We described, for the first time, the metal enhanced fluorescence from the CdTe nanocrystals spin coated on silver island films (SIFs). CdTe nanocrystals show ∼5 fold increase in fluorescence intensity, 3-fold decrease in lifetimes and reduction in blinking on SIFs surfaces that can be observed by both ensemble and single molecule fluorescence studies. The single molecule study also provides further insight on the heterogeneity in the fluorescence enhancement and lifetimes of the CdTe nanocrystals on both glass and SIFs surfaces, which is otherwise not possible to observe using ensemble measurements.Signal detection is a challenging task in chemistry and biology. Most often background noise hinders the detection of the signal of interest. This can be prevailed if one has a signal enhancing and/or directing technique. In this regard, Metal-Enhanced Fluorescence (MEF) is a newly recognized technology, where we study the interactions of fluorophores with metallic colloids or surfaces, which provides fluorescence enhancement. 1 Along with signal enhancement, MEF also provides several other important spectral changes such as increased photostability, decreased lifetime due to increased rates of radiative decay, and increased distance for fluorescence resonance energy transfer (RET). 1c Because of this exceptional improvement in signal detectability provided by the MEF, we were able to increase the intrinsic DNA fluorescence, which is otherwise impractical to perceive. 1d Accordingly we have made considerable progress with MEF studies where we used wide varieties of fluorophores that can be potentially used in different applications. 2Single-molecule fluorescence spectroscopy provides several advantages over ensemble measurements. 3 For instance, it eliminates averaging of the spectral properties over all members of ensemble and can reveal fundamental features otherwise masked in ensemble exp eriments. Accordingly, using single-molecule fluorescence studies we anticipate that the metalfluorophore interactions in the MEF studies can be better revealed, especially at single molecule CdS, 5b CdTe, 5c Si 5d and InP. 5e Also quantum dots fluorescence spectral properties are reported in the presence of metallic nanoparticles at the ensemble level with little details. 6,7In this communication, we describe the ensemble and single-molecule fluorescence spectral properties of CdTe nanocrystals spin coated in PVA matrix on glass and silver island films (SIFs). CdTe nanocrystals were prepared using a modified Weller method (for further details see Supporting Information). 8 Figure 1 shows the fluorescence emission spectra and corresponding intensity decays of CdTe nanocrystals on glass and SIFs surface. Interestingly, the emission spectra from metalized and that from non-metalized area are completely overlapped on each other with a band maxima at about 660 nm ( Figure 1A inset). As seen from the figure, a significant enhancement (of ∼ 5-fold) in fluorescence intensity is observed from the CdTe nanocrystals on SIF surface ...

We report recent achievements in metal-enhanced fluorescence from our laboratory. Several fluorophore systems have been studied on metal particle-coated surfaces and in colloid suspensions. In particular, we describe a distance dependent enhancement on silver island films (SIFs), release of self-quenching of fluorescence near silver particles, and the applications of fluorescence enhancement near metalized surfaces to bioassays. We discuss a number of methods for various shaped silver particle deposition on surfaces.

During the past decade the interactions of fluorophores with metallic particles and surfaces has become an active area of research. These near-field interactions of fluorophores with surface plasmons have resulted in increased brightness and directional emission. However, using metals provide some disadvantages, like quenching at short fluorophore-metal distances, increased rates of energy dissipation due to lossy metals. These unfavorable effects are not expected in dielectrics. In this paper we describe the interactions of fluorophores with one-dimensional (1D) photonic crystals (PCs), which have alternating layers of dielectrics with dimensions that create a photonic bandgap (PBG). Freely propagating light at the PBG wavelength will be reflected. However, similar with metals, we show that fluorophores within near-field distances of the 1DPC interacts with the structure. Our results demonstrated that these fluorophores can interact with both Internal Modes (IM) and Bloch Surface Waves (BSW) of the 1DPC. For fluorophores on the surface of the 1DPC the emission dominantly occurs through the 1DPC and into the substrate. We refer to these two phenomena together as Bragg Grating-Coupled Emission (BGCE). Here we describe our preliminary results on BGCE. 1DPCs are simple to fabricate and can be handled and reused without damage. We believe BGCE provide opportunities for new formats for fluorescence detection and sensing.

We have tested the feasibility of tear glucose sensing using a daily, disposable contact lens embedded with boronic acid-containing fluorophores as a potential alternative to current invasive glucose-monitoring techniques. Our findings show that our approach may, indeed, be suitable for the continuous monitoring of tear glucose levels in the range 50-500 microM, which track blood glucose levels that are approximately 5-10-fold higher. We compare the response of the boronic acid probes in the contact lens to solution-based measurements and can conclude that both the pH and polarity within the contact lens need to be considered with respect to choosing/designing and optimizing glucose-sensing probes for contact lenses.

Three water-soluble fluorescent probes have been specifically designed to determine free cyanide concentrations up to physiologically lethal levels, >20 microM. The probes have been designed in such a way as to afford many notable sensing features, which render them unique with regard to signal transduction, photophysical characteristics, and their application to physiological cyanide determination and safeguard. The probes are readily able to reversibly bind free aqueous cyanide with dissociation constants around 4 microM3. Subsequent cyanide binding modulates the intramolecular charge transfer within the probes, a change in the electronic properties within the probes, resulting in enhanced fluorescence optical signals as a function of increased solution cyanide concentration. The ground-state chelation with cyanide produces wavelength shifts, which also enable the probes to sense cyanide in both an excitation and emission ratiometric manner, in addition to enhanced fluorescence signaling. This has enabled a generic cyanide sensing platform to be realized that is not dependent on fluorescent probe concentration, probe photodegradation, or fluctuations in the intensity of any employed excitation sources, ideal for remote cyanide sensing applications. Further, the >600 nm fluorescence emission of the probes potentially allows for enhanced fluorescence ratiometric cyanide sensing in the optical window of tissues and blood, facilitating their use for the transdermal monitoring of cyanide for mammalian safeguard or postmortem in fire victims, both areas of active research.

In the past several years we have demonstrated the metal-enhanced fluorescence (MEF) and the significant changes in the photophysical properties of fluorophores in the presence of metallic nanostructures and nanoparticles. MEF is largely dependent on several factors, such as chemical nature, size, shape of the nanostructure, and its distance from the interrogating fluorophore. Herein, we elucidate the potential of layer-by-layer (LbL) assembly to understand the distance dependence nature of MEF from sulforhodamine B (SRB) assembled on the plasmonic nanostructured surfaces [in the form of Silver Islands films (SIFs)]. The varied proximity of fluorophores from the SIF surfaces was controlled by constructing different numbers of alternate layers of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). An anionic laser dye SRB could be electrostatically attached to the positively charged PAH layer. Orientation of the SRB probe molecule adsorbed in PSS/PAH-layered assembly was determined by polarized absorption spectroscopy. The observed tilt angle of the probe transition dipole moment with respect to the surface normal was 40°. Our results show that MEF is indeed distance-dependent. Accordingly, we observed a maximum of a ~6-fold increase in the fluorescence intensity from a monolayer of the SRB at a distance of ~9 nm from the metal-nanostructured surface, with the enhancement decreasing down to ~1.5-fold at about a 30 nm separation distance. Consistently, the minimum lifetimes were about 4-fold shorter than those on glass slides without silver, with the lifetimes being about nearly the same for 15 layers of the PSS/PAH assembly. The intensity-time decays were analyzed with a lifetime distribution model to underpin the distance effect on the metal-fluorophore interaction in the nanometric range. The present study provides improved understanding of the interaction between plasmonic nanostructures and fluorophores and, more importantly, their distance dependence nature, where we used a robust, easy, and inexpensive alternate electrostatic LbL assembly as a bottom-up nanofabrication technique to control the probe distance from the surface.

Fluorescence technology pervades all areas of chemical and biological sciences. In recent years, it is being realized that traditional fluorescence can be enriched in many ways by harnessing the power of plasmonic or photonic structures that have remarkable abilities to mold the flow of optical energy. Conventional fluorescence is omnidirectional in nature, which makes it difficult to capture the entire emission. Suitably designed emission directivity can improve collection efficiency and is desirable for many fluorescence-based applications like sensing, imaging, single molecule spectroscopy, and optical communication. By incorporating fluorophores in plasmonic or photonic substrates, it is possible to tailor the optical environment surrounding the fluorophores and to modify the spatial distribution of emission. This promising approach works on the principle of near-field interaction of fluorescence with spectrally overlapping optical modes present in the substrates.In this Account, we present our studies on directional emission with different kinds of planar metallic, dielectric, and hybrid structures. In metal−dielectric substrates, the coupling of fluorescence with surface plasmons leads to directional surfaceplasmon-coupled emission with characteristic dispersion and polarization properties. In one-dimensional photonic crystals (1DPC), fluorophores can interact with Bloch surface waves, giving rise to sharply directional Bloch surface wave-coupled emission. The interaction of fluorescence with Fabry−Peŕot-like modes in metal−dielectric−metal substrates and with Tamm states in plasmonic−photonic hybrid substrates provides beaming emission normal to the substrate surface. These interesting features are explained in the context of reflectivity dispersion diagrams, which provide a complete picture of the mode profiles and the corresponding coupled emission patterns. Other than planar substrates, specially fabricated plasmonic nanoantennas also have tremendous potential in controlling and steering fluorescence beams. Some representative studies by other research groups with various nanoantenna structures are described. While there are complexities to near-field interactions of fluorescence with plasmonic and photonic structures, there are also many exciting possibilities. The routing of each emission wavelength along a specific direction with a given angular width and polarization will allow spatial and spectral multiplexing. Directional emission close to surface normal will be particularly useful for microscopy and array-based studies. Application-specific angular emission patterns can be obtained by varying the design parameters of the plasmonic/photonic substrates in a flexible manner. We anticipate that the ability to control the flow of emitted light in the nanoscale will lead to the development of a new generation of fluorescence-based assays, instrumentation, portable diagnostics, and emissive devices.

We described the effect of fluorophore distance from the silver island films (SIFs) on the metal-enhanced fluorescence (MEF) from two newly developed long-chain nitrobenzoxadiazole derivatives (NBD-C16 and NBD-C18). The well-established Langmuir-Blodgett technique is used to deposit the fluorophores at defined distances from the SIFs surface, and an inert amphiphilic stearic acid is used to control the distance. NBD probes deposited directly on the SIFs surface show the highest metal-enhanced fluorescence of approximately 32-fold, and both of the probes that were studied show a consistent decrease in metal-enhanced fluorescence when increasing the distance from the fluorophore to the SIFs surface. The lowest fluorescence enhancement of approximately 4-fold is observed for the probes located 90 nm from the SIFs surface. Additionally, we also have noticed the shortest fluorescence lifetimes for the NBD probes deposited directly onto the SIFs surface, and the lifetimes are consistently increased when increasing the distances between the fluorophore and SIFs surfaces. These contrasting spectral changes, enhanced fluorescence, and decreased fluorescence lifetimes are in accordance with an increase in the rate of radiative decay for fluorophores near the silver particles. The present study provides significant information on the effect of fluorophore distance on the metal-enhanced fluorescence phenomenon.