From Average to Single Molecule Surface Enhanced Raman Scattering (original) (raw)
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ACS Nano, 2011
In the present work, the combination of chemical immobilization with electron beam lithography enables the production of sensitive and reproducible SERS-active areas composed of stochastic arrangements of gold nanoparticles. The number of nanoparticles was varied from 2 to 500. Thereby a systematic analysis of these SERS-active areas allows us to study SERS efficiency as a function of the number of nanoparticles. We found that the experimental parameters are critical, in particular the size of the SERS-active area must be comparable to the effective area of excitation to obtained reproducible SERS measurements. The sensitivity has also been studied by deducing the number of NPs that generate the enhancement. With this approach we demonstrates that the maximum enhancement, the best sensitivity, is obtained with the smallest number of nanoparticles that is resonant at a given excitation wavelength.
It is now believed that the near-resonance excitation of plasmonic nanoparticles is necessary to increase the Raman signal of nearby molecules. Consequently, for surface-enhanced Raman scattering (SERS) applications, researchers seek to synthesize rationally designed nanoparticles with plasmon resonances (PRs) close to the excitation wavelength. However, existing experiments show contradicting results for the dependence of the SERS enhancement on the PR wavelength. Here, we used the etching method to prepare a set of Au nanorod (AuNR) colloids with a decreasing aspect ratio. The shape morphology of the AuNRs and their concentration and width were kept constant, while the plasmon resonance was progressively decreased from 925 to 650 nm. The AuNRs were functionalized with 1,4nitrobenzenethiol (NBT), and SERS spectra of the colloids were measured under 785 nm laser excitation. The nanorod concentration (∼7 × 10 10 mL −1) was quantified by the atomic absorption spectroscopy and spectrophotometry combined with TEM statistical data and T-matrix simulations. The number of adsorbed NBT molecules per one nanorod (∼10 4) corresponded to the effective footprint of ∼0.55 nm 2 and was close to the monolayer packing density with the topological polar surface area of NBT at 0.468 nm 2. The plasmon peak position correlated weakly with the SERS response; specifically, the ratio between the SERS intensities for on-and off-resonance excitation was below 1.5. This observation contradicts the current understanding of the electromagnetic contribution to the SERS signal. In particular, our simulations agreed with the experimental data for plasmon resonance wavelengths of 785−925 nm, but for shorter wavelengths the simulations predicted an order-of-magnitude decrease in the averaged enhancement factor. In contrast to this finding, the shape morphology strongly affected the SERS response. Specifically, when the initial cigarlike AuNRs were further overgrown to yield dumbbell morphology, their SERS intensity increased 5-fold. Finally, we show that the SERS background spectra can be attributed to both the photoluminescence from AuNR ensemble and the elastic light scattering of a very weak laser background by the same AuNR ensemble.
Journal of Materials Science, 2015
Surface Enhanced Raman Scattering is a sensitive and widely used as spectroscopic technique for chemical and biological structure analysis. One of the keys to increase the sensitivity of SERS sensors is to use nanoparticles/nanostructures. Here, we report on the density effect of gold nanodisks on SERS intensity for a highly sensitive detection of chemical molecules. Various densities of gold nanodisks with a height of 30 nm on gold/glass substrate were fabricated by electron beam lithography in order to have a good uniformity and reproducibility. The evolution of the Enhancement Factor with nanodisk density was quantified and compared to numerical calculations. An enhancement factor as high as 2.6×10 7 was measured for the nanodisk with a diameter of 110 nm and a periodicity of 150 nm which corresponds to the biggest density (42.2%).
Surface-enhanced Raman scattering from finite arrays of gold nano-patches
Journal of Applied Physics, 2013
We experimentally investigate the surface-enhanced Raman scattering (SERS) response of a 2D-periodic array of square gold nano-patches, functionalized by means of a conjugated, rigid thiol. We measure a Raman signal enhancement up to 200 times more intense compared to other plasmon-based nanostructures functionalized with the same molecule, and show that the enhancement is not strictly correlated to the presence of plasmonic resonances. The agreement between experimental and theoretical results reveals the importance of a full-wave analysis based on the inclusion of the actual scattering cross section of the molecule. The proposed numerical approach may serve not only as a tool to predict the enhancement of Raman signal scattered from strongly resonant nanostructure but also as an effective instrument to engineer SERS platforms that target specific molecules. V C 2013 American Institute of Physics.
Applied Spectroscopy, 1998
In agreem ent with previous resu lts reported for colloidal silver clusters, effective surface-enhanced Raman cross sections of about 10 2 16 cm 2 per molecule, corresp onding to enhancem ent factors on the order of 10 14 , have also been obtained for molecules attached to colloidal gold clusters. Spatially isolated nearly spherical colloidal gold particles of about 60 nm size show maximum enhancem ent factors on the order of 10 3 at 514 nm excitation, close to the single plasmon resonance. The enhancement factor increases by eleven orders of m agnitude when colloidal gold clusters are formed by aggregation of the gold colloids and when near-infrared excitation is applied. The large effective surface-en hanced Raman cross section has been estim ated by a straightforward method based on steady-state population redistribution due to the pumping of m olecules to the ® rst excited vibrational state via the strongly enhanced Raman process. Our experim ental ® nding con® rms the im portant role of colloidal clusters for extrem ely large surface-en hanced Raman scattering (SERS) enhancement factors. Simultaneously, it suggests colloidal gold clusters as a substrate for high-sensitivity surface-en hanced Raman scattering, which can provide an enhancement level suf®cient for Raman single m olecule detection. Due to its chemical inactivity, gold m ight have some advantages compared to silver, particularly in biomedical spectroscop y.
The interparticle spacing of the surface-enhanced Raman scattering (SERS) substrate has a strong relationship with its enhancement factor (EF). How to precisely adjust the interparticle gap and produce SERS substrates with excellent quality and high reliability by a facile way is still a challenge. Here, we explored a convenient and cheap method to fabricate gold nanoparticles SERS substrates through photo-deposition of gold nanoparticles using one photon absorption. We show that using one photon absorption with a continuous laser writing technique, it is possible to immobilise and pattern the gold nanoparticle capable of generating SERS activity. Preliminary, optical microscopy, SEM, UV-vis characterization show that our approach represents a powerful alternative to the traditional fabrication of SERS substrates.
2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2015
In recent years, the SERS-related materials research has gradually moved to the much cheaper semiconductor materials against the barriers from noble metals. [9-21] However, compared with noble metals, their relatively weak enhancement factor (EF) is not high enough for molecule to trace detection. Therefore, how to efficiently improve the SERS performance of semiconductors is the focus of a related study. Fortunately, scientists have found that some semiconductive materials could show great Raman scattering enhancement to trace of substances, reaching an ultralow limit of detection (LOD) and an ultrahigh EF. One excellent example is an urchin-like W 18 O 49 reported by Zhao and co-workers; the EF is up to the 3.4 × 10 5 level by means of the surface plasmon resonance. [17] And the single Cu 2 O superstructure particle reported by Guo and coworkers, obtaining an LOD of 10 −9 m and an EF of 8.0 × 10 5 , may be the best enhancement effect among the non-noble metal substrates reported so far. [14] Besides, considering promoting the interfacial charge-transfer process (ICTP) is an important premise to improve the sensitivity of semiconductorbased SERS, Guo's group further developed amorphous ZnO nanocages (a-ZnO NCs). Their study indicates that the remarkable SERS sensitivity can be obtained from the high-efficiency ICTP within the amorphous ZnO NCs-molecules system. [20] Based on previous research, the optimization of the geometry morphology and promotion of the ICTP between the semiconductor and molecules are considered two determining factors for EF improvement in semiconductor SERS. If we obtain an amorphous structure and simultaneously reduce the size of nanoparticles greatly, even to quantum size, we can achieve enhanced SERS. However, achieving both of them simultaneously is challenging. In our previous work, we have prepared 2D MoO 3 nanosheets that indicated excellent LSPR performance. [22] But due to the relatively larger size in the 2D region, its SERS is not so satisfied. In this work, we designed an alternative strategy to fabricate uniform amorphous molybdenum oxide quantum dots. By carrying out a series of efficient regulation strategies on the reaction system, this as-prepared peculiar nanostructure possesses excellent quantum size in uniformity, accompanied with an intensively enhanced plasmonic resonance property. Our experimental results demonstrate the
Gold bulletin, 2013
Surface-enhanced Raman spectroscopy (SERS) has enormous potential for a range of applications where high sensitivity needs to be combined with good discrimination between molecular targets. However, the SERS technique has trouble finding its industrial development, as was the case with the surface plasmon resonance technology. The main reason is the difficulty to produce stable, reproducible, and highly efficient substrates for quantitative measurements. In this paper, we report a method to obtain two-dimensional regular nanopatterns of gold nanoparticles (AuNPs). The resulting patterns were evaluated by SERS. Our bottom-up strategy was divided into two steps: (a) nanopatterning of the substrate by e-beam lithography and (b) electrostatic adsorption of AuNPs on functionalized substrates. This approach enabled us to highlight the optimal conditions to obtain monolayer, rows, or ring of AuNPs, with homogeneous distribution and high density (800 AuNPs/μm 2). The nanostructure distributions on the substrates were displayed by scanning electron microscopy and atomic force microscopy images. Optical properties of our nanostructures were characterized by visible extinction spectra and by the measured enhancements of Raman scattering. Finally, we tried to demonstrate experimentally that, to observe a significant enhancement of SERS, the gold diffusers must be extremely closer. If electron beam lithography is a very attractive technique to perform reproducible SERS substrates, the realization of pattern needs a very high resolution, with distances between nanostructures probably of less than 20 nm.
Deterministic aperiodic arrays of metal nanoparticles for surface-enhanced Raman scattering (SERS)
Opt. Express, 2009
Deterministic Aperiodic (DA) arrays of gold (Au) nanoparticles are proposed as a novel approach for the engineering of reproducible surface enhanced Raman scattering (SERS) substrates. A set of DA and periodic arrays of cylindrical and triangular Au nanoparticles with diameters ranging between 50-110 nm and inter-particle separations between 25-100 nm were fabricated by e-beam lithography on quartz substrates. Using a molecular monolayer of pMA (p-mercaptoaniline) as a Raman reporter, we show that higher values of SERS enhancement factors can be achieved in DA structures compared to their periodic counterparts, and discuss the specific scaling rules of DA arrays with different morphologies. Electromagnetic field calculations based on the semi-analytical generalized Mie theory (GMT) fully support our findings and demonstrate the importance of morphology-dependent diffractive coupling (long-range interactions) for the engineering of the SERS response of DA arrays. Finally, we discuss optimization strategies based on the control of particles sizes and shapes, and we demonstrate that spatially-averaged SERS enhancement factors of the order of ~ 10 7 can be reproducibly obtained using DA arrays of Au nano-triangles. The ability to rigorously design lithographically fabricated DA arrays of metal nanoparticles enables the optimization and control of highly localized plasmonic fields for a variety of chip-scale devices, such as more reproducible SERS substrates, label-free bio-sensors and non-linear elements for nano-plasmonics. 2009 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (240.6695) Surface-enhanced Raman scattering; (050.6624) Subwavelength structures; (290.4020) Mie theory.