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Posted: Jul 25, 2013

Microfluidics device detects drugs in saliva fast

(Nanowerk Spotlight) Surface Enhanced Raman Spectroscopy (SERS) is a powerful analytical method that can detect trace amounts of substances, such as narcotics, toxins, and explosives. The detection is based on the fact that molecules of different substances interacting with light from a laser will scatter the light differently, providing a unique spectrum that can be used to identify the substance, much like a fingerprint.
The key to detection is the enhancement that is provided by small clusters of silver nanoparticles, that concentrate the electric field of the light into 'hot-spots'. A molecule found in such a hot-spot will experience a concentrated electric field and give a very strong SERS signal.
In our recent work, we used a microfluidic device to orchestrate the interactions between silver nanoparticles and methamphetamine molecules in saliva. The microfluidic device allows for the controlled introduction of the sample and the nanoparticles, and the subsequent aggregation of the nanoparticles into hot-spot rich clusters that allow us to detect minute amounts of the drug.
Our findings, which have been published in the July 16, 2013 online edition of ACS NANO ("Rapid Detection of Drugs of Abuse in Saliva Using Surface Enhanced Raman Spectroscopy and Microfluidics"), are a collaborative effort between the microfluidics group of Prof. Meinhart, and the chemistry group of Prof. Moskovits.
Chemical detection using SERS has been around for decades, but without any major real-world applications. In a complex mixture, such as saliva or other biological fluids, there are many molecules and particles that adhere on the silver nanoparticles and prevent the detection of the target molecule when using traditional laboratory based methods.
Also, charged species like salt ions that may be present in the sample will cause the nanoparticles to aggregate too quickly, and will decrease the likelihood of capturing the target analyte in a hot-spot.
Our microfluidic device addresses both of these problems, by partially separating the complex mixture and by controlling the aggregation rate of the nanoparticles. By examining different positions in our microfluidic channel, we can make a "map" of the chemical composition in the device, and find the optimum place for detection of the target molecule.
microfluidics reaction
Schematic of the reaction: Silver nanoparticles (Ag NP), analyte, and salt solution are introduced to the channel from the left and flow toward the right. Analyte molecules resident in the focused stream diffuse laterally into the side flows. Salt ions also diffuse into the colloid stream inducing controlled nanoparticle aggregation, creating SERS-active clusters that convect downstream. Interrogating the region rich in colloid dimers, which provide intense plasmonic enhancement, we are able to achieve optimal SERS-based detection. (Reprinted with permission from American Chemical Society)
Using this method, the detection of methamphetamine is achieved reproducibly, regardless of the presence of other molecules that could be in the saliva, from say, a cigarette, or a recently eaten burger. Additionally, very small amounts of sample and reagents are required.
Microfluidics can be used to miniaturize and automate a lot of processes now conducted in a laboratory, thus allowing complicated analytical methods to be performed in automated portable devices, reducing the time and cost of analysis.
By using microfluidics we can move the sample preparation steps into the chip, creating a device that can deal with unprocessed saliva samples. In order to make a complete, self-contained, and portable detection system chip based laser sources and detectors may be used.
The method developed here can be used for drug screening, for law enforcement and health purposes. This detection method is generalizable and can be expanded for detecting other molecules, such as toxins in drinking water, spoiled food, or disease agents.
Microfluidics is a very powerful tool that has only started to find applications in our daily lives. In the coming years we hope to see many applications that use microfluidics to automate existing analysis methods, and develop new ones, leading the way to cost-effective chemical analysis, disease monitoring, and personalized medicine.
By Chrysafis Andreou, UCSB Microfluidics Lab
 

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