Competition between Molecular Adsorption and Diffusion: Dramatic Consequences for SERS in Colloidal Solutions

Darby, Brendan; Ru, Eric C. Le

doi:10.1021/ja506361d

Cited by 77 publications

(95 citation statements)

References 53 publications

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“…Chemical adsorption can be greatly affected by dilution factors and molecular diffusion, producing inconsistent labeling on silver colloids and leading to the quasi single-molecule-like behavior observed in Figure 1C. 51 Moreover, silver nanoparticles are susceptible to the formation of a shell of silver oxide on the surface as they age. 52 Silver oxide is less conductive than silver metal and may prevent the nanoparticle from acting as an electrode for the NB redox process.…”

Section: ■ Resultsmentioning

confidence: 99%

Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

2015

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Super-resolution surface-enhanced Raman scattering (SERS) is used to investigate local surface potentials on plasmonic gold and silver colloidal aggregates using the redoxactive reporter molecule, Nile Blue A. This molecule is electrochemically modulated between an oxidized emissive state and a reduced dark state. The diffraction-limited SERS emission from Nile Blue on the surface of a single plasmonic nanoparticle aggregate is fit to a two-dimensional Gaussian to track the position of the emission centroid as a function of applied potential. Potential-dependent centroid positions are observed, consistent with molecules experiencing site-specific oxidation and reduction potentials on the nanoparticle electrode surface. Correlated structural analysis performed with scanning electron microscopy reveals that molecules residing in nanoparticle junction regions, or SERS hot spots, appear to be reduced and oxidized at the most negative applied potentials as the potential is cycled. F rom light harvesting 1,2 to trace chemical sensing 3−10 and electrocatalysis, 11−13 plasmonic nanoparticles represent a promising material with a diverse array of applications that are highly dependent upon nanoparticle geometry. Although ensemble studies can elucidate information about a population of nanoparticles, these studies often overlook how subtle differences in nanoparticle structure can influence their properties. To develop a more complete nanoscale understanding of the effect of nanoparticle geometry upon optical or other behaviors, our group and others have used correlated structural and super-resolution optical studies to study molecular interactions with plasmonic substrates. 14−28 In recent work, we used this approach to study the redox reaction of Nile Blue A (NB) at the surface of aggregated silver nanoparticles using surface-enhanced Raman scattering (SERS) as a reporter of the redox state of the molecule. 19 Although the oxidized form of NB is resonant with 642 nm laser excitation and produces strong SERS signal, the reduced form produces little to no SERS. 11,29−31 Thus, NB acts as a simple on/off probe, and potential cycling allows us to move from the ensemble to single-molecule regime with ease. Our earlier studies using super-resolution imaging to localize the SERS emission as the applied potential was modulated suggested that NB molecules in different locations on an aggregated nanoparticle structure experienced different local potentials, indicating a relationship between local redox chemistry and nanoparticle structure. 19

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Section: ■ Resultsmentioning

confidence: 99%

Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

2015

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“…Each set of measurements requires a reference solution. Reference samples were prepared by first mixing 500 µL of Ag colloid solution with 500 µL of 2 mM KCl to displace the citrate capping layer (without inducing aggregation [49]). This solution was then mixed with 1 mL of MilliQ water and the resulting 2 mL (with a final colloid concentration of 8 pM) was pipetted into a cylindrical glass tube.…”

Section: Methodsmentioning

confidence: 99%

Modified optical absorption of molecules on metallic nanoparticles at sub-monolayer coverage

Darby¹,

Auguié²,

Meyer³

et al. 2015

Nature Photon

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138

192

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Enhanced optical absorption of molecules in the vicinity of metallic nanostructures is key to a number of surface-enhanced spectroscopies and of great general interest to the fields of plasmonics and nano-optics. Yet, experimental access to this absorbance has long proven elusive. We here present direct measurements of the intrinsic absorbance of dye-molecules adsorbed onto silver nanospheres, and crucially, at sub-monolayer concentrations where dye-dye interactions become negligible. With a large detuning from the plasmon resonance, distinct shifts and broadening of the molecular resonances reveal the intrinsic properties of the dye in contact with the metal colloid, in contrast to the often studied strong-coupling regime where the optical properties of the dye-molecules cannot be isolated. The observation of these shifts together with the ability to routinely measure them has broad implications in the interpretation of experiments involving resonant molecules on metallic surfaces, such as surface-enhanced spectroscopies and many aspects of molecular plasmonics.Over the last two decades, the optical properties of metallic nanoparticles (NPs) of various sizes and shapes [1] [18][19][20]. Many of these existing and emerging applications are underpinned by the fact that the optical (electronic) absorption of molecules on the surface of metallic NPs is enhanced. But spectral changes induced by molecular adsorption are often ignored because of the experimental challenge of measuring surface absorbance spectra on nanoparticles, despite early attempts more than 30 years ago [21].This question is not directly addressed in the great number of recent studies devoted to the topic of strongcoupling between plasmons and molecules [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]; in this regime, the plasmon-molecule interaction is evidenced by a typical anti-crossing of the two resonances as a function of detuning [25,33], but in such a strongly interacting system the molecular response cannot be isolated. Moreover, in such studies the dye concentration is often large (typically monolayer coverage and above) to maximize dye/plasmon interactions. Dye-dye interactions cannot therefore be neglected and are expected to induce resonance shifts of the dye layer independently of any plas- * Eric.LeRu@vuw.ac.nz monic effects; in fact many studies specifically work with J-aggregates rather than isolated dyes [22, 25-27, 31, 33]. As a result, the intrinsic effect (chemical and/or electromagnetic) of the NP on an isolated adsorbed molecule cannot be elucidated. To this aim, we will show that it is necessary to measure the absorbance of the dye when the dye and plasmon resonances are detuned (to avoid dye-plasmon interaction effects) and at low surface coverage (to avoid dye-dye interaction effects). This low surface coverage poses a significant experimental challenge because the dye absorbance is then very small and obscured by the large optical response (absorption and scattering) of the NPs.We here propose to measure NP/d...

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“…For Au/Ag colloidal NPs to achieve higher SERS activities, high-density "hot spots" with gaps of <10 nm between the Au/Ag-NPs are required; therefore, high-concentration Au/Ag colloidal NPs are needed. However, the high-concentration of NPs can not only waste more of the noble metal but also cause unwanted aggregation that may decrease the "hot spots" density and lead to non-uniform distribution of the "hot spots" [27,43,44]. Thus, fixing "hot spots" in the micro-channels has attracted much attention.…”

Section: Nanostructure Colloids As Sers-sensitive Elementsmentioning

confidence: 99%

Review of microfluidic approaches for surface-enhanced Raman scattering

Zhou

Kim

2016

Sensors and Actuators B: Chemical

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Competition between Molecular Adsorption and Diffusion: Dramatic Consequences for SERS in Colloidal Solutions

Cited by 77 publications

References 53 publications

Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

Modified optical absorption of molecules on metallic nanoparticles at sub-monolayer coverage

Review of microfluidic approaches for surface-enhanced Raman scattering

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