Zhang-Kai Zhou

Plasmonic gold mushroom arrays with refractive index sensing figures of merit approaching the theoretical limit

Localized surface plasmon resonance (LSPR)-based sensing has found wide applications in medical diagnosis, food safety regulation and environmental monitoring. Compared with commercial propagating surface plasmon resonance (PSPR)-based sensors, LSPR ones are simple, cost-effective and suitable for measuring local refractive index changes. However, the figure of merit (FOM) values of LSPR sensors are generally 1-2 orders of magnitude smaller than those of PSPR ones, preventing the widespread use of LSPR sensors. Here we describe an array of submicrometer gold mushrooms with a FOM reaching ~108, which is comparable to the theoretically predicted upper limit for standard PSPR sensors. Such a high FOM arises from the interference between Woods anomaly and the LSPRs. We further demonstrate the array as a biosensor for detecting cytochrome c and alpha-fetoprotein, with their detection limits down to 200 pM and 15 ng ml(-1), respectively, suggesting that the array is a promising candidate for label-free biomedical sensing.

Scalable, full-colour and controllable chromotropic plasmonic printing

Plasmonic colour printing has drawn wide attention as a promising candidate for the next-generation colour-printing technology. However, an efficient approach to realize full colour and scalable fabrication is still lacking, which prevents plasmonic colour printing from practical applications. Here we present a scalable and full-colour plasmonic printing approach by combining conjugate twin-phase modulation with a plasmonic broadband absorber. More importantly, our approach also demonstrates controllable chromotropic capability, that is, the ability of reversible colour transformations. This chromotropic capability affords enormous potentials in building functionalized prints for anticounterfeiting, special label, and high-density data encryption storage. With such excellent performances in functional colour applications, this colour-printing approach could pave the way for plasmonic colour printing in real-world commercial utilization.

Antibody modified gold nano-mushroom arrays for rapid detection of alpha-fetoprotein

Localized surface plasmon resonance (LSPR) combined with immunoassay shows greatly potential in fast detection of tumor markers. In this paper, a highly sensitive LSPR substrate has been fabricated and modified for direct detection of alpha-fetoprotein (AFP). The biosensor was prepared by interference lithography, and modified by covalently immobilizing anti-AFP on the surface of gold nano-mushroom arrays (GNMA). The modification process was investigated by Vis-NIR reflectance spectra and cyclic voltammogram measurements. We revealed the optical properties of the modified GNMA by measuring the Vis-NIR reflectance spectra and simulating its electric intensity field distribution under light illumination. The GNMA substrate was highly sensitive, with a refractive index sensitivity of ~465 nm/RIU. The substrate can be applied to label-free detection of AFP, with the linear range and the limit of detection determined to be 20-200 ng/mL and 24 ng/mL (S/N=3), respectively. We also demonstrated its clinical application by directly detecting AFP in human serum samples. It is expected that our biosensor could be integrated on microfluidic chips for high-throughput detection in portable early diagnosis, post-operative and point-of-care (POC) in clinical applications.

Tuning Triangular Prism Dimer into Fano Resonance for Plasmonic Sensor

We theoretically investigate the plasmonic Fano resonance in a triangular nanoprism dimer. By adjusting the geometry parameters, we have observed a Fano line shape in the scattering spectra, which is induced by the competence of bonding and antibonding modes in the triangular nanoprism dimer. The Fano line shape can be well described by a theoretical model of two harmonic oscillators. A figure of merit value as high as 16.1 is achieved in the triangular nanoprism dimer, which is caused by the Fano resonance. The electric field at the corner of the triangular prisms is the highest among the circular cylinder dimer and square rod dimmers, which shows that the triangular prism dimer is more suitable for the detection of biomolecules. The triangular prism dimer may also used in plasmonic circuits.

Controllably tuning the near-infrared plasmonic modes of gold nanoplates for enhanced optical coherence imaging and photothermal therapy

Tuning the localized surface plasmon modes of gold nanostructures to be in resonance with near-infrared incident light is desirable in various applications such as biosensing, biomedicine/therapy and opto-electronic devices. Unfortunately, current methods for regulating the plasmon modes of gold nanoparticles still suffer from poor controllability and reproducibility. Here, we developed a facile and effective method to precisely tailor the plasmon mode of gold triangular nanoprisms (GTNPs) by simply exposing them to O3 atmosphere. The resonant wavelength of the plasmon mode sustained by the GTNPs can be steadily tuned over a broad spectral range varying from 1010 nm to 780 nm (within the bio-window region), along with their shapes gradually changing from triangular nanoprism into circular nanoplate. By controlling the concentrations of O3, exposure duration, the concentrations of surfactant in suspension and the reaction temperature, GTNPs with various plasmon modes could be efficiently obtained from one original GTNPs sample. To demonstrate the potential applications of these GTNPs, we applied this method to obtain gold nanoplates as-needed for enhanced optical coherence tomography (OCT) and photothermal therapy. The plasmon mode of GTNPs was tuned to match the excitation wavelength of the OCT laser source, and was applied to enhance the signal of OCT imaging. The plasmon mode of GTNPs was also precisely tuned to 808 nm which was well resonant with the wavelength of a near-infrared excitation laser (λex = 808 nm); when the as-obtained GTNPs were used as a photothermal agent, they displayed an enhanced effect of photothermal therapy on Hela cancer cells compared to those without the tuning of the plasmon mode. Considering the simplicity and high controllability of the method for fine-tuning the plasmonic mode of GTNPs, this work has great potential in a wide range of applications such as biomedical imaging and thermotherapy, chemical/biological sensing, surface-enhanced spectroscopy and solar energy harvesting, etc.

Enhance energy transfer between quantum dots by the surface plasmon of Ag island film

We observed enhanced energy transfer between quantum dots which were spin coated on the surface of Ag island film. By adjusting the thickness of Al2O3 spacer layer (4-16 nm) between quantum dots and Ag island film, we studied the distance dependence of the plasmon enhanced energy transfer. Experimental results showed the energy transfer efficiency increased as the quantum dots approached closer to the Ag film. A time-correlated single-photon counting system was also applied to study the dynamics of the plasmon enhanced energy transfer between quantum dots. Obvious decay rate increasing was discovered from time-resolve photoluminescence spectra of both the donor and accepter quantum dots, which clearly confirmed the existence of strong photon-exciton interaction. The theoretical analysis performed by the finite difference time domain method and COMSOL program, further revealed large electronic field and energy transfer rate enchantments on the surface of Ag island film, which uncovered the dominant reasons for the quantum dots’ enhanced energy transfer.

Aluminum nanopyramid array with tunable ultraviolet-visible-infrared wavelength plasmon resonances for rapid detection of carbohydrate antigen 199.

Aluminum-based localized surface plasmon resonance (LSPR) holds attractive properties include low cost, high natural abundance, and ease of processing by a wide variety of methods including complementary metal oxide semiconductor process, making itself having an edge over conventional ones induced by noble metal. However, the inherent drawbacks of plasmonic mode limited on UV-green wavelength, low refractive index sensitivity, as well as heavy-shape-dependence greatly prevent aluminum plasmonics from real-life biosensing. Here, we demonstrated a uniform quasi-3-dimensional Al nanopyramid array (NPA) structure with tunable ultraviolet-visible-infrared (UV-vis-NIR) plasmon resonances for biosensing. By changing the reflection measuring angle, we could easily obtain typical peaks simultaneously exhibited on the reflectance spectrum across UV-vis-NIR wave region. The Al NPAs carried out high refractive index sensitivities which even comparable with that of noble metal, and can be used as a biosensor for directly detecting cytochrome c and carbohydrate antigen 199 in air after the sensing surface was washed cleanly and dried; the limits of detection were determined to be 800 nM and 29 ng/mL, respectively. Our proposed work therefore initiates the low-cost, high-performance biosensing using aluminum plasmonics, which would find wide applications in rapid diagnosis, mobile-healthcare and environmental monitoring.

Flexible Nanowire Cluster as a Wearable Colorimetric Humidity Sensor

Wearable plasmonic devices combine the advantages of high flexibility, ultrathinness, light weight, and excellent integration with the optical benefits mediated by plasmon-enhanced electric fields. However, two obstacles severely hinder further developments and applications of a wearable plasmonic device. One is the lack of efficient approach to obtaining devices with robust antimotion-interference property, i.e., the devices can work independently on the morphology changes of their working structures caused by arbitrary wearing conditions. The other issue is to seek a facile and high-throughput fabrication method to satisfy the financial requirement of industrialization. In order to overcome these two challenges, a functional flexible film of nanowire cluster is developed, which can be easily fabricated by taking the advantages of both conventional electrochemical and sputtering methods. Such flexible plasmonic films can be made into wearable devices that work independently on shape changes induced by various wearing conditions (such as bending, twisting and stretching). Furthermore, due to plasmonic advantages of color controlling and high sensitivity to environment changes, the flexible film of nanowire cluster can be used to fabricate wearable items (such as bracelet, clothes, bag, or even commercial markers), with the ability of wireless visualization for humidity sensing.

Near-UV-enhanced broad-band large third-order optical nonlinearity in aluminum nanorod array film with sub-10 nm gaps

Plasmonic nanostructures with sub-10 nm gaps possess intense electric field enhancements, leading to their high reputation for exploring various functional applications at nanoscale. Till now, although large amounts of efforts have been devoted into investigation of such structures, few works were emphased on the nonlinear optical properties in near-ultraviolet (UV) region. Here, by combining sputtering technique and an optimized anodic aluminum oxide (AAO) template growing method, we obtain aluminum (Al) nanorod array film (NRAF) with average rod diameter and gap size of 50 and 7 nm, respectively. The Al-NRAF exhibits large third-order optical nonlinear susceptibility (χ(3)) and high figure of merit (χ(3)/α) over a broad wavelength range from 360 to 900 nm, and reaches their maximums at the shortest measured wavelength. In addition, comparisons with Au-NRAF and Ag-NRAF samples further confirm that Al-NRAF has better nonlinear optical properties in the blue and near-UV wavelength regions. These results indicate that Al nanostructures are promising candidates for nonlinear plasmonic applications at blue and near-UV wavelengths.

Gold nanoarray deposited using alternating current for emission rate-manipulating nanoantenna

We have proposed an easy and controllable method to prepare highly ordered Au nanoarray by pulse alternating current deposition in anodic aluminum oxide template. Using the ultraviolet–visible-near-infrared region spectrophotometer, finite difference time domain, and Green function method, we experimentally and theoretically investigated the surface plasmon resonance, electric field distribution, and local density of states enhancement of the uniform Au nanoarray system. The time-resolved photoluminescence spectra of quantum dots show that the emission rate increased from 0.0429 to 0.5 ns−1 (10.7 times larger) by the existence of the Au nanoarray. Our findings not only suggest a convenient method for ordered nanoarray growth but also prove the utilization of the Au nanoarray for light emission-manipulating antennas, which can help build various functional plasmonic nanodevices.PACS82.45.Yz, 78.47.jd, 62.23.Pq