Stimuli Responsive Release of Metalic Nanoparticles on Semiconductor Substrates Miguel Santiago-Cordoba, † O ̈ zge Topal, ‡ David L. Allara, † A. Kaan Kalkan, ‡ and Melik C. Demirel* ,† † Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Functional Nanomaterials Laboratory, Oklahoma State University, Stillwater, Oklahoma 74078, United States *S Supporting Information ABSTRACT: Optically active metal nanoparticles have been of recent and broad interest for applications to biomarker detection because of their ability to enable high sensitivity enhancements in various optical detection techniques. Here, we report stimuli responsive release of metallic nanoparticles on a semiconductor thin film array structure based on pH change. The metallic nanoparticles are obtained by a simple redox procedure on the semiconductor surface. This approach allows controlling nanoparticle surface coatings in situ for biomolecule conjugation, such as DNA probes on nano- particles, and rapid stimuli responsive release of these nanoparticles upon pH change. ■ INTRODUCTION Optically active metal nanoparticles, notably those of the coinage metals, have had wide application in optical based molecular sensing, particularly for biologically important molecules in aqueous media. 1-9 The standard approach for biosensing is to prepare nanoparticles in a preliminary step and then transfer into solution as a dispersion or onto substrates as a film. For maximum sensitivity large numbers of nanoparticle- analyte contacts in the analysis solution are required. Solution dispersed metal nanoparticles can have large numbers of contacts but during initial preparation the nanoparticles must be stabilized against aggregation, generally by using surfactant coatings, 10,11 or by the use of adsorbed ions. Surface layers, particularly surfactants, however, block direct electronic coupling with adsorbed analyte molecules and act as a spacer to attenuate the electromagnetic evanescent field extending to the adsorbed analyte during the optical probing. Surfactant layers also can create ambient background noise in an optical detection path, which lowers the measurement sensitivity. Although methods for removal or control of surface coating of nanoparticles have recently been reported, 12 additional pretreatment is required and in order to avoid reaggregation of the clean nanoparticles surface coating must be applied before immersion in solution. Surfactant exchange methods are expensive and laborious. Furthermore, for the case of corrosive metals, e.g., Ag, storage of the prepared metal films is an issue since surface contamination such as carbonates or sulfides can diminish the activity of the surfaces for analyte adsorption and optical probe responses. Metal nanoparticle films directly attached at surfaces, on the other hand, have the advantage of significant additional optical enhancement arising from small interparticle gaps, but this structure creates the problem of slow analyte transport from solution to the interior nanoparticle film surfaces, limiting the sensitivity and detection time. 13 In order to reach the next level of high sensitivity biodetection there is a need for a strategy to control surface coverage, and to prepare fresh mobile nanoparticles in solution that have optimal contact with analyte (i.e., eliminating the transport limit) providing the highest levels of optical response. Toward this end, we explored methods for building pH responsive nanoparticles, which can sequentially undergo stimuli responsive release of mobile metal nanoparticles upon contact with an analyte solution at basic conditions (i.e., pH > 7). Here we report a general configuration based on the use of a thin semiconductor film which is both capable of generating a controlled deposition of silver nanoparticle films by galvanic displacement onto simultaneously formed substrate nanopost arrays and of subsequently launching the nanoparticles, upon exposure to a basic (pH > 7) solution. In a microvolume drop of solution, the nanoparticles capture analyte molecules whose presence is rapidly signaled by optical probing. The nano- particle release occurs by solution-induced dislodging of the nanoparticle with undercutting of the Ag/Ge interface at basic conditions only (i.e., for the acidic or neutral condition, pH ≤ 7, the nanoparticle release is negligible). The outflowing cloud of nanoparticles quickly adsorbs analyte and subsequently, on a slower time scale, aggregates to leave the analyte nested directly in the otherwise bare nanoparticle junctions. An example of this approach, with the development of a nucleic acid biosensor, Received: January 16, 2012 Revised: March 13, 2012 Published: March 19, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 5975 dx.doi.org/10.1021/la3002256 | Langmuir 2012, 28, 5975-5980