Dynamics of Collisions and Adsorption in the Stochastic Electrochemistry of Emulsion Microdroplets.

Current-time recordings of emulsified toluene microdroplets containing 20 mM Ferrocene (Fc), show electrochemical oxidation peaks from individual adsorption events on disk microelectrodes (5 µm diameter). The average droplet diameter (∼0.7 µm) determined from peak area integration was close to Dynamic Light Scattering measurements (∼1 µm). Random walk simulations were performed deriving equations for droplet electrolysis using the diffusion and thermal velocity expressions from Einstein. The simulations show that multiple droplet-electrode collisions, lasting ∼0.11 µs each, occur before a droplet wanders away. Updating the Fc-concentration at every collision shows that a droplet only oxidizes ∼0.58% of its content in one collisional journey. In fact, it would take ∼5.45 × 106 collisions and ∼1.26 h to electrolyze the Fc in one droplet with the collision frequency derived from the thermal velocity (∼0.52 cm/s) of a 1 µm-droplet. To simulate adsorption, the droplet was immobilized at first contact with the electrode while the electrolysis current was computed. This approach along with modeling of instrumental filtering, produced the best match of experimental peaks, which were attributed to electrolysis from single adsorption events instead of multiple consecutive collisions. These results point to a heightened sensitivity and speed when relying on adsorption instead of collisions. The electrochemical current for the former is limited by the probability of adsorption per collision, whereas for the latter, the current depends on the collision frequency and the probability of electron transfer per collision (J. Am. Chem. Soc. 2017, 139, 16923-16931).

Long-range two-photon scattering spectroscopy ruler for screening prostate cancer cells.

Optical rulers have served as a key tool for scientists from different disciplines to address a wide range of biological activity. Since the optical window of state of the art FRET rulers is limited to a 10 nm distance, developing long range optical rulers is very important to monitor real life biological processes. Driven by this need, the current manuscript reports for the first time the design of long-range two-photon scattering (TPS) spectroscopy rulers using gold nano-antenna separated by a bifunctional rigid double strand DNA molecule, which controls the spectroscopy ruler length. Reported data demonstrate that the TPS spectroscopy ruler's working window is a within a 25 nm distance, which is more than twice that of well recognized FRET optical ruler. A possible mechanism for the two-photon spectroscopy ruler's long range capability have been discussed using angle-resolved TPS measurement and FDTD simulations. Solution-phase experimental data demonstrated that a long-range TPS ruler using A9 aptamer can be used for the screening of prostate-specific membrane antigen (PSMA) (+) prostate cancer cells even at 5 cells per mL level. Reported result with PSMA (-) normal skin HaCaT cells indicate that TPS ruler based assay has the capability to enable distinction from non-targeted cell lines. Ultimately, the long range TPS ruler can be used towards better understanding of chemical and biological processes.