Review A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions Matthew A. Cooper * and Victoria T. Singleton Akubio Ltd., 181 Cambridge Science Park, Cambridge CB4 0GJ, United Kingdom, UK The widespread exploitation of biosensors in the analysis of molecular recognition has its origins in the mid-1990s following the release of commercial systems based on surface plasmon resonance (SPR). More recently, platforms based on piezoelectric acoustic sensors (principally ‘bulk acoustic wave’ (BAW), ‘thickness shear mode’ (TSM) sensors or ‘quartz crystal microbalances’ (QCM)), have been released that are driving the publication of a large number of papers analysing binding specificities, affinities, kinetics and conformational changes associated with a molecular recognition event. This article highlights salient theoretical and practical aspects of the technologies that underpin acoustic analysis, then reviews exemplary papers in key application areas involving small molecular weight ligands, carbohydrates, proteins, nucleic acids, viruses, bacteria, cells and lipidic and polymeric interfaces. Key differentiators between optical and acoustic sensing modalities are also reviewed. Copyright # 2007 John Wiley & Sons, Ltd. Keywords: acoustic; affinity; label-free; kinetics; quartz crystal microbalance; QCM; piezoelectric; biosensor; review Revised 15 May 2007; Accepted 15 May 2007 INTRODUCTION To analyse a molecular interaction without the use of reporter labels, it is necessary to couple a molecular recog- nition element (e.g. an antibody or target receptor) to a transducer that converts a chemical or biological interaction into an electrical signal. A biosensor is thus defined as a unique combination of a receptor for molecular recognition and a transducer for transmitting the interaction information into an electrical signal. In turn, a transducer is more specifically defined as a device for converting energy from one form to another for the purpose of measurement of a physical quantity or for information transfer. In theory, there are several different physical phenomena that can be exploited in biosensing: acoustic waves, thermal capacity and heat transfer, photons, neutrons, ions, radioactive parti- cles, electrons, electric fields and magnetic fields. In prac- tice, most analytical platforms in routine use today use optics, with only a few of the more novel transducer moda- lities having transitioned from academic instrument proto- types to robust commercial platforms (Myszka and Rich, 2000; Cooper, 2002; Rich and Myszka, 2006; Rich and Myszka, 2007). However, over the last 5–10 years, there has been an increased recognition of the versatility and broad applicability of acoustic biosensors for the analysis of molecular recognition and associated phenomena (Figure 1). In the period 2001–2005 there were a total of 1404 publications referencing ‘quartz crystal microbalance’ or ‘QCM’ in the Web of Science 1 database, of which 569 specifically involved molecular recognition studies. The number of publications per year grew steadily in this period, from 241 in 2001, to 369 in 2005. The distribution of application areas defined by the class of analyte was generally evenly spread between life sciences research and diagnostic assay development, although interactions invol- ving small molecules, immunoassays, lipids, oligonucleo- tides and polymer coatings were predominant (Figure 1). Acoustic biosensors have been employed in the label-free detection of an incredibly broad range of analytes; from interfacial chemistries and lipid membranes to small mole- cules and whole cells (Marx, 2003). They provide a unique method for observing in situ events involving ‘soft matter’, in which changes in contact mechanics, interfacial dynamics, surface roughness, viscoelasticity, density and mass can be monitored in real time (Thompson and Hayward, 1997; Janshoff et al., 2000; Cooper, 2003, 2006; Cote et al., 2003; Hook and Kasemo, 2006; Steinem and Janshoff, 2006). This level of information content goes beyond that produced by many more widely adopted optical techniques that are used to analyse molecular changes occurring in biochemical processes. Acoustic sensor technology is thus highly inter- disciplinary and has encompassed advances and improve- ments from electrical engineers to cell biologists. This review seeks to impart an awareness of the theory and biological relevance of acoustic physics (in particular for BAW,TSM or QCM sensors; Figure 2), with a focus on the application areas of greatest relevance to biological and biochemical research. We examine published applications of QCM to chemical and biochemical systems over the past JOURNAL OF MOLECULAR RECOGNITION J. Mol. Recognit. 2007; 20: 154–184 Published online in Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/jmr.826 *Correspondence to: M. A. Cooper, Akubio Ltd., 181 Cambridge Science Park, Cambridge CB4 0GJ, UK. E-mail: mcooper@akubio.com Copyright # 2007 John Wiley & Sons, Ltd.