Reactivity in the confinement of self-assembled monolayers:: Chain length effects on the hydrolysis of N-hydroxysuccinimide ester disulfides on gold

Two N-hydroxysuccinimide (NHS) ester disulfides, 16,16'-dithiobis(N-hydroxysuccinimidylhexadecanoate) (NHS-C-15) and 11,11'-dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C-10), were synthesized and adsorbed as self-assembled monolayers (SAMs) on gold surfaces. These SAMs, together with SAMs of 3,3'-dithiobis(N-hydroxysuccinimidylpropionate) (NHS-C-2), 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-C-2 and NHS-C-10 undergo a pseudo-first-order hydrolysis with second-order rate constants of (61 +/- 11) x 10(-2) M-1 s(-1) and (4.5 +/- 0.4) x 10(-2) M-1 s(-1), respectively. SAMs of NHS-C-15 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.