Augmenting Cocaine Signals with Gold

Image by A_Different_Perspective from Pixabay is licensed under CC BY-NC-SA 2.0

Cocaine is a growing problem in the United States. The CDC reports that deaths caused by cocaine and its derivatives have steadily risen since 2012. More recently, deaths involving cocaine and a mixture of opiates and other drugs have surged, only adding to the existing problem. A portable, fast identification technique that does not destroy the sample is needed, in order to test for cocaine when and where a laboratory and its scientific instruments are not readily available.

Currently, if someone is suspected of driving under the influence of cocaine or other drugs excluding alcohol, the police officer collects blood from the suspect. Using this blood sample, technicians analyze the blood using GC-MS to confirm the presence of cocaine. These tests take a lot of time and require extensive preparation, even when there might not be any cocaine in the sample. 

Frank Inscore and his team at Real-Time Analyzers, Inc (RTA) aimed to develop a standard, accurate screening test for cocaine to avoid wasting time and money analyzing negative samples at the forensic laboratory. A screening or presumptive test is the first test performed once a police officer suspects illicit drug involvement. However, the results of this type of test are not specific enough to convict an offender in court.

Therefore a second test, called confirmatory analysis, is performed to confirm the results of the first. For this reason, it is vital that the screening analysis not destroy the sample, so that GC-MS or other confirmatory analysis may be performed afterward.

In addition, forensic researchers are interested in screening saliva samples for several reasons: non-technically trained personnel, can collect the sample non-invasively and if necessary, collection can occur at a crime scene or roadside stop. Another advantage of saliva as the cocaine-containing body fluid is that it does not interfere with RTA’s screening test in the same way blood would.

In a previous ForensicBites article, ultraviolet resonance Raman spectroscopy (UV-Raman) was investigated as a possible method of detection for cocaine. This method was fast, nondestructive and required no sample preparation. However, ultimately UV-Raman did not provide the sensitivity required for forensics – but it did prove that saliva is an ideal body fluid for testing cocaine. Using samples of saliva and the sensitivity of SERS, Inscore’s test provides what UV-Raman was lacking.

Surface-Enhanced Raman Spectroscopy, or SERS, is a technique using the chemical and electromagnetic properties of noble metals (gold, silver) to increase Raman signals. RTA scientists deposited the samples onto a surface containing nanoparticles of gold. One nanoparticle is about one million times smaller than the dot on the exclamation ending this sentence! These very small metal particles amplify vibrations of the bonds between the atoms, thereby increasing the signal of the chemical compound attached to them. It is even possible using SERS to detect a single molecule.

Figure 1: A depiction of what the SPE column attached to the SERS substrate might look like. Credit: Jo Mead (Mouth icon by Smashicons)

To aid in the detection, Inscore and his team developed a one-step method for sample purification using solid-phase extraction (SPE). Scientists drop suspected saliva into a small tube containing porous materials – known as a column. The mixture of ethanol and silica beads in the column interacts with the molecules in saliva – such as salivary mucins, digestive enzymes, and electrolytes– and allow the cocaine to flow out, leaving the saliva behind. Inscore’s integrated system would connect the cocaine isolation method (the SPE part) and the detection method (the SERS) part into one device. This multi-functional device would make it easy to perform accurate cocaine analysis in the field.

The team at Real-Time Analyzers simulated multiple scenarios, using gold (which amplified Raman signals upwards of 3 million times!), and around 80 different substances of abuse. Their main concern was if the device could detect cocaine, even in drug mixtures and real-world samples – which are oftentimes at very low concentrations. The detection limit of the SPE/SERS system was found first using cocaine in water, which served as a check to make sure that the process worked before applying to saliva samples in addition to a standard measure of success of the SPE cleaning procedure.

Real-Time Analyzers could measure cocaine in water at a concentration of 50 parts per billion (ppb) – and sometimes their system detected cocaine as low as 10 ppb. This is the equivalent of being able to detect 10 or 50 drops of ink in a large tanker truck used to haul gasoline.

Figure 2: An oil tanker for scale. Image by Erdenebayar Bayansan from Pixabay is licensed under CC BY-NC-SA 2.0 

The Substance Abuse and Mental Health Services Administration (SAMHSA), a branch of the Department of Health, decides detection limits for forensic analytical methods before they are legally implemented. For cocaine in oral fluid, analytical methods must detect at least as low as 20 ppb for a screening test, as the one Inscore designed. Their prototype measured cocaine in saliva at a concentration of 10 ppb.

In addition to empirical evidence, RTA applied a statistics tool called a Receiver-Operator Curve which plots specificity versus sensitivity of an experimental setup. They determined that the probability of detection for 50 ppb cocaine in saliva was 92%, and for 10 ppb it was 66%. This confirmed that Real-Time Analyzers’ method for testing cocaine when used with a portable Raman instrument could provide accurate screening tests for cocaine. 

Though screening is useful, most analysts aim to produce portable confirmatory tests for use at roadside stops and other ports of entry. To accomplish this, the sensitivity of a technique must be to at or below 8 parts per billion – a significant obstacle. Researchers in Spain using SERS and statistical software can detect cocaine at concentrations as low as 1 ppb. Implementing this technique in crime labs would require training, and not many labs have Raman spectrometers, but the advantages at this time far outweigh the cost. There is still some work to do, but it is very likely that the future of drug detection is here.

TitleDetection of Drugs of Abuse in Saliva by SURFACE-ENHANCED RAMAN SPECTROSCOPY
AuthorsFrank Inscore, Chetan Shende,  Atanu Sengupta, Hermes Huang, and Stuart Farquharson
JournalApplied Spectroscopy
PublisherSage Journals

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