High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle‐reinforced composite networks since deep light penetration of short‐wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet–triplet annihilation upconversion (TTA‐UC) is employed for curing opaque hydrogel composites created by direct‐ink‐write (DIW) 3D printing. TTA‐UC converts low energy red light (λmax = 660 nm) for deep penetration into higher‐energy blue light to initiate free radical polymerizations within opaque objects. As proof‐of‐principle, hydrogels containing up to 15 wt.% TiO2 filler particles and doped with TTA‐UC chromophores are readily cured with red light, while composites without the chromophores and TiO2 loadings as little as 1–2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D‐printed composite polymer networks.

Stimulated Raman scattering (SRS) is a fundamental optical process that was discovered more than 60 years ago. While the early SRS spectroscopy studies have provided valuable insights into materials systems, the advent of SRS microscopy has launched a rapidly growing field in biological imaging. However, a fundamental understanding of the molecular response under SRS is still lacking. Herein we present a new framework to introduce moleculeintrinsic stimulated Raman scattering cross sections, σ SRS , in the unit of Goppert-Mayer (GM). The absolute SRS cross sections determined for real molecular systems challenge the conventional wisdom that Raman spectroscopy is always a weak process. The enormous rate acceleration of SRS, captured by an apparent SRS cross section, stems from a synergistic effect between the field and the molecule. Our new framework goes beyond the conventional opticscentric view and presents a molecule-inclusive perspective, thus offering a comprehensive foundation for the future growth of SRS spectroscopy and microscopy.

Solid lipid nanoparticles (SLNs) are a state-of-the-art lipid-based pharmaceutical drug delivery system. Advantages of SLNs include high biocompatibility, low immunogenicity, superiority in drug encapsulation capacity, and improved colloidal stability. They became widely known in late 2020, as several COVID-19 vaccines are built upon SLNs technology. Despite the increasing impact, the characterization methods of SLNs are currently very limited especially in biological environment, which hinders fundamental understanding of the delivery mechanism and contributes to relatively low success rate in clinical translations. Here, we present close-to-label-free imaging of deuterated SLNs using the emerging stimulated Raman scattering (SRS) microscopy. The introduction of deuterium to lipid structure renders bioorthogonal chemical specificity.Notably, with this approach, we have achieved ultrahigh single-particle sensitivity both in vitro and in vivo, even with particle counting ability. Our bioorthogonal chemical imaging modality by SRS microscopy can be generalized to visualize a wide spectrum of lipid-based drug carriers with high spatiotemporal resolution, chemical specificity, and ultimate sensitivity. This work opens up ways to address critical questions in SLN drug delivery and could also facilitate innovations in lipid nanotechnology and clinical translations. Key points• Direct imaging of lipid nanocarrier that is the basis of the COVID-19 vaccines.• Novel single-particle imaging technique applied to nanomedicine.