Robust Au–PEG/PS Microbeads as Optically Stable Platforms for SERS

Abstract

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for ultrasensitive chemical and biochemical analysis. Among common substrates for SERS, the most important are colloidal nanocrystal suspensions of gold, silver, and their alloys, which are produced by a variety of physical and chemical reduction methods. In a colloidal suspension, the closer interaction between the adsorbent and adsorbate allows the analyte to be naturally retained on the colloid’s surface. These mixed systems may later be studied directly by bulk SERS or cast and air-dried onto an appropriate substrate (glass or silicon wafers) and analyzed by microspectroscopic SERS. The main disadvantage of this method is the low stability of colloidal suspensions; physical (i.e., shape and size) and chemical (i.e., surface charge) properties often vary within days of preparation. Thus, in order to extend the shelf life of the colloidal suspensions, the addition of stabilizers to prevent aggregation and flocculation by increasing the electrostatic potential around the nanoparticles was reported. However, while this process stabilizes the particles, it hinders the interaction between the adsorbent and adsorbate, thus decreasing the SERS output. In recent years, many attempts have been made to deposit optically active nanoparticles on polymer microbeads in order to produce composite materials suitable for SERS. Unfortunately, such materials tend to have low nanoparticle loading and weak SERS because of limited nanoparticle accessibility in the polymer matrix. Recently, Kinoshita et al. established the extraordinary capability of poly(ethylene glycol) (PEG) to selectively retain Au. In the present paper, this property was exploited to produce a gold nanoparticle