Yukun Gao

Size-Dependent Raman Shifts for nanocrystals.

Raman spectroscopy is a very sensitive tool for probing semiconductor nanocrystals. The underlying mechanism behind the size-dependent Raman shifts is still quite controversial. Here we offer a new theoretical method for the quantum confinement effects on the Raman spectra of semiconductor nanocrystals. We propose that the shift of Raman spectra in nanocrystals can result from two overlapping effects: the quantum effect shift and surface effect shift. The quantum effect shift is extracted from an extended Kubo formula, the surface effect shift is determined via the first principles calculations. Fairly good prediction of Raman shifts can be obtained without the use of any adjustable parameter. Closer analysis shows that the size-dependent Raman shifts in Si nanocrystals mainly result from the quantum effect shifts. For nanodiamond, the proportion of surface effect shift in Raman shift is up to about 40%. Such model can also provide a good baseline for using Raman spectroscopy as a tool to measure size.

An ultrasensitive “turn-off” SERS sensor for quantitatively detecting heparin based on 4-mercaptobenzoic acid functionalized gold nanoparticles

An ultrasensitive “turn off” Surface Enhanced Raman Spectroscopy (SERS) sensor was developed for the detection of heparin based on the anti-aggregation of 4-mercaptobenzoic acid stabilized gold nanoparticles. In this paper, protamine, a small protein molecule with positive charges, could induce the aggregation of 4-MBA functionalized gold nanoparticles via surface electrostatic interaction. However, in the presence of heparin, the aggregation of 4-MBA functionalized gold nanoparticles decreased due to the fact that heparin has a strong affinity toward protamine, further causing the loss of the Raman enhanced effect. The normalized SERS intensity of the Raman reporter was proportional to the concentration of added heparin and a good linear detection range was obtained from 0.05 to 20 ng mL−1 (R2 = 0.999) with a calculated detection limit of 0.03 ng mL−1. Moreover, the developed highly selective method is also successfully demonstrated in fetal bovine serum. Our method is specific, simple and cheap, which could be applied to other further research studies of heparin.

A computational study on surface-enhanced Raman spectroscopy of para-substituted Benzenethiol derivatives adsorbed on gold nanoclusters

We presented a computational study on para-substituted Benzenethiol (x-BT, x=H, F, Cl, Br, OH, SH, SeH, NH2, CH3) derivatives interacting with gold cluster for chemical effects related to surface-enhanced Raman spectroscopy (SERS). Density functional theory (DFT) calculations were performed on a series of bridge-type and vertex type x-BT/Au13 complexes for geometric, electronic and excitation properties to determine the key factor in spectral enhancement. Results indicated that off-resonance enhancement factors of bridge-type and vertex-type complexes exhibited different dependency on substitutions, which was greatly influenced by molecule-cluster transitions instead of properties such as interaction energy and charge transfer due to same origination for off-resonance and resonance chemical enhancement.

Creating dynamic SERS hotspots on the surface of pH-responsive microgels for direct detection of crystal violet in solution

A systematic study for the creation of dynamic surface-enhanced Raman scattering (SERS) hotspots is reported. Herein, gold nanoparticles-loaded pH-responsive poly(2-vinylpyridine) (AuNPs/P2VP) composite microgels were prepared efficiently by in situ chemical reduction. The nano-gap among AuNPs could be tuned easily by changing pH values of AuNPs/P2VP microgels solution from 2.0 to 5.0, creating vast SERS hotspots for electromagnetic enhancement. This change increased the SERS intensity remarkably, using 4-mercaptobenzoic acid (PMBA) as the model probe at a concentration of 1 × 10−4 M. Besides, the strongest SERS signal of PMBA was obtained when the gold concentration was 0.6 g L−1, and we systematically studied the relationship between gold concentration of AuNPs/P2VP microgels solution and SERS signal strength. The prepared SERS substrate with the strongest SERS signal was proved to be stable for months. Finally, the new SERS substrate was used for direct detection of crystal violet in solution, considering that crystal violet is the main ingredient in dye wastewater, and the detection limit was about 4 × 10−9 M. This result indicated that the new AuNPs/P2VP microgels are highly effective and can be used as a SERS-active substrate for direct analysis of a variety of objects in solution.

Rapid fabrication of flexible and transparent gold nanorods/poly (methyl methacrylate) membrane substrate for SERS nanosensor application

Flexible substrates have been proposed for daily-life applications in SERS detection due to the prominent sample collection properties such as they can be wrapped around non-planar object surface. Combining the noble metals with polymers, flexible SERS substrates could be fabricated with advantages of light weight, transparency and high SERS sensitivity. Herein, we prepare a gold nanorods (AuNRs)/poly(methyl methacrylate) (PMMA) film as flexible SERS substrate by self-assembling a uniformly AuNRs array layer on PMMA template. This AuNRs/PMMA film performs excellently on thiram trace detection with the lowest detection concentration of 0.5 ppb. The fabricated substrates were applied for practical detection with cucumber by directly covering the AuNRs/PMMA flexible film on the target surface. Furthermore, the high SERS sensitivity as well as great reproducibility present a wide range of prospections for the further application of non-plane surface.

Humidity-responsive nanocomposite of gold nanoparticles and polyacrylamide brushes grafted on Ag film: synthesis and application as plasmonic nanosensor

A general stepwise strategy for the preparation of new humidity-responsive plasmonic nanosensor was described for the first time, based on Ag film functionalization by polyacrylamide (PAAM) brushes via surface-initiated atom transfer radical polymerization (SI-ATRP) method and then assembled with gold nanoparticles (Au NPs). We designed by this way a new plasmonic device made of Au NPs embedded in a humid vapor responsive polymer layer on Ag film and extensively characterized by surface-enhanced Raman scattering (SERS). When the relative humidity (RH) is above 50%, the number of plasmonic hotspots decreases, causing SERS signal reduced noticeably, for the volume expansion of PAAM brushes varied the nano-gap between closely spaced Au NPs, and between Au NPs and Ag film. The reversible optical properties of the prepared nanocomposite tuned by RH were probed through SERS using 4-mercaptopyridine (4-Mpy) as a molecular probe, and the decrease of the RH reversibly induces a significant enhancement of the 4-Mpy SERS signal. By means of the high reversibility, the RH responsive nanocomposite developed in this paper provides a dynamic SERS platform and can be applied as plasmonic nanosensor which is proved to be stable for at least two months.摘要本文报道了一种湿度响应纳米SERS传感器. 通过原子转移自由基聚合技术在银片表面嫁接了具有湿度响应性能的聚丙烯酰胺分子 刷, 并组装金纳米颗粒形成复合结构. 该分子刷湿度响应灵敏, 而且可有效抓取金纳米颗粒, 构成均匀分布的SERS“热点”. 通过调节湿度, 实现了SERS“热点”的可逆调控, 并通过拉曼光谱快速捕捉探针分子特征峰的SERS信号强度变化, 实现湿度响应的SERS传感功能. 湿度低 于50%时, 分子刷收缩, 金纳米颗粒间距降至纳米级, 形成大量热点, 使得SERS增强因子达到2×108; 湿度高于50%, 分子刷舒张, 金纳米颗粒 间距变大, 当湿度大于90%时, SERS“热点”最少, SERS信号最低. 可逆调控湿度变化, 得到可逆的SERS信号变化, 因此该复合材料实现了高 效灵敏的湿度响应SERS传感.

Fabrication of a Flexible Gold Nanorod Polymer Metafilm via a Phase Transfer Method as a SERS Substrate for Detecting Food Contaminants

Surface enhanced Raman scattering (SERS) has been widely used in detection of food safety due to the nondestructive examination property. Here, we reported a flexible SERS film based on a polymer-immobilized gold nanorod polymer metafilm. Polystyrene-polyisoprene-polystyrene (SIS), a transparent and flexible, along with having excellent elasticity, polymer, was chosen as the main support of gold nanorods. A simple phase transfer progress was adopted to mix the gold nanorods with the polymer, which can further be used in most water-insoluble polymers. The SERS film performed satisfactorily while being tested in a series of standard Raman probes, like crystal violet (CV) and malachite green (MG). Moreover, the excellent reproducibility and elastic properties make the film a promising substrate in practical detection. Hence, the MG detection on the fish surface and trace thiram detection on orange pericarp were inspected with detection results of 1 × 10-10 and 1 × 10-6 M, which were below the demand of the National standard of China, exactly matching the realistic application requirements.

Large-scale preparation of flexible and reusable surface-enhanced Raman scattering platform based on electrospinning AgNPs/PCL nanofiber membrane

A flexible and reusable SERS substrates were prepared by electrospinning Ag nanoparticles (AgNPs) in reversed micelle into poly(e-caprolactone) (PCL) nanofibers, forming nanocomposite membrane with high detection sensitivity. The AgNPs/PCL nanofiber membrane was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and surface-enhanced Raman scattering spectroscopy (SERS). SEM and TEM image show that AgNPs dispersed homogeneously in PCL nanofibers. The fabricated substrates based on AgNPs/PCL nanofiber membrane have shown reusability and excellent detection reproducibility in SERS under a low concentration of R6G (10−12 M) and crystal violet (10−11 M), and the detection limit of melamine is as low as 5 ppb, which is far below the safety of FDA and US. Moreover, the substrate showed hydrophobicity and low loss of stress after etching.