Recently, I ended up chasing original reference for self assembled monolayers (SAMs) on gold surface and realized that it was 1989 when Nuzzo and Allara first showed the formation of oriented monolayers on gold using dialkyl disulfides. That’s 25 years or so of SAMs and the way this technology is being used (in biology) this may just be the beginning. George Whitesides has been the early driving force in using SAMs for biological applications. A quick search for SAMs on ACS (American Chemical Society) web site (www.pubs.acs.org) throws up more than 10,000 references-Not bad!
My first exposure to SAMs came around 9 years back while designing biointerfaces for protein arrays. Since then I have used this wonderful technology from designing nanoparticle surface to using them for bio-molecular interaction studies. I didn’t find any celebratory article for 25years of SAMs so as a way of my honoring this wonderful technology I decided to catalogue the application of SAMs in biology. I am certain that I will miss several things given the widespread use of SAMs but something is better than nothing.
Biomaterial Research: U.S. Market for biomaterials in 2000 was 9 billion dollars that show the need for such materials for improving quality of life as well as the commercial importance of Biomaterial research. Perhaps the most significant impact of SAMs in Biomedical research has been in design of model surfaces with specific properties for their interaction with biological system. Surfaces that resist protein binding (non-fouling surfaces) or surfaces that can be tailored to interact with biological system in a specific fashion are important to ensure the success of medical implants. SAMs with poly/oligo-ethylene glycol surfaces have been used to design non-fouling surfaces and to demonstrate that degree of polymerization and polymer density determines the non-fouling properties of the surfaces. SAMs have been used to engineer cell position, shape, function in an effort to model cell cultures to mimic real systems. Stem cell differentiation can be controlled by designing surface cues (chemical, topographical, physical) using SAMs. SAMs will continue to play important role in this area.
Micro-Contact Printing/Soft-Lithography: Soft lithography uses elastomeric stamp typically made of PDMS (Polydimethyl siloxane) to transfer small patterns (micrometer sized) onto substrates. Patterns of SAMs on Gold allow cells to be placed at specific place and shape to control their function. Proteins can be printed using PDMS for making protein arrays. Numerous other applications uses this trick to pattern proteins, DNAs, peptides, aptamers, cells-you name it and it can be printed! Flexible/Plastic circuits uses SAMs for making contacts
Biosensors: SAMs or similar chemistry in gold surfaces is the surface of choice for SPR (surface Plasmon resonance) and QCM (quartz crystal microbalance) biosensors. SAMS are also used for biosensors using microcantilever transducer (see my previous posting). SAMs containing PEGs with reactive end groups are commonly used for specific binding of proteins but at the same time eliminating non-specific binding. Because SAMs enable exquisite control over the density of reactive groups at the surface, the studies on biomolecular interaction as a function of ligand densities have provided useful insight into biology.
Bottom-up fabrication: Self assembled monolayers nanopatterns with reactive end groups are used to grow polymers from the surface.
Dip-Pen Nanolithorgraphy (DPN): Dip pen nanolithography pioneered by Chad Mirkin uses alkanethiol as ink and AFM (Atomic Force Microscopy) cantilever as pen to write on “Gold Coated Paper”. Since first demonstration a wide variety of inks, including small organic molecules, polymers, DNA, proteins, peptides, colloidal nanoparticles, have been patterned on variety of substrates including insulating, semiconducting and metallic substrates. The most visible use of this technology has been Nano-Art. The picture below was created using DPN. Given this is a serious technology I still can’t resist imagining DPN machines replacing “Name on Rice Grain” machines at State Fairs around the country!
Credit: The International Institute for Nanotechnology. The actual size of the image is smaller than a blood cell.
Nanoparticles (Gold/silver) synthesis and chemical functionalization:
- Monolayer protected clusters (MPC). Reduction of HAuCl4 salt in presence of alkanethiol result in gold nanoparticles of 1-3nm. MPC unlike other gold nanoparticles can be repeatedly isolated from and redissolved in common organic solvents without irreversible aggregation or decomposition. The protective MPC can be made reactive by Ligand Exchange Reaction where an alkanethiol with a reactive group replaces the protective alkanethiol from around the gold nanoparticle
- Gold and silver nanostructures and nanoparticles are functionalized using self assembled monolayer’s and used for biosensing. See my previous posting
Send me additional information that I am certain I have missed!