Sensors and Actuators A 184 (2012) 112–118 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical j ourna l h o me pa ge: www.elsevier.com/locate/sna Electronic driver with amplitude and quality factor control to adjust the response of quartz tuning fork sensors in atomic force microscopy applications Laura González, Jorge Otero, Gonzalo Cabezas, Manel Puig-Vidal ∗ SIC-BIO, Bioelectronics and Nanobioengineering Group, Department of Electronics, University of Barcelona, Marti i Franques, 1, 08028, Barcelona, Spain a r t i c l e i n f o Article history: Received 20 January 2012 Received in revised form 19 June 2012 Accepted 19 June 2012 Available online 4 July 2012 Keywords: Atomic force microscopy Quartz tuning fork Q control Self-sensing probe a b s t r a c t Quartz tuning fork-based sensors are self-sensing probes that are gaining popularity in atomic force microscopy. They do not need a laser-photodiode system and they have a higher quality factor in liquid than standard cantilevers. However, the main limitation of tuning fork probes is that they are usually handmade because no commercial probes suitable for a wide range of experiments are available. The handmade devices show considerable variation in dynamic response, thereby hindering the repeatabil- ity of experiments. To overcome this problem, here we develop an electronic driver to simultaneously control the quality factor (Q) and the oscillation amplitude (A) of the device. The driver provides clear advantages over classical Q-control modules where the amplitude of oscillation is modified when the Q factor is changed. Direct measurements on a commercial interferometer showed that the effective Q factor can be adjusted to experimental requirements while maintaining the mechanical amplitude of oscillation constant. Experimental amplitude vs. distance curves confirm that our driver achieves an equivalent dynamic response from distinct handmade sensors (with varying mechanical characteristics) by means of electronic adjustment. The driver is a simple but effective method to ensure the same mea- surement conditions with a range of quartz tuning fork probes and also the reliability and repeatability of experiments. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Atomic force microscopy (AFM) is widely used to image sam- ple surfaces with high accuracy [1,2]. However, despite the wide application of this technique in air, the imaging of soft biological samples in liquid presents several difficulties. Imaging biological samples, such as biomolecules, requires the maintenance of a con- stant, very small interaction force between the tip and the sample surface. Commercial AFM sensors present a very low quality factor (Q) when samples are immersed in liquid and they are very difficult to integrate into multiprobe systems [3]. In contrast, quartz tuning fork (QTF)-based sensors have the capacity to image the topogra- phy of samples with sub-nm resolution [4]. QTFs are self-sensing probes, so they are suitable for use in a multiprobe station [5] work- ing with biological samples [6]. QTF sensors are resonators based on the piezoelectric properties of quartz. Several attempts have been made to study QTFs for their use in scanning probe microscopy [7]. These devices present a very stable resonance frequency and a very narrow band. As they present a higher Q (1000–5000) than standard cantilevers, QTFs show high performance in experiments performed in a liquid environment. For instance, differences of ∗ Corresponding author. Tel.: +34 934039163; fax: +34 934021148. E-mail address: manel.puig@ub.edu (M. Puig-Vidal). 110 nm have been reported in cytospin membrane topography [8] when the cell is in air or immersed in liquid, red blood cell morphology has been accurately measured [9], and recognition events between avidin/biotin and lysozyme/anti-lysozyme have been detected in the phase image at a high signal-to-noise ratio [10]. One of the main drawbacks of QTF sensors is that there are no commercial devices suitable for working in liquid media. Thus these nanotools are custom-made and because of the manufactur- ing process it is very difficult to obtain repeatable probes with a similar response. Given that the lack of repeatability is usually the main source of error in quantitative measurements, it is common to use more than one QTF sensor in the same study because the tip can become contaminated, damaged or even broken. When per- forming an experiment, it is crucial to ensure the same dynamic response of the sensor in order to maintain the conditions in which the measurements are conducted. To overcome the differences between sensors, here we develop a module to control the quality factor (Q) and vibration amplitude (A). The quality factor is controlled analogically and the ampli- tude vibration digitally (with an algorithm). The control module is integrated with the QTF and drives the interface circuitry, which was designed to be transparent to commercial atomic force micro- scopes. All the experiments presented in this study were performed on a commercial Cervantes atomic force microscope (Nanotec 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.06.016