Flexible biosensors are of considerable current interest for the development of portable point-of-care medical products, minimally invasive implantable devices, and compact diagnostic platforms. A new type of flexible electrochemical sensor fabricated by depositing high-density Pt nanoparticles on freestanding reduced graphene oxide paper (rGOP) carrying MnO2 nanowire networks is reported. The triple-component design offers new possibilities to integrate the mechanical and electrical properties of rGOP, the large surface area of MnO2 networks, and the catalytic activity of well-dispersed and small-sized Pt nanoparticles prepared via ultrasonic-electrodeposition. The sensitivity and selectivity that the flexible electrode demonstrates for nonenzymatic detection of H2O2 enables its use for monitoring H2O2 secretion by live cells. The strategy of structurally integrating metal, metal oxide, and graphene paper will provide new insight into the design of flexible electrodes for a wide range of applications in biosensing, bioelectronics, and lab-on-a-chip devices.

Growth of metal-metal oxide nanostructures on freestanding graphene paper for flexible biosensors

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.

Simple SERS substrates: powerful, portable, and full of potential.

Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid–liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.

Tip-enhanced Raman spectroscopy for surfaces and interfaces

A co-modification strategy to improve the electrode performance of SiO-based materials through the use of a carbon coating layer and reduced graphene oxide (RGO) network has been developed. The as-synthesized SiOx/C@RGO nanocomposites showed excellent specific capacity, cycling performance and rate capability when used as an anode in lithium-ion batteries.

A three dimensional SiOx/C@RGO nanocomposite as a high energy anode material for lithium-ion batteries

Individual nanostructured materials: fabrication and surface-enhanced Raman scattering

https://www.byui.edu/Documents/Colleges_and_Departments/Physical_Science_and_Engineering/Chemistry/CARBON%2049%20(2011)%202852–2861.pdf

Synthesis of graphene paper from pyrolyzed asphalt

A simple method for the production of silver nanoparticles on a silicon substrate that is suitable for surface-enhanced Raman spectroscopy (SERS) is presented. The method is based on spontaneous reduction of Ag+ ions by elemental silicon. The oxide layer is removed from the surface of a silicon disk by etching with dilute HF that is present in the same dilute solution of silver nitrate that is used to form the silver nanoparticles. By controlling the concentrations of HF and AgNO3, the morphology of the deposited silver nanostructures can be varied dramatically. The reproducibility of SERS measurements for substrates produced with a given concentration of HF and AgNO3 is good (relative standard deviation ∼ 10%). The application was extended to coating the tips of silicon cantilevers designed for atomic force microscopy (AFM) with silver nanoparticles to permit measurements of tip-enhanced Raman spectra (TERS). The feasibility of TERS measurements with AFM tips prepared in this way is demonstrated.

Electroless deposition of silver onto silicon as a method of preparation of reproducible surface-enhanced Raman spectroscopy substrates and tip-enhanced Raman spectroscopy tips.

A germanium disk on which silver nanoparticles have been deposited by galvanic displacement is shown to be an inexpensive substrate for surface-enhanced Raman spectroscopy (SERS). The preparation, which is based on spontaneous reduction of silver cations at the surface of a germanium disk, is quick and requires nothing more than a Petri dish. The SERS enhancement of silver and gold substrates prepared in this way was measured using benzenethiol and was compared to enhancement of Klarite®, a commercially available gold-coated nanoengineered SERS substrate. The enhancement provided by electrolessly deposited metals was found to be superior over Klarite® and the reproducibility was generally below 15%. Furthermore, unlike the case for nanoengineered substrates, germanium disks can be polished and reused.

http://journals.sagepub.com/doi/pdf/10.1366/000370209787944253

Nanostructural Silver and Gold Substrates for Surface-Enhanced Raman Spectroscopy Measurements Prepared by Galvanic Displacement on Germanium Disks

(Applied. Spectroscopy, 63:396-400)Nanostructural Silver and Gold Substrates for Surface-Enhanced Raman Spectroscopy Measurements Prepared by Galvanic Displacement on Germanium Disks

Silver particles grown on silicon in a galvanic displacement process undergo Ostwald ripening in which small particles merge into bigger ones. In this process the particles are shown by scanning electron microscopy to become smoother. Even though the surface area of the nanoparticles is halved on ripening, the Raman enhancement provided by them is at least five times higher than of the particles that did not undergo the ripening. In several regions, transmission electron microscopy (TEM) images show that the ripened particles resemble agglomerated colloids. In these regions the signal due to surface-enhanced Raman scattering (SERS) is greatly enhanced, suggesting that they are the SERS “hot spots”. The importance of boundaries formed between silver particles to the increased Raman signal is suggested.

Przemysław R. Brejna

Papers

Synthesis of graphene paper from pyrolyzed asphalt

Electroless deposition of silver onto silicon as a method of preparation of reproducible surface-enhanced Raman spectroscopy substrates and tip-enhanced Raman spectroscopy tips.

Nanostructural Silver and Gold Substrates for Surface-Enhanced Raman Spectroscopy Measurements Prepared by Galvanic Displacement on Germanium Disks

(Applied. Spectroscopy, 63:396-400)Nanostructural Silver and Gold Substrates for Surface-Enhanced Raman Spectroscopy Measurements Prepared by Galvanic Displacement on Germanium Disks

Surface-Enhanced Raman Spectroscopy Hot-Spots on Ostwald Ripened Silver Nanoparticles Prepared by Galvanic Displacement