Monolayers of Amino Acid-Synthesized Gold Nanoparticles as SERS Substrates for Trace Chemical Sensing

Abstract

In recent years, extensive studies have been devoted to the applications of surface-enhanced Raman spectroscopy (SERS) in detecting and identifying trace amounts of various analytes. The crucial issue for SERS applications is the selection of reliable and reproducible substrates. Therefore, most studies were dedicated to their fabrication, characterization, and evaluation using selected test compounds. This article reports the results of comparative studies on the fabrication, characterization, and evaluation of SERS substrates in the form of monolayers of amino acid-synthesized gold nanoparticles (AuNPs) on silicon and glass platforms. The combination of AuNPs synthesized with five amino acids and four platforms with smooth and rough surfaces yielded 17 potential SERS substrates. SERS substrates and their components were characterized using microscopy and spectroscopy techniques. The SERS performance of fabricated substrates was evaluated using p-mercaptobenzoic acid and 1,2-bis(4-pyridyl)ethylene as test analytes. The studies showed that not all AuNPs synthesized with amino acids are suitable for fabricating efficient SERS-active substrates. The AuNPs' surface chemistry influences the fabrication and performance of SERS substrates. Among the fabricated SERS-active substrates, the highest enhancement factors (EFs) were estimated for those made with l-serine-synthesized AuNPs.

1

Overview of the fabrication process of AuNP-based SERS substrates.

2

TEM images of gold nanoparticles used in these studies.

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SEM images of SERS substrates fabricated using AuNPs synthesized with l-serine. SERS substrates were made using the following platforms: glass (upper left), frosted glass (upper right), polished silicon (bottom left), and silicon (bottom right).

4

Absorptance spectra of glass platforms (smooth and frosted) and corresponding SERS substrates fabricated using AuNPs synthesized with dl-phenylalanine (A), l-valine (B), l-(4)-hydroxyproline (C), l-serine (D), and l-lysine (E).

5

Absorptance spectra of Si platforms (smooth and rough) and corresponding SERS substrates fabricated using AuNPs synthesized with dl-phenylalanine (A), l-valine (B), l-(4)-hydroxyproline (C), and l-serine (D).

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Graphs of log₁₀ EF obtained based on the results of measurements with pMBA (left) and BPE (right).

7

SERS spectra acquired for substrates made with AuNPs synthesized with l-serine. (A) Background spectra of clean substrates, (B) SERS spectra of pMBA, and (C) SERS spectra of BPE.

8

2D maps of BPE SERS signal intensity at 1200 cm^–1 acquired for SERS substrates fabricated using Ser-synthesized AuNPs. SERS substrates were made using the following platforms: glass (upper left), frosted glass (upper right), polished silicon (bottom left), and silicon (bottom right).