Division of Pharmaceutical Chemistry, University of Helsinki Selective surface modifications of self-assembled monolayers on silicon Niina Suni Division of Pharmaceutical Chemistry, University of Helsinki

Outline Selective surface modifications Aim of the study Self-assmebled monolayers Aim of the study Patterning method Results Applications Acknowledgements

Selective surface modifications Surface modified partially different properties/function on different areas of the surface Hydrophobic/hydrophilic control and switches, biochips, electronics etc.

Chemical surface modifications Self-assembled monolayers (SAMs) Spontaneously formed monomolecular (~nm) layers Alkylsilanes on oxide surfaces Ease of preparation Stability Different structures, head groups

Selective patterning of SAMs selectivce placement µ-contact printing stamp residuals, planar surfaces dip pen nanolithography planar surfaces, slow, special equipment selective removal photolithography resolution limit, masks scanning probe lithographies planar surfaces, slow, special equipment (vacuum) energetic beams: electrons, atoms, ions special equipment, masks, vacuum

Selective surface patterning of SAMs on silicon dioxide... Motivation: Simple method to selectively pattern SAMs With these patterns to control aqueous fluids on microchips without physical wall structures Hydrophobic (C18 ) SAM patterns on hydrophilic Si-SiO2 surface One molecule layer (~3 nm) guides the flow of nL-µL aqueous samples by surface chemistry Silicon Easy to micromachine & derivatize Surface spontaneously oxidized & hydroxylated on which: SAMs from alkylalkoxysilanes or alkylchlorosilanes Compatibility with mass spectrometric techniques: ESI-MS, DIOS-MS

... with electric discharge Small electric currents used to degradate SAMs on silicon (scanning probe microscopes and scanning electron microscopes) Leaves oxidized surface Multiple scans needed, small areas treated, expensive equipment needed, not applicable to high relief structures Atom/ion bombardment used to degradate SAMs on silicon Masks, expensive equipment, vacuum → Simple setup for removing SAMs with electric discharge in atmospheric pressure, applicable also to textured surfaces

Si-SiO2 coated with a hydrophobic SAM top view Planar and textured silicon surfaces Black silicon (efficiently absorbs light) Micropillar silicon (spontaneous fluid flow by capillary forces) Clean oxide layer formed on silicon with piranha solution Trichloro(octadecyl)silane (ODS) SAM formed on silicon dioxide Black silicon ~2.6 nm Micropillar silicon

Electric discharge patterning Platinum tip (diameter 50 µm ) at high voltage brought close to grounded silicon chip Computer-controlled xyz stage to produce different patterns (dots, lines, words..) 0.1 - 3 µA 0.1 - 3 kV 20 – 200 µm 1 mm/s – 5 mm/s

Does the discharge remove the coating? Water contact angle Underlying ”piranha oxide” hydrophilic < 10 DISCHARGE-TREATED PLANAR SURFACE ODS SAM PLANAR SURFACE ODS SAM MICROPILLAR SURFACE 150 110 <10 → Water spreads only in the discharge-treated areas when applied on the surface Micropillar surface supports spontaneous flow → patterns on it serve as surface-guided microfluidic channels 12 µm

Infrared spectroscopy Hydrocarbons (CH stretching vibrations) seen on ODS SAM-surface CH3 (as) 2963 ja ~2881 cm-1 CH2 (as) 2922 ja ~2851 cm-1 No CH stretching seen on discharge-treated surface ODS SAM SURFACE DISCHARGE- TREATED SURFACE

Could the hydrophilicity derive from oxidation of the ODS SAM? X-ray photoelectron spectroscopy oxidized carbon 15% from the total carbon signal No oxidation products detected in the infrared spectrum, either The exact mechanism of the SAM degradation not studied → Discharge removes the coating and exposes silicon dioxide surface Reference piranha oxide surface Discharge-treated surface ODS SAM surface

Does the discharge damage the surface under the coating? A Si AFM B Si LFM C ODS AFM D ODS LFM SEM No damage seen on planar or micro/nanostructured surfaces AFM & LFM No damage observed Friction on discharge- treated surface ~ piranha- treated surface Bigger friction on ODS SAM edge Edge resolution? AFM LFM Discharge- treaed Si Discharge- treated Si Si- SiO2 Discharge- treated ODS Discharge- treated ODS ODS

Optimal patterning parameters? What is the line width? Line width affected by Distance of the tip from the surface Current strength Patterning speed Minimum achieved line width 50 µm Not to damage the surface To thoroughly remove the coating (also from textured surfaces) To create suitable width microfluidic channels To be fast but robust Optimal patterning parameters? → Current 1 µA → Distance of the tip from the surface 50 µm → Scanning speed 1 mm/s  100 µm line width

Applications Surface-guided sample manipulation on analytical microchips → mass spectrometric drug analyses

Surface-guided microchannels in sample introduction to electrospray ionization-mass spectrometry (ESI-MS) Silicon chip with micropillars: spontaneuos flow Voltage at the conical tip sprays the sample into mass spectrometer (Nissilä et al, Rapid Comm Mass Spectrom 2007) Hydrophilic channel patterned on the chip Different width, length, and shaped channels on one chip 1 µM (1 nmol) polar verapamil solution in a surface-guided microchannel electrosprayed into mass spectrometer

Surface-guided sample concentration for desorption/ ionization on silicon-mass spectrometry Small hydrophilic spots on black silicon Concentration of aqueous samples Desorbed/ionized with laser, analyzed with MS Increased analyte signal Patterned black silicon 1 Control samples: Hydrophilic black silicon 2 Hydrophobic black silicon 3

Conclusions Rapid and simple new method to pattern SAMs on conducting substrates Requires only high voltage supply No vacuum, masks, or special equipment With computer control practicly any shapes can be patterned Large areas by dense scan Line width changed by changing the patterning parameters Applicable to textured surfaces Edge resolution enhancement Sharper tips

Results publishes in Angewandte Chemie International Edition 2008, 47, 39, 7442 Markus Haapala, Risto Kostiainen University of Helsinki, Division of Pharmaceutical Chemistry Ari Mäkinen University of Oulu, Department of Physical Sciences Lauri Sainiemi, Sami Franssila Helsinki University of Technology, Department of micro and nanosciences Esa Puukilainen, Elina Färm, Mikko Ritala University of Helsinki, Laboratory of Inorganic Chemistry Acknowledgements NGS-NANO