»
SPECTROSCOPY
» (SERS).31 In fact, SERS has been shown to allow Raman spectroscopy to be performed on single molecules.32
For the initial research into the viability of SERS to detect the presence of N-acetylamantadine in urine, a commercially available gold SERS substrate was evaluated by Dr. Gordon’s and Dr. Hof’s groups at the University of Victoria utilizing mock urine spiked with N-acetylamantadine.
Initial samples were pipetted onto the
gold substrates and, after drying, were analyzed using a fi ber-optic Raman probe coupled with a wavelength stabilized 785-nm laser and miniature spectrometer with a TE cooled back-thinned CCD detector.
Initial results fell well short of the desired detection limit of 1 ng/mL leading the researchers to functionalize the substrates with the capture molecule beta-cyclodextrin. As shown in Figure 2, this particular molecule forms a cone-like shape with a hydrophobic interior and a hydrophilic exterior, which is ideal for this application because it can be dissolved in water while capturing the hydrophobic analyte, keeping it close to the surface of the substrate.
In addition,
they decided to submerge the substrate in the mock urine for 4 hours to allow suffi cient time to be captured by the beta-cyclodextrin. This work, which was recently published in Analyst,17
has successfully
shown detection for concentrations as low as 1 ng/mL in the absence of steroids.
Translational Research
Building on the success of the initial research at the University of Victoria, the translational phase of the research investigates the potential of this technology to take the leap from laboratory bench to clinician’s offi ces. This phase of the research contained 2 main objectives:
1. Identifi cation of a low-cost disposable SERS substrate.
2. Proof of principal demonstration of a compact and transportable SERS reader capable of detecting N-acetylamantadine in solution.
For the fi rst objective, we evaluated a new SERS technology out of the University of Maryland which utilizes gold nanoparticles suspended
in liquid, printed on paper via an inkjet printer cartridge.33-34 form of SERS substrate is called paper SERS, or simply pSERS.
This new
These pSERS substrates off er 2 major advantages for the detection of N-acetylamantadine in urine over traditional gold substrates.
First,
they are signifi cantly cheaper to manufacture, making them ideal for disposable applications. Second, and perhaps more important, due to the capillary eff ect, these substrates naturally absorb liquid samples and concentrate the analyte, allowing the sample preparation to be simplifi ed to a relatively quick dip test.
To validate the ability of pSERS substrates to enhance the Raman signal of N-acetylamantadine in solution, we used slide-mounted pSERS (for convenience) and pipetted a small volume of the sample onto the paper. After drying, we also applied a small amount of hydrochloric acid in order to slightly lower the pH of the sample and increase the effi cacy of the substrate. Next, we collected the spectra shown in Figure 3 using the same set-up as was used at the University of Victoria. From this data, the dominant 740-cm-1 is clearly visible.
vibrational band of the analyte
Finally, a prototype system was developed that was low-cost, transportable, and easy-to-use. For this, the need to both maximize the collection effi ciency while simultaneously minimizing the form factor was recognized, making it imperative to maximize the throughput of the optical systems. As a result, an integrated Raman probe was utilized, including the laser as well as fi ltering and collection (excitation) optics, which had been shown to be 3× to 5× higher throughput than the traditional fi ber Raman probe used in the previous work. The probe was coupled to a high-throughput f/1.3 transmission spectrometer, chosen due to the fact it has approximately10× higher throughput than the spectrometer used for the laboratory tests and is approximately half the size. Next, an inverted slide holder was designed (slide-mounted pSERS were again chosen for convenience), shown in Figure 4(a), to position the pSERS substrate at the focus of the laser. This slide holder also includes a micrometer which can adjust the position of the sample in the z-dimension to assure the sample is always at the optimal focus.
After specifying the appropriate spectrometer, laser, probe optics, and sample holder, the system was integrated into a Class 1 enclosure
Figure 2. Molecular structure of beta-cyclodextrin.
Figure 3. pSERS spectrum of 5 mg N-acetylamantadine.
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| January/February 2015
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