SERS detection of cell surface and intracellular components of microorganisms using nano-aggregated Ag substrate Tibebe Lemma a,b,c, *, Alex Saliniemi a , Ville Hynninen b,c , Vesa P. Hytönen b,c, 1 , J. Jussi Toppari a, 1 a University of Jyvaskyla, Department of Physics, NanoScience Center, P.O. Box 35, FI-40014, University of Jyväskylä, Finland b BioMediTech, University of Tampere, Biokatu 6, FI-33520 Tampere, Finland c Fimlab Laboratories, Biokatu 4, FI-33520 Tampere, Finland A R T I C L E I N F O Article history: Received 27 August 2015 Received in revised form 14 January 2016 Accepted 18 January 2016 Available online 21 January 2016 Keywords: Silver nano-aggregates Bacteria SERS N-acetyl-D-glucosamine (NAG) N-acetylmuramic acid (NAM) A B S T R A C T The intracellular and cell surface composition and structural features of gram-positive and gram- negative bacteria were identified using near-infrared surface-enhanced Raman scattering (SERS). The structural differences of components that reside in the cell envelope are manifested by their SERS spectra, e.g. gram-negative vs. gram-positive. Silver particles were used as a SERS substrate by exploiting the existence of strong local electromagnetic fields (hot spots) within nanoscale aggregates of the particles. The aggregation of silver nanoparticles was induced by magnesium ions. These hot spots reduce the screening length of the double layer. The obtained SERS spectra showed excellent quality having unprecedented high signal to noise ratio, which again enabled identification of numerous cellular components, such as proteins, polysaccharides, lipids and nucleic acids with excellent structure- spectrum correlation. The SERS spectra associated with these features give rise to intense bands that provide information about the chemical composition of the bacterial cell envelope and can also detect low concentration compounds. We demonstrate the high sensitivity of our SERS based method for detection of bacteria and its capability to differentiate between bacterial species based on minor chemical differences on their outer cell envelope. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Rapid detection and identification of clinically relevant micro- organisms (bacteria, yeasts and molds) are critical in many fields such as the food processing, dairy and brewery industries. Microbes (foodborne pathogens) are a major contributor to global disease infections and sporadic outbreaks across industrialized countries. In such outbreaks, hundreds of thousands of food products may have to be recalled, resulting in considerable financial loss (both operational and maintenance), as well as potential criminal liability for hospitals, the healthcare system and the food industry [1]. Although the traditional detection techni- ques, e.g., polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA), have made enormous progress in addressing these issues and are still evolving rapidly via technological innovations, as well as gaining improvements in speed, accuracy, detection, accessibility and data analysis, exces- sive amounts of food are still lost due to microbial spoilage. Current microbial cell detection and identification techniques often focus on culture-dependent methods and expensive reagents, and require a combination of several equipment, which is not ideal for rapid detection. Moreover, several of these analytical techni- ques developed for the detection of microorganisms [2–9] are time consuming, labor-intensive and require highly trained personnel, which poses tremendous financial stress. Therefore, there is strong demand for a novel biosensor technique with high sensitivity, good reliability, low-cost and universal applicability, to enable accurate identification of a variety of microbes based on their structural and biochemical components. Raman scattering and surface-enhanced Raman scattering (SERS) [10,11] have attracted the attention of researchers due to their great potential applications in various fields of science, including biomedical, analytical and physical chemistry. They are used as highly versatile techniques due to unique properties such as (having large cross-sections and) being ultrasensitive, label-free, non-invasive and free of photobleaching, which allow * Corresponding author at: University of Jyvaskyla, Department of Physics, NanoScience Center, P.O. Box 35, FI-40014, University of Jyväskylä, Finland. E-mail address: tlemma@gmail.com (T. Lemma). 1 Denotes equal contribution from both authors. http://dx.doi.org/10.1016/j.vibspec.2016.01.006 0924-2031/ ã 2016 Elsevier B.V. All rights reserved. Vibrational Spectroscopy 83 (2016) 36–45 Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.else vie r.com/locate /vibspec