Rapid functionalization of cantilever array sensors by inkjet printing

> Nanotechnology 15 (2004) 873-880 Background

• Controlled (inkjet printing) deposition of functional layers (thiol chemistry) on microcantilevers for chemical or biochemical sensors.
• Coatings of alkanethiols modified with different end groups.
• Correlation of the deflection signal of a single cantilever with specific biding events.
• Inkjet printing to functionalize cantilever arrays with self-assembled monolayers, polymer solutions, and DNA samples.

Materials and Methdologies

• Cantilever arrays comprising eight identical silicon cantilevers at 250 µm pitch, having a length of 500 µm, a width of 100 µm and a thickness of 0.5–1 µm were used.
• Thiol solutions prepared in ethanol puriss. p. a. (absolute ACS reagent, Fluka) and stored in dard bottles: ODT; HDT (Hexadecane thiol) purified as in a reference; MHA dissolved in dichloromethane, filtered and dried; MUA (Mercaptoundecanoic acid) synthesized as in a reference.
• Solutions for pH-sensing experiments.
• Single-strand DNA oligonucleotides: 3 different 12-mer probe sequences. Thiol modification with a 5'HS(CH2)6 linker. The thiol linker was preserved: dithiolthreit (DTT). Thiolated ssDNA sample solutions in triethylammonium acetate (TEAA) , nonthiolated complimentary DNA in sodium saline citrate (SSC) buffer concentrate diluted 2:1 in deionized water.
• Polymer solutions were prepared with a concentration of 5 mg ml-1 in water, ethanol, dimethylsulfoxide (DMSO) or methylisobutylketone (MIBK).
• MD-P-705-L inkjet dispensing system (Microdrop, Germany) equipped with piezo-driven autopipettes with 70 µm nozzle diameters. Single droplets with diameters in air of 60-80 µm corresponding to volumes of 0.1-0.3 nl were ejected onto selected cantilvers.
• Capillary functionalization: immersion of cantilevers into the open ends of a microcapillary array filled with various coating solutions for functionalization.
• Selective etch by immersing cantilver arrays to remove Au on uncoated surface.

Results and Discussion Alkanethiol functionalization

• Inkjet printing can produce uniform spots of SAMs.
• Aftr printing droplets of solutions onto each cantilever, excess thiol was removed in an ethanol bath.
• A SAM has been formed throught vapor transport from neighboring cantilvers. Air is not a good barrier for volatile thiols.

DNA oligomer sensors

• The 12-mer DNA compounds are less mobile than MUA or HDT, they require longer reaction times to chemisorb with their reactive thiol group on the gold surface and to assemble into a dense monolayer. Withn a few senconds before the evaporation of printed droplets, a dense monolayer formation was observed.

Conclusion

• The combination of drop-on-demand technology with an accurate positioining and alignment system allowed the non-contact coating of microcantilevers.
• Inkjet printing has the advantage that only one side of a cantilver can be coated without contamination of the backside.