Understanding the biophysical characteristics of a single cell is of importance in elucidating cellular diversity and heterogeneity, as well as in determining signaling pathways and networks for self-renewal and differentiation. A particularly important aspect is in uncovering the role of the local mechanical environment, cues from which are transduced into a cascade of biochemical signals ultimately leading to precise biological responses. Many of the existing experimental methods, such as micropipette aspiration and atomic force microscopy (AFM), are invasive and either too slow to scale well to probing cellular structures or suffer from poor resolution. Therefore, the crucial role of the local mechanical environment and the accompanying cell surface biochemical responses remains poorly understood, particularly at the nanoscale. To this end, LBRC is developing plasmonic nanoparticle-coated vertical nanopillar arrays to enable mechanical loading of adherent cells and to incorporate surface-enhanced Raman spectroscopy (SERS) imaging to quantify and correlate biochemical changes to the mechanical deformation.
Mechanical trap SERS platform
Apart from the plasmonic nanoparticle-coated nanopillar platform, LBRC is also designing novel configurations for SERS sensing in other cellular and biomolecular applications. For example, a plasmonic nanostructure, termed composite scattering probes (CSPs), was designed for multimodal biosensing and features localized surface plasmon resonance (LSPR) and SERS modalities [1]. Figure 1(a) shows the fabrication process of the two configurations of CSPs - silver nanoparticles decorated gold nanoparticles (SNPG), and silica and silver thin film-decorated gold nanoparticles (STG) - using electron-beam evaporation and thermal dewetting. As a proof-of-concept study, bovine serum albumin (BSA) was used as a model analyte to demonstrate the capability of refractive index (RI) sensing and molecular analysis. As the LSPR peaks are very sensitive to the dielectric constant of the surrounding medium, the absorption spectra of the CSPs are indicative of the RI of the local environment. In conjunction with partial least squares (PLS) analysis, the CSPs are capable of differentiating the subtle RI changes caused by different concentrations of BSA, as shown in Figure 1(b). On the other hand, the SERS signals were captured using a Raman microscope, and as shown in Figure 1(c), the weak Raman signals were greatly enhanced by the nanostructure.
On the other hand, to overcome the limitation of 2D rigid substrates as used in most conventional biosensors, LBRC takes efforts to achieve 3D spatially resolved label-free biosensing by incorporating SERS with deformable micro-structures. With our collaborator Dr. David Gracias (Johns Hopkins University), the Barman laboratory has developed a graphene-based skin-like biosensing platform that can wrap around soft or irregularly shaped 3D samples [2], as well as a mechanical trap that is able to capture single live cells [3]. Take the mechanical trap as an example, it has four transparent arms composed of glasslike materials with gold nanostars coated on the inner surfaces. When the germanium (Ge) sacrificial layer is dissolved in the cell culture medium, the arms will fold under internal stress and wrap around a live cell, as shown in Figure 2(a). Figure 2(b) shows a scanning electron microscope (SEM) image of a single cell captured in a mechanical trap. An inverted confocal Raman microscope is then employed to image and profile the samples, and Figure 2(c) shows a representative spectrum from the cell. The immunofluorescent images in Figure 2(d) show that the capture cell largely conforms to the boundary of the mechanical trap. Therefore, the mechanical trap platform provides a stable orientation for single cell 3D imaging, and at the same time ensure the high quality of the signal through SERS amplification.