A new type of explosive detector

October 19, 1999

EXPLOSIVES detectors that are used to search for bombs in luggage at airports can miss some devices because the concentration of the explosive material in sampled air is too low. But now a researcher in Maryland has developed a laser-based detector that may achieve the elusive goal of sniffing out single molecules of target substances.

Andrew Pipino, a chemical physicist at the National Institute of Standards and Technology in Gaithersburg, has already been granted a patent on his detector. A commercial prototype, which he hopes to complete in the next two years, would detect the minuscule explosives emissions from landmines, offering an alternative to more dangerous methods such as sniffer dogs. "With this detector we're making an electronic dog," says Pipino. "Next we want to go after chemical and biological weapons detection."

Pipino has demonstrated an experimental system, reported in the current edition of Physical Review Letters (vol 83, p 3093), which is based on an ultra-smooth cube of silica crystal with sides 1 centimetre in length and with a single curved convex face. Two prisms sit next to it on adjacent sides.

A laser pulse is fired into one of the prisms at such an angle that most of it is reflected, except for a minuscule portion of light called an evanescent wave, which passes into the crystal. Once inside the crystal, the tiny pulse undergoes total internal reflection off the cube's inner walls, but is redirected each time it is reflected from the curved side. When the pulse bounces off a wall, a tiny evanescent wave leaks out of the crystal, causing a slight drop in the energy of the pulse. The second prism focuses the evanescent wave leaking from the crystal at that face so that this energy loss can be measured.

After up to a million trips around the crystal, the pulse is all but expired. Researchers call the time this dissipation takes the "ring-down time"-and Pipino can measure it with incredible precision.

If a container of the sample gas is placed against one of the two sides of the crystal without a prism, sample molecules absorb energy from the pulse as it reflects off that face of the cube, decreasing the ring-down time. The difference between ring-down times with and without a sample is unique to each target substance. Different molecules absorb different amounts of light of a given wavelength, so test samples can be analysed over a range of wavelengths to provide a distinctive fingerprint for each.

Pipino claims that his method is about a hundred times more sensitive than rival techniques that also work with films of molecules, particles on surfaces, liquids and similar samples. Samples like these are used in explosives detection and other applications, such as those used to study increasingly miniaturised microchip components.

Pipino believes he can refine his method to achieve an additional 100-fold improvement in sensitivity. "The numbers suggest we might get single-molecule detection."

According to Richard Van Duyne, a chemist at Northwestern University in Evanston, Illinois, the new method has great potential. "This is really a major breakthrough," he says.
Author: Mark Schrope


New Scientist

Related Molecules Articles from Brightsurf:

Finally, a way to see molecules 'wobble'
Researchers at the University of Rochester and the Fresnel Institute in France have found a way to visualize those molecules in even greater detail, showing their position and orientation in 3D, and even how they wobble and oscillate.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

How molecules self-assemble into superstructures
Most technical functional units are built bit by bit according to a well-designed construction plan.

Breaking down stubborn molecules
Seawater is more than just saltwater. The ocean is a veritable soup of chemicals.

Shaping the rings of molecules
Canadian chemists discover a natural process to control the shape of 'macrocycles,' molecules of large rings of atoms, for use in pharmaceuticals and electronics.

The mysterious movement of water molecules
Water is all around us and essential for life. Nevertheless, research into its behaviour at the atomic level -- above all how it interacts with surfaces -- is thin on the ground.

Spectroscopy: A fine sense for molecules
Scientists at the Laboratory for Attosecond Physics have developed a unique laser technology for the analysis of the molecular composition of biological samples.

Looking at the good vibes of molecules
Label-free dynamic detection of biomolecules is a major challenge in live-cell microscopy.

Colliding molecules and antiparticles
A study by Marcos Barp and Felipe Arretche from Brazil published in EPJ D shows a model of the interaction between positrons and simple molecules that is in good agreement with experimental results.

Read More: Molecules News and Molecules Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.