Tetrahydrocannabinol
(THC) is the main active component in marijuana
and the rapid detection of THC in human body fluid plays a critical
role in forensic analysis and public health. Surface-enhanced Raman
scattering (SERS) sensing has been increasingly used to detect illicit
drugs; however, only limited SERS sensing results of THC in methanol
solution have been reported, while its presence in body fluids, such
as saliva or plasma, has yet to be investigated. In this article,
we demonstrate the trace detection of THC in human plasma and saliva
solution using a SERS-active substrate formed by in situ growth of
silver nanoparticles (Ag NPs) on diatom frustules. THC at extremely
low concentration of 1 pM in plasma and purified saliva solutions
were adequately distinguished with good reproducibility. The SERS
peak at 1603 cm–1 with standard deviation of 3.4
cm–1 was used for the evaluation of THC concentration
in a methanol solution. Our SERS measurement also shows that this
signature peak experiences a noticeable wavenumber shift and a slightly
wider variation in the plasma and saliva solution. Additionally, we
observed that THC in plasma or saliva samples produces a strong SERS
peak at 1621 cm–1 due to the stretching mode of
OCO, which is related to the metabolic change of THC
structures in body fluid. To conduct a quantitative analysis, principal
component analysis (PCA) was applied to analyze the SERS spectra of
1 pM THC in methanol solution, plasma, and purified saliva samples.
The maximum variability of the first three principal components was
achieved at 71%, which clearly denotes the impact of different biological
background signals. Similarly, the SERS spectra of THC in raw saliva
solution under various metabolic times were studied using PCA and
98% of the variability is accounted for in the first three principal
components. The clear separation of samples measured at different
THC resident times can provide time-dependent information on the THC
metabolic process in body fluids. A linear regression model was used
to estimate the metabolic rate of THC in raw saliva and the predicted
metabolic time in the testing data set matched well with the training
data set. In summary, the hybrid plasmonic-biosilica SERS substrate
can achieve ultrasensitive, near-quantitative detection of trace levels
of THC in complex body fluids, which can potentially transform forensic
sensing techniques to detect marijuana abuse.
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