**Love Wave Biosensors: A Review**

María Isabel Rocha Gaso, Yolanda Jiménez, Laurent A. Francis and Antonio Arnau

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53077

## **1. Introduction**

In the fields of analytical and physical chemistry, medical diagnostics and biotechnology there is an increasing demand of highly selective and sensitive analytical techniques which, optimally, allow an in real-time label-free monitoring with easy to use, reliable, miniatur‐ ized and low cost devices. Biosensors meet many of the above features which have led them to gain a place in the analytical bench top as alternative or complementary methods for rou‐ tine classical analysis. Different sensing technologies are being used for biosensors. Catego‐ rized by the transducer mechanism, optical and acoustic wave sensing technologies have emerged as very promising biosensors technologies. Optical sensing represents the most of‐ ten technology currently used in biosensors applications. Among others, Surface Plasmon Resonance (SPR) is probably one of the better known label-free optical techniques, being the main shortcoming of this method its high cost. Acoustic wave devices represent a cost-effec‐ tive alternative to these advanced optical approaches [1], since they combine their direct de‐ tection, simplicity in handling, real-time monitoring, good sensitivity and selectivity capabilities with a more reduced cost. The main challenges of the acoustic techniques re‐ main on the improvement of the sensitivity with the objective to reduce the limit of detec‐ tion (LOD), multi-analysis and multi-analyte detection (High-Throughput Screening systems-HTS), and integration capabilities.

Acoustic sensing has taken advantage of the progress made in the last decades in piezoelec‐ tric resonators for *radio-frequency* (rf) telecommunication technologies. The so-called gravi‐ metric technique [2], which is based on the change in the resonance frequency experimented by the resonator due to a mass attached on the sensor surface, has opened a great deal of applications in bio-chemical sensing in both gas and liquid media.

© 2013 Gaso et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Gaso et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Traditionally, the most commonly used acoustic wave biosensors were based on QCM devi‐ ces. This was primarily due to the fact that the QCM has been studied in detail for over 50 years and has become a mature, commercially available, robust and affordable technology [3, 4]. LW acoustic sensors have attracted a great deal of attention in the scientific communi‐ ty during the last two decades, due to its reported high sensitivity in liquid media compared to traditional QCM-based sensors. Nevertheless, there are still some issues to be further un‐ derstood, clarified and/or improved about this technology; mostly for biosensor applica‐ tions.

LW devices are able to operate at higher frequencies than traditional QCMs [5]; typical oper‐ ation frequencies are between 80-300 MHz. Higher frequencies lead, in principle, to higher sensitivity because the acoustic wave penetration depth into the adjacent media is reduced [6]. However, the increase in the operation frequency also results in an increased noise level, thus restricting the LOD. The LOD determines the minimum surface mass that can be de‐ tected. In this sense, the optimization of the read out and characterization system for these high frequency devices is a key aspect for improving the LOD [7].

Another important aspect of LW technology is the optimization of the fluidics, specially the flow cell. This is of extreme importance for reducing the noise and increasing the biosensor system stability; aspects that will contribute to improve the LOD.

The analysis and interpretation of the results obtained with LW biosensors must be deeper understood, since the acoustic signal presents a mixed contribution of changes in the mass and the viscoelasticity of the adsorbed layers due to interactions of the biomolecules. A bet‐ ter understanding of the transduction mechanism in LW sensors is a first step to advance in this issue; however its inherent complexity leads, in many cases, to frustration [8].

The fabrication process of the transducer, unlike in traditional QCM sensors, is another as‐ pect under investigation in LW technology, where features such as: substrate materials, sizes, structures and packaging must be still optimized.

This chapter aims to provide an updated insight in the mentioned topics focused on biosen‐ sors applications.
