Plasmonic enhancement of fluorescence for chemical sensing applications

Field: Chemistry (Physical Sciences)

Fluorescence spectroscopy is a widely used technique, which finds extensive applications in chemical sensing, biochemistry and molecular biology. In many cases, the isotropically emitted fluorescence from samples in free space is observed in the far field optical space which leads to very low emission intensity at the detector. Prolonged exposure during sensing applications can lead to photobleaching effect which can also limit the applications of fluorescence based sensors. These limitations can be overcome by placing the fluorophore in close proximity to a plasmonic surface or nanoparticle to enhance its fluorescence emission or reduce the photobleaching effect which is termed as metal enhanced fluorescence. Substrates whose plasmon resonance wavelength matches the excitation wavelength are preferred for achieving higher enhancement efficiency. However, the commonly used substrates suffer from difficulty in controlling the process parameters to achieve, high reproducibility and involves time consuming and expensive approaches. Silver film over nanosphere substrates can be an ideal substrate for enhancing and controlling the fluorescence. The plasmon resonance of such substrates can be easily simulated and tuned by either varying the dielectric nanosphere diameter or metal film thickness or the metal itself to match the excitation wavelength. The enhancement factor also depends on the distance between the metal and fluorophore layer. Such surfaces are easily reproducible and have tremendous potential in fluorescence based chemical sensing applications.Numerical simulations using finite-difference time domain method were performed on silver film over nanosphere by varying the polystyrene nanosphere size (200, 400 and 600 nm) and silver film thickness to look at the E-field distribution/enhancement and plasmon resonance wavelength. Substrates with 400 nm beads and 100 nm film thickness showed a plasmon resonance at 480 nm which was very close to the excitation wavelength of the fluorophore of interest, fluorescein isothiocyanate. 400 nm nanosphere monolayer substrates prepared by nanosphere lithography technique produced large area of closely packed nanospheres with very few line defects and vacancies as compared to drop coating method. The plasmon resonance wavelength of ~480 nm from the 100 nm silver was determined by minimum reflectivity method and the results matched that of the simulation results. 12 times enhancement was obtained from fluorescein isothiocyanate coated on top of silver film over nanosphere array separated by a spacer layer of poly vinyl alcohol. The fluorescence enhancement on such substrates is attributed to the local field enhancement in addition to the excitation of surface plasmon polaritons along the surface. Metal enhanced fluorescence phenomenon is a distance dependent phenomenon that follows closely with the near-field electric field distribution on the substrate which is strongly dependent on the distance between the fluorophore and the metal surface. In this regard, the spacer layer effect was explored by varying the spacer layer thickness to 8 nm, 40 nm, 65 nm and 100 nm, respectively. The highest enhancement of ~ 16 times was obtained for 8 nm spacer layer. Finally, the metal enhanced fluorescence technique is applied for pH sensing application in the range of 5 to 8. The pH sensitive fluorescein isothiocyanate dye is entrapped in polyvinyl alcohol and coated on top of spacer layer. The fluorescence emission was enhanced by the underlying plasmonically active surface during pH measurements.The inexpensive silver film over nanosphere substrates were shown to be ideal for metal enhanced fluorescence due to the ease in tuning its plasmon resonance. Polyvinyl alcohol acts as a good spacer layer but obtaining a thickness of 10 to 20 nm by spin coating is difficult to achieve. Biochemicals such as streptavidin and avidin can produce a reproducible spacer layer of 8-10 nm but it is expensive and such substrates have limited applications. As a proof of concept the silver film over nanosphere substrate based fluorescence enhancement technique was used for pH sensing application. Though the pH response was comparable to the other reported work still much improvement has to be done to test the specificity, repeatability, response time and thermal effects of the probe. We feel that this work may lead to the development of ion sensing applications as well, where the ions quench the fluorescence as the concentration increases and the metal nanoparticles can enhance the fluorescence by plasmonic effect.