单个体矢量性特征的固体纳米孔道分析研究

引用本文: 林瑶, 应佚伦, 高瑞, 王慧锋, 龙亿涛. 单个体矢量性特征的固体纳米孔道分析研究[J]. 化学学报, 2017, 75(7): 675-678. doi: 10.6023/A17040191 shu

Citation: Lin Yao, Ying Yilun, Gao Rui, Wang Huifeng, Long Yitao. Analysis of Single-entity Anisotropy with a Solid-state Nanopore[J]. Acta Chimica Sinica, 2017, 75(7): 675-678. doi: 10.6023/A17040191 shu

单个体矢量性特征的固体纳米孔道分析研究

林瑶 ^a, ,
应佚伦 ^{a,*,
,} ,
高瑞 ^a, ,
王慧锋 ^{b,*,
,} ,
龙亿涛 ^a,

a.
华东理工大学化学与分子工程学院结构可控先进功能材料及其制备教育部重点实验室上海 200237
b.
华东理工大学信息科学与工程学院上海 200237

通讯作者: 应佚伦, E-mail:yilunying@ecust.edu.cn; 王慧锋, E-mail:whuifeng@ecust.edu.cn

基金项目:
国家自然科学基金 21505043

中央高校基本科研业务费 222201714012

中央高校基本科研业务费 222201717003

国家自然科学基金 21327807

中央高校基本科研业务费 222201718001

国家自然科学基金 21421004

摘要: 固体纳米孔道作为一种高灵敏的单分子检测技术，由于其机械强度高、尺寸可控、易于阵列化集成等方面的显著优势，已经被广泛应用于DNA，蛋白质以及聚合物等小分子的检测.具有矢量性特征的各向异性单个体在纳米孔道中的穿孔行为对具有空间限域效应的纳米孔道离子流特征信号具有显著影响.为解析单个体矢量性特征对纳米孔道分析的影响，本工作利用氮化硅固态纳米孔道，以单个纳米金棒为各向异性的单个体模型，实时观测了其在孔道中的迁移行为.研究发现当纳米金棒穿过纳米孔道时，产生两种不同阻断程度的特征电流信号，通过对电流信号事件的解析，实时获取了具有矢量特征的金棒所导致的两种特征过孔事件；进一步，建立了离子电流模型，分别对这两种各向异性的穿孔事件机制进行了验证.

关键词:

English

Analysis of Single-entity Anisotropy with a Solid-state Nanopore

Lin Yao ^a, ,
Ying Yilun ^{a,*,
,} ,
Gao Rui ^a, ,
Wang Huifeng ^{b,*,
,} ,
Long Yitao ^a,

a.
Key Laboratory for Advanced Materials & School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China
b.
School of Information Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

Corresponding author: Ying Yilun, yilunying@ecust.edu.cn; Wang Huifeng, whuifeng@ecust.edu.cn

Received Date: 30 April 2017
Available Online: 15 July 2017
Fund Project:
the National Natural Science Foundation of China

the Fundamental Research Funds for the Central Universities

the Fundamental Research Funds for the Central Universities

the National Natural Science Foundation of China

the Fundamental Research Funds for the Central Universities

the National Natural Science Foundation of China

Abstract: Solid-state nanopore has emerging as a promising tool for detection and analysis of single molecules due to its advantages of high stability, easy control of diameter and channel length, and the potential for integration into devices and arrays.Therefore, there are intensive studies regarding nanopore-based detection of DNAs, proteins, polymers and other small molecules.The electrochemical confined space of nanopore could efficiently convert the information in single biological molecules with anisotropy characters into measurable electrochemical signatures with high temporal resolution.The anisotropy characters of each analyte, due to its featured physical and chemical properties in different directions, strongly affects the translocation behavior of each single entity (single molecule, single nanoparticle, etc.).To analyze the single-entity anisotropy effects on nanopore translocation, here, we employed gold nanorods (GNRs) as a model for single entities with anisotropy to investigate its translocation behavior through a solid-state nanopore.We performed the GNRs translocation experiments in 10 mmol·L^-1 KCl (pH 8) electrolyte solution with a 100 nm SiN_x solid-state nanopore.The current trace of GNRs translocation through nanopores had been recorded with an ultra-sensitive current amplifier at a sampling rate of 100 kHz filtered at 5 kHz via a low-pass Bessel filter.At applied voltage of-600 mV, two types of characteristic current blockades were observed when single GNRs translocate through the pore.We found this two types of blockades are mainly related to two translocation orientation of GNRs due to its anisotropy.The smaller current blockades are due to the GNR passing through the pore vertically while the larger current blockades are due to the GNR passing through the pore horizontally.To verify our observation of this two types of GNRs translocation events, we employed a simple model which is based on the relationship between the blockade magnitude and the exclude ion volume.The calculated current blockades of two types of GNRs translocation events agree well with the experimental values.These results illustrate that the anisotropy of single entity is an important factor that should be taken into consideration in nanopore translocation.This work will lead to a better understanding of the translocation behavior of single entity with anisotropy in the electrochemical confined space of nanopore.Such understanding is vital to the development of the solid-state nanopore system as a useful single molecule analytical device.

Key words:

1   引言

纳米孔道单分子检测技术由于其高通量、高灵敏度和无需标记等优点^[1]已经广泛地应用于核酸^[2~4]、多肽^{[5, 6]}和蛋白质^[7]等生物分子的研究.纳米孔道具有纳米尺度的空间限域效应, 通过将单个生物分子限域在纳米孔道中产生的特征电流信号, 可获得单个生物分子的大小、带电量、构象变化及其与孔道的相互作用等信息^[8~10].基于单个纳米孔道的空间限域效应在研究P53蛋白与DNA间的弱相互作用中最早被提出^[11], 其通过电化学空间限域的方式放大生物分子间的弱相互作用, 实现生物分子微小构型变化的超灵敏检测.

与蛋白质纳米孔道相比, 固体纳米孔道具有更优越的机械强度、可调控的孔径、可调节的表面特性以及易工艺集成等方面优势^{[12, 13]}.除生物小分子检测外, 固体纳米孔道由于其尺寸可控的优点, 可以应用于各种体积待测物质分析.例如, 通过产生的特征电流信号, 固体纳米孔道可用以研究DNA-蛋白质的复合物, 并获得蛋白质在DNA上每一个结合位点信息^{[14, 15]}.通过在固体纳米孔道表面修饰与蛋白特异性结合的分子, 如DNA适配体等, 延长单个蛋白质通过纳米孔道的时间, 以实现蛋白质与孔道相互作用的实时观测^[16].最近, 固体纳米孔道已被应用于单个纳米颗粒粒径、电量等分析研究^[17~20], 并实现调控纳米颗粒的合成^[21].

具有矢量性特征的单分子/单颗粒在不同方向上的差异会呈现出不同的物理化学性质(如带电量、光学特性、催化特性等).这些矢量性特征直接影响了其在纳米孔道中的行为特征.因此, 亟需解析单个体矢量性特征的纳米孔道行为.本文利用固态纳米孔道, 以单个纳米金棒为各向异性单个体模型, 实时观测了其在纳米孔道中的穿孔行为.通过获得的两种不同阻断特征信号, 实时获取了具有矢量特性的金棒所产生的两种具有各向异性特征的过孔行为; 进一步, 利用构建纳米孔道离子电流模型对这两种穿孔事件机制进行验证, 从而解析了单个金纳米棒矢量性特征.

2   结果与讨论

2.1   纳米孔道检测单个纳米金棒

实验原理示意图如图 1a所示, 固体氮化硅薄膜将电解质溶液分开为cis和tran两个浴池, 薄膜上利用离子束刻蚀技术制备的唯一纳米孔道作为连接两个浴池的离子流孔道.通过在氮化硅固态纳米孔道两端外加电压, 溶液中的离子在电压的作用下产生定向迁移, 从而获得了纳米孔道离子电流.单个纳米金棒在电场力的驱动下进入限域的纳米孔道中, 改变孔道中的离子浓度, 产生特征离子流信号. 图 1b为在10 mmol•L^-1 KCl (pH 8) 电解质溶液中, 对单个氮化硅固态纳米孔道的I~V曲线表征.在-1000 mV到1000 mV范围内, I~V曲线具有良好的线性关系(R²=0.999).扫描电子显微镜(SEM)表征显示纳米孔道直径为100 nm (图 1c).本实验待测物纳米金棒直径为10 nm, 长度为34 nm (图 1d).纳米金棒由于表面包裹着十六烷基三甲基溴化铵分子, 所以表面带正电.为了防止纳米金棒在溶液中聚集, 本研究使用10 mmol•L^-1 KCl作为电解质溶液.通过紫外-可见光谱分析表明纳米金棒在10 mmol•L^-1 KCl电解质中是稳定分散的(图S1).

图 1 (a)固体纳米孔道检测单个纳米金棒示意图.检测池由氮化硅薄膜隔离为cis端和trans端两个浴池, 通过一对铂电极分别连接在浴池两端并施加电压.单个纳米金棒在电压的驱动下进入纳米孔道. (b)固体纳米孔道在10 mmol•L^-1 (pH 8) KCl电解质溶液中的I~V曲线. (c)固体纳米孔道SEM表征图片, 孔道尺寸为100 nm. (d)纳米金棒TEM表征图片, 金棒直径为10 nm, 长度为34 nm Figure 1. (a) Schematic of analyzing the single gold nanorod anisotropy with a SiN_x solid-state nanopore. The nanopre chip was placed to divide two chambers of KCl solution. A pair of Pt electrodes were immersed into two chambers and connected to the amplifier. When a bias voltage was applied, the gold nanorods translocated through nanopore from cis chamber to trans chamber. (b) I~V curve of a solid-state nanopore in 10 mmol•L^-1 KCl electrolyte (pH 8). (c) SEM image of a SiN_x solid-state nanopoe with a diameter of 100 nm. (d) TEM image of gold nanorods with the diameter of 10 nm and the length of 34 nm

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在未加入纳米金棒之前, 100 nm氮化硅固态纳米孔道基线电流如图 2a(ⅰ)所示.当纳米金棒加入cis端浴池后, 纳米孔道离子流出现了明显的瞬时阻断电流信号(图 2a(ⅱ)).如图 2b所示, 以阻断电流幅值(Δi)为纵坐标, 阻断时间(Duration time)为横坐标, 统计获得了-600 mV电压下电流信号的散点分布.可见, 电流信号的散点明显集中在两个区域.我们分别对这两个区域的阻断电流和阻断时间进行了统计, 如图 2c所示, 阻断电流柱状图中有两个高斯峰, 一个具有较大阻断电流的峰, 峰值为435 pA(Δi₁); 一个具有较小阻断电流的峰, 峰值为296 pA(Δi₂).这两个峰所对应的阻断时间分别为0.84 ms和0.82 ms.

图 2 (a) -600 mV电压下, 在cis端浴池中加入纳米金棒前(ⅰ)和加入纳米金棒后( )的时间-电流原始曲线. (b) -600 mV电压下单个纳米金棒穿过固体纳米孔道的阻断时间-阻断电流散点图, 其中横坐标为阻断时间, 纵坐标为阻断电流.图中蓝色和红色散点的分界点为阻断电流柱状图双峰拟合交界点的阻断电流值. (c)图b中蓝色及红色散点分布分别对应的阻断电流柱状图和阻断时间柱状图 Figure 2. (a) Time recording of the current traces acquired at -600 mV. Before nanorods were added, no translocation events were observed (ⅰ). After nanorods were added, the spike-like signals were observed in the current-time trace (ⅱ). (b) Scatter plots of current blockade versus duration time of GNRs translocation events at -600 mV. The boundary current for blue and red scatter plots is 359 pA. (c) Histograms of current blockade and duration time for two populations (blue and red) in the scatter plots. The histograms of current blockade and duration time were fit to the Gaussian function

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2.2   单个纳米金棒矢量性特征的分析

由于Δi反映了单个纳米金棒限域在纳米孔道中时孔道中离子浓度的改变, 因此, 阻断电流的两个峰值可能代表着两种排开不同离子体积的单颗粒特征行为, 即由于具有矢量特征的纳米金棒的各向异性导致其在纳米孔道中的两种主要特征穿孔行为: (Ⅰ)金棒以横轴穿过纳米孔道; (Ⅱ)金棒以纵轴穿过纳米孔道(图 3).为了证实这一各向异性穿孔机制, 本研究构建纳米孔道离子电流模型进行验证.纳米孔道离子电流的改变主要来自两个方面^[22]:一方面是纳米物质进入纳米孔道时由于体积排阻效应导致孔道中的离子浓度减小; 另一方面是纳米物质进入孔道中时由于表面双电层效应导致孔道中的离子浓度增大.当体积排阻效应大于双电层效应时, 孔道总体离子浓度减小, 从而产生阻断电流信号; 当双电层效应大于体积排阻效应时, 孔道总体离子浓度增加, 从而产生增强的电流信号.本研究中所获得的单个纳米金棒的电流信号是阻断电流信号, 说明单个纳米金棒的体积排阻效应明显强于单个金棒表面的双电层效应.在10 mmol•L^-1 KCl电解质溶液中, 纳米金棒表面的双电层厚度约为3 nm^[23], 该厚度相对于本研究中的纳米孔道的尺寸(d=100 nm)是可以忽略不计的.相关研究也表明, 双电层效应占主导因素主要是在低电解质浓度下, 由于待测物质的大小与纳米孔道尺寸相当导致双电层叠加, 从而大大增加孔道中的离子浓度^[22].并且, 随着纳米孔道尺寸的增加, 电流信号可由增强电流信号变成阻断电流信号^[22].

图 3 单个纳米金棒的两种过孔信号以及其相对应的过孔方式.阻断电流较大的信号对应于单个纳米金棒以横轴进入纳米孔道中(Ⅰ), 阻断电流较小的信号对应于单个纳米金棒以纵轴进入纳米孔道中(Ⅱ) Figure 3. Two representative translocation events of single gold nanorod. In type Ⅰ, the GNR translocates through the pore horizontally, which causes a larger current blockade. In type Ⅱ, the GNR translocates through the pore vertically, which causes a smaller current blockade

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因此, 本研究只考虑体积排阻效应, 根据阻断电流与排阻体积的关系式^[24]:

其中, Δi为阻断电流值, σ为电解质溶液的电导率, φ为电极施加的电压值, V为纳米颗粒在孔道中排开的体积大小, H_eff为纳米孔道的有效厚度.因此可以得出, 阻断电流Δi与排阻体积V呈线性关系.本研究中, Δi₁和Δi₂分别对应于单个纳米金棒以横轴穿过纳米孔道和单个纳米金棒以纵轴穿过纳米孔道这两种行为所产生的阻断电流, V₁和V₂分别对应于单个纳米金棒以横轴穿过纳米孔道和单个纳米金棒以纵轴穿过纳米孔道这两种行为所产生的排阻体积, 固体纳米孔道的厚度为约20 nm.我们将Δi₁, V₁带入公式(1), 计算得出纳米孔道的实际有效厚度H_eff=22.3 nm, 同时我们将纳米金棒以纵轴穿过纳米孔道产生的排阻体积V₂带入公式(1), 利用H_eff=22.3 nm计算得到Δi₂^*=451 pA, 与实验值Δi₂=435 pA相比符合.因此, 我们可以认为两种阻断电流的信号是由于单个纳米金棒各向异性所造成的不同穿孔行为, 可用以分析单个金纳米棒的矢量性特征.

3   结论

综上所述, 本文利用氮化硅固态纳米孔道, 以单个纳米金棒为具有矢量特性的单个体模型, 实时观测了其在纳米孔道中的迁移行为.研究发现, 由于纳米金棒的各向异性, 其在纳米孔道迁移过程中, 会产生两种不同阻断程度的特征电流信号, 通过对电流信号事件的分析, 实时获取了纳米金棒的两种特征过孔行为, 并通过离子电流模型对其过孔机制进行了进一步验证.本研究所得纳米金棒的两种特征过孔行为可用于分析单个纳米金棒的矢量特征.研究结果为进一步理解具有矢量性特征的单个体在纳米尺度限域空间内的行为提供了理论模型和依据, 为研究单个体各向异性差异提供了一种直接、免标记的单分子分析工具.

4   实验部分

纳米金棒购自Nanoseedz (Hong Kong, China); 聚二甲基硅氧烷(PDMS)购自Dow Corning (Wiesbaden, Germany); 氯化钾(纯度≥99.5%)购自Aladdin (Shanghai, China); 所有试剂均为分析纯, 所有配制溶液的水均是超纯水(18.2 MΩ•cm), 由Milli-Q制水系统(Billerica, USA)制备.本实验所用的氮化硅固体纳米孔道采用离子束刻蚀技术制备, 由东南大学陈云飞教授课题组加工提供.

固体纳米孔道检测微池主要由三个部分组成:聚四氟乙烯池体、PDMS垫片和氮化硅纳米孔芯片.氮化硅固体纳米孔芯片被夹在由PDMS制成的垫片中间.然后将该芯片对准池子的孔道, 用螺丝将其固定.氮化硅固体孔芯片使用前需要在氧等离子体氛围中处理30 s以保证其表面亲水.检测池安装完毕后, 往微池两个浴槽内分别加入10 mmol•L^-1 KCl电解质溶液.将一对铂电极分别插入到电解质溶液中用于施加电压.实验中产生的电流信号使用Axon 200B (Axon Instruments, Forest City, USA)膜片钳放大器和DigiData 1440A数模转换器(Molecular Devices, Forest City, USA)进行放大和采集.通过5 kHz低通滤波、100 kHz采样频率采集电流信号, 实验所得数据使用自研数据处理软件NANOPORE ANALYSIS (http://people.bath.ac.uk/yl505/nanoporea-nalysis.html)及Origin 9.2 (OriginLab Corporation, Northampton, USA)进行统计分析.
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图 1 (a)固体纳米孔道检测单个纳米金棒示意图.检测池由氮化硅薄膜隔离为cis端和trans端两个浴池, 通过一对铂电极分别连接在浴池两端并施加电压.单个纳米金棒在电压的驱动下进入纳米孔道. (b)固体纳米孔道在10 mmol•L^-1 (pH 8) KCl电解质溶液中的I~V曲线. (c)固体纳米孔道SEM表征图片, 孔道尺寸为100 nm. (d)纳米金棒TEM表征图片, 金棒直径为10 nm, 长度为34 nm

Figure 1 (a) Schematic of analyzing the single gold nanorod anisotropy with a SiN_x solid-state nanopore. The nanopre chip was placed to divide two chambers of KCl solution. A pair of Pt electrodes were immersed into two chambers and connected to the amplifier. When a bias voltage was applied, the gold nanorods translocated through nanopore from cis chamber to trans chamber. (b) I~V curve of a solid-state nanopore in 10 mmol•L^-1 KCl electrolyte (pH 8). (c) SEM image of a SiN_x solid-state nanopoe with a diameter of 100 nm. (d) TEM image of gold nanorods with the diameter of 10 nm and the length of 34 nm

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图 2 (a) -600 mV电压下, 在cis端浴池中加入纳米金棒前(ⅰ)和加入纳米金棒后( )的时间-电流原始曲线. (b) -600 mV电压下单个纳米金棒穿过固体纳米孔道的阻断时间-阻断电流散点图, 其中横坐标为阻断时间, 纵坐标为阻断电流.图中蓝色和红色散点的分界点为阻断电流柱状图双峰拟合交界点的阻断电流值. (c)图b中蓝色及红色散点分布分别对应的阻断电流柱状图和阻断时间柱状图

Figure 2 (a) Time recording of the current traces acquired at -600 mV. Before nanorods were added, no translocation events were observed (ⅰ). After nanorods were added, the spike-like signals were observed in the current-time trace (ⅱ). (b) Scatter plots of current blockade versus duration time of GNRs translocation events at -600 mV. The boundary current for blue and red scatter plots is 359 pA. (c) Histograms of current blockade and duration time for two populations (blue and red) in the scatter plots. The histograms of current blockade and duration time were fit to the Gaussian function

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图 3 单个纳米金棒的两种过孔信号以及其相对应的过孔方式.阻断电流较大的信号对应于单个纳米金棒以横轴进入纳米孔道中(Ⅰ), 阻断电流较小的信号对应于单个纳米金棒以纵轴进入纳米孔道中(Ⅱ)

Figure 3 Two representative translocation events of single gold nanorod. In type Ⅰ, the GNR translocates through the pore horizontally, which causes a larger current blockade. In type Ⅱ, the GNR translocates through the pore vertically, which causes a smaller current blockade

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