Detection of trace tetrahydrocannabinol (THC) through surface enhanced Raman spectroscopy (SERS) on microfabricated platforms

As an increasing number of states legalize marijuana, it will become necessary to create a method for portable detection and quantification of tetrahydrocannabinol (THC), the principal psychoactive component of cannabis. The ability to identify impaired individuals without the need for traditional drug testing could prove invaluable to law enforcement and employers. Toward this end goal, surface enhanced Raman spectroscopy (SERS) with a silver treated, silicon nanowire substrate was investigated as a method for detection. Acid wet etching of silicon with a hydrofluoric acid (HF) / silver nitrate (AgNO3) was the primary investigation method for this work. Various etching parameters were utilized, ranging from 2 M HF/0.02 M AgNO3, 5 M HF/0.02 M AgNO3, 5 M HF/0.10 M AgNO3, 8.15 M HF/0.02 M AgNO3, to 12.2 M HF/0.02 M AgNO3. Bare, unetched silicon and silver sputtered silicon were tested as substrates for SERS for THC detection, but proved to have no signal enhancement. Nanowires were not present at 2 M HF/0.02 M AgNO3 etching conditions and the substrate provided no Raman enhancement. Residual silver from the wet etching was tested to see if it was a viable means of enhancing Raman signal. Measurements spanning from 5 M HF/0.02 M AgNO3 to 12.2 M HF/0.02 M AgNO3 indicate residual silver can be used to enhance signal, but it’s sparse and irregular distribution over the silicon nanowire substrate leads to inconsistent measurements that require aiming the laser at residual silver deposits. The increased AgNO3 etching parameters (5 M HF/0.10 M AgNO3) yielded a residual silver particle film which obstructed the silicon nanowires. The resulting lack of Raman signal enhancement indicated nanowires were necessary in addition to silver for SERS activity. Etching parameters of 8.15 M HF/0.02 M AgNO3 and 12.2 M HF/0.02 M AgNO3 both displayed Raman activity at 1.0 X 107 pg of THC on sputtered chips, and at 3.15 pg on chips with residual silver. The 1.0 X 107 pg of THC tests were performed to determine if the chips could detect a relatively large amount of THC, while the 3.15 pg THC tests were performed to determine if the method could detect THC on the order of magnitude present in breath. The Raman response displayed by the tests indicted qualitative detection is possible. Quantitative tests with THC amounts ranging from 2.4 pg to 10,005.6 pg on the 8.15 M HF/0.02 M AgNO3 (chosen due to its larger signal intensity compared