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Patterning Self-Assembled Monolayers Using Microcontact Printing: a New Technology for Biosensors? Milan Mrksich and George M

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Patterning Self-Assembled Monolayers Using Microcontact Printing: a New Technology for Biosensors? Milan Mrksich and George M 228 reviews Patterning self-assembled monolayers using microcontact printing: a new technology for biosensors? Milan Mrksich and George M. Whitesides Self-assembled monolayers (SAMs), formed upon the adsorption of oJ-substituted alkanethiols on the surface of gold, allow control of the properties of a surface on the molecular scale. A new technique - microcontact printing (@ZP) - can pattern the formation of SAMs, with dimensions on the micron scale. The convenience, low cost, and widespread application offered by SAMs and f&P make this combination of techniques especially suitable for producing and patterning surfaces relevant to biosensors. A biosensor combines two functions: molecular recog- Self-assembled monolayers nition and signal transduction’. The fabrication of a Alkanethiolates on gold biosensor often requires the properties of surfaces to be SAMs of alkanethiolates on gold form when a clean tailored and patterned with complex organic functional surface of gold is exposed to a solution (or vapor) of a groups including, for example, ligands for protein long-chain alkanethiol (RSH, Eqn l), or dialkyldisul- recognition, and attachment points for proteins, pep- fide (RSSR): tides, carbohydrates and other relevant groups. Con- RSH+Au(O),, + RSAu(I)=Au(0),,+)H2(?) (Eqn 1) venient, generally applicable methods for producing surfaces are an important part of biosensor technology. The structure of these SAMs is now well established4 The system of self-assembled monolayers (SAMs) of (Fig. 1). The sulfur atoms coordinate to the gold sur- alkanethiolates on gold is probably the best that is cur- face, and the alkyl chains are close-packed, trans- rently available to accomplish the fimctionalization and extended and tilted at approximately 30” from the per- patterning of surfaces required by many applications in pendicular to the surface. These monolayers are locally biomaterials science (for review, see Ref. 2). The con- well ordered and have few defects that affect the venience and flexibility of SAMs for this purpose has macroscopic properties of the surface at the 100 nm been widely recognized and exploited, especially for scale. The terminal functional group of an w-substi- homogeneous surfaces. Their stability meets the tuted alkanethiolate dominates the properties of the requirements of most biosensors. In this article, we interface between the SAM and a contacting liquid. describe a new technique - microcontact printing The optical characteristics of the system of a SAM sup- (pCP) - that allows SAMs to be patterneds. This pro- ported on gold depend predominantly on the thick- cess can readily generate features down to 1 p,rn in size, ness of the underlying gold. SAMs supported on gold and down to 200 nm with difficulty, and is compatible 5-10 nm in thickness are transparent, whereas SAMs with complex organic functionality. The process also supported on gold thicker than 100 nm are opaque and requires little, or no, access to the photolithographic reflectives. equipment usually required to generate patterns with these dimensions. The combination of SAMs and &P Stability of SAMs provides a remarkably convenient technology for the Monolayers of alkanethiolates on gold are stable for preparation ofpatterned surfaces, giving excellent con- a period of several months in air, or in contact with trol over surface properties at the molecular level. We water or ethanol. While some monolayers desorb on believe that this technology will be useful in the pro- heating to temperatures greater than 70°C, others are duction of biosensors. more stableb. In addition, SAMs are stable barriers against corrosion; for example, a SAM of hexa- decanethiolate protects the underlying gold from dis- M. Mrksich ard G. ,211. W’hitesides are at the Department qf Chem- solution in highly corrosive etchants, such as aqueous istry, Harvard hiversity, Cam/vi&, MA 02138, LISA. CNm/O, (Ref. 3). Monolayers of alkanethiolates on TIBTECH JUNE 1995 P/OL 13) 0 1995, Elswer Saence Ltd 0167 - 7799/95/$9.50 229 reviews gold are sufficiently stable to be useful for many appli- a cations in biosurfaces and biomaterials, and have been used for studies of protein adsorption and cell adhe- sion in aqueous media over periods of several days’-9. Mixed SAMs and complex functionality Adsorption of a mixture of two alkanethiols onto a gold surface allows the production of, so-called, ‘mixed’ SAMs (Ref. 2). The properties of a mixed SAM can be tailored by varying the ratio of the two alkanethiols in the solution from which they adsorb. The properties of SAMs can be specified further by incorporating complex functional groups (for examples, see Table 1). Alternatively, complex functionality can be introduced after the SAM is formed. This strategy is useful for attaching peptides and proteins to organic surfaces, but is usually less well controlled than methods used to assemble fully preformed components (for examples, see Table 2). SAM on other surfaces Alkylsiloxanes The second, widely used class of SAMs is siloxanes. These are obtained by the reaction of a hydroxylated surface (usually the native oxide of silicon) with a sol- ution of alkyltrichlorosilane (or triethoxysilane) 18.19. Figure 1 The reactive trichlorosilane groups condense with Representation of a self-assembled monolayer (SAM) of alkane- hydroxyl groups of the surface, and with neighboring thiolates on the surface of gold. (a) The sulfur atoms (S) of the siloxanes. These SAMs have the advantages that they alkanethiolates coordinate to the hollow three-fold sites of the gold are significantly more thermally stable than alkane- (l,l,l) surface: the gold atoms (open crrcles) are arranged in a thiolates on gold, they do not require evaporation of hexagonal relationship. The alkyl chains are close packed and tilted a layer of metal for preparation of substrates, and they approximately 30” from the normal to the surface. (b) The proper- are optically transparent when supported on glass tres of the SAM are controlled by changing the length of the alkyl slides. Siloxane monolayers have the disadvantages that chain and the terminal functional group X of the precursor alkane- they are less ordered than alkanethiolates on gold, and thiol. that they are chemically inflexible. The alkyl- trichlorosilane groups of the precursors are not com- patible with many functional groups, and the variety including biosensors, implants, chromatographic and of surfaces that can be prepared directly (without car- electrophoretic media, containers for storing and rying out reactions on the surface) is limited; the silox- transferring proteins, and containers for cell and tissue ane headgroup hydrolyzes rapidly, even in mild base. culture. As a result, the mechanism of formation of adsorbed protein layers, and their structures, have been Other organic surjkes studied extensively2z. Out of necessity, much of this Langmuir-Blodgett (L-B) films were the first sys- work has used materials that possess structurally ill- tem of ordered organic monolayers to be studied. defined surfaces, since well-defined, controllable They have been used extensively for the study of bio- surfaces were not available. The ability to control surfaces, and for applications in the materials sciences’“. accurately the nature and density of functional groups The low stability of L-B films, and the lack of methods for patterning their surfaces, limits their use in the pro- duction of biosensors. SAMs obtained by the adsorp- tion of alkylphosphonates on the surface of zirconium Table 1. SAMs containing receptors oxidea”, and hydroxamic acids on the native oxides of several metals (Ag, Al, Cu, Fe, Ti and Zr) (Ref. 21), Receptor Ligand Refs are systems that have recently been studied, and that permit control over the properties of organic surfaces. Porphyrin 0, (reduction) 10 These may be particularly useful when such metal oxides are used. Bis(acetoacetate) Cu(ll) 11 Biotin Interaction of SAW with biological media Streptavidin 12 The adsorption of proteins to surfaces is important Resorcin[4larene Tetrachloroethylene 13 in many materials and systems used in biotechnology, TIBTECH JUNE 1995 (VOL 13) 230 yeviews Table 2. Attachment of proteins to SAMs charides and proteins (i.c. flbronectin, laminin, vitro- nectin, heparin, collagens) that makes LIP a substantial SAM FG Protein (FG) Refs part ofmost tissue - is lnediated by specific interactions between the integrin receptors of the cellular melll- -CO,H Cytochrome c (-NH,) 14 branes and short peptide sequences of the ECM. A common strategy for controlling the attachment of -CO,H Catalase (-NH21 15 cells onto a surface relics on specif+g the adsorption -SSPy Antibody Fab’ (-SH) 16 of ECM proteins onto the surface. The attachment of rat basophilic leukemia cells to SAMs presrllting a -NH, Polyalanine (-CO,H) 17 range of functional groups [methyl, tritluoronlcthyl, alcohol, carboxylic acid, dimethylami~lo, (EC;),,] has Abbreviations: FG, functional group; Py, 2-pyndme. been studied; the cells attached to surfaces that pro- moted adsorption of laminin’). Massia and Hubbell demonstrated that siloxane SAMs presenting the prp- on the surf2ces of SAMs makes the111 particularly well tides Arg-Cly-Asp or Tyr-Ile-Gly-Ser-Arg (the recog- suited for studies of protein adsorption, and for Ftud- nition sequences for fibronectin and laminin, respect- ies of processes dependent on protein adsorption. ively), supported the adhesion and spreading of fibroblast cells without the need for coating the cur- Protein adsorption faces with
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