DNA-embedded Au–Ag core–shell nanoparticles assembled on silicon slides as a reliable SERS substrate† Zhong Zhang, Sha Zhang and Mengshi Lin * This study aimed at developing a sensitive and reliable SERS substrate by assembling DNA-embedded Au–Ag core–shell nanoparticles (NPs) on silicon slides. First, a monolayer of well separated DNA-functionalized Au NPs (40 nm) was decorated on (3-aminopropyl)triethoxysilane modified silicon slides. The DNA-embedded Au–Ag core–shell NPs were assembled on the 40 nm Au–DNA NPs to form a core–satellite structure through DNA hybridization. Using 4-MBA as a Raman dye, the SERS performance of the substrates was evaluated after being cleaned by low oxygen and argon plasma. The Raman intensity of the assembly using DNA-embedded Au–Ag core–shell NPs was 8–10 times higher than the intensity of the assembly using Au NPs as satellites. In addition, the signal-to-noise ratio of the assembly was 2.6 times higher than that of a commercial substrate (Klarite™) when a 785 nm laser was used. The SERS enhancements of the assembled substrates were 2.2 to 2.8 times higher than the Klarite when an acquisition time of 5 s was used at an excitation wavelength of 633 nm. The assembled substrates also show a good spot-to-spot and substrate-to-substrate reproducibility at the excitation wavelengths of 633 and 785 nm. These results demonstrate that the fabrication process is simple and cost-effective for assembling DNA-embedded Au–Ag core–shell NPs on silicon slides that can be used as a reliable SERS substrate. 1. Introduction Surface enhanced Raman scattering (SERS), a technique which can dramatically enhance the Raman signals of analyte mole- cules adsorbed on the metallic nanostructures, has been applied in chemical analysis, detection of food contaminants, biological sensing, and environmental monitoring. 1–5 For example, SERS has been used to detect melamine, pesticides, and pathogens in food using commercial gold substrates. 6–9 SERS was employed to analyze the endocrine disruptor using gold- and silver-decorated microspheres as substrates. 10 SERS was also developed for detection of tumors using labeled gold nanoparticles (NPs) or gold-patterned microarrays as substrates. 11,12 A variety of substrates have been developed in recent years for SERS applications. The substrates fabricated by nano- lithography show promising SERS enhancement and great reproducibility. 13–15 Sub-10 nm metallic nanogap arrays with precise control of the gap morphology have been produced by the nanolithography technique. 16 However, it is still a challenge to fabricate smaller nanogaps (1–2 nm) by nanolithography. Nanolithography also requires specic equipment for the development of nano-patterns and the deposition of a Au or Ag layer. Another lithography method, called nanosphere lithog- raphy, has been adopted by many researchers because it is inexpensive and able to create a large area of ordered nano- structures. 17–19 However, nanosphere lithography still needs to pattern the substrate surface by a layer of nanospheres in advance, which is difficult to be tailored for different SERS detection strategies. In addition, SERS substrates can also be fabricated by assembling Au or Ag NPs on the glass, silicon surface, or liquid/liquid interface. 20–25 Among all the fabrication methods, the self-assembling approach is the most convenient and cost-effective method because it can be performed without using expensive equip- ment. Between the junctions of the self-assembled NPs, numerous hot spots for SERS can be created where an intense electromagnetic eld is generated at this area. However, a problem of self-assembled substrates is that the distance between NPs is difficult to control and may vary in different locations on the same substrate. This could inuence the reproducibility of the substrate and also cause big variations in Raman signals in different batches. To solve this problem, DNA has been used to direct and organize the assembly of Au NPs on the silicon or glass surface modied with (3-aminopropyl)trie- thoxysilane (APTES). 26 The DNA-directed assembly provides an excellent control of the distance between Au NPs using rigid Division of Food Systems & Bioengineering, University of Missouri, Columbia, MO, 65211-5160, USA. E-mail: linme@missouri.edu; Fax: +1-573-884-7964; Tel: +1-573- 884-6718 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3an02116e Cite this: Analyst, 2014, 139, 2207 Received 13th November 2013 Accepted 9th February 2014 DOI: 10.1039/c3an02116e www.rsc.org/analyst This journal is © The Royal Society of Chemistry 2014 Analyst, 2014, 139, 2207–2213 | 2207 Analyst PAPER Published on 13 March 2014. Downloaded by University of Missouri at Columbia on 01/04/2014 14:22:48. View Article Online View Journal | View Issue