Reactivity in the Confinement of Self-Assembled Monolayers: Chain Length Effects on the Hydrolysis of N-Hydroxysuccinimide Ester Disulfides on Gold

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Two N-hydroxysuccinimide (NHS) ester disulfides, 16,16‘-dithiobis(N-hydroxysuccinimidylhexadecanoate) (NHS−C15) and 11,11‘-dithiobis(N-hydroxysuccinimidylundecanoate) (NHS−C10), were synthesized and adsorbed as self-assembled monolayers (SAMs) on gold surfaces. These SAMs, together with SAMs of 3,3‘-dithiobis(N-hydroxysuccinimidylpropionate) (NHS−C2), were used as model systems for an examination of the factors that affect the kinetics of interfacial reactions. The SAMs and the rate of the base-catalyzed hydrolysis of the incorporated NHS ester groups were characterized by grazing incidence reflection Fourier transform infrared (GIR−FTIR) spectroscopy and contact angle measurements. GIR−FTIR spectroscopy shows that SAMs of NHS−C2 and NHS−C10 undergo a pseudo-first-order hydrolysis with second-order rate constants of (61 ± 11) × 10-2 M-1 s-1 and (4.5 ± 0.4) × 10-2 M-1 s-1, respectively. SAMs of NHS−C15 show a sigmoid behavior with a half reaction time of 1700 ± 20 s in 10 mM aqueous NaOH. The rate constants determined based on the contact angle data and application of the Cassie equation are in excellent agreement with the GIR−FTIR spectroscopy results. The increase in conformational order with increasing chain length and the concomitant improvement of packing of the NHS ester end groups, as seen by GIR−FTIR spectroscopy, account for the observed differences in reactivity. Our results imply that surface reactions in SAMs can be controlled via careful design of the adsorbate structure.