Raman and Conductivity Analysis of Graphene for Biomedical Applications

Abstract

In this study, we present a comprehensive investigation of graphene's optical and conductive properties using confocal Raman and a Drude model. A comparative analysis between experimental findings and theoretical predictions of the material's changes and improvements as it transitioned from three-dimensional graphite is also presented and discussed. Besides spectral recording by Raman, which reveals whether there is a single, a few, or multi-layers of graphene, the confocal Raman mapping allows for distinction of such domains and a direct visualization of material inhomogeneity. Drude model employment in the analysis of the far-infrared transmittance measurements demonstrates a distinct increase of the material's conductivity with dimensionality reduction. Other particularly important material characteristics, including carrier concentration and time constant, were also determined using this model and presented here. Furthermore, the detection of micromolar concentration of dopamine on graphene surfaces not only proves that the Raman technique facilitates ultrasensitive chemical detection of analytes, besides offering high information content about the biomaterial under study, but also that carbon-based materials are biocompatible and favorable micro-environments for such detection. Such information is valuable for the development of bio-medical sensors, which is the main application envisioned for this analysis.

Keywords: Drude model; biomaterials; conductivity; confocal Raman mapping; dopamine detection; infrared absorption.

Figure 1

(a–e) Raman spectra recorded in different spots showing the behavior of the G and 2D bands for a single-layer, a few-layers, and multi-layer graphene. The Raman spectrum of bulk HOPG is also presented for comparison.

Figure 2

Confocal Raman mapping images screening: (a,d,g) the location of the G band; and (b,e,h) the location of the 2D band. Yellow pseudo-color in the images corresponds to higher intensity of the filtered Raman feature; (c,f,i) Images preformed with Cluster Analysis software (Ulm, Germany) revealing a mixture of a single-layer and a few-layers of graphene. The arrows mark the presence of a single-layer of graphene in the samples.

Figure 3

(a) Confocal Raman mapping of the G band depicting a microtube-like structure or of graphene whiskers; and (b) the associated Raman spectrum of image (a). New weak Raman vibrations, as labeled, are detected besides the characteristic graphene features.

Figure 4

(a) Far-IR transmission results of three representative samples, as labeled; (b–d) Representative fits demonstrating linear dependence of the ratio of transmittance, T(ω), to [1 − T(ω)] as a function of 1/λ² for the samples in Figure (a).

Figure 5

(a) Representative Raman spectra of standard dopamine powder, graphene and dopamine detected on a graphene surface, as labeled; (b) Confocal Raman mapping of dopamine (red) and graphene (blue). Magenta color is a combination of red and blue indicating the presence of both materials.