Longitudinal Plasmon Band Research Articles

Highly selective localized surface plasmon resonance sensor for selenium diagnosis in selenium-rich soybeans

It is a challenge to determine selenium in acid aqueous for environmental monitoring and selenium-rich agricultural diagnosis. Herein, we developed a novel localized surface plasmon resonance (LSPR) sensor to detect Se(IV) ions based on the extraordinary laterals etching of gold nanorods (AuNRs). The etching started from the laterals in the low amount of Se(IV) ions, and accompanied by an apparent red shift of the longitudinal plasmon band (LPB), and then transformed to the tips etching with the upward of Se(IV) ions, the LPB band immediately shifted to the shorter wavelength. The red shift change (Δλ) of LPB band was utilized to quantitative analysis instead of blue shift or absorbance intensity, which gave a high selectivity for the proposed sensor. More importantly, this sensor could be performed in 0.1 mol/L of HCl solution, which achieved the seamlessly jointing with the pretreatment of complex samples, without time-consuming pH adjustment.Successful selenium detection was demonstrated in complex soybean samples that collected from the maturity after spraying organic chelated selenium at full flower period. The sensor provided a promising way to monitor and diagnose selenium in complex environmental samples and selenium-rich crops.

A multicolor immunosensor for point-of-care testing NTRK1 gene fusion

The gene fusion of Neurotrophic tropomyosin receptor kinase (NTRK) promotes the formation of tumors and is closely related to multiple cancers. Therefore, the detection of NTRK gene fusion markers has been a research hotspot in recent years. However, up to now, there is still lack of a fast and effective detection method for point-of-care testing (POCT). Herein, a multicolor immunosensor for point-of-care testing NTRK1 gene fusion by the naked eye was fabricated. Au nanobipyramids (Au NBPs) can be etched by 3, 3′, 5, 5′-tetramethylbenzidine ions (TMB2+) to produce a variety of aspect ratios, and the blue shift of its longitudinal plasmon band showed different color changes. So, the Au NBPs were combined with horseradish peroxidase (HRP)-TMB immunoassay system for achieving qualitative and semi-quantitative detection of NTRK1 gene fusion marker within 20 min by the naked eyes. The results shows that the relevant linear range for detection of TRKA protein was 7.5 pg/mL to 240 pg/mL. The constructed immunosensor will be expected to be used to quickly detect NTRK gene fusion markers in remote areas with underdeveloped medical conditions.

Photo-Driven Synthesis of Anisotropic Gold Nanoparticles Using Silk Fibroin—Cell Viability Activities in Lymphocyte and Jurkat Cancer Cells

A simple, environmentally benign and low-cost method for the synthesis of different-shaped gold nanoparticles using silk fibroin (SF) under ultraviolet (UV-B) radiation has been reported in this work. No additives such as organic solvents, surfactants, external reducing, stabilizing agents or high temperature were used for the synthesis of the particles. SF acts as a reducing and subsequently, stabilizing agent. The UV-Visible (UV-Vis) absorption spectra support the formation of different-shaped gold nanoparticles by displaying transverse and longitudinal surface plasmon bands at different wavelengths. The size and shape evolution of the nanoparticles was studied using the transmission electron microscope (TEM) to confirm the formation of different-shaped particles. Nano-crystalline structure and FCC crystal family of gold were confirmed by X-ray diffraction (XRD) measurement study. Furthermore, the proliferative activities of the synthesized colloidal gold nanoparticles were tested against lymphocyte and Jurkat cancer cell lines. The obtained results proved that the biogenic different-shaped AuNPs showed better activity in comparison with the spherical-shaped gold nanoparticles.

Control of growth solution on the dimensions of gold nanorods accounted for LSPR sensitivity toward liquid ammonia sensing

In the present study, the effect of dimensions of gold nanorods on its sensing property to detect liquid ammonia was reported. Gold nanorods with two different aspect ratios (GNR1 and GNR2) derived from different lengths and diameters were synthesized using seed-mediated growth method, and the aspect ratio was controlled by changing the silver ion concentration in growth solution. The morphological and size measurement was performed using Transmission Electron Microscopy (TEM), and the average value of aspect ratio (AR) was found to be 3.0 and 3.2 for GNR1 and GNR2, respectively. The characteristics transverse and longitudinal mode of localized surface plasmon resonance (LSPR) have been clearly depicted in UV–vis absorption spectrum of both GNR1 and GNR2. The red shift in longitudinal mode of LSPR from 718 to 732 nm has been observed for GNR with change in aspect ratio from 3.0 to 3.2, respectively. These samples of GNR were tested for liquid ammonia sensing with concentration ranging from 100 to 500 ppm. A clear cut blue shift in longitudinal mode of LSPR of prepared gold nanorod was observed. However, the GNR2 was found to be more sensitive toward liquid ammonia sensing. The origin of such blue shifting and sensitivity of longitudinal mode of LSPR of gold nanorod was explained on the basis of orientation dependence and Dipolar Exciton Coupling Model of coupled plasmon in assemblies of anisotropic plasmonic nanoparticles. With the help of this model, blue shifting in longitudinal plasmon band was correlated with the enhanced formation of H-aggregation induced by dipolar coupling of GNR clusters followed by hydrogen bonding after successive addition of ammonia solution.

Nanoplasmonic sensing of NADH by inhibiting the oxidative etching of gold nanorods

The plasmonic spectral sensing method based on the etching of gold nanorods has been a powerful tool in the field of biomedical detection. Among the plasmonic etching sensors, the most popular sensing strategy is the “positive oxidative etching” plasmonic sensor that is based on plasmon blue-shifting due to nanorod shortening. In this study, we proposed a nanoplasmonic detection of dihydronicotinamide adenine dinucleotide (NADH) based on negative oxidative etching: NADH-mediated inhibition of H2O2 induced gold nanorods etching. In the presence of NADH, the bromine intermediates (Br3− and Br2) used for etching gold nanorods were reduced and consumed, inhibiting the etching rate and resulting in a small blue-shift of the longitudinal plasmon band (LPB). This shift can be utilized to sense the presence of NADH. Under the optimal reaction conditions, the LPB difference (Δλ) of etched gold nanorods and the concentration of NADH had a good linear relationship in the range of 0–4.9 × 10−6 mol/L. The nanoplasmonic sensing showed a satisfactory preference for NADH over other common substances tested. It was successfully applied to the detection of NADH in PBS solution. This system provides a platform for the detection of targets in NADH-generated enzyme catalysis.

Plasmonic nanobiosensor based on Au nanorods with improved sensitivity: A comparative study for two different configurations

Biosensors presenting high sensitivity for the detection of biomolecules are very promising for diseases diagnosis. Nowadays, there is a need for the development of biosensors with fast, trustworthy diagnosis and mostly with low cost, mainly for applications in developing countries. Label-free plasmonic biosensors are good candidates to reach out all these characteristics due to the possibility of spectral tunability, fast sensor response, real-time detection, strong enhancement of the local electric field and excellent adaptability to assemble different nanobiotechnology architectures. In this paper, two different configurations for LSPR based biosensor were developed by using solution-phase gold nanorods (S–P-AuNRs) and AuNRs-chip. The LSPR sensitivities were evaluated by monitoring shifts in the longitudinal plasmon band with changes in the refractive index of the medium surrounding the nanoparticles. AuNRs-chip presented higher sensitivity of 297 nm RIU−1 (refractive index unit) against 196 nm RIU−1 for S–P-AuNRs. Figure of merit (FOM) for AuNRs-chip and S–P-AuNRs were 3.0 and 2.2 RIU−1, respectively. This result was assigned to the coupling of the lower energy longitudinal LSPR mode of propagation for AuNRs-chip among nearby nanoparticles in the film. In addition, an improvement of at least 18% in sensitivity was obtained comparing to others AuNRs based assay with similar aspect ratio. FOM is more appropriate to compare different approaches, in this case, the proposed biosensor reached improvements of at least 114%, presenting higher values even when compared to AuNRs of higher aspect ratio. As a proof of concept, AuNRs surface was chemically modified using mercaptoundecanoic acid followed activation with ethylcarbodiimide and N-hidroxysuccinimide to allow the interaction between Bovine Serum Albumin (BSA) antibody and correspondent antigen. Both configurations studied resulted in efficient plasmonic biosensors, presenting high sensitivity for changes in the refractive index and for surface binding with anti-BSA.

Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures.

This manuscript describes a reversible wet chemical process for the tip-selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip-selective dissolution was achieved by oxidation of AuNRs with a Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on the AuNR surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. A decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in a blue shift in the longitudinal plasmon band (LPB) position, and red shifts in the LPB position were achieved by increasing the AR by 1D growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when the desired AR and/or LPB position is achieved. In addition, the tip-selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long AuNWs.

Enzyme-free multicolor biosensor based on Cu2+-modified carbon nitride nanosheets and gold nanobipyramids for sensitive detection of neuron specific enolase

In this work, we have synthesized Cu2+-modified carbon nitride nanosheets (Cu2+-C3N4) as peroxidase mimic catalytic substance, which is more active than the horseradish peroxidase enzyme in extreme environments. Gold nanobipyramid (Au NBP) is a good chromogenic substrate for multicolor display because the longitudinal plasmon bands of AuNRs can be easily tuned by adjusting their aspect ratios and used as an excellent indicator for colorimetric detection of immunoassays. The generation of TMB2+ from sandwich complex (peroxidase-like catalysis) efficiently etches Au NPBs to produce multicolor variations from brown to olive, green, blue, purple, purple, red, pink, and colorless in presence of varied concentrations of neuron specific enolase (NSE). The experimental results show that the colorimetric detection ranging from 312.5 to 20,000 pM with a detection limit of 92.8 pM, which is higher than other multicolor based sensors. The Cu2+-C3N4 nanosheets and Au NBPs based colorimetric visual (naked eye) semi-quantitative method as a potential platform towards the detection of important biomolecules in clinical and therapeutic applications. The proposed method is simple, low-cost and enzyme-free to detect NSE for the first time.

Field-assisted growth of magnetic nanoparticle supercrystals

We report on the organization of magnetic nanoparticles (Fe3O4 and Fe3O4@Au NPs) into two and three dimensional (2D and 3D) supercrystals (SCs) and which were imbedded in polyethylene glycol (PEG) matrix. The organization of NPs was induced by subjecting their dispersion (in chloroform) to an external magnetic field. The Fe3O4 NPs were prepared by the high temperature decomposition of an Fe-oleate complex. The core–shell Fe3O4@Au NPs were made by treating Fe3O4 seed NPs with chloroauric acid (HAuCl4). Both the bare and Au-coated Fe3O4 NPs dispersed in chloroform were treated, separately, with PEG (molecular weight 6000) and suspended in the presence of an external magnetic field. On evaporation of chloroform, flaky crystals having sizes on the order of tens of millimeters were formed. Examination of the crystals in a transmission electron microscope (TEM) revealed the formation of 2D and 3D stacked NP SCs. The magnetic properties, ascertained by using a vibrating sample magnetometer (VSM), were found to be similar to that due to NPs only. However, the optical signature due to Fe3O4@Au NP SCs indicated the presence of NPs in aggregated forms as displayed through the appearance of longitudinal surface plasmon band in addition to the transverse one.

Prediction of plasmons in silver nanorods using artificial neural networks with back propagation algorithm

This work presents the prediction of plasmon positions in the optical absorption spectra of silver nanorods using artificial neural networks. Silver nanorods were prepared using a modified seed mediated strategy. At first, seed particles were synthesized by reducing silver ions with sodium borohydride in the presence of a stabilizer viz., cetyltrimethylammonium bromide (CTAB). In order to produce silver nanorods, the seeds were added to a growth solution containing a metal salt (AgNO3), a weak reducing agent (ascorbic acid) and a structure directing agent (CTAB). Four parameters viz., volume of metal seed, concentrations of silver nitrate, ascorbic acid and CTAB, which are found to be highly sensitive to plasmon characteristics are varied and grouped into 45 different sets of experiments. Herein, we report a model to predict the transverse and longitudinal plasmon band wavelengths, using a 4-input artificial neural network, having five neurons in the hidden layer and two neurons in the output layer. Levenberg–Marquardt back-propagation algorithm was used for the design. The network architecture has been trained with the 45 sets of experimental data collected and the regression plots with good correlation coefficient values were obtained for training, testing and validation stages.

(Keynote) Surfactant and Halide Control in Gold Nanorod Synthesis

Gold nanorods have been of continuing interest to many scientists due to their brilliant optical properties that are dictated by particle shape. Nanorods exhibit both transverse and longitudinal plasmon bands that are tunable from the visible into the near-infrared. Applications areas include plasmon-based sensors, diagnostics, imaging agents, and even photothermal therapeutics due to reasonably efficient light-to-heat conversion. The primary synthesis of these materials starts with a gold salt, and various reduction steps with various reducing agents, in the presence of particular surfactants and ions, do produce gold nanorods of tunable aspect ratio; yet the details of the synthetic steps and the "true" source of the symmetry-breaking anisotropy to produce rods instead of spheres are still the subject of debate. In this talk we will examine the state of the art in gold nanorod synthesis, including recent work that used a design-of-experiment multifactorial approach to gain deeper mechanistic insights into synergistic effects between the various reagents in the synthesis.

Light-triggered Assembly of Gold Nanorods based on Photoisomerization of Spiropyrans

The self-assembly of gold nanorods (AuNRs) derivatized with photoisomerizable spiropyran ligands was investigated. Under UV irradiation, the AuNRs that were functionalized on the surface by spiropyran with a dithiolane formed end-to-end assemblies accompanied with isomerization of the spiropyran moiety, simultaneously showing a red-shift of the longitudinal plasmon band and the increase in absorbance resulted from the merocyanine form. The networked structure of AuNRs was ultimately built “on demand” by UV irradiation.

Colloidal Dispersion of Gold Nanorods and Gold-Silver Core-Shell Nanorods in Polar Organic Solvents

Abstract Gold nanorods and gold-silver core-shell nanorods were colloidally dispersed in polar organic solvents, acetonitrile, dimethylsulfoxide, and dimethylformamide, and their water mixtures. The gold nanorods showed narrow longitudinal surface plasmon bands in 100% dimethylformamide, 25% dimethylsulfoxide, and 25% acetonitrile solutions, indicating that the gold nanorods were well dispersed in these solutions. For the core-shell nanorods, acetonitrile and its water mixtures gave well-dispersed colloidal solutions. Amphiphilic molecules in the dispersions did not influence the systems; however, thiol-terminated poly(ethylene glycol) (PEG-SH) strongly stabilized the colloidal dispersions. The nanorods in the organic dispersions with the addition of PEG-SH could be redispersed in phosphate buffer saline (PBS) solutions. The zeta potentials of the gold nanorods in 1/10 PBS solutions ranged from −7.5 to +3.3 mV. Steric effects of the bulky PEG chains attached to the nanorod surfaces stabilized their colloidal dispersions in PBS.

Synthesis and optical properties of gold nanorods with controllable morphology

Searching for architectural building blocks with tunable morphology and peculiarity is a prominent challenge for novel diagnostic and therapeutic applications. Here, the aqueous-based seed-mediated methods for preparing highly mono-dispersed Au nanorods with a different aspect ratio are systematically studied by controlling the amounts of Ag ions and seeds. We also explore the effect of pH on the synthesis of gold nanorods. The realization of the overlap of longitudinal plasmon band and excitation source with different degrees is made by changing the aspect ratio of nanorod in order to determine its effect on the overall surface enhancement. In addition, the gold octahedra are prepared by overgrowth on Au nanorods. The SERS effects of Au nanorods are researched and the FDTD simulations are performed to reveal the morphology induced plasmon modes.

Blocked Enzymatic Etching of Gold Nanorods: Application to Colorimetric Detection of Acetylcholinesterase Activity and Its Inhibitors.

The anisotropic morphology of gold nanorods (AuNRs) has been shown to lead to nonuniform ligand distribution and preferential etching through their tips. We have recently demonstrated that this effect can be achieved by biocatalytic oxidation with hydrogen peroxide, catalyzed by the enzyme horseradish peroxidase (HRP). We report here that modification of AuNRs with thiol-containing organic molecules such as glutathione and thiocholine hinders enzymatic AuNR etching. Higher concentrations of thiol-containing molecules in the reaction mixture gradually decrease the rate of enzymatic etching, which can be monitored by UV-vis spectroscopy through changes in the AuNR longitudinal plasmon band. This effect can be applied to develop novel optical assays for acetylcholinesterase (AChE) activity. The biocatalytic hydrolysis of acetylthiocholine by AChE yields thiocholine, which prevents enzymatic AuNR etching in the presence of HRP. Additionally, the same bioassay can be used for the detection of nanomolar concentrations of AChE inhibitors such as paraoxon and galanthamine.

Assemblies of gold nanorods for efficient SALDI mass spectrometry

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry was performed using gold nanorods deposited on ITO plates. The degree of aggregation of the nanorods was controlled on the plates, and the relationship between the SALDI signals and the longitudinal surface plasmon (SP) bands of the gold nanorods were examined. Highly efficient SALDI processes were obtained when the bandwidth of the SP bands was about 300 nm. Optical dark field and SEM observations showed that fusion and ablation of nanorod-assemblies consisting of 4–10 gold nanorods contributed to the efficient SALDI processes.

End-to-end self-assembly of gold nanorods in isopropanol solution: experimental and theoretical studies.

We describe here a modification of properties of colloidal gold nanorods (NRs) resulting from the chemical treatment used to carry out their transfer into isopropanol (IPA) solution. The NRs acquire a tendency to attach one to another by their ends (end-to-end assembly). We focus on the investigation of the change in position and shape of the longitudinal surface plasmon (l-SPR) band after self-assembly. The experimental results are supported by a theoretical calculation, which rationalizes the dramatic change in optical properties when the NRs are positioned end-to-end at short distances. The detailed spectroscopic characterization performed at the consecutive stages of transfer of the NRs from water into IPA solution revealed the features of the interaction between the polymers used as ligands and their contribution to the final stage, when the NRs were dispersed in IPA solution. The efficient method of aligning the NRs detailed here may facilitate applications of the self-assembled NRs as building blocks for optical materials and biological sensing.Graphical

Switching Plasmons: Gold Nanorod-Copper Chalcogenide Core-Shell Nanoparticle Clusters with Selectable Metal/Semiconductor NIR Plasmon Resonances.

Exerting control over the near-infrared (NIR) plasmonic response of nanosized metals and semiconductors can facilitate access to unexplored phenomena and applications. Here we combine electrostatic self-assembly and Cd(2+)/Cu(+) cation exchange to obtain an anisotropic core-shell nanoparticle cluster (NPC) whose optical properties stem from two dissimilar plasmonic materials: a gold nanorod (AuNR) core and a copper selenide (Cu(2-x)Se, x ≥ 0) supraparticle shell. The spectral response of the AuNR@Cu2Se NPCs is governed by the transverse and longitudinal plasmon bands (LPB) of the anisotropic metallic core, since the Cu2Se shell is nonplasmonic. Under aerobic conditions the shell undergoes vacancy doping (x > 0), leading to the plasmon-rich NIR spectrum of the AuNR@Cu(2-x)Se NPCs. For low vacancy doping levels the NIR optical properties of the dually plasmonic NPCs are determined by the LPBs of the semiconductor shell (along its major longitudinal axis) and of the metal core. Conversely, for high vacancy doping levels their NIR optical response is dominated by the two most intense plasmon modes from the shell: the transverse (along the shortest transversal axis) and longitudinal (along the major longitudinal axis) modes. The optical properties of the NPCs can be reversibly switched back to a purely metallic plasmonic character upon reversible conversion of AuNR@Cu(2-x)Se into AuNR@Cu2Se. Such well-defined nanosized colloidal assemblies feature the unique ability of holding an all-metallic, a metallic/semiconductor, or an all-semiconductor plasmonic response in the NIR. Therefore, they can serve as an ideal platform to evaluate the crosstalk between plasmonic metals and plasmonic semiconductors at the nanoscale. Furthermore, their versatility to display plasmon modes in the first, second, or both NIR windows is particularly advantageous for bioapplications, especially considering their strong absorbing and near-field enhancing properties.

Facile synthesis of gold nanorods/hydrogels core/shell nanospheres for pH and near-infrared-light induced release of 5-fluorouracil and chemo-photothermal therapy

We described a facile synthesis of pH and near-infrared (NIR) light dual-sensitive core/shell hybrid nanospheres, consisting of gold nanorods (GNR) as the core and poly(N-isopropylacrylamide-co-methacrylic acid) as the shell, p(NIPAM-MAA). The resultant GNR/p(NIPAM-MAA) nanospheres showed a core/shell structure, with an average diameter of ∼110nm and a strong longitudinal surface plasmon band at NIR region. Due to the photothermal effect of GNR and pH/thermal-sensitive volume transition of p(NIPAM-MAA) hydrogels, the nanospheres with loading of 5-fluorouracil (5-FU) by electrostatic interactions were developed as a smart carrier for pH- and photothermal-induced release of 5-FU. Experimental results testified that the cumulative release of 5-FU from nanospheres was markedly increased in a mild acidic medium. Moreover, a NIR light (808nm) irradiation triggered a greater and faster release of 5-FU, which was further testified by relevant results from in vitro cytotoxicity assay, in vivo tumor growth inhibition and histological images of ex vivo tumor sections. These results revealed significant applications of GNR/p(NIPAM-MAA) nanospheres in controlled release of anticancer agents and photothermal ablation therapy of tumor tissues, accompanied by synergistic effect of chem-photothermal therapy.

Using L-arginine-functionalized gold nanorods for visible detection of mercury(II) ions.

A rapid and simple approach for visible determination of mercury ions (Hg(2+) ) in aqueous solutions was developed based on surface plasmon resonance phenomenon using L-arginine-functionalized gold nanorods (AuNRs). At pH greater than 9, the deprotonated amine group of L-arginine on the AuNRs bound with Hg(2+) leading to the side-by-side assembly of AuNRs, which was verified by transmission electron microscopy images. Thus, when Hg(2+) was present in the test solution, a blue shift of the typical longitudinal plasmon band of the AuNRs was observed in the ultra violet-visible-near infrared (UV-Vis-NIR) spectra, along with a change in the color of the solution, which occurred within 5 min. After carefully optimizing the potential factors affecting the performance, the L-arginine/AuNRs sensing system was found to be highly sensitive to Hg(2+) , with the limit of detection of 5 nM (S/N = 3); it is also very selective and free of interference from 10 other metal ions (Ba(2+) , Ca(2+) , Cd(2+) , Co(2+) , Cs(+) , Cu(2+) , K(+) , Li(+) , Ni(2+) , Pb(2+) ). The result suggests that the L-arginine-functionalized AuNRs can potentially serve as a rapid, sensitive, and easy-to-use colorimetric biosensor useful for determining Hg(2+) in food and environmental samples.