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2.
ACS Nano ; 14(1): 28-117, 2020 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-31478375

RESUMO

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

3.
Anal Chem ; 91(22): 14639-14648, 2019 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-31621299

RESUMO

Metallic nanoparticle solutions are routinely characterized by measuring their extinction spectrum (with UV-vis spectroscopy). Theoretical predictions such as Mie theory for spheres can then be used to infer important properties, such as particle size and concentration. Here we highlight the benefits of measuring not only the extinction (the sum of absorption and scattering) but also the absorption spectrum (which excludes scattering) for routine characterization of metallic nanoparticles. We use an integrating sphere-based method to measure the combined extinction-absorption spectra of silver nanospheres and nanocubes. Using a suite of electromagnetic modeling tools (Mie theory, T-matrix, surface integral equation methods), we show that the absorption spectrum, in contrast to extinction, is particularly sensitive to shape imperfections such as roughness, faceting, or edge rounding. We study in detail the canonical case of silver nanospheres, where small discrepancies between experimental and calculated extinction spectra are still common and often overlooked. We show that this mismatch between theory and experiment becomes much more important when considering the absorption spectrum and can no longer be dismissed as experimental imperfections. We focus in particular on the quadrupolar localized plasmon resonance of silver nanospheres, which is predicted to be very prominent in the absorption spectrum but is not observed in our experiments. We consider and discuss a number of possible explanations to account for this discrepancy, including changes in the dielectric function of Ag, size polydispersity, and shape imperfections such as elongation, faceting, and roughness. We are able to pinpoint faceting and roughness as the likely causes for the observed discrepancy. A similar analysis is carried out on silver nanocubes to demonstrate the generality of this conclusion. We conclude that the absorption spectrum is in general much more sensitive to the fine details of a nanoparticle geometry, compared to the extinction spectrum. The ratio of extinction to absorption also provides a sensitive indicator of size for many types of nanoparticles, much more reliably than any observed plasmon resonance shifts. Overall, this work demonstrates that combined absorption-extinction measurements provide a much richer characterization tool for metallic nanoparticles.

4.
Nanoscale ; 11(25): 12177-12187, 2019 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-31198919

RESUMO

Enhanced interaction between light and molecules adsorbed on metallic nanoparticles is a cornerstone of plasmonics and surface-enhanced spectroscopies. Recent experimental access to the electronic absorption spectrum of dye molecules on silver colloids at low molecular coverage has revealed subtle changes in the spectral shape that may be attributed to a combination of factors, from a chemical modification of the molecule in contact with a metal surface to electromagnetic dye-dye and dye-metal interactions. Here we develop an original model to rigorously address the electromagnetic effects. The dye molecules are described as coupled anisotropic polarisable dipoles and their interaction with the core metal particle is described using a generalised Mie theory. The theory is readily amenable to numerical implementation and yields far-field optical cross-sections that can be compared to experimental results. We apply this model to specific adsorption geometries of practical interest to highlight the effect of molecular orientation on predicted spectral shifts and enhancement factors, as a function of surface coverage. These are compared to experimental results and reproduce the measured spectral changes as a function of concentration. These results have direct implications for the interpretation of surface selection rules and enhancement factors in surface-enhanced spectroscopies, and of orientation and coverage effects in molecular/plasmonic resonance coupling experiments.

5.
Nature ; 568(7750): 36-37, 2019 04.
Artigo em Inglês | MEDLINE | ID: mdl-30944489
6.
Appl Opt ; 57(7): 1581-1588, 2018 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29522005

RESUMO

We use Monte Carlo ray-tracing modeling to follow the stochastic trajectories of rays entering a cylindrical port from inside an integrating sphere. This allows us to study and quantify properties of realistic ports of non-negligible length, as opposed to the common thin-port assumption used in most theoretical treatments, where the port is simply considered as a hole in the spherical wall. We show that most practical ports encountered in integrating sphere applications cannot be modeled as thin ports. Indeed, a substantial proportion of rays entering the port can be reflected back into the sphere, with port reflectances as high as 80% demonstrated on realistic examples. This can have significant consequences on estimates of the sphere multiplier and therefore pathlength inside the sphere, a critical parameter in many applications. Moreover, a nonzero port reflectance is inevitably associated with reduced transmittance through the port, with implications in terms of overall throughput. We also discuss angular redistribution effects in a realistic port and the consequences in terms of detected throughput within a fixed numerical aperture. Those results highlight the importance of real port effects for any quantitative predictions of optical systems using integrating spheres. We believe that those effects can be exploited to engineer ports for specific applications and improve the overall sphere performance in terms of pathlength or throughput. This work carries important implications in our theoretical understanding of integrating spheres and on the practical design of optical systems using them.

7.
Phys Rev E ; 95(3-1): 033307, 2017 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-28415316

RESUMO

We propose a powerful approach to solve Laplace's equation for point sources near a spherical object. The central new idea is to use prolate spheroidal solid harmonics, which are separable solutions of Laplace's equation in spheroidal coordinates, instead of the more natural spherical solid harmonics. Using electrostatics as an example, we motivate this choice and show that the resulting series expansions converge much faster. This improvement is discussed in terms of the singularity of the solution and its analytic continuation. The benefits of this approach are further illustrated for a specific example: the calculation of modified decay rates of light emitters close to nanostructures in the quasistatic approximation. We expect the general approach to be applicable with similar benefits to the solution of Laplace's equation for other geometries and to other equations of mathematical physics.

8.
ACS Omega ; 2(5): 1804-1811, 2017 May 31.
Artigo em Inglês | MEDLINE | ID: mdl-31457544

RESUMO

The wavelength-dependent complex linear polarizability of a dye is a crucial input for the modeling of the optical properties of dye-containing systems. We here propose and discuss methods to obtain an accurate polarizability model by combining absorption spectrum measurements, Kramers-Kronig (KK) tranformations, and density functional theory (DFT) calculations. We focus, in particular, on the real part of the polarizability and its link with static polarizability. In addition, we introduce simple KK-consistent analytic functions based on the theory of critical points as a much more accurate approach to model dye polarizabilities compared with existing models based on Lorentz oscillators. Accurate polarizability models based on critical points and DFT calculations of the static polarizability are derived for five commonly used dyes: Rhodamine 6G, Rhodamine 700, Crystal Violet, Nile Blue A, and Methylene Blue. Finally, we demonstrate explicitly, using examples of Mie Theory calculations of nanoparticle-dye interactions, how an inaccurate polarizability model can result in fundamentally different predictions, further emphasizing the importance of accurate models, such as the one proposed here.

9.
Biomicrofluidics ; 9(1): 014110, 2015 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-25713692

RESUMO

Tunable resistive pulse sensing (TRPS) has emerged as a useful tool for particle-by-particle detection and analysis of microparticles and nanoparticles as they pass through a pore in a thin stretchable membrane. We have adapted a TRPS device in order to conduct simultaneous optical measurements of particles passing through the pore. High-resolution fluorescence emission spectra have been recorded for individual 1.9 µm diameter particles at a sampling period of 4.3 ms. These spectra are time-correlated with RPS pulses in a current trace sampled every 20 µs. The flow rate through the pore, controlled by altering the hydrostatic pressure, determines the rate of particle detection. At pressures below 1 kPa, more than 90% of fluorescence and RPS events were matching. At higher pressures, some peaks were missed by the fluorescence technique due to the difference in sampling rates. This technique enhances the particle-by-particle specificity of conventional RPS measurements and could be useful for a range of particle characterization and bioanalysis applications.

10.
Appl Opt ; 53(31): 7224-9, 2014 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-25402880

RESUMO

We investigate the convergence of the series arising in Mie theory for the solution of electromagnetic scattering by a sphere. In contrast with previous studies that focused only on the scattering cross section, we here consider a wide spectrum of relevant properties, including scattering, extinction, and absorption cross sections, complex scattering amplitudes (i.e., radiation profile), and near-field properties such as surface electric field and average surface field intensity. The scattering cross section is shown to exhibit the fastest convergence, indicating that existing convergence criteria based on this property are not suitable for the majority of other relevant characteristics computed from Mie theory. Criteria are therefore proposed for those properties.

11.
Phys Chem Chem Phys ; 16(43): 23895-9, 2014 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-25277821

RESUMO

We demonstrate the possibility of single molecule (SM) detection via surface-enhanced Raman spectroscopy (SERS) in two seemingly challenging and unexpected cases: first with ultra-low excitation powers of the order of nanowatts and second in as-synthesized and not deliberately-aggregated silver colloid solution. The experiments are carried out using the bi-analyte method on a methylated form of Rhodamine 6G and one of its isotopologues excited at 514 nm close to the electronic resonance. This study spectacularly highlights the fact that SM-SERS detection is much more common and easier to achieve than typically thought, in particular in the case of resonance Raman excitation. As a result, SM-SERS detection in such cases should not be viewed as an indication of good SERS substrate performance as sometimes implicitly assumed.

12.
J Am Chem Soc ; 136(31): 10965-73, 2014 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-25046658

RESUMO

This study highlights a crucial but often overlooked consideration during sample preparation involving surface-adsorbing species: the competition between analyte adsorption and analyte diffusion/mixing strongly affects the distribution of analytes throughout the sample. In cases of fast analyte adsorption, we argue that the use of large-dilution factors, a common approach for sample preparation in surface-enhanced Raman spectroscopy (SERS), may result in an extreme nonuniformity of the surface coverage. This has a direct effect on the aggregation state of the colloidal solution and therefore on the overall SERS signal. Explicitly, we show that the average SERS signal obtained from typical dyes in colloidal solutions can be drastically different for two seemingly equivalent samples, differing only in the method by which the dye molecules were diluted. We, in addition, discuss the implications of such nonuniformity on the statistics of SERS intensities in the context of single-molecule detection. These results vividly highlight the importance of the dilution step in any experiments involving surface-adsorbing species and position SERS as an ideal tool to evidence such effects. In such cases, a simple half-half dilution procedure should be adopted as the standard method to mitigate these effects.


Assuntos
Coloides/química , Análise Espectral Raman , Adsorção , Difusão , Modelos Teóricos , Nanopartículas/química , Reprodutibilidade dos Testes , Soluções , Propriedades de Superfície
13.
J Am Chem Soc ; 135(7): 2809-15, 2013 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-23384115

RESUMO

Single-molecule (SM) electrochemistry studied by surface-enhanced Raman scattering (SERS) with high spectral resolution reveals a picture in which the frequency of Raman modes is correlated with the electrochemical process through the interaction with the surface. Previously unexplored phenomena can be revealed by the synergy of electrochemistry and SM-SERS, which explores in this case subtler spectroscopic aspects (like the frequency of a vibration within the inhomogeneous broadening of a many-molecules Raman peak) to gain the information. We demonstrate, among other things, that the interaction with the surface is correlated both with the molecule vibrational frequencies and with the ability of single molecules to be reduced/oxidized at different potentials along the electrochemical cycle. Qualitative models of the interaction of molecules with surfaces are also touched upon.

14.
Phys Chem Chem Phys ; 15(12): 4233-42, 2013 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-23358525

RESUMO

This work aims to provide simple and accurate closed-form approximations to predict the scattering and absorption spectra of metallic nanospheres and nanoshells supporting localised surface plasmon resonances. Particular attention is given to the validity and accuracy of these expressions in the range of nanoparticle sizes relevant to plasmonics, typically limited to around 100 nm in diameter. Using recent results on the rigorous radiative correction of electrostatic solutions, we propose a new set of long-wavelength polarizability approximations for both nanospheres and nanoshells. The improvement offered by these expressions is demonstrated with direct comparisons to other approximations previously obtained in the literature, and their absolute accuracy is tested against the exact Mie theory.

15.
Anal Chem ; 84(18): 7938-45, 2012 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-22894881

RESUMO

A simple method using standard spectrometers with charge-coupled device (CCD) detectors is described to routinely measure background-corrected spectra in situations where the signal is composed of weak spectral features (such as Raman peaks or absorption lines) engulfed in a much stronger (by as much as ∼10(5)) broad background. The principle of the method is to subtract the dominant fixed-structure noise and obtain a shot-noise limited spectrum. The final noise level can therefore be reduced as desired by sufficient integration time. The method requires multiple shifts of the diffraction gratings to extract the pixel-dependent noise structure, which is then used as a flat-field correction. An original peak-retrieval procedure is proposed, demonstrating accurate determination of peak lineshapes and linewidths and robustness on practical examples where conventional methods would not be applicable. Examples are discussed to illustrate the potential of the technique to perform routine resonant Raman measurements of fluorescent dyes with high quantum yield, using conventional Raman systems. The method can equally be applied to other situations where small features are masked by a broad overwhelming background. An explicit example is given with the measurement of weak absorption lines in atmospheric gases.

16.
Anal Chem ; 84(11): 5074-9, 2012 Jun 05.
Artigo em Inglês | MEDLINE | ID: mdl-22571823

RESUMO

Resonant Raman (RR) spectroscopy, despite its many promising applications in analytical chemistry and biology, remains an experimental challenge (compared to standard Raman) primarily because of the presence of large fluorescence backgrounds overwhelming the RR signals. The observation of RR spectra of fluorophores therefore requires the use of specialized, picosecond-time-resolved setups. Here, we present and demonstrate a method, based on polarization-difference, by which RR spectra and cross sections can be measured using the most standard Raman setup with continuous wave excitation and CCD-based detection. The method is applied to the dyes Nile Blue and rhodamine 6G under resonant excitation. This work should open a new era in RR spectroscopy, where RR spectra can be routinely measured and studied with conventional Raman systems.

17.
Phys Chem Chem Phys ; 14(9): 3219-25, 2012 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-22286367

RESUMO

Single-molecule Surface-Enhanced Raman Scattering (SERS) detection of buckminsterfullerene (C(60)) is achieved by using different isotopologues of the molecule with a distribution around an average isotopic substitution ((12)C → (13)C) of ~30%. The distribution of different isotopologues creates a broad (~20 cm(-1)) average SERS signal within which single-molecule SERS spectra of individual isotopic realizations of the molecule can be distinguished. The SERS enhancement factors for SM-SERS C(60) events are typically in the range of ~10(8), suggesting a limitation imposed by either photobleaching or surface interactions with the (Ag) metallic colloids to reach the highest SERS hot-spots (which can typically have larger maximum enhancements). SM-SERS signals of isotopically substituted C(60) also show broader peaks (FWHM ≈ 4 cm(-1)) than equivalent signals in natural C(60). The latter feature suggests a contribution to the homogeneous broadening coming from isotopic disorder in the molecule; a feature that can only be observed with the ability to detect single-molecule spectra.

18.
Annu Rev Phys Chem ; 63: 65-87, 2012.
Artigo em Inglês | MEDLINE | ID: mdl-22224704

RESUMO

A general overview of the field of single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) as it stands today is provided. After years of debates on the basic aspects of SM-SERS, the technique is emerging as a well-established subfield of spectroscopy and SERS. SM-SERS is allowing the observation of subtle spectroscopic phenomena that were not hitherto accessible. Examples of the latter are natural isotopic substitutions in single molecules, observation of the true homogeneous broadening of Raman peaks, Raman excitation profiles of individual molecules, and SM electrochemistry. With background examples of the contributions produced by our group, properly interleaved with results by other practitioners in the field, we present some of the latest developments and promising new leads in this new field of spectroscopy.

19.
J Phys Chem A ; 116(3): 1000-7, 2012 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-22175443

RESUMO

A novel hybrid spectroscopic technique is proposed, combining surface plasmon resonance (SPR) with surface-enhanced Raman scattering (SERS) microscopy. A standard Raman microscope is modified to accommodate the excitation of surface plasmon-polaritons (SPPs) on flat metallic surfaces in the Kretschmann configuration, while retaining the capabilities of Raman microscopy. The excitation of SPPs is performed as in standard SPR-microscopy; namely, a beam with TM-polarization traverses off-axis a high numerical aperture oil immersion objective, illuminating at an angle the metallic film from the (glass) substrate side. The same objective is used to collect the full Kretschmann cone containing the SERS emission on the substrate side. The angular dispersion of the plasmon resonance is measured in reflectivity for different coupling conditions and, simultaneously, SERS spectra are recorded from Nile Blue (NB) molecules adsorbed onto the surface. A trade-off is identified between the conditions of optimum coupling to SPPs and the spot size (which is related to the spatial resolution). This technique opens new horizons for SERS microscopy with uniform enhancement on flat surfaces.

20.
Nano Lett ; 11(11): 5013-9, 2011 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-21985399

RESUMO

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.


Assuntos
Algoritmos , Misturas Complexas/análise , Misturas Complexas/química , Teste de Materiais/métodos , Ressonância de Plasmônio de Superfície/métodos
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