The detection of a small amount of contaminant in fuel (such as gasoline or diesel) is of practical importance. This can be
conveniently done by terahertz measurements. For example, the detection of chromium salt in regular gasoline was attempted.
Chromium(III) acetate hydroxide (CrAH, [CH3CO2]7 Cr3 [OH]2) (Aldrich) was dissolved in gasoline at 1.9105 mg/mL. A rectangular cuvette with a volume of ~1.5 mL and a pathlength of
~1 mm was used to introduce both the pure and CrAH-contaminated samples in the beam path. Time-resolved spectra of both pure
and mixed gasoline are shown in Figure 8. It is seen that gasoline alone has a higher transmission compared to the CrAH contaminated
one. Figure 9 shows the Fourier transform frequency spectra corresponding to the interferogram of Figure 8. There are significant
identifying features in the respective spectra that can be exploited for discerning the contaminated gasoline from the uncontaminated
one. An interesting observation is that at low frequencies (below ~1.5 THz, Figure 9, red curve) gasoline exhibits higher
transmission than CrAH-contaminated gasoline (blue curve). However, at higher frequencies (above ~1.5 THz) gasoline exhibits
a lower magnitude than that of the CrAH-mixed gasoline (Figure 9). Thus, the frequency-dependent behavior of the two specimens
reveals some interesting molecular interaction between the CrAH and gasoline that needs to be investigated further for molecular
Figure 10: The sequence of reactions yielding the aminated (NH2–terminated) nanodiamond: ND-EDA (ethylenediamine).
Nanodiamond (ND) is a promising material as a nontoxic, carbon-based, solid drug delivery platform. Functional nanodiamonds
are expected to be effective as carriers for the therapeutics. Nanodiamonds have been reported as vectors for in vitro gene
delivery via surface-immobilization with 800 Da polyethyleneimine (PEI800) and covalent conjugation with amine groups (8).
The sequence of reactions yielding the aminated (NH2–terminated) nanodiamond ND-EDA (ethylenediamine) is shown in Figure 10 (9). However, without accurate quantitation of the
degree of amination of a nanodiamond particle surface (amine–nanodiamond), its use in emerging applications such as targeted
drug delivery, diagnostics, pharmaceuticals, and antimicrobials cannot be fully exploited. Fourier transform infrared (FT-IR)
and Raman spectroscopy do not adequately quantify the degree of amination; gas chromatography (GC) and mass spectrometry (MS)
cannot discern sp3 (diamond phase) and sp2 (graphene phase) content. The most promising analytical technique available is nonionizing terahertz spectroscopy (5). Nanodiamond
and amine-terminated nanodiamond samples (samples courtesy of S.C. Picardi) (10) were prepared as follows: 1% dispersant of
the respective materials in powder form were prepared in deionized water at room temperature. Aliquots of 30 μL of each dispersant
were dispensed on glass slides and allowed to dry at 50 °C for ~10 min. The resulting spots were introduced in the spectrometer's
beam path, and the response of the samples were recorded by the front-end. Figure 11 exhibits the Fourier transform absorbance
spectra of both samples. It can be seen that aminated nanodiamond exhibit significantly higher absorbance than the nonaminated
nanodiamond. It is expected that the low frequency offset should scale as a function of the degree of amination per nanodiamond.
Thus, the spectra enable the formulation of criteria to be able to quantify the degree of amination of the nanodiamond particles.
For example, the absorbance magnitude at low frequencies will be proportional to the number of amine molecules attached to
the nanodiamond surface. Also, the shift of the absorbance peaks at different frequencies may be assigned to different molecular
interactions. A systematic investigation should lead to an analytical protocol for routine and deterministic applications.
Figure 11: Broadband terahertz absorbance spectra of nanodiamond and amine-terminated nanodiamond obtained via Fourier analysis
of the temporal signals (not shown). The aminated nanodiamond exhibits significantly higher absorbance.
Terahertz spectroscopy has been discussed in terms of practical examples. An important property of terahertz radiation is
that it can penetrate almost all nonmetallic objects without causing radiation damage. This presents an opportunity to study
intrinsic properties of materials in their native states. A wide broadband terahertz spectrometer was utilized to illustrate
some unique applications of this tool. A brief discussion of the spectrometer calibration was also included. Illustrative
examples exhibit that the method offers very high sensitivity measurements of chemical compounds for DHS applications, characterization
of nanoparticles for quantitation of surface ligand coverage, and the determination of contamination in gasoline, among many
others. It was found that the technique is suitable for interrogating molecular motions that are not easily visible by other
methods such as UV, IR, or Raman spectroscopy. Thus, the terahertz spectrometer not only reproduces the known absorbance peaks
of standard materials, it generates many peaks that were not visible with the previous spectroscopic methods. The difficulty,
however, is that interpretation of these newly discovered peaks may not be readily available. The technique provides a unique
opportunity for discovering critical molecular signatures of a cross-section of materials.
Figure 12: A close-up of Figure 11. The jump in absorbance of aminated nanodiamond can be exploited for formulation of a protocol
for deterministic calculation of degree of amination.
Anis Rahman is with Applied Research & Photonics in Harrisburg, Pennsylvania. Direct correspondence to: email@example.com