Guest Commentary pubs.acs.org/JPCL
Recent Advances in Novel Imaging Modalities
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continuing improvement in acoustic transducers, it has now become possible to design and execute spectroscopic imaging of deep tissue using photoacoustics, as Cheng and co-workers describe in their Perspective. They demonstrate the ability to chemically identify photoacoustic signal through excitation of specific molecular vibrations. Thus, the tracking of pathological changes in, for example, blood vessel walls (i.e., formation of plaques) becomes as real possibility. Some technological challenges remain with respect to endoscopic application of the photoacoustic technology in humans, but these steps toward that goal are a very significant achievement. It is challenging to label individual proteins and other subcellular molecules in a way that provides ease of subwavelength resolution imaging. Weiss’s group presents a Perspective that provides critical advances to achieve this reality. In their work, they exploit the stochastic photoblinking of fluorescence probes in a method coined as super-resolution optical fluctuation imaging (SOFI).7 They rely on high-order analysis of the fluorescence fluctuations, which provides superresolution information at a rate much faster than other superresolution techniques. Thus, subcellular dynamics has the potential to be measured with unprecedented high spatiotemporal resolution. They discuss innovation for the delivery of appropriate probes for SOFI applications. Taken together, the advances in optical imaging described in these three Perspectives represent the cutting edge of optical imaging. They provide examples of technology that will both enhance the fundamental understanding of interfaces (Bachelot Perspective) and be applied to medical pathologies (Cheng Perspective). Finally, with increasing advances in the SOFI technique (Weiss Perspective), there are new insights into the functioning of pathways of single cells on the near horizon.
irect visualization of the natural world has always been a mainstay of science. It is critical to know where the dancers are during the performance. Robert Hooke’s “Micrographia” in 16651 reported astounding imagery only made available through the optical magnification of microscopy. Optical microscopy remained largely unchanged until the advent of two technical breakthroughs. Early in the 20th century, fluorescence microscopy was applied to imaging cells, and with it, came the possibility of tagging individual cellular components with fluorescent dyes.2 This lead eventually to the application of laser excitation of fluorophores and thus the optical sectioning of samples also known as confocal microscopy.3 Subsequently, the technological advances in tagging targets of interest and in so-called super-resolution microscopy have been breathtaking. Fluorescence protein technology has provided scientist with the ability to follow the location and dynamics of individual proteins with subcellular resolution. Additionally, optical and photophysical advances have allowed imaging with spatial resolution that is greater than the limit of light diffraction. Recently, techniques such as stimulated mission depletion (STED) microscopy have been used in imaging of biological samples4 and in optical nanolithography.5 The chemical information content in optical microscopy increased as well. Using molecular vibrations as chemical fingerprints, coherent anti-Stokes Raman scattering (CARS) has been employed to image systems with chemical specificity.6 The following three Perspectives explore new developments in the imaging of complex electromagnetic fields using photochemistry (Plain, J.; Wiederrecht, G. P.; Gray, S. K.; Royer, P.; Bachelot, R. Multiscale Optical Imaging of Complex Fields Based on the Use of Azobenzene Nanomotors. J. Phys. Chem. Lett. 2013, 4, 2124−2132); photoacoustic microscopy with vibrational specificity and deep tissue penetration (Wang, P.; Rajian, J. R.; Cheng, J.-X. Spectroscopic Imaging of Deep Tissue through Photoacoustic Detection of Molecular Vibration. J. Phys. Chem. Lett. 2013, 4, 2177−2185); and higher information content and spatial resolution achieved by exploiting stochastic blinking of fluorescent tags (Xu, J.-M.; Chang, J.; Yan, Q.; Dertinger, T.; Bruchez, M.; Weiss, S. Labeling Cytosolic Targets in Live Cells with Blinking Probes. J. Phys. Chem. Lett. 2013, 4, 2138−2146). The Perspective presented by Bachelot and co-workers on optical imaging of complex electromagnetic fields using lightdriven changes in functionalized polymers describes advances in subwavelength imaging and in understanding the origin of electric fields on solid surfaces. They have developed a topographical transducer of electromagnetic fields based on light-induced cis−trans isomerization of azobenzene compounds impregnated in polymers that coat the surface of interest. Topographical changes are then measured using scanning probe technology. Their results “open up the door to nanolithography of complex structures”. Photoacoustic microscopy relies on local heating in the milliKelvin range to produce detectable sound waves. With the © 2013 American Chemical Society
David T. Cramb*
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Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary AB, Canada T2N 1N4
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
REFERENCES
(1) Hooke, R. Micrographia: Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses, 1st ed.; J. Martyn and J. Allestry: London, 1665. (2) Kasten, F. H. The Development of Fluorescence Microscopy up through World War II. In History of Staining, 3rd ed.; Clark, G., Kasten, F. H., Eds.; Williams & Wilkins: Baltimore, MD, 1983; pp 147−185. (3) Pawley, J. B., Ed. Handbook of Biological Confocal Microscopy, 3rd ed.; Springer: Berlin, Germany, 2006; ISBN 0-387-25921-X. (4) Hell, S. W. Far-Field Optical Nanoscopy. Science 2007, 316, 1153−1158. (5) Fourkas, J. T. Nanoscale Photolithography with Visible Light. J. Phys. Chem. Lett. 2010, 1, 1221−1227.
Published: July 3, 2013 2242
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The Journal of Physical Chemistry Letters
Guest Commentary
(6) Evans, C. L.; Xie, X. S. Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine. Annu. Rev. Anal. Chem. 2008, 1, 883−909. (7) Dertinger, T.; Colyer, R.; Iyer, G.; Weiss, S.; Enderlein, J. Fast, Background-Free, 3D Super-Resolution Optical Fluctuation Imaging (SOFI). Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 22287−22292.
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