Ultramicroscopy 86 (2001) 167–173 Chemical sensors and biosensors in liquid environment based on microcantilevers with amplified quality factor J. Tamayo*, A.D.L. Humphris, A.M. Malloy, M.J. Miles H.H. Wills Physics Laboratory, University of Bristol, Royal Fort, Tyndall Avenue, Bristol BS8 1TL, UK Received 27 July 2000 Abstract A new technique is presented for bio/chemical sensors, based on microcantilevers, for detection in liquid environment. The low quality factor of the cantilever in liquid is increased up to three orders of magnitude by using Q-control. This enables AC detection that is immune to the long-term drift of the DC cantilever response in liquids, and to temperature variations. This technique has been applied for the detection of ethanol in aqueous solution by using the microbalance method, and for antibody/antigen recognition by the surface stress method. The results show the feasibility and very high sensitivity of these novel devices. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.79.L; 07.07.D; 87.80 Keywords: Atomic force microscopy; Chemical sensor; Biosensor 1. Introduction A chemical sensor is an analytical device that combines the specificity of a sensing surface to a target substance with a transducer that produces a signal proportional to the target concentration [1]. In the biosensor devices, the target is a biological substance, and the sensing surface is composed of its specific receptor biomolecule. In a similar way, in an atomic force microscope (AFM), a micro- meter-sized lever (transducer) bends as a conse- quence of the interatomic force between a very sharp tip (sensing surface) and a sample (target) [2]. The cantilever deflection is measured with sub- angstrom resolution by a detector that typically consists of a laser beam reflecting off the cantilever into a segmented photodiode. Thus, forces of the order of 10 pN can be measured. AFM provides the topography of organic and inorganic surfaces at atomic and nanometer scale in air, ultra-high vacuum and in liquids. The large expansion of AFM techniques has widely benefited from the commercialization of microfabricated wafers com- posed of cantilevers of silicon or silicon nitride. This has brought about the birth of a new class of chemical sensors and biosensors based on micro- cantilevers [3–13]. In sensors based on microcantilevers, the cantilever is coated with a sensing film or receptor molecules. As a gaseous or liquid sample solution is flowed over the cantilever, the target molecules attach to the sensitized surface. The cantilever itself works like a transducer, thus in the *Corresponding author. Fax: +34-1-8060701. E-mail address: jtamayo@imm.cnm.csic.es (J. Tamayo). 0304-3991/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0304-3991(00)00082-6