Observation and Analysis of Blinking Surface-enhanced Raman Scattering

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on ...

Kwansei Gakuin University

In blinking surface-enhanced Raman scattering (SERS), probability distributions of the bright and dark events against their duration times are reproduced by a power-law without and with an exponential function, respectively. The truncation at the tail of the power-law suggests not only a potential well but also an energy barrier during a single molecule optical trapping onto the junction.

Power-law statistics in blinking SERS of thiacyanine adsorbed on a single silver nanoaggregate.

In blinking SERRS, probability distributions of the dark events against their duration were mostly reproduced by a power-law with the same exponential function under various excitation laser intensities, although they were given by a power-law with the different exponential functions at other excitation wavelengths.

Analysis of excitation laser intensity dependence of blinking SERRS of thiacarbocyanine adsorbed on single silver nanoaggregates by using a power law with an exponential function.

Blinking statistics in surface-enhanced Raman scattering (SERS) of thiacyanine or thiacarbocyanine adsorbed on single Ag nanoaggregates were analyzed by a power law. A power law reproduces the probability distributions of both the bright and dark SERS occurrences against their duration times. As the localized surface plasmon resonance (LSPR) wavelength of a single Ag nanoaggregate approached the excitation wavelength or the excitation laser intensity increases, the power-law exponents were close to -1.5, a value derived from a one-dimensional random walk model. When the LSPR wavelength left the excitation wavelength or the excitation laser intensity decreases, the power-law exponents deviated from -1.5. The decrease in the power-law exponents in the bright SERS, which indicates a decrease in the probabilities of the long-lived bright SERS, and the increase in the power-law exponents in the dark SERS coincide with the increasing shallowness and narrowing of a optical trapping potential well due to a surface-plasmon-enhanced electromagnetic field around a junction of the Ag nanoaggregates excited at a wavelength apart from the LSPR wavelength or under the low laser intensity, i.e., the low original electromagnetic field, respectively.

Power-law analysis of surface-plasmon-enhanced electromagnetic field dependence of blinking SERS of thiacyanine or thiacarbocyanine adsorbed on single silver nanoaggregates.

On an AgNO3 crystal, an equilateral or a right-angle triangle-shaped Ag trimer was selectively fabricated through near-field photo-reduction and observed in situ by using an apertured cantilever coupled with an atomic force microscope. By using the different triangle-shaped Ag trimers, irradiation wavelength and polarization dependence of surface-enhanced Raman scattering were investigated.

Plasmonic properties of triangle-shaped silver trimers selectively fabricated by near-field photo-reduction using an apertured cantilever for an atomic force microscope.

Surface-enhanced resonance Raman scattering (SERRS) spectra of myoglobin (Mb) with various ligands were measured. In the resonance Raman scattering (RRS) spectra, peaks at around 1610 and 1640 cm(-1) have so far been used to discriminate between the heme iron in a high or low spin state. In the SERRS spectra, however, the spin state cannot be distinguished by the corresponding peaks. Alternatively, the intensity ratio of the SERRS peak at 1560 cm(-1) to that at 1620 cm(-1) was applied to detect the spin states sensitively (1.5 × 10(5) times compared with the RRS); namely, a high ratio was obtained from met-Mb in the high spin state at pH ≤ 7 except for in a strong acid solution. The different marker bands between the SERRS and RRS spectra may be due to the enhancement order from the surface selection rule.

Sensitive marker bands for the detection of spin states of heme in surface-enhanced resonance Raman scattering spectra of metmyoglobin.

For single colloidal Ag nanoaggregates, covered with either large or small amounts of citrate anions, blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine was measured and analyzed by a truncated power law. The power law without and with an exponential function reproduces a probability distribution for bright and dark SERS events versus their duration times, respectively. On the Ag surface, except for junctions of the nanoaggregate with a large or small amount of the citrate anions, two-dimensional fast or one-dimensional slow random walk of the anionic thiacyanine, respectively, was estimated by the exponents and the truncation times in the power law for the dark SERS events. In addition, the power law exponents for the bright SERS events were derived to be of similar values, indicating a similar molecular random walk near the junction, which may be dominated evenly by a surface-plasmon-enhanced electromagnetic field on the same-sized Ag nanoaggregate. Thus, not only the bright SERS, but also the dark SERS molecular behaviour on the Ag surface was investigated by the truncated power law analysis.

Different behaviour of molecules in dark SERS state on colloidal Ag nanoparticles estimated by truncated power law analysis of blinking SERS.

In our SERRS spectra of metmyoglobin by excitation at 514 nm, the peak at 1510 cm(-1), which is assigned to the 6-coordinated heme in the low spin state, was observed by the addition of imidazole and NaN3. Thus, the SERRS likely originates not from the non-native 5-coordinated heme, which is in the high spin state.

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In this review article, we discuss surface-enhanced resonance Raman scattering (SERRS) studies of hemeproteins such as myoglobin, hemoglobin, and cytochrome on various metal substrates; for example, colloidal silver nanoparticles coated with and without self-assembled monolayers (SAM), a roughened silver electrode protected with and without SAM, a sharp silver tip, and colloidal gold nanoparticles coated with and without SAM. Moreover, we classify the studies in terms of an excitation wavelength; namely, excitation at the B- (Soret) band, Q- (α and β) band, and in the near infrared (NIR) range. In the SERRS studies with B band excitation, it has been shown that the hemeprotein on a silver surface takes a non-native form through detachment from the heme pocket in the protein. With Q band excitation, on the other hand, the change in SERRS has been explained by the orientation of the hemeprotein on the surface. Even by excitation in the NIR range, the peak positions are consistent with the assignment of the major vibrational modes of heme despite there being no resonance Raman effect. Thus, the SERRS of hemeproteins is influenced by a resonance Raman effect, LSPR, and interactions with the metal surface such as structural changes, orientation, and selective adsorption. Moreover, we discuss how SERRS has been applied to complicated biological systems such as living cells containing hemeprotein. For mitochondria, a change of the oxidation-state was observed by the electron transport chain in the cell and at different positions. As an example of a biomedical application of SERRS, the sensitive detection of malaria is presented.

Surface-enhanced resonance Raman scattering of hemoproteins and those in complicated biological systems.

Surface-enhanced hyper-Raman scattering (SEHRS) spectra were obtained at desired positions on yeast by focusing a continuous wave near-infrared laser beam while silver nanoparticles (AgNPs) were simultaneously optically trapped. However, the optically trapped colloidal AgNP suspension bubbled up at the focusing point, preventing spectral measurement. In the case of optically trapped AgNPs functionalized with 4-mercaptobenzoic acid (p-MBA), surface-enhanced hyper-Rayleigh scattering was considerably strong, indicating the suppression of the photothermal conversion to form the bubble. Interestingly, the SEHRS peaks that are attributed not only to p-MBA, but also to other species, were very occasionally observed. They may be partly assigned to the β1,3 glucan and protein amide II band. The SEHRS peak at 1366 cm-1 was barely visible in the measurements of conventional baker's yeast even in the suspension (pH 9) despite the effects of high pH on p-MBA. In contrast, the SEHRS peak in the measurements of yeast for biological applications was occasionally observed at 1366 cm-1. This suggests that acidity is correlated with fermentation efficiency. At different positions on single yeast cells, the intensity of the SEHRS peak at 1366 cm-1 varied. This result represents the pH distribution on yeast.

Measurement of pH-dependent surface-enhanced hyper-Raman scattering at desired positions on yeast cells via optical trapping.

In this study, a lycopene-content-based discriminant analysis was performed using a portable near-infrared-excited Raman spectrometer. In the vegetable-juice Raman spectra, the peak intensity of the lycopene band increased with increasing lycopene concentration, but scattering decreased the repeatability of the peak intensity. Consequently, developing a lycopene-concentration regression model using peak intensity is not straightforward. Therefore, a new method known as the product of mean intensity ratio (PMIR) analysis was developed to rapidly identify lycopene-rich samples on-site. In the PMIR analysis, Raman spectra are measured with short exposure times, confirming only the peaks of carotenoids with high concentrations, and thus the lycopene concentrations of vegetable juice samples could be determined successfully. Exposure times of 20ms and 100ms could detect lycopene concentrations of ≥5mg/100g and ≥2mg/100g with 93.2% and 97.7% accuracy, respectively; thus, lycopene-content-based discriminant analysis using the PMIR and a portable Raman spectrometer is feasible.

Use of the product of mean intensity ratio (PMIR) technique for discriminant analysis of lycopene-rich vegetable juice using a portable NIR-excited Raman spectrometer.

Yasutaka Kitahama

Department of Chemistry

Kwansei Gakuin University

Observation and Analysis of Blinking Surface-enhanced Raman Scattering

Yasutaka Kitahama

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Chemistry

Kwansei Gakuin University

Observation and Analysis of Blinking Surface-enhanced Raman Scattering

Department of Chemistry