09 Mar 2021
Jürgen Popp expects the detection abilities of Raman to enhance patient care and public health.
Addressing the SPIE Photonics West Digital Forum, Jürgen Popp of the Leibniz Institute of Photonic Technology (IPHT) outlined recent developments in Raman-related spectroscopy, along with the prospects for translation of the technology into real-world applications.
Popp discussed four specific and currently unmet needs: better monitoring of drug efficacy at the patient's bedside; the ability to detect pathogens spectroscopically without the need for labeling of those species; better measurement of water contaminants as a way to improve public health; and early stage detection of cell agglomerates in the body for the prevention of strokes or cardiovascular disease.
"All these share some common analytics requirements," said Popp. "Molecular specificity, online analysis capability with short detection times, quantifiable and low detection limits, and minimal or label-free sample preparation requirements. On-site or bedside application is also essential."
Raman techniques can meet these requirements, especially when the inherent specificity of its molecular fingerprint spectra is combined with forms of plasmonic effects arising from particular substrates or nanostructures, in surface enhanced Raman spectroscopy (SERS) methods.
Creation of suitable plasmonic-active nanostructures able to act as powerful SERS substrates can now employ several different approaches. Bottom-up preparation methods can achieve complex shaped nanostructures by seed-mediated growth, while self-organization processes and template-based methods can produce assemblies and arrays to particular designs. Top-down approaches can employ electron beam lithography or ion etching, among other techniques.
Popp's first focus application involves monitoring drug efficacy, for example in treatment of bacterial infections in the respiratory or urinary tract which need to strike a balance between dosages high enough to be effective but low enough to avoid toxicity effects. Effectively the treatment should be personalized for each patient at the bedside.
"Therapeutic drug monitoring can be done through chromatographic methods, but the sample preparation times are long and the instrumentation cost is high," noted Jürgen Popp. "We need a low-cost on-site analysis method instead."
One answer is to combine SERS with lab-on-a-chip (LoC) technology, in which plasmonic nanomaterials are incorporated in the LoC's internal structures to produce the desired SERS enhancement. A LoC-SERS technique produces high reproducibility of results and can readily incorporate high sample throughput and automated processing, according to Popp.
The technique has been trialed on the detection of levofloxacin in human urine, as a way to assess how much of the broad spectrum antibiotic is remaining in the body after its use as a treatment and so avoid excessive dosage.
A second area of interest is label-free pathogen detection. Popp described the application of SERS to the detection of phytophthora ramorum, a destructive plant pathogen, and to more complex virus structures, including H1N1 and coxsackievirus. Strategies employing tip-enhanced Raman spectroscopy, where the scattering takes place at an atomically sharp point, have been developed for such virus studies, and the possibility of studying Covid-19 this way has also been investigated.
"These studies involve the first steps towards Raman spectroscopy with single-molecule sensitivity," commented Popp. "Designing the right plasmonic structures can allow us to engineer resonance between the pump, Stokes and anti-Stokes signals, creating surface-enhanced coherent anti-Stokes Raman scattering spectroscopy or SECARS. When this highest overall enhancement is achieved, single virus detection is potentially possible."
Powerful diagnostic tool
Detoxification of water is a pressing public health issue in many locations, and Popp described efforts to monitor the presence of tetrachloroethane (PCE) in groundwater through the SERS analysis of sulfurospirillum bacteria, which is effectively a biomarker for the presence of the chlorocarbon.
"Sulfurospirillum's chemical composition changes in different cultivation conditions, including the presence of abundant PCE," said Popp. "So SERS spectra of the B12 cofactor within a particular enzyme in sulfurospirillum can be used to monitor the amount of PCE present, as a way to monitor the effectiveness of PCE removal techniques."
The final focus area Popp discussed is an immediate human health concern, the build up of cell agglomerates in blood vessels which can lead to atherosclerosis or other cardiovascular diseases. Early stage detection of plaque in arteries and the differentiation of dangerous plaque from less critical build-up could be achieved using SERS, if the technique can identify different macrophages or types of white cell present in different plaques.
Popp described research into the use of SERS labels based on gold nanostars coated in the sugar monomer mannose, the uptake of which by macrophages and subsequent incorporation into plaque can then be monitored by SERS as a possible route to identifying atherosclerosis when it is under way.
"These four areas of research show how SERS can in different applications combine molecular specificity, short detection times, and easy instrumentation for use in portable Raman systems," concluded Popp. "SERS is on the way to becoming a powerful analytical and diagnostic tool."