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.

We report the first in situ quantitative visualization and characterization of electro-induced chitosan hydrogel growth in an aqueous environment. This was enabled with a pair of sidewall electrodes within a transparent fluidic system, which allowed us to resolve the electrogelling mechanism and interpret the dominant causes responsible for the formation and density distribution of the deposited hydrogel. The pH and the time-dependent growth profiles of the chitosan hydrogel were directly visualized, analyzed, and characterized. The results indicate that the gelation and immobilization of chitosan onto the cathode at a pH above its pK_a value (∼6.3) are due to the electrochemically generated concentration gradient of reactant OH ⁻ ions, and their subsequent neutralization of the NH₃ ⁺ groups of chitosan chains in solution near the cathode. The increased gel density around the fringes of the electrodes was demonstrated and correlated with the electrophoretic migration of chitosan cations during deposition. Simulation of the electric potential/field distribution, together with the corresponding dry film topography confirmed the non-uniform, electric field-dependent density distribution of deposited hydrogel. This report provides fundamental understanding towards the mechanism and the kinetics of the electro-induced chitosan gel formation. It also provides important guidelines for pursuing its application in bio-components integrated microsystems. The method in use exemplifies a simple, effective and non-destructive approach for in situ characterization of electro-responsive biopolymers in an aqueous environment.

Hydrogen sensors have been fabricated from single $\mathrm{Sn}{\mathrm{O}}_2$ nanobelts synthesized via catalyst-free thermal evaporation. Ohmic contacts were defined on individual $\mathrm{Sn}{\mathrm{O}}_2$ nanobelts by photolithography and pulsed laser deposition of $\mathrm{Ru}{\mathrm{O}}_2$ . The sensitivity and response time of the sensors, without any catalyst on the surface, to 2% hydrogen at temperatures between 25 and $80°\mathrm{C}$ were measured. Sensitivity greater than 50%, response time $<220\mathrm{s}$ , and power consumption $<10\mathrm{nW}$ at room temperature were demonstrated.

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 CaCO₃ particles and this “buffering” reaction generates a gradient in soluble Ca²⁺ ; and (iii) the locally generated Ca²⁺ 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, Ca²⁺ indicator dyes to observe the Ca²⁺ gradient, and in situ Raman spectroscopy to demonstrate a strong interaction between soluble Ca²⁺ 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.