Aptamer-based biosensors

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Abstract

Nucleic-acid aptamers have attracted intense interest and found wide applications in a range of areas. In this review, we summarize recent advances in the development of aptamer-based biosensors and bioassay methods, most of which have employed electrochemical, optical and mass-sensitive analytical techniques. Aptamers exhibit many advantages as recognition elements in biosensing when compared to traditional antibodies. They are small in size, chemically stable and cost effective. More importantly, aptamers offer remarkable flexibility and convenience in the design of their structures, which has led to novel biosensors that have exhibited high sensitivity and selectivity. Recently, the combination of aptamers with novel nanomaterials has significantly improved the performance of aptamer-based sensors, which we also review in this article. In view of the unprecedented advantages brought by aptamers, we expect aptamer-based biosensors to find broad applications in biomedical diagnostics, environmental monitoring and homeland security.

Introduction

Natural selection is one of the basic mechanisms of evolution, which was discovered by Darwin over 150 years ago. However, the importance of artificial, in-vitro selection was recognized much later.

Aptamers are an excellent example of functional molecules selected in vitro. In 1990, two groups independently developed in-vitro selection and amplification for the isolation of RNA sequences that could specifically bind to target molecules [1], [2]. These functional RNA oligonucleotides were then termed aptamers, derived from the Latin aptus, meaning “to fit” [3]. Later, DNA-based aptamers were also found [4].

Since its discovery, aptamer technology has received tremendous attention in scientific and industrial communities. After nearly 20 years’ endeavor, DNA and RNA aptamers have been identified as binding tightly to a broad range of targets (e.g., proteins, peptides, amino acids, drugs, metal ions and even whole cells), especially with the development of rapid, automated, selection technologies [5]. Aptamers often possess high affinity for their targets, which is derived from their capability of folding upon binding with their target molecule (i.e. they can either incorporate small molecules into their nucleic acid structure or be integrated into the structure of macromolecules (e.g., proteins [6])).

Aptamers have become increasingly important molecular tools for diagnostics and therapeutics. In particular, aptamer-based biosensors possess unprecedented advantages compared to biosensors using natural receptors such as antibodies and enzymes:

  • First, aptamers with high specificity and affinity can in principle be selected in vitro for any given target, ranging from small molecules to large proteins and even cells, thus making it possible to develop a wide range of aptamer-based biosensors.

  • Second, aptamers, once selected, can be synthesized with high reproducibility and purity from commercial sources. Also, in contrast to protein-based antibodies or enzymes, DNA aptamers are usually highly chemically stable.

  • Third, aptamers often undergo significant conformational changes upon target binding. This offers great flexibility in design of novel biosensors with high detection sensitivity and selectivity.

In recent years, in-depth understanding of nucleic-acid aptamers in terms of their conformational and ligand-binding properties has produced intense interest, and led to a range of bioassay methods that rely on aptamer receptors [7], [8], [9], [10]. In line with this trend, below we review recent research advances of aptamer-based sensors employing electrochemical, optical and mass-sensitive transducers.

Section snippets

Targets

Aptamers are also termed “chemical antibodies” because of their artificial process in vitro based on Systematic Evolution of Ligands by EXponential enrichment (SELEX) (Fig. 1). Unlike the preparation of antibodies, which relies on induction of an animal immune system, the SELEX process enables the fabrication of aptamers for non-immunogenic and toxic targets that it is otherwise impossible to obtain by the immune system [3]. Moreover, it is also possible to produce aptamers to specific regions

Assay configuration

Analogous to immunoassays based on the antigen-antibody interaction, aptamer-based bioassays can adopt different assay configurations to transduce bio-recognition events. Since aptamers have been selected to bind very different targets, ranging from small molecules to macromolecules, such as proteins, various assay configurations have been designed and reported. Nevertheless, the majority of these designs fall into two categories of configuration (Fig. 2):

  • single-site binding; and,

  • dual-site

Assay formats

Along with the rapid progress of modern analytical technologies and the application of novel analytical reagents (e.g., nanomaterial-based probes), more and more aptamer-based bioassay formats have been developed in recent years. Aptamer-based sensors (aptasensors) have attracted particular attention.

Apart from the inherent advantages of biosensors (e.g., no need for additional processing steps), aptamer-based biosensors offer the advantage of reusability over antibodies. Furthermore, their

Conclusion and trends

This review has presented an overview on recent advances in the development and the application of aptamer-based sensors. While such aptasensors emerged only about 10 years ago, they have already found broad applications in both basic research and biomedical diagnostics. A range of transducers (e.g., electrochemical, optical and mass-sensitive) have been employed in aptasensors. In particular, label-free sensing formats (e.g., SPR, QCM, SAW and micromechanical cantilevers) offer the promise of

Acknowledgement

This work was supported by National Natural Science Foundation (60537030 and 20725516), National Basic Research Program of China (2006CB933000, 2007CB936000) and Shanghai Municipal Commission for Science and Technology (0652nm006, 0652nm016, 06ZR14106 and 0752nm021).

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