BODIPY derivatives possess unique photophysical properties and for these reasons, they have been used in numerous fields. Among the different applications, they are used in designing chemosensors that has increased in the last years. Here, we report several strategies and examples for detecting analytes of different characteristics: cations, anions, and hazardous and pollutant neutral molecules using BODIPY core as signaling unit.
- neutral molecules
Supramolecular chemistry has become a coherent and alive body of concepts which has recently incorporated new areas of research [1, 2, 3, 4]. The “classical” supramolecular chemistry has developed basic tools and concepts such as coordination of specific substrates to receptor (recognition), chemical reactivity induced by the guests (transformation), and positional controlled changes of atoms or molecules (translocation). On the other hand, another promising area of investigation is the development of “programmed supramolecular systems,” where the recognition process is coupled with a specific action.
Among these programmed systems of supramolecular background the so-called molecular chemical sensors, where the process of recognition is adapted to a process of detection, are of wide interest. The described behavior is achieved by means of the introduction in the ligand (or reactive site) of transducing units which are capable of transmitting information on the molecular recognition process through a change in its physical properties (e.g., optical or electrochemical).
There are three classical approaches for the development of chromogenic-fluorogenic sensors:
Binding site-signaling unit approach: In this approach, the receptor should contain two different subunits kept together by means of a covalent bond. One of such subunits is responsible for the complexation process, while the other transmits the molecular recognition process . As it can be seen in Figure 1, the coordination of the guest induces a change of some properties of the signaling unit, that is, color (chromogenic chemosensors) or fluorescence (fluorescent chemosensors).
Displacement approach: This approach, as well as the former, implies the use of both, specific binding sites and signaling units. However, in this case, both subunits are not covalently linked, but forming a coordination ensemble . In these systems, the addition of a given guest to the solution that contains this “molecular ensemble” favors the displacement reaction: the coordinating unit binds the guest, while the signaling unit is released toward the solution. If this unit shows different optical properties (color or fluorescence) depending on whether it is coordinated or in solution, its release causes a change of the signal. All these systems are based on the use of molecular receptors possessing coordination sites with size and charge distribution suitable to those of the guest (Figure 2).
Chemodosimeter approach: This approach involves the use of specific chemical reactions (generally irreversible) induced by the presence of certain guests that are coupled to a change of color or fluorescent emission [7, 8]. If the reaction is irreversible, the term sensor should not be strictly used, a more appropriate term should be a chromo or fluorogenic reagent or chemodosimeter. Figure 3 shows a scheme of this approach. The ultimate idea is to take advantage of the selective reactivity that determined guests present. The use of reactions induced by determined chemical species has the advantage of presenting a high selectivity and a cumulative effect that is directly related to the concentration.
Depending on the physical property of the signaling unit that changes in the process of complexation, one can readily have systems of different types, that is, electrochemical, fluorescent, colorimetric, etc. Among the different possibilities, fluorescent and colorimetric systems are very interesting due to their high sensibility and the advantage of a possible detection of species of interest to the “naked eye.”
Among the dyes or fluorophores that can be used as signaling unit, the BODIPY core presents several advantages due to its outstanding photophysical properties such as excitation/emission wavelengths in the visible spectral region (≈500 nm), the relatively high molar absorption coefficients and fluorescence quantum yields, fluorescence lifetimes in the nanosecond range, and negligible triplet-state formation. On the other hand, they are relatively insensitive to pH and present good solubility, resistance toward self-aggregation in solution and robustness against light and chemicals [9, 10]. Moreover, the spectroscopic and photophysical profiles can be switched by introducing different electron releasing/withdrawing groups at the appropriate positions of the BODIPY core. Additionally, they usually show good biocompatibility that makes them useful for biological applications.
2. Fluoro- and chromogenic chemosensors and chemodosimeter based on the BODIPY derivatives
2.1. Sensors based on the binding site-signaling unit approach
Among the different substitutions in the BODIPY core, structures like these shown in Figure 4 have been widely used in probe design. There are two main reasons for this selection: (a) the presence of the methyl substituents at 1 and 7 positions of the BODIPY core hinders the free rotation or the phenyl group at 8 which enhances the fluorescence emission and (b) the substitution at 5 or 6 in structure (II) and (III) enlarges electronic delocalization giving rise to possible color changes after the interaction with the analyte.
Many examples of fluorescent sensors based on this type of BODIPY structures were summarized in 2012 by Boens, Leen, Dehaen . For this reason, in the present chapter, only more recent publications will be considered.
In the field of alkaline cation sensors, compounds
Complexation studies of
In relation to heavy metal cations, compound
The sensing properties of
Zinc and cadmium are both elements that play many important roles in our daily life. Zinc is the second most abundant transition metal in the human body, and it is vital for the functions of a large number of enzymes, the stabilization of DNA, gene expression, and neural signal transmission. By contrast, cadmium is a dangerous poison that harms human health and the environment. Two BODIPY-based sensors (Figure 7) able to differentiate these two cations have been described .
The photophysical properties of compound
In contrast, the signaling unit and binding site in probe
2.2. Sensors based on the displacement approach
Most of the sensors following the displacement approach are based on the complexes that in the presence of the analyte, undergoes a decomplexation process that induces strong changes in the optical properties of the system. In some cases, the fluorescence of BODIPY-based compounds is quenched when a complex with the appropriate metal ion is formed. Decomplexation induced by the analyte recovers the ligand fluorescence that can be observed. This approach allows preparing off-on fluorescent chemosensors.
In that sense, compound
Based also on Cu2+ complexes, compound
Based also on the displacement approach, two complexes able to detect the V-nerve agent mimic demeton-S have been described . Acetonitrile solutions of
The behavior of
2.3. Sensors based on the chemodosimeter approach
Due to the selectivity showed by the probes designed following the chemodosimeter approach, there are a large number of applications for detecting different species.
2.3.1. Detection of anions
Following an approach that combines the chemodosimeter and the displacement mechanism, compound
Also, in relation to detecting biothiol compound
2.3.2. Detection of neutral molecules
There are a large number of neutral compounds whose detection has been developed using BODIPY-based chemodosimeters [21, 22, 23, 24, 25, 26, 27, 28]. In this chapter, there are summarized some probes used in detecting dangerous or strongly pollutant analytes.
126.96.36.199. Nerve agents
The sensing units in these compounds were based on the 2-(2-dimethylaminophenyl)ethanol moiety. This moiety has two nucleophilic groups, a dimethylamino group and a primary alcohol (
Acetonitrile solutions of
On the other hand, the sensing unit of compound
Due to the probe structure, compounds
The chromogenic behavior of the acetonitrile solutions of probe
The different chromogenic responses observed upon the addition of DCNP and DFP to
Oximates have been used from the beginning in designing chemosensors for detecting nerve agents and their simulants. Following this idea, compound
Emission spectrum of
188.8.131.52. Pollutant gases
Nitrogen oxides are very dangerous contaminants source of severe environmental problems such as acid rain, smog formation, global warming, and ozone layer weakening. Among these compounds, NO2 is one of the most prevalent and dangerous. Due to the ubiquitous presence of this gas and its health effects, the development of selective and sensitive methods for its detection and quantification has aroused a lot of interest. Thus, compounds
The BODIPY core has been successfully used in the designing of chemosensors following the three more commonly used approaches: binding site-signaling unit, displacement, and chemodosimeter. Depending on the position of the reactive unit in the BODIPY core, chromogenic or fluorescent responses were achieved. In many cases, the analyte induced changes can be observed by the naked eye. Cations, anions, and neutral molecules can be detected in different media: organic or aqueous. The biocompatibility of many of these compounds allows their use in biological applications.
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