**4. Conclusions**

In summary, we have demonstrated a method for a simple and economically viable design of large-scale, highly efficient, ultrasensitive, uniform, and low-cost SERS substrates, especially emphasizing the suppression of the coffee-ring effect. In this method, the glucose acts to achieve this suppression. First, Ag NPs were synthesized by a modified Tollens method in which glucose serves as a reducing agent. Secondly, a Ag NPs/glucose solution was dropped into a hole-shaped PDMS structure. After evaporation, a flat and uniform SERS film array was formed by means of viscidity resistance in a ropy glucose solution, which prevented the suspended particles from reaching the drop edge and ensured a uniform deposition. Thirdly, owing to the protection of glucose, oxidation of the Ag NPs was also avoided, which resulted in long-term storage (at least 6 months) of the SERS substrates. Subsequently, the performance of the SERS substrates fabricated with different concentrations of glucose was investigated, and the result suggested that the optimal concentration of glucose was 0.2 M. Finally, we demonstrated the application of such SERS substrates for detection of R6G and thiram down to 10−<sup>14</sup> M and 10−<sup>10</sup> M, respectively. Thus, such a convenient fabrication method and superior performance of the obtained SERS substrates would provide an opportunity to bring the SERS technology closer to real-world applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/8/7/520/s1, Figure S1: UV-vis absorption spectra of the colloidal Ag NPs synthesized in 0.1–0.5 M glucose; Figure S2: TEM images of the Ag NPs synthesized in 0.1–0.5 M glucose; Figure S3: SERS intensity of 10−<sup>10</sup> M R6G collected at 1509 cm<sup>−</sup><sup>1</sup> from 20 random spots of a pie-shaped SERS substrate; Figure S4: SERS spectra of R6G (10−<sup>10</sup> M) collected from the SERS substrates at different storage times ((a) 0 months, (b) 2 months, (c) 4 months, and (d) 6 months).

**Author Contributions:** Conceptualization, J.Z. and W.W.; Methodology, J.Z.; Software, G.L. and M.W.; Investigation, J.Z. and G.L.; Writing-Original Draft Preparation, J.Z.; Writing-Review & Editing, W.W. and P.L.; Visualization, Z.C.; Project Administration, W.W.; Funding Acquisition, W.W., All authors read and approved the final manuscript.

**Funding:** This research was funded by the National Basic Research Program of China (973 Program, Grant No. 2015CB352100), the National Natural Science Foundation of China (Grant No. 61474006), and the National Key Laboratory Foundation.

**Acknowledgments:** We thank the Electron Microscopy Laboratory of Peking University for the use of their equipment.

**Conflicts of Interest:** The authors declare no conflict of interest.
