Surface-Enhanced Raman Scattering Detection of Amphetamine ...

FIG. 1. Synthetic scheme for the derivatization of amphetamine (R5 H) or methamphetamine (R5 CH 3) with 2-mercaptonicotinic acid.
normalizing the signal. Detection of chlorinated ethylenes
on octadecylthiol-coated surfaces is an example of a passive
coating. 13 Active coatings exhibit a different Raman
spectrum in the presence of the analyte. An example of
an active coating is our work with the pH active coatings
on SERS substrates. 14 This work presents the ® rst step to
a new type of coating. We have termed this type of coating
as reactive. The concept is to put a reactive molecule
onto a SERS substrate and analyze the reaction product
with SERS.
We propose a technique for the detection and quanti-
® cation of amphetamine and methamphetamine utilizing
SERS spectroscopy. This work describes a DCC coupling
reaction whereby an amide is formed by coupling the
amine with 2-mercaptonicotinic acid (2-MNA). 15±17 By
derivatization of the amine with 2-MNA, a new compound
is formed possessing functionalities useful for
SERS analysis. 2-MNA was chosen due to its ability to
bind to noble metals through both the thiol sulfur and the
pyridine nitrogen. 18 We have recently shown that Raman
spectroscopy can be used to examine the responsivity to
alkali m etal cations of diam inodibenzo-18-crown-6,
which had been coupled to 2-MNA and attached to a
SERS surface. 17
The product formed between amphetamine or methamphetamine
with 2-MNA can be coated onto a SERS
substrate and a Raman spectrum obtained from the coated
silver surface. Our detection scheme is based on bands
observed in the Raman spectrum of the amide compared
to the spectra of the starting materials. Quanti® cation of
the drug can be accomplished by adding a known quantity
of pentachlorothiophenol (PCTP) as an internal standard.
The co-adsorption of the amide and PCTP is a competitive
process; however, within the range of our experimental
concentrations the PCTP appears to cover the
surface with a constant fractional coverage. The intensity
of the Raman bands associated with the drug/2-MNA
complex is measured relative to the intensity of a Raman
band associated with the internal standard. An important
advantage of this technique is that the coupling reaction
is not moisture sensitive so that it may be possible to
apply this scheme to the detection of amphetamine and
methamphetamine in biological ¯ uids.
SYNTHESIS OF 2-MNA DERIVATIZED AMINES
Materials and Solvents. D-Amphetamine sulfate and
(1 )m etham phetam ine hydrochloride w ere purchased
from Sigma. Absolute diethyl ether, 2-MNA, and DCC
were purchased from Aldrich. Dichloromethane and ethyl
acetate were purchased from Spectrum and EM Science,
respectively. PCTP was purchased from TCI America.
Absolute ethanol and methanol were purchased from
Pharmco and Spectrum, respectively.
Synthesis. The basic synthetic scheme is illustrated in
Fig. 1. Reaction conditions were varied to examine the
synthesis of the derivatized methamphetamine in chlorinated
organic and aqueous alcohol solvent systems. The
procedure for each will be provided in this section.
Chlorinated Organic Solvent Conditions. Amphetamine
sulfate (363 mg, 1 mmol) or methamphetamine
hydrochloride (186 mg, 1 mmol) was dissolved in 10 mL
of an aqueous saturated solution of K 2CO 3 to neutralize
the sulfate or hydrochloride. The freebase drug was then
extracted from the aqueous solution by using 3 3 2 mL
CH 2Cl 2. 2-MNA (155 mg, 1 mmol) and DCC (206 mg,
1 mmol) were added to the combined extracts (6 mL) of
CH 2Cl 2 containing the drug. A condenser was attached to
the reaction ¯ ask. The reaction mixture was gently re-
¯ uxed overnight under a nitrogen atmosphere. The mixture
was transferred to a separatory funnel with 20 mL
of CH 2Cl 2 and washed with an aqueous saturated solution
of NaHCO 3 (3 3 20 mL) to neutralize unreacted 2-MNA,
followed by H 2O (2 3 10 mL) and a wash with a 10%
HCl (1 3 10 mL) to neutralize unreacted freebase and
DCC, and ® nally, dried over Na 2SO 4. The solvent was
removed under reduced pressure.
Either the amide of AMNA or the amide of MMNA
was re-dissolved in 10 mL of CH 2Cl 2 and then further
diluted with additional CH 2Cl 2 to make standard amide
solutions. The internal standard of PCTP in CH 2Cl 2 was
added to each solution for a ® nal concentration standard
of 3 ppm per each amide solution.
Aqueous/Ethanol Solvent Conditions. Methamphetamine
hydrochloride (186 mg, 1 mmol) was dissolved in
15 mL of saturated aqueous K 2CO 3 to neutralize the hydrochloride.
The freebase of methamphetamine was then
extracted from the aqueous solution by using 3 3 2 mL
diethyl ether. The solvent was removed under reduced
pressure and the methamphetamine redissolved in 8 mL
of ethanol. 2-MNA (169 mg, 1.1 mmol) and DCC (228
mg, 1.1 mmol) in 2 mL H 2O were added to the methamphetamine/ethanol
mixture. A condenser was attached
to the ¯ ask, and the mixture was gently re¯ uxed for 2 h
under a nitrogen atmosphere. The reaction was followed
by thin-layer chromatography (TLC) using a 50% ethanol/ethyl
acetate eluent to monitor the disappearance of
starting materials. The ethanol was removed under reduced
pressure. The reaction mixture was transferred to
a separatory funnel with 10 mL of diethyl ether and extracted
with additional ether (3 3 10 mL). The combined
ether extracts were washed with saturated aqueous
APPLIED SPECTROSCOPY 955