Hybrid gold nanofinger SERS structure for sensing applications
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Hybrid gold nanofinger SERS structure for sensing applications Ansoon Kim and Zhiyong Li* Intelligent Infrastructure Lab, Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304, U.S.A. *[email protected] ABSTRACT We report here a novel hybrid nanostructure for ultra-sensitive sensing applications based on surface-enhanced Raman spectroscopy (SERS). We rationally engineered gold-coated polymer pillar structures, named as gold nanofingers, in analogy to the tweezers at nanoscale, for active molecule capture and detection using SERS technique. Using nanoimprint lithography, we have demonstrated a cost effective manufacturing method of making such hybrid structures over large scale and achieve reliable enhancement factor. In particular, we have demonstrated the sensing application of the nanofinger structures for melamine and chlropyrifos. The limit of detection (LOD) of melamine in water is found to be 10 nM (1.3 ppb), and LOD of chlropyrifos (a pesticide) is found to be 1 nM (0.35 ppb), which is below the EPA tolerance level of 0.1 ppm for chlropyrifos on citrus fruits. INTRODUCTION In this group, we have discovered a novel hybrid structure based on the high-density arrays of gold nanofingers over a large surface area for surface-enhanced Raman scattering (SERS) applications.1-3 The flexibility and high aspect ratio of gold nanofingers enable the fingers to undergo the self-closing process during evaporation of solution. During the closing process, the self-limiting, sub-nm gaps are created between the finger tips to trap molecules. And the molecules coincidently reside in the hot spots formed among the touching finger tips with greatly amplified electromagnetic fields under incident laser illumination.2 In this paper, we will report the sensing applications for molecules of immediate interest to public health, melamine and chlorpyrifos. Melamine is a nitrogen-rich chemical mainly used in the industry to produce kitchenware, commercial filters, flame retardants, etc. The wide use of melamine may cause the presence of trace amounts of melamine in food.4 The melamine was implicated in the pet and human food recalls in 20075, 6 and in the global food safety outbreak in 2008 involving milk and infant formula7, 8. Since milk products contaminated with melamine can lead to kidney disease and even mortality in infants, both the United States and China have set a threshold of 1 part per million (ppm) of melamine in infant formula, and for all other foods, only 2.5 ppm or less are considered risk-free.9, 10 On the other hand, chlorpyrifos is one of the most widely used pesticides, which plays a critical role in protecting fruit or crops. Chlorpyrifos residue was found on 51 foods including fruits, vegetables, grains, beans, and even butter in a report by USDA.11 Research has indicated chlorpyrifos as a neurotoxin, carcinogen and suspected endocrine disruptor, and it has been associated with asthma, reproductive and developmental toxicity, and acute toxicity.12 Tolerances of chlorpyrifos are set at 1 ppm
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