Removal of anti-Stokes emission background in STED microscopy by FPGA-based synchronous detection.

In stimulated emission depletion (STED) microscopy, the role of the STED beam is to de-excite, via stimulated emission, the fluorophores that have been previously excited by the excitation beam. This condition, together with specific beam intensity distributions, allows obtaining true sub-diffraction spatial resolution images. However, if the STED beam has a non-negligible probability to excite the fluorophores, a strong fluorescent background signal (anti-Stokes emission) reduces the effective resolution. For STED scanning microscopy, different synchronous detection methods have been proposed to remove this anti-Stokes emission background and recover the resolution. However, every method works only for a specific STED microscopy implementation. Here we present a user-friendly synchronous detection method compatible with any STED scanning microscope. It exploits a data acquisition (DAQ) card based on a field-programmable gate array (FPGA), which is progressively used in STED microscopy. In essence, the FPGA-based DAQ card synchronizes the fluorescent signal registration, the beam deflection, and the excitation beam interruption, providing a fully automatic pixel-by-pixel synchronous detection method. We validate the proposed method in both continuous wave and pulsed STED microscope systems.

Chromium, cobalt and nickel contents in urban soils of Moa, northeastern Cuba.

Iron, chromium, cobalt and nickel concentration levels in urban soil samples collected from Moa city (Holguín province), northeastern Cuba were determined. Both chromium and nickel contents exceed the Dutch Intervention Value soil quality standard in 2.8-5.4 and 1.3-3.3 times, respectively. Furthermore, cobalt content exceeds the Target Value in 1.3-1.8 times. Metal-to-Iron normalization predicts a natural origin for nickel and cobalt (Enrichment Factor <1), and also a moderate chromium enrichment (Enrichment Factor = 1.5-4.0) in all studied stations. The application of the Urban Environment Entropy Model show that residential area located near to industrial area is slightly affected by industrial chromium emissions and not affected by cobalt and nickel possible emissions. A chromium speciation in soil samples is recommended in order to evaluate the real impact of the current chromium content in Moa urban soils to local urban and suburban agricultures.