Ultramicroscopy 91 (2002) 29–36 Adsorption kinetics and mechanical properties of thiol-modified DNA-oligos on gold investigated by microcantilever sensors Rodolphe Marie, Henriette Jensenius, Jacob Thaysen, Claus B. Christensen, Anja Boisen* Mikroelektronik Centret, Technical University of Denmark, Bldg. 345E, 2800 Lyngby, Denmark Received 29 May 2001; received in revised form 18 December 2001 Abstract ImmobilisedDNA-oligolayersarescientificallyandtechnologicallyappealingforawiderangeofsensorapplications such as DNA chips. Using microcantilever-based sensors with integrated readout, we demonstrate in situ quantitative studiesofsurface-stressformationduringself-assemblyofa25-merthiol-modifiedDNA-oligolayer.Theself-assembly induces a surface-stress change, which closely follows Langmuir adsorption model. The adsorption results in compressivesurface-stressformation,whichmightbeduetointermolecularrepulsiveforcesintheoligolayer.Therate constantoftheadsorptiondependsontheconcentrationoftheoligosolution.Basedonthecalculatedrateconstantsa surfacefreeenergyofthethiol-modifiedDNA-oligoadsorptionongoldisfoundtobe 32.4kJmol 1 .Theadsorption experiments also indicate that first a single layer of DNA-oligos is assembled on the gold surface after which a significantunspecificadsorptiontakesplaceontopofthefirstDNA-oligolayer.Thecantilever-basedsensorprinciple hasawiderangeofapplicationsinreal-timelocalmonitoringofchemicalandbiologicalinteractionsaswellasinthe detection of specific DNA sequences, proteins and particles. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thiol-modified DNA-oligos; Cantilever; Biosensor; Immobilisation; Kinetics; Surface stress; Gold-thiolate bond 1. Introduction Several biological sensors, such as DNA chips [1,2],arebasedonthedetectionofhybridisationto specificDNA-oligosattachedtoasurface.Forthis type of sensors the immobilisation of DNA-oligos iscrucial.Awell-knowntechniqueforastableand reproducible DNA-oligo immobilisation is the chemisorption of thiol-modified DNA-oligos onto a gold surface. The understanding of the proper- ties of DNA-oligo layers is limited, especially regarding the kinetics of the layer formation and the stress formation during immobilisation. Re- cently, acoustic network analysis [3] and quartz balance resonators [4–6] have been used to investigate the kinetics of DNA hybridisation. Moreover,scanningtunnellingmicroscopy[7]and atomic force microscopy [8,9] have been used to study local mechanical and chemical properties of immobilised DNA. However, the data on the *Corresponding author. Tel.: +45-4525-5727; fax: +45- 4588-7762. E-mail address: boisen@mic.dtu.dk (A. Boisen). 0304-3991/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0304-3991(02)00079-7