Read on Twitter
The latest spy thriller! Our new @NatRevChem explores how - with the right chiral molecules, and the right gadgets - hidden patterns can be interlaced into luminescent inks that are otherwise indistinguishable from those currently found in bank notes, passports, etc! (1/n) https://twitter.com/NatRevChem/status/1336019333399646211
An example of "at a glance" authentication of a UK passport using normal security inks. The green glow is an organic dye of some sort and the red glow is a molecule containing a europium complex. Stick 'em under a UV lamp "AKA a blacklight" and you'll see a nice pattern! (2/n)
To encode a hidden fingerprint into the europium complex, you can incorporate them into a left or right handed framework, i.e. make them "chiral".
Here's a representation of chirality in a relatively simple molecule. (3/n)
Here's a representation of chirality in a relatively simple molecule. (3/n)
Light emitted by the molecules (fluorescence/luminescene) is normally a balance of left and right circularly polarised photons. Doing some clever chemistry can introduce a strong bias towards emitting either left or right circularly polarised photons. (4/n)
This is the sort of molecular design that my colleauges in the @PalRobek group and Parker Group at @DurhamChemistry have a long history of. Various efforts from groups around the world result in some very cool looking chiral structures. (5/n)
If you design twin molecules which are biased towards opposite signs of light, you can print them and create features that switch on or off when you view them! (6/n)
The thing is, you don't need a fancy optics-bench set up to see these features! Left and right circularly polarised light filters are used in cheap and common cinema 3D glasses!
Explainer vid:
(7/n)
Explainer vid:
(7/n)
All these 3D glasses would need for "at a glance" of well-designed chiral security inks would be a simple bandpass filter. They could be wearable, or they could be a little view port (e.g. under a counter). (8/n)
Now we peel the next layer. You can go deeper by inspecting the whole chiral emission spectrum. BUT for decades, the tech has been slow, expensive, and cumbersome; taking around an hour for a good scan. Too slow to authenticate anything! (9/n)
Fortunately, we've just built a spectrometer capable of measuring these chiral fingerprints in as little as 10 milliseconds! It could be made fairly cheaply and could also be minaturised into a hand-held device. Allowing us to really look at fine spectral details. (10/n)
Just to give you an idea how insanely small the chiral spectrometer components are getting, here's a minature spectrometer released by Ocean Insight/Ocean Optics! We need a couple of these and some other optical components. (11/n)
So, with the right chiral molecules, it's feasible to create a hand-held rapid scanning spectrometer, which could scan stacks of bank-notes and verify that they were authentic. The question is, which molecules to use? (12/n)
Well, I did a systematic review of molecules with modest to strong chiral emission. Ended up with a database of over 200 entries. I hope it's going to be a really useful reference for the wider-field, as well as for security purposes.
[yes, this is how I spent lockdown] (13/n)
![Well, I did a systematic review of molecules with modest to strong chiral emission. Ended up with a database of over 200 entries. I hope it's going to be a really useful reference for the wider-field, as well as for security purposes.[yes, this is how I spent lockdown] (13/n)](https://pbs.twimg.com/media/EotwfNFWMAE39fj.png "Well, I did a systematic review of molecules with modest to strong chiral emission. Ended up with a database of over 200 entries. I hope it's going to be a really useful reference for the wider-field, as well as for security purposes.[yes, this is how I spent lockdown] (13/n)")
[yes, this is how I spent lockdown] (13/n)
From this meta-analysis we can see how tightly constrained the strength of chiral emission (gem value) is for the various emission transitions of europium and terbium complexes due to theintrinsic properties of these transitions. (14/n)
There are still practical challenges to overcome though. A big one is ensuring populations of chiral molecules can survive high-temperature lamination without changing to a 50/50 mix of left/right molecules and thereby losing the signal. (15/n)
But overall, there are quite a few groups around the world making molecules which could serve as chiral security inks. There's a framework for benchmarking, and the measurement tech is finally good enough too! It looks like we are set for something special to come! (16/n)
Special thanks to @DavidSchilter for a very thorough and detail-orientated editorial process, and to the the rest of the @NatRevChem team for upgrading the artwork etc. It was a great experience as an author! (17/n)
