My favourite measurement is anything that’s prefixed by ‘zepto’. (Note: At 9:04 am PDT I edited my original lede sentence to correct an awkward construction.] I think it’s the ‘z’ (zed) that appeals to me so greatly. Happily for me, scientists at the Indian Institute of Technology Madras (IITM) have published “Selective Visual Detection of TNT at the Sub-Zeptomole Level” [behind a paywall] in the journal Angewandte Chemie.
Here’s more about the research from the Aug. 27, 2012 news item on Nanowerk,
… Indian scientists have now introduced a specific detection method for the explosive TNT that can be used to detect even a single molecule.
Thalappil Pradeep, Ammu Mathew, and P. R. Sajanlal at the Indian Institute of Technology Madras use an ingenious combination of micro- and nanostructures as sensors: gold mesoflowers, flower-shaped gold particles about 4 µm in size, act as supports for silver clusters, tiny clumps of exactly 15 silver atoms embedded in the protein bovine serum albumin. When irradiated with light of the right wavelength, the silver clusters luminesce, giving off red light. The gold of the mesoflower supports intensifies the fluorescence. Their unique shape is a particular advantage, because it is easy to unambiguously identify under an optical microscope, unlike spherical particles.
There are more details and a larger version of the image at Nanowerk.