Sensors and Actuators B 190 (2014) 651–656 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/locat e/snb Self-assembly of phthalocyanines on quartz crystal microbalances for QCM liquid sensing applications Dilek D. Erbahar a , Mika Harbeck a,∗ , Gülay Gümüs ¸ a , Ilke Gürol a , Vefa Ahsen a,b a TÜBITAK Marmara Research Center, Materials Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey b Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey a r t i c l e i n f o Article history: Received 30 May 2013 Received in revised form 4 September 2013 Accepted 5 September 2013 Available online 14 September 2013 Keywords: Phthalocyanine Self-assembled monolayer (SAM) Quartz crystal microbalance (QCM) Chemical sensor Liquid sensing a b s t r a c t After their widespread use in gas sensors, phthalocyanines are applied as sorption matrices for quartz crystal microbalance (QCM) sensors in liquid phase, as well. In order to enhance sensor performance, the application of ultrathin Pc films is envisaged. In this work, QCM liquid sensors functionalized in a self-assembly process with phthalocyanines are proposed and evaluated for the first time. An asymmet- ric tetra substituted nickel phthalocyanine having a thiol linker group to covalently attach to the gold electrode of the QCM and three hexyloxy groups was synthesized. The sensor is assessed in its sensing characteristics in water using selected organic compounds. The sensor responses are fast and recovery after analyte exposure is quick. Detection limits for organic compounds down to sub-ppm levels can be reached. The results demonstrate that the application of phthalocyanines as self-assembled monolayers is very effective to obtain high performing QCM liquid sensors. Besides, layers of alkyloxy substituted phthalocyanines have been previously found unsuitable for the use with the QCM in liquids due to the low adhesion of the Pc layer to the transducer. Thus, the covalent attachment is an excellent way to expand the list of available phthalocyanines as sensitive compounds for such sensors in liquids. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Surface modifications such as the application of ultrathin films down to monolayer thickness are an efficient tool in creating high performance chemical liquid sensors [1,2] or biosensors [3]. Various techniques are available to directly attach the sensitive compound to the transducer surface or to facilitate attachment by providing suitable anchor sites. A common technique for covalent bonding to the substrate via a suitable linker group leads to the formation of monolayers in a self-assembly procedure. Several variations are in use depending on substrate type and the compound to be applied [4]. In case of noble metal substrates covalent attachment is best achieved using thiol functional groups or disulfides, while, e.g., for oxidic surfaces silanes are commonly employed. The use of cova- lent bonding to the substrate surface in contrast to methods using only much weaker physisorption has the advantage of creating sur- face layers of high stability in a reproducible and simple manner. The very stable chemical bonds eliminate the risk of detachment of the sensing material during operation which is an issue espe- cially in liquid sensing applications. Besides, in thin films the kinetic ∗ Corresponding author. Tel.: +90 262 677 3123; fax: +90 262 641 2309. E-mail addresses: dilek.dundar@tubitak.gov.tr (D.D. Erbahar), mika.harbeck@tubitak.gov.tr (M. Harbeck), gulay.gumus@tubitak.gov.tr (G. Gümüs ¸ ), ilke.gurol@tubitak.gov.tr (I. Gürol), ahsen@gyte.edu.tr (V. Ahsen). behavior of the sensors is generally much enhanced due to the direct accessibility of the active sorption sites for the analyte molecules without the necessity of bulk diffusion steps. Phthalocyanines (Pcs) are a class of macromolecules increas- ingly under investigation for chemical sensors operating in liquids, e.g., by serving on quartz crystal microbalance (QCM) sensors as sorption matrices. Typically, spray or spin coating techniques are applied producing films of tens or hundreds of nanometers thick- ness on the transducers [5–7]. However, the Pc molecule can be tailored to undergo self-assembly to form monolayers (self- assembled monolayers, SAMs) or multilayer ultrathin films, as well. For example, Langmuir-Blodgett films of double and triple-decker lanthanide Pcs were used to modify the electrodes of an electro- chemical sensor array [8]. The same technique was also applied to QCM transducers. In the first work using Pcs with the QCM in aque- ous media copper and nickel tetra substituted phthalocyanines and cobalt porphyrin Langmuir-Blodgett films were found to be sensi- tive to various phenols [9]. Later, an octa substituted copper Pc with bulky substituents was characterized in details using spectroscopic techniques and the QCM [10]. For direct covalent attachment of Pcs to gold surfaces a thiol group at the end of a spacer can be introduced to the Pc molecule in a single substituent [11,12] or on multiple substituents [13]. Besides, a sulfur bridge atom can be placed between the substituent and the Pc core for a direct linkage to the substrate [14–17]. Finally, the Pc can be immobilized in a two-step process. Instead of thiol groups 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.09.034