Surface enhanced Raman spectroscopy (SERS) is a powerful spectroscopic technique capable of detecting trace amounts of chemicals and identifying them based on their unique vibrational characteristics. While there are many complex methods for fabricating SERS substrates, there has been a recent shift towards the development of simple, low cost fabrication methods that can be performed in most labs or even in the field. The potential of SERS for widespread use will likely be realized only with development of cheaper, simpler methods. In this Perspective article we briefly review several of the more popular methods for SERS substrate fabrication, discuss the characteristics of simple SERS substrates, and examine several methods for producing simple SERS substrates. We highlight potential applications and future directions for simple SERS substrates, focusing on highly SERS active three-dimensional nanostructures fabricated by inkjet and screen printing and galvanic displacement for portable SERS analysis - an area that we believe has exciting potential for future research and commercialization.
Stimuli-responsive polysaccharides that can undergo a sol-gel transition in response to localized electrical signals provide a unique opportunity to electroaddress biological components at device interfaces. Most polysaccharide electroaddressing mechanisms use electrochemical reactions to generate pH gradients that can locally neutralize the polysaccharide and induce its reversible sol-gel transition to form a hydrogel film adjacent to the electrode surface. The calcium-responsive polysaccharide alginate is an exception; it may electrodeposit without requiring extreme pH gradients and thus may provide a means to electroaddress pH-sensitive biological components. Here, we use a novel device to characterize the mechanism for the anodic electrodeposition of a calcium alginate hydrogel. This device consists of a transparent fluidic channel with built-in sidewall electrodes that allows Ca-alginate electrodeposition to be directly measured by non-destructive optical and spectroscopic methods. We hypothesize a 3-step mechanism for calcium-alginate electrodeposition: (i) water is electrolyzed to locally generate protons (or hydronium ions); (ii) these protons are consumed by reacting with suspended CaCO 3 particles and this ''buffering'' reaction generates a gradient in soluble Ca 2+ ; and (iii) the locally generated Ca 2+ ions interact with alginate to induce its sol-gel transition. We verified this electrodeposition mechanism using pH-responsive dyes to observe the local pH gradients during gel formation, Ca 2+ indicator dyes to observe the Ca 2+ gradient, and in situ Raman spectroscopy to demonstrate a strong interaction between soluble Ca 2+ and alginate. Importantly, these results demonstrate electrodeposition without the need for a substantial pH excursion from neutrality. Thus, calcium alginate appears especially well-suited for electroaddressing labile biological components for applications in biosensors, biofabrication and BioMEMS.
Bacteria often reside at surfaces as complex biofilms in which an exopolysaccharide matrix entraps the population while allowing access to its chemical environment. There is a growing awareness that the biofilm structure and activity are integral to a wide array of properties important to health (the microbiome), disease (drug resistance) and technology (fouling). Despite the importance of bacterial biofilms, few experimental platforms and systems are available to assemble complex populations and monitor their activities. Here, a functionalized alginate composite material for creating in vitro model biofilms suitable for cell-cell signaling studies by entrapping bacterial cells in situ is reported. Biofilm assembly is achieved using device-imposed electrical signals to electrodeposit the stimuli-responsive polysaccharide alginate. This electrodeposition mechanism is versatile in that it allows control of the bacterial population density and distribution. For instance, it is demonstrated that a mixed population can be homogeneously distributed throughout the biofilm or can be assembled as spatially segregated populations within a stratified biofilm. The "electroaddressable" biofilms are visualized using both a planar 2D chip with patterned electrodes and a microfluidic bioMEMS device with sidewall electrodes. Specifically, it is observed that bacteria entrapped within the model biofilm recognize and respond to chemical stimuli imposed from the fluidic environment. Finally, reporter cells are used to demonstrate that bacteria entrapped within this model biofilm engage in intercellular quorum sensing. This work demonstrates the functionality of the stimuli-responsive polysaccharide by biofabricating pseudo-3D cell-gel biocomposites, mimicking the formation of biofilms, for interrogating phenotypes of E. coli bacterial populations. In addition to controlling assembly, the microfluidic device allows the biofilm to be monitored through the fluorescence methods commonly used in biological research. This platform technology should be able to be exploited for monitoring biofilm development, as well as for extending the understanding of the interactions between various bacterial species arranged in controlled patterns.
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