引言(Introduction)可长可短，结构，布局也相对灵活，写作策略、技巧也十分多样，一篇好的引言需要反复的打磨和修改。这里给低年级的研究生(化学、材料背景)介绍一种写引言的方法。一般而言，引言的基本逻辑是摘要的前三点(论文摘要怎么写? - 知乎)：研究背景，问题，文章主旨。对于新手而言，比较安全的写作策略是把每一点展开成一段。下面沪江小编以一篇文章(Syringe-injectable electronics，Jia Liu et al., Nat. Nanotechnol. 10, 629–636 (2015))为例介绍这种写法 (为了使界面清晰简洁，引文省略了参考文献)。这篇文章的研究对象是植入式电子器件，具体是研发了一种可以通过注射器注射植入的柔性电子器件，解决了植入式电子器件的定点植入问题。
引言部分如下:
The emergence of flexible electronics has significantly extended the applications of electronics by allowing intimate interfaces between electronic units and non-planar surfaces for better monitoring and manipulation of their properties. A variety of electronic devices have been integrated on flexible and stretchable substrates, enabling applications from foldable displays to electronic skin. Three-dimensional (3D) interpenetration of flexible electronics within existing structures could further broaden and open up new applications by directly interfacing devices with the internal structures of man-made and biological materials.
第一段交代研究背景。这一段可以分为三个层次：(1)界定研究领域——柔性电子器件，顺便介绍该领域研究的重要意义;(2)一句话概述研究领域的发展现况——柔性电子器件的多样态及广泛应用;(3)引出本文研究对象——三维可植入柔性电子器件。这三个层次是递进关系，逐渐聚焦到本文的研究对象。
Recent work has shown that flexible electronics can be placed into 3D structures through surgical processes or by being attached to and subsequently released from a rigid delivery substrates for biological and biomedical applications. However, direct 3D interpenetration of electronics within these structures is limited by the intrinsic thin-film supporting substrates. We have previously introduced a macroporous mesh paradigm that allows electronics to be combined with polymer precursors and cells to yield 3D interpenetration, although controlled delivery and/or non-surgical placement of these ultraflexible open electronic networks into structures with seamless 3D integration and interpenetration has not been possible.
第二段综述研究进展，指出存在的问题。陈述逻辑是由已知到未知，即“现在已经实现了A，但是还不能实现B/但是A还存在局限”。首句承接上文，综述研究进展，接着以However为标志，展开对问题的讨论，为后文陈述研究工作作铺垫，这是本段的重点。作者还特别交代了在这个问题上的前期工作，目的是(1)突出问题——尽管我们已经做了许多前期工作，这个问题仍然没有解决;(2)突出自己工作的连续性和系统性——我们已经在这个问题上深耕多年;(3)区别前期工作(下一段详述)。
Here, we describe the design and demonstration of macroporous flexible mesh electronics, where the electronics can be precisely delivered into 3D structures by syringe injection, whereupon they subsequently relax and interpenetrate within the internal space of man-made and biological materials. Distinct from previous reports, syringe injection requires the complete release of the mesh electronics from a substrate so that the electronics can be driven by solution through a needle. The syringe-injectable electronics concept involves (1) loading the mesh electronics into a syringe and needle, (2) insertion of the needle into the material or internal cavity and initiation of mesh injection, (3) simultaneous mesh injection and needle withdrawal to place the electronics into the targeted region, and (4) delivery of the input/output region of the mesh outside the material for subsequent bonding and measurements.
第三段概述本文内容，申明论文重点和亮点。这一段也可以分成三层：(1)第一层通常以Here, we show/report/present的固定句式开头，概述本文的研究工作，侧重对思路，策略及方法的讨论，不涉及具体的实验细节。(2)交代本文与(自己的或者是别人的)前期工作的区别——与前期工作相比，本文在哪些方面取得了突破。如果省去不写，第一审稿人有可能认为作者故意隐瞒前期工作以突出本文的创新性(审稿人是会去查相关文献的);第二让本文的工作显得孤立，没有多少积累，底气不足。所以，与其在cover letter里回应审稿人“你们/别人以前都做过了”的质疑，不如一开始就主动交代，划清界限，而且在文字上也有更多处理的空间(也需要更多的文字技巧);(3)明确指出研究工作的重要性和创新性。这里有一个写作技巧值得借鉴：以分点列举的方式陈述工作亮点，编辑/审稿人/读者可以一目了然 (注意：文中的数字序号要加括号，否则会被认为是数学意义上的数字)。
在写作引言时，有几个问题值得注意：
• 引言部分宁短勿长。刚开始写论文的同学最好把引言部分写得简洁精练(如上面介绍的三段式写法)。比如这篇论文(Science, 2016, 353, 1413)只用一段话就能把问题说的很清楚。如果没有多少写作经验很难驾驭长篇大论，极容易在写作过程中攀生枝节，造成逻辑主线模糊，同时影响阅读体验。引言部分最关键的是要保证逻辑线条的清晰。另外关于语言，引言部分多用长句，即以复合句，从句，现在分词做状语及独立主格结构为主。一句话只说一件事，忌构筑复杂的长难句，添加不必要的修饰成分(如插入语、同位语等)(这点和雅思写作的要求是相悖的)，表意清晰最重要。
• 研究背景不要过度引申。研究背景只需要写与本文紧密相关的内容，不要过度引申。比如，写一个用石墨烯作超级电容器的工作既不需要从碳的同素异形体写起，也不需要向读者科普什么叫超级电容器。再如，写一个纤维状太阳能电池的工作不需要牵扯到能源问题。如：
It is not an exaggeration to say that the world will endure a global energy crisis in the near future. The foreseeable exhaustion of fossil fuels has boosted the development of alternative energy sources. As one of the promising candidates, solar power has hitherto attracted massive amounts of attention and thus inspired the exploitation of photovoltaic devices. The traditional silicon-based technology has confronted serious challenges as the heavy bulky packages are impractical for the ever-growing requirements of portable, flexible and low-cost devices.
删除前两句话这一段引言的逻辑也是成立的。
• 切入主题要自然，不要生搬硬套，拐弯抹角。试比较下面两篇引言的开头部分：
Tunable Photothermal Actuators Based on a Pre-programmed Aligned Nanostructure
By organizing their microscopic structures, plants are able to generate diverse mechanical motions in response to external stimuli. For instance, hygroscopic seed dispersal, a type of plant motion, consists of a variety of deformations that share a similar structure and mechanism. The orientation of cellulose fibrils embedded in a matrix of hemicellulose, lignin, pectin, and structural proteins plays a key role in such an actuation, e.g., two-dimensional opening movement in pine cones and three-dimensional twisting motion in Bauhinia variegata pods (Figure 1a,b). Improved understanding of the structure-response relationships in plants has inspired increasing interest in the synthesis of mechanically responsive materials with diverse actuation forms that resemble these biological structures.
作者想从仿生学的角度切入，但是划线部分迟迟不进入主题，并且用了很多专业名词，删除划线部分，引言的逻辑反而更清晰。下面这篇引言同样从生物角度切入，但是从第二句开始句句点题，逻辑清晰，可以顺利实现由自然结构到人工设计的叙事逻辑。
Hierarchically arranged helical fibre actuators driven by solvents and vapours
Mechanical responsiveness to external stimuli, an intrinsic property of plants, enables the performance of vital tasks such as seed dispersal, self-protection and food acquisition. These mechanical responses are derived from the internal helical structure. For instance, torsional movements, including the twisting of wood cells, helical motion of tendrils and coiling of awns, are generated by differential swelling or shrinkage in different parts of the tissue in response to a change in humidity. These swelling and shrinking behaviours are attributed to the helically winding structure of cellulose microfibrils (Fig. 1a,b). (Nat. Nanotechnol., 2015, 10, 1077)
• 研究进展只概括陈述和自己研究紧密相关的内容，不要写成文献综述，也不要指名道姓地陈述别人的工作，文章应该着眼于解决一类问题才能吸引广泛的读者群，这样写会让文章针对性太强 (too specific)，如
Nowadays, the important hinder is the industrial production and application of fiber-shape supercapacitors. Yu et al. has continuously prepared SWNT/rGO hybrid fibers by in situ thermal treatment in an oven at 220 oC for 6 h. Kou et al. developed a coaxial wet-spinning assembly approach to continuously spin polyelectrolyte-wrapped graphene/CNT core-sheath fibers. (Unpublished Manuscript)
比较常用的写法是以列举的方式概括陈述，如
Starting from a widely available GO precursor, the main strategy to assemble porous 3D graphene networks is the self-assembly or gelation of the GO suspension via hydrothermal reduction, chemical reduction or direct crosslinking of the GO sheets. (Nat. Comm., 2015, 6, 6962)Because of their Pt-like catalytic behaviours, Mo-based compounds, such as Mo2C, MoN, MoS2, and others have attracted substantial interest as a new class of electrocatalysts. (Nat. Comm., 2016, 7, 11204)
Recently, fibre-shaped micro-SCs have been developed for flexible and wearable electronics, based on carbon fibres, CNT fibres, rGO fibres and even carbon-based composite fibres containing pseudo-capacitive materials. (Nat. Nanotechnol. 2015, 9, 555)
准确引用参考文献。要重视参考文献的引用，不要随意引用，不要过量引用，也不要缺引和漏引。参考文献一般引用高水平，有代表性的学术文献，引用原始文献，引用最新发表的重要文献，少引或者不引综述性的文章，特别是避免引用很长的综述(因为读者不会去看)，不要在不相关的地方自引。
引言的组织结构有很多种，这里只介绍了其中一种写法，其逻辑比较简单直接，适用于大多数情况。平时多研究一些高水平论文的引言写法会对自己的论文写作有帮助，而要做到写出高屋建瓴的引言非计日程功，需要对自己的研究领域有长期积累和思考。希望上述沪江小编的分享能够帮助大家提升英语论文中引言写作的效果。