We demonstrate a novel optofluidic micropipette device for filter-free fluorescence-based biosensing. The optofluidic micropipette tube composed of a glass capillary microtube and a polymer-based structure designed to load analyte solution using a regular micropipette and serves as an optical waveguide. Ray-tracing simulations suggest that the excitation light can be effectively guided along the glass capillary with a small amount of leakage through the scattering at the solutionair interface. Fluorescence emission of the analyte propagates in the radial direction of the glass capillary which can be efficiently captured by a smartphone camera through a miniaturized objective lens. Fluorescence intensity and spectra were characterized using Rhodamine 6G (R6G) with various concentrations. The emission was collected via a microscope with 5X magnification and a smartphone camera. Both experimental and simulation results suggest that the excitation rays are efficiently coupled into the glass micropipette tube for fluorescence excitation. The fluorescence emissions from the analyte will either pass along the glass tube or propagate in the radial direction collected by the detector. A limited amount of excitation leakage scattered from the liquid-air interface showed a minimal effect on fluorescence detection. We demonstrated the platform that combines the optofluidic micropipette and smartphone camera to detect steroid hormone.
Fluorescent carbon dots (CDs) are one class of carbon-based nanomaterials that exhibit special photoluminescence properties. The unique properties of CDs, such as biocompatibility, tunable emission wavelength, and cost-effective, synthesis, have aroused intense interest. Conventionally, in the same particle size, the emission wavelength of CDs can be controlled by the graphitization of the monomer precursors. To date, it is still challenging to produce long-wavelength emissive CDs because it requires a higher graphitization degree of precursors. Not many results have been reported for the CDs with the emission wavelength longer than 600 nm (red). In this paper, we report a new type of red emissive CDs with the emission peak at 660 nm under ultraviolet light excitation with 30% quantum yield. Different from the conventional CDs with short Stokes shift, the new CDs exhibit 255 nm Stokes shift. This property will benefit applications of biosensors, solid-state lighting, and electronic displays. Furthermore, the carbon dots can be embedded into UV-curable polymer. With the fast photocuring technology, red emissive polymer pattern can be produced immediately by printing, stamping, or plotting. A red emission microLED was fabricated using CD-embedded polymer to generate a color coordinate at (0.56, 0.42).
Nanoparticle-based fluorescence DNA/RNA sensing offers promising applications in both research and medical diagnosis due to the ease of surface chemical modification and sample handling, allowing detection in complex media. The performance of the conventional fluorescence biosensors is often limited by the insufficient fluorescence signal. To overcome the disadvantage, we advanced silver-coated magnetic nanoparticles with strong plasmon resonance to enhance the molecular beacon (MB)-based nucleic acid nanosensors. The silver-coated magnetic nanoparticles were chemically synthesized to compose of 20 nm iron oxide magnetic nanoparticle core and 60 nm thick Ag coating, forming iron oxide/Ag core-shell nanoparticles. Fluorescently labeled DNA MBs were immobilized on the Ag surface which serves as the quencher for the closed MBs and provides fluorescence enhancement for the unfolded MBs in the presence of the complementary target sequence. More importantly, the improved Ag shell mitigates the strong optical absorbance in the visible range associated with the magnetic nanoparticles increasing the fluorescence intensity. The detection was performed by dispersing the nanosensors in a 20 μl analyte solution for 10 minutes for accelerated target capture through 3D diffusion and concentrating them magnetically for enhanced fluorescence signal acquisition. The rapid, label-free DNA detection resulted in a detection limit of 10 pM target DNA.
MicroLED display is emerging as a candidate to drive a new generation of display technology. Full-color microLED based on carbon-dots (CDs) and blue microLED utilizes photoluminescence (PL) of blue-excited red and green emission CDs to achieve large coverage of color gamut and low power consumption. There is a high demand to develop costeffective technologies to enhance CD emission and minimize blue excitation light leakage through the CD layer. Here we demonstrate the use of plasmonic nanoparticles to enhance multicolor CDs in the emitting layer of microLED while suppressing the transmission of blue excitation. Silver nanoparticles are known to have surface plasmon resonances in or close to the blue range. Blue excitation over an emitting layer formed by the mixture of CDs and metal nanoparticles leads to excitation enhancement of CDs and thus the increased quantum efficiency. We studied the emitting layers fabricated by dispersing a mixture of 30 nm silver nanoparticles and CDs at various ratios and obtained a maximum enhancement factor of ~8. The metal nanoparticles also absorbed the blue excitation and reduced the leakage of blue light. Fluorescence lifetime measurements showed negligible changes in the CD emission rate with and without the presence of metal nanoparticles. The analysis implies that the enhanced CD PL is a result of excitation enhancement rather than Purcell effect. This technique offers a low-cost, effective approach to improve the performance of microLED displays.
Purcell enhancement can be realized using hyperbolic metamaterials (HMMs) composed of alternating metal/dielectric multilayers of subwavelength thickness. By adjusting the filling fraction of the metal layer, this structure possesses an effective hyperbolic dispersion and can access to epsilon-near-zero (ENZ) with one of the principal components of the permittivity tensor passes through zero. The unique property theoretically yields a large local density of state (LDOS) enabling to support a high Purcell factor and enhanced spontaneous emission rate of a quantum emitter in the vicinity. However, the property of the fabricated HMM deviates from the ideal characteristics estimated by effective medium theory (EMT) due to the finite thickness of the unit cell. Therefore, the actual LDOS and Purcell factor reduce significantly. Additionally, the outcoupling of the high-k waves from HMM remains challenging. It relies on small-area nanostructure due to the incapability of large-area nanofabrication. In this paper, we experimentally and theoretically study the effect of the unit cell thickness in Ag/ITO HMMs on the enhancement of QD emission. The study on 320 nm thick HMM formed by three different unit cell thicknesses ranging from 80 to 20 nm suggested that the Purcell factor increases as the unit cell thickness decreases. We also demonstrate a large-area outcoupling method using self-assembled nanoparticle monolayer to promote the detectable QD emission in the far field. A maximum enhancement factor of ~40 was observed by incorporating the nanoparticle monolayer. This enhancement technique and large-area outcoupling will find applications in display and biosensing.
Molecular beacon (MB) probe is a fluorophore-labeled oligonucleotide and has been widely used in biological analysis and medical diagnostics by detecting DNA or RNA with specific sequences. The MB initially folds into a loop shape that brings the fluorophore close to a quencher for fluorescence quenching. It opens up upon the binding of target DNA that separates the fluorophore from the quencher to allow fluorescence emission. In this paper, we experimentally demonstrate the use of a silver open-ring nanostructure array (ORA) to enhance both fluorescence emission and quenching of MBs for highly sensitive DNA detection. The ORA displays a broadband resonance spectrum to enhance both the excitation and emission of fluorophores. The fluorescence enhancement is highly dependent on the distance between nanostructure and fluorophore. The couplings of the fluorescence emission and the external excitation with the proximate plasmonic nanostructure result in coherent electron oscillations that in turn act as secondary excitation of the fluorophore in a ~10 nm separation distance, leading to fluorescent enhancement. The resonance feature of ORA also improved the Förster resonance energy transfer between the fluorophore and ORA in an even shorter separation distance that promotes the fluorescence quenching. The enhanced fluorescence emission and quenching amplified the on-off ratio of the detection signal. The sensor was integrated into a microfluidic chamber to handle microliter-volume analyte and achieved a ~300 fM detection limit, an equivalent 360 zmol in a 1.2 μL analyte volume, superior to the detection on plane silver surfaces.
In this paper, we report on two different approaches that have been explored to realize tunable and reconfigurable THz devices for advanced imaging and adaptive wireless communication. The first approach makes use of electronically tunable varactor diodes. Frequency tunable THz antennas based on this approach have been successfully demonstrated for the first time in G-band, enabling the development of spectroscopic THz detectors and focal-plane imaging arrays. The second approach takes advantages of optical THz spatial modulation based on photo-induced free carriers in semiconductors. Using this approach, high-performance tunable THz modulators/attenuators, reconfigurable masks for THz coded aperture imaging, and photo-induced Fresnel-zone-plate antennas for dynamic THz beam steering and forming have been successfully demonstrated. Our recent study also shows that by employing the so-called mesa array technique, sub-wavelength spatial resolution and higher than 100 dB modulation depth can be achieved, making it possible to develop tunable THz devices (e.g., tunable filters) with performance and versatility far beyond those realized by conventional approaches. On the basis of the above investigation, the prospects of high-speed near-field THz imaging, real-time ultra-sensitive heterodyne imaging and prototype adaptive THz wireless communication links will be discussed.
Enhanced photoluminescence (PL) of quantum dots (QD) in visible range using plasmonic nanostructures has potential to advance several photonic applications. The enhancement effect is, however, limited by the light coupling efficiency to the nanostructures. Here we demonstrate experimentally a new open-ring nanostructure (ORN) array 100 nm engraved into a 200 nm thick silver thin film to maximize light absorption and, hence, PL enhancement at a broadband spectral range. The structure is different from the traditional isolated or through-hole split-ring structures. Theoretical calculations based on FDTD method show that the absorption peak wavelength can be adjusted by their period and dimension. A broadband absorption of about 60% was measured at the peak wavelength of 550 nm. The emission spectrum of CdSe/ZnS core-shell quantum dots was chosen to match the absorption band of the ORN array to enhance its PL. The engraved silver ORN array was fabricated on a silver thin film deposited on a silicon substrate using focus ion beam (FIB) patterning. The device was characterized by using a thin layer of QD water dispersion formed between the ORN substrate and a cover glass. The experimental results show the enhanced PL for the QD with emission spectrum overlapping the absorption band of ORN substrate and quantum efficiency increases from 50% to 70%. The ORN silver substrate with high absorption over a broadband spectrum enables the PL enhancement and will benefit applications in biosensing, wavelength tunable filters, and imaging.
We report a technique using photo-induced coded-aperture arrays for potential real-time THz imaging at roomtemperature.
The coded apertures (based on Hadamard coding) were implemented using programmable illumination on
semi-insulating Silicon wafer by a commercial digital-light processing (DLP) projector. Initial imaging experiments
were performed in the 500-750 GHz band using a WR-1.5 vector network analyzer (VNA) as the source and receiver.
Over the entire band, each array pixel can be optically turned on and off with an average modulation depth of ~20 dB
and ~35 dB, for ~4 cm2 and ~0.5 cm2 imaging areas respectively. The modulation speed is ~1.3 kHz using the current
DLP system and data acquisition software. Prototype imaging demonstrations have shown that a 256-pixel image can be
obtained in the order of 10 seconds using compressed sensing (CS), and this speed can be improved greatly for potential
real-time or video-rate THz imaging. This photo-induced coded-aperture imaging (PI-CAI) technique has been
successfully applied to characterize THz beams in quasi-optical systems and THz horn antennas.
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