Designing Receptors for the Next Generation of Biosensors

Nucleotides are amongst the most targeted anionic species for artificial host systems, because they are ubiquitously present in biological systems in which they exercise key roles in many cellular functions and thus offer interesting structural features for the benchmarking of designed host systems in supramolecular chemistry (Voet et al., Fundamentals of biochemistry. Wiley-VCH, Weinheim, 2002). Nucleotides are, for example, involved in DNA synthesis, energy, and electron transfer events (Nath, Bioenerg Biomembr 42:301–309, 2010), cell signaling (Riedl and Salvesen, Nat Rev Mol Cell Biol 8:405–413, 2007), or membrane transport (Tojima et al., Nat Rev Neurosci 12:191–203, 2011). These are complex events which require the molecular recognition of a specific nucleotide. In order to increase our insight into these processes the development of model systems for nucleotide recognition is desirable.

Many physiological functions of life are controlled by the interplay between natural or synthetic agents with their corresponding receptors in the human body (Highlights in bioorganic chemistry: methods and applications. Wiley-VCH, Weinheim, 2000). Such molecular recognition events are based on the combination of many weak attractive non-covalent interactions between receptor and substrate, such as electrostatic, dipole, and dispersion interactions, π-stacking, and hydrogen bonding, together with entropic contributions, as, for example, the liberation of solvent molecules (Core concepts in supramolecular chemistry and nanochemistry. Wiley-VCH, West Sussex, 2007; Angew Chem Int Ed 46:2366–2393, 2007). To gain a more detailed insight into these complex processes, which are currently not entirely understood in all their complexity, the development of synthetic model systems is worthwhile. Within this chapter, selected illustrative examples of artificial receptors for biologically relevant targets such as oligopeptides or protein surfaces will be presented. A directed molecular recognition of these building blocks of life will lead to an increase in knowledge concerning the complex recognition processes taking place within the human body and in the best case will allow for biological processes to be directly targeted and can thus be used for future applications in analytical, biological, or medicinal chemistry.

Considered gold standards for biodetection, immunoassays and nucleic acid-based assays are sensitive, highly selective, and well characterized. However, they are capable of detecting only those targets for which specific reagents (such as antibodies or nucleic acid primers or probes) have been developed. Furthermore, new, emerging, and unexpected pathogens may not be detected. To address the challenge of detecting both known and unknown microbes, assays utilizing antimicrobial peptides (AMPs) are being developed for integration into both biosensors and high-throughput platforms. AMP-based detection represents a new paradigm in sensing—namely, the ability to screen a sample for the presence of many different microbes without target-specific reagents, and to provide broad classification information on the species detected.

Molecular recognition plays an important role in diagnostics, catalysis, separation, and drug development. For years these tasks have been entrusted to antibodies, but recently some viable alternatives are starting to emerge. This review provides a brief overview of natural and synthetic recognition systems, highlighting their advantages and drawbacks. Examples of synthetic receptors include aptamers, engineered proteins, and molecularly imprinted polymers (MIPs). The focus of the present work is on the development and application of a novel class of synthetic receptors—MIP nanoparticles (“plastic antibodies”).

Molecularly imprinted polymers are mostly confined to laboratories and their standardized environments. Chemical sensors based on MIP are no exception to this; however, there are increasing efforts to span the gap toward technological applications and thus exposing the devices to real-life environments and thereby assessing selectivity, sensitivity, and ruggedness of the respective sensors. In some application areas this has already been successful, namely in detecting volatile organics and their mixtures, sensing pesticides in environmental water samples, in assessing oxidation processes, e.g., in engine oils, and in some applications of bioanalysis targeting both signaling molecules/drugs and whole cells, viruses, or bacteria. Here, we summarize the selected aspects for transferring MIP strategies out from lab-bench conditions and highlight some of the successful examples.

In this study, a new method to imprint molecular recognition sites into Au nanoparticles (NPs) composites is described. The method includes the electropolymerization of thioaniline-functionalized Au NPs in the presence of imprint substrates that exhibit affinity interactions with the thioaniline-functionalized Au NPs or with a co-added ligand associated with the electropolymerizable NPs. Exclusion of the imprint substrate from the composite leads to the formation of selective imprinted sites in the Au NPs matrices. The imprinted matrices are implemented for the sensing of explosives, herbicides, saccharides, and ions. π-Donor–acceptor interactions, ionic interactions and H-bonds, or ligand–substrate interactions are used to generate the imprinted sites. The coupling between the localized plasmon of the NPs and the surface plasmon wave of the support is used to amplify the dielectric changes occurring in the NPs matrices upon the binding of the analytes to the imprinted sites, thus enabling the surface plasmon resonance (SPR) transduction of the sensing events. The imprinted Au NPs matrices demonstrate highly selective, stereoselective, and chiroselective sensing performance.

We have successfully completed the development of a glucose sensing system, which is the mission-critical component of an implantable glucose sensor for use by diabetic patients. This proof-of-principle demonstration showed that the closed-cycle, self-contained glucose sensing system can produce a consistent, measurable response to physiologically relevant levels of glucose while functioning under biologically relevant conditions. The sensing system requires the interaction of two components: (1) a competitive agent/signaling component, which is a dendrimer-boronic acid (DBA) construct and (2) a glucose-competitive DBA binding environment, which is an immobilized monosaccharide mimic (iDIOL). The demonstrated sensing system meets our primary stability, sensitivity, and specificity criteria. These results, accompanied by our library of synthetic materials and binding affinity database, provide a firm foundation upon which to optimize the glucose sensing system and incorporate it into the implantable sensor device.

Nature elegantly invokes transition metal ions in a number of fundamental biomolecular processes from guiding protein folding and scaffolding tertiary structures to essential roles in information transmission through ligand binding, electron transfer, and catalysis. Often inspired by such natural systems, innovative transition metal-modified receptors are beginning to emerge with similar functions that hold substantial promise as components of next generation biosensors. This chapter aims to highlight some recent advances in the development of transition metal-modified synthetic receptors. Specifically, systems that incorporate organometallic, monometallic, and supramolecular coordination complexes are reviewed. The text will cover transition metal scaffolded peptide receptors, allosteric supramolecular enzyme mimics, and integrated monolayer-based electroactive receptor systems.