Single molecules

Stipe B C, Rezaei M A and Flo W 1998 Single-molecule vibrational spectroscopy and microscopy Science 280 1732... 

Stipe B C, Rezaei M A, Flo W, Gao S, Persson M and Lundqvist B I 1997 Single-molecule dissociation by tunneling electrons Rhys. Rev. Lett. 78 4410... 

Many of the most interesting current developments in electronic spectroscopy are addressed in special chapters of their own in this encyclopedia. The reader is referred especially to sections B2.1 on ultrafast spectroscopy. Cl.5 on single molecule spectroscopy, C3.2 on electron transfer, and C3.3 on energy transfer. Additional topics on electronic spectroscopy will also be found in many other chapters. 

At the linear level, the microscopic induced dipole vector on a single molecule in the local Cartesian... 

Nie S and Emory S R 1997 Probing single molecules and single nanoparticles by surface-enhanced Raman scattering Science 275 1102-6... 

Kneipp K, Wang Y, Kneipp H, Itzkan I, Dasari R R and Feld M S 1996 Approach to single molecule detection using surface-enhanced Raman scattering ICORS 98 XVth Int. Conf on Raman Spectroscopy ed S A Asher and P B Stein (New York Wley) pp 636-7... 

An important point for all these studies is the possible variability of the single molecule or single particle studies. It is not possible, a priori, to exclude bad particles from the averaging procedure. It is clear, however, that high structural resolution can only be obtained from a very homogeneous ensemble. Various classification and analysis schemes are used to extract such homogeneous data, even from sets of mixed states [69]. In general, a typical resolution of the order of 1-3 mn is obtained today. 

Ribbe A E 1997 Laser scanning confocal microscopy in polymer science Trends Polym. Sc/. 5 333-7 Oliveira M J and Hemsiey D A 1996 Optical microscopy of polymers Sonderb. Prakt. Metallogr. 27 13-22 Nie Sh and Zare R N 1997 Optical detection of single molecules Ann. Rev. Biophys. Biomol. Struct. 26 567-96 Masters B R 1994 Confocal redox imaging of cells Adv. Mol. Cell Biol. 8 1-19... 

Kasas S, Thompson N FI, Smith B L, Flansma P K, Miklossy J and Flansma FI G 1997 Biological applications of the AFM from single molecules to organs Int. J. Im. Syst. Technol. 8 151... 

Betzig E and Chichester R J 1993 Single molecules observed by near-field scanning optical microscopy Science 262 1422... 

Trautman J K, Macklin J J, Brus L E and Betzig E 1994 Near-field spectroscopy of single molecules at room temperature Nature 369 40... 

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. 

Figure Bl.22.11. Near-field scanning optical microscopy fluorescence image of oxazine molecules dispersed on a PMMA film surface. Each protuberance in this three-dimensional plot corresponds to the detection of a single molecule, the different intensities of those features being due to different orientations of the molecules. Sub-diffraction resolution, in this case on the order of a fraction of a micron, can be achieved by the near-field scaiming arrangement. Spectroscopic characterization of each molecule is also possible. (Reprinted with pennission from [82]. Copyright 1996 American Chemical Society.)...

Moerner W E 1996 Fligh-resolution optical spectroscopy of single molecules in solids Acc. Chem. Res. 29 563-71... 

Xie X S 1996 Single-molecule spectroscopy and dynamics at room temperature Acc. Chem. Res. 29 598-606... 

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). 

The approach to, and finally tire achievement of, detection and spectroscopy of single molecules proceeded almost independently from tliree separate directions. 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Figure Cl.5.2. Fluorescence excitation spectra (cps = counts per second) of pentacene in /i-teriDhenyl at 1.5 K. (A) Broad scan of the inhomogeneously broadened electronic origin. The spikes are repeatable features each due to a different single molecule. The laser detuning is relative to the line centre at 592.321 nm. (B) Expansion of a 2 GHz region of this scan showing several single molecules. (C) Low-power scan of a single molecule at 592.407 nm showing the lifetime-limited width of 7.8 MHz and a Lorentzian fit. Reprinted with pennission from Moemer [198]. Copyright 1994 American Association for the Advancement of Science.

Figure Cl.5.3. Near-field fluorescence image 4.5 p.m square) of single oxazine 720 molecules dispersed on die surface of a PMMA film. Each peak (fwhm 100 nm) is due to a single molecule. The different intensities are due to different molecular orientations and spectra. Reprinted widi pennission from Xie 11221. Copyright 1996 American Chemical Society.

Figure Cl.5.4. Comparison of near-field and far-field fluorescence images, spectra and lifetimes for the same set of isolated single molecules of a carbocyanine dye at a PMMA-air interface. Note the much higher resolution of the near-field image. The spectmm and lifetime of the molecule indicated with the arrow were recorded with near-field excitation and with far-field excitation at two different excitation powers. Reproduced with pennission from Trautman and Macklin [125].

It now seems clear tliat, under certain conditions, massive enhancements of what is nonnally a very weak process can be achieved. The ability to obtain vibrational spectra would be a great advance in tlie characterization of single molecules if metliods could be found to reproducibly observe all molecules in a sample, not only tliose tliat happen to bind to special sites on tlie colloid. 

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for cliromophores in cryogenic solids [60], room-temperature surfaces [61], membranes [62] and liquids [63, 64 and 65]. Altliough multiphoton excited fluorescence has been embraced witli great entluisiasm as a teclmique for botli ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe tlian witli ordinary linear excitation, probably due to absorjDtion of more tlian tire desired number of photons from tire intense laser pulse (e.g. triplet excited state absorjDtion) [61],... 

Studies of single molecules in cryogenic solids, while limited to a relatively small number of cliromophore/ matrix combinations (and small variations tliereupon), have covered a wide range of spectroscopic and dynamic processes [66,62]. 

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