Nanowires DOI: 10.1002/smll.200701047 Striped Alloy Nanowire Optical Reflectance Barcodes Prepared from a Single Plating Solution** Andrea Bulbarello, Sirilak Sattayasamitsathit, Agustin G. Crevillen, Jared Burdick, Saverio Mannino, Proespichaya Kanatharana, Panote Thavarungkul, Alberto Escarpa, and Joseph Wang* Nanowires have received considerable recent attention as tagging systems for a variety of product-tracking, identifica- tion, and protection applications. [1,2] Such barcoded nanowire tags are commonly prepared by sequential electro-deposition of different metals within a porous template and display stripes of different metals (commonly silver and gold) that can be distinguished by optical reflectivity microscopy. [3–6] While leading to high coding capacities such nanowires require a time-consuming synthesis involving multiple plating steps from different metal solutions. [7] Recent efforts have demon- strated that encoded single-segment alloy nanowires can be synthesized by a single deposition step and encoded electro- chemically [8] or by X-ray fluorescence (XRF). [9] However, such a greatly simplified preparation route is compromised by the corresponding destructive [8] or expensive [9] readout tools. We demonstrate here that multisegment alloy nanowire barcodes with distinct optical-reflectance striping patterns and large coding capacity can be prepared by a template-assisted electro-deposition from a single gold–silver plating solution mixture in connection to different potentials. The different reduction rates of silver and gold at different plating potentials over the 0.50 to 1.20 V range (versus Ag/AgCl) [10] lead to alloy segments of different Au–Ag compositions and to 3–4 optically distinct, readily decoded alloy segments. Extremely large varieties of optical-reflectance striping patterns can thus be produced by plating these alloy segments in different orders and charges. The new multisegment alloy nanowire preparation route greatly simplifies the code production when compared with the solution-changing sequential deposition of common bimetal nanowire barcodes. [5,7] Scheme 1 illustrates the templa- te-assisted electro-deposition of the multisegment alloy nanowires. The different deposition potentials are applied sequentially in a predetermined order and for different durations (A,B,C) to produce alloy segments of controlled length. This is followed by the template dissolution (D) and optical readout of the reflectance patterns (E). Compared to recently developed single-segment alloy nanowire electro- chemical [8] and XRF [9] barcodes, the new striped alloy nanowires can be readily decoded on the basis of differences in optical reflectivity (which is a faster and cheaper diagnostic tool). The ability to tune the optical properties by adjusting the deposition potential and to generate multisegment alloy nanowires with distinct optical reflectance barcode patterns from a single plating solution is illustrated in Figure 1. For example, Figure 1A displays an optical microscopy image, and the corresponding intensity profile, for a 5-segment nanowire involving four different alloy compositions. This nanowire was prepared from an 85/15 (v/v) Au/Ag plating solution by applying four different potentials in the following sequence: a) 1.20, b) 0.73, c) 0.96, d) 0.50, and e) 1.20 V versus Ag/AgCl. Such sequential deposition from the same solution results in four alloy segments that can be distinguished on the basis of the intensity of their reflectivity. As will be illustrated below, the four intensity levels reflect the stepwise increase of the gold content in the alloy upon increasing the deposition potential from 0.50 to 1.20 V. [10] The corresponding intensity (line) profile (bottom of Figure 1A) clearly illustrates the ability to distinguish between the four segments of different alloy compositions. Each segment yields a char- acteristic intensity level, allowing convenient distinction of adjacent alloy stripes and of the four compositions. A large number of unique codes can thus be prepared by simply varying the deposition conditions while using the same plating solution. For example, Figure 1B–D shows reflectance images (top) and intensity lines (bottom) for three nanowires prepared by applying different potentials (0.50, 0.85, and 1.20 V versus Ag/AgCl) using different preset orders and charges. Such change in the deposition conditions results in distinct striping patterns involving three visibly distinguishable reflectance intensity levels (bright, dark, and intermediate), corresponding to the individual alloy segments. Also shown in Figure 1C is an SEM image of the corresponding nanowire. The individual segments are clearly visible, and as expected, [6] their intensities are the opposite of the corresponding optical-reflectance intensities. [  ] Prof. J. Wang, A. Bulbarello, S. Sattayasamitsathit, A. G. Crevillen, J. Burdick Department of Chemical Engineering Biodesign Institute at ASU 1001 S. McAllister Ave., P.O. Box 875801 Tempe, AZ 85287-5801 (USA) E-mail: joseph.wang@asu.edu Prof. J. Wang, A. Bulbarello, S. Sattayasamitsathit, A. G. Crevillen, J. Burdick Department of Chemistry and Biochemistry Biodesign Institute at ASU 1001 S. McAllister Ave., P.O. Box 875801 Tempe, AZ 85287-5801 (USA) A. Bulbarello, Prof. S. Mannino Department of Food Science and Technology Universita’ degli studi di Milano 20133, Milano (Italy) S. Sattayasamitsathit, Prof. P. Kanatharana, Prof. P. Thavarungkul Faculty of Science, Prince of Songkla University Hat Yai, Songkhla, 90112 (Thailand) A. G. Crevillen, Prof. A. Escarpa Department of Analytical Chemistry, Universidad de Alcala´ Alcala´ de Henares, 28871 (Spain) [  ] This work was supported by the National Science Foundation (Grant number CHE 0506529). A.B., S.S., and A.G.C. acknowledge fellowships from Milan University, from the Thailand Research Fund and from the Spanish Ministry of Education and Science, respectively. small 2008, 4, No. 5, 597–600 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 597