This study presents a biomolecule sensor utilizing ultraviolet plasmonic-enhanced native fluorescence, enhancing sensitivity and selectivity for detecting neurotransmitters (NTs). NTs, like monoamines, fluoresce weakly in the UV range. Plasmonic nanostructures, including Aluminum hole arrays, Aluminum nanocubes, and Al nanotriangles amplify UV fluorescence, and this biosensor improves NT detection, which is critical for understanding neurological disorders. Traditional methods lack multi-NT probing and molecule differentiation. Tested neurotransmitters include Tryptophan, Dopamine, Norepinephrine, and DOPAC. Multi-layered and monolayer silica microspheres increase sensitivity by 28 and 14 times compared to Si wafers. Furthermore, AL hole arrays and AL nanocubes could enhance the FL signal of three neurotransmitters (DA, NE, DOPAC) by 6 to 9 times. This paper highlights UV plasmonic-enhanced fluorescence's potential for distinguishing similar NT structures.
Ultraviolet (UV) plasmonic nanostructures hold promises in enabling label-free sensing of biomolecules using their native fluorescence. Several UV plasmonic structures have been explored to enhance native fluorescence of biomolecules, including metallic thin film, particle array, hole array using aluminum, magnesium, indium, etc. However, the enhancement factor of them is quite small, with less than 80 times for nucleic acids and less than 15 times for amino acids. In order to achieve higher enhancement factor, we study a bowtie nano-antenna (BNA) made of aluminum (Al) in the ultraviolet region. The effect of the native oxide layer on Al is also investigated. The numerical simulation has shown 1026x net enhancement with the optimal geometry.
UV plasmonics has drawn increased attention in recent years, holding promise in enabling label-free sensing of biomolecules such as DNA, peptides, and proteins whose intrinsic fluorescence lies in the UV range. However, these biomolecules exhibit relatively small quantum yields (QY) and extinction cross sections. In order to realize label-free detection of biomolecules, significant enhancement needs to be achieved. Several plasmonic structures have been reported to enhance native fluorescence of DNA and amino acids, with <80 x net enhancement for DNA and <15 x net enhancement for amino acids. Orders of magnitude improvement in the net enhancement factor are needed in order to achieve a detection limit comparable to commercial bioassays. In addition, quantitative fluorescence analysis that can differentiate the contribution of radiative and excitation enhancement is needed for UV studies. Here we report fluorescence enhancement of tryptophan on aluminum hole arrays. By optimizing excitation geometry, the hole size and spacings, we are able to achieve <40 x net enhancement factor, the highest ever observed for tryptophan molecules. We conducted photobleaching experiments and observed 2 x reduction in the fluorescence decay rate on the aluminum hole array compared to an aluminum thin film. The enhancement of total photon yield reaches 17 x, which indicates enhanced radiative rate. The studies we conducted will pave a way for label-free biosensing using UV plasmonics.
Exfoliated gallium oxide (Ga2O3) has been reported as an ultrathin channel material in field-effect transistors. Unlike in 2D materials with van der Waals forces between stacked layers, weak bonding in β-Ga2O3 along the (100) direction enables mechanical exfoliation. The thickness of these exfoliated films has been limited to tens to hundreds of nanometers. Here we summarize our latest work, which followed the process developed by Carey et al., enabling us to attain ~2 nm thick Ga2O3 films over a large surface area (< 1 mm2). The films are characterized using optical microscopy, AFM, XPS, Raman spectroscopy, photoluminescence (PL), and TEM. Optical microscope images showed color changes to the film upon annealing. AFM revealed the film thickness to be as thin as 2 nm over areas << 1 mm2. XPS and Raman spectra revealed characteristic signatures of β-phase Ga2O3. Characteristic PL of Ga2O3 was seen in all samples with overall intensity of luminescence increasing after annealing, attributed to increased crystallinity and grain size. Changes in PL after annealing are associated with an increase in oxygen interstitials and a decrease in oxygen vacancies. Lastly, TEM analysis revealed the film as β-phase Ga2O3 polycrystalline. Overall, our results demonstrate that annealing of thin films obtained from oxide printing of liquid metal Ga is a non-expensive and straightforward process that can lead to β-Ga2O3 films that are nanometer-thin over wafer-scale areas.
There is a growing interest in UV plasmonics due to its potential applications in biomedical field. Native fluorescent of biomolecules reside in the ultraviolet range of the spectrum. However, small quantum yield and poor photo-stability are hindering the development of biosensors based on native fluorescence. UV plasmonics have been shown to improve the quantum yield and photo-stability of biomolecules. In this talk, I will discuss our efforts in studies of UV plasmonic material and nanostructure geometries to engineer the fluorescent properties of biomolecules. UV resonant enhanced Raman holds promise in label free probing of molecule structures. In this talk, I will discuss using resonant enhanced Raman spectroscopy for detecting biomolecules. I will also discuss our efforts in exploring active UV plasmonic material that can enable multiplexed biosensing.
Despite of increasing understandings of UV plasmonic materials, materials that can enable active tuning of UV plasmonic resonance has not been reported. Here, we demonstrate a modification of UV SPR on an aluminum (Al) hole-array by coupling Graphene π plasmon resonance with Al SPR. Graphene monolayer exhibits an abnormal absorption peak in the UV region (270-290nm) due to π plasmon resonance. The location and intensity of the absorption peak depend on the position of Fermi-level, which can be adjusted by electric or chemical doping. Al SPR is shown here to be modified by coupling Graphene π plasmon resonance with Al SPR.
FDTD simulation shows the modification of Al hole-array transmission by adding a single layer of Graphene on top. The shifts of transmission dips after adding a Graphene layer shows a distinct transition at around the Graphene π plasmon position. For transmission dips that are located at shorter wavelength compared to Graphene π plasmon, up to 8nm blue shifts occur after adding Graphene. On the other hand, up to 20nm redshifts occur for transmission dips that are at a longer wavelength relative to Graphene π plasmon. This change in the sign of shifts of transmission dips corresponds to the change in the sign of the real permittivity of Graphene. The amount of shifts diminishes as the transmission dip moves further away from Graphene π plasmon resonance into the visible spectrum. Experimentally we have observed redshifts of SPR dips but not blue shifts possibly due to the poor light collection below 250nm.
UV plasmonic materials such as aluminum and magnesium have drawn attention in recent years due to their potential applications in plasmonic enhanced label free sensing and UV light sources and detectors. In this paper, nanoapertures in Al and Mg thin film have been used to study the lifetime modification and net fluorescence intensity of the freely diffusing UV dye p-terphenyl. Previously, Jiao et al reported lifetime reduction of 3.5 times for Al nanoapertures with diameter 60nm. In this paper, lifetime reduction of 5.4 times is observed for Al nanoapertures with smaller diameter 40nm. For the firs time, lifetime modification in Mg nanoapertures is reported. For the same aperture geometry, Mg nanoaperture show higher Purcell factor and excitation enhancement due to lower loss of Mg in UV range . We observe 7.3 times lifetime reduction for Mg nanoapertures with 40nm diameter.
We also studied numerically bowtie antenna for native fluorescent enhancement and have observed ~180x net enhancement, ~35x radiative rate enhancement. Those results suggest that label free sensing of amino acids and proteins are promising.
Metallic nanostructure can enhance fluorescence through excited surface plasmons which increase the local field as well as improve its quantum efficiency. When coupling to cavity resonance with proper gap dimension, gap hot spots can be generated to interact with fluorescence at their excitation/emission region in UV. A 3D nano-cavity antenna array in Aluminum has been conducted to generate local hot spot resonant at fluorescence emission resonance. Giant field enhancement has been achieved through coupling fundamental resonance modes of nanocavity into surface plasmons polaritons (SPPs). In this work, two distinct plasmonic structure of 3D resonant cavity nanoantenna has been studied and its plasmonic response has been scaled down to the UV regime through finite-difference-time-domain (FDTD) method. Two different strategies for antenna fabrication will be conducted to obtain D-coupled Dots-on-Pillar Antenna array (D2PA) through Focus Ion Beam (FIB) and Cap- Hole Pair Antenna array (CHPA) through nanosphere template lithography (NTL). With proper optimization of the structures, D2PA and CHPA square array with 280nm pitch have achieved distinct enhancement at fluorophore emission wavelength 350nm and excitation wavelength 280nm simultaneously. Maximum field enhancement can reach 20 and 65 fold in the gap of D2PA and CHPA when light incident from substrate, which is expected to greatly enhance fluorescent quantum efficiency that will be confirmed in fluorescence lifetime measurement.
UV plasmonic nanostructures have applications in label free native fluorescence biosensing. Many aluminum
nanostructures have been shown to modify emission properties of UV fluorescence molecules. However, these structures
demonstrate small rate enhancement factors (less than 10x). In this paper, we report FDTD simulation results on
excitation and emission enhancement factors of a pair of aluminum bowtie antenna in ultraviolet region. Our results
show that the optimal geometry is a pair of small bowtie (radius 20nm) with apex angle 60 degrees. The highest radiative
enhancement is 25x (~340nm) and highest total decay rate enhancement is 70x, higher than previously studied
geometries.
Tunable UV devices can enable enhanced functionalities such as multiplexed sensing, wavelength-tunable light emission and so on. Interestingly, in the UV range, graphene shows a tunable optical absorption due to pi-plasmon resonance. In this work we study the UV transmission through monolayer graphene films transferred on top of aluminum hole-arrays. Transmittance though the hole-array was measured before and after graphene transfer. Interaction of graphene pi-plasmons with surface plasmon resonances leads to strong wavelength shifts, i.e. the surface plasmon resonance at the top-interface red-shifts when graphene is added. Furthermore, it is observed that maximum shift occurs in the 280 to 310 nm wavelength range. This is attributed to an enhanced graphene optical conductivity owed to pi-plasmons.
Extraordinary optical transmission (EOT) is a classic phenomenon in plasmonics. The study of plasmonic nanostructures in the ultraviolet (UV) is a relatively uncharted field due to challenges in both engineering (nanostructure design, optimization, and fabrication) and materials science (detailed composition analysis). Our previous research has been mainly focused on UV field enhancement ofdifferent Al nanostructures. In this work, two-dimensional periodic nanohole arrays in Aluminium (Al) and Magnesium (Mg) films were fabricated using Ga focused ion beam (FIB) lithography. Optical transmission through the arrays was obtained in the UV and visible range, with varying array periodicity. Transmission results showed strong resonance enhancement in the UV and visible region resulting from SPP coupling, with corresponding red-shift as the period increases, while waveguide mode peaks remain in place. Comparing Al and Mg EOT results, Al hole-array enabled larger transmission than that of Mg. Dips in transmission through Al arrays occur at similar spectral positions to those of Mg arrays with same periods. Numerical analysis was carried out through finite-difference-time-domain (FDTD) method, which showed far-field transmission consistent with experiments in general. The model was constructed based on transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) of cross-sectioned samples. The effect of Gallium (Ga) implantation from FIB fabrication was qualitatively studied, which indicated Ga implants inside the hole bottom as well as higher implantation within Mg than that within Al. The model also takes into account sidewall geometry and undercut into the substrate.
Ultra-violet (UV) fluorescence lifetime modification by aluminum (Al) and magnesium (Mg) nanoapertures are reported in this manuscript. Nanoapertures with diameter ranging from 30nm to 90nm are fabricated using focused ion beam (FIB). Largest lifetime reduction are observed for apertures with smallest diameters and undercuts into glass substrate. For Al nanoapertures, largest lifetime reduction is ∼5.30×, larger than perviously reported ∼3.50×.1 For Mg nanoapertures, largest lifetime reduction is ∼6.90×, which is the largest lifetime reduction of UV fluorescence dye reported so far in literature. The dependence of count rate per molecule (CRM) on aperture size and undercut is also investigated, revealing that CRM increases with increasing undercut, however, the CRM is small (less than 2) for the entire range of aperture size and undercut we investigated. FDTD simulation were conducted and in order to favorably compare experimental results with simulated results, it is critical to take into account the exact shape and material properties of the nano aperture. Simulation results revealed the fundamental difference between Al and Mg nano aperture under 266nm illumination-Mg nano aperture presents a waveguide mode in which the maximum field enhancement and Purcell factor is within the nano aperture instead of on the surface which is the case for Al nano aperture.
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