Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy

Abstract

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.

Fig. 1

Principle and design of SRS microscopy. (A) Energy diagram for SRS. (B) Input and output spectra of SRS. SRS leads to an intensity increase in the Stokes beam (SRG) and an intensity decrease in the pump beam (SRL). Also shown (not to scale) is the CARS signal generated at the anti-Stokes frequency ω_AS. (C) SRL detection scheme. Stokes beam is modulated at high frequency (MHz), at which the resulting amplitude modulation of the pump beam due to SRL can be detected. (D) SRL microscope with both forward and epi detection. The Stokes beam is modulated by an electro-optic modulator. The transmitted or reflected pump beam is filtered and detected by a large-area photodiode (PD). For epi detection, the backscattered beams are collected by the excitation objective lens (OL) and separated from the excitation beams by a combination of a quarter wave plate (λ/4) and polarizing beam splitter (PBS). The SRL is measured by a lock-in amplifier to provide a pixel of the image. Three-dimensional images are obtained by raster-scanning the laser focus across the sample, and microspectroscopy can be performed by automated tuning of the pump wavelength. (E) The linear dependence of SRL on concentrations of retinol in ethanol at 1595 cm⁻¹. Modulation depth ΔI_p/I_p < 10⁻⁷ can be achieved. Error bars show 1 SD of the signals for a 1-min recording. The detection limit was determined to be 50 µM. (F) Agreement of SRL spectrum (red circles) with the spontaneous Raman spectrum (black line) of the Raman peak (1595 cm⁻¹) of 10 mM retinol in ethanol. The distorted CARS spectrum (blue squares) exhibits a typical dispersive shape. (G) The agreement of the more complex SRL spectrum of methanol (red circles) with the spontaneous Raman spectrum (black line).

Fig. 2

Omega-3 fatty acid uptake by A549 human lung-cancer cells monitored with SRL microscopy and microspectroscopy. (A) Spontaneous Raman spectra of docosahexaenoic acid (DHA, with six C=C bonds), eicosapentaenoic acid (EPA, with five C=C bonds), arachidonic acid (AA, with four C=C bonds), and oleic acid (OA, with a single C=C bond). The strong Raman peak around 3015 cm⁻¹ is characteristic of unsaturated fatty acids. (B) SRL spectra of a lipid droplet (LD, red line) and a region inside the nucleus (blue line). Unlike the nuclear region, the SRL spectrum of the LD shows good correspondence with the spectra from the pure EPA shown in (A). (C) SRL image of a cell at 2920cm⁻¹. (D) SRL image of the same cell at 3015 cm⁻¹. These findings indicate that EPA is taken up by the cells and more strongly enriched in the LDs compared to other cellular organelles. No structural changes of the living cells due to photodamage were observed after repeated scans.

Fig. 3

SRL imaging of fresh mouse tissue. (A) Neuron bundles in corpus callosum of mouse brain imaged at 2845cm⁻¹ highlighting myelin sheaths rich in CH₂. (B) Epi-detected SRL CH₂ image acquired from thick brain tissue. (C) SRL CH₂ images of mouse ear skin in the same area at the indicated depths. From left to right: stratum corneum (4 µm), sebaceous gland (42 µm), and subcutaneous fat layer (105 µm). (D) Comparison of SRL and CARS images of stratum corneum on (2845 cm⁻¹) and off (2780 cm⁻¹) the CH₂ resonance. Unlike CARS, SRL has no nonresonant background.

Fig. 4

Monitoring drug delivery into fresh mouse skin by SRS microscopy. (A) Raman spectra of dimethyl sulfoxide (DMSO, green), retinoic acid (RA, blue), and typical lipids in mouse skin (red). (B) Top view of the penetrated DMSO (green) in the stratum corneum imaged at 670 cm⁻¹ with SRL. (C) SRL DMSO depth profile through the line in (B). (D) Simultaneous two-color SRL image (18) of DMSO (670 cm⁻¹, green) and lipid (2845 cm⁻¹, red) in the subcutaneous fat layer at a depth of ~65 µm through the lower line in (C). (E) Top view of the penetrated RA (blue) in the stratum corneum imaged at 1570 cm⁻¹ with SRL. (F) SRL RA depth profile through the line in (E). SRS allows label-free 3D in situ visualization of two different drug-delivery pathways into the skin.