Noble‐Metal‐Free Materials for Surface‐Enhanced Raman Spectroscopy Detection

Surface‐enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single‐molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble‐metal‐free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble‐metal‐free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene‐based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble‐metal‐free materials for enhancing detection sensitivity can further fuel the development of SERS‐related applications.     

Enhanced Peroxidase-mimicking Activity of Plasmonic Gold-modified Mn3O4 Nanocomposites through Photoexcited Hot Electron Transfer

Enzyme-mimicking artificial nanomaterials often termed nanozymes have broad applications in many fields, including biosensing, pollutant degradation and cancer diagnosis. Herein, we introduce a plasmonic gold nanoparticle-modified Mn₃O₄ nanozyme (Mn₃O₄-Au). Visible or near infrared light excitation into the plasmonic absorption band of the surface-bound gold nanoparticles enhances the catalytic oxidation of tetramethylbenzidine (TMB). The mechanism of light-enhanced peroxidase activity is proposed based on the Mn₃O₄ conduction band mediated hot electron transfer from photoexcited gold nanoparticles to H₂O₂ which undergoes further oxygen-oxygen bond cleavage to yield hydroxyl radical. The surface decoration of plasmonic gold nanoparticles endows Mn₃O₄-Au to be a light-regulated nanozyme.     

Self-assembly 2D plasmonic nanorice film for surface-enhanced Raman spectroscopy

As an ultrasensitive sensing technology, the application of surface enhanced Raman spectroscopy (SERS) is one interesting topic of nano-optics, which has huge application prospectives in plenty of research fields. In recent years, the bottleneck in SERS application could be the fabrication of SERS substrate with excellent enhancement. In this work, a two-dimensional (2D) Ag nanorice film is fabricated by self-assembly method as a SERS substrate. The collected SERS spectra of various molecules on this 2D plasmonic film demonstrate quantitative detection could be performed on this SERS substrate. The experiment data also demonstrate this 2D plasmonic film consisted of anisotropic nanostructures has no obvious SERS polarization dependence. The simulated electric field distribution points out the SERS enhancement comes from the surface plasmon coupling between nanorices. And the SERS signals is dominated by molecules adsorbed at different regions of nanorice surface at various wavelengths, which could be a good near IR SERS substrate for bioanalysis. Our work not only enlarges the surface plasmon properties of metal nanostructure, but also exhibits the good application prospect in SERS related fields.     

A plasmonic waveguide end-coupled to two plasmonic cavities in a non-Hermitian quantum system for dual-band unidirectional reflectionlessness

We demonstrate the possibility of dual-band unidirectional reflectionlessness in a non-Hermitian quantum system composed of a plasmonic waveguide and two end-coupled plasmonic cavities(PCs).Our scheme exhibits dual-band unidirectional reflectionlessness can be obtained at exceptional points by properly adjusting the coupling strength between two PCs,the ratio of decay rates of two PCs,and the ratio of plasmonic cavity-waveguide coupling strengths.As a valuable feature,the quality factor reaches to~175.4 in forward direction,while the backward quality factor is close to~188.2.     

DNA-Based Adaptive Plasmonic Logic Gates

Self-assembled plasmonic logic gates that read DNA molecules as input and return plasmonic chiroptical signals as outputs are reported. Such logic gates are achieved on a DNA-based platform that logically regulate the conformation of a chiral plasmonic nanostructure, upon specific input DNA strands and internal computing units. With systematical designs, a complete set of Boolean logical gates are realized. Intriguingly, the logic gates could be endowed with adaptiveness, so they can autonomously alter their logics when the environment changes. As a demonstration, a logic gate that performs AND function at body temperature while OR function at cold storage temperature is constructed. In addition, the plasmonic chiroptical output has three distinctive states, which makes a three-state molecular logic gate readily achievable on this platform. Such DNA-based plasmonic logic gates are envisioned to execute more complex tasks giving these unique characteristics.     

Plasmonic sensors based on thick metal film perforated with rectangular nanohole arrays

The dependence of optical properties on the ambient medium, the period of nanohole arrays and the metal film thickness in a thick silver film perforated with rectangular nanohole arrays is investigated using the finite‐difference time‐domain technique. As a result of the coupling between top and down surface plasmon polaritons, mediated by localized surface plasmon resonances supported by the metallic rectangular nano‐ holes, interesting light phenomena are observed for varying thickness of the metal film and period of the rectangular nanohole arrays. Based on the dependence of the optical properties on the ambient medium, the possibility of exploiting thick metal rectangular nanohole arrays as plasmonic sensors is further discussed, the potential application as plasmonic sensors is revealed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantum steerability of two qubits mediated by one-dimensional plasmonic waveguides

We study the dynamics of the quantum steering between two separated qubits trapped in a one-dimensional plasmonic waveguide. By numerical methods, we calculate the quantum steerability and other quantum correlations, i.e., entanglement, discord, and coherence, for both cases with and without laser driving fields. It is found that steerability may exhibit a sudden disappearance and sudden reappearance phenomenon. Specifically, there exist time windows with no steerability but finite entanglement. The effects of plasmon wavenumber and the distance between the two qubits on steerability are also examined. Furthermore, we show that quantum steerability is tunable by adjusting the laser driving fields.     

Ultra-wideband surface plasmonic bandpass filter with extremely wide upper-band rejection

An ultra-wideband bandpass filter (BPF) with a wide out-of-band rejection based on a surface plasmonic waveguide (SPW) slotline with ring grooves is designed and analyzed. A paired microstrip-to-slotline transition is designed for quasi-TEM to TM mode conversion by using a microstrip line with a circular pad and the slotline with the same circular slot. The mode conversion between the TM and the surface plasmon polariton (SPP) mode is realized by using a gradient slotline with ring grooves and an impedance matching technique. The upper cut-off frequencies of the passband can be adjusted by using these proposed SPP units, while the lower frequencies of the passband are created by using the microstrip-to-slotline transitions to give an ultra-wideband BPF. The dispersion curves of SPP units, electric field distribution, and the transmission spectra of the proposed ultra-wideband bandpass filter are all calculated and analyzed by the finite-difference time-domain (FDTD) method. The simulated results show that the presented filter has good performance including a wide 3-dB bandwidth of 149% from 0.57 GHz to 3.93 GHz, an extremely wide 40-dB upper-band rejection from 4.2 GHz to 18.5 GHz, and low loss and high selectivity in the passband. To prove the design validity, a prototype of the BPF has been manufactured and measured, showing a reasonable agreement with simulation results. The unique features of the proposed BPF may make it applicable for integrated circuit and plasmonic devices in microwave or THz frequency ranges.     

SHINERS and plasmonic properties of Au Core SiO2 shell nanoparticles with optimal core size and shell thickness

Shell‐isolated nanoparticles (NPs)‐enhanced Raman spectroscopy (SHINERS) can be potentially applied to virtually any substrate type and morphology. How to take a step forward to prepare SHINERS NPs (SHINs) with superior performance is critical for the practical applications of surface‐enhanced Raman scattering (SERS) in the breadth and depth. Here, we present a method to obtain 120 nm diameter gold NPs coated with ultrathin silica shells (1–4 nm). The silica shell can be controlled growth through carefully tuning a series of parameters, such as amount of 3‐aminopropyl triethoxysilane used, pH, reaction time, and reaction temperature. We compare the enhancement factor of the obtained 120 nm Au with a 4 nm silica shell NPs to the 55 nm Au with a 4 nm silica shell NPs, and the activity of a 120 nm SHINs is nearly 24 times that the 55 nm SHIN from a single particle view. We also compare the enhancement factor of 1 nm silica shell Au@SiO₂ NPs with the bare Au NPs. The enhancement factor of 1 nm silica shell Au@SiO₂ NPs was found to be about twice that of the bare particles. For a deeper understanding of the source of the giant enhanced electrical field of the 1 nm silica shell Au@SiO₂ NPs, we study the plasmonic property of single 1 nm silica shell Au@SiO₂ NP on a gold film substrate through correlation of the structure of single NP using SEM with its SPR spectroscopy. We find that the multipolar interaction between the single Au@SiO₂ NP and gold film substrate is important for the SERS. Our studies on the performance of 120 nm SHINs and the plasmonic property of these particles can significantly expand the applications of SHINERS technique and improve the understanding of physical nature of SHINs. Copyright © 2013 John Wiley & Sons, Ltd.     

亚波长之字形MIM等离子波导宽带滤波器

运用时域有限差分（FDTD）方法数值研究了一种亚波长之字形金属-绝缘体-金属（MIM）等离子波导结构的传输属性。之字形波导在连续两个拐角可向外延伸出1~4个短切口。每个切口独立构成一个谐振腔,谐振波长近似与切口深度成线性正比,而与切口方向无关。当任意一个切口满足谐振条件时,该波导结构在对应波长的透射率均趋近于0。随着同深度切口数目的增加,禁带波长区域逐渐展宽,形成一个良好的宽带滤波器。