The
ubiquity of inorganic anions such as fluoride, chloride and phosphate in Nature,
their importance as food additives, agricultural fertilizers and industrial raw
materials, commands considerable attention of the scientific community. The
industrial and agricultural utilization of anions raises
a number of environmental concerns. These issues necessitate the development of
highly sensitive sensor materials. The sensors based on anion-induced changes in
fluorescence appear particularly attractive because they
offer the potential for high sensitivity at low analyte concentration.
There
are several reasons why reliable sensing of anions is a particularly challenging
area of research. Anions are larger than isoelectric cations and therefore have
lower charge-to-radius (surface) ratio, a feature that makes the electrostatic
binding of anions to the receptors less effective. Anions have a wide range of
geometries and are often present in delocalized forms, which results in higher
design complexity of receptors and sensors required for successful recognition
and binding. These and other factors make sensing of anions difficult task.
In our research we focus on the development new compounds and materials that can easily be used to achieve signal amplification in the simple fluorescent anion sensor. This is very important point, because the signal amplification in sensors is expected to yield more sensitive sensors. The first approach to amplified sensing is based on attaching an additional chromophore (donor) capable of resonance energy transfer (RET) to the parent sensor (acceptor). The second approach is to attach moieties to the parent sensor that allow excited state delocalization in the conjugated system.
Both approaches to improved signal transduction are illustrated on 2,3-di(1H-2-pyrrolyl)quinoxaline sensor (DPQ, 1) (Sessler et al, JACS, 1999, 121, 10438). We decided to utilize the DPQ platform and synthesize several compounds and materials that utilize FRET (for example compound 2) and exciton delocalization (compound 3) for amplification of the sensing outcome. The time-resolved fluorescence spectroscopy and quantum yield measurement confirm an efficient FRET (ca 85%) from the pyrene to DPQ moiety (Figure 2). The effective excited state delocalization in pyrene-acetylene-DPQ moieties is confirmed both by the 60 nm red shift of the emission wavelength and twofold increase in the fluorescence lifetime in sensor 3 compared to the parent compound 1. Here is how it works:
You can find more information in the following article: Pohl, R.; Aldakov, D.; Kubát, P.; Jursíková, K.; Marquez, M.; Anzenbacher, Jr., P.: Strategies Toward Improving the Performance of Fluorescence-Based Sensors for Inorganic Anions. Chem. Commun. 2004, 1282-1283. DOI: 10.1039/b315268e
In last few years, we investigated the role of Intramolecular Charge Transfer on sensing of ionic species, namely anions. The mail principle is explained on the following figure: Binding of an anion in an electron-rich receptor results in partial charge transfer from the receptor into the substituents in its vicinity (a feature sometimes called "push"). Presence of suitable acceptor of the overflow electronic density (usually an electron-withdrawing moiety - a "pull" structural feature) could then establish a special synergy that channels electron density over relatively long range (10 angstrom and more). This intramolecular partial charge transfer (ICT) in the so-called "push-pull" chromophores can give rise to dramatic color changes that may be utilized in molecular sensing. Example of successful chromophores-sensors are shown here: DOI: 10.1021/ol0521782.
The sensor molecules and materials made by our group utilize the calix[4]pyrrole receptor shown below. Because pyrrole is electron rich and formation of the complex shown schematically below with anion makes the pyrrole moieties even richer, this compound lends itself to structural modification to establish ICT-based sensors (DOI: 10.1021/ja051421p). The X-ray structure of calixpyrrole-anion complex is here: DOI: 10.1021/ja960307r.
The ICT-calix[4]pyrrole sensors show analyte-specific color change in the presence of various anions (DOI: 10.1021/ja051421p, DOI: 10.1021/ol0521782 ). The next photograph shows several vial with a solution of an ICT-calixpyrrole sensor prior to, and after the addition of anions. The color changes are easy to notice by a naked eye.
Because chemical sensing that requires liquid sensors is not too practical, we fabricate sensor chips comprised of numerous wells (1.00 mm in diameter, 0.25 mm deep), into which the sensors are applied in a suitable solid matrix to form a thin film of sensor in each well. Application of a drop of a liquid analyte (0.20 - 1.00 microliter of water, buffer, etc.) results in a change in color of the sensor film. The change in color may be recorded and mathematically analyzed to identify the analyte in question (DOI: 10.1021/ja0704784). More on the topic of sensors, sensor chips, and chemical analysis that utilizes pattern recognition in array-based sensors can be found here.
The
anion-sensing studies confirmed, that this method is fairly general, and may be
successfully applied to analysis of various anionic species including drugs and
toothpastes.