Self-Assembled Monolayers of Thiolates on Metals as - Whitesides ...

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1124 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. are critical for the development <strong>of</strong> surfaces that present the large, complex ligands and molecules needed for biology and biochemistry. The synthesis <strong>of</strong> functionalized thiols is usually laborious and difficult even for “simple” molecules, and for SAMs comprising alkanethiols linked to a peptide, protein, carbohydrate, or other biomolecules, synthesis can provide a major challenge. Many <strong>of</strong> the strategies adopted for modifying SAMs after their formation derive from precedents established in the 1970s and early 1980s for functionalizing organic films supported on electrodes used for electrochemistry 344 and on solid-phase materials used for chromatography. 345 Some <strong>of</strong> the methods developed for those systems, such as amide-bond formation (section 5.2.1) and nonspecific adsorption <strong>of</strong> polymers (section 5.3), remain important techniques for modifying SAMs. Modification <strong>of</strong> the exposed surface <strong>of</strong> a SAM after formation <strong>of</strong>fers four advantages: (1) it uses common synthetic procedures and thus simplifies the preparation <strong>of</strong> functionalized surfaces (section 5.2); (2) it enables the incorporation <strong>of</strong> ligands into SAMs that are not compatible with thiols or the synthetic methods for preparing them; (3) it can generate multiple samples with different types <strong>of</strong> ligands in a short period <strong>of</strong> time (because the SAMs are easy to prepare); and (4) it preserves the ordered underlying structure <strong>of</strong> the SAM. An important economic advantage <strong>of</strong> modifying the SAM after formation is that the amount <strong>of</strong> ligand required for immobilization is very small (
<strong>Self</strong>-<strong>Assembled</strong> <strong>Monolayers</strong> <strong>of</strong> <strong>Thiolates</strong> Chemical Reviews, 2005, Vol. 105, No. 4 1125 also form triazoles upon reaction with azide compounds. 356 Both methods require at least 3 h for completion <strong>of</strong> the reaction as well as manipulation <strong>of</strong> potentially explosive azides and acetylenes. (The presence <strong>of</strong> copper ion increases the sensitivity <strong>of</strong> both classes <strong>of</strong> compounds to explosion.) Another type <strong>of</strong> reaction that uses terminal azides is the Staudinger reaction: substituted phosphanes react with azides to form amide bonds. This reaction can modify the surfaces <strong>of</strong> cells 357 and immobilize small molecules on glass slides. 358 To the best <strong>of</strong> our knowledge this ligation has not been used to modify SAMs on gold but has proved useful in other types <strong>of</strong> biological surface chemistry. One advantage <strong>of</strong> both ‘click’ chemistry and the Staudinger ligation is that the reactions are highly selective, that is, the reaction is not sensitive to the presence <strong>of</strong> other functional groups, such as amines, hydroxyls, or thiols, in solution or on the surface. An approach for selectively immobilizing proteins is to use SAMs that present ligands that only react when bound to the active site <strong>of</strong> an enzyme. Mrksich et al. have shown that SAMs presenting phosphonates form covalent adducts with an engineered fusion protein comprising cutinase and a protein <strong>of</strong> interest such as calmodulin or sections <strong>of</strong> fibronectin. 359,360 This strategy leaves the enzyme (cutinase) bound to the surface, but the attached protein extends into the ambient solution with a defined orientation. Enzymatic processing <strong>of</strong> ligands presented on SAMs has not been widely explored; 361 further studies should enable a range <strong>of</strong> selective chemical transformations on surfaces. 5.2.2. Activation <strong>of</strong> Surfaces for Reactions An operationally different approach to the functionalization <strong>of</strong> the surfaces <strong>of</strong> SAMs is to form a reactive intermediate, which is then coupled to a ligand (Scheme 3). There are two primary advantages <strong>of</strong> this strategy: (1) the common intermediate can react with a variety <strong>of</strong> ligands and (2) it allows, in principle, spatial discrimination <strong>of</strong> active and inactive regions, that is, the reactivity <strong>of</strong> regions on the surface can be turned ‘on’ or ‘<strong>of</strong>f’. The combination <strong>of</strong> activated intermediates with methods for spatial patterning, e.g., microcontact printing (µCP) (see section 7.1) and scanning probe lithography, 133,362 make it possible to attach ligands in specified locations. 363-365 One <strong>of</strong> the experimentally simplest and most broadly applicable methods developed for modifying SAMs is the formation <strong>of</strong> amide linkages via an interchain anhydride intermediate. 363 In this method, a SAM terminated with carboxylic acids is dehydrated with trifluoroacetic anhydride to yield an interchain anhydride. Exposure <strong>of</strong> this activated surface to amines generates amide bonds. This socalled “anhydride method” produces a SAM (for the best defined cases) with a 1:1 mixture <strong>of</strong> functional groups on the surface (-COOH and -CONHR). Three factors make this reaction very useful for screening structure-property relations for surfaces: (1) the simplicity and rapidity <strong>of</strong> the method, (2) the large number <strong>of</strong> amine-containing organic and orga- Scheme 3. Interfacial Reactions That Involve an Intermediate Functional Group nometallic ligands that are available commercially or that can be synthesized easily, and (3) the high yield normally observed for the coupling reaction. A method familiar to biochemists for modifying free carboxylic acid groups is the activation <strong>of</strong> a free carboxylic acid by N-hydroxysuccinimidyl (NHS) esters and the subsequent reaction with an amine; the reaction yields amide bonds. 366,368 Substituting pentafluorophenol for NHS increases the reactivity <strong>of</strong> the activated ester on the surface by approximately an order <strong>of</strong> magnitude. 366 Using this method, ligands and proteins have been immobilized on mixed SAMs derived from tri(ethylene glycol)-terminated thiols and hexa(ethylene glycol)-carboxylic-acid-terminated thiols. For mixed SAMs with a low fraction <strong>of</strong> acid functional groups (∼10% surface coverage), RAIR spectroscopy indicated that the conversion <strong>of</strong> free acids to NHS esters was nearly quantitative and that the reaction <strong>of</strong> amines with the activated ester generated amides in better than 80% yield. Other approaches for activating the functional surfaces <strong>of</strong> SAMs use external stimuli, such as electrochemical potentials and photoradiation, 369 to transform unreactive functional groups into reactive ones for the subsequent attachment <strong>of</strong> ligands. Mrksich et al. have shown that the electrochemical oxidation <strong>of</strong> SAMs terminated with hydroquinone yields a quinone, which subsequently can react with a diene, e.g., cyclopentadiene, via a Diels-Alder reaction. 367 Cyclic voltammetry can control the degree <strong>of</strong> oxidation/reduction <strong>of</strong> the quinone groups and alter the reactivity <strong>of</strong> the surface dynamically. Koberstein et al. developed a SAM terminated with a tert-butyl ester azobenzene group that undergoes de-esterification photochemically in the presence <strong>of</strong> a photoacid generator (triphenylsulfonium triflate); this reaction
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