SERS Probing of Proteins in Gold Nanoparticle Agglomerates.

Gergo Peter Szekeres,Janina Kneipp

doi:10.3389/fchem.2019.00030

Abstract

The collection of surface-enhanced Raman scattering (SERS) spectra of proteins and other biomolecules in complex biological samples such as animal cells has been achieved with gold nanoparticles that are introduced to the sample. As a model for such a situation, SERS spectra were measured in protein solutions using gold nanoparticles in the absence of aggregating agents, allowing for the free formation of a protein corona. The SERS spectra indicate a varied interaction of the protein molecule with the gold nanoparticles, depending on protein concentration. The concentration-dependent optical properties of the formed agglomerates result in strong variation in SERS enhancement. At protein concentrations that correspond to those inside cells, SERS signals are found to be very low. The results suggest that in living cells the successful collection of SERS spectra must be due to the positioning of the aggregates rather than the crowded biomolecular environment inside the cells. Experiments with DNA suggest the suitability of the applied sample preparation approach for an improved understanding of SERS nanoprobes and nanoparticle-biomolecule interactions in general.

Highlights

The aggregation of silver nanoparticles allows for the Surface-enhanced Raman scattering (SERS) detection of some proteins (Reymond-Laruinaz et al, 2016) at low concentrations in solution (Xu et al, 1999; Han et al, 2009), or re-structuring of nanostructured silver films induced by the protein molecules themselves yielded SERS spectra at surface concentrations on the order of fM per mm2 (Drachev et al, 2005)
The delivery of plasmonic nanostructures into a biological system, such as a live cell, requires the use of individual, non-aggregated nanostructures, which are known to provide very low SERS enhancement (Wang and Kerker, 1981; Joseph et al, 2011), but which are usually later processed by their biomolecular environment and form aggregates that yield high SERS enhancement (Buchner et al, 2014)
The protein concentration is higher in cells than in dilute solutions (Brown, 1991; Milo, 2013), and proteins facilitate the formation of nanoparticle aggregates both in cells and in vitro (Zhang et al, 2009; Bharti et al, 2011; Moerz et al, 2015; Fazio et al, 2016) where they move around freely due to Brownian motion (Jia et al, 2007), additional positioning of nanoaggregates formed by cells was found to occur (Ando et al, 2011; Drescher et al, 2013b)

Summary

INTRODUCTION

Surface-enhanced Raman scattering (SERS) using plasmonic metal nanoparticles has been applied to characterize complex biological samples, ranging from biomacromolecules such as nucleic acids (Kneipp and Flemming, 1986; Barhoumi and Halas, 2010) and proteins (Han et al, 2009; Blum et al, 2012; Bonifacio et al, 2014; Fazio et al, 2016) to living eukaryotic and prokaryotic cells (Kneipp et al, 2006; Ivleva et al, 2010; Aioub and El-Sayed, 2016; Fasolato et al, 2016) and tissues in whole animals (Charan et al, 2011; Kneipp et al, 2012) for several decades now. The aggregation of silver nanoparticles allows for the SERS detection of some proteins (Reymond-Laruinaz et al, 2016) at low concentrations in solution (Xu et al, 1999; Han et al, 2009), or re-structuring of nanostructured silver films induced by the protein molecules themselves yielded SERS spectra at surface concentrations on the order of fM per mm (Drachev et al, 2005). SERS of Proteins application of specially tailored plasmonic substrates (AlvarezPuebla et al, 2011) can be used to conduct efficient SERS experiments with protein solutions These situations are different from probing nanoparticle-biosystem interfaces without protein (or biomolecule) purification. We prepared samples of biomolecule solutions with gold nanoparticles varying in concentration by seven orders of magnitude This preparation allows for the free protein corona formation, in analogy to the situation in cells and culture media. The results enable a discussion about the possible formation of gold nanoaggregates with high SERS enhancement in very crowded molecular environments such as the compartments of animal cells

MATERIALS AND METHODS

Findings

RESULTS AND DISCUSSION