Single molecule measurements

He J, Sankey O, Lee M, Tao N, Li X, Lindsay S (2006) Measuring single molecule conductance with break junctions. Faraday Discuss 131 145-154... 

Furthermore, novel methods for single molecule detection will contribute to improved drug discovery. For instance, fluorescence correlation spectroscopy (FCS) is characterized by the ability to measure single molecules in sub-micro liter sample volumes with a typical range of sensitivity between 10 6 and... 

Figure 7-20. An illustration of the conducting AFM developed by Cui et al. for measuring single molecule conductance.

Tire acceleration of sequencing is being addressed by several approaches to measuring single molecules. A variety of different DNA polymers were electrophoretically driven... 

Why is it necessary to use repetitive excitation pulse sequences to measure single-molecule fluorescence properties ... 

In the second type of experiment that measures single molecule spectral dynamics one performs repeated fluorescence excitation scans of the same molecule. In each scan the line shape is described as above, but now there is the possibility that the center frequency of the line will change from scan to scan because of slow fluctuations. Thus one can measure the center frequency as a function of time, producing what has been called a spectral diffusion trajectory. This trajectory can, in principle, be characterized completely by the spectral diffusion kernel of Eqs. (16) and (19), but of course it must be understood that only the slow Kj < 1 /t) TLSs contribute. In fact, the experimental trajectories are really too short to be analyzed with this spectral diffusion kernel. Instead, it is useful [11, 12] to consider three simpler characterizations of the spectral diffusion trajectories the frequency-frequency correlation function in Eq. (14), the distribution of frequencies from Eq. (15), and the distribution of spectral jumps from Eq. (21). For this application of the theoretical results, in all three of these formulas j should be replaced by s, the labels for the slow TLSs. 

The quantum efficiency for photobleaching, b, may be estimated from the measured single molecule detection rate, Rf, the collection and detection efficiency, D, the fluorescence quantum yield, f, and the bum time, xb/ = x D x tb). 

Li, HT, Ren, XJ, Iflng, LM, Balasubramanian, S, and Klenerman, D, Measuring single-molecule nucleic acid dynamics in solution by two-color filtered ratiometric fluorescence correlation spectroscopy. Proceedings of the National Academy of Sciences of the United States of America 101 (2004) 14425-14430. 

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

A wide variety of measurements can now be made on single molecules, including electrical (e.g. scanning tunnelling microscopy), magnetic (e.g. spin resonance), force (e.g. atomic force microscopy), optical (e.g. near-field and far-field fluorescence microscopies) and hybrid teclmiques. This contribution addresses only Arose teclmiques tliat are at least partially optical. Single-particle electrical and force measurements are discussed in tire sections on scanning probe microscopies (B1.19) and surface forces apparatus (B1.20). 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is tire fitted decay, a single exponential of 480 5 ps convolved witli tire instmment response function of 160 ps fwiim. The decay, which is considerably faster tlian tire natural fluorescence lifetime of cresyl violet, is due to electron transfer from tire excited cresyl violet (D ) to tire conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted witli pennission from Lu andXie [1381. Copyright 1997 American Chemical Society.

Wilkerson C W Jr, Goodwin P M, Ambrose W P, Martin J C and Keller R A 1993 Detection and lifetime measurement of single molecules in flowing sample streams by laser-induced fluorescence Appl. Phys. Lett. 62 2030-2... 

Pirotta M, Guttler F, Gygax FI, Renn A, Sepiol J and Wild U P 1993 Single molecule spectroscopy fluorescence lifetime measurements of pentacene in p-terphenyl Chem. Phys. Lett. 208 379-84... 

Measurements on copper) I) chloride show the vapour to be the dimer of formula CU2CI2, but molecular weight determinations in certain solvents such as pyridine show it to be present in solution as single molecules, probably because coordination compounds such as py -> CuCl (py = pyridine) are formed. 

The ratio 0/0 is thus a measure of the enhancement of the energy of adsorption in a micropore as compared with that on an open surface. In curve (i) of Fig. 4.9 this ratio is plotted as a function of d/r and, as is seen, the enhancement is still appreciable when d = l-Sr, but has almost disappeared when d = 2r , i.e. when the slit is only two molecular diameters wide. Even when d/r = 1, which corresponds to a single molecule tightly packed into the width of the slit, the enhancement is only 1-6-fold. The effect... 

The internal field is that microwave field which is generally the object for solution when MaxweU s equations are appUed to an object of arbitrary geometry and placed in a certain electromagnetic environment. The is to be distinguished from the local field seen by a single molecule which is not necessarily the same (22). The dielectric permittivity as a function of frequency can be described by theoretical models (23) and measured by weU-developed techniques for uniform (homogeneous) materials (24). 

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