Brian G. Saar
Harvard University
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Featured researches published by Brian G. Saar.
Science | 2008
Christian W. Freudiger; Wei Min; Brian G. Saar; Sijia Lu; Gary R. Holtom; Chengwei He; Jason C. Tsai; Jing X. Kang; X. Sunney Xie
Label-free chemical contrast is highly desirable in biomedical imaging. Spontaneous Raman microscopy provides specific vibrational signatures of chemical bonds, but is often hindered by low sensitivity. Here we report a three-dimensional multiphoton vibrational imaging technique based on stimulated Raman scattering (SRS). The sensitivity of SRS imaging is significantly greater than that of spontaneous Raman microscopy, which is achieved by implementing high-frequency (megahertz) phase-sensitive detection. SRS microscopy has a major advantage over previous coherent Raman techniques in that it offers background-free and readily interpretable chemical contrast. We show a variety of biomedical applications, such as differentiating distributions of omega-3 fatty acids and saturated lipids in living cells, imaging of brain and skin tissues based on intrinsic lipid contrast, and monitoring drug delivery through the epidermis.
Science | 2010
Brian G. Saar; Christian W. Freudiger; Jay Reichman; C. Michael Stanley; Gary R. Holtom; X. Sunney Xie
Skin-Deep Raman Spectroscopy Raman spectroscopy allows for molecular identification via vibrational spectra at optical wavelengths. However, if the optical signal is scattered, as occurs when trying to image tissue, the signal becomes very weak, and it becomes difficult to image a sample with high time resolution. Saar et al. (p. 1368) now show that by improving the optics and electronics of the acquisition of the backscattered signal, stimulated Raman scattering spectroscopy can be performed at video rates on human skin, which should enable label-free studies of tissues and, for example, the tracking of the delivery of a drug. Raman spectra can be acquired rapidly from samples that would otherwise scatter the usable signal. Optical imaging in vivo with molecular specificity is important in biomedicine because of its high spatial resolution and sensitivity compared with magnetic resonance imaging. Stimulated Raman scattering (SRS) microscopy allows highly sensitive optical imaging based on vibrational spectroscopy without adding toxic or perturbative labels. However, SRS imaging in living animals and humans has not been feasible because light cannot be collected through thick tissues, and motion-blur arises from slow imaging based on backscattered light. In this work, we enable in vivo SRS imaging by substantially enhancing the collection of the backscattered signal and increasing the imaging speed by three orders of magnitude to video rate. This approach allows label-free in vivo imaging of water, lipid, and protein in skin and mapping of penetration pathways of topically applied drugs in mice and humans.
Optics Letters | 2006
Feruz Ganikhanov; Conor L. Evans; Brian G. Saar; X. Sunney Xie
We demonstrate a new approach to coherent anti-Stokes Raman scattering (CARS) microscopy that significantly increases the detection sensitivity. CARS signals are generated by collinearly overlapped, tightly focused, and raster scanned pump and Stokes laser beams, whose difference frequency is rapidly modulated. The resulting amplitude modulation of the CARS signal is detected through a lock-in amplifier. This scheme efficiently suppresses the nonresonant background and allows for the detection of far fewer vibrational oscillators than possible through existing CARS microscopy methods.
Optics Letters | 2011
Brian G. Saar; Richard S. Johnston; Christian W. Freudiger; X. Sunney Xie; Eric J. Seibel
Coherent Raman scattering methods allow for label-free imaging of tissue with chemical contrast and high spatial and temporal resolution. However, their imaging depth in scattering tissue is limited to less than 1 mm, requiring the development of endoscopes to obtain images deep inside the body. Here, we describe a coherent Raman endoscope that provides stimulated Raman scattering images at seven frames per second using a miniaturized fiber scanner, a custom-designed objective lens, and an optimized scheme for collection of scattered light from the tissue. We characterize the system and demonstrate chemical selectivity in mouse tissue images.
Molecular Pharmaceutics | 2011
Brian G. Saar; L. Rodrigo Contreras-Rojas; X. Sunney Xie; Richard H. Guy
Efficient drug delivery to the skin is essential for the treatment of major dermatologic diseases, such as eczema, psoriasis and acne. However, many compounds penetrate the skin barrier poorly and require optimized formulations to ensure their bioavailability. Here, stimulated Raman scattering (SRS) microscopy, a recently developed, label-free chemical imaging tool, is used to acquire high resolution images of multiple chemical components of a topical formulation as it penetrates into mammalian skin. This technique uniquely provides label-free, nondestructive, three-dimensional images with high spatiotemporal resolution. It reveals novel features of (trans)dermal drug delivery in the tissue environment: different rates of drug penetration via hair follicles as compared to the intercellular pathway across the stratum corneum are directly observed, and the precipitation of drug crystals on the skin surface is visualized after the percutaneous penetration of the cosolvent excipient in the formulation. The high speed three-dimensional imaging capability of SRS thus reveals features that cannot be seen with other techniques, providing both kinetic information and mechanistic insight into the (trans)dermal drug delivery process.
Laboratory Investigation | 2012
Christian W. Freudiger; Rolf Pfannl; Daniel A. Orringer; Brian G. Saar; Minbiao Ji; Qing Zeng; Linda Ottoboni; Wei Ying; Christian Waeber; John R. Sims; Philip L. De Jager; Oren Sagher; Martin A. Philbert; Xiaoyin Xu; Santosh Kesari; X. Sunney Xie; Geoffrey S. Young
Conventional histopathology with hematoxylin & eosin (H&E) has been the gold standard for histopathological diagnosis of a wide range of diseases. However, it is not performed in vivo and requires thin tissue sections obtained after tissue biopsy, which carries risk, particularly in the central nervous system. Here we describe the development of an alternative, multicolored way to visualize tissue in real-time through the use of coherent Raman imaging (CRI), without the use of dyes. CRI relies on intrinsic chemical contrast based on vibrational properties of molecules and intrinsic optical sectioning by nonlinear excitation. We demonstrate that multicolor images originating from CH2 and CH3 vibrations of lipids and protein, as well as two-photon absorption of hemoglobin, can be obtained with subcellular resolution from fresh tissue. These stain-free histopathological images show resolutions similar to those obtained by conventional techniques, but do not require tissue fixation, sectioning or staining of the tissue analyzed.
Angewandte Chemie | 2010
Brian G. Saar; Yining Zeng; Christian W. Freudiger; Yu San Liu; Michael E. Himmel; X. Sunney Xie; Shi You Ding
Research into alternative energy has experienced dramatic growth in recent years, which was motivated by both the environmental impact of current fossil fuels and the unstable and uncertain sources of oil and natural gas. Under ideal conditions, currently unused plant materials, such as agricultural residues, forestry wastes, and energy crops, can be broken down by a series of chemical, enzymatic, and/or microbiological processes into ethanol or other biofuel sources. Biofuels offer an infinitely renewable source of carbon-neutral fuels that can be produced domestically and can make use of waste products from agricultural activity already taking place. The major challenge to be overcome in the widespread adoption of many biofuels is that biomass is intrinsically recalcitrant, making conversion into usable fuels inefficient. This, in turn, means that substantial energy is required to produce the current generation of biofuels, thus decreasing or eliminating their advantages as alternative sources of fuel. The two major chemical species of interest in the biomass conversion process are lignins and polysaccharides such as cellulose and hemicelluloses. Lignins are partly responsible for biomass recalcitrance, but they may also have value as side products in the biorefineries of the future. Cellulose can be broken down to simple sugars, which can then be fermented to produce ethanol. To address the recalcitrance problem presented by lignins, a thermochemical pretreatment process is necessary in current biomass conversion technology. This process uses oxidizing, acidic, or basic conditions along with elevated pressures and/or temperatures to remove or modify lignins and hemicelluloses, thereby enhancing the accessibility for the cellulase enzymes used in the breakdown of cellulose. 6] To optimize the overall conversion efficiency, a detailed understanding of the hydrolysis kinetics of polysaccharides and lignins is critical. For this reason, analytical tools to study the biomass conversion process are needed. Herein, we demonstrate that stimulated Raman scattering (SRS) microscopy, a new imaging method, can offer new information on the biomass conversion processes. The ideal technique for studying the conversion process in situ should offer chemical specificity without exogenous labels, non-invasiveness, high spatial resolution, and real-time monitoring capability. Current analytical methods, such as gas chromatography–mass spectrometry, electron or scanning-probe microscopy, and fluorescence microscopy, cannot satisfy all of these requirements. Microscopy based on infrared absorption offers chemical specificity, but the spatial resolution is limited by the long infrared wavelengths, and penetration depth into aqueous plant samples is limited. Raman microspectroscopy is widely used because it offers label-free chemical contrast with high resolution and chemical specificity. However, the Raman scattering effect is weak, and long pixel dwell times (on the order of 0.1–1 s) are required for imaging plant materials. This means that real-time imaging is challenging, as even a 256! 256 pixel image would require almost two hours at 0.1 s/pixel. Consequently, the dynamic processes involved in the conversion cannot be followed at high spatiotemporal resolution. Coherent Raman microscopy techniques solve many of these problems and offer label-free chemical imaging with high sensitivity and high spatial resolution. Coherent antiStokes Raman scattering (CARS) microscopy is a technique that has been developed over the past ten years and applied to numerous problems of biological or biomedical relevance. However, CARS microscopy suffers from a nonresonant electronic background that can distort the chemical information of interest, making quantitative image interpretation challenging. Herein, we introduce stimulated Raman scattering (SRS) as a tool to study biomass conversion. SRS [*] B. G. Saar, Prof. X. S. Xie Department of Chemistry and Chemical Biology Harvard University, Cambridge, MA (USA) Fax: (+1)617-496-8709 E-mail: [email protected] Y. Zeng, Y. Liu, M. E. Himmel, S. Ding Biosciences Center, National Renewable Energy Laboratory Golden, CO (USA) and Bioenergy Science Center, Oak Ridge National Laboratory Oak Ridge, TN (USA) Fax: (+1)303-384-7752 E-mail: [email protected] C. W. Freudiger Department of Physics and Department of Chemistry and Chemical Biology Harvard University, Cambridge, MA (USA) [**] We thank G. R. Holtom and M. B. J. Roeffaers for helpful discussions. B.G.S. was supported by the Army Research Office through an NDSEG fellowship. C.W.F. was supported by a Boehringer Ingelheim Fonds Ph.D. fellowship. This work is also supported by the US Department of Energy: the instrumentation and data analysis is funded under grant DE-FG02-07ER64500, and the BioEnergy Science Center is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological & Environmental Research in the DOE Office of Science; the delignification process is funded by the Office of the Biomass Program. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201000900. Communications
Optics Letters | 2009
Khanh Kieu; Brian G. Saar; Gary R. Holtom; X. Sunney Xie; Frank W. Wise
We report a high-power picosecond fiber pump laser system for coherent Raman microscopy (CRM). The fiber laser system generates 3.5 ps pulses with 6 W average power at 1030 nm. Frequency doubling yields more than 2 W of green light, which can be used to pump an optical parametric oscillator to produce the pump and the Stokes beams for CRM. Detailed performance data on the laser and the various wavelength conversion steps are discussed, together with representative CRM images of fresh animal tissue obtained with the new source.
Journal of Physical Chemistry B | 2011
Christian W. Freudiger; Maarten B. J. Roeffaers; Xu Zhang; Brian G. Saar; Wei Min; Xiaoliang Sunney Xie
Label-free microscopy based on Raman scattering has been increasingly used in biomedical research to image samples that cannot be labeled or stained. Stimulated Raman scattering (SRS) microscopy allows signal amplification of the weak Raman signal for fast imaging speeds without introducing the nonresonant background and coherent image artifacts that are present in coherent anti-Stokes Raman scattering (CARS) microscopy. Here we present the Raman-induced Kerr effect (RIKE) as a contrast for label-free microscopy. RIKE allows us to measure different elements of the nonlinear susceptibility tensor, both the real and imaginary parts, by optical heterodyne detection (OHD-RIKE). OHD-RIKE microscopy provides information similar to polarization CARS (P-CARS) and interferometric CARS (I-CARS) microscopy, with a simple modification of the two-beam SRS microscopy setup. We show that, while OHD-RIKE microspectroscopy can be in principle more sensitive than SRS, it does not supersede SRS microscopy of heterogeneous biological samples, such as mouse skin tissue, because it is complicated by variations of linear birefringence across the sample.
Optics Express | 2010
Ke Wang; Christian W. Freudiger; Jennifer H. Lee; Brian G. Saar; Xiaoliang Sunney Xie; Chris Xu
We use the time-lens concept to demonstrate a new scheme for synchronization of two pulsed light sources for biological imaging. An all fiber, 1064 nm time-lens source is synchronized to a picosecond solid-state Ti: Sapphire mode-locked laser by using the mode-locked laser pulses as the clock. We demonstrate the application of this synchronized source for CARS and SRS imaging by imaging mouse tissues. Synchronized two wavelength pulsed source is an important technical difficulty for CARS and SRS imaging. The time-lens source demonstrated here may provide an all fiber, user friendly alternative for future SRS imaging.