Molecular sensor

Schematic representation of a chemosensor consisting of a signalling moiety and a recognition moiety that are connected together in some way that facilitates the communication between the two parts.

A molecular sensor or chemosensor is a molecular structure (organic or inorganic complexes) that is used for sensing of an analyte to produce a detectable change or a signal.[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. The application of chemosensors is referred to as chemosensing, which is a form of molecular recognition. All chemosensors are designed to contain a signalling moiety and a recognition moiety, that is connected either directly to each other or through a some kind of connector or a spacer.[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. Chemosensors may also be electrochemically based. Small molecule sensors are related to chemosensors. These are traditionally, however, considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry. Chemosensors are synthetic analogues of biosensors, the difference being that biosensors incorporate biological receptors such as antibodies, aptamers or large biopolymers.

Illustration of the common models used in sensor construction.

Chemosensors describes molecule of synthetic origin that signal the presence of matter or energy. A chemosensor can be considered as type of an analytical device. Chemosensors are used in everyday life and have been applied to various areas such as in chemistry, biochemistry, immunology, physiology, etc. and within medicine in general, such as in critical care analysis of blood samples. Chemosensors can be designed to detect/signal a single analyte or a mixture of such species in solution.[4][8][9][10][11] This can be achieved through either a single measurement or through the use of continuous monitoring. The signalling moiety acts as a signal transducer, converting the information (recognition event between the chemosensor and the analyte) into an optical response in a clear and reproducible manner.

Most commonly, the change (the signal) is observed by measuring the various physical properties of the chemosensor, such as the photo-physical properties seen in the absorption or emission, where different wavelengths of the electromagnetic spectrum are used.[12][13] Consequently, most chemosensors are described as being either colorimetric (ground state) or luminescent (excited state, fluorescent or phosphorescent). Colorimetric chemosensors give rise to changes in their absorption properties (recorded using ultraviolet–visible spectroscopy), such as in absorption intensity and wavelength or in chirality (using circularly polarized light, and CD spectroscopy).[14]

Receptor for selectively binding heparine Receptor for selectively binding tannines

Receptor for selectively binding Saxitoxin

Heparin binding Tannic acid binding Saxitoxin binding

In contrast, then in the case of luminescent chemosensors, the detection of an analyte, using fluorescence spectroscopy, gives rise to spectral changes in the fluorescence excitation or in the emission spectra, which are recorded using a fluorimeter.[15] Such changes can also occur in other excited state properties such as in the excited state life-time(s), quantum yield of fluorescence, and polarisation, etc. of the chemosensor. Fluorescence detection can be achieved at a low concentration (below ~ 10-6 M) with most fluorescence spectrometers. This offers the advantage of using the sensors directly within fibre optic systems. Examples of the use of chemosensors are to monitor blood content, drug concentrations, etc., as well as in environmental samples. Ions and molecules occur in abundance in biological and environmental systems where they are involved/effete biological and chemical processes.[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.

Left: Example of the change observed in the colorimetric azobenzene based chemosensor 1 in pH 7.4 solution upon recognition of copper ion. The recognition/sensing event being communicated as a clear change in colour that is visible to the naked eye. Right: The corresponding changes in the UV-visible absorption spectrum of the chemosensor upon recognition/binding to Cu(II) (shown in blue) and from the free sensor (shown in green). The changes after adding EDTA reverse the changes result in the formation of original spectra (shown in red).

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. above).[17] Chemosensors are designed to contain a signalling moiety and a molecular recognition moiety (also called the binding site or a receptor). Combining both of these components can be achieved in a number of ways, such as integrated, twisted or spaced. Chemosensors are consider as major component of the area of molecular diagnostics, within the discipline of supramolecular chemistry, which relies on molecular recognition. In terms of supramolecular chemistry, chemosensing is an example of host–guest chemistry, where the presence of a guest (the analyte) at the host site (the sensor) gives rise to recognition event (e.g. sensing) that can be monitored in real time. This requires the binding of the analyte to the receptor, using all kinds of binding interactions such as hydrogen bonding, dipole- and electrostatic interactions, solvophobic effect, metal chelation, etc. 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.) and the nature of the solvent (pH, ionic strength, polarity). Chemosensors are normally developed to be able to interact with the target species in reversible manner, which is a prerequisite for continuous monitoring.

One of the first examples of a fluorescent chemosensor developed for anion monitoring (phosphate) in competitive aqueous media. The chemosensors is not emissive in its 'free' form A, but upon recognition of the phosphate by the polyamine receptor moiety (through mixture of electrostatic and hydrogen bonding interactions) B, the fluorescence emission is gradually enhanced, resulting eventually in the formation of a highly fluorescent (host:guest) structure C.

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. Many examples of chemosensors have been developed for observing cellular function and properties, including monitoring ion flux concentrations and transports within cells such as Ca(II), Zn(II), Cu(II) and other physiologically important cations[18] and anions,[19] as well as biomolecules.[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. The term supramolecular analytical chemistry has recently been coined to describe the application of molecular sensors to analytical chemistry.[25] Small molecule sensors are related to chemosensors. However, these are traditionally considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry.

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