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【翻译人：段萌 2011310200426 应化1104】高水溶性银纳米晶体的尺寸可控性制备胡永星，葛建平，林唐娜，张铁瑞，尹亚东（美国加利福尼亚大学河滨分校CA92521，化学系）文 章 信 息：文章历史：2008年1月4日收到来稿，2008年1月30日修改，2008年2月7日在线可用关键词：纳米晶体 银 胶体 表面活性剂 尺寸控制 水溶性摘 要：我们描述了一个改进后的多元醇合成银纳米晶体的制备过程，所合成出的纳米晶体尺寸范围为几毫微米到20nm。聚乙烯吡咯烷酮在传统多元醇合成过程中大大限制了银纳米晶体的生长，阻止颗粒的聚集与融合，导致一个统一整体样品的高溶解度。通过控制反应时间，反应物质的浓度和高分子表面活性剂的链长度，以及反应温度，可以方便的调整纳米晶体的尺寸，均一的尺寸低于20nm的银纳米晶体在高密度光学吸收所需大颗粒应用中是首选，例如，在癌症治疗中的光热光谱分析转换。爱思唯尔公司出版1. 引言银纳米晶体的合成集中研究了很多年，主要是因为其局部表面强所等离子振动引起的不寻常的光学性质。基于银离子的高还原电位发展了大量的化学方法制备各种尺寸，形状以及表面性质的银纳米晶体。已知的一些著名的途径包括照射或辐射还原，辅助表面活性剂对水的减少或其他极性溶剂，溶剂热合成，控制乳胶团的增长，树状分子制模，多元醇过程，以及非极性溶剂中热解。最近的进展使制备从球体到电线、立方体、盘状、棱镜和双椎体等各种形状测距的银纳米晶体成为可能。尽管已经有很多成功的银纳米晶体合成过程，有趣的是，一个详细的文献检索显示，仅有很少的方法能可行的产生水溶性均匀，尺寸控制在20nm以下的银纳米晶体。著名的多元醇过程使用作为表面活性剂，难以制备出具有低于50nm的精密尺寸分布的银粒子。一个很有前景的方法可以解决窄范围尺寸分布涉及到银盐在非极性溶剂中的热解。然而，疏水表面的产品实际上限制了他们的应用，特别是在水溶性作为先决条件的生物系统中。附加步骤一般需要将疏水性纳米颗粒转移到水中。在本文中，我们报告了一个简单修改了传统的多元醇过程，该过程能生成高度水溶性银纳米晶体其尺寸可控制在几毫微米到20nm之间。银纳米晶体在这个尺寸范围有较大不同的特性,例如,他们的吸收主要贡献于消光光谱,而有效的散射产生较大粒子。小型银纳米晶体更高的吸收能量密度可能使他们首选于一些应用如光照疗法。合成过程中关键改进的是用聚丙烯酸(PAA)代替传统使用的PVP作为直接生长银纳米晶体的表面活性剂。对比PVP,PAA由于对银粒子表面羧酸盐集团更高的协调力量，显示更强的尺寸限制效应。此外,不同位的羧酸盐组扩展到水溶液中,赋予粒子在水中一个高水平的分散性，同时也为生物分子通过发达的生物缀合化学的进一步附着提供丰富的参照点。同时发现，银纳米晶体在存在PAA的生长遵循一个简单的单体聚合模型，该模型不同于在PVP案例中的凝固模型。这个发现利用先前对于疏水胶体纳米晶体在非极性有机溶剂散热生长的成熟的思想，方便了尺寸控制。2. 实验部分 2.1试剂与材料硝酸银(AgNO3,99 + %),短链PAA粉(Mw=1800),长链PAA水溶液(Mw=50000,25%)【从西格玛奥德里奇购买】。(EG,99 + %) ，(TEG,99%)【购买自Acros Organics公司】。(DEG,试剂级)【购买自费舍尔科学】。所有的试剂在使用前未经纯化。 2.2银纳米晶体的合成通过减少硝酸银在溶剂中的沸点，使用改性多元醇过程，合成各种尺寸的银纳米晶体。多元醇如乙二醇、二乙二醇和三甘醇被用作溶剂和还原剂。PAA作为表面活性剂来控制银纳米晶体的生长。在一个典型的合成、硝酸银(0.1 g)溶解在乙二醇(3毫升)中，在室温下快速被注入沸腾溶液乙二醇(15毫升)和PAA(0.030M)，同时在氮气保护气下强力搅拌。在注射后的前体溶液，用针管在不同的时间提取等分试样。样品冷却至室温，析出过多的蒸馏水，然后通过离心分离，在11000RPM清洗3次，最后重新分散在蒸馏水中，用稀NaOH溶液的表面羧酸基团中和。 2.3产品描述 使用Tecnai T12透射型电子显微镜（TEM）描述形貌和粒度分布的产品特征。使用Tecnai G2 S-200KV双电子显微镜获得高分辨率透射电子显微镜（HRTEM）图像。在室温下分散在水中的纳米晶体浇铸到碳包覆铜网格中然后在真空下蒸发。通过测量每个超过250个单位的样品得到的纳米粒子的尺寸分布。测定晶体结构采用Bruker D8预先具有CuKa辐射（=1.5418A）的X射线衍射仪（XRD）。在每点1.5秒的扫描速率下，数据的收集从2Y=30-701。海洋光学HR2000CG紫外 - 近红外光谱仪测得的银纳米晶体的水分散液的吸收光谱。 2.4结果与讨论 在传统的多元醇过程中，通过多元醇如乙二醇在高温下减少金属阳离子。在这个过程中，PVP作为控制生长的金属颗粒并防止其结块的表面活性剂已被广泛使用。由于吡咯烷酮的弱配位的金属表面，一个多元醇的过程通常会导致在金属颗粒具有相对大的尺寸。如在图1A中所示就是一个典型的例子，在PVP存在下的元醇还原的银阳离子颗粒，具有从50nm到100nm以上的尺寸范围。此外，如果没有额外的控制手段，产品通常会有一个广泛的粒度分布不均匀的形状。这是因为银粒子的形成并不严格遵循经典的模型，在该模型中，从过饱和的化学反应产生的单体前体，颗粒的进一步增长，实现了吸收新形成的单体成核结果。我们的观察结果显示，使用PVP作为唯一增长的导向剂，颗粒间的聚集和融合过程发生在反应的后期，可以在图-1A中清楚地观察到一些二聚体纳米晶标记的箭头。因此，最终具有很强多晶粒子性的产品，含有大量的缺陷，如双晶面。 在当前方法中的主要改进是使用PAA取代广泛使用PVP作为表面活性剂。羧酸银表面的协调远远强于PVP，PAA能有效地限制纳米晶体生长，防止日益增长的颗粒（图1B）之间的聚集和融合。利用聚丙烯酸作为表面配位体的一个额外的好处是不协调的羧酸酯基团的聚合物链上提供了产品优良的水溶性，以及方便的生物分子附着的共轭点。图1C有计划性地描述了利用聚丙烯酸作为表面活性剂合成的银纳米晶体的表面结构。原文：Size-controlled synthesis of highly water-soluble silver nanocrystalsYongxing Hu, Jianping Ge, Donna Lim, Tierui Zhang, Yadong Yin?Department of Chemistry, University of California, Riverside, CA 92521, USAa r t i c l e i n f oArticle history:Received 4 January 2008Received in revised form30 January 2008Accepted 7 February 2008Available online 4 March 2008Keywords:Nanocrystals Silver Colloid Surfactant Size-control Water solublea b s t r a c tWe describe a modified polyol process for the synthesis of silver nanocrystals with uniform sizes ranging from several nanometers to20nm. The use of polyacrylic acid, in place of polyvinylpyrrolidone in the conventional polyol process, significantly limits the growth of silver nanocrystals, prevents the interparticle aggregation and fusion, and leads to a uniform population of samples with high water solubility. The size of nanocrystals can be conveniently tuned by controlling the reaction time, the concentration and chain length of the polymeric surfactants, and the reaction temperature. Uniform silver nanocrystals within sizes below 20nm are preferred candidates over larger particles for applications where high density of optical absorption is required, for example, for photothermal conversion in cancer therapy.Published by Elsevier Inc.1. IntroductionThe synthesis of silver nanocrystals has been intensively studied for many years mainly because of their unusual optical property as the result of strong localized surface plasmon oscillation. Thanks to the high reduction potential of silver cations, a large number of chemical methods have been developed to the preparation of silver nanocrystals with various size, shape and surface properties. Some notable approaches include photo or radiation reduction , surfactant-assisted reduction in water or other polar solvents, solvothermal synthesis,controlled growth in micelles or microemulsions, templating using dendrimers, polyol process, and thermolysis in nonpolar solvents. Very recent progresses have made it possible to produce silver nanocrystals in various shapes ranging from spheres to wires, cubes, plates, prisms, and bipyramids. Despite so much success in silver nanocrystal synthesis,interestingly, a careful literature search indicates that there are very few robust approaches capable of producing uniform water-soluble silver nanocrystals with tunable sizes below 20nm. The well-known polyol process using polyvinylpyrrolidone (PVP) as the surfactant has difficulties to produce silver particles with a narrow size distribution for sizes below ?50nm. A promising method that can address this size range with narrow size distribution involves the thermolysis of silver salt in nonpolar solvents. However, the hydrophobic surfaces of the products practically limit their applications, particularly in biological systems where water solubility is one of the prerequisites. Additional steps of surface modification are generally required to transfer the hydrophobic nanoparticles to water.In this paper, we report that a simple modification to the traditional polyol process can produce highly water-soluble silver nanocrystals with uniform and tunable sizes from several nanometers to ?20nm. Silver nanocrystals within this size range have some different properties from those larger ones, for example, their extinction spectra are mainly contributed by absorption, while significant scattering occurs for larger particles. The higher density of absorbed energy for small silver nanocrystals may make them preferred candidates for applications such as photothermal therapy. The key improvement in our synthesis is to use polyacrylic acid (PAA) instead of traditionally used PVP as the surfactant to direct the growth of silver nanocrystals. Comparing to PVP, PAA shows stronger size-limiting effect due to the higher coordination power of carboxylate groups to silver particle surfaces. In addition, uncoordinated carboxylate groups extend into the aqueous solution, conferring upon the particles a high degree of dispersibility in water,and also providing abundant anchoring points for further attachment of biomolecules through the well-developed bioconjugation chemistry. It has also been noticed that the growth of silver nanocrystals in the presence of PAA follows a simple monomer-addition growth model which is different from the coagulation model in the PVP case. This finding allowsconvenient size control by taking advantage of concepts developed previously for the thermolytic growth of hydrophobic colloidal nanocrystals in nonpolar organic solvents .2. Experimental section2.1. Chemicals and materialsSilver nitrate (AgNO3, 99+%), short-chain PAA powder (Mw=1800), and long-chain PAA aqueous solution (Mw= 50,000, 25%)were purchased from Sigma-Aldrich. Ethylene glycol (EG, 99+%)and Triethylene glycol (TEG, 99%) were obtained from Acros Organics.Diethylene glycol (DEG, reagent grade) was purchased from Fisher Scientific. All reagents were used without further purification.2.2. Synthesis of Ag nanocrystalsSilver nanocrystals with various sizes were synthesized using a modified polyol process, by reducing AgNO3 at the boiling temperature of the solvent. Polyols such as EG, DEG, and TEG were used as the solvent as well as the reducing agent. PAA was used as a surfactant for controlling the growth of silver nanocrystals. In a typical synthesis, AgNO3 (0.1g) dissolved in EG (3ml) at room temperature was quickly injected into a boiling solution of EG (15ml) and PAA (0.030M) with vigorous stirring under a protective nitrogen atmosphere. Aliquots were extracted using a needle at various times after the injection of precursor solution. Samples were cooled to room temperature, cleaned three times by precipitation with excessive distilled water followed by centrifugation at 11000rpm, and finally re-dispersed in distilled water by neutralization of the surface carboxylic acid groups with dilute NaOH solution.2.3. CharacterizationMorphology and size distribution of the products were characterized using a Tecnai T12 transmission electron microscope (TEM). High-resolution TEM (HRTEM) images were obtained using a Tecnai G2 S-Twin electron microscope operated at 200kV. The nanocrystals dispersed in water were cast onto a carbon-coated copper grid, followed by evaporation under vacuum at room temperature. The size distribution of the nanoparticles was obtained by measuring over 250 units for each sample. Crystal structures were measured on a Bruker D8 advance X-ray diffractometer (XRD) with a CuKa radiation (l 1.5418A).The data were collected from 2y 30701 at a scan rate of 1.5s per point. The absorption spectrum of an aqueous dispersion of silver nanocrystals was measured by an Ocean Optics HR2000CG-UVNIR spectrometer.3. Results and discussionIn a traditional polyol process, metal cations are reduced by polyols such as EG at an elevated temperature. PVP has been extensively used in this process as the surfactant to control the growth of metal particles and prevent their agglomeration. Due to the weak coordination of pyrrolidone to metal surface, a polyol process usually results in metal particles with relatively large size . As shown in a typical example in Fig.1A, the polyol reduction of silver cations in the presence of PVP leads to particles with typical sizes ranging from 50nm to above 100nm 40,41. Also,the products usually have a broad size distribution and nonuniform shapes if no additional means of control is applied. This is because the formation of silver particles does not strictly follow the classic LaMer model in which nucleation results from the supersaturation of monomers produced by chemical reactions of precursors, and the fur