Bioactive paper: Biomolecule immobilization methods and applications in environmental monitoring
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Introduction Low-cost, fast, portable, sensitive, and selective devices are essential to improve point-of-care diagnostics and on-site environmental monitoring.1 Paper is an attractive substrate for constructing such devices as (1) it is extremely cheap and commercially available; (2) liquids can be transported by capillary forces, thus it does not require external power; (3) it is easily patterned; and (4) it consists of porous cellulosic fiber networks that make it biofriendly (see the Introductory article in this issue). Biological entities (enzymes, antibodies, bacteriophages—viruses that infect bacteria, and aptamers— single stranded oligonucleic acids that bind to specific targets) are known to be powerful recognition elements for the construction of sensors, as they naturally exhibit high selectivity for target analytes. Therefore construction of bioactive paper (i.e., paper-based material that incorporates biologically active entities) should lead to extremely efficient, cheap sensors. At present, lateral flow immunoassays based on nitrocellulose (e.g., pregnancy test kit) are one of the only commercially available rapid testing technologies available, but these are generally not quantitative and can detect only one analyte at a time. Recently, a global research effort has started involving the use of cellulose substrates for production of bioactive paper sensors.2 Aspects such as control of liquid flow in paper, development of multianalyte detection capability, new methods
for signal generation, and improved methods for biomolecule immobilization have received renewed attention. In this review, we focus on techniques for the immobilization of biomolecules (enzymes, antibodies, bacteriophages, and aptamers) onto cellulosic paper substrates and provide a few examples of their application for on-site environmental monitoring. The aim of this review is to outline current methods for immobilizing such biomolecules in a manner that is compatible with the requirements for large-scale printing, and to highlight remaining challenges. For a more extensive description, readers should refer to the review written by Pelton.3
Biomolecule immobilization on paper Immobilization is performed with the goal of retaining active biomolecules in a defined region with maximal density and minimal leaching. In the case of paper-based sensors, an added requirement is the need for immobilization by a printing method that allows patterning of biomolecules. Biomolecule immobilization on paper is normally done by one of four different methods: physical immobilization, covalent and affinity-based attachment, immobilization on carriers, and encapsulation.
Physical immobilization Physical immobilization occurs when biomolecules spontaneously adhere onto the surface. The process is driven mainly
Clémence Sicard, Department of Chemistry and Chemical Biology, McMaster University; [email protected] John D. Brennan, Department of Chemistry and Chemical Biology, McMaster University; [email protected] DOI: 10.1557/mrs.2013.61
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