This phenomenon is of particular importance in pharmaceuticals where precise characterisation of an active ingredient is crucial for both safety and efficacy.
Normally crystals are created when a super-saturated solution of the substance has a nucleation point around which crystals can form. If you ever grew copper sulphate crystals on a string, you get the idea. Mechanical shock, ultrasound, or laser energy can also be applied to the super-saturated solution to initiate nucleation.
Existing techniques, however, provide little control over which isomer of a compound is crystallised; it's necessary to carefully control the local conditions, to influence which isomer is produced, adding a layer of complexity and expense to the process.
Like ultrasound and mechanical nucleation, Non-photonic laser-driven nucleation causes thermal cavitation in the solution. Cavitation is the production of a small area of very low pressure and high temperature, so much so that – within the resulting 'bubble' – the liquid vaporises. These small cavitation bubbles provide nucleation points at which crystallisation can happen.
In this new research, investigators from the Universities of Edinburgh and Strathclyde working with Dr Andy Ward from the CLF's Octopus facility have shown that – using a saturated solution of sodium bromide – nucleation induced at the focus of a laser trap resulted in the selective production of one isomer (anhydrous) at a temperature and concentration where another (sodium bromide dihydrate) would have been expected. The trap is formed with a laser beam that is tightly focused using a powerful microscope lens.
This suggests that laser trapping induced nucleation – the technique used – increases the concentration of molecules in a small focal area which is sufficient to start crystal growth of one particular isomers where, otherwise, they could only have done so in more challenging conditions.
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