance between the electronically excited states of two chromophores that the excitation energy is non-radiatively relocated from a donor to an acceptor by non-radiative dipole–dipole coupling (Figure 6). The vibrational transitions in the donor and the acceptor are approximately equivalent. If the FRET is operative in the molecules, they are typically applied to improve the stokes shift artificially. For sensing applications, the emission of the donor at relatively short wavelengths leads to triggering the acceptor emission at longer wavelengths with a ratio of the fluorescence intensities of the donor and acceptor emissions regulated by the target analytes. Elevated FRET efficacy is obtained when the extensive spectral overlaps with the donor emission and the acceptor absorption spectrum [23]. Several FRET-based fluorescent chemosensors were developed following the groundbreaking endeavors for the ratiometric detection of metal ions like Hg²⁺ [24], Zn²⁺ [25].

Figure 6.

Schematic representation of FRET chemosensor in the sensing process.

3. Applications of fluorescent chemosensors

Cyanide (CN⁻) and Fluoride (F⁻) play an essential role in life and are deemed one of very poisonous anions and harmful to human health. They are used in industrial activities like acrylic fiber manufacturing, metallurgy, electroplating, fibers synthesis, and gold extraction. Absorption by the lungs, exposure to the skin, food, and contaminated drinking water are all exposition methods to CN⁻ and F⁻ intoxication. Table 1 summarizes the World Health Organization (WHO) and European (EU) guidelines for anions in drinking water. The WHO set the provisional maximum tolerable daily intake limits to 0.07 mg/L and 1.5 mg/L for CN⁻ and F⁻, respectively, for drinking water. It is consequently of paramount importance to be able to establish their concentration in water at such low levels precisely. The detection of anions is generally effectuated via analytic techniques such as Ionic chromatography. However, these techniques are complex and expensive, where the need to develop new techniques for anions detection that are both accurate and low cost. Recently, fluorescent chemosensors have attracted considerable attention as a promising alternative for anions sensing.

Element	Symbol	WHO standards (mg/L)	EU standards (mg/L)
Cyanide	CN^−	0.07	0.05
Fluoride	F^−	1.5	1.5
Chloride	Cl^−	250	250
Hydrogen sulfate	HSO4^−	No guideline	No guideline
Bromine	Br^−	Not mentioned	0.01
Nitrate	NO3^−	50	50

Table 1.

WHO and EU drinking water standards for anions.

The fluorescent chemosensors have been used in numerous fields, such as biology, physiology, pharmacology, and environmental sciences. Fluorescent chemosensors to detect environmentally critical contaminants such as anions (CN⁻, F⁻) or cations (Hg⁺, Pb²⁺ …) have been widely investigated.

Keleş et al. reported the synthesis of 3,5-dinitro-(N-phenyl)benzamide (DNBA) and it is used as a simple colorimetric and fluorimetric chemosensor for selective determination of CN⁻ in organic (DMSO and ACN) and aqueous solutions (DMSO/H₂O: 8:2, v/v) [26]. The colorimetric and fluorimetric color intensity changed after adding CN due to the interaction between CN⁻ and DNBA. The UV/Vis and fluorescence spectrometry defined the interaction mechanisms between DNBA and CN⁻. Furthermore, spectroscopic results showed that CN⁻ interacts with three different mechanisms; deprotonation, nucleophilic aromatic substitution, and formation of benzisoxazole ring. Figure 7 displays the detection mechanism after adding CN⁻ in a DMSO/H₂O medium.

Figure 7.

CN⁻ detection mechanism reported by Keleş et al. [26].

Zheng et al. studied the use of naphthalimide-based (NIMS) as a fluorescent sensor for F⁻ [27]. The authors reported that F- detection occurs via a desilylation reaction (Figure 8), resulting in a colorimetric/fluorometric spectral response with a broad absorption around 229 nm and a change in color from yellow to blue. Furthermore, it was found that NIMS have high selectivity, rapid response, and sound sensitivity for F⁻. NIMS is a promising sensor for F⁻ detection and quantification in toothpaste.

Figure 8.

Mechanism of F⁻ detection using NIMS sensor.

Sourav Bej et al. reported the synthesis of an oxene-based chemosensor (HyMa) via Knoevenagel condensation to detect multi-analytes such as HSO₄⁻, CN^−, and F⁻ [28]. The authors reported a detection limit of around 38 ppb, 18 ppb, and 94 ppb for HSO₄⁻, CN⁻, and F⁻ respectively. Furthermore, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) displayed a low cytotoxicity and membrane permeability, making it an attractive material for bio-imaging. The mechanism of CN⁻ detection is given in Figure 9.

Figure 9.

The detection mechanism of CN⁻ by HyMa.

Deng et al. reported another chemosensor for the detection of CN⁻. TCNT fluorescent displaying an aggregation-induced emission was synthesized. A red fluorescent emission (596 nm) and significant stokes shift (148 nm) were observed, leading to an increased sensitivity to CN⁻ (detection limit = 0.38 μmol L⁻¹) [29]. The detection was due to a nucleophilic attack of CN⁻ on the vinyl group. Furthermore, the authors proved that TCNT could be used for bioimaging in live organisms.

A donor-π-acceptor structure was synthesized by Sun et al. for the CN⁻ detection (3TT). The detection of CN⁻ occurs via nucleophilic addition to the β-conjugated position of the barbituric acid of 3TT (Figure 10). The authors reported a naked-eye detection with high anti-interference ability and quick response. The detection limit was found to be 39.9 nM [24]. Furthermore, 3TT exhibited exceptional performance in the solid-state, and the 3TT-based test filter paper strips were applied to quickly and effectively detect CN⁻ in water with naked eyes. Likewise, the 3TT was effectively applied to rapidly detect CN⁻ in actual water samples, sour seeds, and food samples, proving its good ability for practical applications in our daily life.

Figure 10.

The detection mechanism of CN⁻ by 3TT.

Coumarin-spiropyran dyad having a hydrogen pyran moiety [2] was synthesized by [25]. The structure has an off–on type fluorescent receptor for detecting CN⁻. Because of the delocalization of π-electrons over the molecule, the receptor has nearly no fluorescence with a quantum yield <0.01. The nucleophilic addition of CN- to the spirocarbon of the molecule leads to a rapid opening of the spirocycle (Figure 11). Hence the localization of π-electrons on the coumarin moiety and a strong light-blue fluorescence occurs at 459 nm, and a high quantum yield (0.52) is obtained. The detection limit was low and equal to 1.0 μM.

Figure 11.

The detection mechanism of CN⁻.

4. Conclusions

Since Goppelsröder stated the first fluorescent chemosensors selective for Al³⁺, the chemosensors field has progressed extensively. Notably, the last 50 years witnessed massive development in the field of fluorometric chemosensors. Hence, numerous chemosensors have been designed by researchers over the last years. Chemosensors present a promising technology for detecting toxic anions and cations in an aqueous medium. This contribution gave a summary of the chemosensors’ chemistry and their applications. Finally, some examples of cyanide and fluoride, among other examples, detection were given.

Acknowledgments

This research was supported by Gazi University. This work was a part of Dr. Mohamed Yahya thesis of Reference No: 10451335.

Conflict of interest

The authors declare no conflict of interest.