This review presents a deep insight into the Surface Generated Acoustic Wave (SGAW) technology for biosensing applications, based on more than 40 years of technological and scientific developments. In the last 20 years, SGAWs have been attracting the attention of the biochemical scientific community, due to the fact that some of these devices - Shear Horizontal Surface Acoustic Wave (SH-SAW), Surface Transverse Wave (STW), Love Wave (LW), Flexural Plate Wave (FPW), Shear Horizontal Acoustic Plate Mode (SH-APM) and Layered Guided Acoustic Plate Mode (LG-APM) - have demonstrated a high sensitivity in the detection of biorelevant molecules in liquid media. In addition, complementary efforts to improve the sensing films have been done during these years. All these developments have been made with the aim of achieving, in a future, a highly sensitive, low cost, small size, multi-channel, portable, reliable and commercially established SGAW biosensor. A setup with these features could significantly contribute to future developments in the health, food and environmental industries. The second purpose of this work is to describe the state-of-the-art of SGAW biosensors for the detection of pathogens, being this topic an issue of extremely importance for the human health. Finally, the review discuses the commercial availability, trends and future challenges of the SGAW biosensors for such applications.

From the first applications of AT-cut quartz crystals as sensors in solutionsmore than 20 years ago, the so-called quartz crystal microbalance (QCM) sensor isbecoming into a good alternative analytical method in a great deal of applications such asbiosensors, analysis of biomolecular interactions, study of bacterial adhesion at specificinterfaces, pathogen and microorganism detection, study of polymer film-biomolecule orcell-substrate interactions, immunosensors and an extensive use in fluids and polymercharacterization and electrochemical applications among others. The appropriateevaluation of this analytical method requires recognizing the different steps involved andto be conscious of their importance and limitations. The first step involved in a QCMsystem is the accurate and appropriate characterization of the sensor in relation to thespecific application. The use of the piezoelectric sensor in contact with solutions stronglyaffects its behavior and appropriate electronic interfaces must be used for an adequatesensor characterization. Systems based on different principles and techniques have beenimplemented during the last 25 years. The interface selection for the specific application isimportant and its limitations must be known to be conscious of its suitability, and foravoiding the possible error propagation in the interpretation of results. This article presentsa comprehensive overview of the different techniques used for AT-cut quartz crystalmicrobalance in in-solution applications, which are based on the following principles:network or impedance analyzers, decay methods, oscillators and lock-in techniques. Theelectronic interfaces based on oscillators and phase-locked techniques are treated in detail,with the description of different configurations, since these techniques are the most used inapplications for detection of analytes in solutions, and in those where a fast sensorresponse is necessary.

This work deals with the design of a high sensitivity DNA sequence detector using a 50 MHz quartz crystal microbalance (QCM) electronic oscillator circuit. The oscillator circuitry is based on Miller topology, which is able to work in damping media. Calibration and experimental study of frequency noise are carried out, finding that the designed sensor has a resolution of 7.1 ng/cm² in dynamic conditions (with circulation of liquid). Then the oscillator is proved as DNA biosensor. Results show that the system is able to detect the presence of complementary target DNAs in a solution with high selectivity and sensitivity. DNA target concentrations higher of 50 ng/mL can be detected.

Introduction. Artifactual variations in the ST segment may lead to confusion with acute coronary syndromes. Objective. To evaluate how the technical characteristics of the recording mode may distort the ST segment. Material and Method. We made a series of electrocardiograms using different filter configurations in 45 asymptomatic patients. A spectral analysis of the electrocardiograms was made by discrete Fourier transforms, and an accurate recomposition of the ECG signal was obtained from the addition of successive harmonics. Digital high-pass filters of 0.05 and 0.5 Hz were used, and the resulting shapes were compared with the originals. Results. In 42 patients (93%) clinically significant alterations in ST segment level were detected. These changes were only seen in “real time mode” with high-pass filter of 0.5 Hz. Conclusions. Interpretation of the ST segment in “real time mode” should only be carried out using high-pass filters of 0.05 Hz.

A deep analysis of the problem associated with oscillators as interface circuits for quartz-crystal-microbalance sensors, reveals that the so-called static capacitance of the sensor is one of the elements that makes the use of oscillators more critical for sensors applications. A phase-locked-loop based circuit specifically designed for compensating the parallel capacitance effects in quartz crystal resonator sensors is presented. This circuit permits the calibration of the external circuitry to the sensor and an accurate determination of the effective capacitive compensation. The system provides a continuous measurement of the motional series resonant frequency and motional resistance. An extension and automation of the proposed system for multiple sensor characterization is introduced. The theoretical analysis of the circuit along with the experimental results presented prove that the proposed system is a good alternative for quartz sensors characterization.