Abstract—In this paper, the design of a QCM sensor for liquid media measurements in vertical position is described. A rugged and low-cost proof holder has been designed, the cost of which is significantly lower than those of traditional commercial holders. The crystal is not replaceable but it can be easily cleaned. Its small volume permits to be used by dipping it in the liquid with the desired location and orientation. The developed design has been experimentally validated by measuring changes in the resonance frequency and resistance of the QCM sensor immersed vertically in different calibrated aqueous glycerol solutions. The obtained results show a great agreement with the Kanazawa theoretical expression. Consequently, the designed QCM sensor would be appropriate for sensing applications in liquids, and might take part of a future on-line multichannel low-cost QCM-based measurement system. Keywords—Holder design, liquid-media measurements, multi- channel measurements, QCM. I. INTRODUCTION HICKNESS-Shear Mode (TSM) sensors, namely QCM, are highly sensitive devices not only able to measure the addition of very small mass, but also non-gravimetric contributions of viscoelastic media, which can be operated in air, aqueous conditions and under vacuum. QCM usually consists of a thin AT-cut quartz wafer with two metallic electrodes deposited uniformly onto both sides of the crystal. Due to the piezoelectric effect, when an AC electrical field is applied perpendicular to the plate, a shear wave is generated that propagates through the quartz crystal inducing its oscillation. This propagation is modified when the faces of the crystal are loaded by a material. QCM sensors are becoming a great alternative to analytical methods of measurement in a wide range of application areas involving small molecular weight ligands, carbohydrates, proteins, nucleic acids, viruses, bacteria, cells, and membrane interfaces [1], [2]. When the QCM sensor is introduced into a liquid, only one of the two crystal faces is in contact to the liquid. For this reason, the traditional design of commercial holders consists of a recipient with the crystal element attached to the bottom by supporting it by means of toroidal rubber O-rings that provide sealing between the liquid and the face with the electrical contacts. These holders use to have several parts and screws to permit the crystal replacement. M. A. Amer is with the Escola Universitària Salesiana de Sarrià, Passeig Sant Joan Bosco 74, 08017 Barcelona, Spain (e-mail: maamer@euss.es). J. A. Chávez, M.J. García-Hernández, J. Salazar, A. Turó are with the Sensor Systems Group. Department of Electronic Engineering. Universitat Politècnica de Catalunya, Jordi Girona 1-3, 08034 Barcelona, Spain. Accordingly, the mounting and the placement of the crystal in the holder is critical as the operation and the sensitivity of the QCM is very dependent on them. In addition, these holders usually are very big and expensive. These facts make these holders unsuitable for some kinds of online measurements in liquid media. Alternatively, in this paper a new QCM holder is proposed. So, the QCM sensor is placed in vertical in the lid of commercial standard low-cost culture plates as measuring cells. For a proper use of the QCM sensor, a specific holder is designed, in which only one of the crystal faces is in contact with the liquid media. The crystal is not replaceable, but the holder may be easily replaced or cleaned. The cost of the whole holder is significantly lower than the commercial ones and can be an interesting alternative for developing a low-cost QCM-based liquid measurement system. II. THEORETICAL A. Electrical Impedance of a Quartz Crystal Resonator. The electrical impedance or admittance [3] of a QCM is influenced by the acoustic load at its faces. It depends on the electrical capacitance formed by the electrodes and the quartz as a dielectric material and the so-called motional impedance, which is contributed by the acoustic load impedance (Z L ), acting at the surfaces of the quartz plate. The acoustic load impedance can consist of a single rigid film, a semi-infinite Newtonian liquid, a single viscoelastic film or a multilayer arrangement. B. The Butterworth van Dyke Equivalent Circuit. Near resonance, the electrical impedance of a QCM can be reduced to the Butterworth van Dyke (BVD) electrical equivalent model [4] as depicted in Fig. 1, where the physical parameters are summarized in lumped equivalent electrical values: electrical capacitance, motional inductance, motional capacitance and motional resistance. According to Fig. 1, the BVD circuit consists of two parallel branches: the upper branch consisting of only the capacitance C 0 represents the fixed dielectric capacitance of the resonator. All of the motional information is contained in the lower branch. M. A. Amer, J. A. Chávez, M. J. García-Hernández, J. Salazar, A. Turó Quartz Crystal Microbalance Holder Design for On-Line Sensing in Liquid Applications T World Academy of Science, Engineering and Technology International Journal of Electrical and Computer Engineering Vol:10, No:5, 2016 684 International Scholarly and Scientific Research & Innovation 10(5) 2016 scholar.waset.org/1307-6892/10004502 International Science Index, Electrical and Computer Engineering Vol:10, No:5, 2016 waset.org/Publication/10004502