Seedless synthesis of gold nanorods by using hydrogen peroxide as a weak reducing agent

Abstract

A new seedless wet chemistry synthesis of gold nanorods by using hydrogen peroxide as the weak reducing agent is reported. A reduced concentration of hexadecyltrimethylammonium bromide is used in our experiment, and the synthesized gold nanorods exhibit tunable longitudinal surface plasmon resonance peaks ranging from 725 to 945?nm. The influence on gold nanorods growth by adjusting the amounts of sodium hydroxide, silver nitrate, sodium borohydride, and hexadecyltrimethylammonium bromide were investigated by the visible-near infrared spectroscopy. Under the proper experimental parameters, the longitudinal surface plasmon resonance peaks can be tuned by varying the hydrogen peroxide amounts. Furthermore, it can be seen that the redshift of the longitudinal absorption peak of the prepared gold nanorods with increasing hydrogen peroxide amount is consistent with the increase tendency of the length-to-width aspect ratio obtained from the transmission electron microscopy images. The method provides a facile pathway to prepare gold nanorods with tunable longitudinal surface plasmon resonance peaks, which have potential applications in biomedicine and nanophotonics.     

金纳米棒状微粒的胶囊模型及吸收光谱

提出了金纳米棒状微粒的胶囊模型,用Waterman发展的T矩阵方法计算了金纳米棒状微粒的吸收光谱.计算谱和实验谱基本符合,520 nm左右处的吸收峰对应于金纳米棒的横向表面等离体子共振（横模）,长波长处的吸收峰对应于金纳米棒的纵向表面等离体子共振（纵模）.随着金纳米棒纵横比的增加,纵模吸收峰表现出显著的红移,横模吸收峰则微弱地蓝移.此外,计算结果表明,金纳米棒状微粒外部介质的介电常数必须随着金纳米棒纵横比的增大非线性地减小.     

银纳米棒光学性质的离散偶极近似计算

利用离散偶极近似 (Discretedipoleapproximation ,简称DDA)的方法 ,从理论上对粒子的形状、尺寸及周围介质等因素对银纳米粒子 ,特别是银纳米棒的光学性质的影响进行了较系统的研究 .计算表明 ,置于空气中的棒状银纳米粒子的光学性质与其形状密切相关 ,纵向表面等离子体共振吸收峰的位置随纳米棒长径比的增加呈现线性红移关系 .给出了空气中银纳米棒纵向表面等离子体共振吸收峰的位置随长径比变化的DDA拟合公式 .如果将金属纳米粒子置于折射率更高的介电环境中 ,其纵向等离子体共振吸收峰的位置进一步呈现线性红移关系 .合成的银纳米粒子的TEM图像及相关的UV VIS消光光谱显示DDA计算结果与实验值相当一致 .DDA算法与Mie′s理论在计算球状银纳米粒子的消光系数时给出很接近的结果 ,这表明用DDA的方法来分析银的光学性质是准确可靠的 ;而DDA算法对银纳米棒消光特性的成功拟合则表明 ,该算法相对Gans′理论而言 ,在研究纳米粒子的光学性质时具有更广的适用性及更高的准确性 .     

Calculated Optical Properties of Dielectric Shell Coated Gold Nanorods

Optical absorption spectra of dielectric shell coated gold nanorods are simulated using the discrete dipole ap- proximation method. The influence of the aspect ratio, shell thickness, dielectric constant of the shell, and surrounding medium on the longitudinal resonance mode is investigated. It is found that the coated dielectric shell does not affect the trend in the dependence of resonance position on the aspect ratio, while it broadens the resonant line width and reduces the sensitivity of plasmon resonance in response to changes of the surrounding medium. F~arthermore, the difference of dielectric constants between the shell and surrounding medium plays an important role in determining the resonance position. The screening effect of the dielectric shell tends to be less apparent for a thicker shell thickness.Optical absorption spectra of dielectric shell coated gold nanorods are simulated using the discrete dipole ap- proximation method. The influence of the aspect ratio, shell thickness, dielectric constant of the shell, and surrounding medium on the longitudinal resonance mode is investigated. It is found that the coated dielectric shell does not affect the trend in the dependence of resonance position on the aspect ratio, while it broadens the resonant line width and reduces the sensitivity of plasmon resonance in response to changes of the surrounding medium. F~urthermore, the difference of dielectric constants between the shell and surrounding medium plays an important role in determining the resonance position. The screening effect of the dielectric shell tends to be less apparent for a thicker shell thickness.     

Seedless synthesis of gold nanorods with longitudinal surface plasmon resonance wavelength of 1200 nm

Gold nanorods with the longitudinal plasmonic resonance peak position up to 1200?nm are prepared using an improved seedless synthetic technique. In this method, dopamine is employed as the weak reducer, whereas cetyltrimethylammonium bromide and sodium oleate are used as the binary surfactant mixture. Gold nanorods produced at various amounts of silver nitrate and hydrochloric acid were characterized by visible-near-infrared spectroscopy and scanning electron microscopy. Results show that the sizes, length-to-width aspect ratios, and the corresponding longitudinal surface plasmon resonance peaks of the synthesized gold nanorods can be tuned by altering the silver nitrate and hydrochloric acid amounts. The present method provides a new procedure for fabrication of gold nanorods with a broad range of plasmonic resonance peaks, which has a great potential for applications such as photothermal therapy and sensing.     

Shape and size dependence of the surface plasmon resonance of gold nanoparticles studied by Photoacoustic technique

We report on the optical absorption properties of as prepared gold naoparticles of different shapes and sizes measured by photoacoustic (PA) method. The gold nanoparticles of two different shapes (dots, rods) have been prepared using the seed mediated growth method. The shape and the size of these different nanoparticles were determined by STM measurements. PA spectra show the splitting of the surface plasmon resonance (SPR) into two modes (transverse and longitudinal) in case of gold nanorods. The increase in the aspect ratio of the nanorods leads to clear redshifts of the longitudinal SPR. These shifts were used to determine the dielectric constant of the surrounding medium and its variation with the aspect ratios.     

Surface plasmon characteristics of tunable photoluminescence in single gold nanorods

Light emission resulting from two-photon excited gold nanoparticles has been proposed to originate from the radiative decay of surface plasmon resonances. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blueshift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tunable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.     

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO₃ using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO₃, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.     

Direct monitoring of gold nanorod growth

The seed-mediated growth of gold nanorods is shown to be strongly dependent on the reaction time and chemical environment of the reaction solution. The versatile seed-mediated approach in aqueous surfactant solutions has been used in this study for the synthesis of gold nanorods. Changes in the aspect ratio of gold nanorods were reflected in shifts of the plasmon resonance peaks and were monitored using UV-Visible absorption spectroscopy (UV-Vis) to follow the different stages of gold nanorod formation as a function of time and varying amounts of silver ion. Unlike the use of strong reducing agents to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. Therefore, cyclic voltammetry was used to electrochemically interrogate the entire reaction from gold seed to gold nanorod as a function of time. Data obtained revealed that time-dependent gold species are involved in gold nanorod formation.     

表面等离激元探针的高介电灵敏度与液体折射率测定

利用金纳米棒在光照射下激发表面等离激元的性质,实验研究了其在不同介电环境下的吸收光谱.通过分析纵向等离子体共振吸收峰峰位随介质折射率的变化,获得了金纳米棒表面等离激元探针测量介质折射率的经验公式为n=(385.59)-1(λ/nm-290.56).利用金纳米棒表面等离激元探针的高介电灵敏度,测试了一些未知液体的折射率,并与阿贝折射仪测量法的结果相比较.结果与分析表明,本方法较之阿贝折射仪测量介质折射率的方法具有更高的精密度.因此,表面等离探针可用于拓展大学物理实验中的介质折射率测量实验.     

Shape dependent full width at half maximum of the absorption band in gold nanorods

The dependence of full width at half maximum (FWHM) of the absorption peak in gold nanorods was investigated as a function of aspect ratio. Numerical calculations based on Drude model and quasi-static theory indicated that, the FWHM of longitudinal absorption peak is wider than that of transverse mode. Further more, with the increasing aspect ratio, the FWHM of longer wavelength absorption peak increasing in a nonlinear way, which is in agreement with the experimental results.     

Optical coherence tomography with plasmon resonant nanorods of gold

We explored plasmon resonant nanorods of gold as a contrast agent for optical coherence tomography (OCT). Nanorod suspensions were generated through wet chemical synthesis and characterized with spectrophotometry, transmission electron microscopy, and OCT. Polyacrylamide-based phantoms were generated with appropriate scattering and anisotropy coefficients (30 cm(-1) and 0.89, respectively) to image distribution of the contrast agent in an environment similar to that of tissue. The observed signal was dependent on whether the plasmon resonance peak overlapped the source bandwidth of the OCT, confirming the resonant character of enhancement. Gold nanorods with plasmon resonance wavelengths overlapping the OCT source yielded a signal-to-background ratio of 4.5 dB, relative to the tissue phantom. Strategies for OCT imaging with nanorods are discussed.     

Influence of amount of CTAB and ascorbic acid concentration on localized surface plasmon resonance property of gold nanorod

Gold nanorods (AuNRs) are prepared through seeded growth approach. Synthesis parameters of the amount of cetyltrimethylammonium bromide (CTAB), and concentration of ascorbic acid (AA) were studied. We were aiming for an aspect ratio of 3 which could be achieved by a nanorod feature in the range of 45 nm length and 15 nm width. The absorption spectra are observed with an UV–visible NIR spectrophotometer and analysed theoretically. It is known that there are two plasmon resonance peaks for gold nanorod corresponding to transverse surface plasmon resonance (TSPR) and longitudinal surface plasmon resonance (LSPR), respectively. It is found that as the concentration of CTAB increases, the yield of NRs increases. As the concentration of AA increased from 0.05 to 0.2 M, LSP shifts to longer wavelength but upon further increasing the concentration, LSP shifts back to shorter wavelength. A linear relationship between LSPR wavelength and surrounding medium dielectric constant is obtained, which is in good agreement with the theoretical results.     

On the extinction multipole plasmons in gold nanorods

On the basis of the T-matrix formalism and numerical simulations, we derive an explicit rule for partial multipole contributions to the plasmon resonances of gold nanorods at a fixed or random orientation. The parity of a given spectral resonance number n coincides with the parity of their multipole contributions l, where l is equal to or greater than n, and the total resonance magnitude is determined by the lowest multipole contribution. We also investigate the dependence of multipole plasmons on the size, shape, and orientation of nanorods with respect to the polarized incident light. It is shown that the multipole resonance wavelengths as a function of the aspect ratio divided by the resonance number collapse onto one linear scaling curve. This scaling is explained by using the plasmon standing wave concept introduced by Schider et al. [Plasmon dispersion relation of Au and Ag nanowires. Phys Rev B 2003;68:155427].     

Surface-enhanced Raman scattering properties of highly ordered self-assemblies of gold nanorods with different aspect ratios

Gold nanorods with aspect ratios of from 1 (particles) to 31.6 were synthesized by the seed-mediated method and packed in a highly ordered structure on a large scale on silicon substrates through capillary force induced self-assembly behaviour during solvent evaporation.The gold nanorod surface exhibits a strong enhancing effect on Raman scattering spectroscopy.The enhancement of Raman scattering for two model molecules (2-naphthalenethiol and rhodamine 6G) is about 5-6 orders of magnitude.By changing the aspect ratio of the Au nanorods,we found that the enhancement factors decreased with the increase of aspect ratios.The observed Raman scattering enhancement is strong and should be ascribed to the surface plasmon coupling between closely packed nanorods,which may result in huge local electromagnetic field enhancements in those confined junctions.     

具有双共振吸收峰的Au纳米粒子制备及其拉曼表征

表面增强拉曼散射(SERS)很大程度的弥补了拉曼散射强度弱的缺点,迅速成为科研工作者们的研究热点,在食品安全、环境污染、毒品以及爆炸物检测等领域应用广泛。纳米技术的发展使得目前对于SERS的研究主要集中于金属纳米颗粒基底的制备,金属纳米粒子的种类、尺寸及形貌对SERS增强和吸收峰峰位均有影响,要获得好的增强效果,需要对金属纳米结构进行工艺优化。特别是,需要结合金属纳米粒子的结构和激励光波长,以期获得更好的增强效果。为了研究SERS增强和吸收峰之间的关系,开展了具有双共振吸收峰的金属纳米粒子的研究。首先利用FDTD Solutions仿真建模,主要针对金纳米颗粒直径、金纳米棒长径比及分布状态对共振吸收峰进行仿真,得到金纳米球理论直径在50 nm左右,金纳米棒理论长径比在3.5~4.5左右时,吸收峰分别分布在532及785 nm附近,符合多波段激励光拉曼增强条件;对于激励光偏振方向,其沿金纳米棒长轴方向偏振时吸收峰位于785 nm附近,沿金纳米球短轴方向偏振时吸收峰位于532 nm附近。然后采用种子生长法,制备了可用于多种波长激励光的双吸收峰表面增强拉曼散射基底。通过改变硝酸银用量(5,10,20,30和40 μL)、盐酸用量(0.1和0.2 mL)以及其生长时间(15,17,21和23 h)等多种工艺参数来控制金纳米棒含量,得到了同时含有金纳米球及金纳米棒的双吸收共振峰金纳米粒子。最后用该样品作为基底,罗丹明6G(R6G)作为探针分子,分别测试其在532,633和785 nm激励光入射时的SERS表征,对分析物R6G最低检测浓度均达到了10-7 mol·L-1,增强因子达到了~105,满足了多波段SERS检测的需要。     

Modeling and identification on nonlinear saturable and reverse-saturable absorptions of gold nanorods using femtosecond Z-scan technique

An improved Z-scan analysis approach is proposed by establishing and solving the saturable absorption (SA) and reverse-SA (RSA) models, respectively. Near-infrared femtosecond Z-scans are carried out on the synthesized gold nanorods (NRs) possessing the average length of 46 nm using a femtosecond laser operated at the wavelength of 800 nm, which is close to the peak position of longitudinal surface plasmon resonance (SPR) (710 nm) of gold NRs. At lower input intensity of less than 400 GW/cm2 , the normalized transmission exhibits only SA phenomenon; however, when it exceeds 400 GW/cm2 , both SA and RSA are observed. By using the presented Z-scan modeling and theory, the three-photon absorption (3PA) is identified in the material, and the 3PA cross-section is determined to be 1.58×1071 cm6s2 .     

Plasmon hybridization in nanorod dimers

Using the plasmon hybridization method we investigate the plasmon modes of nanorod dimers in axial and parallel orientations. We show that the plasmon modes of the system can be viewed as bonding and anti-bonding modes resulting from the hybridization of the plasmon modes of the individual nanorods. The dimer plasmon modes are found to depend sensitively on separation between the nanorods and on their relative spatial orientation. The calculated optical properties agree quantitatively with results from the numerical finite-difference time-domain method. The electric field enhancements are found to depend strongly on aspect ratio defined as the ratio of the major and minor radii, and on the relative orientation of the nanorods. For a nanorod dimer of fixed overall length, the maximum field enhancements are lower than those induced in a solid sphere dimer.     

High Temperature Seedless Synthesis of Au NRs Using BDAC/CTAB Co-surfactant

采用等离子体活化技术制备了一种新型Ni/γ-Al2O3催化剂用于CO2重整CH4反应. 等离子体强化制备的催化剂表现出较高的反应活性和较好的抗积碳能力. 为了达到相同CH4转化率,常规焙烧的催化剂需要比等离子体处理的催化剂高出50 °C 的反应温度. 反应结束后,等离子体处理催化剂的失活率仅为1.7%,而常规催化剂上的失活率为15.2%。通过对催化剂进行BET、H2-TPR、XRD、CO2-TPD和TG等表征分析,结果表明等离子体增强制备方法使催化剂的平均孔直径减小,比表面积增加;催化剂的还原性,镍物种的分散度和催化剂对CO2的吸附量都有显著的提高.     

Plasmon resonance properties of silver-patterned glass substrates fabricated by anodic alumina oxide templates

Ultra-thin anodic aluminum oxide membranes were prepared and served as deposition masks for fabrication of uniformly sized Ag nanodots with different aspect ratios on glass substrates. The surface plasmon resonance (SPR) properties of the supported Ag nanodots were investigated and compared with the predictions of the generalized Maxwell–Garnett theory. By modeling the nanodots as spheroids without adjusting their real geometrical parameters input to the calculation, the resulting theoretical SPR wavelengths are in good agreement with measured extinction peaks. The discrepancy between the theoretical and experimental plasmon resonance peak maxima is within 10 nm for the nanodots with an aspect ratio of less than 1.5. Although this wavelength discrepancy becomes large as the aspect ratio is increased, it is kept at approximately 35 nm for the nanodots with an aspect ratio of 2.44.