Sensors and Actuators B 175 (2012) 123–131 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Surface functionalization studies and direct laser printing of oligonucleotides toward the fabrication of a micromembrane DNA capacitive biosensor G. Tsekenis a , M. Chatzipetrou b , J. Tanner b , S. Chatzandroulis c , D. Thanos a , D. Tsoukalas b , I. Zergioti b,∗ a Biomedical Research Foundation of the Academy of Athens, Soranou Ephessiou 4, 11527 Athens, Greece b National Technical University of Athens, Physics Department, Iroon Polytechneiou 9, 15780 Zografou, Athens, Greece c Institute of Microelectronics NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 15310 Athens, Greece a r t i c l e i n f o Article history: Available online 12 January 2012 Keywords: Micromembranes Microcantilevers Biosensor Laser Induced Forward Transfer a b s t r a c t This work presents a comparative study between two functionalization techniques, gold (Au) and 3-glycidoxypropyl-tri-methoxy silane (GOPTS) that were used to immobilize thiol-modified oligonu- cleotides on low temperature oxide (LTO) on silicon (Si) surfaces toward the fabrication of a micromembrane array capacitive DNA biosensor. In the effort to increase the surface stress that develops upon probe immobilization and target hybridization and thus enhance the sensor’s sensitivity, a number of parameters were investigated such as probe and target concentrations as well as the thickness and roughness of the functionalization layers. Our results indicate that GOPTS is better suited as a function- alization layer for the development of microcantilever or micromembrane-based biosensors due to the enhanced hybridization efficiencies achieved, its relative stability over time and the ability to regenerate the surfaces following analyte recognition. Furthermore, with the use of Laser Induced Forward Transfer, probe oligonucleotides were uniformly deposited at the micron scale on GOPTS-functionalized surfaces, thus allowing for the realization of a micromembrane array capacitive DNA biosensor. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, capacitive biosensors have attracted much attention since they allow the low-cost and label-free detection of numerous bio-recognition events, require simple associated electronics [1] and exhibit fast response, high sensitivity and the potential to be incorporated into parallel arrays [2]. Capacitive biosensors can be further subdivided into permittivity sensors such as interdigitated electrodes [3,4] and electrode–solution interfaces [5,6], and variable distance sensors, also known as micromembrane sensors [7,8]. Micromembrane capacitive sensors rely on the deflection caused by analyte recognition and are the least studied out of the three capacitive sensors mentioned previously. In this detec- tion mode, the micromembrane acts as one of the parallel plates of a capacitor and is functionalized with probe molecules. Upon recognition of the target molecules surface stress variations are induced, which bend the membrane upwards (tensile stress) or downwards (compressive stress). As the membrane deflects, the distance between the two plates changes and this alters the capac- itance of the system [9]. ∗ Corresponding author. Tel.: +30 210 772 3345; fax: +30 210 772 3025. E-mail address: zergioti@central.ntua.gr (I. Zergioti). In order to achieve low detection limits for an analyte, the surface stress that develops across the membranes has to be max- imized. Therefore, only through the in-depth understanding of the complex phenomena and forces that contribute toward the stress observed on the surface of membranes can signal enhance- ment be achieved. The aforementioned forces have been studied extensively by a number of research groups in the context of micro- cantilever biosensors, where the deflection is detected optically [10,11] by exploiting the piezoresistive effect [12] or even by atomic force microscopy (AFM) [13]. The conclusions reached for micro- cantilever biosensors can be applied directly to micromembrane capacitive sensors. A plethora of different explanations have been suggested to account for the deflection of the cantilever. Up to now, no single theory has been proposed to justify the phenom- ena observed both for single stranded DNA (ssDNA) and double stranded DNA (dsDNA). The models that have been put forward are either only theoretical [14,15] and fail to explain the exper- imental data or only deal with particular sub cases such as the model proposed for thiol-modified single-stranded DNA on gold [16]. The design and interpretation of surface hybridization assays is further complicated by the poorly understood aspects of the inter- facial environment that cause both kinetic and thermodynamic behaviors to deviate from those in solution. The observed discrep- ancies depend on a number of factors including packing density, hybridization efficiency [17], concentration, length and sequence 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.01.005