SHG nanoprobes: Advancing harmonic imaging in biology

Abstract

Second harmonic generating (SHG) nanoprobes have recently emerged as versatile and durable labels suitable for in vivo imaging, circumventing many of the inherent drawbacks encountered with classical fluorescent probes. Since their nanocrystalline structure lacks a central point of symmetry, they are capable of generating second harmonic signal under intense illumination - converting two photons into one photon of half the incident wavelength - and can be detected by conventional two-photon microscopy. Because the optical signal of SHG nanoprobes is based on scattering, rather than absorption as in the case of fluorescent probes, they neither bleach nor blink, and the signal does not saturate with increasing illumination intensity. When SHG nanoprobes are used to image live tissue, the SHG signal can be detected with little background signal, and they are physiologically inert, showing excellent long-term photostability. Because of their photophysical properties, SHG nanoprobes provide unique advantages for molecular imaging of living cells and tissues with unmatched sensitivity and temporal resolution.

Taking a deep look: modern microscopy technologies to optimize the design and functionality of biocompatible scaffolds for tissue engineering in regenerative medicine

“…SHG has many features in common with 2-PF ( penetration depth); however, no energy is deposited in the sample (100% energy conversion; figure 3a), and no bleaching or heating effects should occur even with high laser powers or in long-term observations [132]. Nevertheless, the required high laser intensities may cause photodamaging effects derived from still-present AF.…”

“…We obtained rsif.royalsocietypublishing.org J R Soc Interface 10: 20130263 SHG images from these scaffolds ( figure 7b(i)) and by using three-dimensional stacks and reconstructions, it was possible to characterize the scaffolds porous structure (not shown [12,76,141,154], silk [119], gelatin/ collagen gels [13,137] or PHB/PHBHHx [155]. A review by Dempsey et al [132] identifies small particles that are widely used in TE to enhance scaffold mechanical stability or to track cells (e.g. Bioglass, nanodiamonds, Qdots) are susceptible to SHG as well.…”

Single-cell screening and quantification of transcripts in cancer tissues by second-harmonic generation microscopy

“…[18][19][20][21][22] SHG is a nonlinear optical process which converts two incident photons at a frequency ω into one photon at frequency 2ω in a noncentrosymmetrical material. 23 Unlike fluorescence, the SHG process only involves the transition between virtual energy states without any nonradiative loss.…”

Harmonic Nanoparticles for Regenerative Research

“…Compared to fluorescent probes, HNPs possess a series of attractive properties, such as complete absence of bleaching and blinking, narrow emission bands, excitation-wavelength tunability (from ultraviolet to infrared), orientation retrieval capability, and coherent optical response. These unique properties have been recently explained in two comprehensive review papers 11,12 . The possibility of working in the infrared spectral region, which increases imaging depth by minimizing scattering and absorption, also drastically limits sample photo degradation 13,14 .…”