可控还原氧化石墨烯及其复合材料的制备与SERS性能研究
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摘要
表面增强拉曼散射（SERS）是一种具有表面选择性的增强技术，它将会使吸附于某种特定基底上的探针分子的拉曼信号强度得到几个数量级的增强，这项技术的发展极大的推动了拉曼光谱在不同的科研领域和材料结构分析等领域的应用。对于SERS，尽管做过很多的实验研究和理论模拟，但其增强机理仍然存在许多争议，迄今为止都没有一个合理的模型可以对目前存在的多种增强机制进行完整的描绘。通常，被广泛接受的两种机制是电磁机制（EM）和化学机制（CM）。电磁增强机制是由于表面等离子基元被入射光激发所引起的，近几十年中已被研究的较为深入。然而，化学增强机理的根源以及其对SERS的贡献仍然存在争议，因为它和电磁增强同时出现。因此，在对化学增强机制的研究中，寻找一种不存在电磁增强只存在化学增强且独立可调的基底至关重要。
石墨烯是由单层碳原子堆垛而成的蜂窝状晶体层，具有理想的二维结构，是一种具有多项优异性能的特殊材料。石墨烯具有相当光滑的表面，并且在可见光范围内其表面光学透过率可达95%以上，另外，石墨烯的表面等离子基元可达到太赫兹，可见，石墨烯是研究化学增强机理的理想基底。本论文采用氧化还原法制备石墨烯，不同还原程度的氧化石墨烯做基底来研究其SERS性能的变化，并且对GO/Au纳米复合材料做基底时GO所起的作用进行了分析。
本论文第一部分采用Hummers方法获得氧化石墨，经过超声剥离，采用水热法对氧化石墨烯进行还原。分别在酸性条件和碱性条件下通过调节还原温度和还原时间制备不同还原程度的石墨烯，经研究发现，酸性条件下氧化石墨烯的还原速率低于碱性条件，并且两种体系中获得的还原氧化石墨烯样品去除的含氧官能团种类不同。我们采用具有剧烈的荧光效应的罗丹明6G（R6G）分子作为探针分子，对氧化石墨烯和还原的氧化石墨烯进行了拉曼增强效应检测，发现酸性和碱性条件下尽管移除的含氧官能团不同，但拉曼散射峰的强度均减弱，这说明GO中各种含氧官能团对其表面所吸附分子的SERS性能均有贡献，因而只要氧化石墨烯的官能团数量变化，分子拉曼散射信号就会改变。另外还原GO可以抑制探针分子的荧光背底，且随着官能团数量的减少荧光信号降低。
贵金属纳米材料是表面增强拉曼散射常用的基底，它具有优良的物理、化学性能，具有较强的拉曼增强效果。我们知道，分子的拉曼增强散射信号强度依赖于增强区域内分子的数量，但具有苯环结构的分子却很难在贵金属表面吸附，这就限制了拉曼增强的效果。另外，在传统的SERS体系中，金属基底与探针分子直接作用，二者不可避免的相互产生影响，这将使被检测分子的荧光产生或增大或淬灭的现象，人们难以得到本征拉曼散射信号。为了获得一种增强活性高、荧光淬灭效果好、并且表面对不同结构的分子均具有较强吸附性的SERS基底，我们将具有SERS活性、具有较好的荧光淬灭效果并且对芳香结构的分子具有高的亲和性的氧化石墨烯（GO）与Au纳米粒子进行复合，作为拉曼增强的基底，采用浓度为8×10~(-5)M/L的罗丹明6G（R6G）和浓度为1×10~(-4)M/L的2-疏基吡啶（2-MPy）作为探针分子。研究发现：无法在金纳米颗粒表面吸附的R6G分子在加入GO之后则产生SERS效应，并且GO的含量对分子产生的荧光背底有较大的影响；2-MPy作为探针分子时，GO的加入基本没有改变拉曼散射强度，只是被检测分子的拉曼、荧光信噪比增大一个数量级。这部分实验结果说明在石墨烯基金属纳米复合材料的SERS基底中，GO在金属表面和探针分子之间起到了连接桥梁的作用，并且能够使分子产生的荧光得到有效的淬灭。
Surface-enhanced Raman scattering (SERS) is an surface alternativeenhancement technique, it will increase the raman intensities of molecules adsorbed ata certain substrate up to several orders of magnitude, and it greatly promotes the application oframan spectroscopy used for scientific researching and characterizing the structure ofmaterials. For SERS, though there are numerous experimental and theoretical workson it, there are also many controversies about the mechanism and no complete pictureof the enhancement mechanism is available even now. Normally, the two widelyaccepted mechanisms are electromagnetic mechanism (EM) and chemical mechanism(CM). EM is mainly due to the surface plasmons excited by the incident light, andwas achieced some results now. However, the origin and the role of the chemicalenhancement are constantly under debate because of its enhancement with theelectromagnetic enhancement. It is, therefore, important to find a substrate that doesnot possess the electromagnetic enhancement, retaining the chemical enhancementalone and independently tunable.
Graphene, has the ideal2D structure with a monolayer of carbon atoms packedinto a honeycomb crystal plane, is a special material with many spark stellarperformance. Considering graphene, first, its surface is relatively smooth in despite offluctuations that follow from the under lying substrate. Second, the opticaltransmission through the graphene surface in the visible range is higher and is morethan95%. Besides, the surface plasmon on graphene is in the range of terahertz ratherthan in the visible range. So，on the basis of these considerations, graphene does notsupport EM. On the other hand, for CM graphene has possibilities. This paper wasprepared graphene by Chemical methods, the different reduced degrees of grapheneoxide used as substrates to study its SERS performance, and we analyzed the role ofGO in GO/Au nanocomposites when it used for SERS substrate.
In the first part, we synthesized the graphite oxide by the Hummer method,through ultrasound, hydrothermal method was used to reduced graphene oxide(GO). Respectively, under acidic condition and alkaline condition, graphene with differentreduction degrees was obtained by adjusting the reduction temperatures and times. Itis found that the reduction rate of the graphene oxide under acidic condition is slowerthan the alkaline conditions, and the removal of oxygen functional groups aredifferent at this two systems. Rhodamine6G (R6G) with intense fluorescent effectused for probe molecule, and we used GO and reduced GO as substrates for Ramanenhancement experiments, found that the Raman signal intensity became weak withthe reduction degree improving, and the removal of oxygen functional groups weredifferent, but Raman scattering peak positions did not change, indicating that theinteraction between the SERS substrate and the probe molecule have not connectionwith the types of oxygen functional groups in GO, whereas it only impacted by thedegree of the reduction GO, and restore the more thorough, the weaker signalintensities. In addition, reduced GO can inhibit the fluorescence background of theprobe molecules, indicating that reduced GO has fluorescence quenching effect.
The noble metal nanomaterials are commonly used as SERS substrate, they haveexcellent physical and chemical properties, and have strong Raman enhancementactivities. As we know, the Raman scattering signal intensity of molecular depends onthe number of molecules in the enhanced region, but metal surface is difficutlt toadsorb the molecules with benzene ring structure, which limits the Ramanenhancement effect. In the traditional SERS system, metal substrates contact with theprobe molecules directly, there must be exist interactions between both, which willmake the fluorescence background of moleculars to change and difficult to getthisintrinsic Raman scattering signal. In order to obtain an SERS substrate, whichinclude high enhanced activity, perfect fluorescence quenching effect and its surfacehas a strong adsorption for different molecules, therefore, we synthesized GO and Aunanoparticles. The graphene-based nanocomposite used as SERS substrate, andadopted Rhodamine6G (R6G,8×10~(-5)M/L) and2-mercaptopyridine (2-MPy,1×10~(-4)M/L) as probe molecules. The study found that: R6G molecules unable adsorbedon the surface of gold nanoparticles, but Raman signals appear after added into theGO and content of GO influences the molecular fluorescence background greatly. 2-MPy as a probe molecule, the Raman scattering intensity is virtually unchanged andonly suppress the fluorescence signal after added into the GO, indicateing that in thegraphene-based nanocomposite substrate, GO plays as a bridge to connect betweenthe metal surface and the probe molecules, and it has fluorescent quenching effect.
石墨烯是由单层碳原子堆垛而成的蜂窝状晶体层，具有理想的二维结构，是一种具有多项优异性能的特殊材料。石墨烯具有相当光滑的表面，并且在可见光范围内其表面光学透过率可达95%以上，另外，石墨烯的表面等离子基元可达到太赫兹，可见，石墨烯是研究化学增强机理的理想基底。本论文采用氧化还原法制备石墨烯，不同还原程度的氧化石墨烯做基底来研究其SERS性能的变化，并且对GO/Au纳米复合材料做基底时GO所起的作用进行了分析。
本论文第一部分采用Hummers方法获得氧化石墨，经过超声剥离，采用水热法对氧化石墨烯进行还原。分别在酸性条件和碱性条件下通过调节还原温度和还原时间制备不同还原程度的石墨烯，经研究发现，酸性条件下氧化石墨烯的还原速率低于碱性条件，并且两种体系中获得的还原氧化石墨烯样品去除的含氧官能团种类不同。我们采用具有剧烈的荧光效应的罗丹明6G（R6G）分子作为探针分子，对氧化石墨烯和还原的氧化石墨烯进行了拉曼增强效应检测，发现酸性和碱性条件下尽管移除的含氧官能团不同，但拉曼散射峰的强度均减弱，这说明GO中各种含氧官能团对其表面所吸附分子的SERS性能均有贡献，因而只要氧化石墨烯的官能团数量变化，分子拉曼散射信号就会改变。另外还原GO可以抑制探针分子的荧光背底，且随着官能团数量的减少荧光信号降低。
贵金属纳米材料是表面增强拉曼散射常用的基底，它具有优良的物理、化学性能，具有较强的拉曼增强效果。我们知道，分子的拉曼增强散射信号强度依赖于增强区域内分子的数量，但具有苯环结构的分子却很难在贵金属表面吸附，这就限制了拉曼增强的效果。另外，在传统的SERS体系中，金属基底与探针分子直接作用，二者不可避免的相互产生影响，这将使被检测分子的荧光产生或增大或淬灭的现象，人们难以得到本征拉曼散射信号。为了获得一种增强活性高、荧光淬灭效果好、并且表面对不同结构的分子均具有较强吸附性的SERS基底，我们将具有SERS活性、具有较好的荧光淬灭效果并且对芳香结构的分子具有高的亲和性的氧化石墨烯（GO）与Au纳米粒子进行复合，作为拉曼增强的基底，采用浓度为8×10~(-5)M/L的罗丹明6G（R6G）和浓度为1×10~(-4)M/L的2-疏基吡啶（2-MPy）作为探针分子。研究发现：无法在金纳米颗粒表面吸附的R6G分子在加入GO之后则产生SERS效应，并且GO的含量对分子产生的荧光背底有较大的影响；2-MPy作为探针分子时，GO的加入基本没有改变拉曼散射强度，只是被检测分子的拉曼、荧光信噪比增大一个数量级。这部分实验结果说明在石墨烯基金属纳米复合材料的SERS基底中，GO在金属表面和探针分子之间起到了连接桥梁的作用，并且能够使分子产生的荧光得到有效的淬灭。
Surface-enhanced Raman scattering (SERS) is an surface alternativeenhancement technique, it will increase the raman intensities of molecules adsorbed ata certain substrate up to several orders of magnitude, and it greatly promotes the application oframan spectroscopy used for scientific researching and characterizing the structure ofmaterials. For SERS, though there are numerous experimental and theoretical workson it, there are also many controversies about the mechanism and no complete pictureof the enhancement mechanism is available even now. Normally, the two widelyaccepted mechanisms are electromagnetic mechanism (EM) and chemical mechanism(CM). EM is mainly due to the surface plasmons excited by the incident light, andwas achieced some results now. However, the origin and the role of the chemicalenhancement are constantly under debate because of its enhancement with theelectromagnetic enhancement. It is, therefore, important to find a substrate that doesnot possess the electromagnetic enhancement, retaining the chemical enhancementalone and independently tunable.
Graphene, has the ideal2D structure with a monolayer of carbon atoms packedinto a honeycomb crystal plane, is a special material with many spark stellarperformance. Considering graphene, first, its surface is relatively smooth in despite offluctuations that follow from the under lying substrate. Second, the opticaltransmission through the graphene surface in the visible range is higher and is morethan95%. Besides, the surface plasmon on graphene is in the range of terahertz ratherthan in the visible range. So，on the basis of these considerations, graphene does notsupport EM. On the other hand, for CM graphene has possibilities. This paper wasprepared graphene by Chemical methods, the different reduced degrees of grapheneoxide used as substrates to study its SERS performance, and we analyzed the role ofGO in GO/Au nanocomposites when it used for SERS substrate.
In the first part, we synthesized the graphite oxide by the Hummer method,through ultrasound, hydrothermal method was used to reduced graphene oxide(GO). Respectively, under acidic condition and alkaline condition, graphene with differentreduction degrees was obtained by adjusting the reduction temperatures and times. Itis found that the reduction rate of the graphene oxide under acidic condition is slowerthan the alkaline conditions, and the removal of oxygen functional groups aredifferent at this two systems. Rhodamine6G (R6G) with intense fluorescent effectused for probe molecule, and we used GO and reduced GO as substrates for Ramanenhancement experiments, found that the Raman signal intensity became weak withthe reduction degree improving, and the removal of oxygen functional groups weredifferent, but Raman scattering peak positions did not change, indicating that theinteraction between the SERS substrate and the probe molecule have not connectionwith the types of oxygen functional groups in GO, whereas it only impacted by thedegree of the reduction GO, and restore the more thorough, the weaker signalintensities. In addition, reduced GO can inhibit the fluorescence background of theprobe molecules, indicating that reduced GO has fluorescence quenching effect.
The noble metal nanomaterials are commonly used as SERS substrate, they haveexcellent physical and chemical properties, and have strong Raman enhancementactivities. As we know, the Raman scattering signal intensity of molecular depends onthe number of molecules in the enhanced region, but metal surface is difficutlt toadsorb the molecules with benzene ring structure, which limits the Ramanenhancement effect. In the traditional SERS system, metal substrates contact with theprobe molecules directly, there must be exist interactions between both, which willmake the fluorescence background of moleculars to change and difficult to getthisintrinsic Raman scattering signal. In order to obtain an SERS substrate, whichinclude high enhanced activity, perfect fluorescence quenching effect and its surfacehas a strong adsorption for different molecules, therefore, we synthesized GO and Aunanoparticles. The graphene-based nanocomposite used as SERS substrate, andadopted Rhodamine6G (R6G,8×10~(-5)M/L) and2-mercaptopyridine (2-MPy,1×10~(-4)M/L) as probe molecules. The study found that: R6G molecules unable adsorbedon the surface of gold nanoparticles, but Raman signals appear after added into theGO and content of GO influences the molecular fluorescence background greatly. 2-MPy as a probe molecule, the Raman scattering intensity is virtually unchanged andonly suppress the fluorescence signal after added into the GO, indicateing that in thegraphene-based nanocomposite substrate, GO plays as a bridge to connect betweenthe metal surface and the probe molecules, and it has fluorescent quenching effect.
引文
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