PROJECT SUMMARY / ABSTRACT This competitive supplement application requests funds to procure a high speed direct electron counting detector for analytical Energy Filtered TEM (EFTEM). This detector will allow us to accelerate the validation and performance benchmarking of new multi-label, elemental probes under development to scale the impact and accessibility of multicolor electron microscopy (ColorEM). This work will be conducted in the context of an active research project that is advancing the development of a novel method for ColorEM, employing new technologies for ultra-high-speed 4D-STEM (5 R01GM138780-03). This funded project is performing the feasibility studies and surmounting the technical challenges to: 1) make multi-labelled EM specimens with elemental probe combinations optimized for ColorSTEM (and extensible to all methods for ColorEM); 2) troubleshoot the first use of potentially transformative new technologies for STEM detection and associated capabilities for atomic-number (Z) contrast imaging; and, 3) work out the methodology for processing/analysis of very large primary data sets so as to accelerate elemental differentiation/signal interpretation and allow for display of ColorSTEM results. The ultra-fast direct electron detector requested, will allow us to accelerate and sharpen our ability to record, compare, and optimize the performance elemental probe combinations (for both signal-to-noise and differential contrast) as well as provide for a more sensitive and accurate validation of the quantitative performance of our data processing routines, which work hand-in-hand with the labeling chemistry to allow us to reliably and accurately discern the signal recorded from one selectively labeled structure from the specific signal of another. The equipment item requested is an Apollo (Direct Electron, LP), low-noise, high-speed electron-counting, direct detection device (DDD) for transmission electron microscopy, which possesses an industry-leading signal-to- noise and throughput specification that will allow us to more rapidly and accurately validate the performance of new elemental probes, thus accelerating the completion of Aims 1 and 3 of the parent award.