Indicator–displacement assays

doi:10.1016/j.ccr.2006.04.009

Coordination Chemistry Reviews

Volume 250, Issues 23–24, December 2006, Pages 3118-3127

https://doi.org/10.1016/j.ccr.2006.04.009 Get rights and content

Abstract

Indicator displacement assays (IDAs) are now a popular method for converting most any synthetic receptor into an optical sensor. In this review many such assays are highlighted, along with biological counterparts. The focus is upon colorimetric, fluorescent, and metal containing IDAs. The power of the method can be readily appreciated by the large diversity of analytes that have been targeted with this technique. It is clear that the method is now well accepted and will continue to be one of many methods used to create optical detection methods from synthetic receptors.

Introduction

Traditionally, the most widely used approach for chemosensors is the indicator–spacer–receptor approach (ISR). In this approach, an indicator (chromophore or fluorophore) is covalently attached to a receptor through a spacer (Fig. 1). Commonly with organic structures, introduction of an analyte that binds to the receptor would induce measurable changes in fluorescence or absorbance. These measurements can be used to obtain binding constants and stoichiometries of binding [1].

Although it is the most popular, the ISR approach has limitations. The major limitation is that attachment of the indicator to the receptor may require difficult syntheses. An alternate approach that circumvents this problem is the indicator–displacement assay (IDA). Herein, we present a summary of examples, advantages, and applications of IDAs.

In an IDA, an indicator is first allowed to bind reversibly to a receptor. Then, a competitive analyte is introduced into the system causing the displacement of the indicator from the host, which in turn modulates an optical signal [2] (Fig. 2). Based on this principle, the major requirement for an IDA is that the affinity between the indicator and the receptor be comparable to that between the analyte and the receptor.

Signal modulation in an IDA is possible based on several mechanisms: photoinduced electron transfer (PET) [3], [4], fluorescence resonance energy transfer (FRET) [5], electronic energy transfer (EET) [6], [7], or simple changes in local ionic strength or pH [8]. The common interactions between the indicator or analyte and the host are H-bonding [9], [10], [11], [12], [13], [14], [15], electrostatic interactions [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], and complexing with metal centers [28], [29]. These interactions are dependent on the geometry of the guest, its charge, its hydrophobicity, and the solvent system [30].

The IDA offers many advantages over traditional sensing assays. First, the method does not require the indicator to be covalently attached to the receptor. Second, because there are no covalent bonds between the receptor and the indicator, one can employ several different indicators with the same receptor. Third, the assay works well in both organic and aqueous media, and lastly, the assay is easily adapted to different receptors and platforms for quick analysis [2]. In this review we classify three types of IDAs. The colorimetric IDA (C-IDA) which employs colorimetric indicators. The second class is the fluorescent IDA (F-IDA) which uses fluorescent indicators, and the third class is the metal complexing IDA (M-IDA) that utilizes a metal center with either a colorimetric or fluorescent indicator. Hence, an M-IDA is a subset of both a C-IDA and a F-IDA.

IDAs have been used to sense both cations and anions. However, the majority of IDAs have been for anions. Anions play fundamental roles in many phenomena, including biological processes such as the transport of hormones, proteins biosynthesis, DNA regulation, and the activity of enzymes [31]. Recognition or sensing of anions is a current goal of molecular recognition [1]. The important roles of anions have inspired chemists to devote significant efforts toward the designs of practical chemosensors for the detection of various anions, both qualitatively and quantitatively.

Section snippets

Colorimetric indicator–displacement assays

Naked-eye detection of various chemicals has been the inspiration for the development of C-IDAs. In a C-IDA, the indicator's color varies depending on whether it is free or bound to the receptor. Change in the color of the indicator modulates the optical signal, and thus, makes the detection of binding events possible. Prior to the development of this method by our group, there were few examples of C-IDAs in the literature. Two examples we took our lead from were the detection of acetylcholine

Fluorescent indicator–displacement assays

Similar to a C-IDA, in a F-IDA a fluorescent indicator is displaced from a receptor upon the introduction of an analyte. However, unlike a C-IDA, changes in emission of the indicator are measured instead of the absorbance. In general, F-IDA is more sensitive than C-IDA. It can potentially measure concentrations that are one million times smaller than can be determined by an absorbance method [4].

Metal complexing indicator–displacement assays

In a M-IDA, a metal is complexed with a receptor. Then, an indicator (chromophore or fluorophore) is allowed to coordinate with both the metal center and the receptor. Addition of an analyte to the system causes the displacement of the indicator from the metal and the receptor. This results in optical changes that can be measured to derive binding affinity. Zinc and copper have been most effectively used.

Conclusion

In summary, the breadth of the examples given above makes it clear that an IDA is a useful and facile technique for the creation of optical sensors. Receptors designed to exploit hydrogen-bonding, metal coordination, ion-pairing, and hydrophobic interactions have been ameanable to the use of an IDA. This tool provides scientists with an optical interrogation method for the study of many kinds of binding phenomena, followed by extension to a quantitative method. With the demand for accurate and

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ReviewIndicator–displacement assays

Abstract

Introduction

Section snippets

Colorimetric indicator–displacement assays

Fluorescent indicator–displacement assays

Metal complexing indicator–displacement assays

Conclusion

Tetrahedron

Tetrahedron Lett.

Tetrahedron

Curr. Opin. Struc. Biol.

Curr. Opin. Chem. Biol.

Tetrahedron Lett.

Thromb. Res.

FEBS Lett.

Tetrahedron Lett.

Anal. Biochem.

Avd. Protein Chem.

Bioorg. Med. Chem. Lett.

Tetrahedron

Tetrahedron

Tetrahedron Lett.

Molecular Recognition: Chemical and Biochemical Problems

Org. Lett.

Acc. Chem. Res.

Chem. Rev.

Prog. Inorg. Chem.

Transition Metals in Supramolecular Chemistry

Chem. Rev.

Anal. Chem.

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

J. Chem. Soc., Perkin Trans.

Chem. Commun.

Chem. Commun.

J. Org. Chem.

Helv. Chim. Acta

J. Org. Chem.

Chem. Commun.

J. Am. Chem. Soc.

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Acc. Chem. Res.

Angew. Chem. Int. Ed.

Pure Appl. Chem.

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Supramolecular Chemistry of Anions

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Pure Appl. Chem.

J. Org. Chem.

J. Am. Chem. Soc.

Biochemistry

Review
Indicator–displacement assays