Microspot sensing based on surface-enhanced fluorescence from nanosculptured thin films

Alina Karabchevsky
2012 Journal of Nanophotonics  
Nanosculptured thin films (STF) are prepared by the oblique angle deposition technique and take different forms of nano columnar structures. Varieties of STFs were investigated to find the optimum structure for biosensing based on the surface enhanced fluorescence. A comparative study was carried out with STFs containing the nanocolumnar structures that differ in their shape, height (h), and tilt angle with respect to the surface (α), thickness (d), and arrangement. The greatest enhancement of
more » ... he fluorescent signal was found for Ag-based STFs on Si (100), giving an enhancement factor of ×71, where h ¼ 400 nm, d ¼ 75 nm, and α ¼ 23°relative to Ag closed film using fluorescent dye Rhodamine 123. We immobilized the fluorescent receptor to the thiol self-assembly monolayer on Ag-based STF and Ag dense film to demonstrate the applications of STFs for specific biosensing. Upon excitation of the fluorophore by an Hg light source, a CCD camera with controlled exposure time would detect the pattern of fluorescent receptor Anti-Rabbit IgG on the surfaces. A specially designed optical fiber housing attached to the microscope allowed quantitative measurement of the fluorescence spectrum on a microspot parallel to the image grab. Downloaded From: http://nanophotonics.spiedigitallibrary.org/ on 09/18/2014 Terms of Use: http://spiedl.org/terms nanoparticles in a variety of bio-diagnostic applications 6-8 and plasmon modes have been implicated in the extraordinary transmission of light through nanoscale hole 9 and slit 10 arrays. Furthermore, the confined local electric field enhancements that accompany the excitation of LSPRs are used in a variety of near-field enhanced spectroscopy and imaging modes, from near-field scanning optical microscopy 11,12 to surface-enhanced Raman spectroscopy (SERS). 13-18 Although SERS applications have motivated much of the research into surface-enhanced spectroscopy in the past decade, the widespread use of fluorescence-based sensing in biology and the importance of radiative decay near metal electrodes in organic optoelectronics 19,20 are two factors leading to new interest in the study of simple fluorescence near metal nanostructures. 21 Although planar metal films generally quench the emission from nearby fluorophores, 22,23 the effects of metal nanostructures are more complicated. Depending on the details of the system under investigation, fluorescence quenching, 24-26 enhancement, 5,27-30 or both 31 have been reported in experimental studies of fluorescent dyes and quantum dots near nanostructured metals. The increased surface area (and hence the increased quantity of adsorbed dye) of a nanostructured metal surface compared with a planar substrate or dens film might account for some of the reports of enhancement. The observation of enhancement in single molecule experiments, 5,26 planar dye layers with adsorbed nanoparticles, 27 and the observation of fluorescence signal amplification due to the sculptured thin films (STF) 32 indicate that real, nontrivial enhancements of fluorescence using near-field effects are achievable. The origins of such nontrivial fluorescence enhancement effects near nanostructured metal can be understood as arising from two contributions. First, by concentrating the incident light into local nanoantennas, nanostructured surfaces can lead to increased absorption of the incident light by the fluorophores. Second, metal nanostructures can alter the radiative and the nonradiative decay rates of nearby fluorophores, changing both the fluorescence lifetime and quantum yield. Although it has remained difficult to separate the effects of excitation and emission enhancement, both of these local field effects are expected to be extremely sensitive functions of the shape of the metal particle, the orientation of the dye, and the distance between the dye and the metal, 33,34 just as they are for dyes attached to planar metal films. 22, 23 In this paper we present an extensive comparative study of enhanced fluorescence from a variety of nanometallic thin films versus dens films on different substrates. The main purpose of this study is to demonstrate biosensing from the microspot area based on the amplification of fluorescence signal due to the special surface morphology of STF. STFs 35,36 are nanostructured materials with unidirectionally varying properties that can be designed and realized in a controllable manner using variants of physical vapor deposition techniques. The growth mechanism is based on self-organized nucleation during deposition and subsequent highly directional growth due to an atomic shadowing mechanism. A particle flux, incident to the substrate at an angle a (usually a ≥ 80 deg , as measured with respect to the substrate normal), enables preparation of columnar thin films under oblique angle deposition. 36 The ability to instantaneously change the growth direction of their columnar morphology, through simple variations in the direction of the incident vapor flux, leads to a broad spectrum of columnar forms such as spiral, screws, vertical columns, and chevrons. To date, the chief applications of STFs are in optics as polarization filters, Bragg filters, and spectral hole filters. 37,38 At visible and infrared wavelengths, a single-section STF is a unidirectionally nonhomogeneous continuum with direction-dependent properties. Being porous, an STF can act as a sensor for fluids. Recent experimental results 39 and theoretical SPR calculations 40,41 show a great potential for using STFs as SPR sensors. Biomedical applications such as tissue scaffolds, drugdelivery platforms, virus traps, and lab-on-a-chip are also in different stages of development. 42,43 STF are new potential materials for fluorescent enhancement near the surface. 44 In this work we present an extensive study towards biosensing based on a surface-enhanced fluorescence phenomenon from nanostructured metal STFs. A broad comparative study was done using Rhodamine 123 as a fluorophore on a variety of films that differ by their morphology, materials as metals and substrates, inclination angles, height (h), tilt angle with respect to the surface (α), thickness (d), and arrangement to find the best STF candidate for biosensing applications. We used: (1) Rhodamine 123 as a fluorescent dye to stain the special surfaces of STFs. After the staining by Rhodamine 123 we compared different STFs and their references to show the amplification of fluorescent signal due to the metal nanosculptured surfaces. The samples were quantified by means of the fluorescent signal from captured fluorescence images and Karabchevsky et al.: Microspot sensing based on surface-enhanced fluorescence : : :
doi:10.1117/1.jnp.6.061508 fatcat:n5kzq6weergc7gd3i2slg4r444