Zhigao Dai

Low-Cost, Disposable, Flexible and Highly Reproducible Screen Printed SERS Substrates for the Detection of Various Chemicals

Ideal SERS substrates for sensing applications should exhibit strong signal enhancement, generate a reproducible and uniform response, and should be able to fabricate in large-scale and low-cost. Herein, we demonstrate low-cost, highly sensitive, disposable and reproducible SERS substrates by means of screen printing Ag nanoparticles (NPs) on a plastic PET (Polyethylene terephthalate) substrates. While there are many complex methods for the fabrication of SERS substrates, screen printing is suitable for large-area fabrication and overcomes the uneven radial distribution. Using as-printed Ag substrates as the SERS platform, detection of various commonly known chemicals have been done. The SERS detection limit of Rhodamine 6G (R6G) is higher than the concentration of 1 × 10−10 M. The relative standard deviation (RSD) value for 784 points on the detection of R6G and Malachite green (MG) is less than 20% revealing a homogeneous SERS distribution and high reproducibility. Moreover, melamine (MA) is detected in fresh liquid-milk without additional pretreatment, which may accelerate the application of rapid on-line detection of MA in liquid milk. Our screen printing method highlights the use of large-scale printing strategies for the fabrication of well-defined functional nanostructures with applications well beyond the field of SERS sensing.

Large-area, well-ordered, uniform-sized bowtie nanoantenna arrays for surface enhanced Raman scattering substrate with ultra-sensitive detection

We report the fabrication of large-area, well-ordered, uniform-sized noble metal bowtie nanoantenna arrays used as an ultra-sensitive surface-enhanced Raman scattering (SERS) substrate by a refined combination of colloid lithography and subsequent oxygen plasma processing. The tip-to-tip distance between neighboring nanotriangles is made as small as 10 nm by accurately controlling the etching time. Owing to the nanoscale property, the as-fabricated Ag bowtie nanoantenna arrays exhibit a local SERS enhancement larger than 107. The experimental results were confirmed by theoretical calculations.

Large-scale synthesis and screen printing of upconversion hexagonal-phase NaYF4:Yb3+,Tm3+/Er3+/Eu3+ plates for security applications

Up-conversion materials can be applied in anti-counterfeit fields because of their high concealment and high up-conversion fluorescence efficiency. Herein, different lanthanide ions-doped β-NaYF4 up-conversion micro-particles (UCMPs) with uniform hexagonal morphology were synthesized on a large-scale (more than 0.5 g) via a facile hydrothermal method. It is very interesting to find out the fluorescence intensity and the eventual morphology of the UCMPs were highly dependent on the reaction conditions. Then, the optimal UCMPs with uniform morphology and strong fluorescence intensity were selected as promising candidates for preparing ink. Eventually, we successfully printed designable and multicolor fluorescent patterns on flexible substrates (common paper and polyethylene terephthalate, PET) using the as-prepared β-NaYF4:Yb3+,Tm3+/Er3+/Eu3+ UCMPs inks. Under ambient conditions, the printed patterns on conventional paper are invisible, while such printed patterns on PET are white. However, all the patterns could display blue, yellow-green and green fluorescence patterns under the irradiation of a 980 nm laser when using the β-NaYF4:Yb3+,Tm3+/Er3+/Eu3+ UCMPs inks. We envision that the UCMPs fluorescent inks-based flexible and rapid screen printing patterns have enormous potential for anti-counterfeit and security applications.

Enhanced photocatalysis by coupling of anatase TiO2 film to triangular Ag nanoparticle island

In order to overcome the low utilization ratio of solar light and high electron-hole pair recombination rate of TiO2, the triangular Ag nanoparticle island is covered on the surface of the TiO2 thin film. Enhancement of the photocatalytic activity of the Ag/TiO2 nanocomposite system is observed. The increase of electron-hole pair generation is caused by the enhanced near-field amplitudes of localized surface plasmon of the Ag nanoparticles. The efficiently suppressed recombination of electron-hole pair caused by the metal-semiconductor contact can also enhance the photocatalytic activity of the TiO2 film.

Ultrasensitive SERS Substrate Integrated with Uniform Subnanometer Scale “Hot Spots” Created by a Graphene Spacer for the Detection of Mercury Ions

Mercuric ion (Hg2+ ) is one of the most toxic and serious environment polluting heavy metal ions, which can be accumulated in human body through food chains and drinking water, and causes serious damage to human organs. Therefore, development of the efficient and sensitive method for detection of Hg2+ is very necessary. In this study, the high surface sensitivity and fingerprint information about the chemical structures based on surface-enhanced Raman scattering (SERS) for sensing applications are taken advantage of. Au triangular nanoarrays/n-layer graphene/Au nanoparticles sandwich structure with large-area uniform subnanometer gaps are fabricated and used to detect Hg2+ in water via thymine-Hg2+ -thymine coordination; the detection limit of Hg2+ is as low as 8.3 × 10-9 m. Moreover, this SERS substrate is used to detect the Hg2+ -contaminated sandy soil and shows excellent performance. This study indicates the sandwich structure has a great potential in detection of toxic metal ions and environmental pollutants.

Efficiency enhancements in Ag nanoparticles-SiO2-TiO2 sandwiched structure via plasmonic effect-enhanced light capturing

TiO2-SiO2-Ag composites are fabricated by depositing TiO2 films on silica substrates embedded with Ag nanoparticles. Enhancement of light absorption of the nanostructural composites is observed. The light absorption enhancement of the synthesized structure in comparison to TiO2 originated from the near-field enhancement caused by the plasmonic effect of Ag nanoparticles, which can be demonstrated by the optical absorption spectra, Raman scattering investigation, and the increase of the photocatalytic activity. The embedded Ag nanoparticles are formed by ion implantation, which effectively prevents Ag to be oxidized through direct contact with TiO2. The suggested incorporation of plasmonic nanostructures shows a great potential application in a highly efficient photocatalyst and ultra-thin solar cell.

Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform

We have reported that the Ag nanostructure-based substrate is particularly suitable for surface-enhanced Raman scattering when it is coated with monolayer graphene, an optically transparent and chemistry-inertness material in the visible range. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted nanosphere lithography. Our measurements show that atmospheric sulfur containing compounds are powerless to break in the monolayer graphene to vulcanize the surfaces of the Ag bowtie nanoantenna arrays and Ag nanogrids by various means, including scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). Furthermore, the Ag nanostructure substrate coated with the monolayer graphene film shows a larger enhancement of Raman activity and the electromagnetic field than the uncoated substrate. Compared with those of bare Ag nanostructures, the averaged EFs of graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids after one month later in air, respectively. These observations are further supported by theoretical calculations.

Enhanced and polarization dependence of surface-enhanced Raman scattering in silver nanoparticle array-nanowire systems

We report a surface-enhanced Raman scattering (SERS) platform composed of a single Ag nanowire (NW) and nanoparticle array (NW-array). There are three different fundamental combination patterns of NW-array systems. Optical excitation of sandwich nanostructure provides a stronger SERS hot array at the gap between the NW and array. We also have studied the polarization dependence of SERS in the coupled NW-array system for three kinds of pattern. We find SERS are strongly enhanced when the incident light is polarized across the junction between the particle of array and NW, the enhancement is amazingly insensitive to the detailed geometrical structure of coupled pattern.

Wetting properties and SERS applications of ZnO/Ag nanowire arrays patterned by a screen printing method

Three-dimensional (3D) ZnO/Ag nanowire arrays (NWA) SERS substrates are fabricated by a screen-printing technique. The preparation of patterned ZnO NWA involves a two-step growth process. The ZnO nanoparticles-based ink is prepared and printed as seed layers, which are used as templates to fabricate patterned ZnO NWA via a hydrothermal method. During the screen printing process, the patterns of ZnO NWA are controlled by printing plates. The as-obtained NWA with uniform morphology was fabricated at 90 °C and exhibits hydrophilic and lipophilic properties. Subsequently, the sensitive 3D SERS substrates are fabricated through deposition of Ag nanoparticles (NPs) onto the surface of patterned ZnO NWA by magnetron sputtering. The detection limit of Malachite green (MG) can achieve 10−12 M with an enhancement factor of about 2.5 × 1010. Combined with the sensitivity and uniformity, the ZnO/Ag NWA is applied for the quick detection of low concentrations of molecules related to food safety, such as amoxicillin.

Metal ion-mediated synthesis and shape-dependent magnetic properties of single-crystalline α-Fe2O3 nanoparticles

A facile and effective hydrothermal process for the controllable synthesis of uniform single-crystalline hematite (α-Fe2O3) nanoparticles with different shapes is presented. The morphology of the α-Fe2O3 products can be controlled through simply adjusting the metal ions additive. The effects of the different metal ions (atomic number from 25–30) on the size and morphology of the products were investigated. The cubic and thorhombic α-Fe2O3 particles can be generated by adding zinc ions and copper ions to the reaction mixture, respectively. For providing some insight into the correlation between the morphology and physicochemical properties, the magnetic properties of the as-obtained cubic and thorhombic α-Fe2O3 products were investigated. Interestingly, the cubic α-Fe2O3 products exhibited a superparamagnetic properties at T = 300 K. In contrast, the thorhombic α-Fe2O3 products displayed ferromagnetic and low-temperature phase transition behaviours at room temperature. The fundamental understanding of crystal-phase and morphology-tunable nanostructures that are enclosed by shape-dependent magnetic properties is expected to direct the design and development of highly efficient magnetic nanomaterials.