**6. Challenges and outlook**

As a promising advanced analytical technique, SERS has illustrated its potential for a wide range of applications due to its capability of achieving high sensitivity and providing molec‐ ular information. In this review, we mainly summarized the achievements in the selective target trace detection for the environmental application. For a given target, an appropriate strategy should be chosen according to the nature of the target and the sample matrix. Even though a great achievement has been made in this area, there still are some challenges for the real implementation in environmental analysis.

Surface functionalization strategies may be applied to enhance the detection sensitivity and selectivity, but the high cost of the capture element such as antibody and their low stability under harsh analysis conditions are not favorable in the large‐scale practical applications, which need to be improved by further studying. In addition to the sensitivity, for the developing of SERS‐based methods for environmental detection, the attention should also be paid to the guarantee of good accuracy and reproducibility. The highly sensitive SERS response can lead to signal deviation, while reliable quantification becomes difficult due to the relatively poorer reproducibility. Fabrication of substrates with high selectivity, enough sensitivity and reliable reproducibility are the basic demands for real application. On the other hand, advanced data analysis technique such as chemometrics methods can also be applied to further get into the abundant spectrum information for multiplex component analysis. By the way, most of the reported SERS substrates are still based on results obtained in laboratory, and commercial products are still rare. Therefore, more concerning should be paid onto developing substrate that can be manufactured on large scale and used easily during the on‐site analysis. Concerning to large batch production, the cheap, robust and stable substrate such as hydrogel or paper‐based substrate would find more applications.

Detection systems that can be directly applied for infield SERS detection are still rare. For the practical application, the followings are required: (1) the preoperation for the sample preparation should be easy and fast. (2) the target capture, concentration and separation should also be accomplished without additional complex procedures. (3) the detection system should be portable and can be easily extended to the infield detection. In order to meet the demands for real sample which also contains complex matrix, we believe that the future development of SERS‐based detection strategy will be combined with advanced separation or concentration techniques such as fluidic chip devices and test strip techniques. This trend will continue through the future designing of portable integrated systems.
