A limited number of signaling pathways are repeatedly used to regulate a wide variety of processes during development and differentiation. The lack of tools to manipulate signaling pathways dynamically in space and time has been a major technical challenge for biologists. Optogenetic techniques, which utilize light to control protein functions in a reversible fashion, hold promise for modulating intracellular signaling networks with high spatial and temporal resolution. Applications of optogenetics in multicellular organisms, however, have not been widely reported. Here, we create an optimized bicistronic optogenetic system using Arabidopsis thaliana cryptochrome 2 (CRY2) protein and the N-terminal domain of cryptochrome-interacting basic-helix-loop-helix (CIBN). In a proofof-principle study, we develop an optogenetic Raf kinase that allows reversible light-controlled activation of the Raf/MEK/ERK signaling cascade. In PC12 cells, this system significantly improves lightinduced cell differentiation compared with co-transfection. When applied to Xenopus embryos, this system enables blue lightdependent reversible Raf activation at any desired developmental stage in specific cell lineages. Our system offers a powerful optogenetic tool suitable for manipulation of signaling pathways with high spatial and temporal resolution in a wide range of experimental settings.

Highlights d Non-neuronal optogenetics allows for the control of receptor tyrosine kinase activity d Tyrosine 490 of TrkA directly activates the ERK signaling pathway d Tyrosine 785 of TrkA activates ERK signaling through the PLCg-PKC pathway d Tyrosines 490 and 785 of TrkA both contribute to PC12 cell differentiation

For robust single particle optical detection, a high sensitivity in photoluminescence (PL) of Carbon Dots (CDs) must be achieved. PL sensitivity can be successfully correlated with their surface chemistry but requires high synthetic control without altering their basic surface properties. Here we describe conditions for the controlled synthesis of CDs that resulted in a PL sensitivity at the single-particle level. We report that a stoichiometric catalyst N-methyl morpholine-N-oxide (NMMO) can be used as a 'sacrificial' single additive to aid nanoscale surface oxidation. A 24 h NMMO-mediated oxidation increased coverage of oxidized nanoscale surface 3% to 20.9%. NMMO-oxidized CDs (CD-NMMOs) display superior particle brightness, as evidenced by the increase of light absorbance and an enhancement of quantum yield which is characterized by a series of physicochemical and biophysical experiments. We also demonstrate that CD-NMMOs is well suited for intracellular and single-particle imaging.