Electrical Switching of the Off-Resonance Room-Temperature Valley Polarization in Monolayer MoS2 by a Double-Resonance Chiral Microstructure.

Enhancing and expanding the manipulated range of room-temperature valley polarization at off-resonance wavelength is extremely crucial to developing various functional valleytronic devices. Although these have been realized through the double-resonance strategy or twist-angle engineering, the demand for electrical control over the concepts remains elusive. Here, we fabricate a gate-tunable double-resonance chiral microstructure using a molybdenum disulfides (MoS2) monolayer. On the basis of the varied interface charge density, we demonstrate the huge photoluminescence (PL) tuning ability of this configuration. Furthermore, benefiting predominately from the screening of long-range e-h exchange interactions and the chiral Purcell effect, the electrical switching of the room-temperature valley polarization at off-resonance wavelength is also realized. Our work enriches the functions of TMDs-based optoelectronic devices and may create important applications in future valley-polarized encode and information processing devices.

Broadband and compact polarization beam splitter in LNOI hetero-anisotropic metamaterials.

In this paper, theoretical modeling and numerical simulations of a high-performance polarization beam splitter (PBS) based on hetero-anisotropic metamaterials are proposed on the lithium-niobate-on-insulator (LNOI) platform. The hetero-anisotropic metamaterials constructed by sub-wavelength gratings (SWGs) can be regarded as effective anisotropy medium, which exhibits strong birefringence without breaking the geometrical symmetry, contributing to the formation of PBS. Rather than the principle of PBS based on beat-length difference of transverse electric (TE) polarization and transverse magnetic (TM) polarization, the device can realize polarization beam splitting in single beat length, and the footprint of the proposed PBS can be reduced to 8 µm × 160 µm (with S-bend). The simulation results show that the bandwidth is 185 nm (1450∼1634 nm) for TE polarization while the bandwidth is 85 nm (1490∼1575 nm) for TM polarization when the polarization extinction ratio is >20 dB. Furthermore, the insertion loss is less than 1 dB in the range of 1450 to 1650 nm, for both TE and TM polarization. Additionally, the proposed device proves strong robustness of the fabrication tolerance.

A straightforward and sensitive "ON-OFF" fluorescence immunoassay based on silicon-assisted surface enhanced fluorescence.

A straightforward immunoassay based on silicon-assisted surface enhanced fluorescence (SEF) has been demonstrated using a silicon-based fluorescent immune substrate and silver-antibody nanoconjugate (SANC). The P-doped, (100) oriented silicon wafers are used for both fluorophore attachment and antigen immobilization. The silicon substrate offers a very low blank signal in the "OFF" state, due to its fluorescence quenching effect. In the detection process, the capture of the SANCs by the surface-immobilized antigen leads to an effectively enhanced fluorescence to produce an "ON" state. The analytical performance of the presented scheme has been investigated and a limit of detection of 31.4 pg mL-1 has been obtained. Besides the broadened application range compared with the conventional immunoassays, the presented scheme is straightforward, cost effective and sensitive, and is hence expected to find widespread applications in immunoassays as well as other fluorescence-based assays.

Silicon-assisted surface enhanced fluorescence toward improved assay performances.

A novel scheme of silicon-assisted surface enhanced fluorescence (SEF) is presented for SEF-based assays, where the blank signal suppression and the fluorescence signal enhancement is combined. The P-doped, (100) oriented silicon substrate is used to quench the fluorescence of Rose Bengal (RB) molecules attached to it, resulting in an effectively suppressed background signal, which is useful for a lower limit of detection (LOD). When a proper quantity of silver nanoparticles (AgNPs) is deposited on the RB-attached silicon substrate, a significant fluorescence enhancement of up to around 290 fold is obtained, which helps to improve the sensitivity in fluorescence-based assays. Besides, conventional gold nanoparticles (AuNPs) have also been demonstrated to exhibit excellent SEF effect using the presented scheme, providing improved stability and biocompatibility. The mechanism of the observed SEF effect has been investigated, and both the decreased apparent quantum yield and the silicon-induced electric field redistribution are considered to play important roles. The experimental results suggest that the presented scheme holds great potential in the SEF-based assays aiming at higher sensitivity and lower LOD.

Highly efficient and controllable photoluminescence emission on a suspended MoS2-based plasmonic grating.

Being a new class of materials, transition metal dichalcogenides are paving the way for applications in atomically thin optoelectronics. However, the intrinsically weak light-matter interaction and the lack of manipulation ability has lead to poor light emission and tunable behavior. Here, we investigate the fluorescence characteristic of monolayer molybdenum disulfide on a metal narrow-slit grating, where a highly efficient, 471 times photoluminescence enhancement are realized, based on the hybrid surface plasmon polaritons resonances and the decreased influence of substrate. Moreover, the emitted intensity and polarization are controllable due to the polarization-dependent characteristic and anisotropy of grating. The manipulations of light-matter interactions in this special system provide a new insight into the fluorescent emission process and open a new avenue for high-performance low dimensional materials devices designs.

An online pH detection system based on a microfluidic chip.

An online pH detection system is very critical in monitoring the sudden change of pH, especially in strongly acidic and alkaline conditions. We developed a pH sensing chip which works in the range of 5 M [H+]-pH 3.0 and pH 6.0-2 M [OH-] with response time of 90 s. The sensing chip was formed by coating a pH sensing membrane onto the wall of a microfluidic chamber. The pH sensing membrane was prepared by chemically immobilizing m-Cresol purple in polyvinyl alcohol (PVA). The pH detection system consisted of light source, pH sensing chip and photodiode (PD). Once the pH of fluid flowing in the sensing chip changed, the intensity of transmitted light changed. The intensity of transmitted light was converted to voltage, which was the function of pH value, by the PD. A feed-forward artificial neural network (ANN) with error back-propagation training algorithm was employed to model the behavior of the pH sensor and read out pH values of unknown solutions. The pH detection system shows high stability with increasing the ionic strength. It also possesses properties of repeatability, reversibility and long life-time. These advantages make the proposed pH detection system a promising solution for online detection of pH values in harsh conditions.

Manipulating "Hot Spots" from Nanometer to Angstrom: Toward Understanding Integrated Contributions of Molecule Number and Gap Size for Ultrasensitive Surface-Enhanced Raman Scattering Detection.

Narrower gaps between metal nanoparticles (so-called "hot spots") in surface-enhanced Raman scattering (SERS) substrates contribute to stronger electromagnetic (EM) enhancement; however, the accompanying steric effect hinders analyte molecules entering hot spots to access the benefit. To comprehensively understand integrated contributions of the gap size and molecule number accommodated in hot spots and then optimize design of SERS substrates, the thermal shrinking method was employed to manipulate hot spots and the "hottest zone" was defined to evaluate the integrated contributions to SERS intensity of the two factors. In the conventional shrink-adsorption mode, the contributions of the molecule number and gap size are competitive when the gap width is comparable with the target molecule size, which leads to oscillating behavior of SERS intensity versus gap size, and it is analyte molecule size dependent. This result suggests that engineering hot spots should be target molecule directed to achieve ultrasensitive detection. In the proposed adsorption-shrink mode, the contributions of the molecule number and gap size are synergistic, which makes the detection ability of the adsorption-shrink mode attains a single-molecule (SM) level. Excellent performance of the adsorption-shrink SERS strategy benefits detection of trace level pollutants in complex environments. Detection ranges for contaminants with different metal affinity, such as thiram, malachite green (MG), and formaldehyde, are as low as parts per billion, even down to parts per trillion.

Performance improvements of a tunable bandpass microwave photonic filter based on a notch ring resonator using phase modulation with dual optical carriers.

A tunable bandpass microwave photonic filter can be achieved by using a notch ring resonator with optical phase modulation. However, the filter's out of band rejection ratio and shape factor are limited due to the ring resonator's residual phase, which can seriously degrade the filter's performance. By using dual optical carriers and setting their wavelengths oppositely detuned from two resonant frequencies of a notch ring resonator, the residual phase induced by the ring resonator at radio frequencies falling outside the region of the notch stopband is reduced, thus the out-of-band rejection ratio and shape factor of the microwave photonic filter are greatly improved. The proposed microwave photonic filter was both verified theoretically and experimentally. Compared with single optical carrier method, the out-of-band rejection ratio of the filter can be enhanced from 17.7dB to 31.5dB, and the filter's shape factor is improved from 3.05 to 1.78. Besides, the filter's frequency and bandwidth can be tuned by varying the wavelengths of the two optical carriers and the ring resonator's coupling coefficients. Finally, a tunable bandpass microwave photonic filter with frequency tuning range of 2~14GHz, 3dB bandwidth tuning range of 0.673~2.798GHz is demonstrated.

A SERS fiber probe fabricated by layer-by-layer assembly of silver sphere nanoparticles and nanorods with a greatly enhanced sensitivity for remote sensing.

Remote sensing remains a challenge due to its demand for high sensitivity, convenient sampling and rapid response time. Surface enhanced Raman scattering (SERS) spectroscopy is a powerful analytical method for the detection of various samples. Here, aiming at increasing the sensitivity, a novel strategy for the preparation of a SERS probe is demonstrated by using hollow optical fiber tips decorated by layer-by-layer assembly of two kinds of nanoparticles. Specifically, Au@Ag core-shell nanorods and Ag nanospheres with opposite surface charge were assembled layer-by-layer on the tip of hollow optical fibers through electrostatic interaction. Then, much more hotspots are generated due to the close gap between the nanorods and nanospheres in the resultant 3D structure, which can lead to a dramatically enhanced SERS activity of the probe compared with that fabricated by pure silver sphere nanoparticles or nanorods. On the other hand, taking the advantages of the vibration spectroscopic fingerprints property of SERS spectra and the long-distance communication capacity of optical fibers, the remote online detection of biological species including proteins, funguses and cells can be easily achieved within a few minutes. Therefore, such a novel kind of optical fiber-SERS sensor holds great potential for the rapid detection of a wide range of samples due to its superiority of simplicity and high sensitivity.

Super blinking and biocompatible nanoprobes based on dye doped BSA nanoparticles for super resolution imaging.

As one of the super-resolved optical imaging techniques, single molecule localization microscopy (SMLM) received considerable attention due to its impressive spatial resolution. Compared with other fluorescence imaging techniques, SMLM has one particular request for the fluorophores, that is, continuous 'on' and 'off' behaviors of their signals (referred to as 'blinking'). Hence, we present here a kind of super blinking and biocompatible nanoprobes (denoted as SBNs) for SMLM. The SBNs have two main advantages, first, they possess an outstanding fluorescence blinking. Second, they are biocompatible since they are based on bovine serum albumin (BSA). The SBNs are fabricated by doping organic dyes into BSA nanoparticles. We fabricated two kinds of SBNs, one was doped with Alexa Fluor 647 (A647) and the other was doped with Alexa Fluor 594 (A594). Especially for A594 doped SBNs, the improved blinking of A594 doped SBNs induced a better localization precision as compared with A594 alone. Moreover, SMLM imaging of breast cancer cells and exosomes using the SBNs was successfully realized with high spatial resolutions. The work demonstrated here provides a new strategy to prepare novel kinds of super blinking fluorescent agents for SMLM, which broadens the selection of suitable fluorophores for SMLM.

Realization of mid-infrared broadband absorption in monolayer graphene based on strong coupling between graphene nanoribbons and metal tapered grooves.

In this paper, we theoretically propose an effective broadband absorption architecture in mid-infrared region based on strong coupling between the plasmonic resonance of graphene nanoribbons and the waveguide mode of a metal tapered groove. The special architecture facilitates two new hybrid modes splitting with very strong energy distribution on graphene ribbon, which results in the broadband absorption effect. To well explain these numerical results, an analytical dispersion relation of waveguide mode is obtained based on the classical LC circuit model. The fluctuating range of absorption passband is investigated by adjusting the filled medium inside of the grooves. Leveraging the concept and method, a broadband flat-top (bandwidth ≈2.5 µm) absorption with absorption rate over 60% is demonstrated. Such a design not only enhances the intrinsic weak plasmons resonance in mid-infrared spectral region, but also reduces the absorption fluctuations caused by coupling, which are the key features for developing next-generation mid-infrared broadband optical devices.

Single molecule localization imaging of telomeres and centromeres using fluorescence in situ hybridization and semiconductor quantum dots.

Single molecule localization microscopy (SMLM) is a powerful tool for imaging biological targets at the nanoscale. In this report, we present SMLM imaging of telomeres and centromeres using fluorescence in situ hybridization (FISH). The FISH probes were fabricated by decorating CdSSe/ZnS quantum dots (QDs) with telomere or centromere complementary DNA strands. SMLM imaging experiments using commercially available peptide nucleic acid (PNA) probes labeled with organic fluorophores were also conducted to demonstrate the advantages of using QDs FISH probes. Compared with the PNA probes, the QDs probes have the following merits. First, the fluorescence blinking of QDs can be realized in aqueous solution or PBS buffer without thiol, which is a key buffer component for organic fluorophores' blinking. Second, fluorescence blinking of the QDs probe needs only one excitation light (i.e. 405 nm). While fluorescence blinking of the organic fluorophores usually requires two illumination lights, that is, the activation light (i.e. 405 nm) and the imaging light. Third, the high quantum yield, multiple switching times and a good optical stability make the QDs more suitable for long-term imaging. The localization precision achieved in telomeres and centromeres imaging experiments is about 30 nm, which is far beyond the diffraction limit. SMLM has enabled new insights into telomeres or centromeres on the molecular level, and it is even possible to determine the length of telomere and become a potential technique for telomere-related investigation.

Visualization and intracellular dynamic tracking of exosomes and exosomal miRNAs using single molecule localization microscopy.

Exosomes are small membrane vesicles secreted by a wide variety of cells. Studies have demonstrated that exosomal miRNAs can influence the biological processes of recipient cells. Therefore, direct imaging and tracking of exosomal miRNAs in living recipient cells are essential for exosome functional analysis. However, the moderate spatial resolution of conventional fluorescence microscopy limits the precise imaging and tracking of exosomes considering their relatively small size (<100 nm). Here, we took advantage of single molecule localization microscopy (SMLM) to realize the visualization and dynamic tracking of exosomes and exosomal miRNAs in living cells. First, we demonstrated simultaneous SMLM imaging of HeLa-derived exosomes and two kinds of exosomal miRNAs (mir-21 and mir-31) and tracked their movement after cellular uptake. The motion of exosomes within the intercellular filamentous structures was observed successfully. Moreover, dual-color SMLM based dynamic tracking revealed that as one kind of natural carrier of cellular cargoes, exosomes can encapsulate mir-21 inside to prevent enzyme degradation during transfer and then release them into recipient cells. These findings can provide new insights into the pathway of intercellular communication and affirm that SMLM is a powerful technique to track the motion of exosomes and exosomal contents in recipient cells.

Investigating the Intracellular Behaviors of Liposomal Nanohybrids via SERS: Insights into the Influence of Metal Nanoparticles.

The recent proposition to combine liposomes with nanoparticles presents great opportunities to develop multifunctional drug delivery platforms. Although impressive progress has been made, attempts to elucidate the role nanoparticles play in the integral nanohybrids are still rather limited. Here, using surface-enhanced Raman scattering (SERS) technique, we investigate the influence of metal nanoparticles on the liposomal properties, ranging from drug release to intracellular movement. Specifically, we prepared SERS-active nanohybrids by attaching metal nanoparticles to liposomes and employed SERS signals to explore the intracellular behavior of the nanohybrids. Once deposited on the cell membrane, the nanohybrids entered tumor cells via clathrin-mediated endocytosis and then moved to lysosomes. In comparison with pure liposomes, metal nanoparticles in the nanohybrids had little influence on the properties of liposomes. This study fills the gap of the function of nanoparticles in the overall nanohybrids, which provides a significant prerequisite for efficient drug delivery in therapeutic applications.

Joint effect of ferromagnetic and non-ferromagnetic cations for adjusting room temperature ferromagnetism of highly luminescent CuNiInS quaternary nanocrystals.

In this work, highly luminescent quaternary CuNiInS nanocrystals (NCs) are put forward as a good prototype for investigating defect-induced room temperature ferromagnetism. A ferromagnetic Ni cation can preserve the strong luminescence of NCs without introducing intermediate energy levels in the center of the forbidden band. The strong luminescence of NCs is used as an indicator for monitoring the concentration of vacancy defects inside them, facilitating the investigation of the origin of room temperature ferromagnetism in CuNiInS NCs. Our results reveal that the patching of Cu vacancies [Formula: see text] with Ni will result in bound magnetic polarons composed of both [Formula: see text] and a substitution of Cu by Ni [Formula: see text] giving rise to the room temperature ferromagnetism of CuNiInS NCs. Either the ferromagnetic Ni or the non-ferromagnetic Cu cation can tune the magnetism of CuNiInS NCs because of the change of bound magnetic polaron concentration at the altered concentration ratio of [Formula: see text] and [Formula: see text].

Dual-mode tracking of tumor-cell-specific drug delivery using fluorescence and label-free SERS techniques.

We developed a dual-mode detection method for tumor cell specific targeting and intracellular delivery of the chemotherapeutic agent Doxorubicin (DOX) using folic acid functionalized mesoporous silica nanoparticles (FA-MSNs) as carrier systems. In this method, label free surface enhanced Raman scattering (SERS) spectra were utilized to monitor the dynamic release of DOX inside tumor cells in combination with fluorescence images. To investigate the targeting delivery performance of the carrier system, both normal cells (MRC-5) and tumor cells (HeLa) were used as the model cells. The real-time release of DOX from FA-MSNs inside MRC-5 and HeLa cells was monitored. As demonstrated by both fluorescence and SERS results, the DOX loaded FA-MSNs can actively target FA receptor overexpressed tumor cells. Moreover, the releasing behavior of DOX from FA-MSNs in tumor and in normal cells was quantitatively analyzed. Compared with the traditional sole fluorescence or SERS method, this dual-mode detection is more powerful and more accurate, which should have a potential application in drug tracking in living cells.

SERS-based DNA detection in aqueous solutions using oligonucleotide-modified Ag nanoprisms and gold nanoparticles.

Oligonucleotide-modified nanoparticle conjugates show highly promising potential for SERS-based DNA detection. However, it remains challenging to carry out the SERS-based DNA detection in aqueous solutions directly using oligonucleotide-modified nanoparticles, because the Raman reporters would exhibit lower signals when they are dispersed in aqueous solutions than laid on "dry" metal nanoparticles. Here, we synthesized stable oligonucleotide-modified Ag nanoprism conjugates, and performed SERS-based DNA detection in aqueous solution directly by using such conjugates in combination with Raman reporter-labeled, oligonucleotide-modified gold nanoparticles. The experimental results indicate that this SERS-based DNA detection approach exhibited a good linear correlation between SERS signal intensity and the logarithm of target DNA concentration ranging from 10(-11)∼10(-8) M. This sensitivity is comparable to those SERS-based DNA detection approaches with the "dry" process. Additionally, a similar correlation could also be observed in duplex target DNA detection by SERS hybrid probes. Our results suggest that the oligonucleotide-modified Ag nanoprisms may be developed as a powerful SERS-based DNA detection tool.

A straightforward immunoassay applicable to a wide range of antibodies based on surface enhanced fluorescence.

A straightforward immunoassay based on surface enhanced fluorescence (SEF) has been demonstrated using a fluorescent immune substrate and antibody functionalized-silver nanoparticles. Unlike the conventional SEF-based immunoassay, which usually uses the dye-labeled antibodies and the metallic nanostructured-substrates, the presented immune system does not need the antibodies to be labeled with dye molecules. Thus, this immunoassay can be easily applied to the detection of a wide range of target antigens, which is of great importance for its practical application. The experimental results show that this immunoassay has a good specificity as well as the capacity of quantitative detection. Basically, the surface density of the immuno-adsorbed silver nanoparticles increases with the increased amount of target antigens, resulting in a fluorescence enhancement up to around 7 fold. The dose-responsive performance of the immunoassay has been investigated and the limit of detection (LOD) is 1 ng/mL. Due to its simple preparation method and the wide range of detectable antigens, this presented immunoassay is expected to be helpful for extending the SEF-based application.

Immunoassays based on surface-enhanced fluorescence using gap-plasmon-tunable Ag bilayer nanoparticle films.

A novel gap-plasmon-tunable Ag bilayer nanoparticle film for immunoassays is demonstrated. Different from a traditional Ag monolayer nanoparticle film, a desired number of polyelectrolyte (PEL) layers are deposited on the nanoparticles before the self-assembly of a second Ag nanoparticle layer. Interestingly, by controlling the number of the PEL interlayers, the gap plasmon between the two Ag nanoparticle layers can be tuned across the visible spectral range. The ability of the presented Ag bilayer nanoparticle films in fluorescence enhancement has been examined experimentally. A maximal enhancement of around 15.4 fold was achieved when 7 layers of polyelectrolyte were used. When this optimal Ag bilayer nanoparticle film was applied to fluorescence immunoassay, a performance with approximately 3.3-fold enhancement was obtained compared with that performed on a traditional glass substrate. The experimental results suggest that the presented gap-plasmon tunable Ag bilayer nanoparticle films have great potential in fluorescence-based immunoassays. The method of the bilayer-film construction presented here also provides new insights into the rational design of the plasmonic substrates.

A multiplex and straightforward aqueous phase immunoassay protocol through the combination of SERS-fluorescence dual mode nanoprobes and magnetic nanobeads.

A novel aqueous phase immunoassay protocol was demonstrated, using surface enhanced Raman scattering (SERS)-fluorescence dual mode nanoprobes combined with magnetic nanobeads (MBs). Here, the dual mode nanoprobes provide an excellent multiplexing ability while the MBs greatly simplify the immunoassay process. Basically, the nanoprobes were acquired by assembling the Raman reporter tagged Au@Ag core-shell nanorods and quantum dots onto the silica nanospheres. When the specific antigens are presented in the immunoassay system containing antibody modified nanoprobes and MBs, the nanoprobes are captured by the MBs and further precipitated by a magnet. Consequently, both SERS and fluorescence signals are detected in the precipitates. Sandwich type immunoassay was conducted to examine the practicability of this protocol. Experimental results confirmed that the presented immunoassay protocol can accomplish highly specific and sensitive recognition of the target antigens. The detection limit was found out to be 0.1 pg/mL. We anticipate that high throughput bioanalysis can be fulfilled using the proposed immunoassay protocol, as the dual mode nanoprobes provide a great multiplexing capability while the MBs facilitate the convenient aqueous phase detection of the analytes.