SERS-enhanced piezoplasmonic graphene composite for biological and structural strain mapping

Abstract

Thin-film optical strain sensors have the ability to map small deformations with spatial and temporal resolution and do not require electrical interrogation. This paper describes the use of graphene decorated with metallic nanoislands for sensing of tensile deformations of less than 0.04% with a resolution of less than 0.002%. The nanoisland-graphene composite films contain gaps between the nanoislands, which when functionalized with benzenethiolate behave as hot spots for surface-enhanced Raman scattering (SERS). Mechanical strain increases the sizes of the gaps; this increase attenuates the electric field, and thus attenuates the SERS signal. This compounded, SERS-enhanced "piezoplasmonic" effect can be quantified using a plasmonic gauge factor, and is among the most sensitive mechanical sensors of any type. Since the graphene-nanoisland films are both conductive and optically active, they permit simultaneous electrical stimulation of myoblast cells and optical detection of the strains produced by the cellular contractions.

Figure 1

Schematic diagram for strain sensing using metallic nanoislands on graphene. The metallic nanoislands have a self-assembled monolayer of benzenethiolate chemisorbed on the surface, which produced the SERS spectra shown in the diagram. The peaks in the SERS spectra corresponding to the vibrational modes of the benzenethiolate are highlighted in blue boxes. The polarization of incident laser light was in the direction of the long axis of the substrates. The substrate on the right was subjected to a bending strain, with the graphene/nanoisland film on the top surface of the bend. The tensile strain produced by bending increased the distance between the nanoislands; increased separation decreased the plasmonic coupling and thus attenuated the SERS signal.

Figure 2

Morphology and optical spectra of nanoislands on graphene. Transmission electron micrographs (TEM) and electron diffraction patterns (insets) of gold nanoislands (AuNI, a) and silver nanoislands (AgNI, b) to provide a comparison on crystallinity. Scale bars, 100 nm. Reflectance spectra of gold (c) and silver (d) nanoislands with spectra of unstructured, continuous 100 nm metal films for comparison. The dips in reflectance of gold are lower in energy than those of silver because of unequal relativistic contractions between the two metals.

Figure 3

Polarization response of nanoislands under strain. (a) SERS spectra of gold nanoislands under 0.074% strain with incident laser light polarized parallel and perpendicular to strain. The peak of interest in chemisorbed benzenethiol is highlighted between the dashed lines (999 cm⁻¹ peak). Radial plots of gold (b) and silver (c) nanoislands, which reveal the polarization response of the SERS signal with the graphene-nanoisland film under strain. The radial plots are normalized to maximum SERS intensity of the peak indicated in panel (a).

Figure 4

Piezoplasmonic characterization of noble metal nanoislands. Plasmonic gauge factors of gold nanoislands (AuNI) and silver nanoislands (AgNI) are plotted as a function of strain.

Figure 5

SERS mapping of strain gradients using a bent glass substrate bearing silver nanoislands on graphene. (a) Bright-field image of silver nanoislands on a bent glass substrate (scale bar, 500 μm). The yellow line near the center of the image is the selected area (2100 μm × 6 μm) that was mapped using a Raman microscope. The apex of the bent substrate is marked by the dotted white lines and has a strain of 0.032%. (b) A moving average of the SERS intensity along the x-axis in the selected area under three different polarizations (0°, 45°, and 90° in respect to the x-axis). Note that the signal decreases as the polarization aligns with the principle direction of deformation (x-direction).

Figure 6

Optical detection of strain induced by electrical stimulation of C2C12 myoblast cells supported by a substrate consisting of silver nanoislands on graphene. (a) A bright-field image of C2C12 cells on the substrate, with the incident polarization of the Raman laser indicated by the black arrow. The area of illumination by the Raman laser is indicated by the black box. (b) Scanning electron micrographs of critical-point dried C2C12 cells on the substrate demonstrate the interaction of the cells with the underlying nanoislands substrate. Note the stretch marks on the substrate caused by strain. Cells were false colored blue in the micrograph. (c) A fluorescence image of C2C12 cells on the substrate with nuclei (blue), actin (green), and myosin heavy chain (red) clearly visible. (d) The SERS intensity of benzenethiolate chemisorbed to silver nanoislands in the illumination area is depicted as a function of time. The gray areas indicate time periods when the C2C12 cells were stimulated by a pulsed voltage, causing them to contract and pull silver nanoislands apart which translated to a decreased SERS signal (black squares). As a control, silver nanoislands without cells were also stimulated by pulsed voltage (red circles).