On the origin of electrochemical surface-enhanced Raman spectroscopy (EC-SERS) signals for bacterial samples: the importance of filtered control studies in the development of new bacterial screening platforms

The field of point-of-need (PON) screening is currently lacking a rapid, sensitive and cost-effective method for bacterial detection. Such a method would prove invaluable for the efficient identification of pathogenic bacteria that result in a host of human infections and plague the healthcare sector. Surface-enhanced Raman spectroscopy (SERS) has increasingly been put forward as a viable candidate for a bacterial detection and identification platform due to its excellent sensitivity and the corresponding rich vibrational signature. Recently, there has been significant debate within the SERS community as to the origin of the observed SERS signals for cultured bacterial samples. This uncertainty stems from the remarkable similarity among many of the observed SERS signals for a vast array of bacteria, suggesting the spectral features may have a common origin. Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) has recently been shown to largely improve upon the sensitivity of SERS for detection of bacteria. In this report, we will highlight the importance of conducting control studies using filtered bacterial samples. Using a 0.2 μm syringe filter to remove all bacterial cells from the examined sample, which is confirmed via scanning electron microscopy, this study demonstrates an excellent control for determining the origin of the bacterial signal in EC-SERS measurements. Our findings strongly support the argument that SERS spectra observed for bacterial cultures originate from small molecule metabolites released by the bacteria in response to environmental stressors.