Detecting Neurotransmitters Using SERS and SESORS

Bhavya Sharma

Surface-enhanced Raman spectroscopy (SERS) and surface-enhanced spatially offset Raman spectroscopy (SESORS) have been used in medical research for the detection of neurotransmitters such as melatonin, serotonin, and epinephrine. These techniques can assist in the diagnosis of neurological diseases and provide information that can lead to more effective treatment methods. Bhavya Sharma an assistant professor in the department of chemistry at the University of Tennessee (Knoxville, Tennessee), has been using SERS and SESORS to detect neurotransmitters and probe subsurface layers through the skull. Here, she describes the advantages of these techniques and how they are used in biological applications.   

In a recent paper (1), you described the effect of the metal used as the surface-enhanced Raman spectroscopy (SERS) substrate and the excitation wavelength used for detection of seven neurotransmitters. First, How does SERS compare to other techniques, such as high performance liquid chromatography (HPLC), as a detection technique for neurotransmitters, and why is it preferable?

SERS is advantageous in that it provides very rapid sample analysis time versus other techniques including HPLC, mass spectrometry (MS), and fluorescence. SERS also involves little to no sample preparation, no necessary labeling, and results in a “molecular fingerprint” for each analyte even for analytes with very similar chemical structures, making identification more straightforward. If you have analytes that are electrochemically active, you can get clear cyclic voltammograms for individual analytes, except in the cases where samples have similar oxidation potentials; in those cases, misidentification can occur. SERS allows for specific identification of these same electrochemical analytes. 

In that study, you used either gold or silver nanoparticles as substrates for the seven neurotransmitters. What results did you obtain for the two types of nanoparticles with the neurotransmitters?

We found that the affinity of the analyte to the surface of the nanoparticle is based on the metal composition of the nanoparticle, as well as the surface charge. For gold nanoparticles (AuNPs), neurotransmitters with aromatic rings, including melatonin, serotonin, dopamine, epinephrine, and norepinephrine, have a higher affinity for the gold surface. Silver nanoparticles (AgNPs), however, interact more strongly with the carboxyl and amine groups, like those found on amino acid-based neurotransmitters. This stronger interaction between the amino acid-based neurotransmitters and the AgNPs results in a more intense signal and lower limits of detection for molecules such as gamma-aminobutyric acid (GABA) and glutamate. The results also show that we are able to achieve the best enhancement for AgNPs at an excitation wavelength of 633 nm and for AuNPs at 785 nm.

You have also used surface-enhanced spatially offset Raman spectroscopy (SESORS)-which combines the advantages of SERS with spatially offset Raman spectroscopy (SORS)-to detect neurotransmitters through the skull in a noninvasive way (2). What are the advantages of using SESORS for this type of study rather than traditional Raman spectroscopy?

Traditional Raman spectroscopy is a surface selective technique. For example, if you have a multilayered sample, and you take a normal Raman measurement, the main contribution to your signal will be from the surface layer. SESORS takes advantage of the spatial component of light, where the photons that travel deeper into a material typically migrate laterally before scattering back out of the surface layer. So, if you collect your Raman-scattered light at a point that is spatially offset from the incident laser spot, you can acquire signal from those deeper layers in a non-invasive way.

What can be learned by using this technique in biological applications like the one in your study?

For our specific application, we were targeting the detection of neurotransmitters in a brain tissue mimic through an animal skull. We accomplished this by infusing the brain tissue mimic with AuNPs and neurotransmitters and then placing this brain tissue mimic behind an animal skull. The laser is then incident onto the animal skull for detection of these embedded neurotransmitters. The ultimate goal is to develop a non-invasive, or minimally invasive technique for the real-time, in vivo detection of neurotransmitters in the brain. Other groups have used the SORS technique for studies such as the detection of bone disease (3) and SESORS to detect breast cancer (4).

In an in vivo study, how would you apply the nanoparticles to the tissue being studied?

It is well established in the literature that non-specific injections of nanoparticles into an animal model generally end up being cleared rapidly by the body and deposited in the liver and spleen. In order to have specific deposition of nanoparticles, we will need to functionalize the nanoparticles with a molecule that will bind to our target analyte, such as an antibody against a particular neurotransmitter. If we intend to cross the blood brain barrier, we will need to include additional functionalization on the surface of the particles.

What are your next steps in the use of SESORS in the detection of neurotransmitters and neurochemicals?

Our goal is to establish limits of detection for various neurotransmitters that are commonly known to be involved in specific neurological diseases and then to detect these neurotransmitters in the brain. We are moving towards detection of neurochemicals in animal models to demonstrate the true applicability of SESORS as an in vivo detection technique.

References

1)     A.S. Moody and B. Sharma, ACS Chem. Neurosci. 9(6), 1380–1387 (2018). doi:10.1021/acschemneuro.8b00020

2)     A.S. Moody, P.C. Baghernejad, K.R. Webb, and B. Sharma, Anal. Chem. 89(11), 5688–5692 (2017). doi:10.1021/acs.analchem.7b00985

3)     K. Buckley, J. G. Kerns, P. D. Gikas, H. L. Birch, J. Vinton, R. Keen, A. W Parker, P. Matousek, and A. E. Goodship, IBMS BoneKEy,11, Article number: 602 (2014). doi:10.1038/bonekey.2014.97

4)     F. Nicolson, L. E. Jamieson, S. Mabbott, K. Plakas,  N. C. Shand,  M. R. Detty,  D. Graham, and K. Faulds, Chem. Sci., 9, 3788–3792 (2018). doi: 10.1039/c8sc00994e