Flexible and water-stable graphene-based electrodes for long-term use in bioelectronics.

We present the first demonstration of bioelectrodes made from laser-reduced graphene oxide (rGO) on flexible polyethylene terephthalate (PET) substrates that overcome two main issues: using hydrogel on skin interface with standard Ag/AgCl bioelectrodes vs. low signal to noise ratio with capacitance or dry electrodes. Today we develop a dry rGO bioelectrode technology with long-term stability for 100 h in harsh environments and when in contact with skin. Reliability tests in different buffer solutions with pH from 4.8 to 9.2 tested over 24 h showed the robustness of rGO electrodes. In terms of signal to noise ratio, our bioelectrodes performance is comparable to that of commercial ones. The bioelectrodes demonstrate an excellent signal to noise ratio, with a signal match of over 98% with respect to state-of-the-art electrodes used as a benchmark. We attribute the unique stability of our bioelectrodes to the rGO/PET interface modification and composite formation during laser processing used for GO reduction. The rGO/PET composite formation assertion is confirmed by mechanical stripping experiments and visual examination of re-exposed PET. The method developed here is simple, cost-effective, maskless, and can be scaled-up, allowing sustainable manufacture of arbitrary-shaped flexible electrodes for biomedical sensors and wearables.

Localized surface curvature artifacts in tip-enhanced nanospectroscopy imaging.

Tip-enhanced Raman spectroscopy (TERS) allows the chemical analysis with a spatial resolution at the nanoscale, well beyond what the diffraction limit of light makes possible. We can further boost the TERS sensitivity by using a metallic substrate in the so-called gap-mode TERS. In this context, the goal of this work is to provide a generalized view of imaging artifacts in TERS and near-field imaging that occur due to tip-sample coupling. Contrary to the case of gap-mode with a flat substrate where the size of the enhanced region is smaller than the tip size when visualizing 3D nanostructures the tip convolution effect may broaden the observed dimensions due to the local curvature of the sample. This effect is particularly critical considering that most works on gap-mode TERS consider a perfectly flat substrate which is rarely the case in actual experiments. We investigate a range of substrates to evidence these geometrical effects and to obtain an understanding of the nanoscale curvature role in TERS imaging. Our experimental results are complemented by numerical simulations and an analogy with atomic force microscopy artifacts is introduced. As a result, this work offers a useful analysis of gap-mode TERS imaging with tip- and substrate-related artifacts furthering our understanding and the reliability of near-field optical nanospectroscopy.

Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals.

Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir-Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10(3) was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10(4). This is an important step towards the detection of the phonon spectrum from a single QD.

Compact metal probes: a solution for atomic force microscopy based tip-enhanced Raman spectroscopy.

There are many challenges in accomplishing tip-enhanced Raman spectroscopy (TERS) and obtaining a proper tip is probably the greatest one. Since tip size, composition, and geometry are the ultimate parameters that determine enhancement of intensity and lateral resolution, the tip becomes the most critical component in a TERS experiment. However, since the discovery of TERS the cantilevers used in atomic force microscopy (AFM) have remained basically the same: commercial silicon (or silicon nitride) tips covered by a metallic coating. The main issues of using metal-coated silicon cantilevers, such as wearing off of the metal layer or increased tip radius, can be completely overcome by using all-metal cantilevers. Until now in TERS experiments such probes have only been used in a scanning tunneling microscope or in a tuning fork-based shear force microscope but not in AFM. In this work for the first time, we show the use of compact silver cantilevers that are fully compatible with contact and tapping modes in AFM demonstrating their superb performance in TERS experiments.