Raman detection of localized transferrin-coated gold nanoparticles inside a single cell

We investigated the cellular uptake behavior of non-fluorescent metal nanoparticles (NPs) by use of surface-enhanced Raman scattering (SERS) combined with dark-field microscopy (DFM). The uptake of Au NPs inside a single cell could also be identified by DFM first and then confirmed by z-depth-dependent SERS at micrometer resolution. Guided by DFM for the location of Au NPs, an intracellular distribution assay was possible using Raman dyes with unique vibrational marker bands in order to identify the three-dimensional location inside the single cell by obtaining specific spectral features. Au NPs modified by 4-mercaptobenzoic acid (MBA) bearing its –COOH surface functional group were used to conjugate transferrin (Tf) protein using the 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) reaction. The protein conjugation reaction on Au surfaces was examined by means of color change, absorption spectroscopy, and SERS. Our results demonstrate that DFM techniques combined with SERS may have great potential for monitoring biological processes with protein conjugation at the single-cell level.     

Nanoparticle-containing structures as a substrate for surface-enhanced Raman scattering

Metallic nanostructures were prepared through the alternate immersion of derivatized glass slides in solutions of gold nanoparticles (NPs) and a propanedithiol linker molecule. Nanostructures consisting of 1-17 depositions of gold NPs were synthesized, and these substrates were characterized using UV-vis spectroscopy and atomic force microscopy. Subsequently, the surface-enhanced Raman scattering (SERS) of oxazine 720 was obtained at two excitation wavelengths (632 and 785 nm) from all substrates. Maximum SERS enhancement was observed for 9 and 13 NP depositions for 632 and 785 nm excitations, respectively. The difference in the number of NP depositions required for maximum enhancement is attributed to different wavelengths which can excite distinct aggregate structures within the metallic substrate. Therefore, these NP-containing structures can be "tuned" to yield maximum SERS enhancement for the excitation source being used by varying the number of NP depositions.     

内部具有可移动金核的中空银纳米颗粒的制备、表征及其表面增强拉曼效应

采用振荡法和种子生长技术制备出核壳结构的Au@SiO2纳米颗粒及夹层结构的Au@SiO2@Ag纳米颗粒, 用HF将Au@SiO2@Ag NPs夹层的SiO2溶解, 得到内部带有粒径为30 nm的可移动金核、壳层厚度约为30 nm的中空银纳米颗粒(Au@air@Ag NPs). 用扫描电子显微镜和透射电子显微镜对所得到的纳米微球的形貌进行了表征, 并以罗丹明B为探针分子研究了Au@air@Ag 纳米颗粒的表面增强拉曼(SERS)效应, 发现Au@air@Ag 纳米颗粒是一种可应用于SERS的理想材料.     

SERS in PAH-Os and gold nanoparticle self-assembled multilayers

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+(Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (approximately 530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.     

Silica-void-gold nanoparticles: temporally stable surface-enhanced Raman scattering substrates

Reproducible detection of a target molecule is demonstrated using temporally stable solution-phase silica-void-gold nanoparticles and surface-enhanced Raman scattering (SERS). These composite nanostructures are homogeneous (diameter = 45 +/- 4 nm) and entrap single 13 nm gold nanoparticle cores inside porous silica membranes which prevent electromagnetic coupling and aggregation between adjacent nanoparticles. The optical properties of the gold nanoparticle cores and structural changes of the composite nanostructures are characterized using extinction spectroscopy and transmission electron microscopy, respectively, and both techniques are used to monitor the formation of the silica membrane. The resulting nanostructures exhibit temporally stable optical properties in the presence of salt and 2-naphthalenethiol. Similar SERS spectral features are observed when 2-naphthalenethiol is incubated with both bare and membrane-encapsulated gold nanoparticles. Disappearance of the S-H Raman vibrational band centered at 2566 cm(-1) with the composite nanoparticles indicates that the target molecule is binding directly to the metal surface. Furthermore, these nanostructures exhibit reproducible SERS signals for at least a 2 h period. This first demonstration of utilizing solution-phase silica-void-gold nanoparticles as reproducible SERS substrates will allow for future fundamental studies in understanding the mechanisms of SERS using solution-phase nanostructures as well as for applications that involve the direct and reproducible detection of biological and environmental molecules.     

盐诱导金纳米粒子自组装和沉降制备具有宽波段吸收特性的黑金

通过向金溶胶中加入无机盐诱导金纳米粒子自组装,并自然沉降金纳米粒子组装体得到黑金薄膜.研究结果表明,该黑金薄膜在400~1600 nm的宽带范围内表现出很强的吸收能力（>80%）,在400~800 nm的可见范围内能达到94%,表现出高光热转换能力和宽波段的高表面增强拉曼散射（SERS）活性.     

Controlled interparticle spacing for surface-modified gold nanoparticle aggregates

Aggregation of gold nanoparticles of increasing size has been studied as a consequence of adsorption of 2-aminothiophenol (ATP) on gold nanoparticle surfaces. The capping property of ATP in the acidic pH range has been accounted from UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS) studies. The effect of nanoparticle size (8-55 nm) on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects among the gold particles have been critically examined. Various techniques such as transmission electron microscopy, X-ray diffraction, zeta-potential, and average particle size measurement were undertaken to characterize the nanoparticle aggregates. The aggregate size, interparticle distances, and absorption band wavelengths were found to be highly dependent on the pH of the medium and the concentration of the capping agent, ATP. The acquired SERS spectra of ATP relate the interparticle spacing. It has been observed that the SERS signal intensities are different for different sized gold nanoparticles.     

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation–reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.     

Controlled Self-Assembly of Metallacycle-Bridged Gold Nanoparticles for Surface-Enhanced Raman Scattering

In this work, well-defined two-dimensional metallacycles have been successfully employed for the well-controlled self-assembly of gold nanoparticles (AuNPs) into discrete clusters such as dimers, trimers, tetramers, pentamers and even hexamers at the water–oil interface for the first time. Furthermore, the modular construction of metallacycle molecules allows precise control of spacing between the gold nanoparticles. Interestingly, it was found that interparticle spacing below 5 nm created by molecular metallacycles in the resultant discrete gold nanoparticle clusters led to a strong plasmon coupling, thus inducing great field enhancement inside the gap between the NPs. More importantly, different discrete clusters with precise interparticle spacing provide a well-defined system for studying the hot-spot phenomenon in surface-enhanced Raman scattering (SERS); this revealed that the SERS effects were closely related to the interparticle spacing.     

Aqueous Stable Gold Nanostar/ZIF‐8 Nanocomposites for Light‐Triggered Release of Active Cargo Inside Living Cells

A plasmonic core–shell gold nanostar/zeolitic‐imidazolate‐framework‐8 (ZIF‐8) nanocomposite was developed for the thermoplasmonic‐driven release of encapsulated active molecules inside living cells. The nanocomposites were loaded, as a proof of concept, with bisbenzimide molecules as functional cargo and wrapped with an amphiphilic polymer that prevents ZIF‐8 degradation and bisbenzimide leaking in aqueous media or inside living cells. The demonstrated molecule‐release mechanism relies on the use of near‐IR light coupled to the plasmonic absorption of the core gold nanostars, which creates local temperature gradients and thus, bisbenzimide thermodiffusion. Confocal microscopy and surface‐enhanced Raman spectroscopy (SERS) were used to demonstrate bisbenzimide loading/leaking and near‐IR‐triggered cargo release inside cells, thereby leading to DNA staining.     

Size-dependent adsorption of 1,4-phenylenediisocyanide onto gold nanoparticle surfaces

Adsorption of 1,4-phenylenediisocyanide (PDI) has been studied for different-sized gold nanoparticles with mean diameters of 6, 14, 23, 40, 57, and 97 nm using UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS). The SERS enhancement was found to be relatively weak for 6-nm particles due to less aggregation between PDI and gold particles. Concentration-dependent SERS spectra show that PDI was assumed to bridge two different gold particles at low concentrations of PDI, but as the concentration was increased, the bridge appeared to be broken, and PDI bonded to the gold particle only via one of its two isocyanide groups. For the 57- and 97-nm particles, however, the nu(NC)(free) stretching band in the SERS spectrum almost disappeared, even at a high bulk concentration of PDI, differently from the case of the smaller sizes (14, 23, and 40 nm). The 57- and 97-nm particles appeared to cross-link through the pendent isocyanide group even at a high bulk concentration. UV-vis absorption spectra indicated that PDI appeared to aggregate more extensively with increasing size in agreement with Raman data. Our result shows an example that the adsorption scheme of an aromatic diisocyanide may be varied depending on particle size as well as the bulk concentration.     

Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three‐dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self‐assembly. Nanostructures consisting of one to 20 depositions of 13 nm‐diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection–absorption spectroscopy. Surface‐enhanced polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) is observed from Au NPs modified by the probe molecule 4‐hydroxythiophenol. The limitation of this kind of substrate for surface‐enhanced PM‐IRRAS is discussed. The surface‐enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au‐NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au‐NP depositions, and the absolute SERS intensities from different batches are reproducible within 20 %. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between ?800 and +200 mV (vs. Ag/AgCl/sat. Cl^?).     

Gold nanoparticle multilayer films based on surfactant films as a template: preparation, characterization, and application

Highly ordered gold nanoparticle multilayer films were achieved conveniently using didodecyldimethylammonium bromide (DDAB) films as a template. The template was produced by casting DDAB chloroform solution onto the surface of a (3-aminopropyl)trimethoxysilane-modified indium tin oxide substrate and then evaporating the organic solvent. Gold nanoparticle multilayer films were prepared by soaking the template in 2.6 nm colloidal gold solution for 120 min. The well-ordered superlattice structure of the DDAB template and the gold nanoparticle multilayer films was identified by x-ray diffraction. The characterizations of the gold nanoparticle multilayer films by UV-vis spectroscopy, atomic force microscopy, and cyclic voltammerty were described in detail. The application of the as-prepared gold nanoparticle multilayer films in surface-enhanced Raman spectroscopy (SERS) was investigated by using Rhodamine 6G as a probe molecule. It was found that the colloidal gold nanoparticle multilayer films exhibit remarkable enhancement ability and can be used as SERS substrates.     

Glutathione Functionalized Gold Nanoparticles as Efficient Surface Enhanced Raman Scattering Substrate for Poly Chlorinated Biphenyl Detection

Polychlorinated biphenyls (PCBs) are harmful even at trace level in the environment, and they are difficult to detect. This work presents a simple method for preparation of glutathione (GSH) functionalized gold nanoparticles (Au NPs) (GSH-Au NPs) for the detection of PCBs and its isomers based on surface enhanced Raman scattering (SERS). The prepared Au NPs show the surface plasmon band around 533 nm. The crystallinity and formation of GSH-Au NPs were confirmed by using X-ray diffraction and vibrational studies. Transmission electron microscopic analysis showed the average particle size of GSH-Au NPs is around 16 nm. The morphology of the GSH-Au NPs indicates dumbbell-shaped structures with “hot spots” present. These hot spots increase the SERS activity significantly. Gas chromatography–mass spectrum showed that the soil extract contained PCBs, which, has also been detected using SERS. SERS based detection is simple and powerful for identifying the PCBs, as established here for PCBs in the real soil sample.Hence, from this investigation, a rapid, sensitive, cost-effective sensing method for detecting toxic PCBs in the environment was demonstrated.     

Size-controllable synthesis of surface-enhanced Raman scattering-active gold nanoparticles coated on TiO2

As shown in the literature, gold nanoparticles (NPs) were popularly used in the fields of catalyst and surface-enhanced Raman scattering (SERS). In this work, size-controllable Au NPs coated on TiO(2) are synthesized by adjusting the pH of solutions based on sonoelectrochemical methods. The size-controlled Au NPs on TiO(2), ranging from 2 to 80 nm in diameter, can be obtained by varying the pH of solutions from 3 to 7 and placing the sample for 3 h before sonoelectrochemical reductions. The optimal particle sizes of Au NPs on TiO(2) to obtain the strongest SERS effects under an irradiation of 785 nm for probe molecules of adsorbed Rhodamine 6G (R6G) and deposited polypyrrole (PPy) are all ca. 60 nm.     

金纳米棒负载的静电纺丝纤维的制备及其表面增强拉曼光谱性能

为简单有效地制备高活性表面增强拉曼光谱(Surface-enhanced Raman Spectroscopy,SERS)基底。本文采用静电纺丝聚乙烯醇(PVA)/聚丙烯酸(PAA)纳米纤维为支撑材料,通过直接浸泡的方法,利用金纳米棒与电纺纤维之间的静电力,使纳米棒在纤维表面自组装,得到了性能优异的SERS基底。通过透射电子显微镜、扫描电子显微镜对金纳米棒以及不同状态下的电纺纤维的形貌进行表征,结果表明,金纳米棒均匀且密集地负载在纤维表面。通过设置不同的浸泡时间确定了金纳米棒组装平衡的时间为12 h,并通过调控纺丝时间和金纳米棒的浓度发现随着纺丝时间和金纳米棒浓度的增加,复合纤维膜SERS增强效果随之提升。该复合纤维膜具有优异的SERS均匀性,并且能够检测到浓度低至10~(-10)mol/L的4-氨基苯硫酚的存在。     

Phthalocyanine macrocycle as stabilizer for gold and silver nanoparticles

A one-step process was used for the preparation of gold and silver nanoparticles stabilized by an aminophthalocyanine macrocycle. The resultant nanoparticles were characterized by absorption spectra, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The nanoparticles were found to possess relatively narrow size distribution. The gold nanoparticles have an average diameter of ~2 nm, while silver particles have 4–5 nm. Preliminary studies on fluorescence and surface enhanced Raman spectroscopy were carried out using these nanoparticles. Fluorescence studies indicate that gold nanoparticles do not quench the fluorescence, while silver nanoparticles do. The stabilized nanoparticles showed enhancement of the Raman signals, thus revealing that they are good substrates for surface enhanced Raman scattering studies.     

Silica-coated gold nanostars for combined surface-enhanced Raman scattering (SERS) detection and singlet-oxygen generation: a potential nanoplatform for theranostics

This paper reports the synthesis and characterization of surface-enhanced Raman scattering (SERS) label-tagged gold nanostars, coated with a silica shell containing methylene blue photosensitizing drug for singlet-oxygen generation. To our knowledge, this is the first report of nanocomposites possessing a combined capability for SERS detection and singlet-oxygen generation for photodynamic therapy. The gold nanostars were tuned for maximal absorption in the near-infrared (NIR) spectral region and tagged with a NIR dye for surface-enhanced resonance Raman scattering (SERRS). Silica coating was used to encapsulate the photosensitizer methylene blue in a shell around the nanoparticles. Upon 785 nm excitation, SERS from the Raman dye is observed, while excitation at 633 nm shows fluorescence from methylene blue. Methylene-blue-encapsulated nanoparticles show a significant increase in singlet-oxygen generation as compared to nanoparticles synthesized without methylene blue. This increased singlet-oxygen generation shows a cytotoxic effect on BT549 breast cancer cells upon laser irradiation. The combination of SERS detection (diagnostic) and singlet-oxygen generation (therapeutic) into a single platform provides a potential theranostic agent.     

Biocompatible Triplex Ag@SiO2@mTiO2 Core–Shell Nanoparticles for Simultaneous Fluorescence‐SERS Bimodal Imaging and Drug Delivery

Herein, we report the synthesis of biocompatible triplex Ag@SiO₂@mTiO₂ core–shell nanoparticles (NPs) for simultaneous fluorescence‐surface‐enhanced Raman scattering (F‐SERS) bimodal imaging and drug delivery. Stable Raman signals were created by typical SERS tags that were composed of Ag NPs for optical enhancement, a reporter molecule of 4‐mercaptopyridine (4‐Mpy) for a spectroscopic signature, and a silica shell for protection. A further coating of mesoporous titania (mTiO₂) on the SERS tags offered high loading capacity for a fluorescence dye (flavin mononucleotide) and an anti‐cancer drug (doxorubicin (DOX)), thereby endowing the material with fluorescence‐imaging and therapeutic functions. The as‐prepared F‐SERS dots exhibited strong fluorescence when excited by light at 460 nm whilst a stable, characteristic 4‐Mpy SERS signal was detected when the excitation wavelength was changed to longer wavelength (632.8 nm), both in solution and after incorporation inside living cells. Their excellent biocompatibility was demonstrated by low cytotoxicity against MCF‐7 cells, even at a high concentration of 100 μg mL^?1. In vitro cell cytotoxicity confirmed that DOX‐loaded F‐SERS dots had a comparable or even greater therapeutic effect compared with the free drug, owing to the increased cell‐uptake, which was attributed to the possible endocytosis mechanism of the NPs. To the best of our knowledge, this is the first proof‐of‐concept investigation on a multifunctional nanomedicine that possessed a combined capacity for fast and multiplexed F‐SERS labeling as well as drug‐loading for cancer therapy.     

Chemically bound gold nanoparticle arrays on silicon: assembly, properties and SERS study of protein interactions

A highly reproducible and facile method for formation of ordered 2 dimensional arrays of CTAB protected 50 nm gold nanoparticles bonded to silicon wafers is described. The silicon wafers have been chemically modified with long-chain silanes terminated with thiol that penetrate the CTAB bilayer and chemically bind to the underlying gold nanoparticle. The silicon wafer provides a reproducibly smooth, chemically functionalizable and non-fluorescent substrate with a silicon phonon mode which may provide a convenient internal frequency and intensity calibration for vibrational spectroscopy. The CTAB bilayer provides a potentially biomimetic environment for analyte, yet allows a sufficiently small nanoparticle separation to achieve a significant electric field enhancement. The arrays have been characterized using SEM and Raman spectroscopy. These studies reveal that the reproducibility of the arrays is excellent both between batches (<10% RSD) and across a single batch (<5% RSD). The arrays also exhibit good stability, and the effect of temperature on the arrays was also investigated. The interaction of protein and amino acid with the nanoparticle arrays was investigated using Raman microscopy to investigate their potential in bio-SERS spectroscopy. Raman of phenylalanine and the protein bovine pancreatic trypsin inhibitor, BPTI were studied using 785 nm excitation, coincident with the surface plasmon absorbance of the array. The arrays exhibit SERS enhancements of the order of 2.6 x 10(4) for phenylalanine, the standard deviation on the relative intensity of the 1555 cm(-1) mode of phenylalanine is less than 10% for 100 randomly distributed locations across a single substrate and less than 20% between different substrates. Significantly, comparisons of the Raman spectra of the protein and phenylalanine in solution and immobilized on the nanoparticle arrays indicates that the protein is non-randomly orientated on the arrays. Selective SERS enhancements suggest that aromatic residues penetrate through the bilayer inducing conformational changes in the protein.