Zhang Shuo, Maidenberg Yanir, Luo Kai, Koberstein Jeffrey T
Department of Chemical Engineering, Columbia University , New York, New York 10027, United States.
Langmuir. 2014 Jun 3;30(21):6071-8. doi: 10.1021/la501233w. Epub 2014 May 20.
Azide-alkyne click chemistry has emerged as an important and versatile means for tethering a wide variety of guest molecules to virtually any substrate. In many of these applications, it is important to exercise control over the areal density of surface functional groups to achieve a desired areal density of the tethered guest molecule of interest. We demonstrate herein that the areal density of surface azide groups on flat germanium surfaces and nanoparticle substrates (silica and iron oxide) can be controlled kinetically by appropriately timed quenching of the S(N)2 substitution reaction of bromo-alkane-silane monolayers induced by the addition of sodium azide. The kinetics of the azide substitution reaction on monolayers formed on flat Ge substrates, determined by attenuated total reflection infrared spectroscopy (ATR-IR), are found to be identical to those for monolayers formed on both silica and iron oxide nanoparticles, the latter determined by transmission infrared spectroscopy. To validate the method, the percentages of surface bromine groups converted to azide groups after various reaction times were measured by quenching the S(N)2 reaction followed by analysis with ATR-IR (for Ge) and thermogravimetric analysis (after a subsequent click reaction with an alkyne-terminal polymer) for the nanoparticle substrates. The conversions found after quenching agree well with those expected from the standard kinetic curves. The latter result suggests that the kinetic method for the control of azide group areal density is a versatile means for functionalizing substrates with a prescribed areal density of azide groups for subsequent click reactions, and that the method is universal for any substrate, flat or nanoparticle, that can be modified with bromo-alkane-silane monolayers. Regardless of the surface geometry, we find that the azide substitution reaction is complete within 2-3 h, in sharp contrast to previous reports that indicate times of 48-60 h required for completion of the reaction.
叠氮化物-炔烃点击化学已成为一种重要且通用的方法，可将各种各样的客体分子连接到几乎任何底物上。在许多此类应用中，控制表面官能团的面密度以实现所需的目标客体分子的面密度非常重要。我们在此证明，通过适当定时淬灭由叠氮化钠引发的溴代烷烃硅烷单层的S(N)2取代反应，可以动力学方式控制平坦锗表面和纳米颗粒底物（二氧化硅和氧化铁）上表面叠氮基团的面密度。通过衰减全反射红外光谱（ATR-IR）测定，在平坦锗底物上形成的单层上叠氮取代反应的动力学与在二氧化硅和氧化铁纳米颗粒上形成的单层的动力学相同，后者通过透射红外光谱测定。为了验证该方法，通过淬灭S(N)2反应，然后用ATR-IR（用于锗）和热重分析（在与炔基末端聚合物进行后续点击反应之后）对纳米颗粒底物进行分析，测量了不同反应时间后转化为叠氮基团的表面溴基团的百分比。淬灭后得到的转化率与标准动力学曲线预期的转化率非常吻合。后一结果表明，控制叠氮基团面密度的动力学方法是一种通用手段，可用于以规定的叠氮基团面密度对底物进行功能化，以进行后续的点击反应，并且该方法对于任何可被溴代烷烃硅烷单层修饰的底物（平坦的或纳米颗粒的）都是通用的。无论表面几何形状如何，我们发现叠氮取代反应在2-3小时内完成，这与之前报道的反应完成需要48-60小时形成鲜明对比。
