© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com REVIEW Gold and silver nanomaterials (NMs) such as nanoparticles (NPs) and nano- clusters (NCs) possessing interesting optical properties have become popular sensing materials. With strong surface plasmon resonance (SPR) absorp- tion, extraordinary stability, ease in preparation, conjugation, and biocom- patibility, Au NPs are employed to develop sensitive and selective sensing systems for a variety of analytes. However, small sizes of Au and Ag NCs with interesting photoluminescence (PL) properties are used in many PL-based sensing systems for the detection of important analytes. In addition, many bimetallic AuM NMs possessing strong catalytic activity are used to develop highly sensitive fluorescent sensors. This review article is categorized in four sections based on the NMs used in the sensing systems, including Au NPs, bimetallic AuM NMs, Au NCs, and DNA–Ag NCs. In each section, synthetic strategies and optical properties of the NMs are provided briefly, followed by emphasis on their analytical applications in the detection of small molecules, metal ions, DNA, proteins, and cells. Current challenges and future prospects of these NMs-based sensing systems will be addressed. 1. Introduction Integration of nanotechnology and biotechnology has opened many exciting avenues for developing sensitive and selective sensing approaches in analytical chemistry. For example, nano- material (NM)-based sensing systems have been developed for monitoring the levels of ions and biomolecules (e.g., proteins and DNA) in environmental and biological samples. Unlike traditional analytical approaches, high-cost instruments, large amounts of samples and reagents, and/or experienced opera- tors are usually not needed when operating these systems. Noble metal-based sensing NMs such as gold (Au) and silver (Ag) have attracted considerable attention because of their many distinctive physical and optical properties, including stability, biocompatibility, high surface area, size- and shape- dependent optical properties, and/or molecular-like photolu- minescence (PL). [1–3] The optical properties of Au and Ag NMs can be categorized according to their size. [4] Spherical Ag and Au nanoparticles (NPs) having sizes larger than 5 nm possess strong surface plasmon resonance (SPR) absorption bands at DOI: 10.1002/ppsc.201400043 Gold and Silver Nanomaterial-Based Optical Sensing Systems Po-Cheng Chen, Prathik Roy, Li-Yi Chen, Rini Ravindranath, and Huan-Tsung Chang* P.-C. Chen, Dr. P. Roy, L.-Y. Chen, R. Ravindranath, Prof H.-T. Chang Department of Chemistry National Taiwan University 1, Section 4, Roosevelt Road, Taipei 106, Taiwan E-mail: changht@ntu.edu.tw the wavelengths close to 420 and 520 nm, respectively, mainly depending on their sizes. Having such strong SPR character- istics in the UV–vis region, they are also great substrates for surface-enhanced Raman scattering techniques that have been used for the detection of various ana- lytes and cell imaging. [5] By contrast, Au and Ag NMs having sizes smaller than 5 nm possess molecule-like PL behaviors. These smaller NMs are called nanoclus- ters (NCs) or nanodots to distinguish from those large counterparts. Nanodots usually refer to fluorescent NMs consisting of sev- eral hundreds of atoms, NCs refer to those for several to several tens of atoms. For simplicity, fluorescent Au and Ag NMs are presented as NCs in this article. In addi- tion to size, the optical properties of Au and Ag NMs can be controlled by varying their shape, composition, surface, and sur- rounding environment. Spherical Au NPs are ready for conjugation with various recognition elements such as amphiphilic polymers, silanols, sugars, nucleic acids, and proteins through strong Au–S or Au–N bonding or through physical adsorption to provide greater stability and/or selectivity. [6,7] Analyte-induced aggregation of Au NPs, leading to changes in the color (absorption), has been commonly applied for the detection of small molecules such as metal ions and anions, and biopolymers such as peptides, proteins, and DNA. The color change of Au NPs (10–50 nm) from ruby red to blue/purple is easily observed by the naked eye. Aggregation either occurs through non-cross-linking or interparticles cross-linking. [8–11] Other than developing Au NP-based colorimetric assays, Au NPs-based enzyme mimics have become interesting activities toward reactions between substrates and hydrogen peroxide (peroxidase-mimicking) or oxygen (oxidase-like), leading to changes in absorbance and flu- orescence. [12] Thus, they can be used to develop highly sensitive and selective colorimetric and fluorescence assays. Ag NPs likewise display a distance-dependent color, usually from yellow to red or brown as Ag NPs aggregate. By utilizing the distinct color between the dispersed and aggregated Ag NPs, various colorimetric sensing based on Ag NPs for different analytes such as metal ions, [13] small molecules, [14,15] DNA, [16] and proteins [17] have been developed. However, the examples of utilizing Ag NPs as colorimetric sensing probes are fewer compared with Au NPs, mainly due to the instability and tox- icity of Ag NPs, caused by the oxidation and potential leaching of Ag + ion, respectively. [18,19] Contrary to Ag NMs, Au NMs Part. Part. Syst. Charact. 2014, DOI: 10.1002/ppsc.201400043