**1. Introduction**

After several decades of development since it was discovered on electrochemically roughened silver in 1973 [1,2], surface-enhanced Raman scattering (SERS) has become a powerful analytical tool for applications of chemical and biological molecule detection, environmental monitoring, and food safety [3–8]. SERS is able to identify molecules through vibrational fingerprint signals and can even detect single molecules [9,10]. It is well accepted that a Raman signal can be enormously enhanced by noble metal nanostructures with sub–10 nm gaps between them, which we call 'hot spots' [9] Over the past decades, significant efforts in the areas of electron beam lithography [11], colloidal lithography [12], chemical synthesis [13–15], and self-assembly [16–18] have been made to develop highly active SERS substrates. All of these efforts are been focused on sufficiently high electromagnetic field enhancement, good SERS signal stability, and convenience in fabrication and manipulation. However, the above-mentioned requirements are hardly being met simultaneously. Electron beam lithography, nanoimprint lithography, and colloidal lithography can fabricate highly-uniform Ag or Au nanostructures leading to stable and reproducible SERS signals, but these methods are generally expensive and time consuming for large-scale fabrication. Ag or Au nanoparticles (NPs) formed

by chemical synthesis is a very popular approach to SERS substrate preparation, and tremendous Raman enhancement could be achieved effortlessly by rich 'hot spots'. However, the downside of this kind of method is that the stability cannot be guaranteed because of the uneven distribution of 'hot spots'. For example, the NPs often aggregate in solution, which is not conducive to long-term preservation [19]. With regard to the SERS measurement, a general and simple method is mixing analytes with NP solutions, and then measuring the mixtures directly [20]. However, this method is incapable of trace detection. An improved method is to dry the mixtures. It is true that the NPs will be closely packed after drying [21–24], but this gives rise to a challenge of terrible aggregation caused by the coffee ring effect, also resulting in signal instability. Although, a coating method has been proposed to realize uniform and high-density Ag NPs distribution in drying process [25], this method suffers from oxidation of Ag NPs as time goes on. Therefore, it is still a grea<sup>t</sup> challenge to fabricate large-scale SERS substrates with uniform and high-density hot spots via simple and low-cost strategies.

The coffee-ring is a pattern left by a puddle of a particle-laden liquid after evaporation, which is almost familiar to everyone [26]. It is difficult to eliminate this ubiquitous effect from many applications, including the printing, assembly, and distribution of nano/molecular materials [27,28]. Closely packing Ag or Au NPs is the easiest way to obtain SERS substrates that might have a substantial enhancement of detection signals. The coffee-ring effect will make the Ag or Au NPs form as a ring, so that the distribution of the 'hot spots' is nonuniform and uncontrollable [22,23,29].

Herein, we present a convenient and inexpensive strategy to fabricate large-scale SERS substrates with stable and ultrasensitive performance. It involves a green chemistry synthesis method of Ag NPs and a facile approach of dropping the Ag NPs/glucose solution to form a flat film array for SERS detection. Viscous forces from the ropy glucose suppresses the coffee-ring effect, and thus leads to a uniform and compact deposition, but not aggregation of Ag NPs. Due to the wettability of the Ag NPs/glucose film, uniform distribution of analytes is also realized. These make the SERS signal more consistent and sensitive. In this strategy, no toxic chemicals or sophisticated instruments are required to fabricate the SERS substrate. In addition, thanks to the protection of glucose, oxidation of the Ag NPs is avoided, which results in their long-term storage (at least 6 months). Finally, we demonstrate the application of such SERS substrates for detection of R6G (Rhodamine 6G) and thiram (pesticide) down to 10−<sup>14</sup> M and 10−<sup>10</sup> M, respectively.

### **2. Materials and Methods**
