Trapping Proteins within Gold Nanoparticle Assemblies: Dynamically Tunable Hot-spots for Nanobiosensing Abdennour Abbas & Max Fei & Limei Tian & Srikanth Singamaneni Received: 1 June 2012 / Accepted: 12 August 2012 / Published online: 24 August 2012 # Springer Science+Business Media, LLC 2012 Abstract The combination of stimuli-responsive materials with localized surface plasmon resonance nanotransducers provides new leverages in hot spot-based nanosensing. We introduce a simple and effective biodetection method based on the hydro-responsive property of (3-aminopropyl)-trie- thoxysilane (APTES). Gold nanoparticles were adsorbed onto hydro-responsive APTES thin film. The exposure of the film surface to an aqueous solution results in opening inter-particle gaps, allowing analyte binding. A subsequent drying of the sensor surface closes the gap by bringing the nanoparticles to the initial position, thereby trapping the analyte in the most sensitive regions (electromagnetic hot spots). In this reversible configuration, the generation and tuning of the hot spots are independent from both the presence of the analyte and the functionalization of the nanoparticles, which yields highly resolved coupled plasmon bands and provide a general and flexible nano- sensing modality. Furthermore, the intensity of the hot spots can be easily and reversibly tuned to obtain pico- molar sensitivity. Keywords Gold nanoparticles . Localized surface plasmon resonance . Plasmon coupling . Stimuli-responsive materials . Nanobiosensors Introduction Since theoretical and experimental results have shown that assembling nanoparticles or sharpening their features gives rise to hot spots in the electromagnetic field distribution [1–4], a close attention has been dedicated to hot spot- based nanosensing to improve the performance of localized surface plasmon resonance (LSPR) and surface-enhanced spectroscopies [1, 5–8]. A number of strategies have been implemented to overcome the limits of LSPR transducers by taking advantage of the plasmon hot spots in gold nano- particles, including the selective immobilization of the target molecule to the edges of sharpened nanoparticles such as nanotriangles [9]. To further enhance the performance, in- creasing number of reports have been focused on the use of the hot spots generated by the plasmon coupling of assem- bled nanoparticle dimers, chains, or arrays [10, 11]. Assem- bled nanoparticles provide a significant enhancement in the sensing abilities, but adversely make the most relevant re- gion of the electromagnetic hot spots inaccessible for the analyte. This is of a significant importance as both the intensity of the electromagnetic field and the sensitivity decrease exponentially with increasing distance from the nanoparticle surface [ 2, 6]. Additionally, in surface- enhanced Raman scattering, the most sensitive region for biodetection is localized in the close vicinity (∼5 nm) of the nanoparticles [12–14]. A more effective approach has been reported in recent years, based on the functionalization of nanoparticles with single strand DNA to detect a complementary strand [15, 16] or the immobilization of the ligands and corresponding receptors on two different metal nanoparticles [17, 18]. Binding of the target molecule leads to the formation of nanoparticle dimers, thus yielding a spectral blue shift due to coupled plasmon resonance. This approach combines the Electronic supplementary material The online version of this article (doi:10.1007/s11468-012-9431-8) contains supplementary material, which is available to authorized users. A. Abbas : M. Fei : L. Tian : S. Singamaneni (*) Department of Mechanical Engineering and Materials Science, Washington University, St. Louis, MO 63130, USA e-mail: singamaneni@wustl.edu Plasmonics (2013) 8:537–544 DOI 10.1007/s11468-012-9431-8