PH-Triggered Molecular Alignment for Reproducible SERS Detection via an AuNP/Nanocellulose Platform.

Haoran Wei,Peter J Vikesland

doi:10.1038/srep18131

Abstract

The low affinity of neutral and hydrophobic molecules towards noble metal surfaces hinders their detection by surface-enhanced Raman spectroscopy (SERS). Herein, we present a method to enhance gold nanoparticle (AuNP) surface affinity by lowering the suspension pH below the analyte pKa. We developed an AuNP/bacterial cellulose (BC) nanocomposite platform and applied it to two common pollutants, carbamazepine (CBZ) and atrazine (ATZ) with pKa values of 2.3 and 1.7, respectively. Simple mixing of the analytes with AuNP/BC at pH < pKa resulted in consistent electrostatic alignment of the CBZ and ATZ molecules across the nanocomposite and highly reproducible SERS spectra. Limits of detection of 3 nM and 11 nM for CBZ and ATZ, respectively, were attained. Tests with additional analytes (melamine, 2,4-dichloroaniline, 4-chloroaniline, 3-bromoaniline, and 3-nitroaniline) further illustrate that the AuNP/BC platform provides reproducible analyte detection and quantification while avoiding the uncontrolled aggregation and flocculation of AuNPs that often hinder low pH detection.

Highlights

Coupling of AuNPs with BC may be a promising approach for the development of a stable low pH SERS platform since the AuNPs are immobilized within the BC matrix and both components are acid-resistant
SERS detection of carbamazepine and atrazine was achieved using this platform by lowering the solution pH to a value below the analyte’s pKa
The sampling and detection time is short (< 1 min) and the preparation procedure is simple, making this platform promising for real world application

Summary

Introduction

Coupling of AuNPs with BC may be a promising approach for the development of a stable low pH SERS platform since the AuNPs are immobilized within the BC matrix and both components are acid-resistant. AuNP/BC nanocomposites were synthesized by boiling HAuCl4-treated BC in sodium citrate (Na3Cit) solution to produce AuNPs that are fully intercalated within the BC network. These nanocomposites were tested as a SERS platform under acidic pH conditions. ATZ is widely used for weed control in corn acreage and is the most frequently detected herbicide in surface water[31]. Each of these analytes are moderately hydrophobic Each of these analytes are moderately hydrophobic (Kow > 1.5; refs. 34,35) with low pKa values and exhibit low AuNP surface affinity under neutral pH conditions

Methods

Results

Conclusion