Nanoparticle Oscillations and Fronts
Nanoparticle Oscillations and Fronts
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DOI：
10.1002/anie.201004231
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Grzybowski, Bartosz A.
中科院分区：
文献类型：
--
作者：
Lagzi, Istvan;Kowalczyk, Bartlomiej;Grzybowski, Bartosz A.
热力学平衡之外的自组织在生命的背景下是重要的，[1，2]并且激发了人工动态材料和系统的发展[3，4]，从分子[5-9]到纳米[10-12]和微观[13，14]到宏观。[15细胞或有机体中的非平衡自组织的一个奇异特征是它们能够将几个子系统耦合成更大的动态机器。[2]在人造集合体中，这种合成能力在很大程度上是缺乏的，尽管有一些有趣的例子，其中分子尺度的非平衡系统，如化学振荡器控制凝胶或聚合物的尺寸/收缩性[17，18]或周期性地移动络合或沉淀平衡。[19-22]本工作的动机是设计和实现一个化学系统，其中分子尺度子系统将耦合并控制纳米部分的动态自组织。[23]该子系统是pH振荡器[24-26]（pH“时钟”），其控制金属纳米颗粒（NP）表面上的酸性头基的解离。我们发现，通过适当控制NP之间的静电力和货车范德华力（vdW），[27]时钟可以导致NP有节奏的组装/拆卸。此外，在空间分布的介质中，pH振荡可以转化为NP聚集前沿或表面现象的传播，例如基于NP的表面涂层的沉积和去除。虽然概念上很简单（图1a），但将pH振荡与可逆NP聚集耦合的能力需要在分子尺度上仔细设计粒子间力。需要考虑的两个关键效应是纳米颗粒之间的vdW吸引力和在纳米颗粒表面上形成自组装单层（SAM）的分子之间的静电排斥[28，29]。具体而言，当pH时钟“尖峰”到高pH并使SAM去质子化时，静电排斥应该能够克服vdW吸引力;相反，当振荡器处于低pH状态时，vdW力应该足够强以影响NP聚集。为了匹配这些标准，1）覆盖NP的SAM的pKa应在振荡器的pH范围内（使得振荡可以显著地影响SAM内质子化/去质子化分子的分数，以及2）作用在系统中的静电力和vdW力的大小应该是相称的;如我们先前所示，[30]第二点对于直径为几纳米的纳米颗粒是正确的，对于这些纳米颗粒，静电和vdW相互作用的量级为几kT。[31]由于相互作用能相对较小，因此可以预期聚集/分散现象对SAM的厚度敏感（确定NP金属芯之间的距离，从而确定颗粒之间的vdW相互作用的最大幅度）。[27]考虑到这些设计准则，我们考虑了一个系统的基础上，所谓的亚甲基二醇/亚硫酸盐/异戊内酯（MGSG）振荡器。[24-26]我们的选择是出于这样一个事实，即该振荡器涵盖了相对较宽的pH范围（约1.5）。6.8至约9.3)并且基于不期望干扰典型SAM的简单化学反应。然后，我们测试了在不同样品中平均直径d= 6.5-10 nm（标准偏差：σ= 10-15%）并且用在酸性基团中封端的烷硫醇盐官能化的Au和/或Ag NP;例如，巯基十一烷酸，MUA（在溶液中的pKa约为1.5 - 1.5）。4.8，SAM中的pKa报告在6和8之间;[32]对巯基苯酚，（溶液中的pKa约为4.8）。9.3 ...
Self-organization outside the thermodynamic equilibrium is important in the context of life,[1, 2] and has inspired development of artificial dynamic materials and systems [3, 4] on scales from molecular,[5–9] through nanoscopic [10–12] and microscopic,[13, 14] to macroscopic.[15, 16] One of the singular features of non-equilibrium self-organization in cells or organisms is their ability to couple several subsystems into larger, dynamic machinery.[2] In man-made ensembles, such synthetic ability is largely lacking, though there are some interesting examples where molecular-scale, non-equilibrium systems such as chemical oscillators control dimensions/contractility of gels or polymers [17, 18] or periodically shift complexation or precipitation equilibria.[19–22] The motivation of the present work is to design and implement a chemical system in which a molecular-scale subsystem would couple to and control dynamic self-organization of nanoscopic part.[23] This subsystem is a pH oscillator [24–26](pH “clock”) that controls the dissociation of acidic head-groups on the surface of metal nanoparticles (NPs). We show that with a proper control over electrostatic and van der Waals (vdW) forces between the NPs,[27] the clock can then cause a rhythmic assembly/disassembly of the NPs. Additionally, in spatially distributed media, the pH oscillations can translate into the propagation of NP aggregation fronts or surface phenomena, such as deposition and removal of NP-based surface coatings. Although conceptually straightforward (Figure 1a), the ability to couple pH oscillations with reversible NP aggregation requires careful engineering of the interparticle forces at a molecular scale. The two key effects to consider are the vdW attractions between the NPs and the electrostatic repulsions between the molecules forming self-assembled monolayers (SAMs)[28, 29] on nanoparticle surfaces. Specifically, when the pH clock “spikes” to high pH and deprotonates the SAMs, the electrostatic repulsions should be able to overcome vdW attractions; conversely, when the oscillator is in a low-pH state, the vdW forces should be strong enough to affect NP aggregation. To match these criteria, 1) the pKa of the SAMs covering the NPs should be within the pH range of the oscillator (such that the oscillations can significantly affect the fractions of protonated/deprotonated molecules within the SAM, and 2) the magnitudes of electrostatic and vdW forces acting in the system should be commensurate; as we have shown previously,[30] the second point is true for NPs that are a few nanometers in diameter, for which the magnitudes of electrostatic and vdW interactions are on the order of several kT.[31] As the interaction energies are relatively small, the aggregation/dispersion phenomena might be expected to be sensitive to the thickness of the SAMs (determining the distance between the NPs metal cores and thus the maximal magnitude of vdW interactions between the particles).[27] With these design guidelines in mind, we considered a system based on the so-called methylene glycol/sulfite/gluconolactone (MGSG) oscillator.[24–26] Our choice was motivated by the fact that this oscillator covers a relatively broad range of pH (ca. 6.8 to ca. 9.3) and is based on simple chemical reactions that are not expected to interfere with typical SAMs. We then tested Au and/or Ag NPs with average diameters in different samples d= 6.5–10 nm (standard deviations: σ= 10–15%) and functionalized with alkane thiolates terminated in acidic groups; for example, mercaptoundecanoic acid, MUA (pKa in solution ca. 4.8, pKa in a SAM reported between 6 and 8;[32] para-mercaptophenol,(pKa in solution ca. 9.3 …