Microbial Raman

Surface-enhanced Raman scattering (SERS) can be used in a new microarray approach to microbial detection that is label-free, according to researchers writing in the April issue of the journal Analytical Chemistry.

Maria Knauer, Natalia Ivleva, Xiangjiang Liu, Reinhard Niessner, and Christoph Haisch of the Institute of Hydrochemistry and Chair for Analytical Chemistry, at Technische Universitaet Muenchen, Germany, explain how their SERS research has led to chemically synthesized nanoparticles being successfully applied to an immunoassay for label-free detection of single microorganisms.

Growing concerns surrounding microbial contamination of drinking water have become a sharper focus of research in recent years. Yet, the development of a simple, fast, but also highly selective high-throughput detector has remained elusive. The TU team explain that most current methods for water monitoring are time-consuming and based on prior spiking of analytes. More recently, immunoassays on microarray technologies have come to the fore with a wide range of biomolecules including nucleic acids, oligonucleotides, proteins, peptides, carbohydrates, toxins, and even microorganisms being amenable to characterization in water, tissue, and blood samples.

Sophisticated labelling methods remain crucial to many of these modes of detection, however. It is the labelling process that requires larger reactant volumes and additional preparation steps and is one of the most time-consuming aspects of an analysis. However, determination of microorganisms in water need not involve enrichment steps or labelling.

The team explains that power of Raman spectroscopy: "RS is a vibrational spectroscopic method that provides highly specific information about material properties and is, compared to existing microarray technology as well as to infrared spectroscopy, more appropriate for analyzing biological molecules in aqueous samples." But, they add, conventional Raman is hampered by limited sensitivity.

Researchers have attempted to surmount this limitation by using a variety of metal structures, silver, gold, and copper in the form of metal plates, colloids, rods, and coatings, to induce the SERS effect. The reproducible production of SERS substrates of specific shape and size is challenging but crucial to the process.

The TU researchers have now modified a silver colloid preparation procedure for SERS of microorganisms. With these nanoparticles in a colloidal sol they were able to achieve an enhancement factor of over 10⁸. The main advantages of their procedure also include it being "straightforward, fast, and inexpensive" as well as highly reproducible. By optimizing the rate of silver particle agglomeration, the team was also able to form hot spots on the surface of the microorganisms and so could detect two different strains with high spatial resolution; again without labelling.

"To our knowledge, there exists no literature for both qualitative and quantitative results of a label- free in situ detection of microorganisms on microarrays in an aqueous environment using SERS," the team asserts. "A fast and non-destructive readout method is crucial for microorganism analysis in order to carry out a gentle detection and characterization. In this work, we give proof of principle that identification and quantification is possible with our technique."