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Distinct Effects of Multivalent Macroion and Simple Ion on the Structure and Local Electric Environment of a Weak Polyelectrolyte in Aqueous Solution
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2017-09-08 00:00:00 , DOI: 10.1021/acs.jpcb.7b05387 Chen Qu 1 , Benxin Jing 1, 2 , Shengqin Wang 3 , Yingxi Zhu 1, 2
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2017-09-08 00:00:00 , DOI: 10.1021/acs.jpcb.7b05387 Chen Qu 1 , Benxin Jing 1, 2 , Shengqin Wang 3 , Yingxi Zhu 1, 2
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Adding ionic species can critically affect the structure of weak polyelectrolyte (PE) chains, whose charge density in aqueous solution can be greatly regulated by bathing solution conditions such as pH and added ions. Distinct from simple ions that can be treated as point charges, multivalent macroions of finite size, including many charged nanoparticles and biopolymers, could show strong electrostatic coupling with PEs and effectively modify the conformation and assembly of PEs in aqueous solution. In this work, we have compared the effects of hydrophilic multivalent macroion of finite size and simple divalent ion on the conformational transition of a model weak polybase, poly(2-vinylpyridine) (P2VP), in dilute aqueous solution. By using fluorescence correlation spectroscopy combined with photon counting histogram analysis, we have examined the swollen-to-collapsed conformational transition and local electric potential of a P2VP chain in ionic aqueous solution at a single-molecule level. Adding inorganic polytungstate ([W12]) macroion bearing eight negative charges per [W12] of ∼0.8 nm in diameter at increased concentration from 10–9 to 10–5 mM can lead to a shift of the critical conformational transition pH, pHcr, of P2VP to lower pH values, in an opposite trend to the previously reported effect of adding simple monovalent anion. Conversely, adding simple divalent sulfate anion can lead to a nonmonotonic change of pHcr when increasing its concentration from 0.03 to 15 mM. Additionally, at pH < pHcr where P2VP is highly protonated and adopts a swollen conformation, a monotonic decrease of P2VP size is observed with increased sulfate ionic concentration, exhibiting the typical ionic screening effect. In contrast, the size of the P2VP chain shows little change with increasing [W12] concentration before the precipitation of P2VP from water. To investigate the distinct effects of multivalent ion and macroion on the conformational transition of P2VP in aqueous solution, we have also measured the local proton concentration in the vicinity to a P2VP chain by an attached pH-sensitive fluorescence probe. In both cases, we have observed the monotonic reduction of the local electric potential of a swollen P2VP chain with increased ionic concentration, despite the increased protonation degree of P2VP. The results suggest that counterion condensation of multivalent ion and macroion can modify the effective net charge density of P2VP chains in dilute aqueous solution. However, possibly due to its high multivalency and finite size, multivalent [W12] macroion is much more effective in modifying the local electric environment and structure of P2VP chains at 3–7 orders of magnitude lower concentrations than simple sulfate counterion.
中文翻译：
多价Macroion和简单离子对水溶液中弱聚电解质的结构和局部电环境的不同影响
添加离子物种会严重影响弱聚电解质（PE）链的结构，该弱聚电解质链在水溶液中的电荷密度可以通过沐浴溶液条件（例如pH值和添加的离子）来极大地调节。与可被视为点电荷的简单离子不同，有限大小的多价大分子离子，包括许多带电的纳米粒子和生物聚合物，可表现出与PE的强静电耦合，并有效地修饰水溶液中PE的构象和组装。在这项工作中，我们比较了有限尺寸的亲水性多价大分子离子和简单的二价离子对稀水溶液中模型弱多碱聚（2-乙烯基吡啶）（P2VP）构象转变的影响。通过使用荧光相关光谱结合光子计数直方图分析，我们已经研究了在单分子水平上离子水溶液中P2VP链的溶胀到塌陷的构象转变和局部电势。添加无机聚钨酸盐（[W12 ]）的宏离子，每[W 12 ]带有八个负电荷，直径从0.8〜10 nm增加到10 –9至10 –5 mM，可导致P2VP的关键构象转变pH pH cr降低。 pH值与以前报道的添加简单单价阴离子的作用相反。相反，当将其浓度从0.03增加到15 mM时，添加简单的二价硫酸根阴离子会导致pH cr的非单调变化。此外，在pH <pH cr在P2VP高度质子化并采用溶胀构象的情况下，随着硫酸盐离子浓度的增加，观察到P2VP尺寸的单调减小，表现出典型的离子筛选效果。相反，P2VP链的大小随[W 12P2VP从水中沉淀之前的浓度。为了研究多价离子和大分子离子对水溶液中P2VP构象转变的独特影响，我们还通过连接的pH敏感荧光探针测量了P2VP链附近的局部质子浓度。在这两种情况下，我们都观察到，尽管P2VP的质子化程度增加，但随着离子浓度的增加，溶胀的P2VP链的局部电势单调降低。结果表明，多价离子和大分子离子的抗衡离子缩合可以改变稀水溶液中P2VP链的有效净电荷密度。然而，可能由于其高价和有限的大小，多价[W 12]相比于简单的硫酸盐抗衡离子，Macroion在改变局部电环境和P2VP链结构方面要有效得多，其浓度降低了3-7个数量级。
更新日期：2017-09-08
中文翻译：
多价Macroion和简单离子对水溶液中弱聚电解质的结构和局部电环境的不同影响
添加离子物种会严重影响弱聚电解质（PE）链的结构，该弱聚电解质链在水溶液中的电荷密度可以通过沐浴溶液条件（例如pH值和添加的离子）来极大地调节。与可被视为点电荷的简单离子不同，有限大小的多价大分子离子，包括许多带电的纳米粒子和生物聚合物，可表现出与PE的强静电耦合，并有效地修饰水溶液中PE的构象和组装。在这项工作中，我们比较了有限尺寸的亲水性多价大分子离子和简单的二价离子对稀水溶液中模型弱多碱聚（2-乙烯基吡啶）（P2VP）构象转变的影响。通过使用荧光相关光谱结合光子计数直方图分析，我们已经研究了在单分子水平上离子水溶液中P2VP链的溶胀到塌陷的构象转变和局部电势。添加无机聚钨酸盐（[W12 ]）的宏离子，每[W 12 ]带有八个负电荷，直径从0.8〜10 nm增加到10 –9至10 –5 mM，可导致P2VP的关键构象转变pH pH cr降低。 pH值与以前报道的添加简单单价阴离子的作用相反。相反，当将其浓度从0.03增加到15 mM时，添加简单的二价硫酸根阴离子会导致pH cr的非单调变化。此外，在pH <pH cr在P2VP高度质子化并采用溶胀构象的情况下，随着硫酸盐离子浓度的增加，观察到P2VP尺寸的单调减小，表现出典型的离子筛选效果。相反，P2VP链的大小随[W 12P2VP从水中沉淀之前的浓度。为了研究多价离子和大分子离子对水溶液中P2VP构象转变的独特影响，我们还通过连接的pH敏感荧光探针测量了P2VP链附近的局部质子浓度。在这两种情况下，我们都观察到，尽管P2VP的质子化程度增加，但随着离子浓度的增加，溶胀的P2VP链的局部电势单调降低。结果表明，多价离子和大分子离子的抗衡离子缩合可以改变稀水溶液中P2VP链的有效净电荷密度。然而，可能由于其高价和有限的大小，多价[W 12]相比于简单的硫酸盐抗衡离子，Macroion在改变局部电环境和P2VP链结构方面要有效得多，其浓度降低了3-7个数量级。
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