A universal sensor for mercury (Hg, Hg(I), Hg(II)) based on silver nanoparticle-embedded polymer thin film.

Detection of mercury at concentration levels down to parts-per-billion is a problem of fundamental and practical interest due to the high toxicity of the metal and its role in environmental pollution. The extensive research in this area has been focused primarily on specific sensing of mercuric (Hg(2+)) ion. As mercury exists in the oxidation states, +2, +1 and 0 all of which are highly toxic, a universal sensor covering all the three while ensuring high sensitivity, selectivity, and linearity of response, and facilitating in situ as well as ex situ deployment, would be very valuable. Silver nanoparticle-embedded poly(vinyl alcohol) (Ag-PVA) thin film fabricated through a facile protocol is shown to be a fast, efficient and selective sensor for Hg(2+), Hg(2)(2+) and Hg in aqueous medium with a detection limit of 1 ppb. The sensor response is linear in the 10 ppb to 1 ppm concentration regime. A unique characteristic of the thin film based sensor is the blue shift occurring concomitantly with the decrease in the surface plasmon resonance absorption upon interaction with mercury, making the sensing highly selective. Unlike the majority of known sensors that work only in situ, the thin film sensor can be used ex situ as well. Examination of the thin film using microscopy and spectroscopy through the sensing process provides detailed insight into the sensing event.

Real time monitoring of the in situ growth of silver nanoparticles in a polymer film under ambient conditions.

Direct monitoring of the formation and growth of nanoparticles by microscopy in real time is of fundamental interest in understanding the chemical and self assembly processes involved. Such studies are difficult to implement in solution, but have been carried out on solid substrates, mostly under specialized conditions, including ultra high vacuum. The present study illustrates the facility with which in situ growth of metal nanoparticles in thin polymer films under ambient conditions can be monitored by real time atomic force microscopy. Formation of silver nanoparticles inside spin-coated thin films of poly(vinylpyrrolidone) containing silver nitrate, under ambient conditions, is revealed by the emergence and growth of surface plasmon resonance absorption extending over several hours. Atomic force microscopy allows 'direct observation' of structures growing near the surface; individual nanostructures can be monitored in the case of very thin films. A plausible mechanism is proposed for the chemical reactions occurring inside the film with the polymer itself acting as the reducing agent leading to the formation and growth of the nanoparticles. The present study opens up new avenues to carry out investigations into the mechanisms and kinetics of nanoparticle growth.

Polymer thin films embedded with in situ grown metal nanoparticles.

Metal nanoparticle-polymer composites are versatile materials which not only combine the unique characteristics of the components, but also manifest mutualistic effects between the two. Embedding inside polymer thin films facilitates immobilization and organization of the metal nanoparticles and tuning of their electronic and optical responses by the dielectric environment. The embedded metal nanoparticles in turn can impact upon the various material attributes of the polymer matrix. Some of the most convenient and attractive routes to the fabrication of metal nanoparticle-embedded polymer thin films involve in situ generation of the nanoparticles through reduction or decomposition of appropriate precursors inside the solid film. In this tutorial review we present an overview of the different methodologies developed using this general concept and describe the environment-friendly protocol we have optimized for the fabrication of noble metal nanostructures inside polymer thin films, using aqueous media for the synthesis and deploying the polymer itself as the reducing as well as stabilizing agent. A variety of techniques that have been exploited to characterize the precursor to product transformation inside the polymer film are discussed. The unique control provided by the in situ fabrication route on the size, shape and distribution of the nanostructures, and application of the polymer thin films with the in situ generated metal nanoparticles in areas such as nonlinear optics, surface enhanced Raman scattering, e-beam lithography, microwave absorption, non-volatile memory devices and random lasers, illustrate the versatility of these materials. A brief appraisal of the avenues for future developments in this area is presented.

Microwave absorber based on silver nanoparticle-embedded polymer thin film.

Silver nanoparticle-embedded poly(vinyl alcohol) films are fabricated through a simple in situ process. The nanocomposite films are a few hundred nanometers thick with silver concentrations below 10% and the nanoparticles 5-10 nm in diameter. These films are shown to exhibit appreciable microwave absorption in the 8-12 GHz range; the return and insertion losses are found to be sensitive to the nanoparticle content.