Molecular sensor
[1][2][3][4] The action of a chemosensor, relies on an interaction occurring at the molecular level, usually involves the continuous monitoring of the activity of a chemical species in a given matrix such as solution, air, blood, tissue, waste effluents, drinking water, etc.
[5][6][7] The signalling is often optically based electromagnetic radiation, giving rise to changes in either (or both) the ultraviolet and visible absorption or the emission properties of the sensors.
[16] The development of molecular chemosensors as probes for such analytes is an annual multibillion-dollar business involving both small SMEs as well as large pharmaceutical and chemical companies.
Chemosensors were first used to describe the combination of a molecular recognition with some form of reporter so the presence of a guest can be observed (also referred to as the analyte, c.f.
The recognition/binding moiety is responsible for selectivity and efficient binding of the guest/analyte, which depend on ligand topology, characteristics of the target (ionic radius, size of molecule, chirality, charge, coordination number and hardness, etc.)
Optical signalling methods (such as fluorescence) are sensitive and selective, and provide a platform for real-time response, and local observation.
As chemosensors are designed to be both targeting (i.e. can recognize and bind a specific species) and sensitive to various concentration ranges, they can be used to observed real-live events on the cellular level.
As each molecule can give rise to a signal/readout, that can be selectively measured, chemosensors are often said to be non-invasive and consequently have attracted significant attentions for their applications within biological matter, such as within living cells.
[20][21] The design of ligands for the selective recognition of suitable guests such as metal cations[22] and anions[23][24] has been an important goal of supramolecular chemistry.
In the 1980s the development of chemosensing was achieved by Anthony W. Czarnik,[26][27][28] A. Prasanna de Silva[29][30][31] and Roger Tsien,[32][33][34] in the book Fluorescent Chemosensors for Ion and Molecule Recognition.
They focused on the analysis of various types of luminescent probes for ions and molecules in solutions and within biological cells, for real-time applications.
[35] Czarnik introduced the term ‘chemosensor’ to describe synthetic compounds capable of binding to analytes and providing a reversible signaling response.
The work of Lynn Sousa in the late 1970s, on the detection of alkali metal ions, possibly resulting in one of the first examples of the use of supramolecular chemistry in fluorescent sensing design,[37] as well as that of J.-M. Lehn, H. Bouas-Laurent and co-workers at Université Bordeaux I, France.
When the sensing event results in the formation of a signal that is visible to the naked eye, such sensors are normally referred to as colorimetric.
An extension to this approach is the development of molecular beacons, which are oligonucleotide hybridization probes based on fluorescence signalling where the recognition or the sensing event is communicated through enhancement or reduction in luminescence through the use of Förster resonance energy transfer (FRET) mechanism.
These are integrated directly or connected with a short covalent spacer depending on the mechanism involved in the signalling event.
[5] The so-called UT taste chip (University of Texas) is a prototype electronic tongue and combines supramolecular chemistry with charge-coupled devices based on silicon wafers and immobilized receptor molecules.
are designed so that the excited state of the fluorophore component of the chemosensor is quenched by an electron transfer when the sensor is not complexed to these ions.
By complexing the sensor with a cation, the conditions for electron transfer are altered so that the quenching process is blocked, and fluorescence emission is 'switched on'.
The driving force for PET is represented by ΔGET, the overall changes in the free energy for the electron transfer can be estimated using the Rehm-Weller equation.
[52][53] Chemosensors were one of the first examples of molecules that could result in switching between 'on' or 'off' states through the use of external stimuli and as such can be classed as synthetic molecular machine, to which the Nobel Prize in Chemistry was awarded to in 2016 to Jean-Pierre Sauvage, Fraser Stoddart and Bernard L. Feringa.
The application of these same design principles used in chemosensing also paved the way for the development of molecular logic gates mimics (MLGMs),[54][55] being first proposed using PET based fluorescent chemosensors by de Silva and co-workers in 1993.
The field has advanced from the development of simple logic systems based on a single chemical input to molecules capable of carrying out complex and sequential operations.
Other receptors are sensitive not to a specific molecule but to a molecular compound class, these chemosensors are used in array- (or microarray) based sensors.
Interaction of saxitoxin with the sensor's crown ether moiety kills its PET process towards the fluorophore and fluorescence is switched from off to on.
