Single-molecule SERS

Figure 10.3. Schematic experimental set-up for single-molecule SERS. Insert (top) shows a typical Stokes and anti-Stokes Raman spectrum. Insert (bottom) shows an electron microscope image of SERS-active colloidal clusters. (With permission from Ref. 21.)...

Figure 10.10. SEM micrographs of Ag nanoparticles. The images show (a) overview of the particles shapes and sizes, (b) Ag particle dimers after incubation in hemoglobin solution, and (c), (d) hot dimers and corresponding single-molecule SERS spectra. The double arrows in (c) and (d) indicate the polarization of the incident laser radiation. (With permission from Ref. 35.)...

Maruyama, Y., and Futamata, M. (2005). Elastic scattering and emission correlated with single-molecule SERS. J. Raman Spectrosc. 36 581-592. 

Vlckova B, Pavel I, Sladkova M, Slskova K, Slouf M (2007) Single molecule SERS perspectives of analytical applications. J Mol Struct 834-836 42 7... 

Otto A (2006) On the significance of Shalaev s hot spots in ensemble and single-molecule SERS by adsorbates on metallic films at the percolation threshold. J Raman Spectrosc 37 937... 

This changed with the observation of single-molecule SERS in 1997 by two independent studies by Nie and Emory [22] and Kneipp et al. [23], which sparked... 

Le Ru et al. demonstrated that a careful consideration of the minimum signal required for detection, the Raman cross section of the molecules involved, and the characteristics of the hot spot were required to determine the minimum enhancement factor necessary for single-molecule detection using SERS [95]. Through this process they concluded that a minimum enhancement factor of 2 x 10 was required for detection of single molecules with bare Raman cross sections of 10 cm sr This is 10 times smaller than the 10 " initially claimed to be essential for single-molecule SERS. The bare Raman cross section of many... 

But what about the mechanisms involved in single-molecule SERS The electromagnetic mechanism has been shown to theoretically provide enhancement factors up to 10 °-10 for LSPR hot spots. This would enable detection of both resonant and noiu-esonant single molecules, and Dieringer et al. recently confirmed that the electromagnetic mechanism is all that is required in order to explain singlemolecule sensitivity [97]. 

Vlckova et al. demonstrated single-molecule SERS in self-assembled dimers of silver nanoparticles [161]. The self-assembly process was driven by nanoparticle functionalization with a bifunctional molecular linker (4,4 -diaminoazobenzene) which forms a molecular bridge between the particles. Molecules forming the molecular bridge are then precisely positioned within the hot spot with the molecular population being controlled by careful optimization of the linker to... 

Etchegoin PG, Le Ru EC (2008) A perspective on single molecule SERS current status and future challenges. Phys Chem Chem Phys 10(40) 6079-6089... 

Otto A (2002) What is observed in single molecule SERS, and why J Raman Spectrosc 33(8) 593-598... 

Etchegoin PG, Meyer M, Le Ru EC (2007) Statistics of single molecule SERS signals is there a Poisson distribution of intensities Phys Chem Chem Phys 9(23) 3006-3010... 

Otto A, Bruckbauer A, Chen YX (2003) On the chloride activation in SERS and single molecule SERS. J Mol Struct 661-662 501-514... 

One issue in the comparison of theory and experiment for single molecule SERS is that the experiments have mostly been done for aggregates of particles. It is well known that fields can be enhanced at the junction between two closely spaced particles, so a key question is whether the additional field associated with small clusters of particles is... 

Because the use of colloidal silver or gold as a substrate for single molecule SERS, as observed with confocal microscopy, has met with some criticism (the chemically prepared colloids are exposed to the molecules to be adsorbed and are subsequently deposited onto a substrate, e.g. an ITO-coated glass slide during this... 

In February 1997, Nie and Emory were the first to report on Raman spectroscopy of single molecules adsorbed at optically selected nanoparticles [15]. For this system, giant enhancement factors of lO " to 10 had been estimated, which were substantially larger than the ensemble averaged enhancement factors. It was also important that the single-molecule SERS spectra were both more intense and more stable than single-molecule fluorescence spectra [15]. 

A. Otto, What Is Observed in Single Molecule SERS, and Why , ]. Raman Spec-trosc. 33, 593 (2002)... 

Much of the interest in SERS to date has focused on analytical applications. The technique has been used in a sensitive biosensor of glucose [72] and to assay lysophosphatidic acid, a biomarker for ovarian cancer [73]. With the advent of single-molecule SERS, biophysical studies comparable to the single-molecule fluorescence experiments described in Sect. 5.9 should be interesting. 

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